2 The CDF Experiment at Fermilab Contents - Harvard University ...

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).