We present preliminary results on the production of intermediate mass dimuons in 158 A·GeV In-In collisions at the CERN SPS. NA38 and NA50 observed a strong excess in this region in S-U and Pb-Pb interactions with respect to the dimuon production rate expected from p-A data. Thanks to the use of a pixel vertex tracker, NA60 can separate the prompt dimuons from the pairs resulting from open charm decays and show that the excess dimuons are of prompt origin

Arnaldi, R

Averbeck, R

Banicz, K

Castor, J

Chaurand, B

Cicalò, C

Colla, A

Cortese, P

Damjanovic, S

David, A

De Falco, A

Devaux, A

Drees, A

Ducroux, L

Enyo, H

Ferretti, A

Floris, M

Force, P

Förster, A

Guettet, N

Guichard, A

Gulkanian, H R

Heuser, J

Keil, M

Kluberg, L

Loureno, C

Lozano, J

Manso, F

Martins, P

Masoni, A

Neves, A

Ohnishi, H

Oppedisano, C

Parracho, P

Pillot, P

Puddu, G

Radermacher, E

Ramalhete, P

Rosinsky, P

Scomparin, E

Seixas, J

Serci, S

Shahoyan, Ruben

Sonderegger, P

Specht, H J

Tieulent, R

Usai, G

Veenhof, R

Wöhri, H K

CERN Document Server

Intermediate mass dimuon production inIndium-Indium collisions at the CERN SPSRuben Shahoyan∗ for the NA60 CollaborationCFTP, Instituto Superior Técnico, Lisbon, PortugalE-mail: Ruben.Shahoyan@cern.chR. Arnaldi 10, R. Averbeck 9, K. Banicz 2,4, J. Castor 3, B. Chaurand 7, C. Cicalò 1,A. Colla 10, P. Cortese 10, S. Damjanovic 4, A. David 2,5, A. De Falco 1, A. Devaux 3,A. Drees 9, L. Ducroux 6, H. En’yo 8, A. Ferretti 10, M. Floris 1, P. Force 3,A. ,Förster 2, N. Guettet 2,3, A. Guichard 6, H. Gulkanian 11, J. Heuser 8, M. Keil 2,5,L. Kluberg 2,7, C. Lourenço 2, J. Lozano 5, F. Manso 3, A. Masoni 1, P. Martins 2,5,A. Neves 5, H. Ohnishi 8, C. Oppedisano 10, P. Parracho 2, P. Pillot 6, G. Puddu 1,E. Radermacher 2, P. Ramalhete 2, P. Rosinsky 2, E. Scomparin 10, J. Seixas 2,5,S. Serci 1, P. Sonderegger 5, H.J. Specht 4, R. Tieulent 6, G. Usai 1, R. Veenhof 2,5,H.K. Wöhri 2,51Univ. di Cagliari and INFN, Cagliari, Italy,2CERN, Geneva, Switzerland,3LPC, Univ. Blaise Pascal and CNRS-IN2P3, Clermont-Ferrand, France,4Univ. Heidelberg, Heidelberg, Germany,5CFTP, Instituto Superior Técnico, Lisbon, Portugal6IPN-Lyon, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Lyon, France,7LLR, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France,8RIKEN, Wako, Saitama, Japan,9SUNY Stony Brook, New York, USA,10Univ. di Torino and INFN, Italy,11YerPhI, Yerevan, ArmeniaWe present preliminary results on the production of intermediate mass dimuons in 158 A·GeVIn-In collisions at the CERN SPS. NA38 and NA50 observed a strong excess in this region inS-U and Pb-Pb interactions with respect to the dimuon production rate expected from p-A data.Thanks to the use of a pixel vertex tracker, NA60 can separate the prompt dimuons from the pairsresulting from open charm decays and show that the excess dimuons are of prompt origin.International Europhysics Conference on High Energy PhysicsJuly 21st - 27th 2005Lisboa, Portugal∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(HEP2005)131Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS Ruben ShahoyanOne of the most interesting findings of the experiments studying heavy ion collisions in viewof Quark-Gluon Plasma search is the excess of dimuons in the intermediate mass region (IMR,1.2–2.7 GeV/c2) observed in S-U and Pb-Pb collisions by NA38/NA50 [1] and in S-W collisionsby HELIOS-3 [2]. The continuum between the φ and J/ψ peaks is well described in p-A collisionsby the superposition of Drell-Yan dimuons and muon pairs from simultaneous semi-muonic decaysof D and D mesons [1]. Since the Drell-Yan contribution can be normalized from the high massregion, an excess in heavy-ion collisions is more likely to be due to the increase of open charmproduction (difficult to explain in the framework of pQCD) or to the appearance of the long-soughtthermal dimuons, considered to be one of the signatures of QGP formation [3]. It could also be dueto the modification of the Drell-Yan mass spectrum due to higher-twist effects in nucleus-nucleuscollisions [4]. Thus it is crucial to separate the open charm contribution from prompt dimuons.The NA60 experiment can do this by