Mass Splitting Research Articles

SU(2)D lepton portals for the muon g−2 , W -boson mass, and dark matter

We propose a novel model for lepton flavor and dark matter based on the $SU(2)_D$ gauge symmetry and vector-like leptons in its fundamental representations. We introduce a dark $SU(2)_D$ Higgs doublet and a Higgs bi-doublet for the mass mixing between the vector-like lepton and the lepton. As a result, the seesaw lepton masses are generated and there are sizable one-loop contributions to the muon $g-2$ via the $SU(2)_D$ gauge bosons and the relatively heavy vector-like lepton, as indicated in Fermilab E989. The tree-level mass mixing between the $Z$ boson and the isospin neutral gauge boson of $SU(2)_D$ in our model accounts for the shift in the $W$ boson mass, being consistent with Tevatron CDFII. Finally, we show that the isospin charged gauge boson of $SU(2)_D$ becomes a plausible candidate for dark matter with a small mass splitting tied up to the modified $W$ boson mass, and there is a viable parameter space where the favored corrections to the muon $g-2$ and the $W$ boson mass and the dark matter constraints are simultaneously fulfilled.

W -boson mass in the triplet seesaw model

The CDF collaboration has recently published a precision measurement of the W-boson mass that differs from the Standard Model prediction by seven standard deviations. This result can be explained with additional electroweak multiplets that either break the custodial symmetry or contribute to oblique parameters at loop level. Here, we study one of the best-motivated scenarios involving new multiplets: the type-II seesaw model, which involves a scalar triplet that generates Majorana neutrino masses and can furthermore resolve the W-boson mass discrepancy. This favors a doubly-charged scalar with mass between 100 and 200 GeV as well as other scalars with a fixed mass splitting. The entire preferred parameter space is testable at the LHC.

Inflation, electroweak phase transition, and Higgs searches at the LHC in the two-Higgs-doublet model

Combining the Higgs searches at the LHC, we study the Higgs inflation in the type-I and type-II two-Higgs-doublet models with non-minimally couplings to gravity. After imposing relevant theoretical and experimental constraints, we find that the Higgs inflation imposes stringent constraints on the mass splitting between A, H±, and H, and they tend to be nearly degenerate in mass with increasing of their masses. The direct searches for Higgs at the LHC can exclude many points achieving Higgs inflation in the region of mH (mA) < 450 GeV in the type-I model, and impose a lower bound on tan β for the type-II model. The Higgs inflation disfavors the wrong sign Yukawa coupling region of type-II model. In the parameter space achieving the Higgs inflation, the type-I and type-II models can produce a first order electroweak phase transition, but vc/Tc is much smaller than 1.0.

Three-body molecules D¯D¯*Σc : Understanding the nature of Tcc , Pc(4312) , Pc(4440) , and Pc(4457)

The nature of the three pentaquark states, $P_{c}(4312)$, $P_{c}(4440)$ and $P_{c}(4457)$, discovered by the LHCb Collaboration in 2019, is still under debate, although the $\bar{D}^{(\ast)}\Sigma_{c}$ molecular interpretation seems to be the most popular. In this work, by adding a $\bar{D}$ meson into the $\bar{D}^{\ast}\Sigma_{c}$ pair, we investigate the mass and decay width of the three-body molecules $\bar{D}\bar{D}^{\ast}\Sigma_{c}$ and explore the correlation between the existence of the $\bar{D}\bar{D}^{\ast}\Sigma_{c}$ molecules with the existence of $\bar{D}^{(\ast)}\Sigma_{c}$ and $\bar{D}^{\ast}\bar{D}$ two-body molecules. The latter can be identified with the doubly charmed tetraquark state $T_{cc}$ recently discovered by the LHCb Collaboration. Based on the molecular nature of $P_{c}(4312)$, $P_{c}(4440)$, $P_{c}(4457)$, and $T_{cc}$, our results indicate that there exist two three-body bound states of $\bar{D}\bar{D}^{\ast}\Sigma_{c}$ with $I(J^{P})=1(1/2^{+})$ and $I(J^{P})=1(3/2^{+})$, and binding energies $37.24$ MeV and $29.63$ MeV below the $\bar{D}\bar{D}^{\ast}\Sigma_{c}$ mass threshold. In addition, we find that the mass splitting of these two three-body molecules are correlated to the mass splitting of $P_{c}(4440)$ and $P_{c}(4457)$, which offers a non-trivial way to reveal the molecular nature of these states. The partial widths of two $\bar{D}\bar{D}^{\ast}\Sigma_{c}$ molecules decaying into $J/\psi p \bar{D}$ and $J/\psi p \bar{D}^{\ast}$ are found to be several MeV. We recommend the experimental searches for the $\bar{D}\bar{D}^{\ast}\Sigma_{c}$ molecules in the $J/\psi p \bar{D}$ and $J/\psi p \bar{D}^{\ast}$ invariant mass distributions.

