1. High Energy Dilepton Experiments
Introduction

2. M. Riordan and W. Zajc, Sci. Am., May 2006, 34-41
Outline
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?
M. Riordan and W. Zajc, Sci. Am., May 2006, 34-41
emphasis for today (very simplified)
what is the objective?
what are the experimental boundary conditions?

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)

4. Chirality what is chirality? simplification of chirality: helicity
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

5. Chirality conservation
massive 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
right-handed
left-handed

6. QCD and chiral symmetry
the QCD Lagrangian:
free gluon field
interaction of quarks
with gluon
free quarks of
mass mn
formal definition of chirality in QCD
chirality operators: PR, PL
PR = ½ (1 + g5), PL = ½ (1 - g5)
with g5 = i g0 g1 g2 g3, 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

7. Chiral symmetry breaking
explicit chiral symmetry breaking
mass term mnynyn 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 r (JP=1-): Dm≈500 MeV
even worse for the nucleon:
N* (½-) and N (½+): Dm≈600 MeV
 (small) current quark masses don’t explain this
chiral symmetry is also spontaneously broken
spontaneously = dynamically

8. Origin of mass current quark mass constituent quark mass
generated by spontaneous symmetry breaking (Higgs mass)
contributes ~5% to the visible (our) mass
constituent quark mass
~95% generated by spontaneous chiral symmetry breaking (QCD mass)

9. Chiral symmetry restoration
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

10. How does CSR manifest itself?
Chiral Symmetry Restoration
expect modification of hadron spectral properties (mass m, width G)
explicit relation between (m,G) 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

11. Theoretical guidance
predictions for in-medium properties of the r meson
one example where experiments have the potential to guide the theory
mass of r
width of r
Pisarski 1982
Leutwyler et al 1990 (p,N)
Brown/Rho 1991 ff
Hatsuda/Lee 1992
Dominguez et. al1993
Pisarski 1995
Rapp 1996 ff

12. CSR and low mass dileptons
what are the best probes for CSR?
requirement: carry hadron spectral properties from (T, rB) to detectors
relate to hadrons in medium
leave medium without final state interaction
dileptons from vector meson decays
best candidate: r meson
short lived
decay (and regeneration) in medium
properties of in-medium r and of medium itself not well known
f meson (mf≈2xmK)  ee/KK branching ratio!
m [MeV] Gtot [MeV]  [fm/c] BRe+e-
r x 10-5
w x 10-5
f x 10-4

13. One experimentalists dream
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 rB (continuous)  feasibility of systematic studies!

14. Another ones nightmare
a lot of ‘horrible’ experimental difficulties
prepare a system with (T,rB)≠(0,r0) 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

15. Stage is set for a first class drama
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

16. Probing strongly interacting matter
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

17. High energy heavy-ion accelerators
fixed-target machines
~1 AGeV beam energy  √sNN ~ 2 GeV
~10 AGeV beam energy  √sNN ~ 5 GeV
(no dileptons)
~160 AGeV beam energy  √sNN ~ 17 GeV
future: ~30 AGeV beam energy  √sNN ~ 8 GeV
colliders
~100 AGeV beam energy  √sNN = 200 GeV
future: 2.25 AGeV beam energy  √sNN = 5.5 TeV

18. Not all collisions are the same
Participants
Spectators
impact parameter b
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
from a “Glauber”
MonteCarlo calculation

19. Experimental determination of geometry
Paddle signal (a.u.)
Paddles/BBC
ZDC Au
Central Multiplicity Detectors
5% Central
STAR

20. The experimental challenge (RHIC)
STAR
ONE central Au+Au collision at max. energy
PHENIX
production of MANY secondary particles

21. Anatomy of a Au+Au collision
time g e m p K L
Jet cc g f
Time
freeze-out
expansion
hadronization
formation and
thermalization
of quark-gluon
matter?
hard parton scattering
Space Au