measuring the muon’s offset from the interaction vertex,exploiting the long lifetime of D mesons: cτ = 312 µm for D+ and 123 µm for D0 [5].The NA60 setup [6] consists of a Muon Spectrometer (MS) and a Zero Degree Calorimeter(ZDC), both inherited from NA50, a Vertex Tracker (VT) made from silicon pixel planes [7, 8]embedded in a 2.5 T dipole magnetic field, and a Beam Tracker (BT) measuring the transverseposition of the incoming ion before its interaction in the target.By matching the muons reconstructed in the MS to the tracks measured in the VT we obtainthe muon kinematics unaffected by the scattering in the hadron absorber. The matching is doneby computing the weighted distance squared (χ 2) between these two tracks in the space of anglesand inverse momenta, taking into account the error matrices. This procedure improves the massresolution from 70–80 MeV/c2 to 20–25 MeV/c2 at M∼1 GeV/c2 and allows us to relate the muonto the interaction vertex. The latter is reconstructed with resolution better than 10 µm in X (bendingplane) and 15 µm in Y over most of the multiplicity range. The quality of the vertexing can becontroled using the BT measurement of the incoming ion extrapolated to the interaction point withcentrality independent resolution of 20 µm. The precision of the measurement of the muon offsetwith respect to the vertex can be estimated using the muons from J/ψ decays, which at our energyare always prompt and have negligible background. The estimated offset resolution (includingerrors of the vertexing) for the muons of 15–20 GeV/c momentum typical for the IMR dimuons, is∼40 µm in X and ∼55 µm in Y directions. To account for the momentum dependence of the offsetresolution, the weighted offset is used, defined as ∆µ =√∆x2V−1xx +∆y2V−1yy +2∆x∆yV−1xy for thesingle muons and ∆µµ =√(∆2µ1 +∆2µ2)/2 for dimuons, where the V−1 is the inverted error matrixfrom the vertex fit and the muon extrapolation.There are two types of background in NA60. Muons from uncorrelated pi and K decays createthe combinatorial background (CB). Additionally, some fraction of the muons from the MS can bewrongly matched to hadronic tracks in the VT, thus creating the fake matches background. Bothtypes of background are subtracted by mixed events technique. First, the CB spectrum is built bycombining into muon pairs two single muons from different like-sign dimuons in such a way thatthe obtained dimuons respect the acceptance and trigger conditions of NA60. Since this proceduredoes not distinguish between the fake and correct matches, after the subtraction of this spectrumonly signal dimuons (both with correct and fake matches) remain. Then the spectrum of dimuonscontaining the fake matches (FB), is created by matching the MS muons from one event with VT2131/2PoS(HEP2005)131Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS Ruben Shahoyantracks from a different event and combining these muon matches with original matches from thedata. This spectrum contains the fake matches both for the signal and combinatorial pairs. Toseparate the former contribution, we build the fake combinatorial dimuons (FCB) in the same wayas it is done for the CB but using the single muons from the like-sign FB. The correctly matchedsignal spectrum is obtained as Data - CB - (FB - FCB). Fig. 1a shows the measured opposite-signdimuons’ mass spectrum together with the two different types of background and the extractedsignal. Only matches with χ 2 < 1.5 are selected.)2Mass (GeV/c1 2 3 4 5110210310410510a)DataComb.Bckg.Signal FakesSignal)2Mass (GeV/c1 2 3 4 5110210310410510)µµData (Like Sign Generated (Mixed Comb.Bckg) b) / ndf 2χ  24.71 / 14p0        0.001± 0.994  Generated / Data0.5 1 1.5 2 2.5 30.911.1Figure 1: (a) Opposite-sign dimuon data, combinatorial background (CB), fake signal and final correctlymatched signal; (b) Measured and generated like-sign CB spectra.The quality of the mixed CB is checked by comparing the generated like-sign spectrum withthe measured one, Fig. 1b. As one can see, the latter is reproduced within ∼1%, which, taking intoaccount a signal to background ratio of ∼1/10, means 10–20% systematic error on the final signal.To fit the obtained spectra, the Drell-Yan