dielectrons at SPS

1
di-electrons at SPS
Harald Appelshäuser Institut für
Kernphysik Universität Frankfurt for the CERES
Collaboration
2
di-electrons at SPS

• introduction
• CERES experiment at the SPS
• electron pair analysis
• recent results from run2000 158 AGeV/c Pb-Au

3
dilepton mass spectrum (schematic)

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

• CERES (e pairs)
• low mass region, r,w,f
• chiral symmetry restoration
• thermal radiation

4
CERES history
Eur.Phys.J. C4 (1998) 231, 249
p-induced reactions

• consistent with expectations from
• known hadronic sources
• the hadronic cocktail

our best knowledge of cross sections, form
factors, branching ratios in the absence of
new physics, folded with detector acceptance
and resolution
5
CERES history
Phys. Rev. Lett. 75 (1995) 1272
HI-induced reactions
pt gt 0.2 GeV/c qee gt 35 mrad 2.1lthlt2.65 ltdNch/dhgt
125

• significant enhancement
• in the low mass region
• E 5.0 /- 0.7(stat) /- 2.0 (syst)
• for meegt0.2 GeV/c2

6
CERES history
CERES coll. NPA 661(1999), PLB 422(1998), NPA
715 (2003)
Pb-Au at 158 AGeV/c
combined 95/96 data

• E 2.4 /- 0.2(stat) /- 0.6(syst)
• for meegt0.2 GeV/c2
• w.r.t. HI-cocktail

thermal particle yields (f!) and mass dependence
of flow
7
CERES history
Pb-Au at 158 AGeV/c
combined 95/96 data
calculations R. Rapp
8
CERES history
CERES coll. PRL 91 (2003) 042301
Pb-Au at 40 AGeV/c
E 5.9 /- 1.5 (stat) /- 1.2 (syst)/- 1.8
(decays) for meegt 0.2 GeV/c2
T vs. mB?
9
the upgraded CERES experiment
acceptance 8 lt q lt 15 deg 2.1 lt h lt 2.65
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Silicon Drift Detectors
SDD1 and SDD2

• charged particle tracking
• vertex reconstruction

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RICH
RICH1 and RICH2 electron ID via ring signature
field-free operation allows combined mode
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Time Projection Chamber

• radial TPC
• charged particle tracking
• momentum determination
• electron ID via dE/dx

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TPC tracking efficiency
pt 0.1 GeV/c

• radial TPC
• charged particle tracking
• momentum determination
• electron ID via dE/dx

TPC efficiency (MC)
plab (GeV/c)
TPC efficiency (MC)
q (rad)
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momentum and mass resolution
Ks0 ? pp- (with SDD vertex)
mK0 498.7 /- 0.1 MeV/c2 s 11.3 MeV/c2
W. Ludolphs PhD in prep.
? mass resolution 4 for f?ee
15
electron pair analysis
data set from Run 2000
Pb-Au 80 AGeV/c s/sgeo 30 0.5M
Events Pb-Au 158 AGeV/c s/sgeo 30 3 M
Events Pb-Au 158 AGeV/c s/sgeo 8 30 M
Events
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electron pair analysis

• charged particle tracking
• electron identification
• rejection of combinatorial background

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charged particle tracking
angular matching of TPC and SDD tracks
sq(mrad)
sf(mrad)
pt(GeV/c)
pt(GeV/c)
vertex condition suppresses late conversions
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electron identification
energy loss dE/dx in TPC
ring signature in RICH
s(dE/dx) / (dE/dx) 10
e
p
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electron identification
RICH
TPC
e.g. at 1.5 GeV/c 0.03 (TPC) x 0.003 (RICH)
10-4 p-efficiency at 0.9 (TPC) x 0.65
(RICH) 60 e-efficiency
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background rejection
dominant sources are p0-Dalitz and g-conversions
still a large number of tracks remaining from
unrecognized p0-Dalitz pairs and g-conversions!
21
background rejection
on the remaining tracks kinematical and
topological cuts

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

22
background rejection
kinematical cut

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

better ptgt0.1 GeV/c ? mt,ee gt 0.2 GeV
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background rejection
topological cuts

