- Email: [email protected]
Indirect searches for dark matter
Physics of the Dark Universe 4 (2014) 41–43
Contents lists available at ScienceDirect
Physics of the Dark Universe journal homepage: www.elsevier.com/locate/dark
Indirect searches for dark matter Fiorenza Donato ∗ Physics Department, Università di Torino and INFN, Torino, Italy
article
info
Keywords: Dark matter Cosmic rays Antimatter
abstract The indirect detection of particle dark matter (DM) is based on the search for anomalous components in cosmic rays (CRs) due to the annihilation of DM pairs in the galactic halo, on the top of the standard astrophysical production. We revise here the most recent results on the DM searches for γ -rays, antiprotons, antideuterons and positrons in CRs. © 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction The nature of the dark matter (DM) in the Universe keeps being one of the most intriguing problems in fundamental physics and cosmology. The most common hypothesis for particle DM consists of weakly interacting massive particles (WIMPs), supposed to be cold thermal relics after the Big Bang and to build up the galactic dark haloes. The so-called indirect DM investigation technique relies in the search for its stable annihilation products in the halo of galaxies, and in particular in the Milky Way. The indirect DM detection focuses on rare components in charged cosmic rays (CRs) [1] as well as in γ -rays and, more generally, in multiwavelength photons [2,3]:
χ + χ → qq¯ , W + W − , . . . → p¯ , D¯ , e+ , γ , & ν ′ s.
(1)
The investigation of these annihilation products is pursued by means of ground based telescopes, balloon borne detectors and space-based experiments. The charged CRs reach eventually the Earth after diffusing on the galactic magnetic fields. The lowest energy (below O(10) GeV) CRs, moreover, suffer the effect of the solar wind which drifts particles out of the heliosphere. On the other side, γ -rays and photons in general are not distorted neither significantly absorbed along their way to the Earth, and are therefore searched in the sky portions where they are predicted to be overabundant with respect to the astrophysical background. 2. Annihilation into photons The γ -ray flux dΦγ /dEγ from DM annihilating particles as a function of observed energy Eγ , when looking at the direction ψ
∗
Tel.: +39 3281418869. E-mail address: [email protected].
and θ (longitude and latitude in Galactic coordinates, respectively) in the sky, by an experiment with spatial resolution α and under a solid angle ∆Ω = 2π (1 − cos α), may be expressed as [4,5]: dΦγ dEγ
·
(Eγ , ψ, θ , ∆Ω ) =
dNγi dEγ
∆Ω
0
dΩ
l.o.s
1 ⟨σann v⟩ 4π
2m2χ
·
Bi
i
ρ 2 (r (s, ψ, θ ))ds.
(2)
The DM mass is mχ , and ⟨σann v⟩ is the averaged annihilation cross section times the relative velocity. Bi is the branching ratio into the final state i and dNγi (Eγ )/dEγ is the photon spectrum per annihilation (which depends on the annihilation channels). The sum is in principle performed over all the annihilation channels. The last term in Eq. (2) contains the (squared) DM density ρ(r ) (r being the galactocentric distance) integrated along a distance s from the Earth in the direction along the line of sight (l.o.s). The first part of Eq. (2) includes the details of the assumed particle physics model for the WIMPs. Often, analyses are carried in generic models, in which the ⟨σann v⟩ is fixed to a representative value and the annihilation in supposed to occur via a unique channel. The second part of Eq. (2) has an astrophysical origin and it is usually modeled according to the results of cosmological collisionless simulations, generally predicting a steepening of the DM profile in the inner parts of the resolved halos (e.g. [6,7] and references therein). The search for γ -rays of DM origin has been intensively pursued in these last years, thanks to the excellent performances of the Large Area Telescope (LAT) on the Fermi γ -ray space Telescope (Fermi). One of the most favored targets have been identified with especially dense regions such as the galactic center [8,9], and Milky Way satellites like faint and ultra faint dwarf galaxies [10,11]. The latter targets provide quite constraining upper limits on the ⟨σann v⟩,
http://dx.doi.org/10.1016/j.dark.2014.06.001 2212-6864/© 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
42
F. Donato / Physics of the Dark Universe 4 (2014) 41–43
