Exotic Decay Research Articles

Higgs physics at ILC

International Linear Collider (ILC) is a viable option for realization of a future Higgs factory. With proven accelerator technology and technically mature detector design, ILC offers well-understood physics program in the top, EW and Higgs sectors. Clean environment of electron–positron collisions, together with beam polarization of both beams and center-of-mass reach at a TeV scale, will make ILC a precision tool to probe realizations of new physics. The above will be illustrated on examples of fully simulated measurements in the Higgs sector, including Higgs couplings, Higgs self-coupling, CP violation measurements and Higgs exotic decay studies at ILC.

Topportunities at the LHC: rare top decays with light singlets

The discovery of the top quark, the most massive elementary particle yet known, has given us a distinct window into investigating the physics of the Standard Model and beyond. With a plethora of top quarks to be produced in the high luminosity era of the LHC, the exploration of its rare decays holds great promise in revealing potential new physics phenomena. We consider higher-dimensional operators contributing to flavour-changing-neutral-current top decays in the SMEFT and its extension by a light singlet species of spin 0, 1/2, or 1, and exhibit that the HL-LHC (and other future colliders) may observe many exotic top decays in a variety of channels. Light singlets which primarily talk to the SM through such a top interaction may also lead to distinctive long-lived particle signals. Searching for such long-lived particles in top-quark decays has the additional advantage that the SM decay of the other top quark in the same event provides a natural trigger.

Search for Light Long-Lived Particles in pp Collisions at sqrt[s]=13 TeV Using Displaced Vertices in the ATLAS Inner Detector.

A search for long-lived particles (LLPs) using 140 fb^{-1} of pp collision data with sqrt[s]=13 TeV recorded by the ATLAS experiment at the LHC is presented. The search targets LLPs with masses between 5 and 55GeV that decay hadronically in the ATLAS inner detector. Benchmark models with LLP pair production from exotic decays of the Higgs boson and models featuring long-lived axionlike particles (ALPs) are considered. No significant excess above the expected background is observed. Upper limits are placed on the branching ratio of the Higgs boson to pairs of LLPs, the cross section for ALPs produced in association with a vector boson, and, for the first time, on the branching ratio of the top quark to an ALP and a u/c quark.

Anatomy of vector-like bottom-quark models in the alignment limit of the 2-Higgs doublet model type-II

Expanding upon our ongoing investigation of Vector-Like Quark (VLQ) phenomenology within a 2-Higgs Doublet Model (2HDM) framework, in this paper, we complement a previous one dedicated to Vector-Like Top-quarks (VLTs) by studying Vector-Like Bottom-quarks (VLBs), specifically focusing on their behavior in the alignment limit of a Type-II Yukawa structure. We examine the potential for detecting VLBs at the Large Hadron Collider (LHC) and analyze their decay signatures, encompassing both Standard Model (SM) processes and exotic decays. The objective is to differentiate among singlet, doublet, and triplet configurations of VLBs by identifying distinct decay patterns, thereby providing insights into the structure of Beyond the SM (BSM) physics.

Search for decays of the Higgs boson into a pair of pseudoscalar particles decaying into bb¯τ+τ− using pp collisions at s=13 TeV with the ATLAS detector

This paper presents a search for exotic decays of the Higgs boson into a pair of new pseudoscalar particles, H→aa, where one pseudoscalar decays into a b-quark pair and the other decays into a τ-lepton pair, in the mass range 12≤ma≤60 GeV. The analysis uses pp collision data at s=13 TeV collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 fb−1. No significant excess above the Standard Model (SM) prediction is observed. Assuming the SM Higgs boson production cross section, the search sets upper limits at 95% confidence level on the branching ratio of Higgs bosons decaying into bb¯τ+τ−, B(H→aa→bb¯τ+τ−), between 2.2% and 3.9% depending on the pseudoscalar mass. © 2024 CERN, for the ATLAS Collaboration 2024 CERN

Exploring exotic decays of the Higgs boson to multi-photons at the LHC via multimodal learning approaches

