NA60 results on the spectral function in IndiumIndium collisions

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NA60 results on the ? spectral function
in Indium-Indium collisions
Sanja Damjanovic NA60 Collaboration
Asilomar, 14 June 2006
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Outline

• Isolation of excess dimuons above hadron decays
• Mass spectra (published in PRL)
• Shape analysis of mass spectra (new)
• Comparison to theory
• Acceptance-corrected pT spectra (new)
• Comparison to theory

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Event sample Indium-Indium

• 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
• present analysis 1/2 of total data

4
Main steps of the data analysis
talk of Andre David
? reconstruction of the event vertex within the
segmented target ? matching of tracks from
muon spectrometer and silicon vertex
telescope ? assessment of combinatorial
background by event mixing ? assessment
of fake matches by overlay MC and/or event
mixing
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Subtraction of combinatorial background and fakes
Net data sample 360 000 events
Fakes / CB lt 10
For the first time, ? and ? peaks clearly visible
in dilepton channel even ??µµ seen
w
f
h
Mass resolution23 MeV at the ? position
Progress over CERES statistics factor
gt1000resolution factor 2-3
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Associated track multiplicity distribution
Track multiplicity from VT tracks for triggered
dimuons for
opposite-sign pairs combinatorial background
signal pairs
4 multiplicity windows
new some part of the analysis also in 12
multiplicity windows
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Understanding the Peripheral data
Fit hadron decay cocktail and DD to the data
5 free parameters to be fit h/w, r/w, f/w,
DD, overall normalization (h?/h 0.12,
fixed) do the fits for all pT and three bins
in pT
Extrapolate fit parameters to full phase space
(using particle generator Genesis)
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Comparison of hadron decay cocktail to data
9
Full-phase-space particle ratios from the
cocktail fits

• h/w and f/w nearly
• independent of pT
• 10 variation due to
• the w
• enhanced r/w, mostly
• at low pT (due to pp
• annihilation, see later)

• General conclusion
• peripheral bin very well described in terms of
  known sources
• low M and low pT acceptance of NA60 under control

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Isolation of an excess in the more central
data
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Goal Find excess above hadron decays
without fits Conservative approach Use
particle yields so as to set a lower limit
to a possible excess
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Comparison of data to conservative cocktail
all pT
Cocktail definition see next slide
?/? fixed to 1.2
? data -- sum of cocktail sources
including the ?
Clear excess of data above cocktail, rising with
centrality
But how to recognize the spectral shape of the
excess?
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Isolate possible excess by subtracting cocktail
(without ?) from the data
? set upper limit, defined by
saturating the measured yield in the
mass region close to 0.2 GeV ? leads to
a lower limit for the excess at very low
mass ? and f fix yields such as to get,
after subtraction, a smooth underlying
continuum
difference spectrum robust to mistakes even on
the 10 level, since the consequences of such
mistakes are highly localized.
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Phys.Rev.Lett. 96 (2006) 162302
Excess spectra from difference data - cocktail
all pT
No cocktail ? and no DD subtracted
Clear excess above the cocktail ?, centered at
the nominal ? pole and rising with
centrality Similar behaviour in the other pT bins
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Systematics
Illustration of sensitivity ? to correct
subtraction of combinatorial background
and fake matches ? to variation of the ? yield
Systematic errors of continuum 0.4ltMlt0.6 and
0.8ltMlt1GeV 25
Structure in ? region completely robust
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Shape analysis of excess mass spectra
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Excess mass spectra in 12 centrality windows
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Shape vs. centrality
nontrivial changes of all three variables at
dNch/dygt100 ?
r
3/2(LU) continuum RC-1/2(LU) peak
RR peak/continuum
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RMS of total excess
r
Consistency with shape analysis ?Further rise
starting at dNch/dy 100
significant! (bad fit (c23) for linear rise
above dNch/dy30)
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Comparison of data to model predictions
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Two alternatives how to compare data to
predictions
? use predictions in the form decay
the virtual photons g into mm- pairs, propagate
these through the NA60 acceptance filter and
compare results to uncorrected data at
the output (presently done for mass spectra
in selected pT regions) ? correct data for
acceptance in 3-dim. space M-pT-y and
compare directly to predictions at the input
(presently done for pT spectra in selected
mass regions)
? conclusions as to agreement or disagreement
of data and predictions are independent
of whether comparison is done at input or
output
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Acceptance filtering of theoretical prediction
understanding the spectral shape at the output
all pT
input thermal radiation based on white spectral
function
output white spectrum !
By pure chance, for the M-pT characteristics of
direct radiation, without pT selection,the NA60
acceptance roughly compensates for the
phase-space factors and directly measures the
ltspectral functiongt
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Comparison of data to RW, BR and Vacuum ?
Predictions for In-In by Rapp et al. (2003) for
ltdNch/d?gt 140, covering all scenarios
Data and predictions as shown, after acceptance
filtering, roughly mirror the respective spectral
functions, averaged over space-time and momenta.
Theoretical yields normalized to data in mass
interval lt 0.9 GeV
Only broadening of ? (RW) observed, no mass
shift (BR)
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New theoretical developments since QM05
Brown and Rho, comments on BR scaling,
nucl-th/0509001Brown and Rho, formal aspects of
BR scaling, nucl-th/0509002
Rapp and van Hees, parameter variations for 2p,
hep-ph/0604269Rapp and van Hees, 4p, 6p
processes, hep-ph/0603084
Renk and Ruppert, finite T broadening, Phys. Rev.
C71 (2005) Renk and Ruppert, finite T
broadening and NA60, hep-ph/0603110 Renk,
Ruppert, Müller, BR scaling and QCD Sum Rules,
hep-ph/0509134 Renk and Ruppert, What the NA60
dilepton data can tell, hep-ph/0605330

