Search

The hot and dense environment of the supernova core serves as an extraordinary factory for new feebly-interacting particles. Low-energy supernovae, a class of supernovae with low explosion energy, are particularly intriguing due to their stringent constraints on the energy transfer caused by new particles from the supernova core to the mantle. We investigate low-energy supernova constraints on millicharged particles by considering three production channels in the core: plasmon decay, proton bremsstrahlung, and electron-positron annihilation processes. We find that the electron-positron annihilation process, previously omitted in supernova studies on millicharged particles, is the dominant production channel in the high-mass region. By studying the energy deposition due to Coulomb scatterings with protons in the supernova mantle, we find that low-energy supernovae impose the most stringent constraints on millicharged particles in the mass range of $\sim(10-200)$ MeV, surpassing the energy loss limit from SN1987A by nearly one order of magnitude., Comment: 14 pages, 11 figures

Gravitational wave (GW) signals arising from binary neutron star mergers offer new, sensitive probes to ultralight mediators. Here we analyze the GW signals in the GW170817 event detected by the LIGO/Virgo collaboration to impose constraints on the ultralight isospin-violating mediator that has different couplings to protons and neutrons. Neutron stars, which primarily consist of neutrons, are the ideal places to probe the isospin-violating mediator. Such a mediator can significantly alter the dynamics of the binary neutron star mergers, through both the long-range Yukawa force and the new dipole radiation. We compute the gravitational waveform by taking into account the new physics effects due to the isospin-violating mediator and use the Bayesian inference to analyze the gravitational wave data in the GW170817 event. We find that although the current fifth force experiments (including MICROSCOPE and EW) often provide more stringent constraints than the GW170817 data, in the parameter space where the isospin-violating force is completely screened by the Earth (namely, the Earth is charge neutral under this force), the GW170817 data offer the leading constraints: the upper bound on the neutron coupling is $f_n \lesssim 10^{-19}$ in the mediator mass range of $\simeq(3\times10^{-16},\,5\times10^{-14})$ eV., Comment: 26 pages, 12 figures. v2: version accepted for publication in JHEP

A small component of dark matter (DM) that is strongly interacting with the standard model sector is consistent with various experimental observations. Despite the small abundance, strongly-interacting DM can lead to pronounced signals in DM direct detection experiments. We study Belle II sensitivity on strongly-interacting DM that has a MeV-GeV mass and couples with electrons. By taking into account the substantial interactions between DM and electrons within detectors, we compute the ``ceiling'' of the mono-photon signature at Belle II, beyond which the mono-photon channel loses its sensitivity, and visible ECL clusters due to DM scatterings assume significance. We study two ECL signatures for strongly-interacting DM: the mono-cluster and the di-cluster channels. To carry out detailed calculations and to compare with other constraints, we consider DM models with light mediators, as they naturally lead to sizable interaction cross sections. We compute exclusion regions for the di-cluster, mono-cluster, and mono-photon channels. We find that Belle II (with currently accumulated data of 362 fb$^{-1}$) can rule out a significant portion of the parameter space above the ceilings of the constraints from various DM direct detection and neutrino experiments, for the vector mediator case with mass $\gtrsim 10$ MeV. Belle II also offers superior constraints on new light particles compared to PBH for the scalar mediator with mass $\gtrsim 10$ MeV., Comment: 17 pages, 16 figures. v2: version accepted for publication in PRD

Atmospheric collisions can copiously produce dark sector particles in the invisible dark photon model, leading to detectable signals in underground neutrino detectors. We consider the dark photon model with the mass mixing mechanism and use the Super-K detector to detect the electron recoil events caused by the atmospherically produced dark sector particles within the model. We find that the combined data from four Super-K runs yield new leading constraints for the invisible dark photon in the mass range of $\sim(0.5-1.4)$ GeV, surpassing the constraints from NA64, BaBar, and searches for millicharged particles., Comment: 6 pages, 2 figures

Measurement of cosmic photons may reveal their propagation in the interstellar environment, thereby offering a promising way to probe axions and axion-like particles (ALPs). Numerical methods are usually used to compute the propagation of the photon-ALP beam due to the complexity of both the interstellar magnetic field and the evolution equation. However, under certain conditions, the evolution equation can be greatly simplified so that the photon-ALP propagation can be analytically solved. By using analytic methods, we find two distinct photon-ALP propagation modes, determined by the relative magnitude of the photon-ALP mixing term in comparison to the photon attenuation term. In one mode, the intensity of photons decreases with the increasing distance; in the other mode, it also exhibits oscillatory behavior. To distinguish the two propagation modes, we compute the observable quantities such as the photon survival probability and the degree of polarization., Comment: 19 pages, 8 figures

