Dilepton production has been measured with HADES, the "High Acceptance
DiElectron Spectrometer". In pp collisions at 2.2GeV kinetic beam energy,
exclusive eta production and the Dalitz decay eta -> gamma e+e- has been
reconstructed. The electromagnetic form factor is well in agreement with
existing data. In addition, an inclusive e+e- spectrum from the C+C reaction at
2AGeV is presented and compared with a thermal model.Comment: 11 pages, 3 figures, proceedings of the IVth International Conference
  on Quarks and Nuclear Physics, Madrid, June 5th-10th, submitted to
  Eur.Phys.J.

A. Balanda

A. Belyaev

A. Blanco

A. Ierusalimov

A. Ivashkin

A. Kozuch

A. Kugler

A. Kurepin

A. Mangiarotti

A. Reshetin

A. Rustamov

A. Sadovsky

A. Schmah

Agakichiev

Amsler

Arnaldi

Averbeck

B. Kämpfer

B. Michalska

B. Ramstein

B. Sailer

B. Spruck

B. W. Kolb

Balestra

Bratkovskaya

C. Agodi

C. Gilardi

C. Müntz

C. Sturm

Cassing

Ch. Hadjivasiliou

Collaboration)

D. Belver

D. González-Dıaz

D. Kirschner

D. Mishra

E. Castro

E. Grosse

E. Moriniere

Ernst

F. Dohrmann

F. Guber

F. Krizek

G. Agakishiev

G. Bellia

H. Ströbele

H. Tsertos

H. Wen

I. Fröhlich

I. Iori

I. Koenig

I. Veretenkin

J. A. Garzón

J. Dıaz

J. Friese

J. L. Boyard

J. Lamas-Valverde

J. Marın

J. Markert

J. Mousa

J. Otwinowski

J. Pietraszko

J. Stroth

J. Wüstenfeld

K. Kanaki

K. Teilab

L. Fabbietti

L. Lopes

L. Naumann

Landsberg

M. Böhmer

M. Destefanis

M. Golubeva

M. Heilmann

M. Jurkovic

M. Palka

M. Roy-Stephan

M. Sudol

M. Traxler

M. Wisniowski

O. Fateev

O. Pechenova

P. Braun-Munzinger

P. Cabanelas

P. Finocchiaro

P. J. R. Fonte

P. Salabura

P. Tlusty

P. Zumbruch

R. Gernhäuser

R. Holzmann

R. Kotte

R. Krücken

R. Novotny

R. Simon

R. Trebacz

S. Chernenko

S. Lang

The European Physical Journal A

English

arXiv

5 pages, 4 figures.-- PACS nrs.: 13.25.Jx, 13.40.Hq, 14.40.Aq, 25.40.Ve.-- ISI Article Identifier: 000245667300102.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-ex/0610048Contributed to: The IVth International Conference on Quarks and Nuclear Physics - QNP 2006 (Madrid, Spain, June 5-10, 2006).e(+) e(-) production was studied using the High Acceptance DiElectron Spectrometer (HADES). In pp collisions at 2.2GeV kinetic beam energy, the exclusive η production and the Dalitz decay η -> γe(+)e(-) have been reconstructed. The electromagnetic form factor of the latter decay was found to be in good agreement with the existing theoretical predictions. In addition, an inclusive e(+)e(-) invariant-mass spectrum from the C-12 + C-12 reaction at 2AGeV is presented and compared with a simplified thermal model.The HADES collaboration gratefully acknowledges the support by BMBF grants 06TM970I, 06GI146I, 06F-140, and 06DR120 (Germany), by GSI (TM-FR1,GI/ME3,OF/STR), by grants GA CR 202/00/1668 and GA AS CR IAA1048304 (Czech Republic), by grant KBN 1P03B05629 (Poland), by INFN (Italy), by CNRS/IN2P3 (France), by grants MCYT FPA2000-2041-C02-02 and XUGA PGID T02PXIC20605PN (Spain), by grant UCY-10.3.11.12 (Cyprus), by INTAS grant 03-51-3208 and EU contract RII3-CT-2004-506078.Peer reviewe

Frohlich, I.

Agakishiev, G.

Agodi, C.

Balanda, A.

Bellia, G.

Belver, D.

