1. Effect of equilibrium phase transition on multiphase transport in relativistic heavy ion collisions
喻 梅 凌
华中师范大学粒子物理研究所
2019/2/24
第十届全国粒子物理大会 桂林

2. outline
Introduction and motivation
AMPT model and the time evolution
Collective phase transition and its effect
Conclusion

3. I. Introduction and motivation
Stages of heavy ion collision and different theoretical approach

4. Basic ideas of Monte Carlo simulation about transport theory
Transport theory describes the evolution of parton distribition in phase space
transport equations are numerically solved by simulating the dynamical evolution of the parton distributions as a succession of binary parton-parton collisions
The mechanics about thermalization and formation of QGP are studied in microscopic
Necessity to combine transport model with phase transition
In heavy ion simulation, when collective phenomenon emerge, transport isn’t enought
Parton phase is in pQCD vacuum, while hadron phase is in physical vacuum

5. II. AMPT model and the time evolution
Components of A MultiPhase Transport Model (AMPT v2.11)
Z.W.Lin et al. PRC 72(2005)064901
Initial conditions: (x,p) distributions of minijet partons from hard process and strings from soft process ,strings are melted to be partons
Partonic transport:
only two-body elastic scatterings
two partons collide if
Hadronization:
Hadron transport:
AMPT v2.11 was successful in elliptic flow and HBT but failed to describe hadron rapidity and transverse momentum spectra
AuAu collision at 200 AGeV, impact parameter b<3fm, parton cross section 10mb

6. Parton and hadron time evolution in AMPT v2.11
Parton formation time:
When parton interaction cease, hadrons are produced one by one
The percentage of parton and hadron varies with time:
When t<5fm/c, parton dominate, system in deconfined phase
When t>30fm/c, hadron dominate, system in confined phase
Problems occur:
1) some partons have unreasonable long lifetime
2) deconfined partons exist in confined physical vacuum
3) parton-wise hadronization but no collective phase transition

7. III. Collective phase transition and its effect
Extract temperature from particle spectrum
Assume locally thermal equilibrium, in a thermal + transverse radial flow model, the transverse mass distribution is
T: kinetic freezeout temperature,
Lattice predicted critical temperature

8. Collective phase transition following a super-cooling stage
To solve the problem of AMPT, we have a physical picture:
1)High temperature QGP expands and evaporates hadrons
2)The system supercools to a point (model parameter) lower than
3)A sudden phase transition turns all the left partons to hadrons at this point
hadron phase
parton phase
The time evolution of parton and hadron temperature with phase transition at t=5fm/c

9. The effect of collective phase transition on final state distribution
1) hadron rapidity distribution
AMPT with phase transition describes data better

10. 2) Elliptic flow

11. Collective phase transition is necessary for transport model
IV. Conclusion
Unreasonable long lifetime of partons is found in transport model
To solve the problem, a collective phase transition is added to the transport model AMPT to replace the parton-wise hadronization
Better model and data agreement are achieved for the longitudinal distribution and elliptic flow
Collective phase transition is necessary for transport model

13. II. AMPT model and the time evolution
Components of A MultiPhase Transport Model (AMPT v2.11)
Initial conditions: spatial and momentum distributions of minijet partons from hard process and strings from soft process ,strings are melted to be partons
Partonic transport: only two-body elastic scatterings are considered with
cross section Two partons will undergo scattering when the closest
distance between them is smaller than
Hadronization:
Hadron transport:
AMPT v2.11 was successful in elliptic flow and HBT but failed to describe hadron rapidity and transverse momentum spectra

14. Basic ideas of Monte Carlo simulation about transport theory
Semi-classically, parton density distribution in phase space can be described by Boltzman equation
Boltzman equation are solved by simulating the interaction and evolution of partons in detail
The mechanics about thermalization and formation of QGP are studied microscopic
Necessity to combine transport model with phase transition
In transport model, partons hadronize one by one with unreasonable long lifetime
Parton phase is in pQCD vacuum, while hadron phase is in physical vacuum