Tuesday evening lecture with Stan Bentvelsen
CC Attribution - NonCommercial - NoDerivatives 3.0 Germany:
You are free to use, copy, distribute and transmit the work or content in unchanged form for any legal and non-commercial purpose as long as the work is attributed to the author in the manner specified by the author or licensor.
Bentvelsen, Stan
Professor Stan Bentvelsen is programme leader of the FOM programme 'Exploration of new phenomena at the highest energy frontier with D0 and ATLAS' at Nikhef and is closely involved in the Higgs research at CERN. Bentvelsen studied Theoretical Physics at the University of Amsterdam and completed his PhD cum laude in experimental high-energy physics there in 1994. From 1994 until 2000 he was a staff researcher at CERN in Geneva. He then became a senior scientist at FOM-Nikhef. Since 2005 he has been Professor of 'Collider physics at the LHC' at the University of Amsterdam and programme leader at FOM. Bentvelsen is also a member of the FOM Governing Board.
 The big question is we have two particles tonight the glass is it half empty or half full and Stamm-Benfeldsen will tell us all about it We move on to a to a second particle That I'd like to present to you. I'm very happy that I am able here to present to you the results that we obtained With our detector the atlas detector And I'll be speaking on behalf of our atlas group and what you see here on the cover is Is the well the observation of the Higgs particle? I put some question mark there because we are not so sure maybe as a previous speaker that this is the The particle that you are looking for but the evidence that was also alluded to is indeed something which is close to six sigmas and not five and But I think here we indeed play a completely different kind of game This is indeed large science Last year we were celebrating the 20th birthday of the atlas collaboration and imagine that you had to have the vision in 1992 to make the letter of intent for atlas It was approved by the council of CERN in 1994 We started installing some ten years ago or so and it was done in 2008 and then we took data and I'd like to share with you some of the things that we saw in that data But before doing that I also like to present The group of people from the Netherlands that are involved in this kind of research and really experimental particle physics is teamwork I'm here on behalf of my team, but the people that I that are really doing the hard work I would like to refer to the tomorrow sessions Focus session Higgs into focus and the parallel session Subatomic physics because there you can hear all the details and all the nitty-witty Kind of hard work that has been going on in order to achieve this this fact So we are a collaboration of a form Nick F the robot University and and the University of Amsterdam So let's start by doing some particle physics and particle physics. I mean particles without any size without any form factor without just point like particles that can be described by mathematical points and We have to dive deep inside the the the nucleon to To encounter the quarks here the up and the down quark as you well know and the leptons the neutrino and the electron But those are not the only particles that we know of because in the in the last many years decades via all sorts of accelerators we know that this has to be extended by by New families of particles that have been discovered over the years for example the charm and the strains and the top and the bottom the top was only discovered in 1995 at the Tevoton in Chicago and The the town of Trino was only made visible Five years later. So but these are then the basic constituents of of matter interesting but maybe much more interesting is how they interact with each other and We know that there are four fundamental forces playing a role in in physics I listed them here from you for you Electromagnetism weak interactions strong interactions and gravitation and over the years again. We have created Quantum field theories to in order to describe them and for example in 1948 by Feynman and others to monago and Swinger managed to write down a quantum field theory of Electromagnetism where you see that the messenger for the force is the photon Well known much later Also, the strong interaction was made in a quantum field theory where the gluon also Marslas is mediating the strong interaction but weak interaction posed more problems and it was only in 1967 or so that glass how Weinberg and Salah managed to write down a theory describing in one go Electromagnetism and weak interaction by introducing the W and the Z bosons and the W and the Z bosons They are not so light. They are actually pretty massive 80 91 times the proton Mars or so and and That caused problems the problems are partly solved. No, let me first Discuss with you That this this model is actually the standard model we call it the standard model it you can write it down on a cup of tea and and and it very elegantly summarizes all these very fundamental forces based on symmetries on gate symmetries and The model was made consistent around 1970 or so By the renormalization program of Feldman and at Holt and they rightfully were awarded the Nobel Prize in 1999 and when they were doing at our their Renormalization program they discovered that they can that can easily be done not easily but could be done for Marslas particles But not for the massive particles. So the W and the Z bosons being massive were causing problems but then it was noted that somewhat earlier already in 1964 and Another way of dynamically generating Mars was on the market described by Peter Hicks François Englert and Robert Brout and That was put in the standard model in order to give the W's and the Z bosons and also the fermions later on actually a mass and By the discovery of the Z and the W bosons in 1983 That standard model as it was then called was made into a big success And also there was a Nobel Prize given to a Dutch colleague by Simon van der Meer and also Carlo Rubia So what is then this Hicks idea and the idea is what I told is based on on Symmetries and the idea is that you need the gate symmetry in order to preserve Renormalization and that we call in an unbroken phase But in an unbroken symmetric phase the particles cannot have a mass and that would mean that they would move with the speed of light Every particle that we know of in an empty vacuum The idea is then in a broken phase in the vacuum there can exist a field the Hicks field here visualized by this somewhat wavy kind of of background field and particles do actually interact with this background field and by Interacting with this background field they act effectively acquire a mass and one particle is interacting a bit stronger with this background field than the other one and thereby Effectively acquiring a different mass than the other one so masses are described now in a completely different way Masters are described as interactions interactions with an omnipresent Hicks field But in order to see if this description makes sense because here you have now a description with which you can solve the problems of renormalization in the standard model, but The consequence of this description is that also the vacuum itself can fluctuate and This is then