Belyaev, A.

Blanco, Alfonso

Böhmer, M.

Boyard, J. L.

Braun-Munzinger, P.

Müntz, C.

Naumann, L.

Novotny, R.

Otwinowski, J.

Pachmayer, Y. C.

Palka, M.

Pechenov, V.

Pechenova, O.

Cavalcanti, T. P.

Pietraszko, J.

Przygoda, W.

Ramstein, B.

Reshetin, A.

Roy-Stephan, M.

Rustamov, A.

Sadovsky, A.

Sailer, B.

Salabura, P.

Schmah, A.

Simon, R.

Spataro, S.

Spruck, B.

Strobele, H.

Stroth, J.

Sturm, C.

Sudol, M.

Teilab, K.

Tlusty, P.

Traxler, M.

Trebacz, R.

Tsertos, H.

Veretenkin, I.

Wagner, V.

Wen, H.

Wisniowski, M.

Wojcik, T.

Wustenfeld, J.

Zanevsky, Y.

Zumbruch, P.

Cabanelas, P.

Castro, E.

Chernenko, S.

Christ, T.

Destefanis, M.

Díaz Medina, José

Dohrmann, F.

Eberl, T.

Fabbietti, Laura

Fateev, O.

Finocchiaro, P.

Fonte, P.

Friese, J.

Galatyuk, T.

Garzón, Juan Antonio

Gernhäuser, R.

Gilardi, C.

Golubeva, M.

González Díaz, Diego

Grosse, E.

Guber, F.

Hadjivasiliou, C.

Heilmann, M.

Hennino, T.

Holzmann, R.

Ierusalimov, A.

Iori, I.

Ivashkin, A.

Jurkovic, M.

Kampfer, B.

Kanaki, K.

Karavicheva, T.

Kirschner, D.

Koenig, I.

Koenig, W.

Kolb, B. W.

Kotte, R.

Kozuch, A.

Krizek, F.

Krücken, R.

Kugler, A.

Kuhn, W.

Kurepin, A.

Lamas-Valverde, J.

HADES Collaboration

Lang, S.

Lange, J. S.

Lopes, L.

