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 to …

Lecture II: Experiments galore

Lecture III: HADES at GSI

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

Lecture II:

Dilepton experiments: An overview(with emphasis on HIC)


Evolution of the universe

Rafelski 2005

hadronizationρ ≈ few times ρ0

T ≈ 100 MeV

Such conditions can berealized in heavy-ion collisions,but treac ≈ 10-23 s << 10-6 sand finite volume only !


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

Typical dilepton mass spectrum

Physics issues:

Intermediate mass:- thermal radiation ?- charm modification

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

Characteristic features of resultingdilepton mass distribution:

In this lecture focus ison low mass region!


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 beginning: “anomalous” dilepton yields

H.J. Specht in Lecture Notes on Physics 221/1985

ω

φ

QM1985

Compilation of results ondilepton production

(from Fermilab, SLAC, ISR, KEK 1975-1985)

“anomalous” excess at low mass over known* Dalitz yields (by x3!)

Starting point of the CERN SPS dilepton expts. (HELIOS, CERES, NA38/50 and DLS at LBNL)

* ”known” means known in 1985!


The early HI experiments (1990 - 2000)

HELIOS/NA34 at the CERN SPS (e+e-, μ+μ-, γ)

CERES/NA45 at the CERN SPS (e+e-)

NA38/NA50 at the CERN SPS (μ+μ-)

DLS at the Bevalac (e+e-)


HELIOS at the CERN SPS HELIOS

HELIOS measured μ+ μ -

e+e-

and photons

The NA34 apparatus in 1989

p + W

data from Masera et al., NPA 590 (1995) 93BUU calc. Cassing et al., PLB 377 (1996) 5

Data agree with cocktail of free meson decays No “anomalous” yield!S + W

First observationof in-medium excess


CERES at the CERN SPS

CERES, a e+e- spectrometer with:

RICH-in-RICH design ( symmetry) dE/dx in Si drift for bkgd rejection 4% mass resolution (with TPC upgrade)

< year 2000 > year 2000

HELIOS findings fully confirmed by CERES:

in p+A: cocktail of in-vacuum decays

in A+A: large excess yield at Mee> 0.2 GeV

Agakishiev et al., EPJC 41 (2005) 475


CERES: low-mass e+e- excess in Pb+Au

Central A+A collisions exhibit a strong enhancement of low-mass dilepton productionas compared to p+A reactions (HELIOS & CERES)

Vacuum properties of mesons do not suffice to describe these data, needed are: pion annihilation (accounts for part only) in-medium modifications of vector meson properties e.g. broadening and/or mass shift of the rho meson

EPJ C41 (2005) 475EPJ C41 (2005) 475 PRL 91 (2003) 042301PRL 91 (2003) 042301


CERES: Comparison with theory

H. Hees & R. Rapp, PRL 97 (2006) 102301

In particular e+e- yield between and favours -broadening over dropping-mass scenario, i. euse in-medium spectral function

dropping mass

in-medium hadronic spectral function

e+e- pair yield after subtraction of hadronic cocktail, excluding :

Many more calculations done: thermal models (e.g. Kämpfer et al.) transport models (HSD, UrQMD)

► See Rez 2008 school for nice intros to transport models (E. Bratkovskaya), HMB theory (R. Rapp) statistical models (PBM)

HMB = hadronic many-body models


The DLS spectrometer at the Bevalace+e- production at 1-2 AGeV

DLS at the Bevalac (1987-1993)Data: R.J. Porter et al.: PRL 79(97)1229

Model: E.L. Bratkovskaya et al.: NP A634(98)168,

BUU with vacuum spectral function

Excess of e+e- yield observed over cocktail of in-vacuum meson decays. But, reaction is baryon dominated ► nucleon resonances (Δ, N*) ► bremsstrahlung (pp vs. pn)

DLS: pair excess at 1 AGeV

Transport calc.: Frankfurt UrQMD

Ernst et al. PRC 58 (1998) 447

DLS yield excessAt M= 0.2 - 0.6 GeVin all A+A systems.

Lecture II: 15


In-medium Vector Meson spectroscopy

NA60 at the CERN SPS: In+In → μ+μ-

HADES at GSI: p+p, p+A, A+A → e+e-

E325 at the KEK PS: p+Cu → e+e-

CLAS at JLAB: γ+A → e+e-

CB/TAPS at ELSA: γ+A → ω → π0γ (photons)

LEPS at SPring-8: γ+A → → K+K- (kaons)

ANKE at COSY: p+A → → K+K- (kaons)

The advent of high-resolution & high-statistics experiments:

Have to deal with final state interactionsof VM decay products

And, of course

PHENIX & STAR at RHIC

ALICE, ATLAS & CMS at LHC


17m

The vertex region (2 detectors):

Zero degree calorimeter(centrality measurements)

Muon Spectrometer

hadron absorber

and trackingmuon trigger

magnetic field

iron w

all

muonother

targets

2.5 T dipole magnet

beam tracker vertex tracker

hadron absorber

The NA60 experiment at CERN

Fixed target dimuon experiment at the CERN SPS (NA50 upgrade) Apparatus composed of 4 main detectors

Origin of muons can be accurately determined (less bkgd from weak decay muons) Improved dimuon mass resolution (~20 MeV/c2 at High rate because of radiation-hard Si pixel det.

Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them (in both angles and momentum) to muon tracks measured in the spectrometer behind the absorber.


NA60: Subtraction of CB and fakes

For the first time, and peaks clearly visible in dilepton channel; even μμ seen

Net data sample: 360 000 events

Mass resolution:23 MeV at the position

Fakes / CB < 10 %

Progress over CERES:

statistics: factor >1000 resolution: factor 2-3


NA60: hadron decay cocktailin peripheral In+In collisions

log


● data

-- sum of cocktail sources

including the

NA60: data & cocktail vs. centrality

Clear excess of data above cocktail, rising with centrality


NA60: Excess = data - cocktail

Clear excess above the cocktail , centered at the nominal pole and rising with centrality

Similar behaviour in all pT bins

No cocktail and no DD subtracted

all pT


NA60: Comparison to HMB models

Two theoretical scenarios for in-medium spectral function

a.) broadening Rapp/Wambach Ad.N.P. 25 (2000) 1

b.) dropping mass Brown/Rho Phys. Rep. 363 (2002) 85

evaluated for the same fireball evolution (R. Rapp 2003 keeping the original normalization)

dropping mass scenario fails to reproduce the NA60 data

Data not yet acounted for in mass range M > 1 GeV

HMB = hadronic many-body models


Dimuon pairs with invariant mass > 1 GeV

H. van Hees & R. Rapp, PRL 99 (2006) 102301

yield above 1 GeV attributed to hadronic processs like 4 annihilation (possibly augmented by chiral mixing)

J. Ruppert et al., hep-ph/0706.1934

yield above 1 GeV dominated by partonic processes like qq+-


NA60: radial flow of muon pairs

R. Arnaldi et al., PRL 100 (2008) 022302

nearly linear rise of Teff with dimuon mass for M < 1GeV consistent with radial flow of a hadronic source:

drop of Teff at M 1 GeV

interpreted as transition to low-flow source of e.g. partonic origin

► First observation of radial flow of thermal di-leptons; mass dependence of Teff tool to identify nature of the emitting source;

mostly partonic radiation for M> 1 GeV ?

hadrons show expected flow pattern: Teff M however, hierarchy of freeze out: different hadrons have different couplings to pions: max. for ; min. for

LMR lMR

Thermal vs. radial expansion: What’s the deal ?

Equilibrated fireball (Maxwell-Boltzmann distribution):

<Ekin> = 3/2 kT where T = same for all particles

with radial expansion (blast wave):

<Ekin> = 3/2 kT + 1/2 M β2 = 3/2 kTeff (T and β same

for all particles)

► Teff ~ M (for M>>T)

Teff from muon pair Pt slopes:


two forward muon spectrometers

forward spectrometers muon measurement in range:

1.2 < |h| < 2.4

p 2 GeV/c

central spectrometer measurement in range:

|h| 0.35 p 0.2 GeV/c

two central electron/photon/hadron spectrometers

The PHENIX detector at RHIC


e e+

PHENIX: Tracking and particle id


PHENIX: Dielectrons at

Observed dilepton yield reproduced bycocktail from known hadronic sources

p + p Au+Au PRC 81 (2010) 034911

signal-to-background 1/200

combinatorial background must be determined to 0.25% !!

► in 2009 installed HBD, expect S/B=1/10

arXiv: 0802.0050


PHENIX: Au+Au vs. p+p data

pp data and cocktail normalized to Au+Au data for mee<100 MeV

• PHENIX low mass continuum (150 < mee<750 MeV): enhancement compared to cocktail: 4.7 0.4(stat) 1.5(sys) 0.9(model)


q

q

Gluon Compton

q

g q

e+

e-

Au+Au data compared to calculations

hadronic source:

partonic sources

►observed enhancement not reproduced by current calculations !!!

STAR claims a much weaker effect (QM2011) :

Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI Lecture I: 33

The HADES experiment at GSI

symmetry

hadron-blind RICH

2-3% mass resolution

For more see Lecture III …

large excess !


The ubiquitous dilepton excess in HIC

An excess of dilepton excess yield (over a calculated cocktail of “known” sources) has been observed for M = 0.2 – 0.6 GeV, at all beam energies, from 1 AGeV to 100+100 AGeV !