the smoking gun for this whole description Which means that there is an excitation of this vacuum field and this is what we call the Hicks particle and The Hicks particle is actually the only particle because it's a scalar. It's a boson That does have a mass in in the standard model, but the trouble is that we don't know the mass It's not predicted by theory So that means that we basically have to look everywhere in order to see if this whole picture that I present here Doesn't make sense or not So in order to do that You well aware and it was also alluded to previously already We have this magnificent accelerator being constructed many years ago already with 27 kilometers in circumference and and the big Detectors Visualizing and detecting the collisions the up last detector and the counterpart the CMS detector because we do want to be pretty sure if we have a measurement that it's also measured independently by an independent group of scientists in our case the CMS people and There are two big challenges in the construction of this of this Accelerator the first challenge is really to get ultimate high energies in order to create Heavy new resonances heavy particles and indeed it's in the TEV range The design is seven thousand to seven thousand GV proton to proton collisions and in order to do so you have to really Keep the particles in their path in the circular path and therefore you need the magnets and here you see one of the Diplomagnets that is doing that one of the 1234 Diplomagnets that are installed in the accelerator It will be or they're cooled to one point nine Kelvin with the liquid helium and they ultimately give a magnetic field of eight point three Tesla and to impress you a little bit one third of the helium supply of of the whole world is actually in this machine and Here you see on the on the right downside you see the two little holes something like a five centimeters in diameter One with a magnetic field going upward the other one with a magnetic field going downward so that the protons can move in opposite directions through each of these tubes and Then comes the second challenge and the second challenge is really to get an awful lot of collisions and We call that luminosity the brightness of the beam The amount of collisions that that we'd like to observe You have to imagine that if you collide two protons head on the chances of actually producing an Higgs particle there is Ten orders of magnitude below having a cross-section at all so it's really looking for a needle in something like ten million haystacks or something and So in order to increase on the luminosity The particles are actually accelerated in bunches each bunch consisting of ten to eleven protons and the buns Is followed by another buns some 15 nanoseconds or something like 16 meters or so further down And so in this machine a whole train of bounces are going clockwise and a whole train of bounces are going anti-clockwise and the bounces of protons they actually they Go past each other having a chance of a proton for one buns hitting a proton of the other buns and the rest Continuing to make a new chance in the next round so What do you do when you operate such a machine? It's you operate it 24 hours seven days a week Between March and December. It means you have to fill the machine at low energy at injection energy You have to ramp it up to the energy the high ultimate energy you start the collisions And that is a stable configuration of something like ten hours or so and after the ten hours the Intensity of the beams has degraded a little bit due to the lifetime the particles has just disappeared And then you recycle and the recycling takes typically a few hours and then you restart the operation again and And all the time the Atlas detector is taking data taking collisions whenever LHC gives you a call saying hey listen we have a stable beam here and So what what you're doing then is looking at at the end of the day individual collisions between protons but protons are of course build out of of quarks and and partons and the interesting physics that we Obtain here is when actually two quarks or two partons of this proton make the collision that makes the heart collision Where some new physics is is hopefully Happening and you see on the right side the plot where the time goes to the right how we visualize the two protons Hitting each other producing new physics Now you have to detect the decay products of the collision And for that we build the Atlas detector and here you see a Part of the Atlas detector with the beam a perpendicular Here's a perpendicular to the beam pipe with the beam pipe at the downside of this picture and like onion rings Various sub detectors subsequently putting one after the other So in the very heart of the detector you find the tracking chambers Which are measuring the direction and the momentum of the charged particles Because they are metatinar magnetic field then the calorimeters where you stop the particles like electrons and photons in order to measure their Energy and then there is one particular particle the muon 200 times as heavy as an electron Basically that is able to penetrate through the calorimeters and hitting at the very outside of the detector the muon chambers Now this is a simile or this is a picture of the Atlas detector. You have to mention 44 meters long and 22 meters high detector where you see here a simulation of a of an event. This is not a real event and The the involvement of of Nikf in this project is Both an industry in instrumentation in R&D for developing new ways of detecting particles We we are heavily involved in electronics in the calibration Now we are heavily involved in the analysis in the software development in the computing Etc. So there is whole bunch of activities that we are actually doing in this detector And if you make it more concrete it means that we have built part of the muon precision chambers. We have The end cap toys from the industry we have a semi conducting tracker and cap Made it in Amsterdam and put it downstairs In Geneva involved in the trigger and the data acquisition tracking and in an analysis wise We're involved in supersymmetry searches Higgs physics as I will tell you about and top quark physics. So Ten years ago or so We were in the face that we could really enter the the cavern and start building the thing and here is a very Quick movie on this period So here you see the the barrel toy is being built up that is a part of the muon system And that gives that very characteristic a view of the Atlas detector where you inside see the calorimeter being installed And There's some movements up and down in order to be able to put everything in together here you see the calorimeter inside the the magnet being put in so that's a quite a large Effort and also as I told you at Nick F. We have done our Not only fair share but actually quite a lot which you see here some impression on the on the people that have been doing that for Some some time and I couldn't resist showing this plot of stanko Which is one of our PSD students that are actually installing one of the calibration systems for the for the muon chambers so at one point in 2008 or so the the detector and I'm seen were all done and then we had this this famous