Searching for exotic production of Higgs boson + X to map out new physics
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79 ASTRONOMY AND ASTROPHYSICS↗
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Development of systematic uncertainty-aware neural network trainings for binned-likelihood analyses at the LHC
We propose a neural network training method capable of accounting for the effects of systematic variations of the data model in the training process and describe its extension towards neural network multiclass classification. The procedure is evaluated on the realistic case of the measurement of Higgs boson production via gluon fusion and vector boson fusion in the τ τ decay channel at the CMS experiment. The neural network output functions are used to infer the signal strengths for inclusive production of Higgs bosons as well as for their production via gluon fusion and vector boson fusion. We observe improvements of 12 and 16% in the uncertainty in the signal strengths for gluon and vector-boson fusion, respectively, compared with a conventional neural network training based on cross-entropy.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Scientific Summary Report for Early Career Award "High Precision Event Simulation for the LHC"
Particle physics has entered a new era with the discovery of the Higgs boson at the CERN Large Hadron Collider (LHC). To determine all properties of the boson and to reveal possible signatures of physics beyond the Standard Model it is crucial to make reliable predictions for both signal and background reactions. Monte-Carlo event generators produce such predictions fully differentially at the level of observed particles. They rely on various approximations to cope with the many-body dynamics of the problem and on models to deal with the non-abelian, nonlinear nature of Quantum Chromodynamics (QCD), the quantum-field theoretical model of the strong nuclear force. Dependence on the model parameters is one of the main uncertainties affecting measurements and searches at the LHC. Whenever possible such dependence must be reduced or eliminated.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
An Effective Field Theory Approach to neutrinoless double beta decay [Slides]
The Standard Model of Particle Physics includes: 1. describes nature in a economic and elegant way (spontaneously broken) gauge symmetry; 2. generates fermion and boson masses via the Higgs mechanism.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Modelling Z→TT processes in ATLAS with T-embedded Z →μμ data
We describe the concept, technical realisation and validation of a largely data-driven method to model events with Z→ττ decays. In Z→μμ events selected from proton-proton collision data recorded at √s=8 TeV with the ATLAS experiment at the LHC in 2012, the Z decay muons are replaced by τ leptons from simulated Z →ττ decays at the level of reconstructed tracks and calorimeter cells. The τ lepton kinematics are derived from the kinematics of the original muons. Thus, only the well-understood decays of the Z boson and τ leptons as well as the detector response to the τ decay products are obtained from simulation. All other aspects of the event, such as the Z boson and jet kinematics as well as effects from multiple interactions, are given by the actual data. This so-called τ-embedding method is particularly relevant for Higgs boson searches and analyses in ττ final states, where Z→ττ decays constitute a large irreducible background that cannot be obtained directly from data control samples. In this paper, we discuss the relevant concepts based on the implementation used in the ATLAS Standard Model H→ττ analysis of the full datataset recorded during 2011 and 2012.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for resonant and nonresonant Higgs boson pair production in the $ \mathrm{b}\overline{\mathrm{b}}\mathit{\ell \nu \ell \nu } $ final state in proton-proton collisions at $ \sqrt{s}=13 $ TeV
Searches for resonant and nonresonant pair-produced Higgs bosons (HH) decaying respectively into ℓνℓν, through either W or Z bosons, and $ \mathrm{b}\overline{\mathrm{b}} $ are presented. The analyses are based on a sample of proton-proton collisions at $ \sqrt{s}=13 $ TeV, collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{−1}$. Data and predictions from the standard model are in agreement within uncertainties. For the standard model HH hypothesis, the data exclude at 95% confidence level a product of the production cross section and branching fraction larger than 72 fb, corresponding to 79 times the standard model prediction. Constraints are placed on different scenarios considering anomalous couplings, which could affect the rate and kinematics of HH production. Upper limits at 95% confidence level are set on the production cross section of narrow-width spin-0 and spin-2 particles decaying to Higgs boson pairs, the latter produced with minimal gravity-like coupling.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
