Performance of the STAR Heavy Flavor
Tracker in Measuring Charged B Meson
through charged B  J/Ψ + X Decay
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 Background
Information:
› Goal of RHIC: Find the QGP
› Au + Au collisions at RHIC
 Goals
for the Summer
› Learn C++
› Study B meson measurement using the
HFT
 Process
› “What I did this summer”
 Results
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Quarks come in six
varieties: up, down,
strange, charm,
bottom, and top
 Gluons bind quarks
into mesons (2 quarks)
and baryons (3 quarks)
– this is called
“confinement”.

<http://www.fnal.gov/pub/inquiring/matter/madeof/index.html>
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


Scientists believe that quarks
were free from
“confinement” during the
first few moments after the
Big Bang, and formed
quark-gluon plasma
During heavy-ion collisions,
this freedom is briefly
recreated
The QGP is expected to
form in heavy-ion collisions,
but not in p+p collisions
<http://www.bnl.gov/RHIC/QGP.htm>
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Scientists expected that the matter
produced in high energy Au+Au
collisions would behave like a gas
 Instead, the matter resembles a perfect
liquid

› Strong Interactions between particles
› “Flows” like a liquid
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
Why does RHIC have
different types of
collisions?
› p+p
 To act as a standard
for comparison
› d+Au
 To study the
conventional nuclear
effect
› Au+Au
 To study the QGP
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Most Central
Less Central
Not Central (Peripheral)
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Currently 2 experiments running
 STAR records the tracks of
particles created in collisions
using several layers of detectors
 Gold ions are accelerated in
opposing directions around the
accelerator– to 99.95% the
speed of light!
 Maximum Energy for Au+Au
collisions: 200 GeV / nucleon
(recall : Gold nuclei have 197
nucleons)

(Scientific American , 2006)
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
Two layers of pixel detectors
› 100 million pixels (30 x 30 μm)
› Resolution: ~10 μm

This is the detector whose performance I
analyzed in measuring B mesons.
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Studying the QGP with Heavy Quarks
•
Up
1.5 to 4
MeV
Charm
1,150 to 1,350
MeV
Down
4 to 8
MeV
Bottom
4,100 to 4,400
MeV
Strange
80 to 130
MeV
Top
178,000 ± 4,300
MeV
Heavy Quarks are ~1000 times heavier than light quarks
o Experience less acceleration from the collision
o Have long life that can be measured by Silicon Detectors
• cτ of charm quarks: ~100-300 microns
• cτ of bottom quarks: ~500microns.
•
I will study B meson production through the J/Ψ + X decay
channel
• The major background in this study is the direct J/Ψ particles
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J/Ψ = 1 charm quark + 1 anticharm quark
B meson = 1 bottom (anti)quark + 1 other quark

This is one of the major decay channels
used to study the B quark
e-
J/Ψ
e+
Gold ions
********
Gold ions
*************
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1)
2)
3)
Learn C++
Create an analysis code to analyze
simulated collision data
Determine the performance of the STAR
Heavy Flavor Silicon Tracker in
measuring the B meson through B  J/Ψ
+ X decay
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Goal: to analyze simulated collision data
› p+p collisions (what you saw in June)
› signal events with 1 B decay J/Ψ or
direct J/Ψ per event
› mixed events: gold-gold collisions with 20
B decay J/Ψ or direct J/Ψ per event
embedding
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-p+p collisions
(J/Ψ +X)
Event Generator
-Au+Au collisions
(embedded
p+p collisions)
Analysis!
(my task)
GEANT –
Detectors
OFF
Detector
Response-Detectors ON
(Reconstruction)
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Refit helix
 Make sure each track has hits on both
layers of the HFT
 Recreate mass of J/Ψ particle

Use helix
characteristics and
magnetic field to refit
the path of the particle
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Refit helix
 Make sure each track has hits on both
layers of the HFT
 Recreate mass of J/Ψ particle

