Physics of heavy ion collison: V and VIOpen Charm

J/Low mass di-lepon

Thermal photon

Y. Akiba (RIKEN)Beijing Summer School 2004

Lecture plan

I. Theroy backgroundII. ExperimentsIII. Soft Hadron PhysicsIV. Hard Probe

V. Charm J/

VI. Low mass dileptonthermal radiationstatus of the field

Thomas Peitzmann

YasuyukiAkiba

Hadronization (Freeze-out) + Expansion

Pre-equilibrium

QGP phase

Thermalization

Mixed phase ?

Probes of the heavy ion collisionProbes of the heavy ion collision

p K

e

Space (z)

Tim

e

A A

etc..High pthadrons( jets )

• Soft hadrons: probes of the final stage (freeze-out)• charm, J/, di-leptons, photons: probe of the initial stage

c

What is charm

• Because of its large mass, it can be produced only at the initial stage probe of the initial stage of the collision probe of the medium formed in the collision

• Production rate of charm is much smaller than light quarks (~1/10,000 at SPS and ~1/300 at RHIC energy)

Quarks (PDG 2002) current quark mass u 1-5 MeV d 3-9 MeV s 75-170 MeV c 1.15 – 1.35 GeV b 4.0 – 4.4 GeV t 174 +- 5.3 GeV (178 GeV)

Light quarksp, , K, , etc

Heavy quarksD,D*,J/B,B*,(no bound state

m << QCD (u,d) orm ~ QCD (s)Non-perturbativepQCD is not applicable

m > QCD

pQCD is applicable

Physics with open charm production

• pp and hadron-hadron collisions– Comparison with pQCD calculation– Measurement of gluon density G(x)– Intrinstic <kt>– Base line for charm physics in pA and AA

• pA– Gluon shadowing– Energy loss of gluons in cold nuclear matter– Parton multiple scattering and <kt>– Base line (normal nuclear effect) for charm physics in AA

• AA– Gluon shadowing– Energy loss of charm in high density matter– Thermal production of charm in high temperature QGP

Measurement of open charm

• Charmed meson (D, D, D) can be measured from its decay products as peak in invarant masssD0KD+KD*D(K)

• Charmed mesons has a large leptonic branching ratio (D0: 7 %, D+:17%). Charm production can be measured indirectly by single lepton in 0.5< pt<3 GeV/c and lepton pairs in 1<M<3 GeV.In higher pT and mass, b-quark decay becomes dominant.

c c

0DK

0D

K

indirect signal:single leptonlepton pairs

direct signal:DK, DK

+

Open charm at fixed target (cross section)

• Charm cross section by fixed target experiments are reasonably reproduced by LO pQCD event generator (PYTHIA) with large K-factor, or by NLO pQCD calculation (HVQMNR). Note that pQCD may or may not be applicable to charm production because charm mass is small (~1.5GeV)

• In the left figure, world pi+N data and p+N data are compared with PYTHIA calculation. The s1/2 dependence of the calculation mainly reflects the underlying PDF.

s1/2(GeV)

cc in pN,N

Open charm at fixed target (<Pt>)

• FNAL E791 and CERN WA92 measured hight statistics Pt distribution of D-mesons in +N collision. (No high statistics data in p+A is available)

• In the left figure, E791 data and WA92 data are compared with PYTHIA calculation with different <kt> values. <kt> ~ 1.5 GeV/c seems to be required to reproduces the data.

• Comparison with NLO pQCD calculation also suggests that observed pt distribution is harder than predicted, and large (>1GeV/c) intrinsic <kt> is needed to reproduce the data.

• No published data on nuclear dependence in <pt2> but it can be similar to that of J/

pT2 (GeV2/c4)

E791 d/dpT2

pT2 (GeV2/c4)

WA92 d/dpT2

Nuclear dependence of Open charm cross section

• Open charm cross section has little or no nuclear dependence = 1.00± 0.05 (E769 250GeV +A) = 0.92 ±0.06 (WA82 340GeV +A) = 1.02 ±0.03 ±0.02 (E789 800GeV p+A)

• This is consistent with that there is little nuclear shadowing at the x region probed by the fixed target charm experiments.

• Significant nuclear suppression is reported by large xf region (WA78, =0.81 ± 0.05). This can be due to nuclear shadowing in small x, but this is not well understood.

• At RHIC, we can prove x region much smaller than in fixed target. We may observe nuclear shadowing effect in charm production.

E769 250 GeV ± PRL 70,722 (1993)

= 1.00 ± 0.05

Open Charm enhancement (?) at SPS Pb+Pb

• NA50 measured di-muon distribution in 1.5<M<2.5 GeV.• The mass distribution is well reproduced by DD pairs + Drell-Yan components in p+A

(@450GeV) with charm cross section cc/A = 36.2±9.1 b, consistent with charm cross sections of other experiments

• In Pb+Pb, an excess of dimuon signal is observed. The excess can be explained by enhancement of charm production by factor of ~3 in central Pb+Pb.

