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Roberta ArnaldiINFN Torino (Italy) on behalf of the NA60 Collaboration

J/ production in Indium-Indium collisions: new results from NA60

• Introduction• Updated In-In event sample• Results on (J/)/Drell-Yan• Results on J/• Comparison with other nuclear systems• Evaluation of systematic errors• Conclusions

2nd International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions

June 9 – 16 2006, Asilomar Conference Grounds

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J/ production studied in p-A, S-U and Pb-Pb collisions by NA38/NA50

(CERN SPS)

J/suppression one of the most direct signatures of QGP formation (Matsui-Satz 1986)

J/ suppression from p-A to Pb-Pb collisions

NA60 : study In-In collisions

Light systems,peripheral Pb-Pb collisions

J/ suppression scales with L

Central Pb-Pb collisionsL-scaling broken

Anomalous suppression

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MUON FILTER

BEAMTRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

BEAM

IC

not to scale

• Origin of muons can be accurately determined• Improved dimuon mass resolution

Matching in coordinate and in momentum space

ZDC allows studies vs. collision centrality

NA60 has a high granularity and radiation-hard silicon tracking telescope in the vertex region muons are measured before multiple scattering and energy loss in the absorber

beam

~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal Magnet(ACM)

IronwallHadron absorber

ZDC

Target area

NA60’s detector concept

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What’s new?

QM2005 J/ results obtained with a first preliminary data reconstruction

• Updated reconstruction now available

• Much better alignment (a few m accuracy) see A. David’s talk

• Tracking quality in the vertex spectrometer improved

• Full statistics now available (factor 2 increase for the J/)

Preliminary J/ results from In-In collisions Quark Matter 2005

Tracks 2

QM2005 reconstruction

new reconstruction

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measured yield expected yield in case of pure nuclear absorption of the J/

For both analysis we compare, for the various centrality bins,

Analysis methods

Two different (complementary) approaches:

less sensitive to systematic effects

limited by high-mass DY statistics

few centrality bins

more sensitive to systematic effects

very small statistical errors

several centrality bins

b) direct study of the J/ centrality distribution

a) ratio of J/ and Drell-Yan production cross sections

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Event selection

• In-In interaction in one of the 7 targets (z of the collision determined with ~ 200 µm accuracy)

• Phase space window (remove acceptance edges) -0.5 < cosCS < 0.5 & 0 < yCM < 1

• Beam pile-up removed (in ±12 ns window)

• Matching between muon spectrometer and vertex telescope tracks

• Require the dimuon production point to coincide with the most upstream interaction vertex

Accuracy: a few hundred m

Available statistics (after cuts):~ 29000 J/

• Muon spectrometer target cut: based on the distance between the beam axis and the extrapolated muon tracks at the target position.

Reject events where the dimuon is NOT produced in the target region (where the primary interaction took place)

Accuracy: a few cm

Available statistics (after cuts): ~ 45000 J/ ~ 320 Drell-Yan (M >4.2 GeV)

J/: can use stricter quality cuts

J/ / DY: keep maximum number of events

Most cuts are common to the two analysis methods

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J/ / DY analysis

Set 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) • level of muon spectrometer target cut < 3%

Set B (higher ACM current)

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Centrality estimate

Npart

Centrality of the collisions:

• Charged track multiplicity

• EZDC used in this analysis

158ZDC spect partE N GeV N

Take into account

• the small contribution of secondary particles emitted in the ZDC angular acceptance (η > 6.3)

• the smearing due to the ZDC experimental resolution (~9% at the Indium peak)

Npart distribution for various EZDC bins(bin width 1 TeV)

Target

Projectile

Target

Projectile

EZDC (GeV)

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• Data points have been normalized to the expected J/ normal nuclear absorption, calculated with

as measured with p-A NA50 data

J/ / DY vs. centrality

J/abs = 4.18 0.35 mb

• Qualitative agreement with NA50 results plotted as a function of Npart

bin1 Npart = 63

bin2 Npart = 123

bin3 Npart = 175

B. Alessandro et al., Eur. Phys. J. C39(2005) 335

3 centrality bins

Anomalous suppression

present in Indium-Indium

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J/ EZDC distribution: event samples

• good compatibility between Sets A and B

• in the following, we will show results obtained summing up the two event samples

• a (small) EZDC bias due to nuclear fragment reinteractions is corrected for

2/dof = 0.8

ratio Set B / Set Afirst bin

peripheralcentral

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Comparison with nuclear absorption

Data are compared to the expected J/ centrality distribution, calculated assuming nuclear absorption (with abs =4.18 mb) as the only suppression source

Nuclearabsorption

Normalization of the nuclear absorption curve

we require the ratio measured/expected, integrated over centrality, to be equal to the same quantity for the J//DY analysis (0.87 ± 0.05)

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Measured / Expected vs. Npart

Departure from the expected normal nuclear absorption already in peripheral eventsSaturation in more central events ?