Interpretation of XENON1T excess with MeV boosted dark matter * *The work of T. Li was supported by the National Natural Science Foundation of China (11975129, 12035008) and "the Fundamental Research Funds for the Central Universities", Nankai University (63196013). L.-B. Jia acknowledges support from the Longshan academic talent research supporting program of SWUST (18LZX415)

The XENON1T excess of keV electron recoil events may be induced by the scattering of electrons and long-lived particles with an MeV mass and high speed. We consider a tangible model composed of two scalar MeV dark matter (DM) particles, and , to interpret the XENON1T keV excess via boosted . A small mass splitting is introduced, and the boosted can be produced using the dark annihilation process of via a resonant scalar ϕ. electron scattering is intermediated by a vector boson X. Although the constraints from Big Bang nucleosynthesis, cosmic microwave background (CMB), and low-energy experiments set the mediated electron scattering cross section to be , the MeV scale DM with a resonance enhanced dark annihilation today can still provide sufficient boosted and induce the XENON1T keV excess. The relic density of is significantly reduced by the s-wave process , which is permitted by the constraints from CMB and 21-cm absorption. A very small relic fraction of is compatible with the stringent bounds on un-boosted -electron scattering in DM direct detection, and the -electron scattering is also allowed.

Correlating gravitational waves with W-boson mass, FIMP dark matter, and Majorana seesaw mechanism

We study a minimal extension of the standard model by introducing three right-handed neutrinos and a new scotogenic scalar doublet, in which the mass splittings between neutral and charged components are responsible for the W-boson mass newly measured by the CDF Collaboration. This model can not only generate non-vanishing Majorana neutrino masses via the interaction of right-handed neutrinos and scotogenic scalars, but also explain the Universe’s missing matter in the form of FIMP dark matter. We also study the influence of the mass splitting on the first order electroweak phase transition, and find that it can further enhance the transition strength and thus induce gravitational waves during the phase transition, which may be detected in the forthcoming detectors such as U-DECIGO.

Scalar dark matter and radiative Dirac neutrino mass in an extended U(1)B−L model

We explore a gauged U(1)B−L extension of the standard model with inclusion of three right-handed neutrinos of exotic B−L charges to cancel the gauge anomaly. Non-trivial transformation of new particles under B−L symmetry forbids the generation of neutrino mass at tree level and hence a small Dirac mass can be generated radiatively at one loop with scalar, fermion doublets and singlet scalars. We also discuss the phenomenology of a scalar dark matter, which can be obtained from the mixing of neutral CP even component of a doublet scalar and a real singlet scalar. An adhoc Z2 symmetry is required in the current framework to stabilize the dark matter candidate. Presence of new particles with Z2 odd charges and small mass splitting, makes the phenomenology more interesting by governing the relic density with co-annihilation processes. We explore the spin-independent direct detection constraints on dark matter via the scalar mediation. Constraints from lepton flavor violating decay processes like μ→eγ are also discussed.

Milky Way Satellite Census. IV. Constraints on Decaying Dark Matter from Observations of Milky Way Satellite Galaxies

