Standard Model Particles Research Articles

Constraining Dark Boson Decay Using Neutron Stars

Inspired by the well-known anomaly in the lifetime of the neutron, we investigated its consequences inside neutron stars. We first assessed the viability of the neutron decay hypothesis suggested by Fornal and Grinstein within neutron stars, in terms of the equation of state and compatibility with observed properties. This was followed by an investigation of the constraint information on neutron star cooling that can be placed on the decay rate of the dark boson into standard model particles, in the context of various BSM ideas.

Stellar cooling limits on light scalar boson revisited

We revisit the stellar cooling limits on the light scalar boson whose coupling to the Standard Model particles is described by its mixing with the Higgs boson. Strong constraints have been obtained from the electron-nucleus bremsstrahlung process (Ne→NeS) and the resonant plasma effect in the medium. We find that the bremsstrahlung contribution from the electron and nucleus scattering is of similar magnitude to the plasma mixing effect including the off-resonant mixing. The constraints on the scalar coupling are found to be about three orders of magnitude weaker than the previous evaluations because of the missing factor of the electron-nucleon mass ratio. For white dwarfs, the stellar cooling constraint is even more suppressed due to the Pauli blocking effect. We obtain limits on the Higgs-scalar mixing angle of 10−10–10−9, which are given by white dwarfs or red giants up to the luminosity limits, in the region where the scalar mass is lighter than about 10 keV.

Exploring maverick top partner decays at the LHC

In this work, we have considered an extension of the standard model (SM) with a $SU(2{)}_{L}$ singlet vectorlike quark (VLQ) with electric charge $Q=+2/3$. The model also contains an additional local $U(1{)}_{d}$ symmetry group and the corresponding gauge boson is the dark photon. The VLQ is charged while all the SM particles are neutral under the new $U(1{)}_{d}$ gauge group. Even though in this model the VLQ possesses many properties qualitatively similar to that of the traditional top partner (${T}_{p}$), there are some compelling differences as well. In particular, its branching ratio to the traditional modes (${T}_{p}\ensuremath{\rightarrow}bW,tZ,th$) are suppressed which in turn helps to evade many of the existing bounds, mainly coming from the LHC experiments. In an earlier work, such a VLQ is referred to as ``maverick top partner''. It has been shown that the top partner in this model predominantly decays to a top quark and a dark photon/dark Higgs pair (${T}_{p}\ensuremath{\rightarrow}t{\ensuremath{\gamma}}_{d},t{h}_{d}$) over a large region of the parameter space. The dark photon can be made invisible and consequently, it gives rise to the missing transverse energy (${\overline{)E}}_{T}$) signature at the LHC detector. We have mainly focused on the LHC signatures and future prospects of such top partners. In particular, we have studied the $t\overline{t}+{\overline{)E}}_{T}$ and $t+{\overline{)E}}_{T}$ signatures in the context of the LHC via pair and single productions of the top partner, respectively at 13 and 14 TeV LHC center of mass energies assuming that the dark photon either decays into an invisible mode or it is invisible at the length scale of the detector. We have shown that one can exclude $\mathrm{sin}{\ensuremath{\theta}}_{L}\ensuremath{\sim}0.025$ (0.05) for ${m}_{{T}_{p}}\ensuremath{\le}2.0\text{ }(2.6)\text{ }\text{ }\mathrm{TeV}$ at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ with an integrated luminosity of $3\text{ }\text{ }{\mathrm{ab}}^{\ensuremath{-}1}$ using the single top partner production channel.

Early kinetic decoupling effect on the forbidden dark matter annihilations into standard model particles

The early kinetic decoupling (eKD) effect is an inevitable ingredient in calculating the relic density of dark matter (DM) for various well-motivated scenarios. It appears naturally in forbidden dark matter annihilation, the main focus of this work, which contains fermionic DM and a light singlet scalar that connects the DM and standard model (SM) leptons. The strong suppression of the scattering between DM and SM particles happens quite early in the DM depletion history, where the DM temperature drops away from the thermal equilibrium, $T_\chi < T_{\rm SM}$, leading to the decreased kinetic energy of DM. The forbidden annihilation thus becomes inefficient since small kinetic energy cannot help exceed the annihilation threshold, naturally leading to a larger abundance. To show the eKD discrepancy, we numerically solve the coupled Boltzmann equations that govern the evolution of DM number density and temperature. It is found that eKD significantly affects the DM abundance, resulting in almost an order of magnitude higher than that by the traditional calculation. We also discuss the constraints from experimental searches on the model parameters, where the viable parameter space shrinks when considering the eKD effect.

