Physics Scenarios Research Articles

Decoding the B→Kνν excess at Belle II: Kinematics, operators, and masses

An excess in the branching fraction for B+→K+νν recently measured at Belle II may be a hint of new physics. We perform thorough likelihood analyses for different new physics scenarios such as B→KX with a new invisible particle X, or B→Kχχ through a scalar, vector, or tensor current with χ being a new invisible particle or a neutrino. We find that vector-current three-body decay with mX≃0.6 GeV—which may be dark matter—is most favored, while two-body decay with mX≃2 GeV is also competitive. The best-fit branching fractions for the scalar and tensor cases are a few times larger than for the two-body and vector cases. Past measurements provide further discrimination, although the best-fit parameters stay similar. Published by the American Physical Society 2024

CPT, Majorana fermions, and particle physics beyond the Standard Model

Abstract After reviewing charge conjugation and the CPT theorem, we define Majorana fermions and clarify the relationship of Majorana, Weyl, and Dirac fields. Appearance of Majorana fermions in various scenarios of physics beyond the Standard Model is discussed, including neutrino masses, baryon asymmetry of the universe, grand unified theories, and supersymmetry.

Feasibility of acousto-electric tomography

In acousto-electric tomography (AET) the goal is to reconstruct the electric conductivity in a domain from electrostatic boundary measurements of corresponding currents and voltages, while the domain is perturbed by a time-dependent acoustic wave, thus taking advantage of the acousto-electric effect. We approach the AET reconstruction in two steps: First, the interior power density is obtained from boundary measurements by solving a linear inverse and ill-posed problem; second, the interior conductivity is reconstructed from the power density by solving a non-linear and well-posed problem. Mathematically these inverse problems are fairly well understood, and reconstruction methods work well on synthetic data. This is in contrast to experimental findings. An effect can indeed be observed and data can be collected. However, the acousto-electric coupling is very weak, and consequently, the change in the measured voltage due to the acoustic perturbation might be too small compared to the background noise for viable reconstructions. In this paper, we take one step towards understanding the feasibility of AET. We provide an in-silico model of the coupled physics scenario based on standard models for the individual phenomena. Moreover, we formulate and implement numerically a full reconstruction method for the inverse problem via the two steps. We perform computational experiments with realistically chosen parameters from the context of medical imaging. The focus is on understanding the role of the acousto-electric coupling parameter and the signal-to-noise ratio (SNR). The critical signal strength is analyzed and the omnipresent Johnson–Nyquist noise is estimated. We obtain both positive and negative findings; we can reconstruct features even under severe noise conditions, but we also find that the SNR one is likely to face in practice is too low to obtain useful reconstructions.

Electrobaryonic axion: hair of neutron stars

Axion-like particles are predicted in many physics scenarios beyond the Standard Model (SM). Their interactions with SM particles may arise from the triangle anomaly of the associated global symmetry, along with other SM global and gauge symmetries, including anomalies with the global baryon number and electromagnetic gauge symmetries. We initiate the phenomenological study of the corresponding “electrobaryonic axion”—a particle that couples with both the baryon chemical potential and the electromagnetic field. Neutron stars, particularly magnetars, possessing high baryon density and strong magnetic fields, can naturally develop a thin axion hair around their surface. In this study, we calculate this phenomenon, considering the effects of neutron star rotation and general relativity. For axion particles lighter than the neutron star rotation frequency, the anomalous interaction can also induce the emission of axion particles from the neutron star. In the light axion regime, this emission can significantly contribute to the neutron star cooling rate.

SMEFT analysis of charged lepton flavor violating B -meson decays

Charged lepton flavor violation (cLFV) processes, potentially important for various beyond the Standard Model physics scenarios are analyzed in the Standard Model effective field theory framework. We consider the most relevant two-quark–two-lepton (2q2ℓ) operators for the leptonic and semileptonic LFV B-decay (LFVBD) processes Bs→μ+e−,B+→K+μ+e−,B0→K*0μ+e−, and Bs→ϕμ−e+. We analyze the interplay among the Wilson coefficients responsible for these LFVBDs and other cLFV processes, like CR(μ→e), ℓi→ℓjγ, ℓi→ℓjℓkℓm, and Z→ℓiℓj, to find the maximal possible LFV effects in B-meson decays. We probe the scale of new physics in relation to the constraints imposed by both classes of the LFV decays while considering both the present bounds and future expectations. In view of proposed experiments at LHCb-II and Belle II to study charged LFV processes, we have also provided the upper limits on the indirect constraints on such LFVBDs. For the processes where the B meson is decaying to μ± and e∓, we show that new physics can be constrained by an enhancement of 2–4 orders of magnitude on the current sensitivities of the branching ratios of B+→K+μ+e−,B0→K*0μ+e−, and Bs→ϕμ±e∓. Published by the American Physical Society 2024

