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Search for Higgs bosons decaying to charm quarks

Periodic Reporting for period 4 - HIGCC (Search for Higgs bosons decaying to charm quarks)

Reporting period: 2022-03-01 to 2023-08-31

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.

During this project new methods for the detection of charm quarks in the CMS experiment have been developed. The new methods are making use of modern pattern recognition and machine learning concepts. In combination with new techniques for data analysis and interpretation, an unprecedented sensitivity for the measurement of Higgs to charm decays has been achieved. The project has shown that the actual observation of this process is coming into reach and can be expected for the High-Luminosity LHC. The methods and analyses developed in this project will be employed in future data analyses of the CMS Experiment. A spectacular result of this project was the first observation of the decay of the Z boson into charm quarks at a hadron collider. This observation marks a major milestone and demonstrates that the developed analysis provides an unprecedented precision. The project was paving the way for a new era in constraining the coupling of the Higgs boson to charm quarks.
At the start of the project, the focus was in the search for the Higgs boson decay to bottom quarks (H->bb). Members of the project 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 observation of the H->bb decay with an observed (expected) significance of 5.6 (5.5) standard deviations and a signal strength of 1.04 plus/minus 0.20. This milestone confirmed the validity of the Standard Model of particle physics in this specific context.

In parallel, the project 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 have been developed, in particular the methods for identification of charm quark jets and their calibration. Several scientific publications originated from this stage of the project already. 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 with an observed (expected) upper limit of 70 (37) times the standard model prediction.

During the second stage of the project, the methods for charm jet identification have been further refined and associated systematic uncertainties have been reduced. The full dataset of LHC Run 2, taken until 2018, has been analyzed. New, unprecedented precision has been achieved. A particular success was the first observation of the decay of the Z boson in to charm quarks at a hadron collider, which was facilitated in this project with a significance of 5.7 standard deviations in agreement with the expectations from the standard model. The observed (expected) upper limit on the signal strength of the Higgs decay into charm quarks, produce in association with vector-bosons is 14 (7.6 with an uncertainty of plus 3.4/minus 2.3) times the standard model prediction. For the first time this result has been interpreted in terms of a limit on the coupling modifier kappa-charm which has been found to be 1.1 < kappa-charm < 5.5.

In addition to the vector-boson associated production mode (VH) an inclusive analysis, targeting the gluon-fusion production process (ggH), has been conducted for the first time as well. As the gluon fusion process does not feature any leptons that could be used for triggering or event selection, a number of particular challenges needed to be addressed. For example, this channel is only accessible through topologies with high Lorentz-boosts for which dedicated double-charm tagging algorithms were be developed. Also the inclusive analysis resulted in an observation of the Z->cc decay for the first time in this topology.

In summary the HIGCC project kicked off a new era for constraining the Higgs boson coupling to charm quarks at the LHC. This research area will be a highly attractive and productive topic in future LHC runs, especially at the High-Luminosity LHC.

The project also spawned several other threads of research which are also still being followed today, for example in the area of AIsafety.
A novel method for the calibration of the charm quark jet identification performance is being developed. In parallel, several threads to improve the performance of the charm jet identification algorithms themselves have been followed successfully. During the project the full dataset of LHC Run 2 has been analysed. The new data analysis methods provided an unprecedented sensitivity to the Higgs-Charmquark coupling, much better than originally anticipated. The project contributed a substantial enhancement of our understanding of the Higgs sector. The observation of the Z->cc decay has also been observed for the fist time at the LHC, marking a major success.
Event Display Higgs Boson Decay into Bottom Quarks