Measurement of the Average $B^{0}_{s}$ Lifetime in the Decay $B^{0}_{s} \to J/\Psi\Phi$

Abstract

The lifetime difference between the long (CP odd) and short (CP even) lived components of the Bg meson is currently predicted to be of the order of 10 % in the Standard Model. It has been suggested that the decay Bg->• J/\|> 4) is predominantly CP even and thus the measured average lifetime could be shorter than the lifetime measured in the inclusive decay modes. We present a measurement of the average lifetime of the 6° meson in its decay Eg->• J/4> cj), with J/\|)-> M.+ M.~ and cj)->• K+K-. During January 2002 and August 2003 the CDF experiment at the Tevatron has been exposed to about 135 pb" 1 of pp collisions with a centre-of-mass energy of A/S = 1.96 TeV. In the data sample collected with the J/\Jj dimuon trigger we fully reconstruct about 125 Bg-> J/\J) (J) candidates with precision silicon information. This is currently the largest exclusive Bg sample. We perform a fit to the proper decay time information to extract the average Bg lifetime and simultaneously use the mass information to disentangle signal from background. For cross-checks we measure the lifetime in the higher statistics modes Bj-» J/\J> K* and B°-> J/4> K*°, which both have similar decay topologies and kinematics. We obtain r(B°s-> J/\|> cf>) = (1.31±5:l3(stat.) ± 0.02(syst.)) ps , which is currently the best single measurement of the Bg lifetime and is consistent with other measurements. This result is not accurate enough to establish the existence of a possible significant lifetime difference between the CP odd and even states. This thesis would not have been possible without the help and contributions of many friends and colleagues. I want to take the opportunity to thank all of them, and in particular: • Todd Huffman for giving me the opportunity to work with the CDF group at Oxford and dealing with the administrative side of my D.Phil. I am especially grateful for the opportunity to stay at Fermilab for 1.5 years. • Jeff Tseng for competent guidance, patient supervision and his constant interest in my work. His sharp physical understanding provided vital input at any time. Together with Farrukh Azfar for the great culinary expeditions all across Chicagoland. • Joe Boudreau for his great experience in particle physics and many stimulating discussions on B lifetimes.