Outline:

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Dimuon production in Indium-Indium collisionsat
the CERN SPS

• Outline
• Introduction detector concept performance
  backgrounds etc.
• Analysis of the intermediate mass dimuons
• Is there an IMR excess in the Indium data?
• If so, is it due to prompt dimuons or to charm
  decays?
• Analysis of the low mass dimuons
• Are there in-medium modifications of the r
  meson?
• If so, do we see a broadening or a mass shift of
  the r?
• Analysis of the J/y suppression
• Is there anomalous suppression in Indium-Indium
  collisions?
• How does it compare to the Pb-Pb pattern and to
  model predictions?

nexttalks
EPSHEP 2005, Lisbon, July 2005 R.
Shahoyan on behalf of the NA60 Collaboration
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Searching for the QCD phase transition at the
CERN SPS

• Since 1986, many experiments studied high-energy
  nuclear collisions at the CERN SPSto search for
  the QCD phase transition from hadronic matter to
  a deconfined state,where chiral symmetry is
  restored
• Some of the predicted signatures of new physics
  required measuring lepton pairs and motivated
  NA38, CERES, HELIOS-3 and NA50
• the production of thermal dimuons directly
  emitted from the new phase
• 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 exceeded
• Some of the measurements done by these
  experiments are consistent with the behavior
  predicted in case a Quark-Gluon Plasma phase is
  formed
• However, these observations left some questions
  open and motivated a new experiment

NA60
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Standard way of measuring dimuons NA50
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Standard way of measuring dimuons NA50
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Standard way of measuring dimuons NA50
Muons are measured after passing the long
absorber (gt70 X0) ? Bad kinematics resolution
(80 MeV/c2 at the ?)No information on muon
vertex
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Standard way of measuring dimuons NA50
Muons are measured after passing the long hadron
absorber (gt70 X0) ? Bad kinematics resolution
(80 MeV/c2 at the ?)No information on muon
vertex
Concept of NA60 place a silicon tracking
telescope in the vertex region to measure the
muons before they suffer multiple scattering in
the absorber
7
Standard way of measuring dimuons NA50
2.5 T dipole magnet
vertex tracker
beam tracker
Muons are measured after passing the long hadron
absorber (gt70 X0) ? Bad kinematics resolution
(80 MeV/c2 at the ?)No information on muon
vertex
Concept of NA60 place a silicon tracking
telescope in the vertex region to measure the
muons before they suffer multiple scattering in
the absorber
8
Standard way of measuring dimuons NA50
2.5 T dipole magnet
vertex tracker
beam tracker
Muons are measured after passing the long hadron
absorber (gt70 X0)?Bad kinematics resolution
(80 MeV/c2 at the ?)No information on muon
vertex
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 muons measured
in the spectrometer
9
Standard way of measuring dimuons NA50
2.5 T dipole magnet
vertex tracker
beam tracker
Muons are measured after passing the long hadron
absorber (gt70 X0)?Bad kinematics resolution
(80 MeV/c2 at the ?)No information on muon
vertex
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 muons measured
in the spectrometer ?Improved kinematics (20
MeV/c2 at the ?)
10
Standard way of measuring dimuons NA50
2.5 T dipole magnet
vertex tracker
beam tracker
Muons are measured after passing the long hadron
absorber (gt70 X0)?Bad kinematics resolution
(80 MeV/c2 at the ?)No information on muon
vertex
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 muons measured
in the spectrometer ?Improved kinematics (20
MeV/c2 at the ?)Origin of muons can be
accurately determined
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The muon spectrometer

• Hadron absorber
• 8 MWPCs
• 4 trigger hodoscopes
• Toroidal magnet

dipolemagnet
beam
hadron absorber

• provides
• muon identification, trigger, tracking and
  momentum measurement

• trigger selectivity
• around 1 dimuon per 106 pp collisions

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The NA60 target region in the 2003 Indium run
2.5 T dipole magnet
Beam Tracker
operated at 130 K(improves radiation hardness)
Pixel detectors
eight 4-chip and eight 8-chip planes
NA60-ALICEpixel planes(10 MHz)
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New silicon detectors for the 2004 proton run
Faster tracking planes, for high luminosity
running (1 collision every 25 ns, on average)
7 differentnuclear targets
NA60-ATLASpixel planes(40 MHz)
NA60-ATLASstrip planes(40 MHz)
See the talk of Andre David on Friday at 9h30, in
the Detectors and Data Handling session,for many
more details on the NA60 silicon detectors
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Data sets collected by NA60

