טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPanikashvili Natalia
SubjectMeasurement of the lambda/b Lifetime with the D/phi Detector
Development of an Algoritm for Selection of J/psi
Particles at the Second Level Trigger of
The ATLAS Detector. A Study...
DepartmentDepartment of Physics
Supervisors Professor Shlomit Tarem
Professor Neal Homer
Full Thesis textFull thesis text - English Version


Abstract

We present a new measurement of the Λb lifetime through the exclusive decay channel ΛbàJ/ψΛ. The measurement was done in the DØ experiment, based on an integrated luminosity of 1.2fb-1. The measurement of the b hadron lifetime is important since to understand the influence of the light quarks in b hadron decays. At the leading order, Heavy Quark Effective Theory predicts the lifetime of all b hadrons to be the same, since the light quarks are considered spectators. Including the interactions between the b quark and the light quark, the lifetime of b hadrons should satisfy the following relation: τ(B+)≥ τ(B0)≈ τ(Bs)>τ(Λb)>> τ(Bc). We found the lifetime of the Λb to be τ(Λb) = 1.218 +0.130 -0.115 (stat) ± 0.042 (syst) ps, and the B0 lifetime to be τ(B0) = 1.501 +0.078 -0.074 (stat) ± 0.050 (syst) ps, yielding a lifetime ratio of τ(Λb)/τ(B0) = 0.811 +0.096-0.087 (stat) ± 0.034 (syst), which is consistent with the word average of τ(Λb)/τ(B0) = 0.844 ± 0.043 and supports the most recent theoretical prediction.


The b production cross section at the LHC is very high, but it is just a small fraction of the total inelastic cross section (0.7%), therefore the trigger for B physics is required to be very selective. The output rate of the first level trigger at luminosity of about L = 1031cm-2s-1 is expected to contain around 227Hz of events with at least one energetic muon. Most of these events are from π's and K's decays, and only 35Hz are b events. At the second level trigger this rate of events must be reduced to about 20Hz. This goal is achieved by di-muon selection algorithm, which I developed during my PhD studies. The algorithm exhibits substantially better selection for the "gold" channels (J/ψ) as compared to the existing ATLAS algorithms.


For nearly thirty years it has been experimentally known that Λ hyperons produced inclusively in proton-proton collisions can be polarized. However up to now no theory has given a comprehensive explanation to this phenomenon, so this puzzle still remains unsolved.

We present an assessment of the feasibility of extracting the inclusive Λb polarization, its parity violating alpha parameter through the full reconstruction of the four final state charged particles resulting from its decay. Our results are based on the full reconstruction of generated Λb events of fixed polarization in the ATLAS detector.