Lepton-pair production in nuclear collisions past, present, future Hans J. Specht...

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


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


Motivation


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)


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


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


‹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> ?


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


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


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


2nd generation experiments SPS


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


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


Present

3rd generation experiments SPS

NA60

1st generation experiments RHIC

PHENIX


NA60


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


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


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)



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.



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%



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%


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


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


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%


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


Mass spectra of excess dimuons


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


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


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


The spectral function


*(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


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()(/


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


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


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


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


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


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


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:


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 )


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


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?


Transverse momentum spectra


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


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)


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 )


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


Excess pT spectra for three centrality bins

hardly any centrality dependence, but significant mass dependence

sum over centralities

(spectra arbitrarily normalized)


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)


(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


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


What can we learn from pT spectra?

Radial Flow

Origin of dileptons


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)


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


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)


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.


Disentangling the mT spectra of the peak and the continuum


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.


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


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)


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)


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


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


Acceptance-corrected mass spectra


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


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


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


PHENIX


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


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


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…


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; 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…..


Explicit connection between broadening and the chiral condensate

Quantitative description of fireball dynamics;present models ad hoc, hydrodynamics far off

Wish list to theory

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Lepton-pair production in nuclear collisions past, present, future Hans J. Specht...

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