and open charm contributions were generated byPythia 6.2 using CTEQ6M PDFs with EKS98 nuclear modifications. To fix the relative normal-ization of these contributions, a K-factor of 1.8 was applied to Pythia’s Drell-Yan cross section, inorder to reproduce the high-mass cross section measured by NA50 [9]. The cc¯ cross-section wastaken to be 12 µb/nucleon, to reproduce the NA50 analysis of the 450 GeV p-A data. Note thatthis is almost twice higher than the world average cross section at this energy [10]. The absoluteDrell-Yan normalization as a function of the collision centrality is obtained from the number ofthe observed J/ψ events, the J/ψ suppression pattern and the expected ψ /DY ratio. The data areanalyzed in the kinematic domain 0 < yCM < 1, |cosθCS|< 0.5 and 1.2 < Mµµ < 2.7.First we check if the superposition of the expected Drell-Yan and open charm contributionscan describe the mass spectrum of NA60. Fig. 2a shows the result of the fit with the normalizationfactors fixed within ±10% of their expected values. There is a significant excess in the IMR, around80% of the expected DY yield (Fig. 2b). Letting the open charm normalization free improves thefit, leading to almost twice the expected charm, as shown in Fig. 2c. This is consistent with theobservation of NA50 that a charm yield enhanced by a factor of 3 is needed to describe the dimuonmass spectrum of central Pb-Pb collisions.In the next step we fit the signal dimuons weighted offset distribution as a superposition of theprompt and open charm contributions. For the former the shape of the sum of the measured J/ψand φ offsets is used, while for charm we use the Monte-Carlo shape, smeared to account for thedifference between the Monte-Carlo and data observed in the J/ψ and φ signals. Fig. 3a shows theresult of the fit with the prompt contribution normalized to the expected Drell-Yan and the opencharm left free. The latter is too flat to describe the data. Letting both contributions vary freely, see3131/3PoS(HEP2005)131Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS Ruben Shahoyan)2Mass (GeV/c2 3 4 5110210310 Prompt: 1.1 (fixed)Charm : 1.1 (fixed)/NDF:  8.02χFit   a))2Mass (GeV/c1.5 2 2.50.40.60.810.07± 0.79→[Data - Sources]/DY b))2Mass (GeV/c2 3 4 5110210310 Prompt: 1.1 (fixed)0.09±Charm : 1.81/NDF:  1.82χFit   c)Figure 2: Fit of IMR mass spectrum to the superposition of Drell-Yan and open charm contributions: (a)both normalizations are fixed to be within 10% of expected value; (b) ratio of the excess from (a) to theDrell-Yan contribution; (c) open charm yield is let free.Fig. 3b, gives a good description of the data, preferring the yield extrapolated from the NA50 p-Adata, but requiring almost twice as many prompt dimuons as the expected Drell-Yan.Weighted offset0 1 2 3 4 5 6110210310Prompt: 1.10 (fixed)0.10±Charm : 1.74/NDF:  4.72χFit   a)Weighted offset0 1 2 3 4 5 61102103100.11±Prompt: 1.910.16±Charm : 0.95/NDF:  0.92χFit   b)Figure 3: Fit of IMR dimuon weighted offset spectrum by the superposition of prompt (dashed) and opencharm (dotted) contributions: (a) the prompts are fixed to the expected Drell-Yan level and the charm is free;(b) both contributions are free;In conclusion, this preliminary analysis of In-In collisions at 158 A·GeV shows an excess ofIMR dimuons over the expected Drell-Yan and open charm contributions. It is caused by promptdimuons and has a steeper mass distribution than Drell-Yan.This work was partially supported by the Fundação para a Ciência e Tecnologia (Portugal)under the SFRH/BPD/5656/2001 contract.References[1] M.C. Abreu et al. (NA50 Coll.), Eur. Phys. J. C14 (2000) 443.[2] A.L.S. Angelis et al. (HELIOS-3 Coll.), Eur. Phys. J. C13 (2000) 433.[3] R. Rapp and E. Shuryak, Phys. Lett. B473 (2000) 13.[4] J.W. Qiu and X.F. Zhang, Phys. Lett., B525 (2002) 265.[5] S. Eidelman et al. (PDG), Phys. Lett. B592 (2004) 1.[6] R.Arnaldi et al. (NA60 Coll.), Eur. Phys. J. C43 (2005) 167.[7] K. Banicz et al., Nucl. Instrum. Meth. A539 (2005) 137.[8] G. Usai et al. (NA60 Coll.), Eur. Phys. J. C43 (2005) 415.[9] M.C. Abreu et al. (NA50 Coll.), Phys. Lett. B410 (1997) 337.[10] H. Wöhri and C. Lourenço, J. Phys. G30 (2004) S315.4131/4PoS(HEP2005)131

Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS

NA60International audienceWe present preliminary results on the production of intermediate mass dimuons in 158 A-GeV In-In collisions at the CERN SPS. NA38 and NA50 observed a strong excess in this region in S-U and Pb-Pb interactions with respect to the dimuon production rate expected from p-A data. Thanks to the use of a pixel vertex tracker, NA60 can separate the prompt dimuons from the pairs resulting from open charm decays and show that the excess dimuons are of prompt origin

Shaoyan, R.

Arnaldi, R.

Averbeck, R.

Banicz, K.

Castor, J.

Chaurand, B.

Cicalo, C.

Colla, A.

Cortese, P.

Damjanovic, S.

David, A.

de Falco, A.

Devaux, A.

Drees, A.

Ducroux, L.

En'Yo, H.

Ferretti, A.

Floris, M.

Force, P.

Guettet, N.

Guichard, A.

Gulkanian, H.

Heuser, J.

Kneil, M.

Kluberg, L.

Lozano, J.

Manso, F.

Masoni, A.

Martins, P.

Neves, A.

Ohnishi, H.

Oppedisano, C.

Parracho, P.

Pillot, Philippe

Puddu, G.

Radermacher, E.

Ramalhete, P.

Rosinsky, P.

Scomparin, E.

Seixas, J.

Serci, S.

Sonderegger, P.

Specht, H.J.

Tieulent, R.

Usai, G.

Veenhof, R.

Wöhri, H.K.

