INT 10-2A, July 13, 2010

Itzhak Tserruya

Dileptons: outstanding issues and prospects

Itzhak Tserruya INT 10-2A, July 13, 2010 2

Outline Introduction

SPS results Low-mass region (CERES and NA60) Intermediate mass region (NA50, NA60)

RHIC results first results from PHENIX

Prospects with the HBD

Low energy (DLS and HADES)

meson Summary

Introduction The Quark Gluon Plasma created in relativistic heavy ion

collisions is characterized by two fundamental properties:

DeconfinementChiral Symmetry Restoration

Electromagnetic probes (real or virtual photons) are sensitive probes of both properties and in particular lepton pairs are unique probes of CSR.

Thermal radiation emitted in the form of dileptons (virtual photons) provides a direct fingerprint of the matter formed: QGP (qqbar annihilation) and dense HG (+- annihilation)

What have we learned in almost 20 years of dilepton measurements?

PHENIX + HBD STAR?

Dileptons in A+A at a Glance:

Itzhak Tserruya

90 95 1000 0585

PHENIX

Time Scale

= Period of data taking

4INT 10-2A, July 13, 2010

Energy Scale

10 158 [A GeV]

17 [GeV]√sNN200

// // //

PHENIXMPD

SPS Low-masses(m 1GeV/c2)

Itzhak Tserruya 5INT 10-2A, July 13, 2010

Consistent story between CERES and NA60 results

CERES Pioneering Results (I) Strong enhancement of low-mass e+e- pairs

(wrt to expected yield from known sources)

Enhancement factor (0.2 <m < 1.1 GeV/c2 ):

2.45 ± 0.21 (stat) ± 0.35 (syst) ± 0.58 (decays)

No enhancement in pp

nor in pA

Last CERES result (2000 Pb run PLB 666(2008) 425)

Itzhak Tserruya INT 10-2A, July 13, 2010

CERES Pioneering Results (II)

Strong enhancement of low-mass e+e- pairs in all A-A

systems studied

First CERES result PRL 75, (1995) 1272

Last CERES result PLB 666 (2008) 425

Eur. Phys J. C41 (2005) 475 PRL 91 (2003) 042301

Better tracking and better mass resolution (m/m = 3.8%) due to: Doublet of silicon drift chambers close to the vertex Radial TPC upgrade downstream of the double RICH spectrometer

pT and Multiplicity Dependencies

Enhancement is mainly at low pT

Increases faster than linearly with multiplicity

Itzhak Tserruya

Dropping Mass or Broadening (I) ? Interpretations invoke:

* +- * e+e-

thermal radiation from HG

dropping meson mass (Brown et al)

* in-medium modifications of : broadening spectral shape (Rapp and Wambach)

CERES Pb-Au 158 A GeV 95/96 dataCERES Pb-Au 158 A GeV 95/96 data

* vacuum ρ not enough to reproduce data

Dropping Mass or Broadening (I) ? Interpretations invoke:

* +- * e+e-

thermal radiation from HG

* in-medium modifications of :

broadening spectral shape (Rapp and Wambach)

dropping meson mass (Brown et al)

CERES Pb-Au 158 A GeV 2000 dataCERES Pb-Au 158 A GeV 2000 data

* vacuum ρ not enough to reproduce data

Data favor the broadening scenario.

NA60 Low-mass dimuons in In-In at 158 AGeV

, and even peaks clearly visible in dimuon channel

S/B = 1/7

Mass resolution:23 MeV at the position

Real data !

Superb data!

11Itzhak Tserruya INT 10-2A, July 13, 2010

Dimuon Excess PRL 96 (2006) 162302

Dimuon excess isolated by subtracting the hadron cocktail (without the )

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

Excess centered at the nominal ρ pole

confirms & consistent with, the CERES results

Excess rises and broadens with centrality

More pronounced at low pT

NA60 low mass: comparison with models

• All calculations normalized to data at m < 0.9 GeV performed by Rapp et al., for <dNch/d> = 140

Excess shape consistent with broadening of the

(Rapp-Wambach)

Mass shift of the (Brown-Rho) is ruled out

Is this telling us something

about CSR?

Subtract the cocktail from the data (without the )

PRL 96 (2006) 162302

Intermediate masses(m = 1-3 GeV/c2)

Thermal radiation from the partonic phase?