A search for heavy Higgs bosons decaying into vector bosons in same-sign two-lepton final states in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector
A search for heavy Higgs bosons produced in association with a vector boson and decaying into a pair of vector bosons is performed in final states with two leptons (electrons or muons) of the same electric charge, missing transverse momentum and jets. A data sample of proton–proton collisions at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018 is used. The data correspond to a total integrated luminosity of 139 fb –1 . The observed data are in agreement with Standard Model background expectations. The results are interpreted using higher-dimensional operators in an effective field theory. Upper limits on the production cross-section are calculated at 95% confidence level as a function of the heavy Higgs boson’s mass and coupling strengths to vector bosons. Limits are set in the Higgs boson mass range from 300 to 1500 GeV, and depend on the assumed couplings. The highest excluded mass for a heavy Higgs boson with the coupling combinations explored is 900 GeV. Limits on coupling strengths are also provided.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for the associated production of a Higgs boson with a single top quark in proton-proton collisions at $ \sqrt{s}=8 $ TeV
Our paper presents the search for the production of a Higgs boson in association with a single top quark, using data collected in proton-proton collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 fb -1 . The search exploits a variety of Higgs boson decay modes resulting in final states with photons, bottom quarks, and multiple charged leptons, including tau leptons, and employs a variety of multivariate techniques to maximize sensitivity to the signal. Furthermore, our analysis is optimized for the opposite sign of the Yukawa coupling to that in the standard model and corresponding to a large enhancement of the signal cross section. In the absence of an excess of candidate signal events over the background predictions, 95% confidence level observed (expected) upper limits on anomalous tHq production are set, ranging between 600 (450) fb and 1000 (700) fb depending on the assumed diphoton branching fraction of the Higgs boson. This is the first time that results on anomalous tHq production have been reported.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Higgs to b b ¯ from vector boson fusion for high-scale physics
Vector boson fusion is arguably the most direct collider probe of electroweak symmetry breaking. Typically, the signature includes two forward/backward jets with low transverse momenta with a scale that is set by the mass of the vector boson. For this reason, an upper cut is used when searching for vector boson fusion processes in the Standard Model. Alternatively, the upper cut on the forward jets can be removed and the high-momentum exchange region of vector boson fusion can be studied. This phase space region has sensitivity to new physics via higher dimensional operators and form factors. In this work, we study the high-momentum region of the vector boson fusion channel where the Higgs decays to b b ¯ . We show that, depending on the form of new physics, the limits on the new physics scale range from 0.5 to 1.8 TeV. Published by the American Physical Society 2024
Astronomy & Astrophysics↗
Search for long-lived particles using out-of-time trackless jets in proton-proton collisions at $\sqrt{s}$ = 13 TeV
A search for long-lived particles decaying in the outer regions of the CMS silicon tracker or in the calorimeters is presented. The search is based on a data sample of proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded with the CMS detector at the LHC in 2016–2018, corresponding to an integrated luminosity of 138 fb −1 . A novel technique, using nearly trackless and out-of-time jet information combined in a deep neural network discriminator, is employed to identify decays of long-lived particles. The results are interpreted in a simplified model of chargino-neutralino production, where the neutralino is the next-to-lightest supersymmetric particle, is long-lived, and decays to a gravitino and either a Higgs or Z boson. This search is most sensitive to neutralino proper decay lengths of approximately 0.5 m, for which masses up to 1.18 TeV are excluded at 95% confidence level. The current search is the best result to date in the mass range from the kinematic limit imposed by the Higgs boson mass up to 1.8 TeV.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Dynamics of dark matter misalignment through the Higgs portal
