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86 Section 2: CDF Physics Analysis 2.4.6 Di-Jet Mass Resolution (Prof. Schmitt & Dr. Dorigo) Our ability to extract a higgs signal over a large QCD background depends critically on the resolution we can attain on the mass as calculated directly from the two b-jets. If the resolution is poor, then it will be dicult to observe a peak from the higgs in the dijet mass spectrum. The better the resolution, the smaller the mass window we can dene for a signal, and the smaller the number of background events for a given luminosity. The expected resolution for a jet-jet resonance in Run I was roughly Mjj =0:1 M jj . A relative improvement of this number by 30% would signicantly extend our discovery reach for the Higgs boson in Run II (see for example Ref. [17]). In order to achieve that improvement we must study in detail the characteristics of b-quark jets emitted in the Higgs decay, and exploit fully the information provided by the all relevant detector components. For example, three-dimensional tracking in the new SVX II detector will allow us to infer the momentum of the escaping neutrinos in semileptonic b-quark decays, greatly improving the energy measurement of the resulting jets. This plan will work well in Run II, given the larger acceptance for charged leptons from semileptonic decays provided by the new plug calorimeter and the completion of the central muon system. Furthermore, the possibility of measuring track momenta up to a rapidity jj = 2:0, thanks to the new COT and silicon detectors, will allow a fruitful use of tracking information to improve the calorimetric measurement of jets. While searching for evidence of Z ! bb decays Dorigo succeeded in improving the dijet mass resolution for b-jets. He found that the most useful quantities were the muon momentum, the projection of missing transverse energy along the jet axes, and the charged fraction of the jets. The muon momentum is needed in the correction of the jet originated from the semileptonic decay of the parent b-quark, because the minimum ionizing muons do not contribute linearly to the energy measured in the calorimeter. The missing E T , projected along the jet directions in the transverse plane, provides useful information on the neutrino momentum and on possible uctuations of the energy measurement. The charged fraction of the jets, dened as the ratio between the total momentum of charged tracks belonging to a jet and the energy measured in the calorimeter, also helps to reduce the error in the energy measurement. By properly accounting for the value of these observables, it was possible to reduce the resolution of the dijet mass, M =M jj , by nearly 50% (see Fig. 42).
Section 2: Di-Jet Mass Resolution 87 We are presently studying the application of these methods to search for a Higgs boson signal in Run II. PYTHIA Z → bb: Mass Reconstruction 2500 2000 All Corrections µ and Missing E T Corrections µ Corrections Standard Corrections Constant 2367. Mean 90.01 Sigma 12.27 1500 1000 500 0 20 40 60 80 100 120 140 Dijet Invariant Mass (GeV/c 2 ) Figure 42: The four gaussian ts show the improvement of the mass reconstruction for simulated Z ! b b events (PYTHIA V5.7) when the observable characteristics of the b-quark decays are properly taken into account in the mass reconstruction.
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