The Fermi Large Area Telescope is providing the measurement of the high energy (20 GeV to 1TeV) cosmic-ray electrons and positrons spectrum with unprecedented accuracy. This measurement represents a unique probe for studying the origin and diffusive propagation of cosmic rays as well as for looking for possible evidences of dark matter. In this framework, we discuss possible interpretations of Fermi results in relation with other recent experimental data on energetic electrons and positrons and in the searches of gamma-ray fluxes coming from WIMP pair annihilations in the sky

Morselli, A.

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DOI 10.1393/ncc/i2010-10604-3Colloquia: LC09IL NUOVO CIMENTO Vol. 33 C, N. 2 Marzo-Aprile 2010The quest for dark matter with the Fermi experimentA. MorselliINFN, Sezione di Roma Tor Vergata - Rome, Italy(ricevuto il 17 Febbraio 2010; approvato il 25 Marzo 2010; pubblicato online il 18 Giugno 2010)Summary. — The Fermi Large Area Telescope is providing the measurement ofthe high energy (20GeV to 1TeV) cosmic-ray electrons and positrons spectrum withunprecedented accuracy. This measurement represents a unique probe for studyingthe origin and diffusive propagation of cosmic rays as well as for looking for possibleevidences of dark matter. In this framework, we discuss possible interpretations ofFermi results in relation with other recent experimental data on energetic electronsand positrons and in the searches of gamma-ray fluxes coming from WIMP pairannihilations in the sky.PACS 96.50.sb – Composition, energy spectra and interactions.PACS 95.35.+d – Dark matter (stellar, interstellar, galactic, and cosmological).PACS 95.85.Ry – Neutrino, muon, pion, and other elementary particles; cosmicrays.PACS 98.70.Sa – Cosmic rays (including sources, origin, acceleration, and interac-tions).1. – DescriptionRecently the experimental information available on the Cosmic Ray Electron (CRE)spectrum has been dramatically expanded as the Fermi-LAT Collaboration [1, 2] hasreported a high-precision measurement of the electron spectrum from 20GeV to 1TeVperformed with its Large Area Telescope (LAT) [3]. The spectrum shows no prominentspectral features and it is significantly harder than that inferred from several previousexperiments. These data together with the PAMELA data on the rise above 10GeV ofthe positron fraction [4] are quite difficult to explain with just secondary production [5-7].The temptation to claim the discovery of dark matter is strong but there are competingastrophysical sources, such as pulsars, that can give strong flux of primary positrons andelectrons (see [8-11] and references therein). At energies between 100GeV and 1TeV theelectron flux reaching the Earth may be the sum of an almost homogeneous and isotropiccomponent produced by Galactic supernova remnants and the local contribution of a fewpulsars with the latter expected to contribute more and more significantly as the energyincreases.c© Societa` Italiana di Fisica 4950 A. MORSELLIFig. 1. – PAMELA data and a possible contribution from Monogem and Geminga pul-sars [11]. Black-dotted line shows the background from secondary positrons in cosmic raysfrom GALPROP.Two pulsars, Monogem, at a distance of d = 290 pc and Geminga, at a distance ofd = 160 pc, can give a significant contribution to the high energy electron and positronflux reaching the Earth and with a set of reasonable parameters of the model of electronproduction we can have a nice fit of the PAMELA positron fraction [4] and Fermi data (seefigs. 1 and 2), but it is true that we have a lot of freedom in the choice of these parametersbecause we still do not know much about these processes, so further study on high energyemission from pulsars is needed in order to confirm or reject the pulsar hypothesis.Fig. 2. – (Colour on-line) Electron-plus-positron spectrum (blue continuous line) for the samescenario as in fig. 1. The gray band represents systematic errors on the Fermi-LATdata [3].THE QUEST FOR DARK MATTER WITH THE FERMI EXPERIMENT 51Fig. 3. – (Colour on-line) The parameter space of particle dark matter mass vs. pair-annihilationrate, for models where dark matter annihilates into monochromatic e±. Models inside theregions shaded in gray and cyan over-produce e± from dark matter annihilation with respect tothe Fermi-LAT and H.E.S.S. measurements, at the 2-σ level. The red and blue contours outlinethe regions where the χ2 per degree of freedom for fits to the PAMELA and Fermi-LAT data isat or below 1.Nevertheless a dark-matter interpretation of the Fermi-LAT and of the PAMELAdata is still an open possibility. In fig. 3 we show the parameter space of particle darkmatter mass vs. pair-annihilation rate, for models where dark matter annihilates intomonochromatic e± [11]. The