Transcript f Meson Production in 158 AGeV In

f Meson Production in In-In Collisions and
Highlights from NA60
Michele Floris1
for the NA60 Collaboration
Strangeness in Quark Matter 2007
1University
and INFN, Cagliari, Italy
Outline
The NA60 Experiment
 Detector Concept
Phi Meson Production in In-In Collisions
 Analysis details
 pT, y and decay angular distributions
 f/w ratio
Highlights from NA60
 In medium modification of the r
 Intermediate mass range excess: prompt or
charm?
 Centrality dependence of J/y suppression
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The NA60 Experiment
• Fixed target dimuon
experiment
2.5 T dipole
magnet at the CERN SPS
muon trigger and tracking (NA50)
• Apparatus
composed
of 4tracker
main detectors
vertex
beam
magnetic field
tracker
targets
Zero degree calorimeter
(centrality measurements)
Muon Spectrometer
hadron absorber
<1m
17m
>10m
The vertex region (2 detectors):
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 (in both angles and momentum) to the tracks measured in the
spectrometer
Origin of muons can be accurately determined
Improved dimuon mass resolution
(~20 MeV/c2 at w instead of 80 MeV/c2)
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High luminosity mm experiment:
possible with radiation tolerant
detectors and high speed DAQ
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Data Sample
 InIn collisions at 158 AGeV Incident beam energy
 5 weeks in Oct.-Nov. 2003
 ~ 4 ∙ 1012 ions delivered
 ~ 230 million dimuon triggers
 Data analysis
 Select events with
only one reconstructed vertex
in target region
(avoid re-interactions)
 Match muon tracks from
Muon Spectrometer with
charged tracks from
Vertex Tracker (candidates
selected using weighted distance squared  matching c2)
 Subtract Background
 Two data samples
 Different current settings in the Muon Spectrometer magnet
 Different acceptances
 High current setting suppresses LMR
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f Meson Production: Motivation
 Strangeness production in Heavy Ion Collisions
 Mass and Width changes of the f in the medium
 Differerences between f  KK and f  mm
 K meson modification in the medium
 pT dependence of K suppression
 Two channels have been studied in 158 AGeV PbPb :
 f  mm
 Muons not influenced by the medium
 Previous SPS measurements: NA50
Acceptance limited to high pT
 f  KK
 Better mass resolution
 No physical BG
 Previous SPS measurements: NA49
 f puzzles:
 Broad pT coverage, but dominated by low pT
 Absence of in-matter modifications of width in KK
 Discrepancy between absolute yields
 Discrepancy between T slope: radial flow (NA49) or no radial flow (NA50)?
 Measurements from NA60 in In-In collisions
 NA60 measures the mm channel with good pT coverage (0-2.6 GeV)
 Rapidity, decay angular distribution, pT and f/w ratio
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Spectra: Analysis Procedure
f
total
background
We select the events on the f peak and
use two side mass windows to estimate
the pT,y and decay angle distribution of
the continuum under the peak
5 centrality bins
Acceptance would require correction with 2D
matrices: pT vs y and decay angle vs pT
After tuning MC to data (iterative procedure)
 1D acceptance correction
Systematic error: variation of analysis cuts
and parameters
4000 A data set only
Rapidity Distribution
sgauss
sgaus
s = 1.13 ± 0.06 ± 0.05
reflected
All centralities
Width estimated with a Gaussian fit
Constant within errors
Agreement with previous measurements
in other colliding systems at the same
energy
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Decay Angular Distributions
The angular distribution of the positive muon can be measured with respect
to 3 different quantization axes
1. Collins – Soper
2. Gottfried – Jackson
3. Helicity
The 3 frames are identical for pT  0
First measurement in HI collisions at the SPS
Gottfried-Jackson
Collins-Soper
Helicity
y
pµ+
pµ+ Hel
x
x
GJ
z axis
axis
fCS zzaxis
pprojectile
pprojectile
ptarget
y p
target
Viewed from f rest frame
Angular distributions fitted with the function:
dN
 1   cos 2 
d cos 
polarization
Provides information on the production mechanism
Previous measurements:
ACCMOR (h-Be) and Sixel et al. (K- - p / p- - p)
 Non negligible  (GJ Frame)
Heavy Ion
 Global polarization?
