Study of the J/Study of the J/ production and suppression production and suppression in Indium-Indium collisions at the CERN SPSin Indium-Indium collisions at the CERN SPS

Alberto Colla - INFN Torino, Italy

for the NA60 Collaboration

Hot Quarks 2004

July 18 - 24, Taos Valley, NM, USA

Outline of the presentation:

• Physics motivation of NA60

• Overview of the detector concept

• First results from the 2003 Indium run

Heavy ion collisions and dilepton probes (a)

• Dileptons produced in heavy-ion collisions are very useful probes for the study of the QCD phase transition from hadron to quark-gluon matter

• Since 1986 a systematic measurement of dilepton spectra from high energy collisions has been carried out at the CERN SPS by various experiments (NA38, NA50, CERES …).

• Many interesting results were found:

M (GeV)

CERESPb-Au 158 GeV

The low mass dielectron data collected in heavy-ion collisions (S-Au, Pb-Au) exceeds the expected sum of light meson decays, which describes the proton data

1) 3)

DD_

DY

NA50Pb-Pb 158 GeV

central collisions

charm DY

The yield of intermediate mass dimuons seen in heavy-ion collisions (S-U, Pb-Pb) exceeds the sum of Drell-Yan and D meson decays, which describes the proton data

2)

M (GeV)

Pb-Pb data: J/ anomalously suppressed in central collisions

L. Ramello (NA50 Coll.), QM 2002

abs

abs

The J/ production is suppressed in heavy-ion collisions (Pb-Pb) with respect to the yields extrapolated from proton-nucleus data

p-A and S-U data: J/ absorption in “normal” nuclear matter, with (*) mb 3034 .. abs

Heavy ion collisions and dilepton probes (b)

NA50

(*) G. Borges (NA50 Coll.), QM 2004

3)

Specific questions that remain open

What is the origin of the low mass dilepton excess? need much more statistics, better signal to background ratio and mass

resolution resolve the peak study the signal versus pT and collision centrality

Is the intermediate mass excess due to thermal dimuons from a quark-gluon plasma?What is the open charm yield in nucleus-nucleus collisions?

measure secondary vertices with ~ 50 µm precision separate prompt dimuons from D meson decays

What is the physics variable driving the J/ suppression? L, Npart, energy density?Are the charmonium states broken by deconfined quarks and gluons?

measure the J/ pattern in Indium-Indium and compare it with Pb-Pb

Which fraction of J/ comes from c decays (c → J/ + ?

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

NA60New and accurate measurements are needed

http://cern.ch/na60

Idea: place a high granularity and radiation-hard silicon tracking telescope in the vertex regionto measure the muons before they suffer multiple scattering and energy loss in the absorber

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,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof, D. Walker and H. Wöhri

Lisbon

CERN

Bern

Torino

Yerevan

CagliariLyon

Clermont

Riken

Stony Brook

Palaiseau

Heidelberg

BNL

The NA60 Experiment

~ 60 people13 institutes8 countries

NA60’s detector concept

~ 1m Muon Spectrometer

MWPC’s

Trigger Hodoscopes

Toroidal Magnet

IronwallHadron absorber

ZDC

Target areabeam

or

prompt dimuon muon pair fromdisplaced vertices

Matching in coordinate and momentum space

Origin of muons can be accurately determined Improved dimuon mass resolution

ZDC

dimuon studies vs. collision centrality

MUON FILTER

BEAMTRACKER

TARGETBOX

VERTEX TELESCOPE

Dipole field2.5 T

BEAM

IC

not on scale

Pixel detectors Beam Tracker

The NA60 target region: reality

2.5 T dipole magnet

~ 100 pixel detectors (radiation tolerant)in 11 tracking points; cells = 50 × 425 µm2

Two stations of 50 m pitch micro-strip detectors

Operated at 130 K increased radiation hardness

target boxwindows

7 In targets

z-vertex (cm)

A first look at the Indium data

6500 A

4000 A

Indium beam

158 A GeV

5-week long run in Oct.–Nov. 2003 Indium beam of 158 GeV/nucleon ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape

Transverse vertexingwith 20 µm accuracy(and < 200 µm in Z)

M (GeV)

dN

/dM

(

Eve

nt s

/ 50

MeV

)

(80% of collected statistics) Beam

tracker station

EZDC

N.

