1. The Experimental Quest for In-Medium Effects
Romain Holzmann
GSI Helmholtzzentrum für Schwerionenphysik, Darmstadt at
23rd Indian-Summer School of Physics
and
6th HADES Summer School:
FAIR
October 3-7, 2011 in Rez/Prague, Czech Republic
Lecture I: Pedestrian’s approach
Lecture II: Experiments galore
Lecture III: HADES at GSI

2. Lecture I: A pedestrian’s approach to medium effects
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

3. Mass of composite systems
Naively, the mass of a composite object is
the sum of the masses of its constituents.
Binding energy reduces the mass slightly:
molecules, atoms: 10-8 effect nuclei: 10-2 effect
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

4. The origin of hadron masses
M   mi
binding energy
effect  10-8
atom
10-10 m
atomic nucleus m
M   mi
binding energy
effect  10-2
M » Σ mi
nucleon
10-15 m
1 GeV >> 20 MeV
nucleon: mass not determined by sum of current quark masses !!!
► Could say: mass given by energy stored in motion of quarks
and by energy of gluon fields (m = E/c2)
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

5. Masses of quarks and leptonsc u b s d m e nt nm ne
10-3
10-2
10-1 1 10
102
103
104
105
10-6
10-5
10-4
M l,q [MeV/c2]
0.511
1777
106
2-4
4-8
80-140
~1200
~4600
~175000
“mass” means here
current mass = weak mass
Masses of elementary particles
(quarks, leptons) are generated
by interaction with the Higgs field
search for Higgs LHC
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

6. Phenomenology of quark masses
Quark masses are not directly observable,
they are parameters in models fitted to
hadron properties.
Systematics of (current) quark masses
(from PDG full report, 2000):
Picture taken from
Zhu et al., PLB647 (2007) 366
each dot represents
one model fit!
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

7. The evolution of the universe
T time
15 billion
years
3 oK
Two steps
in mass generation:
Electro-weak transition
(Higgs mechanism)
► weak mass
= current mass
Chiral transition
(hadronization)
► strong mass
We observe the
constituent mass:
M = Mw + Ms
1 billion
years
20 oK
years
3.000 oK
From the Big Bang
to the galaxies:
expansion & cooling
109 oK
~100 MeV
3 minutes 2.
1 millionth
of a second
(1 μs)
1012 oK
~100 GeV 1.
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

8. Mass generation in QCD-inspired model
Weak masses
through interaction
with Higgs boson
Constituent quark
masses
u,d s c b
mq= mweak + mstrong
(p = momentum of quark)
C. Fischer et al., Ann. Phys. 324 (2008) 106
non-perturbative ansatz for
momentum-dependent quark mass function
Dyson-Schwinger approach:
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

9. The strong interaction
Hadron physics deals with phenomena mediated by
the strong force …
… the theory of which is Quantum Chromo Dynamics (QCD)
Nucleus
(R  1-10 fm; M  A x GeV)
Quarks
(R < 10-4 fm; M  10 MeV)
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

10. QCD: running coupling constant αs
Quarks are confined!
Coupling strength between two quarks
perturbative
QCD: aS << 1
non-perturbative
QCD: aS  1
Coupling strength between two quarks
perturbative
QCD: aS << 1
non-perturbative
QCD: aS  1
Coupling strength between two quarks
perturbative
QCD: aS << 1
non-perturbative
QCD: aS  1 f
~1 fm
Asymptotic freedom
(Physics Nobel prize 2004)
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

11. QCD: quarks  jets Jet production in e+e- collisions
The quark-quark potential
increases at large distances:
Jet production
in e+e- collisions
Quarks are confined and
by trying to separate them
jets of hadrons materialize
► first experimental confirmation
in e+e- collisions at SLAC
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

12. Non-pertubative QCD At low energy the QCD equations
cannot be solved explicitely:
fall back on models
solve on the lattice
explore symmetries of LQCD
perturbative
QCD: aS << 1
non-perturbative
QCD: aS  1
with Nf = 3
Chiral symmetry:
In the limit of zero mass
left- and right-handed quarks decouple
But: M(quark) > 0  symmetry broken !
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

