A Journey Towards Mont Blanc

At the LHC, top quark production cross section is 10 times larger than Tevatron. At Tevatron pair production cross section is 67 pb while at LHC the cross section is 860 pb. So it is fair to say that top quark decay background is dominant in higgs searches at LHC.

Top quarks always decay to W boson and b quark. So the decay channel is depending on W boson decay. W boson decay to lepton and neutrino with 1/9. Since there are three kind of leptons, the rate of decaying to lepton is going to be 3/9. However tau decays eventually to electron or muon which are soft enough to be removed by applying transverse momentum requirement in the analysis. Experimentally total rate of decaying to lepton will be 2/9. The decay to hadron is 6/9.

Now think about 3 cases. Single lepton plus jets, dileptons and multi-jet.

1. Single lepton plus jets : one W decays leptonically and the other W decays hadronically. Then the rate is 2 x 2/9 x 6/9 = 29.6% of all top quark pair production. 2 is needed because there are two cases.

2. Dileptons : each W decays leptonically. 2/9 x 2/9 = 4.9%. It provides clean samples but there are two neutrinos which makes it hard to reconstruct top mass.

3. Multi-jet : The lagest sample of top quark pair production is 6 jets decay from fully hadronic decay. The branching ratio is 6/9 x 6/9 = 44.4%. However these events suffer from a very large background from multi-jet events.

I am posting it not to forget the branching ratio.

In March 2009, D0 and CDF reported electroweak single top decay discovery at Tevatron. The cross section of single top quark is 244 pb, 60 pb or 10 pb for W-gluon fusion(t-channel), tW-channel or s-channel at LHC. It will be also interesting to see the single top result from LHC.