Low-mass dielectron measurement at RHIC-PHENIX

Yoshihide Nakamiya (for the PHENIX collaboration)Hiroshima University, Japan

1

2

Outline Physics goals for low-mass dielectron measurement

Difficulties and Challenges for di-electron analysis in Au+Au collisions

Low-mass vector mesons () at low pT Mass shape Yield comparison among several decay modes.

Low-mass continuum at low pT. Continuum yield

Summary

3

Physics of low-mass dielectronR. Rapp J. Phys G31 (2005) S217

M.Harada et. al. Phys.Rev.D74:114006,2006.

In hadronic phase

In partonic phase

RHIC

High energy heavy-ion collisions produced extremely high-temperature medium.

Several kinds of model calculations predict significant change of spectral function (but tendency of the change is different between them)

My motivation is what behavior is experimentally observed ?

KEK-PS/E325

Low-mass vector mesons as a probe Mass spectrum shape

Careful investigation of mass shape of light vector mesons are essential to discuss chiral symmetry restoration.

It is important to measure transparent probes like e+e- decay modes because electrons can pass through the medium with few interactions and therefore carry original information on the meson production.

Yield difference between e+e- and K+K-

Branching ratio between e+e- and K+K- may be sensitive to mass modification, since Mphi is approximately 2 MK.

Need to compare yield of e+e- and K+K-

T.Hatsuda and S.Lee Phys.Rev.C46-1 1992

mKK

m

4

CERES/NA45 (past experiment)

Enhancement of di-electron continuum had been discovered at CERES/NA45.

Some theoretical comparisons had been done. ex) dropping, collision broadening …

PHENIX experiments

Detailed analysis become possible at PHENIX since continuum distribution can be separated from mass peak of light vector mesons.

Low-mass dielectron continuum as a probe

Rapp-Wambach, 2000

--- dropping--- collision broadening

5

Systematic studies give much information about properties of hot medium.

several collision species - p+p (as baseline) - d+Au (examine cold nuclear effects) - Au+Au (examine effects in partonic matter)

several decay channels

All data shown in this presentation are taken in 200 GeV (were taken in run3 - run5)

Fundamental strategy for low-mass dielectron measurement at PHENIX

mesons

-> e+e- BR = 10-4 -> K+K- BR = 50%

mesons -> e+e- BR = 10-5

-> 0 BR = 9% -> 0+- BR = 90%

6

Particle detection at PHENIX

e+

e

Acceptance -0.35 < η < 0.35 2 x 90°in azimuthal

angle for two arms Electron detection

precision tracking : DC/PC

electron id : RICH, EMCal etc

e separation is 1/10-4

at pT < 4.7 GeV/c

The PHENIX spectrometers are excellent devices to measure electrons. (can also measure hadron/photon at the same time, it make systematic measurement possible) 7

Uncorrelated combinatorial pair

Difficulty(1) High multiplicity dNch/d ~ 650 ( in most central Au+Au collision at mid-rapidity)

Difficulty(2) Huge combinatorial background Dalitz decay (e+e-) photon conversion

Difficulty(3) Small fraction to dielectron decay mode from

Branching Ratio (BR) -> e+e- BR = 10-4 -> K+K- BR = 50%

-> e+e- BR = 10-5

-> 0 BR = 9%-> 0+- BR = 90%

0

PRL 91 052303(2003) 5

-5

0e+e-

e+e-

Difficulties in di-electron analysis in Au+Au 200GeV

ex)

It is a key to overcome their difficulties by analysis efforts. 8

Key challenges for di-electron analysis in Au+Au 200GeV

Difficulty(1) High multiplicity

accidental charged hadron contamination unphysical correlated background

Difficulty(2) Huge combinatorial background

background by uncorrelated pairs

Difficulty(3) Small fraction to dielectron decay mode

not avoidable (decided by physics)

9

Challenge against each item are shown from the next slide.

Number of PMT distribution in RICH is different between real data and simulation due to random association tracks.

electron ID by RICH Charged hadron tracks are detected as electrons by random associations under high multiplicity environment.

Black : electron identified tracks (real data)Red : electrons (simulation)

Electron identification in high multiplicity environment (Au+Au 200GeV)

10

Black : electron identified tracks (real data)Blue : random associated tracks (real data)

Detector response under high multiplicity environment is reasonably understood and evaluated.

Black : electron identified tracks after subtracting random associated tracks Red : electrons (simulation)

electron ID by RICH Charged hadron tracks are detected as electrons by random associations under high multiplicity environment.