13. Chiral symmetry breaking in a nutshell
The QCD Lagrangian is invariant against
independent global SU(3) flavor rotations
of left- and right-handed quarks:
 left- and the right-handed worlds decouple
This symmetry is explicitly broken by the finite
masses of the current (u,d,s) quarks.
On top of this, chiral symmetry is spontaneously broken,
and much more strongly so, because of the existence of
a non-vanishing vacuum expectation value
of the scalar quark condensate:
Analogy: the spontaneous orientation of the
elementary magnetic dipoles in a ferromagnet qL
gluon (g) qR
gluon (g) qL qR
gluon (g)
Coupling to the condensate
generates hadron masses
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

14. Phase transition: ferromagnetism  paramagnetism
Restoration of full rotational symmetry:
vanishing of magnetisation
Ferromagnetic:
rotational symmetry
about 1 axis
Paramagnetic:
full rotational symmetry (3d)
TCurie
magnetisation M
ferro magnetic
para magnetic
temperature T
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

15. Spontaneous chiral symmetry breaking
The ground state of QCD (i.e. vacuum) does not share the chiral symmetry
of the QCD Lagrangian. The vacuum is populated by scalar quark-antiquark
pairs in 3P0-states: quark-antiquark pairs with J=0+:
Non-zero chiral condensate:
A left-handed quark qL can be converted into a right-handed quark qR
by interaction with a scalar qq pair: ► chiral symmetry breaking = +
annihilate
Due to the condensate chiral symmetry is broken!
But, it can be restored for
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

16. Chiral symmetry breaking in the hadronic sector
If chiral symmetry were to hold in the hadronic sector we would expect
chiral partners with same spin but opposite parity to be degenerate in mass:
1232
1535
1520
938
≈ 600
≈ 290
nucleon
Observation:
135
600
≈ 470
scalar meson
1260
770
≈ 490
vector meson
Mass split is large, comparable to hadron masses !
 Chiral symmetry is broken in the hadronic sector
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

17. The chiral condensate as order parameter
The chiral condensate in QCD is an order parameter for
the breaking (or restoration) of chiral symmetry (like magnetization in ferromagnet!)
Hadron masses determined in a non-trivial way by chiral symmetry
breaking, i.e. via the interplay with the condensate ► calculated within models!
If chiral condensate could be changed by external parameters - like , T - and
if it were possible to study how this affects hadron masses, then
Deeper understanding of chiral symmetry breaking and restoration,
and of hadron mass generation
, . p - beams
heavy ion reactions:
A+AV+X
mV (>>0;T>>0)
elementary reaction:
,   V+X
mV (=0;T=0)
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

18. Quark condensates 2-quark condensate 4-quark condensate
J. Wambach et al.
SIS 18
SIS 300
SPS
SIS 18
SIS 300
SPS
freeze-out regions
S. Leupold, Trento Workshop 2005
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

19. QCD sum rules + higher order terms However, is not an observable!!
QCD sum rules provide a link between hadronic observables and condensates:
(T. Hadsuda and S. Lee, PRC 46 (1992) R34; S. Leupold and U. Mosel, PRC58 (1998) 2939)
+ higher order terms
hadronic spectral function:
Chiral condensate related only to integral over hadronic spectral functions;
 spectral function are constrained, but not determined
Hadronic models are still needed for specific predictions
of hadron properties !!
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

20. Model predictions for in-medium masses of mesons
V. Bernard and U.-G. Meißner
NPA 489 (1988) 647
NJL-model
K. Saito, K. Tushima, and A.W. Thomas
PRC 55 (1997) 2637
Quark-meson coupling model (QMC)
decrease of  mass by 15%
at normal nuclear matter density
mass degeneracy of chiral partners
reached at high baryon densities
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

21. Model calculations of the ω spectral function
for rB:
F. Klingl et al. NPA 610 (1997) 297
NPA 650 (1999) 299
lowering of in-medium mass + broadening of resonance
P. Mühlich et al., NPA 780 (2006) 187
 spectral function
(structure due to coupling to
S11,P13 resonances)
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

22. Calculation of the ρ spectral functions
vacuum
hadronic medium
e.g. Leupold, Mosel, Post et al.
NPA 741 (2004) 81, NPA 780 (2006) 187
+ other calc.
vacuum ρ
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

