Recent results from NA60 and future plans Gianluca Usai –University of Cagliari and INFN (Italy) Hard Probes - Eilat 15/010/2010

Outline Thermal dimuons and hadrons: r spectral function and thermal radiation in the IMR region Transition form factors of h and w Dalitz decays in In-In and pA …collisions Nuclear dependence of r, w, f cross sections vs A Charmonia and open charm: absorption cross section in pA collisions and anomalous . ..suppression in AA collisions J/y absolute cross sections in pA collisions open charm production in pA collisions Present ideas about a NA60-like future experiment

NA60 setup and data samples 2.5 T dipole magnet hadron absorber targets beam tracker Si-pixel tracker muon trigger and tracking magnetic field Track matching in coordinate and momentum space  Improved dimuon ...mass resolution - Distinguish prompt from decay dimuons Additional bend by the dipole field  acceptance coverage down to low pT Radiation-hard silicon pixel detectors  high luminosity Data samples In-In collisions at 158 GeV/nucleon p-A collisions at 158 and 400 GeV 9 nuclear targets, Al-U-W-Cu-In-Be1-Be2-Be3-Pb (mixed A-order to limit possible z-dependent systematics) 3

Excess dimuons in In-In collisions at the SPS Phys. Rev. Lett. 96 (2006) 162302 Peripheral data: well described by meson decay cocktail (η, η’, ρ, ω, f) and DD More central data (shown): existence of excess dimuons isolation of excess by subtraction of the measured decay cocktail (without r), based solely on local criteria for the major sources η Dalitz, ω and f

The r spectral function Rapp-Wambach: hadronic model predicting strong broadening/no mass shift Brown/Rho scaling: dropping mass due to dropping of chiral condensate Predictions for In-In by Rapp et al (2003) for dNch/d = 140, covering all scenarios Theoretical yields normalized to data in mass interval < 0.9 GeV After acceptance filtering, data and predictions mirror the r spectral function, averaged over space-time and momenta Only broadening of  (RW) observed, no mass shift (BR)

Excess mass spectrum up to 2.5 GeV All known sources (hadro-cocktail, open charm, Drell-Yan) subtracted Acceptance corrected spectrum (pT>0.2 GeV) Absolute normalization → comparison to theory in absolute terms! M<1 GeV: thermal pp r  mm Strong r broadening – no mass shift In-medium effects dominantly driven by baryons M>1 GeV: thermal qq g  mm suggested to be dominant by Teff vs M (supported by R/R, D/Z) and/or multipion processes (H/R) Eur. Phys. J. C 59 (2009) 607 How to distinguish? 6 6 6

The rise and fall of radial flow of thermal dimuons Phys. Rev. Lett. 100 (2008) 022302 Teff: extracted from fits to mT=(Mmm2+pT2)1/2 with thermal function exp(-mT/Teff) Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV Rise consistent with radial flow of a hadronic source (here pp→r→mm) , taking the freeze-out ρ as the reference Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→mm) 7

EM transition form factors for w and h Dalitz decays EM decay of light unflavored mesons ABl+l- Described by QED for point-like particles A form factor accounts for the more complex em structure arising at the vertex of the transition Believed to be dominated by photon-hadron resonance interaction (vector meson dominance) Old experimental results (Lepton-G experiment) showed, for +-0, a deviation from VMD at ~ 3 level Use the high statistics In-In and pA data to investigate this anomaly VMD

Isolating the Dalitz region in the peripheral data Phys. Lett. B 677 (2009) 260 subtraction of - η, ω, ɸ resonance decays also - η’ Dalitz decay (η’/η=0.12) - uncorr. μ+μ- from DDbar (both nearly negligible ;  systematic errors) correct for acceptance fit remaining sources η, ω and ρ; χ2/ndf~1, globally and locally anomaly of ω form factor directly visible in the spectrum 9

Results on form factors Phys. Lett. B 677 (2009) 260 Perfect agreement of NA60 and Lepton G, confirming ω anomaly Large improvement in accuracy; for ω, deviation from VMD 3  10 σ 10

NA60 results in the new edition of the PDG First result from a heavy-ion experiment in the PDG ever 11 11

