HBT two-pion correlations at LHC Qingfeng Li ( 李庆峰 ) (Huzhou Teachers College)

Q.Li for NN2012 in San Antonio2 Outline LHC physics UrQMD updates (cascade & dynamic modes) HBT Correlations Calculation Results at LHC Summary Mainly From: Q.Li, G. Graef, M. Bleicher, PRC 85, (2012); G. Graef, M. Bleicher, Q.Li, PRC, 85, (2012); G. Graef, Q.Li, M. Bleicher, JPG 39, (2012).

Q.Li for NN2012 in San Antonio3 LHC physics TeV physics To find: Higgs;micro black holes; extra dimensions (Kaluza – Klein Theory); dark matters …a large number of unsolved questions in fundamental physics CERN

Q.Li for NN2012 in San Antonio4 What to do now… The extracted bulk properties of the high temperature fireball created in such ultrarelativistic collisions have provided unprecedented information for fundamental investigations of the phase diagram of quantum chromodynamics. 1.to explore collective features of the strong interaction in high multiplicity pp events; 2.to explore expansion properties of the created matter by investigating the spatial shape of the fireball from AA collisions; 3.to explore the spatial structure of the source created in collisions of various heavy ions at different energies and centralities to shed light on the observed scaling violation when going from pp to AA collisions at the LHC.

Q.Li for NN2012 in San Antonio5 UrQMD a microscopic many-body approach to p-p, p-A, and A-A interactions at energies ranging from SIS up to LHC. It is based on the covariant propagation of mesons and baryons. Furthermore it includes rescattering of particles, the excitation and fragmentation of color strings, and the formation and decay of hadronic resonances. At LHC, the inclusion of hard partonic interactions in the initial stage is important and is treated via the PYTHIA model. The model can be downloaded from ……

Q.Li for NN2012 in San Antonio6 Main updates in recent years 1.In cascade mode: the newest version is 3.3 (to include LHC physics); Phys. Rev. C 84, (2011); 2.In the Boltzmann+hydrodynamics hybrid mode: the newest version is 2.3p1 (can be downloaded from the website); Phys. Rev. C 78, (2008); 3.In the “ mean-field potential ” version: based on v2.1, adding mean field potentials for hadrons. V1.3+PYTHIA  V2.1+hydrodynamics  V2.3+LHC collisions  V3.3 Q.Li version  SPS  RHIC  LHC

Q.Li for NN2012 in San Antonio7 The HBT correlation and paramerization The quotient of two-particle and one-particle spectra The two-particle correlator C(q,K) is related to the emission function s(x,K), which is the Wigner phase-space density of the particle emitting system and can be viewed as the probability that a particle with average momentum K is emitted from the space-time point x in the collision region.  the two-particle relative wave function. Experimentally: Theoretically: The correlator is constructed with the help of the CRAB program HBT=Robert Hanbury-Brown and Richard Q. Twiss

Q.Li for NN2012 in San Antonio8 Cont ’ d CRAB analyzing program: me.html Three-dimensional Gaussian parameterization LCMS is employed in usual calculations Coulomb effect in FSI is considered for charged two-kaon correlation with a Bowler-Sinyukov method non-Gaussian effect can be discussed under the Edgeworth expansion The fitting work can be done by the ROOT or the ORIGIN software (using  -squared method)

Q.Li for NN2012 in San Antonio9 Calculation Results at LHC 1.Q1: p+p collisions at √s NN =7 TeV 2.Q2: : Pb+Pb collisions at √s NN =2.76 TeV 3.Q3: Examination of scaling of HBT radii with charged particle multiplicity

Q.Li for NN2012 in San Antonio10 Q1: p+p collisions at √s NN =7 TeV It seems like in massive nucleus-nucleus collisions, a strongly interacting medium is created even in pp collisions, that exhibits similar bulk properties such as space momentum correlations and collective behaviour; While it is often argued, that the particle emitting system in p+p collisions is too small to create a medium that exhibits bulk properties, this should be different at a center of mass energy of √s= 7 TeV. an essential quantity that influences the particle freezeout radii is the formation time in flux tube fragmentation  the recent LHC data on pp collisions allows to determine the formation time in the flux tube break-up From JPG 39, (2012)

Q.Li for NN2012 in San Antonio11 Formation time in UrQMD For the Lund model the formation times are proportional to the transverse mass of the created hadron and inversely proportional to the string tension. For simplicity UrQMD uses a constant formation time of t f = 0.8 fm/c for hard collisions. The average dN ch/d  from UrQMD is 15% smaller than ALICE data

Q.Li for NN2012 in San Antonio12 Projections of Correlation function Non-Gaussian effect is visible in out and long directions and at large q

Q.Li for NN2012 in San Antonio13 K T dependence of HBT radii for diff. dN classess and diff. formation times the present ALICE data allows to constrain the formation time to values of t f ≈ fm/c. An additional momentum dependence in t f is needed.

