In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40
GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a
trigger selection corresponding to approximately the most central 30% of the
geometrical cross section. Results on low-mass electron pair analysis are
presented.
  The upgrade of the experimental setup with the radial drift TPC has allowed
to enhance hadron physics capabilities of the experiment. New results on hadron
spectra (including Lambda) and flow are presented.Comment: Talk at the International Nuclear Physics Conference INPC2001,
  Berkeley, CA, July 29th - August 3rd 200

Adamová, D.

Agakichiev, G.

Appelshäuser, H.

Belaga, V.

Braun-Munzinger, P.

Cherlin, A.

Damjanović, S.

Dietel, T.

Drees, A.

Esumi, S.I.

Filimonov, K.

Fomenko, K.

Fraenkel, Z.

Garabatos, C.

Glässel, P.

Hering, G.

Kushpil, V.

Lenkeit, B.

Maas, A.

Marı́n, A.

Milov, A.

Milošević, J.

Miśkowiec, D.

Panebrattsev, Y.

Petchenova, O.

Petraček, V.

Pfeiffer, A.

Rak, J.

Ravinovich, I.

Rehak, P.

Sako, H.

Schmitz, W.

Schukraft, J.

Sedykh, S.

Seipp, W.

Shimansky, S.

Slı́vová, J.

Specht, H.J.

Stachel, J.

Tilsner, H.

Tserruya, I.

Wessels, J.P.

Wienold, T.

Windelband, B.

Wurm, J.P.

Xie, W.

Yurevich, S.

Yurevich, V.

Šumbera, M.

English

arXiv

K. Filimonov

D. Adamová

G. Agakichiev

H. Appelshäuser

V. Belaga

P. Braun-Munzinger

A. Cherlin

S. Damjanović

T. Dietel

A. Drees

S. I. Esumi

K. Fomenko

Z. Fraenkel

C. Garabatos

P. Glässel

G. Hering

V. Kushpil

B. Lenkeit

A. Maas

A. Marı́n

J. Milošević

A. Milov

D. Miśkowiec

Y. Panebrattsev

O. Petchenova

V. Petraček

A. Pfeiffer

J. Rak

I. Ravinovich

P. Rehak

H. Sako

W. Schmitz

J. Schukraft

S. Sedykh

W. Seipp

S. Shimansky

J. Slı́vová

H. J. Specht

J. Stachel

M. Šumbera

H. Tilsner

I. Tserruya

J. P. Wessels

T. Wienold

B. Windelband

J. P. Wurm

W. Xie

S. Yurevich

V. Yurevich

Crossref

New results on Pb-Au collisions at 40 AGeV from the CERES/NA45 experiment

Adamová, D.

Esumi, S. I.

Glässel, P.

Milošević, J.

Miśkowiec, D.

Petraček, V.

Appelshäuser, H.

Sedykh, Sergei

Slı́vová, J.

Specht, H. J.

Šumbera, M.

Wessels, J. P.

Wurm, J. P.

Damjanović, S.

GSI Repository

In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb - Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including $\Lambda$) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40 GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a trigger selection corresponding to approximately the most central 30% of the geometrical cross section. Results on low-mass electron pair analysis are presented. The upgrade of the experimental setup with the radial drift TPC has allowed to enhance hadron physics capabilities of the experiment. New results on hadron spectra (including Lambda) and flow are presented.Low-mass dilepton spectra are expected to provide information on the early stage of the relativistic heavy-ion collisions which may experience an onset of deconfinement and/or chiral symmetry restoration. The CERES/NA45 measurements of e+e−-pair production in 158 AGeV Pb-Au collisions revealed a significant excess of the dielectron yield over known hadronic sources in the invariant mass region 0.25<mee<0.7 GeV/c2 [1]. The enhancement is mostly observed for pairs with low transverse momentum ptee<0.5 GeV/c and increases stronger than linearly with the event multiplicity. These experimental data have stimulated extensive theoretical discussions on the mechanisms of low-mass dilepton production and emission from hot and dense hadronic medium (for a review see [2]). The thermal radiation from the interaction phase of the hadronic fireball via annihilation processes, predominantly π+π−→ρ→e+e−, may account for a large fraction of the observed enhancement. The spectral shape in the mass region of the excess, however, cannot be described and requires introducing in-medium effects on the ρ spectral function. Two scenarios that take into account in-medium modifications of the vector meson properties have been particularly successful in reproducing the CERES results. One assumes a reduction of the ρ-meson mass in the hot and dense medium as a precursor of chiral symmetry restoration [3], following Brown-Rho scaling [4]. Another approach uses a ρ-meson spectral function which takes into account modifications of meson properties due to interactions with the surrounding hadrons [5]. Dilepton production rates and shapes of the invariant mass spectra in these two calculations are very similar pointing to a shift of the hadron-parton duality threshold towards lower masses [6]

