Observed Positron Excess Research Articles

Cosmic-ray positrons strongly constrain leptophilic dark matter

Cosmic-ray positrons have long been considered a powerful probe of dark matter annihilation. In particular, myriad studies of the unexpected rise in the positron fraction have debated its dark matter or pulsar origins. In this paper, we instead examine the potential for extremely precise positron measurements by AMS-02 to probe hard leptophilic dark matter candidates that do not have spectral features similar to the bulk of the observed positron excess. Utilizing a detailed cosmic-ray propagation model that includes a primary positron flux generated by Galactic pulsars in addition to a secondary component constrained by He and proton measurements, we produce a robust fit to the local positron flux and spectrum. We find no evidence for a spectral bump correlated with leptophilic dark matter, and set strong constraints on the dark matter annihilation cross-section that fall below the thermal annihilation cross-section for dark matter masses below 60 GeV and 380 GeV for annihilation into τ+τ- and e+e-, respectively, in our default model.

GeV Observations of the Extended Pulsar Wind Nebulae Constrain the Pulsar Interpretations of the Cosmic-Ray Positron Excess

Abstract It has long been suggested that nearby pulsars within ∼1 kpc are the leading candidate of the 10–500 GeV cosmic-ray positron excess measured by PAMELA and other experiments. The recent measurement of the surface brightness profile of TeV nebulae surrounding Geminga and PSR B0656+14 by the High-Altitude Water Cherenkov Observatory (HAWC) suggests inefficient diffusion of particles from the sources, giving rise to a debate on the pulsar interpretation of the cosmic-ray positron excess. Here we argue that GeV observations provide more direct constraints on the positron density in the TeV nebulae in the energy range of 10–500 GeV and hence on the origin of the observed positron excess. Motivated by this, we search for GeV emission from the TeV nebulae with the Fermi Large Area Telescope (LAT). No spatially extended GeV emission is detected from these two TeV nebulae in the framework of two-zone diffusion spatial templates, suggesting a relatively low density of GeV electrons/positrons in the TeV nebulae. A joint modeling of the data from HAWC and Fermi-LAT disfavors Geminga and PSR B0656+14 as the dominant sources of the positron excess at ∼50–500 GeV for the usual Kolmogorov-type diffusion, while for an energy-independent diffusion, a dominant part of the positron excess contributed by them cannot be ruled out by the current data.

Measuring the local diffusion coefficient with H.E.S.S. observations of very high-energy electrons

The HAWC Collaboration has recently reported the detection of bright and spatially extended multi-TeV gamma-ray emission from Geminga, Monogem, and a handful of other nearby, middle-aged pulsars. The angular profile of the emission observed from these pulsars is surprising, in that it implies that cosmic-ray diffusion is significantly inhibited within ~25 pc of these objects, compared to the expectations of standard Galactic diffusion models. This raises the important question of whether the diffusion coefficient in the local interstellar medium is also low, or whether it is instead better fit by the mean Galactic value. Here, we utilize recent observations of the cosmic-ray electron spectrum (extending up to ~20 TeV) by the H.E.S.S. Collaboration to show that the local diffusion coefficient cannot be as low as it is in the regions surrounding Geminga and Monogem. Instead, we conclude that cosmic rays efficiently diffuse through the bulk of the local interstellar medium. Among other implications, this further supports the conclusion that pulsars significantly contribute to the observed positron excess.

Cosmic ray signatures of a 2–3 Myr old local supernova

The supernova explosion which deposited $^{60}$Fe isotopes on Earth 2-3 million years ago should have also produced cosmic rays which contribute to the locally observed cosmic ray flux. We show that the contribution of this "local source" explains the "anomalies" observed in the positron and antiproton fluxes and explains why their spectral shape agrees with that of the proton flux. At the same time, this local source component accounts for the difference in the slopes of the spectra of cosmic ray nuclei as the result of the slightly varying relative importance of the "local" and the average component for distinct CR nuclei. Such a "local supernova" model for the spectra of nuclei can be tested via a combined measurement of the energy dependence of the boron-to-carbon (primary-to-secondary cosmic rays) ratio and of the antiproton spectrum: While the antiproton spectrum is predicted to extend approximately as a power law into the TeV range without any softening break, the B/C ratio is expected to show a "plateau" at a level fixed by the observed positron excess in the 30-300 GeV range. We discuss the observability of such a plateau with dedicated experiments for the measurement of the cosmic ray composition in 10 TeV energy range (NUCLEON, ISS-CREAM).