Q.Li for NN2012 in San Antonio14 Q2: : Pb+Pb collisions at √s NN =2.76 TeV Non-Gaussian effect is stronger at LHC than at lower energies Calculated non-Gaussian effect is more obvious than data From PRC 85, (2012)

Q.Li for NN2012 in San Antonio15 K T dependence of HBT radii 1,Strong kT dependence  substantial expansion of the source 2,As RHIC  LHC, HBT radii (esp. R L ) rise. 3,At LHC, R L and R O are larger than data, separately & R O /R S ratio is larger than data.

Q.Li for NN2012 in San Antonio16 x-t correlation even in the cascade calculation, there exists a visibly positive correlation between the emission time and position. The most important contribution to R O comes from the emission duration term

Q.Li for NN2012 in San Antonio17 Contribution of emission duration to the HBT radii To lead to smaller R O values in all k T bins but leaves R S unchanged; Overall it results in an improved agreement with the data of the ratio.

Q.Li for NN2012 in San Antonio18 Some hints 1.The overestimation of both R O and R L can be attributed to the known fact that the pressure in the early stage is not strong enough in the cascade model calculations. 2.A higher pressure would lead to a more explosive expansion, a stronger phase-space correlation, and a faster decoupling of the system, thus leading to smaller regions of homogeneity. 3.A more satisfactory solution is possible in the near future by improving the dynamic processes for both QGP and HG phases.

Q.Li for NN2012 in San Antonio19 Q3: Examination of scaling of HBT radii with charged particle multiplicity Same: 1) charged particle multiplicity at midrapidity: |  |<1.2 for pp; |  |<0.8 for other classes. 2) K T bin: MeV/c. Different solutions: To change: a) beam energy: Pb+Pb at √s=2760, 200, 130, 62.4 GeV and E lab = 158 GeV; b) centrality: Pb+Pb within 0-5%, 5-20%, 20-50% and 50-80% centralities; c) colliding system: Pb+Pb, Cu+Cu, C+C, p+p. From PRC, 85, (2012)

Q.Li for NN2012 in San Antonio20 Scaling of the HBT radii 1,The scaling is good if the change in N ch is caused by a change of centrality at a fixed energy. 2,A small offset on the order of 2-3 fm is visible for different system sizes, due to the finite size of the nuclei. 3,Increasing the center-of-mass energy leads to a reduction of the radii at a given fixed N ch -bin.

Q.Li for NN2012 in San Antonio21 Some hints 1.The scaling of the source size with (dN ch /d  ) 1/3 for different centralities is a hint that the underlying physics, e.g. pion production via resonance decay versus production via string fragmentation, is nearly unchanged by changes in the collision geometry. 2.A change in √s on the other hand results not only in different weights of the production mechanisms, but also in changed expansion dynamics towards a more violent expansion with increased energy. Different pp and AA result is attributed to the strongly different particle production mechanisms in AA and pp. I.e., bulk emission vs. string/jet dominated emission.

Q.Li for NN2012 in San Antonio22 Freeze-out time By fitting the hydrodynamic expression: 1, a shorter decoupling time with increased energy 2, UrQMD overestimates the source lifetime by a factor of ∼ 2–3 when compared to LHC data  back to the duration time

Q.Li for NN2012 in San Antonio23 Summary Two-pion HBT correlations at LHC are calculated by using the UrQMD v the present ALICE pp data allows to constrain the formation time to values of t f ≈ fm/c. 2.The overestimation of both R O and R L from Pb+Pb central collisions can be attributed to the known fact that the pressure in the early stage is not strong enough in the cascade model calculations. 3.The scaling of the source size with (dN ch /d  ) 1/3 for different centralities is a hint that the underlying physics, e.g. pion production via resonance decay versus production via string fragmentation, is nearly unchanged by changes in the collision geometry, while change in √s on the other hand results not only in different weights of the production mechanisms, but also in changed expansion dynamics towards a more violent expansion with increased energy. Different pp and AA result is attributed to the strongly different particle production mechanisms. I.e., bulk emission vs. string/jet dominated emission.

Q.Li for NN2012 in San Antonio24 Thank you for your attention! Using the for more Thank you for your attention! Using the for more