J/ production in In-In collisions at SPS energiesJ/ production in In-In collisions at SPS energies

Philippe Pillot — IPN Lyonfor the NA60 Collaboration

QCD @ work 2005 June 16–20

Outline:

• Reminder of the physics motivation

• A glimpse of the detector concept

• J/ production in Indium-Indium collisions

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Physics motivationsPhysics motivations

QCD predicts that, above a critical temperature or energy density, strongly interacting matter undergoes a phase transition to a new state (QGP) where the quarks and gluons are no longer confined in hadrons, and chiral symmetry is restored.

Since 1986, many experiments have studied high-energy nuclear collisions at the CERN SPS to search for this QCD phase transition. Some of the theory-driven “signatures” required measuring lepton pairs and motivated NA38, CERES, HELIOS-3 and NA50:

changes in the spectral function (mass shifts, broadening, disappearance) when chiral symmetry restoration is approached

the production of thermal dimuons directly emitted from the new phase, if in thermal equilibrium

the suppression of charmonium states (J/, ’, c) dissolved when critical thresholds are exceeded

Some of the measurements done by these experiments are consistent with the expectations derived from the theoretical predictions in case a QGP phase is formed.

This talk will be focused on the suppression of the J/

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J/L

Projectile

Target

“normal nuclear absorption”: Survival probability of the J/ exp(-Labs)

abs = 4.18 ± 0.35 mb• No anomalous suppression is seen in light-ion collisions (O-Cu, O-U and S-U)• Anomalous suppression is seen in Pb-Pb

Experimental observations / Questions that remain openExperimental observations / Questions that remain open

• From p-A collisions at 200, 400, 450 GeV (NA3, NA38, NA50 and NA51):

What is the physics mechanism behind the J/ suppression seen in Pb-Pb collisions? What is the impact of the c feed-down on

the observed J/ suppression pattern? Is the normal nuclear absorption of the c

identical to the one of the J/? Can we compare proton-nucleus data at 450 GeV with Pb-Pb data at 158 GeV?

New measurements are needed

NA60

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What is the physics behind the abnormal suppression ?What is the physics behind the abnormal suppression ?

Study of the J/ abnormal suppression as a function of different variables, with different colliding systems, in order to find the good observable: L, Npart, .

For instance, results obtained in the three colliding systems will either overlap when plotted as a function of L or when plotted as a function of Npart

NA60 collected data in indium-indium collisions.To be compared with the S-U and Pb-Pb results of NA38/NA50

The correlation between the different variables, as a function of the centrality of the collision, depends on the colliding system

Npart

L (

fm)

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~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal Magnet

IronwallHadron absorber

Target areabeam

ZDC

NA60’s detector in the 2003 indium runNA60’s detector in the 2003 indium run

MUON FILTER

BEAMTRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

IC

not on scale

5-week long run in Oct.–Nov. 2003: ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape

Two data sets collected, with 4000 A and 6500 A in the toroidal magnet

L and Npart calculated using the Glauber modelEZDC Nspectators 158 GeV

0 10 20 TeV

target

ProjectileCentral collisions

Peripheral collisions

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The vertex telescopeThe vertex telescope

Measurement accuracy of the interaction vertex: < 20 µm perpendicularly to the beam axis < 200 µm along the beam axis

Beam Tracker

Silicon stripsoperated at 130 K

(improves radiation hardness)

16 silicon pixel planes50425 µm2 pixel

size~ 800 000 pixels

Vertex Telescope

The interaction must take place in one of the seven targets

Dipole

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~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal Magnet

IronwallHadron absorber

Target areabeam

Matching in coordinate and momentum space

µ

µ

Matching in coordinate and momentum space origin of muons can be accurately determinedMatching in coordinate and momentum space origin of muons can be accurately determined dimuon mass resolution is improved

Spectrometer onlySpectrometer + telescope

J/

’

M (J/) :105 70 MeV

Matching between the muons and the vertex tracksMatching between the muons and the vertex tracks

Matching in coordinate and momentum space origin of muons can be accurately determined dimuon mass resolution is improved but matching efficiency is only 70% (at the )

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6500 Ano matching

J/ production in indium-indium collisionsJ/ production in indium-indium collisions

A multi-step fit is performed:a) M > 4.2 GeV: normalize the DY

Background Combinatorial background from and K decays determined from the measured like-sign pairs

preliminary

Phase space window:-0.5 < cos CS < 0.5 and 0.0 < ycms < 1.0

Acceptances:J/ : 12.4 % (6500 A); 13.8 % (4000 A)DY : 13.2 % (6500 A); 14.1 % (4000 A)

Bµµ (J/) / (DY2.9−4.5) :21.3 ± 1.3 (6500 unmatched)19.0 ± 1.2 (4000 unmatched)