11

Electron identification in high multiplicity environment (Au+Au 200GeV)

Blue : opening angle between two tracks on RICH plane(same event) Red : opening angle between two tracks on RICH plane(event mixing)

Effect from unphysical correlated tracks are understood and evaluated .

RICH plane

Unphysical correlated background in high multiplicity environment (Au+Au 200GeV) unphysical correlated background in RICH

When a points to the same ring as an electron, it is associated to the same ring. This happens for a typical values of opening angle which folded with the average momentum of the electron corresponded to a particular invariant mass.

12

Multiplicity effects to E/p are reasonably evaluated and calibrated.

hadron rejection by Energy-momentum matching Energy-Momentum matching (E/P) is distributed about 1.

• small electron mass compared to measured momentum scale• hadrons deposit only MIP energy in EMCal.

Black : electron identified tracks (real data)Red : random associated tracks (real data)

Black : electron identified tracks after subtracting random associated tracks

13

Electron identification in high multiplicity environment (Au+Au 200GeV)

Combinatorial background (Au+Au 200GeV)

--- same event N+-

--- mixed event N+-

shape from mixed event

Good agreement for uncorrelated distribution in same event

normalization between same event and mixed event

Normalize B++ and B to N++ and N .

Normalize mixed +- pairs to 2N N N

→ee

→ee

J

14

Combinatorial background from uncorrelated pairs are evaluated.

15

Results of low-mass vector mesons

16

Result of mass shape analysis for e+e- in Au+Au 200 GeV

It is successful to extract signals in Au+Au 200 GeV … so far we cannot discussed mass shape due to lack of

statistics and signal to background ratio. need more statistics (3 times as many statistics as run4 has

already taken at run7 ) need to improve S/N (upgrade devices )

Future improvement of dielectron measurement by Hadron Blind detecor (HBD ) Remaining problems

We made analysis effort and extract the signal in Au+Au 200 GeV. But the following cases cannot be improved by analysis. case1) Counter part of electron pair go out from the PHENIX acceptance case2) Very low pT electrons cannot escape the magnetic field

Solutions HBD detector has capability to identify Dalitz decaying electrons via their opening angle. S/B will be expected to improve a factor 10 to 100.

17

e+

e-0e+e-

Case 1

e+

e-

Case 2

0e+e-

HBD

Extended K+K- analysis

The three independent kaon analyses (double kaon id, no kaon ID and single kaon ID methods) are consistent with each other. Spectra between K+K- and e+e- are reasonable agreementin p+p.

d+Au0-20%

p+p

No ID Single ID Double ID

M.B.

Consistency check for mesons in p+p/d+Au 200 GeV

d+Au0-20%

p+p

No ID Single ID Double ID e+e-

M.B.

18

Consistency check for mesons in p+p/d+Au 200GeV

spectra also show good agreement among several decay channels.

p+pd+Au

0 dAu MB (PRC75 151902)0+- dAu MB(PRC75 151902)e+e- pp MB (PHENIX preliminary)0 pp MB (PRC75 151902)0+- pp MB(PRC75 151902)0 pp ERT (PHENIX preliminary)0+- pp ERT (PHENIX preliminary)

19

e+e- AuAu MB e+e- 20-40% x 10-3 e+e- 40-92% x 10-1 K+K- AuAu MB (no PID)K+K- AuAu MB (double PID) K+K- AuAu MB (PRC72 014903) K+K- 20-40% x 10-3 (double PID)K+K- 40-92% x 10-1 (double PID) K+K- 40-92% x 10-1 (PRC72 014903)

It’s successful to measure pT spectra of e+e- in Au+Au.

… so far we cannot make any clear statement about the comparison between spectra for e+e- and

More statistics and improvements of S/N are needed.

40-92%20-40%

Au+Au

pT spectra comparison for mesons in Au+Au 200 GeV

M.B.

K+K-

20

pT spectra for mesons in Au+Au 200GeV

measurement are done up to 10 GeV/c.

Long awaited ee, along with ee

0-20%

20-60%60-92%

M.B.

21

22

Results of low-mass dielectron continuum

Hadronic source of ee “Cocktail” of ee from hadron decays

Dalitz decays

0ee, 0ee

Direct decays

eeJee

Semileptonic decays of heavy flavor

Drell Yan process

Estimate contribution from hadron decay Fit 0and± data

For ,J/ mesons, measured yields are used(in Au+Au/p+p)

For ther hadrons, we use normalization A under assumption they follows “mT scaling”

Charm production c= Ncoll x 567±57±193b from single electron measurement.

