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