We propose a new ``disappearing positron track'' channel at Belle II to search for dark matter, in which a positron that is produced at the primary interaction vertex scatters with the electromagnetic calorimeter to produce dark matter particles. Such scatterings can occur via either annihilation with atomic electrons, or the bremsstrahlung process with target nuclei. The main backgrounds are due to photons and neutrons that are produced in the same scatterings and then escape detection. We require a large missing energy and further veto certain activities in the KLM detector to suppress such backgrounds. To illustrate the sensitivity of the new channel, we consider a new physics model where dark matter interacts with the standard model via a dark photon, which decays predominantly to dark matter; we find that our proposed channel can probe some currently unexplored parameter space, surpassing both the mono-photon channel at Belle II and the NA64 constraints., Comment: 7 pages, 2 figures. v2: version accepted for publication in JHEP

Light millicharged particles can be copiously produced from meson decays in cosmic ray collisions with the atmosphere, leading to detectable signals in large underground neutrino detectors. In this paper we study a new channel to produce millicharged particles in the atmosphere, the proton bremsstrahlung process. We find that the proton bremsstrahlung channel can produce a much larger flux of millicharged particles than the previously studied meson decay channel, resulting in an improvement on the SuperK limit by nearly one order of magnitude. Consequently, SuperK can probe new parameter space beyond the current leading limits from ArgoNeuT. We further note that the study on the proton bremsstrahlung process can be extended to other atmospherically produced light particles, and to millicharged particle searches in proton accelerators., Comment: 11 pages, 8 figures

Dark matter that interacts strongly with baryons can avoid the stringent dark matter direct detection constraints, because, like baryons, they are likely to be absorbed when traversing the rocks, leading to a suppressed flux in deep underground labs. Such strongly interacting dark matter, however, can be probed by dark matter experiments or other experiments operated on the ground level or in the atmosphere. In this paper we carry out systematic analysis of two of these experiments, XQC and CSR, to compute the experimental constraints on the strongly interacting dark matter in the following three scenarios: (1) spin-independent and spin-dependent interactions; (2) different velocity dependent cross sections; (3) different dark matter mass fractions. Some of the scenarios are first analyzed in the literature. We find that the XQC exclusion region has some non-trivial dependencies on the various parameters and the limits in the spin-dependent case is quite different from the spin-independent case. A peculiar region in the parameter space, where the XQC constraint disappears, is also found in our Monte Carlo simulations. This occurs in the case where the interaction cross section is proportional to the square of the velocity. We further compare our XQC and CSR limits to other experimental constraints, and find that a large parameter space is allowed by various experiments if the dark matter mass fraction is sufficiently small, $f_\chi\lesssim 10^{-4}$., Comment: 26 pages, 13 figures. v2: added references, more CMB and Lyman-alpha constraints, conclusion unchanged. v3: journal version

We propose an explanation to the new W mass measurement recently reported by the CDF collaboration, which is larger than the standard model expectation by about 7 standard deviations. To alleviate the tensions that are imposed on the electroweak sector by the new W mass measurement, we carry out an analysis in the Stueckelberg extended standard model where a new neutral gauge boson appears which mixes with the two neutral gauge bosons in the electroweak sector both via the Stueckelberg mass terms and via the gauge invariant Stueckelberg-Higgs portal interaction and spoils the custodial symmetry at the tree level so that the simple relation between the W boson mass and the Z boson mass does not hold. We find that such an extension increases the W boson mass if the new gauge boson mass is larger than the Z boson mass. We further show that there exists a significant part of the parameter space in the extended model which includes the CDF mass anomaly and is consistent with the various observables at the Z pole and consistent with the ATLAS dilepton limits. The Stueckelberg $Z'_{\rm St}$ boson, which resolves the CDF W mass anomaly, should be searchable in future LHC experiments., Comment: v1, 6 pages, 2 figures. v2, refs added

