Highlights from NA60 experiment by jsf12239


									            Highlights from NA60 experiment

 Study of dimuon production
  in p-A and A-A collisions
       at CERN SPS


                                               Alessandro Ferretti
                                         (University of Torino and INFN)
• NA60 concept                             for the NA60 collaboration
• In-medium modification of vector mesons
• Intermediate mass range excess: prompt or charm?
• J/ψ production in p-A and A-A                                            1
                    Layout of the NA60 experiment
                                                    Muon trigger and tracking:
                      2.5 T dipole magnet           NA10/38/50 spectrometer
    beam                   vertex tracker

                                                                       magnetic field

                                                                                                     Iron wall

                                            hadron absorber
                Matching in coordinate                                                           Muon
                and momentum space!

NA60 concept: place a radiation-hard silicon tracking telescope in the vertex region
to measure the muon tracks before they suffer multiple scattering in the absorber and
match them to the tracks measured in the muon spectrometer.
• Origin of muons can be accurately determined
• Mass resolution @ :~20 MeV/c2 (vs. 80 MeV/c2)                        or
• Mass resolution @ J/:~70 MeV/c2 (vs. 105 MeV/c2)                  displaced                     !
Low dimuon masses – In-In data
     Search for in-medium modifications of vector mesons
                                     Peripheral data: well reproduced
                                     by the hadronic cocktail
                                     Central data: isolate the excess
                                    by subtracting the cocktail
                                     Phys. Rev. Lett. 96 (2006) 162302

                                 and :
                                     fix yields such as to get, after
                                     subtraction, a smooth underlying
                               :
                                     = set upper limit, defined by
                                     saturating the measured yield in
                                     the mass region close to 0.2 GeV
                                     (lower limit for excess).
                                     = use yield measured for
                                     pT > 1.4 GeV/c (where the excess
                                     is small)                      3
Excess spectra from difference
Fine analysis in 12 centrality bins
                                           data – cocktail
                                               (all pT)
                                          •No cocktail  and no
                                          DD subtracted

                                          • Clear excess above the
                                          cocktail , centered at
                                          the nominal  pole and
                                          rising with centrality

                                          • Excess even more
                                          pronounced at low pT

                      cocktail / =1.2                           4
Evolution of the excess shape as a function of centrality
Quantify the peak and the broad symmetric continuum with a mass interval C around the
peak (0.64 <M<0.84 GeV) and two equal bins L, U on either side

                                              “continuum” = 3/2(L+U)
                                              “peak” = C-1/2(L+U)

                                               Fine analysis in 12 centrality bins


Peak/cocktail  drops by a factor 2
from peripheral to central:
the peak is not simply the cocktail 
residing over a broad continuum.

nontrivial changes of all three                              peak/continuum
variables at dNch/dy>100?                                                             5
                         Comparison to theory

• Predictions for In-In by Rapp et al.
  (2003) for <dNch/d> = 133,
  covering all scenarios

• Data and predictions as shown,
  after acceptance filtering, roughly
  mirror the respective spectral
  functions, averaged over space-
  time and momenta

• Theoretical yields normalized to
  data in mass interval < 0.9 GeV/c2

    Only broadening of  (RW)
   No mass shift (BR) observed
Description of the mass region above 1 GeV
 Rapp / Hees, hep-ph/0604269 (2006)      Ruppert / Renk, Phys.Rev.C (2005)

Mass region above 1 GeV described in
                                         Mass region above 1 GeV described
terms of hadronic processes, 4 p …,
                                         in terms of partonic processes,
sensitive to vector-axialvector mixing   dominated by q-qbar annihilation
and therefore to chiral symmetry

                         Hadron-parton duality?                              7
First look at transverse momentum distributions
• Weak centrality dependence             • Differential fits with gliding
                                         windows of pT=0.8 GeV  local
• Trend at small mT different to what    slope Teff
expected from radial flow (not for !)
                                         • At high pT, -like region hardest,
• High mass interval shows steepest      high mass region softest!
slope  smaller T slope
                                         • Not yet explained by theory

