di-electrons at sps

Harald Appelshäuser Hirschegg Workshop 2005

di-electrons at SPS

Harald Appelshäuser

Institut für KernphysikUniversität Frankfurt

for the

CERES Collaboration


di-electrons at SPS

• introduction

• CERES experiment at the SPS

• electron pair analysis

• recent results from run2000 158 AGeV/c Pb-Au


dilepton mass spectrum (schematic)

NA50, NA60 ( pairs):HMR and IMR:• J/ suppression• open charm enhancement• thermal radiation• NA60: +low mass

CERES (e pairs):low mass region, • chiral symmetry restoration• thermal radiation


CERES history

p-induced reactions:

• consistent with expectations from known hadronic sources

• the hadronic cocktail:

Eur.Phys.J. C4 (1998) 231, 249

our best knowledge of cross sections, form factors, branching ratios… in the absence of new physics, folded with detector acceptance and resolution


CERES history

• significant enhancement in the low mass region

• E = 5.0 +/- 0.7(stat) +/- 2.0 (syst) for mee>0.2 GeV/c2

HI-induced reactions:Phys. Rev. Lett. 75 (1995) 1272

pt > 0.2 GeV/cee > 35 mrad2.1<<2.65<dNch/d> = 125


CERES history

Pb-Au at 158 AGeV/c:

CERES coll. NPA 661(1999), PLB 422(1998), NPA 715 (2003)

• E = 2.4 +/- 0.2(stat) +/- 0.6(syst) for mee>0.2 GeV/c2

w.r.t. HI-cocktail:

thermal particle yields (!) and mass dependence of flow

combined 95/96 data


CERES history

Pb-Au at 158 AGeV/c:

enhanced vacuum- due to -annihilation

modified -spectral function

Brown-Rho scaling

calculations: R. Rapp

combined 95/96 data


CERES history

Pb-Au at 40 AGeV/c:

enhanced vacuum- due to -annihilation


Brown-Rho scaling

E = 5.9 +/- 1.5 (stat) +/- 1.2 (syst)+/- 1.8 (decays) for mee> 0.2 GeV/c2

CERES coll. PRL 91 (2003) 042301

T vs. B?


the upgraded CERES experiment

radial TPC

RICHes

Silicon DriftDetectors

acceptance:8 < < 15 deg2.1 < < 2.65


Silicon Drift Detectors

SDD1 and SDD2:

• charged particle tracking

• vertex reconstruction

=205 mRun 2000 Pb-Au


RICH

RICH1 and RICH2: electron ID via ring signature

field-free operation allows combined mode


Time Projection Chamber

radial TPC:


• momentum determination

• electron ID via dE/dx


TPC tracking efficiency

radial TPC:


• momentum determination

• electron ID via dE/dx

plab (GeV/c)

(rad)

TP

C e

ffic

ien

cy (

MC

)T

PC

eff

icie

ncy

(M

C)

pt ~ 0.1 GeV/c


momentum and mass resolution

Ks0 +- (with SDD vertex)

mass resolution ~4% for ee

mK0 = 498.7 +/- 0.1 MeV/c2

= 11.3 MeV/c2

W. LudolphsPhD in prep.


electron pair analysis

data set from Run 2000:

Pb-Au 80 AGeV/c geo= 30% 0.5M Events

Pb-Au 158 AGeV/c geo= 30% 3 M Events

Pb-Au 158 AGeV/c geo= 8% 30 M Events



1. charged particle tracking

2. electron identification

3. rejection of combinatorial background


charged particle tracking

angular matching of TPC and SDD tracks:

(m

rad

)

(m

rad

)pt(GeV/c) pt(GeV/c)

vertex condition suppresses late conversions


electron identification

energy loss dE/dx in TPC:ring signature in RICH:

e

dE/dx) / (dE/dx) = 10%


electron identification

RICH TPC

e.g. at 1.5 GeV/c: 0.03 (TPC) x 0.003 (RICH) = 10-4 -efficiency at 0.9 (TPC) x 0.65 (RICH) = 60% e-efficiency


background rejection

dominant sources are 0-Dalitz and -conversions

Dalitz recognition:

