rhic ii workshop, 11/11/051 em probes at rhic ii zhangbu xu (bnl) for the rhic-2 em-probes working...

RHIC II Workshop, 11/11/05 1

EM Probes at RHIC II

Zhangbu Xu (BNL)

For the RHIC-2 EM-Probes Working Group(co-convenors: G. David, R. Rapp, XZB)

http://www.phenix.bnl.gov/WWW/publish/david/rhicii_emTalks by members at April Workshop

R. Rapp (Nov. PAC meeting)Simulations from individuals

First Draft of Working Group Document


1. Motivations: a) Chiral Symmetry Restoration

b) QGP Radiation -- Temperature

2. Experimental Measurements• Vector Meson In-medium Properties

• Thermal Dilepton Spectra

• Direct Photons

3. Challenges: • Chiral partners?

• Understanding backgrounds

• What do we learn from SPS results

4. Conclusions

Outline


Search for Chiral Symmetry Restoration

scholar.google.com: 4,130www.google.com: 76,700

Responsible for >90% mass of the visible world


QCD Phase DiagramA thermodynamic stateA thermodynamic state is specified by a set of values of all the thermodynamic parameters necessary for the description of the system. --- statistical mechanics by K. HuangTemperature (T), chemical potential (, pressure(P) …

1. Chemical/thermal Equilibrium at certain stage of the evolution

2. At the predicted QCD phase boundary

3. persistent from SPS to RHIC

4. Below the phase boundary atAGS and SIS

5. Can we put a point above?


Experimental Signatures

• Vector Meson Properties• Thermal Dileptons• Direct Photons

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

RHIC


EM Emission and Chiral Symmetry

at Tc: Chiral

Restoration

)Im(Ims

dsf AV

2

)Im(Imsds)qq( AVs 2 [Brown-rho, Rapp…]

nucl-th/0409054


Low-Mass Vector Mesons at RHIC

• hadron liquid close to Tc: , “melt”, more robust?

• baryon+antibaryons important Energy Scan!!


Dilepton from HI

smooth transition HG → QGP liquid?Integrated yields from several stages

R. Rapp, hep-ph/0010101


QGP Temperature and Composition

• thermal window: q0 , Mee≈ 2GeV

• photons: jet-QGP interactions compromise T-sensitivity• less so for dileptons; charm?

Direct Photons Intermed.-Mass Dileptons

Energy Scan


Detector Upgrades

• PHENIX – Hadron Blind Detector (HBD)

reject conversion and Dalitz decay– Silicon Vertex Tracking Detector

Measure charm background

• STAR– Time-of-Flight

TPC+TOF+EMC Electron/muon Identification– Heavy Flavor Tracker (HFT)

TPC+HFT reject conversion, Dalitz decay and HFe

– Data Acquisition System (1KHz minbias rate)


Measuring Low Mass Electron Pairs in PHENIX

Hardware• Compensate magnetic field with an inner coil to preserve e+e- pair opening angle (foreseen in original design B0 for r 50-60cm)• Compact HBD in inner region

Strategy • Identify signal electrons with p > 200 MeV/c from vector mesons in the outer PHENIX detectors

• Identify low momentum electrons with p < 200 MeV/c (mainly from Dalitz pairs and conversions) in the HBD

• Reject pair if opening angle < 200 mrad (for ~ 90% rejection).

HBD

HBD

Requirements * Electron efficiency 90% * Double hit recognition 90% * Modest rejection ~ 200C. Woody, RHICII EM probes group meeting, 04/30/05


Resulting Low Mass Pair Spectrum with the HBD

• Combinatorial background is reduced by more than two orders of magnitude• limited by combinatorial background from open charm• A precision measurement of charm using the Silicon Vertex Tracking Detector.

C. Woody, RHICII EM probes group meeting, 04/30/05


Rates for Vector Meson Production

• ’s produced in || < 0.5• e+,e- into PHENIX central arm acceptance• pT,e > 200 MeV/c

Consider Run 8 (200 GeV Au x Au, 4x Design Luminosity) :

• Lpeak = 30 x 1026 cm-2 s-1

• L ave store = 8 x 1026 cm-2 s-1

• 20 KHz peak min. bias rate• 5.4 KHz avg min. bias rate• 10 week dedicated HBD run (central field in configuration)• RHIC x PHENIX = 0.5 x 0.5 = 0.25

