Lepton-pair production in nuclear collisions – past, present, future
Hans J. Specht
Ruprecht-Karls-Universität Heidelberg
Prague, October 26, 2007
H. J. Specht, Prague October 26 2007 2
Past
QM Bielefeld 1982 (1st generation exp. SPS)
proton-proton in the 1970s
2nd generation experiments SPSNA45/CERESNA38/HELIOS 3NA50
H. J. Specht, Prague October 26 2007 3
Proton-proton collisions in the 1970s
d/d
M (n
b/G
eV)
M (GeV)
Summary of lepton pair data in the low-mass region (LMR) (H.J.S., QM Helsinki 1984)
Unsuitable data, but milestones in theoretical interpretation !
Lepton pair data from FNAL in intermediate-mass region (IMR)
(Branson et al., PRL 1977)
E.Shuryak, Phys.Lett.B ‘79thermal radiation from ‘Quark-gluon plasma’
Bjorken/Weisberg, Phys.Rev.D ‘76 dileptons from partons produced in collision > than Drell-Yan (10-100)
‘anomalous pairs’
Ti=500 MeV
J
H. J. Specht, Prague October 26 2007 4
‘First’ Quark Matter Conference (1982)
First systematic discussion, between particle and nuclear physicists, on the theoretical end experimental aspects of QGP formation in ultra-relativistic nucleus-nucleus collisions
Milestones
Basic physics ideas on all observables, including lepton pairs in all mass regions (but not yet J/, jets, CGC,…)
Basic instrumental ideas on the 1st generation experiments at the CERN SPS
H. J. Specht, Prague October 26 2007 5
Motivation
H. J. Specht, Prague October 26 2007 6
ℓ +
ℓ -
γ
*
Lepton Pairs: basic motivation
dileptons more rigorous and more rich than photons
lowest order rate ~ ems
lowest order rate ~ em
1 variable: pT
2 variables: M, pT
production sources for thermal radiation
LMR: M<1 GeV
IMR: M>1 GeV
hadronic: → * → ℓℓprime probe of chiral symmetry restoration (R. Pisarski, PLB ‘82)
hadronic: ???partonic: qq → ℓℓnaïve expectation 1982: prime probe of deconfinement (Kajantie, McLerran, al. ’82 ff)
H. J. Specht, Prague October 26 2007 7
The as a probe for chiral symmetry restoration (Pisarski, 1982)
Principal difficulty :
properties of in hot anddensematter unknown (related to the mechanism of mass generation)
properties of hot and dense medium unknown (general goal of studying nuclear collisions)
coupled problem of two unknowns: need to learn on both
H. J. Specht, Prague October 26 2007 8
Origin of the masses of light hadrons?
Expectation
Mh~10-20 MeV approximate chiral SU(nf)L× SU(nf)R symmetry chiral doublets, degenerate in mass
Observed
MN~1 GeV spontaneous chiral symmetry breaking <qq> ≠ 0 M ~ 0.77 GeV ≠ Ma1 ~ 1.2 GeV
General question of QCD
H. J. Specht, Prague October 26 2007 9
‹qq›-
1.0 T/Tc
mL
L
1.0 T/Tc
Many different theoretical approaches including Lattice QCD still very much under development
Lattice QCD
(for B=0 andquenched approx.)
two phase transitions at the same critical temperature Tc deconfinement chiral symmetry
transition restoration
hadron spectral functions on the lattice only now under study
explicit connection between spectral properties of hadrons (masses,widths) and the value of the chiral condensate <qq> ?
H. J. Specht, Prague October 26 2007 10
measure lepton pairs (e+e- or μ+μ-)
no final state interactions; continuous emission during the whole space-time evolution of the collision system dominant component at low invariant masses: thermal radiation, mediated by the vector mesons ,(,)
tot [MeV] (770) 150 (1.3fm/c)
8.6 (23fm/c)
4.3 (46fm/c)
in-medium radiation dominated by the :1. life time τ =1.3 fm/c << τcollision > 10 fm/c2. continuous “regeneration” by
Principal experimental approach
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Low-mass dileptons + chiral symmetry
• How is the degeneration of chiral partners realized ?• In nuclear collisions, measure vector+-, but axial vector?
