goethe university frankfurt gsi helmholtzzentrum für...

23/11/2016 Alberica Toia 1

Nuclear and Particle Physics 4aElectromagnetic Probes

Alberica ToiaGoethe University Frankfurt

GSI Helmholtzzentrum für Schwerionenforschung

Lectures and Exercise Winter Semester 2016-17


Organization● Language: English● Lecture:

● Wednesday 09:00 (c.t.) - 11:00 ● Phys 01.402

● Marks / examination→ only if required / desired● Seminar presentation → schein ● Oral Exam → grade

● Office hours: tbd on demand


Info: Email and Website● E-Mail:

[email protected]

● Website: https://web-docs.gsi.de/~alberica/lectures/KT4a_WS1617.html

mailto:[email protected]


Content● 1) Introduction: Heavy Ion Physics● 2) Detectors● 3) Dielectrons: low energy● 4) Dielectrons: intermediate energy● 5) Dielectrons: high energy● *** 07.12 Dielectrons Theory (H. van Hees) ***● 6) Photons: intermediate energy● 7) Photons: high energy● 8) Dark Matter● 9) ElectroWeak Probes

OCTOBER

NOVEMBER

DECEMBER

JANUARY


Dielectrons: intermediate energy● Motivation

● Dileptons and the QGP ● Dileptons and the Hadron Gas: Chirality, chiral symmetry

breaking and chiral symmetry restoration

● Experimental challenges● combinatorial background

● Dileptons in heavy-ion collisions experiments● Intermediate-energy

– CERES– NA60


The little bang in the lab

• High energy nucleus-nucleus collisions: – fixed target – colliders

• QGP formed in a tiny region

(10-14m) for very short time (10-23s)– Existence of a mixed phase?– Later freeze-out

• Collision dynamics: different observables sensitive to different reaction stages


Probing the QGP

penetrating beam(jets or heavy particles) absorption or scattering pattern

QGP

Rutherford experiment atom discovery of nucleus SLAC electron scatteringe proton discovery of quarks

Penetrating beams created by parton scattering before QGP is formed High transverse momentum particles jets Heavy particles open and hidden charm or bottom

Probe QGP created in Au+Au collisions Calculable in pQCD Calibrated in control experiments: p+p (QCD vacuum), p(d)+A (cold medium)

Produced hadrons lose energy by (gluon) radiation in the traversed medium QCD Energy loss → medium properties

Gluon density Transport coefficient

probe

bulk


Electromagnetic Radiation Thermal black body radiation

Real photons Virtual photons * which appear as dileptonseeor

No strong final state interaction Leave reaction volume undisturbed and reach detector

Emitted at all stages of the space time development Information must be deconvoluted

time

hard parton scattering

AuAu

hadronization

freeze-out

formation and thermalization of quark-gluon

matter?

Space

Time Jet cc e p K


What can we learn from dilepton emission?Emission rate of dilepton per volume

Boltzmann factortemperature

EM correlatorMedium property

ee decay

Hadronic contributionVector Meson Dominance

qq annihilation

Medium modification of mesonChiral restoration

From emission rate of dilepton, one can decode

• medium effect on the EM correlator • temperature of the medium

arXiv:0912.0244

Thermal radiation frompartonic phase (QGP)

q

q

e+

e-

e+

e-

+

-

Photonself-energy

QGP

HadronGas


Theory predictions for dilepton emission rate

Vacuum EM correlatorHadronic Many Body theoryDropping Mass Scenarioq+q → →ee (HTL improved)(q+g → q+→qee not shown)

Theory calculation by Ralf Rapp

dMdydpp

dN

tt

ee at y=0, pT=1.025 GeV/cUsually the dilepton emission is measured and compared as dN/dpTdM

The mass spectrum at low pT is distorted by the virtual photon → ee decay factor 1/M, which causes a steep rise near M=0

qq annihilation contribution is negligible in the low mass region due to the M2 factor of the EM correlator

In the caluculation, partonic photon emission processq+g → q+ → qe+e- is not included

1/M*→ee

qq → * → e+e-

≈(M2e-E/T)×1/M

arXiv:0912.0244


The mass of composite systems

mass given by energy stored in motion of quarks and by energy in colour gluon fields

