arXiv subjects

Wei-Hong Zhang

Publications and source records attributed to Wei-Hong Zhang.

5 recordsLinked to original sources

arXiv2018

Constraints on the Sommerfeld-enhanced dark matter annihilation from the gamma rays of subhalos and dwarf galaxies

The substructures of the Galactic dark matter halo such as dark matter subhalos and dwarf galaxies have very low velocity dispersions, which makes them useful in constraining the scenario of Sommerfeld-enhanced dark matter annihilation. We calculate the velocity distribution of dark matter particles in dark matter halo substructures using the Eddington's formula with NFW density profile. We parametrize the effect of Sommerfeld enhancement of s-wave dark matter annihilation on the gamma-ray flux as the Sommerfeld-enhanced J-factors, and explicitly calculate their values for 15 known dwarf spheroidal galaxies. Using the Fermi-LAT 3FGL data on the unassociated point-sources and the N-body simulation results on the dark matter subhalo distribution, we derive upper limits on the dark matter annihilation cross sections with Sommerfeld enhacement. Similar upper limits are derived from the Ferm-LAT gamma-ray data on the dwarf spheroidal galaxies. We find that in a wide region of parameter space, the constraints can be a few orders of magnitude more stringent than that in the case without the Sommerfeld enhancement. For dark matter annihilation dominantly into $e^+e^-$, $μ^+μ^-$, $τ^{+}τ^{-}$ and $b\bar b$, those constraints can exclude thermal relic dark matter for the dark matter mass below about 1 TeV.

astro-ph.HE↗

arXiv2018

Origins of sharp cosmic-ray electron structures and the DAMPE excess

Nearby sources may contribute to cosmic-ray electron (CRE) structures at high energies. Recently, the first DAMPE results on the CRE flux hinted at a narrow excess at energy ~1.4 TeV. We show that in general a spectral structure with a narrow width appears in two scenarios: I) "Spectrum broadening" for the continuous sources with a delta-function-like injection spectrum. In this scenario, a finite width can develop after propagation through the Galaxy, which can reveal the distance of the source. Well-motivated sources include mini-spikes and subhalos formed by dark matter (DM) particles $χ_{s}$ which annihilate directly into e+e- pairs. II) "Phase-space shrinking" for burst-like sources with a power-law-like injection spectrum. The spectrum after propagation can shrink at a cooling-related cutoff energy and form a sharp spectral peak. The peak can be more prominent due to the energy-dependent diffusion. In this scenario, the width of the excess constrains both the power index and the distance of the source. Possible such sources are pulsar wind nebulae (PWNe) and supernova remnants (SNRs). We analysis the DAMPE excess and find that the continuous DM sources should be fairly close within ~0.3 kpc, and the annihilation cross sections are close to the thermal value. For the burst-like source, the narrow width of the excess suggests that the injection spectrum must be hard with power index significantly less than two, the distance is within ~(3-4) kpc, and the age of the source is ~0.16 Myr. In both scenarios, large anisotropies in the CRE flux are predicted. We identify possible candidates of mini-spike (PWN) sources in the current Fermi-LAT 3FGL (ATNF) catalog. The diffuse gamma-rays from these sources can be well below the Galactic diffuse gamma-ray backgrounds and less constrained by the Ferm-LAT data, if they are located at the low Galactic latitude regions.

astro-ph.HE↗

arXiv2016

Antiprotons from dark matter annihilation through light mediators and a possible excess in AMS-02 $\bar{p}/p$ data

We show that in the scenario where dark matter (DM) particles annihilate through light mediators, the energy spectra of the final state cosmic-ray particles depend strongly on the mediator mass. For final state antiprotons, a spectrum with relatively narrow peak occurs when the mediator mass is comparable to the $\pbar p$ production threshold. Of interest, the latest AMS-02 data on the $\pbar/p$ flux ratio hint at a bump-like excess over the expected background in the energy range $\sim100-450$ GeV. We show that such a light mediator scenario is favoured by the latest AMS-02 data over the scenarios of DM direct annihilation into the standard model particles and that of antiprotons produced from inside supernova remnants (SNRs), and is consistent with the upper limits derived from the Fermi-LAT data on the gamma rays towards the dwarf spheroidal galaxies. The $\pbar/p$ flux ratio with energy above 450 GeV is predicted to fall with energy quickly, which can be easily distinguished from the other two scenarios as they predict the ratio to be flattening or rising up to multi-TeV region.

hep-ph↗

arXiv2016

Connecting the LHC diphoton excess to the Galatic center gamma-ray excess

The recent LHC Run-2 data have shown a possible excess in diphoton events, suggesting the existence of a new resonance $ϕ$ with mass $M\sim 750$~GeV. If $ϕ$ plays the role of a portal particle connecting the Standard Model and the invisible dark sector, the diphoton excess should be correlated with another photon excess, namely, the excess in the diffuse gamma rays towards the Galactic center, which can be interpreted by the annihilation of dark matter(DM). We investigate the necessary conditions for a consistent explanation for the two photon excesses, especially the requirement on the width-to-mass ratio $Γ/M$ and $ϕ$ decay channels, in a collection of DM models where the DM particle can be scalar, fermionionic and vector, and $ϕ$ can be generated through $s$-channel $gg$ fusion or $q\bar q$ annihilation. We show that the minimally required $Γ/M$ is determined by a single parameter proportional to $(m_χ/M)^{n}$, where the integer $n$ depends on the nature of the DM particle. We find that for the scalar DM model with $ϕ$ generated from $q\bar q$ annihilation, the minimally required $Γ/M$ can be as low as $\mathcal{O}(10^{-3})$. For the scalar DM model with $ϕ$ generated from $gg$ fusion and fermionic DM model with $ϕ$ from $q\bar q$ annihilation, the required $Γ/M$ are typically of $\mathcal{O}(10^{-2})$. The vector DM models, however, require very large $Γ/M$ of order one. For the DM models which can consistently explain both the excesses,the predicted cross sections for gamma-ray line are typically of $\mathcal{O}(10^{-31}-10^{-29})~\text{cm}^{3}\text{s}^{-1}$, which are close to the current limits from the Fermi-LAT experiment.

hep-ph↗

arXiv2016

A 750 GeV dark matter messenger at the Galactic Center

The first data from the LHC Run-2 have shown a possible excess in diphoton events with invariant mass $\sim 750$ GeV, suggesting the existence of a new resonance which may decay dominantly into dark matter (DM) particles. We show in a simple model that the reported diphoton excess at the LHC is consistent with another photon excess, the $2$ GeV excess in cosmic gamma-ray fluxes towards the Galactic Center observed by the Fermi-LAT. Both the excesses can be simultaneously explained by a $\sim 60$ GeV scalar DM particle annihilating dominantly into two gluons with a typical thermal annihilation cross section, which leads to the prediction of a large width to mass ratio $Γ/M\approx \mathcal{O}(10^{-2})$ of the resonance. The upper limit on the dijet search at LHC Run-1 leads to a $lower$ limit on the predicted cross section for DM annihilating into $γγ$ final states $\langleσv\rangle_{γγ} \gtrsim\mathcal{O}(10^{-30})~\mbox{cm}^{3}\mbox{s}^{-1}$. Both the predictions can be tested by the LHC, Fermi-LAT and future experiments.

hep-ph↗