A light singlet scalar field feebly coupled through the super-renormalizable Higgs portal provides a minimal and well-motivated realization of ultra-light bosonic dark matter. We study the cosmological production of dark matter in this model by elucidating the dynamics of two sources of scalar field misalignment generated during the radiation era. For large scalar masses (above $\mathcal{O}$ (10 −3 eV)), dark matter is produced through thermal misalignment, by which the scalar field is driven towards large field values as a result of the finite-temperature effective potential. The dominance of thermal misalignment in this mass range leads to a sharp relic abundance prediction which is, to a significant extent, insensitive to the initial conditions of the scalar field. On the other hand, for low mass scalars (below $\mathcal{O}$ (10 −5 eV)), dark matter is produced via VEV misalignment, which is caused by the induced scalar field vacuum expectation value triggered by the electroweak phase transition. We show that the relic abundance in this low mass range is sensitive to the scalar field initial conditions. In the intermediate mass range, the relic abundance is a consequence of a competition between thermal misalignment and VEV misalignment, which can potentially lead to novel forced resonance effects which cause a recurring enhancement and suppression in the late time oscillation amplitude as a function of the scalar mass. We compare our relic abundance predictions with constraints and projections from equivalence principle and inverse square law tests, stellar cooling, resonant molecular absorption, and observations of extra-galactic background light and diffuse X-ray backgrounds. New experimental ideas are needed to probe most of the cosmologically motivated regions of parameter space.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Inflection-point inflation in a hyper-charge oriented U ( 1 ) X model
Inflection-point inflation is an interesting possibility to realize a successful slow-roll inflation when inflation is driven by a single scalar field with its value during inflation below the Planck mass (ΦI≲M Pl ). In order for a renormalization group (RG) improved effective λΦ 4 potential to develop an inflection-point, the running quartic coupling λ(Φ) must exhibit a minimum with an almost vanishing value in its RG evolution, namely λ(Φ I )≃0 and β λ (ΦI)≃0, where β λ is the beta-function of the quartic coupling. Here in this paper, we consider the inflection-point inflation in the context of the minimal gauged U(1) X extended Standard Model (SM), which is a generalization of the minimal U(1) B$-$L model, and is constructed as a linear combination of the SM U(1) Y and U(1) B$-$L gauge symmetries. We identify the U(1) X Higgs field with the inflaton field. For a successful inflection-point inflation to be consistent with the current cosmological observations, the mass ratios among the U(1) X gauge boson, the right-handed neutrinos and the U(1) X Higgs boson are fixed. Focusing on the case that the extra U(1) X gauge symmetry is mostly aligned along the SM U(1) Y direction, we investigate a consistency between the inflationary predictions and the latest LHC Run-2 results on the search for a narrow resonance with the di-lepton final state.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Observation of W W γ Production and Search for H γ Production in Proton-Proton Collisions at s = 13 TeV
The observation of W W γ production in proton-proton collisions at a center-of-mass energy of 13 TeV with an integrated luminosity of 138 fb − 1 is presented. The observed (expected) significance is 5.6 (5.1) standard deviations. Events are selected by requiring exactly two leptons (one electron and one muon) of opposite charge, moderate missing transverse momentum, and a photon. The measured fiducial cross section for W W γ is 5.9 ± 0.8 ( stat ) ± 0.8 ( syst ) ± 0.7 ( modeling ) fb , in agreement with the next-to-leading order quantum chromodynamics prediction. The analysis is extended with a search for the associated production of the Higgs boson and a photon, which is generated by a coupling of the Higgs boson to light quarks. The result is used to constrain the Higgs boson couplings to light quarks. © 2024 CERN, for the CMS Collaboration 2024 CERN
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Opportunities in Flavour Physics at the HL-LHC and HE-LHC
