Electromagnetic Measurements at RHIC

Electromagnetic Measurements at RHIC

Hideki Hamagaki

Center for Nuclear StudyUniversity of Tokyo

2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 2

Prologue• EM probe = penetrating probe?

– correct– EM probes provide information on when and where they

are produced– EM probe? = leptons or photons; promptly produced

without intermediate state (hadron resonances); thermal photons/pairs, direct photons, Drell-Yan pairs, pairs from annihilation (quark-level and hadron-level),

• Physics quantities with EM measurements have many other sources

– hadron’s leptonic or photonic decays (as I will show you later)

– some of hadrons work as penetrating probes, depending on their life-time and when and where they are produced;


Prologue -- cont.• J/

– production: initial stage, and hadronization stage (recombination)

– life-time: c = 2264 fm; decay far outside the source

– long life time: 0, , D and B

• low-mass vector mesons; , , – production: creation in hadron phase + coalescence in the

freeze-out stage– life-time: c() = 1.3 fm, c() = 23.3 fm, c() = 46.2 fm– a part of and decays inside the source (modifications),

and the rest outside the source (free decay)– (recreation; thermalized): ee (escape from

the source) is dominated by the decay inside, can be used as a fast clock, and is a “pseudo-penetrating probe”


Outline of My Talk

• Photon measurements• Lepton measurements

– Single electron measurements leptonic decay of open quarks

– J/ measurements– Thermal pairs and vector mesons

• Summary and Outlook


Direct photons• Direct photon is a unique probe, which provides direct

information of its birth, because of penetrating property• photons can come from every stage of collisions, and can

have various origins• direct photons = not from “hadron decay”

direct photons

photons from hadron decay

non-thermalinitial hard scattering

pre-equilibrium

thermalQGP

Hadron gas


Quick Look of Various Photon Sources

• for thermal and hard photon measurements, hadron decay is a non-trivial background source– strong suppression of high pT

hadrons would improve the ratio, in particular, for hard direct photons

• a window for QGP thermal photons; pT = 1 ~ 3 GeV/c

thermal

decay

hard


Success in the 1st round• Direct photon in high pT

region in Au+Au collisions– suppression of high pT 0

yield makes the 0 ratio larger

– comparable to a pQCD calculation (Ncoll scaling), which means;

no strong initial state effect; modification of structure function

0 suppression is the final state effect

• 2nd round thermal photons


Is thermal photon yield suppressed?

• Comparison of the present result with calculations with kT broadening and jet QGP bremsstrahlung– fast quarks passing through QGP, which is a

significant photon source for pT < 6 GeV/c

• At present, it is too early to claim anything significant

• Gluon plasma (GP) and photon yield– chemically non-equilibrium state; GP

QGP– hotter than QGP ( smaller degeneracy)

• hot glue scenario?– If GP stays long, it will have impact to

photon production in hot matter; photon yield will be related to the evolution from GP to QGP

– Other way to say: photon suppression is the signature of GP; native no-QGP hadron picture can be ruled out


Single electrons are from many sources

• Photonic– Photon conversions

• from decay mesons• from direct photons

– Dalitz decays of 0, , ’, , • Non-photonic

– leptonic decay of D & B– Thermal lepton-pairs– Di-electron decays of ,,– Kaon decay (Ke3 0e)

• Leptonic decay of D & B is dominant among the “signals”

– Today’s signal is tomorrow’s BG

• Subtraction is a tricky business


Extraction of non-photonic componentsNe

0

1.1% 1.7%

Dalitz : 0.8% X0 equivalent

0

With a converter in

W/O converter Conversion at the detectors

0.8%

Non-photonic

1

1

RNNRN

RNNN

NNRNNNN

sim

C

e

NC

esimNp

e

sim

NC

e

C

eP

e

Np

e

P

esim

C

e

Np

e

P

e

NC

e

without Conv.with Conv.

En

trie

s/N

evt

En

trie

s/N

evt

pT[GeV/c]Mininum Bias Au+Au in sNN=200GeV

N(tot)/N(photonic)


Centrality dependence of electron yield

• Ncoll scaling seems to work well• NAuAu(pT;b) = Npp(pT)Ncoll

= 0.938+-0.075(stat) +- 0.018(sys) = 0.958+-0.035(stat) with p-p data included

• Binary scaling seems to work well

• keen interest has been in the behavior of heavy quarks in dense matter

– thermalization? – energy loss?


Hints of communication

• v2 for non-photonic electron– prediction based on a quark

coalescence model v2(D) ≈ v2(light) + v2 （ charm ）

– “thermal” is preferred

• RAA(pT) for non-photonic single electrons in Au+Au

– significant reduction at high pT– suggest sizable energy loss!