New results on form factors from the NA60 p-A data 400 GeVpA data: 178000 muon pairs without target selection 5.5104 -Dalitz, 1.2104 -Dalitz (600 and 60 for Lepton-G) Further improvement of a factor 10 with respect to In-In peripheral analysis preliminary preliminary

r, w, f cross section and particle ratios in p-A at 400 GeV Fit of the raw mass spectrum for each nuclear target Cross section as a function of the mass number A, normalizing the cross sections to Be targets Targets simultaneously exposed to the beam: uncertainties on the luminosity cancel out  acceptance and recostruction efficiencies do not completely cancel out (these quantities and their time evolution are computed for each target)

Fits of single targets Events per 20 MeV/c2 Events per 20 MeV/c2 p-Be χ2/ndf = 111/55 p-Be χ2/ndf = 87/55 p-Cu χ2/ndf = 171/55 p-In Events per 20 MeV/c2 χ2/ndf = 152/55 p-W χ2/ndf = 116/55 p-Pb χ2/ndf = 123/55 p-U

Cross section ratios Fit excluding Be target αw (no Be) = 0.72 ± 0.035 ± 0.017 αf (no Be) = 0.92 ±0.030 ±0.015 preliminary preliminary αr = 0.769 ± 0.041± 0.020 αw = 0.768 ± 0.013± 0.020 αf = 0.841 ± 0.011± 0.013 Clear effect present when excluding Be: aw decreases towards 2/3 (full absorption) limit while af increases closer to 1

Particle ratios f meson has significantly higher alpha w.r.t the ω preliminary preliminary NA27 Consistent with ρ/ω = 1 Clear evidence of a rising trend as a function of A f meson has significantly higher alpha w.r.t the ω

r/w interference? Sides of the r/w peak not properly described Suggestion of r/w coherent production? Statistics seems enough to be sensitive to a quantitative study

Charmonia suppression: pA, AA Study of charmonium production/suppression in pA/AA collisions THE hard probe at SPS energy AA collisions Color screening and charmonium suppression > 20 year long history pA collisions Production models (CSM, NRQCD, CEM, ....) Initial/final state nuclear effects (shadowing, dissociation,...) Reference for understanding dissociation in a hot medium

A-dependence of relative cross sections 400 GeV 158 GeV  increasing suppression of the J/ yield as a function of A larger suppression at 158GeV using a Glauber fit to extract abs 158 GeV: abs J/ = 7.6 ± 0.7 ± 0.6 mb 400 GeV: abs J/ = 4.3 ± 0.8 ± 0.6 mb NA60 Coll., arXiv:1004.5523 very good agreement with NA50 (4.6 ± 0.6mb) shadowing neglected, as usually done at fixed target (but not correct!) using 158 GeV: = 0.882 ± 0.009 ± 0.008 400 GeV:  = 0.927 ± 0.013 ± 0.009 systematic errors due to : target thicknesses (<1.5%)‏ J/ y distribution (<1.5%)‏ reconstruction eff. calculation (<3%)‏

Anomalous suppression In-In 158 GeV (NA60) Pb-Pb 158 GeV (NA50) Using the previously defined reference: Central Pb-Pb:  still anomalously suppressed In-In: almost no anomalous suppression B. Alessandro et al., EPJC39 (2005) 335 R. Arnaldi et al., Nucl. Phys. A830 (2009) 345 R.Arnaldi, P. Cortese, E. Scomparin Phys. Rev. C 81 (2009), 014903

New result: J/y cross section in pA J/ production cross sections for pA data Preliminary Systematic error on (absolute) luminosity estimation quite high Relative luminosity estimate between 158 and 400 GeV much better known (~2-3% systematic error) Normalize NA60 400 GeV cross section ratios to NA50 results 158 GeV cross sections constrained by the relative normalization

New reference using J/y cross sections Alternative approach for the normalization of the pA reference curve based on the pA J/ absolute cross section To fully profit from this approach, a measurement of the absolute J/ cross section in In-In would be needed. For the moment… J//DY values are obtained rescaling the DY cross section measured at 450 GeV by NA50 (not enough statistics at 158 GeV) Main advantage: no assumption on SU, since it is not used anymore in the fit Preliminary No practical consequence on anomalous J/Ψ suppression difference with previous CNM reference ~1% well within errors

