Forward physics in CMS : simulation of PMT hits in HF and Higgs mass reconstruction methods with a focus on forward jet tagging

Abnormally high energy events were seen in the Hadronic Forward (HF) calorimeter for pion and muon data during testbeam in 2004. Analysis of testbeam data suggested that such events were caused by particles traveling the entire length of HF and striking the photomultiplier (PMT) windows in the readout box behind HF. Charged particles traversing the window of the PMT emit Cerenkov radiation, which creates abnormally high energy events in the data. To further study these events, a modification of the existing official CMS HF simulation was created that added the PMT windows to the simulation as sensitive detectors. In agreement with testbeam data, abnormally high energy events in the PMTs were seen in the simulation for muons and pions. The simulation was then extended to jets simulated with Pythia, and then for collision like events as well. PMT hits were seen in both of these cases. Energy sharing between PMTs for long and short fibers in HF as well as timing differences between normal HF events and PMT events were investigated as methods to tag such abnormal events. While both methods were somewhat successful, it was determined that they were not sufficient. The simulation was also modified to use thinner PMT windows. Reducing the thickness of the window reduced the number of PMT hits, and drastically reduced the energy of these hits, bringing most of them below standard jet energy thresholds. These results led to the replacement of the existing PMTs with new PMTs with a smaller, thinner window. Higgs mass reconstruction methods were applied to Monte Carlo datasets for