Light Pseudoscalars Research Articles

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.

Form factor for Dalitz decays from J/ψ to light pseudoscalars* *Supported by the National Natural Science Foundation of China (11935017, 12293060, 12293065, 12293061, 12293062, 12293063, 12075253, 12192264, 12175063, 12205311, 12070131001 (CRC 110 by DFG and NNSFC)), the National Key Research and Development Program of China (2020YFA0406400), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34030302). The Chroma software

We calculate the form factor for the Dalitz decay with being the SU( ) flavor singlet pseudoscalar meson. The difference among the partial widths at different can be attributed in part to the and quark mass dependences induced by the anomaly dominance. in both is well described by the single pole model . Combined with the known experimental results of the Dalitz decays , the pseudoscalar mass dependence of the pole parameter is approximated by with and . These results provide inputs for future theoretical and experimental studies on the Dalitz decays .

Direct production of light scalars in the type-I two-Higgs-doublet model at the lifetime frontier of the LHC

A light pseudoscalar A in the sufficient large tanβ region of type-I two-Higgs-doublet model can be naturally a long-lived particle. We focus on H±A, HA, and AA pair productions via the electroweak processes mediated by the bosons at the LHC, including pp→W±/Z→H±/HA and pp→h→AA at the 14 TeV LHC. The possibility of probing A as a LLP at the FASER-2, FACET, MoEDAL-MAPP-2, and MATHUSLA is discussed. We find that FASER-2 fails to probe any parameter space within 0.2 GeV&lt;mA&lt;10 GeV for all the considered processes. For 130&lt;mH±=mH&lt;400 GeV, FACET, MoEDAL-MAPP-2, and MATHUSLA can probe tanβ≲104−6 for mA≲3 GeV, and tanβ≲106−8 for 3 GeV≲mA&lt;10 GeV from pp→W±/Z→H±/ZA processes. And pp→h→AA process covers similar parameter space. All processes can surpass the current limits. Published by the American Physical Society 2024

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.

Muon g−2 in a type-X 2HDM assisted by inert scalars: A test at the ILC

A two-Higgs doublet model can predict the observed muon $g\ensuremath{-}2$ for an appropriately light pseudoscalar that now faces tight constraints. However, it was shown recently in the companion paper [N. Chakrabarty, Muon $g\ensuremath{-}2$ in a type-X 2HDM assisted by inert scalars: A test at the LHC, preceding paper, arXiv:2112.13126.] that augmenting the two-Higgs doublet model by an additional inert doublet can lead to an explanation to the muon $g\ensuremath{-}2$ anomaly for a much heavier pseudoscalar. In this study, we probe such a framework at the proposed International Linear Collider using beam polarization for $\sqrt{s}=1\text{ }\text{ }\mathrm{TeV}$. Using multivariate techniques, we analyze the signals ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}+$ missing transverse energy and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}+$ missing transverse energy in the lepton- and muon-specific versions of the framework, respectively. Our analysis reveals that the ${e}^{+}{e}^{\ensuremath{-}}$ machine operating at a $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ luminosity predicts a $5\ensuremath{\sigma}$ discovery of a pseudoscalar as heavy as 400 GeV. Comparing with the companion paper, it is concluded that the International Linear Collider is a much more potent machine than the LHC in this regard.

Muon g−2 in a type-X 2HDM assisted by inert scalars: A test at the LHC

A scenario augmenting the well-known type-X two-Higgs doublet model with an additional inert doublet is proposed. The type-X two-Higgs doublet model is known to offer a solution to the muon $g\ensuremath{-}2$ anomaly for a light pseudoscalar. We show that the proposed framework can accommodate a heavier pseudoscalar on account of two-loop Barr-Zee contributions to muon $g\ensuremath{-}2$ stemming from the inert doublet. We subsequently explore an interesting ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}+$ missing transverse energy signal that can be used to probe the present scenario at the 14 TeV LHC.

Improved Limits on Lepton-Flavor-Violating Decays of Light Pseudoscalars via Spin-Dependent μ→e Conversion in Nuclei.

