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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 …

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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

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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? )

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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

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The Suppression is the Same for and – Parton level effect

No suppression photons don’t participate!

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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.]

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Intermediate pT, large p/ratio

Unexpected large p/ ratio in central Au+Au collisions

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

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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?

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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!

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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

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-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.

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-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.

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’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.

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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.

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Strange and down quark distribution

Strange quark distributions are flatter than light quarks!

arXiv:0801.2265

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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

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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.

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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

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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