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NA60 results on the spectral function in Indium-Indium collisions. Sanja Damjanovic NA60 Collaboration. Asilomar, 14 June 2006. Outline. Isolation of excess dimuons above hadron decays Mass spectra (published in PRL) Shape analysis of mass spectra (new) - PowerPoint PPT Presentation
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NA60 results on the spectral function in Indium-Indium collisions
Sanja Damjanovic NA60 Collaboration
Asilomar, 14 June 2006
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Outline
Isolation of excess dimuons above hadron decays
Mass spectra (published in PRL)
Shape analysis of mass spectra (new)
Comparison to theory
Acceptance-corrected pT spectra (new)
Comparison to theory
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
4
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 combinatorial background by event mixing
assessment of fake matches by overlay MC and/or event mixing
talk of Andre David
5
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
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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
new: some part of the analysis also in 12 multiplicity windows
7
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”)
8
Comparison of hadron decay cocktail to data
all pT
Very good fit quality
log
9
Full-phase-space 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
10
Isolation of an excess in the more central data
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Goal Find excess above hadron decays without fits
Conservative approach Use particle yields so as to set a lower limit to a possible excess
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● 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?
13
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.
14
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
Phys.Rev.Lett. 96 (2006) 162302
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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
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Shape analysis of
excess mass spectra
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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 ?
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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)
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Comparison of data
to model predictions
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Two alternatives how to compare data to predictions
use predictions in the form
decay the virtual photons * into +- pairs, propagate these through the NA60 acceptance filter and compare results to uncorrected data at the output (presently done for mass spectra in selected pT regions)
correct data for acceptance in 3-dim. space M-pT-y and compare directly to predictions at the input (presently done for pT spectra in selected mass regions)
dydMdp
Nd
T2
*3
conclusions as to agreement or disagreement of data and predictions are independent of whether comparison is done at input or output
22
output:
white spectrum !
understanding the spectral shape at the output
By pure chance, for the M-pT characteristics of direct radiation, without pT selection,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()(/
Acceptance filtering of theoretical prediction:
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Predictions for In-In by Rapp et al. (2003) for ⟨dNch/d⟩ = 140, covering all scenarios
Theoretical yields 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
Data and predictions as shown, after acceptance filtering, roughly mirror the respective spectral functions, averaged over space-time and momenta.
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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, hep-ph/0604269Rapp and van Hees, 4, 6… processes, hep-ph/0603084
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 and Ruppert, What the NA60 dilepton data can tell, hep-ph/0605330
Skokov and Toneev, BR scaling and NA60, Phys. Rev. C73 (2006) Dusling and Zahed, Chiral virial approach and NA60, nucl-th/0604071
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Modification of BR bychange of the fireball parameters
Parameter variations for Brown/Rho scaling
even switching out all temperature effects does not lead to agreement between BR and the data
))/(1)(1( 2
0
0*cTTCmm
Van Hees and Rapp, hep-ph/0604269
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Comparison of data to RH(2+4+QGP)
Vector-Axialvector Mixing: interaction with real ’s (Goldstone bosons). Use only 4 and higher parts of the correlator V in addition to 2
)0,(
),(
2
1)1( 00*
cAVV T
T
Use 4, 6 … and 3, 5… (+1) processes from ALEPH data, mix them, time-reverse them and get +- yields
van Hees and Rapp, hep-ph/0603084
27
Comparison of data to RH(2+4+QGP)
In this model, the yield above 0.9 GeV is sensitive to the degree of vector-axialvector mixing and therefore to chiral symmetry restoration!
whole spectrum reasonably well described, now even in absolute terms
direct connection to IMR results >1 GeV from NA60
Van Hees and Rapp, hep-ph/0603084
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pT- dependences
Comparison of data to RW(2+4+QGP)
theoretical results plotted in absolute terms
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Comparison of data to RR
Spectral function only based onhot pions (Dyson-Schwinger) , no baryon interactions included
Continuum contribution from partons, dominating the region >1GeV
broadening described, except for low-mass tail
Theoretical results obtained in absolute terms
talk of Joerg RuppertRenk and Ruppert, hep-ph/0603110
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Comparison of data to RR
pT dependences
talk of Joerg Ruppert
theoretical results plotted in absolute terms
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Acceptance-corrected excess pT spectra
Preliminary
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reduce 3-dimensional acceptance correction in M-pT-y to 2-dimensional correction in M-pT, using measured y distribution as an input
use slices of m = 0.1 GeV and pT = 0.2 GeV
resum to three extended mass windows
0.4<M<0.6 GeV 0.6<M<0.9 GeV 1.0<M<1.4 GeV
Present strategy of acceptance correction
subtract charm from the data (based on NA60 IMR results)before acceptance correction
33
Experimental results on the y distribution of the excess
use measured mass and pT spectrum as input to the acceptance correction in y (iteration procedure)
agreement betweenthe three pT bins
results close torapidity distribution of pions (from NA49) for the same √s, as expected (RR)
34
Excess pT spectra for three centrality bins
hardly any centrality dependence
significant mass dependence
(spectra arbitrarily normalized)
35
Centrality-integrated excess pT spectra
significant mass dependence (also vs. mT, see below )
possible origin:
different physics sources
radial flow
p-dependence of in-medium spectral function
(arbitrarily normalized at pT=1GeV)
36 at high pT, rho like region hardest, high-mass region softest !