• The other explanation is possible: e.g. thermal dimuon pair produciton.

NA50 NA50

Charm and single electron at RHIC

• At RHIC, it is expected that charm decay can be the dominant component of single electron in pt > 1.5 GeV/c– Large production cross section of

charm ( 300-600 ub)– Production of the high pt pions

is strongly suppressed relative to binary scaling

– Production of charm quark roughly scale with binary collisions.

• PHENIX observed “excess” in single electron yield over expectation from light meson decays and photon conversions Observation of charm signal at RHIC

Simulation before RHIC

How to extract charm signal from single electron (1)

• A large fraction of the observed electrons are background from Dalitz decays and photon conversions.Dalitz decay:

0, e+e-

0e+e-

e+e-

Photon conversion: 0, e+e-

They are related to corresponding photon decay mode (0 0

• In Cocktail method, these electron background are calculated from the hadron spectra (“cocktail”) and subtracted from the observed electron data. (See left plot).

• The cocktail method is used in PHENIX RUN-1 Au+Au (130 GeV), RUN-2 p+p (200 GeV), and STAR RUN3 (d+Au and pp).

conversion

0 ee

ee, 30

ee, 0ee

ee, ee

ee

’ ee

How to extract charm signal from single electron (2)

Ne

0

1.1% 1.7%

Dalitz : 0.8% X0 equivalent

0

With converter Conversion in converter

W/O converter Conversion in the beam pipe, etc

0.8%

Non-photonic (charm)

The yield Ne of Dalitz decay and photon conversion is proportional to the yield of real photon. The yield Y per photon of these “photonic” electron is expressed as , where is the branching ratio of the Dalitz decay relative to the two photon decay mode, and t is the thickness of the conversion material in term of the raditaiton length. The factor ½ is to make the ratio per photon, and the factor 7/9 is the photon conversion probability per radiation length. For 0, =1.2%, and the Dalitz decay is equivalent to 1.2/2×(9/7)=0.8% radiation length of conversion material. By adding a thin additional converter, the yield of the conversion electron increases. Since yield of charm decay electron is independent of t, we can determine its fraction by comparing the data with and without the converter.

Material thickness (rad. Length)

)97

21( tY

RHIC RUN1: first observation of charm signal by PHENIX

• PHENIX observed excess of single electron yield over the contribution from light meson decays and photon conversoins

• Spectra of single electron signal is compared with the calculated charm contribution.

• Charm contribution calculated asEdNe/dp3 = TAAEd/dp3

– TAA: nuclear overlap integral– Ed/dp3: electron spectrum from

charm decay calculated using PYTHIA

• The agreement is reasonably good.

0 10%cc 380 60 200 b and 0 92%

cc 420 33 250 b

Assuming that all single electron signal is from charm decay and the binary scaling, charm cross section at 130 GeV is obtained as

PHENIX PRL88 192303

Comparison with other experiments

• PHENIX single electron cross section is compared with the ISR data single electron data

• Charm cross section derived from the electron data is compared with fixed target charm data

• Single electron cross sections and charm cross sections are compared with – Solid curves: PYTHIA– Shaded band: NLO QCD

Assuming binary scaling, PHENIX data are consistent with s systematics (within large uncertainties)!

charm data in pp,dAu, and AuAu @ 200 GeV

• STAR– First observation of D meson in d+Au at 200 GeV– Single electron measurement in pp and d+Au

• PHENIX– Single electron measurement in p+p, d+Au, and AuAu

• Baseline cross section in p+p• Binary scaling in d+Au: cold nuclear matter• Binary scaling in Au+Au

– Elliptic flow of single electrons from charm decay

D meson and single e measurement by STAR

• reconstruction of D K pi mesons in minimum bias dAu collisions– A large combinatorial background (S/B ~ 1/600) but the signal is visible and significant

• Single electron measurements in pp and d+Au.– Confirmed the PHENIX single electron measurement– A large photon ocnversion background is tagged and subtracted

D0+D0

0 < pT < 3 GeV/c, |y| < 1.0

d+Au minbias

STAR charm measurement results

• STAR D-meson measurement (d+Au) and single electron measurement (pp and d+Au) gives a consistent charm cross section (1.3±0.2±0.4 mb)

• The charm cross section by STAR is somewhat higher than NLO pQCD energy dependence curve tuned with the low energy data.

• PHENIX result (0.7±0.1±0.3 mb in pp: see next slides) is lower than STAR and closer to the NLO pQCD curve.

NNcc

PHENIX: Single electron in pp at 200GeV

• Inclusive invariant cross section for the electrons

• We observe deviation from the cocktail prediction which indicates significantNon-photonic component : electrons originated from the processes not included in the cocktail

p+p s=200 GeV PHENIX Preliminary

base line measurement in pp @ 200 GeV

PHENIX PRELIMINARY

• PHENIX single electron data (after background subtraction) is compared with PYTHIA calcuation of ce and be spectrum. The PYTHIA under-predict the single electron yield at high pT. This can be due to harder fragmentation of cD and bB

• The total charm cross section in pp at 200 GeV is extracted from a fit to the single electron data. The obtained cross section is lower than STAR. But the two measurement is consistent with the large systematic uncertainties.

