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Performance of the STAR Heavy Flavor Tracker in Measuring Charged B Meson
through charged B J/Ψ + X Decay
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 2
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
• 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
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
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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
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Gold ions
e+
B Meso
n
J/Ψ
Gold ions
e-
* * * * * * * * * * * * * * * * * * * * *
collision!
1) Learn C++2) Create an analysis code to analyze
simulated collision data3) 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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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
θp1
p2P cos121/ PPM J
P = p2 + p2
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L
Gold ions
e+
B Meso
n
J/Ψ
Gold ions
e-
collision!
* * * * * * * * * * * * * * * * * * * * *
DCA
TpL ˆLxy
Txy p
MLc '
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Unlike-sign pairsLike-sign pairs
Lots of background!
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p+p collision: perfect electron ID Au+Au collision: perfect electron ID
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:
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Γ = 6 Kev (for J/Ψ + X e+ + e-)ΔEΔt = ћ/2Δt = 1.1 x 10-19 scΔt = 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.
Cross Section (σ): the likelihood of interaction between two particles
Scale factor for direct J/Ψ : ~72. The ratio of BJ/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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cτ’
B decay J/ΨDirect J/Ψ
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Need to remove these wiggles!
B decay J/ΨDirect J/Ψ
cτ’
Unlike-sign pairsLike-sign pairsNet (Unlike – Like)
cτ’
cτ’
Unlike-sign pairsLike-sign pairsNet (Unlike – Like)
B
J/
Ψ
e+ +
e-
dire
ct
J/Ψ
e
+ +
e-
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B decay J/ΨDirect J/Ψ
cτ
DCA
B decay J/ΨDirect J/Ψ
cτ
with 40x more direct J/Ψ this time
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Direct J/ΨB decay J/Ψ
cτ’
pT > 1.25 GeV
Signal/Background
Efficiency
cτ’(cut)
cτ’(cut)
B decay J/ΨDirect J/Ψ
cτ
cτ’ comparison
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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 time27
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: Fmagnetic = qv x BNewton’s 2nd law: F=ma
R= mv/qB (B and v are the same for both kinds of particles) RAu ≈ 197/79 ≈ 2Rd ≈ 2/1 = 2Rp ≈ 1/1 = 1 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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B decay J/ΨDirect J/Ψ
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)
1)
2)
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