lepton-pair production in nuclear collisions – past, present, future

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


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


‘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


ℓ +

ℓ -

γ

*

qq

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)


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


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


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


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


Present

3rd generation experiments SPS

NA60

1st generation experiments RHIC

PHENIX


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


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


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


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 ?


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)


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 )


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


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


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?


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


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?


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


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


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


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…


Future

NA60 ???

2nd generation experiments RHICPHENIX

1st generation experiments LHCALICE, CMS, ATLAS

3th generation experiments SPS

1th generation experiments FAIRCBM


Conclusions

Finally, after 25 years:

Spectral function of in-medium identified

Thermal radiation from partons identified

The field takes a very long breath…..


BKP

Fine analysis in 12 centrality bins


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


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


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


Comparison of hadron decay cocktail to data

all pT

Very good fit quality

log


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()(/


Transverse momentum distribution of thermal dimuons

hardly any centrality dependence

significant mass dependence

(spectra arbitrarily normalized)


Evolution of inverse slope parameter Teff with Mass Fine analysis in 12 centrality bins


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?


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


The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins


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


CERESNA60


Dilepton yields : well-described by superposition of leptonic decay of final state hadrons

CERES/NA45 at the CERN SPS; results on p-Be/Au

lepton-pair production in nuclear collisions – past, present, future

Documents