        Conclusions I (low dimuon masses)

Mass spectra
• Pion annihilation seems to be a major contributor to the lepton
  pair excess at SPS energies
• Strong broadening, but no significant mass shift of the 

pT spectra
• Strong mass dependence of pT spectra
• Spectra behave opposite to expected from radial flow
• pT spectra could serve as a handle to disentangle partonic from
  hadronic sources (breaking parton-hadron duality)

Intermediate dimuon masses (IMR – 1.1<mmm<2.5 GeV)
   Observed IMR excess in In-In over expected Charm and Drell-Yan yields.
   Where it comes from?
NA60 measures the muon offsets m: distance between interaction vertex and track impact point

             Dimuon offset     (2m 1  2m 2 ) / 2

  Fix prompt contribution to the expected               Leave DY free and fix open charm
  DY– leave open charm free                             according to expectations

            Bad Fit                                           Good Fit

The excess is a prompt source ~2 times higher than the expected DY yield                        10
Centrality dependence of the excess

 Slight increase as a function of number of
 participants with respect to Drell-Yan


                 Corrected for acceptance     All data

Mass spectrum (1.16<M<2.56 GeV/c2)
Contributions to IMR corrected for the acceptance in
  -0.5 < cos  < 0.5
  2.92 < ylab < 3.92
(both 4000 and 6500 A data sample used)


pT dependence of the excess (1.16<M<2.56 GeV/c2)
                                             No acceptance correction

  High pT tail strongly depends on the
  correctness of Drell-Yan description
  by Pythia
  Teff fits are performed in 0< pT <2.5
  GeV/c                                     6500 A

           Corrected for acceptance

                                             Corrected for acceptance

Towards a “unification” of low and intermediate dimuon
mass regions: evolution of excess Teff vs. Mmm


      Conclusions II (intermediate mass range)
• Prompt dimuons production is ~2 times higher than the expected
Drell-Yan in Indium-Indium collisions at 1.16 < m < 2.56 GeV/c2.
• Charm production is compatible with expectations.
• Prompts/Drell-Yan slightly increases with number of participants.
• Excess contribution is dominated by low pT’s, reaching a factor
  3.50.4 for pT<0.5 GeV/c.
• The effective temperature of the excess (~190 MeV) is considerably
lower than the temperatures observed at lower masses (both for the
resonances and the low-mass excess)

     Results are preliminary, since they depend on the correctness
     of used Drell-Yan and Charm contribution pT distributions.
                     Need to be verified with pA data
Direct J/ sample – In-In data
The measured J/ vs. EZDC distribution is compared to the distribution expected in case
of pure nuclear absorption.

    Normalization of the
    nuclear absorption curve:
    the ratio measured/expected, integrated over centrality, is fixed to the one
    resulting from J/ψ/DY analysis (0.87 ± 0.05).
(J/)/DY in p-A collisions at 158 GeV

               (J/)/DY = 29.2  2.3
               L = 3.4 fm

                                             B   μμ σ J/ψ σ DY ) L 
                                                                     158 GeV, meas

                                                                                         1.03  0.08
                                             μμ   σ J/ψ σ DY ) L
                                                                450 (400) GeV, extrap

• Preliminary NA60 result shows that the rescaling of the J/
  production cross section from 450(400) GeV to 158 GeV is correct!
• Next step: obtain absJ/ at 158 GeV                                                            17
                                       J/ polarization
• Quarkonium polarization  test of production models
    • Color Singlet Model: transverse polarization
    • Color Evaporation Model: no polarization
    • Non-Relativistic QCD: transverse polarization at high pT
• Deconfinement should lead to a higher degree of polarization
(Ioffe,Kharzeev PRC 68(2003) 094013)

       0.5 < pT < 5 GeV                             0 < pT < 5 GeV
       0.1 < yCM < 0.6                              0.4 < yCM < 0.75