An analysis is given of the Belle II sensitivities and NA64 constraints on the sub-GeV Dirac dark matter that interacts with charged leptons. We consider two different types of interactions between sub-GeV Dirac dark matter and the charged leptons: the EFT operators and the light vector mediators. We compute the Belle II mono-photon sensitivities on sub-GeV dark matter with 50 ab$^{-1}$ data which are expected to be accumulated in the full Belle II runs. Although the Belle II mono-photon sensitivities on the EFT operators are of similar size as the LEP constraints, Belle II can probe new parameter space of the light vector mediator models that are unexplored by LEP. For both the EFT operators and the light vector mediator models, the Belle II mono-photon sensitivities can be several orders of magnitude stronger than the current dark matter direct detection limits, as well as the white dwarf limits. The light vector mediator can also be directly searched for by reconstructing the invariant mass of its di-lepton decay final states at Belle II, which is found to be complementary to the mono-photon channel. We compute the NA64 constraints on the sub-GeV Dirac dark matter and provide analytic expressions of the dark matter cross section in the Weizs\"acker-Williams approximation, for the EFT operators, and for the light vector mediator models. We find that the current NA64 data (with $2.84 \times 10^{11}$ electron-on-target events) provide strong constraints on sub-GeV dark matter. Although the NA64 constraints are found to be about one order of magnitude smaller than the Belle II sensitivities for the EFT operators, NA64 can probe some regions of the parameter space in the light vector mediator models that are beyond the reach of Belle II., Comment: v1: 14 pages, 13 figures. v2: 33 pages, 23 figures; NA64 constraints added

Long-lived particles that are present in many new physics models beyond the standard model, can be searched for in a number of newly proposed lifetime frontier experiments at the LHC. The signals of the long-lived dark photons can be significantly enhanced in a new dark photon model in which dark photons are copiously produced in the hidden radiation process. We investigate the capability of various lifetime frontier detectors in probing the parameter space of this model, including the far forward detectors FACET and FASER, the far transverse detector MATHUSLA, and the precision timing detector CMS-MTD. We find that the accessible parameter space is significantly enlarged by the hidden radiation process so that FACET, MATHUSLA, and CMS-MTD can probe a much larger parameter space than the so-called minimal model. The parameter space probed by FACET is found to be much larger than FASER, which is largely due to the fact that the former has a larger decay volume and is closer to the interaction point. There also exists some parameter space that can be probed both by the far detectors and by precision timing detectors, so that different experiments can be complementary to each other. A brief overview of the lifetime frontier detectors is also given., Comment: 17 pages, 17 figures; v2: PRD version

We propose a two-component dark matter explanation to the EDGES 21 cm anomalous signal. The heavier dark matter component is long-lived whose decay is primarily responsible for the relic abundance of the lighter dark matter which is millicharged. To evade the constraints from CMB, underground dark matter direct detection, and XQC experiments, the lifetime of the heavier dark matter has to be larger than $0.1\, \tau_U$, where $\tau_U$ is the age of the universe. Our model provides a viable realization of the millicharged dark matter model to explain the EDGES 21 cm, since the minimal model in which the relic density is generated via thermal freeze-out is ruled out by various constraints., Comment: 13 pages, 2 figures

In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment., Comment: 82 pages, 31 figures. This is the version of the article before peer review or editing, as submitted by an author to Reports on Progress in Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi:10.1088/1361-6633/ac4649

We show that a single vector leptoquark can explain both the muon $g-2$ anomaly recently measured by the Muon g-2 experiment at Fermilab, and the various $B$ decay anomalies, including the $R_{D^{(*)}}$ and $R_{K^{(*)}}$ anomalies which have been recently reported by the LHCb experiment. In order to provide sizeable positive new physics contributions to the muon $g-2$, we assume that the vector leptoquark particle couples to both left-handed and right-handed fermions with equal strength. Our model is found to satisfy the experimental constraints from the large hadron collider., Comment: 6 pages, 3 figures

We present a dark matter model to explain the excess events in the electron recoil data recently reported by the Xenon1T experiment. In our model, dark matter $\chi$ annihilates into a pair of on-shell particles $\phi$ which subsequently decay into $\psi \psi$ final state; $\psi$ interacts with electron to generate the observed excess events. Due to the mass hierarchy, the velocity of $\psi$ can be rather large and can have an extended distribution, which provides a good fit to the electron recoil energy spectrum. We estimated the flux of $\psi$ from dark matter annihilations in the galaxy and further determined the interaction cross section which is sizable but small enough to allow $\psi$ to penetrate the rocks to reach the underground labs., Comment: 4 pages, 3 figures

