Standard Model Prediction Research Articles

Measurement of the D* longitudinal polarization in B0→D*−τ+ντ decays

The longitudinal polarization fraction of the D* meson is measured in B0→D*−τ+ντ decays, where the τ lepton decays to three charged pions and a neutrino, using proton-proton collision data collected by the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV and corresponding to an integrated luminosity of 5 fb−1. The D* polarization fraction FLD* is measured in two q2 regions, below and above 7 GeV2/c4, where q2 is defined as the squared invariant mass of the τντ system. The FLD* values are measured to be 0.52±0.07±0.04 and 0.34±0.08±0.02 for the lower and higher q2 regions, respectively. The first uncertainties are statistical and the second systematic. The average value over the whole q2 range is FLD*=0.41±0.06±0.03. These results are compatible with the Standard Model predictions. © 2024 CERN, for the LHCb Collaboration 2024 CERN

First Indication of Solar B8 Neutrinos through Coherent Elastic Neutrino-Nucleus Scattering in PandaX-4T

The PandaX-4T liquid xenon detector at the China Jinping Underground Laboratory is used to measure the solar B8 neutrino flux by detecting neutrinos through coherent scattering with xenon nuclei. Data samples requiring the coincidence of scintillation and ionization signals (paired), as well as unpaired ionization-only signals (US2), are selected with energy threshold of approximately 1.1 keV (0.33 keV) nuclear recoil energy. Combining the commissioning run and the first science run of PandaX-4T, a total exposure of 1.20 and 1.04 tonne·year are collected for the paired and US2, respectively. After unblinding, 3 and 332 events are observed with an expectation of 2.8±0.5 and 251±32 background events, for the paired and US2 data, respectively. A combined analysis yields a best-fit B8 neutrino signal of 3.5 (75) events from the paired (US2) data sample, with ∼37% uncertainty, and the background-only hypothesis is disfavored at 2.64σ significance. This gives a solar B8 neutrino flux of (8.4±3.1)×106 cm−2 s−1, consistent with the standard solar model prediction. It is also the first indication of solar B8 neutrino “fog” in a dark matter direct detection experiment. Published by the American Physical Society 2024

Testing the CKM unitarity at high energy via the W+W− production at the LHC and future colliders

We propose a novel test to assess the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, VCKM, at present and future collider experiments. Our strategy makes use of the W+W− production cross section to directly probe the VCKM†VCKM product, which regulates the high-energy behavior of the observable. The violation of unitarity is signaled by an anomalous behavior of the cross section that grows quadratically with the W+W− invariant mass with respect to the Standard Model prediction. By using the recent ATLAS measurements of the W+W− cross section we are able to constrain the maximal unitarity violation allowed by current data, producing a bound complementary to the results of flavor physics experiments. Forecasts for the high luminosity phase of the LHC and for the future 100 TeV hadron collider are also discussed.

Test of lepton flavor universality with a measurement of R(D*) using hadronic B tagging at the Belle II experiment

The ratio of branching fractions R(D*)=B(B¯→D*τ−ν¯τ)/B(B¯→D*ℓ−ν¯ℓ), where ℓ is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of 189 fb−1 at the SuperKEKB asymmetric-energy e+e− collider. Data is collected at the ϒ(4S) resonance, and one B meson in the ϒ(4S)→BB¯ decay is fully reconstructed in hadronic decay modes. The accompanying signal B meson is reconstructed as B¯→D*τ−ν¯τ using leptonic τ decays. The normalization decay, B¯→D*ℓ−ν¯ℓ, produces the same observable final-state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields R(D*)=0.262−0.039+0.041(stat)−0.032+0.035(syst). This result is consistent with the current world average and with Standard Model predictions. Published by the American Physical Society 2024

Rare B and K decays in a scotogenic model

A scotogenic model can radiatively generate the observed neutrino mass, provide a dark matter candidate, and lead to rare lepton flavor-violating processes. We aim to extend the model to establish a potential connection to the quark flavor-related processes within the framework of scotogenesis, enhancing the unexpectedly large branching ratio (BR) of B+→K+νν¯ observed by the Belle II Collaboration. Meanwhile, the model can address tensions between some experimental measurements and standard model (SM) predictions in flavor physics, such as the muon g−2 excess and the higher BR of Bs→μ−μ+. In the model, we introduce the following dark particles: a neutral singlet Dirac-type lepton (N); two inert Higgs doublets (η1,2), with one carrying a lepton number; a charged singlet dark scalar (χ+); and a singlet vectorlike up-type dark quark (T). The first two entities are responsible for the radiative neutrino mass, and χ+ couples to right-handed quarks and leptons and can resolve the tensions existing in muon g−2 and Bs→μ−μ+. Furthermore, the BR of B+→K+νν¯ can be enhanced up to a factor of 2 compared to the SM prediction through the mediations of the dark T and the charged scalars. In addition, we also study the impacts on the K→πνν¯ decays. Published by the American Physical Society 2024

Theory-Driven Evolution of the Weak Mixing Angle.