Skokov and Toneev, BR scaling and NA60, Phys.
Rev. C73 (2006)

Dusling and Zahed, Chiral virial approach and
NA60, nucl-th/0604071

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Parameter variations for Brown/Rho scaling
Modification of BR bychange of the fireball
parameters
Van Hees and Rapp, hep-ph/0604269
even switching out all temperature effects does
not lead to agreement between BR and the data
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Comparison of data to RH(2p4pQGP)
van Hees and Rapp, hep-ph/0603084

Vector-Axialvector Mixing ?interaction with real
ps (Goldstone bosons). Use only 4p and higher
parts of the correlator PV in addition to 2p
Use 4p, 6p and
3p, 5p (1p) processes from ALEPH data, mix
them, time-reverse them and get mm- yields
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Comparison of data to RH(2p4pQGP)
Van Hees and Rapp, hep-ph/0603084

whole spectrum reasonably well described, now
even in absolute terms direct connection to IMR
results gt1 GeV from NA60
In this model, the yield above 0.9 GeV is
sensitive to the degree of vector-axialvector
mixing and therefore to chiral symmetry
restoration!
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Comparison of data to RW(2p4pQGP)
pT- dependences
theoretical results plotted in absolute terms
29
Comparison of data to RR
Renk and Ruppert, hep-ph/0603110

talk of Joerg Ruppert
Spectral function only based onhot pions
(Dyson-Schwinger) , no baryon interactions
included
Theoretical results obtained in absolute terms
Continuum contribution from partons, dominating
the region gt1GeV
r broadening described, except for low-mass tail
30
Comparison of data to RR
talk of Joerg Ruppert
theoretical results plotted in absolute terms
pT dependences
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Acceptance-corrected excess pT spectra
Preliminary
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Present strategy of acceptance correction
? reduce 3-dimensional acceptance correction
in M-pT-y to 2-dimensional correction in
M-pT, using measured y distribution as an
input ? use slices of ?m 0.1 GeV
and ?pT 0.2 GeV ? resum to three
extended mass windows 0.4ltMlt0.6
GeV 0.6ltMlt0.9 GeV
1.0ltMlt1.4 GeV
subtract charm from the data (based on NA60 IMR
results)before acceptance correction
33
Experimental results on the y distribution of
the excess
use measured mass and pT spectrum as input to
the acceptance correction in y (iteration
procedure)
agreement betweenthe three pT bins
results close to rapidity distribution of pions
(from NA49) for the same vs, as expected (RR)
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Excess pT spectra for three centrality bins

(spectra arbitrarily normalized)
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Centrality-integrated excess pT spectra

(arbitrarily normalized at pT1GeV)

• significant mass dependence (also vs. mT, see
  below )
• possible origin
• different physics sources
• radial flow
• ? p-dependence of in-medium spectral
  function