Discovery Sorry No, the discovery it is a disaster One of the now it's pretty serious actually the two of these long dipole magnets they are joined up by by electric by by an electric joint or yeah, and There's something went wrong and was a spark developing and the spark Actually started to blow up or melt down part of the pipe where the liquid helium was in and liquid helium started to escape and it was a pressure wave inside the Collider itself and something like 400 meters were of the accelerator were destroyed and It took us a year or so to recover from this some hundred dipole magnets were really effective affected by this by this incident That means that after a year when we were fully recovered The LHC is not running at the design energy of 7000 gv per beam But half of its energy three and a half thousand and in this year or sorry in 2012 4,000 gv and We will be stopping at the program next month in order to install safety valves and to make More adjustments in order to restart the machine at the end of 2014 with a full energy So we're going in a shutdown period for a while Nevertheless 2011 it was extremely successful the the Expectations for the luminosity were exceeded by a factor of five or so and again last year in 2012 The luminosity was exceeded by by expectations. It means that in total we have something now like 1.8 times tends to the 15 collisions Viewed by the Atlas detector the detector did an excellent Was performing excellently data taking efficiencies up to 94 percent 96 percent of the channels operational. These are all the things that need to be did gritty details that we are worrying about and What makes this so successful and and and look at the harsh environment here? You see actually an overlapping Bunch crossing I was telling you that in the bounce crossing you have a chance of one proton hitting another proton But that is actually not the case at all Here you see an overlapping 20 protons or 27 collisions in one bounce crossing and one of them is actually doing some interesting physics in one of them a Zebo zone is being produced, but now you have to figure out what it is and you have to get rid of the underlying physics So what we do when we analyze our data is the following we have something like 20 to 20 million collisions a second This these are observed with the Atlas detector We have an online triggering system which spits out at a rate of 600 Hertz Events that are selected by one of the many many trigger menus over a thousand trigger Possibilities that we have and that means that we are storing something like a gigabyte of data per second This data is then spread out over the world in ten centers one of them being Sarah and Nick have in Amsterdam And so that we can then look at the data from there and do our analysis And this is what the PhD students typically are doing when they make the analysis Now here's the here's an example This is actually quite a cute example because here you see in the detector if you see the Atlas detector as I told you the Mughals they are penetrating through coming Completely out of the detector. So these are represented by the headlines in this in this event display. This is real data and You see that the two muons actually are a signature for a zebo zone So here's a zebo zone produced and what you see is if you calculate the event mass of the two muons You actually get a mass very close to the zebo zone and so That's a nice observation, but we're not so interested in zebo zones We know that the zebo zone is there and we have found it back, but we wanted to look for the Higgs particle now So here's the status of of the Higgs particle before we started doing collisions on the line below you see somewhere between zero and 500 gv Let's say the range in which we expect the Higgs particle to exist Indicating the mass of the proton the mass of the W and the zebo zone a gold atom and also the red area The area which has been excluded already before LHC started up to 114 gv or so so When you make Higgs particles or when you search for Higgs particles you first have to produce Higgs particles if it's there now the prediction with the data for 2012 is Depending on its mass which we don't know something like 200,000 or 77,000 Of those Higgs particles out in this 10 to the 18 million collisions now the Higgs particle once you have A created one doesn't exist for a long time it actually decays and It has various possibilities of decaying so one of the promising golden channels is the decay to two z particles that Subsequently decay to four Laptons and you see here on the bottom side you see let's say the sensitivity of this channel as a function of the Higgs mass So if it's somewhere in the blue curve on the blue line We will be sure that we find the Higgs particle if it has a mass in that area But that's not the only channel another possibility is when the Higgs decays into two photons and you see that that has a completely different sensitivity actually what we call a low mass sensitivity around 140 120 or so and then a third channel which is where Higgs decays into two W bosons decaying into two left ones and there is a bit of a problem Because the W's also decaying to neutrinos and that you cannot measure so we measure the neutrinos only through missing energy now you see also the nicker flags for the channels that we have put Emphasis on within our group so the nine Megan group is very active for example, so in the Higgs to the WW and Higgs to CC as well Now in the first round of data in 2011 we were able to exclude the existence of this Higgs particle for a very long range of its possible mass before you started off Somewhere between hundred and forty one up to something like four hundred and seventy gv We knew from the data that we took in 2011 that it was not there so that leaves only a tiny gap Let's say in between hundred and forty nine hundred and forty one to see if there is a new particle Now here's an example of a display where you see a Higgs to WW Going to two leptons in this case an electron and a muon and two neutrinos which you cannot see So that is what we call missing energy and from here you have to reconstruct back the kinematics in order to see if you can Find back the Higgs mass not so trivial much more straightforward is when the Higgs is decaying into two Z bosons and the two Z bosons Subsequently decayed to two four leptons of four muons and here you see a beautiful example actually of a collision where indeed you see four of those muons in the final state and And when you calculate the invariant mass of these four muons you will find that this adds up to 125.1 gv But this is one event and that's not Significant enough as you can imagine but we found others here's another one where you have again four of those muons In in the final state and it's in this case the mass of the four muons add up 224.6 gv or so So Having having all this data in your hands you start to make distributions of course and you know that we are looking in a certain window for masses so Here is then the plot of the invariant mass on the horizontal axis of the four leptons I gave you two examples and here's the plot that is covering the range between zero and 250 gv and What we do is we blind our data first and not in order to To be to biased