Binary scaling (Ncoll) to Au+Au.

n0T2TT

3

3

pp)bpapexp(

A

pd

σdE

23

Continuum Extraction in p+p 200 GeV

Signal/Background ~ 1. Peaks have well separated due to high resolution. The main components of backgrounds are taken into account. 　　

Uncorrelated background by event mixing Contamination from “jet pair” and “cross pair” is taken

into account.

π0π0

e+

e-

e+

e-

γ

γ

π0

e-γ

e+

“Jet pairs”

“Cross pairs”0

e e

e e

X

Correlated Signal = Data-Mix

Mixed events

“Jet pairs”

“Cross pair”

24

Low-mass dielectron continuum in p+p 200 GeV

Excellent agreement has seen between data and hadron decay contribution from “cocktail” analysis. Charm cross section ( integration after cocktail subtraction)

c=544 ± 39 (stat) ± 142 (sys) ± 200 (model) b25

Continuum Extraction in Au+Au 200 GeV

Signal/Background ~ 1/200.

Peaks have well separated due to high resolution

Signal is also extracted in Au+Au 200 GeV.

Mixed events

Correlated Signal = Data-Mix

26

Low-mass dielectron continuum in Au+Au 200 GeV

Charm from PYTHIAfiltered by acceptancec= Ncoll x 567±57±193b

Charm “thermalized” filtered by acceptancec= Ncoll x 567±57±193b

Continuum enhancement is observed in 150 < mee<750 MeV. Intermediate-mass dielectron continuum are consistent with PYTHIA if charm production from thermal radiation is taken into account.

27

0<pT<0.7 GeV/c

0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c

0<pT<8.0 GeV/c

p+pAu+Au

arXiv: 0802.0050arXiv: 0706.3034

Low mass excess in Au+Au is concentrated in low pT. (Medium dependent effects)

pT dependence of continuum enhancement

28

0 < pT < 8.0 GeV/c 0 < pT < 0.7 GeV/c

0.7 < pT < 1.5 GeV/c 1.5 < pT < 8.0 GeV/c

Centrality dependence of continuum enhancement

Mass window

(a) 150 < mee< 750 MeV (b) 0 < mee< 100 MeV

0 region (mee<100 MeV)

production scales approximately with Npart

Excess region (150<mee<750 MeV)

production from hot matter.scattering processes likeor qq annihilation) a yield rising faster than proportional to Npart

29

Mass dependent dielectron pT spectra

p+p Au+Au

p+p data are consistent with “cocktail” up tp 3 GeV/c Au+Au data enhanced for all pT at the range above m

30

Comparison to theoretical model (in Au+Au)

Both dropping and collision broadening models had beenwell reproduced in CEREC/NA45. Any models are not described continuum enhancement in PHENIX !?

Theoretical models

vacuum spectral function dropping collision broadening

Rapp-Wambach, 2000CERES/NA45

--- dropping--- collision broadening

PHENIX

31

Di-electron analysis Difficulties of dielectron analysis in Au+Au 200 GeV are identified, attacked and

overcome.

Low-mass vector meson Make sure that pT spectra between several decay channels are consistent in p+p.

It is successful to extract signals in Au+Au 200 GeV.

(but statistical and systematic errors are large so far) 3 times as many statistics as at run4 has already taken at run7 signal/background will be improved by HBD.

Low-mass dilelectron continuum Dielectron mass spectra are well reproduced by “cocktail” analysis in p+p 200GeV.

Enhancement of continuum yields is observed in 150 < M ee < 750 MeV/c2 in Au+Au 200GeV.

Effective theoretical models to explain the enhancement are needed.

32

Summary

33

I’m glad to join in Nagoya workshop !I’d like to say thanks to all participants !

Back up

34

35

Generated electrons in Au+Au 200GeV

36

• After several experimental checking, we found integrated information is not enough to determine the difference between e+e- and K+K-.•Need a careful check of mT spectra.

Yield and temperature for mesons

37

The invariant mass spectra for φ mesons e+e- p+p e+e- d+Au e+e- Au+Au

φφ

K+K- (double PID) d+Au K+K- (double PID) Au+Au

38

The invariant mass spectra for φ mesonsno PID) AuAUsingle PID) ppno PID) pp

no PID) dAu

39

The invariant mass spectra for ω mesons e+e- p+p e+e- d+Au e+e- Au+Au

ωω

p+p 0 p+p 0Au+Au