Clu

ste

rs i

n V

T

N.

Clu

ste

rs i

n V

T Broad centrality coveragemeasured by the ZDC and

by the number of clustersseen in the Vertex Telescope

After event selection

Before event selection

(~47% statistics left)

EZDC

• Opposite-sign dimuon mass distributions

• Before quality cuts• No muon matching• Two spectrometer settings

(100% of collected statistics)

EZDC distributions and data selection for high mass dimuon analysis

all events

after rejecting beam pile-up

& non-interacting Indium ions

all events

after rejecting beam pile-up& non-interacting beam ions

after muon quality cuts & in dimuon phase space window

Minimum bias (ZDC) trigger Dimuon trigger

• Beam pile-up is rejected using Beam Tracker timing information

• Non-interacting beam ions are rejected using Interaction Counter

• Severe quality cuts have been used in this preliminary analysis (statistics will increase in the future, especially for peripheral collisions)

• Dimuon data analysis performed for events with EZDC < 15 TeV

and in the phase space window: 0 < ycms < 1 ; |cos CS| < 0.5

Understanding the opposite-sign dimuon mass distribution

Dimuon data from the 6500 A event sampleNo muon track matching used in this analysisMass resolution at the J/ : ~100 MeV

Combinatorial background from & K decays estimated from the measured like-sign pairs

Signal mass shapes from Monte Carlo: PYTHIA with MRS A (Low Q2) parton densities GEANT 3.21 for detector simulation reconstructed as the measured data

Acceptances from Monte Carlo simulation: for J/ : 12.4 % for DY : 13.4 % (in window 2.9–4.5 GeV)

J/

’

DY

Background

Charm

A multi-step (max likelihood) fit is performed:a) M > 4.2 GeV : normalise the DYb) 2.2<M<2.5 GeV: normalise the charm (with DY fixed)

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

DY yield = 162 ± 131302 ± 104 in range 2.9–4.5 GeV

J/ yield = 23532 ± 298

J/ / Drell-Yan in Indium-Indium collisions

B (J// (DY) = 19.5 ± 1.6

In-In collisions of EZDC < 15 TeV L = 7.0 fm

(from Glauber fit to the minimum bias EZDC distribution)

0.87 ± 0.07 w.r.t. the absorption curve

and Npart = 133

all data rescaled to 158 GeV

Stability checks: • Background increase by 10% : less than 3% change• Different event selection or fitting procedure : less than 8% change• Using GRV parton densities instead of MRS : 0.87 ± 0.07 0.93 ± 0.08

J/

L

Projectile

Target

preliminary

Comments and on-going analysis - 1

study of the centrality dependence of the J/ suppression in several binscannot use the measured Drell-Yan events, due to the low statistics

alternative analysis in progress, with the Drell-Yan yield estimated from a Glauber analysis of the Minimum Bias EZDC distribution

2 independent Minimum Bias triggers in NA60:

minimum amount of signal in the ZDC Indium ion crossing the Beam Tracker

Beam Tracker Trigger

Dimuon trigger

After event selection

After the corrections, we see the same trend inboth EZDC spectra, for central events

EZDC spectra of Beam Trackerand Dimuon triggers

The J/and “DY” EZDC distributions will be obtained with

two different triggers it is crucial to verify:

• the time stability of the dimuon and MB triggers• the influence of any possible trigger timing bias on the ZDC signal acquisition.

An analysis of the ZDC signals in the three NA60 triggers was performed:

Stability of the trigger timing confirmedSmall (~5%) timing biases found and corrected.

