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production in proton-nucleus and indium-indium collisions
Michele FlorisUniversity and INFN, Cagliari, Italy.
on behalf of the NA60 Collaboration
ISMD 2005, Kromeriz, Czech Republic, August, 14th 2005.
Outline Motivation Apparatus Collected data Results in p-A Results in In-In Ongoing work for KK
14/08/2005 ISMD 2005 2
Motivation The study of meson production in heavy ion collisions
carries information about strangeness production Two channels have been studied:
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
The discrepancy between the values of the inverse slope T measured by these experiments led to the so-called
puzzle New measurements from NA60 in In-In collisions
NA60 measures the channel with very good pT coverage Has access to the KK channel
17m
The vertex region:
The NA60 experiment
• Fixed target dimuon experiment at the CERN SPS• Apparatus composed of 4 main detectors
Zero degree calorimeter(centrality measurements)
Muon Spectrometer
Analysis strategy• Create sample of matched muons• Subtract combinatorial BG via event
mixing• Estimate fakes via overlay MC• Fit experimental spectra with
expected sources • Hadronic cocktail (Genesis)• IMR continuum (open charm and
Drell-Yan or empirical)
14/08/2005 ISMD 2005 4
Data Taking
Two samples discussed in this talk p-A Collisions (400 GeV protons)
Six targets, three nuclei (Be, In, Pb) Microstrips silicon vertex detector 4 days in 2002, 600 000 dimuons collected (at “low”
beam intensity: 1–3 x 108 protons/burst) New high statistics sample collected in 2004, still to be
analyzed In-In collisions (158 GeV ions)
Seven indium targets Silicon pixels vertex detector 5-week-long run in 2003, very good statistics (~ 230
million dimuon triggers on tape) ~ 50% statistics discussed in this talk
14/08/2005 ISMD 2005 5
Detector performance
Dimuon mass resolution at M ~ 1 GeV:In-In: ~ 23 MeV (independent of centrality)p-A: ~ 30 MeVPrevious dimuon experiments: ~ 80 MeV
p beam
400 GeV
In Pb
3 x BeBe
Clear separation of all targets (Z vertex resolution ~ 600–900 m in p-A, better than 200 m in In-In)
target boxwindows
7 In targets
z-vertex (cm)
Transverse vertexingwith 20 µm accuracy
Beam tracker station
Indium beam
158 A GeV
The acceptance of NA60 extends, all the way down to small M and pT
A (
%)
14/08/2005 ISMD 2005 6
BG+charm
production in proton-nucleus collisions
• Mass spectra described as a superposition of the low mass resonances decays into muons + charm + DY
• Fit to the mass spectra to extract the ratio
p-Be 0.062
0.04
p-In 0.083 0.07
p-Pb 0.081 0.06
14/08/2005 ISMD 2005 7
production in indium-indium collisions4 Centrality bins:• Npart estimated from a Glauber fit to
the EZDC spectrum
Fit input:• Hadron cocktail (genesis)• Low-level (empirical) continuum
source with exponential fall-off (to mimic continuum under the vector mesons)
• Parameters allowed to vary: , , and the continuum
• Arbitrarily normalize to the
Total sample: • 570 000 events after BG subtraction • 50% of the full statistics• S/B = 1/4
peripheralall pT
Signal
Cocktail
14/08/2005 ISMD 2005 8
Peripheral Bin
Peripheral bin studied in three pT bins The normalizations of the hadron decay cocktail and of the
continuum are independently fit in each pT bin and ratios are
nearly pT independent The peripheral
bin is well described in terms of expected sources but:
“Too many” low pT mesons Peripheral In-In is
not quite pp, it’s more like CC or OO
Effect of pion annhilation cannot be neglected
14/08/2005 ISMD 2005 9
/ cross section ratio – Vacuum
Vacuum contribution ( annihilation) important at low pT even in peripheral collisions
Effect becomes dramatic in more central collisions complicated
continuum below the
However, the excellent mass resolution of NA60 allows us to extract a
robust yield
Vacuum Cocktail Cocktail
14/08/2005 ISMD 2005 10
/ cross section ratio – Centrality Dependence
As a function of centrality:
We restrict analysis topT > 1 GeV
Increase of a factor ~2 from peripheral to central collisions
14/08/2005 ISMD 2005 11
/ comparison to NA50
A direct comparison is impossible, due to the contribution from pion annihilation, which must be even higher in Pb-Pb
collisions, and which NA50 cannot isolate
NA50 points converted to the windowpT>1.1 GeV/c assuming T=228 MeV
used
(lower limit for NA50 ratio)
14/08/2005 ISMD 2005 12
/ comparison to NA49 /
Same trend as a function of Npart constant
If we set the ratio to 0.07–0.08, as suggested by statistical models, then
the NA60 yield is a factor 1.5–2 higher than