ALEPH data: VacuumAt Tc: Chiral Restoration
M2n [GeV]2
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In-medium changes of the properties (relative to vacuum)
Selected theoretical references
mass of width of Pisarski 1982 Leutwyler et al 1990 (,N)
Brown/Rho 1991 ff
Hatsuda/Lee 1992
Dominguez et. al1993
Pisarski 1995
Rapp 1996 ff
very confusing, experimental data crucial
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2nd generation experiments SPS
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Measuring electron pairs in CERES/NA45: concept
Pioneering experiment, built 1989-1992; data production 1993-1996
TPC (not shown),added 1998/99; data production 1999-2000
Original set-up (S-Au): puristic hadron-blind tracking with 2 RICH detectorsLater addition (Pb-Au): 2 SiDC detectors + pad (multi-wire) chamber
low field (air coils), limited tracking → limited resolution slow detectors, no trigger → very limited statistics
H. J. Specht, Prague October 26 2007 15
CERES/NA45 at the CERN SPS: results for S-Au Phys.Rev.Lett.75 (1995)
strong excess of dileptons above meson decays enormous boost to theory ( ~ 400 citations) surviving interpretation: → → e+e-, but in-medium effects required lasting ambivalence (10 a): mass shift (BR) vs. broadening (RW) of
Brown/Rho
Rapp/Wambach
Vacuum
First clear sign of
new physics
inLMR
H. J. Specht, Prague October 26 2007 16
Rapp-WambachBrown/RhoKaempfer
2000 data (TPC)
resolution and statistical accuracy remained insufficient to unambiguously determine the in-medium spectral properties of the
CERES/NA45 at the CERN SPS: results for Pb-Au PLB ’98; NPA ’99, EPJC ‘05 NPA ’06 (QM05); tbp
H. J. Specht, Prague October 26 2007 17
NA34-3, QM95, EPJC ’98 and ‘00
Other SPS results: HELIOS / NA34-3 and NA50
Rapp/Shuryak PLB 2000
NA50, EPJC ’00, NPA QM01
Excess dileptons described as a1(4) → +- via chiral (V-A) mixing
S-W p-W
LMR IMR
Excess dileptons also described as thermal radiation from dominantly hadronic processes
Li/Gale, PRL 1998
First clear sign of
new physics
inIMR
Enhanced open charm as origin of the excess only ruled out by NA60 in ‘05
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Vector- Axialvector (V-A) Mixing
interaction with real ’s (Goldstone bosons) use only 4 and higher parts of the correlator V in addition to 2
)0,(),(
21)1( 00*
cAVV T
T
Use 4, 6 … and 3, 5… (+1) processes from ALEPH data, mix them, time-reverse them and get +- yields
H. J. Specht, Prague October 26 2007 19
Present
3rd generation experiments SPS
NA60
1st generation experiments RHIC
PHENIX
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NA60
H. J. Specht, Prague October 26 2007 21
Improved dimuon mass resolution Distinguish prompt from decay dimuons
Track matching in coordinate and momentum space
2.5 T dipole magnet
hadron absorber
targets
beam tracker
vertex trackermuon trigger and tracking (NA50)
magnetic field
Measuring dimuons in NA60: concept
>10m<1m
Radiation-hard silicon pixel detectors (LHC development) High luminosity of dimuon experiments maintained
H. J. Specht, Prague October 26 2007 22
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: full statistics
Event sample: Indium-Indium
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Main steps of the data analysis
reconstruction of the event vertex within the segmented target
Beam Trackersensors
windows
The interaction vertex is identified with better than 10-20 m accuracy in the transverse plane and 200 m along the beam axis.
(note the log scale)
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Main steps of the data analysis
matching of tracks from muon spectrometer and silicon vertex telescope
matching done using the weighted distance (2) in slopes and inverse momenta.
a certain fraction of muons is matched to closest non-muon tracks (fakes); only events with2 < 3 are selected.