depending on the annihilation channel and, of course, the WIMP mass. Searches have been performed in cosmological structures as well [12–16]. A recent analysis of Fermi-LAT data of the galactic center region has unveiled the feature of a narrow line close to 130 GeV [17–19]. The Fermi-LAT Collaboration sets its significance to a mere 1.6σ (in somewhat different target regions) when considering a larger set of re-processed data and a 2D modeling of the energy dispersion [20,9]. The interpretation in terms of annihilating DM would in any case require a loop-suppressed direct annihilation into photons anomalously larger. Only a quite larger exposure will allow to clarify the statistical robustness and potential instrumental effects of the line feature. The new observation strategy for Fermi approved by the Collaboration1 could get this reachable by the end of 2014 [21]. At high galactic latitudes, a faint γ -ray irreducible emission has been measured, and shown to be isotropic at a high degree [22]. The Fermi-LAT has already reported the detection of a non-zero angular power spectrum (APS) above the noise level [22]. Given the low background level, the high latitude emission can provide stringent bounds on the DM annihilation strength [23–25], even if a detailed knowledge of both the galactic and the extragalactic background has proven to be crucial [26]. 3. Dark matter into charged CRs The DM annihilation may occur also to charged light antimatter, such as antiprotons, antideuterons, and positrons. An oversimplified picture of the Galaxy consists of a thin stellar disk, where astrophysical sources are located; a diffusive, cylindrical magnetic halo extending beneath and above the galactic disk, where diffusion takes place. A much more extended spherical DM halo embeds the diffusive halo and may be the source of charged particles produced by WIMP DM pair annihilation. Once charged particles produced in DM annihilation are released in the diffusive halo, they experience the same diffusion and propagation history as all the galactic CRs [27]. However, given the DM sources are mostly located in the halo of the Galaxy – at variance with the CRs from astrophysical sources, located in the galactic disk – the exotic (DM induced) and astrophysical originated CRs are sensitive to propagation parameters in different ways. We remind to [1,28], and refs. therein, for all the details relating to propagation of CRs originated from DM annihilation. 3.1. Annihilation into antiprotons The differential rate of antiproton production as a function of space coordinates r and z, and antiproton kinetic energy Tp¯ is defined as: qDM p¯ (r , z , Tp¯ ) = ⟨σann v⟩
F,h
(F)
Bχ h
dNp¯h dEp¯
ρχ (r , z ) mχ
2
.
(3)
For the solution of the transport equation for a generic source term qDM (r , z , E ) we remind to [29,28]. The secondary antiproton flux is due to spallation reactions among p and He CRs and the H and He in the interstellar medium (ISM). Antiprotons from astrophysical sources have been shown to be highly compatible with the most recent Pamela data [30]. Presently, the antiproton flux tool has been shown to be extremely powerful in setting limits to DM annihilation [30–33]. The major obstacle is still the uncertainty due to CRs propagation, which severely affects the prediction of primary fluxes. A major improvement is expected by the boron-to-carbon data collected by the AMS-02 experiment, which will improve the constraints on the CR propagation parameters.
1 http://fermi.gsfc.nasa.gov/ssc/proposals/alt_obs/obs_modes.html.
3.2. Annihilation into antideuterons In the seminal paper [34], it was proposed cosmic antideuterons as a possible indirect signature for galactic DM. It was shown that the antideuteron spectra deriving from DM annihilation are expected to be flatter than the secondary astrophysical component at kinetic energies as low as Td . 