the experimental quest for in-medium effects

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:

Physics @ FAIR

October 3-7, 2011 in Rez/Prague, Czech Republic

Lecture I: Pedestrian’s approach

Lecture II: Experiments galore

Lecture III: HADES at GSI

Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI Lecture I: 2

Lecture I:

A pedestrian’s approach to medium effects


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


The origin of hadron masses

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)

M mi

binding energyeffect 10-8

atom 10-10 m

M » Σ mi

nucleon 10-15 m

atomic nucleus 10-14 m

M mi

binding energyeffect 10-2

1 GeV >> 20 MeV


Masses of quarks and leptons

Masses of elementary particles (quarks, leptons) are generated by interaction with the Higgs field

search for Higgs particle @ LHC

Leptons Quarkst

c

u

b

s

d

e

e

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

2-44-8

80-140

~1200~4600

~175000

“mass” means herecurrent mass = weak mass


Phenomenology of quark masses

Picture taken from

Zhu et al., PLB647 (2007) 366

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):

each dot representsone model fit!


The evolution of the universe

15 billion years3 oK

20 oK

3.000 oK

109 oK~100 MeV

1012 oK~100 GeV

1 billion years

300.000 years

3 minutes

1 millionth of a second (1 μs)

From the Big Bangto the galaxies:expansion & cooling

Two stepsin mass generation:

1. Electro-weak transition

(Higgs mechanism)

► weak mass = current mass

2. Chiral transition (hadronization)

► strong mass

We observe theconstituent mass:

M = Mw + Ms

1.

2.

T time


mq= mweak + mstrong

Mass generation in QCD-inspired model

Weak massesthrough interaction with Higgs boson

Constituent quarkmasses

u,d

s

cbb

csu,d

(p = momentum of quark)

C. Fischer et al., Ann. Phys. 324 (2008) 106

non-perturbative ansatz formomentum-dependent quark mass function

Dyson-Schwinger approach:


The strong interaction

Hadron physics deals with phenomena mediated bythe 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)


Coupling strength between two quarks

perturbativeQCD: aS << 1

non-perturbativeQCD: aS 1 f



non-perturbativeQCD: aS 1

QCD: running coupling constant αs




Quarks are confined!

Krr

crV s

3

4)(

~1

fm

Asymptotic freedom(Physics Nobel prize 2004)


QCD: quarks jets

The quark-quark potential

increases at large distances:

Krr

crV s

3

4)(

Jet productionin e+e- collisions

Quarks are confined andby trying to separate themjets of hadrons materialize

► first experimental confirmation in e+e- collisions at SLAC


Non-pertubative QCD



At low energy the QCD equationscannot be solved explicitely:

fall back on models solve on the lattice explore symmetries of LQCD

Chiral symmetry:

In the limit of zero mass left- and right-handed quarks decouple

But: M(quark) > 0 symmetry broken !

with Nf = 3


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

Coupling to the condensategenerates hadron masses

qL

qL qL

qLgluon (g)

qR

qR qR

qRgluon (g)

qL

qL qR

qRgluon (g)


Phase transition: ferromagnetism paramagnetism

Restoration of full rotational symmetry:vanishing of magnetisation

Paramagnetic:

full rotational symmetry (3d)

Ferromagnetic:

rotational symmetry about 1 axis

TCurie

ma

gn

etis

atio

n M

ferro magnetic

para magnetic

temperature T


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+:

021 pp

121

RqLq

1L

03P;0J Non-zero chiral condensate:

Due to the condensate chiral symmetry is broken!

But, it can be restored for 00|qq|0

A left-handed quark qL can be converted into a right-handed quark qR

by interaction with a scalar qq pair: ► chiral symmetry breaking

RqLq

=

RqLq

+

annihilate


135

0: 600

≈ 47

0

0:

scalar meson

1260

770

≈ 49

0

1:

1:a1

vector meson

21

21

23

21,2

1 1232

1535 1520

938

≈ 60

0

≈ 29

0

23

nucleon

Observation:

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:

Mass split is large, comparable to hadron masses !

Chiral symmetry is broken in the hadronic sector

Chiral symmetry breaking in the hadronic sector


The chiral condensate in QCD is an order parameter for the breaking (or restoration) of chiral symmetry (like magnetization in ferromagnet!)

The chiral condensate as order parameter

Hadron masses determined in a non-trivial way by chiral symmetry breaking, i.e. via the interplay with the condensate ► calculated within models!

, . p - beams

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

heavy ion reactions:A+AV+X

mV (>>0;T>>0)

elementary reaction:, V+X

mV (=0;T=0)


Quark condensates

J. Wambach et al.

SIS 18SIS 300SPS

SIS 18SIS 300SPS

freeze-out regions

SIS 18SIS 300 SPS

S. Leupold, Trento Workshop 2005

2-quark condensate 4-quark condensate


Hadronic models are still needed for specific predictions of hadron properties !!

