Dark Matter Particle Explorer observations of high-energy cosmic ray electrons plus positrons and their physical implications

The DArk Matter Particle Explorer(DAMPE) is a satellite-borne, high-energy particle and γ-ray detector, which is dedicated to indirectly detecting particle dark matter and studying high-energy astrophysics. The first results about precise measurement of the cosmic ray electron plus positron spectrum between 25 Ge V and 4.6 Te V were published recently. The DAMPE spectrum reveals an interesting spectral softening arount 0.9 Te V and a tentative peak around 1.4 Te V. These results have inspired extensive discussion. The detector of DAMPE, the data analysis, and the first results are introduced. In particular, the physical interpretations of the DAMPE data are reviewed.     

Low-scale neutrino seesaw mechanism and scalar dark matter

Recently the AMS-02 experiment has released the data of positron fraction with a very small statistical error. Because of the small error, it is no longer easy to fit the data with single dark matter for a fixed diffusion model and dark matter profile. In this paper, we propose a new interpretation of the data: that it originates from decay of two-component dark matter. This interpretation gives a rough threshold of the lighter DM component. When DM decays into leptons, the positron fraction in the cosmic rays depends on the flavor of the final states, and this is fixed by imposing a non-Abelian discrete symmetry on our model. By assuming two gauge-singlet fermionic decaying DM particles, we show that a model with non-Abelian discrete flavor symmetry, e.g. $T_{13}$ , can give a much better fitting to the AMS-02 data compared with a single-component dark matter scenario. Few dimension-six operators of the universal leptonic decay of DM particles are allowed in our model, since its decay operators are constrained by the $T_{13}$ symmetry. We also show that the lepton masses and mixings are consistent with current experimental data, due to the flavor symmetry.     

Explanations of the DAMPE high energy electron/positron spectrum in the dark matter annihilation and pulsar scenarios

Many studies have shown that either the nearby astrophysical source or dark matter(DM)annihilation/decay can be used to explain the excess of high energy cosmic ray(CR)e~±,which is detected by many experiments,such as PAMELA and AMS-02.Recently,the dark matter particle explorer(DAMPE)collaboration has reported its first result of the total CR e~±spectrum from 25 Ge V to 4.6 Te V with high precision.In this work,we study the DM annihilation and pulsar interpretations of this result.We show that the leptonic DM annihilation channels toτ~+τ~-,4μ,4τ,and mixed charged lepton final states can well explain the DAMPE e~±spectrum.We also find that the mixed charged leptons channel would lead to a sharp drop structure at～Te V.However,the ordinary DM explanations have been almost excluded by the constraints from the observations of gamma-ray and CMB,unless some exotic DM models are introduced.In the pulsar scenario,we analyze 21 nearby known pulsars and assume that one of them dominantly contributes to the high energy CR e~±spectrum.Involving the constraint from the Fermi-LAT observation of the e~±anisotropy,we find that two pulsars could explain the DAMPE e~±spectrum.Our results show that it is difficult to discriminate between the DM annihilation and single pulsar explanations of high energy e~±with the current DAMPE result.     

PAMELA,FGST and sub-TeV dark matter

PAMELA's observation that the cosmic ray positron fraction increases rapidly with energy implies the presence of primary sources of energetic electron–positron pairs. Of particular interest is the possibility that dark matter annihilations in the halo of the Milky Way provide this anomalous flux of antimatter. The recent measurement of the cosmic ray electron spectrum by the Fermi Gamma Ray Space Telescope, however, can be used to constrain the nature of any such dark matter particle. In particular, it has been argued that in order to accommodate the observations of Fermi and provide the PAMELA positron excess, annihilating dark matter particles must be as massive as ∼1 TeV or heavier. In this Letter, we revisit Fermi's electron spectrum measurement within the context of annihilating dark matter, focusing on masses in the range of 100–1000 GeV, and considering effects such as variations in the astrophysical backgrounds from the presence of local cosmic ray accelerators, and the finite energy resolution of the Fermi Gamma Ray Space Telescope. When these factors are taken into account, we find that dark matter particles as light as ∼300 GeV can be capable of generating the positron fraction observed by PAMELA.     