We studied centrality and pT dependence of  in the 3 frames
Further developments: azimuthal distributions, study wrt the reaction plane
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Helicity Distribution
 = 0.1 ± 0.1 ± 0.1
pT > 0.2 GeV/c
All centralities
 = 0, independent of centrality
Analysis repeated at pT < 1 GeV and pT > 1GeV.
No evidence for  ≠ 0.
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Gottfried – Jackson Distribution
 = 0. ± 0.1 ± 0.04
pT > 0.2 GeV/c
All centralities
 = 0, independent of centrality
Analysis repeated at pT < 1 GeV and pT > 1GeV.
No evidence for  ≠ 0.
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Collins – Soper Distribution
 = -0.2 ± 0.2 ± 0.2
pT > 0.2 GeV/c
All centralities
Hint for  < 0 in peripheral events? Not significant (2s).
Acceptance limits fit range Large errors on 
Analysis repeated at pT < 1 GeV and pT > 1GeV.
No evidence for  ≠ 0.
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mT Distribution
Spectra fitted with the function:
1 dN
 k  e mT / T
mT dmT
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Depends on the fit range in presence of radial flow
 Effective temperature
Centrality dependence stronger at low pT
Linear mass dependence at low pT
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Tslope: Fit Range Dependence
NA60 In-In (pT < 1.6 GeV/c)
NA49 Pb-Pb
NA50 Pb-Pb
NA60 In-In (pT > 1.1 GeV/c)
NA49 Pb-Pb
NA50 Pb-Pb
Low pT (NA49): Agreement with NA49 when the fit is performed in the same range
High pT (NA50): Lower T absolute values, flatter rise with centrality.
No agreement with NA50
 Difference between NA50 and NA49 was not due to different decay channel
 Hint for the presence of radial flow → Blast Wave analysis
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f/w ratio: Analysis Procedure
pT > 1 GeV/c
MC
Data
f/w ratio extracted from a fit of the mass
distribution with expected sources:
Parameters allowed to vary:
h/w, r/w, f/w and the continuum
• 2-body and Dalitz decays of low mass
mesons
• Open charm continuum (low level)
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r region in central bins parameterised
to reproduce the NA60 excess data.
Little dependence on the
parameterisation (~ 5%)
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f/w ratio
pT > 1 GeV/c
Full pT and y
f yield increases from peripheral to central collision by a factor ~ 3
(Consistent with previous measurement)
Absolute yield measurement in progress
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IMR Excess: Previous Measurements
 NA38/NA50 was able to describe the IMR dimuon spectra in p-A (Al, Cu, Ag, W) 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 (factor ~2 excess for central Pb-Pb).
 The study of this excess was one of the main objectives of the NA60 experiment at SPS.
NA38/NA50 proton-nucleus data
central
collisions
M (GeV/c2)
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NA60 Measurement of the IMR excess
NA60 can separate the prompt and open charm contribution on a statistical
basis by measuring the dimuon offset with respect to the primary vertex
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
(2m1  2m 2 ) / 2
Dimuon weighted offset
Single muon weighted offset
Offset resolution ≈ ~40 mm, < ct
(D+ : 312 mm, Do : 123 mm)
J/y muons
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IMR: Expected Sources
Analysis of the mass spectra in the range 1.16 GeV/c2 < Mmm < 2.56 GeV/c2
Coverage in Collins-Sopper angle: | cos qCS | < 0.5
Analysis repeated for the 2 samples (4000 A and 6500 A) and for
different cuts on the matching c2 (c2match < 1.5 and c2match < 3.0)
Sources (open charm and Drell Yan) simulated using Pythia
Monte Carlo dimuons reconstructed on top of a real event
Relative normalization:
Drell Yan: Reproduce high mass cross section measured by NA3 and NA50
Open Charm: Cross section which reproduces the NA50 p-A dimuon data
 Yield of expected sources in units of expected cross section in the
following
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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.
Fit range
4000 A, c2match <1.5
4000 A, c2match <1.5
Fit range
6500 A, c2match <1.5
6500 A, c2match <1.5
Offset fits with free prompt and charm

~2.4 times more prompts are required than what Drell-Yan provides.

