Constraints on mixing and CP-violation in the neutral charmed meson system at LHCb

QC Physics

Alexander, Michael T. ; Chris, Parkes

University:University of Glasgow

Department:School of Physics and Astronomy

关键词: LHCb, charm, CP violation, mixing, yCP, AGamma, D0, lifetime, VELO, impact parameter resolution;   

Others  :  http://theses.gla.ac.uk/3549/1/2012AlexanderPhD.pdf

来源: University of Glasgow

【 摘 要 】

This thesis presents measurements of the charm sector mixing and CP-violation parameters yCP and AGamma, made using data collected in 2010 by the LHCb experiment at the LHC at s^1/2 = 7 TeV. yCP is defined as the difference from unity of the ratio of the effective lifetime of the D0 meson decaying to a CP-undefined final state to its lifetime when decaying to a CP-eigenstate. AGamma is the CP-asymmetry of the effective lifetimes of the D0 and D0bar when decaying to a CP-eigenstate. In the absence of CP-violation yCP will be consistent with the mixing parameter y, and AGamma will be consistent with zero.CP-violation in the charm sector is predicted to be very small in the SM, though first evidence for direct CP-violation in D0 decays has recently been observed by LHCb. Observation of significantly more CP-violation than is allowed in the SM would be a strong indication of new physics. The current world best measurements of yCP and AGamma show no evidence of CP-violation.The methods used to measure the effective lifetime of the D0 are presented, together with a detailed study of the impact parameter resolutions achieved by Vertex Locator (VELO) sub-detector. A resolution of < 36 μm on the x and y components of impact parameter measurements is measured for particles with pT > 1 GeV. The final dataset on which yCP and AGamma are measured comprises 28.0 +/- 2.8 pb−1, from which 226,110 D0 to Kpi and 30,481 D0 to KK candidates are selected. The dominant sources of systematic uncertainty arise from combinatorial backgrounds and D0 produced in decays of B mesons. The final results are found to beAGamma = (−5.9 +/- 5.9 (stat.) +/- 2.1 (syst.)) x 10−3, yCP = (5.5 +/- 6.3 (stat.) +/- 4.1 (syst.)) x 10−3.yCP is consistent with the world average of y and with zero, and AGamma is consistent with zero. Thus, these results show no evidence for CP-violation or mixing in the D0 system.