first measurement of the spectral function in high-energy nuclear collisions

S. Damjanovic, Hot Quarks 2006 1

First measurement of the spectral function in high-energy nuclear collisions

Sanja Damjanovic NA60 Collaboration

Villasimius, Sardinia, 18 May 2006


Outline

Experimental set-up

Data analysis Understanding the peripheral data

Isolation of an excess in the more central data

Comparison of the excess to model predictions

Conclusions


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

Event sample: Indium-Indium


Subtraction of combinatorial background and fakes

For the first time, and peaks clearly visible in dilepton channel ; even μμ seen

Net data sample: 360 000 events

Mass resolution:23 MeV at the position

Fakes / CB < 10 %

Progress over CERES: statistics: factor >1000resolution: factor 2-3


Phase space coverage in mass-pT plane

Final data after subtraction of combinatorial background and fake matches

The acceptance of NA60 extends (in contrast to NA38/50) all the way down to small mass and small pT

MC


Track multiplicity from VT tracks for triggered dimuons for

Centrality bin multiplicity ⟨dNch/dη⟩3.8

Peripheral 4–28 17

Semi-Peripheral

28–92 63

Semi-Central 92–160 133

Central > 160 193

Associated track multiplicity distribution

4 multiplicity windows:

opposite-sign pairs combinatorial background signal pairs


Results


Understanding the Peripheral data

Fit hadron decay cocktail and DD to the data

5 free parameters to be fit:

DD, overall normalization

(0.12fixed)

do the fits for all pT and three bins in pT

Extrapolate fit parameters to full phase space

(using particle generator “Genesis”)


Comparison of hadron decay cocktail to data

all pT

Very good fit quality

log


The region (small M, small pT)

is remarkably well described

Comparison of hadron decay cocktail to data

→ the (lower) acceptance of NA60

in this region is well under control

pT < 0.5 GeV


Particle ratios from the cocktail fits

and nearly

independent of pT; 10% variation due to the

enhanced mostly at low pT (due to ππ annihilation, see later)

General conclusion: peripheral bin very well described in terms of known sources low M and low pT acceptance of NA60 under control


Isolation of an excess in the more central data


Understanding the cocktailfor the more central data

Need to fix the contributions from the hadron decay cocktail

Cocktail parameters from peripheral data?

How to fit in the presence of an unknown source?

Nearly understood from high pT data, but not yet used

Goal of the present analysis:

Find excess above cocktail (if it exists) without fits


Conservative approach

Use particle yields so as to set a lower limit to a possible excess


● data

-- sum of cocktail sources

including the

Cocktail definition: see next slide

all pT

Comparison of data to “conservative” cocktail

Clear excess of data above cocktail, rising with centrality

fixed to 1.2

But: how to recognize the spectral shape of the excess?


Isolate possible excess by subtractingcocktail (without ) from the data

set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV

leads to a lower limit for the excess at very low mass

and : fix yields such as to get, after subtraction, a smooth

underlying continuum

difference spectrum robust to mistakes even on the 10% level, since the consequences of such mistakes are highly localized.


Excess spectra from difference: data - cocktail

all pT

Clear excess above the cocktail , centered at the nominal pole and rising with centrality

Similar behaviour in the other pT bins

No cocktail and no DD subtracted


Enhancement relative to cocktail use mass range 0.2<m<0.9 GeV to normalize to

Total data, no DD subtracted

Non-linear rise with centrality, steeper for low pT


Systematics

Systematic errors of continuum 0.4<M<0.6 and 0.8<M<1GeV 25%

Illustration of sensitivity to correct subtraction of combinatorial background and fake matches; to variation of the yield

Structure in region completely robust


Comparison of excess

to model predictions


Acceptance filtering of theoretical prediction

all pT

Output: spectral shape much distorted relative to input, but somehow reminiscent of the spectral function underlying the input; by chance?

Input (example):

thermal radiation based on RW spectral function

functionspectralTMMfdMdN )/exp()(/


Output:

white spectrum !

Understanding the spectral shape at the output

By pure chance, for all pT and the slope of the pT spectra of the direct radiation, the NA60 acceptance roughly compensates for the phase-space factors and directly “measures” the <spectral function>

Input:

thermal radiation based on white spectral function

all pT functionspectralTMMfdMdN )/exp()(/


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


Comparison of data to RW, BR and Vacuum

pT dependence same conclusions


New theoretical developments since QM05

Brown and Rho, comments on BR scaling, nucl-th/0509001Brown and Rho, formal aspects of BR scaling, nucl-th/0509002

Rapp and van Hees, parameter variations for 2, unpublished Rapp and van Hees, 4, 6… processes , hep-ph/0603084 Rapp and van Hees, 4, 6… processes , hep-ph/0604269

Renk and Ruppert, finite T broadening, Phys. Rev. C71 (2005) Renk and Ruppert, finite T broadening and NA60, hep-ph/0603110 Renk, Ruppert, Müller, BR scaling and QCD Sum Rules, hep-ph/0509134 Renk, Ruppert, Müller, theoretical thoughts on NA60, unpublished Skokov and Toneev, BR scaling and NA60, Phys. Rev. C73 (2006) Dusling and Zahed, Chiral virial approach and NA60, nucl-th/0604071 Bratkovskaya and Cassing, HSD and NA60, in progress


Brown/Rho scaling ?


Modification of DM by

• change of the fireball parameters

Dropping Mass (DM) vs Rapp/Wambach

Results of Rapp (2/2006):(now in absolute terms and propagated through the NA60 acceptance filter)

even switching out all temperature effects does not lead to agreement between DM and the data

))/(1)(1( 2

0

0*cTTCmm


Dropping Mass (DM) vs Rapp/Wambach

Still same conclusions in all pT windows

(χ2 at low pT !)

loss of sensitivity at low pT because of acceptance cut


Chiral Virial Approach Dusling/Zahed

First attempt to describe the centrality dependence of the excess data.

Reasonable description, but increasing overestimate of central peak


Shape analysis of

excess mass spectra

Is there still more in the data to help understanding the origin of the broadening?


Excess mass spectra in 12 centrality windows


Shape vs. centrality

3/2(L+U) “continuum”

R=C-1/2(L+U) “peak”

RR peak/continuum

nontrivial changes of all three variables at dNch/dy>100 ?


RMS of total excess

Consistency with shape analysis

Further rise starting at dNch/dy =100 significant!

(bad fit (2=3) for linear rise above dNch/dy=30)


Conclusions (I) : data

• pion annihilation seems to be a major contribution to the lepton pair excess in heavy-ion collisions at SPS energies

• no significant mass shift of the intermediate

• only broadening of the intermediate


Conclusions (II) : interpretation

• all models predicting strong mass shifts of the intermediate including Brown/Rho scaling, are not confirmed by the data

• models predicting strong broadening roughly verified; not completely clear whether broadening due to T or baryon density

• theoretical investigation on an explicit connection between broadening and the chiral condensate clearly required


http://cern.ch/na60

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~ 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

first measurement of the spectral function in high-energy nuclear collisions

Documents

hot quarks

high pt data

central data comparison

ptcomparison of data

cocktail definition

seennet data sample

masspt plane final data

cocktail fits hw