J/Y,%20Charm%20and%20intermediate%20mass%20dimuons%20in%20Indium-Indium%20collisions

1
J/Y, Charm andintermediate mass dimuonsin
Indium-Indium collisions
, not RHIC!

• Results from recent data (year 2003) from SPS

• Time is limited. I will focus on open
  charmintermediate mass dimuons, first. then
  move to J/y analysis, if time allowed

• Hiroaki Ohnishi, RIKEN/JAPAN
• For the NA60 collaboration

XXXV International Symposiumon Multiparticle
Dynamics 2005
KROMERÍŽ, CZECH REPUBLIC, August 9-15, 2005   
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Search for the QCD phase transition
QCD predicts that strongly interacting matter,
above a critical temperature, undergoes a phase
transition to a state where the quarks and gluons
are no longer confined in hadrons, and chiral
symmetry is restored Such a phase transition
should be seen through dilepton signals

• the suppression of strongly bound heavy
  quarkonium states dissolved when certain
  critical thresholds are exceeded
• the production of thermal dimuons
• changes in the r spectral function (mass
  shifts, broadening, disappearance) when chiral
  symmetry restoration is approached

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Intermediate mass dimuon measurement from p-A to
Pb-Pb

• NA50 was able to describe the IMR dimuon spectra
  in p-A collisions as a sum of Drell-Yan and Open
  Charm contributions (but charm production
  cross-section higher than the world average)

NA38/NA50 proton-nucleus data
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Intermediate mass dimuon measurement from p-A to
Pb-Pb

• NA50 was able to describe the IMR dimuon spectra
  in p-A collisions as a sum of Drell-Yan and Open
  Charm contributions (but charm production
  cross-section higher than the world average)

• The yield of intermediate mass dimuons measured
  in heavy-ion collisions exceeds the sum of
  expected sources (Charm and DY)

NA50 Pb-Pbcentral collisions
NA38/NA50 proton-nucleus data
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Explanation of intermediate mass dimuon

• The intermediate mass dimuon yields in heavy-ion
  collisions can be reproduced by

• by scaling up the Open Charm contribution by up
  to a factor of 3

• by adding thermal radiation from a
  quark-gluon-plasma

• To identify the source of enhancement, we need
  to separate D meson decays and prompt dimuons

We need to measure secondary vertices with 50
mm precision
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NA60 detector concept
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 muon measured
in the spectrometer
Matching in coordinate and in momentum space
?

• Improved dimuon mass resolution
• Origin of muons can be accurately determined

Prompt dimuon
Displaced dimuon
12 tracking planes made with Rad-hard silicon
pixel detector
OR
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Data set

• Two muon spectrometer settings

• 5-week long run in 2003In-In _at_ 158 GeV/nucleon

Events/50 MeV

• Centrality selection using
• beam spectator energy in the ZDC
• or charged multiplicity in the vertex
• spectrometer

Raw ??- invariant mass spectrum
mµµ (GeV/c2)

• 41012 ions on target
• 2108 dimuon triggers collected

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Muon track offset resolution

• Offset resolution is evaluated with prompt dimuon
  (J/y) 4050 ?m

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Background subtraction

• Combinatorial background
• Significantly reduced by the track matching
  procedure
• Nevertheless, still the dominant dimuon source
  for m?? lt 2 GeV/c2

Cannot use

• NA60 acceptance quite asymmetric ?

Nback 2vNN--

• Mixed event technique developed ? accurate to
  12

• Fake matches background muon matched to a wrong
  vertex telescope track
• Evaluated with mixed events ? complicated but
  rigorous approach

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Background subtraction resulting mass
distribution
Detail will be discussedfollowing
presentationby M. Floris
Data integrated over centrality (Matching ?2 lt
1.5)
This talk focuses on

(if possible)
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Intermediate mass dimuon analysis
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NA60 Signal analysis simulated sources

• Charm and Drell-Yan contributions are calculated
  by overlaying Pythia events on real data (using
  CTEQ6M PDFs with EKS98 nuclear modifications and
  mc1.3 GeV/c2)The fake matches in the MC events
  are subtracted as in the real data