Anomalous suppression: SPS vs RHIC SPS results compared with RHIC RAA results normalized to RAA(CNM) Both Pb-Pb and Au-Au seem to depart from the reference curve at Npart~200 For central collisions more important suppression in Au-Au with respect to Pb-Pb M. Leitch (E866), workshop on “Quarkonium in Hot Media:From QCD to Experiment”, Seattle 2009 Agreement between SPS and RHIC results as a function of the charged particle multiplicity

Open charm production in pA collisions Open charm share initial state effects with charmonium  a measurement of open charm in p-A collisions may help in understanding J/ suppression Recent results from SELEX and E866 suggest rather strong nuclear effects on open charm E866/NuSea Preliminary A. Blanco et al. (SELEX), EPJC64(2009) 637 M. Leitch (E866), workshop on “Heavy Quarkonia Production in Heavy-Ion Collisions”, ECT* 2009

Open charm dimuons in pA: NA60 NA50 tried to evaluate DD production studying the IMR in pA Large background levels (S/B ~0.05 at m = 1.5 GeV/c2) NA50 had to impose a constant DD/DY vs A (i.e. DD= DY ~1 ) M.C. Abreu et al., EPJC14(2000) 443 NA60 is much better placed, thanks to the muon matching  S/B is ~60 times more favourable

Open charm dimuons in pA: results Kinematic range: 3.2<ylab<3.7 (-0.17<ycm<0.33) Study the ratio /DD (reduce systematic errors) Preliminary Systematic errors Fit starting point Track 2 /DY (norm. and J/) Background normalization Using the measured J/ value one gets DD = 0.960.03 Anti-shadowing would suggest DD >1

New Pb-Pb and/or other systems at lower energy Future high precision dilepton measurements with an NA60-like setup QCD phase diagram poorly known in the region of highest baryon densities and moderate temperatures – is there a critical point? Marek’s “horn” in <K+>/<p+>: the critical point might be located within √sNN~5-10 GeV Heavy ion beams should be available at the SPS again in next years New Pb-Pb and/or other systems at lower energy

Low mass dileptons: top to low SPS energies Decrease of energy 160 to 40 AGeV: predicted net r in-medium effects, in particular for M<0.4 GeV, increase by a factor 2 because of baryons! Pioneering measurement by CERES at 40 AGeV: enhancement increases! Seems to confirm importance of baryonic effects Might not be just coincidental with expectation of emergence of the critical point Compelling to continue research into the regime of maximal baryon density experimentally accessible

QCD critical point search – experimental landscape partly complementary programs planned at CERN SPS 2011 BNL RHIC 2010 DUBNA NICA 2013 GSI SIS-CBM 2016 NA60-like experiment: Dilepton measurements in region not covered by other experiments High precision muon pair measurements: - high luminosity → statistics - very good mass resolution …- acceptance down to low pT - background subtraction …. ….easier than in e+e- Flexibility to change energy (and A) Complementary to NA61

NA60-like apparatus layout for lower energies Thoroidal magnet R=150 cm Compress the spectrometer reducing the absorber Enlarge transverse dimensions Angular coverage: 240 mrad Thoroidal magnet R=240 cm Original NA60:Angular coverage: 160 mrad

Acceptance coverage of low energy setup <y> 30 AGeV pT (GeV) <y> 158 AGeV <y> 30 AGeV y y   +- 158 AGeV   +- 30 AGeV Standard NA60 setup y range 0-1 in the cms system @ 158 GeV Low energy setup

Combinatorial background: key point Pions entering the absorber all interact within the first 1-2 interaction lengths Additional muons produced in the absorber from secondary pions are all soft and with tracks not pointing to the interaction vertex CB due to pions and kaons decaying into muons before the hadron 2.5 T dipole field Muon filter µ ~10 cm vertex , K  µ offset Beam Tracker Vertex Detector Detector concept: essential to keep distance from interaction point to absorber as small as possible

Summary NA60 physics analysis still ongoing pA low mass dimuons: Transition form factors of h and w Dalitz decays: confirmation of Lepton G with significant improvement of errors Light resonance cross sections vs A: ar,w close to 2/3, af closer to 1 pA high mass dimuons: J/psi cross sections: new CNM reference with pA data taken in the same energy/kinematic domain of AA Open charm in pA: DD close to 1 Ion beams will be circulating and might be available for experiments at the SPS in the incoming years Feasibility of new dimuon experiment and setup of a future collaboration being pursued