       H = 0.03  0.06                             CS = -0.03  0.17
       2/ndf =1.01                                 2/ndf =1.42

pT2 vs centrality
• If pT broadening is due to gluon scattering in the initial state
       pT2 = pT2pp + gN · L

                                                 • NA60 In-In points are in fair
                                                   agreement with Pb-Pb results

                                                 • We get
                                                  gNInIn = 0.067  0.011 (GeV/c)2/fm
                                                  pT2ppInIn = 1.15  0.07 (GeV/c)2
                                                  2/ndf = 0.62

                                                       to be compared with
                                                 gNPbPb = 0.073  0.005 (GeV/c)2/fm
                                                 pT2ppPbPb = 1.19  0.04 (GeV/c)2
                                                 2/ndf = 1.22
                                                        (NA50 2000 event sample)

           pT broadening consistent with initial state gluon scattering                 19
’ suppression in In-In and pA collisions
• Study limited by statistics in In-In (N’ ~ 300)
• Normalized to Drell-Yan yields

                                                     • Most peripheral point
                                                       (Npart ~ 60) does not show
                                                       an anomalous suppression
                                                     • Good agreement with
                                                        Pb-Pb results


                                                      Also the ’ value measured
                                                      by NA60 at 158 GeV is in
                                                      good agreement with
                          Preliminary!                the normal absorption
                                                      pattern, calculated from
                                                      450 (400) GeV data
                Conclusions III (J/ψ suppression)
• NA60 has performed a high-quality study of J/ production in In-In
collisions at the SPS which confirms, for a much lighter system, the
anomalous suppression seen in Pb-Pb collisions by NA50

• Preliminary results from p-A collisions at 158 GeV show that the
normalization of the absorption curve is correct

• Peripheral In-In and Pb-Pb results are compatible with p-A

• Absence of J/ polarization in the kinematical window probed by NA60

• pT distributions sensitive to initial state effects

• Study of J/ suppression for other collision systems, with the accuracy
  allowed by a vertex spectrometer, would be very interesting

                         The NA60 collaboration

                                        CERN       Heidelberg                   ~ 60 people
                                                                                13 institutes
                            Palaiseau                      Bern                  8 countries

                           BNL                                  Riken

                  Stony Brook                                     Yerevan

                       Lisbon                                   Torino

                                 Clermont                Cagliari

   R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen, B. Cheynis,
  C. Cicalò, A. Colla, P. Cortese, S. Damjanović, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees,
 L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigoryan, J.Y. Grossiord, N. Guettet, A. Guichard,
H. Gulkanyan, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço, J. Lozano, F. Manso, P. Martins, A. Masoni,
  A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot, T. Poghosyan, G. Puddu, E. Radermacher,
 P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, H.J. Specht,
                   R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri

spare slides

                              J/ / DY analysis
                    Set A (lower ACM current)              Set B (higher ACM current)

• Combinatorial background (p, K decays) from event mixing method (negligible)
• Multi-step fit:
    a) DY (M>4.2 GeV), b) IMR (2.2<M<2.5 GeV), c) charmonia (2.9<M<4.2 GeV)
•   Mass shape of signal processes from MC (PYTHIA+GRV94LO pdf)

• Results from set A and B statistically compatible  use their average in the following

• Stability of the J/ / DY ratio:
    • Change of input distributions in MC calculation  0.3% (cos), 1% (rapidity)
    • Tuning of quality cut for muon spectrometer tracks  < 3%                         24
                  Smooth effect or sharp drop ?