• The “pp best fit” is used in the comparison of d+Au and Au+Au data in the following slides.

PHENIX PRELIMINARY

JPG30(2004)1189

Cold nuclear matter: dAu @ 200 GeV

• PHENIX measured single electron spectrum in d+Au and compared it with the pp best fit curve.

• In the plot, the d+Au data is divided the nuclear overlap function TAA, and it represents the invariant cross section per binary N+N collisions.

• The single e data in d+Au min. bias is consistent with the binary scaled p+p single electron data.

PHENIX PRELIMINARY

1/T A

BE

dN/d

p3 [m

b G

eV-2]

JPG30(2004)1189

Centrality dependence in dAu (PHENIX)

• single electrons from non-photonic sources agree well with pp fit and binary scaling

PHENIX PRELIMINARYPHENIX PRELIMINARY

PHENIX PRELIMINARYPHENIX PRELIMINARY

1/T AB

1/T AB

1/T AB

1/T AB

1/T A

BE

dN/d

p3 [m

b G

eV-

2 ]1/

T ABE

dN/d

p3 [m

b G

eV-

2 ]

1/T A

BE

dN/d

p3 [m

b G

eV-

2 ]1/

T ABE

dN/d

p3 [m

b G

eV-

2 ]

JPG30(2004)1189

Hot nuclear matter: AuAu @ 200 GeV1/

T ABE

dN/d

p3 [m

b G

eV-2]

PHENIX

• PHENIX measured single electron in Au+Au at 200 GeV as well. The Dalitz/conversion background is subtracted by the converter method.

• The AuAu data is divided by nuclear overlap integral TAB to obtain the cross section per binary N+N collisions.

• The Au+Au data is compared with the best fit curve to the PHENIX p+p data.

• AuAu ≈ scaled pp (within errros) for pT ≤ 1.5 GeV/c

• The data in pT>2 GeV is somewhat lower than the best fit curve. Is this indication for “high” pT suppression? However, the statistics is limited to draw a definite conclusion here.

JPG30(2004)1189

1/T AA

1/T AA

1/T AA

Centrality dependence in AuAu (PHENIX)

• uncertainties too large for definite statements regarding (small) deviations from binary scaled pp results

1/T AA

1/T AA

1/T A

BE

dN/d

p3 [m

b G

eV-2]

1/T A

BE

dN/d

p3 [m

b G

eV-2]

1/T A

BE

dN/d

p3 [m

b G

eV-2]

1/T A

BE

dN/d

p3 [m

b G

eV-2]

1/T A

BE

dN/d

p3 [m

b G

eV-2]

1/T A

BE

dN/d

p3 [m

b G

eV-2]

S. Kelley et. al. (PHENIX) JPG30(2004)1189

Binary collision scaling in AuAu (PHENIX)

0.906 < < 1.042

dN/dy = A (Ncoll)

• binary collision scaling of pp result works VERY WELL for non-photonic electrons in AuAu charm cross section scale with the bianry collision.

• Note that the integrated yield in low pT single electron is not sensitive to the change of the D-meson momentum distribution (see next slide).

D meson spectrum shapeSingle electron from charm/beauty decay in Au+Au scale with the number of binary collisions. Does this mean that there is no “high pT suppression” in charm sector?The answer is no. Single electron spectrum from PYTIHIA calculation and from D-mesons with thermal distribution + radial flow model (hydo in the plot) has almost identical shape in the measured pT (pT<2.5 GeV/c). Even if D meson production at high pT is suppressed, the present data is not sensitive to the suppression effect.Higher pT data is needed (RUN4) Elliptic flow of charm can be a better way to probe energy loss of charm in the medium (see next)

D from PYTHIA

D from Hydro

B from PYTHIA

B from Hydro

e from PYTHIA

e from Hydro

See poster S.Batsouli

Ecm=130 GeV

S. Batsouli et. al.,PLB557(2003)26

Does charm flow? electron v2 (PHENIX)

• In Au+Au collision at RHIC, a strong suppression of high pT particle production is observed. The leading (and almost unique) explanation of the suppression is energy loss of quark/gluon in the dense medium formed in the collision.

• If charm quark also lose energy in the medium, the momentum distribution of D-meson and its decay electron should be modified. However, as seen in the previous slide, the data can not distinguish no energy loss scenario (no change from pp) and thermal + flow model (strong energy loss)

• The elliptic flow is a probe of early thermalization. If charm quark interact strongly with the dense medium and thermalized, it should also obtain elliptic flow (v2). If charm has v2, single electron from charm decay should also has v2.

• Data shown here is PHENIX preliminary (QM04)

Elliptic flow of single e: dn/d of e distribution

Miss ID e+(e-)

e+(e-)candidate

Electron v2 is estimated by using reaction plane method. (measure azimuthal angle of each electrons with respect to the reaction plane )

dn/d = N(1+2v2cos(2* (φ-rp)))

The bottom distribution is after miss ID electrons are subtracted.