so what you see here is the blinded data before Unblinded What you see here is the data itself in the red histogram that you see is Calculations that we did Where we also have four muons in the final state of four leptons I should say in the final state But they are not coming from a Higgs particle at all. They are the background So this is the backgrounds for continuous zz background And you see some sort of a little Feynman diagram on the top there which which shows you that you can have also Contributions where there's no Higgs at all involved in the game, but you still have the same signature in your detector so you have to disentangle the two and at one point we started to unblind and then Low and behold we found something which you can say is a small signal And significance of 3.6 sigma you see that it basically consists of something like 11 events that are above the red background which is Which is significant 3.6, but we couldn't claim a discovery We were looking also at other channels here is a channel where you have a Higgs decaying into two of these photons and these photons They are They don't carry any charge So what they do is they leave their signature in the calorimeter. These are the yellow blobs that you see in this in this event display and the with the red circles and what you see is the the yellow blobs you can measure the energy of the photons by this absorption in the Chlorimeter and because you know the direction you can calculate the event mass again and what you find is here a mass of 126 GeV this is not at all trivial because there are many processes that also lead to Two photons in the final state we background again And there are basically two important ways of background you have fake photons for example a pion if you know what a pion is a Pi zero can decay into two photons that are very collinear and it's not so easy to disentangle a POMP photon from from a decaying pion also you have prompt photons again not from Higgs Decays as I show him in the plot here down below So effectively what you do if if you calculate the event mass of the whole set you get a distribution like this and you see that It is a completely different behavior that was what I showed before in a few for mules But again as a function of the Higgs mass you see now a continuum, but we're looking for peaks here and indeed if you fit the continuum and subtracted from From the observation you will see a little peak here with a significance of something like four and a half sigma Now You can combine all this information at this one and the previous channels and then at some point You can calculate the data significance for each mass and this is the situation of July 4th in 2012 where you see the data significance as a function of the Higgs mass all the range between 110 GV to 500 something GV The P zero value. I think you know that And you see a dip there a dip which is Reaching something like six sigma and the dip means that you cannot describe the data without having anything anything new in it And this is what we then call a discovery five sigma But by claiming a truth Discovery we also have to look at our friends from the CMS collaboration and on that date We were extremely curious to what they were showing and indeed they were also having at the very same spot 126 or so GV a Significance of five sigma so that led to this to this observation of the CERN director general I would now say I Think we have it You agree With your For me it's Really an incredible thing that it happened in my lifetime I know Pete Higgs a little bit that he was completely cute Flabberg acid by the fact that we found this particle so soon and also in in the Netherlands We had our we had our fair share of celebrating and we have to suddenly deal with media which we didn't do so much before So what happened since July 4th now the question is is it the standard model Higgs we now in the in the phase in which we are really in very much detail Go and find if if the predictions of the standard model once we know the mass are actually Valid so what you see here is a plot. It's difficult to read maybe but you see the different decay channels and Higgs to gamma gamma and Higgs to zizi and and the expectation from the standard model is given by the Vertical line and you see that the data is not on the vertical line Very nicely so we are now in an intermediate states like is it something that we don't understand or do we just need more collisions Realize that we have something like two-thirds of the total data set unblinded So we still have new data on the shelves that we haven't looked at something else very intriguing What we found actually not so long ago is if you look at the masses of the Higgs particle when it decays into two photons or the masses of the Higgs particle when it decays to four left ons we don't find the same mass back and The mass difference is something like three three and a half gv a two and a something sigma Is that significant enough to claim that maybe we are not dealing with a simple standard model Higgs something more complex We don't know yet, but this is clearly something that's for which we need more data. I think that This this discovery is actually one of the most important discoveries in particle physics for the last 42 years or so We have definitely observed the particle But if it's really Higgs, we don't know completely for sure yet But in the whole LHC program imagine that only 1% of the data has been delivered at this moment The program continues for many more years to come in a near future The energy will be doubled We will doing in-depth Higgs studies and we will search for beyond the standard model physics and in the far future Our ambitions to is going for very high luminosities at the LHC and maybe doing high precision physics in an E plus E minus collider So that leads me to my very last slide I think that this Discovery was really a major leap in understanding, but there are new questions that are now Getting very urgent. We really have to find out what are the quantum numbers of the particles Are there more Higgs particles or are there any other particles? Is it elementary or is it the composite? how does it couple to itself and What we also don't know because it's a scalar particle What actually screens it mass from being at the Planck scale and there's all there's a whole connection to cosmology And maybe it's nice to quote what Feldman always used to tell in his in his lectures on Higgs physics You know you have this Higgs vacuum had this Higgs field which also contains energy and if you start calculating What is the curvature of this of this? Effect of having energy in our universe it will mean that the universe would shrink to the size of a football and It clearly means that we lack understanding of what's really going on at this very fundamental level. So thank you very much
