dilepton and medium effect on particle properties

Zhangbu Xu, USTC 2009 lecture 1

Dilepton and medium effect on particle properties

Zhangbu Xu

Outline•共振态在 QGP 态下的性质

•双轻子谱•探测方法


Brown-Rho Scaling Broadening Low Mass Dilepton

measuring properties

Last Call for RHIC PredictionsNucl.Phys. A661 (1999) 205-260

• Chiral UA(1) symmetry restoration (J. Schaffner, D. Kharzeev, et al.)手征对称

Medium Effect in Dense Nuclear Matter

R. Rapp, et al


A little bit of historyWith the establishment of the hydrogen bubble-chamber, entirely new possibilities for research into high-energy physics present themselves. Results have already been apparent in the form of newly-discovered elementary particles. The first, very short-lived, so called, "resonance particle" was found in 1960. The Nobel Prize in Physics 1968

First time Invariant Mass was used (bump hunting)

(1952?)(1385) (1960)(892) (1961)(1405)

1411 (1967)


Measurement Technique 强子道的测量

• Mixed-event, Like-sign Background subtraction

• P-Wave Breit-Wigner function or Modified BW

ISRK*0 from p+p Collisions


All that Matters: X-section 作用截面

Different by 5

Rescattering>Regeneration at later stageChemical freeze-out

Kinetic freeze-out

K*

lost

K*

measured

K

K* K

K*

K*

K

K*

K K K*

measured

强子衰变道 : 散射 ( 大于 ) 再产生


重核子碰撞的时空演化 Stages in the collision

1. Pre-equilibriumHard parton scattering processes

2. EquilibrationAfter ≤ 1 fm/c partons materialise

and either hadronise or rescatter.

3. Thermal quark-gluon plasmaHydrodynamic expansion

4. Hadronization (phase transition)Quark coalescence + gluon

fragmentation … or … String fragmentation

5. Hadron gasHadrons continue to interact

6. Particle freeze-out Chemical freeze-out 化学冻结

粒子数不再变化 Thermal freeze-out 热冻结

不再有强相互作用

a) Without QGP b) With QGP

Hadron gas

Mixed phaseHadron gas

Parton formation and thermalisation.

Hadron formation.

t

z

, K, N, …, K, N, …

f

f

0 = h

(H)

(Q)

3.

4.

5.

6.

4.

6.

QGP

2.

1.

5.

A A

1.

0 = q


共振态有化学冻结吗 ?

Au+Au, p+p, Thermal Fit; Feed-down

STARPHENIX

Strangeness Enhancement Resonance Suppression

K. Schweda, O. Barannikova


Finite Interaction Cross Section Breakdown of Ideal fluid? Different Cross Section regeneration or rescattering ? Different Cross Section Flow at hadron stage

Resonance driven by Cross Section

Statistical and systematic errors added in quadrature

M. Bleicher et al. J. Phys. G 25 (1999) 1859

σ(Kπ)

σ(ππ)

K. Schweda, P. Fachini, C. Markert, H. Zhang


质量 or 重子 / 介子 ?

RCP flattens at intermediate pT

Two groups (2<pt<6GeV/c):

- ,K0s, K, K*, mesons

- p,, , baryons

Meson/Baryon Effect!

Collective velocity is not the only

scale

quark coalescence ?

介子共振态的特征 : 质量大


Quark Contents

K+K*? s+uK*?

Constituent Quark Scaling?

n=20.3

C. Nonaka, B. Muller et al. , Phys.Rev.C69:031902,2004


+ - Invariant Mass Distribution

2.1106 Au+Au minimum bias events and 4.7106 pp events

Breit-Wigner fixed width 0 = 150 MeV and f0 = 75 MeV fixed f0 masse 0.98 GeV (AuAu) and 0.96 GeV (pp)

0 mass = 0.698 ± 0.013 GeV (AuAu)

0 mass = 0.729 ± 0.006 GeV (pp)

AuAu 40% to 80%0.2 pT 0.9

GeV/c

|y| 0.5

0

f0

K0S

K*0

STAR Preliminary

STAR Preliminary

0

f0

K0S

K*0

0.2 pT 0.8 GeV/c

|y| 0.5

pp

Statistical error only Statistical error only

sNN = 200 GeV

P. Fachini (STAR)


<qq>, TQGP

The Lost Sword (Fable, 《呂氏春秋》 ~250B.C.)

RHIC

<qq>, TQGP

The SwordA+A CollisionsMoving Boat

Freeze-outFerry

HadronsMark

Detecting SignalsJumping into water

Chiral Symmetry Restoration e+e-, thermal Radiation g+ge+e-


Brown-Rho Scaling Rho Broadening Low Mass Dilepton

measuring properties

Last Call for RHIC PredictionsNucl.Phys. A661 (1999) 205-260

• Chiral UA(1) symmetry restoration (J. Schaffner, D. Kharzeev, et al.)

Medium Effect in Dense Nuclear Matter

R. Rapp, et al


How to Probe Early Stage?

