NeutrinotelescopesearchesfordarkmatterintheSun · 2017. 10. 17. · 10 1 10 2 10 3 10 4 Dark matter mass m (GeV) 10 42 10 41 10 40 10 39 10 38 10 37 10 36 10 35 10 34 10 33 cross-section

Neutrino telescope searches for dark matter in the Sun

Pat ScottFundamental Physics Section, Department of Physics, Imperial College London, Blackett Laboratory,

Prince Consort Road, London SW7 2AZ, UK

I give a brief review of a few recent developments and future directions in the search fordark matter using high-energy neutrinos from the Sun. This includes the ability to recastneutrino telescope limits on nuclear scattering of dark matter to arbitrary new theories, andnew calculations of the solar atmospheric background relevant to such searches. I also touchon applications to global searches for new physics, and prospects for improving searches forasymmetric dark matter in the Sun.

1 Current status

High-energy neutrinos from the Sun provide one of the cleanest potential discovery channels forweakly-interacting dark matter (DM). Weakly-interacting DM particles passing through the Sunare expected to scatter on solar nuclei. Some of these collisions reduce the kinetic energy ofthe DM particle enough for it to become gravitationally bound to the Sun, causing it to returnon a bound orbit and undergo subsequent scattering, eventually thermalising and settling downto the solar core. If DM is able to annihilate, either with itself of with anti-DM captured in asimilar manner, high-energy SM particles will be produced in the solar core. Even if neutrinosare not amongst those particles produced in the annihilation hard process, they will still begenerated with quite high energies in the decay and subsequent interaction of other SM particleswith nuclei in the Sun. Unlike the other SM particles, these GeV-scale neutrinos are then ableto travel unhindered from the centre of the Sun to the surface, and across space to Earth, wherethey may be detected with terrestrial experiments.

The directionality of the signal is the primary means by which it can be distinguished fromthe atmospheric neutrino background, caused by cosmic ray interactions with the Earth’s at-mosphere. The only known background to the signal is therefore the analogous production ofhigh-energy neutrinos in the atmosphere of the Sun, due to interactions of cosmic rays with solarnuclei.

The capture of dark matter by the Sun typically becomes the rate-limiting step in the pro-duction of any signal, rather than the annihilation. Searches for high-energy neutrinos fromthe Sun are therefore most useful for constraining the interaction cross-section of dark matterwith nuclei. Spin-dependent interactions are particularly relevant, as the Sun consists mostly ofhydrogen, which possesses nuclear spin.

Current limits from neutrino telescope and direct searches for dark matter scattering areshown in Fig. 1. The IceCube neutrino telescope presently provides the leading sensitivity tospin-dependent scattering with protons at high DM masses1, whereas Super-Kamiokande2 andPICO-603 have the leading sensitivity at low masses. ANTARES4 also provides complementaryconstraints. Direct searches lead the way for spin-independent interactions, and spin-dependentinteractions with neutrons.

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Figure 1 – Current limits on nuclear scattering of dark matter from neutrino telescopes and direct detection. Thetop row shows limits from the leading neutrino telescopes on both the spin-dependent scattering cross-sectionwith protons, and the spin-independent cross-section (with any nucleon). The lower row compares limits on thespin-dependent interactions with protons and neutrons, illustrating the role of various direct detection and colliderexperiments, as well as the evolution of corresponding supersymmetric theory predictions over time. Figures fromIceCube1 (top row) and LUX5 (bottom row).

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Figure 2 – Current limits on spin-dependent nuclear scattering of DM, compared to older calculations of theneutrino floor for neutrino searches toward the Sun; more accurate calculations of the floor are now available9,10,but give broadly similar results. Figure from Ng et al.11

2 Improved background calculations

Previous predictions of the background rate of high-energy neutrinos from the Sun, due tointeractions of cosmic rays with nuclei in the solar atmosphere, were computed more than adecade ago.6,7,8 However, two more recent recalculations have appeared.9,10 Compared to theolder predictions, the new calculations make use of modern knowledge on neutrino oscillations,production and interaction cross-sections. One of these10 also makes use of up-to-date modelsof the solar composition and structure, and carries out extensive Monte Carlo simulations ofneutrino production, interaction and oscillation. Both studies (and another at the same time,based on the old flux estimates11) show that the solar atmospheric background lies barely an orderof magnitude below current sensitivity limits for some models (Fig. 2). This suggests that futureneutrino telescopes might be able to directly measure this irreducible ‘neutrino floor’, and thatthe improved calculations of the background rates should be included in future phenomenologicalstudies of DM scattering and annihilation in the Sun.

3 Recasting efforts

One of the major difficulties in interpreting the results of neutrino searches for DM in the Sunis the model-dependence of most published limits. This arises from the fact that the signal pre-diction is highly model-dependent, as the capture rate, annihilation rate, annihilation branchingfractions and resulting neutrino spectrum predicted at Earth all enter the calculation of thepredicted signal in a significant way. Traditional presentations from experiments1,4,15 give limitson scattering cross-sections as a function of DM mass, under various limiting assumptions aboutthe dominant annihilation channel. Recently, a more general and flexible method for presentingthe results of such searches has been developed16,12, allowing existing searches to be easily recastto provide detailed and consistent constraints on alternative DM models. This allows existingresults to be converted to limits on different annihilation channels than those assumed in theoriginal analysis (Fig. 3), and for them to be applied to much more complex models (Fig. 4),including arbitrary combinations of different annihilation final states and nuclear interactions(spin-dependent, spin-independent, and even more general forms). Public software exists to

