Ralf Averbeck Department of

Physics & Astronomy

High Energy Dilepton Experiments

Introduction

Ralf Averbeck,2

Outline

M. Riordan and W. Zajc, Sci. Am., May 2006, 34-41

chirality, chiral symmetry, and chiral symmetry breaking an experimentalist‘s

humble approach

setting the (general) stage for experiments what‘s the deal?

the “soup kitchen“ basics what are we dealing with?

emphasis for today (very simplified) what is the objective? what are the experimental boundary conditions?

Ralf Averbeck,3

Nuclear matter as QCD laboratory “ordinary” nuclear matter

3 (light) constituent quarks quarks interact via the exchange

of gluons gluons carry color charge! (→

complicated vacuum)

key observations isolated quarks are NEVER

observed (“confinement“) quark masses: ~1% of the nucleon mass

– hadron masses >> sum of quark masses– related to chiral symmetry breaking

properties of the strong interaction, theoretically described in QCD (Quantum Chromo Dynamics)

Ralf Averbeck,4

Chiralitywhat is chirality?

origin: the greek word for hand: “” when does an object/system have “chirality”?

– when it differs from its mirror image– L(eft) and R(ight) versions of this object/system

simplification of chirality: helicity helicity = projection of a particle’s spin on its

momentum direction high energy limit

– helicity = chirality

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Chirality conservationmassive particles P

left and right handed components must exist m>0 particle moves with v<c

– if P looks left handed in the laboratory– P will look right handed in a rest frame moving

faster than P but in the same direction chirality is NOT a

conserved quantity

in a massless word chirality is conserved

– careful: m=0 is a sufficient but not a necessary condition left-handed

right-handed

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the QCD Lagrangian:QCD and chiral symmetry

free gluon field interaction of quarkswith gluon

free quarks ofmass mn

formal definition of chirality in QCD chirality operators: PR, PL

– PR = ½ (1 + 5), PL = ½ (1 - 5)– with 5 = i 0 1 2 3, i.e. the product of Dirac matrices

R and L projections of wave fct. u: uR,L = PR.Lu

mass is problematic mu ≈ 4 MeV, md ≈ 7 MeV mnucleon ≈ 1 GeV ≈ 20 x current quark mass

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explicit chiral symmetry breaking mass term mnnn in the QCD Lagrangian

chiral limit: mu = md = ms = 0 chirality would be conserved all states have a ‘chiral partner’

(opposite parity and equal mass)

real life a1 (JP=1+) is chiral partner of (JP=1-): m≈500 MeV even worse for the nucleon:

– N* (½-) and N (½+): m≈600 MeV (small) current quark masses don’t explain this

chiral symmetry is also spontaneously broken spontaneously = dynamically

Chiral symmetry breaking

Ralf Averbeck,8

current quark mass generated by spontaneous symmetry breaking

(Higgs mass) contributes ~5% to the visible (our) mass

Origin of mass

1

10

100

1000

10000

100000

1000000

u d s c b t

QCD Mass

Higgs Mass

constituent quark mass ~95% generated

by spontaneous chiral symmetry breaking (QCD mass)

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Chiral symmetry restoration

3250MeVqq

0qq

spontaneous symmetry breaking gives rise to a nonzero ‘order parameter’ QCD: quark condensate many models (!):

hadron mass and quark condensate are linked

numerical QCD calculations at high temperature and/or

high baryon density deconfinement and

approximate chiral symmetry restoration (CSR) constituent mass approaches current mass

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Chiral Symmetry Restoration expect modification of hadron spectral

properties (mass m, width )

explicit relation between (m,) and <qq>?QCD Lagrangian parity doublets are

degenerate in mass how is this degeneracy realized? do the masses drop to zero (or where else)? do the widths increase (melting resonances)?

good questions, without (obvious) good answers

How does CSR manifest itself?

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predictions for in-medium properties of the meson

one example where experiments have the potential to guide the theory

Theoretical guidance

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

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what are the best probes for CSR? requirement: carry hadron spectral properties

from (T, B) to detectors relate to hadrons in medium leave medium without final state interaction

dileptons from vector meson decays

best candidate: meson– short lived – decay (and regeneration) in medium– properties of in-medium and of medium itself not well known

meson (m≈2xmK) ee/KK branching ratio!

CSR and low mass dileptons

m [MeV] tot [MeV] [fm/c] BRe+e-

770 150 1.3 4.7 x 10-5

8.6 23 7.2 x 10-5

4.4 44 3.0 x 10-4

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a fundamental question! how are hadron masses generated?

theoretical guidance but no crisp answer! availability of a sensitive probe (dilepton

decays of low-mass vector mesons)! strong variation of <qq>

with T (above critical TC) and B (continuous) feasibility of systematic studies!