An alternate left-right symmetric model with Dirac neutrinos

We study a different variant of left-right symmetric model, incorporating Dirac type neutrinos. In the absence of the bi-doublet scalars, the possibility of a universal seesaw type of mass generation mechanism for all the Standard Model charged fermions have been discussed. The model has been constructed by extending the Standard Model particle spectrum with heavy vector-like fermions as well as different scalar multiplets. We have shown that this model can generate non zero neutrino mass through loop mediated processes. The parameters which are involved in neutrino mass generation mechanism can satisfy the neutrino oscillation data for both normal and inverted hierarchy. The lightest charged Higgs plays a crucial role in neutrino mass generation mechanism and can have mass of {mathcal {O}}[text {GeV}]. We have systematically studied different constraints which are relevant for the charged Higgs phenomenology. In addition to that we also briefly discuss discovery prospects of the charged Higgs at different colliders.

Constraints on the Higgs boson self-coupling from single- and double-Higgs production with the ATLAS detector using pp collisions at [formula omitted] TeV

Constraints on the Higgs boson self-coupling are set by combining double-Higgs boson analyses in the bb¯bb¯, bb¯τ+τ− and bb¯γγ decay channels with single-Higgs boson analyses targeting the γγ, ZZ⁎, WW⁎, τ+τ− and bb¯ decay channels. The data used in these analyses were recorded by the ATLAS detector at the LHC in proton–proton collisions at s=13 TeV and correspond to an integrated luminosity of 126–139 fb−1. The combination of the double-Higgs analyses sets an upper limit of μHH<2.4 at 95% confidence level on the double-Higgs production cross-section normalised to its Standard Model prediction. Combining the single-Higgs and double-Higgs analyses, with the assumption that new physics affects only the Higgs boson self-coupling (λHHH), values outside the interval −0.4<κλ=(λHHH/λHHHSM)<6.3 are excluded at 95% confidence level. The combined single-Higgs and double-Higgs analyses provide results with fewer assumptions, by adding in the fit more coupling modifiers introduced to account for the Higgs boson interactions with the other Standard Model particles. In this relaxed scenario, the constraint becomes −1.4<κλ<6.1 at 95% CL.

The end of the particle era?

The discovery of the Higgs boson in 2012 at CERN completed the experimental confirmation of the Standard Model particle spectrum. Current theoretical insights and experimental data are inconclusive concerning the expectation of future discoveries. While new physics may still be within reach of the LHC or one of its successor experiments, it is also possible that the mass of particles beyond those of the Standard Model is far beyond the energy reach of any conceivable particle collider. We thus have to face the possibility that the age of “on-shell discoveries” of new particles may belong to the past and that we may soon witness a change in the scientists' perception of discoveries in fundamental physics. This article discusses the relevance of this questioning and addresses some of its potential far-reaching implications through the development, first, of a historical perspective on the concept of particle. This view is prompt to reveal important specificities of the development of particle physics. In particular, it underlines the close relationship between the evolution of observational methods and the understanding of the very idea of particle. Combining this with an analysis of the current situation of high-energy physics, this leads us to the suggestion that the particle era in science must undergo an important conceptual reconfiguration.

Implications on cosmology from Dirac neutrino magnetic moments

The mechanism for generating neutrino masses remains a puzzle in particle physics. If neutrino masses follow from a Dirac mass term, then neutrino states exist with opposite chirality compared to their weakly-interacting counterparts. These inactive states do not interact with their active counterparts at measurable scales in the standard model. However, the existence of these states can have implications for cosmology as they contribute to the radiation energy density at early times, and the matter energy density at late times. How Dirac neutrinos may populate thermal states via an anomalous magnetic moment operator is the focus of this work. A class of models where all neutrinos have a magnetic moment independent of flavor or chirality is considered. Subsequently, the cross sections for neutrinos scattering on background plasma particles are calculated so that the relic inactive neutrino energy is derived as a function of plasma temperature. To do so, one needs cross sections for scattering on all electrically charged standard-model particles. Therefore, the scattering cross section between a neutrino and $W$-boson via the magnetic moment vertex is derived. Current measurements put a constraint on the size of the neutrino magnetic moment from the cosmological parameter $N_{\rm eff}$ and light-element primordial abundances. Finally, how the extra Dirac states contribute to the matter energy density at late times is investigated by examining neutrino free-streaming.

