Relativistic Heavy Ion Collider (RHIC)

Direct J/Ψ (signal)B decay J/Ψ (background)

Performance of the STAR Heavy Flavor Tracker in measuring the charged B meson through B J/Ψ + X decay

Elizabeth BrostDepartment of Physics, Grinnell College / Purdue University Physics REU program

The Solenoidal Tracker at RHIC (STAR) detector, which is located at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, gathers data from particle collisions that occur at relativistic speeds. STAR’s main task is to study the characteristics of the matter produced in these collisions, particularly the quark-gluon plasma (QGP), which is expected to have been created a few microseconds after the “Big Bang”. Among all probes used to study the properties of the QGP, heavy quarks are unique. Their mass is generated mainly from the Higgs mechanism and is not affected by the surrounding medium. They are produced instantly after the collision and provide direct access to the initial state properties of the medium. All of these features make them ideal for studying the QGP. The Heavy Flavor Tracker is the core of the future STAR heavy flavor physics program and will soon enable STAR to directly measure heavy flavor mesons. One way to study heavy quarks is the B (J/Ψ e+ + e-) + X decay channel, and this channel was the focus of my research. Using simulated central Au+Au collision data containing electron-positron pairs from charged B J/Ψ decay and electron-positron pairs from prompt J/Ψ decay, I was able to reconstruct the displaced vertices (Lxy)for the J/Ψ particles. Then, I made a distribution of cτ’ for charged B decay (signal) and prompt (background) J/Ψ particles. Finally, after making successive cuts of cτ’ , I created signal-to-background and efficiency distributions for measuring the charged B mesons in central Au+Au collisions through this decay channel.

The National Science Foundation – for sponsoring the REU program / Purdue University /

Prof. Wei Xie – for being my advisor this summer / Quan Wang – for creating the simulated collision data

This decay channel is commonly used to study the bottom quarkoThe major background in this study is the direct J/Ψ particlesoOther background (e.g. correlated charm quark pairs) is neglected

Bottom mesons have a long life that can be directly measured by silicon detectors o cτ ~ 500 microns

cτ’ (using the mass and pT of the J/Ψ particle– where c is the speed of light and τ is the quark’s lifetime in its rest frame) can be used to approximate cτ at pT(J/)>1.25GeV/c, since the J/Ψ particle decays at essentially the same location as the B meson , and carries most of the momentum from the decay [1][2]. I used cτ’ for this analysis.

Signal / Background as a function of cτ’ (cut)

Efficiency as a function of cτ’(cut)

The relative population of direct J/Ψ (red) to B decay J/Ψ (blue) at RHIC is ~72, based on their respective production cross sections. The optimum signal to background ratio can be located by applying successive cuts to the cτ’ distribution.

-One possible cut, cτ’ ~ 0.05 cm (the dotted blue line) results in a S/B ratio of 2, and an efficiency of ~27%

Abstract

The B J/Ψ + X e+ + e- decay channel

Quark-Gluon Plasma (QGP)

Solenoidal Tracker at RHIC (STAR)

References

Acknowledgements

1. Measurement of the J/Ψ Meson and b-Hadron Production Cross Sections in p-pbar Collisions at √s = 1960GeV D. Acosta, et al., arXiv:hep-ex/0412071v1 (2004).2. Measurement of B hadron lifetimes using J/ Ψ final states at CDF F. Abe., et al., Phys. Rev. D, 57, 5382 (1998).3. A proposed STAR microvertex detector using Active Pixel Sensors with some relevant studies on APS performance S. Kleinfelder, et al., Nucl. Instr. and Meth. A565, 132 (2006).

cτ’ (cm)

Sign

al /

Bac

kgro

und

cτ’ (cm)

cτ’ distr

ibution

Effi

cien

cy

cτ’ (cm)

Txy p

MLc ')(

B

B

Txy p

MLc

IST silicon strip detector Two layers of pixel detectors

o100 million pixels (30 x 30 μm)o Resolution: ~10 μm

Will be added to the STAR detector in 2012~2013.This is the detector whose performance I analyzed in measuring B mesons.

Analysis

Goal

Make successive cuts of cτ’

Gold ions

e+

B Meso

n

J/Ψ

Gold ions

e-

* * * * * * * * * * *

* * * * * * * *

TpL ˆLxy

L

Heavy quarks make good probes because: Heavy Quarks are up to ~1000 times heavier than light quarks

o They experience less acceleration from the collisiono They are created instantly after the collision, and therefore provide access to the initial state conditions of the medium.

The mass of a heavy quark is not affected by the surrounding medium – it comes mainly from the Higgs mechanism

Studying the QGP with Heavy Quarks

To analyze the performance of the Heavy Flavor Tracker at STAR in measuring the charged B meson through the B J/Ψ + X e+ + e- decay channel

B J/Ψ + X e+ + e- decay

Scientists believe that quarks were free from “confinement” during the first few moments after the Big Bang, and formed quark-gluon plasma The QGP is expected to form in relativistic heavy-ion collisions Like water, QCD matter can experience phase transitions:

B decay J/ΨDirect J/Ψ

J/Ψ = 1 charm quark + 1 anticharm quark ----- B meson = 1 bottom (anti)quark + 1 other quark

A central Au+Au collision, as seen by STAR’s Time Projection Chamber (TPC).

Heavy ions or protons are accelerated in opposing directions around the accelerator – to 99.95% the speed of light Maximum energy for heavy ion collisions: 200 GeV / nucleon Currently 2 experiments running (STAR and PHENIX)

The Heavy Flavor Tracker (HFT) [3]