• search for remnants of the lost partner

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invariant mass distributions
1.8x107 Pb-Au events at 158 AGeV/c
signal
mixed-event bg
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electron pair analysis
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electron pair analysis
combinatorial background
ptgt0.1 GeV/c
ptgt0.2 GeV/c

• for BG subtraction
• same-event BG for meelt 0.2 GeV/c
• normalized mixed-event BG for meegt 0.2 GeV/c

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mass spectrum
Run 2000 Pb-Au 158 AGeV/c

• normalization to cocktail at p0-Dalitz

Enhancement for meegt0.2 GeV/c2 E
3.3 /- 0.3 (stat)
no systematic errors yet!
A.Marin (QM04) J.Phys.G30 (2004)
28
mass spectrum
Run 2000 Pb-Au 158 AGeV/c
thermal radiation (Kämpfer et al.)
A.Marin (QM04) J.Phys.G30 (2004)
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centrality dependence
meelt0.2 GeV/c2
0.2ltmeelt0.6 GeV/c2
meegt0.6 GeV/c2
ltdNch/dhgt

• pair yield increases stronger than linear with
  ltdNch/dhgt

30
multiplicity dependence
meelt0.2 GeV/c2
0.2ltmeelt0.6 GeV/c2
meegt0.6 GeV/c2
ltdNch/dhgt

• pair yield increases stronger than linear with
  ltdNch/dhgt
• most central point confirmed by 2000 data

31
ptgt0.1 GeV/c mass spectrum

• spectra agree for meegt0.7 GeV/c2
• pt gt 0.1 GeV/c adds sensitivity to low
• masses and pair pt (mtgt0.2 GeV)

32
ptgt0.1 GeV/c mass spectrum
Run 2000 Pb-Au 158 AGeV/c
with ptgt0.1 GeV/c selection

• enhancement extends to p0-peak

• E 5.6 /- 0.4 (stat)
• for mgt0.2 GeV/c2

S. Yurevitch, PhD in prep.
33
ptgt0.1 GeV/c mass spectrum
Run 2000 Pb-Au 158 AGeV/c
modified r-spectral function
Brown-Rho scaling
S. Yurevitch, PhD in prep.
34
mass spectrum and models
WA98 direct photons PRL85 (2000) calculations
Turbide, Rapp, Gale PRC (2004)
Run 2000 Pb-Au 158 AGeV/c
modified r-spectral function
Brown-Rho scaling
S. Yurevitch, PhD in prep.
35
transverse momentum spectra
mee lt 0.2 GeV/c2
pt gt 0.2 GeV/c
pt gt 0.1 GeV/c
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transverse momentum spectra
0.2 lt mee lt 0.7 GeV/c2
pt gt 0.1 GeV/c
pt gt 0.2 GeV/c

• enhancement located at low pt

• larger enhancement due to increased low pt
  acceptance

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transverse momentum spectra
mee gt 0.7 GeV/c2
pt gt 0.1 GeV/c
pt gt 0.2 GeV/c

• moderate enhancement at high masses

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thermal source parametrization
Thomas, Gallmeister, Zschocke, Kämpfer

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

• same source is consistent with low pt photons

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

39
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
2mp
40
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

41
low mass enhancement at SPS

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

• complementary information from
• ee and mm experiments

Axel Drees, Hard Probes 2004
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summary and outlook

• preliminary results of ee- production in 158
  AGeV/c Pb-Au
• from run2000 have been presented
• low mass enhancement with ptgt0.2 GeV/c
  consistent with
• highest centrality in 95/96
• larger enhancement observed with ptgt0.1 GeV/c
  selection
• new data production just finished larger
  significance
• expected (r/w, f)
• evaluation of efficiency and systematic errors

43
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 Republic GSI Darmstadt,
Germany Frankfurt University, Germany Heidelberg
University, Germany JINR Dubna, Russia Weizmann
Institute, Rehovot, Israel SUNY at Stony Brook,
USA CERN, Switzerland BNL, Upton, USA Münster
University, Germany MPI Heidelberg, Germany
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Mass spectrum
Comparison to 96 data
s/stot 8
s/stot 30
E(96) 3.7 /- 1.0 (stat) /- 1.5 (syst) E(00)
5.6 /- 0.4 (stat) /- ?
96 data G. Hering, PhD thesis, nucl-ex/0203004