Without centrality selection:L = 6.6 fm and Npart = 123 nucleons~ 70 000 J/ events~ 520 events above M = 4.2 GeV

c) 2.9 < M < 4.2 GeV: get the J/ and ’ yields (with DY & charm fixed)

b) 2.2 < M < 2.5 GeV: normalize the charm (with DY fixed)

Signal mass shapes from Monte Carlo:• PYTHIA with GRV94_LO parton densities• GEANT 3.21 for detector simulation • reconstructed as the measured data

DY

J/

’

Charm

Stricter cuts are needed for the analysis versus centrality: ~ 60% of the events are kept 3 centrality bins

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Comparison with previous measurementsComparison with previous measurements

The J/ is “anomalously” suppressed in indium-indium collision In-In, S-U and Pb-Pb points do not overlap as a function of L: the physics behind the “anomalous” J/ suppression does not depend on L Npart seems to be a much better scaling variable

preliminary

Studies as a function of energy density and other variables are under way Number of centrality bins will increase when the “minimum bias analysis” will be available

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Study of the transverse momentum of the J/Study of the transverse momentum of the J/

NA50 – Quark Matter 2002

Data rescaled to 158 GeV/c

The observed increase of the pT2 values has

been interpreted in terms of initial-state parton multiple scattering:the number of g-N scatterings grows linearly with L, resulting in a higher pT

2 for the J/

NA3, NA38 and NA50 studied the transverse momentum distributions of the J/ from p-A up to Pb-Pb collisions.

L: length of nuclear matter traversed

pT2pp is the value that

a cc pair has in absence of any scattering

pT2 = pT

2pp + agN L

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pT distribution analysispT distribution analysis

Analysis versus centrality: ~ 40 000 J/ events 7 centrality bins have been defined

For this study, we select dimuons in the mass range [2.9,3.3] GeV

The pT distribution is corrected for the acceptance of the experimental apparatus, calculated from Monte-Carlo simulations

The pT2 value is extracted

6500 Ano matching

1/p T

d

N/d

p T

pT (GeV/c)

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Results on the pT2 of the J/Results on the pT2 of the J/

pT2pp ((GeV/c)2) agN ((GeV/c)2/fm) 2/ndf

Pb-Pb 1.11 0.03 0.081 0.003 0.6

In-In 1.11 0.08 0.080 0.011 0.4

Pb-Pb + In-In 1.10 0.02 0.081 0.003 0.5

pT2 = pT

2pp + agN L

pT

2 J/ (

GeV

/c)2

L (fm)

Compatible results between NA50 and NA60

The Indium points are well reproduced by the initial-state parton multiple scattering model

preliminary

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Summary and outlookSummary and outlook

• The J/ / DY cross section ratio in Indium-Indium collisions, in 3 centrality bins, shows that the In and Pb patterns overlap reasonably well in Npart but not in L

• Work is on-going to increase the number of centrality bins, deriving the DY reference from the measured minimum bias events, and to study the overlap of the In and Pb patterns using other physics variables (energy density, etc)

• The <pT2> of the J/ in In-In follows the pattern defined by the Pb-Pb points as a

function of L, reinforcing the interpretation of initial-state parton multiple scattering

• To understand the production and suppression of charmonium states in heavy-ion collisions, a solid reference baseline from proton-nucleus data is needed. In 2004, NA60 took data with proton beams incident on 7 different nuclear targets:

• at 400 GeV: to study the impact of c production on the J/ suppression• at 158 GeV: to extract the normal nuclear absorption of the J/ at the energy of the heavy ion data

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Backup slides

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Stability checksStability checks

We changed several steps in the analysis procedure to check the stability of the results:• data selection• background estimation• parameterization of the signal shapes• fitting procedure

Systematical uncertainties are still under study; should be similar to the statistical errors when no centrality selection is done and smaller in the analysis versus centrality

preliminary

Matching

after muon track matching6500 A

Furthermore, the analysis of the dimuon mass spectra after muon track matching leads to essentially the same numerical values.

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pT distribution analysispT distribution analysis

6500 Ano matching

~ 40 000 J/ events 7 centrality bins have been defined

For this study, we select dimuons in the mass range [2.9,3.3] GeV

The pT distribution is corrected for the acceptance of the experimental apparatus, calculated from Monte-Carlo simulations

Monte Carlo events have been generated using cosCS and y distributions which reproduce the acceptance-corrected experimental data

Since the different kinematical variables are related, the acceptance in one variable can have a strong dependence on other variables. To take into account these correlations, two methods have been used:

The two methods lead essentially to the same numerical values

Monte Carlo events have been generated with flat pT, cosCS and y distributions 3-D acceptance matrices

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