8301145
Higgs mechanism and bulk gauge boson masses in the Randall-Sundrum model
Assuming the breaking of gauge symmetries by the Higgs mechanism, we consider the associated bulk gauge boson masses in the Randall-Sundrum background. With the Higgs field confined on the TeV-brane, the W and Z boson masses are naturally an order of magnitude smaller than their Kaluza-Klein excitation masses. The electroweak precision data require the lowest excited state to lie above about 30 TeV, with fermions on the TeV-brane. This bound is reduced to about 10 TeV if the fermions reside sufficiently close to the Planck-brane. Thus, some tuning of parameters is needed. We also discuss the bulk Higgs case, where the bounds are an order of magnitude smaller.
16353140
Singlets and the Electroweak Phase Transition
Singlet extensions of the Standard Model (SM) allow for a strongly first order electroweak phase transition, because of trilinear terms in the tree-level potential. We present a systematic procedure to study the parameter space of the Next-to-Minimal Supersymmetric SM (NMSSM). We find that this model is consistent with electroweak baryogenesis for a wide range of parameters, allowing Higgs masses up to at least 115 GeV.
14730206
Reducing constraints in a higher dimensional extension of the Randall and Sundrum model
In order to investigate the phenomenological implications of warped spaces in more than five dimensions, we consider a 4 + 1 + δ dimensional extension to the Randall and Sundrum model in which the space is warped with respect to a single direction by the presence of an anisotropic bulk cosmological constant. The Einstein equations are solved, giving rise to a range of possible spaces in which the δ additional spaces are warped. Here we consider models in which the gauge fields are free to propagate into such spaces. After carrying out the Kaluza Klein (KK) decomposition of such fields it is found that the KK mass spectrum changes significantly depending on how the δ additional dimensions are warped. We proceed to compute the lower bound on the KK mass scale from electroweak observables for models with a bulk SU(2) × U(1) gauge symmetry and models with a bulk SU(2)R × SU(2)L × U(1) gauge symmetry. It is found that in both cases the most favourable bounds are approximately MKK ≳ 2 TeV, corresponding to a mass of the first gauge boson excitation of about 4 – 6 TeV. Hence additional warped dimensions offer a newwayof reducing the constraints on the KK scale.
16265499
Bubble wall velocity in the minimal supersymmetric light stop scenario
We build on existing calculations of the wall velocity of the expanding bubbles of the broken symmetry phase in a first-order electroweak phase transition within the light stop scenario (LSS) of the MSSM. We carry out the analysis using the 2-loop thermal potential for values of the Higgs mass consistent with present experimental bounds. Our approach relies on describing the interaction between the bubble and the hot plasma by a single friction parameter, which we fix by matching to an existing 1-loop computation and extrapolate to our regime of interest. For a sufficiently strong phase transition (in which washout of the newly created baryon asymmetry is prevented) we obtain values of the wall velocity, vw≈0.05, far below the speed of sound in the medium, and not very much deviating from the previous 1-loop calculation. We find that the phase transition is about 10% stronger than suggested by simply evaluating the thermal potential at the critical temperature. We also comment on the relevance of our results to extended models, such as the NMSSM.
15025301
Electroweak constraints on warped geometry in five dimensions and beyond
Here we consider the tree level corrections to electroweak (EW) observables from standard model (SM) particles propagating in generic warped extra dimensions. The scale of these corrections is found to be dominated by three parameters, the Kaluza-Klein (KK) mass scale, the relative coupling of the KK gauge fields to the Higgs and the relative coupling of the KK gauge fields to fermion zero modes. It is found that 5D spaces that resolve the hierarchy problem through warping typically have large gauge-Higgs coupling. It is also found in D> 5 where the additional dimensions are warped the relative gauge-Higgs coupling scales as a function of the warp factor. If the warp factor of the additional spaces is contracting towards the IR brane, both the relative gauge-Higgs coupling and resulting EW corrections will be large. Conversely EW constraints could be reduced by finding a space where the additional dimension’s warp factor is increasing towards the IR brane. We demonstrate that the Klebanov Strassler solution belongs to the former of these possibilities.
16805200
Flavor violation and warped geometry
Extra dimensions have interesting consequences for flavor physics. We consider a setup where the standard model fermions and gauge fields reside in the bulk of a warped extra dimension. Fermion masses and mixings are explained by flavor dependent fermion locations, without relying on hierarchical Yukawa couplings. We discuss various flavor violating processes induced by (Kaluza–Klein) gauge boson exchange and non-renormalizable operators. Experimental constraints are satisfied with a Kaluza–Klein scale of about 10 TeV. Some processes, such as muon–electron conversion, are within reach of next generation experiments.
