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Register a new userGamma-Ray Constraint on Galactic Positron Production by MeV Dark Matter
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John F. Beacom
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The Galactic positrons, as observed by their annihilation gamma-ray line at 0.511 MeV, are difficult to account for with astrophysical sources. It has been proposed that they are produced instead by dark matter annihilation or decay in the inner Galactic halo. To avoid other constraints, these processes are required to occur invisibly, such that the eventual positron annihilation is the only detectable signal. However, electromagnetic radiative corrections to these processes inevitably produce real gamma rays (``internal bremsstrahlung); this emission violates COMPTEL and EGRET constraints unless the dark matter mass is less than about 20 MeV.
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The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare to the observed diffuse Galactic gamma-ray data. Even in a simplified but conservative treatment, we find that the positron injection energies must be $lesssim 3$ MeV, which strongly constrains models for Galactic positron production.
MeV dark matter (DM) particles annihilating or decaying to electron-positron pairs cannot, in principle, be observed via local cosmic-ray (CR) measurements because of the shielding solar magnetic field. In this letter, we take advantage of spacecraft Voyager 1s capacity for detecting interstellar CRs since it crossed the heliopause in 2012. This opens up a new avenue to probe DM in the sub-GeV energy/mass range that we exploit here for the first time. From a complete description of the transport of electrons and positrons at low energy, we derive predictions for both the secondary astrophysical background and the pair production mechanisms relevant to DM annihilation or decay down to the MeV mass range. Interestingly, we show that reacceleration may push positrons up to energies larger than the DM particle mass. We combine the constraints from the Voyager and AMS-02 data to get novel limits covering a very extended DM particle mass range, from MeV to TeV. In the MeV mass range, our limits reach annihilation cross sections of order $langle sigma vrangle sim 10^{-28}{rm cm^3/s}$. An interesting aspect is that these limits barely depend on the details of cosmic-ray propagation in the weak reacceleration case, a configuration which seems to be favored by the most recent boron-to-carbon ($B/C$) data. Though extracted from a completely different and new probe, these bounds have a strength similar to those obtained with the cosmic microwave background --- they are even more stringent for $p$-wave annihilation.
We propose a possible explanation for the recently observed anomalous 511 keV line with a new millicharged fermion. This new fermion is light [${cal O}({rm MeV})$]. Nevertheless, it has never been observed by any collider experiments by virtue of its tiny electromagnetic charge $epsilon e$. In particular, we constrain parameters of this millicharged particle if the 511 keV cosmic $gamma$-ray emission from the galactic bulge is due to positron production from this new particle.
It has been proposed that during the formation of the first generation stars there might be a dark star phase in which the power of the star comes from dark matter annihilation. The adiabatic contraction process to form the dark star would result in a highly concentrated density profile of the host halo at the same time, which may give enhanced indirect detection signals of dark matter. In this work we investigate the extragalactic $gamma$-ray background from dark matter annihilation with such a dark star formation scenario, and employ the isotropic $gamma$-ray data from Fermi-LAT to constrain the model parameters of dark matter. The results suffer from large uncertainties of both the formation rate of the first generation stars and the subsequent evolution effects of the host halos of the dark stars. We find, in the most optimistic case for $gamma$-ray production via dark matter annihilation, the expected extragalactic $gamma$-ray flux will be enhanced by 1-2 orders of magnitude. In such a case, the annihilation cross section of the supersymmetric dark matter can be constrained to the thermal production level, and the leptonic dark matter model which is proposed to explain the positron/electron excesses can be well excluded. Conversely, if the positron/electron excesses are of a dark matter annihilation origin, then the early Universe environment is such that no dark star can form.
GRAMS (Gamma-Ray and AntiMatter Survey) is a novel project that can simultaneously target both astrophysical observations with MeV gamma rays and an indirect dark matter search with antimatter. The GRAMS instrument is designed with a cost-effective, large-scale LArTPC (Liquid Argon Time Projection Chamber) detector surrounded by plastic scintillators. The astrophysical observations at MeV energies have not yet been well-explored (the so-called MeV-gap) and GRAMS can improve the sensitivity by more than an order of magnitude compared to previous experiments. While primarily focusing on MeV gamma-ray observations, GRAMS is also optimized for cosmic ray antimatter surveys to indirectly search for dark matter. In particular, low-energy antideuterons will provide an essentially background-free dark matter signature. GRAMS will be a next generation experiment beyond the current GAPS (General AntiParticle Spectrometer) project for antimatter survey.
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