Searching for spectral features in the g-ray sky. Alejandro Ibarra Technische Universität München

Transcription

1 Searching for spectral features in the g-ray sky Alejandro Ibarra Technische Universität München Oslo 5 November 2014

2 Outline Motivation Indirect dark matter searches with gamma-rays. Overcoming backgrounds Gamma-ray spectral features A simple model generating spectral features. Conclusions

3 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc Clusters of galaxies Mpc Observable Universe Gpc distance

4 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc Clusters of galaxies Mpc Observable Universe Gpc distance

5 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc M87 Clusters of galaxies Mpc Observable Universe Gpc distance

6 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc Clusters of galaxies Mpc Observable Universe Gpc distance Segue 1 (discovered by the SDSS in 2006)

7 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc Clusters of galaxies Mpc Abell 1689 Observable Universe Gpc distance

8 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc Clusters of galaxies Mpc Observable Universe Gpc distance

9 There is evidence for particle dark matter in a wide range of distance scales Solar system pc Galaxies kpc Clusters of galaxies Mpc Observable Universe Gpc distance The discovery of the dark matter was one (among the many) great discoveries in Physics of the 20th century. In fact, it was one of the first particles for which there was evidence: Electron - Thomson, 1897 Proton - Rutherford, 1919 Neutron - Chadwick, 1932 Positron Anderson, 1932 First evidence for dark matter - Zwicky, 1933

10 DARK MATTER???????

11 DARK MATTER?????? Goal for the 21st century: identify the properties of the dark matter particle?

12 Roszkowski

13 Roszkowski

14 WIMP dark matter SM SM annihilation scattering production

15 WIMP dark matter SM SM annihilation scattering production

16 WIMP dark matter SM SM annihilation scattering production Assuming that the dark matter particles were in thermal equilibrium with the SM in the Early Universe, their relic abundance reads:

17 WIMP dark matter SM SM annihilation scattering production Assuming that the dark matter particles were in thermal equilibrium with the SM in the Early Universe, their relic abundance reads: Correct dark matter abundance, h2 0.1, if

18 WIMP dark matter SM SM scattering production Assuming that the dark matter particles were in thermal equilibrium with the SM in the Early Universe, their relic abundance reads: annihilation ~ weak interaction Correct dark matter abundance, h2 0.1, if

19 WIMP dark matter SM SM scattering production Assuming that the dark matter particles were in thermal equilibrium with the SM in the Early Universe, their relic abundance reads: annihilation ~ weak Correct dark matter abundance, h2 0.1, if interaction SM SM (provided )

20

21 Dark matter searches with gamma-rays e n p g

22 Dark matter searches with gamma-rays e n p g Expected gamma-ray flux in a given direction: Source term (particle physics) Line-of-sight integral (astrophysics)

23 Dark matter searches with gamma-rays e n p g Expected gamma-ray flux in a given direction: Source term (particle physics) Line-of-sight integral (astrophysics) Which s v? A well motivated choice: As required by thermal production. First milestone for exclusion.

24 Problem for discovery: for typical channels and typical masses, the expected flux lies well below the background. E 2 GeV cm 2 s 1 sr bb m=500 GeV E GeV Do we understand backgrounds to the ~1% accuracy?

25 modelling of the diffuse emission Inverse Compton bremmstrahlung p0-decay

26 Great progress in understanding the diffuse gamma-ray emission, but unfortunately a detailed picture is still lacking. Always possible to use the gamma-ray data to set constraints on the dark matter properties (and should be done).

27 Great progress in understanding the diffuse gamma-ray emission, but unfortunately a detailed picture is still lacking. Always possible to use the gamma-ray data to set constraints on the dark matter properties (and should be done). However, to convincingly claim a dark matter signal it is necessary to convincingly subtract the astrophysical background.

28 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal

29 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal Kuhlen, Diemand, Madau

30 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Segue 1: Optical image

31 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Segue 1: Optical image

32 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Segue 1: Optical image

33 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Mass-to-light ratio ~ 3400 M /L Most -dominated object known so far! Segue 1: Optical image

34 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Segue 1: Gamma-ray image (simulated!)

