NA60 results on the spectral function in Indium-Indium collisions

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


Outline




• Origin of muons can be accurately determined• Improved dimuon mass resolution

Matching in coordinate

and momentum spaceor!

Measuring dimuons in NA60: concept

hadron absorberMuonOther

and trackingMuon triggerA

CM

magnet

Iron wall

NA50 spectrometer2.5 T dipole magnet

targets

beam tracker

vertex tracker

40 cm >10 m

prompt

displaced


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


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%


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


Outline


hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected pT spectra Comparison to theoretical models


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.


Outline




Excess mass spectra for 12 centrality bins

Cocktail from /=1.2Excess always centered at nominal -pole, while width increases


Outline




Shape modification vs. centrality

Divide region around peak in 3 windows labeled L, C and U:

Peak/ρ

Ratio

Continuum/ρ

UL21CPeak""

UL23Continuum""

Continuum""Peak""Ratio

“Peak” decreases with respect to cocktail ρ by ~2 from first to last pointChange in trend for dNch/dy > 100


Outline




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


Outline




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 GeV0.6<M<0.9 GeV1.0<M<1.4 GeV

Subtract charm from the data (based on NA60 IMR results) before acceptance correction


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.

uni

ts


Excess pT spectra: 3 centrality bins

Hardly any centrality dependenceBUT

Significant mass dependence


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%


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


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 )


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


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)


http://cern.ch/na60

Lisbon

CERN

Bern

Torino

Yerevan

CagliariLyonClermont

RikenStony 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


Backup


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)


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

Documents

NA60 results on the spectral function in Indium-Indium collisions