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Figure 4 – 25-parameter supersymmetric benchmark models from Silverwood et al.13 and Cahill-Rowley et al.14,colour-coded according to how strongly disfavoured they are by the recast limit from the 79-string IceCube searchfor DM annihilation in the Sun. The borders of the grey regions show the recast limits for typical ‘hard’ (τ+τ−)and ‘soft’ (bb̄) spectra seen in supersymmetric models. Figure from IceCube.12

perform the recast (nulike: http://nulike.hepforge.org).This recastable form also has the advantage of providing substantial additional information

compared to the traditional presentation, as it provides event-level information on neutrino/muonarrival angles and energies, and a detailed approximation to the full likelihood form of theexperiment. This is particularly important when including searches for high-energy neutrinosfrom the Sun in global analyses of new physics scenarios such as supersymmetry. IceCube searcheshave been used in this format in the most recent reference global analyses of supersymmetricDM17,18 and scalar singlet DM19, done in the context of the GAMBIT project.20,21,22,23,24,25

4 Prospects for combined analysis and application to new models

Whilst the recastable form12 of the 79-string IceCube data15 is far more flexible and generallyuseful to the phenomenological community, there is some amount of overhead required to reducethe data to the necessary form. For this reason, recastable versions of the 86-string IceCubesearch1, and the latest results from ANTARES4 and Super-Kamiokande2 are not yet available.It is hoped that this will soon change. One significant driver for such a development is theprospect that the data of all three neutrino telescopes could be seamlessly combined, to givea single unified and strengthened limit. Indeed, as soon as each of the individual datasets isavailable in recastable form, the combination would be extremely straightforward to perfomvia the composite likelihood method. Including the combined constraint in global analyses ofsearches for new physics would be similarly straightforward.

A related recasting application will be to rigorously apply neutrino telescope limits to modelsof asymmetric DM that exhibit both symmetric and asymmetric components26, allowing strongconstraints to be placed on their asymmetry parameter r∞. Here the capture rates of such modelsneed to be carefully determined via a low-energy effective operator treatment27, and the fullrange of possible operators for both scattering and annihilation, along with their interferences,taken into account. The results of such an exercise will be especially interesting to compareto helioseismological and low-energy solar neutrino observables, given recent suggestions of apossible signal of DM from this sector. 28,29,30,31,32,33,a

Acknowledgments

I am supported by STFC (ST/K00414X/1, ST/P000762/1, ST/L00044X/1), and thank my co-authors on a number of the works discussed here.

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aSee also A. Vincent, these proceedings.

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13. H. Silverwood, P. Scott, et. al., Sensitivity of IceCube-DeepCore to neutralino dark matterin the MSSM-25, JCAP 3 (2013) 27, [arXiv:1210.0844].

14. M. W. Cahill-Rowley, J. L. Hewett, A. Ismail, M. E. Peskin, and T. G. Rizzo, pMSSMBenchmark Models for Snowmass 2013, arXiv:1305.2419.

15. IceCube Collaboration: M. G. Aartsen, R. Abbasi, et. al., Search for Dark MatterAnnihilations in the Sun with the 79-String IceCube Detector, Phys. Rev. Lett. 110(2013) 131302, [arXiv:1212.4097].

16. P. Scott, C. Savage, J. Edsjö, and the IceCube Collaboration: R. Abbasi et al., Use ofevent-level neutrino telescope data in global fits for theories of new physics, JCAP 11(2012) 57, [arXiv:1207.0810].

17. GAMBIT Collaboration: P. Athron, C. Balázs, et. al., Global fits of GUT-scale SUSYmodels with GAMBIT, Eur. Phys. J. C in press (2017) [arXiv:1705.07935].

18. GAMBIT Collaboration: P. Athron, C. Balázs, et. al., A global fit of the MSSM withGAMBIT, Eur. Phys. J. C in press (2017) [arXiv:1705.07917].

19. GAMBIT Collaboration: P. Athron, C. Balázs, et. al., Status of the scalar singlet darkmatter model, Eur. Phys. J. C 77 (2017) 568, [arXiv:1705.07931].

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21. GAMBIT Collider Workgroup: C. Balázs, A. Buckley, et. al., ColliderBit: a GAMBITmodule for the calculation of high-energy collider observables and likelihoods,Eur. Phys. J. C in press (2017) [arXiv:1705.07919].

22. GAMBIT Dark Matter Workgroup: T. Bringmann, J. Conrad, et. al., DarkBit: AGAMBIT module for computing dark matter observables and likelihoods, Eur. Phys. J. Cin press (2017) [arXiv:1705.07920].

23. GAMBIT Models Workgroup: P. Athron, C. Balázs, et. al., SpecBit, DecayBit andPrecisionBit: GAMBIT modules for computing mass spectra, particle decay rates andprecision observables, submitted to Eur. Phys. J. C (2017) [arXiv:1705.07936].

24. GAMBIT Flavour Workgroup: F. U. Bernlochner, M. Chrząszcz, et. al., FlavBit: AGAMBIT module for computing flavour observables and likelihoods, Eur. Phys. J. C inpress (2017) [arXiv:1705.07933].

25. GAMBIT Scanner Workgroup: G. D. Martinez, J. McKay, et. al., Comparison ofstatistical sampling methods with ScannerBit, the GAMBIT scanning module,Eur. Phys. J. C in press (2017) [arXiv:1705.07959].

26. K. Murase and I. M. Shoemaker, Detecting asymmetric dark matter in the Sun withneutrinos, Phys. Rev. D 94 (2016) 063512, [arXiv:1606.03087].

27. R. Catena and B. Schwabe, Form factors for dark matter capture by the Sun in effective

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105 (2010) 011301, [arXiv:1003.4505].29. M. Taoso, F. Iocco, G. Meynet, G. Bertone, and P. Eggenberger, Effect of low mass dark

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1 Current status2 Improved background calculations3 Recasting efforts4 Prospects for combined analysis and application to new models