We construct a model in which the standard model is extended by a hidden sector with two gauge $U(1)$ bosons. A Dirac fermion $\psi$ charged under both $U(1)$ fields is introduced in the hidden sector which can be a subcomponent of the dark matter in the Universe. Stueckelberg mass terms between the two new gauge $U(1)$ fields and the hypercharge gauge boson mediate the interactions between the standard model sector and the hidden sector. A remarkable collider signature of this model is the enhanced long-lived dark photon events at the LHC than the conventional dark photon models; the long-lived dark photons in the model can be discriminated from the background by measuring the time delay signal in the precision timing detectors which are proposed to be installed in the LHC upgrades and have an ${\cal O} (10)$ pico-second detection efficiency. Searches with current LHCb data are also investigated. Various experimental constraints on the model including collider constraints and cosmological constraints are also discussed., Comment: 10 pages, 11 figures; v2: JHEP version

We propose to search for millicharged particles in electron colliders operated with the center-of-mass energies at ${\cal O}$(1-10) GeV, which include Belle II, BESIII, BaBar, and also the proposed experiment STCF. We use the monophoton final state at electron colliders to probe the parameter space of millicharged particles, that is spanned by millicharge $\epsilon$ and mass $m$. We find that electron colliders have sensitivity to the previously unexplored parameter space for millicharged particles with MeV-GeV mass: $\epsilon \lesssim {\cal O}(10^{-1})$ for $0.5$ GeV $\lesssim m \lesssim 3.5$ GeV in BaBar, $\epsilon \lesssim {\cal O}(10^{-3})$ for $0.1$ GeV $\lesssim m \lesssim 1.5$ GeV in BESIII, $\epsilon \lesssim 10^{-3}-10^{-2}$ for $0.1$ GeV $\lesssim m \lesssim 4$ GeV in Belle II, and $\epsilon \lesssim {\cal O}(10^{-4})$ for $1$ MeV $\lesssim m \lesssim 1$ GeV in STCF., Comment: 8 pages, 9 figures; v2: journal version

We investigate the capability of the future electron collider CEPC in probing the parameter space of several dark matter models, including millicharged dark matter models, $Z'$ portal dark matter models, and effective field theory dark matter models. In our analysis, the monophoton final state is used as the primary channel to detect dark matter models at CEPC. To maximize the signal to background significance, we study the energy and angular distributions of the monophoton channel arising from dark matter models and from the standard model to design a set of detector cuts. For the $Z'$ portal dark matter, we also analyze the $Z'$ boson visible decay channel which is found to be complementary to the monophoton channel in certain parameter space. The CEPC reach in the parameter space of dark matter models is also put in comparison with Xenon1T. We find that CEPC has the unprecedented sensitivity to certain parameter space for the dark matter models considered; for example, CEPC can improve the limits on millicharge by one order of magnitude than previous collider experiments for ${\cal O}(1)-100$ GeV dark matter., Comment: v1: 21 pages, 31 figures; v2: journal version

The cosmic electron energy spectrum recently observed by the DAMPE experiment exhibits two interesting features, including a break around 0.9 TeV and a sharp resonance near 1.4 TeV. In this analysis, we propose a dark matter explanation to both exotic features seen by DAMPE. In our model, dark matter annihilates in the galaxy via two different channels that lead to both a narrow resonance spectrum near 1.4 TeV and electron excess events over an extended energy range thus generating the break structure around TeV. The two annihilation channels are mediated by two gauge bosons that interact both with dark matter and with the standard model fermions. Dark matter annihilations through the s-channel process mediated by the heavier boson produce monoenergetic electron-positron pairs leading to the resonance excess. The lighter boson has a mass smaller than the dark matter such that they can be on-shell produced in dark matter annihilations in the galaxy; the lighter bosons in the final state subsequently decay to generate the extended excess events due to the smeared electron energy spectrum in this process. We further analyze constraints from various experiments, including HESS, Fermi, AMS, and LHC, to the parameter space of the model where both excess events can be accounted for. In order to interpret the two new features in the DAMPE data, dark matter annihilation cross sections in the current galaxy are typically much larger than the canonical thermal cross section needed for the correct dark matter relic abundance. This discrepancy, however, is remedied by the nonperturbative Sommerfeld enhancement because of the existence of a lighter mediator in the model., Comment: 23 pages, 21 figures

