Post on 19-Jan-2016
Quark Matter 2006 November 14-20, Shanghai
NA60 results on the spectral function in Indium-Indium collisions
J. SeixasNA60 Collaboration
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 2
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Conclusions
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 3
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Conclusions
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 4
• Origin of muons can be accurately determined• Improved dimuon mass resolution
Matching in coordinate
and momentum space
or!
Measuring dimuons in NA60: concept
hadron absorberMuon
Other
and trackingMuon triggerA
CM
magnet
Iron wall
NA50 spectrometer2.5 T dipole magnet
targets
beam tracker
vertex tracker
40 cm >10 m
prompt
displaced
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 5
Event sample
5-week long run in Oct.–Nov. 2003 Indium beam of 158 GeV/nucleon ~
4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape
Present analysis: ~1/2 of total data
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 6
Main steps of the data analysis
Reconstruction of the event vertex within the segmented target
Matching of tracks from muon spectrometer and silicon vertex telescope
Assessment of fake matches (incorrect match between tracks in the muon spectrometer and silicon vertex telescope) by overlay MC and/or event mixing: agreement ~5%
Assessment of combinatorial background (pairing of uncorrelated muon pairs from and K decays) by event mixing: accuracy ~1%
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 7
Mass spectrum
Net data sample: 360 000 events
Fakes / CB < 10 % ω and peaks clearly
visible in dilepton channel; even μμ seen
Mass resolution:23 MeV at the position
Progress over CERES: statistics: factor >1000resolution: factor 2-3
ω
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 8
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Comparison to theoretical models
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 9
Isolation of the excess
and : fix yields such as to get, after subtraction, a smooth underlying continuum
: () set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV/c2 (lower limit for excess).
() use yield measured for pT > 1.4 GeV/c
difference spectrum
robust to mistakes
on the 10% level, since
the consequences
of such mistakes are
Highly localized.
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 10
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Conclusions
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 11
Excess mass spectra for 12 centrality bins
Cocktail from /=1.2
Excess always centered at nominal -pole, while width increases
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 12
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Conclusions
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 13
Shape modification vs. centrality
Divide region around peak in 3 windows labeled L, C and U:
Peak/ρ
Ratio
Continuum/ρ
UL21
CPeak""
UL23
Continuum""
Continuum""Peak""
Ratio
“Peak” decreases with respect to cocktail ρ by ~2 from first to last point
Change in trend for dNch/dy > 100
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 14
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Conclusions
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 15
Comparison to theory [talks by R. Rapp, V. Metag, G. Usai]
Predictions for In-In by Rapp et al. (2003) for <dNch/d> = 133, covering all scenarios
Data and predictions as shown, after acceptance filtering, roughly mirror the respective spectral functions, averaged over space-time and momenta.
Theoretical yields normalized to data in mass interval < 0.9 GeV/c2.
Only broadening of (RW)
No mass shift (BR) observed
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 16
Outline
Event sample Isolation of excess dimuons above
hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Conclusions
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 17
Strategy for acceptance correction
Reduce 3-dimensional acceptance correction in M-pT-y to a 2-dimensional correction in M-pT, using measured y distribution as an input. Use for control.
Use slices of m = 0.1 GeVpT = 0.2 GeV
Check behaviour on 3 extended mass windows
0.4<M<0.6 GeV
0.6<M<0.9 GeV
1.0<M<1.4 GeVSubtract charm from the data (based on NA60 IMR results) before acceptance correction
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 18
Rapidity distribution of the excess
Agreement betweenthe three pT bins
Results close to rapidity distribution of ’s (from NA49 and NA60) for the same √s
arb
. unit
s
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 19
Excess pT spectra: 3 centrality bins
Hardly any centrality dependenceBUT
Significant mass dependence
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 20
Systematics of low-pT data: Combinatorial background
Enhanced yield at low-pT seen at all centralities, including the peripheral bin
Errors at low pT, due to subtraction of combinatorial background:
Enhanced yield at low pT not due to incorrect subtraction of combinatorial background
peripheral 1%semiperipheral 10%semicentral 20%central 25%
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 21
Excess pT spectra: mass dependence
Since we do not observe any centrality dependence, we sum over centralities and keep dependence in mass
Differential fits to pT spectra: assume locally 1-par. mT scaling (~exp(-mT/Teff)) and use gliding windows of pT=0.8 GeV; get Teff for each bin
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 22
Excess pT spectra: mass dependence
Systematic errors: < 15 MeV
-like region hardest high-mass region softest
Different sources in different mass regionsHadronic or partonic?
low Teff @ low pT
Trend opposite to what
expected from radial flow
(see )
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 23
Conclusions (Mass spectra)
Pion annihilation seems to be a major contributor to the lepton pair excess at SPS energies
Strong broadening, but no significant mass shift of the intermediate
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 24
Conclusions (pT spectra)
Strong mass dependence of pT spectra
Spectra behave oposite to expected from radial flow
pT spectra could serve as a handle to disentangle partonic from hadronic sources (breaking parton-hadron duality)
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 25
http://cern.ch/na60
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. Damjanović, 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, 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
The NA60 experiment
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 27
Backup
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 28
RMS of total excess
Consistency with shape analysis
Further rise starting atdNch/dy =100
Bad fit (2=3) for linear rise above dNch/dy=30)
J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 30
Systematics of low-pT due to the cocktail subtraction
radical (unrealistic) increase of Dal subtraction 20% decrease of yield in mass window 0.6<M<0.9
enhanced yield at low pt cannot be due to incorrect subtraction
Fit:0-0.4 GeV
Teff=82 MeV
Teff=97 MeV
Teff=118MeV
enhanced yield at low pt even after radical increase (by nearly a factor of 2) of the Dalitz yield
Systematics of low-pT data due to the cocktail subtraction