Post on 09-Jan-2016
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Meson Production in In-In Collisions and
Highlights from NA60Michele Floris1
for the NA60 CollaborationStrangeness in Quark Matter 2007
1University and INFN, Cagliari, Italy
June 28, 2007 Strangeness in Quark Matter 2
OutlineThe NA60 Experiment Detector Concept
Phi Meson Production in In-In Collisions Analysis details pT, y and decay angular distributions ratio
Highlights from NA60 In medium modification of the Intermediate mass range excess: prompt or
charm? Centrality dependence of J/ suppression
June 28, 2007 Strangeness in Quark Matter 3
The NA60 Experiment• Fixed target dimuon experiment at the CERN SPS• Apparatus composed of 4 main detectors
17m
The vertex region (2 detectors):
Zero degree calorimeter(centrality measurements)
Muon Spectrometer
Origin of muons can be accurately determinedImproved dimuon mass resolution (~20 MeV/c2 at instead of 80 MeV/c2)
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
High luminosity experiment: possible with radiation tolerant detectors and high speed DAQ
2.5 T dipole magnet
hadron absorber
targets
beam tracker
vertex trackermuon trigger and tracking (NA50)
magnetic field
>10m<1m
June 28, 2007 Strangeness in Quark Matter 4
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 2)
Subtract Background Two data samples
Different current settings in the Muon Spectrometer magnet Different acceptances High current setting suppresses LMR
June 28, 2007 Strangeness in Quark Matter 5
Meson Production: Motivation Strangeness production in Heavy Ion Collisions Mass and Width changes of the in the medium
Differerences between →KK and → K meson modification in the medium pT dependence of K suppression
Two channels have been studied in 158 AGeV PbPb :
Muons not influenced by the medium Previous SPS measurements: NA50
Acceptance limited to high pT
KK Better mass resolution No physical BG Previous SPS measurements: NA49
Broad pT coverage, but dominated by low pT
puzzles: 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 channel with good pT coverage (0-2.6 GeV) Rapidity, decay angular distribution, pT and ratio
Spectra: Analysis Procedure
We select the events on the peak and use two side mass windows to estimate the pT,y and decay angle distribution of
the continuum under the peak
background
total
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
5 centrality bins
4000 A data set only
June 28, 2007 Strangeness in Quark Matter 7
Rapidity Distribution
Width estimated with a Gaussian fitConstant within errorsAgreement with previous measurements in other colliding systems at the same energy
gau
s
All centralities
= 1.13 ± 0.06 ± 0.05
reflected
gau
ss
June 28, 2007 Strangeness in Quark Matter 8
Decay Angular Distributions
pprojectile ptarget
z axisCS
pµ+
y
x
Viewed from rest frame
Collins-Soper
pprojectile ptarget
z axisGJ
pµ+
y
Viewed from rest frame
Gottfried-Jackson
pprojectile ptarget
z axisHel
pµ+
x
Helicity
y
The angular distribution of the positive muon can be measured with respect to 3 different quantization axes
1. Collins – Soper 2. Gottfried – Jackson3. Helicity
Angular distributions fitted with the function:
2cos1cos
d
dN
polarization
Provides information on the production mechanismPrevious measurements: ACCMOR (h-Be) and Sixel et al. (K- - p / - - p)
Non negligible (GJ Frame)Heavy Ion
Global polarization?
The 3 frames are identical for pT 0First measurement in HI collisions at the SPS
We studied centrality and pT dependence of in the 3 framesFurther developments: azimuthal distributions, study wrt the reaction plane
June 28, 2007 Strangeness in Quark Matter 9
Helicity Distribution
All centralities
= 0.1 ± 0.1 ± 0.1
= 0, independent of centralityAnalysis repeated at pT < 1 GeV and pT > 1GeV.
No evidence for ≠ 0.
pT > 0.2 GeV/c
June 28, 2007 Strangeness in Quark Matter 10
Gottfried – Jackson Distribution
All centralities
= 0. ± 0.1 ± 0.04
= 0, independent of centralityAnalysis repeated at pT < 1 GeV and pT > 1GeV.
