120786763
Is constituent quark scaling a unique sign of parton recombination?
We study the transverse momentum dependence of the anisotropic flow v2 for π, K, nucleon, Λ, Ξ and Ω for Au+Au collisions at $\sqrt{s_{NN}} = 200\,\mathrm{GeV}$ within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured v2, they both predict the particle-type dependence of v2 as observed by the RHIC experiments: v2 exhibits a hadron-mass hierarchy in the low pT region and a number-of-constituent-quark (NCQ) dependence in the intermediate pT region. The failure of the hadronic models to reproduce the absolute magnitude of the observed v2 indicates that transport calculations of heavy-ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependences observed in heavy-ion collisions at intermediate pT might be related to the hadronic cross sections in vacuum rather than to the hadronization process itself.
116897708
Measurements of $K^0_s$, $\Lambda$ and $\Xi$ from Au+Au collisions at \sqrt{s_NN} = 7.7, 11.5 and 39 GeV in STAR
We report on the measurements of Ks, Lambda and Xi spectra at mid-rapidity (|y|<0.5) in the most central (0-5%) Au+Au collisions at \sqrt{s_NN} = 7.7, 11.5 and 39 GeV from the STAR experiment. The extracted yields and the corresponding data from Pb+Pb collisions measured by the NA49 and CERES experiments at SPS are consistent. The Lambda, Anti-Lambda, Xi- and Xi+ to pion ratios agree well with the predictions from a statistical hadronization model at all three energies.
119431648
Recent results on strangeness production at STAR
We report on the measurements of strange hadrons (K±, K0S, Λ, Ξ, Ω and ) production at mid-rapidity in Au+Au collisions at = 7.7 - 39 GeV from the STAR beam energy scan (BES) program. The collision energy dependence of strange hadron yields are presented. The strange hadron related particle ratios are compared with calculations from statistical and hadronic cascade models. The nuclear modification factors and baryon to meson ratios (/K0S and Ω/) are presented. The physics implications for collision dynamics are discussed.
122820296
Influence of the non-flow effects and fluctuations on the v 2 measurements at RHIC
The cumulant method is applied to study elliptic flow (v2) in Au+Au collisions at √s = 200 AGeV, with the UrQMD model. We find that the four and six-particle cumulants are good measures of the real elliptic flow over a wide range of centralities except for the most central and very peripheral events. There the cumulant method is affected by the v2 fluctuations. In mid-central collisions, the four and six-particle cumulants are shown to give a good estimation of the true differential v2, especially at large transverse momentum, where the two-particle cumulant method is heavily affected by the non-flow effects.
123188749
Layout of bunch compressor for Beijing XFEL test facility
In this paper, we describe the layout of the bunch compressor for the Beijing XFEL test facility (BTF). Our bunch compressor setup is different from the usual one due to the space limit. The compensation X-BAND cavity and the first bunch compressor are separate in distance. The electron bunch is decelerated first and then accelerated to enter the first bunch compressor. The simulation result shows that our setup works well, and the nonlinear term is well compensated. Also, we present the result about the CSR emittance dilution study. Finally, we develop a program to study microbunch instability in the second BTF bunch compressor.
118882297
1D Longitudinal Beam Dynamics of Laser Plasma Wakefield Accelerator
In this paper, we get the 1D approximate analytical solution of the plasma electrostatic wake driven by the laser, and get the oscillating frequency of the wake modified by the nonlinear high order terms, and find that the frequency depends on the oscillating amplitudes of the wake and the laser which is the general nonlinear phenomenon. Finally we analyze the longitudinal beam dynamics in this electrostatic wake, and find that the high order terms don't change the topology of the longitudinal phase space.