35 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Gamma-ray image taken with the MAGIC telescopes

36 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies MAGIC coll. arxiv:

37 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies

38 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies Fermi-LAT coll. arxiv:

39 Overcoming backgrounds Strategy 1: Search for a gamma-ray excess with the spatial morphology expected from an annihilation signal A promising target for detection: dwarf galaxies B. Anderson Fermi Symposium October 2014

40 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Idea: dn/de Monochromatic signal at E=100 GeV E

41 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Idea: dn/de Assume power-law background E

42 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Idea: dn/de Total spectrum Fit data to E

43 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Data don't really look like a power law...

44 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Data don't really look like a power law... Signal concentrated in a narrow energy range

45 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Data don't really look like a power law... In a narrow energy window, the background resembles a power-law ( Taylor's theorem) Signal concentrated in a narrow energy range

46 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Data don't really look like a power law... Repeat the search with different windows postulating a signal at different masses. sliding energy window Signal concentrated in a narrow energy range

47 Overcoming backgrounds Strategy 2: Search for a gamma-ray excess with an energy spectrum qualitatively different from the background. Fermi. Coll. arxiv:

48 Overcoming backgrounds Strategy 3: Combine both methods. Search for gamma-ray spectral features in regions where it is most likely to find a signal. Traditional approach: select a geometrically simple region of the sky and search for features. e.g region b >10 plus a square centered at the Galactic Center (Fermi coll.)

49 Overcoming backgrounds Strategy 3: Combine both methods. Search for gamma-ray spectral features in regions where it is most likely to find a signal. Traditional approach: select a geometrically simple region of the sky and search for features. e.g region b >10 plus a square centered at the Galactic Center (Fermi coll.) Disadvantage: in the chosen region the background could be too large and bury the signal Instead, choose regions where, for a given dark matter profile, the signal-to-background ratio is maximized

50 Overcoming backgrounds Consider a generalized NFW profile Target regions which maximize the signal-to-background ratio: a=1.0 a=1.1 a=1.2 a=1.4 Bringmann, Huang, AI, Vogl, Weniger arxiv:

51 Bringmann, Huang, AI, Vogl, Weniger arxiv:

52 Bringmann, Huang, AI, Vogl, Weniger arxiv: Hint for a line-like gamma ray excess at 130 GeV! (with a local significance of more than 4s!) See also Weniger, arxiv: Su, Finkbeiner, arxiv: Tempel, Hektor, Raidal, arxiv:

53 Latest news on the 130 GeV excess Fermi-LAT collaboration arxiv: Local fit significance 2s global Significance reduced to 3.3s (1.6s with LEE) The 130 GeV excess was probably a statistical fluke

54 Latest news on the 130 GeV excess Fermi-LAT collaboration arxiv:

55 Latest news on the 130 GeV excess A. Albert Fermi Symposium October 2014

56 Gamma-ray spectral features in Particle Physics Three gamma-ray spectral features have been identified: Gamma ray line Srednicki, Theisen, Silk '86 Rudaz '86 Bergstrom, Snellman '88 Internal bremsstrahlung Bergstrom '89 Flores, Olive, Rudaz '89 Bringmann, Bergstrom, Edsjo '08 Gamma ray box AI, Lopez Gehler, Pato '12

57 Gamma-ray lines The dark matter particle is electrically neutral. The annihilation g g arises at the one loop level SM SM Monochromatic line Very distinctive spectrum!

58 Gamma-ray lines The dark matter particle is electrically neutral. The annihilation g g arises at the one loop level SM SM Monochromatic line Very distinctive spectrum! However, with a very suppressed rate:

59 Fermi-LAT collaboration arxiv:

60 Canonical value of sv Fermi-LAT collaboration arxiv:

61 Canonical value of sv Expected cross section Fermi-LAT collaboration arxiv:

62 Internal bremsstrahlung SM med SM Assume a model where the dark matter particle annihilates into Standard Model light particles via the interaction with a mediator in the t-channel

63 Internal bremsstrahlung Diagrams contributing to the process SM SM g: SM med g SM

64 Internal bremsstrahlung Diagrams contributing to the process SM SM g: SM med g SM

65 Internal bremsstrahlung Diagrams contributing to the process SM SM g: SM med g SM In the case m mmed the scalar propagator gets enhanced when ESM is small. Enhancement of the amplitude (and the rate) when Eg is close to the kinematic end-point.