arXiv:nucl-ex/0610048v1  31 Oct 2006Dilepton production in pp and CC collisionswith HADESI.Fröhlich10, G.Agakishiev11, C.Agodi1, A.Balanda5, G.Bellia1,2, D.Belver19,A.Belyaev9, A.Blanco3, M.Böhmer15, J.L.Boyard17, P.Braun-Munzinger6,P.Cabanelas19, E.Castro19, S.Chernenko9, T.Christ15, M.Destefanis11, J.Díaz20,F.Dohrmann7, I.Durán19, T.Eberl15, L.Fabbietti15, O.Fateev9, P.Finocchiaro1,P.J.R.Fonte3,4, J.Friese15, T.Galatyuk6, J.A.Garzón19, R.Gernhäuser15,C.Gilardi11, M.Golubeva14, D.González-Díaz6, E.Grosse7,8, F.Guber14,Ch.Hadjivasiliou16, M.Heilmann10, T.Hennino17, R.Holzmann6, A.Ierusalimov9,I.Iori12,13, A.Ivashkin14, M.Jurkovic15, B.Kämpfer7, K.Kanaki7, T.Karavicheva14,D.Kirschner11, I.Koenig6, W.Koenig6, B.W.Kolb6, R.Kotte7, F.Krizek18,R.Krücken15, A.Kugler18, W.Kühn11, A.Kurepin14, J.Lamas-Valverde19, S.Lang6,S.Lange6, L.Lopez3, A.Mangiarotti3, J.Marín19, J.Markert10, V.Metag11,B.Michalska5, D.Mishra11, E.Moriniere17, J.Mousa16, M.Münch6, C.Müntz10,L.Naumann7, R.Novotny11, J.Otwinowski5, Y.C.Pachmayer10, M.Palka5,V.Pechenov11, T.Pérez11, J.Pietraszko6, R.Pleskac18, V.Pospísil18, W.Przygoda5,B.Ramstein17, A.Reshetin14, M.Roy-Stephan17, A.Rustamov6, A.Sadovsky7,B.Sailer15, P.Salabura5, A.Schmah6, P.Senger6, K.Shileev14, R.Simon6,S.Spataro1, B.Spruck11, H.Ströbele10, J.Stroth10,6, C.Sturm6, M.Sudol10,6,K.Teilab10, P.Tlusty18, M.Traxler6, R.Trebacz5, H.Tsertos16, I.Veretenkin14,V.Wagner18, H.Wen11, M.Wisniowski5, T.Wojcik5, J.Wüstenfeld7, Y.Zanevsky9,P.Zumbruch61) Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud, 95125Catania, Italy2) Dipartimento di Fisica e Astronomia, Università di Catania, 95125, Catania,Italy3) LIP-Laboratório de Instrumentação e Física Experimental de Partículas,Departamento de Física da Universidade de Coimbra, 3004-516 Coimbra,PORTUGAL.14) ISEC Coimbra, Portugal5) Smoluchowski Institute of Physics, Jagiellonian University of Cracow, 30059Cracow, Poland6) Gesellschaft für Schwerionenforschung mbH, 64291 Darmstadt, Germany7) Institut für Kern- und Hadronenphysik, Forschungszentrum Rossendorf, PF510119, 01314 Dresden, Germany8) Technische Universität Dresden, 01062 Dresden, Germany9) Joint Institute of Nuclear Research, 141980 Dubna, Russia10) Institut für Kernphysik, Johann Wolfgang Goethe-Universität, 60486 Frank-furt, Germany11) II.Physikalisches Institut, Justus Liebig Universität Giessen, 35392 Giessen,Germany12) Istituto Nazionale di Fisica Nucleare, Sezione di Milano, 20133 Milano, Italy13) Dipartimento di Fisica, Università di Milano, 20133 Milano, Italy14) Institute for Nuclear Research, Russian Academy of Science, 117312 Moscow,Russia15) Physik Department E12, Technische Universität München, 85748 Garching,Germany16) Department of Physics, University of Cyprus, 1678 Nicosia, Cyprus17) Institut de Physique Nucléaire d’Orsay, CNRS/IN2P3, 91406 Orsay Cedex,France18) Nuclear Physics Institute, Academy of Sciences of Czech Republic, 25068Rez, Czech Republic19) Departamento de Física de Partículas. University of Santiago de Compostela.15782 Santiago de Compostela, Spain20) Instituto de Física Corpuscular, Universidad de Valencia-CSIC,46971-Valencia, SpainAbstractDilepton production has been measured with HADES, the “High Accep-tance DiElectron Spectrometer”. In pp collisions at 2.2GeV kinetic beamenergy, exclusive η production and the Dalitz decay η → γe+e− has beenreconstructed. The electromagnetic form factor is well in agreement withexisting data. In addition, an inclusive e+e− spectrum from the 12C+12Creaction at 2AGeV is presented and compared with a thermal model.21 IntroductionOne of the main open questions in QCD is the origin of the hadron masses. Besidethe so-called Goldstone bosons like pi and η, the typical mass scales of hadronsin the vacuum is in the order of GeV, whereas the current quark masses mu, mdare 5-15MeV. Based on this fundamental question it has been proposed that themass of the hadrons is related to the spontanous breaking of chiral symmetry,which should be partially restored at finite baryon and energy densities. In con-nection to this question, one of the topics in the modern hadron physics is howhadrons behave in strong interacting medium. This means that their properties -like mass and width - have to be measured inside either cold, dense or hot nuclearmatter. In this context, vector mesons have been proposed as an ideal probe forsuch studies, since they decay via an intermediate virtual photon γ∗ into dileptons(e+e− or µ+µ−) which do not undergo strong interaction. Recently, the NA60collaboration [1] has extracted the ρ − ω line shape using the dimuon channelin In+In collisions at 158AGeV, which would correspond to a hot environment.These measurement indicates broadening of the line shape rather than a droppingof the mass.However, for a complete understanding of the hadronic properties it is impor-tant to measure not only the hot, but also the dense region of the phase space.At beam energies of 1-2 AGeV, which corresponds to moderate densities (2-3ρ0), the production of mesons is dominated by multi-step exitations of a limitednumber resonances and their subsequent decays, like ∆+,0 → Npi0 → Nγe+e−(pi-Dalitz), ∆ → Ne+e− (∆-Dalitz), N∗(1535) → Nη → Nγe+e− (η-Dalitz)and the decay of virtual resonances in N(ω, ρ). Here, most of the productionmechanisms are at or even below threshold, which means that mesons are morelikely produced in the dense phase of the fireball evolution.The result of such experiments is usually a dilepton invariant mass spectrumcontaining all these sources (dilepton-cocktail). Before a conclusion on the prop-erties of ρ and ω can be drawn, the contribution of Dalitz decays have to be sub-tracted.2 Properties of the virtual photonFor a more detailed view, it is very helpful to know that a virtual photon (decayinginto 2 stable particles) has 6 degrees of freedom: First, its invariant mass M invγ∗ .Moreover the momentum PXγ∗ , the polar θXγ∗ and the azimuthal angle φXγ∗ of the3virtual photon in the rest frame of the source X . Angles are defined with respectto the production plane and momentum transfer. Finally, the 2 decay angles of thephoton into the dilepton pair, which are usually described with the helicity angleθeee (which is the angle of one lepton in the dilepton rest frame, with respect tothe direction of the photon), and the Treiman-Yang angle φeee (the orientation ofthe decay plane around the photon direction). Since a detector has a finite accep-tance as a function of total phase space, for an interpretation of the invariant massspectrum M invγ∗ the additional distribution functions have to be known. For thepseudoscalar mesons, these are well under control: For a spin-less state, no align-ment information can be carried from the production mechanism to the decay, soθXγ∗ , φXγ∗ and φeee are isotropic. The helicity angle is proposed to be 1+cos2 θeee [2].For a given mass of γ∗ its momentum is fixed by the mass of the meson. The onlydegree of freedom is the mass spectrum, which is based on a well-know electro-magnetic form factor [3]. In total, the pseudoscalar mesons have no uncertaintyand are a good choice for a detector performance test.This is very different for the ∆ Dalitz decay, because for particles carryingspin, production and decay do not factorize. In addition, the form factor as well asthe branching ratio are based on calculations [4] and the helicity angle has quitesome uncertainties [2]. For the direct decay of vector mesons, the helicity anglehas to be isotropic, but the Treiman-Yang angle could contain higher order contri-butions. Moreover, the ω → pi0e+e− transition form factor cannot be consistentlydescribed within the vector meson dominance model. This shows, that it is notonly important to measure the dilepton production in heavy ion collision, but col-lect also information of the individual sources to avoid systematic errors based onthe detector acceptance. This is one of the goals of the HADES detector (GSI,Darmstadt). Therefore, its program spans from pp, pip to piA and AA collisions.First data has been taken in CC collisions at 1 and 2AGeV, ArK + Cl collisionsat 1.78AGeV, and pp at 1.25 and 2.2GeV.A more detailed view of the detector can be found in [5, 6]. Shortly, HADESis a magnetic spectrometer, consisting of up to 4 mini drift chambers (MDC) withan toroidal magnetic field. Particle identification is done based on time-of-flightmeasurements. In addition, a Ring Imaging Cherenkov detector (RICH) and anelectromagnetic Pre-Shower detector adds lepton identification features.43 The pp→ ppη reaction at 2.2GeVIn January 2004 the first pp run has been carried out at a kinetic beam energyof 2.2GeV, in order to study the performance of the detector using the exclu-sive reaction pp → ppη which has been measured by the DISTO collaborationbefore [7] at kinetic beam energies of 2.15, 2.5 and 2.85GeV. Since the decaysη → pi+pi−pi0 [8] and η → γe+e− [3] are known, it served as a calibration re-action and first test if HADES is able to measure electromagnetic form factors.The analysis techniques are described in detail