Measurement of Single Electron
from Semileptonic Decay
of Charm/Bottom Quarks
in RHIC-PHENIX
Fukutaro Kajihara
(CNS, Univ. of Tokyo)
1
Introduction
RHICで行われた二つの代表的な測定
楕円型フロー
ジェット・クェンチング
完全流体性
高密度状態
Next Step は？
これまでの成果は Soft probe (p, K, p 等) による結果
反応初期状態を直接的に probe する観測量が必要
熱的電磁輻射、Heavy quarks (charm/bottom)
「閉じ込めの破れ」の検証 J/y, Y, Heavy quark v2
Soft probe から Hard probe へ
Heavy quark の測定
2
Heavy Quark Production
Production
gg->QQ “gluon fusion”
Sensitive to the initial
gluon density
Mass is large as(mC2) ~ 0.3
 can use pQCD
hA
g
c
_
c
(a)
hB
Cold nuclear matter effect
(a) Cronin effect
p-p, d-Au
(b) (Anti-) shadowing
(c) Absorption
Hot/dense matter effect
Au-Au
(c) Energy loss
Need systematic study for entanglement.
g
(b)
(c)
3
How do We Measure Heavy Quarks?
間接測定: Single Electron/Prompt muon
Semileptonic decayからのレプトンを測定
直接測定:
DKp, DKpp
K



D0
c c
K
D
p+
0
比較的大きな branching ratio
Meson
D±,D0
Mass
1869(1865) GeV
BR D0 --> Kp
(3.85 ± 0.10) %
BR --> e +X
D±: 17.2, D0: 6.7 %
4
History of Single Electron Measurement
Single electronは1970年代の初期にCERN-ISRにおいて測定された.
当時は charm quarkがまだ発見されていなかった.
F. W. Busser et al, PLB53, 212
F. W. Busser et al, NPB113, 189
後にcharm quarkのsemileptonic decayから生成された電子であると判明
I. Hunchlife and C. H. Llewellyn Smith, PLB61,472
M. Bourquin and J.-M. Gaillard, NPB114,334
5
Heavy Quark Measurement at RHIC
PHENIX
Single electron
measurements in p+p,
d+Au, Au+Au
sNN = 130,200,62.4 GeV
Phys. Rev. Lett. 88, 192303 (2002)
STAR
Direct D mesons hadronic
decay channels in p+p/d+Au
D0Kp
D±Kpp
D*±D0p
Single electron
measurements in p+p, d+Au
6
実験とデータ解析
7
8
The PHENIX detector
A composite detector to
measure leptons,
photons and hadrons.
9
The PHENIX detector
Event trigger is defined by
beam-beam counters.
Beam-beam counters
10
The PHENIX detector
Central arms
Tracking chambers
RICH counters
Central
arm
EM calorimeters
TOF counters
11
Cross-section of PHENIX
PHENIX central arm:
|h| < 0.35
Df = 2 x p/2
p > 0.2 GeV/c
vertex: |zvtx| < 20 cm
Charged particle
tracking analysis using
DC and PC → p
Electron identification
Ring Imaging Cherenkov
detector (RICH)
Electro- Magnetic
Calorimeter (EMC)
→ energy E
12
Most hadrons
do not emit
Cerenkov light
Electron ID
mirror
Cerenkov
photons from
e+ or e- are
detected by
array of PMTs
ＲＩＣＨ
Electrons are identified by
RICH and EMCal E/p
matching, position matching, shower shape cut.
PMT array
PMT array
Au-Au data
r [cm]
All
Electrons
Central Magnet emit
charged tracks
Cerenkov
photons
in RICH.
Apply RICH cut
Real
Net signal
z [cm]
RICH ring shape
(signal accumulated)
Accidental
background
Energy-Momentum [GeV]
13
Background for Inclusive Electron
Main source
Random combinations of EMC cluster and RICH ring
pT independent
Minor source
d-electrons knocked by the hadron in RICH active volume
d/h<10-6
14
E/p in Au+Au collisions
Purity of e± sample excellent after
subtraction of “random association” background
E/p cut
15
Singnal and Background
Photonic Electron
Photon Conversion
Main photon source: p0h → gg
In material: g → e+e- (Major contribution of photonic electron)
Dalitz decay of light neutral mesons
p0h → g e+e- (Large contribution of photonic)
The other Dalitz decays are small contributions
Direct Photon (is estimated as very small contribution)
Non-photonic Electron
Heavy flavor electrons (the most of all non-photonic)
Weak Kaon decays
Ke3: K± → p0 e± e (< 3% of non-photonic in pT > 1.0 GeV/c)
Vector Meson Decays
wfJ/y → e+e- (< 2-3% of non-photonic in all pT.)
16
Background Subtraction: Cocktail Method
Most sources of background
have been measured in
PHENIX
Decay kinematics and
photon conversions can be
reconstructed by detector
simulation
Then, subtract “cocktail” of
all background electrons from
the inclusive spectrum
Advantage is small statistical
error.
17
Background Subtraction: Converter Method
We know precise radiation length (X0) of each detector material
The photonic electron yield can be measured by increase of
additional material (photon converter was installed)
Advantage is small systematic error in low pT region
Background in non-photonic is
subtracted by cocktail method
7
p  L C
9
Photon Converter (Brass: 1.7% X0)
Ne
Electron yield
converter
0.8% 0.4%
1.7%
With converter
Photonic
W/O converter
Dalitz : 0.8% X0 equivalent
radiation length
Non-photonic
0
18
Material amounts: 0
Consistency Check of Two Methods
Both methods were
checked each other
Left top figure shows
Converter/Cocktail ratio
of photonic electrons
Left bottom figure shows
non-photon/photonic ratio
19
Results and Discussion
20
Result of p+p at s = 200 GeV
PRL, 97, 252002 (2006)
Heavy flavor electron
compared to FONLL
Data/FONLL = 1.71
+/- 0.019 (stat) +/- 0.18 (sys)
D0
~factor 2
Upper limit of FONLL
CDF, PRL 91, 241804 (2003)
Tevatronの実験結果
21
Drell-Yan process
FONLL:
electron spectrum may be
~50% c + ~50% b
for 3 < pT < 8 GeV
Drell-Yan component
investigated as well:
< 10% up to 10 GeV
FONLL calculation: Cacciari, Nason, Vogt, PRL95 (2005) 122001
Drell-Yan from: Gavin et al., hep-ph/9502372
Comparison: Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257
22
Result of d+Au at sNN=200 GeV
No strong modification compared to p+p
PHENIX PRELIMINARY
23
Result of Au+Au at sNN = 200 GeV
Submitted to PRL (nucl-ex/0611018)
Heavy flavor electron
compared to binary scaled
p+p data (FONLL*1.71)
MB
Clear high pT suppression
in central collisions
S/B > 1 for pT > 2 GeV/c
(according to inside figure)
p+p
24
Energy Loss of Heavy Quark
In vacuum, q < mQ/EQ ではGluonの制動放射が抑制される
 “dead cone” effect
Heavy quarkのenergy lossは小さい (Dokshitzer-Kharzeev, 2001):
放射されたgluonのエネルギー分布 wdI/dw は放出角依存性
があり、抑制される
Gluonsstrahlung probability

