The main results obtained from the analysis of the NA60 data during 2007 are reviewed. Also the future perspectives for the analysis activity during 2008 are considered

CERN. Geneva

CERN. Geneva. SPS Experiments Committee

SPSC

CERN Document Server

EUROPEAN LABORATORY FOR PARTICLE PHYSICSCERN-SPSC-2007-032SPSC-SR-026October 3, 2007NA60 status reportThe NA60 CollaborationAbstractIn this report the main results obtained from the analysis of the NA60 data during2007 are reviewed. Also the future perspectives for the analysis activity during 2008are considered.1 The low and intermediate dimuon mass regionsin Indium-Indium collisions: first evidence ofdilepton radial flowAs discussed in the previous report, in the mass region below 2.6 GeV an excess overthe expected sources has been isolated.The left panel of Fig. 1 shows in detail the excess in the low mass region resultingfrom the subtraction of the known pseudoscalar and vector mesons contributions [1].In the intermediate mass region, the detailed analysis of the offset distributions(also discussed in the previous report) showed that the observed open charm yield isconsistent with the extrapolation from the NA50 p-A data. Thus, no enhanced opencharm production is observed. The excess comes from a prompt source and it is 2.4times the expected Drell-Yan yield. The left panel of Fig. 2 shows the dimuon excessmass distribution observed in the intermediate mass region, which is steeper than theDrell-Yan.0 0.2 0.4 0.6 0.8 1 1.2 1.40100002000030000No centrality selectiondN/dM per 20 MeVM (GeV)Tall pγµµ→η µµ→ωµµ→φ0piµµ→ωµµ→ηγµµ→’ηIn-In0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8510610710810910 (arb.un.)T dN/dmT1/m0.2<M<0.4 GeV0.4<M<0.6 GeV0.6<M<0.9 GeVφ1.0<M<1.4 GeV>30η/dchdN In-In 3)×( 6)×(-M (GeV)TmFigure 1: Left: Isolation of an excess above the electromagnetic decays of neutralmesons in the low mass region. Total data (open circles), individual cocktail sources(solid), difference data (thick triangles), sum of cocktail sources and difference data(dashed). Right panel: Transverse mass spectra of the excess for four mass windowssummed over centralities (excluding the peripheral bin) in comparison to the φ.ForM < 1 GeV the dimuon excess yield is dominated by the process pi+pi− → ρ→1)2Mass (GeV/c1.2 1.4 1.6 1.8 2 2.2 2.4dN/dM, acc. corr.310410510 2± 22 ±199  1± 17 ±193  2± 24 ±171  (MeV)effTDrell-YanOpen charmExcess (GeV/c)0 - MTM0 0.2 0.4 0.6 0.8 1T dN/dMT1/M10210310410510]     T [MeV]2M [GeV/c3 ±21±1.16 - 1.40    1992±16±1.40 - 2.0      1933±23±2.0   - 2.56    171Figure 2: Left: excess mass distribution in the intermediate mass region and com-parison to Drell-Yan and open charm. Right: Transverse mass spectra of the excessfor three mass windows summed over centralities.µ+µ− with a strongly modified intermediate ρ. Different many-body hadronic modelsare able to describe the data with a strongly broadened intermediate ρ without anymass shift.On the other hand, there is not a unique interpretation for masses above 1 GeV.There are models able to describe this region in terms of partonic processes (thelong sought thermal radiation from the plasma) [2], others where the predominantprocesses are hadronic (4 pion annihilation or higher processes) [3]. This ambivalencereflects what is usually called parton-hadron duality.In order to try to clarify this situation, NA60 has now focused the attention onthe transverse momentum spectra, which encode information on the source temper-ature and on the collective motion (radial flow) that takes place during the fireballexpansion. When the spectra are fitted with the simple Boltzmann-like functiondN/dpT = pT exp(−mT/Teff ), the effective temperature extracted from the spectrais not any longer the source temperature. It is blue shifted (boosted) by the collec-tive motion and acquires a linear dependence on mass. Since dimuons are emittedcontinuously during the whole fireball lifetime, they probe regions where the sourcetemperature was high and the flow