We propose to search for millicharged particles at the BESIII detector which is operated at the Beijing Electron Positron Collider. We compute the monophoton signal events at the BESIII detector due to millicharged particle production, as well as due to standard model irreducible/reducible backgrounds. By utilizing all the data accumulated at the BESIII detector since 2011, we derive new leading upper limits on millicharge, $\varepsilon \lesssim (0.86-2.5) \times10^{-3}$, for the mass range, $0.1~\text{GeV} \lesssim m \lesssim 1~\text{GeV}$. Furthermore, projections with more data to be collected at the BESIII detector are also made. Our analysis significantly reduces the parameter region of millicharge to account for the anomalous 21 cm absorption signal near redshift $z\simeq 17$ recently observed by the EDGES experiment

The recent high energy electron and positron flux observed by the DAMPE experiment indicates possible excess events near 1.4 TeV. Such an excess may be evidence of dark matter annihilations or decays in a dark matter subhalo that is located close to the solar system. We give here an analysis of this excess from annihilations of Dirac fermion dark matter which is charged under a new $U(1)_X$ gauge symmetry. The interactions between dark matter and the standard model particles are mediated the $U(1)_X$ gauge boson. We show that dark matter annihilations from a local subhalo can explain the excess with the canonical thermal annihilation cross section. We further discuss the constraints from the relic density, from the dark matter direct detection, from the dark matter indirect detection, from the cosmic microwave background, and from the particle colliders., Comment: 6 pages, 3 figures

Weakly interacting massive particles are a widely well-probed dark matter candidate by the dark matter direct detection experiments. Theoretically, there are a large number of ultraviolet completed models that consist of a weakly interacting massive particle dark matter. The variety of models makes the comparison with the direct detection data complicated and often non-trivial. To overcome this, in the non-relativistic limit, the effective theory was developed in the literature which works very well to significantly reduce the complexity of dark matter-nucleon interactions and to better study the nuclear response functions. In the effective theory framework for a spin-1/2 dark matter, we combine three independent likelihood functions from the latest PandaX, LUX, and XENON1T data, and give a joint limit on each effective coupling. The astrophysical uncertainties of the dark matter distribution are also included in the likelihood. We further discuss the isospin violating cases of the interactions. Finally, for both dimension-five and dimension-six effective theories above the electroweak scale, we give updated limits of the new physics mass scales., Comment: 33 pages, 11 figures, PandaX run10 data included and version accepted in JHEP, "code is available at the LikeDM website, https://likedm.hepforge.org/"

More than one Higgs boson may be present near the currently discovered Higgs mass, which can not be properly resolved due to the limitations in the intrinsic energy resolution at the Large Hadron Collider. We investigated the scenarios where two $CP$-even Higgs bosons are degenerate in mass. To correctly predict the Higgs signatures, quantum interference effects between the two Higgs bosons must be taken into account, which, however, has been often neglected in the literature. We carried out a global analysis including the interference effects for a variety of Higgs searching channels at the Large Hadron Collider, which suggests that the existence of two degenerate Higgs bosons near 125 GeV is highly likely. Prospects of distinguishing the degenerate Higgs case from the single Higgs case are discussed., Comment: 15 pages, 5 figures

Jinping Neutrino Experiment (Jinping) is proposed to significantly improve measurements on solar neutrinos and geoneutrinos in China Jinping Laboratory - a lab with a number of unparalleled features, thickest overburden, lowest reactor neutrino background, etc., which identify it as the world-best low-energy neutrino laboratory. The proposed experiment will have target mass of 4 kilotons of liquid scintillator or water-based liquid scintillator, with a fiducial mass of 2 kilotons for neutrino-electron scattering events and 3 kilotons for inverse-beta interaction events. A number of initial sensitivities studies have been carried out, including on the transition phase for the solar neutrinos oscillation from the vacuum to the matter effect, the discovery of solar neutrinos from the carbon-nitrogen-oxygen (CNO) cycle, the resolution of the high and low metallicity hypotheses, and the unambiguous separation on U and Th cascade decays from the dominant crustal anti-electron neutrinos in China., Comment: Proposal for the Jinping Neutrino Experiment