2–3 GeV/n. This argument motivated the proposal of a space-borne experiment [35,36], looking for cosmic antimatter (antiproton and antideuteron) and having the potential to discriminate between standard and exotic components for a wide range of DM models. A prototype flight has been undertaken recently [37]. The present experimental upper limit [38] is still far from the expectations on the secondary antideuteron, flux which are produced by spallation of cosmic rays on the interstellar medium [39,40], but perspectives for the near future are very encouraging. The production of cosmic antideuterons is based on the fusion process of a p¯ and n¯ pair. One of the simplest but powerful treatment of the fusion of two or more nucleons is based on the socalled coalescence model [34,41], while more complex modelings have been recently considered (see [42] and refs. therein). The secondary antideuteron flux is the sum of the six contributions corresponding to p, He and p¯ CR fluxes impinging on H and He in the ISM, and is affected by a non-negligible uncertainty band [40]. The primary DM induced source relies on the fusion of nucleon produced according to Eq. (3). As shown in [41,42], the antideuteron flux induced by DM annihilation largely exceeds the secondary contribution for energies .GeV/nucleon, and is therefore a quite promising indirect DM inspection left to next generation space borne detectors. The predictions, similarly to the antiproton flux, are affected by huge astrophysical uncertainties but, again, they could be significantly reduced by the next-to-come AMS-02 nuclear data. 3.3. Annihilation into positrons The annihilation of DM into positrons occurs both directly and from hadronization and decay processes [43]. In the case of positrons and electrons, the diffusive equation (Eq. (2)) is dominated by energy losses and space diffusion. In particular, above a few GeV the energy losses by synchrotron radiation in the galactic magnetic fields and by inverse Compton scattering on stellar light and on CMB photons dominate. Like for antiprotons, a background of secondary positrons is produced by the spallation of the interstellar medium by impinging high-energy particles [44]. The dominant mechanism is the collision of protons with hydrogen atoms at rest producing charged pions π ± which decay into muons µ± . The latter are also unstable and eventually lead to electrons and positrons. The positron fraction has been recently measured by the Pamela [45,46] and confirmed by Fermi-LAT [47] and AMS-02 [48] experiments. A rising trend of the positron fraction has been confirmed by all the experiments at high energies. It has been shown that the data may be accommodated within astrophysical models predicting electron and positron emissions from pulsars and near supernovae remnants (see [49–51] and refs. therein). A possible DM contribution cannot be excluded, but it requires huge boost factors and peculiar annihilation final states [30,52]. 4. Conclusions The indirect dark matter detection through photons (mainly
γ -rays) and charged cosmic antimatter is discussed. We have outlined the present state-of-the-art, showing that this kind of research is experiencing an intense experimental activity, which requires a detailed modeling for the data interpretation. The forthcoming measurements will be crucial to ascertain the presence of WIMPs in the halo of our Galaxy and to hint at their particle physics nature.
F. Donato / Physics of the Dark Universe 4 (2014) 41–43
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[27] D. Maurin, F. Donato, R. Taillet, P. Salati, Cosmic rays below z = 30 in a diffusion model: new constraints on propagation parameters, Astrophys. J. 555 (2001) 585–596. [28] F. Donato, N. Fornengo, D. Maurin, P. Salati, R. Taillet, Antiprotons in cosmic rays from neutralino annihilation, Phys. Rev. D 69 (6) (2004) 063501. [29] A. Barrau, G. Boudoul, F. Donato, D. Maurin, P. Salati, R. Taillet, Antiprotons from primordial black holes, A&A 388 (2002) 676–687. [30] F. Donato, D. Maurin, P. Brun, T. Delahaye, P. Salati, Constraints on WIMP dark matter from the high energy PAMELA p¯ /p data, Phys. Rev. Lett. 102 (7) (2009) 071301. [31] N. Fornengo, L. Maccione, A. Vittino, Constraints on particle dark matter from cosmic-ray antiprotons, J. Cosmol. Astropart. Phys. 1404 (2014) 003. [32] C. Evoli, I. Cholis, D. Grasso, L. Maccione, P. Ullio, Antiprotons from dark matter annihilation in the galaxy: astrophysical uncertainties, Phys. Rev. D 85 (2012) 123511. [33] M. Cirelli, G. Giesen, Antiprotons from dark matter: current constraints and future sensitivities, J. Cosmol. Astropart. Phys. 4 (2013) 15. [34] F. Donato, N. Fornengo, P. Salati, Antideuterons as a signature of supersymmetric dark matter, Phys. Rev. D 62 (4) (2000) 043003. [35] K. Mori, C.J. Hailey, E.A. Baltz, W.W. Craig, M. Kamionkowski, W.T. Serber, P. Ullio, A novel antimatter detector based on X-ray deexcitation of exotic atoms, Astrophys. J. 566 (2002) 604–616. [36] C.J. Hailey, W.W. Craig, F.A. Harrison, J. Hong, K. Mori, J. Koglin, H.T. Yu, K.P. Ziock, Development