 The big question is we have two particles tonight the glass is it half empty or half full and  Stamm-Benfeldsen will tell us all about it  We move on to a to a second particle  That I'd like to present to you. I'm very happy that I am  able here to present to you the results that we obtained  With our detector the atlas detector  And I'll be speaking on behalf of our atlas group and what you see here on the cover is  Is the well the observation of the Higgs particle?  I put some question mark there because we are not so sure maybe as a previous speaker that this is the  The particle that you are looking for but the evidence that was also alluded to is indeed something which is  close to six sigmas and not five and  But I think here we indeed play a completely different kind of game  This is indeed large science  Last year we were celebrating the 20th birthday of the atlas collaboration and imagine that you had to have the vision in  1992 to make the letter of intent for atlas  It was approved by the council of CERN in 1994  We started installing some ten years ago or so and it was done in  2008 and then we took data and  I'd like to share with you some of the things that we saw in that data  But before doing that I also like to present  The group of people from the Netherlands that are involved in this kind of research and really experimental particle physics is teamwork  I'm here on behalf of my team, but the people that I that are really doing the hard work  I would like to refer to the tomorrow sessions  Focus session Higgs into focus and the parallel session  Subatomic physics because there you can hear all the details and all the nitty-witty  Kind of hard work that has been going on in order to achieve this this fact  So we are a collaboration of a form Nick F  the robot University and and the University of Amsterdam  So let's start by doing some particle physics and particle physics. I mean particles without any  size without any form factor without just point like  particles that can be described by mathematical points and  We have to dive deep inside the the the nucleon to  To encounter the quarks here the up and the down quark as you well know and the leptons the neutrino and the electron  But those are not the only particles that we know of because in the in the last many years decades  via all sorts of accelerators we know that this has to be extended by by  New families of particles that have been discovered over the years for example the charm and the strains and the top and the bottom  the top was only discovered in 1995 at the Tevoton in Chicago and  The the town of Trino was only made visible  Five years later. So but these are then the basic constituents of of matter  interesting but maybe much more interesting is how they interact with each other and  We know that there are four fundamental forces  playing a role in in physics  I listed them here from you for you  Electromagnetism weak interactions strong interactions and gravitation and over the years again. We have  created  Quantum field theories to in order to describe them and for example in 1948 by Feynman and others to monago and Swinger  managed to write down a quantum field theory of  Electromagnetism where you see that the messenger for the force is the photon  Well known much later  Also, the strong interaction was made in a quantum field theory where the gluon also Marslas is  mediating the strong interaction  but weak interaction posed more problems and it was only in  1967 or so that glass how Weinberg and Salah managed to write down a theory describing in one go  Electromagnetism and weak interaction by introducing the W and the Z bosons and the W and the Z bosons  They are not so light. They are actually pretty massive 80 91 times the proton Mars or so and  and  That caused problems the problems are partly solved. No, let me first  Discuss with you  That this this model is actually the standard model we call it the standard model it you can write it down on a cup of tea and  and and it very elegantly  summarizes all these very fundamental forces based on symmetries on gate symmetries and  The model was made consistent around 1970 or so  By the renormalization program of Feldman and at Holt and they rightfully were awarded the Nobel Prize in 1999 and  when they were doing at our  their  Renormalization program they discovered that they can that can easily be done not easily but could be done for Marslas particles  But not for the massive particles. So the W and the Z bosons being massive were causing problems  but then it was noted that somewhat earlier already in 1964 and  Another way of dynamically generating Mars was on the market  described by Peter Hicks  François Englert and Robert Brout and  That was put in the standard model in order to give the W's and the Z bosons and also the fermions later on  actually a mass and  By the discovery of the Z and the W bosons in 1983  That standard model as it was then called was made into a big success  And also there was a Nobel Prize given to a Dutch colleague by Simon van der Meer and also Carlo Rubia  So what is then this Hicks idea and the idea is what I told is based on on  Symmetries and the idea is that you need the gate symmetry in order to preserve  Renormalization and that we call in an unbroken phase  But in an unbroken symmetric phase the particles cannot have a mass and that would mean that they would move with the speed of light  Every particle that we know of in an empty vacuum  The idea is then  in a broken phase in the vacuum there can exist a field the Hicks field here  visualized by this somewhat wavy kind of of background field and  particles do actually interact with this  background field and by  Interacting with this background field they act effectively acquire a mass and one particle is interacting a bit stronger with this background field than the other one and  thereby  Effectively acquiring a different mass than the other one so masses are described now in a completely different way  Masters are described as interactions interactions with an omnipresent Hicks field  But in order to see if this description makes sense because here you have now a description  with which you can solve the problems of renormalization in the standard model, but  The consequence of this description is that also the vacuum itself can fluctuate and  This is then the smoking gun for this whole description  Which means that there is an excitation of this vacuum field and this is what we call the Hicks particle and  The Hicks particle is actually the only particle because it's a scalar. It's a boson  That does have a mass in in the standard model, but the trouble is that we don't know the mass  It's not predicted by theory  So that means that we basically have to look everywhere in order to see if this whole picture that I present here  Doesn't make sense or not  So in order to do that  You well aware and it was also alluded to previously already  We have this magnificent accelerator being constructed many years ago already with  27 kilometers in circumference and and the big  Detectors  Visualizing and detecting the collisions the up last detector and the counterpart the CMS detector because we do want to be pretty sure  if we have a measurement that it's also measured independently by an independent group of scientists in our case the CMS people and  There are two big challenges in the construction of this of this  Accelerator the first challenge is really to get ultimate high energies in order to create  Heavy new resonances heavy particles and indeed it's in the TEV range  The design is seven thousand to seven thousand GV proton to proton collisions and in order to do so