Year-4 RUN results with high statistics are definitely needed to confirm these


Quarkonium in HI collisions

• Novel idea of J/ suppression– by Matsui and Satz (1986; before

experimental results)– a good probe of deconfinement

• suppresion due to Debye screening in deconfined phase

• History in Brief– observation of suppression in S + A

turned out to be similar to p + A

– anomalous suppression observed in Pb + Pb

a result to evident QGP at SPS


Recent Progress in Theory (I)

• Lattice-QCD told us;

– confining potential starts to disappear at low temperature far below TC

– J/ does not melt easily• M. Asakawa, T. Hatsuda; Phys.

Rev. Lett. 92 (2004) 012001

– Impact to the scenarios• Is dissociation dynamical or

static?


Recent Progress in Theory (II)• Enhancement of J/ yield

– basic idea is to add recombination of charms to scenario of the direct production with subsequent suppressions

– statistical hadronization model• coalescence in the final stage

– L. Grandchamp et al., Nucl.Phys. A715 (2003) 545

– A. Andronic et al., Nucl.Phys.A715 (2003) 529

– kinetic formation model• reproduction in QGP +

coalescence in the final stage– R.L. Thews et al., Phys. Rev. C 63

(2001) 054905


J/ in Au-Au collisions at RHIC

• results from Year-2 RUN– very poor

• but is already inconsistent with large enhancement scenarios

– e.g. kinetic formation model, cf. PRC 63, 054905 (2001)

• In Run-4, 240 b-1 recorded with improved detector performance

– ~ 100 times more J/signals expected than in Run-2


Thermal radiation and low-mass vector mesons

• Still missing at RHIC

• Experimentally, combinatorial background is very large and must be subtracted properly

• Large physics background comes from charm.– Charm production is

measured with ~15% accuracy by single electron measurements.

A prediction

R. Rapp, nucl-ex 0204003


Loss-mass pairs• Famous CERES results

– enhancement in the pair yield

• A probe of Chiral symmetry restoration and/or medium modification

• Only hadronic decays so far at RHIC–

• STAR (Phys. Rev. Lett. 92, 092301)

– K+K- • PHENIX (nucl-ex/0410012)• STAR (nucl-ex/0406003)

• Lepton-pair measurement is demanding– should provide information on space-

time evolution at RHIC, which might be very different from SPS

– mass shift vs broadening

R. Rapp (Nucl. Phys A661(1999) 238c


Combinatorials is overwhelming

• Combinatorial background is determined with ~1% accuracy in Run3 and Run2 using a mixed event method

• Higher statistics from Run4 data should help, but it may not be enough for low-mass vector mesons

Real and Mixed e+e- Distribution

Run2 AuAuMinimum Bias

0

Real - Mixed with systematic errors

1 2 3 [GeV/c2] 0 1 2 3 [GeV/c2]


How to measure low-mass pairs• At PHENIX, R&D efforts to develop

a Dalitz rejecter, in order to manage combinatorial backgrounds

• HBD (hadron Blind Detector) is a strong candidate– UV photon detector

• with CsI cathode• CF4 gas radiator• Ne(Cherenkov) > Ne(ionization)

S/B ~ 1/500

“combinatorial pairs”

total background

Irreducible charm backgroundall signal

charm signal


Summary and Outlook• Direct photons

– thermal photons may not be abundant

• Single electron measurement in Au+Au collisions– dominant contributions from charm and bottom– yield at mid-rapidity scales with Ncoll

no significant effect from initial state

– possible finite v2 and high pT suppression• flow and stopping effect -- to be determined

• J/ result for HI collisions at RHIC– inconclusive result from Year-2 RUN– good statistics in Year-4 RUN stay tuned

• Thermal pairs and low-mass vector mesons– combinatorials, charms, ...– HBD is to buster combinatorials


Heavy flavor production• Charm (or bottom) production

– Leading order at low x = ’’gluon fusion’’

A good probe of; • Initial state (probably OK)

– Parton distribution functions– kT broadening

• Early stage of evolution (?)– Parton energy loss?– Thermalization & Flow? – Thermal production?

• How to measure?– exclusive– leptonic decay measure electrons


The result implies;

• no significant initial state effect in the mid-rapidity at RHIC energy– gluon shadowing & anti-shadowing are not large

• no anomalous enhancement due to thermal production

• No energy loss?• No thermalization? next slide

Pb / p


Is Heavy quarks immune to Hot matter?

• The answer is “not definite”• Electron pT distribution is not

sensitive to modification of pT distribution of the parent, due to decay kinematics

• Need to extend to higher pT, in order to see “flow’’ or “energy loss’’ effects

Year-4 RUN

– A caveat

D from PYTHIA

D from Hydro

B from PYTHIA

B from Hydro

e from PYTHIA

e from Hydro

S. Batsouli et al.: Phys.Lett. B557 (2003) 26


Limitation of inclusive e measurement

• High pT region in e– c decay b decay– not high pT/m for parent c and

b

• Exclusive measurement– D0 K+ + - ... bySTAR– OK in p+p & d+Au– combinatorial will be a killer in

Au+Au

need vertex determination

Electromagnetic Measurements at RHIC

Documents

hard photon measurements

em measurements

photon production

rounddirect photon

significant photon source

qgp thermal photons

fm decay

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