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Main steps of the data analysis
assessment of combinatorial background (CB) by event mixing
CB mostly from ,K→v decays
agreement of data and mixed CB over several orders of magnitude accuracy of agreement ~1%
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Main steps of the data analysis
assessment of fake matches by overlay MC and/or event mixing
Fake matches of the CB automatically subtracted as part of the mixed-events technique
Fake matches of the signal pairs (<10% of CB) are obtained in two different ways: Overlay MC or Event Mixing; agreement to within 5%
H. J. Specht, Prague October 26 2007 27
Low-mass data sample for 158 AGeV In-In
For the first time, and peaks clearly visible in dilepton channel ; even μμ seen
Net sample: 440 000 events
Mass resolution:20 MeV at the position
Progress over CERESstatistics: factor >1000resolution: factor 2-5
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Track multiplicity from VT tracks for triggered dimuons for
Centrality bin multiplicity ⟨dNch/dη⟩3.8
Peripheral 4-30 17
Semi-Peripheral
30-110 70
Semi-Central 110-170 140
Central 170-240 190
Associated track multiplicity distribution
4 multiplicity windows:
opposite-sign pairs combinatorial background signal pairs
some part of the analysis also in 12 multiplicity windows
H. J. Specht, Prague October 26 2007 29
Understanding the peripheral data
well described by meson decay cocktail:
same analysis pT differential:
2-body decays: , Dalitz decays: , open charm: DD
particle ratios and , extrapolated to full pT and full rapidity space (using particle generator GENESIS), found to be independent of pT
acceptance of NA60 understood to within <10%
H. J. Specht, Prague October 26 2007 30
Particle ratios from the cocktail fits
and nearly independent of pT , y, cosCS
acceptance of NA60 understood to within <10% over complete M, pT, y, cosCS range
H. J. Specht, Prague October 26 2007 31
Mass spectra of excess dimuons
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Excess dimuonsPhys. Rev. Lett. 96 (2006) 162302
accuracy 2-3%, but results robust to mistakes even at the 10% level
isolation of excess by subtraction of measured decay cocktail (without ), based solely on local criteria for the major sources and
and : fix yields such as to get, after subtraction, a smooth underlying continuum
fix yield at pT >1 GeV profiting from the very high sensitivity of the spectral shape of the Dalitz decay to any underlying admixture from other sources; lower limit from peripheral data
H. J. Specht, Prague October 26 2007 33
Excess mass spectra in 12 centrality windows
clear excess above the cocktail (bound to the with =1.2)
excess centered at the nominal pole rising with centrality
similar behaviour pT - differentially
no cocktail and no DD subtracted
Eur.Phys.J.C 49 (2007) 235
all pT
H. J. Specht, Prague October 26 2007 34
Centrality dependence of spectral shape
peak: R=C-1/2(L+U) continuum: 3/2(L+U)
nontrivial changes of all variables at dNch/dy>100 (onset of anomalous J/ suppression)
“melting” of the
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The spectral function
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*(q)
(T,B) μ+
μ-
Dilepton Rate in a strongly interacting medium
dileptons produced by annihilation of thermally excited particles:
+- in hadronic phase qq in QGP phase
photon selfenergy
at SPS energies + - →*→μ+μ- dominant
Vector-Dominance Model
hadron basis
spectral function
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Physics objective
Goal: Study properties of the rho spectral function Im D
in a hot and dense medium
Procedure:Spectral function accessible through rate equation, integrated over space-time and momenta
Limitation:Continuously varying values of temperature T and baryon density B, (some control via multiplicity dependences)
functionspectralTMMfdMdN )/exp()(/
H. J. Specht, Prague October 26 2007 38
spectral function in vacuum
vacuum spectral function
2
21 g)(gintL
Introduce as gauge boson into free + Lagrangian
1)0(2)0(2)0( )]()([)( MmMMD
is dressed with free pions
(like ALEPH data V(→ 2
H. J. Specht, Prague October 26 2007 39
spectral function in hot and dense hadronic matter (I)
Dropping mass scenario Brown/Rho et al., Hatsuda/Lee
universal scaling law
))/(1)(1( 2
0
2/10
2/1, cT TTCqqqq
2/10
2/1,
0* / qqqqmm T
explicit connection between hadron masses and chiral condensate
continuous evolution of pole mass with T and broadening atfixedignored
H. J. Specht, Prague October 26 2007 40
spectral function in hot and dense hadronic matter (II)
Hadronic many-body approach Rapp/Wambach et al., Weise et al.