We present the first purely theoretical calculation of the weak mixing angle in the MS[over ¯] scheme at low energies by combining results from lattice QCD with perturbation theory. We discuss its correlation with the hadronic contribution to the anomalous magnetic moment of the muon and to the energy dependence of the electromagnetic coupling. We also compare the results with calculations using cross-section data as input. Implications for the standard model prediction of the mass of the W boson are also discussed.

How viable is a QCD axion near 10 MeV?

There has been an attempt to revive the visible QCD axion at the 10 MeV scale assuming that it exclusively couples to the first-generation quarks and the electron. This variant of the QCD axion is claimed to remain phenomenologically viable, partly due to a clever model construction that induces tree-level pion-phobia and exploits uncertainties inherent in the chiral perturbation theory. We confront this model with the cosmological domain wall problem, the quality issue and constraints arising from the electron electric dipole moment. It is also pointed out that the gluon loop-generated axion-top coupling can provide a very large contribution to rare B-meson decays, such that the present LHCb data for B0 → K*0e+e− rule out the model for the axion mass larger than 30 MeV. There is a strong motivation for pushing the experimental analysis of B → K(*)e+e− to a lower e+e− invariant mass window, which will conclusively determine the fate of the model, as its contribution to this branching ratio significantly exceeds the Standard Model prediction.

Search for production of a single vectorlike quark decaying to tH or tZ in the all-hadronic final state in pp collisions at s=13 TeV

A search for electroweak production of a single vectorlike T quark in association with a bottom (b) quark in the all-hadronic decay channel is presented. This search uses proton-proton collision data at s=13 TeV collected by the CMS experiment at the CERN LHC during 2016–2018, corresponding to an integrated luminosity of 138 fb−1. The T quark is assumed to have charge 2/3 and decay to a top (t) quark and a Higgs (H) or Z boson. Hadronic decays of the t quark and the H or Z boson are reconstructed from the kinematic properties of jets, including those containing b hadrons. No deviation from the standard model prediction is observed in the reconstructed tH and tZ invariant mass distributions. The 95% confidence level upper limits on the product of the production cross section and branching fraction of a T quark produced in association with a b quark and decaying via tH or tZ range from 1260 to 68 fb for T quark masses of 600–1200 GeV. © 2024 CERN, for the CMS Collaboration 2024 CERN

Probing axion-like particles in leptonic decays of heavy mesons

Abstract We study the possibility to find the axion-like particles (ALPs) through the leptonic decays of heavy mesons. The Standard Model (SM) predictions of the branching ratios of the leptonic decays of heavy mesons are less than corresponding experimental upper limits. This provides some space for the existence of decay channels where the ALP is one of the products. Three scenarios are considered: first, the ALP is only coupled to one single charged fermion, namely, the quark, the antiquark, or the charged lepton; second, the ALP is only coupled to quark and antiquark with the same strength; third, the ALP is coupled to all the charged fermions with the same strength. The constraints of the coupling strength in different scenarios are obtained by comparing the experimental data of the branching ratios of leptonic decays of $B^-$, $D^+$, and $D_s^+$ mesons with the theoretical predictions which are achieved by using the Bethe-Salpeter (BS) method. These constraints are further applied to predict the upper limits of the leptonic decay processes of the $B_c^-$ meson in which the ALP participates.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

Measurement of single top-quark production in association with a W boson in pp collisions at s=13 TeV with the ATLAS detector

The inclusive cross section for the production of a single top quark in association with a W boson is measured using 140 fb−1 of proton-proton collision data collected with the ATLAS detector at s=13 TeV. Events containing two charged leptons and at least one jet identified as originating from a b-quark are selected. A multivariate discriminant is constructed to separate the tW signal from the tt¯ background. The cross section is extracted using a profile likelihood fit to the signal and control regions and it is measured to be σtW=75−14+15 pb, in good agreement with the Standard Model prediction. The measured cross section is used to extract a value for the left-handed form factor at the Wtb vertex times the Cabibbo-Kobayashi-Maskawa matrix element |fLVVtb| of 0.97±0.10. © 2024 CERN, for the ATLAS Collaboration 2024 CERN