elsewhere [9]. Basically, for bothreaction types kinematical contraints were used to identify the missing particle pi0or γ, respectively. In addition, a kinematic refit reduced the background.3.1 The hadronic decay η → pi+pi−pi0By using a selection on cos(θCMη ) and fitting the corresponding pp missing-massspectrum, the angular distribution of the η meson emission has been evaluated.The acceptance correction has been done using a full Monte-Carlo simulationwith generated events of pp → pM → ppη where the shape M → pη was takenfrom [7]. Only the alignment of the ppη plane has been left according to phasespace. Together with the fact that the beam axis has rotational symmetry, only twoangles are left for the alignment of the ppη plane, which are almost independent,thus allowing to compare directly 1-dimensional angular distributions. After thefull analysis chain, generated and accepted events have been divided as a functionof cos(θCMη ). Results are shown in Fig. 1 together with the found distribution byDISTO with an anisotropy coefficient (2nd-order Legendre polynomial) of c2 =−0.32 ± 0.10. A fit on the data gives c2 = −0.14 ± 0.09, which tends more toan isotropic η production, but is still consistent within error bars with the previousresult, showing that the hadron efficiency is understood as a function of phasespace.3.2 The Dalitz decay η → γe+e−Similar methods have been used for the η Dalitz decay. In addition, a selectionon the opening angle of larger 4o was used to reject contributions by η → γγ anda subsequent conversion in the detector material. The invariant mass was usedas the observable which was corrected for acceptance and efficiency. Again, byfitting the pp missing-mass spectrum for each invariant mass slice, the yield hasbeen extracted. For the acceptance correction, the known production angles have5been fixed in the simulation, and the known η decay properties have been usedas described in Sec. 2. Fig. 2 shows the corrected invariant mass spectrum [10]together with functions using the QED as well as vector meson dominance model(VDM) form factor [3] which have been normalized to the number of η’s found byexperiment. In addition, each data point has been corrected in position accordingto the strong slope of the models. It can be seen that HADES is not sensitiveto distinguish these 2 models, but at least agrees to the predictions within thegiven errors. This demonstrates that the efficiency is understood as a functionof the invariant mass, which is the main important observable in the heavy iondata. In addition, the ratio between the 2 η decay channels (R=Nη→pi+pi−pi0Nη→γe+e−) havebeen calculated for a full-cocktail simulation and the data. The result is Rexp =15.3 ± 1.8stat and Rsim = 15.6 ± 0.9stat, a nice agreement confirming that thelepton response of the HADES spectrometer is well understood [9].4 Results on C+C at 2AGeVThe first result on dilepton production in C+C collisions at a kinetic beam en-ergy of 2AGeV is ready for publication [11]. Details of the basic analysis stepscan also be found in [12]. Basically, leptons are identified via the RICH detec-tor and reconstructed using the MDCs. An opening angle of the pair of 9o hasbeen applied to remove background from conversion. Finally, the combinatorialbackground (i.e. pairs mixed from different sources) has been removed by thelike-sign method. For events with masses (M invee > 0.5GeV/c2) the combinatorialbackground has been evaluated using the event-mixing technique.In contrast to exclusive reactions, in heavy ion reactions only the final dilep-ton cocktail can be shown, without any selection on the different sources. As ex-plained above, the virtual photon properties might vary, thus the extrapolation to4pi is difficult, since 6 independent variables should be taken into account. Whilethe integration over these variables cannot be done, a different method has beenchosen. First, a single track efficiency as a function of plabe , θlabe , φlabe has been ex-tracted by simulation and applied to the data. This reduces the acceptance to abinary value. In order to compare the efficiency corrected data to any model, anacceptance filter has been developed [13]. Fig. 3 shows the invariant mass spec-trum after all analysis steps. Absolute normalization has been done by a measure-ment of the charged pions, whose mean is the number of pi0 in a isospin-symmeticreaction.In order to make a statement for the contribution of the vector mesons, a sim-6ulation has been made based on a simple thermal model [14] and