One experimentalists dream

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a lot of ‘horrible’ experimental difficulties prepare a system with (T,B)≠(0,0) and control

(or determine) these parameters lepton measurements are difficult

– ‘needle in the haystack’ compared to hadrons– many lepton sources beyond

vector meson decays lepton pair measurements

suffer from combinatorial background, i.e. pairs not originating from the same parent

interpretation is difficult due to ‘other’ medium effects

Another ones nightmare

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what is the real theory of CSR? Volker

what have experiments observed at and close to the nuclear ground state? Piotr

what have experiments observed far away from the nuclear ground state (in particular along temperature axis)? Ralf

Stage is set for a first class drama

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nuclear matter close to the ground state electromagnetic probes (photon or electron beams) hadronic probes (pion or proton beams)

excited strongly interacting matter relativistic nuclear

collisions

accessible regions high temperature at

low net baryon density colliders

moderate temperature at high net baryon density fixed-target machines

Probing strongly interacting matter

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fixed-target machines ~1 AGeV beam energy √sNN ~ 2 GeV

– Bevalac@LBNL, SIS@GSI ~10 AGeV beam energy √sNN ~ 5 GeV

– AGS@BNL (no dileptons) ~160 AGeV beam energy √sNN ~ 17 GeV

– SPS@CERN future: ~30 AGeV beam energy √sNN ~ 8 GeV

– SIS300@FAIR

colliders ~100 AGeV beam energy √sNN = 200 GeV

– RHIC@BNL future: 2.25 AGeV beam energy √sNN = 5.5 TeV

– LHC@CERN

High energy heavy-ion accelerators

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Not all collisions are the same

Participants

Spectators

Spectators small impact parameter

(b~0) high energy density large volume large number of

produced particles

measured as: fraction of cross section

“centrality” number of participants number of nucleon-

nucleon collisions

impact parameter

b

from a “Glauber”MonteCarlo calculation

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Experimental determination of geometry

5% Central

Paddles/BBCZDC ZDC

Au Au

Paddles/BBC Central

Multiplicity Detectors

Paddle signal (a.u.)

STAR

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The experimental challenge (RHIC)STAR ONE central

Au+Au collision at max. energy

production of MANY secondary particles

PHENIX

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Anatomy of a Au+Au collisiontime

hard parton scattering

AuAu

hadronization

freeze-out

formation and thermalization of quark-gluonmatter?

Space

Time

expansion

Jet cc e pK

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electromagnetic radiation: , e+e, rare, no strong interaction

– probe all time scales– in-medium properties

of light vector mesons probe for chiral symmetry

restoration effects

hadrons: , K, p, … abundant, final state

–yields, spectra → energy density, thermalization, hadrochemistry

–correlations, fluctuations, azimuthal asymmetries → collective behavior

Different probes tell different stories

cc

J

qq

ee

“hard” probes: jets, heavy quarks, direct rare, produced initially (before

quark-gluon matter forms!)–probe hot and dense matter

investigate evolution of a system that “lives” for ~10-22 s (~100 fm/c) in a volume ~10-42 m3 (~1000 fm3) with energy ~6 x 10-6 J (~40 TeV)

p

p

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one particle ratio (e.g. p/p) determines B/T

a second ratio (e.g. /p) then determines T predict all other hadron abundances and ratios

do the huge yields of various hadron species in the final state reflect a THERMAL distribution?

abundances in hadrochemical equilibrium

Final state hadrochemistry

1

1

2 /3

3

22

Tmp

hhBh

e

pdVgN

lesantipartic and

,.......,,,,,,,,,, DdpKKh

spin isospindegeneracy

temperature atchemical freezeout

baryochemicalpotential

final state: hadron gas close to phase boundary

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How close to the phase boundary? final state at RHIC (and elsewhere!):

hadronic black body consistent with chemical equilibrium

very close to phase boundary between hadronic and quark-gluon matter

B → 0 means B/B → 1 (early universe)

T = 177 MeV provides lower limit for initial temperature

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4-vector of particle:

More practical variables: transverse momentum Lorentz invariant

related transverse mass

rapidity Lorentz transformation:

related pseudo rapidity

Particle counting kinematics

22

cosT

T T

p p

m m p

1

ln2

ln tan2

L

L

E py

E p

p m y

0 90

lab cms beam

ocms cms

y y y

y

mass mmomentum p

polar angleazimuth

beam axis

measure:

p and are not Lorentz invariant!!