M mi

binding energyeffect 10-8

atom 10-10 m

M » mi

nucleon 10-15 m

atomic nucleus 10-14 m

M mi

binding energyeffect 10-3

the role of chiral symmetry breaking

• chiral symmetry = fundamental symmetry of QCD for massless quarks

• chiral symmetry broken on hadron level


Chirality

left-handed

right-handed

Chirality (from the Greek word for hand: “”)when an object differs from its mirror image

simplification of chirality: helicity(projection of a particle’s spin on its momentum direction)

massive particles P left and right handed components must exist m>0 → particle moves w/ v<c

– 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 m

u = m

d = m

s = 0

– chirality is conserved


QCD and chiral symmetry breaking the QCD Lagrangian:

free gluon field free quarks ofmass mn

explicit chiral symmetry breaking mass term mnnn in the QCD Lagrangian

chiral limit: mu = md = ms = 0 chirality would be conserved

left–handed u,d,s, quarks remain left-handed forever all states have a ‘chiral partner’

(opposite parity and equal mass)

real life: mu and m

d are so small (m

u≈ 4 MeV m

d≈ 7 MeV)

that our world should be very close to chiral limit 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

interaction of quarkswith gluon


Origin of mass constituent quark mass

~95% generated by spontaneous chiral symmetry breaking (QCD mass)

current quark mass generated by spontaneous

symmetry breaking (Higgs mass) contributes ~5% to the visible

(our) mass


Chiral symmetry restoration spontaneous symmetry breaking gives rise to a nonzero ‘order

parameter’ QCD: quark condensate <qq> ≈ -250 MeV3

many models (!): hadron mass and quark condensate are linked

numerical QCD calculations at high temperature and/or high baryon density

→ deconfinement and <qq> → 0 approximate chiral symmetry restoration (CSR)

→ constituent mass approaches current mass

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 the degeneracy of chiral partners realized ? ● do the masses drop to zero? ● do the widths increase (melting resonances)?


Hadron massesG. Brown & M. Rho, PRL (1991) 2720

Brown-Rho scaling

Vacuum: Vector and Axial spectral functionswell separated (ALEPH data)

At Tc: Chiral symmetry restoration


CSR and low mass dileptons 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 a special probe for CSR, long lifetime but m(Ф)≈ 2 m(K) simultaneous measurement of φ → ee and φ → KK could be a

powerful tool to evidence in-medium effects

m [MeV] tot [MeV] [fm/c] BR → e+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


Dilepton Signal● What is its temperature?

→ measure thermal photons

● Does it restore chiral symmetry?→ modification of the vector mesons

● How does it affect heavy quarks?→ modification of the intermediate mass region

● All these questions can be answeredby measuring dileptons (e+e− or μ+μ−)

● no strong final state interactions:

● leave collision system unperturbed

● emitted at all stages: need todisentangle contributions


Dilepton Signal Dileptons characterized by 2 variables: M, pT

M: spectral functions and phase space factors pT: three contributions to pT spectra

pT - dependence of spectral function (dispersion relation)T - dependence of thermal distribution of “mother” hadron/partonM - dependent radial flow () of “mother” hadron/parton

Note I: M Lorentz-invariant, not changed by flow

Note II: final-state lepton pairs themselves only weakly coupled

dilepton pT spectra superposition of ‘hadron-like’ spectra at fixed T

early emission: high T, low T

late emission: low T, highT

final spectra from space-time folding over T- T history from Ti → Tfo

→ handle on emission region, i.e. nature of emitting source

mT

1/m

T d

N/d

mT

light

heavyT

purely thermalsource

explosivesource

T,

mT

1/m

T d

N/d

mT

light

heavy


Dilepton signal Low Mass Region:

mee < 1.2 GeV/c2

Dalitz decays of pseudo-scalar mesons Direct decays of vector mesons In-medium decay of mesons in the hadronic gas phase

Intermediate Mass Region: 1.2 < mee < 2.9 GeV/c2

correlated semi-leptonic decays of charm quark pairs

Dileptons from the QGP

High Mass Region: mee> 2.9 GeV/c2

Dileptons from hard processes–Drell-Yan process–correlated semi-leptonic decays of heavy quark pairs

