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J/J/, Charm and, Charm andintermediate mass dimuonsintermediate mass dimuonsin Indium-Indium collisionsin Indium-Indium collisions

Hiroaki Ohnishi, RIKEN/JAPANHiroaki Ohnishi, RIKEN/JAPAN

For the NA60 collaborationFor the NA60 collaboration

XXXV International Symposiumon Multiparticle Dynamics 2005

KROMĚŘÍŽ, CZECH REPUBLIC,

August 9-15, 2005   

• Results from recent data (year 2003) from SPS

• Time is limited. I will focus on open charm+intermediate mass dimuons, first. then move to J/ analysis, if time allowed

, not RHIC!

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QCD predicts that strongly interacting matter, above a critical temperature, undergoes a phase transition to a state where the quarks and gluons are no longer confined in hadrons, and chiral symmetry is restored

Such a phase transition should be seen through dilepton signals:

Search for the QCD phase transitionSearch for the QCD phase transition

• the suppression of strongly bound heavy quarkonium states dissolved when certain critical thresholds are exceeded

• the production of thermal dimuons

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

This talk focuson this aspect!

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Intermediate mass dimuon Intermediate mass dimuon measurement from p-A to Pb-Pbmeasurement from p-A to Pb-Pb

• NA50 was able to describe the IMR dimuon spectra in p-A collisions as a sum of Drell-Yan and Open Charm contributions (but: charm production cross-section higher than the “world average”)

NA38/NA50 proton-nucleus data

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Intermediate mass dimuon Intermediate mass dimuon measurement from p-A to Pb-Pbmeasurement from p-A to Pb-Pb

• The yield of intermediate mass dimuons measured in heavy-ion collisions exceeds the sum of expected sources (Charm and DY)

NA50 Pb-Pbcentral collisionsNA38/NA50 proton-nucleus data

• NA50 was able to describe the IMR dimuon spectra in p-A collisions as a sum of Drell-Yan and Open Charm contributions (but: charm production cross-section higher than the “world average”)

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Explanation of Explanation of intermediate mass dimuonintermediate mass dimuon

• The intermediate mass dimuon yields in heavy-ion collisions can be reproduced by• by scaling up the Open Charm contribution by up to a factor of 3

• by adding thermal radiation from a quark-gluon-plasma

• To identify the source of enhancement, we need to separate D meson decays and prompt dimuons

We need to measure secondary vertices with ~ 50 m precision

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NA60 detector conceptNA60 detector concept

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

beam

~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal MagnetIronwallHadron absorber

ZDC

Target area

Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorberand match them to the muon measured in the spectrometer

MUON FILTER

BEAMTRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

BEAM

IC

not to scale Prompt dimuon Displaced dimuon

OR

Matching in coordinate and in momentum space

12 tracking planes made withRad-hard silicon pixel detector

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Data setData set• 5-week long run in 2003

In-In @ 158 GeV/nucleon

• Two muon spectrometer settings

• Centrality selection using• beam spectator energy in the ZDC• or charged multiplicity in the vertex spectrometer

• ~ 4×1012 ions on target• ~ 2×108 dimuon triggers collected

Raw +-

invariant massspectrum

m µµ (GeV/c2)

Events

/50

MeV

• Set B (high muon magnet current)• Good resolution at high mass

• Used for J/ analysis

• Set A (low Muon magnet current)• Good acceptance at low mass• Used for LMR and IMR analysis

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Muon track offset resolutionMuon track offset resolution

• Offset resolution is evaluated with prompt dimuon (J/) ~ 40–50 m

J/

J/

Weighted Offset () 100

Off

set

reso

luti

on

(m

)

• To eliminate the momentum dependence of the offset resolution,

we use the offset weighted by the error matrix of the fit:

2/)2( 11212

xyyyxx VyxVyVx

2/)( 22

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for single muons

for dimuons

J/

Weighted Offset () 100

Off

set

reso

luti

on

(m

)

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Background subtractionBackground subtraction• Combinatorial background

• Significantly reduced by the track matching procedure• Nevertheless, still the dominant dimuon source for m < 2 GeV/c2

• NA60 acceptance quite asymmetric Cannot use

• Fake matches background: muon matched to a wrong vertex telescope track• Evaluated with mixed events complicated but rigorous approach

Nback= 2√N++N--

• Mixed event technique developed accurate to 1–2%

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Real

data !