Lepton-flavor-violating decays of light pseudoscalars, P=π^{0},η,η^{'}→μe, are stringently suppressed in the standard model up to tiny contributions from neutrino oscillations, so that their observation would be a clear indication for physics beyond the standard model. However, in effective field theory such decays proceed via axial-vector, pseudoscalar, or gluonic operators, which are, at the same time, probed in spin-dependent μ→e conversion in nuclei. We derive master formulas that connect both processes in a model-independent way in terms of Wilson coefficients and study the implications of current μ→e limits in titanium for the P→μe decays. We find that these indirect limits surpass direct ones by many orders of magnitude.

Searches for the light invisible hypothetic pseudoscalar in K + → π + π 0 P decay

A high statistics data sample of the K + decays was recorded in 2012 and 2013 by the OKA collaboration. The missing mass analysis was performed to search for the light invisible pseudoscalar P in the decay K + → π + π 0 P for two alternative scenarios, namely for the pseudoscalar sgoldstino and for the axion. No signal is observed, the upper limits for the branching ratio of the decay are derived for both scenarios. The 90% CL upper limit for the case of sgoldstino is estimated to be in the region from 2 · 10−5 to 1 · 10−6 for the sgoldsino mass from 20 to 200 MeV and for the case of axion the upper limit is changing from 1 · 10−5 to 1 · 10−6 for the same mass range.

Neutral pseudoscalar and vector meson masses under strong magnetic fields in an extended NJL model: Mixing effects

Mixing effects on the mass spectrum of light neutral pseudoscalar and vector mesons in the presence of an external uniform magnetic field $\vec B$ are studied in the framework of a two-flavor NJL-like model. The model includes isoscalar and isovector couplings both in the scalar-pseudoscalar and vector sectors, and also incorporates flavor mixing through a 't Hooft-like term. Numerical results for the $B$ dependence of meson masses are compared with present lattice QCD results. In particular, it is shown that the mixing between pseudoscalar and vector meson states leads to a significant reduction of the mass of the lightest state. The role of chiral symmetry and the effect of the alignment of quark magnetic moments in the presence of the magnetic field are discussed.

Chapter 5 Dark Matter and New Physics Beyond the Standard Model with LHAASO

In order to reveal the nature of dark matter, it is crucial to detect its non-gravitational interactions with the standard model particles. The traditional dark matter searches focused on the so-called weakly interacting massive particles. However, this paradigm is strongly constrained by the null results of current experiments with high precision. Therefore there is a renewed interest of searches for heavy dark matter particles above TeV scale. The Large High Altitude Air Shower Observatory (LHAASO) with large effective area and strong background rejection power is very suitable to investigate the gamma-ray signals induced by dark matter annihilation or decay above TeV scale. In this document, we review the theoretical motivations and background of heavy dark matter. We review the prospects of searching for the gamma-ray signals resulted from dark matter in the dwarf spheroidal satellites and Galactic halo for LHAASO, and present the projected sensitivities. We also review the prospects of searching for the axion-like particles, which are a kind of well motivated light pseudo-scalars, through the LHAASO measurement of the very high energy gamma-ray spectra of astrophysical sources.

Exploring the flavor content of light and heavy-light pseudoscalars

The electroweak properties of light and charmed $D$ and ${D}_{s}$ pseudoscalar mesons are investigated within a unified covariant constituent quark model. The quark-antiquark-meson vertices are assumed to have a symmetric form by the exchange of quark momenta, which is successful in describing the light pseudoscalar meson properties. The flavor decomposition of the elastic electromagnetic form factors, electromagnetic charge radii, and weak decay constants are calculated. Based on the results a discussion on the SU(3) and SU(4) symmetry breaking is made and a comparison with the pion and kaon properties to highlight the Higgs contribution to the structure of these mesons.

On the Interpretation of the CMS Excess at an Invariant Mass of the Muon Pair of 28 GeV in the Minimal Supersymmetry

The possibility of interpretation of the excess in the pp → μ+μ–b $$\bar {b}$$ channel observed by the CMS Collaboration (LHC, CERN) at an invariant mass of the muon pair of 28 GeV ( $$\sqrt s $$ = 8 and 13 TeV) as a signal of one of the Higgs bosons in the minimal supersymmetry has been considered. It has been shown that parametric scenarios with a light pseudoscalar that satisfy the perturbative unitarity conditions in the Higgs sector exist in the regime of large radiative corrections with a light CP-even scalar h corresponding to the observed Higgs boson.