–Charmonia –Upsilons

→ HMR probe the initial stage Little contribution from thermal radiation

• LMR: mee < 1.2 GeV/c2 o LMR I (pT >> mee)

quasi-real virtual photon region. Low mass pairs produced by higher order QED correction to the real photon emission

o LMR II (pT<1GeV)Enhancement of dilepton discovered at SPS (CERES, NA60)


HI low mass dileptons at a glance

(KEK E235) CERES

DLS

NA60

HADES

CBM

90 95 1000 0585

PHENIX

● Time scale of experiments

● Energy scale of experiments

(KEK E235)

CERES

DLS

NA60

HADES

CBM PHENIX

10 158 [A GeV]

17 [GeV]√sNN200

// // //

// // //

ALICE

[A TeV]

1


Dilepton Analysis Challenges● Experimental Challenge

● Need to detect a very weak source of pairs ~ 10-6 /π0 ● in the presence of hundreds of charged particles in central AA

collision● and several pairs per event from trivial origin

π0 Dalitz decays ~ 10-2/π0 + γ conversions (assume 1% radiation length) 2x10-2 /π0 → huge combinatorial background (dN∝

ch/dy)2

● Analysis Challenge

● Electron pairs are emitted through the whole history of the collision (from the QGP phase, mixed phase, HG phase and after freeze-out)

– need to disentangle the different sources.

– need excellent reference pp and dA data.

– need independent information about the known sources in nuclear collisions


Dilepton Analysis Steps● Tracking + Momentum reconstruction → Resolution

(position, momentum)● Particle Identification → Purity● Rejection close pairs → Significance,

Signal/Background

● Pairing: mee

= [2 p1p

2 (1-cosθ)]1/2

● Subtraction of Background(mixed events, like-sign)

● Efficiency Correction● Mass Spectrum


Remarks on S/B● how is the signal obtained?

● unlike-sign pairs: F● combinatorial background: B (like-sign pairs or event mixing)

→ S = F – B

● statistical error of S● depends on magnitude of B, not S

S ≈ √2B (for S<<B)

● “background free equivalent” signal Seq

● signal with same relative error in a situation with zero backgroundSeq = S * S/2Bexample: S = 104 pairs with S/B = 1/250 → Seq = 20

● systematic uncertainty of S● dominated by systematic uncertainty of B

S/S = B/B * B/Sexample: B/B = 0.25% precision, S/B = 1/250 → S/S = ~60% systematic uncertainty of S

S2 = F2 + B2 = S2 + B2 + B2

≈ 2B2 = 2B2

B = √B

Seq

/ Seq

= √2B / S

Seq

= √Seq

/ √S

eq = √2B / S

/ S

eq = 2B / S2


Separating Signal from Background


SPS @ CERN SuperProtonSynchrotron (since 1976)

parameters– circumference: 6.9 km– beams for fixed target

experiments– protons up to 450 GeV/c– lead up to 158 GeV/c

past– SppS proton-antiproton

collider discovery of vector bosons W±, Z

now– injector for LHC

experiments– Switzerland: west area (WA)– France: north area (NA) dileptons speak

french!


Experimental Setup: CERES-1

1992 setup – minimal configuration, no particle trackingOptimized for minimum response to hadrons and photons of hadronic origindouble RICH spectrometer (

thr=32), for eID and hadron rejection (hadron blind)

Magnetic field → azimuthal deflection → momentum measurement between the two radiators→ conversion pairs: double rings in RICH1, open up in RICH2


Experimental Setup: CERES-1bis

1995 setup – tracking: doublet of SiDC – RICH1 – RICH2 - PCRadial drift Silicon Detector: high resolution vertex and tracking

RICH1

RICH2


Target region

13

segmented target 13 Au disks (thickness: 25 m; diameter: 600 m)

Silicon drift chambers: provide vertex: z = 216 m provide event multiplicity ( = 1.0 – 3.9) powerful tool to recognize conversions at the target