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indication for chemical equilibrium good chance for kinetic equilibrium as well

first guess: a thermal Boltzmann source:

but we are dealing with a system of interacting particles expanding into vacuum flow natural ordering of particles occurs with the highest

velocity being present at the system’s ‘outer edge’ particle spectra represent a convolution of

– thermal motion– radial expansion of the source, i.e. radial flow

Tym

TTE

TT

TE T

eymEedyddmm

Nd

dp

NdEe

dp

Nd )cosh(3

3

3

3

3

)cosh(;

Particle spectra (basics)

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blast wave model if a thermal source is boosted radially with a

velocity boost and evaluated at y=0

with simple assumption: uniform sphere of radius R

and boost velocity varies linearly with r:

T

mK

T

pIm

dm

dN

mTT

TTT

)cosh()sinh(110

boost 1tanh

R

rr

T

mK

T

pIdrmr

dm

dN

m

MAXT

RTT

TTT

1

0 102

tanh)(

)cosh()sinh(1

Particle spectra (radial flow)

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What does this mean?

mT1/m

T d

N/d

mT

light

heavyT

purely thermalsource

explosivesource

T,

mT1/m

T d

N/d

mT

light

heavy

mT = (pT2 + m2)½

different spectral shapes for particles of different mass strong collective radial flow

reasonable agreement with hydrodynamic prediction at RHIC Tfo ~ 100 MeV <r> ~ 0.55 c

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Kinetic freeze-out systematics r

increases continuously

Tfo

saturates around AGS energy

strong collective radial expansion at RHIC high pressure high rescattering rate thermalization likely

Slightly model dependenthere: blast wave model

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translates into momentum anisotropy

in final state Fourier expansion

elliptic flow strength

Elliptic flow → early thermalization initial state of non-central

Au+Au collision spatial asymmetry asymmetric pressure gradients

x

zNon-central Collisions

in-plane

out-of-plane

y

Au nucleus

Au nucleus

3 3

R30T T

2 cosnn

d N d NE v nd p p d dp dy

2 Rcos 2v

shape “washes out” during expansion, i.e. elliptic flow is “self quenching” v2 reflects early interactions and pressure gradients

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Hadron v2 and hydrodynamicsobservations at RHIC

v2 is large and for soft hadrons in reasonable agreement with ideal hydrodynamics (not true at lower energies)

mesons

baryons

indication for thermalization at a time when quark degrees of freedom are important!

PHENIX: nucl-ex/0608033

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g

g

medium

One view behind the curtain tomography - one way to establish properties of a system

calibrated probe (e±, X-rays with known beam energy & direction) calibrated interaction (known interaction mechanism!) suppression/absorption pattern reveals details about the interior

“hard probe” tomography at RHIC probe has to be “auto generated” initial state hard parton (quark, gluon)

scattering jets calibration: p+p collisions

Ralf Averbeck,33

Nuclear modification of jets “shooting” hard probes through the medium nuclear modification factor:

direct photons “shine” through the medium (no strong interaction)

hadrons (from jet fragmentation) don’t

ppinYieldN

AuAuinYieldR

binaryAA

very large density of strongly interacting scattering centers!

Ralf Averbeck,34

High energy heavy-ion collisions central HI collisions @ RHIC

hot and dense phase of strongly interacting matter!

central HI collisions @ SPS quantitatively and qualitatively different conditions

major in-medium effects due to the onset of chiral symmetry restoration expected @ SPS & RHIC

experiments are promising

Ralf Averbeck,35

Lepton-pair physics: topics

chiral symmetry restoration continuum enhancement modification of vector mesons

thermal radiation

suppression (enhancement)

known sources of lepton pairs

modifications expected due to the QCD phase transition(s) lepton pairs are

– rich in physics– experimentally

challenging

emitted over the full evolution of the collision

reach detectors undistorted from strong FSI

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The double challenge the experimental challenge

example: central Au+Au collisions @ √sNN = 200 GeV– need to detect a weak e+e- source

– hadron decays (mee > 200 MeV/c2; pT > 200 MeV/c) ~4x10-6/0

– in the presence of many charged particles dNch/dy ~ 700– and several pairs/event from trivial origin

– Dalitz decay of 0 ~10-2/0

– conversion of (assuming 1% radiation length) ~2x10-2/0

huge combinatorial background (dNch/dy)2

the analysis challenge e+e- emitted through the whole history of the collision

– need to disentangle the different sources– need excellent p+p and p(d)+A reference data

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HI low-mass dileptons at a glance time scale of experiments

= period of data taking

ALICE

(KEK E235)

CERES

DLS

NA60

HADES

CBM

90 95 1000 0585

PHENIX

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HI low-mass dileptons at a glance energy scale of experiments

(KEK E235)

CERES

DLS

NA60

HADES

CBM PHENIX

10 158 [A GeV]

17 [GeV]√sNN200

// // //

// // //

ALICE

[A TeV]

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Summary and Outlook chiral symmetry restoration (CSR)

a topic of fundamental interest a topic that can be studied via

(challenging) dilepton experiments high energy HI collisions

produce strongly interacting matter in which major CSR effects are expected

dilepton experiments have been done and have produced results 2nd lecture: SPS experiments 3rd lecture: RHIC experiments & future