N e+e- = 1.2 x 105 produced in PHENIX acceptance

8.2 x 109 min. bias events produced

N e+e- = 1.5 x 10-5 / min.bias evente+e- PHENIX1 : 0.9 : 0.7

C. Woody, RHICII EM probes group meeting, 04/30/05

Lower energy 30GeV2 = 44


STAR MRPC-TOFr Electron PID

Electron identification: TOFr |1/ß-1| < 0.03 TPC dE/dx electrons!!!2 in azimuth, |eta|<1

electrons

Clean electron PID can be obtained up to PT< 3 GeV/c. This is used to measure the semileptonic decay of open charm.Hadron rejection power at 10-5. M. Shao et al., nucl-ex/0505026Higher pT: TPC+TOF+EMC

STAR Collaboration, PRL 94, 2005, 062301


Reject conversionBackground: e+e-

HFT discriminates

background !

Need low mass detector

Also: DD e+e-

e+e-


Mass(e+e-)<0.15 GeV/c2

TPC e+e- invariant massγ conversion π0 Dalitz decay

Dalitz Decay

pT (GeV/c)

Ang

le

Find the pair from Dalitz,reject both leptons


Charm Spectrum Measurement

• Au + Au, 50M central events

• D0 K +

• Stat. uncertainties small

•We can subtract DDbar e+e-

•Alternative Approach: reject electrons event-by-event(~50% for electron pair)•HFT resolution <10m


STAR Dilepton Capability

TOF: vector meson TOF+HFT: thermal radiation


Rate Estimate1|/ dydn 15.0 02.0Assume: (pp)

5107)( eeBr 4103)( eeBr From PDG:

MeV15 MeV8300|/ dydn (AuAu)

TOF match+PID eff ≈ 80% TPC+HFT eff ≈ 60% (?)

Au+Au #events for ω

with 3σ signal

#events for Φ with 3σ signal

TOF+TPC 7M 2M

TOF+TPC+HFT 200K 100K

Preliminary estimation for the requested data

Acceptance: N e+e- = 8 x 10-4 / min.bias event


Muon ID at low pT

e

STAR Preliminary:0.15<pT<0.25 GeV/c, DCA<3cm

less Dalitz decay and conversion (<x10), Decays from ,K rejected by HFT DCADilepton mass: 0.2<m<0.6 GeV


Direct Photons

• Direct Measurement of direct photons

All – known sources

• Virtual photon (via low-mass dilepton)

HBT


VirtualDirect Photons

S/B=~1

0

0

direct

data

incl.

direct

*

*

RR

RR

calculated from Dalitz formula

measuredRdata ÷

R

R

Rdirect

incl.

direct

measured with EMCal

Here we are…


Comparison to Conventional result

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

( + 1 )

Conventional method:

all-


HBT

• Thermal model for direct photon production

• Assuming =1• EMC+TPC efficiencies:

50%+(5—9%) • 16 million Au+Au events

Sandweiss/Chikanian (Yale)


Challenges

• Lack of measurements of Chiral Partners

• Backgrounds on dilepton, direct photon measurements

• What we learn from SPS Measurements


Chiral Partner ( a1)

UrQMD minimum bias Au+Au

at √sNN = 200 GeV

h1 3 π B.R. = 0.1h1 ρ π B.R. = 0.9Mass = 1.170 GeV/c2

Γ = 360 MeV/c2

3σ signal a1 π± 54M events ! Interesting but difficult (Type B)

Cocktail

P. Fachini


Vector MesonCSR

• Experimental observation of modification of Vector Spectral Function significant by itself

• Model calculation of V-A Spectral Function:Calculate vector and axialvector spectral functions as a function of temperature and density (including as many of the constraints, Weinberg sum rule) in a chirally invariant model.

• Compare results with Lattice QCD

• Detailed Experimental-Thoery comparison vs energy/density scanPerform detailed comparisons of the in-medium effects on the vector correlator with dilepton data (centrality, excitation function, mass and q_t-spectra), which requires a realistic expansion model (e.g. hydro/transport).