Illustration of mass dependence of pT spectra
differential fits to pT
spectra, assuming locally 1-parameter mT scaling and using gliding windows of pT=0.8 GeV local slope Teff
very low Teff at low pT, enormous dynamic range, hardly compatible with flow alone (for only one component) systematic errors <15 MeV
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Systematics of low-pT data: acceptance
pT spectrum of at low pT much flatter (higher Teff)
acceptance of inbetweenthat of the twomass windows
enhanced yield at low pT not due to incorrect acceptance
38
Systematics of low-pT data: combinatorial background
enhanced yield at low-pT seen at all centralities, including the peripheral bin
estimate of errors at low pT, due to subtraction of combinatorial background: peripheral 1%semiperipheral 10% semicentral 20%central 25%
enhanced yield at low pT not due to incorrect subtraction of combinatorial background
39
Comparison of data
to model predictions
40
Comparison to theory: mass window 0.6<M<0.9 GeV
1-parameter differential mT fits
at low pT: better description by RH at higher pT much better description by RR (freeze-out ρ incl.)
(arbitrarily normalized at pT=1 GeV)
in detail, data different from any theoretical prediction
41
Comparison to theory: mass window 1.0<M<1.4 GeV
1-parameter differential mT fits
(arbitrarily normalized at pT=1 GeV)
RH: dominantly hadronic processes (4); lower T; role of flow? RR: dominantly partonic processes (qq); high T; low flow
comparison to data inconclusive
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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
• strong mass dependence of pT spectra
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models predicting strong broadening roughly verified
Conclusions (II) : interpretation
all models predicting strong mass shifts of the intermediate including Brown/Rho scaling, are not confirmed by the data
theoretical investigation on an explicit connection between broadening and the chiral condensate clearly required
pT spectra not yet fully described by any theory, rich in detail, promising handle on emission sources and fireball dynamics
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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
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BACKUPBACKUP
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Peripheral DataPeripheral Data
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enhanced yield at low-pT seen at all centralities, including the peripheral bin
Excess pT spectra for four centrality bins
peripheral bin at high-pT slightly softer
49
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
50
Comparison of hadron decay cocktail to data
Again good agreement
between cocktail and data
0.5 < pT < 1 GeV
pT > 1 GeV
51
Pt spectraPt spectra
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very low Teff at low pT, enormous dynamic range, hardly compatible with flow alone (for only one component)
at high pT, rho like region hardest, high-mass region softest !
Illustration of mass dependence of pT spectra
integral fits to pT
spectra, assuming 1-parameter mT scaling and varying the upper cut in pT (bad χ2 for full pT range)
53
Comparison to theory: mass window 0.4<M<0.6 GeV
1-parameter differential mT fits
(arbitrarily normalized at pT=0.7 GeV)
at low pT: better description by RH/DZ at higher pT much better description by RR
in detail, data different from any theoretical prediction
54
Teff from differential fits to pT spectra
55
Shape analysis
56
RMS of continuum
If observed peak subtracted, the remaining continuum described by RMS of a flat distribution, independent of dNch/dy
Subtraction of cocktailcreates dip in the middle RMS>RMSflat
57
of peak (from Gaussian fit)
Sigma of peak consistent with sigma of cocktail/vacuum independent of dNch/dy
58
Further theoretical results
59
Comparison of data to RW, BR and Vacuum
pT dependence same conclusions
60
Modification of DM by
• “fusion” of the two scenarios
Dropping Mass (DM) vs Rapp/Wambach
Results of Rapp (8/2005):(not propagated through NA60 acceptance filter)
Neither fusion nor parameter change able to make DM scenario unobservable
ncTTCmm ))/(1)(1( 2
0
0*
• change of the fireball parameters
61
*2*2 gmgm
Skokov and Toneev on BR scaling
ncTTmm ))/(1)(15.01( 2
0
0*
full dynamical model including deconfinement transition
n=0.3
n=0
standard scaling of pole mass, no broadening
same modification to the vector dominance coupling
theoretical results normalized to data
even for n=0, no agreement with the data (like Rapp)
62
Chiral Virial Approach Dusling/Zahed
First attempt to describe the centrality dependence of the excess data.
