Searching for dark matter annihilation lines with HESS II. Knut Dundas Morå for the HESS collaboration

Transcription

1 Searching for dark matter annihilation lines with HESS II Knut Dundas Morå for the HESS collaboration

2 Outline Indirect Dark Matter detection Dark Matter signatures Dark Matter distributions Fermi Line Search The HESS Experiment HESS 1 Line Search HESS 2 Line Telescope Background techniques Instrument Response Line search Conclusions 2

3 Indirect Dark Matter of neutralinos toby gamma-rays Annihilation of neutralinos Annihilation to gamma-rays Predicted SUSY modelspredicted by Signatures mm χ χ In the case of a direct two-body annihilation to one or two 25/06/2014 Matthieu25/06/2014 Kieffer photons, a distinctive spectral peak would appear Gamma-ray signal from DM Background mχ 25/06/2014 mχmχ Gamma-ray signal from DM Background mχ Gamma-ray signal from DM Background The particle physics factor contains the thermally averaged cross-section, as well as the energy spectrum of dark matter to gamma rays The expected flux of gamma rays d d particle (E, ) = astro ( ) (E) from dark matter annihilation may de de be factorized into an d particle < v > dn astrophysical and particle physics = by SUSY(E) Annihilation of neutralinos to(e) gamma-rays Predicted models factor. 2 de 8 m de MatthieuTeVPA/IDM Kieffer Amsterdam TeVPA/IDM Co 2 Amsterdam Conference mχ 3 Matthieu Kieffer Amsterdam TeVPA/IDM Conference mχ

4 Dark matter distribution The astrophysical factor contains the line-of-sight integral of the squared dark matter density. astro( ) = Z d Z Line of sight 2 (r)dl Many models of dark matter densities are peaked towards the center- leading to a large signal from the center of a dark matter halo. Typical targets are either high signal sources, such as the galactic center, or have a large fraction of inferred dark matter versus ordinary matter- such as dwarf galaxies. Via Lactea n-body simulation of a galaxy NFW = 0 r R s (1 + r R s ) 2 4

5 The Fermi Line An analysis by Weniger of Fermi- LAT data(arxiv: v2) showed a line feature in the spectrum at 130 GeV from the galactic center, with a 3.2σ significance. arxiv: v2 [hep-ph] 8 Aug 2012 A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope Christoph Weniger Max-Planck-Institut für Physik, Föhringer Ring 6, München, Germany weniger@mppmu.mpg.de Abstract. The observation of a gamma-ray line in the cosmic-ray fluxes would be a smokinggun signature for dark matter annihilation or decay in the Universe. We present an improved search for such signatures in the data of the Fermi Large Area Telescope (LAT), concentrating on energies between 20 and 300 GeV. Besides updating to 43 months of data, we use anewdata-driventechniquetoselectoptimizedtargetregions depending on the profile of the Galactic dark matter halo. In regions close to the Galactic center, we find a 4.6σ indication for a gamma-ray line at E γ 130 GeV. When taking into account the lookelsewhere effect the significance of the observed excess is 3.2σ. Ifinterpretedintermsof dark matter particles annihilating into a photon pair, the observations imply a dark matter mass of m χ =129.8 ± GeV and a partial annihilation cross-section of σv χχ γγ = ( 1.27 ± ) 27 cm 3 s 1 when using the Einasto dark matter profile. The evidence for the signal is based on about 50 photons; it will take a few years of additional data to clarify its existence. The feature corresponded to a cross section of <σv>=1.27e-27 cm^3/s (NFW) Su and Finkbeiner (arxiv: ) located the signal at 1.5 west of the galactic center, and found a 5σ detection. The significance has been seen to be decreasing with more data. Figure 4. Upper sub-panels: the GeV measured events with statistical erro horizontal bars show the best-fit models with (red ) and without DM ( indicates the corresponding Nγ= line TS=36.11 flux component alone. In the Nγ= lower.31 s after subtracting the model with line contribution. Note that we rebin 15 after performing the fits in order to produce the plots and calculate th χ 2 r χ2 /dof. The counts are listed in Tabs. 1, 2 and Gal l [deg] GeV Gal l [ Nγ= TS=28.42 Nγ=

6 The HESS telescope The High Stereoscopic System is an Air Cherenkov Telescope located in the Namibian Khomas Highlands. Four -meter telescopes collect Cherenkov light of showers from high energy particles impacting the atmosphere The HESS array detect gamma rays from 200GeV to >30TeV for energy spectrum analysis. 6

7 The HESS 1 Line search v1 A HESS search for a spectral line was published in 2012 For a fixed line mass, a gaussian with the width of the energy dispersion was fit to the data together with a modified power law using a binned likelihood: Signal Exclusion Gaussian(μ x,σ x ) sr -1 ) s -1 (TeV 1.7 m -2-3 Bckg. P(x) Bckg. G(x) Bckg. sum simulated line (E γ = 2 TeV) dn/de γ E γ bins Expected number of counts in bin i Number of reconstructed counts in bin i -5 1 E γ (TeV) 7 FIG. 1. Reconstructed flux spectrum of the CGH region, us-

8 HESS 1 results v1 The HESS collaboration published a limit based on 112 hours of data. Below, the computed limit from HESS (red points) and Fermi-LAT are shown. /s) 3 (95% CL) (cm HESS Einasto Fermi-LAT Einasto <σv> χ χ γ γ m χ (TeV) 8