Its origin is thought to be: baryon dominated at SIS energies (resonance matter!) meson dominated at SPS energies (+ baryonic, + partonic?) with a strong (?) partonic component at RHIC energies

A comprehensive & quantitative theoretical description is still missing

What happens at LHC ? ALICE will tell us …

At FAIR energies (2 - 25 AGeV) Nmeson ≈ Nbaryon

► HADES & CBM will survey this region and provide high-quality data


In-medium Vector Meson spectroscopy

NA60 at the CERN SPS: In+In → μ+μ-

HADES at GSI: p+p, p+A, A+A → e+e-

E325 at the KEK PS: p+Cu → e+e-

CLAS at JLAB: γ+A → e+e-

CB/TAPS at ELSA: γ+A → ω → π0γ (photons)

LEPS at SPring-8: γ+A → → K+K- (kaons)

ANKE at COSY: p+A → → K+K- (kaons)

The advent of high-resolution & high-statistics experiments:

Have to deal with final state interactionsof VM decay products

And, of course

PHENIX & STAR at RHIC

ALICE, ATLAS & CMS at LHC

Cold vs. hot nuclear matter

Meson

Mass[MeV/c2]

[MeV/c2]

c [fm/c]

BR(Ve+e-) BR in hadrons

0 770 150 1.3 4.4x10-5 + - (~100%)

782 8.4 23.4 7.1x10-5 0 (8%)

1020 4.4 44.4 3.1x10-5 K+K- (50%)

p () + A

e-

e+

F

No FSI ! FSI !

tot

eemesoneeee AAN

fireballeepartee AN

A + A

Lecture II: 37

Experimental approaches using elementary probes

1. Measurement of the in-medium meson line shape:

► in-medium mass shift? broadening? structures?

2. Measurement of the transparency ratio: in A

► in-medium width

(note that TA is similar to the RAA of HICs)

VXNγ

VXAγA σA

σT

3. Measurement of the excitation function: (study threshold behaviour)

► in-medium mass shift, broadening

Lecture II: 38

J.G.Messchendorp et al., Eur. Phys. J. A 11 (2001) 95

p

A + X

0

2ppm

advantage: 0 large branching ratio (8.3 ·10-2 ) no contribution ( 0 : 7 10-4)

disadvantage:

• 0 rescattering, reduced by removing low energy 0

(T > 150 MeV)

CBELSA/TAPS: lineshape on Nb and LH2

No significant difference between

lineshapes from Nb

and LH2 !!!

Sensitivity toin-medium modifications ?

Re-analysis: M. Nanova

Lecture II: 39

ω in-medium width & collisional broadening

Comparison to transport model calculations:

M. Kaskulov, E. Hernandez, E. OsetEPJ A 31 (2007) 245

P. Mühlich and U. MoselNPA 773 (2006) 156

normalized to C!

in-medium inelastic width proportional to absorption: (,|p|) vabs

in the medium: additional inelastic channels remove mesons, e.g. N N► shortening of lifetime; increase in width

transparency ratio:

XC

XAA AT

12/

Results in (0,<|p|> 1.1 GeV/c) 130-150 MeV is broadened in the medium by a factor 16!!in-medium ω width comparable to free ρ width !!

Data: M. Kotulla et al.,

PRL 100 (2008) 192302

Role of regeneration in TA : , ,’ CBELSA/TAPS

absorption NOT affected by secondary reactions

„fast” are absorbed similarly to , slow are enhanced by secondary production NN , NN(1535)

weaker absorption of ’ (’- N coupling) :

’ in-medium : 25-30 MeV

M. Nanova: Hadron2011

0 AE (1.2 - 2.2)

ANKE: Transparency from p +A

p + A : in-medium ~ 33 -50 MeV @ <p > =1.1 GeV/c and 0:

+A (Spring-8, JLab) : in-medium ~ 23-100 MeV @ <p > 1.7 GeV/c

28 MeV from predictions (Valencia/Giessen) - kaon loop modification

ANKE : p @ 2.83 GeV

Phys. Lett. B 695, 74-77 (2011).

K+ K-

0 < < 8 0

0.6 <p < 1.6 GeV/c

lab

A. Polyanskiy : Hadron2011

Preliminary

e+e- pairs from p+A: HADES

Large acceptance for pe+e- < 1GeV/c :

not measured by CLAS, KEK-E325

CLAS,KEK

N*/ ?

0 VM

Preliminary

cocktail : HADES + PYTHIA

HADES: / region

HADES: 3.5 GeV p + Nb vs. p + p

for pe+e- <0.8 GeV/c excess over pp: secondary from N(1520), (1700),…

►Slow pairs show strong in-medium effect

absorption compatible with CBTAPS:GiBuu gives in-medium 150 MeV

p>0.8 GeV/c

p<0.8 GeV/c

To be compared with theory …

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The Experimental Quest for In-Medium Effects