was small (early phases of the fireball with alow effective temperature close to the real source temperature), or regions where thetemperature was low and the flow was high (later phases of the fireball lifetime with20 0.5 1 1.5 2 2.5100150200250300350 (MeV)effTM (GeV))φ, ω, ρ, ηhadrons (dimuonsLMRLMR, w/o DYIMR,  w/o DYIn-In>30η/dchdNFigure 3: Inverse slope parameter Teff vs. dimuon mass M for dNch/dη > 30.a larger effective temperature because the increasing flow boost predominates overthe fireball cooling). In this way, dimuon transverse momentum spectra can be usedas a diagnostic tool to access the emission region and may differentiate between ahadronic or a partonic nature of the source.The acceptance-corrected transverse momentum spectra of the excess were mea-sured for several mass intervals from threshold up to 2.6 GeV. The results are shown indetail in the right panels of Fig. 1 and Fig. 2. They were fitted with the Boltzmann-like exponential function given above. The effective temperature Teff as a function ofthe dimuon invariant mass is shown in Fig. 3. For comparison, also the temperatureof the stable hadrons η, ω and φ, as measured by the same experiment, are shown(open circles). Several striking features are seen. First, the effective temperatureof the excess shows an almost linear rise with mass up to M = 1 GeV, as for thehadrons. As mentioned above, this is a rather clear evidence of radial flow.Second, the effective temperature exhibits a sudden drop of almost 50 MeV atM = 1 GeV. In addition, for masses beyond 1 GeV, Teff remains at an approximatelyconstant value. This feature reflects seemingly a condition of rather small or no flow.This observation could suggest, for M > 1 GeV, that dimuons are emitted from asource which was dominant in an early stage of the fireball evolution, as the partonicprocess qq¯ → µ+µ−.All these results have been finalised. A paper has been submitted to PhysicalReview Letters. A more extensive paper mainly focused on the excess found in the3intermediate mass region is ready for submission.2 J/ψ suppression in Indium-Indium collisionsThe results on the J/ψ suppression, discussed in the previous report, have beenfinalised and published in Physical Review Letters. The suppression pattern is shownin the left panel of Fig. 4 and compared with the NA50 results relative to Pb-Pbcollisions. The two suppression patterns are compatible within the common range ofnumber of participants and clearly show that the J/ψ is suppressed beyond normalnuclear absorption at SPS energies. A similar effect has been recently observed alsoin Au-Au collisions at RHIC (√s = 200 GeV/nucleon).partN0 50 100 150 200 250 300 350 400 yieldψMeasured / Expected J/0.50.60.70.80.911.11.21.3)/DY normalizationψError on (J/Error related to centrality determination (GeV/c)Tp0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 (a.u.)T dN/dpT1/p-410-310-210-110110210310410510610710810910All centralities> = 194part<N> = 175part<N> = 151part<N> = 126part<N> = 102part<N> = 78part<N> = 58part<N> = 33part<NFigure 4: Left: Comparison between the In-In (NA60, circles) and Pb-Pb (NA50,triangles) suppression patterns. The box on the left shows the 6% systematic errorrelated to the normalization procedure of the In-In points. Right: J/ψ transversemomentum spectra for several centrality bins.Further results have been obtained from the study of the differential spectra. Thetransverse momentum distributions have been studied in several centrality bins andare shown in the right panel of Fig. 4. The effective temperature, integrated overcentrality, is TJ/ψ = 231±2 MeV. A broadening of the transverse momentum spectraas a function of centrality is observed, consistent with the broadening due to gluonscattering in the initial state. Also the J/ψ rapidity distribution has been measured.Data are consistent with a gaussian distribution roughly independent of centralityand having a width of 0.68± 0.02.The J/ψ polarization in the helicity reference frame has