Periodic Reporting for period 4 - ExclusiveHiggs (Search for New Physics in First and Second Generation Quark Yukawa Couplings through Rare Exclusive Decays of the Observed Higgs Boson)
Periodo di rendicontazione: 2021-09-01 al 2023-08-31
In the last century great advances have been achieved in our understanding of the microcosm. These culminated in the development of the Standard Model of particle physics, which describes the fundamental building blocks of nature, the elementary matter particles, and their interactions, mediated by force carrier particles. The experimental observation of the particles of the Standard Model was completed with the discovery of the Higgs boson, which is connected to the mechanism proposed to explain the mass generation for the force carriers, in 2012 by the ATLAS and CMS Collaborations at the CERN Large Hadron Collider.
Following the discovery, a substantial body of accumulated experimental evidence suggests that this mechanism, as postulated in the Standard Model, is realised in nature and results in the observed masses of the force carriers. Within the Standard Model, the elementary matter particles, the quarks and the leptons, acquire their mass through a separate mechanism, which also involves the Higgs boson. Alas, no explanation of the large variations of the observed masses of the elementary matter particles is provided. Case in point, the top-quark - the heaviest matter particle - is approximately 330,000 times heavier than the electron - the lightest matter particle. Several theories beyond the Standard Model predict different mechanisms for mass generation, aspiring to explain the observed mass hierarchy.
The ExclusiveHiggs project explored the interactions of elementary matter particles with the Higgs boson using the ATLAS detector. Searches for rare exclusive Higgs boson decays and the direct search for Higgs boson decays to second generation quark--anti-quark pairs were undertaken. These new channels can shed light to the least studied part of the Standard Model, and map the challenges and the opportunities in tackling these questions in future facilities. At the same time, an extensive set of searches for analogous processes in the decays of the massive force carriers was performed, most of these for the first time, further enhancing the scientific value of the proposed research programme. Progress in this area is crucial to complete our understanding of the Standard Model and elucidating the origin of mass.
Although no deviation from the Standard Model was observed to-date, within the ExclusiveHiggs programme a legacy of new searches and measurements was delivered. These results prompted other experiments to perform similar searches and the theory community to investigate the properties of these decays. Thus, the ExclusiveHiggs project established a sub-field of research into the Higgs sector. The developed techniques are now used in a variety of physics investigations, and the way was paved for future research that may potentially lead to the observation of new physics phenomena.
Following the discovery, a substantial body of accumulated experimental evidence suggests that this mechanism, as postulated in the Standard Model, is realised in nature and results in the observed masses of the force carriers. Within the Standard Model, the elementary matter particles, the quarks and the leptons, acquire their mass through a separate mechanism, which also involves the Higgs boson. Alas, no explanation of the large variations of the observed masses of the elementary matter particles is provided. Case in point, the top-quark - the heaviest matter particle - is approximately 330,000 times heavier than the electron - the lightest matter particle. Several theories beyond the Standard Model predict different mechanisms for mass generation, aspiring to explain the observed mass hierarchy.
The ExclusiveHiggs project explored the interactions of elementary matter particles with the Higgs boson using the ATLAS detector. Searches for rare exclusive Higgs boson decays and the direct search for Higgs boson decays to second generation quark--anti-quark pairs were undertaken. These new channels can shed light to the least studied part of the Standard Model, and map the challenges and the opportunities in tackling these questions in future facilities. At the same time, an extensive set of searches for analogous processes in the decays of the massive force carriers was performed, most of these for the first time, further enhancing the scientific value of the proposed research programme. Progress in this area is crucial to complete our understanding of the Standard Model and elucidating the origin of mass.
Although no deviation from the Standard Model was observed to-date, within the ExclusiveHiggs programme a legacy of new searches and measurements was delivered. These results prompted other experiments to perform similar searches and the theory community to investigate the properties of these decays. Thus, the ExclusiveHiggs project established a sub-field of research into the Higgs sector. The developed techniques are now used in a variety of physics investigations, and the way was paved for future research that may potentially lead to the observation of new physics phenomena.
The CERN Large Hadron Collider completed its second data taking campaign in 2018 and, following maintenance and upgrades, the third data taking campaign is ongoing, with a total of approximately twenty quadrillion proton collisions recorded. The ExclusiveHiggs team contributed in ensuring that interesting events in this dataset have been selected and recorded with high quality. Moreover, the team led the design, development, maintenance, and improvement of the highly selective algorithms dedicated to select and record interesting events for the rare exclusive decays of the Higgs boson and the carriers of the weak force. These algorithms enabled the collection of an unprecedented dataset that would otherwise be lost.
Based on the collected dataset, the ExclusiveHiggs team led the search for several rare exclusive decays of the Higgs, Z, and W bosons. Novel techniques were developed to overcome the experimental challenges encountered, including the modelling of other processes resulting in similar signatures but not related to the sought physics signal - the background. These innovative background modelling techniques rely solely on the observed dataset, minimising dependence on the physics description and detector simulation, and enhancing the search sensitivity.