Electron Identification: RICH

main tool for electron ID use the number of hits per ring (and their analog sum)

to recognize single and double rings


pA results dielectron mass spectra and expectation from a ‘cocktail’ of

known sources Dalitz decays of neutral mesons (→ e+e- and ’ dielectron decays of vector mesons ( → e+e-) semileptonic decays of particles carrying charm quarks

dielectron production in p+p and p+A collisions at SPS well understood in terms of known hadronic sources


AA resultsCERES PRL 92 (95) 1272

discovery of low mass e+e- enhancement in 1995 significant excess in S-Au (factor ~5 for m>200 MeV)


AA results dielectron excess at low and

intermediate masses in HI collisions is well established

onset at ~2 m → - annihilation?

maximum below meson near 400 MeV

→ hint for modified meson in dense matter

e-

e+

CERES Eur.Phys.Jour. C41(2005)475


CERES-1 → CERES-2

addition of a TPC to CERES improved momentum

resolution improved mass resolution dE/dx → hadron identification

and improved electron ID particle ID and momentum

measurement are separated inhomogeneous magnetic field

→ a nightmare to calibrate


CERES-2 results the CERES-1

results persists strong

enhancement in the low-mass region

enhancement factor (0.2 <m < 1.1 GeV/c2 ) → 3.1 ± 0.3 (stat.)

but the improvement in mass resolution isn’t outrageous


CERES at low-energy PRL 91 (2003) 042301

data taking in 1999 and 2000

improved mass resolution improved background

rejection results remain statistics

limited

Pb-Au at 40 AGeV enhancement for

mee> 0.2 GeV/c2

– 5.9±1.5(stat)±1.2(sys)±1.8(decay)

strong enhancement at lower sor larger baryon density

vacuum Brown-Rho scalingbroadening of


pT dependence

low mass e+e- enhancement at low pT

qualitatively in a agreement with annihilation pT distribution has little discriminative power

mee<0.2 GeV/c2 0.2<mee<0.7 GeV/c2 mee>0.7 GeV/c2

hadron cocktailBrown-Rho scalingbroadening of


Thermal radiation from HG low mass enhancement due to annihilation?

Thermal radiation from HG spectral shape dominated meson

vacuum vacuum values of width and mass

in-medium Brown-Rho scaling

– dropping masses as chiral symmetry is restoredconjecture that links hadron masses to the quark condensate.Effective QCD Lagrangian, quarks are the relevant d.o.f.

Rapp-Wambach melting resonances– ρ-meson scatters off particles in

the high density medium→ collision broadening of spectral function

– Pure hadronic model, only indirectly related to CSR

medium modifications driven by baryon density

model space-time evolution of collision

e-

e+

0

1/3

0

ρ

ρ

ρ

ρ

ρ0.161

qq

qq

m

m


Theory comparison attempt to attribute the

observed excess to vacuum meson ( )

– inconsistent with data– overshoot in region– undershoots @ low mass

modification meson – needed to describe data– data do not distinguish

between– broadening or melting of

-meson (Rapp-Wambach)– dropping masses (Brown-

Rho)

indication for medium modifications, but data are not accurate enough to distinguish models

largest discrimination between and → need mass resolution!


Theory comparison● Interpretation invoke+- → → * → e+e-Thermal radiation from hadron gas● Vacuum not enough to reproduce the data● In medium-modifications of :

● Broadening spectral fc. (Rapp-Wambach)● Dropping mass (Brown-Rho)● Thermal radiation: e+e- yield

from qq annihilation in pQCD (Kaempfer)

Data favor broadening scenarioUncertainties are large for a firm conclusion


Centrality dependence

naïve expectation: quadratic multiplicity dependence medium radiation particle density squared

more realistic: smaller than quadratic increase density profile in transverse plane life time of reaction volume

F=

yiel

d/c

ock

tail

mee<0.2 GeV/c2 0.2<mee<0.6GeV/c2 mee>0.6 GeV/c2

CERESpT > 200 MeV/c

1995/962000

Nch

strong centrality dependence→ challenge for theory !