No evidence for ≠ 0.
pT > 0.2 GeV/c
June 28, 2007 Strangeness in Quark Matter 11
Collins – Soper Distribution
All centralities
= -0.2 ± 0.2 ± 0.2
Hint for < 0 in peripheral events? Not significant (2).Acceptance limits fit range Large errors on
Analysis repeated at pT < 1 GeV and pT > 1GeV. No evidence for ≠ 0.
pT > 0.2 GeV/c
June 28, 2007 Strangeness in Quark Matter 12
Tm
TT
Tekdm
dN
m/1
Spectra fitted with the function:
mT Distribution
Depends on the fit range in presence of radial flow Effective temperatureCentrality dependence stronger at low pT
Linear mass dependence at low pT
June 28, 2007 Strangeness in Quark Matter 13
Tslope: Fit Range Dependence
NA60 In-In (pT < 1.6 GeV/c)NA49 Pb-PbNA50 Pb-Pb
NA60 In-In (pT > 1.1 GeV/c)NA49 Pb-PbNA50 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
June 28, 2007 Strangeness in Quark Matter 14
ratio: Analysis Procedure
ratio extracted from a fit of the mass distribution with expected sources:
MCData
• 2-body and Dalitz decays of low mass mesons• Open charm continuum (low level)
Parameters allowed to vary: and the continuum
region in central bins parameterised to reproduce the NA60 excess data. Little dependence on the parameterisation (~ 5%)
pT > 1 GeV/c
June 28, 2007 Strangeness in Quark Matter 15
ratio
pT > 1 GeV/c
Full pT and y
yield increases from peripheral to central collision by a factor ~ 3(Consistent with previous measurement)Absolute yield measurement in progress
June 28, 2007 Strangeness in Quark Matter 16
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
centralcollisions
M (GeV/c2)
IMR Excess: Previous Measurements
June 28, 2007 Strangeness in Quark Matter 17
NA60 Measurement of the IMR excessNA60 can separate the prompt and open charm contribution on a statistical basis by measuring the dimuon offset with respect to the primary vertex
2/)2( 11212
xyyyxx VyxVyVx
2/)( 22
21
Single muon weighted offset
Dimuon weighted offset
To eliminate the momentum dependence of the offset resolution, we use the muon offset weighted by the error matrix of the fit:
J/ muons
Offset resolution ≈ ~40 m, < c(D+ : 312 m, Do : 123 m)
June 28, 2007 Strangeness in Quark Matter 18
Sources (open charm and Drell Yan) simulated using PythiaMonte Carlo dimuons reconstructed on top of a real event
IMR: Expected Sources
Analysis of the mass spectra in the range 1.16 GeV/c2 < M < 2.56 GeV/c2 Coverage in Collins-Sopper angle: | cos CS | < 0.5
Analysis repeated for the 2 samples (4000 A and 6500 A) and for different cuts on the matching 2 (2
match < 1.5 and 2match < 3.0)
Relative normalization:Drell Yan: Reproduce high mass cross section measured by NA3 and NA50Open Charm: Cross section which reproduces the NA50 p-A dimuon data Yield of expected sources in units of expected cross section in the following
Fit range
4000 A, 2match <1.5
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,2match <1.5
6500 A, 2match
<1.5
Fit range
6500 A, 2match <1.5
Data integrated in collision centralities and in pT
and the fit to the offset spectra shows that the excess is prompt.