Pt dependence of raw electron v2

The figure shows pt dependence electron v2. The electrons v2 are including background & charm/beauty decay (signal) electrons v2.

Dalitz decaysDi-electron decays Photon conversionsKaon decaysThermal dileptons

charm decay beauty decay

“background”

“signal”

page4

Does charm flow?PHENIX PRELIMINARY

• Since hadrons (pi0, eta, kaon) etc has v2, the background electrons from these hadrons also shows finite v2.

• From the v2(pT) of the inclusive electrons (previous slide), the contribution from the background electrons are subtracted.

• The data corresponds to the v2 of “signal” electrons (i.e form charm and beauty decays). The data is compared with the models with and without charm quark flow in the medium.

• definite answer: AuAu RUN-04 at RHIC!

Charm: Summary

• Charm is an important probe of the dense matter created in the nucleus-nucleus collision.

• Charm measurements at RHIC started producing a wealth of data. The binary scaling of the charm production in d+Au and Au+Au collision is now established. The charm measurement at RHIC is now moving to looking for energy loss/high pT suppression of charm and the elliptic flow of charm. These measurements are important to understand the property of the dense medium formed at RHIC.

• The first detailed study of charm production in Au+Au collision will be available in near future from the high statistics data of RHIC RUN-4.

J/

• J/ in pp and pA• J/ at SPS: NA50 results• J/ at RHIC

– First measurement in ppJ/ at 200 GeV– J/ in d+Au collisions– J/ in Au+Au

• J/ at RHIC: suppression or enhancement• RUN2 result• RUN4 outlook

J/ suppression• J/ is a bound state of c and cbar quark

(charmonium). Experimentally, it is observed as a sharp peak in invariant mass of lepton pairs (e+e- and +-).

• Matsui and Satz predicted that the production of J/ is suppressed due to Debye screening of color force in a quark-gluon plasma J/ suppression is an evidence of deconfinement.

• Because of the prediction by Matsui and Satz, J/ production in p+A and AA collisions are extensively studied in the past ~20 years. The predicted suppression was observed by NA50 at SPS.

• However, J/ is also subject to various nuclear effect. It requires a careful systematic study of J/ production in pp, pA and AA collision to draw definite conclusion on the deconfinement.

Perturbative Vacuum

cc

Color Screening

cc

Physics with Charmonium production

• pp : study of production mechanism– Color evapolation model, Color singlet model, Color Octet model– Production cross section and its pt and s1/2 dependence– Polarization– Production of J/, (2S), c, states– Base line for pA and AA

• pA : study of “normal nucleus effect”– Nuclear dependence of (J/) ….A or abs (nuclear absorption)– Nuclear dependence of <Pt2> …multiple scattering effect?– Base line for AA

• AA : study of “medium effect” in high density matter– J/ suppression : signature of QGP (Matsui/Satz)– J/ formation by c quark coalescence at RHIC/LHC ?

J/ at fixed target and the ISR

• Many experimental data in the last 30 years near threshold to s1/2 = 63 GeV

• Rapid increase of production cross section to s1/2 ~ 20 GeV followed by a gradual increase in higher energy.

• The s1/2 dependence is similar to that of charm production cross section, suggesting that J/ and charm ratio is almost constant with energy

• Detailed, high statistics study of A dependence by E772/789/866 at 38 GeV and p+A and A+A measurement by NA38/50

J/ at fixed target: A dependence

• Significant nuclear dependence of cross section is observed• Power law parameterization = A

= 0.92 (E772. PRL66(1991) 133) (limited pt acceptance bias?) = 0.919 ± 0.015 (NA38. PLB444(1998)516) = 0.954 ± 0.003 (E866 @ xF=0. PRL84(2000),3258 ) = 0.934 ± 0.014 (NA50, QM2001)

• Absorption model parameterization = 6.2 mb (NA38/50/51) to 4.4 mb (NA50, QM2002)

• Small difference in between J/ and (2S) (E866)(J/) – (S) ~ 0.02-0.03 @ xF = 0

J/ at fixed target: <Pt2>

• <pt2> of J/ increases with beam energy– <kt> increase with energy ?– Gluon radiation effect? ( pQCD )

• <pt2> of J/ increases with L (path length in target nuclear matter) or A1/3

– Consistent with multiple scattering of incoming partion ( Cronin effect)

<pt2> vs L <pt2> vs A<pt2> vs s1/2

J/ suppression at SPS

• NA50 observed anomalous suppression of J/ in Pb+Pb collisions at 158 GeV

• Deviation from the nuclear absorption model (with ~4 to 6 mb) is seen in ET > 40 GeV or >2.5 GeV/fm3

• If the suppression is due to J/break-up in high energy density matter, this is a strong evidence for QGP formation

• If J/ suppression is due to QGP formation, almost all of initially produced J/ should be suppressed at RHIC energy

NA50

NA50

J/ at RHIC

• J/ in pp collision: baseline• J/ in d+Au collision: gluon shadowing?• J/ in Au+Au collision: suppression or enhancement

J/ee and J/ in pp @ 200 GeV

e+e- |y|<0.35 - 1.2 < y < 2.2

First measurement at s1/2=200 GeV by PHENIX Limited statistics (~100/nb). More data in later runs

ppJ/ at RHIC (PHENIX)

PHENIX measured the pT distribution and rapidity distribution of J/ at 200GeVThe result is consitent with the extrapolation from the lower energy data.