NA50 IMR Results 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 +

3.6OC nicely reproduces the data).

Drell Yan + Open charm

Drell Yan + 3.6 x Open charm

charm enhancement?

Itzhak Tserruya

NA60: IMR excess in agreement with NA50

IMR yield in In-In collisions enhanced compared to expected yield from DY and OC

Can be fitted with fixed DY (within 10%) and OC enhanced by a factor of ~3 Fit range

4000 A, 2 <1.5

2.90.14

2.750.14 1.120.17

Free prompt and open charm scaling factors

Full agreement with NA50

… But the offset distribution (displaced vertex) is not compatible with this assumption

Fixed prompt and free open charm

NA60: IMR excess is a prompt source

Origin of the IMR Excess

17Itzhak Tserruya

Hees/Rapp, PRL 97, 102301 (2006) Renk/Ruppert, PRL 100,162301 (2008)

Dominant process in mass region m > 1 GeV/c2:

INT 10-2A, July 13, 2010

hadronic processes, 4 … partonic processes, qq annihilation

Quark-Hadron duality?

NA60 excess: absolutely normalized mass spectrum

pT distributions Low-mass region Intermediate mass region

Fit in 0.5<PT<2 GeV/c(as in LMR analysis)

The mT spectra are exponential, the inverse slopes do not depend on mass.

The mT spectra are exponential, the inverse slopes depend on mass.

Radial Flow

Thermal radiation from partonic phase?

Itzhak Tserruya

RHIC results

Dileptons in PHENIX: p+p collisions

22Itzhak Tserruya

Mass spectrum measured from m = 0 up to m = 8 GeV/c2

Very well understood in terms of: hadron cocktail at low masses heavy flavor + DY at high masses

INT 10-2A, July 13, 2010

Dileptons in PHENIX: Au+Au collisions

Low masses: strong enhancement in the mass range

m = 0.2 – 0.7 GeV/c2. Enhancement extends down to very low masses Enhancement concentrated at central collisions

No enhancement in the IMR ?

Low mass region: evolution with pT

Excess present at all pair pT but more pronounced at low pair pT

mT distribution of low-mass excess

PHENIX

The excess mT distribution exhibits two clear components

It is well described by the sum of two exponential distributions with inverse slope parameters:

T1 = 92 11.4stat 8.4syst MeV

T1 = 258.3 37.3stat 9.6syst MeV

All this is very different from the SPS

results

Comparison to theoretical model (Au+Au)PHENIX

All models that successfully described the SPS data fail in describing the PHENIX results

Low-mass pair excess at RHIC The low-mass pair enhancement observed in Au+Au at

√sNN = 200 GeV implies at least two sources.

Source I: e+ e- (with intermediate modified in the medium mainly through scattering off baryons) as observed at CERN, must be present at RHIC also.

Pion annihilation (Rapp – Van Hees) is insufficient to describe the data

Source II - The remaining excess – Origin not at all clear

Obvious question: when does this second source appear?

Au+Au vs Cu+Cu

Npart = 98

Is there enhancement in the IMR also?

Cu+Cu Centrality Spectra

Au+Au vs Cu+Cu: surprising results

In Cu+Cu like in Au+Au the enhancement is observed only in most central collisions.

But for all observables I know, there is no difference in the results from Cu+Cu and Au+Au when compared at the same number of participants (global observables, J/ suppression, …. )

Are low-mass electron pairs different?

IMR: no enhancement in Au+Au. Is there an enhancement in Cu+Cu?

Prospects at RHIC

Dileptons in PHENIX: Au+Au collisions

All pairsCombinatorial BGSignal • BG determined by event mixing

technique, normalized to like sign yield

• Green band: systematic error w/o error on CB

Integral:180,000 above p0:15,000

PHENIX has mastered the event mixing technique to unprecedented precision (±0.25%). But with a S/B ≈ 1/200 the

statistical significance is largely reduced and the systematic errors are large

Min bias Au+Au √sNN = 200 GeVarXiv: [nucl-ex]

Matching resolution in z and

HBD Installed and fully operational in Run9 and Run10

Single vs double e separation

Hadron blindnessh in F and R bias e-h separation h rejection

What can we expect from Run-10

In Run-10 PHENIX accumulated a large sample of Au+Au collisions at:

√sNN = 200 GeV

Better quality data over the entire mass range

Significant improvement of S/B in the LMR

Further characterization (better centrality dependence) of the low mass excess

Good quality data on LVM, RAA of and , in particular comparison of

KK and ee.