Exploring color-octet scalar parameter space in minimal R-symmetric models

In this work we study the collider phenomenology of color-octet scalars (sgluons) in minimal supersymmetric models endowed with a global continuous R symmetry. We systematically catalog the significant decay channels of scalar and pseudoscalar sgluons and identify novel features that are natural in these models. These include decays in nonstandard diboson channels, such as to a gluon and a photon; three-body decays with considerable branching fractions; and long-lived particles with displaced vertex signatures. We also discuss the single and pair production of these particles and show that they can evade existing constraints from the Large Hadron Collider, to varying extents, in large regions of reasonable parameter space. We find, for instance, that a 725 GeV scalar and a 350 GeV or lighter pseudoscalar can still be accommodated in realistic scenarios.

Search for Axionlike Particles Produced in e^{+}e^{-} Collisions at Belle II.

We present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e^{+}e^{-}→γa, a→γγ in the mass range 0.2<m_{a}<9.7 GeV/c^{2} using data corresponding to an integrated luminosity of (445±3) pb^{-1}. Light pseudoscalars interacting predominantly with standard model gauge bosons (so-called axionlike particles or ALPs) are frequently postulated in extensions of the standard model. We find no evidence for ALPs and set 95%confidence level upper limits on the coupling strength g_{aγγ} of ALPs to photons at the level of 10^{-3} GeV^{-1}. The limits are the most restrictive to date for 0.2<m_{a}<1 GeV/c^{2}.

Light Scalars in NMSSM and $$B \to K{\kern 1pt} *{\kern 1pt} ll$$ Angular Observables

Most of the Beyond-the-SM models predict scalar particles. Due to this, it is important to study all possible manifestations of the latter in different kind of experiments. In this talk we consider NMSSM scenarios with light (pseudo) scalars, which have masses ranging from 10 to 100 GeV. They escape the current experimental bounds since they have large singlet component. The main goal of the study is to revisit the $${{q}^{2}}$$ ‑dependent angular $$B \to K{\kern 1pt} *{\kern 1pt} ll$$ observables, which are sensitive to scalar contributions, and estimate the magnitude of the latter in the considered scenarios. In addition, prospects of experimental study of the effects are discussed.

Contribution of neutral pseudoscalar mesons to aμHLbL within a Schwinger-Dyson equations approach to QCD

A continuum approach to Quantum Chromodynamics (QCD), based upon Schwinger-Dyson (SD) and Bethe-Salpeter (BS) equations, is employed to provide a tightly constrained prediction for the $\gamma^{*} \gamma^{*} \rightarrow \{ \pi^0, \eta, \eta', \eta_c, \eta_b \}$ transition form factors (TFFs) and their corresponding pole contribution to the hadronic light-by-light (HLbL) piece of the anomalous magnetic moment of the muon ($a_\mu$). This work relies on a practical and well-tested quark-photon vertex Ansatz approach to evaluate the TFFs for arbitrary space-like photon virtualities, in the impulse approximation. The numerical results are parametrized meticulously, ensuring a reliable evaluation of the HLbL contributions to $a_\mu$. We obtain: $a_{\mu}^{\pi^0-\textrm{pole}} = (6.14 \pm 0.21) \times 10^{-10}$, $a_{\mu}^{\eta-\textrm{pole}} = (1.47 \pm 0.19) \times 10^{-10}$, $a_{\mu}^{\eta'-\textrm{pole}} = (1.36 \pm 0.08) \times 10^{-10}$, yielding a total value of $a_{\mu}^{\pi^0+\eta+\eta'-\textrm{pole}} = (8.97 \pm 0.48) \times 10^{-10}$, compatible with contemporary determinations. Notably, we find that $a_{\mu}^{\eta_c+\eta_b-\textrm{pole}} \approx a_{\mu}^{\eta_c-\textrm{pole}} = (0.09 \pm 0.01) \times 10^{-10}$, which might not be negligible once the percent precision in the computation of the light pseudoscalars is reached.

