Study of the J/y production and suppression
in Indium-Indium collisions at the CERN SPS
Outline of the presentation:
• Physics motivation of NA60
• Overview of the detector concept
• First results from the 2003 Indium run
m
m
Alberto Colla - INFN Torino, Italy
for the NA60 Collaboration
Hot Quarks 2004
July 18 - 24, Taos Valley, NM, USA
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:
1)
CERES
Pb-Au 158 GeV
2)
NA50
Pb-Pb 158 GeV
charm_
DD
3)
DY
DY
central collisions
M (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
M (GeV)
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
Heavy ion collisions and dilepton probes (b)
3)
The J/y production is suppressed in heavy-ion collisions (Pb-Pb) with respect to the
yields extrapolated from proton-nucleus data
sabs
L. Ramello (NA50 Coll.), QM 2002
p-A and S-U data: J/y absorption in “normal”
nuclear matter, with sabs  4.3  0.3 mb (*)


Pb-Pb data: J/y anomalously suppressed
in central collisions
(*) G. Borges (NA50 Coll.), QM 2004
sabs
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 w 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/y suppression? L, Npart, energy density?
Are the charmonium states broken by deconfined quarks and gluons?
 measure the J/y pattern in Indium-Indium and compare it with Pb-Pb
Which fraction of J/y comes from c decays (c → J/y + g ?
What is the impact of the c feed-down on the observed J/y suppression pattern?
 study the nuclear dependence of c production in p-A collisions
New and accurate measurements are needed
The NA60 Experiment
http://cern.ch/na60
Idea: place a high granularity and radiation-hard silicon tracking telescope in the vertex region
to measure the muons before they suffer multiple scattering and energy loss in the absorber
CERN
Heidelberg
~ 60 people
13 institutes
8 countries
Bern
Palaiseau
BNL
Riken
Yerevan
Stony Brook
Torino
Lisbon
Clermont
Lyon
Cagliari
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
NA60’s detector concept
~ 1m
beam
Muon Spectrometer
Target
area
Hadron absorber
MWPC’s
Iron
wall
Toroidal Magnet
m
m
Trigger Hodoscopes
ZDC
Dipole field
2.5 T
Matching in coordinate
and momentum space
TARGET
BOX
BEAM
MUON
FILTER
BEAM
TRACKER
TELESCOPE
IC

 Origin of muons can be accurately
determined
 Improved dimuon mass resolution
muon pair from
displaced vertices
prompt dimuon
or
ZDC

dimuon studies vs.
collision centrality
The NA60 target region: reality
2.5 T dipole magnet
Beam Tracker
Two stations of 50 mm pitch micro-strip detectors
Operated at 130 K  increased radiation hardness
Pixel detectors
~ 100 pixel detectors (radiation tolerant)
in 11 tracking points; cells = 50 × 425 µm2
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
• Opposite-sign dimuon
mass distributions
• Before quality cuts
• No muon matching
• Two spectrometer settings
Before
event selection
N. Clusters in VT




