e+e--pair production in Pb-Au collisions at 158 GeV per nucleon

Agakichiev, G.; Tel-Zur, G.; Holl, P.; Sampietro, M.; Damjanovic, S.; Ravinovich, I.; Hering, G.; Rehak, P.; Cherlin, A.; Wessels, J.P.; Voigt, C.; Ullrich, T.; Nix, O.; Lenkeit, B.; Jung, Ch.; Stachel, J.; Miskowiec, D.; Razin, S.; Voloshin, S.; Yurevich, V.; Panebrattsev, Yu.; Appelshauser, H.; Sako, H.; Weber, C.; Glassel, P.; Messer, F.; Wurm, J.P.; Schmitz, W.; Pfeiffer, A.; Messer, M.; Fraenkel, Z.; Baur, R.; Shimanskiy, S.; Wienold, T.; Marin, A.; Fuchs, Ch.; Rak, J.; Faschingbauer, U.; Seipp, W.; Schukraft, J.; Tserruya, Itzhak; Specht, H.J.; Braun-Munzinger, P.; Drees, A.; Richter, M.; Socol, E.; Gatti, E.; Saveljic, N.; de los Heros, C.P.; Esumi, S.; Bielcikova, J.

doi:10.1140/epjc/s2005-02272-3

Article
Report number	nucl-ex/0506002
Title	e+e--pair production in Pb-Au collisions at 158 GeV per nucleon
Related title	GEV
Author(s)	Agakichiev, G. (Darmstadt, GSI) ; Appelshauser, H. (Heidelberg U.) ; Bielcikova, J. (Heidelberg U. ; Heidelberg, Max Planck Inst.) ; Baur, R. (Heidelberg U.) ; Braun-Munzinger, P. (Darmstadt, GSI) ; Cherlin, A. (Weizmann Inst.) ; Damjanovic, S. (Heidelberg U.) ; Drees, A. (Stony Brook U.) ; Esumi, S. (Heidelberg U.) ; Faschingbauer, U. (Heidelberg, Max Planck Inst. ; Heidelberg U.) ; Fraenkel, Z. (Weizmann Inst.) ; Fuchs, Ch. (Heidelberg, Max Planck Inst.) ; Gatti, E. (Milan Polytechnic) ; Glassel, P. (Heidelberg U.) ; Hering, G. (Darmstadt, GSI) ; de los Heros, C.P. (Weizmann Inst.) ; Holl, P. (Brookhaven Natl. Lab.) ; Jung, Ch. (Heidelberg U.) ; Lenkeit, B. (Heidelberg U.) ; Marin, A. (Darmstadt, GSI) ; Messer, F. (Heidelberg U. ; Heidelberg, Max Planck Inst.) ; Messer, M. (Heidelberg U.) ; Miskowiec, D. (Darmstadt, GSI) ; Nix, O. (Heidelberg, Max Planck Inst.) ; Panebrattsev, Yu. (Dubna, JINR) ; Pfeiffer, A. (Heidelberg U.) ; Rak, J. (Heidelberg, Max Planck Inst.) ; Ravinovich, I. (Weizmann Inst.) ; Razin, S. (Dubna, JINR) ; Rehak, P. (Brookhaven Natl. Lab.) ; Richter, M. (Heidelberg U.) ; Sampietro, M. (Milan Polytechnic) ; Sako, H. (Darmstadt, GSI) ; Saveljic, N. (Heidelberg U.) ; Schmitz, W. (Heidelberg U.) ; Schukraft, J. (CERN) ; Seipp, W. (Heidelberg U.) ; Shimanskiy, S. (Dubna, JINR) ; Socol, E. (Weizmann Inst.) ; Specht, H.J. (Heidelberg U.) ; Stachel, J. (Heidelberg U.) ; Tel-Zur, G. (Weizmann Inst.) ; Tserruya, Itzhak (Weizmann Inst.) ; Ullrich, T. (Heidelberg U.) ; Voigt, C. (Heidelberg U.) ; Voloshin, S. (Heidelberg U.) ; Weber, C. (Heidelberg U.) ; Wessels, J.P. (Heidelberg U. ; Munster U.) ; Wienold, T. (Heidelberg U.) ; Wurm, J.P. (Heidelberg, Max Planck Inst.) ; Yurevich, V. (Dubna, JINR) CERES Collaboration Show all 51 authors
Publication	2005
Imprint	1 Jun 2005
Number of pages	39
In:	Eur. Phys. J. C 41 (2005) 475-513
DOI	10.1140/epjc/s2005-02272-3
Subject category	Nuclear Physics
Abstract	We present the combined results on electron-pair production in 158 GeV/n {Pb-Au} ( $\sqrt{s}$ = 17.2 GeV) collisions taken at the CERN SPS in 1995 and 1996, and give a detailed account of the data analysis. The enhancement over the reference of neutral meson decays amounts to a factor of 2.31 $\pm0.19 (stat.)\pm0.55 (syst.)\pm0.69 (decays)$ for semi-central collisions (28% $\sigma/\sigma_{geo}$ ) when yields are integrated over $m>$ 200 MeV/ $c^2$ in invariant mass. The measured yield, its stronger-than-linear scaling with $N_{ch}$ , and the dominance of low pair $p_t$ strongly suggest an interpretation as {\it thermal radiation} from pion annihilation in the hadronic fireball. The shape of the excess centring at $m\approx$ 500 MeV/ $c^2$ , however, cannot be described without strong medium modifications of the $\rho$ meson. The results are put into perspective by comparison to predictions from Brown-Rho scaling governed by chiral symmetry restoration, and from the spectral-function many-body treatment in which the approach to the phase boundary is less explicit.

$CERES inclusive $e^+e^-$ mass spectra of 450~GeV p-Be, p-Au, and 200~GeV/n S-Au collisions ~\cite{neutral-meson-pBe,prl1995}. Plotted is the number of electron pairs per charged particle, both in the acceptance and per event. Contributions from various hadron decays as expected from p-p collisions are shown together with their sum (thick line), and the systematic error on the latter is indicated by the shaded area. In S-Au a higher single-electron $p_t$ cut reduces the $\pi^\circ$ Dalitz component. See text and footnote.$

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