Summary of CERES first systematic study of e+e- production in elementary

and HI collisions at SPS energies pp and pA collisions are consistent with the expectation

from known hadronic sources a strong low-mass low-pT enhancement is observed in

HI collisions

→ consistent with in-medium modification of the meson→ data can’t distinguish between two scenarios dropping mass as direct consequence of CSR collisional broadening of in dense medium

WHAT IS NEEDED FOR PROGRESS? STATISTICS MASS RESOLUTION


How to overcome these limitations more statistics

run forever → not an option higher interaction rate

– higher beam intensity– thicker target

needed to tolerate this– extremely selective hardware trigger– reduced sensitivity to secondary interactions, e.g. in target

→ can’t be done with dielectrons as a probe, but dimuons are just fine!

better mass resolution stronger magnetic field detectors with better position resolution → silicon tracker embedded in strong magnetic field!


The NA60 Experiment a huge hadron absorber

and muon spectrometer (and trigger!)

and a tiny, high resolution, radiation hard vertex spectrometer


Standard detection: NA50MuonOther

hadron absorber

target

beammagnetic field

thick hadron absorber to reject hadronic background trigger system based on fast detectors to select muon

candidates (1 in 104 PbPb collisions at SPS energy) muon tracks reconstructed by a spectrometer (tracking

detectors+magnetic field) extrapolate muon tracks back to the target taking into account

multiple scattering and energy loss, but … poor reconstruction of interaction vertex (z ~10 cm) poor mass resolution (80 MeV at the )

muon trigger and tracking


The NA60 experimentBased on the NA50 spectrometer with the addition of a Si tracker

Additional bend by the dipole fieldTrack matching in coordinate and momentum space• Improved dimuon mass resolution• Distinguish prompt from decay dimuonsDimuon coverage extended to low pT


The NA60 pixel vertex detectorDIPOLE MAGNET2.5 T

HADRON ABSORBER

TARGETS

~40 cm

1 cm

12 tracking points with good acceptance 8 small 4-chip planes 8 large 8-chip planes in 4 tracking stations

~3% X0 per plane 750 m Si readout chip 300 m Si sensor ceramic hybrid

800000 readout channels in 96 pixel assemblies


Vertexing in NA60

Beam Trackersensors

windows

z ~ 200 m along the beam directionGood vertex identification with 4 tracks

X

Extremely clean target identification (Log scale!)

Resolution ~ 10 - 20 m in the transverse plane


Contribution to mass resolution two components

multiple scattering in the hadron absorber– dominant at low momentum

tracking accuracy– dominant at high momentum

high mass dimuons (~3 GeV/c2) absorber doesn’t matter

low mass dimuons (~1 GeV/c2) absorber is crucial momentum measurement before

the absorber promises huge improvement in mass resolution

→ track matching is critical for high resolution low mass dimuon measurements!


Muon Track Matching

track matching has to be done in position space momentum space

to be most effective the pixel telescope has to be a spectrometer!

Muon spectrometer Pixel telescope

1p )1(

2p

2p

1z 2z

Absorber

Measured points Measured points


In+In: LMR Min. Bias unlike sign dimuon mass

distribution before quality cuts and without muon track matching

S/B ~ 1/7 , and even peak clearly

visible Mass resolution: 23 MeV at the

peak

drastic improvement in mass resolution

still a large unphysical background

BR = 5.8x10-6!


Fake matches

hadron absorber

muon trigger and tracking

targetfake

correct

Hadron absorber

Muon spectrometer

fake match: matched to wrong track in pixel telescope important in high multiplicity events

how to deal with fake matches keep track with best 2 (but is it right?) embedding of muon tracks into other event identify fake matches and determine the fraction of these relative to correct

matches as function of– centrality– transverse momentum


Event mixing: like-sign pairs compare measured and mixed like-sign pairs

accuracy in NA60: ~1% over the full mass range


In+In: LMR peripheral

Eur.Phys.J.C 49 (2007) 235

Well described by meson decay ‘cocktail’: η, η’, ρ, ω, f and DD contributions(Genesis generator developed within CERES and adapted for dimuons by NA60).