Background subtraction: resulting mass distributionData integrated over centrality (Matching 2 < 1.5)

Low mass dimuons

This talk focuses on

Intermediate J/+

Detail will be discussedfollowing presentationby M. Floris

(if possible)

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Intermediate mass Intermediate mass dimuon analysisdimuon analysis

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NA60 Signal analysis: NA60 Signal analysis: simulated sourcessimulated sources

• Charm and Drell-Yan contributions are calculated by overlaying Pythia events on real data 　 (using CTEQ6M PDFs with EKS98 nuclear modifications and mc=1.3 GeV/c2)The fake matches in the MC events are subtracted as in the real data

• Relative normalizations:– for DY: K-factor of 1.8;

to reproduce DY cross-sections of NA3 and NA50– for charm: we use the cross-section needed to reproduce

the NA50 p-A dimuon data 　(a factor 2 higher than the “world average” of direct charm measurements)

• Absolute normalization: 　 The expected DY contribution, as a function of the collision centrality, is obtained from the number of observed J/ events and the suppression pattern　　　　　 A 10% systematical error is assigned to this normalization

The fits to mass and weighted offset spectra are reported in terms ofthe DY and Open Charm scaling factors needed to describe the data

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• Procedure: Fix the Charm and DY contributions to the expected yields and see if their Sum describes the measured Data

An excess is clearly present !

The expected Charm and DY yields, plus 10%, cannot explain the measured data

IMR mass dimuons analysisIMR mass dimuons analysisa la NA50a la NA50

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Question: Is it compatible with the NA50 observation?

• Procedure: Try to describe the measured mass spectrum by leaving the Charm normalization as a free parameter

Answer: Yes, leaving the Charm yield free describes the In-In data, with ~ 2 times more charm than needed by the NA50 p-A data

NA50 would require a factor 3.5 of Charm enhancement incentral Pb-Pb collisions…

NA38+NA50p-A

S-U

Pb-Pb

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Question: Is this validated by the offsets information?

• Procedure: Fix the prompt contribution to the expected DY yield and see if the offset distribution can be described with enhanced Charm

Answer:

No, Charm is too flat to describe

the remaining spectrum……

we need more prompts!

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Question: How many more prompts do we need?

• Procedure: Leave both contributions freeand see if we can describe the offset distribution

Answer: A good fit requires two times more prompts than the expected Drell-Yan yield

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Question: Is the prompt yield sensitive to the Charm level?

• Procedure: Change the Charm contribution by a factor of 2and see how that affects the level of prompts

Answer: No, we always need two times more prompts than the expected Drell-Yan, within 10%

(the Charm contribution is too small to make a difference)

If we decrease the Charm yield to 0.55,the level of the Prompts contribution

changes from 1.91 ± 0.11 to 2.08 ± 0.07

If we increase the Charm yield by a factor of 2, the description of the data deteriorates significantly

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Question: What is the mass shape of the excess?

• Procedure: Fix the DY and Charm contributions to their expected yields and see how the excess, relative to DY or Charm, depends on the dimuon mass

Answer: The mass spectrum of the excess dimuons is steeper than DY and flatter than Open Charm

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Centrality dependence of the Excess = Data - DY - Charm

very

preliminary

very

preliminaryThe yield of excess dimuons increases faster than linearly with Nparticipants

If the excess dimuons are due to a hard process, they should have the same centrality dependence as the expected sources (DY + Charm).

Not excluded by the data, at this time.

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SummarySummary

• There is an excess of intermediate mass dimuons in Indium-Indium collisions• The offset distribution requires a factor 2 more prompts than expected from DY

The excess is not due to open charm enhancement• The excess grows faster than linearly with the number of participants

IMR dimuons

　 Results are very robust with respect to variations of the 　　 matching 2 cut: changing 　 the Signal / Background ratio

　　 by a factor of 2 changes the results by less than 10%

The excess cannot be due to a bias in the background subtraction

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J/J/ suppression suppression

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J/J/ production in p-A to Pb-Pb production in p-A to Pb-Pb• The study of J production in p-A collisions at 200, 400 and 450 GeV, by

NA3, NA38, NA50 and NA51, gives a “J/ absorption cross-section in normal nuclear matter” of 4.18 ± 0.35 mb.

NA38/NA50

J/ normal nuclear

absorption curve

J/

L

Projectile

Target

Survival probability ofthe J/: exp(-Labs)

• In the more central Pb-Pb collisions the L scaling is broken and an “anomalous suppression” sets in

• In p-A, light-ion, the data follow this normal nuclear absorption which scales with “the length of nuclear matter crossed by the (pre-resonant) J/”, L.

• peripheral Pb-Pb collisions also follows L scaling

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J/

’

DY

Background

Charm

A multi-step fit (max likelihood) is performed:

a) M > 4.2 GeV : normalize the DY

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

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

Combinatorial background from and K decays estimated from like-sign pairs(less than 3% under the J/)

Signal mass shapes from Monte Carlo:• PYTHIA and GRV 94 LO parton densities• GEANT 3.21 for detector simulation • reconstructed as the measured data

Acceptances from Monte Carlo simulation:• for J/ : 12.4 % (6500 A); 13.8 % (4000 A)• for DY : 13.2 % (6500 A); 14.1 % (4000 A) (in mass window 2.9–4.5 GeV)

without matching6500 data set

no centrality selection

The J/The J/ standard analysis standard analysis

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Centrality dependence Centrality dependence (standard analysis)(standard analysis)