The two data sets, with different systematics, are consistent with each other

Obtained Charm contribution is lower than extrapolation from NA50 p-A data. Statistics is not enough
for its study vs centrality and pT, it will be fixed to 0.7  0.15 (average of 4 and 6.5 kA data)
4000 A, c2match < 3
4000 A, c2match <1.5
6500 A, c2match < 3
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6500 A, c2match <1.5
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cc Cross Section
Our statistics is not enough to study the
s cc
differentially in centrality.
But bulk of existing measurements is in agreement with expectation of its scaling with number of binary
collisions, characteristic for hard process:  = 1 in s cc  s A
pA
0
H.Woehri and C.Lourenco,
Phys.Rep. 433 (2006) 127-180
In further analysis the normalization factor for
NA50 pA data) leading to 9.5±2 mb/nucleon
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cc
will be fixed to 0.7  0.15 (wrt extrapolation from
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sCC: comparison with other measurements
Effect of nuclear modification of PDFs
NA60
H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180
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Excess corrected for acceptance
Define the excess as Signal – [ Drell-Yan (1 ± 0.1) + Open Charm (0.7±0.15) ]
Systematic errors account for
uncertainty in Drell-Yan and
Charm normalization factors
The excess acceptance correction is done differentially in M and pT
Assuming: flat cosq CS distribution for decay angle and rapidity distribution similar to
Drell-Yan (sy~1)
Once the excess is corrected for acceptance, the two data sets can be summed up
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Centrality and pT dependence of excess
Excess/Nparticipants(arb. scale)
Excess already present in
peripheral collisions, scales
faster than NPart
Excess stronger at low pT
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pT Spectra of the excess
Fit in PT<2.5 GeV/c
Fit in 0.5<PT<2 GeV/c
Drell Yan
TEFF is rather low compared both to the
Drell-Yan and to the Low Mass Region
(T ~ 250 MeV)
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Summary
f Meson Production
 T slope increases with centrality and depends
on fit range → hint for radial flow
 Compatible with NA49
 Absolute yields measurement in progress
IMR excess
 Excess is prompt
 Open charm yield agrees with NA50 p-A
 Excess is qualitatively different from Drell-Yan
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The NA60 Collaboration
CERN
56 people
13 institutes
8 countries
Heidelberg
http://na60.cern.ch/
Bern
Palaiseau
BNL
Riken
Yerevan
Stony Brook
Torino
Lisbon
Clermont
Lyon
Cagliari
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. En’yo, 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,
G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,
P. Sonderegger, H.J. Specht, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof, D. Walker and H. Wöhri
June 28, 2007
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BACKUP
f/w ratio
pT > 1 GeV/c
Full phase space
NA60
NA50 (Arb. Rescaled)
Compared to NA50 f/(rw):
NA50 data have a common mT > 1.5 GeV/c2 cut
Extrapolated to pT > 1 GeV/c using Tslope measured by NA50
Ambiguity: need to assume r/w to extract f/w
(Arbitrary rescaling of NA50 data)
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pT Distribution
central
peripheral
1 dN
 k  e mT / T
Spectra fitted with the function:
pT dpT
to extract the Tslope
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Depends on the fit range!
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Acceptance would require correction
with 2D matrices: pT vs y and decay
angle vs pT.
After tuning MC to data (iterative
procedure)
 1D acceptance correction
Systematic error: variation of
analysis cuts and parameters:
• c2 cut of Matched Dimuon
• Fake subtraction method
• Side windows Offset
• Mass window width
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Vertex resolution (in the transverse plane)
BT
The BT measurement
(s = 20 mm at the target)
allows us to control the
vertexing resolution and
systematics
s (mm)
BT
Beam Tracker measurement vs. vertex reconstructed with Vertex Tracker
The interaction vertex is identified with
a resolution of 10–20 mm accuracy in the
transverse plane
30
Dispersion between beam track and
VT vertex
20
10
Vertex resolution (deconvoluting sBT=20 mm)
0
Number of tracks
NA60 Signal Analysis: simulated sources
 Charm and Drell-Yan contributions are obtained by overlaying real event data on dimuons
generated by Pythia 6.326 (CTEQ6L PDFs with EKS98 nuclear modifications. 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, by event mixing.
 Relative normalizations:
 Drell-Yan: K-factor of 1.9. Reproduces In-In data at M>4 GeV/c2 and cross sections measured
by NA3 and NA50 (J. Badier et al. (NA3 Coll.), Z.Phys.,C26: (1985) 489.
M.C. Abreu et al. (NA50 Coll.), Phys. Lett. B410 (1997) 337).
 Charm: s = 13.6 mb/nucleon. Obtained from the cross section describing NA50 p-A dimuon
cc
data at 450 GeV by its rescaling to 158 GeV using Pythia.
Note: this is factor ~2 higher than the extrapolation from the “world average” cross section
(H.Woehri and C.Lourenco, J.Phys. G30 (2004) 315)
Possible explanation: both NA60 and NA50 detect
dimuons only in |cosq|<0.5, while DD  m  m 
shows very strong rise at large cosq

