相对论重离子碰撞中 f 介子的产生

1

相对论重离子碰撞中介子的产生

陈金辉中国科学院上海应用物理研究所

QCD相变与重离子碰撞物理国际暨第七届全国研讨会@USTC2008年 7月 10号 -12号

Many thanks to: X. Cai, S. Blyth, F. Jin, H. Huang, G. Ma, J. Ma,

B. Mohanty, N. Xu …

2

Outline

Introduction– What we have learnt from RHIC

Motivation– Why we focus on -meson?

Results– -meson elliptic flow measurement– -meson spectra measurement– / and / ratio

Conclusion and Outlook

3

pT Scales and Physical ProcessesRCP

Three PT Regions:

-- Fragmentation (high pT jet energy loss)

-- multi-parton dynamics (recombination or coalescence or …)

-- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? )

4

High pT suppression

Very dense matter has been created in central Au+Au collisions

The dense matter is responsible for the suppression of high pT particles and the disappearance of back-to-back correlation

5

The Suppression is the Same for and – Parton level effect

No suppression photons don’t participate!

6

High pT phenomena at RHIC

Very dense matter has been created in central Au+Au collisions!

This dense matter is responsible for the suppression of high pT particles and the disappearance of back-to-back correlation!

The energy loss observed at RHIC is in parton level, but the mechanism for parton energy loss is yet to be understood! [Won’t elaborate in this talk.]

7

Intermediate pT, large p/ratio

Unexpected large p/ ratio in central Au+Au collisions

– The hadronization scheme should be different from e+e- !

8

Intermediate pT, v2 and RCP grouping

STAR

PHENIX

Baryon

Meson

V2 and RCP for PID measurement shown a B/M grouping behavior

– partonic degree of freedom?

9

What can we learn from those phenomena?

At RHIC intriguing experimental features:– enhanced baryon over meson production– strong elliptic flow– grouping behavior of v2 and RCP for PID

? Hadronization of bulk dense matter created at RHIC should be different from e+e- collisions! ? Quark Coalescence/Recombination ? Evidence for Deconfinement ? Possible for mass-effect rather than B/M type

are particularly important probes for these issues!

10

Why -meson ?

[1] A. Shor, Phys. Rev. Lett. 54 (1985) 1122

K+

K-

K-

K+

φ

φφ

K+K-

QGP The -meson is a clean probe from early time:● Small for interactions with non-strange

particles[1]

● Relatively long-lived (41 fm/c) → decays outside the fireball

The -mesoncan provide info on particle production mechanisms /medium constituents:● Theis a meson but as heavy as , p

baryons (mass vs. particle type?)

An interesting probe to understand the strangeness dynamics:● No net strangeness in the initial colliding nuclei

11

-meson measurement, v2 and RCP

For v2 and RCP measurement, -meson follows the trend observed in the Ks, mesons rather than in the p baryons

– clear signature for the Coa./Reco. hadronization mechanism.STAR Col. Phys. Rev. Lett. 99, (2007) 112301.

12

-meson production at RHIC

<pT>

/K-

STAR Col. Phys. Lett. B 612, (2005) 181,

Phys. Rev. Lett. 99, (2007) 112301.

1. Evolution in the centrality dependence;

2. <pT>, -meson may decouple early;

3. N()/N(K), ruled out the K-coalescence.

13

’sare mostly from bulk s quarks

N()/N() vs. pT is consistent with a model based on the recombination of thermal s quarks up to pT ~ 4.0 GeV/c, but disagrees at higher pT.

v2() shows similar behavior as PID’s, positive signature for partonic collectivity at RHIC.

STAR Col. Phys. Rev. Lett. 99, (2007) 112301.

14

Parton pT distributions at Hadronization?

)2/(

)3/(

)2/(

)3/(

T

T

T

T

p

pd

p

ps

Can we extract the strange quark pT distribution from multi-strange hadron data? If baryons of pT are mostly formed from coalescence of partons at pT/3 and mesons of pT are mostly formed from coalescence of partons at pT/2

and particles have no decay feed-down contribution!These particles will freeze-out earlier from the system and have small hadronic rescattering cross sections[1,2].

[1] A. Shor, PRL 54 (1985) 1122;[2] H. Van Hecke et al., PRL 81 (1998) 5764.

15

Strange and down quark distribution

Strange quark distributions are flatter than light quarks!

arXiv:0801.2265

16

Test on s/d ratio at hadronization

s/d quark ratios

=

= Yes! but with large uncertainties due to decay feed-down corrections in

arXiv:0801.2265

X. Wang

17

Summary and Conclusion

N()/N(K) vs. Npart rules out the Kaon coalescence as a dominant channel for production at RHIC;

N()/N() vs. pT favors the model prediction that s are made via thermalied s-quarks coalescence at RHIC;

v2() vs. pT conclude that the partonic collectivity has been formed at RHIC;

N()/N() and N()/N() vs. pT/nq indicate that strange quarks may have developed a stronger collective radial flow than the light quarks during the initial parton evolution at RHIC;

Since mesons are made via coalescence of seemingly thermalized s quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC.

18

Outlook: Extend PID Capability

ToF detector updated:

— 5 trays of ToF system installed in Run 8, commissioned, and used for physics.

— 90 (of 120) ToF trays to be installed for Run 9 and will be completed before Run 10.

/K separation to 1.6 GeV/c (0.65 TPC) (+K)/p to 3 GeV/c (1.1 TPC) Clean electron ID down to 0.2 GeV

19

The location of the QCD Critical Point

Outlook: RHIC is ready for the Beam Energy Scan

Hadron gas

QGP

sketch by P. Sorensen

Key measurements

— PID spectra and v2

— K/ , <pT> … fluctuation