lepton-pair production in nuclear collisions – past, present, future
DESCRIPTION
Lepton-pair production in nuclear collisions – past, present, future. Hans J. Specht Ruprecht-Karls-Universität Heidelberg. INPC07 Tokyo, Japan, June 3-8, 2007. proton-proton in the 1970s. QM Bielefeld 1982 (1 st generation exp. SPS). 2 nd generation experiments SPS. - PowerPoint PPT PresentationTRANSCRIPT
Lepton-pair production in nuclear collisions – past, present, future
Hans J. Specht
Ruprecht-Karls-Universität Heidelberg
INPC07 Tokyo, Japan, June 3-8, 2007
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 2
Past
QM Bielefeld 1982 (1st generation exp. SPS)
proton-proton in the 1970s
2nd generation experiments SPSNA45/CERESNA38/HELIOS 3NA50
Hans J. Specht; INPC Tokyo, 3-8 June, 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
Hans J. Specht; INPC Tokyo, 3-8 June, 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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 5
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)
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 6
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 7
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 8
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 9
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 10
Present
3rd generation experiments SPS
NA60
1st generation experiments RHIC
PHENIX
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 11
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 12
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 13
Excess dimuons
Phys. Rev. Lett. 96 (2006) 162302; Eur.Phys.J.C 49 (2007) 235
accuracy 2-3%, but difference spectrum robust to mistakes even at the 10% level, since the consequences are highly localized
existence of excess dimuons
Peripheral data:
well described by meson decay cocktail ()
More central data:
isolation of excess by subtraction of measured decay cocktail (without ), based solely on local criteria for the major sources and
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 14
Centrality dependence of excess mass spectrafrom / =1.2
continuum: 3/2(L+U) peak: C-1/2(L+U)
nontrivial change for dNch/dy>100 ?
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 15
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)
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 16
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 )
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 17
Dropping in the vector manifestation of chiral symmetry
presently, dropping mass in this approach not favored
Simulations by Rapp et al. (2006); thermal fireball same as before, here with absolute normalization
- dropping related to Brown Rho scaling in the vector manifestation
- modification of vector dominance
HLS (Hidden Local Symmetry):
Harada and Sasaki; hep-ph/0702205
Under preparation: broadening effects; moving a1
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 18
van Hees and Rapp, Phys.Rev.Lett. 97 (2006) 102301
In this model, low-mass tail requires baryon interactions
Comparison of data to Hees/Rapp: role of baryons
Whole spectrum reasonably well described, even in absolute terms
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 19
Ruppert Renk, hep-ph/0702012
Mass region above 1 GeV described in terms of hadronic processes, 4 …
Rapp/Hees 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?
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 20
Dilepton transverse momentum spectra: basics
Dileptons characterized by 2 variables: M, pT
Note I: M Lorentz-invariant, not changed by flowNote II: final-state lepton pairs themselves only weakly coupled
→ handle on emission region, i.e. nature of emitting source
pT: pT - dependence of spectral function (dispersion relation) T - dependence of thermal distribution of “mother” hadrons/partons M - dependent radial flow () of “mother” hadrons/partons
M: spectral functions and phase space factors
dilepton pT spectra superposition of ‘hadron-like’ spectra at fixed T
early emission: high T, low T
late emission: low T, highT final spectra from space-time folding over T- T history from Ti → Tfo
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 21
Transverse mass distributions of thermal dimuons
effTTT
TmdmdN
mexp~1
Steepening at low mT contrary to expectation for radial flow; relation to pion spectra?
Note: does not exist for
Fit mT spectra for pT>0.4 GeV with
Monotonic flattening of spectra with mass up to M=1 GeV, steepening again above !
Signs for mass-dependent radial flow?
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 22
mT spectra for very peripheral collisions
No steepening of spectra at low m T
No difference in T eff between and (same mass)
No influence of processes left
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 23
The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins
Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV
Rise consistent with radial flow of a hadronic source (here →→)
Note: Teff of (from separate analysis of peak and continuum)>Teff of dimuons, as required
Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→)
Combining M and p T of thermal dileptons breaks hadron-parton duality
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 24
Measuring dielectrons in PHENIX: first results
Start-up with insufficient rejection tools → S/B ~1000
Electron-pair measurements notoriously difficult due to combinatorial background dueto 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
HQ06, QNP06: EPJC ’07
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 25
Hadron Blind Detector (HBD) in inner field-free region (double Helmholtz coils)
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…
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 26
Future
NA60 ???
2nd generation experiments RHICPHENIX
1st generation experiments LHCALICE, CMS, ATLAS
3th generation experiments SPS
1th generation experiments FAIRCBM
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 27
Conclusions
Finally, after 25 years:
Spectral function of in-medium identified
Thermal radiation from partons identified
The field takes a very long breath…..
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 28
BKP
Fine analysis in 12 centrality bins
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 29
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 30
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 31
Predictions for In-In by Rapp et al. (11/2005) for ⟨dNch/d⟩ = 140 Comparison of data to RW(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,(),(
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
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 32
Comparison of hadron decay cocktail to data
all pT
Very good fit quality
log
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 33
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()(/
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 34
Transverse momentum distribution of thermal dimuons
hardly any centrality dependence
significant mass dependence
(spectra arbitrarily normalized)
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 35
Evolution of inverse slope parameter Teff with Mass Fine analysis in 12 centrality bins
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 continuoulsy during fireball expansion (reduced flow), while hadrons are emitted at final freeze-out (maximal flow);how can Teff be similar?
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 36
Radial flow of thermal dimuonsFine analysis in 12 centrality bins
From separate analysis of peak and continuum: Teff of peak 300+-17 MeV. Identify with freeze-out
Subtract contribution of freeze-out in each mass bin to obtain Teff of the pure in-medium part.
The other hadrons ( ) freeze out earlier than the T eff of thermal dimuons well below the hadron line defined by the
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 37
The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins
Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV
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 breaks hadron-parton duality
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 38
CERESNA60
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 39
CERESNA60
Hans J. Specht; INPC Tokyo, 3-8 June, 2007 40
Dilepton yields : well-described by superposition of leptonic decay of final state hadrons
CERES/NA45 at the CERN SPS; results on p-Be/Au