Relativistic Heavy Ion Collider (RHIC)Direct J/ (signal)B decay J/ (background)Performance of the STAR Heavy Flavor Tracker in measuring the charged B meson through B J/ + X decayElizabeth BrostDepartment of Physics, Grinnell College / Purdue University Physics REU programThe 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. STARs 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/ 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/ decay and electron-positron pairs from prompt J/ decay, I was able to reconstruct the displaced vertices (Lxy)for the J/ particles. Then, I made a distribution of c for charged B decay (signal) and prompt (background) J/ particles. Finally, after making successive cuts of c , 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 quarkThe major background in this study is the direct J/ particlesOther background (e.g. correlated charm quark pairs) is neglected Bottom mesons have a long life that can be directly measured by silicon detectors c ~ 500 microns c (using the mass and pT of the J/ particle where c is the speed of light and is the quarks lifetime in its rest frame) can be used to approximate c at pT(J/y)>1.25GeV/c, since the J/ particle decays at essentially the same location as the B meson , and carries most of the momentum from the decay . I used c for this analysis.
Signal / Background as a function of c (cut)Efficiency as a function of c(cut) The relative population of direct J/ (red) to B decay J/ (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 c distribution.-One possible cut, c ~ 0.05 cm (the dotted blue line) results in a S/B ratio of 2, and an efficiency of ~27%AbstractThe B J/ + X e+ + e- decay channelQuark-Gluon Plasma (QGP)Solenoidal Tracker at RHIC (STAR)ReferencesAcknowledgements1. Measurement of the J/ 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/ 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).
c (cm)Signal / Background c (cm) c distribution Efficiency IST silicon strip detector Two layers of pixel detectors100 million pixels (30 x 30 m) Resolution: ~10 m Will be added to the STAR detector in 2012~2013.This is the detector whose performance I analyzed in measuring B mesons.
AnalysisGoalMake successive cuts of c
Heavy quarks make good probes because: Heavy Quarks are up to ~1000 times heavier than light quarks They experience less acceleration from the collision 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 QuarksTo analyze the performance of the Heavy Flavor Tracker at STAR in measuring the charged B meson through the B J/ + X e+ + e- decay channel
B J/ + 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/Direct J/ J/ = 1 charm quark + 1 anticharm quark ----- B meson = 1 bottom (anti)quark + 1 other quark
A central Au+Au collision, as seen by STARs 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)