June 28, 2007 Strangeness in Quark Matter 20
~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, 2match < 3
6500 A, 2match < 3
4000 A, 2match <1.5
6500 A, 2match <1.5
Offset fits with free prompt and charm
June 28, 2007 Strangeness in Quark Matter 21
Our statistics is not enough to study the 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
In further analysis the normalization factor for will be fixed to 0.7 0.15 (wrt extrapolation from
NA50 pA data) leading to 9.5±2 b/nucleon
cc
AccpA 0
H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180
cc
cc Cross Section
June 28, 2007 Strangeness in Quark Matter 22
NA60
H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180
Effect of nuclear modification of PDFs
CC: comparison with other measurements
June 28, 2007 Strangeness in Quark Matter 23
The excess acceptance correction is done differentially in M and pT
Assuming: flat cos CS distribution for decay angle and rapidity distribution similar to Drell-Yan (y~1)
Once the excess is corrected for acceptance, the two data sets can be summed up
Systematic errors account for uncertainty in Drell-Yan and
Charm normalization factors
Excess corrected for acceptanceDefine the excess as Signal – [ Drell-Yan (1± 0.1) + Open Charm (0.7±0.15) ]
June 28, 2007 Strangeness in Quark Matter 24
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
June 28, 2007 Strangeness in Quark Matter 25
Fit in 0.5<PT<2 GeV/cFit in PT<2.5 GeV/c
pT Spectra of the excess
TEFF is rather low compared both to the Drell-Yan and to the Low Mass Region (T ~ 250 MeV)
Drell Yan
June 28, 2007 Strangeness in Quark Matter 26
Summary 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
June 28, 2007 Strangeness in Quark Matter 27
Lisbon
CERN
Bern
Torino
Yerevan
CagliariLyon
Clermont
BNL Riken
Stony Brook
Palaiseau
Heidelberg
BNL
56 people13 institutes 8 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. 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
The NA60 Collaborationhttp://na60.cern.ch/
BACKUP
June 28, 2007 Strangeness in Quark Matter 29
ratio
pT > 1 GeV/c
Full phase space
Compared to NA50 : NA50 data have a common mT > 1.5 GeV/c2 cut
Extrapolated to pT > 1 GeV/c using Tslope measured by NA50Ambiguity: need to assume to extract
(Arbitrary rescaling of NA50 data)
NA60NA50 (Arb. Rescaled)
June 28, 2007 Strangeness in Quark Matter 30
pT Distribution
peripheral
central
Tm
TT
Tekdp
dN
p/1 Spectra fitted with the function:
to extract the Tslope Depends on the fit range!
June 28, 2007 Strangeness in Quark Matter 31
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:
• 2 cut of Matched Dimuon• Fake subtraction method • Side windows Offset• Mass window width
Vertex resolution (in the transverse plane)
The interaction vertex is identified witha resolution of 10–20 m accuracy in the transverse plane
Dispersion between beam track andVT vertex
Vertex resolution (deconvoluting BT=20 m)
10
20
30
0
(
m)
Number of tracks
Beam Tracker measurement vs. vertex reconstructed with Vertex Tracker
BTBT
The BT measurement
( = 20 m at the target) allows us to control the vertexing resolution and systematics
June 28, 2007 Strangeness in Quark Matter 33
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: = 13.6 b/nucleon. Obtained from the cross section describing NA50 p-A dimuon
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 |cos|<0.5, while
shows very strong rise at large cos Our full phase space acceptance for charm is
very sensitive to the correctness of kinematic
distribution from Pythia
DD
cc
NA60 Signal Analysis: simulated sources
June 28, 2007 Strangeness in Quark Matter 34
Absolute normalization: the expected Drell-Yan contribution, as a function of the collision centrality, is obtained from the number of observed J/ events and the 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/ 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 DY accounting for the nuclear absorption of the J/
A 10% systematical error (mostly due to the uncertainty of the J/ nuclear absorption cross section) is assigned to this normalization.
Signal shapes used to fit the dimuon weighted offset distributions are:
prompt : mixture of J/ and data (open charm contamination is < 1%)
charm: Monte Carlo smeared by amount needed for J/ and MC to reproduce data
The fits to mass and weighted offset spectra are reported in terms ofthe DY and Open Charm scaling factors needed to describe the data
DD
NA60 Signal analysis: simulated sources
June 28, 2007 Strangeness in Quark Matter 35
A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with 2 < 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)
Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Tracker (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.
Muon track matching
June 28, 2007 Strangeness in Quark Matter 36
Combinatorial BackgroundCB (uncorrelated muon pairs coming from 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 () and like sign () pairs. Quality of CB is assessed comparing LS spectra. Accuracy ~1% over several orders of magnitude!
Fakes in CB also subtracted!
June 28, 2007 Strangeness in Quark Matter 37
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
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
hadron absorber
muon trigger and tracking
target
fake
correct
June 28, 2007 Strangeness in Quark Matter 38
= 23 MeV
fake = 110 MeV
Example of Overlay MC: the
Fakes calculation with Overlay MC and Mixing
method agree in absolute level and shape within 5%!
June 28, 2007 Strangeness in Quark Matter 39
Fakes/CB < 10 %
For the first time and peaks clearly visible in dilepton channel
(23 MeV mass resolution at the
also visible
Clean Spectrum