• Relative normalizations
• for DY K-factor of 1.8 to reproduce DY
  cross-sections of NA3 and NA50
• for charm we use the cross-section needed to
  reproduce the NA50 p-A dimuon data (a factor 2
  higher than the world average of direct charm
  measurements)

• Absolute normalization The expected DY
  contribution, as a function of the collision
  centrality, is obtained from the number of
  observed J/? events and the ? suppression
  pattern ? A 10 systematical error is
  assigned to this normalization

The fits to mass and weighted offset spectra are
reported in terms ofthe DY and Open Charm
scaling factors needed to describe the data
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IMR mass dimuons analysisa la NA50

• Procedure Fix the Charm and DY contributions to
  the expected yields and see if their Sum
  describes the measured Data

The expected Charm and DY yields, plus 10,
cannot explain the measured data
An excess is clearly present !
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Question Is it compatible with the NA50
observation?

• Procedure Try to describe the measured mass
  spectrum by leaving the Charm normalization as a
  free parameter

NA50 would require a factor 3.5 of Charm
enhancement incentral Pb-Pb collisions
Answer Yes, leaving the Charm yield free
describes the In-In data, with 2 times more
charm than needed by the NA50 p-A data
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Question Is this validated by the offsets
information?

• Procedure Fix the prompt contribution to the
  expected DY yield and see if the offset
  distribution can be described with enhanced Charm

Answer No, Charm is too flat to describe the
remaining spectrum
we need more prompts!
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Question How many more prompts do we need?

• Procedure Leave both contributions freeand see
  if we can describe the offset distribution

Answer A good fit requires two times more
prompts than the expected Drell-Yan yield
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Question Is the prompt yield sensitive to the
Charm level?

• Procedure Change the Charm contribution by a
  factor of 2and see how that affects the level of
  prompts

If we decrease the Charm yield to 0.55,the level
of the Prompts contribution changes from 1.91
0.11 to 2.08 0.07
If we increase the Charm yield by a factor of 2,
the description of the data deteriorates
significantly
Answer No, we always need two times more
prompts than the expected Drell-Yan, within
10 (the Charm contribution is too small to make
a difference)
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Question What is the mass shape of the excess?

• Procedure Fix the DY and Charm contributions to
  their expected yields and see how the excess,
  relative to DY or Charm, depends on the dimuon
  mass

Answer The mass spectrum of the excess dimuons
is steeper than DY and flatter than Open Charm
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Centrality dependence of the Excess Data - DY -
Charm
The yield of excess dimuons increases faster than
linearly with Nparticipants
If the excess dimuons are due to a hard process,
they should have the same centrality dependence
as the expected sources (DY Charm).
Not excluded by the data, at this time.
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Summary
IMR dimuons

• There is an excess of intermediate mass dimuons
  in Indium-Indium collisions
• The offset distribution requires a factor 2 more
  prompts than expected from DY ? The excess
  is not due to open charm enhancement
• The excess grows faster than linearly with the
  number of participants

• Results are very robust with respect to
  variations 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 due to a bias in the background
  subtraction

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J/Y suppression
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J/Y production in p-A to Pb-Pb

• The study of J/y production 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 4.18 0.35 mb.

• In p-A, light-ion, the data follow this normal
  nuclear absorption which scales with the length
  of nuclear matter crossed by the (pre-resonant)
  J/y, L.

• peripheral Pb-Pb collisions also follows L
  scaling

• In the more central Pb-Pb collisions the L
  scaling is broken and an anomalous
  suppression sets in

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The J/Y standard analysis
Background
without matching 6500 data set no centrality
selection

• Combinatorial background from ? and K decays
  estimated from like-sign pairs(less than 3
  under the J/y)
• Signal mass shapes from Monte Carlo
• PYTHIA and GRV 94 LO parton densities
• GEANT 3.21 for detector simulation
• reconstructed as the measured data
• Acceptances from Monte Carlo simulation
• for J/y 12.4 (6500 A) 13.8 (4000 A)
• for DY 13.2 (6500 A) 14.1 (4000 A)
• (in mass window 2.94.5 GeV)

