Outline:

1
Accurate measurements of dimuon productionin
proton-nucleus and nucleus-nucleus collisions at
the SPS

• Outline
• Looking for the QCD phase transition
• Observations of previous experiments
• The NA60 detector and performance
• First results from the 2002 and 2003 data
• Details on the operation of pixel detectors ?
  Markus Keil

Carlos LourençoBonn, November 2004
2
The QCD phase transition
QCD predicts that, above a critical temperature
or energy density, strongly interacting matter
undergoes a phase transition to a new state where
the quarks and gluons are no longer confined in
hadrons, and chiral symmetry is restored
Basic idea collide heavy nuclei at high energies
to create very high energy densities and break in
the lab the links that confine the quarks inside
the nucleons ever since the Big Bang
3
The deconfinement of the skiers
A skier is confined inside snow patches (like the
quarks in hadrons)
the skier can move further ? a new phase develops
the skier can move freely over long distances
deconfinement sets in
4
Two labs to recreate the Big Bang

• AGS 19862000
• Si and Au beams up to 14.6 A GeV
• only hadronic variables

• RHIC 2000 ?
• Au beams up to ?s 200 GeV
• 4 experiments

BNL
CERN

• SPS 19862003
• O, S, Pb and In beams lt 200 A GeV
• hadrons, photons and dileptons

• LHC 2008 ?
• Pb beams up to ?s 5.5 TeV
• ALICE, CMS ATLAS experiments

plus GSI new facility, from 2012 onwards
5
The CERN SPS heavy ion physics program

• 1986 1987 Oxygen at 60 and 200 GeV/nucleon
• 1987 1992 Sulphur at 200 GeV/nucleon
• 1994 2000 Lead at 40, 80 and 158
  GeV/nucleon
• 2002 Lead at 20 and 30 GeV/nucleon
• 2003 Indium at 158 GeV/nucleon

and p-A collisionsto establish the (very
important) reference baseline
dimuons
2004
NA60
In
hadrons
multistrange
dielectrons
NA49
2000
NA45 Ceres
NA50
NA57
photons hadrons
strangelets
hadrons
Pb
NA44
WA98
WA97
NA52
1994
dimuons
hadrons
1992
NA34/3 Helios-3
NA38
WA94
WA93
NA35
NA36
S
NA34/2 Helios-2
WA85
WA80
O
1986
6
Observations of previous experiments

• Since 1986, many experiments studied high-energy
  nuclear collisions at the CERN SPS to search for
  the QCD phase transition.Some of the
  theory-driven signatures required measuring
  lepton pairs and motivated NA38, CERES, HELIOS-3
  and NA50
• the production of thermal dimuons directly
  emitted from the new phase, if in thermal
  equilibrium
• changes in the r spectral function (mass
  shifts, broadening, disappearance) when
  chiral symmetry restoration is approached
• the suppression of charmonium states (J/y, y,
  cc) dissolved when certain critical
  thresholds are exceededSome of the measurements
  done by these experiments are consistent with the
  expectations derived from the theoretical
  predictions if a QGP phase is formed

7
Excess production of intermediate mass dimuons

• 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.

proton-nucleus data
Pb-Pb data
NA38/NA50
8
Thermal dimuon production or charm enhancement?

• The intermediate mass dimuon yields in heavy-ion
  collisions can be reproduced
• by scaling up the open charm contribution by up
  to a factor of 3 (!)
• or by adding thermal radiation from a
  quark-gluon-plasma phase ? the first direct
  evidence of a thermalized pre-hadronization phase
• The data collected by NA38/NA50 cannot
  distinguish among the two alternatives.

9
Low mass dilepton production

• The low mass di-electron data collected in
  heavy-ion collisions (S-Au, Pb-Au) exceeds the
  expected sum of light meson decays, which
  describes the proton data
• The excess electron pairs are concentrated at
  low pT and scale with Nch2

CERES
Which one is theright explanation?
no r vacuum r dropping r mass (Brown-Rho)
medium dependent chiral condensate broadening of
the r spectral function (Rapp-Wambach) r-hadron
interactions
10
J/y production from p-A to S-U collisions
The study of the J/y absolute production
cross-sections in p-A collisions at 200, 400 and
450 GeV, by NA3, NA38, NA50 and NA51, gives a
J/y absorption cross-section in normal nuclear
matter of 4.11 0.43 mb. No anomalous
suppression is seen in light-ion
collisionsO-Cu, O-U and S-U, while the Pb-Pb
value is significantly lower, even when
integrated over all centralities.
NA38/NA50
Survival probability ofthe J/y exp(-rLsabs)
sabs 4.11 0.43 mb
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J/y suppression in S-U and Pb-Pb collisions vs.
centrality
S-U 200 GeV
Pb-Pb 158 GeV
No anomalous suppression is seen in S-U
collisions, while the Pb-Pb values depart from
the normal nuclear absorption curve for energy
densities just above the most central S-U data?
evidence of the QCD phase transition ?
12
Questions raised by these interesting observations

• Is the observed low mass di-lepton excess a
  signal of chiral symmetry restoration? ?
  Improved statistics, mass resolution and signal
  to background ratio are needed resolve the
  w peak study the signal versus collision
  centrality and pT
• Is the observed intermediate mass excess due to
  thermal dimuons from a QGP?
• Or is the open charm yield enhanced in
  nucleus-nucleus collisions? ? Measure secondary
  vertices with 50 mm precision separate
  prompt dimuons from D meson decays
• Is the J/y suppressed in the QGP phase or in a
  precursor state?
• What is the physics variable driving the J/y
  suppression? L, Npart, energy density? ?
  Measure the J/y suppression pattern in
  Indium-Indium and compare it with Lead-Lead
• What is the impact of the cc feed-down on the
  observed J/y suppression pattern? ? Study the
  nuclear dependence of cc production in p-A
  collisions

