Transcript Document 7301157

NA60 results on charm and intermediate mass dimuon
production in In-In 158 GeV/A collisions
 Introduction
 NA60 apparatus and data treatment
 Intermediate mass region (IMR) analysis
 Summary
Quark Matter 2006, Shanghai
R. Shahoyan, IST (Lisbon)
on behalf of the NA60 collaboration
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Previous measurements of the
intermediate mass (1.2<M<2.7 GeV/c2) dimuons
 NA38/NA50 was able to describe the IMR (between  and J/) dimuon spectra in p-A collisions at
450 GeV as the sum of Drell-Yan and Open Charm contributions.
 However, the yield observed by NA50 in heavy-ion collisions (S-U, Pb-Pb) exceeds the sum of DY
and Open Charm decays, extrapolated from the p-A data.
 The study of this excess was one of the main objectives of the NA60 experiment at SPS.
The preliminary analysis of Indium-Indium collisions at 158 GeV/A showed that the excess is caused
by prompt dimuons and not by charm (QM05).
NA38/NA50 proton-nucleus data
central
collisions
M (GeV/c2)
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Concept of NA60
muon trigger
2.5 T dipole magnet
targets
vertex tracker
hadron absorber
muon
other tracks
Concept of NA60: place a silicon tracking telescope in the vertex region to measure
the muons before they suffer multiple scattering in the absorber
and match them to the tracks measured in the muon spectrometer

Improved kinematics; dimuon mass resolution at the :
~20 MeV/c2 (instead of 80 MeV/c2 in NA50)
Origin of muons can be accurately determined
iron wall
magnetic field
beam tracker
and tracking
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… but
a muon might 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)
Beam Tracker
sensors
windows
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)
BT
The interaction vertex is identified with
a resolution of 10–20 mm accuracy in
the transverse plane
The BT measurement
(with 20 mm resolution
at the target) allows us
to control the vertexing
resolution and
systematics
s (mm)
BT
Beam Tracker measurement vs. vertex reconstructed with Vertex Telescope
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Dispersion between beam track and
VT vertex
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Vertex resolution (assuming sBT=20 mm)
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Number of tracks
Offset resolution
 J/ muons are almost background free and come
from the interaction vertex (no B decays at SPS
energies)  used to monitor the resolution of the
transverse distance of the track at the vertex: ~40 mm
 Data were realigned after QM05 - the non-Gaussian
tails caused by misalignment are reduced.
Good enough to separate prompt dimuons from
Open Charm off-target decays !
svertex  simpact < c (D+ : 312 mm, Do : 123 mm)
 To eliminate the momentum dependence of the
offset resolution, we use the muon offset weighted by
the error matrix of the fit:
 m  ( x 2 Vxx1   y 2 Vyy1  2 x  y Vxy1 ) / 2
Dimuon weighted offset:
(2m1  2m 2 ) / 2
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Background Subtraction
Our measured dimuon spectra consist of:
correctly matched signal
signal muons from the spectrometer are
associated with their tracks in the Ver.Tel.
wrongly matched signal (fakes)
at least one of the muons is matched to
an alien track
correctly matched combinatorial pairs
muons from ,K decays are associated
with their tracks or with the tracks of their
parent mesons
wrongly matched combinatorials (fakes)
association between the ,K decay muon
and an alien track
All these types of background
are subtracted by
Event Mixing
in narrow bins in centrality, for each target,
and magnetic field polarities (~2300 samples)
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Background Subtraction
CB
mixing
Combinatorial Background (mainly from uncorrelated  and K decays)
Subtracted by building a sample of mm pairs using muons from different Like Sign events.
Mixing procedure accounts for correlations in the data due to the dimuon trigger.
The normalization for the Opposite Sign sample is computed analysticaly.
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Background Subtraction
CB
mixing
Subtracting the Mixed CB from the data we obtain the Signal (correct and fake) in m+msample and zero (or residual background) in the Like Sign dimuons sample.
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Background Subtraction
CB
mixing
Fakes
mixing
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
CB
mixing
Fakes
mixing
Fakes
CB
mixing
In order to extract from the fake matches the signal contribution we repeat
the Combinatorial Mixing procedure for the generated fakes sample,
obtaining the combinatorial fake matches
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Background Subtraction
CB
mixing
Fakes
mixing
Fakes
CB
mixing
Subtracting the combinatorial fakes from all fakes we obtain the fake signal
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Background Subtraction
CB
mixing
Fakes
mixing
Fakes
CB
mixing
Subtracting the fake signal from the total matched signal leads to the correct signal spectrum
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Background Subtraction
The quality of combinatorial subtraction can be
controlled by comparing the built mixed event
Like Sign dimuon spectra to the corresponding
measured data.
Accuracy is better than 1%
The residual background of Like-Sign dimuons
sample is accounted as a systematic error
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.
event
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event
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mixed event
1.16<M<2.56
Background Subtraction: results
Signal Fake Matches
2 data samples with different current
settings in the Muon Spectrometer magnet
(changes acceptance)
Kinematical domain
selected for this study:
1.16 < M < 2.56 GeV/c2
0 < yCM < 1
|cos| < 0.5
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Signal/Background dependence on matching cut
Variation of the matching 2 from 3 to 1.5 increases the
Signal/Background by factor 2 (at the expense of ½ of the signal)