of the gaseous antiparticle spectrometer for spacebased antimatter detection, Nucl. Instrum. Methods Phys. Res. B 214 (2004) 122–125. [37] P. von Doetinchem, T. Aramaki, N. Bando, S.E. Boggs, H. Fuke, F.H. Gahbauer, C.J. Hailey, J.E. Koglin, S.A.I. Mognet, N. Madden, S. Okazaki, R.A. Ong, K.M. Perez, T. Yoshida, J. Zweerink, The flight of the GAPS prototype experiment, Astropart. Phys. 54 (2014) 93–109. [38] H. Fuke, T. Maeno, K. Abe, S. Haino, Y. Makida, S. Matsuda, H. Matsumoto, J.W. Mitchell, A.A. Moiseev, J. Nishimura, M. Nozaki, S. Orito, J.F. Ormes, M. Sasaki, E.S. Seo, Y. Shikaze, R.E. Streitmatter, J. Suzuki, K. Tanaka, K. Tanizaki, T. Yamagami, A. Yamamoto, Y. Yamamoto, K. Yamato, T. Yoshida, K. Yoshimura, Search for cosmic-ray antideuterons, Phys. Rev. Lett. 95 (8) (2005) 081101. [39] P. Chardonnet, J. Orloff, P. Salati, Production of anti-matter in our galaxy, Phys. Lett. B 409 (1997) 313–320. [40] R. Duperray, B. Baret, D. Maurin, G. Boudoul, A. Barrau, L. Derome, K. Protasov, M. Buénerd, Flux of light antimatter nuclei near Earth, induced by cosmic rays in the Galaxy and in the atmosphere, Phys. Rev. D 71 (8) (2005) 083013. [41] F. Donato, N. Fornengo, D. Maurin, Antideuteron fluxes from dark matter annihilation in diffusion models, Phys. Rev. D 78 (4) (2008) 043506. [42] N. Fornengo, L. Maccione, A. Vittino, Dark matter searches with cosmic antideuterons: status and perspectives, J. Cosmol. Astropart. Phys. 9 (2013) 31. [43] T. Delahaye, R. Lineros, F. Donato, N. Fornengo, P. Salati, Positrons from dark matter annihilation in the galactic halo: theoretical uncertainties, Phys. Rev. D 77 (2008) 063527. [44] T. Delahaye, F. Donato, N. Fornengo, J. Lavalle, R. Lineros, et al., Galactic secondary positron flux at the Earth, Astron. Astrophys. 501 (2009) 821–833. [45] O. Adriani, et al., An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV, Nature 458 (2009) 607–609. [46] O. Adriani, et al., Cosmic-ray positron energy spectrum measured by PAMELA, Phys. Rev. Lett. 111 (8) (2013) 081102. [47] M. Ackermann, et al., Measurement of separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope, Phys. Rev. Lett. 108 (2012) 011103. [48] M. Aguilar, et al., First result from the alpha magnetic spectrometer on the international space station: precision measurement of the positron fraction in primary cosmic rays of 0.5350 GeV, Phys. Rev. Lett. 110 (2013) 141102. [49] D. Hooper, P. Blasi, P.D. Serpico, Pulsars as the sources of high energy cosmic ray positrons, J. Cosmol. Astropart. Phys. 0901 (2009) 025. [50] M. Di Mauro, F. Donato, N. Fornengo, R. Lineros, A. Vittino, Interpretation of AMS-02 electrons and positrons data, J. Cosmol. Astropart. Phys. 1404 (2014) 006. [51] P.D. Serpico, Astrophysical models for the origin of the positron ‘excess’, Astropart. Phys. 39–40 (2012) 2–11. [52] M. Cirelli, M. Kadastik, M. Raidal, A. Strumia, Model-independent implications of the e+-, anti-proton cosmic ray spectra on properties of dark matter, Nuclear Phys. B 813 (2009) 1–21.
Contents lists available at ScienceDirect
Physics of the Dark Universe journal homepage: www.elsevier.com/locate/dark
Indirect searches for dark matter Fiorenza Donato ∗ Physics Department, Università di Torino and INFN, Torino, Italy
article
info
Keywords: Dark matter Cosmic rays Antimatter
abstract The indirect detection of particle dark matter (DM) is based on the search for anomalous components in cosmic rays (CRs) due to the annihilation of DM pairs in the galactic halo, on the top of the standard astrophysical production. We revise here the most recent results on the DM searches for γ -rays, antiprotons, antideuterons and positrons in CRs. © 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction The nature of the dark matter (DM) in the Universe keeps being one of the most intriguing problems in fundamental physics and cosmology. The most common hypothesis for particle DM consists of weakly interacting massive particles (WIMPs), supposed to be cold thermal relics after the Big Bang and to build up the galactic dark haloes. The so-called indirect DM investigation technique relies in the search for its stable annihilation products in the halo of galaxies, and in particular in the Milky Way. The indirect DM detection focuses on rare components in charged cosmic rays (CRs) [1] as well as in γ -rays and, more generally, in multiwavelength photons [2,3]:
χ + χ → qq¯ , W + W − , . . . → p¯ , D¯ , e+ , γ , & ν ′ s.