filtered withthe detector acceptance. The resulting curves are shown also in Fig. 3. The solidline contains the pi and η Dalitz decays with the decay properties as describedabove and the known production cross sections and distribution from TAPS [15].It can be seen, that the pi0 region is well described, however already in the η regionmodel and data disagree. It is clear that additional sources are needed.For a more complete view, ∆ production has been added by pi0 scaling and thevector meson production by mT scaling [16]. However, still a factor of around 2remains in the mass region of 0.2GeV/c2 < M invee < 0.7GeV/c2, where the ∆decay is part of the contribution, as well as the low mass tail of the ρ meson. Ithas to be clarified if this enhancement is due to a different ∆ yield and/or decayproperties, or modified vector meson shapes in the dense medium. This meansthat the HADES data has to be compared to advanced model calculations[17, 18,19], which on the other hand needs as much constraints as possible extracted byelementary collisions.5 Summary and OutlookIn summary, results for dilepton production obtained with HADES in elementaryas well as heavy ion collisions have been presented. While the analysis of thebeam times described in this work is almost finished, the CC collision at 1AGeVis under analysis. In addition, the pp run at 1.25GeV (below the η threshold) ded-icated to ∆ production has collected in spring 2006 promising statistics. System-atic studies using elementary reactions will be continued, which is of particularimportance for the interpretation of heavy ion inclusive mass spectra.The collaboration gratefully acknowledges the support by BMBF grants 06TM970I,06GI146I, 06F-140, and 06DR120 (Germany), by GSI (TM-FR1,GI/ME3,OF/STR), bygrants GA CR 202/00/1668 and GA AS CR IAA1048304 (Czech Republic), by grantKBN 1P03B05629 (Poland), by INFN (Italy), by CNRS/IN2P3 (France), by grantsMCYT FPA2000-2041-C02-02 and XUGA PGID T02PXIC20605PN (Spain), by grantUCY-10.3.11.12 (Cyprus), by INTAS grant 03-51-3208 and EU contract RII3-CT-2004-506078.References[1] R. Arnaldi et al., Phys.Rev.Lett. 96, (2006) 162302.7[2] E.L. Bratkovskaya, O.V. Teryaev and V.D. Toneev, Phys.Lett. B348, (1995)283.[3] L.G. Landsberg Phys.Repts. 128, (1985) 301.[4] C. Ernst et al., Phys.Rev.C 58, (1998) 447.[5] R. Schicker et al., Nucl.Instr.Meth A380, (1996) 586.[6] A. Agakichiev et al., Nucl.Instr.Meth, to be published.[7] F. Balestra et al., Phys. Rev. C69 (2004) 064003.[8] C. Amsler et al., Phys. Lett. B 346 (1995) 203.[9] S. Spataro for the HADES collaboration, proceedings of the Meson 2006,Int.J.Mod.Phys, to be published.[10] B. Spruck, PhD thesis, University of Gießen.[11] A. Agakichiev et al., Phys.Rev.Lett, submitted, nucl-ex/0608031[12] Th. Eberl for the HADES collaboration, Eur.Phys.J, to be published.[13] R. Holzmann, unpublished. (requests to r.holzmann@gsi.de)[14] M.A. Kagarlis, GSI Report 200-03 (2000), unpublished.[15] R. Averbeck at al., Z. Phys. A 359 (1997) 65.[16] E.L. Bratkovskaya, W. Cassing and U. Mosel, Phys.Lett. B424, (1998) 244.[17] W. Cassing and E.L. Bratkovskay, Phys.Rept. 308, (1999) 65.[18] K. Shekhter et al, Phys.Rev. C68, (2003) 014904. M.D. Cozma, C. Fuchs,E. Santini, A. Faessler, Phys.Lett. B640, (2006) 150.[19] D. Schumacher, S. Vogel and M. Bleicher, nucl-th/0608041.8)CMηθcos(-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1Normalized Yields00.020.040.060.080.10.12exp (HADES)fitDISTOPreliminaryFigure 1: Distribution of the η polar production angle in the center of mass frame(statistical errors only). The plot is corrected for acceptance. The solid line rep-resents a fit to the data, whereas the dashed lines shows a parametrization for theexisting [7] in the same beam energy regime.9]2 [MeV/c-e+eM0 100 200 300 400 500 600a.u.210310410510Preliminaryexp (HADES)VMDQEDFigure 2: Invariant mass distribution M invee for the decay η → γe+e− [10] (sta-tistical errors only). The dashed line is showing the prediction for a simple QEDfactor, while the solid one represents the full VDM calculation [3].10]2 [GeV/ceeM0 0.2 0.4 0.6 0.8 1]2  [1/ GeV/cee  dN/dM0 pi1/N-810-610-410-210C  2 AGeV12C + 12 o > 9-e+eθ0piη∆0ρωFigure 3: Invariant mass distribution M invee for CC at 2AGeV [12]. The datapoints contain statistical error only. The lines are a thermal model simulation asdescribed in the text11

Dilepton production in pp and CC collisions with HADES

Dilepton production in pp and CC collisions with HADES

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