Possible evidence for the stochastic acceleration of secondary antiprotons by supernova remnants

The antiproton-to-proton ratio in the cosmic-ray spectrum is a sensitive probe of new physics. Using recent measurements of the cosmic-ray antiproton and proton fluxes in the energy range of 1-1000 GeV, we study the contribution to the $\bar{p}/p$ ratio from secondary antiprotons that are produced and subsequently accelerated within individual supernova remnants. We consider several well-motivated models for cosmic-ray propagation in the interstellar medium and marginalize our results over the uncertainties related to the antiproton production cross section and the time-, charge-, and energy-dependent effects of solar modulation. We find that the increase in the $\bar{p}/p$ ratio observed at rigidities above $\sim$ 100 GV cannot be accounted for within the context of conventional cosmic-ray propagation models, but is consistent with scenarios in which cosmic-ray antiprotons are produced and subsequently accelerated by shocks within a given supernova remnant. In light of this, the acceleration of secondary cosmic rays in supernova remnants is predicted to substantially contribute to the cosmic-ray positron spectrum, accounting for a significant fraction of the observed positron excess.

Higgs portal dark matter and neutrino mass and mixing with a doubly charged scalar

We consider an extension of the Standard Model involving two new scalar particles around the TeV scale: a singlet neutral scalar ϕ, to be eventually identified as the Dark Matter candidate, plus a doubly charged SU(2)L singlet scalar, S++, that can be the source for the non-vanishing neutrino masses and mixings. Assuming an unbroken Z2 symmetry in the scalar sector, under which only the additional neutral scalar ϕ is odd, we write the most general (renormalizable) scalar potential. The model may be regarded as a possible extension of the conventional Higgs portal Dark Matter scenario which also accounts for neutrino mass and mixing. This framework cannot completely explain the observed positron excess. However a softening of the discrepancy observed in conventional Higgs portal framework can be obtained, especially when the scale of new physics responsible for generating neutrino masses and lepton number violating processes is around 2 TeV.

Can we explain AMS-02 antiproton and positron excesses simultaneously by nearby supernovae without pulsars or dark matter?

We explain the excess of the antiproton fraction recently reported by the AMS-02 experiment by considering collisions between cosmic-ray protons accelerated by a local supernova remnant (SNR) and the surrounding dense cloud. The same "$pp$-collisions" provide the right ratio of daughter particles to fit the observed positron excess simultaneously in the natural model parameters. The supernova happened in relatively lower metalicity than the major cosmic-ray sources. The cutoff energy of electrons marks the supernova age of $\sim 10^{5}$ years, while the antiproton excess may extend to higher energy. Both antiproton and positron fluxes are completely consistent with our predictions in Fujita, Kohri, Yamasaki and Ioka (2009).

High-energy positrons and gamma radiation from decaying constituents of a two-component dark atom model

We study a two-component dark matter candidate inspired by the minimal walking technicolor (WTC) model. Dark matter consists of a dominant strongly interactive massive particle (SIMP)-like dark atom component made of bound states between primordial helium nuclei and a doubly charged technilepton and a small WIMP-like component made of another dark atom bound state between a doubly charged technibaryon and a technilepton. This scenario is consistent with direct search experimental findings because the dominant SIMP component interacts too strongly to reach the depths of current detectors with sufficient energy to recoil and the WIMP-like component is too small to cause significant amount of events. In this context, a metastable technibaryon that decays to [Formula: see text], [Formula: see text] and [Formula: see text] can, in principle, explain the observed positron excess by AMS-02 and PAMELA, while being consistent with the photon flux observed by FERMI/LAT. We scan the parameters of the model and we find the best possible fit to the latest experimental data. We find that there is a small range of parameter space that this scenario can be realized under certain conditions regarding the cosmic ray propagation and the final state radiation (FSR). This range of parameters fall inside the region where the current run of large hadron collider (LHC) can probe, and therefore it will soon be possible to either verify or exclude conclusively this model of dark matter.

Origin of the spectral upturn in the cosmic-rayC/FeandO/Feratios

The observed spectrum of Galactic cosmic rays has several exciting features such as the rise in the positron fraction above $\ensuremath{\sim}10\text{ }\text{ }\mathrm{GeV}$ of energy and the spectral hardening of protons and helium at $\ensuremath{\gtrsim}300\text{ }\text{ }\mathrm{GeV}/\mathrm{n}\mathrm{ucleon}$ of energy. The ATIC-2 experiment has recently reported an unexpected spectral upturn in the elemental ratios involving iron, such as the $\mathrm{C}/\mathrm{Fe}$ or $\mathrm{O}/\mathrm{Fe}$ ratios, at energy $\ensuremath{\gtrsim}50\text{ }\text{ }\mathrm{GeV}$ per nucleon. It is recognized that the observed positron excess can be explained by pion production processes during diffusive shock acceleration of cosmic-ray hadrons in nearby sources. Recently, it was suggested that a scenario with nearby source dominating the GeV-TeV spectrum may be connected with the change of slope observed in protons and nuclei, which would be interpreted as a flux transition between the local component and the large-scale distribution of Galactic sources. Here I show that, under a two-component scenario with nearby source, the shape of the spectral transition is expected to be slightly different for heavy nuclei, such as iron, because their propagation range is spatially limited by inelastic collisions with the interstellar matter. This enables a prediction for the primary/primary ratios between light and heavy nuclei. From this effect, a spectral upturn is predicted in the $\mathrm{C}/\mathrm{Fe}$ and $\mathrm{O}/\mathrm{Fe}$ ratios in good accordance with the ATIC-2 data.