36
Illustration of mass dependence of pT spectra
differential fits to pT spectra, assuming locally
1-parameter mT scaling and using gliding windows
of ?pT0.8 GeV ? local slope Teff
very low Teff at low pT, enormous dynamic range,
hardly compatible with flow alone (for only one
component)
systematic errors lt15 MeV
at high pT, rho like region hardest,
high-mass region softest !
37
Systematics of low-pT data acceptance
pT spectrum of f at low pT much flatter (higher
Teff)
acceptance of f in betweenthat of the two mass
windows
enhanced yield at low pT not due to incorrect
acceptance
38
Systematics of low-pT data combinatorial
background
enhanced yield at low-pT seen at all
centralities, including the peripheral bin
estimate of errors at low pT, due to subtraction
of combinatorial background peripheral
1semiperipheral 10 semicentral
20central 25
enhanced yield at low pT not due to incorrect
subtraction of combinatorial background
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Comparison of data to model predictions
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Comparison to theory mass window 0.6ltMlt0.9
GeV
(arbitrarily normalized at pT1 GeV)
1-parameter differential mT fits
at low pT better description by RH at higher
pT much better description by RR (freeze-out ?
incl.)
in detail, data different from any theoretical
prediction
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Comparison to theory mass window 1.0ltMlt1.4
GeV
(arbitrarily normalized at pT1 GeV)
1-parameter differential mT fits
RH dominantly hadronic processes (4p...)
lower T role of flow? RR dominantly partonic
processes (qq) high T low flow
comparison to data inconclusive
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• Conclusions (I) data
• pion annihilation seems to be a major
  contribution to the lepton pair excess in
  heavy-ion collisions at SPS energies
• no significant mass shift of the intermediate ?
• only broadening of the intermediate ?
• strong mass dependence of pT spectra

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Conclusions (II) interpretation
? all models predicting strong mass shifts of
the intermediate r, including Brown/Rho
scaling, are not confirmed by the data
? models predicting strong broadening roughly
verified
? pT spectra not yet fully described by any
theory, rich in detail, promising handle on
emission sources and fireball dynamics
? theoretical investigation on an explicit
connection between broadening and the chiral
condensate clearly required
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The NA60 experiment
http//cern.ch/na60
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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BACKUP
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Peripheral Data
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Excess pT spectra for four centrality bins

peripheral bin at high-pT slightly softer
enhanced yield at low-pT seen at all
centralities, including the peripheral bin
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Comparison of hadron decay cocktail to data
pT lt 0.5 GeV
The ? region (small M, small pT) is
remarkably well described
? the (lower) acceptance of NA60 in this region
is well under control
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Comparison of hadron decay cocktail to data
0.5 lt pT lt 1 GeV pT gt 1 GeV
Again good agreement between cocktail and data
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Pt spectra
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Illustration of mass dependence of pT spectra
integral fits to pT spectra, assuming 1-parameter
mT scaling and varying the upper cut in pT (bad
?2 for full pT range)
very low Teff at low pT, enormous dynamic range,
hardly compatible with flow alone (for only one
component)
at high pT, rho like region hardest,
high-mass region softest !
53
Comparison to theory mass window 0.4ltMlt0.6
GeV
(arbitrarily normalized at pT0.7 GeV)
1-parameter differential mT fits
at low pT better description by RH/DZ at
higher pT much better description by RR
in detail, data different from any theoretical
prediction
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Teff from differential fits to pT spectra
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Shape analysis
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RMS of continuum
If observed peak subtracted, the remaining
continuum described by RMS of a flat
distribution, independent of dNch/dy
Subtraction of cocktail r creates dip in the
middle ? RMSgtRMSflat
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s of peak (from Gaussian fit)
Sigma of peak consistent with sigma of
cocktail/vacuum r, independent of dNch/dy
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Further theoretical results
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Comparison of data to RW, BR and Vacuum ?
same conclusions
pT dependence
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Dropping Mass (DM) vs Rapp/Wambach

• Modification of DM by
• fusion of the two scenarios

• change of the fireball parameters

Results of Rapp (8/2005)(not propagated through
NA60 acceptance filter)
Neither fusion nor parameter change able to make
DM scenario unobservable
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Skokov and Toneev on BR scaling
full dynamical model including deconfinement
transition
standard scaling of pole mass, no broadening
n0.3
n0
same modification to the vector dominance coupling
theoretical results normalized to data
even for n0, no agreement with the data (like
Rapp)
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Chiral Virial Approach Dusling/Zahed
First attempt to describe the centrality
dependence of the excess data. Reasonable
description, but increasing overestimate of
central ? peak
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DZ Integrated Dimuon Rates
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Acceptance NA60 vs CERES
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Comparison of lepton pair acceptance
CERES
CERES
reduction of acceptance at low M and low pT
similar
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Acceptance comparison of CERES and NA60
SourceRapp/Wambachlow pT dominated
? acceptance variation of NA60 between ltRWgt and
ltBRgt factor of 3 ? acceptance variation between
0.4 and 0.8 GeV different by a factor of 2
between CERES and NA60
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Phase space coverage in pT-mass plane
NA60
CERES
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Comparison of NA60 to CERES
Suppression of low mass part of RW similar in
CERES and in NA60
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Phase space coverage in y-pT plane
Comparison to CERES MC simulation with RW
(low-mass and low-pt dominated) both acceptances
shifted relative to midrapidity, but difference
only dy0.3
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Phase space coverage in y-pT plane
Examples from MC simulations
Optimal acceptance at high mass, high pT
ltygt 3.5 at low mass, low pT ltygt
3.8
Shift of acceptance away from midrapidity not
much different from CERES
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Experimental set-up
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Measuring dimuons in NA60 concept
2.5 T dipole magnet
beam tracker
vertex tracker
targets
Matching in coordinate and momentum space

