arXiv subjects

Xiao-Jun Bi

Publications and source records attributed to Xiao-Jun Bi.

At least 145 records · Page 8Linked to original sources

arXiv2011

Probing Light Stop Pairs at the LHC

In this work, we study the light stop pair signals at the LHC. We explore the SUSY parameter space with non-universal gaugino and third generation masses at the GUT scale. Recent LHC SUSY search results based on 35pb$^{-1}$ and 1fb$^{-1}$ 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$^{-1}$.

hep-ph↗

arXiv2011

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 antiproton-to-proton 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 antiproton-to-proton data can actually be more sensitive than the direct detection or relic density for light dark matter (less than 70 GeV).

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arXiv2011

Cosmic ray spectral hardening due to dispersion of source injection spectra

The cosmic ray (CR) energy spectra measured with ATIC, CREAM and PAMELA showed that there is remarkable hardening for rigidity of several hundred GV. We propose that this hardening is due to the superposition of spectra from a population of sources, e.g., supernova remnants (SNRs), whose injection spectral indices have a dispersion. Adopting proper model parameters the observational data can be well explained. It is interesting that the injection source parameters are similar with that derived from gamma-ray observations of SNRs, which may support the SNR-origin of CRs. Furthermore this mechanism provides an alternative explanation of the "ankle-cutoff" structure of the ultra high energy CR spectra.

astro-ph.HE↗

arXiv2011

A statistical model to explain the gamma-ray variability and flares of the Crab nebula

Recently the AGILE and Fermi/LAT detectors uncovered giant $γ$-ray flares from the Crab nebula. The duration of these flares is a few days. The Fermi/LAT data with monthly time binning further showed significant variability of the synchrotron tail of the emission, while the inverse Compton component was stable. The simultaneous or follow-up observations in X-ray, optical, infrared and radio bands did not find significant flux variation. Based on these observations, we propose that the $γ$-ray variability and flares are due to statistical fluctuations of knots that can accelerate electrons to $\sim$PeV energies. The maximum achievable energy of electrons is adopted to be proportional to the size of the knot, which is assumed to follow a power-law distribution. Thus the low energy electron flux will be stable due to the large number of small knots, while the high energy electron flux may experience large fluctuations. Monte Carlo realization of such a picture can reproduce the observational data quite well given proper model parameters.

astro-ph.HE↗

arXiv2011

Cosmic ray spectral hardening due to dispersion in the source injection spectra

Recent cosmic ray (CR) experiments discovered that the CR spectra experience a remarkable hardening for rigidity above several hundred GV. We propose that this is caused by the superposition of the CR energy spectra of many sources that have a dispersion in the injection spectral indices. Adopting similar parameters as those of supernova remnants derived from the Fermi $γ$-ray observations, we can reproduce the observational CR spectra of different species well. This may be interpreted as evidence to support the supernova remnant origin of CRs below the knee. We further propose that the same mechanism may explain the "ankle" of the ultra high energy CR spectrum.

astro-ph.HE↗

arXiv2011

Breit-Wigner Enhancement Considering the Dark Matter Kinetic Decoupling

In the paper we study the Breit-Wigner enhancement of dark matter (DM) annihilation considering the kinetic decoupling in the evolution of DM freeze-out at the early universe. Since the DM temperature decreases much faster (as $1/R^2$) after kinetic decoupling than that in kinetic equilibrium (as 1/R) we find the Breit-Wigner enhancement of DM annihilation rate after the kinetic decoupling will affect the DM relic density significantly. Focusing on the model parameters that trying to explain the anomalous cosmic positron/electron excesses observed by PAMELA/Fermi/ATIC we find the elastic scattering $Xf\to Xf$ is not efficient to keep dark matter in kinetic equilibrium, and the kinetic decoupling temperature $T_{kd}$ is comparable to the chemical decoupling temperature $T_f\sim O(10) GeV$. The reduction of the relic density after $T_{kd}$ is significant and leads to a limited enhancement factor $\sim O(10^2)$. Therefore it is difficult to explain the anomalous positron/electron excesses in cosmic rays by DM annihilation and give the correct DM relic density simultaneously in the minimal Breit-Wigner enhancement model.