Used to check the systematics of the background subtraction
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NA60 Signal analysis: simulated sources
Charm and Drell-Yan contributions are obtained by overlaying real data on Pythia 6.326 events
(CTEQ6L PDFs with EKS98 and mc=1.5 GeV/c2. kT=0.8 for Drell-Yan and 1 for Charm)
The fake matches in the MC events are subtracted as in the real data
Relative normalizations:
 for Drell-Yan: K-factor of 1.9; to reproduce measured cross-sections of NA3 and NA50
 for Charm: 13.6 mb/nucleon needed to reproduce the extrapolated cross section from
NA50 p-A dimuon data at 450 GeV
Note: this is factor 2 larger than the “world average”, but:
 we detect only in |cos|<0.5
 DD  m  m  shows very strong rise at large cos

Our full phase space acceptance of charm strongly depends on the correctness of Pythia.
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 (talk of E.Scomparin).
A 10% systematical error is assigned to this normalization.
Signal shapes used to fit the weighted offset distributions are:
prompt dimuons:
mixture of J/ and  data
charm:
MC smeared by amount needed to reproduce J/ and  by MC
The fits to mass and weighted offset spectra are reported in terms of
the DY and Open Charm scaling factors needed to describe the data
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Data integrated in collision centralities and in PT
Fit of the mass spectra with prompts fixed to Drell-Yan (within 10%) shows that the dimuon yield in IMR is
higher than expected …
4000 A, 2 <1.5
Fit range
6500 A, 2 <1.5
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Data integrated in collision centralities and in PT
Fit of the mass spectra with prompts fixed to Drell-Yan (within 10%) shows that the dimuon yield in IMR is
higher than expected … and the fit to the offset spectra shows that the excess is prompt.
4000 A, 2 <1.5
4000 A,2 <1.5
Fit range
6500 A, 2 <1.5
6500 A, 2 <1.5
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Offset fits with free prompt and charm normalizations:

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
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Requires ~2.4 times more prompts than what Drell-Yan provides.
Obtained Charm contribution is lower than extrapolation from NA50 p-A data
The two data sets, with different systematics, are consistent with each other
Charm will be fixed to 0.7 0.15
4000 A, 2 < 3
4000 A, 2 <1.5
6500 A, 2 < 3
6500 A, 2 <1.5
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The acceptance correction of the excess is done differentially in M and PT
(does not require the knowledge of true distributions) and assuming
 flat cos distribution for decay angle
 rapidity distribution similar to Drell-Yan (sy~1)
Centrality dependence of the excess
Slight increase w.r.t. Drell-Yan, versus number of participants
preliminary
Corrected for acceptance
All data
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PT dependence of the excess (1.16<M<2.56
GeV/c2)
PT spectrum of the excess, especially at high PT, strongly depends
on the correctness of Drell-Yan description by Pythia (and charm on lesser extent)

TEFF. fits are performed in 0< PT <2.5 GeV/c
No acceptance correction
Drell-Yan
(T in GeV)
Charm
Excess
Corrected for acceptance
preliminary
6500 A
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Relative excess vs PT at 1.16<M<2.56
GeV/c2
preliminary
Corrected for acceptance
All data
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Contributions to IMR in
-0.5 < cos  < 0.5
2.92 < ylab < 3.92
(both 4000 and 6500 A data sample used)
preliminary
Corrected for acceptance
TEFF in MeV ( 0 < pT < 2.5 GeV/c) for excess
Estimated systematic errors on TEFF are ~20 MeV
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Comparison with TEFF extracted from low mass region
Summary
1. Prompt dimuons production is ~2.5 higher than the expected Drell-Yan in
Indium-Indium collisions at 1.16 <M < 2.56 GeV/c2.
2. Charm production is compatible with expectations.
3. Prompts/Drell-Yan slightly increases with number of participants.
4. Excess contribution is dominated by low PT’s, reaching a factor
3.50.4 for PT<0.5 GeV/c.
5. The effective temperature of the excess (~190 MeV) is considerably
lower than the temperatures observed at lower masses
(both for the resonances and the low-mass excess)
Results are preliminary, since they depend on the correctness
of used Drell-Yan and Charm contribution PT distributions.
Need to be verified with pA data