CERN Document Server

arXiv:nucl-ex/0109017v1  26 Sep 2001New Results on Pb-Au Collisions at 40 AGeV from theCERES/NA45 ExperimentK. Filimonov11∗, D. Adamová†, G. Agakichiev∗∗, H. Appelshäuser∗, V. Belaga‡,P. Braun-Munzinger∗∗, A. Cherlin§, S. Damjanovic´∗, T. Dietel∗, A. Drees¶,S. I. Esumi∗, K. Fomenko‡, Z. Fraenkel§, C. Garabatos∗∗, P. Glässel∗, G. Hering∗∗,V. Kushpil†, B. Lenkeit‖, A. Maas∗∗, A. Marín∗∗, J. Miloševic´∗, A. Milov§,D. Mis´kowiec∗∗, Y. Panebrattsev‡, O. Petchenova‡, V. Petracˇek∗, A. Pfeiffer‖,J. Rak∗∗, I. Ravinovich§, P. Rehak††, H. Sako∗∗, W. Schmitz∗, J. Schukraft‖,S. Sedykh∗∗, W. Seipp∗, S. Shimansky‡, J. Slívová∗, H. J. Specht∗, J. Stachel∗,M. Šumbera†, H. Tilsner∗, I. Tserruya§, J. P. Wessels∗, T. Wienold∗, B. Windelband∗,J. P. Wurm‡‡, W. Xie§, S. Yurevich∗ and V. Yurevich‡∗Universität Heidelberg, Germany†NPI/ASCR, ˇRež, Czech Republic∗∗GSI, Darmstadt, Germany‡JINR, Dubna, Russia§Weizmann Institute, Rehovot, Israel¶Department for Physics and Astronomy, SUNY Stony Brook, USA‖CERN, Geneva, Switzerland††Brookhaven National Laboratory, Upton, USA‡‡Max-Planck-Institut für Kernphysik, Heidelberg, GermanyINTRODUCTIONLow-mass dilepton spectra are expected to provide information on the early stage of the relativistic heavy-ioncollisions which may experience an onset of deconfinement and/or chiral symmetry restoration. The CERES/NA45measurements of e+e−-pair production in 158 AGeV Pb-Au collisions revealed a significant excess of the dielectronyield over known hadronic sources in the invariant mass region 0.25<mee < 0.7 GeV/c2 [1]. The enhancement is mostlyobserved for pairs with low transverse momentum peet < 0.5 GeV/c and increases stronger than linearly with the eventmultiplicity. These experimental data have stimulated extensive theoretical discussions on the mechanisms of low-massdilepton production and emission from hot and dense hadronic medium (for a review see [2]). The thermal radiationfrom the interaction phase of the hadronic fireball via annihilation processes, predominantly p + p −→ r → e+e−, mayaccount for a large fraction of the observed enhancement. The spectral shape in the mass region of the excess, however,cannot be described and requires introducing in-medium effects on the r spectral function. Two scenarios that takeinto account in-medium modifications of the vector meson properties have been particularly successful in reproducingthe CERES results. One assumes a reduction of the r -meson mass in the hot and dense medium as a precursor of chiralsymmetry restoration [3], following Brown-Rho scaling [4]. Another approach uses a r -meson spectral function whichtakes into account modifications of meson properties due to interactions with the surrounding hadrons [5]. Dileptonproduction rates and shapes of the invariant mass spectra in these two calculations are very similar pointing to a shiftof the hadron-parton duality threshold towards lower masses [6]. The measurement of dilepton spectra at lower beam1 Present address: LBNL, Berkeley, USAenergy of 40 AGeV allows to study the effect of varying the baryon density and provides additional constraints for themodel calculations. It is also the first measurement after upgrading the CERES spectrometer with a Time ProjectionChamber to provide a better resolution in the mass region of the narrow resonances w and f .EXPERIMENTAL SETUPThe CERES spectrometer (Figure 1) is optimized to measure electron pairs at midrapidity (2.1 < h < 2.6) with fullazimuthal coverage. Two silicon drift chambers (SIDC1,2) located 10 cm and 13.8 cm behind a segmented Au targetprovide a precise angular measurement of charged particles and vertex reconstruction. The electrons are identifiedby the two Ring Imaging Cherenkov detectors (RICH1,2) operated at a g th = 32, which rejects 95% of all chargedhadrons. The momentum of all charged particles is measured by the radial drift Time Projection Chamber (TPC) [7].The TPC has an active length of 2 m and an outer diameter of 2.6 m and provides the measurement of 20 space pointsin a magnetic field with a maximal radial component of 0.5 T.beamUV detector 2UV detector 1W-shieldtargetSiDC1/SiDC2radiator 1 mirror 1main coilscorrection coilsradiator 2mirror 28o15oTPC drift gas volumeTPC read-out chamberTPC coils-1 0 1 2 3 4 5m 1/r electric fieldvoltage dividerHV cathodeFIGURE 1. The upgraded CERES spectrometer at the CERN-SPSThe data taking period in the fall of 1999 was the first run of CERES after the TPC upgrade. Due to problems withthe new readout the data set is limited in statistics and TPC efficiency. In total we recorded 8.7M Pb-Au events at 40AGeV with centrality s / s geo ≈ 30%.ELECTRON-POSITRON PAIR ANALYSISThe dominant sources of e+e−-pairs are photon conversions and p 0-Dalitz decays which are characterized by thesmall pair opening angle and invariant masses below 200 MeV/c2. The large number of such pairs (Ne+e−(m<200MeV/c2)/Ne+e− (m>200 MeV/c2)∼ 104) combined with limited track reconstruction efficiency and acceptance resultsin a combinatorial background for events with two partially reconstructed low-mass pairs. This combinatorial back-ground can be significantly reduced by pairing only tracks with transverse momentum pt > 200 MeV/c. To removethe hadronic contamination in accidental matching between RICH and TPC we use dE/dx information from the TPC.Conversions and Dalitz pairs with opening angle less than 10 mrad which are not recognized as two individual rings inthe RICH detectors are rejected by a double energy loss cut in the two SIDC detectors. To account for a limited TPCefficiency in the 1999 data set, we also remove tracks which have a SIDC-RICH electron candidate within 70 mrad.Finally, identified Dalitz pairs (mee < 200 MeV/c2) are excluded from further combinatorics. The signal is extractedby subtracting like-sign pairs from unlike-sign pairs. The total number of open pairs (mee > 200 MeV/c2) is 180±48(stat.) with a signal-to-background ratio of 1/6.The measured e+e− invariant mass spectrum is compared to the hadronic cocktail in Figure 2 (left). The data were0 0.2 0.4 0.6 0.8 1 1.210-910-810-710-610-510-4-1 )2) (100 MeV/ch/dch)/(dNeedmh/deeN2(d)2 (GeV/ceemCERES/NA45        Pb-Au 40 AGeVPreliminary 30 %»geos/s>=210h/dch<dN<2.65h2.1<>200 MeV/ctp>35 mradeeQgeefi0pgeefih0peefiwgeefi⁄h eefireefiweefif0 0.2 0.4 0.6 0.8 1 1.2 1.410-910-810-710-610-510-4-1 )2) (100 MeV/ch/dch)/(dNeedmh/deeN2(d)2 (GeV/ceemCERES/NA45        Pb-Au 40 AGeVPreliminary 30 %»geos/s>=210h/dch<dN<2.65h2.1<>200 MeV/ctp>35 mradeeQFIGURE 2. Inclusive e+e− invariant mass spectrum at 40 AGeV in comparison to expectations from known hadronic sourcesin heavy ion collisions (‘hadronic cocktail’) (left) and in comparison to model calculations (right) assuming the vacuum r spectralfunction (dash-dotted), a dropping r mass (dotted) and including medium modifications of the r spectral function (thick solid line).Also shown is the hadronic cocktail without r (thin solid line). The dashed line refers to a lowest order pQCD rate calculation. Allcalculations are from [8].normalized to the hadronic cocktail in the p 0-Dalitz region. Similar to the observations at 158 AGeV, the measurede+e− yield exhibits an enhancement by a factor of 5.0± 1.5 (stat.) in the mass range mee>200 MeV/c2 compared tothe expectations from known hadronic sources in heavy-ion collisions. The excess is most pronounced for pairs withsmall transverse momentum. The results are also compared to different theoretical calculations in