Search for di-higgs production with CMS data at $\sqrt{s}$ = 13 tev at the LHC

A search for $HH$ production in the $HH \to \tau \tau \tau \tau $ final state is presented in the thesis in the context of BSM physics where a spin-0 Radion (X) decays to a Higgs boson pair. The $HH$production cross section in the SM is very small. Nevertheless, different BSM theories consider either anomalous Higgs self-coupling or presence of a resonant particle that decays to $HH$, that may lead to a significant enhancement of the production cross section. This analysis is performed using pp collision data recorded by CMS in 2016, which correspond to 35.9 fb$^{-1}$ . The final states of $HH \to \tau \tau \tau \tau $ are selected to achieve the highest S/B value. In this analysis, events where each of the Higgs bosons decays to a pair of tau leptons and subsequently two tau leptons decay leptonically and other two decay hadronically, are considered. Though the Higgs decay branching ratio to the tau lepton pair is small, e.g ∼ 6%, the absence of any $b$-jets in the final state reduces the background contamination. These final states were further divided into opposite-signed (OS) and same-signed (SS) channels based on leptonic decay of two OS or SS taus. This was done keeping in mind that in the SS channels, the background contribution will be smaller compared to the OS channels. Background contributions are estimated from MC simulated events, except for the multijet contribution. The multijet contribution, where jets fake $\tau_{h} $, has been estimated using a data-driven technique. The contribution of multijet in the signal region is extrapolated from the $\tau_{h} $ anti-isolated region where one or two $\tau_{h} $s are fake. 124 The fake rate estimation method is validated in a region which is away from the signal search region and good agreement is seen between the estimated and observed numbers. No excess in data over the expected SM background contribution is observed in the signal region. The 95% CL upper limits on $\sigma\left ( pp \to X \right ) \times BR(HH \to \tau \tau \tau \tau )$ has been calculated where the observed limit is consistent with the expected one. The result has also been compared with the other major search channels, namely, $b\bar{b}b\bar{b}$, $bb\gamma \gamma $, $bb\tau \tau $, $bbWW$. It has been observed that the channel under consideration can be competitive with $bbWW$ at low mass. The sensitivity of the search is limited by statistics. With a lot more data expected after LHC Run-III and eventually during the High-Luminosity LHC running period, improved analysis techniques to separate signal and background and more robust background estimation, the sensitivity of the search is expected to improve significantly. A calibration procedure for the CMS HCAL and estimation of calibration constants using 2016 data has also been described in the thesis. Reliable estimate of the calibration constants plays an important role in the measurement of energy of hadrons, jets as well as missing transverse energy with high precision. Two different methods, namely iterative and method of moments, have been discussed. In both the methods, either the mean energy or the mean variance is equalized in all the segments along $\phi $ direction for a particular $\eta$ segment utilizing the fact that the HCAL detector is symmetric in $\phi $ direction and also the energy deposition from minimum bias events is $\phi $ dependent. A simulation study using helicity angles sensitive to $HH$ production to distinguish vector boson fusion process from gluon-gluon fusion process and use of the helicity angles in CMS search for $HH \to b\bar{b}\gamma \gamma $ has been presented.