QCD sum rules

Chiral condensate related only to integral over hadronic spectral functions; spectral function are constrained, but not determined

qq 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)

242222

2

24

111

16

1

24Gqqm

QQs

sRds

Q sq

s

+ higher order terms

222

2

ssMs

ss1F~sR

hadronic spectral function:


Model predictions for in-medium masses of mesons

V. Bernard and U.-G. MeißnerNPA 489 (1988) 647

NJL-model

mass degeneracy of chiral partnersreached at high baryon densities

K. Saito, K. Tushima, and A.W. ThomasPRC 55 (1997) 2637

Quark-meson coupling model (QMC)

decrease of mass by 15%

at normal nuclear matter density

smallfor)1(

00 mm


Model calculations of the ω spectral function

P. Mühlich et al., NPA 780 (2006) 187

spectral function(structure due to coupling to S11,P13 resonances)

for B:

F. Klingl et al. NPA 610 (1997) 297 NPA 650 (1999) 299

lowering of in-medium mass + broadening of resonance


Calculation of the ρ spectral functions

e.g. Leupold, Mosel, Post et al.NPA 741 (2004) 81, NPA 780 (2006) 187

vacuum ρ+ other calc.

vacuum hadronic medium


Evidence for in-medium changes

Nucleon resonances excited in photoabsorption on nuclei:

"melting" of the resonances above the 33

Bianchi et al.Phys. Rev. C 54 (1996) 1688

In the nuclear medium: Fermi motion collisional broadening final-state effects

33

D13F15


Kaons in the medium

D.B. Kaplan et al., PLB 175 (1986) 57G.E Brown et al., NPA 567 (1994) 937T. Waas et al., PLB 379 (1996) 34J. Schaffner-Bielich et al., NPA 625 (1997) 325G. 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

2

1222

122*

2

2

12

2222

),(

8

3

8

3),(

kmUUkmk

fffkmk

KVSKNK

NNS

KNKNK


K- spectral function in nuclear matter

Self-consistent coupled channel calculations

(1405)

K-K-

N-1L. Tolos, A. Ramos, E. Oset, arXiv:nucl-th/0702089


Basic experimental approach:hadron decay in the medium:

221H pppT,ρ,m

reconstruct the invariant mass from 4-momenta of decay products:

compare 0 pT,ρ,mH

with vacuum Hm (listed in PDG)

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

ensure that decays occur in the medium:

select shortlived mesons ( cut on low meson momenta

: 1.3 fm; : 23 fm; : 46 fm );c

c


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.

Pros and cons of HIC vs. elementary


Evolution of the universe

Rafelski 2005

hadronizationρ ≈ few times ρ0

T ≈ 100 MeV

Such conditions can berealized in heavy-ion collisionsbut treac ≈ 10-23 s << 10-6 s !


Dileptons as probes in heavy-ion reactions

explosion of collision zone:freeze-out of yields

photons and dileptons are undistorted probes of strongly interacting matter

new forms of matter?

medium modifications of hadrons? hot & dense fireball:

e+,+

e-,-

dileptons:probe the full space-time evolution of the collision,being emitted through all stages of the reaction

e+

e-

two collidingnuclei

A+A @ 2 AGeV

bremsstrahlung

formation of highly compressed and

heated collision zone

e+,+

e-,-


Dilepton emitting processes

Semi-leptonic D decays:

D or D → leptons + meson(s)

Mll ≤ 1 GeV:

Drell-Yan process:

eeqq

qq

Direct decays of VM:

ρ,

Dalitz decays

R

N

e-

e+

N

η

RN

N

N

N

Bremsstrahlung:

of mesons: of baryons:

Mll > 1 GeV: direct decays of cc, bb +


Low mass:- continuum enhancement ?- modification of vector mesons ?

Dilepton invariant mass spectra

Physics issues

Intermediate mass:- thermal radiation ?- charm modification

High mass:- J/ suppression ? enhancement ?- Drell-Yan

Characteristic features ofdilepton invariant mass spectra

In these lectures focus ison low mass region!


Dimuon sprectrum from p+p at LHC

light quarkstates

cc bb


The experimental challenge ...

Must detect e+e- pairs μ+μ- pairs

among large hadronic background!

► See next lecture…

e+

e-

N

N

NN


Overview (of HI expts.)

Time + advance in technology

LHCLHC

RHICRHIC

SPSSPS

SIS 300SIS 300

SIS18SIS18BevalacBevalac

SIS 100SIS 100AGSAGS

at SIS100

HELIOS 3

s

En

erg

y

CMSATLAS

1990 2000 20182010

the experimental quest for in-medium effects

Documents

experimental quest

medium effects

medium effectsrez

theconstituent mass

herecurrent mass

masses of quarks

sum of current quark

pedestrians approach