66 Internal bremsstrahlung Bringmann, Huang, AI, Vogl, Weniger arxiv:

67 Internal bremsstrahlung Expected annihilation cross section for the 2 3 process. SM SM +...

68 Limits on the annihilation cross section from the Fermi-LAT data NFW NFW Bringmann, Huang, AI, Vogl, Weniger arxiv:

69 Limits on the annihilation cross section from the Fermi-LAT data Canonical value of sv Canonical value of sv NFW NFW Bringmann, Huang, AI, Vogl, Weniger arxiv:

70 Limits on the annihilation cross section from the Fermi-LAT data Canonical value of sv Canonical value of sv NFW Limit on the total annihilation cross section from dwarf galaxy observations = Number of photons with E=1-100 GeV Geringer-Sameth, Koushiappas, arxiv: NFW

71 Gamma-ray box Assume on shell production of an intermediate scalar, f.

72 Gamma-ray box Assume on shell production of an intermediate scalar, f. Assume that the scalar decays into two photons Photon spectrum in the rest frame of the scalar

73 Gamma-ray box Assume on shell production of an intermediate scalar, f. Assume that the scalar decays into two photons Photon spectrum in the rest frame of the scalar Photon spectrum in the galactic frame

74 box-shaped spectrum m=100 GeV mf=90 GeV mf=70 GeV

75 New aspect: the spectral feature arises from a tree level 2 2 annihilation. The strength of the signal could be unsuppressed, depending on the cross section ff and BR(f gg). AI, Lopez Gehler, Pato arxiv:

76 AI, Lee, Lopez Gehler, Park, Pato arxiv:

77 Expected if BR=1 Expected if BR=1 AI, Lee, Lopez Gehler, Park, Pato arxiv:

78 Gamma-ray spectral features in Particle Physics Recapitulation Smoking gun for dark matter: no (known) astrophysical process can produce a sharp feature in the gamma-ray energy spectrum Three gamma-ray spectral features have been identified: Gamma ray line Internal bremsstrahlung Gamma ray box

79 Gamma-ray spectral features in Particle Physics Recapitulation Smoking gun for dark matter: no (known) astrophysical process can produce a sharp feature in the gamma-ray energy spectrum Three gamma-ray spectral features have been identified: Gamma ray line Internal bremsstrahlung Gamma ray box

80 Gamma-ray spectral features in Particle Physics Recapitulation Smoking gun for dark matter: no (known) astrophysical process can produce a sharp feature in the gamma-ray energy spectrum Three gamma-ray spectral features have been identified: Gamma ray line Internal bremsstrahlung Gamma ray box Rather suppressed rates... Could the observation of spectral features be precluded by other experiments?

81 A simplified model generating spectral features Consider a toy model consisting on a Majorana dark matter particle, c, an intermediate charged scalar particle, h, and a light SM fermion, f. Interaction Lagrangian: Simple model, but rich phenomenology Gamma-ray spectral features Antimatter production in annihilations. Signals at direct detection experiments. Signals at colliders. High energy neutrinos from the Sun.

82 Limits on the model parameters from XENON100 and from the LHC translate into limits on the production rate of spectral features. Garny, AI, Pato, Vogl arxiv:

83 The limits from XENON100 and from the LHC are weaker for leptophilic models. Garny, AI, Pato, Vogl arxiv:

84 Conclusions The indirect search for dark matter is hindered by the existence of large (and still poorly understood) astrophysical backgrounds. In order to claim a dark matter signal, it is necessary to devise strategies to suppress the backgrounds. A promising approach consists in searching for sharp features in the gamma-ray spectrum. No known astrophysical process can produce such a signal in the 100 GeV - TeV range smoking gun for detection. From the particle physics side, spectral features are predicted in simple models. The predicted rates are usually fairly small. Other observations already constrain the possibility of observing gamma-ray spectral features. Important to assess the prospects to observe a signal in future experiments.

85 Conclusions The indirect search for dark matter is hindered by the existence of large (and still poorly understood) astrophysical backgrounds. In order to claim a dark matter signal, it is necessary to devise strategies to suppress the backgrounds. A promising approach consists in searching for sharp features in the gamma-ray spectrum. No known astrophysical process can produce such a signal in the 100 GeV - TeV range smoking gun for detection. From the particle physics side, spectral features are predicted in simple models. The predicted rates are usually fairly small. Other observations already constrain the possibility of observing gamma-ray spectral features. Important to assess the prospects to observe a signal in future experiments. Thank you for your attention!