124884650
Two-Particle Correlations with Neutral Pion and Direct Photon Triggers in pp and Pb+Pb Collisions with ALICE at the LHC
Since a long time ago, the ultimate constituents of matter have always puzzled the mankind and been researching. It began with the theory of atomism (indestructible atom) speculated by Democritus, a philosopher of ancient Greece, followed by the modern atomic theory proposed by John Dalton with the law of multiple proportions in 1803. In the early 20th century, the electron was discovered by J. J. Thomson through the measurement ofmass to charge ratio in his explorations on the properties of cathode rays, and Ernest Rutherford theorized that atoms have their charge concentrated in a very small nucleus through his discovery and interpretation of Rutherford scattering with the gold foil experiment. With more experiments built for researching, more particles, such as proton, neutron and quark, are discovered in succession. All the discoveries let us clearly know that the atom consists of the nucleus and electrons, the nucleus is made up of protons and neutrons which are composed of fundamental particles, quarks and gluons. Currently, it is widely known that the ultimate constitutes of the matter are three generation quarks, their anti-quarks and three leptons. These fundamental particles have strong, weak and electromagnetic interactions by mediating gluons, W± and Z bosons, and photons. A famous theory, Standard Model(SM), established to describe the electromagnetic, weak, and strong nuclear interactions and the fundamental particles, is success in explaining a wide variety of experimental results. The strong force of quarks and gluons is described by a theory of Quantum Chromodynamics (QCD). Three signi cant features, con nement, asymptotic freedom and chiral symmetry restoration, reveal the main characteristics of QCD. The strong interaction increases or the coupling constant αs describing the strong interaction strength becomes larger with the momentum transfer decreasing. Therefore, the quarks and gluons are con ned in the hadrons in the normal world with low momentum transfers, known as con nement. The second feature is called asymptotic freedom. According to the asymptotic freedom, the coupling constant αs becomes smaller and the interaction is perturbative with the momentum transfer increasing (equivalently at short distances). The third characteristic associated with QCD is chiral symmetry restoration. The chiral symmetry exists as an exact symmetry only when the mass parameter of a quark is strictly zero. At low energy region, Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics theory of quarks and gluons. According to the Lattice QCD prediction, a new matter, Quark-Gluon Plasma (QGP), which consists of deconv ned quarks and gluons, is excepted to be created at extremely high temperature and/or high baryons density. More results demonstrate that the QGP matter may be created in the universe after a microsecond of the Big Bang due to the formed extremely high temperature or in the interior of the neutron stars with high baryons density. Therefore, it is great signi cant to search for the characteristic signatures of the QGP and research its properties for understanding the evolution and formation of the early stages of the universe. However, it is impossible to extract directly the signatures of the early stages of the universe due to its long time evolution and explore the interior of the neutron stars. So how to create the QGP matter under the normal laboratory conditions is a great challenge. Ultra-relativistic heavy-ion collision experiment is considered as an available approach to producing the QGP phase. In the heavy-ion collisions, two Lorentz contracted nuclei approach to each other with velocities nearly equal to the velocity of light and have colliding. In the colliding instant, both contracted nuclei pass through each other in the region of geometrical overlap. Many processes of partonparton hard scatterings occur in the overlap region, which result in depositing a large amount of energy in a limit volume. The energy density is so high that a new matter state consisting of de ned quarks and gluons is created. Since 1960's, a series of heavy ion accelerators, such as Alternating Gradient Synchrotron (AGS), Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC), have been built to search for the QGP signatures and research its properties. In European Organization for Nuclear Research (CERN), the current biggest accelerator, Large Hadron Collider (LHC), was designed at 1998 and run successfully at the end of 2009. As one of four experiments, A Large Ion Collider Experiment (ALICE), whose aim is to study the physics of the strongly interacting matter at extreme energy densities. The centre-of-mass energy of √ sNN = 2.76 TeV of Pb+Pb collisions running in 2010 is about 14 higher than the highest energy of RHIC. Hence, it is excepted that the QGP created at LHC has longer lifetime and larger volume than at RHIC. This provides much better conditions for searching for the QGP and studying its properties. In the heavy-ion collisions, the formed QGP only exists in a short time and then fragment into a great variety of nal hadrons. In this case, we can only study the QGP phase by di erent measurements from the nal particles. Up to now, some main measurements which are considered as the signatures of the formation of the QGP are strangeness enhancement, suppression of J/ψ production, direct photons and thermal di-leptons, jet quenching, collective ow, and so on. Two-particles correlation is considered as a powerful probe for understanding the vi properties of the strongly interacting hot and dense medium. In such an analysis, a particle is chosen from higher pT region and called the trigger particle, which is presumably from jet fragmentations. The so called associated particles from lower pT region are always from the other fragmentation of the jet, or another production, such as collective ow. At RHIC and LHC, the measurements of the azimuthal angle distribution from two-particle correlations