‘Excess’ of primary cosmic ray electrons

With the accurate cosmic ray (CR) electron and positron spectra (denoted as Φe− and Φe+, respectively) measured by AMS-02 Collaboration, the difference between the electron and positron fluxes (i.e., ΔΦ=Φe−−Φe+), dominated by the propagated primary electrons, can be reliably inferred. In the standard model, the spectrum of propagated primary CR electrons at energies ≥30GeV softens with the increase of energy. The absence of any evidence for such a continuous spectral softening in ΔΦ strongly suggests a significant ‘excess’ of primary CR electrons and at energies of 100–400GeV the identified excess component has a flux comparable to that of the observed positron excess. Middle-age but ‘nearby’ supernova remnants (e.g., Monogem and Geminga) are favored sources for such an excess.

Neutrino mass and dark matter in light of recent AMS-02 results

We study a simple extension of the Standard Model supplemented by an electroweak triplet scalar field to accommodate small neutrino masses by the type-II seesaw mechanism, while an additional singlet scalar field can play the role of cold dark matter (DM) in our Universe. This DM candidate is leptophilic for a wide range of model parameter space, and the lepton flux due to its annihilation carries information about the neutrino mass hierarchy. Using the recently released high precision data on positron fraction and flux from the AMS-02 experiment, we examine the DM interpretation of the observed positron excess in our model for two kinematically distinct scenarios with the DM and triplet scalar masses (a) non-degenerate ($m_{\rm DM}\gg m_{\Delta}$), and (b) quasi-degenerate ($m_{\rm DM} \simeq m_\Delta$). We find that a good fit to the AMS-02 data can be obtained in both cases (a) and (b) with a normal hierarchy of neutrino masses, while the inverted hierarchy case is somewhat disfavored. Although we require a larger boost factor for the normal hierarchy case, this is still consistent with the current upper limits derived from Fermi-LAT and IceCube data for case (a). Moreover, the absence of an excess anti-proton flux as suggested by PAMELA data sets an indirect upper limit on the DM-nucleon spin-independent elastic scattering cross section which is stronger than the existing DM direct detection bound from LUX in the AMS-02 preferred DM mass range.

Decaying Dark Atom Constituents and Cosmic Positron Excess

We present a scenario where dark matter is in the form of dark atoms that can accommodate the experimentally observed excess of positrons in PAMELA and AMS-02 while being compatible with the constraints imposed on the gamma-ray ux from Fermi/LAT. This scenario assumes that the dominant component of dark matter is in the form of a bound state between a helium nucleus and a-2particle and a small component is in the form of a WIMP-like dark atom compatible with direct searches in underground detectors. One of the constituents of this WIMP-like state is a+2metastable particle with a mass of 1 TeV or slightly below that by decaying toe+e+,μ+μ+andτ+τ+produces the observed positron excess. These decays can naturally take place via GUT interactions. If it exists, such a metastable particle can be found in the next run of LHC. The model predicts also the ratio of leptons over baryons in the universe to be close to-3.

Cosmic ray positrons from a local, middle-aged supernova remnant

We argue that the cosmic ray positron excess observed in ATIC-2, Fermi LAT, PAMELA, HESS and recently in the precision AMS-02 experiment can be attributed to production in a local, middle-aged supernova remnant (SNR). Using the prediction of our model of cosmic ray acceleration in SNR we estimate that the SNR responsible for the observed positron excess is located between 250 and 320pc from the Sun and is 170–380kyear old. The most probable candidate for such a source is the SNR which gave birth to the well-known Geminga pulsar, but is no longer visible. Other contenders are also discussed.

Dark matter in natural supersymmetric extensions of the standard model

We explore the dark matter sector in extensions of the minimal supersymmetric standard model that can provide a good fit to the PAMELA cosmic-ray positron excess, while at the same time addressing the little-hierarchy problem of the minimal supersymmetric standard model. Adding a singlet Higgs superfield $S$ can account for the observed positron excess, as recently discussed in the literature, but we point out that it requires a fine-tuned choice for the parameters of the model. We find that including an additional singlet $\ensuremath{\Psi}$ in a model with gauge-mediated supersymmetry breaking is a minimal extension which allows both a reduction of the weak-scale fine-tuning and an interpretation of the cosmic-ray observations in terms of dark matter annihilations in the galactic halo. Our setup contains a light axion but does not require light $CP$-even scalars in the spectrum.