Q
 m  1 
dI
dI
Q
w
w
 1    2 
E  q
dw HEAVY
dw LIGHT
  Q 
2
Dokshitzer, Khoze, Troyan, JPG 17 (1991) 1602.
Dokshitzer and Kharzeev, PLB 519 (2001) 199.
1
[q 2  (mQ / EQ ) 2 ]2
2
25
Nuclear Modification Factor: RAA
3
p+p reference:
d 3 N AA dP
RAA  pT  
N coll  d 3 N pp dp 3 Data (converter) for pT<1.6 [GeV/c]
1.71*FONLL
for pT>1.6 [GeV/c]
Suppression level
is the almost same
as p0 and h in
high pT region
26
RAA vs. Npart
横運動量で積分し、Npartの関数として計算した
Binary scaling works well for pT>0.3 GeV/c integration
Clear suppression is seen for pT>3.0 GeV/c integration
Total error from p+p
27
Dielectron Continuum Measurements
in √sNN = 200GeV Au+Au
schematic dilepton mass distribution
28
Radiative Energy Loss
Radiative Energy Loss with reasonable gluon
densities do not explain the observed suppression
Djordjevic, Phys. Lett. B632 81 (2006)
Armesto, Phys. Lett. B637 362 (2006)
DGLV Radiative
Energy Loss Model
dNg/dy = 1000
29
Collisional Energy Loss
Collisional energy loss may be significant for heavy
quarks
Wicks, nucl-th/0512076
van Hess, Phys. Rev. C73 034913 (2006)
DGLV Radiative +
Elastic Scattering
dNg/dy = 1000
van Hee & Rapp
Elastic Scattering
30
Other models
Charm alone seems to describe better the
suppression at high-pT
Dead cone is more significant for bottom quark
 Larger collisional (relative) Energy loss
DGLV Radiative +
Elastic Scattering
For Only Charm
Larger Dead Cone
and Larger
Collisional E-loss
For Bottom Quark
31
Heavy Flavor RAA at LHC
>100 cc pairs and >5 bb pairs per central Pb-Pb collision
Baseline: PYTHIA to reproduce c and b pT distributions from
NLO pQCD
qˆ
 7  qˆ
 25  100 GeV 2 /fm
LHC
RHIC
Eskola, Kajantie, Ruuskanen, Tuominen,
Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. NPB 570 (2000) 379.
MNR: Mangano, Nason, Ridolfi, NPB 373 (1992) 295.
32
Summary
sNN=200 GeV における Au+Au 衝突実験において、


mid rapidity
0.3 < pT < 9.0 GeV/c
Heavy quarkからの寄与と考えられる電子を測定した
Integrated yield (pT > 0.3 GeV/c) がBinary scaling している
RAA が high pT 領域において強い抑制効果を示した
理論計算との比較


典型的なRadiative Energy Loss のModelが成り立たない
更なる発展には、D/Bの識別測定が必要不可欠
Outlook
D meson measurement in p+p by electron （ Kp measurement ）
High statistic Cu+Cu analysis
Single m measurement in forward rapidity
D/B direct measurement by Silicon Vertex Tracker
33
Backup slides
34