Motivated by the success of the flavour physics programme carried out over the last decade at the Large Hadron Collider (LHC), we characterize in detail the physics potential of its High-Luminosity and High-Energy upgrades in this domain of physics. We document the extraordinary breadth of the HL/HE-LHC programme enabled by a putative Upgrade II of the dedicated flavour physics experiment LHCb and the evolution of the established flavour physics role of the ATLAS and CMS general purpose experiments. We connect the dedicated flavour physics programme to studies of the top quark, Higgs boson, and direct high-$p_T$ searches for new particles and force carriers. We discuss the complementarity of their discovery potential for physics beyond the Standard Model, affirming the necessity to fully exploit the LHC's flavour physics potential throughout its upgrade eras.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for heavy resonances that decay into a vector boson and a Higgs boson in hadronic final states at $\sqrt{s} = 13$ $\,\text {TeV}$
A search for heavy resonances with masses above 1 $\,\text {TeV}$ , decaying to final states containing a vector boson and a Higgs boson, is presented. The search considers hadronic decays of the vector boson, and Higgs boson decays to b quarks. The decay products are highly boosted, and each collimated pair of quarks is reconstructed as a single, massive jet. The analysis is performed using a data sample collected in 2016 by the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 $\,\text {TeV}$ , corresponding to an integrated luminosity of 35.9 $\,\text {fb}^{-1}$ . The data are consistent with the background expectation and are used to place limits on the parameters of a theoretical model with a heavy vector triplet. In the benchmark scenario with mass-degenerate $\mathrm{W^{'}}$ and $\mathrm{Z}' $ bosons decaying predominantly to pairs of standard model bosons, for the first time heavy resonances for masses as high as 3.3 $\,\text {TeV}$ are excluded at 95% confidence level, setting the most stringent constraints to date on such states decaying into a vector boson and a Higgs boson.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for Non-resonant Higgs Boson Pair Production in the Four Bottom Quark Decay Channel With the CMS Experiment
This dissertation presents a search for non-resonant Higgs boson pair production, focusing on the four bottom quark decay channel. It explores the gluon fusion and vector boson fusion production mechanisms. The analysis is performed with a dataset of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS detector at the LHC, corresponding to an integrated luminosity of around 138 inverse femtobarns. Innovative techniques in the areas of object identification, event categorization, signal identification, and background modeling are used to maximize the analysis sensitivity. No excess of signal events are observed relative to the background-only expectation, and 95% CL upper limits on the production cross section are set. The observed upper limit on the standard model production cross section is set at 3.6 times the theoretical expectation. At the time of writing this dissertation, it is the most stringent constraint at the LHC from an individual channel. Furthermore, the observed constraint on the coupling modifier of the Higgs boson self-interaction is set between -2.3 and 9.4. The observed constraint on the coupling modifier of the di-vector-boson-di-Higgs-boson interaction is set between -0.1 and 2.2.
Guerrero Ibarra, Guerrero Fernando↗
Search for a Higgs boson produced in association with a charm quark and decaying to a W boson pair in proton-proton collisions at $ \sqrt{s}=13 $ TeV
This paper presents a search for a Higgs boson produced in association with a charm quark (cH) which allows to probe the Higgs-charm Yukawa coupling strength modifier κ$_{c}$. Higgs boson decays to a pair of W bosons are considered, where one W boson decays to an electron and a neutrino, and the other W boson decays to a muon and a neutrino. The data, corresponding to an integrated luminosity of 138 fb$^{−1}$, were collected between 2016 and 2018 with the CMS detector at the LHC at a center-of-mass energy of $ \sqrt{s}=13 $ TeV. Upper limits at the 95% confidence level (CL) are set on the ratio of the measured yield to the standard model expectation for cH production. The observed (expected) upper limit is 1065 (506), corresponding to an observed (expected) constraint of |κ$_{c}$| < 211 (95). When combined with the previous search for cH in the diphoton decay channel of the Higgs boson, the limits are interpreted as observed (expected) constraints at 95% CL on the value of κ$_{c}$, |κ$_{c}$| < 47 (51).[graphic not available: see fulltext]
Hayrapetyan, Aram [Yerevan Phys. Inst.]↗
Future particle-physics projects in the United States
Here, basic proposals of experiments aimed at precision measurements of Standard Model parameters and at searches for new particles, including dark-matter particles, are described along with future experimental projects considered by American Physical Society at the meeting in the summer of 2013 and intended for implementation within the next ten to twenty years.
43 PARTICLE ACCELERATORS↗