Motivated by excess diphoton events reported by ATLAS and CMS, we show that composite pseudoscalars, bound states of heavy fermions under a new confining gauge theory, can be pair-produced by Drell-Yan production if the constituent particles are electrically charged. The decays of these pion-like bound states into two photons can explain the observed events near 750 GeV. The model predicts that there should be significant numbers of 4-photon events from decays of the pseudoscalar pairs, as well as same-sign dilepton pairs at a somewhat lower invariant mass, or multijet signals, coming from decays of charge-2 pseudoscalars. These states and their decays are necessary for avoiding stable charged baryon-like relics in the new confining sector. A neutral baryon-like dark matter candidate of mass $\sim N_c\times 375$ GeV is a further prediction of the model, for SU($N_c$) confining gauge group., Comment: 4 pages, 2 figures; v2: added figure and changed operator in eq. (13). Paper withdrawn because resonant production of heavy vector meson always dominates over the considered mechanism

We discuss the diboson excess seen by the ATLAS detector around 2 TeV in the LHC run I at $\sqrt{s}=8$ TeV. We explore the possibility that such an excess can arise from a $Z'$ boson which acquires mass through a $U(1)_X$ Stueckelberg extension. The corresponding $Z'$ gauge boson is leptophobic with a mass of around 2 TeV and has interactions with $SU(2)_L$ Yang-Mills fields and gauge fields of the hypercharge. The analysis predicts $Z'$ decays into $WW$ and $ZZ$ as well as into $Z\gamma$. Further three-body as well as four-body decays of the $Z'$ such as $WWZ, WW\gamma, WWZZ$ etc are predicted. In the analysis we use the helicity formalism which allows us to exhibit the helicity structure of the $Z'$ decay processes in an transparent manner. In particular, we are able to show the set of vanishing helicity amplitudes in the decay of the massive $Z'$ into two vector bosons due to angular momentum conservation with a special choice of the reference momenta. The residual set of non-vanishing helicity amplitudes are identified. The parameter space of the model compatible with the diboson excess seen by the ATLAS experiment at $\sqrt s=8$ TeV is exhibited. Estimate of the diboson excess expected at $\sqrt s= 13$ TeV with 20 fb$^{-1}$ of integrated luminosity at LHC run II is also given. It is shown that the $WW$, $ZZ$ and $Z\gamma$ modes are predicted to be in the approximate ratio $1:\cos^2\theta_W (1+ \alpha \tan^2\theta_W)^2/2: (1-\alpha)^2\sin^2\theta_W/2$ where $\alpha$ is the relative strength of the couplings of hypercharge gauge fields to the couplings of the Yang-Mills gauge fields. Thus observation of the $Z\gamma$ mode as well as three-body and four-body decay modes of the $Z'$ will provide a definite test of the model and of a possible new source of interaction beyond the standard model., Comment: 12 pages, 6 figures

Tentative evidence for excess GeV-scale gamma rays from the galactic center has been corroborated by several groups, including the Fermi collaboration, on whose data the observation is based. Dark matter annihilation into standard model particles has been shown to give a good fit to the signal for a variety of final state particles, but generic models are inconsistent with constraints from direct detection. Models where the dark matter annihilates to mediators that subsequently decay are less constrained. We perform global fits of such models to recent data, allowing branching fractions to all possible fermionic final states to vary. The best fit models, including constraints from the AMS-02 experiment (and also antiproton ratio), require branching primarily to muons, with a $\sim 10-20\%$ admixture of $b$ quarks, and no other species. This suggests models in which there are two scalar mediators that mix with the Higgs, and have masses consistent with such a decay pattern. The scalar that decays to $\mu^+\mu^-$ must therefore be lighter than $2m_\tau \cong 3.6$ GeV. Such a small mass can cause Sommerfeld enhancement, which is useful to explain why the best-fit annihilation cross section is larger than the value needed for a thermal relic density. For light mediator masses $\sim (0.2-2)$ GeV, it can also naturally lead to elastic DM self-interactions at the right level for addressing discrepancies in small structure formation as predicted by collisionless cold dark matter., Comment: 18 pages, 14 figures; v2: updated CMB constraint and added references; v3 corrected direct detection cross section