Indirect searches for dark matter with AMS-02

AMS-02 is a multi-purpose spectrometer with superconducting magnet, and is designed for 3 years of data taking aboard the International Space Station. Its high performance regarding particle identification and energy measurement will allow performing indirect searches for dark matter (DM) in different channels simultaneously: gamma rays, positrons and antiprotons. AMS-02 sensitivity to those signals are presented and – provided the positron excess is due to DM signal – it is shown that it allows to probe new physics models in detail. Its high sensitivity could even be a unique opportunity to reach the Majorana nature of the DM particle through final state polarization effects. PACS  95.35.+d; 95.55.Vj     

Antiproton identification below threshold with the AMS-02 RICH detector

The Alpha Magnetic Spectrometer(AMS-02),which is installed on the International Space Station(ISS),has been collecting data successfully since May 2011.The main goals of AMS-02 are the search for cosmic anti-matter,dark matter and the precise measurement of the relative abundance of elements and isotopes in galactic cosmic rays.In order to identify particle properties,AMS-02 includes several specialized sub-detectors.Among these,the AMS-02 Ring Imaging Cherenkov detector(RICH) is designed to provide a very precise measurement of the velocity and electric charge of particles.We describe a method to reject the dominant electron background in antiproton identification with the use of the AMS-02 RICH detector as a veto for rigidities below 3 GV.A ray tracing integration method is used to maximize the statistics of  with the lowest possible e background,providing 4 times rejection power gain for e background with respect to only 3% of  signal efficiency loss.By using the collected cosmic-ray data,e contamination can be well suppressed within 3% with β≈ 1,while keeping 76% efficiency for  below the threshold.     

Pseudo-familon dark matter

Motivated by a model of pseudo-Majoron dark matter, we show how the breaking of a global symmetry that acts nontrivially in lepton generation space can lead to a viable pseudo-familon dark matter candidate. Unlike the pseudo-Majoron, the pseudo-familon in our model decays primarily to charged leptons and can account for the excess observed in the cosmic ray electron and positron spectra.     

Lepton-portal dark matter in hidden valley model and the DAMPE recent results

We study the recent e±cosmic ray excess reported by DAMPE in a Hidden Valley Model with lepton-portal dark matter. We find the electron-portal can account for the excess well and satisfy the DM relic density and direct detection bounds, while electron+muon/electron+muon+tau-portal suffers from strong constraints from lepton flavor violating observables, such as μ→3 e. We also discuss possible collider signatures of our model, both at the LHC and a future 100 Te V hadron collider.     

Origin of hardening in spectra of cosmic ray nuclei at a few hundred GeV using AMS-02 data

Many experiments have confirmed spectral hardening at a few hundred GeV in the spectra of cosmic ray(CR)nuclei.Three different origins have been proposed:primary source acceleration,propagation,and the superposition of different kinds of sources.In this work,a broken power law has been employed to fit each of the spectra of cosmic ray nuclei from AMS-02 directly,for rigidities greater than 45 GeV.The fitting results of the break rigidity and the spectral index differences less than and greater than the break rigidity show complicated relationships among different nuclear species,which cannot be reproduced naturally by a simple primary source scenario or a propagation scenario.However,with a natural and simple assumption,the superposition of different kinds of sources could have the potential to explain the fitting results successfully.Spectra of CR nuclei from a single future experiment,such as DAMPE,will provide us the opportunity to do cross checks and reveal the properties of the different kinds of sources.     

On the cosmic-ray spectra of three-body lepton-flavor-violating dark matter decays

We consider possible leptonic three-body decays of spin-1/2, charge-asymmetric dark matter. Assuming a general Dirac structure for the four-fermion contact interactions of interest, we study the cosmic-ray electron and positron spectra and show that good fits to the current data can be obtained for both charged-lepton-flavor-conserving and flavor-violating decay channels. We find that different choices for the Dirac structure of the underlying decay operator can be significantly compensated by different choices for the dark matter mass and lifetime. The decay modes we consider provide differing predictions for the cosmic-ray positron fraction at energies higher than those currently probed at the PAMELA experiment; these predictions might be tested at cosmic-ray detectors like AMS-02.     