Our full phase space acceptance for charm is
very sensitive to the correctness of kinematic
distribution from Pythia
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NA60 Signal analysis: simulated sources
Absolute normalization: the expected Drell-Yan contribution, as a function of the collision centrality, is
obtained from the number of observed J/y events and the y suppression pattern:

Data is split in 12 bins in collision centrality (number of participants obtained from the measured
charged multiplicity in the Vertex Tracker).

In each bin the number of J/y events is extracted and corrected for the anomalous suppression
(E.Scomparin, proceedings of Quark Matter 2006, Shanghai)

Expected number of events Drell-Yan events at 2.9<M<4.1 GeV/c2 is extracted from y/DY
accounting for the nuclear absorption of the J/y.
A 10% systematical error (mostly due to the uncertainty of the J/y nuclear absorption cross section)
is assigned to this normalization.
Signal shapes used to fit the dimuon weighted offset distributions are:
prompt : mixture of J/y and  data (open charm contamination is < 1%)
charm:
DD  mm Monte Carlo smeared by amount needed for J/y and  MC to reproduce data
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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Muon track matching
Matching between the muons in the Muon Spectrometer (MS) and the
tracks in the Vertex Tracker (VT) is done using the weighted distance (c2)
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
c2 < 3 are selected (standard
analysis).
Fake matches are subtracted by
a mixed-events technique and an
overlay MC method (only for
signal pairs, see below)
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Combinatorial Background
CB (uncorrelated muon pairs coming from p and K decays) is estimated with
an Event Mixing technique
Take muons from different events and calculate their invariant mass. Takes account of charge
asymmetry, correlations between the two muons (induced by magnetic field sextant
subdivision: detector geometry), trigger conditions
Apparatus triggers both opposite sign (mm) and like sign (mm , mm) pairs. Quality of
CB is assessed comparing LS spectra.
Accuracy ~1% over several orders of magnitude!
Fakes in CB also subtracted!
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Fake Matches
 “Fake Matches” are those tracks
where a muon track from the
Muon Spectrometer is matched
to the wrong track from
the Vertex Tracker
muon trigger and
tracking
fake
target
correct
hadron absorber
 Fake matches of the signal pairs (<10% of CB) can be
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. Start with
hadron decay cocktail + continuum; improve by iteration.
 Event mixing
More rigorous, but more complicated. Less statistics
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Example of Overlay MC: the f
sf
= 23 MeV
sfake = 110 MeV
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Fakes calculation with
Overlay MC and Mixing
method agree in absolute
level and shape within 5%!
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Clean Spectrum
For the first time w and f peaks
clearly visible in dilepton channel
(23 MeV mass resolution at the f)
w
h
hmm also visible
f
Fakes/CB < 10 %
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