Similar cocktail describes NA60 data: p-Be, p-Pb, In-In peripheral

Eur.Phys.J.C 43 (2005) 407


Digression: EM transition form factor

56

In-In, peripheral

hep-ph/0902.2547

Acceptance-corrected data (after subtraction of , and peaks) fitted by three contributions:

2/1

2

2

2

23

2

2

22

41

211

)(

3

2)(

m

m

m

m

m

m

mdm

d 22 )( mF

TM

eMTMmM

M

m

M

m

M

m

m

Md

Rd

2/3

22222

2

22/1

2

22/3

2

2

4

42

2

21

41

41

)2(3

)(

22 )( mF

2/3

22

222

22

22/1

2

2

2

2

2

0

2

0

00

41

41

21

)(

3

)(

mm

mm

mm

m

m

m

m

m

mdm

d

Confirmed anomaly ofF wrt VDM prediction.

Improved errors wrt the Lepton-G results.

Removes FF ambiguity in the ‘cocktail’

pole approximation:

22222 /1)( mmF

0


Cocktail subtraction (w/o )

ω and : fix yields such as to get, after subtraction, a smooth underlying continuum

:

- set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV (lower limit for excess).- use yield measured for pT > 1.4 GeV/c

how to nail down an unknown source? → try to find excess above cocktail without fit constraints


Excess vs centralityData – cocktail (all p

T)

● No cocktail and DD subtracted● Clear excess above cocktail , - centered at the nominal pole - rising with centrality● Excess even more pronounced at low p

T

Confirm CERES data!


Excess shape vs centralityQuantify the peak and the broad symmetric continuum with a mass interval C around the peak (0.64 <M<0.84 GeV) and two equal side bins L, U

continuum = 3/2(L+U) peak = C-1/2(L+U)

Peak/cocktail drops by a factor 2 from peripheral to central:

the peak seen is not the cocktail

nontrivial changes of all three variables at dNch/dy>100 ?

peak/

continuum/

peak/continuum

Fine analysis in 12 centrality bins


Theory comparison

data consistent with

broadening of (RW),mass shift (BR) not needed

Rapp & Wambach hadronic model with strong broadening but no mass shift

Brown & Rho dropping mass due to dropping chiral condensate

calculations for all scenarios in In-In for dNch/d = 140 (Rapp et al.)

spectral functions after acceptance filtering, averaged over space-time and momenta

Keeping original normalization


Acceptance corrected spectrum

Mass spectrumcorrected for acceptancein M-p

T


In+In: IMR hadron-parton duality

Rapp / van Hees Ruppert / Renk

dominant at high M hadronic processes 4

dominant at high M partonic processes mainly qqbar annihilation


IMR: the NA50 measurement

centralcollisions

M (GeV/c2)

NA50: excess observed in IMR in central Pb-Pb collisions

charm enhancement? thermal radiation?

answering this question was one of the main motivations for building NA60

Drell-Yan and Open Charm are the main contributions in the IMR• p-A is well described by the sum of these two contributions (obtained from Pythia)• The yield observed in heavy-ion collisions exceeds the sum of DY and OC decays,extrapolated from the p-A data.• The excess has mass and pT shapes similar to the contribution of the Open Charm (DY + 3.6xC nicely reproduces the data).


IMR: disentangling the sources

D0

K-

+

e

D0

100m

charm quark-antiquark pairs are mainly produced in hard scattering processes in the earliest phase of the collisions

c c

0DK

l

0D

l

K+

-

charmed hadrons are “long” lived → identify the typical offset (“displaced vertex”) of D-meson decays (~100 m)

need superb vertexing accuracy (20-30 m in the transverse plane) → NA60


IMR: disentangling the sources measure for vertex displacement

primary vertex resolution momentum dependence of secondary vertex resolutions → “dimuon weighted offset”

charm decays (D mesons) → displaced J/→prompt

vertex tracking is well under control!