Search for physics beyond the standard model in multilepton final states in proton-proton collisions at sqrt{s} = 13 TeV

A search for physics beyond the standard model in events with at least three charged leptons (electrons or muons) is presented. The data sample corresponds to an integrated luminosity of 137 fb−1 of proton-proton collisions at sqrt{s} = 13 TeV, collected with the CMS detector at the LHC in 2016–2018. The two targeted signal processes are pair production of type-III seesaw heavy fermions and production of a light scalar or pseudoscalar boson in association with a pair of top quarks. The heavy fermions may be manifested as an excess of events with large values of leptonic transverse momenta or missing transverse momentum. The light scalars or pseudoscalars may create a localized excess in the dilepton mass spectra. The results exclude heavy fermions of the type-III seesaw model for masses below 880 GeV at 95% confidence level in the scenario of equal branching fractions to each lepton flavor. This is the most restrictive limit on the flavor-democratic scenario of the type-III seesaw model to date. Assuming a Yukawa coupling of unit strength to top quarks, branching fractions of new scalar (pseudoscalar) bosons to dielectrons or dimuons above 0.004 (0.03) and 0.04 (0.03) are excluded at 95% confidence level for masses in the range 15–75 and 108–340 GeV, respectively. These are the first limits in these channels on an extension of the standard model with scalar or pseudoscalar particles.

Photon-dark photon conversions in extreme background electromagnetic fields

The mixing of photons with light pseudoscalars in the presence of external electromagnetic fields has been used extensively to search for axion-like-particles. A similar effect for dark photon propagating states is usually not considered due to the Landau-Yang theorem. We point out that mixing between photon and dark photon propagating states in background electromagnetic fields can indeed occur, in non-linear QED, through a four-photon vertex by integrating out the electron box diagram. Starting from the Schwinger Lagrangian, we derive the equations of motion for dark photons interacting with the Standard Model photon through gauge kinetic terms. We provide expressions for the perpendicular and parallel refractive indices in series expansions in the critical field strength, valid both in the strong and weak background field limits. We then consider mixing between the photon-dark photon propagating system in the presence of pure electric and magnetic background fields, and work out the probability of conversion when the background fields are homogeneous. We indicate outlines of the calculation in the inhomogeneous case, and finally express our results in the active-sterile basis, where we find that the mixing induced by background fields can lead to corrections to the tree-level mixing in the zero field limit that is usually considered to probe such systems. Our results may find applications for probing photon-dark photon conversions in the vicinity of neutron stars and in table-top petawatt laser experiments.

Search for an exotic decay of the Higgs boson to a pair of light pseudoscalars in the final state with two muons and two b quarks in pp collisions at 13 TeV

A search for exotic decays of the Higgs boson to a pair of light pseudoscalar particles a1 is performed under the hypothesis that one of the pseudoscalars decays to a pair of opposite sign muons and the other decays to bb‾. Such signatures are predicted in a number of extensions of the standard model (SM), including next-to-minimal supersymmetry and two-Higgs-doublet models with an additional scalar singlet. The results are based on a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb−1, accumulated with the CMS experiment at the CERN LHC in 2016 at a centre-of-mass energy of 13 TeV. No statistically significant excess is observed with respect to the SM backgrounds in the search region for pseudoscalar masses from 20 GeV to half of the Higgs boson mass. Upper limits at 95% confidence level are set on the product of the production cross section and branching fraction, σhB(h→a1a1→μ+μ−bb‾), ranging from 5 to 33 fb, depending on the pseudoscalar mass. Corresponding limits on the branching fraction, assuming the SM prediction for σh, are (1–7)×10−4.

Model-independent bounds on light pseudoscalars from rare B-meson decays

New light pseudoscalars, such as axion-like particles, appear in many well-motivated extensions of the Standard Model and provide an exciting target for present and future experiments. We study the experimental sensitivity for such particles by revising the CHARM exclusion contour, updating bounds from LHCb and presenting prospects for NA62 and SHiP. We first consider a simplified model of a light pseudoscalar A and then propose a model-independent approach applicable to any spin-0 boson light enough to be produced in B-meson decays. As illustration, we provide upper bounds on BR(B→KA)×BR(A→μ+μ−) as a function of the boson lifetime and mass for models that satisfy minimal flavour violation. Our results demonstrate the important complementarity between different experiments resulting from their different geometries.