We use a recent census of the Milky Way (MW) satellite galaxy population to constrain the lifetime of particle dark matter (DM). We consider two-body decaying dark matter (DDM) in which a heavy DM particle decays with lifetime τ comparable to the age of the universe to a lighter DM particle (with mass splitting ϵ) and to a dark radiation species. These decays impart a characteristic “kick velocity,” V kick = ϵ c, on the DM daughter particles, significantly depleting the DM content of low-mass subhalos and making them more susceptible to tidal disruption. We fit the suppression of the present-day DDM subhalo mass function (SHMF) as a function of τ and V kick using a suite of high-resolution zoom-in simulations of MW-mass halos, and we validate this model on new DDM simulations of systems specifically chosen to resemble the MW. We implement our DDM SHMF predictions in a forward model that incorporates inhomogeneities in the spatial distribution and detectability of MW satellites and uncertainties in the mapping between galaxies and DM halos, the properties of the MW system, and the disruption of subhalos by the MW disk using an empirical model for the galaxy–halo connection. By comparing to the observed MW satellite population, we conservatively exclude DDM models with τ < 18 Gyr (29 Gyr) for V kick = 20 kms−1 (40 kms−1) at 95% confidence. These constraints are among the most stringent and robust small-scale structure limits on the DM particle lifetime and strongly disfavor DDM models that have been proposed to alleviate the Hubble and S 8 tensions.

Experimental Search for Neutron to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly.

An unexplained >4σ discrepancy persists between "beam" and "bottle" measurements of the neutron lifetime. A new model proposed that conversions of neutrons n into mirror neutrons n^{'}, part of a dark mirror sector, can increase the apparent neutron lifetime by 1% via a small mass splitting Δm between n and n^{'} inside the 4.6T magnetic field of the National Institute of Standards and Technology Beam Lifetime experiment. A search for neutron conversions in a 6.6T magnetic field was performed at the Spallation Neutron Source which excludes this explanation for the neutron lifetime discrepancy.

Phenomenology of one zero texture Yukawa matrix in a flavor symmetric scotogenic model

The scotogenic model is well known for accommodating small neutrino mass and dark matter. Here, we have realized the scotogenic model with the help of discrete flavor symmetry [Formula: see text]. We have obtained three one zero textures of Yukawa coupling matrices from the model and have studied its impact on neutrino phenomenology and related aspects of cosmology. On the basis of [Formula: see text] symmetry, we have further discarded two structures of one zero texture Yukawa coupling matrix. We further analyze if the effective mass of active neutrinos obtained by the virtue of the Yukawa coupling matrix is consistent with the KamLAND-Zen limit for [Formula: see text]. Also different lepton flavor violating (LFV) processes such as [Formula: see text] and [Formula: see text] are implemented and their influence on neutrino phenomenology is studied corresponding to the Yukawa coupling matrix. The entire work is carried out considering the dark matter mass [Formula: see text] in the region 450–750 GeV. We have also obtained some significant results for baryon asymmetry of the Universe in agreement with the one zero texture of the coupling matrices. Furthermore, interesting results for relic abundance on the basis of distinct mass splittings are between the inert scalars.

Gluon string breaking and meson spectrum in the holographic Soft Wall model

We propose a general method for finding a string-like meson spectrum which is based on a certain condition for the breaking of closed gluon string. A model is constructed that successfully realizes the proposed method. The model is based on the holographic Soft Wall model for QCD and the use of the Wilson confinement criterion. We applied our approach to the vector and scalar cases and obtained numerical predictions for the intercepts of corresponding Regge like radial meson spectra. A good agreement is obtained both with the existing experimental data and with some other known phenomenological approaches. Remarkably, our closed string breaking condition has two branches. We argue that they should correspond to states of opposite parity. The Wilson confinement criterion leads then to a natural mass splitting between parity partners. The constructed model represents thus the first example of bottom-up holographic model in which the effects of chiral symmetry breaking can emerge automatically, i.e., without additional assumptions taken outside of the holographic approach.

Leptogenesis in type Ib seesaw models

We study leptogenesis in three different realizations of the type Ib seesaw mechanism, where the effective masses of the neutrinos are obtained by the spontaneous symmetry breaking of two different Higgs doublets. In the minimal type Ib seesaw model, where two right-handed neutrinos form a Dirac mass, we show that it is impossible to produce the correct baryon asymmetry, even including a pseudo-Dirac mass splitting. In an extended type Ib seesaw model, with a third very heavy Majorana right-handed neutrino, together with the low-scale Dirac pair of right-handed (RH) neutrinos and an extra singlet Higgs boson, we find that the imbalance of matter and antimatter can be explained by resonant leptogenesis. In the resulting low-scale effective type Ib seesaw mechanism, we derive the allowed range of the seesaw couplings consistent with resonant leptogenesis. Dark matter may also be included via the right-handed neutrino portal. The Dirac RH neutrino masses may lie in the 1--100-GeV mass range, accessible to the future experiments Search for Hidden Particles (SHiP) and Future electron-positron Circular Collider (FCC-$ee$), allowing the type Ib seesaw mechanism with leptogenesis and dark matter to be tested.