you have to really  Keep the particles in their path in the circular path and therefore you need the magnets and here you see one of the  Diplomagnets that is doing that one of the  1234  Diplomagnets that are installed in the accelerator  It will be or they're cooled to one point nine Kelvin with the liquid helium and they ultimately give a magnetic field  of eight point three Tesla and to impress you a little bit  one third of the  helium supply of of the whole world is actually in this machine and  Here you see on the on the right downside you see the two little holes something like  a five centimeters in diameter  One with a magnetic field going upward the other one with a magnetic field going downward  so that the protons can move in opposite directions through each of these tubes and  Then comes the second challenge and the second challenge is really to get an awful lot of collisions and  We call that luminosity the brightness of the beam  The amount of collisions that that we'd like to observe  You have to imagine that if you collide two protons head on the chances of actually producing an Higgs particle there is  Ten orders of magnitude below having a cross-section at all  so it's really looking for a needle in something like ten million haystacks or something and  So in order to increase on the luminosity  The particles are actually accelerated in bunches each bunch consisting of ten to eleven protons and the buns  Is followed by another buns some 15 nanoseconds or something like 16 meters or so further down  And so in this machine a whole train of bounces are going clockwise and a whole train of bounces are going anti-clockwise  and the bounces of protons they actually they  Go past each other having a chance of a proton for one buns hitting a proton of the other buns and the rest  Continuing to make a new chance in the next round  so  What do you do when you operate such a machine? It's you operate it 24 hours seven days a week  Between March and December. It means you have to fill the machine at low energy at injection energy  You have to ramp it up to the energy the high ultimate energy you start the collisions  And that is a stable configuration of something like ten hours or so and after the ten hours the  Intensity of the beams has degraded a little bit due to the lifetime the particles has just disappeared  And then you recycle and the recycling takes typically a few hours and then you restart the operation again and  And all the time the Atlas detector is taking data taking collisions  whenever LHC gives you a call saying hey listen we have a stable beam here and  So what what you're doing then is looking at at the end of the day individual collisions between protons  but protons are of course  build out of of quarks and and partons and the  interesting physics that we  Obtain here is when actually two quarks or two partons of this proton make the collision that makes the heart collision  Where some new physics is is hopefully  Happening and you see on the right side the plot where the time goes to the right how we visualize the two protons  Hitting each other producing new physics  Now you have to detect the decay products of the collision  And for that we build the Atlas detector and here you see a  Part of the Atlas detector with the beam a perpendicular  Here's a perpendicular to the beam pipe with the beam pipe at the downside of this picture and like onion rings  Various sub detectors subsequently putting one after the other  So in the very heart of the detector you find the tracking chambers  Which are measuring the direction and the momentum of the charged particles  Because they are metatinar magnetic field then the calorimeters where you stop the particles like electrons and photons in order to measure their  Energy and then there is one particular particle the muon  200 times as heavy as an electron  Basically that is able to penetrate through the calorimeters and hitting at the very outside of the detector the muon chambers  Now this is a simile or this is a picture of the Atlas detector. You have to mention 44 meters  long and 22 meters high detector where you see here a simulation of a of an event. This is not a real event and  The the involvement of of Nikf in this project is  Both an industry in instrumentation in R&D for developing new ways of detecting  particles  We we are heavily involved in electronics in the calibration  Now we are heavily involved in the analysis in the software development in the computing  Etc. So there is whole bunch of activities that we are actually doing in this detector  And if you make it more concrete it means that we have built part of the muon precision chambers. We have  The end cap toys from the industry we have a semi conducting tracker and cap  Made it in Amsterdam and put it downstairs  In Geneva involved in the trigger and the data acquisition tracking and in an analysis wise  We're involved in supersymmetry searches Higgs physics as I will tell you about and top quark physics. So  Ten years ago or so  We were in the face that we could really enter the the cavern and start building the thing and here is a very  Quick movie on this period  So here you see the the barrel toy is being built up that is a part of the muon system  And that gives that very characteristic a view of the Atlas detector where you inside see the  calorimeter being installed  And  There's some movements up and down in order to be able to put everything in together here you see the calorimeter inside the  the magnet  being put in so that's a quite a large  Effort and also as I told you at Nick F. We have done our  Not only fair share but actually quite a lot which you see here some impression on the on the people that have been doing that for  Some some time and I couldn't resist showing this plot of stanko  Which is one of our PSD students that are actually installing one of the calibration systems for the for the muon chambers  so at one point in  2008 or so the the detector and I'm seen were all done and then we had this this famous  Discovery  Sorry  No, the discovery it is a disaster  One of the now it's pretty serious actually the two of these  long dipole magnets they are joined up by  by electric by by an electric joint or yeah, and  There's something went wrong and was a spark developing and the spark  Actually started to blow up or melt down part of the pipe where the liquid helium was in and liquid helium  started to escape and it was a pressure wave inside the  Collider itself and something like 400 meters were of the accelerator were destroyed and  It took us a year or so to recover from this some hundred dipole magnets were really effective affected by this  by this incident  That means that after a year when we were fully recovered  The LHC is not running at the design energy of 7000 gv per beam  But half of its energy three and a half thousand and in this year or sorry in 2012 4,000 gv and  We will be stopping at the program next month  in order to  install safety valves and to make  More adjustments in order to restart the machine at the end of 2014 with a full energy  So we're going in a shutdown period for a while  Nevertheless 2011 it was extremely successful the the  Expectations for the luminosity were exceeded by a factor of