J/y
Charm
y
DY
A multi-step fit (max likelihood) is
performed a) 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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Centrality dependence (standard analysis)
An anomalous suppression is present in the
Indium-Indium data
The small statistics of high mass dimuons limits
the number of centrality bins
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Direct J/Y analysis

• Idea directly compare the measured J/? sample
  (only matched dimuons), as a function of
  centrality, with the yield expected from the
  normal nuclear absorption
• The integrated ratio Measured / Expected is
  imposed to be the same as in the standard analysis

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Comparison with previous results
S, In and Pb data points do not overlap in
the L variable the physics behind the
anomalous J/? suppression does not depend on
L
The In-In and Pb-Pb J/y suppression
patterns are in fair agreement as a function
of the Npart variable
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Direct J/? sample comparison with theoretical
models
It is important to emphasize that these models
were previously tuned on the p-A, S-U and Pb-Pb
suppression patterns obtained by NA38 and NA50
We consider models for which we have predictions
specifically made for In-In collisions
J/y absorption by produced hadrons (comovers)
Capella and Ferreiro, hep-ph/0505032 J/y
suppression in the QGP and hadronic phases
including thermal regeneration and in-medium
properties of open charm and charmonium states
Grandchamp, Rapp, Brown, Nucl.Phys. A715
(2003) 545 Phys.Rev.Lett. 92 (2004) 212301
hep-ph/0403204 cc suppression by deconfined
partons when geometrical percolation sets in
Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004)
547.
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Suppression by produced hadrons (comovers)
The model takes into account nuclear absorption
and comovers interaction with sco 0.65 mb
(Capella-Ferreiro)
In-In _at_ 158 GeV
J/y / NColl
nuclear absorption
comover nuclear absorption
(E. Ferreiro, private communication)
NA60 In-In 158 GeVpreliminary
Pb-Pb _at_ 158 GeV
The smeared form (dashed line) is obtained taking
into account the resolution on NPart, due to our
experimental resolution
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QGP hadrons regeneration in-medium effects
The model simultaneously takes into account
dissociation and regeneration processes in both
QGP and hadron gas (Grandchamp, Rapp, Brown)
In-In _at_ 158 GeV
fixed thermalization time
fixed thermalization time
centrality dependent thermalization time
BmmsJ/y/sDY
centrality dependent thermalization time
Nuclear Absorption
Suppression Regeneration
QGPhadronic suppression
Regeneration
Number of participants
NA60 In-In 158 GeVpreliminary
The smeared form (dashed line) is obtained taking
into account the resolution on NPart, due to our
experimental resolution
Pb-Pb _at_ 158 GeV
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Suppression due to a percolation phase transition
Model based on percolation (Digal-Fortunato-Satz)
Sharp onset (due to the disappearance of the cc
meson) at Npart 125 for Pb-Pb and 140 for
In-In
The dashed line includes thesmearing due to the
ZDC resolution
Pb-Pb _at_ 158 GeV
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Summary
IMR dimuons

• There is an excess of intermediate mass dimuons
  in Indium-Indium collisions
• The offset distribution requires a factor 2 more
  prompts than expected from DY ? The excess
  is not due to open charm enhancement
• The excess grows faster than linearly with the
  number of participants

J/Y suppression

• The J/y shows an anomalous suppression already
  in Indium-Indium
• The suppression is centrality dependent and sets
  in at 90 Npart

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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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Comparison with previous results
very preliminary
Bjorken energy density, estimated from VENUS
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Specific questions that remain open

• Is the anomalous suppression also present in
  lighter nuclear systems?

- Study collisions between other systems, such as
Indium-Indium

• Which is the variable driving the suppression?
  L, Npart, energy density?

- Study the J/? suppression pattern as a function
of different centrality variables, including data
from different collision systems

• What is the normal nuclear absorption cross
  section at the energy of the heavy ion data?

- Study J/? production in p-A collisions at 158
GeV

• 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
- Study the nuclear dependence of cc production
in p-A collisions