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A. Förster,CERNPH 1
Andreas Förster
CERN-PH
on behalf of the NA60 Collaboration
Latest Results of NA60
A. Förster,CERNPH 2
Introduction
- NA60 is a second generation experiment, designed to answer specific questions left open, in the leptonic sector, by the previous round of SPS experiments, finished in 2000
- It has been designed in order to reach unprecedented accuracy in the measurement of muon pair production in HI collisions
- After its approval in 2000, NA60 has taken data in 2002 (p+A), 2003 (In+In) and 2004 (p+A), now being analyzed
A. Förster,CERNPH 3
Experimental Setup
MUON FILTERBEAM
TRACKER
TARGETBOX
VERTEX TELESCOPE
Dipole field2.5 T
BEAM
IC
not to scale
Matching in coordinate and in momentum space: - improved dimuon mass resolution - origin of muons can be accurately determined
!or
beam
~ 1m Muon Spectrometer
MWPC’s
Trigger Hodoscopes
Toroidal MagnetIronwallHadron absorber
ZDC
Target area µ
µ
A. Förster,CERNPH 4
Vertex and Offset Resolution
Beam Trackersensors
Vacuum boxwindows
Vertex reconstruction
J/
- Vertex resolution: ~200 m in z ~10-20 m in x,y- Good target ID even for most peripheral collisions (> 3 tracks)
- Offset resolution of the dimuon vertex: 40 - 50 m
A. Förster,CERNPH 5
Centrality Estimation
Target
Projectile
Target
Projectile
EZDC (GeV)
- Collision centrality estimated from the energy released in the Zero Degree Calorimeter (ZDC)
Number of participants
- Number of participants (Npart
) and
other centrality variables estimated using a Glauber calculation
A. Förster,CERNPH 6
In+In 158 AGeV
LMR IMR HMR
Analysis Topics:
1) Dimuons: a) Low Mass Dimuons (LMR) - vector mesons in medium b) Intermediate Mass Dimuons (IMR) - open charm/thermal radiation c) High Mass Dimuons (HMR) - anomalous J/suppression
2) Charged Particles a) → KK b) Elliptic Flow
Indium beam of 158 GeV/nucleon ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape
A. Förster,CERNPH 7
Dimuons:Comb. Background Subtraction
dimuon mass [GeV]
- Most pairs are comb. backgr.- Subtraction by event mixing- Quality controlled by comparing mixed event like sign dimuon spectra to measured like sign dimuon spectra
- and peaks visible with a mass resolution of 20 - 23 MeV
signal statistics:360 000 events
A. Förster,CERNPH 8
Low Mass Dimuons
CERES/NA45 - CERES: well established excess in Pb+Au above the hadronic decay cocktail (that describes p+Be data)
- Different theoretical explanations: mass shift, broadening, ...
- Good resolution and statistics needed
A. Förster,CERNPH 9
Peripheral Collisions
- 4 centrality bins, defined by Nch
- Fit independently in 3 bins in pt
- Free parameters: DD, overall normalization
- The peripheral data are well reproduced by the hadronic cocktail - Good fit quality down to low mass and low p
t
(low acceptance region well under control)
A. Förster,CERNPH 10
Excess in Central Collisions
• data sum of the cocktail sources including the ρ
- Comparison to a “conservative” cocktail, definition: see next slide
- ρ/ω fixed to 1.2
- Clear excess of data above cocktail, rising with centrality
A. Förster,CERNPH 11
Isolating the Excess
- Conservative approach: Use particle ratios such as to set a lower limit to a possible excess: - fix yields such as to get a smooth underlying continuum after subtraction
- set upper limit, defined by “saturating” the measured yield around 0.2 GeV → leads to lower limit for the excess at low masses
- not subtracted
A. Förster,CERNPH 12
Comparison to Model Predictions
- Predictions for In+In ( ⟨dNch
/d⟩=140 )
by Rapp et al. (2003) for different scenarios
- Theoretical yields folded with NA60 acceptance and normalized to data for M < 0.9 GeV
- Excess shape consistent with broadening of the (Rapp,Wambach RW)
- Models predicting a mass shift clearly disfavoured (Brown,Rho BR)
A. Förster,CERNPH 13
Low Mass Dimuons: Summary
Lepton pair excess at SPS energies confirmed
Mass shift of the intermediate clearly disfavoured
Broadening of the intermediate describes the data
A. Förster,CERNPH 14
High Mass Dimuons: J/ Suppression
- Anomalous J/ suppression discovered by NA50 in Pb+Pb collsions
- Generally considered one of the most direct signatures of deconfinement
- NA60: is anomalous J/ suppression as well present in the lighter system In+In?