Search for an exotic vectorlike X-quark in same-sign dilepton final states at the future 3 TeV CLIC*

Abstract Vectorlike quarks (VLQs) with masses at the TeV-scale have been predicted in many new physics scenarios beyond the Standard Model (SM). Based on a simplified doublet model including the exotic vectorlike X quark (VLQ-X) with an electric charge , we study the production of the VLQ-X decaying into at the future Compact Linear Collider (CLIC) with TeV including the initial state radiation and beamstrahlung effects. We focus on the final signals, including same-sign dileptons (electrons or muons), at least one b-tagged jet, and large missing transverse momentum. By performing detailed signal-to-background analyses and detector simulations, we obtain the exclusion capabilities and discovery reaches, respectively, on the mass of the VLQ-X as well as the relevant parameters ( for GeV and the coupling strength for GeV) for some typical luminosities at the 3 TeV CLIC.

Revisiting models that enhance B+→K+νν¯ in light of the new Belle II measurement

Belle II recently reported the new measurement B(B+→K+νν¯)=(2.3±0.7)×10−5 [I. Adachi (Belle-II Collaboration), ] which is two times larger than their previous result (although consistent within errors) and about 2.7σ higher than the SM prediction. We reexamine new physics scenarios we discussed previously, which can enhance this rate to determine if they can accommodate the higher value reported in the new measurement. We use consistency with existing bounds on B→K*νν¯, b→sℓ+ℓ−, B→D(*)ℓν¯, and Bs mixing to limit possible explanations for the excess. For the case of lepton flavor violating neutrino couplings, we find that only two leptoquarks remain viable requiring a large C9′ττ=−C10′ττ. For models with different types of light dark matter particle pairs (scalar, fermion, or vector), the preliminary q2 distribution from Belle II, which shows that the excess appears mostly for bins with 3≤q2≤7 GeV2 [I. Adachi (Belle-II Collaboration), ], implies only the vector current operators with scalar or vector dark matter particles with masses in the hundreds of MeV can match the anomaly. Published by the American Physical Society 2024

Non-resonant anomaly detection with background extrapolation

Complete anomaly detection strategies that are both signal sensitive and compatible with background estimation have largely focused on resonant signals. Non-resonant new physics scenarios are relatively under-explored and may arise from off-shell effects or final states with significant missing energy. In this paper, we extend a class of weakly supervised anomaly detection strategies developed for resonant physics to the non-resonant case. Machine learning models are trained to reweight, generate, or morph the background, extrapolated from a control region. A classifier is then trained in a signal region to distinguish the estimated background from the data. The new methods are demonstrated using a semi-visible jet signature as a benchmark signal model, and are shown to automatically identify the anomalous events without specifying the signal ahead of time.

Searching for lepton flavor violation with the CMS experiment

Searches for Lepton Flavor Violation (LFV) stand at the forefront of experimental particle physics research, offering a sensitive probe to many scenarios of physics beyond the Standard Model. The high proton–proton collision energy and luminosity provided by the CERN Large Hadron Collider (LHC) and the excellent CMS detector performance allow for an extensive program of LFV searches. This paper reviews a broad range of LFV searches conducted at the CMS experiment using data collected in LHC Run 2, including [Formula: see text] decays, Higgs boson decays, and top quark production and decays. In each analysis, the online and offline event selections, signal modeling, background suppression and estimation, and statistical interpretation are elucidated. These searches involve various final state particles in a large transverse momentum range, showcasing the capability of the CMS experiment in exploring fundamental questions in particle physics.

Polarised cross sections for vector boson production with Sherpa

Measurements of vector boson polarisation in vector boson production processes offer a powerful probe of the electroweak symmetry breaking mechanism, scrutinising the Standard Model and new physics scenarios alike. Since massive vector bosons can only be observed as intermediate particles, polarised cross section templates from simulation are necessary to extract their polarisation from measurable unpolarised distributions. In this work we present an extension of the Sherpa Monte-Carlo event generator allowing the simulation of polarised cross sections for vector boson production processes. Based on the narrow-width approximation, polarised cross sections of all possible polarisation combinations for an arbitrary number of intermediate vector bosons can be simulated in a single simulation run. In addition, it is possible to directly predict the interference between different intermediate polarisation states, and various differing polarisation definitions can be studied simultaneously. Besides the simulation of polarised cross sections at fixed LO and LO+PS accuracy as well as in multijet-merged calculations, we also present parton-shower-matched polarised cross sections with approximate NLO QCD corrections in the vector boson production processes. We demonstrate that the differences of this approximation to full NLO QCD predictions are small and it thus opens up the possibility for fully-simulated calculations at the hadron level including polarisation information and higher-order QCD effects for the first time.

