Higgs Search

1. Introduction

   If we believe the recent theoretical predictions, the SM Higgs boson is just around the corner. This means, that the mass of the Higgs boson should be around 120 GeV/c². A Higgs boson with this mass prefers to decay to a pair of b-quarks (>80%). So it is natural to search for the Higgs in final states with two b-jets.

   Unfortunately it is impossible to extract a signal out of simple bb^~ events, because the QCD bb^~ background is too large. One solution is to look for bb^~ events associated with other particles (W^±,Z⁰, tt^~).

   

   The backgrounds of these particles together with the bb~ pair are much lower than the QCD bb^~ background, but are still orders higher than the signals. Strategies to reduce this backgrounds efficient have to be developed.

2. Event Reconstruction

   In the W^± H⁰ channel with the W^± decaying to an electron or muon and a neutrino the reconstruction of the event is rather easy: first the lepton has to be identified as a lepton of the W^± decay; therefore it has to be isolated and together with the missing energy it`s transverse mass has to come close to W^± mass. Then there are exactly two b-tagged jets required of which the invariant mass is calculated. Here it is worth to say, that the b-tagging is essential to reduce the huge non-b-jet backgrounds.

   

   The case of tt^~ H⁰ is more complicated than the the previous: since the top quarks almost always decay to a W boson and a b-quark, the final stated contains four b-jets. At the first moment it is not clear which two jets should be used for the Higgs mass reconstruction. In the case of the semileptonic top decay (one W boson decays to a lepton and a neutrino, the other to two jets) the confusion is even increasing due to the two additional non-b-jets. To solve this combinatorial problem one can reconstruct masses of the W bosons and top quarks, so that at the end there are only two b-tagged jets for the Higgs mass reconstruction are left. For this purpose we define a likelihood function containing information of the invariant W (the leptonic W^± is not reconstructed, because the W mass is used to calculate the z momentum of the neutrino) and top masses and of the b-tagging information of each jet. All possible combinations of the various jets are now calculated and the combination with the highest likelihood is considered as the true configuration. To separate the signal from the background events a cut on this likelihood values is used. Beside the b-tagging here it is also important to reconstruct the resonances with a high mass resolution. This is only possible with corrections on the jet energies back to the quark energies.

3. Preliminary Results

   In the W^± H⁰ channel a complete signal to background study has been done: The result for the three years of low luminosity is not very promising. For an assumed Higgs mass of 100 GeV/c² the significance is only 2.2 - for higher masses this means even worse results, because the production cross section decreases.

   The case of Z⁰ H⁰ is also not promising. The production cross section here is only half of the one of the W^± H⁰ channel. The only chance of these channels is at high luminosity with harder cuts.

   The tt^~ H⁰ channel is still under work. It is worth to mention, that the combinatorial problem is already solved in an acceptable way and that the expected irreducible background is lower than in the case of the other two channels. In the case of tt^~ H⁰ the number of signals events can be roughly doubled by using also events with both W bosons decaying hadronically.

   

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