HAL-IN2P3

Intermediate mass dimuon production in indium-indiumcollisions at the CERN SPSR. Shaoyan, R. Arnaldi, R. Averbeck, K. Banicz, J. Castor, B. Chaurand, C.Cicalo, A. Colla, P. Cortese, S. Damjanovic, et al.To cite this version:R. Shaoyan, R. Arnaldi, R. Averbeck, K. Banicz, J. Castor, et al.. Intermediate mass dimuonproduction in indium-indium collisions at the CERN SPS. International Europhysics Confer-ence on High Energy Physics HEP2005, Jul 2005, Lisboa, Portugal. Proceedings of Science,pp.PoS(HEP2005)131, 2005. <in2p3-00025243>HAL Id: in2p3-00025243http://hal.in2p3.fr/in2p3-00025243Submitted on 14 Mar 2007HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Intermediate mass dimuon production inIndium-Indium collisions at the CERN SPSRuben Shahoyan∗ for the NA60 CollaborationCFTP, Instituto Superior Técnico, Lisbon, PortugalE-mail: Ruben.Shahoyan@cern.chR. Arnaldi 10, R. Averbeck 9, K. Banicz 2,4, J. Castor 3, B. Chaurand 7, C. Cicalò 1,A. Colla 10, P. Cortese 10, S. Damjanovic 4, A. David 2,5, A. De Falco 1, A. Devaux 3,A. Drees 9, L. Ducroux 6, H. En’yo 8, A. Ferretti 10, M. Floris 1, P. Force 3,A. ,Förster 2, N. Guettet 2,3, A. Guichard 6, H. Gulkanian 11, J. Heuser 8, M. Keil 2,5,L. Kluberg 2,7, C. Lourenço 2, J. Lozano 5, F. Manso 3, A. Masoni 1, P. Martins 2,5,A. Neves 5, H. Ohnishi 8, C. Oppedisano 10, P. Parracho 2, P. Pillot 6, G. Puddu 1,E. Radermacher 2, P. Ramalhete 2, P. Rosinsky 2, E. Scomparin 10, J. Seixas 2,5,S. Serci 1, P. Sonderegger 5, H.J. Specht 4, R. Tieulent 6, G. Usai 1, R. Veenhof 2,5,H.K. Wöhri 2,51Univ. di Cagliari and INFN, Cagliari, Italy,2CERN, Geneva, Switzerland,3LPC, Univ. Blaise Pascal and CNRS-IN2P3, Clermont-Ferrand, France,4Univ. Heidelberg, Heidelberg, Germany,5CFTP, Instituto Superior Técnico, Lisbon, Portugal6IPN-Lyon, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Lyon, France,7LLR, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France,8RIKEN, Wako, Saitama, Japan,9SUNY Stony Brook, New York, USA,10Univ. di Torino and INFN, Italy,11YerPhI, Yerevan, ArmeniaWe present preliminary results on the production of intermediate mass dimuons in 158 A·GeVIn-In collisions at the CERN SPS. NA38 and NA50 observed a strong excess in this region inS-U and Pb-Pb interactions with respect to the dimuon production rate expected from p-A data.Thanks to the use of a pixel vertex tracker, NA60 can separate the prompt dimuons from the pairsresulting from open charm decays and show that the excess dimuons are of prompt origin.International Europhysics Conference on High Energy PhysicsJuly 21st - 27th 2005Lisboa, Portugal∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(HEP2005)131Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS Ruben ShahoyanOne of the most interesting findings of the experiments studying heavy ion collisions in viewof Quark-Gluon Plasma search is the excess of dimuons in the intermediate mass region (IMR,1.2–2.7 GeV/c2) observed in S-U and Pb-Pb collisions by NA38/NA50 [1] and in S-W collisionsby HELIOS-3 [2]. The continuum between the φ and J/ψ peaks is well described in p-A collisionsby the superposition of Drell-Yan dimuons and muon pairs from simultaneous semi-muonic decaysof D and D mesons [1]. Since the Drell-Yan contribution can be normalized from the high massregion, an excess in heavy-ion collisions is more likely to be due to the increase of open charmproduction (difficult to explain in the framework of pQCD) or to the appearance of the long-soughtthermal dimuons, considered to be one of the signatures of QGP formation [3]. It could also be dueto the modification of the Drell-Yan mass spectrum due to higher-twist effects in nucleus-nucleuscollisions [4]. Thus it is crucial to separate the open charm contribution from prompt dimuons.The NA60 experiment can do this by measuring the muon’s offset from the interaction vertex,exploiting the long lifetime of D mesons: cτ = 312 µm for D+ and 123 µm for D0 [5].The NA60 setup [6] consists of a Muon Spectrometer (MS) and a Zero Degree Calorimeter(ZDC), both inherited from NA50, a Vertex Tracker (VT) made from silicon pixel planes [7, 8]embedded in a 2.5 T dipole