Search for Asymmetric Fission in Mass 200 Region: A Case Study in 197Tl

Asymmetric fission mode in mass 200 region has been looked into by carrying out a systematic study in 197Tl at ~42 MeV excitation energy. The experimental mass distribution reported at this energy does not show any indication of asymmetric fission. However, the semi-empirical Monte-Carlo code GEF predicts a well distinct asymmetric mass split. The present study underlines the importance of N/Z ratio as well as the excitation energy of the fissioning nucleus in determining the onset of asymmetric mass split.

Minimal dark matter model for muon g−2 with scalar lepton partners up to the TeV scale

The E989 experiment at the Fermi National Laboratory reported a $4.2\ensuremath{\sigma}$ discrepancy between the measured magnetic dipole moment of the muon, and its prediction in the Standard Model (SM). In this study, we address the anomaly by considering a minimal and generic extension to the SM which also provides for a dark matter (DM) candidate. The extra states in this framework are: a SM singlet Majorana fermion, referred to as the Bino, playing the role of DM; and muonic scalars, referred to as sleptons. The couplings between the sleptons, SM muons and the Bino can account for the muon $g\ensuremath{-}2$ anomaly if the scalar muon partners, or smuons, mix chirality. On the other hand, the DM relic density is satisfied primarily through coannihilation effects involving the Bino and the lighter sleptons. The viable parameter space of our model includes regions with relatively light coannihilating particles, similar to what has been found in previous scans of the Minimal Supersymmetric Standard Model (MSSM). Relaxing the assumption of minimal flavor violation typically assumed in the MSSM, we see that scenarios with sizable smuon mixing and large mass splittings between the smuons can satisfy both the muon $g\ensuremath{-}2$ anomaly and the DM relic density for coannihilating particle masses up to and beyond the TeV scale. When we specify the origin of the left-right smuon mixing to be trilinear couplings between the smuons and the SM Higgs boson, the constraints on these scenarios arising from perturbative unitarity and electroweak vacuum stability confine the coannihilating particle masses to be $\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{TeV}$. We demonstrate that next generation direct detection experiments are only marginally sensitive to the viable parameter space of our model, and, thus, a future lepton collider could be the essential probe necessary to distinguish our model from other solutions to the muon $g\ensuremath{-}2$ anomaly which involve extra states beyond the SM.

Determination of |Vcb| using N3LO perturbative corrections to Γ(B → Xcℓν) and 1S masses

We determine |Vcb| using the third-order perturbative series for the inclusive semileptonic B decay width and for the masses of the bottomonium 1S states. We use the masses of ηb(1S) and ϒ(1S) as short-distance masses and point out that there is a sizable difference of |Vcb| between the two 1S mass schemes. This is the dominant error of our determination and stems from insufficiency to describe theoretically the observed mass splitting of the bottomonium 1S states. We also study the significance of the O(ΛMS‾2) non-perturbative effects in HQET with respect to the current perturbative accuracy. Our result |Vcb|=0.0425(11) is consistent with the PDG value determined from the inclusive decays and has a slightly larger error.

Revising inelastic dark matter direct detection by including the cosmic ray acceleration

The null signal from collider and dark matter (DM) direct detector experiments makes the interaction between DM and visible matter too small to reproduce the correct relic density for many thermal DM models. The remaining parameter space indicates that two almost degenerated states in the dark sector, the inelastic DM scenario, can co-annihilate in the early universe to produce the correct relic density. Regarding the direct detection of the inelastic DM scenario, the virialized DM component from the nearby halo is nonrelativistic and not able to excite the DM ground state, even if the relevant couplings can be considerable. Thus, a DM with a large mass splitting can evade traditional virialized DM direct detection. In this study, we connect the concept of cosmic-ray accelerated DM in our Milky Way and the direct detection of inelastic scattering in underground detectors to explore spectra that result from several interaction types of the inelastic DM. We find that the mass splitting delta <mathcal{O}left(1 mathrm{GeV}right) can still be reachable for cosmic ray accelerated DM with mass range 1 MeV < {m}_{chi_1} < 100 GeV and sub-GeV light mediator using the latest PandaX-4T data, even though we conservatively use the astrophysical parameter (effective length) Deff = 1 kpc.

