What does the rho do? NA60’s dimuon experiment and in-medium modifications of vector mesons

J. Ruppert Quark Matter 2006, Nov. 2006

What does the rho do? NA60’s dimuon

experiment and in-medium modifications

of vector mesonsJörg Ruppert

Nuclear Theory, Department of Physics,McGill University, Montreal, Quebec, Canada

In collaboration with:

Thorsten RenkDepartment of Physics,

University of Jyväskylä, Jyväskylä, Finland


Outline

1. Dilepton emission in URHIC2. Dynamical Evolution3. In-Medium modifications4. NA60’s 158 AGeV In-In di-muon

spectra 5. Conclusions


Focus on dileptons in low mass region in this talk

EM probes in URHICEM probes in URHIC


What to learn from dilepton emission?

What to learn from dilepton emission?

Averaged in-medium photon spectral function(EM correlator)

In-medium EM current-current correlator and dynamical evolution of the system are folded in order to predict the spectrum.

On top of that contributions from non-thermal sources (hadron decays, Drell-Yan, open charm etc.)

(modulus factor accounting for finite lepton masses)

(Equilibrium description, for firstnon-equilibrium studies see Schenke, Greiner)


Evolution vs. EM correlator

Evolution vs. EM correlator


QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Reliable (extensively tested for PbPb) fireball evolution model as basis for In-In calculations!

T. Renk (2005)

Dynamical evolution model for URHIC

Dynamical evolution model for URHIC


“Vacuum rho” vs. “cocktail rho”“Vacuum rho” vs. “cocktail rho”

Concept introduced and importance of vacuum rho contribution

pointed out in Renk, Ruppert, hep-ph/0605130.

Contribution is important for all centrality classes!


Flow profile and pt Flow profile and pt


Vector-meson dominanceVector-meson dominance

The current-field identity(J. J. Sakurai)

μυυμμυυμρρ Π⇒⇒⇒ ImImImIm JJD Spectral density

Dilepton emission in URHIC in the low mass region can teach us about in-medium modifications of vector mesons spectral densities once the dynamical evolution is fixed!


In-Medium vector mesons

In-Medium vector mesons

Vector-meson spectral densities as calculated in hadronicmany-body approaches. E.g. Rapp & Wambach (1999), Lichard & Gale (1994), Renk&Mishra (2004), Lichard & Juran (2006)

Vector-meson spectral densities as inferred from experiment. E.g. Shuryak (1991); Eletsky & Ioffe (1997); Eletsky, Belkacem, Ellis, Kapusta (2001); Martell & Ellis (2004), Klingl, Kaiser, Weise (1997)


Hot meson gas in Phi-functional approach

Hot meson gas in Phi-functional approach

Nota Bene: Results for in-medium modifications of the rho-meson based onRuppert, Renk, Phys.Rev.C71:064903,2005 are to be reinvestigated (Calculations

for an erratum are in progress. Prepints using this spectral function for

dilepton studies will be updated.) Thanks to Knoll, Riek, and van Hees for pointingout a wrong numerical factor in the self-energy formulas and discussions regarding

the influence of spurious modes on the results.

Phi-Functional approach [Baym, Luttinger, Ward, Cornwall, Jackiw, Tomboulis]

All results presented here don’t rely on this approach.


Vector Meson spectral densities as inferred from experiment

Vector Meson spectral densities as inferred from experiment

Eletsky, Belkacem, Ellis, Kapusta (2001)

Contribution to retarded self-energy from rho/particle pion/nucleon scattering

Scattering amplitude in cm-frame from rho/ particle pion/nucleon scattering

Shuryak (1991), Eletsky & Ioffe (1997)


Comparison to NA60 data

Comparison to NA60 data

QGP quasi-particle picture,(Schneider et al. (2002))

In-medium Rho spectral function(Eletsky et. al. (2001))

In-medium Omega spectral function(Martell & Ellis (2004))

(Results were obtained by folding with the schematic acceptance matrix,

not the full MC acceptance simulation.)

In-medium Phi not (yet)included

Renk, Ruppert (2006)

Contributions included in our calculation:


Pt-cut M-spectraPt-cut M-spectra

(Results were obtained by folding with the schematic acceptance matrix,not the full MC acceptance simulation. Theory curves for high p_T are integratedup to p_T<1.5 GeV.)