Interpretation of the DAMPE 1.4 TeV peak according to the decaying dark matter model

Highly accurate measurements of cosmic ray electron flux by the dark matter particle explorer(DAMPE) ranging between 25 Ge V and 4.6 Te V have recently been published. A sharp peak structure was found at ～ 1.4 Te V. This unexpected peak structure can be reproduced by the annihilation/decay of a nearby dark matter(DM) halo. In this study, we adopt the decaying-DM model to interpret the ～ 1.4 Te V peak. We found that the decay products of the local DM subhalo could contribute to the DMAPE peak with mDM= 3 Te V and τ～ 10~(28) s. We also obtain constraints on DM lifetime and the distance of the local DM subhalo by comparison with DAMPE data.     

Leptophilic dark matter confronts AMS-02 cosmic-ray positron flux

With the measurement of positron flux published recently by AMS-02 collaboration, we show how the leptophilic dark matter fits the observation. We obtain the percentages of different products of dark matter annihilation that can best describe the flux of high energy positrons observed by AMS. We show that dark matter annihilates predominantly into ττ pair, while both ee and μμ final states should be less than 20%. When gauge boson final states are included, the best branching ratio of needed ττ mode reduces.     

Nearby source interpretation of differences among light and medium composition spectra in cosmic rays

Recently the AMS-02 reported the precise measurements of the energy spectra of medium-mass compositions (Neon, Magnesium, Silicon) of primary cosmic rays, which reveal different properties from those of light compositions (Helium, Carbon, Oxygen). Here we propose a nearby source scenario, together with the background source contribution, to explain the newly measured spectra of cosmic ray Ne, Mg, Si, and particularly their differences from that of He, C, O. Their differences at high energies can be naturally accounted for by the element abundance of the nearby source. Specifically, the abundance ratio of the nearby source to the background of the Ne, Mg, Si elements is lower by a factor of ∼ 1.7 than that of the He, C, O elements. Such a difference could be due to the abundance difference of the stellar evolution of the progenitor star or the acceleration process/environment, of the nearby source. This scenario can simultaneously explain the high-energy spectral softening features of cosmic ray spectra revealed recently by CREAM/NUCLEON/DAMPE, as well as the energy-dependent behaviors of the large-scale anisotropies. It is predicted that the dipole anisotropy amplitudes below PeV energies of the Ne, Mg, Si group are smaller than that of the He, C, O group, which can be tested with future measurements.     

Tasting nuclear pasta made with classical molecular dynamics simulations

Recently the AMS-02 reported the precise measurements of the energy spectra of medium-mass compositions (Neon, Magnesium, Silicon) of primary cosmic rays, which reveal different properties from those of light compositions (Helium, Carbon, Oxygen). Here we propose a nearby source scenario, together with the background source contribution, to explain the newly measured spectra of cosmic ray Ne, Mg, Si, and particularly their differences from that of He, C, O. Their differences at high energies can be naturally accounted for by the element abundance of the nearby source. Specifically, the abundance ratio of the nearby source to the background of the Ne, Mg, Si elements is lower by a factor of ~ 1.7 than that of the He, C, O elements. Such a difference could be due to the abundance difference of the stellar evolution of the progenitor star or the acceleration process/environment, of the nearby source. This scenario can simultaneously explain the high-energy spectral softening features of cosmic ray spectra revealed recently by CREAM/NUCLEON/DAMPE, as well as the energy-dependent behaviors of the large-scale anisotropies. It is predicted that the dipole anisotropy amplitudes below PeV energies of the Ne, Mg, Si group are smaller than that of the He, C, O group, which can be tested with future measurements.

     

On the contribution of a hard galactic plane component to the excesses of secondary particles

The standard model of cosmic ray propagation has been very successful in explaining all kinds of galactic cosmic ray spectra. However, high precision measurement have recently revealed an appreciable discrepancy between data and model expectations, from spectrum observations of gamma-rays, e+/e- and probably the B=C ratio starting from ~10 GeV energy . In this work, we propose that a hard galactic plane component, supplied by the fresh cosmic ray sources and detained by local magnetic elds, can contribute additional secondary particles interacting with local materials. By properly choosing the intensity and spectral index of the harder component up to multi-T eV energy , a two-component gamma-ray spectrum is obtained and agrees very well with the observation. Simultaneously , the expected neutrino numbers from the galactic plane could contribute ~60% of IceCube observed neutrino number below a few hundreds of TeV under our model. In addition to these studies, we nd that the same pp-collision process responsible for the excess gamma ray emission could account for a signi cant amount of the positron excess, but a more detailed mechanism is needed for a full agreement. It is expected that the excesses in the p=p and B=C ratio will show up when energy is above ~10 GeV. We look forward this model being tested in the near future by new observations from AMS02, IceCube, AS-gamma, HA WC and future  experiments such as LHASSO, HiSCORE and CTA.     