hep-ph↗

arXiv2011

Neutrino emission of Fermi supernova remnants

The Fermi $γ$-ray space telescope reported the observation of several Galactic supernova remnants recently, with the $γ$-ray spectra well described by hadronic $pp$ collisions. The possible neutrino emissions from these Fermi detected supernova remnants are discussed in this work, assuming the hadronic origin of the $γ$-ray emission. The muon event rates induced by the neutrinos from these supernova remnants on typical km$^3$ neutrino telescopes, such as the IceCube and the KM3NeT, are calculated. The results show that for most of these supernova remnants the neutrino signals are too weak to be detected by the on-going or up-coming neutrino experiment. Only for the TeV bright sources RX J1713.7-3946 and possibly W28 the neutrino signals can be comparable with the atmospheric background in the TeV region, if the protons can be accelerated to very high energies. The northern hemisphere based neutrino telescope might detect the neutrinos from these two sources.

astro-ph.HE↗

arXiv2011

A statistical model for the $γ$-ray variability of the Crab nebula

A statistical scenario is proposed to explain the $γ$-ray variability and flares of the Crab nebula, which were observed recently by the Fermi/LAT. In this scenario electrons are accelerated in a series of knots, whose sizes follow a power-law distribution. These knots presumably move outwards from the pulsar and have a distribution in the Doppler boost factor. The maximal electron energy is assumed to be proportional to the size of the knot. Fluctuations at the highest energy end of the overall electron distribution will result in variable $γ$-ray emission via the synchrotron process in the $\sim 100$ MeV range. Since highly boosted larger knots are rarer than smaller knots, the model predicts that the variability of the synchrotron emission increases with the photon energy. We realize such a scenario with a Monte-Carlo simulation and find that the model can reproduce both the two $γ$-ray flares over a period of $\sim$ year and the monthly scale $γ$-ray flux fluctuations as observed by the Fermi/LAT. The observed $γ$-ray spectra in both the steady and flaring states are also well reproduced.

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arXiv2010

On the Detectability of Galactic Dark Matter Annihilation into Monochromatic Gamma-rays

Monochromatic gamma-rays are thought to be the smoking gun signal for identifying the dark matter annihilation. However, the flux of monochromatic gamma-rays is usually suppressed by the virtual quantum effects since dark matter should be neutral and does not couple with gamma-rays directly. In the work we study the detection strategy of the monochromatic gamma-rays in a future space-based detector. The monochromatic gamma-ray flux is calculated by assuming supersymmetric neutralino as a typical dark matter candidate. We discuss both the detection focusing on the Galactic center and in a scan mode which detects gamma-rays from the whole Galactic halo are compared. The detector performance for the purpose of monochromatic gamma-rays detection, with different energy and angular resolution, field of view, background rejection efficiencies, is carefully studied with both analytical and fast Monte-Carlo method.

astro-ph.HE↗

arXiv2010

Galactic diffuse gamma rays --- recalculation based on the new measurements of cosmic electron spectrum

In this work, we revisit the all-sky Galactic diffuse $γ$-ray emission taking into account the new measurements of cosmic ray electron/positron spectrum by PAMELA, ATIC and Fermi, which show excesses of cosmic electrons/positrons beyond the expected fluxes in the conventional model. Since the origins of the extra electrons/positrons are not clear, we consider three different scenarios to account for the excesses: the astrophysical sources such as the Galactic pulsars, dark matter decay and annihilation. Further, new results from Fermi-LAT of the (extra-)Galactic diffuse $γ$-ray are adopted. The background cosmic rays without the new sources give lower diffuse $γ$ rays compared to Fermi-LAT observation, which is consistent with previous analysis. The scenario with astrophysical sources predicts diffuse $γ$-rays with little difference with the background. The dark matter annihilation models with $τ^{\pm}$ final state are disfavored by the Fermi diffuse $γ$-ray data, while there are only few constraints on the decaying dark matter scenario. Furthermore, these is always a bump at higher energies ($\sim$ TeV) of the diffuse $γ$-ray spectra for the dark matter scenarios due to final state radiation. Finally we find that the Fermi-LAT diffuse $γ$-ray data can be explained by simply enlarging the normalization of the electron spectrum without introduce any new sources, which may indicate that the current constraints on the dark matter models can be much stronger given a precise background estimate.