Positrons in cosmic rays from dark matter annihilations for uplifted Higgs regions in the MSSM

We point out that there are regions in the MSSM parameter space which successfully provide a dark matter (DM) annihilation explanation for observed positron excess (e.g. PAMELA), while still remaining in agreement with all other data sets. Such regions (e.g. the uplifted Higgs region) can realize an enhanced neutralino DM annihilation dominantly into leptons via a Breit-Wigner resonance through the CP-odd Higgs channel. Such regions can give the proper thermal relic DM abundance, and the DM annihilation products are compatible with current antiproton and gamma ray observations. This scenario can succeed without introducing any additional degrees of freedom beyond those already in the MSSM.

Neutralinos in an extension of the minimal supersymmetric standard model as the source of the PAMELA positron excess

We consider a scenario within the minimal supersymmetric standard model (MSSM) extended by a singlet chiral superfield, in which neutralino dark matter annihilates to light singletlike Higgs bosons, which proceed to decay to either electron-positron or muon-antimuon pairs. Unlike neutralino annihilations in the MSSM, this model can provide a good fit to the PAMELA cosmic ray positron fraction excess. Furthermore, the singletlike Higgs bosons can induce a large Sommerfeld enhancement and provide an annihilation rate sufficient to accommodate the observed positron excess.

Origin of the Positron Excess in Cosmic Rays

We show that the positron excess measured by the PAMELA experiment in the region between 10 and 100 GeV may well be a natural consequence of the standard scenario for the origin of Galactic cosmic rays. The "excess" arises because of positrons created as secondary products of hadronic interactions inside the sources, but the crucial physical ingredient which leads to a natural explanation of the positron flux is the fact that the secondary production takes place in the same region where cosmic rays are being accelerated. Therefore secondary positrons (and electrons) participate in the acceleration process and turn out to have a very flat spectrum, which is responsible, after propagation in the Galaxy, for the observed positron excess. This effect cannot be avoided though its strength depends on the values of the environmental parameters during the late stages of evolution of supernova remnants.

PAMELA/ATIC anomaly from the meta-stable extra dark matter component and the leptophilic Yukawa interaction

We present a supersymmetric model with two dark matter (DM)components explaining the galactic positron excess observed byPAMELA/HEAT and ATIC/PPB-BETS: One is the conventional (bino-like)lightest supersymmetric particle (LSP) χ, and the other is aTeV scale meta-stable neutral singlet ND, which is a Diracfermion (N,Nc). In this model, ND decays dominantly intoχe+e− through an R parity preserving dimension 6operator with the life time τN ∼ 1026 sec. We introducea pair of vector-like superheavy SU(2) lepton doublets (L,Lc)and lepton singlets (E,Ec). The dimension 6 operator leading tothe ND decay is generated from the leptophilic Yukawainteractions by W ⊃ NecE+LhdEc+m3/2l1Lc with thedimensionless couplings of order unity, and the gauge interactionby ℒ ⊃ (2)1/2g′ẽc*ecχ+h.c. Thesuperheavy masses of the vector-like leptons (ML,ME ∼ 1016 GeV) are responsible for the longevity of ND. The lowenergy field spectrum in this model is just the MSSM fields andND.Even for the case that the portion of ND is much smaller thanthat of χ in the total DM density [\U0001d4aa(10−10) ≲ nND/nχ], the observed positron excess can beexplained by adopting relatively lighter masses of the vector-likeleptons (1013 GeV ≲ ML,E ≲ 1016 GeV).The smallness of the electron mass is also explained. This modelis easily embedded in the flipped SU(5) grand unification, whichis a leptophilic unified theory.

Kaluza-Klein dark matter and the positron excess

The excess of cosmic positrons observed by the HEAT experiment may be the result of Kaluza-Klein dark matter annihilating in the galactic halo. Kaluza-Klein dark matter annihilates dominantly into charged leptons that yield a large number and hard spectrum of positrons per annihilation. Given a Kaluza-Klein dark matter particle with a mass in the range of 300-400 GeV, no exceptional substructure or clumping is needed in the local distribution of dark matter to generate a positron flux that explains the HEAT observations. This is in contrast to supersymmetric dark matter that requires unnaturally large amounts of dark substructure to produce the observed positron excess. Future astrophysical and collider tests are outlined that will confirm or rule out this explanation of the HEAT data.