9 The HESS 2 telescope The HESS 2 telescope is an additional, larger (28m) telescope located in the center of the HESS array The larger telescope (6X the area) collects more light from fainter air showers, allowing HESS 2 to reach lower energies- below 0 GeV for a spectrum analysis. The camera features 2000 photomultipliers, which may be read out at 3600 images/secondten times the speed of HESS1 This allows HESS 2 to complement the Fermi line search. 9

10 Cherenkov telescope background estimation The largest background in Cherenkov telescopes is cosmic rays that are misidentified as gamma rays As the atmosphere, as well as the background from e.g. star and moonlight changes, it is necessary to have background (OFF) regions. True ON-OFF OFF ON OFF RA One may either define separate OFF pointings, or point the telescope at an offset, and define on and off regions in the camera field of view The first method allows for larger signal regions, up to the size of the FOV, at the expense of using 2/3 of the time off the signal region FOV Signal Background Wobble

11 Computing Sensitivities A simplified sensitivity procedure has been made to a full likelihood method. The method uses sidebands both spatially, as well as in energy to compute an estimate of the background. log_(e) -1.1 to -1-1 to to 0 ON OFF Lower Sideband Signal Region FOV ON OFF Background Region Upper Sideband 11

12 Computing Sensitivities ON-OFF/Beta + fit In the energy sidebands, the program fits a power-law (convolved with the instrument response) to any broad-spectrum excess. ON-OFF/Beta 2 Together with the OFF-counts in the energy search region, an estimate of the expected number of events in the signal region is computed: log_(e) -1.1 to -1-1 to to 0 ON OFF log(e) Lower Sideband Signal Region Background Region Upper Sideband n ON bkg estimate = n ON OFF fit (E search )+n OFF bkg (E search )/ estimate = q 2 ON OFF fit + n OFF (E search / 2 12

13 The Model (Mono) Reconstruction Plots from H.E.S.S. Highlights TevPa talk by Christian Stegeman Reconstructing shower events from a single telescope image is a challenge HESS-2 results have so far been produced with Model Mono The Model reconstruction uses a semi-analytical shower model, that is then fitted to the image 13

14 IRF Plots from H.E.S.S. Highlights TevPa talk by Christian Stegeman The efficiency of HESS is expressed as the effective area it presents to gamma-rays HESS-2 is expected to give a marked improvement in the energy threshold from ~300GeV, down to ~80 GeV. ] 2 Effective area [m 5 H.E.S.S. Preliminary In addition to Mono and HESS-1 effective area curves, a hybrid reconstruction, with CT5 plus one or more of the smaller telescopes for a stereoscopic reconstruction. 4 CT5 (standard analysis) CT5 (pulsar/grb analysis) CT1-4 (standard analysis) MAGIC I mono (Albert et al. (2008)) log (E [TeV]) true 14

15 IRF Plots from H.E.S.S. Highlights TevPa talk by Christian Stegeman In addition to the acceptance, the bias (right) and dispersion in energy need to be taken into account. Spatial resolution (lower right). Radial acceptance- the reduction in effective area with increasing distance to the center of the telescope Lastly, all instrument response functions are a function of pointing, zenith angle and night-sky background 15

16 The HESS 2 Line search R.O.I. : Fermi Line region (l = -1.5 ), θroi = 0.25 Sum over data events in the energy range [80GeV ; 1TeV] Probability density functions Observation time : 50 h Unbinned approach, based spectral MC on events : shapes A new line search using the HESS Number of line events derived from Fermi 130GeV line flux Minimization of the -2ln(L) function<σ v>=2.3 cm.s ) (C. Weniger, 2 telescope is underway Reconstruction of the Line energy and Line rate η ann Line signal (130 reconstruct: η = S/(S+B) GeV) % emission According to H.E.S.S. 2 MonoDiffuse reconstruction (preliminary) Since 2013, data from the galactic Hadrons Use of test statistic estimator TS (TeV) center has been collected. A total of ~670 hours are available for signal : Identify a line-like data in- LPNHE the presence of14 18/04/2014 Matthieu from Kieffer H.E.S.S.2 Réunion du Vendredi observation at2zenith angles below different types of background 35 Reconstruction works well Line energy (TeV) Lineforeseen) rate to (+ fit of background components * Hadrons (profile known from control sample or MC) Two main sources of background: reconstruction Diffuse gamma ray emission in the galactic plane (profile from Line (few % errors, <TS> 40) may be forestimated rates > 2% from hadrons thatpossible Expected signal on [80 GeV ; 0 GeV] off-regions Detection (5σ) for rates > as 7% Diffuse sources of gamma-rays, Gaussian line (smeared by response functions), 130GeV expec Procedure repeated 00 times well as contamination from known η sources 25/06/2014 Matthieu Kieffer Amsterdam TeVPA/IDM Conference 15 Probability Sum over data events in the energy range [80GeV ; 1TeV] 16

17 Conclusions The HESS 2 line paper will extend the HESS limits to lower energies. Based on Monte Carlo and OFF data, the choices of ROI are made to minimize the expected upper limit. The HESS data comprises galactic center data, as well as data on the Fermi Hotspot. There are around 600 hours of possible dark-time for the Galactic Center every summer, which is attenuated by both the instrument uptime and weather. In addition, this is the most requested observation period The analysis has an expected significance of over 5 sigma for a benchmark signal with the mass and flux as reported by Weniger. 17