Cross-correlation power spectra and cosmic birefringence of the CMB via photon-neutrino interaction

In the context of the standard model of particles, the weak interaction of cosmic microwave background (CMB) and cosmic neutrino background (CνB), can generate non-vanishing TB and EB power spectra in the order of one loop forward scattering, in the presence of scalar perturbation, which is in contrast with the standard scenario cosmology. Comparing our results with the current experimental data may provide, significant information about the nature of CνB, including CMB-CνB forward scattering for TB, TE, and EB power spectra. To this end, different cases were studied, including Majorana CνB and Dirac CνB. On the other hand, it was shown that the mean opacity due to cosmic neutrino background could behave as an anisotropic birefringent medium and change the linear polarization rotation angle. Considering the contributions from neutrino and anti-neutrino forward scattering with CMB photons (in the case of Dirac neutrino), we introduce relative neutrino and anti-neutrino density asymmetry (δν = Δnν /nν = nν -nν̅ /nν ). Then, using the cosmic birefringence angle reported by the Planck data release β = 0.30° ± 0.11° (68%C.L.), some constraints can be put on δν . Also, the value of cosmic birefringence due to Majorana CνB medium is estimated at about β| ν ≃ 0.2 rad. In this respect, since Majorana neutrino and anti-neutrino are exactly the same, both CB contributions will be added together. However, this value is at least two orders larger than the cosmic birefringence angle reported by the Planck data release, β = 0.30° ± 0.11° (68%C.L.). Finally, we shortly discussed this big inconsistency. It is noteworthy that to calculate the contribution of photon-neutrino forward scattering for cosmic birefringence, we just consider the standard model of particles and the standard scenario of cosmology.

An EFT hunter’s guide to two-to-two scattering: HEFT and SMEFT on-shell amplitudes

We derive the contact terms contributing to the four-point amplitudes of the standard model particles, keeping terms with up to quartic energy growth. Imposing just the unbroken low-energy symmetry, and treating the electroweak gauge bosons and the Higgs as independent degrees of freedom, we obtain the most general four-point contact-term amplitudes, corresponding to the Higgs Effective Field Theory (HEFT) framework. The contact terms are spanned by a basis of Stripped Contact Terms, which carry the polarization information, multiplied by polynomials in the Mandelstam invariants. For terms with quadratic energy growth, we also derive the low-energy Standard Model Effective Field Theory (SMEFT) predictions, via on-shell Higgsing of the massless SMEFT contact terms. We discuss several aspects of bottom-up versus top-down on-shell derivations of the HEFT and SMEFT amplitudes, highlighting in particular the simple counting of HEFT dimensions in the on-shell approach and the transparent relation between perturbative unitarity and gauge-invariance in the little-group covariant massive spinor formalism. Our results provide a formulation of Effective Field Theory analyses directly in terms of observable quantities. For terms with quadratic energy growth, we also provide the mapping to the Warsaw basis.

A derivation of the standard model particles from internal spacetime

‘Internal spacetime’ is a modification of general relativity that was recently introduced as an approximate spacetime geometric model of quantum nonlocality. In an internal spacetime, time is stationary along the worldlines of fundamental (dust) particles. Consequently, the dimensions of tangent spaces at different points of spacetime vary, and spin wavefunction collapse is modeled by the projection from one tangent space to another. In this paper, we develop spinors on an internal spacetime, and construct a new Dirac-like Lagrangian [Formula: see text] whose equations of motion describe their couplings and interactions. Furthermore, we show that hidden within [Formula: see text] is the entire standard model: [Formula: see text] contains precisely three generations of quarks and leptons, the electroweak gauge bosons, the Higgs boson, and one new massive spin-[Formula: see text] boson; gluons are considered in a companion paper. Specifically, we are able to derive the correct spin, electric charge, and color charge of each standard model particle, as well as predict the existence of a new boson.

Probing a textrm{Z}^{prime } with non-universal fermion couplings through top quark fusion, decays to bottom quarks, and machine learning techniques

The production of heavy neutral mass resonances, text {Z}^{prime }, has been widely studied theoretically and experimentally. Although the nature, mass, couplings, and associated quantum numbers of this hypothetical particle are yet to be determined, current LHC experimental results have set strong constraints assuming the simplest beyond Standard Model (SM) hypotheses. We present a new feasibility study on the production of a text {Z}^{prime } boson at the LHC, with family non-universal couplings, considering proton–proton collisions at sqrt{s} = 13 and 14 TeV. Such a hypothesis is well motivated theoretically and it can explain observed differences between SM predictions and experimental results, as well as being a useful tool to further probe recent results in searches for new physics considering non-universal fermion couplings. We work under two simplified phenomenological frameworks where the textrm{Z}^{prime } masses and couplings to the SM particles are free parameters, and consider final states of the text {Z}^{prime } decaying to a pair of textrm{b} quarks. The analysis is performed using machine learning techniques to maximize the sensitivity. Despite being a well motivated physics case in its own merit, such scenarios have not been fully considered in ongoing searches at the LHC. We note the proposed search methodology can be a key mode for discovery over a large mass range, including low masses, traditionally considered difficult due to experimental constrains. In addition, the proposed search is complementary to existing strategies.