been studied as a functionof transverse momentum, rapidity and centrality. This measurement has consider-able interest since the quarkonium polarization is a key test for different productionmodels. Moreover, it was speculated that deconfinement could lead to a significant4polarization. The present results show that the polarization is consistent with zero,independent of centrality or any kinematic variable.In addition, also the ψ′ suppression pattern has been measured. While it ispresently thought that the J/ψ could survive well above the deconfinement criti-cal temperature (up to 2Tc), the relatively loose bound states ψ′ and χc should meltfor temperatures around Tc. This study is limited by the statistics which amounts to300 ψ′ in the total sample. The yield is normalized to the Drell-Yan and studied inthree centrality bins. An anomalous suppression is observed, in good agreement withthe Pb-Pb results.Presently the suppression pattern is being studied as a function of rapidity andtransverse momentum. In addition, also the measurement of the J/ψ absolute crosssection is under way.3 φ production in In-In collisionsThe results on the φ differential spectra have been finalised and a paper draft is inpreparation. The inverse T slopes extracted from transverse momentum spectra, asdiscussed in the previous report, are in rather good agreement with NA49. Whenthe fit is restricted to the NA50 transverse momentum range (pT > 1 GeV), theobserved slopes are slightly smaller (a hint of radial flow) though not enough toagree with the NA50 values. The study of the differential spectra was completedby the measurement of the rapidity distribution and the polarization. The data areconsistent with a gaussian rapidity distribution roughly independent of centrality andhaving a width of 1.15±0.06. The polarization was extensively measured in differentreference systems (Collins-Soper, Gottfried-Jackson and helicity). In all of the threereference frames the polarization is compatible with zero.The largest effort in the past months was devoted to the absolute yield measure-ment. This is important in order to try to shed light on the long-standing discrep-ancy, know as φ puzzle, between the results found in Pb-Pb collisions by NA49 (whichmeasured the φ in the kaon channel) and NA50 (which measured the φ in the muonchannel). The yield measured in the muon channel by NA50 is 2-3 times higher thanthe yield measured in the kaon channel by NA49. Theoretical models tried to explainit assuming kaon absorption and rescattering in the hot and dense medium, whichwould suppress the observed φ yield in the kaon channel. In addition, the absoluteyield measurement in NA60 now provides a normalization also for other processesin the low mass region, which is essential for quantitative comparisons to theoreticalmodels.The measurement is performed following two independent methods. The first isbased on the luminosity determination, exploiting the beam counters provided bythe ZDC and the Beam Tracker detectors. In the second approach, the number ofobserved J/ψ (corrected for anomalous suppression and nuclear absorption) or Drell-Yan events are used to estimate the number of interactions by using the known J/ψ5or Drell-Yan elementary cross sections (cross sections per binary collision). The firstpreliminary results seem to indicate that the φ yield in In-In (in the muon channel)is also higher than the one measured by NA49. On the other hand, the observed φenhancement in In-In saturates already at a number of participants corresponding toPb-Pb semi-peripheral collisions. This fact, together with the smaller system size inIn-In which would lead to a smaller φ yield with respect to Pb-Pb, could indicatequalitative agreement with the NA50 results. The first preliminary results from NA60may be shown in the oral presentation.4 Elliptic flow of charged particles and J/ψ In-IncollisionsThe results on the charged particles and J/ψ elliptic flow discussed in the previousreport have been