Reasonable description, but increasing overestimate of central peak
63
DZ Integrated Dimuon Rates
225
150
B
T
a1a1
64
AcceptanceAcceptance
NA60 vs CERESNA60 vs CERES
65
CERES
reduction of acceptance at low M and low pT similar
Comparison of lepton pair acceptance
CERES
66
Acceptance comparison of CERES and NA60
acceptance variation of NA60 between <RW> and <BR> factor of 3
acceptance variation between 0.4 and 0.8 GeV different by a factor of 2 between CERES and NA60
Source:Rapp/Wambach
low pT dominated
67
CERES NA60
Phase space coverage in pT-mass plane
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Comparison of NA60 to CERES
Suppression of low mass part of RW similar in CERES and in NA60
69
Phase space coverage in y-pT plane
Comparison to CERES
MC simulation with RW (low-mass and low-pt dominated):
both acceptances shifted relative to midrapidity, but difference only δy=0.3
70
Phase space coverage in y-pT plane
Examples from MC simulations
Optimal acceptance:
at high mass, high pT
<y> = 3.5
at low mass, low pT
<y> = 3.8
Shift of acceptance away from midrapidity not much different from CERES
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Experimental set-up
72
2.5 T dipole magnet
hadron absorber
• Origin of muons can be accurately determined• Improved dimuon mass resolution
Matching in coordinate
and momentum space
targets
beam tracker
vertex trackermuon trigger and tracking
magnetic field
or
!
Measuring dimuons in NA60: concept
73
BackgroundBackground
74
Analysis Topics
LMR IMR HMR
1) Low Masses:vector mesons + continuum rad. (LMR)
2) Intermediate Masses: charm + continuum rad. (IMR)
3) High Masses:J/, ’ and DY (HMR)
75
Selection of primary vertex
Beam Trackersensors
windows
The interaction vertex is identified with better than 20 m accuracy in the transverse plane and 200 m along the beam axis.
(note the log scale)
Present analysis (very conservative):
Select events with only one vertex in the target region,
i.e. eliminate all events with secondary interactions
76
A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with 2 < 3 are selected.
Fake matches are subtracted by a mixed-events technique (CB) and an overlay MC method (only for signal pairs, see below)
Muon track matching
Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Telescope (VT) is done using the weighted distance (2) in slopes and inverse momenta. For each candidate a global fit through the MS and VT is performed, to improve kinematics.
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Determination of Combinatorial Background
Basic method:
Event mixing
takes account of
charge asymmetry
correlations between the two muons, induced by magnetic field sextant subdivision trigger conditions
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Combinatorial Background from ,K→ decays
Agreement of data and mixed CB over several orders of magnitude
Accuracy of agreement ~1%
79
Fake Matches Fake matches of the combinatorial background are automatically subtracted as part of the mixed-events technique for the combinatorial background
Fake matches of the signal pairs (<10% of CB) are obtained in two different ways:
Overlay MC : Superimpose MC signal dimuons onto real events. Reconstruct and flag fake matches. Choose MC input such as to reproduce the data.
Event mixing : More complicated, but less sensitive to systematics
80
Fake-match background
example from overlay MC: the fake-match contribution localized in mass (and pT) space: = 23 MeV, fake = 110 MeV; fake prob. 22%
complete fake-match mass spectrum agreement between overlay MC and event mixing, in absolute level and in shape, to within <5%
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Signal and background in 4 multiplicity windows
S/B
2 1/3
1/8 1/11
Decrease of S/B with centrality, as expected
82
Mixing events
The event mixing requires 12 “pools” of single muons Two muon charges, times Six spectrometer trigger sextants
And we can only mix single muons from The same target The same centrality class The same field polarities, running conditions, etc(~6000 event samples)
For the mixing we only use single muons from the like-sign dimuon triggers
The mixed event technique gives both shape and absolute scale
83
Combinatorial background: normalization Assume that the probability to accept the
pair ij is Pij = Pi Pj,
Pi and Pj are the single muon probabilities for sextants i and j (as if NA60 would have collected single muon triggers)
The observed number of pairs ij is Nij
Then the single muon underlying probabilities are
And the absolute probability to have a given muon is
Pi
N
146
3 52
jiij PPNN
ij
jiij
jiiii PPNPPNN )1(
PPP
ii
i 1,
Muon spectrometertrigger system sextants
84
Sensitivity of Sensitivity of
difference proceduredifference procedure
85
Sensitivity of the difference procedure
Change yields of , and by +10%:
enormous sensitivity, on the level of 1-2%, to mistakes in the particle yields.
The difference spectrum is robust to mistakes even on the 10% level, since the consequences of such mistakes are highly localized.
86
Excess spectra from difference data-cocktail
No cocktail and no DD subtracted
pT < 0.5 GeV
Clear excess above the cocktail , centered at the nominal pole and rising with centrality
Similar behaviour in the other pT bins
87
medium modification medium modification of of
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Masking of by pion annihilation
Central Bin
The chances are better at high pT but no effect is visible there!
90
Medium modification of
Flattening of the pt distributions develops very fast with centrality.
Compatible with radial flow?
Or indication of broadening in the medium??
91
results from KEK-E325results from KEK-E325
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Results from KEK-E325
ee pairs from 12 GeV pA collisions
superb mass resolution of 10 MeV
Tail below the interpreted as , shifted down in mass
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Alternative interpretation: / interference
Rob Veenhof, Winter Workshop on Nucl.Dynamics, Breckenridge 2005
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