The Standard Model (SM) Higgs boson, the most recently discovered elementary particle, may still serve as a mediator between the SM sector and a new physics sector related to dark matter (DM). The Large Hadron Collider (LHC) has not yet fully constrained the physics associated with the Higgs boson, leaving room for such possibilities. Among the various potential mass scales of the dark sector, the sub-GeV mass range is particularly intriguing. This parameter space presents significant challenges for DM direct detection experiments that rely on nuclear recoils. Various innovative experimental methods are currently under investigation to explore this sub-GeV dark sector. The LHC, functioning as a Higgs factory, could explore this sector once the challenge of identifying DM signals is resolved. Due to the significantly lower mass of particles in the dark sector compared to the Higgs boson, these particles are expected to be highly boosted following the Higgs boson’s decay. However, detecting and identifying these highly boosted particles remains a considerable challenge at the LHC, despite their eventual decay into SM particles. We employ a well-motivated leptophobic ZB′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {Z}_B^{\\prime } $$\\end{document} model as a prototype to analyze the distinctive signatures from Higgs boson exotic decays into multi-photons. These signatures consist of collimated photons that fail to meet the photon isolation criteria, forming jet-like objects. Conventional analyses relying solely on the purity of energy deposits in the electromagnetic calorimeter would fail to detect these signatures, as they would be overwhelmed by background events from Quantum Chromodynamics. To effectively distinguish between such novel signal signatures and SM background events, we leverage advanced machine learning techniques, specifically the transformer encoder in a multimodal network structure. This neural network successfully separates dark sector signals from SM backgrounds, outperforming traditional event selection analyses.

Improving probes of hAA coupling in the Type-X two Higgs doublet model scenario: The crucial role of τ-jet charge identification

The exotic decay modes of the already discovered 125-GeV scalar into a pair of light pseudoscalars are a good probe of those new physics scenarios where such pseudoscalars exist. Searches in the mass region where the pseudoscalar (A) is lighter than 62.5 GeV have yielded null findings so far. No search has yet examined mA>62.5 GeV where the cross section is suppressed by the off-shell pseudoscalar. We point out a possible enhancement of the sensitivity of probing hAA coupling in the context of a Type-X two Higgs doublet model. We focus on h→AA(⁎)→4τ, and select events with two same-sign τ-jets along with a pair of same-sign leptons. This enables much more effective background elimination than in the erstwhile proposed channels. Taking two values of the ττA coupling into account, we obtain limits on the hAA coupling that can be probed at 2σ and 3σ significance, for mA ranging up to 85 GeV. For mA<62.5 GeV, too, we find the probe through our suggested channel exhibits considerable improvement upon the usual 2μ2τ-based searches conducted at the LHC. Within this region, achieving the reach of λhAA coupling at 3000 fb−1 luminosity using our strategy would require approximately ∼3.6×105 fb−1 luminosity using conventional 2μ2τ-based searches.

Search for light long-lived neutral particles from Higgs boson decays via vector-boson-fusion production from pp collisions at s=13\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s}=13$$\\end{document} TeV with the ATLAS detector

A search is reported for long-lived dark photons with masses between 0.1 GeV and 15 GeV, from exotic decays of Higgs bosons produced via vector-boson-fusion. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are probed. This search uses the full LHC Run 2 (2015–2018) data sample collected in proton–proton collisions at s=13\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s}=13$$\\end{document} TeV, corresponding to an integrated luminosity of 139 fb-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$fb^{-1}$$\\end{document}. Dominant backgrounds from Standard Model processes and non-collision sources are estimated using data-driven techniques. The observed event yields in the signal regions are consistent with the expected background. Upper limits on the Higgs boson to dark photon branching fraction are reported as a function of the dark photon mean proper decay length or of the dark photon mass and the coupling between the Standard Model and the potential dark sector. This search is combined with previous ATLAS searches obtained in the gluon–gluon fusion and WH production modes. A branching fraction above 10% is excluded at 95% CL for a 125 GeV Higgs boson decaying into two dark photons for dark photon mean proper decay lengths between 173 and 1296 mm and mass of 10 GeV.

Illuminating all-hadronic final states with a photon: Exotic decays of the Higgs boson to four bottom quarks in vector boson fusion plus gamma at hadron colliders

We investigate the potential to detect Higgs boson decays to four bottom quarks through a pair of pseudoscalars, a final state that is predicted by many theories beyond the Standard Model. For the first time, the signal sensitivity is evaluated for the final state using the vector boson fusion (VBF) production with and without an associated photon, for the Higgs at mH=125 GeV, at hadron colliders. The signal significance is 0.5σ to 6σ, depending on the pseudoscalar mass ma, when setting the Higgs decay branching ratio to unity, using an integrated luminosity of 150 fb−1 at s=13 TeV. This corresponds to an upper limit of 0.3, on the Higgs branching ratio to four bottom quarks, with a nonobservation of the decay. We also consider several variations of selection requirements—input variables for the VBF tagging and the kinematic variables for the photon—that could help guide the design of new triggers for the Run-3 period of the LHC and for the HL-LHC. Published by the American Physical Society 2024

Jet-Origin Identification and Its Application at an Electron-Positron Higgs Factory.