Fiducial and differential cross-section measurements of electroweak Wγjj\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W\\gamma jj$$\\end{document} production in 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

The observation of the electroweak production of a W boson and a photon in association with two jets, using pp collision data at the Large Hadron Collider at a centre of mass energy of 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, is reported. The data were recorded by the ATLAS experiment from 2015 to 2018 and correspond to an integrated luminosity of 140 fb-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}. This process is sensitive to the quartic gauge boson couplings via the vector boson scattering mechanism and provides a stringent test of the electroweak sector of the Standard Model. Events are selected if they contain one electron or muon, missing transverse momentum, at least one photon, and two jets. Multivariate techniques are used to distinguish the electroweak Wγjj\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W\\gamma jj$$\\end{document} process from irreducible background processes. The observed significance of the electroweak Wγjj\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W\\gamma jj$$\\end{document} process is well above six standard deviations, compared to an expected significance of 6.3 standard deviations. Fiducial and differential cross sections are measured in a fiducial phase space close to the detector acceptance, which are in reasonable agreement with leading order Standard Model predictions from MadGraph5+Pythia8 and Sherpa. The results are used to constrain new physics effects in the context of an effective field theory.

Electric dipole moments as probes of the RD(*) anomaly

The measurements of the lepton flavor universality (LFU) in B(B¯→D(*)lν¯) indicate a significant deviation from the standard model prediction at a 3–4σ level, revealing a violation of the LFU (RD(*) anomaly). It is known that the RD(*) anomaly can be easily accommodated by an SU(2)L-singlet vector leptoquark (LQ) coupled primarily to third-generation fermions, whose existence is further motivated by a partial gauge unification. In general, such a LQ naturally leads to additional CP-violating phases in the LQ interactions. In this paper, we point out that the current RD(*) anomaly prefers the CP-violating interaction although B(B¯→D(*)lν¯) are CP-conserving observables. The CP-violating LQ predicts a substantial size of the bottom-quark electric dipole moment (EDM), the chromo-EDM, and also the tau-lepton EDM. Eventually at low energy, the nucleon and electron EDMs are radiatively induced. Therefore, we conclude that the RD(*) anomaly with the SU(2)L-singlet vector LQ provides unique predictions: neutron and proton EDMs with opposite signs and a magnitude of O(10−27) e cm, and suppressed electron EDM. Furthermore, we show that a similar EDM pattern is predicted in an SU(2)L-doublet scalar LQ scenario that can accommodate the RD(*) anomaly as well. These EDM signals could serve as crucial indicators in future experiments. Published by the American Physical Society 2024

Global fit to b→cτν anomalies as of Spring 2024

Recently, several new experimental results of the test of lepton flavor universality (LFU) in B→D(*) semileptonic decays were announced: the first result of RD from the LHCb run 1 data, the first results of RD and RD* from the LHCb run 2 data, and the first result of RD* from the Belle II collaboration. Including these new data, a global analysis still prefers the violation of the LFU between the tau and light leptons. A new world average of the data from the , LHCb, Belle, and Belle II collaborations is RD=0.342±0.026 and RD*=0.287±0.012. Including this new data, we update a circumstance of the b→cτν¯ measurements and their implications for new physics (NP). Incorporating recent developments for the B→D(*) form factors in the Standard Model, we observe a 4.4σ deviation from the Standard Model prediction. Our updates also include model-independent NP formulas for the related observables and the global fittings of parameters for leptoquark scenarios as well as single NP operator scenarios. Furthermore, we show future potential to indirectly distinguish different NP scenarios with the use of the precise measurements of the polarization observables in B→D(*)τν¯ at the Belle II and the high-pT flavored-tail searches at the LHC. We also discuss an impact on the LFU violation in ϒ→l+l−. Published by the American Physical Society 2024

Search for Higgs boson pair production with one associated vector boson in proton-proton collisions at 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

A search for Higgs boson pair (HH) production in association with a vector boson V (W or Z boson) is presented. The search is based on proton-proton collision data at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb−1. Both hadronic and leptonic decays of V bosons are used. The leptons considered are electrons, muons, and neutrinos. The HH production is searched for in the bb¯bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{b}\\overline{\ extrm{b}}\ extrm{b}\\overline{\ extrm{b}} $$\\end{document} decay channel. An observed (expected) upper limit at 95% confidence level of VHH production cross section is set at 294 (124) times the standard model prediction. Constraints are also set on the modifiers of the Higgs boson trilinear self-coupling, kλ, assuming k2V = 1, and vice versa on the coupling of two Higgs bosons with two vector bosons, k2V. The observed (expected) 95% confidence intervals of these coupling modifiers are −37.7 < kλ < 37.2 (−30.1 < kλ < 28.9) and −12.2 < k2V < 13.5 (−7.2 < k2V < 8.9), respectively.