Third-family lepton-quark fusion

We analyze the signatures of new physics scenarios featuring third-family quark-lepton unification at the TeV scale in lepton-quark fusion at hadron colliders. Working with complete UV dynamics based on the SU(4) gauge symmetry in the third-family fermions, we simulate the resonant production of a vector leptoquark at the next-to-leading order, including its decay and matching to the parton showers. The precise theoretical control over this production channel allows us to set robust bounds on the vector leptoquark parameter space which are complementary to the other production channels at colliders. We emphasize the importance of the resonant channel in future searches and discuss the impact of variations in the model space depending on the flavor structure of the vector leptoquark couplings.

Same-sign taus signatures of maximally flavor-violating scalars at the LHC

We explore single and double flavor-violating scalar (flavon) production at the 13 and 14 TeV LHC in an effective field theory formulation where flavons always change the flavor of the Standard Model fermions. When those scalars couple to mass, their flavor-changing couplings to top quarks and tau leptons are favored. Focusing on the mass region below the top-quark mass, we find couplings that fit the muon (g−2) discrepancy and avoid several current experimental constraints. We determine the potential of the LHC to exclude or discover such a new physics scenario with clean signatures consisting of same-sign tau leptons and the simultaneous observation of resonances in the tau plus electron or muon invariant mass. We found that in the double production mode, effective couplings down to order 10−2 TeV−1 can be probed for flavon masses in the 10–170 GeV range at the 14 TeV HL-LHC, but couplings down to 0.1 TeV−1 can already be excluded at 95% confidence level with data collected from the 13 TeV LHC in the same mass interval. We also explore the impact of sizeable diagonal flavon couplings on the prospects of LHC for the signals we propose. Published by the American Physical Society 2024

Beyond the Standard Model prospects for kaon physics at future experiments

Rare kaon decays offer a powerful tool for investigating new physics in s → d transitions. Currently, many of the interesting decay modes are either measured with rather large uncertainties compared to their theoretical predictions or have not yet been observed. The future HIKE programme at CERN will provide unprecedented sensitivity to rare kaon decays, allowing for strong constraints on new physics scenarios with lepton flavour universality violation. We present the overall picture that emerges from a study of the different decay modes with a global analysis considering projections based on the HIKE programme, both with and without KOTO-II future measurements. We also highlight the most relevant decays and identify that in addition to the “golden channel”, K+→π+νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {K}^{+}\ o {\\pi}^{+}\ u \\overline{\ u} $$\\end{document}, the rare KL→π0ℓℓ¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {K}_L\ o {\\pi}^0\\ell \\overline{\\ell} $$\\end{document} decay, especially in the electron sector offers strong constraints on short-distance physics.

The automation of SMEFT-assisted constraints on UV-complete models

The ongoing Effective Field Theory (EFT) program at the LHC and elsewhere is motivated by streamlining the connection between experimental data and UV-complete scenarios of heavy new physics beyond the Standard Model (BSM). This connection is provided by matching relations mapping the Wilson coefficients of the EFT to the couplings and masses of UV-complete models. Building upon recent work on the automation of tree-level and one-loop matching in the SMEFT, we present a novel strategy automating the constraint-setting procedure on the parameter space of general heavy UV-models matched to dimension-six SMEFT operators. A new Mathematica package, match2fit, interfaces MatchMakerEFT, which derives the matching relations for a given UV model, and SMEFiT, which provides bounds on the Wilson coefficients by comparing with data. By means of this pipeline and using both tree-level and one-loop matching, we derive bounds on a wide range of single- and multi-particle extensions of the SM from a global dataset composed by LHC and LEP measurements. Whenever possible, we benchmark our results with existing studies. Our framework realises one of the main objectives of the EFT program in particle physics: deploying the SMEFT to bypass the need of directly comparing the predictions of heavy UV models with experimental data.

