1 j/ production in in-in and p-a collisions gianluca usai university of cagliari and infn...

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J/ production in In-In and p-A collisions

Gianluca UsaiUniversity of Cagliari and INFN

• Introduction• Centrality dependence of J/ suppression in In-In collisions• Preliminary results on J/ production in p-A collisions• Outlook/conclusions

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J/ suppression in nuclear collisions

• Previous knowledge• 1986 – 1992: NA38 experiment (light ions and protons)• 1994 – 2000: NA50 experiment (Pb ions and protons)

•CERN SPS energy (s ~ 20 GeV/nucleon) Study the onset of deconfinement (Matsui and Satz, 1986)

• Main topics• Normal vs anomalous suppression

needs accurate p-A data • Scaling variables(s) for the onset of the anomaly

needs comparison between different colliding systems• J/ vs c vs ’ suppression

needs high statistics (’) needs a sophisticated apparatus (c J/ )

First two issues addressed by NA60

from H. Satz,hep-ph/0609197

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Results from p-A and Pb-Pb

• Absorption in cold nuclear matter (p-A) can explain S-U data• Anomalous suppression sets in for semi-peripheral Pb-Pb collisions

• But• p-A data taken in a different energy/kinematic range• Is there anomalous suppression for systems lighter than Pb-Pb ?

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The NA60 experiment

hadron absorberMuonOther

and trackingMuon trigger

magnetic field

Iron wall

NA10/38/50 spectrometer2.5 T dipole magnet

Matching in coordinate and momentum space

targets

beam tracker

vertex tracker

• Improved dimuon mass resolution (from 100 to 70 MeV for J/)• Origin of muons can be accurately determined• Better control of systematics related to

• centrality determination (EZDC, Nch)• background from out-of-target interactions

(important for the study of peripheral events)

ZDC

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Event sample (A-A collisions) and quality cuts

• In-In @ 158 GeV/nucleon• ~ 4×1012 ions on target• ~ 2×108 dimuon triggers collected

• 2 event samples• Set A (low ACM current) mass resolution @ J/ ~ 125 MeV• Set B (high ACM current) mass resolution @ J/ ~ 105 MeV

• After muon matching mass resolution @ J/ ~ 70 MeV

• Both sets are used for J/ analysis maximize statistics

• General quality cuts• Pile-up rejection (using beam tracker)• Interaction vertex in one of the 7 In subtargets• 0 < yCM < 1, -0.5 < cosCS < 0.5 (remove acceptance edges)

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Further selection criteria

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

as measured with p-A NA50 data at 400 and 450 GeV

J/ / DY vs. centrality (analysis a)

J/abs = 4.18 0.35 mb

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

bin1 Npart = 63 (EZDC< 7 TeV)

bin2 Npart = 123 (7< EZDC< 11 TeV)

bin3 Npart = 175 (EZDC> 11 TeV)

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

3 centrality bins,defined through

EZDC

Anomalous suppression

present in Indium-Indium

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J/ yield vs nuclear absorption (analysis b)• Compare data to the expected J/ centrality distribution, calculated assuming nuclear absorption (with abs =4.18 mb) as the only suppression source

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

Nuclearabsorption

extrpA

centr all DYJ/ψμμ

meas

centr all DYJ/ψμμ

ZDCZDCnucl.abs.J/ψ

ZDCZDCmeasJ/ψ

σσB

σσB

dE dEdN

dE dEdN

Normalization of thenuclear absorption curve

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Results and systematic errorsSmall statistical errors

Careful study of systematicerrors is needed

• Uncertainty on normal nuclear absorption parameters (abs(J/) and pp(J/))• Uncertainty on relative normalization between data and absorption curve• Uncertainty on centrality determination (affects relative position of data and abs. curve)

• Glauber model parameters• EZDC to Npart

• ~10% error centrality indep. does not affect shape of the distribution• Partly common to analyses a and b• (Most) Central points affected by a considerable error

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Comparison with previous results (vs Npart)

• Good agreement with PbPb• S-U data seem to show a different behavior

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The nuclear absorption cross section• Nuclear absorption reference obtained so far from the NA50 p-A data at 400/450 GeV

a rescaling is needed

• Main assumptions used up to now

• absJ/ not depending on energy ( same abs

J/ at 158 GeV) • Energy dep. of J/ production cross section normalization of the nuclear absorption reference rescaled by using data sets at 200 GeV and a parameterization (“Schuler”) of cross section energy and kinematic dep.