The small statistics of high mass dimuons limits the number of centrality bins

An “anomalous suppression” is present in the Indium-Indium data

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Direct J/Direct J/ analysis analysis• Idea: directly compare the measured J/ sample (only matched

dimuons), as a function of centrality, with the yield expected from the normal nuclear absorption

• The integrated ratio Measured / Expected is imposed to be the same as in the standard analysis

EZDC (TeV)

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Comparison with previous resultsComparison with previous results

S, In and Pb data points do not overlap in the L variable: the physics behind the

“anomalous” J/ suppression does not depend on L

The In-In and Pb-Pb J/ suppression patterns are in fair agreement as a function of the Npart variable

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Direct J/ sample: comparison with theoretical models

It is important to emphasize that these models were previously tuned on the p-A, S-U and Pb-Pb suppression patterns obtained by NA38 and NA50We consider models for which we have predictions specifically made for In-In collisions:

J/ absorption by produced hadrons (comovers) Capella and Ferreiro, hep-ph/0505032;

J/ suppression in the QGP and hadronic phases including thermal regeneration and in-medium properties of open charm and charmonium states Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; hep-ph/0403204

c suppression by deconfined partons when geometrical percolation sets in Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

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Suppression by produced hadrons (“comovers”)

In-In @ 158 GeV The model takes into account nuclear absorption and comovers interaction with co = 0.65 mb (Capella-Ferreiro)

J/

NC

oll

nuclear absorption

comover + nuclear absorption

Pb-Pb @ 158 GeV

(E. Ferreiro, private communication)

The smeared form (dashed line) is obtained taking into account the resolution on NPart, due to our experimental resolution

NA60 In-In 158 GeVpreliminary

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QGP + hadrons + regeneration + in-medium effects

The smeared form (dashed line) is obtained taking into account the resolution on NPart, due to our experimental resolutionPb-Pb @ 158 GeV

NA60 In-In 158 GeVpreliminary

B

J/

/D

Y

Nuclear Absorption

Regeneration

QGP+hadronic suppression

Suppression + Regeneration

In-In @ 158 GeV

Number of participants

fixed thermalization timecentrality dependent thermalization time

fixed thermalization timecentrality dependent thermalization time

The model simultaneously takes into account dissociation and regeneration processes in both QGP and hadron gas (Grandchamp, Rapp, Brown)

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Suppression due to a percolation phase transition

The dashed line includes thesmearing due to the ZDC resolution

Sharp onset (due to the disappearance of the c mes

on) at Npart ~ 125 for Pb-Pb and ~ 140 for In-In

Model based on percolation (Digal-Fortunato-Satz)

Pb-Pb @ 158 GeV

NA60 In-In 158 GeVpreliminary

The measured data show a similar pattern but the anomalous suppression sets in at N

part ~ 90

NA60 In-In 158 GeVpreliminary

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SummarySummary

• The J/y shows an anomalous suppression already in Indium-Indium

• The suppression is centrality dependent and sets in at ~ 90 Npart

• There is an excess of intermediate mass dimuons in Indium-Indium collisions• The offset distribution requires a factor 2 more prompts than expected from DY

The excess is not due to open charm enhancement• The excess grows faster than linearly with the number of participants

J/ suppression

IMR dimuons

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Background Subtraction: method

Our measured dimuon spectra consist of:

correctly matched signal signal muons from the spectrometer are associated with their tracks in the Ver.Tel.

wrongly matched signal (fakes) at least one of the muons is matched to an alien track

correctly matched combinatorial pairs muons from ,K decays are associated with their tracks or with the tracks of their parent mesons

association between the ,K decay muon and an alien track

All these types of backgroundare subtracted by

Event Mixing(in narrow bins in centrality for each target)

wrongly matched combinatorials (fakes)

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The “mixed” background sample (fake matches and combinatorial) must reproduce the offsets of the measured events: therefore, the offsets of the single muons (from different events) selected for mixing must be replicated in the “mixed” event.

mixed eventevent 1

event 2

Background Subtraction: method (offsets)

(All masses)

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NA50 Pb-PbNA60 In-In

NA50 Pb-PbNA60 In-In

very

preliminary

Bjorken energy density, estimated from VENUS

Comparison with previous resultsComparison with previous results

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Specific questions that remain openSpecific questions that remain open

- Study the J/ suppression pattern as a function of different centrality variables, including data from different collision systems

- Study collisions between other systems, such as Indium-Indium

• Which is the variable driving the suppression? L, Npart, energy density?

• Is the anomalous suppression also present in lighter nuclear systems?

Study the nuclear dependence of c production in p-A collisions

- Study J/ production in p-A collisions at 158 GeV

• What is the normal nuclear absorption cross section at the energy of the heavy ion data?

• What is the impact of the c feed-down on the observed J/ suppression pattern?

- Study the nuclear dependence of c production in p-A collisions