Predicted Gluon Shadowing in d+Au

J/ in dAu collisions at RHIC

• J/ produced by gluon fusion• sensitivity to gluon pdf• 3 rapidity ranges in PHENIX probe different momentum fraction of Au partons

– South (y < -1.2) : large X2 (in gold)– Central (y ~ 0) : intermediate– North (y > 1.2) : small X2 (in gold)

From Eskola, Kolhinen, VogtNucl. Phys. A696 (2001) 729-746.

X1 X2

J/ inNorthy > 0

X1 X2

J/ inSouthy < 0

rapidity y

Aud

sx1x2 ~ MJ/2

Parton saturation at small x

• The parton density depends on the kinematic region.• At small x (= pparton/pbeam), the parton density increase.• In nucleus A, the parton density is ~ A1/3 * (parton density in nucleon)• The effective size of parton is ~s/Q2

• For a large nucleus, the density A1/3 s/Q2 x NNparton appraoches ~ 1

Violate unitarity parton density saturation Breakdown of the binary scaling (shadowing)

Parton densityin low x is veryhigh

J/ in pp and dAu collisions at RHIC

• rapidity distributions: improved pp / first dAu measurements

Aud

J/ dAu/pp versus rapidity at RHIC

S.B. Klein and R. Vogt, nucl-th/0305046

Low x2(shadowing)

• indication for (weak) shadowing and absorption• centrality dependence studied as well!• more statistics desirable to disentangle nuclear

effects (and distinguish models)

Aud

High x2

Low x2

J/ at RHIC: suppression or enhancement

• In the prediction of Matsui and Satz, J/y is suppressed if a deconfined state of matter is formed.

• However, this prediction ignored the possibility that J/y is formed by recombination of c and cbar pairs. Since a large number of charm quark pairs are (~20 pairs per event in the central Au+Au collision) at RHIC, this process can not be ignored.

• In model calculations, J/y at RHIC is enhanced (R. Thews et al) or only slightly suppressed.

• If J/ formation by the recombination is established, this can be considered as a direct evidence of deconfinment.

R.Thews et al, PRC63(2001)054905

Grandchamp, Rapp

J/ at AuAu at RHIC

• PHENIX measured J/ ee in Au+Au collision in RUN2.

• Although the statistics is quite limited, the data disfavored the model of a large enhancement.

• High statistics data from RUN4 Au+Au will provide the first definite measurement of the J/ production at RHIC.

PRC69(2004)014901

J/ measurement by PHENIX RUN1-RUN4Year Ions sNN Luminosity Detectors J/

2000 Au-Au 130 GeV 1 b-1 Central (electrons) 0

2001 Au-Au 200 GeV 24 b-1 Central 13 + 0 [1]

2002 p-p 200 GeV 0.15 pb-1 + 1 muon arm 46 + 66 [2]

2002 d-Au 200 GeV 2.74 nb-1 Central 300+800+600

2003 p-p 200 GeV 0.35 pb-1 + 2 muon arms 100+300+120

2004 Au-Au 200 GeV 240 µb-1 Central

+ 2 muon arms~400+2x1600 ?

Huge increase of data in RUN4

1.5 B events recorded inPHENIX in RUN4

J/ Signal in RUN4 Au+Au (PHENIX)

PHENIX has accumulated ~240µb-1 of Au+Au data and already see clear J/ signal from a small portion of AuAu (data less than 10%).

J/->ee J/->

J/ : summary

• J/ remains an important QCD probe in pp, pA (dA) and AA collisions.

• The anomalous J/ suppression at SPS by NA50 is considered as an intriguing evidence for QGP formation at SPS.

• At RHIC, the first results of J/y in pp and dA are obtained. The data seems to indicates small nuclear absorption at RHIC energy as well as nuclear shadowing effect in forward rapidity.

• If J/ suppression at SPS is due to deconfinement, almost all of initaly produced J/ at RHIC will be suppressed. On the other hand, due to a large charm production at RHIC enegy, J/ can be formed by quark re-combination. The forthcoming J/ data in RHIC RUN-4 will answer whether J/ is suppressed or enhanced at RHIC.