• pA vs A-A• Different energy (158 vs 400/450)• Different kinematic domain (0<yCM<1 vs -0.5<yCM<0.5)

• Direct measurement of pA collisions at 158 GeV essential in order to:• determine abs

J/ at the same energy of the nucleus-nucleus data• reduce the systematic errors on the various rescaling factors

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NA60 p-A data at 158 GeV: first preliminary results

7 different nuclear targets exposed simultaneously to the beam for 3 days

J/ dimuon origin accurately determined

Pb

Be

InCu

W

U

Al

All targets

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The silicon tracker for the pA run

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pA at 158 GeV: PC muons

• Fit of the invariant mass spectrum with a superposition of the various expected sources: Drell-Yan, J/, ’, open charm

2/ndf = 1.24

DYJ/, ’DD

Still significantstatistics for high-mass Drell-Yan events

Possible to extractB J//DY, averaged over all nuclear targets

B J//DY = 30.1 2.3 0.4 with 2.9<mDY<4.5 GeV/c2

NJ/ 2.5 104

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pA at 158 GeV: VT muons

• Target ID available

• Much lower statistics• 9 targets• Average tracking/matching efficiency 40-50%

pW : NJ/ = 1.5103

• Consequences• impossible to extract B J//DY

(poor DY statistics)• Evaluation of NJ/ anyway robust (huge peak over a small continuum)

Evaluate J/ cross sections ratios between different targets

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Cross section ratios

•all targets simultaneously exposed to the beam beam luminosity factors Ni

inc cancel out - no systematic errors

BeBetBe

incBe

JBe

AAtA

incA

JA

JBe

JA

ANN

N

ANN

N

arg

/

arg

/

/

/

• Acceptance and reconstruction efficiencies do not cancel out completely because each target sees the vertex spectrometer under a (slightly) different angle computed (together with time evolution) for each target separately

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Acceptances/efficiencies• acceptance relative to a kinematic window covered by all the targets

Pixel efficiency vs time: example

• Uncertainty on input rapidity distributions taken as a systematic error

• Reconstruction efficiency calculated from the pixel efficiency in each plane on a run-by-run basis

Acceptance

Acceptance reco efficiency

3.2<ylab<3.7 ( 0.3<ycm<0.8) -0.5 < cosCS < 0.5

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Relative cross sections at 158 GeV

Calculate abs J/ using

the Glauber model

abs J/ = 7.1 1.0 mb

Significantly higher than the NA50 value @ 400/450 GeV

• investigated systematic errors:target thicknesses (from 0.3 to 2 %, target dependent)J/ y distribution (up to 7%, target dependent)

Very preliminary

rec. efficiency calculation (< 2 %)

•summed in quadrature with statistical errors, before carrying out the Glauber fit

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NA60: pA @ 400 GeV• data taken immediately after the sample at 158 GeV • data @158 and 400 collected with

Same layout of the apparatus Same data analysis procedure

• Very good agreement with the NA50 result• Use these data as a control experiment

abs J/ = 3.8 0.5 mb

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absJ/ vs √s

• Much debated issue (see C. Lourenco talk later today)

• Compilation from various experiments

Statistical+sytematic errors

E866: M.Leitch, privatecommunication

Hera-B: F. Faccioli, privatecommunication

NA50: published results

NA3Published result

• Relative systematics NA3 vs NA50/NA60 under investigation

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Comparison with nucleus-nucleus (1)

Absorption curve based only on “low energy” data

•Data @158 GeV for B J//DY

p-A at 158 GeV (NA60)S-U at 200 GeV/nucleon (NA38, 6 points)In-In at 158 GeV/nucleon (NA60, 3 points)Pb-Pb at 158 GeV/nucleon (NA50, 8 points)

• Two possible approaches

1) Use only pA data at 158 GeV• Advantage: only pA points are used (i.e. only cold matter effects)• Drawback: error on normalization is high (10%)

2) Include S-U points• Advantage: much smaller error on normalization (7 points are used)• Drawback: make an extra hypothesis, i.e. S-U is “normal”

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• Use only pA points at 158 GeV for calculating the absorption curve (normalization not determined with high accuracy)

• Clear anomalous suppression signal in Pb-Pb collisions• SU points lie parallel and higher by 10% with respect to the abs. curve

• Effect likely dominated by a statistical fluctuation of (J/)/DY in pA

Very preliminary!

