CERN Accelerating science

Thesis
Report number	CERN-THESIS-2024-240
Title	Constraints on Higgs Self-coupling via $HH\rightarrow b\bar{b}\gamma\gamma$ and Joint Interpretation of Single- and Double-Higgs Analyses Using Data Collected with the ATLAS Detector at $\sqrt{s}=13 ~\mathrm{TeV}$
Related title	Contraintes sur l'auto-couplage du Higgs avec $HH\rightarrow b\bar{b}\gamma\gamma$ et interprétation conjointe des analyses simple- et double- Higgs à l'aide de données collectées avec le détecteur ATLAS à $\sqrt{s}=13 ~\mathrm{TeV}$
Author(s)	Shen, Qiuping (Shanghai Jiao Tong University (CN))
Publication	309.
Thesis note	PhD : Shanghai Jiao Tong University and Université Paris Cité : 2024-11-22
Thesis supervisor(s)	Marchiori, Giovanni ; Yang, Haijun
Note	Presented 22 Nov 2024
Subject category	Particle Physics - Experiment ; Detectors and Experimental Techniques
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Abstract	This thesis presents measurements of the Higgs ($H$) boson self-coupling $\lambda$ derived from a search for Higgs boson pair ($HH$) production in the $b\bar{b}\gamma\gamma$ final state, as well as from a joint interpretation of various searches for Higgs boson pair production and measurements of single-Higgs boson production. The measurements utilize $140~\mathrm{fb}^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13~\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider during 2015--2018. Limits are presented on the cross-section of the Higgs boson pair production and on multiplicative modifiers of the Higgs boson self-coupling ($\kappa_{\lambda}$) and the quartic $VVHH$ ($V=W,Z$) vertex coupling ($\kappa_{2V}$). In addition, the thesis presents an optimization work of the algorithm used by the ATLAS experiment to identify photon candidates through their electromagnetic shower shapes in order to improve the algorithm's robustness against pile-up in the harsh environment of the LHC. Photon identification algorithms are crucial in collider experiments to distinguish prompt photons from candidates arising from collimated photon pairs from decays of neutral mesons. These algorithms typically rely on shower shapes reconstructed with an electromagnetic calorimeter. In the ATLAS experiment at the LHC, the large level of pile-up (additional inelastic $pp$ collisions) leads to additional energies deposited in the calorimeter that impact the shower shape distributions. In this thesis, improvements in the ATLAS photon identification algorithm are studied by optimizing the selection criteria on the shower shapes as a function of pile-up. A new set of criteria for the identification of photons is proposed that leads to significantly higher prompt photon identification at high pile-up. The Higgs boson self-coupling induces leading-order Feynman diagrams for Higgs boson pair production in proton-proton collisions. The process $HH\rightarrow b\bar{b}\gamma\gamma$ is one of the most sensitive channels for searching Higgs boson pair production due to the clean diphoton signature and low irreducible backgrounds. Events are selected and classified in orthogonal categories with different signal-to-background ratios using machine learning techniques to distinguish signal from background. An unbinned maximum likelihood fit is performed to the diphoton invariant mass distribution of the selected events to extract constraints on the signal strength (i.e., ratio between measured and predicted values, $\mu_{HH}$) of Higgs boson pair production and on $\kappa_{\lambda}$ and $\kappa_{2V}$. No significant deviation from the expected background predicted by Standard Model processes has been observed. The observed upper limit on the signal strength of the Higgs boson pair production is $\mu_{HH} < 3.96$ at 95% confidence level (CL). The observed 95% CL constraints for the coupling modifiers are $-1.44 < \kappa_\lambda < 6.92$ and $-0.48 < \kappa_{2V} < 2.68$, where the other coupling modifiers are fixed to their SM predictions in the interpretations. The trilinear Higgs boson vertex induced by the Higgs boson self-coupling also modifies the leading-order predictions for single Higgs boson production cross-sections and branching ratios through next-to-leading-order electroweak corrections. The Higgs boson self-coupling can thus be constrained more precisely from a joint interpretation of searches for Higgs boson pair production in various final states (\bbyy, \bbtt, \bbbb) and of measurements of single-Higgs boson production in many decay channels ($\gamma\gamma$, $WW^*$, $ZZ^*$, $\tau^+\tau^-$, $b\bar{b}$). A crucial advantage of this approach is that single-Higgs boson data constrains the values of the modifiers of the Higgs boson to other particles (such as to the top quark) that also affect Higgs boson pair production and that would otherwise need to be fixed to some reference value (typically the SM prediction) in the interpretation of Higgs boson pair data alone. The results of this work are as follows:. The combination of searches for Higgs boson pair production in three decay channels yields an upper limit on the $HH$ signal strength $\mu_{HH}$ < 2.4 at 95% CL. The joint interpretation of single- and di-Higgs analyses constrains the Higgs boson self-coupling to $-0.4 < \kappa_\lambda < 6.3$ at 95% CL, assuming all other coupling modifiers are fixed to their SM predictions, and to $-1.4 < \kappa_\lambda < 6.1$ when the other main coupling modifiers (to top and bottom quarks, $W$ and $Z$ bosons, and $\tau$ leptons) are floated. The results provide significant insights into the fundamental properties of the Higgs boson and its interactions and represent an important milestone towards the observation of Higgs boson pair production and of the Higgs boson self-interaction.

Email contact: qiuping.shen@cern.ch