• rejection of tracks which form a pair ee < 35 mrad

• tracks which form a pair mee < 0.2 GeV/c2 excluded from further pairing

…still a large number of tracks remaining from unrecognized 0-Dalitz pairs and -conversions!



on the remaining tracks: kinematical and topological cuts

• conversion and Dalitz tracks are soft• the lost partner must be close-by

TPC

SDD

RICH



kinematical cut:

better: pt>0.1 GeV/c mt,ee > 0.2 GeV

• single-lepton pt-cut acceptance loss at low mass and pair pt

traditional: pt>0.2 GeV/c

mt,ee > 0.4 GeV



topological cuts:

energy loss in SDD: V-track signature in TPC:

• search for remnants of the lost partner


invariant mass distributions

~1.8x107 Pb-Au events at 158 AGeV/c:

pt>0.2 GeV/c: mee<0.2 GeV/c2: S = 2844+/- 113, B/S = 1.8 mee>0.2 GeV/c2: S = 2174+/-182, B/S = 13

cou

nts

/100

MeV

/c2

pt > 0.2 GeV/c

pt>0.1 GeV/c: mee<0.2 GeV/c2: S = 18030+/-347, B/S = 2.8 mee>0.2 GeV/c2: S = 5511+/-351, B/S = 20

cou

nts

/100

MeV

/c2

pt > 0.1 GeV/c

signal

mixed-event bg



• combination of unlike-sign pairs from the same event

Signal S:

• like-sign combinations from the same event

() same-event correlations (partially) contained() if not S>>B, finite statistics contributes

• unlike-sign (or like-sign) combinations from mixed events

() no statistical limitation() normalization, no correlations

Background B:



pt>0.2 GeV/c pt>0.1 GeV/c

for BG subtraction: • same-event BG for mee< 0.2 GeV/c• normalized mixed-event BG for mee> 0.2 GeV/c

combinatorial background


mass spectrum

Enhancement for mee>0.2 GeV/c2:

E = 3.3 +/- 0.3 (stat)

Run 2000 Pb-Au 158 AGeV/c:

A.Marin (QM04) J.Phys.G30 (2004)

• normalization to cocktail at 0-Dalitz

…no systematic errors yet!


mass spectrum


modified -spectral function(Rapp, Wambach)

Brown-Rho scaling

thermal radiation (Kämpfer et al.)

A.Marin (QM04) J.Phys.G30 (2004)

discrimination soon possible


centrality dependence

• pair yield increases stronger than linear with <dNch/d>

<dNch/d>

mee<0.2 GeV/c2 0.2<mee<0.6 GeV/c2 mee>0.6 GeV/c2


multiplicity dependence

• pair yield increases stronger than linear with <dNch/d>• most central point confirmed by 2000 data

NA60 In-In

mee<0.2 GeV/c2 0.2<mee<0.6 GeV/c2 mee>0.6 GeV/c2

<dNch/d>NA60 In-In


pt>0.1 GeV/c mass spectrum


• both spectra normalized to 0-Dalitz

• spectra agree for mee>0.7 GeV/c2

• pt > 0.1 GeV/c adds sensitivity to low masses and pair pt (mt>0.2 GeV)




with pt>0.1 GeV/c selection:

• enhancement extends to 0-peak

• E = 5.6 +/- 0.4 (stat) for m>0.2 GeV/c2

S. Yurevitch, PhD in prep.





Brown-Rho scaling


low-mass enhancement over models?


mass spectrum and models



Brown-Rho scaling


WA98 direct photons PRL85 (2000)calculations: Turbide, Rapp, Gale PRC (2004)

low pt enhancement over models?