Dark matter and neutrino masses in a Portalino-like model

We explore a Portalino-like model of dark matter and neutrino masses in which right-handed neutrino fields connect gauge neutral operators from the Standard Model and Hidden Sector. Neutrino masses are generated via a seesaw-like mechanism that can explain the light active neutrino masses. The model includes a “Portalino” state that connects the two sectors via the neutrino portal. Dark Matter in this model consists of a hidden sector Dirac fermion that dominantly freezes-out via resonant annihilations into other hidden sector states, which ultimately results in a population of Portalinos. Due to small mixing in the extended neutrino sector these Portalinos tend to be cosmologically long lived, decaying into Standard Model particles leading to constraints on the model from Big Bang Nucleosynthesis and measurements of the Cosmic Microwave Background radiation. Combining these limits with direct constraints on the size of the Portalino–neutrino mixing and the assumptions of the model the viable mass ranges for the Portalino states are found to be {0.02}~hbox {eV}lesssim m_n lesssim {6.4}~hbox {eV} or {489}~{hbox {MeV}} lesssim m_n lesssim TeV. Indirect dark matter signals in the form of highly boosted, mono-energetic Portalinos produced in Dark Matter annihilations provide a target for neutrino telescopes.

Invisible dilaton

We analyse the dynamics of a light scalar field responsible for the $\mu$ term of the Higgs potential and coupled to matter via the Higgs-portal mechanism. We find that this dilaton model is stable under radiative corrections induced by the standard model particle masses. When the background value of the scalar field is stabilised at the minimum of the scalar potential, the scalar field fluctuations only couple quadratically to the massive fields of the standard model preventing the scalar direct decay into standard model particles. Cosmologically and prior to the electroweak symmetry breaking, the scalar field rolls down along its effective potential before eventually oscillating and settling down at the electroweak minimum. These oscillations can be at the origin of dark matter due to the initial misalignment of the scalar field compared to the electroweak minimum, and we find that, when the mass of the scalar field is less than the eV scale and acts as a condensate behaving like dark matter on large scales, the scalar particles cannot thermalise with the standard model thermal bath. As matter couples in a composition-dependent manner to the oscillating scalar, this could lead to a violation of the equivalence principle aboard satellites such as the MICROSCOPE experiment and the next generation of tests of the equivalence principle. Local gravitational tests are evaded thanks to the weakness of the quadratic coupling in the dark matter halo, and we find that, around other sources, these dilaton models could be subject to a screening akin to the symmetron mechanism.

Influence of the deviation of the matter power spectrum at small scales on the global 21-cm signal at cosmic dawn

The matter power spectrum has been strongly constrained by astronomical measurements at large scales, but only weakly at small scales. Compared with the standard scenario, the deviation of the matter power spectrum at small scales has influence on the cosmological structure formation, e.g., the comoving number density of dark matter halos. The thermal history of the intergalactic medium (IGM) can be changed if dark matter is made of weakly interacting massive particles and can annihilate into standard model particles. The changes of the evolution of IGM could leave imprints on the relevant astronomical observations. Taking into account the dark matter annihilation, we investigate the impact of the deviation of matter power spectrum at small scales on the global 21-cm signal. In view of the measurements of the global 21-cm signal by the EDGES experiment, we explore the allowed parameter space of ${m}_{s}$, which describes the degree of deviation, by requiring the differential brightness temperature of the global 21-cm signal $\ensuremath{\delta}{T}_{21}\ensuremath{\le}\ensuremath{-}50\text{ }\text{ }\mathrm{mK}$ at redshift $z=17$.

Direct Detection of Dark Photon Dark Matter Using Radio Telescopes.

Dark photons can be the ultralight dark matter candidate, interacting with Standard Model particles via kinetic mixing. We propose to search for ultralight dark photon dark matter (DPDM) through the local absorption at different radio telescopes. The local DPDM can induce harmonic oscillations of electrons inside the antenna of radio telescopes. It leads to a monochromatic radio signal and can be recorded by telescope receivers. Using the observation data from the FAST telescope, the upper limit on the kinetic mixing can already reach 10^{-12} for DPDM oscillation frequencies at 1-1.5GHz, which is stronger than the cosmic microwave background constraint by about one order of magnitude. Furthermore, large-scale interferometric arrays like LOFAR and SKA1 telescopes can achieve extraordinary sensitivities for direct DPDM search from 10MHz to 10GHz.