Results from PAMELA,ATIC and FERMI: Pulsars or dark matter?

It is well known that dark matter dominates the dynamics of galaxies and clusters of galaxies. Its constituents remain a mystery despite an assiduous search for them over the past three decades. Recent results from the satellite-based PAMELA experiment show an excess in the positron fraction at energies between 10 and 100 GeV in the secondary cosmic ray spectrum. Other experiments, namely ATIC, HESS and FERMI, show an excess in the total electron (e ⁺ + e ⁻) spectrum for energies greater than 100 GeV. These excesses in the positron fraction as well as the electron spectrum can arise in local astrophysical processes like pulsars, or can be attributed to the annihilation of the dark matter particles. The latter possibility gives clues to the possible candidates for the dark matter in galaxies and other astrophysical systems. In this article, we give a report of these exciting developments.     

Higgs portal,fermionic dark matter,and a Standard Model like Higgs at 125 GeV

We show that fermionic dark matter (DM) which communicates with the Standard Model (SM) via the Higgs portal is a viable scenario, even if a SM-like Higgs is found at around 125 GeV. Using effective field theory we show that for DM with a mass in the range from about 60 GeV to 2 TeV the Higgs portal needs to be parity violating in order to be in agreement with direct detection searches. For parity conserving interactions we identify two distinct options that remain viable: a resonant Higgs portal, and an indirect Higgs portal. We illustrate both possibilities using a simple renormalizable toy model.     

Astroparticle physics with AMS-02

The main physics goals of the AMS-02 experiment in the astroparticle domain are searches for antimatter and dark matter. The discovery potential of primordial antimatter by AMS-02 is presented, emphasizing the completeness of the AMS-02 detector for these searches. Meanwhile, antiproton detection suffers from a large secondary interaction background; the anti-⁴He or anti-³He signal would allow one to probe the Universe for existence of antimatter. The expected signal in AMS-02 is presented and compared to results from present experiments. The e⁺ and antiproton channels will contribute to the dark matter detection studies. A SUSY neutralino candidate is considered. The expected flux sensitivities in a three-year exposure for the e⁺/e^? ratio and antiproton yields as a function of energy are presented and compared to other direct and indirect searches.     

Significant gamma lines from inert Higgs dark matter

One way to unambiguously confirm the existence of particle dark matter and determine its mass would be to detect its annihilation into monochromatic gamma-rays in upcoming telescopes. One of the most minimal models for dark matter is the inert doublet model, obtained by adding another Higgs doublet with no direct coupling to fermions. For a mass between 40 and 80 GeV, the lightest of the new inert Higgs particles can give the correct cosmic abundance of cold dark matter in agreement with current observations. We show that for this scalar dark matter candidate, the annihilation signal of monochromatic gammagamma and Zgamma final states would be exceptionally strong. The energy range and rates for these gamma-ray line signals make them ideal to search for with the soon upcoming GLAST satellite.     

Dark Supersymmetry

We propose a model of Dark Supersymmetry, where a supersymmetric dark sector is coupled to the classically scale invariant non-supersymmetric Standard Model through the Higgs portal. The dark sector contains a mass scale that is protected against radiative corrections by supersymmetry, and the portal coupling mediates this scale to the Standard Model, resulting in a vacuum expectation value for the Higgs field and the usual electroweak symmetry breaking mechanism. The supersymmetric dark sector contains dark matter candidates, and we show that the observed dark matter abundance is generated for a natural choice of parameters, while avoiding the current experimental bounds on direct detection. Future experiments can probe this scenario if the dark sector mass scale is not too high.