Search for a Light Pseudoscalar Higgs Boson in the Two Higgs Doublet Model and Operations, Commissioning of the CMS Phase-1 pixel detector

A search is presented for a light pseudoscalar Higgs boson using full Run 2 data (2016-2018) collected by the CMS (Compact Muon Solenoid) experiment at the LHC (Large Hadron Collider), corresponding to an integrated luminosity of 137.60 fb$^{-1}$ and at a $\sqrt{s}$ = 13 TeV. The study looks into the decay Higgs boson (H) via the H$\rightarrow$aa$\rightarrow$$\mu$$\mu$$\tau$$\tau$ channel. The Higgs boson can be both standard-model-like (125 GeV) or heavier. The pseudoscalar mass falls within the range $m$$_{a}$$\epsilon$[2$m$$_{\tau}$, $m$$_{H}$ /2]. The large mass difference between the Higgs and the pseudoscalar means that the final tau lepton decay products are highly boosted in the decay direction and collimated. A modified version of tau reconstruction is used to account for the highly overlapping decay products. The modified reconstruction technique gives higher reconstruction efficiency over the standard tau reconstruction and hence better signal significance and background rejection. In the absence of a significant signal, model-independent limits are set at 95% C.L. on the branching fraction for H$\rightarrow$aa$\rightarrow$ $\mu$$\mu$$\tau$$\tau$ as a function of $m$$_{a}$. The document also discusses the operations and commissioning of the CMS Phase-1 pixeldetector.