1. 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)
for the NA60 collaboration
NA60 concept
In-medium modification of vector mesons
Intermediate mass range excess: prompt or charm?
J/ψ production in p-A and A-A

2. Layout of the NA60 experiment
Muon trigger and tracking:
2.5 T dipole magnet
NA10/38/50 spectrometer
beam tracker
vertex tracker
magnetic field
Iron wall
hadron absorber
targets
ZDC
Matching in coordinate and momentum space!
Muon
Other
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 :~20 MeV/c2 (vs. 80 MeV/c2)
Mass J/:~70 MeV/c2 (vs. 105 MeV/c2)
first of all, let’s give a look to the NA60 experimental setup. NA60 INHERITED THE na50 muon spectrometer, composed of tracking chambers, trigger hodoscopes, a solenoidal magnet and a zdc to measure the spectator nucleons energy which is used to determine the centrality of the collision. in addition to this, we or !
prompt
displaced

3. Search for in-medium modifications of vector mesons
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)   f
 and :
fix yields such as to get, after subtraction, a smooth underlying continuum :
= 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)
the first physics topic I will show you is the study of the dimuon low mass spectrum. In this range, CERES reported an excess w.r.t. pA data and peripheral collisions: this excess could be due to rho broadening or mass shift from chiral symmetry restoration. Because of insufficient mass resolution CERES was not able to individuate the source of the excess, but thanks to its vertex tracker, NA60 can measure the dimuon yields with unprecedented mass resolution: the hadronic states are clearly resolved
NA 60 peripheral data are well … : to isolate the excess present in central bins we subtracted the hadronic cocktail obtained in peripheral data.

4. Excess spectra from difference
Fine analysis in 12 centrality bins
data – cocktail
(all pT)
No cocktail r and no DD subtracted
Clear excess above the cocktail , centered at the nominal r pole and rising with centrality
Excess even more pronounced at low pT
these are the results in 12 centrality bins. Cocktail rho was not subtracted because we expect some modification of its yield, so the rho is inside the excess: cocktail rho is shown for comparison
cocktail / =1.2

5. 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
continuum/r
Peak/cocktail r drops by a factor 2 from peripheral to central: the peak is not simply the cocktail r residing over a broad continuum.
peak/r
nontrivial changes of all three variables at dNch/dy>100?
peak/continuum

6. 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

7. Hadron-parton duality?
Description of the mass region above 1 GeV
Rapp / Hees, hep-ph/ (2006)
Ruppert / Renk, Phys.Rev.C (2005)
Mass region above 1 GeV described in terms of hadronic processes, 4 p …, sensitive to vector-axialvector mixing and therefore to chiral symmetry restoration
Mass region above 1 GeV described in terms of partonic processes, dominated by q-qbar annihilation
Hadron-parton duality?

8. First look at transverse momentum distributions
Differential fits with gliding windows of pT=0.8 GeV  local slope Teff
At high pT, -like region hardest, high mass region softest!
Not yet explained by theory
Weak centrality dependence
Trend at small mT different to what expected from radial flow (not for f!)
High mass interval shows steepest slope  smaller T slope
the transverse mass spectra show a weak centality dependence. Concerning the slope, the trend...

9. 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 r
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)

10. 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 Dm: distance between interaction vertex and track impact point
Dimuon offset
Fix prompt contribution to the expected DY– leave open charm free
Leave DY free and fix open charm according to expectations
An excess in the imr was measured by NA50: it could be explained either by thermal radiation from the medium produced in the collision or by an enhancement of charm production. To discriminate between these two sources it is necessary to measure the offset of the muon tracks w.r.t. interaction vertex: NA60 can do it!
Good Fit
Bad Fit
The excess is a prompt source ~2 times higher than the expected DY yield

11. Centrality dependence of the excess
Slight increase as a function of number of participants with respect to Drell-Yan
All data
preliminary
Corrected for acceptance

12. Mass spectrum (1.16<M<2.56 GeV/c2)
Contributions to IMR corrected for the acceptance in < cos  < < ylab < 3.92
(both 4000 and 6500 A data sample used)
preliminary
Excess is not drell-yan

13. pT dependence of the excess (1.16<M<2.56 GeV/c2)
No acceptance correction
6500 A
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
preliminary
Corrected for acceptance
Corrected for acceptance

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

15. 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 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

16. Direct J/ sample – In-In data
The measured J/ vs. EZDC distribution is compared to the distribution expected in case of pure nuclear absorption.
An anomalous suppression of the j/psi yield w.r.t the normal nuclear absorption has been measured by NA50 in lead-lead collisions: to understand which physics mechanism drive this suppression, NA60 took data with a different colliding system, namely In-In. Here you can see the results of na60
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).