Modification in medium

Decay quicklymatter exists 10-23s

Small or no FSIleptons, photons

Golden: J/

q

q

l

l

s

Small Branching Ratio(10-4), Low Production Rate


NA49 at SPS

A central Pb+Pb Collision1 cm slice centred on beam-axis

B = 1.5 T B = 1.1 T

NA49 Pb+Pb 158 GeV/A

s 17 GeV


-Production: Hadronic versus Leptonic Decay Channel

Different decay channels: NA49: -+

NA50: -+

Transverse momentum spectrum fit: 1/mTdn/dmT ~ exp(-

mT/T)

(NA49: 3.0<y<3.8)significant differences in

slopes and yields

Dieter Röhrich, QM02


Vector Meson at RHIC

AMPT: (l+l-)/(K+K-)=1.5 Experiments’ comparison

c=50fm

K

K

l+l-

K+K-

AMPT, STAR

Nucl-th/0202086

130GeV


The NA45 experiment

Ring Imaging Cherenkov (RICH)Radiator chosen to select electrons

Time Projection ChamberUsed for charged particle tracking

Nee

Nch ~10 5

th 32

95% of charged hadrons do not produced Cherenkov light


Dileptons at SPS

Conventional cocktail of particles Underpredicts dilepton production

Mass shifted??


The NA50/NA60 experimentNA50: di-muon spectrometerNote: absorber to remove all hadrons

muon spectrometer

absorber

ET E0

event selection

Zhangbu Xu, USTC 2009 lecture 21S. Damjanovic, Quark Matter 2008 21

双轻子的质量和动量谱斜率Phys. Rev. Lett. 96 (2006) 162302

drop

-Teff rises in low mass region radial flow of a hadronic source- drop at M~1 GeV suggests partonic nature of equilibrated particles~thermal radiation? - note: yield goes like (dN ch /dy)2


PHENIX from Above


West Arm tracking:

DC,PC1, PC2, PC3 electron ID:

RICH, EMCal

Photons EMCal

East Arm tracking:

DC, PC1, TEC, PC3 hadron& electron ID:

RICH,TEC, TOF, EMC photons:

EMCal

PHENIX Setup

full heavy ion program and first spin physics including electron & muon pairs

South Arm tracking:

MuTr muon ID:

MuID

Other Detectors Vertex

& centrality: ZDC, BBC, MVD

accumulated ~200 106 Au-Au


Measured p+p Mee

Cocktail filtered in PHENIX acceptance

Charm, Bottom, DY contributions from PYTHIA

Excellent agreement!

arXiv:0802.0050


Au+Au Mee compared to Cocktail Data and Cocktail

absolutely normalized Cocktail normalized to

Au+Au measurements Except ccbar

Low mass excess 150 < Mee < 750 X 3.4±0.2(stat.)

±1.3(syst.)±0.7(model) Intermediate mass

agreement

arXiv:0706.3034

Thermal contribution perhaps if ccbar mass spectrum softened by rescatt.


PT Dependence of Au+Au Mee

Low Mass excess is a low pT enhancement Huge excess at

lowest pT

Late Phase info

0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c

0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c

PHENIX Preliminary Still significant at high pT

PHENIX Preliminary QM08


STAR UpgradesMRPC ToF barrel

BBCRPSD

PMD

F0S

FMS

EMC barrelEMC End Cap

- DAQ1000

- TPC FEE

- MTD

- Soft HFT: Si-pixels; IST: Si-strips; FGT: GEM-layers

Integrated Tracking Upgrades

finished

ongoing

MTD


STAR 的目标

To study: 1. vector meson production and in-medium effect 2. heavy Quarkonian production (suppression)

Measuring the dilepton →

Electrons may come from the following sources at low mass region:

γ conversionπ0 , η Dalitz decaysρ ω Φ vector meson decays

Focus on the dilepton spectra at low mass region


What STAR Can Do?

Future upgraded detectors:

Full TOF μVertex detector

Electrons PID Reject electrons not from primary vertex

(gamma conversion)

Simulation:

γ conversion and π0 , η Dalitz decay background

How can μVertex detector deal with γ conversion subtraction?


Electron IdentificationBefore, using TPC dE/dx only to separate electrons from hadrons is complicated

TOF detector lights up this study

|1/β-1|<0.03

With TOF PID cut, electron band can be separated from others readily!

A prototype TOF tray (TOFr) installed this year


DileptonAssume electrons can be identified with full TOF

Total bkgd: ~10-4/25MeV (ω) ~2∙10-5/25MeV (Φ)

Dalitz decay bkgd: ~5∙10-6/25MeV (ω) ~5∙10-7/25MeV (Φ)


Background Rejection

L.J.Ruan

Electrons from gamma conversion

Require TPC+SVT+μVertex

~98% electrons fromgamma conversion rejected

Dalitz decays become dominant sources!!!

Great thanks to Kai!


Upgraded TOF for dileptons

Detectors

TPC+TOF 20K 6K

Background: e+e-

HFT discriminates background

Need low mass detector

Statistics comparable to NA60

Charm background


强作用下的宇称破缺

Vacuum Engineering Big Bang

Zhangbu Xu, USTC 2009 lecture 35D. Kharzeev, QM08


Conclusions

Vector Meson Resonances are sensitive tools to study new state of matter

An important physics program STAR will pursue

TOF+TPC+SVT+ μVertexSTAR RHIC II Upgrade

dilepton and medium effect on particle properties

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