Non-perturbative thermal QCD at all temperatures: the case of mesonic screening masses

We present a strategy based on the step-scaling technique to study non-perturbatively thermal QCD up to very high temperatures. As a first concrete application, we compute the flavour non-singlet meson screening masses at 12 temperatures covering the range from T ∼ 1 GeV up to ∼ 160 GeV in the theory with three massless quarks. The calculation is carried out by Monte Carlo simulations on the lattice by considering large spatial extensions in order to have negligible finite volume effects. For each temperature we have simulated 3 or 4 values of the lattice spacing, so as to perform the continuum limit extrapolation with confidence at a few permille accuracy. Chiral symmetry restoration manifests itself in our results through the degeneracy of the vector and the axial vector channels and of the scalar and the pseudoscalar ones. In the entire range of temperatures explored, the meson screening masses deviate from the free theory result, 2πT, by at most a few percent. These deviations, however, cannot be explained by the known leading term in the QCD coupling constant g up to the highest temperature, where other contributions are still very relevant. In particular the vector-pseudoscalar mass splitting turns out to be of O(g4) in the entire range explored, and it remains clearly visible up to the highest temperature, where the two screening masses are still significantly different within our numerical precision. The pattern of different contributions that we have found explains why it has been difficult in the past to match non-perturbative lattice results at T ∼ 1 GeV with the analytic behaviour at asymptotically high temperatures.

Cosmology and signals of light pseudo-Dirac dark matter

In this paper, we analyze the cosmological evolution, allowed parameter space, and observational prospects for a dark sector consisting of thermally produced pseudo-Dirac fermions with a small mass splitting, coupled to the Standard Model through a dark photon. This scenario is particularly notable in the context of sub-GeV dark matter, where the mass-off-diagonal leading interaction limits applicability of both CMB energy injection constraints and standard direct detection searches. We present the first general study of the thermal history of pseudo-Dirac DM with splittings from 100 eV to MeV, focusing on the depletion of the heavier “excited” state abundance via scatterings and decays, and on the distinctive signals arising from its small surviving abundance. We analyze CMB energy injection bounds on both DM annihilation and decay, accelerator-based probes, and new line-like direct-detection signals from the excited DM down-scattering on either nuclei or electrons, as well as future search prospects in each channel. We also comment on the relevance of this signal to the few-keV Xenon1T electron excess and on possible diurnal modulation of this signal, and introduce a signal-strength parametrization to facilitate the comparison of future experimental results to theoretical expectations.

Detecting long-lived particles trapped in detector material at the LHC

We propose a two-stage strategy to search for new long-lived particles that could be produced at the CERN LHC, become trapped in detector material, and decay later. In the first stage, metal rods are exposed to LHC collisions in an experimental cavern. In the second stage, they are immersed in liquid argon at a different location, where out-of-time decays could be detected. Using a benchmark of pair-produced long-lived gluinos, we show that this experiment would have unique sensitivity to gluino-neutralino mass splittings down to 3 GeV, in previously uncovered lifetimes of days to years.

Generalized equations and their solutions in the (S, 0) + (0, S) representations of the Lorentz group

I present explicit examples of generalizations in relativistic quantum mechanics. First of all, I discuss the generalized spin-1/2 equations for neutrinos. They have been obtained by means of the Gersten-Sakurai method for derivations of arbitrary-spin relativistic equations. Possible physical consequences are discussed. Next, it is easy to check that both Dirac algebraic equation Det( – m) = 0 and Det( + m) = 0 for u – and υ – 4-spinors have solutions with . The same is true for higher-spin equations. Meanwhile, every book considers the equality p0 = Ep for both u – and υ – spinors of the (1/2, 0) ⊕ (0, 1/2) representation, thus applying the Dirac-Feynman-Stueckelberg procedure for elimination of the negative-energy solutions. The recent Ziino works (and, independently, the articles of several others) show that the Fock space can be doubled. We re-consider this possibility on the quantum field level for both S = 1/2 and higher spin particles. The third example is: we postulate the non-commutativity of 4-momenta, and we derive the mass splitting in the Dirac equation. The applications are discussed.