(1)
The investigation of these annihilation products is pursued by means of ground based telescopes, balloon borne detectors and space-based experiments. The charged CRs reach eventually the Earth after diffusing on the galactic magnetic fields. The lowest energy (below O(10) GeV) CRs, moreover, suffer the effect of the solar wind which drifts particles out of the heliosphere. On the other side, γ -rays and photons in general are not distorted neither significantly absorbed along their way to the Earth, and are therefore searched in the sky portions where they are predicted to be overabundant with respect to the astrophysical background. 2. Annihilation into photons The γ -ray flux dΦγ /dEγ from DM annihilating particles as a function of observed energy Eγ , when looking at the direction ψ
∗
Tel.: +39 3281418869. E-mail address: [email protected].
and θ (longitude and latitude in Galactic coordinates, respectively) in the sky, by an experiment with spatial resolution α and under a solid angle ∆Ω = 2π (1 − cos α), may be expressed as [4,5]: dΦγ dEγ
·
(Eγ , ψ, θ , ∆Ω ) =
dNγi dEγ
∆Ω
0
dΩ
l.o.s
1 ⟨σann v⟩ 4π
2m2χ
·
Bi
i
ρ 2 (r (s, ψ, θ ))ds.
(2)
The DM mass is mχ , and ⟨σann v⟩ is the averaged annihilation cross section times the relative velocity. Bi is the branching ratio into the final state i and dNγi (Eγ )/dEγ is the photon spectrum per annihilation (which depends on the annihilation channels). The sum is in principle performed over all the annihilation channels. The last term in Eq. (2) contains the (squared) DM density ρ(r ) (r being the galactocentric distance) integrated along a distance s from the Earth in the direction along the line of sight (l.o.s). The first part of Eq. (2) includes the details of the assumed particle physics model for the WIMPs. Often, analyses are carried in generic models, in which the ⟨σann v⟩ is fixed to a representative value and the annihilation in supposed to occur via a unique channel. The second part of Eq. (2) has an astrophysical origin and it is usually modeled according to the results of cosmological collisionless simulations, generally predicting a steepening of the DM profile in the inner parts of the resolved halos (e.g. [6,7] and references therein). The search for γ -rays of DM origin has been intensively pursued in these last years, thanks to the excellent performances of the Large Area Telescope (LAT) on the Fermi γ -ray space Telescope (Fermi). One of the most favored targets have been identified with especially dense regions such as the galactic center [8,9], and Milky Way satellites like faint and ultra faint dwarf galaxies [10,11]. The latter targets provide quite constraining upper limits on the ⟨σann v⟩,
http://dx.doi.org/10.1016/j.dark.2014.06.001 2212-6864/© 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
42
F. Donato / Physics of the Dark Universe 4 (2014) 41–43
depending on the annihilation channel and, of course, the WIMP mass. Searches have been performed in cosmological structures as well [12–16]. A recent analysis of Fermi-LAT data of the galactic center region has unveiled the feature of a narrow line close to 130 GeV [17–19]. The Fermi-LAT Collaboration sets its significance to a mere 1.6σ (in somewhat different target regions) when considering a larger set of re-processed data and a 2D modeling of the energy dispersion [20,9]. The interpretation in terms of annihilating DM would in any case require a loop-suppressed direct annihilation into photons anomalously larger. Only a quite larger exposure will allow to clarify the statistical robustness and potential instrumental effects of the line feature. The new observation strategy for Fermi approved by the Collaboration1 could get this reachable by the end of 2014 [21]. At high galactic latitudes, a faint γ -ray irreducible emission has been measured, and shown to be isotropic at a high degree [22]. The Fermi-LAT has already reported the detection of a non-zero angular power spectrum (APS) above the noise level [22]. Given the low background level, the high latitude emission can provide stringent bounds on the DM annihilation strength [23–25], even if a detailed knowledge of both the galactic and the extragalactic background has proven to be crucial [26]. 3. Dark matter into charged CRs The DM annihilation may occur also to charged light antimatter, such as antiprotons, antideuterons, and positrons. An oversimplified picture of the Galaxy consists of a thin stellar disk, where astrophysical sources are located; a diffusive, cylindrical magnetic halo extending beneath and above the galactic disk, where diffusion takes place. A much more extended spherical DM halo embeds the diffusive halo and may be the source of charged particles produced by WIMP DM pair annihilation. Once charged particles produced in DM annihilation are released in the diffusive halo, they experience the same diffusion and propagation history as all the galactic CRs [27]. However, given the DM sources are mostly located in the halo of the Galaxy – at variance with the CRs from astrophysical sources, located in the galactic disk – the exotic (DM induced) and astrophysical originated CRs are sensitive to propagation parameters in different ways. We remind to [1,28], and refs. therein, for all the details relating to propagation of CRs originated from DM annihilation. 3.1. Annihilation into antiprotons The differential rate of antiproton production as a function of space coordinates r and z, and antiproton kinetic energy Tp¯ is defined as: qDM p¯ (r , z , Tp¯ ) = ⟨σann v⟩
F,h
(F)
Bχ h
dNp¯h dEp¯
ρχ (r , z ) mχ
2
.