LHC di-lepton searches for Z′ bosons which explain measurements of b → sl+l− transitions

Several current measurements of b→sl+l− processes are in tension with Standard Model predictions, whereas others (e.g. RK and RK⁎) are in reasonable agreement with them. We examine some recent Z′ models that fit the data as a whole appreciably better than does the Standard Model, confronting the models with ATLAS resonance searches in the e+e− and μ+μ− channels. The models are constrained by both channels; we find that the searches rule out less than one fifth of the allowed range of MZ′. We estimate that the HL-LHC can roughly double the current sensitivity in MZ′, but to cover the whole of the parameter space would take a more powerful machine, such as a 100 TeV pp collider.

New BABAR studies for the data-driven predictions of the muon g − 2

Using the full BABAR data sample, a dedicated measurement of additional radiation in e+e−→μ+μ−γ and e+e−→π+π−γ initial-state-radiation events is presented. Results are obtained at next-to-leading and next-to-next-to-leading order, with one and two additional photons, respectively, for radiation from the initial and final states. The comparison with predictions from Phokhara and AfkQed Monte Carlo generators reveals discrepancies for the former in the one-photon rates and angular distributions. While this disagreement has a negligible effect on the e+e−→π+π−(γ) cross section measured by BABAR, the impact on other cross-section measurements may be larger than uncertainties assigned. The findings shed a new light on the long-standing deviation between the muon g−2 measurement and the Standard Model prediction using the data-driven dispersive approach.

New renormalization scheme in the two Higgs doublet models

We propose a new renormalization scheme in the two Higgs doublet models with a softly-broken Z2 symmetry and CP-conservation in the Higgs sector. In this scheme, counterterms for mixing angles of the Higgs bosons are determined by using the decay rates of the discovered Higgs boson h, i.e., h→τ+τ− and h→ZZ⁎→Zℓ+ℓ− at next leading order (NLO) instead of using the renormalized two-point functions which are adopted in the previous scheme. We require that the decay rates at NLO are determined to be the corresponding predictions at NLO in the Standard Model (SM) times square of the scaling factor which describes the deviation of h couplings at tree level from the SM value. The mixing angles then maintain the meaning of the “alignmentness”, i.e., how the properties of h are close to the SM predictions, while they lose such meaning in the previous scheme. We compare the predictions of the decay rates at NLO given in the new scheme and those in the previous scheme.

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

Next-to-leading-order electroweak correction to H→Z0γ

Inspired by the recent observation of the Higgs boson radiative decay into Z0 by and Collaborations, we investigate the next-to-leading-order (NLO) electroweak correction to this rare decay process in Standard Model (SM). Implementing the on shell renormalization scheme, we find that the magnitude of the NLO electroweak correction may reach 7% of the leading-order (LO) prediction, much more significant than that of the NLO QCD correction, which is merely about 0.3%. After incorporating the O(α) correction, the predicted partial width from various α schemes tend to converge to each other. Including both NLO electroweak and QCD corrections, the SM prediction for the branching fraction shifts from the LO value of (1.40–1.71)×10−3 to (1.55±0.06)×10−3, considerably lower than the measured value Bexp[H→Z0γ]=(3.4±1.1)×10−3. Resolving this discrepancy clearly calls for further theoretical investigations, and, more importantly, experimental efforts from and the prospective Higgs factories such as and . Published by the American Physical Society 2024

Calculating the two-photon exchange contribution to KL→μ+μ− decay

We present a theoretical framework within which both the real and imaginary parts of the complex, two-photon exchange amplitude contributing to KL→μ+μ− decay can be calculated using lattice quantum chromodynamics. The real part of this two-photon amplitude is of approximately the same size as that coming from a second-order weak strangeness-changing neutral-current process. Thus a test of the standard model prediction for this second-order weak process depends on an accurate result of this two-photon amplitude. A limiting factor of our proposed method comes from low-energy three-particle ππγ states. The contribution from these states will be significantly distorted by the finite volume of our calculation—a distortion for which there is no available correction. However, a simple estimate of the contribution of these three-particle states suggests their contribution to be at most a few percent allowing their neglect in a lattice calculation with a 10% target accuracy. Published by the American Physical Society 2024