HBT two-pion correlations at LHC

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

2. Outline • LHC physics • UrQMDupdates (cascade & dynamic modes) • HBT Correlations • Calculation Results at LHC • Summary Mainly From: Q.Li, G. Graef, M. Bleicher, PRC 85, 034908 (2012); G. Graef, M. Bleicher, Q.Li, PRC, 85, 044901 (2012); G. Graef, Q.Li, M. Bleicher, JPG 39, 065101(2012). Q.Li for NN2012 in San Antonio

3. LHC physics • TeV physics • http://lhc.web.cern.ch/lhc/ • 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 Antonio

4. 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. • to explore collective features of the strong interaction in high multiplicity pp events; • to explore expansion properties of the created matter by investigating the spatial shape of the fireball from AA collisions; • 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 Antonio

5. 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 http://urqmd.org …… Q.Li for NN2012 in San Antonio

6. LHC RHIC SPS Q.Li version Main updates in recent years • In cascade mode: the newest version is 3.3 (to include LHC physics); Phys. Rev. C 84, 034912 (2011); • In the Boltzmann+hydrodynamics hybrid mode: the newest version is 2.3p1 (can be downloaded from the website); Phys. Rev. C 78, 044901 (2008); • 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 for NN2012 in San Antonio

7. HBT=Robert Hanbury-Brown and Richard Q. Twiss The HBT correlation and paramerization The quotient of two-particle and one-particle spectra Experimentally: Theoretically: 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. The correlator is constructed with the help of the CRAB program Q.Li for NN2012 in San Antonio

8. Cont’d • CRAB analyzing program: http://www.nscl.msu.edu/~pratt/freecodes/crab/home.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 effectcan 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 Antonio

9. Calculation Results at LHC • Q1: p+p collisions at √sNN=7 TeV • Q2: : Pb+Pb collisions at √sNN=2.76 TeV • Q3: Examination of scaling of HBT radii with charged particle multiplicity Q.Li for NN2012 in San Antonio

10. Q1: p+p collisions at √sNN=7 TeV From JPG 39, 065101(2012) • 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 Q.Li for NN2012 in San Antonio

11. 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 tf = 0.8 fm/c for hard collisions. Q.Li for NN2012 in San Antonio The average dNch/d from UrQMD is 15% smaller than ALICE data

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

13. KT 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 tf ≈ 0.3-0.8 fm/c. An additional momentum dependence in tf is needed. Q.Li for NN2012 in San Antonio

14. Q2: : Pb+Pb collisions at √sNN=2.76 TeV From PRC 85, 034908 (2012) Non-Gaussian effect is stronger at LHC than at lower energies Calculated non-Gaussian effect is more obvious than data Q.Li for NN2012 in San Antonio

15. KT dependence of HBT radii 1,Strong kT dependencesubstantial expansion of the source 2,As RHICLHC, HBT radii (esp. RL) rise. 3,At LHC, RL and RO are larger than data, separately & RO/RS ratio is larger than data. Q.Li for NN2012 in San Antonio

16. 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 ROcomes from the emission duration term Q.Li for NN2012 in San Antonio

17. Contribution of emission duration to the HBT radii • To lead to smaller ROvalues in all kTbins but leaves RSunchanged; • Overall it results in an improved agreement with the data of the ratio. Q.Li for NN2012 in San Antonio

18. Some hints • The overestimation of both ROand RLcan be attributed to the known fact that the pressure in the early stage is not strong enough in the cascade model calculations. • 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. • 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 Antonio

19. Q3: Examination of scaling of HBT radii with charged particle multiplicity From PRC, 85, 044901 (2012) • Same: 1) charged particle multiplicity at midrapidity: ||<1.2 for pp; ||<0.8 for other classes. 2) KT bin: 300-400 MeV/c. • Different solutions: • To change: a) beam energy: Pb+Pb at √s=2760, 200, 130, 62.4 GeV and Elab = 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. Q.Li for NN2012 in San Antonio

20. Scaling of the HBT radii 1,The scaling is good if the change in Nch 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 Nch-bin. Q.Li for NN2012 in San Antonio

21. Some hints • The scaling of the source size with (dNch/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. • 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 Antonio

22. 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 Antonio

23. Summary • Two-pion HBT correlations at LHC are calculated by using the UrQMD v3.3. • the present ALICE pp data allows to constrain the formation time to values of tf ≈ 0.3-0.8 fm/c. • The overestimation of both ROand RLfrom Pb+Pb central collisionscan be attributed to the known fact that the pressure in the early stage is not strong enough in the cascade model calculations. • The scaling of the source size with (dNch/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 Antonio

24. Thank you for your attention!Using the e-mail: liqfer@gmail.com for more discussions. Q.Li for NN2012 in San Antonio