N. Clusters in VT
A first look at the Indium data
Broad centrality coverage
measured by the ZDC and
by the number of clusters
seen in the Vertex Telescope
EZDC
After event selection
(~47% statistics left)
EZDC
Transverse vertexing
with 20 µm accuracy
(and < 200 µm in Z)
7 In targets
Indium beam
158 A GeV
4000 A
(80% of
collected statistics)
6500 A
(100% of
collected statistics)
Beam
tracker
station
Mmm (GeV)
target box
windows
z-vertex (cm)
EZDC distributions and data selection for high mass dimuon analysis
Minimum bias (ZDC) trigger
Dimuon trigger
all events
all events
after rejecting beam pile-up
& non-interacting Indium ions
after rejecting beam pile-up
& non-interacting beam ions
after muon quality cuts & in
dimuon phase space window
• 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 sample
No muon track matching used in this analysis
Mass resolution at the J/y : ~100 MeV
Background
J/y
Combinatorial background from  & K decays
estimated from the measured like-sign pairs
Charm
y’
DY
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/y : 12.4 %
 for DY : 13.4 % (in window 2.9–4.5 GeV)
A multi-step (max likelihood) fit is performed:
a) M > 4.2 GeV : normalise the DY
b) 2.2<M<2.5 GeV: normalise the charm (with DY fixed)
c) 2.9<M<4.2 GeV: get the J/y yield
(with DY & charm fixed)
DY yield = 162 ± 13
1302 ± 104 in range 2.9–4.5 GeV
J/y yield = 23532 ± 298
J/y / Drell-Yan in Indium-Indium collisions
B s(J/y / s(DY) = 19.5 ± 1.6
Projectile
J/y
 0.87 ± 0.07 w.r.t. the absorption curve
In-In collisions of EZDC < 15 TeV  L = 7.0 fm and Npart = 133
(from Glauber fit to the minimum bias EZDC distribution)
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
L
Target
Comments and on-going analysis - 1
study of the centrality dependence of the J/y suppression in several bins
cannot 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
EZDC spectra of Beam Tracker
and Dimuon triggers
The J/y 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 confirmed
Small (~5%) timing biases found and corrected.
After the corrections, we see the same trend in
both EZDC spectra, for central events
After event selection
Beam Tracker Trigger
Dimuon trigger
EZDC (GeV)
Comments and on-going analysis - 2
First plots of high mass dimuon spectra after muon track matching
between the Vertex Telescope and the Muon Spectrometer
Without event selection
Before track matching
After track matching
Mmm (GeV)
With event selection
Before track matching
After track matching
Mmm (GeV)
 dimuon matching efficiency: ~ 70% at the J/y
 mass resolution at the J/y 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
from a preliminary analysis
of a very small event sample …
mass resolution :
20–25 MeV at M ~ 1 GeV
less than 1 % of total statistics
no centrality selection
w
f
S/B ~ 1/4
opposite-sign
signal
combinatorial background
With respect to the Pb-Au CERES data:
• Combinatorial background resulting from  and K
decays estimated through a mixed-event
technique, using like-sign muon pairs.
• The normalization is still preliminary.
• 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
Summary and outlook
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 w and f masses
• more than 100 000 reconstructed J/y events (before track matching)
• first results on the analysis of the J/y suppression in In-In and comparison with NA50
• analysis of the centrality dependence of the J/y on the way; results soon available
To understand the heavy-ion results we need a solid reference baseline from p-A data
NA60 is about to take ~ 70 days of 400 GeV protons (*) in 2004, with 7 different
nuclear targets, at high beam intensities (~ 2 × 109 p/burst), to study:
• the impact of c production on the J/y 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 rescaling factors
Together with the proton run of 2004, NA60 should be able to :
• study the production of low mass dimuons, including the r, w and f 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
Backup slides
Standard analysis of J/y/DY : event selection
Pixel Pl.7
Dimuon trigger event selection: 2.9<M<3.3 GeV
+ Beamscope
% diff
+ Int. Counter
% diff
+ Glob.Cut 10%
% diff
+ Y, CosCS
% diff
-27%
+ Beamscope
-13%
+ Int. Counter
-23%
+ Glob.Cut 10%
-4%
47% of initial ev.
EZDC
Impact of c production on the study of J/y suppression
• A big fraction (~30 %) of the measured J/y yield
results from c decays: c → J/y + g
 is the observed J/y suppression due to the c ?
• What is the “normal nuclear absorption” of the c ?
• E866, NA50 : The y’ is more absorbed
• NRQCD : the c should be less absorbed
NA60 will track the converted photons
and will measure ac and the c to J/y
ratio with ~ 2% accuracy
p-A
In-In collisions : low mass phase space coverage
The dipole field in the target region leads to much
better pT coverage than previous dimuon
measurements
dimuons now competitive with respect to dielectrons
Monte-Carlo
without field
A (%)
Acceptance improves
in all M and pT windows
by a factor 50 for
M ~ 500 MeV and
pT ~ 500 MeV/c
with 2.5 T field
A (%)
after muon
track matching