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Refit helix
 Make sure each track has hits on both
layers of the HFT
 Recreate mass of J/Ψ particle

p1
P
P = p2 + p2
θ
p2
MJ / 
P1  P2 1  cos 
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e-
L
J/Ψ
e+
Gold ions
********
c '  Lxy

DCA
Gold ions
*************

M
pT
Lxy


ˆT
 L p
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Unlike-sign pairs
Like-sign pairs
p+p collision: perfect electron ID
Au+Au collision: perfect electron ID
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cτ’ approximates cτ to within a small
correction factor (<15%).
 B decay J/Ψ particles are the signal
 Direct J/Ψ particles are the background

› cτ is effectively zero:
Γ = 6 Kev (for J/Ψ + X  e+ + e-)
ΔEΔt = ћ/2
Δt = 1.1 x 10-19 s
cΔt = 3.3 x 10-11 m  way less than the detector resolution
In this study, other background (e.g. correlated
charm quark pairs) is neglected.
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


Cross Section (σ): the likelihood of interaction
between two particles
Scale factor for direct J/Ψ : ~72.
The ratio of BJ/Ψ over direct J/Ψ is expected to
increase in Au+Au collisions (because direct J/Ψ
experience more suppression than the B decay
J/Ψ).
Branching ratios:
B J/Ψ + X (1.094%)
J/Ψ  e+ + e- (5.94%)
Cross sections:
p+p  B meson (3.8μb)
p+p  J/Ψ  e+ + e- (178nb)
σ(B J/Ψ + X  e+ + e-) = 3.8 μb x 1.094% x 5.94%
σ(J/Ψ  e+ + e-)
178 nb
= 1/72.09
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B decay J/Ψ
Direct J/Ψ
cτ’
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B J/Ψ  e+ + e-
B decay J/Ψ
Direct J/Ψ
Unlike-sign pairs
Like-sign pairs
Net (Unlike – Like)
cτ’
Need to remove these wiggles!
direct J/Ψ  e+ + e-
cτ’
Unlike-sign pairs
Like-sign pairs
Net (Unlike – Like)
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cτ’
B decay J/Ψ
Direct J/Ψ
with 40x more direct J/Ψ this time
Direct J/Ψ
B decay J/Ψ
cτ
pT > 1.25 GeV
B decay J/Ψ
Direct J/Ψ
DCA
cτ
cτ’25
Signal/Background
cτ’ comparison
B decay J/Ψ
Direct J/Ψ
cτ’(cut)
Efficiency
cτ
cτ’(cut)
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
Results:
› Best signal/background at cτ’(cut) ~0.05
› Efficiency at that cut: ~27%

Future plans:
› Take the “pile-up” effect into account in the
analysis code
 Large hit density on the HFT
 60 hits/cm2 on the inner pixel detector
 8 hits/cm2 on the outer pixel detector
› HFT is slow: 200μs integration time
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Wei Xie, for being my advisor this summer
 Quan Wang, for creating the simulated
collisions
 The National Science Foundation, for the
REU program
 Steve Durbin and John Yeazell,
for running the REU program

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Lorentz force:
Newton’s 2nd law:
Fmagnetic = qv x B
F=ma
R= mv/qB
RAu ≈ 197/79 ≈ 2
Rd ≈ 2/1 = 2
Rp ≈ 1/1 = 1
(B and v are the same for both kinds
of particles)
Therefore, d+Au collisions are most practical, since
they will stay at roughly the same radius in the
beam lines, using the same magnetic field.
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1)
B decay J/Ψ
Direct J/Ψ
2)
cτ
1) Measurement of B hadron lifetimes using J/ psi
final states at CDF (PHYSICAL REVIEW D 1 MAY
1998 VOLUME 57, NUMBER 9)
2) Measurement of the J/psi Meson and b-Hadron
Production Cross Sections in p¯p Collisions at sqrt(s )=
1960GeV (arXiv:hep-ex/0412071v1 27 Dec 2004)
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Unlike-sign pairs
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