five or so and again last year in 2012  The luminosity was exceeded by by expectations. It means that in total we have something now  like 1.8 times tends to the 15 collisions  Viewed by the Atlas detector  the detector did an excellent  Was performing excellently data taking efficiencies up to 94 percent  96 percent of the channels operational. These are all the things that need to be did gritty details that we are worrying about and  What makes this so successful and and and look at the harsh environment here? You see  actually an overlapping  Bunch crossing I was telling you that in the bounce crossing you have a chance of one proton hitting another proton  But that is actually not the case at all  Here you see an overlapping  20 protons or 27 collisions in one bounce crossing and one of them is actually doing some interesting physics in one of them a  Zebo zone is being produced, but now you have to figure out what it is and you have to get rid of the underlying physics  So what we do when we analyze our data is the following we have something like 20 to 20 million collisions a second  This these are observed with the Atlas detector  We have an online triggering system which spits out at a rate of 600 Hertz  Events that are selected by one of the many many trigger menus over a thousand trigger  Possibilities that we have and that means that we are storing something like a gigabyte of data per second  This data is then spread out over the world in ten centers one of them being Sarah  and Nick have in Amsterdam  And so that we can then look at the data from there and do our analysis  And this is what the PhD students typically are doing when they make the analysis  Now here's the here's an example  This is actually quite a cute example because here you see in the detector if you see the Atlas detector as I told you the  Mughals they are penetrating through  coming  Completely out of the detector. So these are represented by the headlines in this in this event display. This is real data and  You see that the two muons actually are a signature for a zebo zone  So here's a zebo zone produced and what you see is if you calculate the event mass of the two muons  You actually get a mass very close to the zebo zone  and so  That's a nice observation, but we're not so interested in zebo zones  We know that the zebo zone is there and we have found it back, but we wanted to look for the Higgs particle now  So here's the status of of the Higgs  particle before we started doing collisions on the line below you see somewhere between zero and 500 gv  Let's say the range in which we expect the Higgs particle to exist  Indicating the mass of the proton the mass of the W and the zebo zone a gold atom and also the red area  The area which has been excluded already before LHC started up to 114 gv or so  so  When you make Higgs particles or when you search for Higgs particles you first have to produce Higgs particles if it's there now the prediction with the data for  2012 is  Depending on its mass which we don't know something like 200,000 or 77,000  Of those Higgs particles out in this 10 to the 18 million collisions  now the Higgs particle once you have  A created one doesn't exist for a long time it actually decays and  It has various possibilities of decaying so one of the promising golden channels is the decay  to two z particles that  Subsequently decay to four  Laptons and you see here on the bottom side you see let's say the sensitivity of this channel as a function of the Higgs mass  So if it's somewhere in the blue curve on the blue line  We will be sure that we find the Higgs particle if it has a mass in that area  But that's not the only channel another possibility is when the Higgs decays into two photons and you see that that has a completely different  sensitivity actually what we call a low mass sensitivity around  140 120 or so and then a third channel which is where Higgs decays into two  W bosons decaying into two left ones and there is a bit of a problem  Because the W's also decaying to neutrinos and that you cannot measure so we measure the neutrinos only through missing energy  now you see also the nicker flags for the channels that we have put  Emphasis on within our group so the nine Megan group is very active for example, so in the Higgs to the WW and  Higgs to CC as well  Now in the first round of data in  2011 we were able to exclude the existence of this Higgs particle for a very long range of its possible mass before you started off  Somewhere between hundred and forty one up to something like four hundred and seventy gv  We knew from the data that we took in  2011 that it was not there so that leaves only a tiny gap  Let's say in between hundred and forty nine hundred and forty one to see if there is a new particle  Now here's an example of a display where you see a Higgs to WW  Going to two leptons in this case an electron and a muon and two neutrinos which you cannot see  So that is what we call missing energy and from here you have to reconstruct back the kinematics in order to see if you can  Find back the Higgs mass not so trivial  much more  straightforward is when the Higgs is decaying into two Z bosons and the two Z bosons  Subsequently decayed to two four leptons of four muons and here you see a beautiful example actually of a collision  where indeed you see four of those muons in the final state and  And when you calculate the invariant mass of these four muons you will find that this adds up to  125.1 gv  But this is one event and that's not  Significant enough as you can imagine but we found others here's another one where you have again four of those muons  In in the final state and it's in this case the mass of the four muons add up  224.6 gv or so  So  Having having all this data in your hands you start to make distributions of course and you know that we are looking in a certain window for masses  so  Here is then the plot of the invariant mass on the horizontal axis of the four leptons  I gave you two examples and here's the plot that is covering the range between zero and 250 gv and  What we do is we blind our data first and not in order to  To be to biased so what you see here is the blinded data before  Unblinded  What you see here is the data itself in the red histogram that you see is  Calculations that we did  Where we also have four muons in the final state of four leptons I should say in the final state  But they are not coming from a Higgs particle at all. They are the background  So this is the backgrounds for continuous zz background  And you see some sort of a little Feynman diagram on the top there which which shows you that you can have also  Contributions where there's no Higgs at all involved in the game, but you still have the same signature in your detector  so you have to disentangle the two and  at one point we started to unblind and then  Low and behold we found something which you can say is a small signal  And significance of 3.6 sigma you see that it basically consists of something like 11  events that are above the red background which is  Which is significant 3.6, but we couldn't claim a discovery  We were looking also at other channels here is a channel where you have a Higgs decaying into two of these photons and these photons  They are  