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
S. Damjanovic, Hot Quarks 2006 2
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
S. Damjanovic, Hot Quarks 2006 3
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
S. Damjanovic, Hot Quarks 2006 4
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
S. Damjanovic, Hot Quarks 2006 5
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
S. Damjanovic, Hot Quarks 2006 6
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
S. Damjanovic, Hot Quarks 2006 7
Results
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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”)
S. Damjanovic, Hot Quarks 2006 9
Comparison of hadron decay cocktail to data
all pT
Very good fit quality
log
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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
S. Damjanovic, Hot Quarks 2006 11
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
S. Damjanovic, Hot Quarks 2006 12
Isolation of an excess in the more central data
S. Damjanovic, Hot Quarks 2006 13
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
S. Damjanovic, Hot Quarks 2006 14
Conservative approach
Use particle yields so as to set a lower limit to a possible excess
S. Damjanovic, Hot Quarks 2006 15
● 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?
S. Damjanovic, Hot Quarks 2006 16
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.
S. Damjanovic, Hot Quarks 2006 17
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
S. Damjanovic, Hot Quarks 2006 18
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
S. Damjanovic, Hot Quarks 2006 19
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
S. Damjanovic, Hot Quarks 2006 20
Comparison of excess
to model predictions
S. Damjanovic, Hot Quarks 2006 21
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()(/
S. Damjanovic, Hot Quarks 2006 22
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()(/
S. Damjanovic, Hot Quarks 2006 23
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
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Comparison of data to RW, BR and Vacuum
pT dependence same conclusions
S. Damjanovic, Hot Quarks 2006 25
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
S. Damjanovic, Hot Quarks 2006 26
Brown/Rho scaling ?
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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
S. Damjanovic, Hot Quarks 2006 28
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
S. Damjanovic, Hot Quarks 2006 29
Chiral Virial Approach Dusling/Zahed
First attempt to describe the centrality dependence of the excess data.
Reasonable description, but increasing overestimate of central peak
S. Damjanovic, Hot Quarks 2006 30
Shape analysis of
excess mass spectra
Is there still more in the data to help understanding the origin of the broadening?
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Excess mass spectra in 12 centrality windows
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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 ?
S. Damjanovic, Hot Quarks 2006 33
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)
S. Damjanovic, Hot Quarks 2006 34
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
S. Damjanovic, Hot Quarks 2006 35
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
S. Damjanovic, Hot Quarks 2006 36
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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