• 2002 400 GeV protons incident on Be, In and Pb
  targets (only 4 days)
• a first study of low mass dimuon production in
  p-A collisions
• 2003 5 weeks of Indium ions at 158 GeV/nucleon
  topic of this talk
• 4 1012 ions delivered in total
• 230 million dimuon triggers on tape
• two data sets ACM 4000 and 6500 A
• 2004 proton-nucleus data with 7 targets (Be, Al,
  Cu, In, W, Pb, U) and two beam energies
• 400 GeV 300 000 reconstructed J/y events ? to
  study cc production
• 158 GeV 30 000 J/y events ? to measure
  sabs(J/y) with an accuracy around 23

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Muon Matching
Muons from the Muon Spectrometer are matched to
the Vertex Telescope tracks by comparing the
slopes and momenta. Each candidate passing a
matching ?2 cut is refitted using both track and
muon measurements, to improve kinematics. Most
background muons from ? and K decays are rejected
in this matching step
buta muon may be matched to an alien track
(or to its proper track which picked too many
wrong clusters) ? Fake matches, additional
source of background
By varying the cut on the matching ?2 we can
improve the signal/background ratio
?
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Vertex resolution (along the beam axis)
Good target identification even for the most
peripheral collisions (? 4 tracks) The
interaction vertex is identified with better than
200 mm accuracy along the beam axis
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Vertex resolution (in the transverse plane)
Beam Tracker measurement vs. vertex reconstructed
with Vertex Telescope
The BT measurement (with 20 ?m resolution at the
target) allows us to control the vertexing
resolution and systematics
The interaction vertex is identified witha
resolution of 1020 mm accuracy in the transverse
plane
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Offset resolution
Using the muons from J/? decays (no background,
from the interaction point) we determine the
resolution of the impact parameter of the track
at the vertex (offset) 4050 ?m The
non-Gaussian tails are caused by imperfect
alignment (to be improved)
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Offset resolution
Good enough to separate prompt dimuons from Open
Charm off-target decays ! ?vertex ? ?impact lt
c? (D 312 ?m, Do 123 ?m) To
eliminate the momentum dependence of the offset
resolution, we use the offset weighted by
the error matrix of the fit for single
muons and for dimuons
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Background Subtraction method
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Background Subtraction method
Combinatorial Background (mainly from
uncorrelated ? and K decays)Subtracted by
building a sample of ?? pairs using muons from
different Like Sign events.Mixing procedure
accounts for correlations in the data due to the
dimuon trigger.
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Background Subtraction method
Subtracting the Mixed CB from the data we obtain
the Signal (correct and fake) in ??- sample and
zero (or residual background) in the Like Sign
dimuons sample.
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Background Subtraction method
The Fake Matches Background is subtracted by
Monte Carlo (used for the Low Mass Analysis) or
by matching the muons from one event to tracks
from another one a special weighting procedure
is used to account for the mixed fake matches
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Background Subtraction method
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Background Subtraction method
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Background Subtraction method
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Background Subtraction method (offsets)
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.
(All masses)
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Background Subtraction systematics
The quality of combinatorial subtraction can be
controlled by comparing the built mixed event
Like Sign dimuon spectra to the corresponding
measured data.
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Accounting for residual background
The background on the OS dimuon spectrum should
be similar to the LS spectra? use residual LS
background as an estimate of the unsubtracted OS
background. It is accounted as a systematic
error the errors on the background are globally
scaled up to guarantee that the residual LS
background is zero within 3 standard deviations.
Because of the high background level, a 1 error
in the background estimate leads to 10
systematic error on extracted signal
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Background Subtraction resulting mass
distribution
Data integrated over centrality Matching ?2 lt 1.5
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Background Subtraction resulting offset
distribution
Dimuon weighted offsets
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preliminary
Analysis of the intermediate mass dimuons
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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
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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 ? direct evidence
  of a thermalized pre-hadronization phase