the NA49 value
14/08/2005 ISMD 2005 13
transverse momentum spectrum
We select the events on the peak and use two side mass windows to estimate the pT distribution of the continuum
under the peak
Then we correct for the acceptance, calculated (by Monte Carlo) as a 2-dim matrix: pT and y
background
total
14/08/2005 ISMD 2005 14
There is no significant variation of the extracted inverse slope parameter, T, with rapidity
There is a clear increase from peripheral to central collisions
With full statistics, extension up to pT > 3 GeV/c should be feasible
pT spectrum versus y and centrality
Comparison to NA49/NA50
Pb-Pb
In-In
Si-Si
C-C
pp
The In-In measurement of NA60follows the NA49 systematics,
The disagreement between NA49 and NA50 is not due to the different decay channel
Average NA60 value
1. All pT: 253 ± 2 MeV
2. NA50 range (mT > 1.65 GeV/c): 244 ± 5 MeV
3. NA49 range (pT < 1.5 GeV/c): 260 ± 5 MeV Only small variations…
14/08/2005 ISMD 2005 16
pT (GeV/c)
MC
KK, Analysis strategy KK can be studied using charged tracks
reconstructed in vertex telescope Brute force method (no PID):
Assume all tracks are kaons Make invariant mass from all track pairs
Huge combinatorial BG Subtracted by event mixing
technique Kinematical cuts
Single tracks avoid phase space boundaries pT and p improve Signal/BG
Pair Opening angle improve Signal/BG
14/08/2005 ISMD 2005 17
Signal
Very clean signal!
MC
KK, Status
Measured spectrumCombinatorial BG
DATA
Something yet to be tuned (BG?)
Subtraction
• Detailed Monte Carlo (Venus) studies (full detector description)
• First attempts to get a signal out of the data
MC
Semi-peripheral In-In collisions
14/08/2005 ISMD 2005 18
Summary
NA60 is well suited to help understanding the “ puzzle” New In-In measurements Better pT coverage than previous experiments Capability to measure KK
ratio: Rise with Npart consistent with NA49 and NA50 Absolute values between NA49 and NA50
Inverse slope T of the pT-distribution: Agreement between NA49 and NA60 The difference between NA49 and NA50
is not due to the different channels probed KK
Full MC simulation shows the feasibility of the study Final tuning still needed for background subtraction in real data
14/08/2005 ISMD 2005 19
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/
14/08/2005 ISMD 2005 21
Measuring DimuonsMWPC &
trigger hodosMWPC &
trigger hodosToroidalmagnet
Iron WallHadron absorber
TargetLast trigger
station
Limiting Factor:• Enegy loss• Multiple Scattering
Tracking before the hadron absorber
Vertex Detector
High multiplicity+
High luminosity
Rad-hard silicon pixels
14/08/2005 ISMD 2005 22
hadron absorber
and trackingmuon trigger
magnetic field
iron w
all
muonother
targets
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 muon measured in the spectrometer
Improved kinematics (~20 MeV/c2 at instead of 80 MeV/c2 in NA50)Origin of muons can be accurately determined
2.5 T dipole magnet
beam tracker vertex tracker
Concept of NA60
14/08/2005 ISMD 2005 23
Peripheral bin
0.5 < pT < 1.0 GeV/c pT > 1.0 GeV/cpT < 0.5 GeV/c
SignalCocktail
14/08/2005 ISMD 2005 24
Clear increase of the extracted slope parameter T with multiplicity
Purely statistical errors
pT spectra vs multiplicity
14/08/2005 ISMD 2005 25
Fit of the pT distribution in different pT ranges Differential” fits
Fix fit interval at pT=0.8 GeV and move the extremes
Dynamic range of 40 MeV in Teff (all centralities)
Flat trend for
peripheral
collisions Indication for flow
in In-In collisions
14/08/2005 ISMD 2005 26
Agreement of data and mixed CB over several orders of magnitude
Accuracy of agreement ~1%
Combinatorial Background from ,K→ decays
14/08/2005 ISMD 2005 27
A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with 2 < 3 are selected.
Fake matches are subtracted by a mixed-events technique (CB) 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 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.
Muon track matching
14/08/2005 ISMD 2005 28
Fake-match contribution localized in mass (and pT) space
= 23 MeV fake = 110 MeV
Example of overlay MC: the
14/08/2005 ISMD 2005 30
Mass spectrum in semi-central In-In collisionsComplicated continuum under the in more central collisionsHowever, the excellent mass resolution of NA60 allows us to extract a robust yield
14/08/2005 ISMD 2005 31
Predictions for In-In by Rapp et al (2003) for <dNch/d > = 140, covering all scenarios Theoretical yields, folded with acceptance of NA60 and normalized to data in mass interval < 0.9 GeV
Only broadening of (RW) observed, no mass shift (BR)
Comparison of data to RW, BR and Vacuum