                                                                 Step position   Npart

                                            Step position: Npart = 86 ± 8
                                            ( Bj ~ 1.6 GeV/fm3 )
                                            A1= 0.98 ± 0.02
                                            A2= 0.84 ± 0.01
                                            2/dof = 0.7

• Taking into account the EZDC resolution
  data are compatible with a sharp drop
• An onset smoother than our resolution on Npart (~20) is disfavored
• Work in progress to extend our Npart range towards more peripheral events
                             T vs centrality

 Fitting functions
                                         • Used by NA50
1) dN/dpT = pT mT K1(mT/T)               • Gives slightly
2) dN/dpT = pT e     -mT/T                 higher T values (~ 7 MeV)
      Comparison with recent results (HERA-B, E866)

                    Helicity                   Helicity                       Collins

• HERA-B, in p-A collisions at 920 GeV, sees (mostly in the
 Collins-Soper reference system) a significant
 longitudinal polarization at low pT (P. Faccioli et al., Hard Probes 2006)

• No polarization in NA60, which covers a higher xF region

• E866, at still larger xF, sees a (slight) transverse polarization
 (T.H. Chang et al., PRL 91(2003), 211801)
         Azimuthal distribution of the J/
             central                            peripheral

More peripheral data  hint for a non isotropic emission pattern?
Only 50% of the statistics analyzed

                          Event selection
• 2 event selections have been used for J/ analysis
• No matching required
• Extrapolation of muon tracks must lie in the target region
        Higher statistics
        Poor vertex resolution (~1 cm)
• Matching between muon tracks and vertex spectrometer tracks
• Dimuon vertex in the most upstream interaction vertex
 (MC correction to account for centrality bias due to fragment reinteraction)
        Better control of systematics
        Good vertex resolution (~200 mm)
        Lose 40% of the statistics

 • After quality cuts  NJ/ ~ 45000 (1), 29000 (2)
• 2 analyses
a) Use selection 1 and normalize to Drell-Yan
b) Use selection 2 and normalize to calculated J/ nuclear absorption
              J/ kinematical distributions
• Study of differential distributions important in order to assess
    • Role of initial state effects  pT distributions
    • Production mechanisms and/or deconfinement  polarization
• Technique
    • 3-D acceptance correction (pT, y, cos)
    • Fine binning (0.1 GeV/c pT, 0.05 y-units, 0.1 cos-units)
    • Define fiducial region (zone with local acceptance >1%)
         -0.1<cosH<0                                          Viewed from
                                              y               J/ rest frame     Collins
                                                                  CS            Soper
                                                  ptarg                     z
                                                                                Frames for
                                                                        z         studies
                                          x       J/              H
                                  pproj                   ptarg                   Helicity
                                                   y                                       30
            Low mass dimuons

                     • Net data sample:
                       360 000 events
                     • Fakes / CB < 10 %
                     • ω and  peaks clearly visible
                      in dilepton channel; even
                       μμ seen
                    • Mass resolution:
                       23 MeV at the  position

                     • Progress over CERES:
                       statistics: factor >1000
                       resolution: factor 2-3

                               Role of baryons

 Calculations for In-In by Rapp et al. (11/2005) for <dNch/d> = 140

• Improved model:
    • Fireball dynamics                       Baryons important in the
    • 4 p processes
    • spectrum described in absolute terms
                                              low mass tail
              pT spectra - acceptance correction
  • Reduce 3-dimensional acceptance correction in M-pT-y
    to a 2-dimensional correction in M-pT, using measured
    y distribution as an input. Use  for control
  • Use slices of
    m = 0.1 GeV
    pT = 0.2 GeV
  • Check behaviour on 3
    extended mass windows

        0.4<M<0.6 GeV
        0.6<M<0.9 GeV
        1.0<M<1.4 GeV

Subtract charm from the data (based on NA60
IMR results) before acceptance correction

               Systematics of low-pT data:
                combinatorial background

• Enhanced yield at low-pT
  seen at all centralities,
  including the peripheral
• Errors at low pT, due to
  subtraction of

   peripheral      1%
   semiperipheral 10%
   semicentral    20%
   central        25%

         Enhanced yield at low pT not due to incorrect
         subtraction of combinatorial background
Excess pT spectra: 3 centrality bins

      Hardly any centrality dependence
        Significant mass dependence

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