Xiao-Jun Bi

The 750 GeV diphoton excess at the LHC and dark matter constraints

The recent reported 750 GeV diphoton excess at the 13 TeV LHC is explained in the framework of effective field theory assuming the diphoton resonance is a scalar (pseudoscalar) particle. It is found that the large production rate and the broad width of this resonance are hard to be simultaneously explained if only visible final states are considered. Therefore an invisible decay channel to dark matter (DM) is strongly favored by the diphoton resonance with a broad width, given a large coupling of the new scalar to DM. We set constraints on the parameter space in this scenario using the results from LHC Run 1, DM relic density, and DM direct and indirect detection experiments. We find that the DM searches can exclude a large portion of the parameter regions accounting for the diphoton excess with a broad width.

Constraints and Tests of the OPERA Superluminal Neutrinos

The superluminal neutrinos detected by OPERA indicate Lorentz invariance violation (LIV) of the neutrino sector at the order of 10(-5). We study the implications of the result in this work. We find that such a large LIV implied by OPERA data will make the neutrino production process π → μ + ν(μ) kinematically forbidden for a neutrino energy greater than about 5 GeV. The OPERA detection of neutrinos at 40 GeV can constrain the LIV parameter to be smaller than 3×10(-7). Furthermore, the neutrino decay in the LIV framework will modify the neutrino spectrum greatly. The atmospheric neutrino spectrum measured by the IceCube Collaboration can constrain the LIV parameter to the level of 10(-12). The future detection of astrophysical neutrinos of galactic sources is expected to be able to give an even stronger constraint on the LIV parameter of neutrinos.

Constraining the interaction strength between dark matter and visible matter: I. Fermionic dark matter

In this work we study the constraints on the dark matter interaction with the standard model particles, from the observations of dark matter relic density, the direct detection experiments of CDMS and XENON, and the indirect detection of the (p) over bar /p ratio by PAMELA. A model independent way is adopted in the study by constructing the effective interaction operators between dark matter and standard model particles. The most general 4-fermion operators are investigated. We find that the constraints from different observations are complementary with each other. Especially the spin independent scattering gives very strong constraints for corresponding operators. In some cases the indirect detection of (p) over bar /p data can actually be more sensitive than the direct detection or relic density for light dark matter (less than or similar to 70 GeV)

Implications of the AMS-02 positron fraction in cosmic rays

The AMS-02 collaboration has just released its first result of the cosmic positron fraction e(+)/(e(-) + e(+)) with high precision up to similar to 350 GeV. The AMS-02 result shows the same trend with the previous PAMELA result, which requires extra electron/positron sources on top of the conventional cosmic ray background, either from astrophysical sources or from dark matter annihilation/decay. In this paper we try to figure out the nature of the extra sources by fitting to the AMS-02 e(+)/(e(-) + e(+)) data, as well as the electron and proton spectra by PAMELA and the (e(-) + e(+)) spectrum by Fermi and HESS. We adopt the GALPROP package to calculate the propagation of the Galactic cosmic rays and the Markov Chain Monte Carlo sampler to do the fit. We find that under the conventional assumptions about the background and the extra source of the e(-) + e(+), we cannot fit the AMS-02 and Fermi/HESS data well simultaneously. The AMS-02 data require less electrons/positrons from the extra sources than that required by Fermi/HESS. It may indicate that the model needs to be refined or the data between these experiments have systematic uncertainties. The pulsar scenario generally fits the data better than the DM scenario. Furthermore, the constraints from gamma-rays also disfavor the DM scenario to explain the cosmic ray lepton data

Constraints on the dark matter annihilation scenario of Fermi 130 GeV gamma-ray line emission by continuous gamma-rays, Milky Way halo, galaxy clusters and dwarf galaxies observations

It was recently reported that there may exist monochromatic γ-ray emission at ~ 130 GeV from the Galactic center in the Fermi Large Area Telescope data, which might be related with dark matter (DM) annihilation. In this work we carry out a comprehensive check of consistency of the results with the DM annihilation scenario, using the 3.7 yrs Fermi observation of the inner Galaxy, Galactic halo, clusters of galaxies and dwarf galaxies. The results found are as follows. 1) Very strong constraints on the DM annihilation into continuous γ-rays from the Galactic center are set, which are as stringent as the natural scale assuming thermal freeze-out of DM. Such limit sets strong constraint on the DM models to explain the line emission. 2) No line emission from the Galactic halo is found in the Fermi data, {and the constraints on line emission is marginally consistent with} the DM annihilation interpretation of the ~ 130 GeV line emission from the inner Galaxy. 3) No line emission from galaxy clusters and dwarf galaxies is detected, although possible concentration of photons from clusters in 120–140 GeV is revealed. The constraints from clusters and dwarf galaxies are weak and consistent with the DM annihilation scenario to explain the ~ 130 GeV line emission.