The LHC searches for the CP-odd Higgs boson is studied (with masses from 300 GeV to 1 TeV) in the context of the general two-Higgs-doublet model. With the discovery of the 125 GeV Higgs boson at the LHC, we highlight one promising discovery channel of the hZ. This channel can become significant after the global signal fitting to the 125 GeV Higgs boson in the general two-Higgs-doublet model. It is particularly important in the scenario where two CP-even Higgs bosons in the two-Higgs-doublet model have the common mass of 125 GeV. Since the final states involve a Standard-Model-like Higgs boson, we apply the jet substructure analysis of the fat Higgs jet in order to eliminate the Standard Model background sufficiently. After performing the kinematic cuts, we present the LHC search sensitivities for the CP-odd Higgs boson with mass up to 1 TeV via this channel., Comment: 26 pages, 12 figures, submitted to Phys. Rev. D

An emission line with energy of $E\sim 3.5$ keV has been observed in galaxy clusters by two experiments. The emission line is consistent with the decay of a dark matter particle with a mass of $\sim 7$ keV. In this work we discuss the possibility that the dark particle responsible for the emission is a real scalar ($\rho$) which arises naturally in a $U(1)_X$ Stueckelberg of MSSM. In the MSSM Stueckelberg extension $\rho$ couples only to other scalars carrying a $U(1)_X$ quantum number. Under the assumption that there exists a vectorlike leptonic generation carrying both $SU(2)_L\times U(1)_Y$ and $U(1)_X$ quantum numbers, we compute the decay of the $\rho$ into two photons via a triangle loop involving scalars. The relic density of the $\rho$ arises via the decay $H^0\to h^0+ \rho$ at the loop level involving scalars, and via the annihilation processes of the vectorlike scalars into $\rho + h^0$. It is shown that the galactic data can be explained within a multicomponent dark matter model where the 7 keV dark matter is a subdominant component constituting only $(1-10)$\% of the matter relic density with the rest being supersymmetric dark matter such as the neutralino. Thus the direct detection experiments remain viable searches for WIMPs. The fact that the dark scalar $\rho$ with no interactions with the standard model particles arises from a Stueckelberg extension of a hidden $U(1)_X$ implies that the 3.5 KeV galactic line emission is a signal from the hidden sector., Comment: 20 pages, 4 figures, 1 table. To appear in Physical Review D

One of the simplest hidden sectors with signatures in the visible sector is fermionic dark matter $\chi$ coupled to a $Z'$ gauge boson that has purely kinetic mixing with the standard model hypercharge. We consider the combined constraints from relic density, direct detection and collider experiments on such models in which the dark matter is either a Dirac or a Majorana fermion. We point out sensitivity to details of the UV completion for the Majorana model. For kinetic mixing parameter $\epsilon \le 0.01$, only relic density and direct detection are relevant, while for larger $\epsilon$, electroweak precision, LHC dilepton, and missing energy constraints become important. We identify regions of the parameter space of $m_\chi$, $m_{Z'}$, dark gauge coupling and $\epsilon$ that are most promising for discovery through these experimental probes. We study the compatibility of the models with the galactic center gamma ray excess, finding agreement at the 2-3$\sigma$ level for the Dirac model., Comment: 13 pages, 10 figures; v.2 added reference; v.3 minor clarifications, published version

The recently discovered 3.5 keV X-ray line from extragalactic sources may be evidence of dark matter scatterings or decays. We show that dark atoms can be the source of the emission, through their hyperfine transitions, which would be the analog of 21 cm radiation from a dark sector. We identify two families of dark atom models that match the X-ray observations and are consistent with other constraints. In the first, the hyperfine excited state is long-lived compared to the age of the universe, and the dark atom mass is relatively unconstrained; dark atoms could be strongly self-interacting in this case. In the second, the excited state is short-lived and viable models are parameterized by the value of the dark proton-to-electron mass ratio $R$: for $R = 10^2-10^4$, the dark atom mass is predicted be in the range $350-1300$ GeV, with fine structure constant $\alpha'\cong 0.1-0.6$. In either class of models, the dark photon must be massive with $m_{\gamma'} \gtrsim$ 1 MeV and decay into $e^+ e^-$. Evidence for the model could come from direct detection of the dark atoms. In a natural extension of this framework, the dark photon could decay predominantly into invisible particles, for example $\sim 0.5$ eV sterile neutrinos, explaining the extra radiation degree of freedom recently suggested by data from BICEP2, while remaining compatible with BBN., Comment: 5 pages, 1 figure; v.2: added references, published version, except v.2 includes improved fig. 1