D(M,q;B,T)=[M2-m2--B-M ]-1
B /0 0 0.1 0.7 2.6
hot and baryon-rich matter hot matter
is dressed with: hot pions baryons(N,..) mesons (K,a1..)
“melts” in hot and dense matter
- pole position roughly unchanged - broadening mostly through baryon interactions
H. J. Specht, Prague October 26 2007 41
Final mass spectrum
),;,()( 440
3
0
i
therm
FB
therm
TqMqxdd
dNqqMdVd
dMdN fo
integration of rate equation over space-time and momenta required
continuous emission of thermal radiation during life time of expanding fireball
example: broadening scenario
B /0 0 0.1 0.7 2.6
H. J. Specht, Prague October 26 2007 42
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
correct data for acceptance in 3-dim. space M-pT-y and compare directly to predictions at the input
dydMdpNd
T2
*3
conclusions as to agreement or disagreement of data and predictions are independent of whether comparison is done at input or output
H. J. Specht, Prague October 26 2007 43
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
H. J. Specht, Prague October 26 2007 44
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:
H. J. Specht, Prague October 26 2007 45
Predictions by Rapp (2003) for all scenarios
Comparison of data to RW, BR and Vacuum
Data and predictions as shown, after acceptance filtering, roughly mirror the spectral function, averaged over space-time and momenta.(Eur.Phys.J.C 49 (2007) 235)
Theoretical yields normalized to data for M<0.9 GeV
Only broadening of (RW) observed, no mass shift (BR)
H. J. Specht, Prague October 26 2007 46
Modification of BR by change 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
van Hees and Rapp, hep-ph/0604269
modeling now in absolute terms (without freeze-out )
H. J. Specht, Prague October 26 2007 47
Renk/Ruppert, hep-ph/0702012Hees/Rapp Phys.Rev.Lett. (2006)
Comparison to more recent theoretical developments
Dusling/Zahed
Dusling/Zahed Phys.Rev.C (2007)
attempts to make mass shifts compatible with the data so far failed
all these results favour broadening without mass shift
H. J. Specht, Prague October 26 2007 48
Renk/Ruppert, hep-ph/0702012
Mass region above 1 GeV described in terms of hadronic processes, 4 …
Hees/Rapp Phys.Rev.Lett. (2006)
Hadron-Parton Duality for M >1 GeV
Mass region above 1 GeV describedin terms of partonic processes, qq…
How to distinguish?
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Transverse momentum spectra
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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
● assume uniform cosCS distributions (Collins-Soper frame) throughout, as measured
use slices of m = 0.1 GeV and pT = 0.2 GeV
keep m =0.1 GeV or resum to extended mass windows
subtract charm from the data (based on NA60 IMR results) before acceptance correction
Strategy of acceptance correction
H. J. Specht, Prague October 26 2007 51
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 NA60
NA60 (=1.5)
H. J. Specht, Prague October 26 2007 52
Experimental results on cosCS distributions: and
errors purely statistical
pprojectile ptarget
z axisCS
pµ+
y
x
Viewed fromrest frame
θcos α1cosθ ddσ 2
Collins Soper frame
integration over azimuth angle
First measurement of angular distributions for and in nuclear collisions
Polarization consistent with zero(for found before to be 0 in C reactions, 1977 )
H. J. Specht, Prague October 26 2007 53
For the first time, the polarization of thermal radiation is measured and found to be zero (different form DY), as anticipated since decades. This is also used in all theoretical generators.
Experimental results on cosCS distributions: excess
pprojectile ptarget
z axisCS
pµ+
y
x
Viewed from dimuonrest frame
θcos α1cosθ ddσ 2
Collins Soper frame
integration over azimuth angle
errors purely statistical
H. J. Specht, Prague October 26 2007 54
Excess pT spectra for three centrality bins
hardly any centrality dependence, but significant mass dependence
sum over centralities
(spectra arbitrarily normalized)
H. J. Specht, Prague October 26 2007 55
Centrality-integrated excess mT spectra
effTTT
TmdmdN
mexp~1
steepening at low mT; not observed for hadrons (like
fit mT spectra for pT>0.4 GeV with
monotonic flattening of spectra with mass up to M=1 GeV, followed by a steepening above
Signs for mass-dependent radial flow?
