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Search for Higgs bosons decaying to charm quarks

Periodic Reporting for period 2 - HIGCC (Search for Higgs bosons decaying to charm quarks)

Reporting period: 2019-03-01 to 2020-08-31

In the standard model of particle physics (SM), the Higgs boson explains the existence of mass of the elementary particles. However, the model suffers from severe weaknesses: radiative corrections drive the theoretical mass of the Higgs boson to extremely high, unnatural values, while the observed mass is rather low (the famous hierarchy problem). Unknown mechanisms of physics beyond the standard model (BSM) must exist to avoid this unnatural situation. Such BSM mechanisms modify the predicted properties and decay patterns of the Higgs boson. The experimental collaborations at the LHC are measuring these decay patterns as precisely as possible.

Among these decay patterns is the decay into bottom quarks, which has been discovered, as expected, in the year 2018. A much bigger challenge, however, is the search for the decay into charm quarks. This is interesting because the small decay fraction into charm quarks of 2.9% makes it susceptible to BSM modifications, if they exist. A measurement of this charm decay fraction would either unravel new physics that has been sought for more than 60 years, or constrain BSM scenarios to enhance the understanding of the fundamental theory of matter.

However, the decay of the Higgs boson into charm quarks has been considered to be experimentally inaccessible at the LHC, because of the difficulties to distinguish charm quarks from other quarks. It is the goal of this project to overcome these experimental obstacles with new methods for the detection of charm quarks in the CMS detector. The new methods will be based on decay vertex reconstruction algorithms that make use of modern pattern recognition concepts. In combination with new techniques for data analysis and interpretation, this will facilitate the first observation of the Higgs to charm decay, and the measurement of its branching fraction, if it is anomalously enhanced through BSM contributions. With this strategy the first indication for physics beyond the standard model may be found. Even in the absence of a discovery, the project will be able to determine the world's best constraints on the Higgs to charm quark decay branching fraction.
At the start of the project, the HIGCC team joined the analysis teams at CERN and at other research institutions to search for the Higgs boson decay to bottom quarks (H->bb). Members of the HIGCC team made critical contributions to this analysis, which was the highest priority at the LHC at the time. In 2018 the effort culminated in the historic discovery of the H->bb decay, which confirmed the validity of the Standard Model of particle physics in this specific context. In parallel, the HIGCC team started to establish the search for Higgs decays into charm quarks (H->cc) and took leadership of this effort within the CMS collaboration. Various necessary ingredients and fundamental components for the search for H->cc needed to be developed, in particular the methods for identification of charm quark jets and their calibration. During the first two years of the project, these developments have been followed with highest priority. A first demonstration of the feasibility of the project has been made and first results on the search for H->cc have been published in early 2020, using LHC data from 2016. The world's most stringent limits on the H->cc decay have been obtained in the first iteration of this analysis. Deviations from the predictions of the Standard Model have not been observed so far.
Currently, a novel method for the calibration of the charm quark jet identification performance is being developed. The method is about to be published in 2020. In parallel, several threads to improve the performance of the algorithms themselves are being followed. Furthermore, the LHC dataset from 2017 and 2018 is available for analysis now as well. It is therefore the main objective of the second half of the project to combine the new methods with the larger LHC dataset to obtain the best possible sensitivity to the Higgs-Charmquark coupling. We expect to substantially enhance our understanding of the Higgs sector and we are confident that deviations from the predictions will be detected with this analysis. Furthermore, we expect evidence for the observation of the Z->cc decay for the fist time at the LHC with this dataset as well.