astro-ph.HE↗

arXiv2010

Gamma rays and neutrinos from dark matter annihilation in galaxy clusters

The $γ$-ray and neutrino emissions from dark matter (DM) annihilation in galaxy clusters are studied. After about one year operation of Fermi-LAT, several nearby clusters are reported with stringent upper limits of GeV $γ$-ray emission. We use the Fermi-LAT upper limits of these clusters to constrain the DM model parameters. We find that the DM model distributed with substructures predicted in cold DM (CDM) scenario is strongly constrained by Fermi-LAT $γ$-ray data. Especially for the leptonic annihilation scenario which may account for the $e^{\pm}$ excesses discovered by PAMELA/Fermi-LAT/HESS, the constraint on the minimum mass of substructures is of the level $10^2-10^3$ M$_{\odot}$, which is much larger than that expected in CDM picture, but is consistent with a warm DM scenario. We further investigate the sensitivity of neutrino detections of the clusters by IceCube. It is found that neutrino detection is much more difficult than $γ$-rays. Only for very heavy DM ($\sim 10$ TeV) together with a considerable branching ratio to line neutrinos the neutrino sensitivity is comparable with that of $γ$-rays.

astro-ph.HE↗

arXiv2009

Clumpiness enhancement of charged cosmic rays from dark matter annihilation with Sommerfeld effect

Boost factors of dark matter annihilation into antiprotons and electrons/positrons due to the clumpiness of dark matter distribution are studied in detail in this work, taking the Sommerfeld effect into account. It has been thought that the Sommerfeld effect, if exists, will be more remarkable in substructures because they are colder than the host halo, and may result in a larger boost factor. We give a full calculation of the boost factors based on the recent N-body simulations. Three typical cases of Sommerfeld effects, the non-resonant, moderately resonant and strongly resonant cases are considered. We find that for the non-resonant and moderately resonant cases the enhancement effects of substructures due to the Sommerfeld effect are very small ($\sim \mathcal{O}(1)$) because of the saturation behavior of the Sommerfeld effect. For the strongly resonant case the boost factor is typically smaller than $\sim \mathcal{O}(10)$. However, it is possible in some very extreme cases that DM distribution is adopted to give the maximal annihilation the boost factor can reach up to $\sim 1000$. The variances of the boost factors due to different realizations of substructures distribution are also discussed in the work.

astro-ph.HE↗

arXiv2009

Cosmology from very high energy $γ$-rays

In this work we study how the cosmological parameter, the Hubble constant $H_0$, can be constrained by observation of very high energy (VHE) $γ$-rays at the TeV scale. The VHE $γ$-rays experience attenuation by background radiation field through $e^+e^-$ pair production during the propagation in the intergalactic space. This effect is proportional to the distance that the VHE $γ$-rays go through. Therefore the absorption of TeV $γ$-rays can be taken as cosmological distance indicator to constrain the cosmological parameters. Two blazars Mrk 501 and 1ES 1101-232, which have relatively good spectra measurements by the atmospheric Cerenkov telescope, are studied to constrain $H_0$. The mechanism constraining the Hubble constant adopted here is very different from the previous methods such as the observations of type Ia supernovae and the cosmic microwave background. However, at $2σ$ level, our result is consistent with other methods.

astro-ph↗

arXiv2009

Cosmic e^\pm, \bar p, γand neutrino rays in leptocentric dark matter models

Dark matter annihilation is one of the leading explanations for the recently observed $e^\pm$ excesses in cosmic rays by PAMELA, ATIC, FERMI-LAT and HESS. Any dark matter annihilation model proposed to explain these data must also explain the fact that PAMELA data show excesses only in $e^\pm$ spectrum but not in anti-proton. It is interesting to ask whether the annihilation mode into anti-proton is completely disallowed or only suppressed at low energies. Most models proposed have negligible anti-protons in all energy ranges. We show that the leptocentric $U(1)_{B-3L_i}$ dark matter model can explain the $e^\pm$ excesses with suppressed anti-proton mode at low energies, but at higher energies there are sizable anti-proton excesses. Near future data from PAMELA and AMS can provide crucial test for this type of models. Cosmic $γ$ ray data can further rule out some of the models. We also show that this model has interesting cosmic neutrino signatures.