IMR: confirm whether or not the yield is enhanced

Additional measurement of charm cross section using high pT electrons with less background, different systematic and smaller errors

√sNN = 62.4 GeV (and 39 GeV?)

Onset of the second source?

Thermal Radiation at RHIC

Itzhak Tserruya

Thermal radiation at RHIC (I) Search for the thermal radiation in the dilepton spectrum Avoid the huge physics background inherent to a real photon

measurement. Capitalize on the idea that every source of real photons should also

emit virtual photons. At m0, the yield of virtual photons is the same as real photon

Real photon yield can be measured from virtual photon yield, observed as low mass e+e- pairs

36INT 10-2A, July 13, 2010

Enhancement of (almost real photons) low-mass dileptons

Restricted kinematic window: Low mass e+e- pairs m<300MeV & 1<pT<5 GeV/c

p+p:• Good agreement of p+p data

and hadronic decay cocktail • Au+Au:• Clear enhancement visible

above mp =135 MeV for all pT

1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV

Excess Emission of almost real photons

Thermal radiation from the QGP at RHICe+e- invariant mass excess: - transformed into a spectrum of real photons under the assumption that the excess is entirely due to internal conversion of photons.- compared to direct (real) photon measurement (pT>4GeV)

NLO pQCD (W. Vogelsang)

Good agreement in range of overlap

pQCD consistent with p+p down to pT=1GeV/c

Au+Au data are above Ncoll scaled p+p for pT < 2.5 GeV/c

Fit Au+Au excess with exponential function + ncoll scaled p+p

Tave = 221 19stat 19syst MeV corresponds to

Tini = 300 to 600 MeV t0 = 0.15 to 0.6 fm/c

exp + ncoll scaled pp

Low-energies:

DLS and HADES

DLS “puzzle”

Strong enhancement over hadronic cocktail with “free” spectral function

DLS data: Porter et al., PRL 79, 1229 (1997)

Calculations: Bratkovskaya et al., NP A634, 168 (1998)

Enhancement not described by in-medium spectral function All other attempts to reproduce the DLS results failed Main motivation for the HADES experiment

HADES confirms the DLS results

Mass distribution pT distribution

Putting the puzzle together (I)

Spectra normalized to 0 measured in C+C and NN

C+C @ 1 AGeV: <M>/Apart = 0.06 ± 0.07

N+N @ 1.25 GeV (using pp and pd measurements)<M

NN>/Apart = 1/4(pp+2pn+nn)/2 = 1/2(pp+pn) = 0.0760.015

C+C @ 1 AGeV – pp & pd @ 1.25 GeV

Dielectron spectrum from C+C consistent with superposition of NN collisions!

No compelling evidence for in-medium effects in C+C

Putting the puzzle together (II)

Recent transport calculations:

enhanced NN bremsstrahlung , in line with recent OBE calculations

HSD: Bratkovskaya et al. NPA 807214 (2008)

The DLS puzzle seems to be reduced to an understanting of the elementary contributions to NN reactions.

The meson l+l- and K+K-

Inconclusive results

Inconclusive results SPS

PHENIX

Uncertainties in the e+e- channel too large for a conclusive statement. Waiting for HBD improved results

The reanalyzed NA50 results in and the CERES results in the ee are compatible within 1-2σ and within errors there is room for some effect.

Summary Consistent and coherent picture from the SPS: Low-mass pair enhancement: thermal radiation from the HG Approach to CSR proceeds through broadening (melting) of the resonances IMR enhancement: thermal radiation from partonic phase

RHIC results very intriguing: Strong enhancement of low-mass pairs down to very low masses Enhancement observed only in central Au+Au and Cu+Cu collisions No enhancement in the IMR ? Challenge for theoretical models Looking forward to more precise results with the HBD

DLS puzzle solved in C+C. Dilepton spectrum understood as mere superposition of NN collisions. Is that so also for heavier system? Onset of low-mass pair enhancement?

meson – elusive probe

INT 10-2A, July 13, 2010

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