Transcript "Dileptons: outstanding issues and prospects"

Dileptons: outstanding
issues and prospects
INT 10-2A, July 13, 2010
Itzhak Tserruya
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
Itzhak Tserruya
INT 10-2A, July 13, 2010
2
Introduction

The Quark Gluon Plasma created in relativistic heavy ion
collisions is characterized by two fundamental properties:
 Deconfinement
 Chiral 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?
Dileptons in A+A at a Glance:
Time Scale
Energy
Scale
CBM
NA60
HADES
CBM
MPD
NA60
PHENIX + HBD
PHENIX
STAR?
HADES
CERES
PHENIX
CERES
DLS
DLS
MPD
//
85
901
//
95
10
//
158
00
//
//
05
10
[A GeV]
//
17
200
√sNN [GeV]
= Period of data taking
Itzhak Tserruya
INT 10-2A, July 13, 2010
4
SPS Low-masses
(m  1GeV/c2)
 Consistent story between CERES and NA60 results
Itzhak Tserruya
INT 10-2A, July 13, 2010
5
CERES Pioneering Results (I)
Strong enhancement of low-mass e+e- pairs
(wrt to expected yield from known sources)
Last CERES result
(2000 Pb run PLB 666(2008) 425)
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
Itzhak Tserruya
INT 10-2A, July 13, 2010
6
CERES Pioneering Results (II)
First CERES result
Last CERES result
PRL 75, (1995) 1272
PLB 666 (2008) 425
Strong enhancement
of low-mass e+eBetter tracking and better mass resolution (m/m = 3.8%) due to:
pairs in all A-A
 Doublet of silicon drift chambers
systems close
studiedto the vertex
 Radial TPC upgrade downstream of the double RICH spectrometer
Eur. Phys J. C41 (2005) 475
Itzhak Tserruya
PRL 91 (2003) 042301
INT 10-2A, July 13, 2010
7
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:
CERES Pb-Au 158 A GeV
95/96 data
* +-  *  e+e-
thermal radiation from HG
* vacuum ρ not enough to
reproduce data
* in-medium modifications of :

broadening  spectral shape
(Rapp and Wambach)

dropping  meson mass
(Brown et al)
Itzhak Tserruya
INT 10-2A, July 13, 2010
9
Dropping Mass or Broadening (I) ?
Interpretations invoke:
CERES Pb-Au 158 A GeV 2000 data
* +-  *  e+e-
thermal radiation from HG
* vacuum ρ not enough to reproduce data
* in-medium modifications of :
broadening  spectral shape

(Rapp and Wambach)
dropping  meson mass

(Brown et al)
Itzhak Tserruya
INT 10-2A, July 13, 2010
Data favor the broadening
scenario.
10
NA60 Low-mass dimuons in In-In at 158 AGeV
Superb data!



Mass resolution:
23 MeV at the  position


S/B = 1/7
,  and even  peaks
clearly visible in dimuon
channel

Itzhak Tserruya
INT 10-2A, July 13, 2010
11
Dimuon Excess
Eur.Phys.J.C
PRL 96 (2006)49162302
(2007) 235
Dimuon excess isolated by subtracting
the hadron cocktail (without the )
Excess centered at the
nominal ρ pole

Excess rises and broadens
with centrality


More pronounced at low pT
confirms & consistent with,
the CERES results
NA60 low mass: comparison with models
PRL 96 (2006) 162302


Subtract the cocktail from the data
(without the )

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?
All calculations normalized to data at m < 0.9 GeV
performed by Rapp et al., for <dNch/d> = 140
•
Itzhak Tserruya
INT 10-2A, July 13, 2010
13
SPS
Intermediate masses
(m = 1-3 GeV/c2)
 Thermal radiation from the partonic phase?
Itzhak Tserruya
INT 10-2A, July 13, 2010
14
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 + 3.6 x Open charm
Drell Yan + Open charm
charm enhancement?
Itzhak Tserruya
NA60: IMR excess in agreement with NA50
2.90.14