A. Förster,CERNPH 15
J/ Suppression in In+In
J/ψ
ψ’
DYBackground
Charm
- NA50: Normalization to Drell-Yan → scales with number of NN-collisions → no sizeable final state effects
- NA60: very limited Drell-Yan statistics (~ 300 for M
> 4.0 GeV)
→ only three bins
A. Förster,CERNPH 16
Direct J/ Sample
Nuclearabsorption
- directly compare measured J/ yield with the theoretical distribution as expected in the case of pure nuclear absorption
A. Förster,CERNPH 17
Comparison to Model Predictions
Satz, Digal, FortunatoRapp, Grandchamp, BrownCapella, Ferreiro
J/ψ absorption by produced hadrons (comovers) - Capella and Ferreiro, Eur.Phys.J. C42 (2005) 419
J/ψ suppression in the QGP and hadronic phases - including thermal regeneration and in- medium properties of open charm and charmonium states Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; J.Phys.G 30 (2004) S1355
χc suppression by deconfined partons when geometrical percolation sets in - Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.
A. Förster,CERNPH 18
High Mass Dimuons : Summary
Anomalous J/ supression observed in In+In
Centrality dependent, with an onset around Npart
= 90
Theoretical predictions (tuned on Pb+Pb) do not properly describe the In+In data
A. Förster,CERNPH 19
Charged Particles:Reconstruction of the Reaction Plane
- NA60 acceptance: ~ 0 < ycm
< 1
- Use elliptic flow v2 to estimate the event
plane (v1 = 0 at midrapidity)
- Determination from charged particle tracks as measured in the silicon pixel vertex tracker
- Reaction Plane: Plane defined by the beam direction and the impact parameter vector
A. Förster,CERNPH 20
Determination of the Event Plane
- Event Plane: estimate of Reaction Plane from experimental data using the flow itself- has to be corrected for the influence of the resolution
- Emission angles of all particles i are related to the event plane angle
via the event flow vector Q
- we use n=2 (v2), since v
1 = 0 at midrapidity
Qn , x = Qn cosnn =∑i
pt , i cosni
Qn , y = Qn sin nn =∑i
pt , i sin nin =
1n arctan Qn , y
Qn , x
A. Förster,CERNPH 21
The Azimuthal Acceptance of NA60
- The NA60 acceptance is highly assymetric in the azimuthal angle - Beam not perfectly centered in the experimental coordinate system
A. Förster,CERNPH 22
<cos(2)>
Recentering(Poskanzer,Voloshin PRC58 (1998) 1671 , NA49, PRC 68 (2003) 034903)
- Recentering performed per track in (pt,y)-acceptance bins (like in NA49)
Qx = Q cos2=∑i
pt , icos2i−⟨cos2⟩ pt , y
Qy = Q sin 2=∑i
pt , isin 2i−⟨sin 2⟩ pt , y
- Separately for each of the 7 subtargets- Only (p
t,y)-bins with
correction factors < 0.2 used
A. Förster,CERNPH 23
Flattening
uncorrected after recentering▴ after flattening
- Recentering by definition only removes distortions in the same Fourier order as used for the event plane determination- Higher orders removed by a flattening procedure (Poskanzer,Voloshin PRC58 (1998) 1671)- This imposes that the event plane distribution has to be flat
A. Förster,CERNPH 24
Event Plane Resolution- Resulting values for v'
2 have to be corrected for the event plane resolution
- v2 = v'
2 / <cos[2()]>
- Correction factors determined from subevents using the method of Ollitrault
A. Förster,CERNPH 25
MonteCarloSimulation
- Generation of + and -
with an assumed v2(p
t)
- Tracking through experimental setup using Geant- Using the same data analysis as for real data
- Input is reproduced very well
A. Förster,CERNPH 26
Azimuthal Distributions
h±
- Fit to the data: dN/d ~ 1 + (2 v'
2 cos[2])
- v2 = v'
2 / <cos[2()]>
A. Förster,CERNPH 27
pt - Dependence of v
2
- Results are not corrected for non-flow correlations due to HBT effect- Therefore, we only show results for p
t > 0.5 GeV
- ~ 50% of total measured statistics
- v2 saturates for large p
t
A. Förster,CERNPH 29
Azimuthal Distributions of J/
J/ J/
- Large statistical errors (~50% of the total measured statistics)
A. Förster,CERNPH 30
Azimuthal Distributions : Summary
NA60 can measure azimuthal particle emission patterns
Charged particles show a significant v2 , saturating at high p
t
Azimuthal distributions of J/have large statistical errors (50% of total statistics analyzed so far)
A. Förster,CERNPH 31
Summary
Low Mass Dimuons: - Lepton pair excess at SPS energies confirmed - Mass shift of the intermediate clearly disfavoured - Broadening of the intermediate describes the data
High Mass Dimuons: - Anomalous J/ supression observed in In+In - Centrality dependent, with an onset around N
part = 90
- Theoretical predictions (tuned on Pb+Pb) do not properly describe the In+In data
Charged Particles: - NA60 can measure azimuthal particle emission patterns - Charged particles show a significant v
2 , saturating at high p
t
- Azimuthal distributions of J/have large statistical errors (50% of total statistics analyzed so far)
A. Förster,CERNPH 32
The NA60 Collaboration
Lisbon
CERN
Bern
Torino
Yerevan
CagliariLyon
Clermont
Riken
Stony Brook
Palaiseau
Heidelberg
BNL
~ 60 people13 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. Damjanovi , A. David, A. de Falco, N. de Marco,ć
A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A.Förster,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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