magnetic field, and a Beam Tracker (BT) measuring the transverseposition of the incoming ion before its interaction in the target.By matching the muons reconstructed in the MS to the tracks measured in the VT we obtainthe muon kinematics unaffected by the scattering in the hadron absorber. The matching is doneby computing the weighted distance squared (χ 2) between these two tracks in the space of anglesand inverse momenta, taking into account the error matrices. This procedure improves the massresolution from 70–80 MeV/c2 to 20–25 MeV/c2 at M∼1 GeV/c2 and allows us to relate the muonto the interaction vertex. The latter is reconstructed with resolution better than 10 µm in X (bendingplane) and 15 µm in Y over most of the multiplicity range. The quality of the vertexing can becontroled using the BT measurement of the incoming ion extrapolated to the interaction point withcentrality independent resolution of 20 µm. The precision of the measurement of the muon offsetwith respect to the vertex can be estimated using the muons from J/ψ decays, which at our energyare always prompt and have negligible background. The estimated offset resolution (includingerrors of the vertexing) for the muons of 15–20 GeV/c momentum typical for the IMR dimuons, is∼40 µm in X and ∼55 µm in Y directions. To account for the momentum dependence of the offsetresolution, the weighted offset is used, defined as ∆µ =√∆x2V−1xx +∆y2V−1yy +2∆x∆yV−1xy for thesingle muons and ∆µµ =√(∆2µ1 +∆2µ2)/2 for dimuons, where the V−1 is the inverted error matrixfrom the vertex fit and the muon extrapolation.There are two types of background in NA60. Muons from uncorrelated pi and K decays createthe combinatorial background (CB). Additionally, some fraction of the muons from the MS can bewrongly matched to hadronic tracks in the VT, thus creating the fake matches background. Bothtypes of background are subtracted by mixed events technique. First, the CB spectrum is built bycombining into muon pairs two single muons from different like-sign dimuons in such a way thatthe obtained dimuons respect the acceptance and trigger conditions of NA60. Since this proceduredoes not distinguish between the fake and correct matches, after the subtraction of this spectrumonly signal dimuons (both with correct and fake matches) remain. Then the spectrum of dimuonscontaining the fake matches (FB), is created by matching the MS muons from one event with VT2131/2PoS(HEP2005)131Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS Ruben Shahoyantracks from a different event and combining these muon matches with original matches from thedata. This spectrum contains the fake matches both for the signal and combinatorial pairs. Toseparate the former contribution, we build the fake combinatorial dimuons (FCB) in the same wayas it is done for the CB but using the single muons from the like-sign FB. The correctly matchedsignal spectrum is obtained as Data - CB - (FB - FCB). Fig. 1a shows the measured opposite-signdimuons’ mass spectrum together with the two different types of background and the extractedsignal. Only matches with χ 2 < 1.5 are selected.)2Mass (GeV/c1 2 3 4 5110210310410510a)DataComb.Bckg.Signal FakesSignal)2Mass (GeV/c1 2 3 4 5110210310410510)µµData (Like Sign Generated (Mixed Comb.Bckg) b) / ndf 2χ  24.71 / 14p0        0.001± 0.994  Generated / Data0.5 1 1.5 2 2.5 30.911.1Figure 1: (a) Opposite-sign dimuon data, combinatorial background (CB), fake signal and final correctlymatched signal; (b) Measured and generated like-sign CB spectra.The quality of the mixed CB is checked by comparing the generated like-sign spectrum withthe measured one, Fig. 1b. As one can see, the latter is reproduced within ∼1%, which, taking intoaccount a signal to background ratio of ∼1/10, means 10–20% systematic error on the final signal.To fit the obtained spectra, the Drell-Yan and open charm contributions were generated byPythia 6.2 using CTEQ6M PDFs with EKS98 nuclear modifications. To fix the relative normal-ization of these contributions, a K-factor of 1.8 was applied to Pythia’s Drell-Yan cross section, inorder to reproduce the high-mass cross section measured