finalised and a paper draft is in preparation.5 Results from p-A 2002 dataThe results from p-A data collected in 2002, mainly focused on the ω and φ productionhave been finalised and a paper is ready for submission.6 p-A 2004 reconstruction and analysisThe p-A data can be divided in two samples, both collected with seven differenttargets (Al, Be, Cu, In, W, Pb and U). Data were taken with a 158 GeV/c beam(four days) using a vertex tracker made of 13 ALICE pixel planes, 2 ATLAS pixelplanes and 2 BNL microstrip planes. In the remaining data taking period, a 400GeV/c beam was used, with a few different setups of the vertex tracker includingalso ATLAS microstrip planes.The analysis of the data collected with the 158 GeV/c beam is ongoing. Theobjective is to measure the nuclear dependence of the J/ψ absorption cross sectionat the same energy as the In-In data. The total sample consists of ∼ 11000 J/ψdimuons. In a preliminary analysis, the ratio σJ/ψ/σDY averaged over the targetshas been measured and compared to previous results as a function of L, the meanpath of the J/ψ in nuclear matter. In the left panel of Fig. 5 a compilation ofpA measurements, taken at various energies and rescaled at 158 GeV/c, is shown.The new NA60 point is in rather good agreement. This seems to indicate that therescaling of the elementary production cross section, which was so far used in thedetermination of the nuclear absorption reference, is indeed correct. The next stepwill be the measurement of the absorption cross section.Concerning the 400 GeV/c data sample, approximately 15% of the total statisticshas been reconstructed and the analysis is ongoing. This fraction corresponds to a6set-up where the first ATLAS microstrip planes were replaced by pixel planes. Thefirst physics results are expected by the end of this year. Selecting events with onedimuon correctly associated to a vertex in the target region, the total statistics ofopposite sign muon pairs amounts to ∼ 100000, 6 times more than the statisticscollected in 2002. The right panel of Fig. 5 shows the preliminary dimuon invariantmass spectra for the different nuclear targets. Fitting the ω, φ and J/ψ peaks withgaussian functions, the measured mass resolution is σω = 35 MeV, σφ = 37 MeV,σJ/ψ = 104 MeV. In the same figure the like-sign mass distribution (∼ 5000 events)is also shown.For what concerns the reconstruction of the remaining data sample, the align-ment of the microstrip planes proved to be a rather delicate problem. In particular,the usual alignment method based on the minimization of residuals failed almostcompletely. A new method has been developed, based on the simultaneous searchof correlations in the x, y coordinates between tracks reconstructed with the pixelplanes and microstrips. The results obtained with this new method are satisfactoryand a semi-automatic tool for the final alignment is under development. We expectto increase significantly the reconstructed data sample by the end of the year.L(fm)0 1 2 3 4 5)2.9-4.5(DYσ)/Ψ(J/σ µµB203040NA51, pp,pd, 450 GeVNA50 p-A 450 GeV LINA50 p-A 450 GeV HINA50 p-A, 400 GeV HINA60, p-A, 158 GeV )2Mass (GeV/c0.5 1 1.5 2 2.5 3 3.510210310=35 MeVωσ=37 MeVφσ=104 MeVψJ/σFigure 5: Left panel: Compilation of the σJ/ψ/σDY values measured in pA andnucleus-nucleus collisions at the SPS, rescaled, when necessary, to 158 GeV/c2. Thelines indicate the results of a Glauber fit to the p-A data and the size of the error.The full circle indicates the preliminary NA60 result for pA collisions at 158 GeV/c2.Right panel: Dimuon invariant mass spectra, integrated over the different nucleartargets, for 15% of the total 400 GeV/c statistics.The lack of sufficient manpower prevented so far the analysis of the proton datain parallel with the indium data. Since the analysis of the indium data will be largelyterminated by the end of 2008, the proton data analysis will then receive the fullattention and the collaboration will put the highest effort into the completion of thecorresponding physics program.77 Future plans7.1 Near Term Priority topics1. In-In analysis. The time estimate to complete each of the following analyses,which will lead to about 10 paper, is 1− 1.5 person-years.