• The data collected by NA38/NA50 cannot
  distinguish among these two alternatives.
• ? We need to measure secondary vertices with
  50 mm precision to separate prompt dimuons
  from D meson decays

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NA60 Signal analysis simulated sources
Charm and DY contributions are generated with
Pythia (mc1.3 GeV/c2, CTEQ6M PDFswith EKS98
nuclear modifications) overlaid on real data. The
fake matches in MC are subtracted as for the real
data.
Absolute normalization The expected DY
contribution, in narrow centrality bins, is
obtained from the number of observed J/? events,
using the known ?/DY function (see the talk of C.
Lourenço) as a function of the collision
centrality. ? A 10 systematic uncertainty band
is assumed on this DY normalization
The fits to mass and weighted offset spectra are
done in terms of scaling factors for DY and Open
Charm needed to describe the data.
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IMR analysis is there an excess?
Do the expected Charm and DY contributions
describe the data?Assume prompts to be DY and
normalize charm from theworld average
NA50 p-A
dimuon data? ?
Kinematical domain studied 1.2ltMlt2.7 GeV/c2,
0ltyCMlt1 and cos?lt0.5
None of these normalizations describes the
data There is an IMR excess in In-In collisions
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IMR analysis a la NA50 mass fits
Can we describe the data by leaving the Open
Charm normalization as a free parameter? NA50
could, with up to a factor of 3 Charm enhancement
in central Pb-Pb collisions
Letting the Charm contribution free describes the
data(even though with a relatively bad ?2)with
a charm enhancement factor around 2 in NA50
units ?Our observations, so far, are consistent
with previous resultsIs this validated by the
offsets information?
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IMR analysis offsets
To account for the imperfect alignment of the
real data, the offsets of the reconstructed MC
muons were smeared to reproduce the offsets
measured for the J/? muons. Since the J/? and ?
dimuons show very similar offset distributions,
their normalized sum was used as shape for the
prompt (DY) contribution. For Open Charm, the
smeared MC distribution of weighted offsets was
used in the fits.
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IMR analysis weighted offset fits
Fix prompt contribution to DY (with 10
uncertainty on its normalization)? Fit fails
Charm is too flat to describe remaining
spectrum ?we need more prompts
Letting both contributions free ? Good fit
quality, but 2 times more PromptsResulting
Charm yield is close to the value expected from
the NA50 p-A data
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IMR analysis weighted offset fits
Fix Charm contribution to world average value
or
Fix Charm contribution to NA50 p-A expected
value
?
Fit always requires 2 timesmore prompts
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IMR analysis excess
The offset fits show that the prompt contribution
is enhanced by a factor 2. We quantify the
excess in the mass spectrum with respect to DY by
fixing the Charm contribution to the expected
world average and NA50 reference values.
The mass spectrum of the excess dimuons is
steeper than DY(and flatter than Open Charm)
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• Conclusions
• There is an IMR excess the dimuon mass and
  offset distributions cannot be described by
  the sum of Open Charm and LO DY dimuons
• The offset distribution accommodates both the
  world average and the NA50 p-A expectations
  for the open charm yield. The latter option is
  preferred by the fit.
• In both cases, the measured offset distribution
  always requires a factor 2 more prompts than
  provided by the DY contribution.
• ?
• The excess is prompt
• Results are very robust with respect to variation
  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 caused by a bias in the background
  subtraction.
• Outlook
• Explore all In-In 2003 statistics (50 of the
  data is not yet reconstructed)
• Reprocess already analyzed data after improving
  the detectors alignment
• Perform similar analysis on p-A 2004 data will
  clarify the Open Charm reference yield

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BACKUP
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The background is evaluated for each subtarget in
narrow multiplicity bins
Signal/Background
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Fix DY contribution and assume the rest is charm
? mediocre fit quality
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NA50-like analysis
Let DY and Open Charm free fit improves
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Weighted offset fit
DY is constrained fit fails
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Excess / DY
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