EZDC (GeV)

Comments and on-going analysis - 2

First plots of high mass dimuon spectra after muon track matchingbetween the Vertex Telescope and the Muon Spectrometer

M (GeV)

dN

/dM

(

Eve

nt s

/ 50

MeV

)

M (GeV)d

N/d

M

(E

ven

t s/ 5

0 M

eV)

Without event selection With event selection

Before track matchingAfter track matching

Before track matchingAfter track matching

dimuon matching efficiency: ~ 70% at the J/ mass resolution at the J/improves from ~100 MeV to ~70 MeV track matching useful to get rid of combinatorial background and out-of-target events

cleaner spectrum

Low mass dimuon production in Indium-Indium collisions

S/B ~ 1/4

no centrality selection

opposite-sign

signal

combinatorial background

less than 1 % of total statistics

from a preliminary analysis of a very small event sample …

mass resolution :20–25 MeV at M ~ 1 GeV

With respect to the Pb-Au CERES data:

• factor ~ 700 higher effective statistics

• Mass resolution ~2%, better by a factor 2

• Full information on associated track multiplicity

• Completely different systematic uncertainties

• The 2.5 T dipole field allows for good pT

coverage down to very low dimuon masses

• Combinatorial background resulting from and K

decays estimated through a mixed-event

technique, using like-sign muon pairs.

• The normalization is still preliminary.

Summary and outlook

Together with the proton run of 2004, NA60 should be able to :

• study the production of low mass dimuons, including the , and resonances

• clarify the cause of the excess of intermediate mass dimuons in heavy-ion collisions

• improve the understanding of the production and suppression of charmonium states

Harvest from the 5-week long Indium run in Oct.–Nov. 2003 :

• ~ 1 million signal low mass dimuons (after track matching)

• mass resolution ~ 20–25 MeV at the and masses

• more than 100 000 reconstructed J/ events (before track matching)

• first results on the analysis of the J/ suppression in In-In and comparison with NA50

• analysis of the centrality dependence of the J/ on the way; results soon available

To understand the heavy-ion results we need a solid reference baseline from p-A dataNA60 is about to take ~ 70 days of 400 GeV protons, for an expected integrated luminosity of ~ 5•104 nb-1, with 7 different nuclear targets, to study:

• the impact of c production on the J/ suppression

• the nuclear dependence of open charm production

• the intermediate mass prompt dimuons

• the low mass dimuons with unprecedented accuracy

(*) latest news: 1 week with a 158 GeV proton beam, to compare p-A, In-In and Pb-Pb data without introducing model-dependent rescaling factors

Backup slides

Standard analysis of J/y/DY : event selection

+ Beamscope

+ Beamscope + Int. Counter

+ Int. Counter + Glob.Cut 10%

+ Glob.Cut 10% + Y, CosCS

-27%

-13%

-23%

-4%

% diff

% diff

% diff

% diff

47% of initial ev.

Dimuon trigger event selection: 2.9<M<3.3 GeVP

ixel

Pl.7

EZDC

• A big fraction (~30 %) of the measured J/ yield

results from c decays: c → J/ + → J/ +

e+e- is the observed J/ suppression due to the c

disappearance ?• What is the “normal nuclear absorption” of

the c?

• E866, NA50 : The ’ is more absorbed

• NRQCD : the c should be less absorbedNA60 will track the converted

photons and will measure cand

the c to J/ ratio with ~ 2% accuracy

p-A

Impact of c production on the study of J/ suppression

In-In collisions : low mass phase space coverage

The dipole field in the target region leads to much better pT coverage than previous dimuon

measurementsDimuons now competitive with respect to dielectrons

with 2.5 T field

without field

A (%)

A (%)

Monte-Carlo

Acceptance improvesin all M and pT windows

by a factor 50 forM ~ 500 MeV andpT ~ 500 MeV/c

after muontrack matching