• Low Mass / thermal radiation- Acceptance-corrected mass spectra and spectral moments- Polarisation of the ω and the continuum- Elliptic flow of the continuum- Hadrons: electromagnetic transition form factors (η, ω Dalitz)Responsible: S. Damjanovic, H.J. Specht (part time)• Low mass / hadrons- Absolute calibration of NA60- φ analysis- Charged particle multiplicity- Flow analysisResponsible: M. Floris, G. Usai• J/ψ- Anomalous suppression as a function of pT and y.- Finalisation of differential spectra and polarization- Absolute cross sectionResponsible: R. Arnaldi, E. Scomparin2. Calibration and reconstruction of p-A dataResponsible: R. Shahoyan, P. Martins (PhD), P. Parracho (PhD), P. Ramalhete(PhD), P. Cortese, M. Floris, C. Lourenco, J. Seixas, F. MansoTime estimate: 1/2 - 3/4 year87.2 Further important analysis topics1. p-A analysis• Low mass hadrons- Nuclear dependence of particle ratios- ρ/ω interference• J/ψ- Nuclear dependence at 400 and 158 GeV- Transverse momentum spectra• Open charm in p-A- Cross sections - Nuclear dependence• χc in p-A- Nuclear dependence2. Ultra-peripheral collisions in In-In3. Λ and D0 production in In-InThere are more topics than people presently available for these analyses.8 Requests to CERNThe NA60 collaboration will continue the analysis activity at least until the end of2009. For this reason, we would like to request the support from the CERN ITdivision at the present level. In particular, the analysis work makes use of the rawdata stored on the CASTOR system. In addition, we would like to keep the officeswhich are presently assigned.References[1] R. Arnaldi et al. (NA60), Phys. Rev. Lett. 96 (2006) 162302.[2] J. Ruppert, T. Renk, arXiv:hep-ph:0612113[3] H. van Hees and R. Rapp. Phys. Rev. Lett. 97 (2006) 1023019Papers published and in progress - 20071. J/ψ production in Indium-Indium collisions at 158 GeV/nucleon. Phys. Rev.Lett. 99 (2007) 132302. Available in the arXiv: arXiv:0706.43612. First measurement of radial flow of thermal dileptons in high energy nuclearcollisions. Submitted to Physics Review Letters.3. Production of dimuons in the mass region between the φ and the J/ψ in indium-indium collisions at the SPS. Paper in preparation, to be submitted to EuropeanPhysics Journal.4. φ meson production in indium-indium collision at 158 AGeV incident beamenergy. Paper in preparation, to be submitted to European Physics Journal.5. φ, ω and dimuon production in proton-nucleus collisions at 400 GeV. Paper inpreparation, to be submitted to The Journal of High Energy Physics.6. Elliptic flow of charged particles and J/ψ in indium-indium collisions. Paperin preparation, to be submitted to European Physics Journal.7. New results from NA60 and other SPS experiments (G. Usai et al.), J. Phys.G 34 (2007) S2338. NA60 results on charm and inermediate mass dimuons production in In-In 158GeV collisions, (R. Shahoyan et al.), J. Phys. G 34 (2007) S10299. New results on the ρ spectral function in Indium-Indium collisions, (J. Seixaset al.), J. Phys. G 34 (2007) S102310. J/ψ production in In-In and p-A collisions. (E. Scomparin et al.), J. Phys. G34 (2007) S46311. Quantifying the prompt intermediate mass dimuon excess in the In-In collisionsmeasured by NA60. (A. David et al.), Nucl. Phys. A 783 (2007) 20512. J/ψ suppression in In-In collisions at 158-GeV/nucleon. (R. Arnaldi et al.),Nucl. Phys. A 783 (2007) 26113. NA60 results on the rho spectral function in In-In collisions. (S. Damjanovic etal.), Nucl. Phys. A 783 (2007) 32714. First Measurement of the ρ Spectral Function in Nuclear Collisions (S. Dam-janovic et al.). Eur. Phys. J. C 49 (2007) 23515. φ production in NA60 (M. Floris et al.) Eur. Phys. J. C 49 (2007) 25510

NA60 Status Report

Abstract

Similar works

Full text

Available Versions

CERN Document Server