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

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Background
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Analysis Topics

• Low Massesvector mesons continuum rad. (LMR)
• Intermediate Masses charm continuum rad.
  (IMR)
• High MassesJ/y, y and DY (HMR)

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Selection of primary vertex
The interaction vertex is identified with better
than 20 mm accuracy in the transverse plane and
200 mm along the beam axis.
(note the log scale)
Beam Trackersensors
windows
Present analysis (very conservative) Select
events with only one vertex in the target
region, i.e. eliminate all events with secondary
interactions
76
Muon track matching
Matching between the muons in the Muon
Spectrometer (MS) and the tracks in the Vertex
Telescope (VT) is done using the weighted
distance (?2) in slopes and inverse momenta. For
each candidate a global fit through the MS and VT
is performed, to improve kinematics.
A certain fraction of muons is matched to closest
non-muon tracks (fakes). Only events with ?2 lt 3
are selected. Fake matches are subtracted by a
mixed-events technique (CB) and an overlay MC
method (only for signal pairs, see below)
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Determination of Combinatorial Background
Basic method Event mixing

• takes account of
• charge asymmetry
• correlations between the two muons,
  induced by magnetic field
  sextant subdivision trigger conditions

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Combinatorial Background from ?,K?? decays
Agreement of data and mixed CB over several
orders of magnitude
Accuracy of agreement 1
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Fake Matches

• Fake matches of the combinatorial
  background are
• automatically subtracted as part of the
  mixed-events
• technique for the combinatorial background
• Fake matches of the signal pairs (lt10 of
  CB)
• are obtained in two different ways
• Overlay MC
• Superimpose MC signal dimuons onto real
  events.
• Reconstruct and flag fake matches. Choose
  MC
• input such as to reproduce the data.
• Event mixing
• More complicated, but less sensitive to
  systematics

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Fake-match background
example from overlay MC the f fake-match
contribution localized in mass (and pT) space ??
23 MeV, ?fake 110 MeV fake
prob. 22
complete fake-match mass spectrum agreement
between overlay MC and event mixing, in absolute
level and in shape, to within lt5
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Signal and background in 4 multiplicity windows
S/B
Decrease of S/B with centrality, as expected
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Mixing events

• The event mixing requires 12 pools of single
  muons
• Two muon charges, times
• Six spectrometer trigger sextants
• And we can only mix single muons from
• The same target
• The same centrality class
• The same field polarities, running conditions,
  etc
• (6000 event samples)
• For the mixing we only use single muons from the
  like-sign dimuon triggers
• The mixed event technique gives both shape and
  absolute scale

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Combinatorial background normalization

• Assume that the probability to accept the pair
  ?i?j is Pij Pi Pj,
• Pi and Pj are the single muon probabilities for
  sextants i and j (as if NA60 would have
  collected single muon triggers)
• The observed number of pairs ?i?j is Nij
• Then the single muon underlying probabilities are
• And the absolute probability to have a given muon
  is

Muon spectrometertrigger system sextants
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Sensitivity of
difference procedure
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Sensitivity of the difference procedure
Change yields of ?, ? and ? by 10 ? enormous
sensitivity, on the level of 1-2, to mistakes
in the particle yields.
The difference spectrum is robust to mistakes
even on the 10 level, since the consequences of
such mistakes are highly localized.
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Excess spectra from difference data-cocktail
pT lt 0.5 GeV
No cocktail ? and no DD subtracted
Clear excess above the cocktail ?, centered at
the nominal ? pole and rising with
centrality Similar behaviour in the other pT bins
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medium modification
of w ?
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Masking of w by pion annihilation
Central Bin
r/w1.2
The chances are better at high pT but no effect
is visible there!
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Medium modification of w?
Flattening of the pt distributions develops very
fast with centrality. Compatible with radial
flow? Or indication of ? broadening in the
medium??
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results from KEK-E325
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Alternative interpretation ?/? interference
Rob Veenhof, Winter Workshop on Nucl.Dynamics,
Breckenridge 2005