To enhance the scientific discovery power of high-energy collider experiments, we propose and realize the concept of jet-origin identification that categorizes jets into five quark species (b,c,s,u,d), five antiquarks (b[over ¯],c[over ¯],s[over ¯],u[over ¯],d[over ¯]), and the gluon. Using state-of-the-art algorithms and simulated νν[over ¯]H,H→jj events at 240GeV center-of-mass energy at the electron-positron Higgs factory, the jet-origin identification simultaneously reaches jet flavor tagging efficiencies ranging from 67% to 92% for bottom, charm, and strange quarks and jet charge flip rates of 7%-24% for all quark species. We apply the jet-origin identification to Higgs rare and exotic decay measurements at the nominal luminosity of the Circular Electron Positron Collider and conclude that the upper limits on the branching ratios of H→ss[over ¯],uu[over ¯],dd[over ¯] and H→sb,db,uc,ds can be determined to 2×10^{-4} to 1×10^{-3} at 95%confidence level. The derived upper limit for H→ss[over ¯] decay is approximately 3 times the prediction of the standard model.

Nanosecond anomaly detection with decision trees and real-time application to exotic Higgs decays

We present an interpretable implementation of the autoencoding algorithm, used as an anomaly detector, built with a forest of deep decision trees on FPGA, field programmable gate arrays. Scenarios at the Large Hadron Collider at CERN are considered, for which the autoencoder is trained using known physical processes of the Standard Model. The design is then deployed in real-time trigger systems for anomaly detection of unknown physical processes, such as the detection of rare exotic decays of the Higgs boson. The inference is made with a latency value of 30 ns at percent-level resource usage using the Xilinx Virtex UltraScale+ VU9P FPGA. Our method offers anomaly detection at low latency values for edge AI users with resource constraints.

ALP-assisted strong first-order electroweak phase transition and baryogenesis

Axion-like particles (ALPs) can be naturally lighter than the electroweak scale. We consider an ALP that couples to the Standard Model Higgs to achieve the strong first-order electroweak phase transition. We discuss the two-field dynamics of the phase transition and the associated computation in detail and identify the viable parameter space. The ALP mass can be from the MeV to GeV scale. Baryon asymmetry can be explained by local baryogenesis without violating the current electron and atom electric dipole moment bound in most of the viable parameter space. The viable parameter space can be probed through Higgs exotic decay, rare kaon decay, the electron and atomic electric dipole moment, and the effective number of neutrinos in the cosmic microwave background in the future. The gravitational-wave signal is too weak to be detected.

Rare and exclusive few-body decays of the Higgs, Z, W bosons, and the top quark

Abstract We perform an extensive survey of rare and exclusive few-body decays ---defined as those with branching fractions $\mathcal{B} \lesssim 10^{-5}$ and two or three final particles--- of the Higgs, Z, W bosons, and the top quark. Such rare decays can probe physics beyond the Standard Model (BSM), constitute a background for exotic decays into new BSM particles, and provide precise information on quantum chromodynamics factorization with small nonperturbative corrections. We tabulate the theoretical $\mathcal{B}$ values for almost 200 rare decay channels of the four heaviest elementary particles, indicating the current experimental limits in their observation. Among those, we have computed for the first time ultrarare Higgs boson decays into photons and/or neutrinos, H and Z radiative decays into leptonium states, radiative H and Z quark-flavour-changing decays, and semiexclusive top-quark decays into a quark plus a meson, while updating predictions for a few other rare H, Z, and top quark partial widths. The feasibility of measuring each of these unobserved decays is estimated for p-p collisions at the high-luminosity Large Hadron Collider (HL-LHC), and for $e^+e^-$ and p-p collisions at the future circular collider (FCC).

Search for an exotic decay of the Higgs boson into a Z boson and a pseudoscalar particle in proton-proton collisions at [formula omitted

A search for an exotic decay of the Higgs boson to a Z boson and a light pseudoscalar particle (a), decaying to a pair of leptons and a pair of photons, respectively, is presented. The search is based on proton-proton collision data at a center-of-mass energy of s=13TeV, collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 138fb−1. The analysis probes pseudoscalar masses ma between 1 and 30 GeV, leading to two pairs of well-isolated leptons and photons. Upper limits at 95% confidence level are set on the Higgs boson production cross section times its branching fraction to two leptons and two photons. The observed (expected) limits are in the range of 1.1–17.8 (1.7–17.9)fb within the probed ma interval. An excess of data above the expected standard model background with a local (global) significance of 2.6 (1.3) standard deviations is observed for a mass hypothesis of ma=3GeV. Limits on models involving axion-like particles, formulated as an effective field theory, are also reported.

