RevModPhys.84.1477】Tests of the standard electroweak model at the energy frontier

Consistent with b hadron production is roughly 15

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Unformatted text preview: 11 57:2 Æ 4:2ðestÞ " tt samples from a subset of the the final state and whether or not Observed yield b tag required? Expt. Ref. 246 98 39 17 13 8 6 284 87 200 0 !1 no CDFa D0b !1 ¼1 !2 !1 CDFc D0d a Aaltonen et al., 2009b. Abazov et al., 2009i. c Aaltonen et al., 2009c. d Abazov et al., 2008f. b These loose initial selections have minimal top-quark mass bias and form the basis for further analysis. Typical signal and background estimates are given in Table XXII for various channels. In the selections, tradeoffs are made between signal acceptance and purity. In particular, the requirement that an event have one or more jets consistent with being initiated by b quarks considerably improves the signal purity. After the initial selections, various methods are used to determine the top mass. Among these are (1) template methods in which data are fit to the predicted signal plus background distribution for a variable that is sensitive to the top mass, (2) ME methods similar to those discussed in " Sec. III.E.4 in which tt event kinematics determined by matrix element calculations are convoluted with detector resolution functions to predict measured kinematic distributions as a function of the top mass used in the matrix element calculations, and (3) other weighting methods in which missing information is supplied by trying possible reconstructed " jet and true tt decay parton assignments and computing a probability for the particular assignment as a function of top mass. The ME and various weighting methods compute event-by-event likelihoods as a function of top mass, and then compute the total likelihood for a data set as the products of the event likelihoods. Combinations of these methods are also used. For lepton þ jet events, the momenta of the four jets and the lepton are fully measured, and the neutrino momentum in the plane transverse to the beam is inferred from the event ET . 6 This gives 17 measured values. Two constraints are available. The first is that the invariant mass of one pair of reconstructed jets should be consistent with that of the W boson, and the second is that the reconstructed mass of the two top quarks should be consistent with each other. This information allows a 1 degree-of-freedom fit to be performed in which the top mass is a free parameter. The resulting event top mass values are not simply averaged to determine the mass, but provide an input variable to the various methods described above. The fit 2 can also be used to determine (or weight) the most likely parton-jet assignment in an event. Complications to this fit Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 arise from the presence of intrinsic transverse momentum of " the tt system ptt" and from final-state gluon radiation giving T rise to additional jets. The ptt" can be incorporated by adding T an additional variable for the unknown transverse momentum ~ X to the fit and constraining it using a priori estimates...
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