Comparison with nucleus-nucleus (2)

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Comparison with nucleus-nucleus (3)•pA and SU look compatible (normalization and slope)

Slope fixed by pA points Normalization as a weighted average of the pA and SU points

Uncertainties on the reference curve:

3% due to absolute normalization3% on average, slightly dependent on centrality, due to abs

J/ uncertainty (not shown)

Very preliminary!

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• SU shows no anomalous suppression (by construction)• Pb-Pb shows a clear anomalous suppression in central events• In-In exhibits a smaller effect(?)

• For In-In results obtained without Drell-Yan• Slight rising tendency for semi-central events to be understood• Systematic effects of this (more complex) analysis being re-checked

Very preliminary!

Comparison with nucleus-nucleus (4)

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Conclusions

• The preliminary result for the J/ nuclear absorption cross section at 158 GeV is abs

J/ = 7.1 1.0 mb, larger than the one measured at 400/450 GeV by NA50

• The suppression seen in In-In is qualitatively similar to what observed by NA50 in Pb-Pb collisions

• An anomalous suppression in A-A is still present even with the (higher) abs

J/ @158 GeV

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Outlook

Almost every new piece of experimental information on quarkonium production presents a new “puzzle”

D. Kharzeev

• Previous measurements seem to indicate no or small energy dependence

Physics explanation? L dependence of abs J/ ? Or (trivially) some experiment

is wrong?

The comparison NA60 vs NA50 at 400 GeV seems to give confidence on the results, but, before drawing any final conclusion, we want to be very cautious ...

Stay tuned for the final results in the incoming months

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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

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J/ transverse momentum distributions• The pT distributions of the J/ have been obtained using a 1D acceptance correction method• The input distributions for the other kinematical variables (y, cosCS) have been obtained starting from a 3D correction algorithm and then adjusted iteratively on the data

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pT2 vs L for pA at 158 GeV

BeAl

CuIn

W

Pb

U

• Systematic errors • Choice of the generated y and cos distributions in the acceptance calculations ( 1%)• Various choice of kinematic selection connected with the detector geometry ( 3.5 %)

similar tostatistical errors

• Wrt QM08 results, a small systematic effect due to a 5 mm stretching of the vertex telescope has now been corrected (2.8% increase in pT)

<pT2>= <pT

2>pp+ gN L (Cronin effect)<pT

2>pp=1.20 ± 0.07 (GeV/c)2

gN=0.030 ± 0.020 (GeV/c)2/fm

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pT spectra: comparison A-A vs p-A“Control experiment”: pA at 400 GeV: comparison with NA50 is OK

• Linear increase of pT2 vs L for p-A and A-A, slope smaller in p-A 158 GeV

• L scaling broken between p-A and A-A• Initial state parton scattering cannot be the only source of transverse momentum broadening. Final state effects ?

Very preliminary!

• Systematic errors 4% for the NA60 points <1% for the NA38 points 2% for the NA50 points

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pT spectra: some more data points

• In the literature, one can find a few more measurements of pT

2 in this energy range (NA3, NA38 at 200 GeV)

• These experiments seem to suggest a higher pT2 ( 15%) with respect

to the NA60 points now checking relative systematics in detail

Systematicerrors explicitlyquoted, when

available

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/DY at 400 GeV (NA60 vs NA50)

• Analyzing NA60 data at 400 GeV, one can get J/ / DY, averaged over the various nuclear targets, and compare it with the values measured by NA50 at the same energy

Again a very good

agreement

NA60 (1 day after 158 GeV data taking!)NA50

Relative systematicsNA60 vs NA50 wellunder control also

for J/ / DY

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Comparison with nucleus-nucleus (3, backup)

Use this newreference curve

to look for anomaloussuppression

3% due to absolute normalization3% on average, slightly dependent on centrality, due to abs

J/ uncertainty

Uncertainties on the reference curve:

• Seen the compatibility between pA and SU (normalization and slope) Slope fixed by pA points

Normalization as a weighted average of the pA and SU points