NA60
New and accurate measurements are needed
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The NA60 Experiment
http//na60.cern.ch
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 hadron
absorber
60 people 13 institutes8 countries
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. Damjanovic, A. David, A. de Falco,
N. de Marco,A. Devaux, A. Drees, L. Ducroux, H.
Enyo, 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, P.
Pillot,G. Puddu, E. Radermacher, P. Ramalhete,
P. Rosinsky, E. Scomparin, J. Seixas, S. Serci,
R. Shahoyan,P. Sonderegger, H.J. Specht, R.
Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R.
Veenhof and H. Wöhri
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Standard way of measuring dimuons NA50, PHENIX,
ALICE,
and tracking
muon trigger
magnetic field
iron wall
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How NA60 measures dimuons
2.5 T dipole magnet
beam tracker
vertex tracker
targets
matching in coordinate and momentum space
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The NA60 target region and muon spectrometer
dipolemagnet
muon spectrometer
hadron absorber
beam
ironwall
toroidalmagnet
targets
tracking chambers
pixels
beam tracker
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Requirements on the detectors
dimuon production(small cross-sections)
high statistics
high precision
high-energy(nuclear) collisions
high multiplicities
high luminosities
very clean and selective trigger
accurate time tagging and fast readout
very highradiation tolerance
very good granularity
state of the art silicon detectors
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An event from the Indium-Indium data
Mmm1.048 GeV
muon trackmatching
Mmm1.017 GeV
Run 6935Burst 33 Event 5987
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Low mass dimuons in p-nucleus collisions
p-A 400 GeV
Thanks to the silicon tracking telescope in the
target region, the dimuon mass resolution is
significantly improved
matching
NA60
20
Low mass dimuon production in heavy-ion collisions
For the first time in heavy-ion collisions, w and
f meson peaks are clearly visible (23 MeV mass
resolution at the f).The h?mm channel is also
seen! ? Breakthrough in low mass dilepton
physics
21
Transverse momentum distribution of f dimuons
Good acceptance down to zero pT, for the f and
lower dimuon masses
22
f production NA60 versus NA50 and NA49
NA50 f ? mm- NA49 f ? KK-
Average T(f) for In-In collisions 1) all pT
252 3 MeV 2) pT lt 1.50 GeV (NA49 range)
256 6 MeV 3) mT gt 1.65 GeV (NA50 range) 245
5 MeV NA60 will solve the discrepancy
betweenthe previous NA49 and NA50 results
23
Vertexing capabilities of NA60
The interaction vertex is identified with better
than 20 mm accuracy in the transverse plane and
200 mm along the beam axis.
24
Separating prompt dimuons from charm decays
Ratio between displaced and prompt dimuons
Looking at the distance between the extrapolated
muon tracks and the collision vertex, and also at
the distance between the two muon tracks, we can
separate prompt dimuons from open charm
decays. We will be able to see if the excess
dimuon production seen by NA38/NA50 occurs in the
prompt dimuon sample (signal of thermal dimuon
production) or in the displaced vertex sample
(anomalous enhancement of D meson production).
charm signal(where expected)
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J/y production in Indium-Indium collisions
Background
J/y
Charm
after muon track matching
y
DY
A multi-step fit is performeda) M gt 4.2 GeV
normalize the DY
b) 2.2 lt M lt 2.5 GeV normalize the charm
(with DY fixed)
c) 2.9 lt M lt 4.2 GeV get the J/y yield
(with DY charm fixed)
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J/y / Drell-Yan in Indium-Indium Collisions
B s(J/y) / s(DY) 19.2 1.2for L 6.8 fm or
Npart 128
? 0.84 0.05 w.r.t. the normal
nuclear absorption
After acceptance corrections Acc(J/y) 12.4
Acc(DY) 13.4
preliminary
regions which will be exploited by the centrality
study in Indium-Indium collisions
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J/y transverse momentum in Indium-Indium
collisions
1/pT dN/dpT
3.2 lt yLAB lt 3.8-0.4 lt cos?CS lt 0.4
pT (GeV/c)
NA50 Pb-Pb 1996 NA60 In-In
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J/y polarization in Indium-Indium collisions
The J/y polarization is important to understand
quarkonium production- CEM no polarization -
NRQCD transverse polarization at high
pT Measurements so far no polarization
seen. Obtained from the angular distribution of
the ? from the J/? ? ??- decays, in the rest
frame of the J/?
l
l
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Summary and outlook
The Indium-Indium data collected in 2003, at 158
A GeV, allows NA60 to 1) Study low mass dilepton
production with unprecedented accuracy in terms
of statistics, mass resolution and signal to
background ratio 2) Understand the origin of the
intermediate mass dimuon excess 3) Identify the
physics variable driving the suppression of the
J/y meson Furthermore, we will also be able
to 4) Measure the absolute cross-section of D
meson production in Indium collisions 5) Study f
production in mm- and KK- decays, down to zero
pT 6) Study the onset of the y suppression, in a
single collision system The proton-nucleus data
collected in 2004, at 400 and 158 GeV, will allow
NA60 to set a very robust reference baseline,
with respect to which the heavy-ion specific
features can be extracted. It should also allow
us to measure the fraction of J/y mesons
resulting from decays of the cc states, and the
nuclear dependence of this feed-down. NA60 would
like to run in the years 2006-2008 with proton
beams and in 2008-2009 with Lead ion beams but
we need to attract new collaborators