Exploring quark-gluon plasma in relativistic heavy-ion collisions

Transcription

1 Exploring quark-gluon plasma in relativistic heavy-ion collisions Guang-You Qin 秦广友 Duke University of Science and Technology of China July 12 th, 2011

2 Outline Introduction Collective flow of hot QGP Jet-medium interaction Summary

3 QCD basics Quantum Chromodynamics (QCD) A quark field theory of the strong interaction Elementary fields: quarks and gluons Responsible for most of ordinary matter u u d Corlor confinement and asymptotic freedom Quarks and gluons carry colors Only color-neutral particles in vacuum Coupling diverges at large distance (small Q 2 ) Quarks and gluons are free at small distance (large Q 2 )

4 QCD basics Quantum Chromodynamics (QCD) A quark field theory of the strong interaction Elementary fields: quarks and gluons Responsible for most of ordinary matter u u d Corlor confinement and asymptotic freedom Quarks and gluons carry colors Only color-neutral particles in vacuum Coupling diverges at large distance (small Q 2 ) Quarks and gluons are free at small distance (large Q 2 ) Gross Wilczek Politzer 2004 Nobel Prize

5 QCD thermodynamics from Lattice F. Karsch, Prog. Theor. Phys. Suppl. 153, 106 (2004) S. Borsanyi et al., JHEP 1011, 077 (2010) QGP: a soup of deconfined quarks and gluons Searching for QGP = Heating/melting QCD matter T c =175MeV (Gupta, Luo, Mohanty, Ritter, Xu, Science 24, June 2011)

6 Phases of matter QGP

7 QGP and early Universe A few microseconds after Big Bang, the entire Universe was in QGP phase Collide beams of ultrarelativistic heavy ions (little bangs) to heat QCD matter Nature Experiment

8 Little Bangs in laboratories Relativisitic Heavy Ion Collider (RHIC) at BNL: Colliding two Au/Cu GeV per nucleon pair Larger Hadron Collider (LHC) at CERN: Colliding two Pb 2.76TeV/5.5TeV per nucleon pair

9 Little Bangs in laboratories Relativisitic Heavy Ion Collider (RHIC) at BNL: Colliding two Au/Cu GeV per nucleon pair STAR Larger Hadron Collider (LHC) at CERN: Colliding two Pb 2.76TeV/5.5TeV per nucleon pair

10 Evolution of a heavy ion collision initial state QGP and hydrodynamic expansion hadronic phase and freeze-out pre-equilibrium hadronization 0.1fm/c 1fm/c 10fm/c t Two Lorentz-contracted nuclei approach each other Hard collisions produce high-p T particles Soft collisions thermalize the QGP The QGP expands hydrodynamically and hadronizes Hadron gase continues to expand until freeze-out Hadrons reach the detector

11 Tools for studying HIC

12 Collective flow of hot QGP Reaction plane z p y p x y Eccentricity: x y y x 2 x Elliptic flow: v 2 p p 2 x 2 x p p 2 y 2 y Hydrodynamic evolution translates initial spatial anisotropy into final momentum anisotropy

13 Relativistic hydrodynamics Based on conservation law T ( x) 0 For ideal fluid: T id eu u P( g u u ) For viscous fluid: T T id Describe the transport of macroscopic degrees of freedom Assume local thermal equilibrium Inputs: equation of state: e = e(p) initial energy momentum tensor (energy/entropy density, flow u ) Need transport coefficients (shear viscosity, bulk viscosity ) Freeze-out conditions

14 v 2 at RHIC compared with ideal hydro 0 Bulk of matter (> 99% particles) behaves like nearly perfect fluid η/s = 1/4π for certain quantum field theories using AdS/CFT (Kovtun, Son, Starinets, PRL, 2005) How perfect is such fluid? How low is the viscosity?