hep-ph↗

arXiv2009

On the e$^+$e$^-$ excesses and the knee of the cosmic ray spectra -- hints of cosmic rays acceleration at young supernova remnants

Supernova remnants have long been regarded as sources of the Galactic cosmic rays up to petaelectronvolts, but convincing evidence is still lacking. In this work we explore the common origin of the subtle features of the cosmic ray spectra, such as the knee of cosmic ray 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 cosmic ray spectra can be well reproduced in a scenario with e$^+$e$^-$ pair production by interactions between high energy cosmic rays and background photons in an environment similar to the young supernova remnant. The success of such a coherent explanation serves in turn as an evidence that at least a portion of cosmic rays might be accelerated at young supernova remnants.

astro-ph.HE↗

arXiv2009

ATIC and PAMELA Results on Cosmic e^\pm Excesses and Neutrino Masses

Recently the ATIC and PAMELA collaborations released their results which show the abundant e^\pm excess in cosmic rays well above the background, but not for the \bar{p}. Their data if interpreted as the dark matter particles' annihilation imply that the new physics with the dark matter is closely related to the lepton sector. In this paper we study the possible connection of the new physics responsible for the cosmic e^\pm excesses to the neutrino mass generation. We consider a class of models and do the detailed numerical calculations. We find that these models can natually account for the ATIC and PAMELA e^\pm and \bar{p} data and at the same time generate the small neutrino masses.

hep-ph↗

arXiv2009

Perspective of Galactic dark matter subhalo detection on Fermi from the EGRET observation

The perspective of the detectability of Galactic dark matter subhaloes on the Fermi satellite is investigated in this work. Under the assumptions that dark matter annihilation accounts for the "GeV excess" of the Galactic diffuse $γ$-rays discovered by EGRET and the $γ$-ray flux is dominated by the contribution from subhaloes of dark matter, we calculate the expected number of dark matter subhaloes that Fermi may detect. We show that Fermi may detect a few tens to several hundred subhaloes in 1-year all sky survey. Since EGRET observation is taken as a normalization, this prediction is independent of the particle physics property of dark matter. The uncertainties of the prediction are discussed in detail. We find that the major uncertainty comes from the mass function of subhaloes, i.e., whether the subhaloes are "point like" (high-mass rich) or "diffuse like" (low-mass rich). Other uncertainties like the background estimation and the observational errors will contribute a factor of $2\sim 3$.

astro-ph↗

arXiv2009

Parameters in a Class of Leptophilic Dark Matter Models from PAMELA, ATIC and FERMI

In this work we study a class of leptophilic dark matter models, where the dark matter interacts with the standard model particles via the $U(1)_{L_i-L_j}$ gauge boson, to explain the $e^{\pm}$ excess in cosmic rays observed by ATIC and PAMELA experiments, and more recently by Fermi experiment. There are three types of $U(1)_{L_i-L_j}$ models: a) $U(1)_{L_e - L_μ}$, b) $U(1)_{L_e - L_τ}$, and c) $U(1)_ {L_e-L_τ}$. Although ATIC or Fermi data is consistent with PAMELA data separately, ATIC and Fermi data do not agree with each other. We therefore aim to identify which of the three models can explain which data set better. We find that models a) and b) can give correct dark matter relic density and explain the ATIC and PAMELA data simultaneously recur to the Breit-Wigner enhancement. Whereas model c) with a larger $Z^\prime$ mass can explain Fermi and PAMELA data simultaneously. In all cases the model parameters are restricted to narrow regions. Future improved data will decide which set of data are correct and also help to decide the correct dark matter model.

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