Reducible backgrounds

 The reducible backgrounds to a signal of a heavy Higgs particle are from processes with a large transverse momentum in the hard scattering process. Even with a requirement of very high energy jets in the final state, the QCD-jet production still has by far the largest cross section. Other processes that require special attention are t$\bar{t}$, jW and jZ production. To reduce the time required for the simulation in PYTHIA a general cut was placed on the transverse momentum in the hard scattering process, ensuring that events with a transverse momentum in the hard scattering below this cut were never simulated. While a too low cut would cause the simulation of a large number of events that eventually would be rejected, a too large cut would introduce a bias in the simulated events and cause a too low prediction in the size of the background.

Samples of t$\bar{t}$ and jW events were simulated with different values of the p_T cut in the hard scattering. In fig. 8.1a the t$\bar{t}$ and jW cross sections from the simulation are shown as a function of the p_T cut, while the same cross section is shown in fig. 8.1b after requiring a jet with transverse energy above 350 GeV transverse and an isolated lepton with transverse momentum above 100 GeV. It can be seen that for a p_T cut on the hard scattering above 350 GeV the cross sections after the identification cuts are strongly biased. Except for the simulations of the Higgs signal all simulations concerning the identification of a heavy Higgs are made with a p_T cut of 300 GeV. It is estimated that this at the most adds a 10% systematic uncertainty to the level of the background.

  

Figure: The cross sections of the t$\bar{t}$ and jW backgrounds as a function of the p_T cut on the hard scattering in the simulation. In (a) before any identification and in (b) after the identification of a jet with transverse energy above 350 GeV and an isolated lepton with p_T > 100 GeV.

The main tools to reject the reducible background are lepton identification, jet identification, forward tag jets, and a low amount of tracks outside the jets from the vector bosons. All the methods will be described in detail in connection with the different Higgs decay channels described below.