15 Extracting η/s from viscous hydro Song, Bass, Heinz, Hirano, Chen, PRL, 2011 / s (1 2.5)/(4 ) 20-30% e 2 uncertainty

16 Shear viscosity for QCD matter Chen, Deng, Dong, Wang, arxiv:

17 Is that all? Initial conditions are lumpy due to various fluctuations Need event-by-event hydro for the fireball evolution GYQ, Petersen, Bass, Muller, PRC, 2010

18 Initial state fluctuations Even moments are strongly correlated with RP, with one maximum along y direction GYQ, Petersen, Bass, Muller, PRC, 2010 Odd moments are not correlated with RP

19 Harmonic flows Need more detailed study GYQ, Petersen, Bass, Muller, PRC, 2010 Ma, Wang, PRL, 2011

20 Jet-medium interaction Jets can provide valuable information for the hot matter Jets get modified by the medium The medium responds to jet propagation Jet quenching GYQ, Majumder, PRL, 2010 Hard scales involved, perturbative approach applicable, well controlled in reference pp collisions

21 Evidence for jet RHIC Hadrons are suppressed in Au+Au No suppression for photons No suppression for d+au R AA = 1 N coll dn AA /dp T dy dn pp /dp T dy

22 Jet energy loss in medium Pj j ' jet jet

23 pqcd-based jet energy loss models Modeling of medium A collection of static scattering centers (BDMPS, Zakharov, GLV, ASW) Thermally equilibrated, perturbative medium (AMY) General nuclear medium with a short correlation length (Higher Twist) Resummation schemes Sum over all possible soft interactions (BDMPS, AMY) Path integral representation of hard parton propagation (Zakharov, ASW) Opacity expansion (GLV) Evolution scheme (multiple emissions) Poisson independent emissions (BDMPS, GLV, ASW) Transport rate equations (AMY) Modified DGLAP equations (Higher Twist) For detailed comparison between models see the brick report : arxiv: 1106:1106

24 General idea of jet quenching calculation h A j c b a d B d h f f d D a / A b / B ab jd h / j

25 General idea of jet quenching calculation h A c b a d B d f f d D h a / A b / B ab jd h / j ~

26 General idea of jet quenching calculation a b c d h A B ' / ' / / j h j j jd ab B b A a h D P d f f d j j h j h jd ab B b A a h D d f f d / / / ~

27 Jet RHIC GYQ, Ruppert, Gale, Jeon, Moore, Mustafa, PRL, 2008 Bass, Gale, Majumder, Nonaka, GYQ, Renk, Ruppert, PRC, 2009 ˆ T 2 d p 1 dt 400MeV 5 GeV 2 fm

28 arxiv: 1106:1106

29 Jet the LHC pqcd-based energy loss picture qualitatively agrees with measurements Need more quantitative studies

30 Full jet reconstruction Jet quenching is a partonic process Best to study jet in-medium modification at partonic level Full jet reconstruction Get all the sprays of hadrons to reconstruct the energy/momentum of the parent parton Different algorithms (SisCone, k T, antik T ) Different ordering in recombining final fragments (cone/sequential) Different sensitivity to the event background Cacciari, Salam, Soyez, JHEP, 2008 R FastJet: 2 ( J ) ( J ) 2

31 Jet LHC ATLAS CMS

32 Dijet LHC ATLAS, PRL, 2010 R=0.4, anti-k T, E 1 >100GeV, E 2 >25GeV, Δφ>π, η <2.8 Di-jet asymmetry increases with centrality Di-jet angular distribution is largely unchanged (similar results from CMS: arxiv: ) A J E E T,1 T,1 E E T,2 T,2 How do we understand this?

33 A partonic jet shower in medium out rad, E L E, rad L in coll E L coll E g broad E g Leading parton: Transfers energy to medium by elastic collisions Radiates gluons due to scatterings in the medium (inside and outside jet cone) Radiated gluons (vacuum & medium-induced): Transfer energy to medium by elastic collisions Be kicked out of the jet cone by multiple scatterings after emission

34 Dijet asymmetry calculation GYQ, Muller, PRL, 2011 GYQ, arxiv: Loktin, Belyaev, Snigirev, EPJC, 2011 Young, Schenke, Jeon, Gale, 2011 He, Vetev, Zhang, 2011

35 Where does the lost energy go? Integrating over the whole event final state the momentum balance is restored The momentum difference in the dijet is balanced by low p T particles at large angles relative to the away side jet axis

36 Where does the lost energy go? Need to combine jet transport and medium evolution simultaneously Integrating over the whole event final state the momentum balance is restored The momentum difference in the dijet is balanced by low p T particles at large angles relative to the away side jet axis

37 Summary The bulk matter produced in relativistic heavy ion collisions is a nearly perfect fluid (strongly interacting QGP) Higher harmonic flow measurements consistent with the picture of hydrodynamic evolution from fluctuating initial states Jet quenching measurements consistent with perturbative QCD energy loss picture Full jet/dijet measurements allow us to study jet-medium interaction at the partonic level