Low mass di-lepton

• Motivation– Chiral symmetry restoration– Duration time

• CERES results– S+Au (200 GeV)– Pb+Au (158 GeV)– Pb+Au (40 GeV)

• More recent results from SPS– CERES (with TPC)– NA60

• RHIC/PHENIX– Theory prediction– RUN2 AuAu

Low mass di-lepton --- Motivation

• Low mass di-lepton (Mee < 1 GeV)– The low mass di-electron pair is dominated by Daliz decays of 0, (e+e-

) and di-lepton decay of the vector mesons ( e+e-, +- ). The • Chiral symmetry restoration

– Mass and width of the vector mesons (r,w,f) can be modified in medium by partial restoration of chiral symmetry. The modified mass/width of these mesons in the medium can be measured through its di-lepton decay mode.

• Thermal di-leptons– If long lived hadronic source or QGP is formed, the hot medium should

emit thermal di-lepton pairs. It is expected that the dominant contribution to the thermal pair is hadronic gas or mixed phase.

• The meson regeneration in a long lived hadronic gas– The r meson has a very short (1.2 fm/c) life time. If a long lived hadronic

gas is formed, the meson is re-generated in the medium. This is then observed by an enhancement of the yield of ee compared with .

Low mass vector meson as a probe of dense matter

• Vector mesons has short life time and it can decay inside of the dense matter.

• In di-lepton decay mode, the decay lepton can escape from the medium without any interaction direct probe of the property of meson in the hot/dense matter.

• The property (mass, width) of the mesons can be modifed in the medium due to (partial) chiral symmetry restoration.

e+

e-

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

CERES (NA45)

• Designed to measure low mass e+e- pair at SPS. Observed low mass pair enhancement.• Two sets of Ring Imaging CHerenkov detector (RICH) for hadron-blind tracking

– Only electrons and very high pt hadron emits Cerenkov light in a very narrow cone around the track. The Cerenkov light is reflected by a spherical mirror and focues on UV photon sensitive detector. The emission angle of the track is determined by RICH-1, and the angle of the track after the magnetic field is determined by RICH-2.

• Measure e+e- pairs in mid-rapidity of p+A, S+Au, and Pb+Au collisions in Mee<1.5GeV

• Major results– Low mass e+e- pairs in S+Au at 200 GeV (PRL75(1995),1272)– Low mass e+e- pairs in Pb+Au at 158 GeV (PLB442(1998)405)– Low mass e+e- pairs in Pb+Au at 40 GeV (PRL91(2003)042301)

CERES results (1) pA and SAu

• CERES experiment observed a large (by a factor of 5) enhancement of the yield of low mass di-electron pairs (0.2<Mee<1.5 GeV) relative to the expectation from normal hadronic source (so-called “hadronic cocktail”) in S+Au collision at 200 GeV (sNN

1/2=20 GeV)

• No such enhancement is observed in p+Be nor p+Au collisions at 450 GeV

Enhancement in S+Au No enhancement in p+A

CERES results: Pb+Au

• CERES observed similar enhancement in Pb+Au collisions at 158 GeV (sNN1/2=17

GeV) and 40 GeV (sNN1/2=9 GeV).

• Enhancement factors:S+Au 200 GeV 5.0±0.7(stat)±2.0(sys) (PRL75(1995)1271Pb+Au 158 GeV 3.5±0.4(stat)±0.9(sys) (95 dataPLB422(1998)4052.9±0.3(stat)±0.6(sys) (95/96 data. QM2002)Pb+Au 40 GeV 5.9±1.5(stat)±1.2(sys) (PRL91(2003)042301

Pb+Au 158 GeV

95/96 combined data (QM02)2670±260 pairs in m>0.2 GeV

95 data in PLB422648 ± 105 pairs in m>0.2 GeV

PRL912670±260 pairs in m>0.2 GeV

Pb+Au 40 GeV

Theory comparison• The leading theory explanation

of the CERES low mass pair enhancement is thermal production of e+e- pair through the meson in the medium

e+e-

with modification of the meson mass/width.

• The data is not consistent with normal hadronic cocktail nor vaccuum spectral function (with long reaction time)

• The data do not discern between theories of in-medium modifications

More recent development in SPS• New CERES data in Pb+Au (158GeV)• A TPC (3-D tracking detector) was added to

CERES spectrometer higher mass resoluiton.

• Enhancement factor: 2.9xxx (preliminary)• ee is observed

Nch< 40

• NA60 data in In+In collision• Si pixel tracker is added in the target region of

NA38/NA50 di-muon spectromter• Low mass region is measured in di-muon

channel. Better mass resolution and wider kinematic overage than NA50

• Clear peaks of w ee and f ee are observed. The experiment can provide an independent measurement of low mass continuum.

NA60. 1% of data

RHIC: theory expectation

A large contribution from charm decay

R. Rapp nucl-th/0204003

Low mass enhancement due to modification

Prediction at RHIC At RHIC energy, it is predicted that the shape of the r meson is broadened, leading to a large enhancement of low mass di-leptons in M<1 GeV. The p meson width is also increased.

In the mass region above 1 GeV, a large yield of thermal di-lepton from QGP and the mixed phase is predicted.

Due to a large charm production, the di-leptons from correlated charm pair is a severe background in this mass region.

