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Highlights from NA60 experiment

Study of dimuon production in p-A and A-A collisions

at CERN SPS

• NA60 concept• In-medium modification of vector mesons • Intermediate mass range excess: prompt or charm?• J/ψ production in p-A and A-A

Alessandro Ferretti(University of Torino and INFN)

for the NA60 collaboration

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Layout of the NA60 experiment

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)

• Mass resolution @ J/:~70 MeV/c2 (vs. 105 MeV/c2)

hadron absorberMuonOther

Muon trigger and tracking:

magnetic field

Iron wall

NA10/38/50 spectrometer2.5 T dipole magnet

Matching in coordinate and momentum space!

targets

beam tracker

vertex tracker

ZDC

or!

prompt

displaced

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Peripheral data: well reproducedby 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 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)

Low dimuon masses – In-In data

Search for in-medium modifications of vector mesons

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

• Excess even more pronounced at low pT

Excess spectra from difference

•No cocktail and no DD subtracted

data – cocktail(all pT)

cocktail / =1.2

Fine analysis in 12 centrality bins

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Evolution of the excess shape as a function of centralityQuantify 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

peak/

continuum/

peak/continuum

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

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 variables at dNch/dy>100?

Fine analysis in 12 centrality bins

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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

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Ruppert / Renk, Phys.Rev.C (2005)

Mass region above 1 GeV described in terms of partonic processes, dominated by q-qbar annihilation

Description of the mass region above 1 GeV

Mass region above 1 GeV described in terms of hadronic processes, 4 …, sensitive to vector-axialvector mixing and therefore to chiral symmetry restoration

Hadron-parton duality?

Rapp / Hees, hep-ph/0604269 (2006)

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• 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

• Trend at small mT different to what expected from radial flow (not for

• High mass interval shows steepest slope smaller T slope

First look at transverse momentum distributions

• Weak centrality dependence

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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)

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

Intermediate dimuon masses (IMR – 1.1<m<2.5 GeV)

NA60 measures the muon offsets : distance between interaction vertex and track impact point

2/)( 22

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

Good FitBad Fit

Leave DY free and fix open charm according to expectations

Observed IMR excess in In-In over expected Charm and Drell-Yan yields. Where it comes from?

Dimuon offset

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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

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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)

Mass spectrum (1.16<M<2.56 GeV/c2)

preliminary

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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

Corrected for acceptance

No acceptance correction

6500 A

Corrected for acceptance

pT dependence of the excess (1.16<M<2.56 GeV/c2)

preliminary

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

preliminary

•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 3.50.4 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

Conclusions II (intermediate mass range)

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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).

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(J/)/DY in p-A collisions at 158 GeV

0.081.03

σσB

σσBextrap GeV, (400) 450

LDYJ/ψμμ

meas GeV, 158

LDYJ/ψμμ

(J/)/DY = 29.2 2.3L = 3.4 fm

Preliminary!

• 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 abs

J/ at 158 GeV

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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

CS = -0.03 0.17

2/ndf =1.42

0 < pT < 5 GeV0.4 < yCM < 0.75

H = 0.03 0.06

2/ndf =1.01

0.5 < pT < 5 GeV0.1 < yCM < 0.6

• Deconfinement should lead to a higher degree of polarization (Ioffe,Kharzeev PRC 68(2003) 094013)

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pT2 vs centrality

gNInIn = 0.067 0.011 (GeV/c)2/fm

pT2pp

InIn = 1.15 0.07 (GeV/c)2

2/ndf = 0.62

• If pT broadening is due to gluon scattering in the initial state pT

2 = pT2pp + gN · L

gNPbPb = 0.073 0.005 (GeV/c)2/fm

pT2pp

PbPb = 1.19 0.04 (GeV/c)2

2/ndf = 1.22

pT broadening consistent with initial state gluon scattering

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

• We get

to be compared with

(NA50 2000 event sample)

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’ 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

Preliminary!

In-In:

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

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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

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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. 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

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spare slides

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J/ / DY analysisSet A (lower 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%

Set B (higher ACM current)

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Smooth effect or sharp drop ?

Npart

Mea

s/E

xp

1

Step position

A1A2

Step position: Npart = 86 ± 8

( Bj ~ 1.6 GeV/fm3 )

A1= 0.98 ± 0.02A2= 0.84 ± 0.012/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

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T vs centrality

1) dN/dpT = pT mT K1(mT/T)

2) dN/dpT = pT e -mT/T

• Used by NA50• Gives slightly higher T values (~ 7 MeV)

Fitting functions

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Comparison with recent results (HERA-B, E866)

• 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)

Helicity Helicity CollinsSoper

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Azimuthal distribution of the J/

central peripheral

More peripheral data hint for a non isotropic emission pattern?

Only 50% of the statistics analyzed

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Event selection

• 2 event selections have been used for J/ analysis1)• 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

• 2 analysesa) Use selection 1 and normalize to Drell-Yanb) Use selection 2 and normalize to calculated J/ nuclear absorption

• After quality cuts NJ/ ~ 45000 (1), 29000 (2)

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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<cosH<0

pproj

ptargz

CS

+y

x

Viewed from J/ rest frame

y

z

x H

+

pproj ptarg

J/

CollinsSoper

Helicity

Frames forpolarization

studies

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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 >1000resolution: factor 2-3

ω

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

Role of baryons

Baryons important in the low mass tail

• Improved model:• Fireball dynamics• 4 processes• spectrum described in absolute terms

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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 GeVpT = 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

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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:

Enhanced yield at low pT not due to incorrect subtraction of combinatorial background

25%central20%semicentral10%semiperipheral

1%peripheral

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Excess pT spectra: 3 centrality bins

Hardly any centrality dependenceBUT

Significant mass dependence