Figure 2 (right). Themeasurements clearly disfavor a purely hadronic scenario assuming the vacuum r spectral function.HADRON SPECTRA AND YIELDSThe addition of the TPC allowed for a systematic investigation of hadronic observables around midrapidity. Thematching requirement to the SIDC system leads to a very efficient suppression of non-vertex tracks. On the otherhand, using SIDC detectors as a veto improves the signal-to-background ratio for the reconstruction of L and K0s .The measured transverse mass distributions of negatively charged hadrons h− and proton-like positive net charges‘(+)− (−)’ are shown in Figure 3 in different bins of rapidity for events with centrality s / s geo<15%. The solid linesrepresent the best fits to the spectra using an exponential 1/mt dN/dmt µ exp(−mt/T ). The rapidity averaged slopes(Th− = 176±5 MeV and T(+)−(−) = 278±12 MeV) are similar to those observed at top AGS and SPS energies, withonly a weak dependence on centrality. The large difference between h− and (+)−(−) slopes indicates the presence ofstrong radial flow also at 40 AGeV. The midrapidity yield of h− as function of Npart (Figure 3, right) rises significantlystronger than linear, while a rise with N1.07±0.04part at top SPS energy has been reported recently [9]. This points toa change in the particle production mechanism going from 40 AGeV to 158 AGeV. In fact, a strongly non-linearbehaviour has also been observed at the AGS [10]. The non-linear rise of the midrapidity (+)− (−)-yield with Npartis possibly caused by an increasing amount of stopping, in agreement with the UrQMD prediction.We have also identified L hyperons by invariant mass reconstruction [11]. The measurements cover the rapidityrange from y =2.0 to 2.4 and transverse momenta from pt = 0.9 GeV/c to 2.5 GeV/c. L transverse momentumspectra have been fitted by an exponential in the three centrality intervals (Figure 4, left). The extracted inverse slopeparameters are shown in Figure 4 (right). The L slopes measured at 40 AGeV are within errors consistent with those10-210-111010 210 310 410 50 0.5 1 1.540 AGeV Pb+Au→h-+Xx 104s / s geo 15%mT-m0 (GeV)1/N ev 1/mT dn2 /dmTdy (GeV-2 )yp=2.15yp=2.25yp=2.35yp=2.45yp=2.55CERES Preliminary10-210-111010 210 310 410 50 0.5 1 1.540 AGeV Pb+Au→(+)-(-)+Xx 104s / s geo 15%mT-m0 (GeV)1/N ev 1/mT dn2 /dmTdy (GeV-2 )yp=2.05yp=2.15yp=2.25yp=2.35yp=2.45CERES Preliminary0501001500 50 100 150 200 250 300 350 400nParticipantsdN/dy midCERES Preliminaryh-(+)-(-)UrQMD:h-(+)-(-)pFIGURE 3. Left panel: Measured h− (left) and proton-like positive net charges (right) transverse mass spectra for events withcentrality s /s geo<15%. Beginning with the lowest rapidity bin the spectra have been multiplied by a successive factor of 10. Thelines are the exponential fits to the data. Right panel: Midrapidity yields of h− and (+)−(−) as a function of number of participants.Also shown are results from UrQMD.10 310 410 510 6dN/dp t (a.u.)10 310 410 510 610 310 410 510 60.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3zvertex > 50 cmno zvertex cutpt (GeV/c)01002003004000 100 200 300 400NpartInverse Slope TL (MeV)E891, 11.6 AGeVCERES, 40 AGeVWA97, 158 AGeVFIGURE 4. Left: Measured L transverse momentum spectra for the three centrality intervals. The solid lines are the exponentialfits to the data. Right: L inverse slope parameters as a function of number of participants.at the top SPS energy [12] and AGS [13]. They are also very similar to the parameters extracted from spectra ofproton-like positive net charges. By integrating the spectra where the data are available and using the results from theexponential fits to extrapolate to infinity and to pt=0 we obtained the yield of L -hyperons in the measured rapidityinterval: dN/dyL= 11±2 for centrality s / s geo<5%. We have also measured the ratios of L /p = 0.22± 0.05 and¯L / L = 0.024±0.010 for the same centrality.DIRECTED AND ELLIPTIC FLOWAnisotropies in the azimuthal distribution of particles, also called anisotropic (directed, elliptic, etc.) transverse flow,have proven to be sensitive to the initial pressure in the collision region and the degree of thermalization during theexpansion stage. Our flow analysis is based on the azimuthal hit distributions in the SIDC1,2. The two coefficients v1and v2, respectively quantifying the strength of directed and elliptic flow, are obtained from a Fourier decompositionof the azimuthal charged particle distribution with respect to the orientation of the reaction plane [14]. The v1 andv2 values have been corrected for the finite reaction plane resolution, determined by the subevent method. Thepseudorapidity dependence of directed and elliptic flow measured at 40 AGeV is presented in Figure 5 (left) for three -0.100.1v1  centrality 35 - 15 %SIDC1SIDC2  centrality 15 - 5 %     centrality 5 - 0 %-0.100.12 3v2    2 3     2 3h  -0.100.110 -1 1 10 10 2 10 3 10 4Ebeam/A (GeV)v 2 FOPIPlastic BallLANDEOSE895E877 pions, protonsNA49 pions, protonsCERES  charged part.STAR  charged part.protonsin-planeout-of-planeFIGURE 5. Left: Pseudorapidity dependence of directed (v1) and elliptic (v2) flow at 40 AGeV for three centrality intervals.Right: v2 as a function of the beam kinetic energy for semi-central collisions of Pb or Au nuclei.different centralities. While v1 exhibits the characteristic zero-crossing at midrapidity, v2 is positive and independent ofpseudorapidity. We have also performed the measurements of elliptic flow at 80 and 158 AGeV. The v2-values increasewith beam energy (Figure 5, right) and fit smoothly into the measured systematics.OUTLOOKIn the fall of 2000 we took a sample of 33M central Pb-Au events at 158 AGeV with a very good overall performanceof the spectrometer. The analysis of the 2000 data will provide a long-awaited dilepton invariant mass spectrum withthe mass resolution D m/m<2% in the region of w and f resonances. The systematic hadron analysis of data taken at 40,80 and 158 AGeV is in progress, including results on HBT-interferometry and mean transverse momentum fluctuations[15].REFERENCES1. Agakichiev, G., et al. (CERES Collaboration), Nucl. Phys. A, 661, 23 (1999).2. Rapp, R., and Wambach, J., Adv. Nucl. Phys., 25, 1 (2000).3. Li, G., Ko, C., and Brown, G., Phys. Rev. Lett., 75, 4007 (1995).4. Brown, G., and Rho, M., Phys. Rev. Lett., 66, 2720 (1991).5. Rapp, R., Chanfray, G., and Wambach, J., Nucl. Phys. A, 617, 472 (1997).6. Rapp, R., Nucl. Phys. A, 661, 33 (1999).7. Agakichiev, G., et al. (CERES Collaboration), Nucl. Phys. A, 661, 673 (1999).8. Rapp, R. (2001), private communication.9. Aggarwal, M., et al. (WA98 Collaboration), Eur. Phys. J. C, 18, 651 (2001).10. Ahle, L., et al. (E802 Collaboration), Phys. Rev. C, 59, 2173 (1999).11. Schmitz, W., L Production in Pb-Au Collisions at 40 AGeV, Ph.D. thesis, Heidelberg University (2001).12. Sandor, L., et al. (WA97 Collaboration), Nucl. Phys. A, 661, 481 (1999).13. Ahmad, S., et al. (E891 Collaboration), Phys. Lett. B, 382, 35 (1996).14. Voloshin, S., and Zhang, Y., Z. Phys. C, 70, 665 (1996).15. Adamová, D., et al. (CERES Collaboration), Nucl. Phys. A (2001), proceedings of Quark Matter 2001, in press.

New results on Pb - Au collisions at 40 AGeV from the CERES/NA45 experiment

New Results on Pb-Au Collisions at 40 AGeV from the CERES/NA45
  Experiment

New Results on Pb-Au Collisions at 40 AGeV from the CERES/NA45 Experiment

Abstract

Similar works

Full text

Available Versions

Crossref

Crossref

Crossref

GSI Repository

CERN Document Server