Results for central collisions

Results for central collisions

(Results were obtained by folding with the schematic acceptance matrix,not the full MC acceptance simulation.)



pt - spectrapt - spectra

•Good description in low-mass, rho-like, and higher-mass region can already be obtained with the original version of the scaled evolution (hep-ph/0605130). Adjustment in flow profile lead only to a ~10 MeV change in slope. Model accounts for the data above p_T>0.5 GeV.


What to learn from pt - spectra?

What to learn from pt - spectra?

•Rather insensitive to intrinsic momentum dependences of in-medium hadronic spectral-functions and QGP rate, sensitive to emission temperature and flow => Tool to characterize emission region.

•pT-spectrum above ~1 GeV (in 0.6<M<0.9 GeV integrated region)and above ~1.25 GeV (in 0.4<M<0.6 GeV) dominated by vacuum-rho,naturally small contribution to the M>1.0 GeV region.

•Effective T* is significantly lower in 1.0<M<1.4 GeV integrated region. This is an strong indication of a different source in comparison to lower mass regions.

•pT-spectrum in 1.0<M<1.4 GeV integrated region is in our evolutiondominated by a partonic source.

J. Ruppert Quark Matter 2006, Nov. 2006Compilation by S. Damjanovic

pt -spectra model-comparison

pt -spectra model-comparison


What about 4pi contribution?What about 4pi contribution?Suggested as dominant source due to chiral mixing in the M>1 GeV region by van Hees & Rapp (2006) (for details of their approach see next talk).

We employed a different rate to study 4-pi contributions in our evolution, namely

4-pion annihilation rates by Lichard (2006) (cmp. also Lichard & Juran (2006)).

Assumption: 4pi-processes are contributing as 2pi-processes all the way down to Tf=130 MeV (augmented by the corresponding fugacity factor) .

Gives an upper limit of the contribution.

Lichard (2006), hadronic interaction adjusted to describe BaBar Data

In-medium-Phi not(yet) included.



Discrimination of dominant source in M>1.0 GeV region via pt

- spectra

Discrimination of dominant source in M>1.0 GeV region via pt

- spectra Different mechanisms:Experimentum crucis•QGP dominant, contributes at higher T, low flow. If this is the dominant source=> lower effective T* in dilepton pt - spectra (1.0<M<1.4 GeV

integrated)

•4pi-annihilation, contributes significantly close to Tf << Tc

(augmented by fugacity factor), considerable flow has built up.

If dominant source => higher effective T* in pt - spectra (1.0<M<1.4 GeV integrated)

General argument based on flow is not specific to one evolution model!


SummarySummary•Reliable dynamical evolution essential to infer information about in-medium modifications of EM-current-current correlator

•Different sources (Vacuum rho, in-medium vector mesons, QGP, 4pi) built up dilepton-spectrum! Still clear message from low mass dileptons => Substantial in-medium broadening of the Rho-meson necessary to describe low mass dimuon enhancement.

• Substantial broadening of Rho-meson caused by scattering off from nucleons and pions (Eletsky et al. (2002)), partial contribution from pion scattering in In-In substantial. In-medium broadening including scattering off by nucleons and pions seem to be on the same order as Rapp/Wambach (1999) approach. (see talks R. Rapp and H. van Hees).

•pt - spectra can probe different stages of the medium-evolution and reveal information about the dominant sources in different mass regions. Important:flow must be implemented consistently.

•Experimental pt - spectra indicates substantial contribution from partonic component in the 1 GeV<M<1.5 GeV region.

•Outlook: Calculations for the IMR region 1 Gev<M<3 GeV.

Special thanks to

S. Damjanovic, C. Gale,

J. Kapusta,

P. Lichard, B. Müller,

B. Neufeld, H. Specht



4-pion annihilation as inferred from hadronic

interactions

4-pion annihilation as inferred from hadronic

interactions

Lichard (2006)

Example: Annihilation of four chargedpions into a rho-meson and viaVMD into dileptons.

Comparison between model prediction an inverse process Measurement byBaBaR.


Documents

What does the rho do? NA60’s dimuon experiment and in-medium modifications of vector mesons