17. (J/)/DY in p-A collisions at 158 GeV
L = 3.4 fm
Preliminary!
To claim that an anomalous suppression of the j psi as been observed, j/psi production in p-A collision must be accurately known. Up to now,this knowledge came from the pA collisions with a proton energy of 400 GeV, and to allow a comparison we had to rescale the cross section for elementary collisions to the rather different energy an kinematical domain of the 158 GeV collisions.
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

18. 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) )
CS =  0.17
2/ndf =1.42
0 < pT < 5 GeV
0.4 < yCM < 0.75
H = 0.03  0.06
2/ndf =1.01
0.5 < pT < 5 GeV
0.1 < yCM < 0.6

19. 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 =  (GeV/c)2/fm
pT2ppInIn = 1.15  0.07 (GeV/c)2
2/ndf = 0.62
to be compared with
gNPbPb =  (GeV/c)2/fm
pT2ppPbPb = 1.19  0.04 (GeV/c)2
2/ndf = 1.22
gNPbPb96 =  (GeV/c)2/fm
pT2ppPbPb96 = 1.10  0.03 (GeV/c)2
2/ndf = 1.38
(NA event sample)
pT broadening consistent with initial state gluon scattering

20. ’ suppression in In-In and pA collisions
Study limited by statistics in In-In (N’ ~ 300)
Normalized to Drell-Yan yields
In-In:
Preliminary!
Most peripheral point
(Npart ~ 60) does not show
an anomalous suppression
Good agreement with
Pb-Pb results
pA:
Also the ’ value measured
by NA60 at 158 GeV is in
good agreement with
the normal absorption
pattern, calculated from
450 (400) GeV data

21. 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

22. The NA60 collaboration http://cern.ch/na60 Lisbon CERN Bern Torino
Yerevan
Cagliari
Lyon
Clermont
Riken
Stony Brook
Palaiseau
Heidelberg
BNL
~ 60 people
13 institutes 8 countries
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

23. spare slides

24. Set A (lower ACM current) Set B (higher ACM current)
J/ / DY analysis
Set A (lower ACM current)
Set B (higher ACM current)
Combinatorial background (, 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%

25. Smooth effect or sharp drop ?
Npart
Meas/Exp 1
Step position A1 A2
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

26. T vs centrality Fitting functions Used by NA50 Gives slightly
higher T values (~ 7 MeV)
1) dN/dpT = pT mT K1(mT/T)
2) dN/dpT = pT e -mT/T

27. Comparison with recent results (HERA-B, E866)
Helicity
Helicity
Collins
Soper
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), )

28. Azimuthal distribution of the J/
central
peripheral
More peripheral data  hint for a non isotropic emission pattern?
Only 50% of the statistics analyzed

29. Event selection 2 event selections have been used for J/ analysis 1)
No matching required
Extrapolation of muon tracks must lie in the target region
Higher statistics
Poor vertex resolution (~1 cm) 2)
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 m)
Lose 40% of the statistics
After quality cuts  NJ/ ~ (1), (2)
2 analyses
a) Use selection 1 and normalize to Drell-Yan
b) Use selection 2 and normalize to calculated J/ nuclear absorption

30. 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
pproj
ptarg z
CS + y x
Viewed from
J/ rest frame H
J/
Collins
Soper
Frames for
polarization
studies
Helicity

31. 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 ω  

32. Role of baryons Baryons important in the low mass tail
Calculations for In-In by Rapp et al. (11/2005) for <dNch/d> = 140
Improved model:
Fireball dynamics
4  processes
spectrum described in absolute terms
Baryons important in the low mass tail

33. 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

34. Systematics of low-pT data: combinatorial background
Enhanced yield at low-pT seen at all centralities, including the peripheral bin
Errors at low pT, due to subtraction of combinatorial background:
peripheral 1%
semiperipheral
10%
semicentral
20%
central
25%
Enhanced yield at low pT not due to incorrect subtraction of combinatorial background

35. Excess pT spectra: 3 centrality bins
Hardly any centrality dependence
BUT
Significant mass dependence