RHIC II Workshop, 11/11/05 28Mass (GeV)

subtracted spectrumthickness of linerepresents statistical error

Vacuum Vector mesons

QGPQGP

15% at m=2GeVNeeds ~+-2%

R. Seto, RHICII EM probes group meeting, 04/30/05

crude charm RHIC 2

Charm Background in PHENIX Dilepton


Charm/Dalitz Background in STAR

DDbar e+e- correlations can be measured (<+-10%)Dalitz rejection: Inner Detector tracking: SSD+HFT+Vertexin progress


Direct Photons Temperature

• pQCD+QGP+HG+Jet• Extract temperature

difficult • Energy Scan important• Detailed model

comparison


Residual Correlation from HBT

Background = 4x signalSubtract from known ±± HBTSandweiss/Chikanian (Yale) in progress

0

0

0

e+

e-

0

e+

e-

Similar effect for virtual photons?Negligible with HBD


drop. mass (norm.)

[RR+Wambach ’99]

• -meson “melting” , dropping mass?• address: absolute norm., pt-dep., M > 0.9GeV (4→!)• “cocktail-“ (+smooth signal)? vector dominance? • precision data essential to rule out models

Recent Advances at SPS: Power of Precision

drop. mass (norm.)drop. mass (norm.)

NA60


Electromagnetic Probes at SPSDileptons

MediumEffects!10% QGP

[RR+ Shuryak ’99]

HG: 4→

30% QGP

[Turbide,RR+Gale’04]

Direct Photons

[Liu+ RR’05]

Central Pb(158AGeV)+Pb


What needed at the SPS?

Experimentally:• Low-mass e+e: drop. mass vs. broad., energy scan [run time]

• Cronin enhancement vs. temperature [systematic p-A]

• Isolation of charm dileptons [vertex detector]

• Redundancy

Physics: • QGP subdominant ?!

• early thermalization (v2) ?!

RHIC II (dedicated QCDLab) has it all


Conclusion• EM probes are directly sensitive to the

chiral symmetry properties and to the temperatures of the partonic system

• There are three major measurements related to the questions above

• Each with complemental/redundant techniques• Challenges:

not only confirm a theory, but rule out incorrect ones requires precision data in M, qt , centrality and √s

• RHIC (detectors) positioned to meet the challenges: - Dalitz suppression / charm ID; redundancy - energy-scan (006-200GeV!?)

• Theory Homework: chiral theory with baryons, sQGP, dilepton components, …


Remarks

• Am I too negative? 哀兵必胜“ pessimistic” troops win

• Continue pushing our detectors to their limits

• Understand the strength and weakness

• Am I too positive?I don’t think of all the misery, but of all the beauty that still remains. – Anne Frank

• Continue exploration of techniques and simulations

• Understand and reduce the background

CHARM DILEPTON

I believe we can fly, RHIC II will provide us with wings


2.2 EM Emission and Chiral Symmetry

~ “- a1(1260)” (chiral partners)

Axial-/Vector in Vacuum

pQCD cont.

)Im(Ims

dsf AV

2

at Tc: Chiral

Restoration

)T(fMqxdd

dN Bee23

2

44

Imem ~ [ImD+ImD/10+ImD/5]

• Low-Mass Dilepton Rate:

-mesondominated!

• Axialvector Channel: ± invariant mass-spectra ~ Im Da1(M) ?!


T=1.4Tc

[Tokyo]

Spectral Functions

Thermalization at RHIC study the phase diagram: • (highest) temperature of the matter• chiral symmetry restoration (mass generation!)• medium effects on spectral properties above + below Tc

Introduction I: EM-Probes -- The Basic Questions

Inevitable consequences of QGP, link to lattice QCD

1.0 T/Tc

m‹qq›-

[Bielefeld]

Chiral CondensateQCD Phase Diagram


(ii) Heavy Partons + Multiple Bound States

→ based on finite-T lattice potentials approach to “zero-binding line” ~ stable-mass-resonance

[Shuryak,Zahed, Brown, …]

• thermal parton scattering through bound states?• composite interactions? quark scaling?

Dilepton Radiationratio to pert. qq rate

Mee/mq

_


Cocktail comparison

• Data and cocktail absolutely normalized

•Cocktail from hadronic sources•Charm from PYTHIAPredictions are filtered in PHENIX acceptance

•Good agreement in 0 Dalitz•Continuum:hint for enhancement not significant within systematics

•What happens to charm?•Single e pt suppression•angular correlation???

• LARGE SYSTEMATICS!

rhic ii workshop, 11/11/051 em probes at rhic ii zhangbu xu (bnl) for the rhic-2 em-probes working...

Documents

rhic slide

rhic hadron liquid close

hepph0010101 slide

visible world slide

rapp nuclth0409054 slide

lowmass vector mesons

khz minbias rate slide

low mass electron pairs