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Comparison with other SPS results

The J/ suppression patterns are in fair agreement when plotted against Npart

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Comparison with the extreme case of a step-like function

Npart

Mea

s/E

xp

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Step position

A1A2

Step position: Npart = 82 ± 9

A1= 0.98 ± 0.03

A2= 0.85 ± 0.01

2/dof = 2.0

Resolution on Npart estimate (due to the measured EZDC resolution) taken into accountA certain amount of physics smearing can be accommodated by the data

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Contrary to the J/ / DY analysis this new approach gives negligible statistical errors (< 2%)

systematic errors must be carefully evaluated

Systematic errors

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Study of systematic errors: event selection

An alternative (less severe) event selection has been tested:

• require J/ to be produced in the target region, but not necessarily in the upstream vertex• apply a Monte-Carlo correction for events where the J/ is produced by re-interaction of a nuclear fragment in the target region

The suppression patterns obtainedwith the two different event selections agree within 1-2%

Event selection well under control

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Study of systematic errors: inputs to Glauber model

The expected J/ nuclear absorption, as a function of Npart, is obtained by meansof a Glauber calculation

The choice of the inputs to the Glauber calculation may influence the shape of thecalculated J/ centrality distribution

Only for very central events, corresponding to our most central bin (EZDC<3 TeV)an effect is clearly visible

Try different nucleardensity distribution functions

with respect to the default parameterization (De Vries)

De Vries et al. Atomic Data and Nuclear Data Tables 36, 495-536 (1987)Landolt – Bornstein Numerical Data…(Springer-Verlag 1967)

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EZDC = 158 x NSpect + EsecLink EZDC - centrality given by:

Study of systematic errors: centrality assignment

Esec energy released in ZDC by forward (secondary particles estimated by means of a fit to the EZDC distribution of minimum bias events

We study the influence on the nuclearabsorption curve of a 10% systematic error on the evaluation of Esec

Expected to affect mainly central events(higher relative contribution of secondaries)

Sizeable effect for EZDC < 3 TeV

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ppJ/ψσ (450 GeV) = 5.6 ± 0.1nb

abs

Jσ =4.18 ± 0.35 mb

Dominant source of uncertainty is due to the rescaling from 450 to 158 GeV

Study of systematic errors: abs

J/pp(450 GeV) and abs

J/ from NA50 (G. Borges et al., EPJ C43(2005)161)

Rescale to 158 GeV (taking into account phase space factors)

pp ppJ/ψ J/ψσ (200 GeV)/σ (450 GeV) = 0.319 ± 0.025pp ppJ/ψ J/ψσ (158 GeV)/σ (200 GeV) = 0.752 ± 0.011

using NA50 and NA3/NA38 data

from s and xF parametrization of J/pp

Combining the two errors we have a ~ 10%uncertainty, (almost) independent of centrality

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Summary on systematic errors

Various sources of systematic errors have been investigated and their effect on the measured suppression pattern is the following:

The most central bin is affected by a sizeable systematic error relatively to the others. There is also a ~10% systematic error, independent on centrality

We can accurately evaluate the shape of the suppression pattern, but its absolute normalization is more uncertain

• Event selection 1-2%

• Input to Glauber model (In density distributions)

• Link EZDC – Npart

• Error on J/pp(450 GeV) 8% centrality independent

• Error on abs 3-4 % (almost) centrality independent

• Error due to the J//DY normalization ~ 6% centrality independent

>10% for EZDC < 3 TeVnegligible elsewhere

5 -10 % for EZDC < 3 TeVnegligible elsewhere

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Work in progress: Azimuthal distribution of J/

More peripheral data hint for a non isotropic emission pattern?

Only 50% of the statistics analyzed

central peripheral

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Conclusions

• NA60 has measured J/ production as a function of centrality in In-In collisions

• Updated results, obtained with much improved alignment/reconstruction

• The full statistics has been used (~ factor 2 with respect to Quark Matter 2005)

• Results show that the J/ is anomalously suppressed in In-In collisions, with a drop followed by a plateau…

• Accurate study of the systematic errors affecting the suppression pattern has been performed

• Next step: results on the J/ production in p-A collisions at 158 GeV reduce systematic error on the normalization of the absorption curve

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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. Grigorian, J.Y. Grossiord,N. Guettet, A. Guichard, H. Gulkanian, 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,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