RHIC RUN-2 low mass data (PHENIX)

]/[c 10)()(2.74.13)PHX into(/ 252.124.8 GeVsysstatdmdN LMR

Low Mass Region (LMR) = 0.3-1.0 GeV

PHENIX measured low mass single electron pairs in Au+Au at 200 GeV. The data are compare with a calculation of hadronic cocktail. In the IMR, the charm yield from PYTHIA scaled by the number of binary collisions.

Predictions: LMR = 9.2 x 10-5

IMR = 1.5 x 10-5

Statistics is too limited to get significant singal.Higher statistics in RUN4 will provide a first measurement of the low mass di-lepton at RHIC.

]/[c 10)()(60.238.0)PHX into(/ 2540.181.0 GeVsysstatdmdN IMR

Intermediate Mass Region (IMR) = 1.1-2.5 GeV

PHENIX Preliminary

PHENIX Preliminary

and at RHIC

• PHENIX observed a clean and sharp ee and ee signal in d+Au collision at 200 GeV.

• In RUN-4 data, signficant measurement of and in ee decay channel is expected. The modification of mass and witdh of and in the medium can be tested from the data.

d+Au at 200 GeV

Low mass di-lepton: summary

• In hot/dense matter, properties of light vector mesons can be modified from their vacuum values.

• CERES experiment at SPS observed enhancement of low mass di-electrons in 0.2<Mee<1 GeV. The enhancement can be due to broadening of r in the medium.

• If this is due to change of r spectral shape, even larger enhancement is expected at RHIC.

• At RHIC, no significant measurement of low mass dileptons so far. The first data will come out from RHIC RUN4 Au+Au data.

photons: direct probe of hot/dense matter• real photon sources in AA collisions

– “trivial” background• decays of light hadrons ()• dominant at low / intermediate pT (few

GeV/c)– “thermal” black body radiation from hot

medium• partonic and/or hadronic medium• expected at low pT (<3-4 GeV/c)• Direct measurement of the temperature of

the hot matter– “direct” photons from initial state hard

scattering• Compton scattering dominates, i.e. probe for

gluon distribution• calculable in pQCD• Dominant at high pT (>3-4 GeV/c)• no fragmentation of photon!• A well calibrated probe of initial parton

density reference to the jet suppression

Direct photon in SPS (WA98)

• WA98 experiment observed direct photon in Pb+Pb collision at 158 GeV in pT>1.5 GeV/c

• The observed direct photon signal can be either– Themral photon from

QGP or hadronic gas– pQCD photon with

additional boost by <kT>

Subtraction method,upper limit

Correlation method:

Subtraction method

Predictionshadronic gasQGP

sumpQCD

Most probable yield (Ro=6 fm)The lowest yield (Ro=0)

Thermal photon at RHIC: theory prediction

At RHIC, the initial temperature of the matter should be higher than at SPS.T ~ 350 MeV is expected.

Higher yield of thermal photon.

In the model calculation shown in the left figure, the thermal photon from QGP is the dominant source of photons in 1<pT< 3 GeV/c

The signal level is 5 to 10% of photons from hadron decays (0, etc)

Theroy prediciton of Thermal photonS. Turbide, R. Rapp, C. Gale (hep-ph/0308085)

RHIC: direct photon in Au+Au (PHENIX)

In the measured high pT range, pQCD direct photon is dominant. Good agreement with unsuppressed, binary scaled pp NLO pQCD calculation. ( G(x), q(x) is not

modified in Au) Other source of direct photon (e.g. jet-medium interaction) is not ruled out Better systematic error control is needed to look for thermal photons in 1<pT<3 GeV/c

1 + ( pQCD direct x Ncoll) / ( phenix pp backgrd x Ncoll)

1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang NLO

1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang, mscale = 0.5, 2.0

PHENIX Preliminary PbGl / PbSc Combined

AuAu 200 GeV Central 0-10%

Summary: thermal photon

• Thermal photon is the most direct evidence that a hot matter is formed in nucleus-nucleus collison. The temperature of the system can be determined from the yield of the thermal photon.

• WA98 at SPS observed direct photon in Pb+Pb collision at 158 GeV. It is, however, not conclusive if the observed signal is thermal photon or pQCD direct photon.

• At RHIC, the first observation of direct photon at high pt is reported by PHENIX. The data indicates that pQCD direct photon follows binary scaling, as expected. Improved systematic uncertainty is required to observe thermal photon signal in low pT region.

Status of the field

• Summary of the results from RHIC in the first 3 years

• Does a QGP produced at RHIC

results from RHIC

• Since the start of the RHIC in the year 2000, a large amount of the data have been collected and analyzed by the four RHIC experiments.RUN1 (2000): Au+Au collisions at 130 GeVRUN2 (2001-2): Au+Au collisions at 200 GeV

p+p collisions at 200 GeVRUN3 (2003) d+Au collisions at 200 GeV

p+p collisions at 200 GeVRUN4 (2004) Au+Au collisions at 200 GeV

(>>10 times of RUN2 data) Au+Au collisions at 62 GeV (> RUN2 data)

• About 100 journal publications (total citation ~4000) from the four RHIC experiments

• The data from RUN1-RUN3 have already provide evidence that an extreme dense matter is formed at RHIC huge excitement in the field.