Neutrino magnetic moment portal and supernovae: New constraints and multimessenger opportunities

We scrutinize the hypothesis that gauge singlet fermions -- sterile neutrinos -- interact with Standard Model particles through the transition magnetic moment portal. These interactions lead to the production of sterile neutrinos in supernovae followed by their decay into photons and active neutrinos which can be detected at $\gamma$-ray telescopes and neutrino detectors, respectively. We find that the non-observation of active neutrinos and photons from sterile-neutrino decay associated to SN1987A yields the strongest constraints to date on magnetic-moment-coupled sterile neutrinos if their masses are inside a $0.1-100$ MeV window. Assuming a near-future galactic supernova explosion, we estimate the sensitivity of several present and near-future experiments, including Fermi-LAT, e-ASTROGAM, DUNE, and Hyper-Kamiokande, to magnetic-moment-coupled sterile neutrinos. We also study the diffuse photon and neutrino fluxes produced in the decay of magnetic-moment coupled sterile neutrinos produced in all past supernova explosions and find that the absence of these decay daughters yields the strongest constraints to date for sterile neutrino masses inside a $1-100$ keV window.

Dark matter perspective of left-right symmetric gauge model

We consider an incarnation of left-right symmetric model with a local gauge symmetry of SU(3)C⊗SU(2)L⊗U(1)L⊗SU(2)R⊗U(1)R. Heavy scalars and fermions present in the 27 of E6 are included in the matter sector along with the Standard Model (SM) fermions. Two such colour singlet fermions, N and LS, transforming as bi-doublet and singlet under SU(2) s respectively, can be potential candidates for Dark Matter (DM). Assignment of U(1) charges for the matter fields restricts some of the exotic fermions to interact with the SM fermions. We study in some details the prospect of such fermionic dark matters by calculating relic densities and direct detection cross-sections by treating both these particles as relics. In such a two component Dark Matter scenario, LS having smaller interaction with the SM, will dominantly contribute to relic density. However, it cannot be detected at earth bound experiments with their present sensitivity. On the contrary, N having higher rate of interaction with the SM particle has too large annihilation cross-section thus contributes very little to relic density. In fact, its interaction with the SM is too high such that N-nucleon cross-section for a wide range of N mass is higher than the experimental limits from XENON, LUX or PICO. However, such a high N-nucleon cross-section can be tamed by assuming additional dimension-6 operators involving N and SM quarks. We derive limits on the strengths of such interactions from experimental data.

Origin of neutrino masses, dark matter, leptogenesis, and inflation in a seesaw model with triplets

We consider a new physics model, where the Standard Model (SM) is extended by hyperchargeless $Y=0$ triplet fermions and a Higgs triplet with hypercharge $Y=2$. The first two generation fermion triplets are even under the ${Z}_{2}$ transformation. In contrast, the third fermion triplet and scalar triplet are odd under the same ${Z}_{2}$ transformation. It is a unifying framework for the simultaneous explanation of neutrino mass and mixing, dark matter, baryogenesis, inflation, and reheating temperature of the Universe. The two ${Z}_{2}$ even neutral fermions explain the neutrino low energy variables, whereas the third one can serve as a viable dark matter candidate, explaining the exact relic density. The scalar triplet is coupled nonminimally to gravity and forms the inflaton. We calculate the inflationary parameters and find them consistent with the new Planck-2018 constraints. We also do the reheating analysis for the inflaton decays/annihilations to relativistic SM particles. The triplet fermions associated with ${Z}_{2}$ even sector can provide the observed baryon asymmetry of the Universe at the TeV scale.

Building the Standard Model particles and fields within a sphere fiber bundle framework

In this paper a sphere fiber bundle structure underlying the Standard Model of Particle Physics is developed. The developed bundle framework is shown to contain mathematical structures naturally suited for modeling the Standard Model’s elementary particles and fields.This modeling yields numerous results across the spectrum of physics, including: a resolution to the family problem, explanation of the 6-6 lepton–quark split in nature, a spacetime structural explanation of dark matter and energy, prediction of repulsive gravitational effects occurring in localized regions of space maintained at energies O(1015K), a modeling of the SM elementary particles as vibrating closed loops in spacetime, elimination of the fermionic QFT VEV catastrophe, the unveiling of a grand unification model at odds with standard GUT models, prediction of Dirac-style magnetic monopoles and of a quark-gluon and quasi-quark-gluon plasma near inception of the universe, a topological explanation for quark confinement, resolution to the cosmological constant problem, and a presentation of the model’s topology of quantum entanglement which reconciles relativistic locality with quantum non-locality.