111119743
Upgrade of the COMPASS calorimetric trigger
In 2009 COMPASS performed a short measurement of neutral Primakoff reactions, characterised by highly energetic photons in one of the two electromagnetic calorimeters. A digital trigger was implemented in the existing readout electronics which calculates the energy released in the central region of the calorimeter. In 2012 a long measurement of these processes has been performed. In order to extend the kinematic range to lower energetic photons the trigger system has been upgraded in a way to be more selective to specific physics channels. The new ADC firmware preserves hit information and provides it to newly developed backplane trigger modules. There hits from all three thousand channels are processed and the trigger decision is made.
8636737
The universal sampling ADC readout system of the COMPASS experiment
The COMPASS experiment (COmpact Muon Proton Apparatus for Structure and Spectroscopy) is a fixed target experiment located at the CERN Super Proton Synchrotron. The physics program is focused on the study of hadron structure and hadron spectroscopy with high intensity muon and hadron beams, up to 160 GeV/c for muons and 190 GeV/c for hadrons respectively. To allow the tracking of charged particles with very low and as well very high momentum, COMPASS comprises two magnetic spectrometer stages extending to a total length of 60 m. From the data acquisition point of view, about 200000 analog detector channels have to be read along the complete experiment. Depending on the detector signal characteristics and the number of channels, this task is realized by frontend electronics using either dedicated ASICs and/or sampling analog-to-digital (ADC) or time-to-digital (TDC) components. The sampling ADC based readout system of the COMPASS experiment comprises today over 127k channels equipped with the APV25 frontend ASIC and 5728 direct sampling channels. An important feature from the beginning was the combination of data transfer, clock and trigger distribution and configuration access within a standardized serial interface between the different ADC modules and the first stage of data concentrator modules. By choosing between a copper or fiber realization for this interface, either a low cost interconnect or a link with galvanic decoupling can be realized. The ongoing development of the sampling ADC electronics is focused on the migration toward the Advanced Telecom Computing Architecture (ATCA) crate standard, to overcome the backplane bandwidth limitations in VME systems. In addition, the ATCA standard provides better cooling and monitoring capabilities. To simplify the transition to ATCA, the MSADC module was already realized as a mezzanine card, which can be mounted on an ATCA based carrier card as well. In addition, the MSADC card fits also to the MicroTCA form factor, which allows to provide a handy building block for laboratory based data acquisition systems.
47544178
A digital trigger for the electromagnetic calorimeter at the COMPASS experiment
At the COMPASS experiment at CERN's SPS many physics channels containing neutral particles are studied. These particles are identified by two electromagnetic calorimeters covering different transfers regions. For certain physics the main signature is described by these neutral particles which requires a calorimetric trigger system. The method described here is fully based on the existing front end electronics and uses digital pulse shape analysis techniques. This approach allows to implement a flexible trigger system as well as to reduce cost by avoiding to produce new electronic components. The implementation of this new trigger as well as the performance measured during the 2009 run will be discussed.
118063177
Measurement of the Pion Polarizability at COMPASS
The value of the pion polarizability is predicted with high precision by Chiral Perturbation Theory. However, the existing experimental values are at tension with this prediction as well as among themselves. The COMPASS experiment at the CERN SPS accesses pion-photon reactions via the Primakoff effect, where high-energetic pions react with the quasi-real photon field surrounding the target nuclei. Flagship channel is the Primakoff reaction in which a single real photon is produced, giving access to pion Compton scattering. Using this process the pion polarizability is extracted from the measured cross-section shape. End of 2009 COMPASS performed a measurement of the pion polarizability using a nickel target. The large amount of data collected in combination with the possibility to study systematic effects using the analogous reaction with a muon beam, the most precise experimental value to date was determined.
113403181
New data acquisition system for the COMPASS experiment
The modern market offers low cost high performance FPGA integrated circuits equipped with dozens of multi gigabit serial links making them ideal devices for data transmission and data sorting applications. Therefore we have designed the new DAQ system that would perform the detector readout and event building in a custom made FPGA based hardware. The software part will provide the control and monitoring function. Currently, the prototypes of the new FPGA card are being tested and the control and monitoring software is being prepared for the tests with the real hardware.