IMR excess is prompt approach

Dix Drell-Yan (within 10%)

Fix prompt component Charm can't describe the small

offset region

Fix charm component Good description of offset

→ IMR excess is a prompt component

Fit range

DD

DY

1.120.17

DataPrompt: 2.290.08Charm: 1.160.16Fit 2/NDF: 0.6

DD

Prompt

Eur.Phys.J. C59 (2009) 607

DD

Prompt

~50m ~1mm


Analysis of mT

decomposition of low mass region

contributions of mesons (,,) continuum plus meson extraction of vacuum

hadron mT spectra for ,, vacuum

dilepton mT spectra for low mass excess intermediate mass excess

Phys. Rev. Lett. 96 (2006) 162302


Interpretation of Teff

interpretation of Teff from fitting to exp(-mT/Teff) static source: Teff interpreted as the source temperature radially expanding source:

– Teff reflects temperature and flow velocity

– Teff depends on the mT range

– large pT limit: common to all hadrons

– low pT limit: mass ordering of hadrons

final spectra: space-time history Ti→Tfo & emission time hadrons

– interact strongly– freeze out at different times depending on cross section with pions– Teff → temperature and flow velocity at thermal freeze out

dileptons– do not interact strongly– decouple from medium after emission– Teff → temperature and velocity evolution averaged over emission time

mpTT TT

Tfeff

v1

v1

mpmTT TTfeff v2

1 2


Mass ordering of hadronic slopes

158 AGeV Central collisions

Pb-Pb

In-In

Si-Si

C-C

pp

separation of thermal and collective motion reminder

blast wave fit to all hadrons simultaneously

simplest approach

slope of <Teff> vs. m isrelated to radial expansion

baseline is related tothermal motion

works (at least qualitatively) at SPS

mpmTT TTfeff v2

1 2


Example of hydrodynamic evolution

vT =

0.1

vT =

0.4

(specific for In-In: Dusling et al.)

hadronphase

partonphase

monotonic decrease of T from

early times to late times

medium center to edge

monotonic increase of vT from

early times to late times

medium center to edge

dileptons may allow to disentangle emission times

early emission (parton phase)– large T, small vT

late emission (hadron phase)– small T, large vT


mT distributions

Fit with: effT Tm

TT

edmm

dN /

Extract Teff

from each mass slice

Extract -peakSame-side window:continuum = 3/2(L+U) peak = C-1/2(L+U)

IMR

LMR

-region


Dilepton Teff

systematics hadrons ()

Teff depends on mass Teff smaller for

→ decouples early Teff large for

→ decouples late

low mass excess clear flow effect visible follows trend set by hadrons possible late emission

intermediate mass excess no mass dependence indication for early emission Close to T

c, critical

temperature where phase transition occursEur.Phys.J. C (2009), in press, nucl-ex/0812.3053


Digression: Polarization

73

NA60 also measured the polarization (in the Collins-Soper frame) for m≤ m

Lack of any polarization in excess (and in hadrons) supports emission from thermalized source.

2cossin2

cos2sincos11 22

dd

Submitted to PRL, nucl-ex/0812.3100


Digression: in medium effects?

Flattening of the pT distributions at low pT, developing very fast with centrality.

Low-pT ω’s have more chances to decay inside the fireball ?

Appearance of that yield elsewhere in the spectrum, due to ω mass shift and/or broadening, unmeasurable due to masking by the much stronger contribution.

Disappearance of yield out of narrow ω peak in nominal pole position

Can only measure disappearance

Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053


Digression: in medium effects?Determine suppression vs pT with respect to (extrapolated from pT>1GeV/c)

Account for difference in flow effects using the results of the Blast Wave analysis

effTT TmdmdN exp~/ 2

Reference line: ω/Npart = 0.131 f.ph.s.

Strong centrality-dependent suppression at pT<0.8 GeV/c ,

beyond flow effects

Eur.Phys.J. C (2009),nucl-ex/0812.3053

Reference line: /Npart = 0.0284 f.ph.s. (central

coll.)

Consistent with radial flow effects


Summary NA60 high statistics & high precision dimuon spectra decomposition of mass spectra into “sources” LMR:

– access to in-medium spectral function

– data consistent with broadening of the – data do not require mass shift of the

IMR:– large prompt component at intermediate masses

dimuon mT spectra promise to separate time scales low mass dimuons shows clear flow contribution indicating late

emission intermediate mass dimuons show no flow contribution hinting

toward early emission

goethe university frankfurt gsi helmholtzzentrum für...

Documents