They don't carry any charge  So what they do is they leave their signature in the calorimeter. These are the yellow blobs that you see in this in this  event display and  the  with the red circles and what you see is the the yellow blobs you can measure the energy of the photons by this absorption in the  Chlorimeter and because you know the direction you can calculate the event mass again and what you find is here a mass of  126 GeV this is not at all trivial because there are many processes that also lead to  Two photons in the final state we background again  And there are basically two important ways of background you have fake photons for example a pion if you know what a pion is a  Pi zero can decay into two photons that are very collinear and it's not so easy to disentangle a  POMP photon from from a decaying pion also you have prompt photons again not from Higgs  Decays as I show him in the plot here down below  So effectively what you do if if you calculate the event mass of the whole set you get a distribution like this and you see that  It is a completely different behavior that was what I showed before in a few for mules  But again as a function of the Higgs mass you see now a continuum, but we're looking for peaks here  and  indeed if you fit the continuum and  subtracted from  From the observation you will see a little peak here with a significance of something like four and a half sigma  Now  You can combine all this information at this one and the previous channels and then at some point  You can calculate the data significance for each mass and this is the situation of July 4th in  2012 where you see the data significance as a function of the Higgs mass all the range between 110 GV to 500 something GV  The P zero value. I think you know that  And you see a dip there a dip which is  Reaching something like six sigma and the dip means that you cannot describe the data without having anything anything new in it  And this is what we then call a discovery five sigma  But by claiming a truth  Discovery we also have to look at our friends from the CMS collaboration and on that date  We were extremely curious to what they were showing and indeed they were also having at the very same spot  126 or so GV a  Significance of five sigma so that led to this to this observation of the CERN director general  I would now say I  Think we have it  You agree  With your  For me it's  Really an incredible thing that it happened in my lifetime  I know Pete Higgs a little bit that he was completely cute  Flabberg acid by the fact that we found this particle so soon and also in in the Netherlands  We had our we had our fair share of celebrating and we have to suddenly deal with media which we didn't do so much before  So what happened since July 4th now the question is is it the standard model Higgs we now in the in the phase in which we are really in very much detail  Go and find if if the predictions of the standard model once we know the mass are actually  Valid so what you see here is a plot. It's difficult to read maybe but you see the different decay channels and  Higgs to gamma gamma and Higgs to zizi and and the  expectation from the standard model is given by the  Vertical line and you see that the data is not on the vertical line  Very nicely so we are now in an intermediate states like is it something that we don't understand or do we just need more collisions  Realize that we have something like two-thirds of the total data set unblinded  So we still have new data on the shelves that we haven't looked at something else very intriguing  What we found actually not so long ago is if you look at the masses of the Higgs particle when it decays into two photons  or the masses of the Higgs particle when it decays to four left ons we don't find the same mass back and  The mass difference is something like three three and a half gv  a two and a something sigma  Is that significant enough to claim that maybe we are not dealing with a simple standard model Higgs something more complex  We don't know yet, but this is clearly something that's for which we need more data. I think that  This this discovery is actually one of the most important discoveries in particle physics for the last 42 years or so  We have definitely observed the particle  But if it's really Higgs, we don't know completely for sure yet  But in the whole LHC program imagine that only 1% of the data has been delivered at this moment  The program continues for many more years to come in a near future  The energy will be doubled  We will doing in-depth Higgs studies and we will search for beyond the standard model physics and in the far future  Our ambitions to is going for very high luminosities at the LHC and maybe doing high precision physics in an E plus E minus collider  So that leads me to my very last slide  I think that this  Discovery was really a major leap in understanding, but there are new questions that are now  Getting very urgent. We really have to find out what are the quantum numbers of the particles  Are there more Higgs particles or are there any other particles? Is it elementary or is it the composite?  how does it couple to itself and  What we also don't know because it's a scalar particle  What actually screens it mass from being at the Planck scale and there's all there's a whole connection to cosmology  And maybe it's nice to quote what Feldman always used to tell in his in his lectures on Higgs physics  You know you have this Higgs vacuum had this Higgs field which also contains energy and if you start calculating  What is the curvature of this of this?  Effect of having energy in our universe it will mean that the universe would shrink to the size of a football and  It clearly means that we lack understanding of what's really going on at this very fundamental level. So thank you very much
PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_0_995.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_996_2471.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_2472_4727.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_4728_9839.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_9840_10955.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_10956_13331.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_13332_15827.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_15828_16499.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_16500_16883.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_16884_17135.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_17136_21683.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_21684_25583.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_25584_26327.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_26328_26411.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_26412_28703.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_28704_29567.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_29568_32339.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_32340_32399.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_32400_36887.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_36888_47399.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_47400_48947.png PhysicsFOMVeldhoven2013StanBentvelsenTuesdayeveninglecture_48948_51156.png