transverse mass: mT = (pT2 + M2)1/2
Phys. Rev. Lett. (2007)
H. J. Specht, Prague October 26 2007 56
(analysis done by Ruben Shahoyan)
measurement of muon offsets :distance between interaction vertex and track impact point
charm not enhanced; excess prompt; 2.4 × DY
excess similar to open charmsteeper than Drell-Yan
excess mass spectrum (characteristics simillar to PbPb NA50)
Extension to intermediate mass region
H. J. Specht, Prague October 26 2007 57
Transverse mass distributions of thermal dimuons
agreement within the mass region of overlap (1<M<1.4 GeV)
IMRLMR
extract T eff from fits to m T spectra throughout
H. J. Specht, Prague October 26 2007 58
What can we learn from pT spectra?
Radial Flow
Origin of dileptons
H. J. Specht, Prague October 26 2007 59
Freeze-OutQGPA+A Hadron GasNN-coll.
Time evolution of a nucleus-nucleus collision
radial expansion under pressure
expansion velocity vT
evolution of vT : small in QGP phase (short),building up in hadron phase (long)
H. J. Specht, Prague October 26 2007 60
Radial flow and pT spectra
Teffm
TT
TedmmdN /
thermalization due to interactions collective (flow) velocity vT same for all particles
two components in pT spectra: thermal and flow pT = pT
th + M vT
2Tfeff vmTT
hadron pT spectra: determined at freeze-out mass ordering
m T-M(GeV)
2-parameter fits Tf, vT 1-parameter fits Teff
NA49
H. J. Specht, Prague October 26 2007 61
Dilepton transverse momentum spectra
three contributions to pT spectra
note: final-state lepton pairs themselves only weakly coupled
→ handle on emission region, i.e. nature of emitting source
T - dependence of thermal distribution of “mother” hadrons/partons M - dependent radial flow (v) of “mother” hadrons/partons pT - dependence of spectral function, weak (dispersion relation)
dilepton pT spectra superposition from all fireball stages
early emission: high T, low vT
late emission: low T, high vT
final spectra from space-time folding over T-vT history from Ti → Tf
(including low-flow partonic phase)
hadron pT spectra determined at freeze-out Tf (restricted information)
H. J. Specht, Prague October 26 2007 62
Evolution of inverse slope parameter Teff with mass
Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV
Rise reminiscent of radial flow of a hadronic source
But:thermal dimuons emitted continuously during fireball expansion (reduced flow), while hadrons are emitted at final freeze-out (maximal flow);how can Teff be similar?
Systematic errors studied in great detailed (CB, cocktail subtraction, acceptance, y and cosCS distributions, subtraction of DY and open charm on level <= statistical errors.
H. J. Specht, Prague October 26 2007 63
Disentangling the mT spectra of the peak and the continuum
H. J. Specht, Prague October 26 2007 64
peak: C-1/2(L+U) continuum: 3/2(L+U)
Shape analysis and pT spectra
mT spectra very different for the peak and continuum: Teff of peak higher by 70 MeV than that of the continuum !
use same side-window subtractionmethod as discussed before
identify the peak with the freeze-out in the dilute final stage,when it does not experience further in-medium influences.