Pair production of the singlet vector-like B quark at the CLIC

Vector-like quarks (VLQs) are a common feature of many scenarios of new physics beyond the Standard Model (SM), which generally decay into a SM third-generation quark with a SM gauge boson or a Higgs boson. The presence of a new exotic decay mode of VLQs will reduce the branching ratios of these standard decay modes and thus relax the current mass exclusion limits from LHC experiments. Based on a model-independent framework, we investigate the prospect of discovering the pair production of the weak-singlet VLQ-B at the future 3-TeV Compact Linear Collider (CLIC), by focusing on the final states including one Z boson and four b-jets via two types of modes: Z→ℓ+ℓ-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z\\rightarrow \\ell ^{+}\\ell ^{-}$$\\end{document} and Z→νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z\\rightarrow \ u {\\bar{\ u }}$$\\end{document}. By performing a rapid detector simulation of the signal and background events, and considering the initial state radiation and beamstrahlung effects, the exclusion limit at the 95% confidence level and the 5σ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$5\\sigma $$\\end{document} discovery prospects are respectively obtained on the branching ratio of B→bZ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B\\rightarrow bZ$$\\end{document} and the VLQ-B masses at the future 3-TeV CLIC with an integrated luminosity of 5 ab-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}.

Two proton emission and other exotic decays in the 48Ni region with ACTAR TPC

Since the first indirect measurements of the two proton radioactivity in 2002 [1], some theoretical descriptions have been able to reproduce experimental results until the 2-proton radioactivity was established for 67Kr at RIKEN [2], with a lifetime about 20 times lower than expected. New theory inputs have recently been developed [3][4], predicting angular distributions of the emitted protons for some of the two proton emitters, that need to be confirmed experimentally. An experiment at GANIL/LISE3 facility was performed in May 2021 aiming to produce one of the 2 proton emitters (48Ni) and measure the angular distribution of the emitted protons. We used the ACTAR TPC detector to implant the ions and perform the tracking of their proton decays. In addition, other exotic nuclei in the 48Ni region were produced. For several of them we have found a first evidence of exotic decays such as β delayed 3- and 2-proton emission or low energy proton branches for β delayed single proton emission. The analysis of this decays will provide helpful experimental information about the structure and the partial decay schemes of these unstable nuclei.

Search for the exotic decay of the Higgs boson into two light pseudoscalars with four photons in the final state in proton-proton collisions at sqrt{s} = 13 TeV

A search for the exotic decay of the Higgs boson to a pair of light pseudoscalars, each of which subsequently decays into a pair of photons, is presented. The search uses data from proton-proton collisions at sqrt{s} = 13 TeV recorded with the CMS detector at the LHC that corresponds to an integrated luminosity of 132 fb−1. The analysis probes pseudoscalar bosons with masses in the range 15–62 GeV, coming from the Higgs boson decay, which leads to four well-isolated photons in the final state. No significant deviation from the background-only hypothesis is observed. Upper limits are set on the product of the Higgs boson production cross section and branching fraction into four photons. The observed (expected) limits range from 0.80 (1.00) fb for a pseudoscalar boson mass of 15 GeV to 0.26 (0.24) fb for a mass of 62 GeV at 95% confidence level.

Higgs rare and exotic decays at CMS and ATLAS

Higgs rare and exotic decays at CMS and ATLAS

Search for H → aa → 4b exotic decays and VBF HH → 4b with CEPC and ATLAS

Search for H → aa → 4b exotic decays and VBF HH → 4b with CEPC and ATLAS

New Geiger-Nuttall law for cluster radioactivity half-lives* *Supported in part by the National Natural Science Foundation of China (12175100, 11975132), the construct program of the key discipline in hunan province, the Research Foundation of Education Bureau of Hunan Province, China (18A237), the Shandong Province Natural Science Foundation, China (ZR2022JQ04), the Opening Project of Cooperative Innovation Center for Nuclear Fuel Cycle

In this study, derived from Balasubramaniam's formula [Phys. Rev. C 70, 017301 (2004)] and further considering the effect of the parent nucleus mass, blocking effect, and effect of reduced mass on cluster radioactivity half-lives, we propose a new Geiger-Nuttall law that is model-independent to systematically evaluate the half-lives of this process for 16 even-even nuclei and 10 odd-A nuclei. For comparison, a single universal curve for cluster radioactivity and α decay proposed by Poenaru [Phys. Rev. C 83, 014601 (2011)], a scaling law proposed by Horoi [J. Phys. G: Nucl. Part. Phys. 30, 945 (2004)], an extension of the Viola-Seaborg formula from α decay to cluster radioactivity proposed by Ren et al. [Phys. Rev. C 70, 034304 (2004)], a new semi-empirical formula for exotic cluster decay proposed by Balasubramaniam et al. [Phys. Rev. C 70, 017301 (2004)], and a unified formula for the half-lives of α decay and cluster radioactivity proposed by Ni et al. [Phys. Rev. C 78, 044310 (2008)] are also used. The calculated results of our new Geiger-Nuttall law are in good agreement with the experimental half-lives, with the least rms being 0.606, and are better than the compared values. Moreover, we extend this formula to predict the cluster radioactivity half-lives of 51 nuclei whose decay energies are energetically allowed or observed but not yet quantified in NUBASE2020.