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
… But the offset
Full agreement with NA50
NA60:
distribution (displaced vertex) is not compatible with this assumption
IMR excess is a prompt source
Fixed prompt and free open charm
2.750.14
Free prompt and open charm scaling factors
1.120.17
Origin of the IMR Excess
Hees/Rapp, PRL 97, 102301 (2006)
Renk/Ruppert, PRL 100,162301 (2008)
Dominant process in mass region m > 1 GeV/c2:
hadronic processes, 4 …
partonic processes, qq annihilation
Quark-Hadron duality?
Itzhak Tserruya
INT 10-2A, July 13, 2010
17
NA60 excess: absolutely
normalized mass spectrum
Itzhak Tserruya
INT 10-2A, July 13, 2010
18
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 depend on
mass.
The mT spectra are
exponential, the inverse
slopes do not depend
on mass.
 Radial Flow
Thermal radiation
from partonic phase?
Itzhak Tserruya
RHIC results
Itzhak Tserruya
INT 10-2A, July 13, 2010
21
Dileptons in PHENIX: p+p collisions


Itzhak 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
22
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
The excess mT distribution exhibits
two clear components
 It is well described by the sum of
two exponential distributions with
inverse slope parameters:
PHENIX

T1 = 92  11.4stat  8.4syst MeV

T1 = 258.3  37.3stat  9.6syst MeV
All this is very
different from the SPS
results
Itzhak Tserruya
INT 10-2A, July 13, 2010
25
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?
Itzhak Tserruya
INT 10-2A, July 13, 2010
27
Au+Au vs Cu+Cu
Npart = 98
Is there enhancement in the IMR also?
28
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?
Itzhak Tserruya
INT 10-2A, July 13, 2010
30
Prospects at RHIC
Itzhak Tserruya
INT 10-2A, July 13, 2010
31
Dileptons in PHENIX: Au+Au collisions
Min bias Au+Au √sNN = 200 GeV
arXiv:
[nucl-ex]
All pairs
Combinatorial BG
Signal
Integral:180,000
above 0:15,000
• BG determined by event mixing technique,
normalized to like sign yield
• Green band: systematic error w/o error on CB
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
Matching resolution in z and 
Single vs double e separation
HBD
Installed and fully operational in Run9 and Run10
Hadron blindness
h 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?
Itzhak Tserruya
INT 10-2A, July 13, 2010
34
Thermal Radiation
at RHIC
Itzhak Tserruya
INT 10-2A, July 13, 2010
35
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 m0, 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
Itzhak Tserruya
INT 10-2A, July 13, 2010
36
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 m =135 MeV for all pT
Itzhak Tserruya
1 < pT < 2 GeV
2 < pT < 3 GeV
3 < pT < 4 GeV
4 < pT < 5 GeV
Excess  Emission of almost
real photons
INT 10-2A, July 13, 2010
37
Thermal radiation from the QGP at RHIC
exp + ncoll scaled pp
e+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)
Good agreement in range of overlap
 pQCD consistent with p+p down to
pT=1GeV/c
NLO pQCD (W. Vogelsang)
 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
Low-energies:
DLS and HADES
Itzhak Tserruya
INT 10-2A, July 13, 2010
39
DLS “puzzle”
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
 Allenhancement
other attempts
to reproduce
the DLS
results
Strong
over
hadronic cocktail
with
“free”failed
 spectral function
 Main motivation for the HADES experiment
HADES confirms the DLS results
Mass distribution
Itzhak Tserruya
pT distribution
INT 10-2A, July 13, 2010
41
Putting the puzzle together (I)
C+C @ 1 AGeV – pp & pd @ 1.25 GeV
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) = 0.0760.015
Dielectron spectrum from C+C consistent with
superposition of NN collisions!
INT 10-2A, July 13, 2010
No compelling evidence
for in-medium effects in C+C42
Itzhak Tserruya
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.
Itzhak Tserruya
INT 10-2A, July 13, 2010
43
The  meson

+
ll
and  
+
KK
 Inconclusive results
Itzhak Tserruya
INT 10-2A, July 13, 2010
44
Inconclusive results
SPS
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.
PHENIX
Uncertainties in the   e+e- channel too large
for a conclusive statement. Waiting for HBD
improved results
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 lowmass pair enhancement?
 meson – elusive probe