Quantitative study of the AMS-02 electron/positron spectra: Implications for pulsars and dark matter properties

The Alpha Magnetic Spectrometer (AMS-02) has just published the unprecedentedly precise measurement of the cosmic electron and positron spectra. In this paper, we try to give a quantitative study on the AMS-02 results by a global fitting to the electron and positron spectra, together with the updated positron fraction data. The Markov chain Monte Carlo algorithm is adopted to do the fitting. The primary electron spectrum and the parameters for pulsars or dark matter that contribute extra positrons are determined simultaneously. We find that there is a hardening of the primary electron spectrum at similar to 60 GeV. With such a new feature at the background spectrum, both the pulsars and dark matter can explain the AMS-02 results very well. The dark matter scenario shows a drop at the positron fraction at similar to 300 GeV but suffers very strong constraints from Fermi gamma-ray observations. The fitting results also suggest that the propagation model with convection may be more favored by the lepton data than the reacceleration model.

Probing light stop pairs at the LHC

In this work, we study the light stop pair signals at the LHC. We explore the supersymmetry parameter space with nonuniversal gaugino and third-generation masses at the grand unified theory scale. Recent LHC supersymmetry search results based on 35 pb(-1) and 1 fb(-1) of data are implemented to put the limits on stop pair events. The dark matter relic density and direct detection constraints are also taken into account. Detailed simulations on the signals and background for some benchmark points are performed, and it is found that the stop pair signals usually escape the LHC search if the present cut conditions are used. We also explore the potential and sensitivity of ILC to probe such scenarios. It is found that the ILC can detect them with an integrated luminosity of a few tens of fb(-1).

Constraining the interaction strength between dark matter and visible matter: II. Scalar, vector and spin-3/2 dark matter

We investigate the constraints on the scalar, vector and spin-3/2 dark matter interaction with the standard model particles, from the observations of dark matter relic density, the direct detection experiments of COMS and XENON, and the indirect detection of the (p) over bar /p ratio by PAMELA. A model independent way is adopted by constructing general 4-particle operators up to dimension 6 for the effective interaction between dark matter and standard model particles. We find that the constraints from different experiments are complementary with each other. Comparison among these constraints may exclude some effective models of dark matter and limit some parameters of others. The spin-independent direct detection gives strong constraints for some operators, while the indirect detection of (p) over bar /p data can be more sensitive than direct detection or relic density for light dark matter (whose mass less than or similar to 70 GeV) in some cases. The constraints on some operators for spin-3/2 dark matter are shown to be similar to those on their analogous operators for Dirac fermionic dark matter. There are still some operators not sensitive to the current dark matter direct and indirect search experiments

Pulsar interpretation for the AMS-02 result

The AMS-02 Collaboration has just published a high-precision measurement of the cosmic positron fraction e(+)/(e(-) + e(+)), which rises with energy from similar to 5 GeV to similar to 350 GeV. The result indicates the existence of primary electron/positron sources to account for the positron excess. In this work, we investigate the possibility that the nearby mature pulsars with ages of O(10(5)) yr are the primary positron sources. By fitting the data we find that the positrons from a single nearby pulsar, such as Geminga or Monogem, with the spectral index alpha similar to 2 can interpret the AMS-02 result. We also investigate the possibility that high-energy positrons are generated by multiple known pulsars in the ATNF catalogue. Such a scenario can also fit the AMS-02 data well. Future precise measurements of fine structures in the positron spectrum would be a support to the pulsar scenario.

On the e+e– Excesses and the Knee of the Cosmic Ray Spectra—Hints of Cosmic Ray Acceleration in Young Supernova Remnants

Supernova remnants (SNRs) have long been regarded as sources of the Galactic cosmic rays (CRs) up to petaelectronvolts, but convincing evidence is still lacking. In this work we explore the common origin of the subtle features of the CR spectra, such as the knee of CR spectra and the excesses of electron/positron fluxes recently observed by ATIC, H.E.S.S., Fermi-LAT, and PAMELA. Numerical calculation shows that those features of CR spectra can be well reproduced in a scenario with e+e– pair production by interactions between high-energy CRs and background photons in an environment similar to the young SNR. The success of such a coherent explanation serves in turn as evidence that at least a portion of CRs might be accelerated in young SNRs.