in A+A collisions show a strong suppression even disappeared at the high pT and enhancement with double-peak at the low pT on the away side, and ridge structure in pseudo-rapidity direction at the low pT on the near side compared to pp collisions. All the measurements can be explained as the e ects of the hot and dense medium, and imply the QuarkGluon Plasma is indeed formed in the heavy-ion collisions. When the direct photon is selected as the trigger particle, the correlations probably tag the γ-jet events produced from the QCD Compton scattering process, q+g → q+γ and q+ q̄ → g+γ annihilation process. In these processes, the photons momenta in the center-of-mass frame are approximately balanced by that of the recoil partons. The photons do not occur energy loss when going through the medium due to only electromagnetic interactions happen between photons and other particles because of the large mean free path of photons. The fragments of the recoil partons have rich information, such as the parton fragmentation function with the medium e ects, due to the interactions of the recoil partons and medium. In this thesis, the medium e ects and the parton fragmentation function are measured by π-hadron correlations and direct photon-hadron correlations, where the π and photons are detected by the electromagnetic calorimeters (EMCal) and the charged hadrons are reconstructed by the central barrel detector system. In the π-hadron correlations, the azimuthal angle distribution of the correlations and the per-trigger yield modi cation factor, IAA = Y PbPb Y pp , on the near side and away side are measured in pp and Pb+Pb collisions at √ sNN = 2.76 TeV. In central Pb+Pb collisions, an away side suppression from in-medium energy loss is observed (IAA ≈ 0.6), which is from the e ects of partons energy loss. Moreover, there is an enhancement above unity of (IAA ≈ 1.2) on the near side which has not been observed with any signi cance at lower collision energies. The signi cant near side enhancement of IAA in the pT region observed shows that the near side parton is also subject to medium e ects. IAA is sensitive to (i) a change of the fragmentation function, (ii) a possible change of the quark/gluon jet ratio in the nal state due to the di erent coupling to the medium, and (iii) a bias on the parton pT spectrum after energy loss due to the trigger particle selection. In the direct photon-hadron vii correlations, both isolation technique and statistical subtraction method are used to extract the direct photons and measure the azimuthal angle distribution of the correlations and the parton fragmentation function in pp at √ s = 7 TeV. The isolation technique used for the analysis is based on the physics that there is no particle or only a few particles around the leading order direct photons. The parton fragmentation function is measured and compared to the theory calculations at 8.0 < p T < 25.0 GeV/c. The statistical subtraction method is based on the fact that all photons consist of direct photons and decay photons from hadrons decay. Since there is no enough statistics of pp collisions at √ s = 7 TeV, it is impossible to extract signi cant results with the statistical subtraction method. But the work in this thesis develops the method in ALICE data analysis, which can be used quickly for measuring the parton fragmentation function and studying the medium e ects in the next running. This thesis is organized as follows: Chapter 1 presents the Standard Model of the particle physics including the description of the Higgs boson and the Quantum Chromodynamics, the Lattice QCD predication and the QCD phase diagram. The space-time evolution of the heavy-ion collisions and some signi cant measurements for searching for the QGP phase from SPS, RHIC and LHC are summarized in Chapter 2. Chapter 3 gives an overview of the ALICE experiment and a description of the ALICE online
56119429
Centrality dependence of particle production in p-Pb collisions at $\sqrt{s_{\rm NN} }$= 5.02 TeV
We report measurements of the primary charged-particle pseudorapidity density and transverse momentum distributions in p-Pb collisions at sNN=5.02TeV and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined by using different event-activity estimators, i.e., charged-particle multiplicities measured in three different pseudorapidity regions as well as the energy measured at beam rapidity (zero degree). The procedures to determine the centrality, quantified by the number of participants (Npart) or the number of nucleon-nucleon binary collisions (Ncoll) are described. We show that, in contrast to Pb-Pb collisions, in p-Pb collisions large multiplicity fluctuations together with the small range of participants available generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy-centrality classes, the Npart dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb participants, an approximate independence of the multiplicity per participating nucleon measured at mid-rapidity of the number of participating nucleons is observed. Furthermore, at high-pT the p-Pb spectra are found to be consistent with the pp spectra scaled by Ncoll for all centrality classes. Our results represent valuable input for the study of the event-activity dependence of hard probes in p-Pb collisions and, hence, help to establish baselines for the interpretation of the Pb-Pb data.
119304211
Hybrid model approach for strange and multistrange hadrons in 2.76A TeV Pb+Pb collisions
Using the VISHNU hybrid model, we calculate the multiplicity, spectra, and elliptic flow of $\Lambda$, $\Xi$ and $\Omega$ in 2.76 A TeV Pb+Pb collisions. Comparisons between our calculations and the ALICE measurements show that the model generally describes the soft hadron data of these strange and multi-strange hadrons at several centrality bins. Mass ordering of elliptic flow among $\pi$, K, p, $\Lambda$, $\Xi$ and $\Omega$ has also been studied and discussed. With a nice description of the particle yields, we explore chemical and thermal freeze-out of various hadrons species at the LHC within the framework of the VISHNU hybrid model.