(3)
For the solution of the transport equation for a generic source term qDM (r , z , E ) we remind to [29,28]. The secondary antiproton flux is due to spallation reactions among p and He CRs and the H and He in the interstellar medium (ISM). Antiprotons from astrophysical sources have been shown to be highly compatible with the most recent Pamela data [30]. Presently, the antiproton flux tool has been shown to be extremely powerful in setting limits to DM annihilation [30–33]. The major obstacle is still the uncertainty due to CRs propagation, which severely affects the prediction of primary fluxes. A major improvement is expected by the boron-to-carbon data collected by the AMS-02 experiment, which will improve the constraints on the CR propagation parameters.
1 http://fermi.gsfc.nasa.gov/ssc/proposals/alt_obs/obs_modes.html.
3.2. Annihilation into antideuterons In the seminal paper [34], it was proposed cosmic antideuterons as a possible indirect signature for galactic DM. It was shown that the antideuteron spectra deriving from DM annihilation are expected to be flatter than the secondary astrophysical component at kinetic energies as low as Td . 2–3 GeV/n. This argument motivated the proposal of a space-borne experiment [35,36], looking for cosmic antimatter (antiproton and antideuteron) and having the potential to discriminate between standard and exotic components for a wide range of DM models. A prototype flight has been undertaken recently [37]. The present experimental upper limit [38] is still far from the expectations on the secondary antideuteron, flux which are produced by spallation of cosmic rays on the interstellar medium [39,40], but perspectives for the near future are very encouraging. The production of cosmic antideuterons is based on the fusion process of a p¯ and n¯ pair. One of the simplest but powerful treatment of the fusion of two or more nucleons is based on the socalled coalescence model [34,41], while more complex modelings have been recently considered (see [42] and refs. therein). The secondary antideuteron flux is the sum of the six contributions corresponding to p, He and p¯ CR fluxes impinging on H and He in the ISM, and is affected by a non-negligible uncertainty band [40]. The primary DM induced source relies on the fusion of nucleon produced according to Eq. (3). As shown in [41,42], the antideuteron flux induced by DM annihilation largely exceeds the secondary contribution for energies .GeV/nucleon, and is therefore a quite promising indirect DM inspection left to next generation space borne detectors. The predictions, similarly to the antiproton flux, are affected by huge astrophysical uncertainties but, again, they could be significantly reduced by the next-to-come AMS-02 nuclear data. 3.3. Annihilation into positrons The annihilation of DM into positrons occurs both directly and from hadronization and decay processes [43]. In the case of positrons and electrons, the diffusive equation (Eq. (2)) is dominated by energy losses and space diffusion. In particular, above a few GeV the energy losses by synchrotron radiation in the galactic magnetic fields and by inverse Compton scattering on stellar light and on CMB photons dominate. Like for antiprotons, a background of secondary positrons is produced by the spallation of the interstellar medium by impinging high-energy particles [44]. The dominant mechanism is the collision of protons with hydrogen atoms at rest producing charged pions π ± which decay into muons µ± . The latter are also unstable and eventually lead to electrons and positrons. The positron fraction has been recently measured by the Pamela [45,46] and confirmed by Fermi-LAT [47] and AMS-02 [48] experiments. A rising trend of the positron fraction has been confirmed by all the experiments at high energies. It has been shown that the data may be accommodated within astrophysical models predicting electron and positron emissions from pulsars and near supernovae remnants (see [49–51] and refs. therein). A possible DM contribution cannot be excluded, but it requires huge boost factors and peculiar annihilation final states [30,52]. 4. Conclusions The indirect dark matter detection through photons (mainly
γ -rays) and charged cosmic antimatter is discussed. We have outlined the present state-of-the-art, showing that this kind of research is experiencing an intense experimental activity, which requires a detailed modeling for the data interpretation. The forthcoming measurements will be crucial to ascertain the presence of WIMPs in the halo of our Galaxy and to hint at their particle physics nature.
F. Donato / Physics of the Dark Universe 4 (2014) 41–43
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