Probing new physics with charge asymmetries in 2 same-sign leptons plus jets final states at the LHC

We study the impact of new physics models in the charge asymmetry defined for LHC final states consisting of two same-sign leptons (2SSl, with l = e, μ) plus jets (Njets ≥ 2), with a center-of-mass energy of s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV, where the main SM contribution is tt¯W\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}W $$\\end{document} production. Concretely, we consider three different new physics sources for the charge asymmetries: a heavy neutral scalar/pseudoscalar arising from the general two Higgs doublet model, an effective theory with dimension-6 four-quark operators, and a simplified R-parity-violating supersymmetric model with electroweakino production (higgsino-like or wino-like). We propose measuring the charge asymmetries differentially with respect to several kinematic observables, and inclusively/exclusively with the number of b-tagged jets in the final state (Nb ≥ {1, 2, 3}). Results are compared with the SM prediction using the χ2 criteria, expressing sensitivity in terms of the normal Gaussian significance. We show that some of the proposed new physics scenarios can be probed at the LHC even for the already recorded integrated luminosity of 139 fb−1. Finally, we also estimate the prospects for the potential LHC sensitivity to the considered new physics models in its high-luminosity phase.

Students’ scaling of axes when constructing qualitative graphs that represent a physics scenario

We have studied how first-year university science students construct graphs based on hypothetical qualitative physics scenarios. We gave students a questionnaire that asked them to complete two Cartesian graphs in one of three different scenarios (a ball rolling down a track, a beaker being filled with water, the resistance between different points on a metal bar) given as a written piece of text accompanied by a diagram of a hypothetical experiment that included three evenly spaced points on the set-up. Two of the three points were also indicated on the position axis of the partially drawn graph. We found that students can find it hard to translate equal spatial intervals in the experiment to a line graph. We found that most students either did not explain why they put the third point on the graph where they did, or did not plot the point at all. Some students drew unequal intervals on the position axis to indicate unequal time or resistance intervals. The difficulties became more prevalent as the levels of abstraction increased. Our findings suggest that constructing a scale on a qualitative graph requires significant mental effort from the students.

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}.

Electron-positron, parton-parton, and photon-photon production of τ -lepton pairs: Anomalous magnetic and electric dipole moments spin effects

Anomalous contributions to the electric and magnetic dipole moments of the τ lepton from new physics scenarios have brought renewed interest in the development of new charge-parity violating signatures in τ-pair production at Belle II energies, and also at higher energies of the Large Hadron Collider and the Future Circular Collider. In this paper, we discuss the effects of spin correlations, including transverse degrees of freedom, in the τ-pair production and decay. These studies include calculating analytical formulas, obtaining numerical results, and building semirealistic observables sensitive to the transverse spin correlations induced by the dipole moments of the τ lepton. The effects of such anomalous contributions to the dipole moments are introduced on top of precision simulations of e−e+→τ−τ+, qq¯→τ−τ+ and γγ→τ−τ+ processes, involving multibody final states. The τ decays are simulated along with radiative corrections, in particular electroweak box contributions of WW and ZZ exchanges are taken into account. Respective extensions of the Standard Model amplitudes and the reweighting algorithms are implemented into the Monte Carlo, which is used to simulate τ-pair production in e−e+ collisions, and the aupinner program, which is used to reweight events with τ pair produced in pp collisions. Published by the American Physical Society 2024

Emerging jet probes of strongly interacting dark sectors

A strongly interacting dark sector can give rise to a class of signatures dubbed dark showers, where in analogy to the strong sector in the Standard Model, the dark sector undergoes its own showering and hadronization, before decaying into Standard Model final states. When the typical decay lengths of the dark sector mesons are larger than a few centimeters (and no larger than a few meters) they give rise to the striking signature of emerging jets, characterized by a large multiplicity of displaced vertices.In this article we consider the general reinterpretation of the CMS search for emerging jets plus prompt jets into arbitrary new physics scenarios giving rise to emerging jets. More concretely, we consider the cases where the SM Higgs mediates between the dark sector and the SM, for several benchmark decay scenarios. Our procedure is validated employing the same model than the CMS emerging jet search. We find that emerging jets can be the leading probe in regions of parameter space, in particular when considering the so-called gluon portal and dark photon portal decay benchmarks. With the current 16.1 fb−1 of luminosity this search can exclude down to O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(20)% exotic branching ratio of the SM Higgs, but a naive extrapolation to the 139 fb−1 luminosity employed in the current model-independent, indirect bound of 16 % would probe exotic branching ratios into dark quarks down to below 10 %. Further extrapolating these results to the HL-LHC, we find that one can pin down exotic branching ratio values of 1%, which is below the HL-LHC expectations of 2.5–4 %. We make our recasting code publicly available, as part of the LLP Recasting Repository.

Research on Creating Teaching Scenarios for High School Physics Based on Core Competencies

Research on Creating Teaching Scenarios for High School Physics Based on Core Competencies