Beyond exclusive rare decays, the team made significant contributions to other approaches probing the mass generation for matter particles, including the first search for Higgs boson decays to a pair of charm-quarks and the first ATLAS search Higgs boson decays to electron-positron pairs and anomalous decays to electrons and muons. These were complemented by novel searches for additional Higgs bosons, covering new ground in the understanding of the structure of the Higgs sector.
The team also contributed to the ATLAS upgrade for the High Luminosity Large Hadron Collider. Team members were involved in research and development efforts for the ATLAS Inner Tracker, including qualification of irradiation facilities and inter-facility comparisons, detector component assembly and characterisation, and evaluating the upgraded detector physics potential. The team contributed to the world-wide discussions for the future direction of particle physics, and the potential of proposed future colliders.
Based on the collected dataset, the ExclusiveHiggs team led the search for several rare exclusive decays of the Higgs, Z, and W bosons. Novel techniques were developed to overcome the experimental challenges encountered, including the modelling of other processes resulting in similar signatures but not related to the sought physics signal - the background. These innovative background modelling techniques rely solely on the observed dataset, minimising dependence on the physics description and detector simulation, and enhancing the search sensitivity.
Beyond exclusive rare decays, the team made significant contributions to other approaches probing the mass generation for matter particles, including the first search for Higgs boson decays to a pair of charm-quarks and the first ATLAS search Higgs boson decays to electron-positron pairs and anomalous decays to electrons and muons. These were complemented by novel searches for additional Higgs bosons, covering new ground in the understanding of the structure of the Higgs sector.
The team also contributed to the ATLAS upgrade for the High Luminosity Large Hadron Collider. Team members were involved in research and development efforts for the ATLAS Inner Tracker, including qualification of irradiation facilities and inter-facility comparisons, detector component assembly and characterisation, and evaluating the upgraded detector physics potential. The team contributed to the world-wide discussions for the future direction of particle physics, and the potential of proposed future colliders.
The matter particles, the quarks and the leptons, are organised in three generations, each a heavier copy of the previous one. To-date, only the Higgs boson interactions with matter particles of the third generation, which is also the heaviest, have been established. More recently, evidence has been observed for Higgs boson interactions with leptons of the second generation, but not for the corresponding quarks. The ExclusiveHiggs project led the way in the study of the Higgs boson interactions with matter particles of the first two generations.
The ExclusiveHiggs team was instrumental in the publication of a suite of dedicated searches for flavour-conserving and flavour-violating exclusive Higgs boson decays to a meson - a system composed by a quark and an anti-quark - and a photon, as well as searches for similar decays of the Z and W bosons, the first direct search for Higgs boson decays to a pair of charm-quarks, the first ATLAS search for Higgs boson decays to electron-positron pairs and anomalous decays to electrons and muons, direct searches for flavour-violating interactions of the Higgs boson involving top-quarks and charm-quarks, as well as searches for Higgs boson decays into a Z boson and a light resonance and into pairs of light resonances. These searches, to a large extent, set the state of the art in our current understanding of the Higgs boson interactions with matter particles of the first and second generations.
The team developed and maintained highly-selective dedicated trigger algorithms that recorded the dataset to search these exclusive topologies. The team contributed improvements to the trigger at large, e.g. improving tau-lepton trigger track reconstruction. These physics searches were enabled by a suite of new methods, including non-parametric data-driven background estimation methods relying on ancestral sampling or on neural networks, and neural network-based reweighting of the simulation to accurately describe the data. In particle reconstruction and identification, the team contributed in the development of charm-quark identification algorithms, in understanding the detector’s capability to disentangle topologies with nearby charged particles, and in exploiting the internal structure of hadronic jets originating from decays of energetic low-mass resonances.
The ExclusiveHiggs team was instrumental in the publication of a suite of dedicated searches for flavour-conserving and flavour-violating exclusive Higgs boson decays to a meson - a system composed by a quark and an anti-quark - and a photon, as well as searches for similar decays of the Z and W bosons, the first direct search for Higgs boson decays to a pair of charm-quarks, the first ATLAS search for Higgs boson decays to electron-positron pairs and anomalous decays to electrons and muons, direct searches for flavour-violating interactions of the Higgs boson involving top-quarks and charm-quarks, as well as searches for Higgs boson decays into a Z boson and a light resonance and into pairs of light resonances. These searches, to a large extent, set the state of the art in our current understanding of the Higgs boson interactions with matter particles of the first and second generations.
The team developed and maintained highly-selective dedicated trigger algorithms that recorded the dataset to search these exclusive topologies. The team contributed improvements to the trigger at large, e.g. improving tau-lepton trigger track reconstruction. These physics searches were enabled by a suite of new methods, including non-parametric data-driven background estimation methods relying on ancestral sampling or on neural networks, and neural network-based reweighting of the simulation to accurately describe the data. In particle reconstruction and identification, the team contributed in the development of charm-quark identification algorithms, in understanding the detector’s capability to disentangle topologies with nearby charged particles, and in exploiting the internal structure of hadronic jets originating from decays of energetic low-mass resonances.