1Roberta Arnaldi for the NA60 collaboration

• NA60 concept

• In medium modification of the • Intermediate mass range excess: prompt or charm? • Centrality dependence of J/ suppression

Recent results on muon pair production from the NA60 experiment

Study of dimuon production at CERN SPS in

• p-A @ 158 and 400 GeV • In-In @ 158 GeV

2

LMR IMR HMR

NA60 physics topics

Origin of the intermediate mass excess, connected with thermal dilepton production?

centralcollisions

Origin of the J/ suppression, comparing results obtained in several colliding systems

Origin of the low-mass excess, connected with chiral symmetry restoration?

3

hadron absorberMuonOther

magnetic field

Iron wall

2.5 T dipole magnet

targets

beam tracker

vertex tracker

ZDC

NA60 experimental apparatus

Muon trigger and tracking(NA10/NA38/NA50 spectrometer)

Matching in coordinate and momentum space

Origin of muons can be accurately determined ~20 MeV/c2 (vs. 80 MeV/c2)

J/ ~70 MeV/c2 (vs. 105 MeV/c2)

Improved dimuon mass resolution:

or!

prompt

displaced

4

Low Mass regionSearch for in-medium modifications of vector mesons in In-In collisions

Peripheral data: well reproduced by the hadron decay cocktail ()

Central data: excess is isolated by subtracting the cocktail Phys. Rev. Lett. 96 (2006) 162302

5

Comparison to theory

• Dropping mass (BR) disfavoured

• Hadronic models predicting strong broadening/no mass shift (RW) in fair agreement with data

• Prediction by Rapp et al. (2003) for all scenarios

• Theoretical yields normalized to data for M<0.9 GeV/c2

Data and predictions, after acceptance

filtering, can be interpreted as the spectral function, averaged over space-time and momenta

Clear excess above the cocktail - rising with centrality. - more pronounced at low pT

6

Transverse mass distributions

• Weak centrality dependence

• Strong mass dependence of mT spectra

• From the fit to the mT spectra:

• Strong steepening at low mT, opposite to what expected from radial flow

effTTT

Tmdm

dN

mexp~

1

• M<1 GeV: monotonic flattening of the slopes

Teff increases• M>1 GeV: slope steepens again small Teff

7

Intermediate Mass region

The excess is a prompt source 2.2 times higher than the expected DY

The fit to the mass spectra, with charm and Drell-Yan contributions fixed to the expected yields shows an excess in IMR

NA60 measures the muon offsets :

distance between interaction vertex and track impact point

Fix prompt contribution to the expected DY – leave open charm free

Bad Fit Good Fit

Fix charm contribution according to expectations – leave prompts free

Study of the origin of the excess over expected charm and Drell-Yan yields

8

Centrality and pT dependence of the excess

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

Excess contribution dominates at low pT, (factor ~3.5 for pT<0.5GeV/c)

Corrected for acceptance

9

Transverse mass distributions

Teff are extracted from fits performed in the region corresponding to 0.5< PT <2 GeV/c

effTTT

Tmdm

dN

mexp~

1

10

Teff rises with dimuon mass, followed by a decrease for M>1GeV

Rise up to the mass consistent with radial flow

of hadronic source

Decrease at masses ~1GeV indicates a non-flow situation, maybe due to a different dimuon source, as partonic processes

Teff of >Teff of dimuons

Towards an unification between low and intermediate mass regions: evolution of Teff vs. dimuon mass

11

J/ mass region

Nuclearabsorption

Expected J/ yield as a function of centrality has been calculated for In-In collisions

Study the onset of deconfinement (Matsui and Satz, 1986)

Nuclear absorption cross section J/

abs=4.18 0.35 mb is extracted from p-A data

J/ absorption in cold nuclear matter plays a role. Its contribution must be quantitatively assessed

There is a suppression beyond nuclear absorption

Preliminary!

12

Results

~10% error centrality independent not affecting the shape of the pattern

• Small statistical errors• Careful study of systematic errors needed: In-In and Pb-Pb show a

compatible pattern in the common Npart region

Anomalous suppression sets in at 50 < Npart <100,corresponding to Bj ~ 1.5 GeV/fm3 (0=1 fm/c)

(J//DY)meas /(J//DY)nucl. abs

(dNJ//dEzdc)meas/(dNJ//dEzdc)nucl. abs

13

• Size of the anomalous suppression reasonably reproduced• Quantitative description not satisfactory

Theoretical models and comparison between SPS and RHIC

R.Rapp, EPJ C43(2005) 91S. Digal et al. EPJ C32(2004) 547

A. Capella et al. EPJ C42(2005) 419

• J/ suppression apparently does not depend on collisions energy.• Many competing mechanisms involved theoretical interpretation not straightforward

14

Conclusions NA60 has performed a high-quality study of dimuon production

in Indium-Indium collisions at the SPS

• The anomalous suppression, seen in Pb-Pb collisions by NA50, is confirmed for a lighter system (onset at Bj ~ 1.5 GeV/fm3)• At SPS energies the anomalous suppression seems to scale with Npart

J/

Low masses• Strong broadening, but no significant mass shift of the intermediate • Evidence for radial flow of hadronic origin up to the mass, followed by a non-flow region (in the IMR)

Intermediate masses• Excess is prompt, open charm production agrees with expectations• Excess dominated by low pT (factor 3.50.4 for pT<0.5 GeV/c)

15

The NA60 Collaborationhttp://cern.ch/na60

Lisbon

CERN

Bern

Torino

Yerevan

CagliariLyon

Clermont

Riken

Stony Brook

Palaiseau

Heidelberg

BNL

~ 60 people13 institutes8 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.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