22. Different probes tell different stories
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 K
cc
J/ p q
hadrons: p, K, p, …
abundant, final state
yields, spectra → energy density, thermalization, hadrochemistry
correlations, fluctuations, azimuthal asymmetries → collective behavior e- e+ g
electromagnetic radiation: g, e+e-, m+m-
rare, no strong interaction
probe all time scales
in-medium properties of light vector mesons
 probe for chiral symmetry restoration effects
dies ist eine schematische darstellung einer SI Kollision. Zwei kerne terffen sich mit einem stossparameter b. Je kleiner b ist um so mehr nucleonen nehmen and er kollision teil, um so mehr kollsisionen zwischen nucleonen finden statt und um so mehr teilchen werden erzeugt.
In centralen kollisionen, d.h. mit sehr kleinem Stossparameter b, werden viele tausend teilchen erzeugt - die meisten werden senkrecht zur strahlachse erzeugt. Dies bezeichent man als mid rapidity oder rapiditaet null.
die meisten teilen sind hadronen pi,k,p. Sie werden bei der hadronsynthese produziert und tragen information vor allem von der letzten phase der kollision von kurz bevor die teilechen aufhoeren strak zu ww.
Aus den eigenschaften der hadronen koennen wir auf die anfangs energie dichte zurueckschliessen. Testen ob im reactions volumen thermishen und chemischen gleichgewicht herscht. Collectives Verhalten - expansion unter druck. Eine besondere Bedeutung kommt Teilechen mit strange quarks zu. eine erhoete production dieser teilchen ist eine der schluessel vorhersagen fuer die erzeugung von qm.
Um Information ueber die heisse und dichte phase zu erhalten braucht man ander observabelen. Die sensitive sind auf fruehe phase der kollision. Hier unterscheiden man zwei gruppen von observablen.
Electromagnetische Strahlung - photonen oder virtuelle photonen, lepton paare. Werden zu jeder zeit abgestrahlt ww dann aber nicht mehr stark und tragen daher information von der fruehen phase zum detector.
Zum einen kann man sich das als schwarzkoerperstrahlung vorstellen die dann die anfangs temperatur wiederspiegelt.
Dies war auch eine der schluesselvorhersagen. Zum anderen hat man ueber em strachlung aber auch zugriff auf die eigenschaften von hadronen in dichter heisser materie dies ist vermutlich der einziege zugriff auf chirale symmtry wiederherstellung die man hat.
Die ander klasse von observablen sind sogenannte harte oder durchdringenede proben. Wie charmonium oder bottonium J/psi und upsilon sowie jets. diese sonden werden in den ersten collisionen noch for der formation von qm erzeugt. Sie durchdringen die heisse materie und sind deshalb sensitive auf deren eigenschaften.
Hier gibt es zwei schluesselvorhersagen
(i) abschirmung der farbladungen im plasma fuehrt zur underdrueckung aller J/psi resonancen
(ii) starker energieverlust in farbiger materie fuehrt zum verschwinden von jets
Alle experimentellen beobachtungen sind consistent mit diesen erwartungen. Das moechte ich ihnen nun im folgenden
Zeigen. zunaecht fuer Ergebnisse vom SPS und dann fuer erste Ergebnisse vom RHIC.
“hard” probes: jets, heavy quarks, direct g
rare, produced initially (before quark-gluon matter forms!)
probe hot and dense matter

23. Final state hadrochemistry
do the huge yields of various hadron species in the final state reflect a THERMAL distribution?
abundances in hadrochemical equilibrium
spin isospin
degeneracy
temperature at
chemical freezeout
baryochemical
potential
one particle ratio (e.g. p/p) determines mB/T
a second ratio (e.g. p/p) then determines T
predict all other hadron abundances and ratios
final state: hadron gas close to phase boundary

24. 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
mB → 0 means B/B → (early universe)
T = 177 MeV provides lower limit for initial temperature

25. Particle counting  kinematics
4-vector of particle:
More practical variables:
transverse momentum Lorentz invariant
related transverse mass
rapidity Lorentz transformation:
related pseudo rapidity h
mass m
momentum p
polar angle q
azimuth f
beam axis
measure:
p and q are not Lorentz invariant!!

26. Particle spectra (basics)
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

27. Particle spectra (radial flow)
blast wave model
if a thermal source is boosted radially with a velocity bboost and evaluated at y=0
with
simple assumption: uniform sphere of radius R and boost velocity varies linearly with r:

28. What does this mean?
different spectral shapes for particles of different mass  strong collective radial flow
purely thermal
source mT
1/mT dN/dmT
light
heavy T
reasonable agreement with hydrodynamic prediction at RHIC
Tfo ~ 100 MeV
<br> ~ 0.55 c
explosive
source
light
T,b
1/mT dN/dmT
heavy mT
mT = (pT2 + m2)½

29. Kinetic freeze-out systematics
increases continuously
Tfo
saturates around AGS energy
strong collective radial expansion at RHIC
high pressure
high rescattering rate
thermalization likely
Slightly model dependent
here:
blast wave model

30. Elliptic flow → early thermalization
initial state of non-central Au+Au collision
spatial asymmetry
asymmetric pressure gradients x z
Non-central Collisions
in-plane
out-of-plane y
Au nucleus
translates into
momentum anisotropy in final state
Fourier expansion
elliptic flow strength
shape “washes out” during expansion, i.e. elliptic flow is “self quenching”
v2 reflects early interactions and pressure gradients

31. Hadron v2 and hydrodynamics
observations at RHIC
v2 is large and for soft hadrons in reasonable agreement with ideal hydrodynamics (not true at lower energies)
mesons
baryons
PHENIX: nucl-ex/
indication for thermalization at a time when quark degrees of freedom are important!

32. 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 g
medium

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
very large density of strongly interacting scattering centers!

34. High energy heavy-ion collisions
central HI RHIC
hot and dense phase of strongly interacting matter!
central HI SPS
quantitatively and qualitatively different conditions
major in-medium effects due to the onset of chiral symmetry restoration SPS & RHIC
experiments
are promising

35. Lepton-pair physics: topics
known sources of lepton pairs
emitted over the full evolution of the collision
reach detectors undistorted from strong FSI
chiral symmetry restoration
continuum enhancement
modification of vector mesons
thermal radiation
suppression
(enhancement)
modifications expected due to the QCD phase transition(s)
lepton pairs are
rich in physics
experimentally challenging

36. The double challenge the experimental challenge the analysis challenge
example: central Au+Au √sNN = 200 GeV
need to detect a weak e+e- source
hadron decays (mee > 200 MeV/c2; pT > 200 MeV/c) ~4x10-6/p0
in the presence of many charged particles dNch/dy ~ 700
and several pairs/event from trivial origin
Dalitz decay of p ~10-2/p0
conversion of g (assuming 1% radiation length) ~2x10-2/p0
 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

37. HI low-mass dileptons at a glance
time scale of experiments
(KEK E235)
CERES
DLS
NA60
HADES
CBM 90 95 10 00 05 85
PHENIX
ALICE
= period of data taking

38. 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]
√sNN
200 //
ALICE
[A TeV]

39. 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