H. J. Specht, Prague October 26 2007 65
Understanding the difference in Teff between peak and continuum
continuum emission (very schematic)
post freeze-out emission
softened by 1/unsoftened average over Tth,vT, fixed at T=Tf with max vT
detailed balance between formation and decay N fixed
1BN
dtdN
free exponential decay of N
/)(1 ftteBN
dtdN decay rate:
time integral:
cf tt
BNN
BNN
pT spectra:
softening and averaging contribute about equally to the 70 MeV
H. J. Specht, Prague October 26 2007 66
for a given hadron M, the measured T eff defines a line in the T fo-v T plane
crossing of hadrons with defines T f, v T max reached at respective hadron freeze-out
Hierarchy in hadron freeze-out
different hadrons have different coupling to pions ( maximal) clear hierarchy of freeze-out (also for light-flavored hadrons)
use of Blast wave code
large difference between and (same mass)
H. J. Specht, Prague October 26 2007 67
Comparison of hadron hierarchy and thermal dimuons
Teff of thermal dimuons well below the hadron line defined by the
Teff of continuum corrected for peak contribution → pure in-medium radiation
the freeze-out earlier than
Phys. Rev. Lett. (2007)
H. J. Specht, Prague October 26 2007 68
The rise and fall of radial flow of thermal dimuons Strong rise of Teff with dimuon mass even persists for the pure in-medium part. Sudden drop for M>1 GeV now even more sharply defined
Rise consistent with radial flow of a hadronic source (here →→)
Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→)
Combining M and p T of thermal dileptons seems to break hadron-parton duality
H. J. Specht, Prague October 26 2007 69
mT spectra for very peripheral (“pp-like”) collisions
Asymptotic limit reached only in very peripheral collisions: no steepening of spectra at low m T
no difference in T eff between and (same mass) no yield enhancement of no broadening of (rms)
No influence of processes left
T eff=198+-6 MeVT eff=201+-4 MeV
H. J. Specht, Prague October 26 2007 70
Acceptance-corrected mass spectra
H. J. Specht, Prague October 26 2007 71
Data in comparison to theory : 0<pT<0.8 GeV
One common absolute normalization factor, fixed for 0.6<M<0.9 GeV 0<pT <2.4 GeV
Differences at low mass mostly reflect differences in the low-mass tail of the spectral functions
H. J. Specht, Prague October 26 2007 72
Data in comparison to theory : 1.6<pT<2.4 GeV
One common absolute normalization factor fixed for 0.6<M<0.9 GeV 0<pT <2.4 GeV
Differences at higher masses and higher pT mostly reflect differences in the flow
H. J. Specht, Prague October 26 2007 73
Final steps of the data analysis
● Acceptance corrected mass/pT spectra:
isolation of the spectral functions in slices of pT, by unfolding of the phase space factors and radial flow
in-medium dispersion relation of the
moments of the spectral function (Weise, 2007)
● Elliptic flow of thermal dileptons ?
(all results including final steps unique, never measured before)
● Polarization of vector mesons and thermal radiation in
● Electromagnetic Transition Form Factors for -Dalitz
● In-medium effects for the and the
H. J. Specht, Prague October 26 2007 74
PHENIX
H. J. Specht, Prague October 26 2007 75
Measuring dielectrons in PHENIX: first results
start-up with insufficient rejection tools → S/B ~1000
electron-pair measurements notoriously difficult due to combinatorial background from unrecognized Dalitz and conversion pairs
nevertheless, encouraging first results, but systematic uncertainties presently too large to draw any conclusions
Next-generation experiment with proper rejection mandatory
subm. to PRL, nucl-ex/0706.3034
H. J. Specht, Prague October 26 2007 76
Measuring dielectrons in PHENIX: first results
Next-generation experiment with proper rejection mandatory
IMR LMR
drop (or constant?) of yield/Ncoll
nonlinear rise of the yield/Npart for LMR
subm. to PRL, nucl-ex/0706.3034
H. J. Specht, Prague October 26 2007 77
Hadron Blind Detector (HBD) in inner field-free region (double coils with field compensation inside, P.G. & H.J.S (1987))
tracking in outer PHENIX detectors (not shown) with matching to HBD
analysis strategy also similar: cuts in single- electron pT, pair opening angle, etc.
Measuring dielectrons in PHENIX: upgrade conceptmutation of CERES + tracking into a collider detector…
H. J. Specht, Prague October 26 2007 78
Future
NA60 ???
2nd generation experiments RHICPHENIX
1st generation experiments LHCALICE, CMS, ATLAS
3th generation experiments SPS
1th generation experiments FAIRCBM
H. J. Specht, Prague October 26 2007 79
Conclusions
Finally, after 25 years:
Spectral function of in-medium identified; only broadening, no mass shift
Thermal radiation from partons identified;use of radial flow of thermal dileptons as diagnostic tool
The field takes a very long breath…..
H. J. Specht, Prague October 26 2007 80
Explicit connection between broadening and the chiral condensate
Quantitative description of fireball dynamics;present models ad hoc, hydrodynamics far off
Wish list to theory