Does a QGP is formed at RHIC?

• The main purpose of RHIC is to create a de-confined state of matter, a quark-gluon plasma, and to study its property.

• There is an on-going debate if the RHIC has actually produced a QGP.– Many theorists in the field think yes.– Most of experimentalists are more conservative and we are

evaluating the experimental evidences for the QGP in the data.– It is a general consensus that there is compelling evidence that

RHIC collisions produces a very dense matter that thermalize quickly. The basic point of the debase is whether it is appropriate to call it a Quark-Gluon Plasma.

– The evidences are presented in the past lectures, but here I summarize them from the perspective of the QGP search.

What is a QGP

• One of the problem is that the term “QGP” is not well defined in terms of its experimental signature and properties.

• Some of the properties that a QGP should have – High energy density ( > 1 GeV/fm^3)– High temperature (T > 170 MeV)– It is thermalized (otherwise, its property such as T, P, etc are not

very meaningful)– It is de-confined. It is made of quark and gluons, not colorless

hadrons• Questions are then

– Is there evidence for high density?– Is there evidence for thermalization?– Is there evidence for de-confinement?

Is a dense matter formed

• The answer is yes. There are several pieces of evidence that the RHIC collisions produces matter with energy density far above the critical energy density =1 GeV/fm3

d+Au

Au+Au

dNch/dh and dET/dh measurement shows that the energy density at =1 fm/c is > 5 GeV/fm3

The hydrodynamic calculation indicates that the the theralization time is < 1 fm/c Initial energy density 10-15 GeV/fm3

The high pt suppression data provide direct evidence that dense matter is formed at RHIC. Presently, parton energy loss is the only mechanism that can explain the data. The energy density estimation by the energy loss model is ~ 15 GeV/fm3

Is the system thermalized?

• Soft hadron data provide evidence that thermalization is achieved– Hadron Spectra: thermal freeze-out at 110 MeV with radial flow <T>=0.5 – Hadron ratio: chemical freeze-out at 170 MeV ~ Tc– Elliptic flow: Direct evidence of rapid themalization. t(therm) < 1 fm/c.

• Impressive success of hydrodynamic model (of parton fluid) that requires rapid thermalization.– Problem: failure in reproducing HBT measurement data

Hadron ratios Radial flow Elliptic flow

Is system de-confined? Is T > Tc?

• There is no direct evidence so far on the de-confinement at RHIC.– Anomalous anti-baryon/pion ratio at high pT provide a hint that

the hadron formation is dominated by quark recombination. However, theoretical interpretation of the phenomena is not conclusive

– Forthcoming J/y data from RHIC RUN-4 may provide the evidence of de-confinement

• There is no direct measurement of initial temperature at RHIC– The measurement of thermal radiation can provide the direct

determination of initial temperature.

Do we have a single consistent theory?

A slide by T. Hallman (STAR spokesperson) at ICHEP04

Is unicorn captured?

• The experimental data from the first 3 years of RHIC operation provide compelling evidence that a dense matter is formed. It is difficult to describe the matter in terms of colorless hadrons

• There is no direct experimental evidence of de-confinement nor high temperature. Because of these lack of evidence, we think it is still premature to declare the discovery of QGP.

• However, the dragnet is closing, and there is a good chance that the missing evidence will be captured in near future.

RHIC

From slides of R. Stock and M. Gylassyin Quark Matter 2004

Explanation to the question: Binary scaling• At high energy, nucleus behaves like

dilute gas of partons• High energy parton has small cross

section. It pass through target nucleus without interatciton. Sometime, it can make hard scattering, but the probability is very small.

Effectively, the target nucleus is a dilute gas of target partons

the flux of the incident parton does not decrease in target nucleus

Probability of a parton making a hard collision is proportional to number of collision.(But the value of the probability even in a large nucleus target is << 1)

Size of parton ~ 1/Q2

Q>>1 GeV r < 0.2 fm point-like

No interaction

If Nparton becomes very largeand Nparton*parton ~ 1, the binaryscaling breaks down (shadowing)

More explanation on binary scaling

• Partons in the target are “confined” in nucleon, but confinement is meaningful only in a large scale (> 1 fm).

• At RHIC collision, the thickness of the target nucleus is small due to Lorentz contraction. In the time scale the hard scattering takes place, the partons in the target nuclei don’t know where they are --- which nucleon it belongs.

• The large inelastic cross section of N+N collision (~40mb at RHIC energy) is due to the effect of confinement. At the parton level, almost no interaction is taking place.

• For fast partons, a nucleus is like a dilute gas of target partons.

g, q

Size of target nucleiin cms of RHIC< 6 fm * 2 / fm(Lorentz factor) = 100