23. Evidence for in-medium changes
Nucleon resonances excited in photoabsorption on nuclei:
"melting" of the resonances above the 33
33
D13
F15
Bianchi et al.
Phys. Rev. C 54 (1996) 1688
In the nuclear medium:
Fermi motion
collisional broadening
final-state effects
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

24. Kaons in the medium
D.B. Kaplan et al., PLB 175 (1986) 57
G.E Brown et al., NPA 567 (1994) 937
T. Waas et al., PLB 379 (1996) 34
J. Schaffner-Bielich et al., NPA 625 (1997) 325
G. Mao et al., PRC 59 (1999) 3381
Dispersion relation:
Repulsive (attractive) potential for K+ (K-)
Models predict same trend, but differ quantitatively
Uncertainty on production cross section of K in the medium
Observables: yields (AA vs. NN), flow, pt distributions
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

25. K- spectral function in nuclear matter
Self-consistent coupled channel calculations
L. Tolos, A. Ramos, E. Oset, arXiv:nucl-th/
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

26. Basic experimental approach:
hadron decay in the medium:
reconstruct the invariant mass from 4-momenta of decay products:
compare
with vacuum
(listed in PDG)
ensure that decays occur in the medium:
 select shortlived mesons (
 cut on low meson momenta
: 1.3 fm; : 23 fm; : 46 fm )
avoid distortion of 4-momenum vectors by final-state interactions
 dilepton spectroscopy: ρ, ω,   e+e- (or μ+μ- )
real photons (and K+) are useful as well
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

27. Pros and cons of HIC vs. elementary
Advantage:
sizable effects due to high
densities and temperatures
(regeneration of mesons)
Disadvantage:
any signal represents an integration
over the full space-time history of
the heavy-ion collision with strong
variations in densities and
temperatures
Heavy-ion collisions: A+A
Advantage:
well controlled conditions:
important for theoretical interpretation
no time dependence of baryon density:
B B(t); T=0;
Disadvantage:
small medium effects since
  0 and T=0
Elementary reactions: , p, -beams
Goal (in both approaches):
Test concepts for hadron mass generation by comparing predictions
based on these concepts with experimental observations how hadron
properties are changed in a strongly interacting environment.
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

28. Evolution of the universe
Rafelski 2005
hadronization
ρ ≈ few times ρ0
T ≈ 100 MeV
Such conditions can be
realized in heavy-ion collisions
but treac ≈ s << 10-6 s !
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

29. Dileptons as probes in heavy-ion reactions
two colliding
nuclei
2 AGeV
bremsstrahlung 
formation of highly
compressed and
heated collision zone
e+,+
e-,- 
explosion of collision zone:
freeze-out of yields
e+,+
e-,- 
new forms of matter?
medium modifications of hadrons?
hot & dense fireball:
dileptons:
probe the full space-time evolution of the collision,
being emitted through all stages of the reaction
photons and dileptons are undistorted probes of strongly interacting matter
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

30. Dilepton emitting processes
Mll ≤ 1 GeV:
Mll > 1 GeV: direct decays of cc, bb +
Dalitz decays
Drell-Yan process:
of mesons: of baryons: R g* N p e- e+
πo, γ w g*
πo,η
Direct decays of VM:
w,ρ, g* R N
Bremsstrahlung:
Semi-leptonic D decays:
D or D → leptons + meson(s)
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

31. Dilepton invariant mass spectra
Characteristic features of
dilepton invariant mass spectra
Physics issues
Low mass:
continuum enhancement ?
modification of vector mesons ?
Intermediate mass:
thermal radiation ?
charm modification
High mass:
J/ suppression ?
enhancement ?
Drell-Yan
In these lectures focus is
on low mass region!
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

32. Dimuon sprectrum from p+p at LHCbb
light quark
states cc
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

33. The experimental challenge ...
Must detect
e+e- pairs
μ+μ- pairs
among large hadronic background!
► See next lecture…
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI

34. CERES Overview (of HI expts.) CMS ATLAS s Energy HELIOS 3
LHC
s Energy
RHIC
SPS
CERES
HELIOS 3
SIS 300
SIS 100 AGS
at SIS100
SIS18
Bevalac
1990
2000
2010
2018
Time + advance in technology
Rez The Experimental Quest for In-Medium Effects R. Holzmann, GSI