transverse momentum spectra

mee < 0.2 GeV/c2 :

pt > 0.1 GeV/c pt > 0.2 GeV/c

modified

B-R scaling

cocktail



0.2 < mee < 0.7 GeV/c2 :


modified

B-R scaling

cocktail

• enhancement located at low pt

• larger enhancement due to increased low pt acceptance



mee > 0.7 GeV/c2 :


modified

B-R scaling

cocktail

• moderate enhancement at high masses


thermal source parametrization

Thomas, Gallmeister,Zschocke, Kämpfer

• inclusion of colder source describes pt > 0.1 GeV/c data

• same source is consistent with low pt photons

• pt > 0.1 and pt > 0.2 GeV/c data are not described simultaneously


soft dileptons from DCC

Koch, Randrup, Wang, Kluger, APCTP Seoul, Korea, 1997 nucl-th/9712061

• rapid cooling of chirally restored state leads to a misaligned vacuum (Disoriented Chiral Condensate)

• system will decay into normal vacuum by emission of soft pions

annihilation of soft pions may be significant source of

low pt,ee dileptons pairs with mee ~ 2m


soft dileptons from DCC

Koch, Randrup, Wang, Kluger, APCTP Seoul, Korea, 1997 nucl-th/9712061

• DCC source is softer than thermal source• only visible at small mee and pair pt


low mass enhancement at SPS

5.6

NA60

• low mass dilepton enhancement most pronounced at mid-rapidity and small mt

Axel Drees, Hard Probes 2004

• complementary information from ee and experiments


summary and outlook

• preliminary results of e+e- production in 158 AGeV/c Pb-Au from run2000 have been presented

• low mass enhancement with pt>0.2 GeV/c consistent with highest centrality in 95/96

• larger enhancement observed with pt>0.1 GeV/c selection

• new data production just finished – larger significance expected ()

• evaluation of efficiency and systematic errors


CERES Collaboration

D. Adamova, G. Agakichiev, D. Antonczyk, A. Andronic, H. Appelshäuser, V. Belaga, J. Bielcikova, P. Braun-Munzinger, O. Busch, A. Castillo, A. Cherlin,

S. Damjanovic, T. Dietel, L. Dietrich, A. Drees, S. Esumi, K. Filimonov, K. Fomenko,Z. Fraenkel, C. Garabatos, P. Glässel, G. Hering, J. Holeczek, V. Kushpil, B. Lenkeit,

W. Ludolphs, A. Maas, A. Marin, J. Milosevic, A. Milov, D. Miskowiec, R. Ortega, Yu. Panebrattsev, O. Petchenova, V. Petracek, A. Pfeiffer, S. Radomski, J. Rak,

Ravinovich, P. Rehak, W. Schmitz, J. Schukraft, H. Sako, S. Shimansky, S. Sedykh, J. Stachel, M. Sumbera, H. Tilsner, I. Tserruya, G. Tsiledakis, T. Wienold, B. Windelband, J.P. Wessels, J.P. Wurm, W. Xie, S. Yurevich, V. Yurevitch

NPI ASCR, Rez, Czech RepublicGSI Darmstadt, Germany

Frankfurt University, GermanyHeidelberg University, Germany

JINR Dubna, RussiaWeizmann Institute, Rehovot, Israel

SUNY at Stony Brook, USACERN, SwitzerlandBNL, Upton, USA

Münster University, GermanyMPI Heidelberg, Germany


Mass spectrum

Comparison to 96 data:

96 data: G. Hering, PhD thesis, nucl-ex/0203004

/tot = 8%

/tot = 30%

E(96) = 3.7 +/- 1.0 (stat) +/- 1.5 (syst)E(00) = 5.6 +/- 0.4 (stat) +/- ?

di-electrons at sps

Documents

agevc ssgeo

low mass region e

thermal particle

electron pair analysisdata

dataceres historypbau

mass resolutionks0 p

dalitz tracks

low massceres e pairs