by NA50 [9]. The cc¯ cross-section wastaken to be 12 µb/nucleon, to reproduce the NA50 analysis of the 450 GeV p-A data. Note thatthis is almost twice higher than the world average cross section at this energy [10]. The absoluteDrell-Yan normalization as a function of the collision centrality is obtained from the number ofthe observed J/ψ events, the J/ψ suppression pattern and the expected ψ /DY ratio. The data areanalyzed in the kinematic domain 0 < yCM < 1, |cosθCS|< 0.5 and 1.2 < Mµµ < 2.7.First we check if the superposition of the expected Drell-Yan and open charm contributionscan describe the mass spectrum of NA60. Fig. 2a shows the result of the fit with the normalizationfactors fixed within ±10% of their expected values. There is a significant excess in the IMR, around80% of the expected DY yield (Fig. 2b). Letting the open charm normalization free improves thefit, leading to almost twice the expected charm, as shown in Fig. 2c. This is consistent with theobservation of NA50 that a charm yield enhanced by a factor of 3 is needed to describe the dimuonmass spectrum of central Pb-Pb collisions.In the next step we fit the signal dimuons weighted offset distribution as a superposition of theprompt and open charm contributions. For the former the shape of the sum of the measured J/ψand φ offsets is used, while for charm we use the Monte-Carlo shape, smeared to account for thedifference between the Monte-Carlo and data observed in the J/ψ and φ signals. Fig. 3a shows theresult of the fit with the prompt contribution normalized to the expected Drell-Yan and the opencharm left free. The latter is too flat to describe the data. Letting both contributions vary freely, see3131/3PoS(HEP2005)131Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS Ruben Shahoyan)2Mass (GeV/c2 3 4 5110210310 Prompt: 1.1 (fixed)Charm : 1.1 (fixed)/NDF:  8.02χFit   a))2Mass (GeV/c1.5 2 2.50.40.60.810.07± 0.79→[Data - Sources]/DY b))2Mass (GeV/c2 3 4 5110210310 Prompt: 1.1 (fixed)0.09±Charm : 1.81/NDF:  1.82χFit   c)Figure 2: Fit of IMR mass spectrum to the superposition of Drell-Yan and open charm contributions: (a)both normalizations are fixed to be within 10% of expected value; (b) ratio of the excess from (a) to theDrell-Yan contribution; (c) open charm yield is let free.Fig. 3b, gives a good description of the data, preferring the yield extrapolated from the NA50 p-Adata, but requiring almost twice as many prompt dimuons as the expected Drell-Yan.Weighted offset0 1 2 3 4 5 6110210310Prompt: 1.10 (fixed)0.10±Charm : 1.74/NDF:  4.72χFit   a)Weighted offset0 1 2 3 4 5 61102103100.11±Prompt: 1.910.16±Charm : 0.95/NDF:  0.92χFit   b)Figure 3: Fit of IMR dimuon weighted offset spectrum by the superposition of prompt (dashed) and opencharm (dotted) contributions: (a) the prompts are fixed to the expected Drell-Yan level and the charm is free;(b) both contributions are free;In conclusion, this preliminary analysis of In-In collisions at 158 A·GeV shows an excess ofIMR dimuons over the expected Drell-Yan and open charm contributions. It is caused by promptdimuons and has a steeper mass distribution than Drell-Yan.This work was partially supported by the Fundação para a Ciência e Tecnologia (Portugal)under the SFRH/BPD/5656/2001 contract.References[1] M.C. Abreu et al. (NA50 Coll.), Eur. Phys. J. C14 (2000) 443.[2] A.L.S. Angelis et al. (HELIOS-3 Coll.), Eur. Phys. J. C13 (2000) 433.[3] R. Rapp and E. Shuryak, Phys. Lett. B473 (2000) 13.[4] J.W. Qiu and X.F. Zhang, Phys. Lett., B525 (2002) 265.[5] S. Eidelman et al. (PDG), Phys. Lett. B592 (2004) 1.[6] R.Arnaldi et al. (NA60 Coll.), Eur. Phys. J. C43 (2005) 167.[7] K. Banicz et al., Nucl. Instrum. Meth. A539 (2005) 137.[8] G. Usai et al. (NA60 Coll.), Eur. Phys. J. C43 (2005) 415.[9] M.C. Abreu et al. (NA50 Coll.), Phys. Lett. B410 (1997) 337.[10] H. Wöhri and C. Lourenço, J. Phys. G30 (2004) S315.4131/4PoS(HEP2005)131

Intermediate mass dimuon production in indium-indium collisions at the CERN SPS

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Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS

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