preferred range for the dark matter mass lies between400GeV and 1–2TeV, with larger masses increasingly constrained by the H.E.S.S. results.The required annihilation rates, when employing the dark matter density profile implytypical boost factors ranging between 20 and 100, when compared to the value 〈σv〉 ∼3× 10−26 cm3/s expected for a thermally produced dark matter particle relic.How can one distinguish between the contributions of pulsars and dark matter anni-hilations? Most likely, a confirmation of the dark matter signal will require a consistencybetween different experiments and new measurements of the reported excesses with largestatistics. The observed excess in the positron fraction should be consistent with cor-responding signals in absolute positron and electron fluxes in the PAMELA data andall lepton data collected by Fermi. Fermi has a large effective area and long projectedlifetime, 5 years nominal with a goal 10 years mission, which makes it an excellent de-tector of cosmic-ray electrons up to ∼ 1TeV. Future Fermi measurements of the totallepton flux with large statistics will be able to distinguish a gradual change in slope witha sharp cutoff with high confidence [12]. The latter can be an indication in favor of thedark matter hypothesis. A strong leptonic signal should be accompanied by a boost inthe γ-ray yield providing a distinct spectral signature detectable by Fermi.The Galactic Center (GC) is expected to be the strongest source of γ-rays fromDM annihilation, due to its coincidence with the cusped part of the DM halo density52 A. MORSELLIEnergy (MeV)310 410counts/MeV−410−310−210−110110210Fig. 4. – (Colour on-line) Spectra from the likelihood analysis of the Fermi/LAT data (numberof counts vs. reconstructed energy) in a 7◦ × 7◦ region around the Galactic Center (number ofcounts vs. reconstructed energy). The likelihood analysis is the standard one used with the LATdata. The main analysis steps are: 1) to select data of high quality (selection cuts on eventsenergy, zenith angle, reconstruction and classification quality); 2) to build an emission modelof the region, based on the previous knowledge and experimental evidence of new excesses withenough statistical significance; 3) to apply the likelihood analysis to the data and the consideredmodel. For each model component a fit of the free parameters and the computation of thestatistical significance is obtained. Here in the plot, from above to below: the black points arethe observed data; the black line is the sum of all the components; the pink line is the Galacticdiffuse emission; the lower black line is the isotropic extragalactic; the other components are thesources detected. These results are preliminary.profile [13, 14]. A preliminary analysis of the data, taken during the first 11 months ofthe Fermi satellite operations, is shown in figs. 4 and 5.The reported results were obtained with a binned likelihood analysis, performed bymeans of the tools developed by the Fermi/LAT Collaboration (gtlike, from the Fermianalysis tools [15]). For this analysis:– a ROI of 7◦ × 7◦ was considered. This ROI was used in order to minimize thebackground contribution and to avoid significative leakage of the gamma-ray signalunder study.Fig. 5. – (Colour on-line) Residuals ((exp.data - model)/model) of the above likelihood analysis.The blue area shows the systematic errors on the effective area. These results are preliminary.THE QUEST FOR DARK MATTER WITH THE FERMI EXPERIMENT 53– The ROI was centred at the position RA = 266.46◦, Dec = −28.97◦, i.e. theposition of the brightest source.– The data taken during the first 11 months (8/2008-7/2009) have been used.– The events were selected to have energy between 400MeV and 100GeV.– Only events classified of “diffuse” class were and which converted in the front partof the tracker have been selected for the analysis. The selection in energy, eventclassification and conversion provided us with events with very well reconstructedincoming direction and data have been binned into a 100× 100 bins map.– The IRF and the events classification are those relative to the Pass6V3 version ofthe Fermi/LAT analysis software.In order to perform the likelihood analysis for the LAT data, a model of the alreadyknown sources and the diffuse background should be built. The model in use for thepresented analysis contains 11 sources in the Fermi 1 year catalog (to be published)which are located into or very close to the considered ROI. These sources have a point-like spatial model and a spectrum in the form of a power law. The model also containsthe diffuse gamma-ray background which is made up of two components:1) the Galactic diffuse gamma-ray background. The observed Galactic Diffuse emis-sion was modelled by means of the GALPROP code [16] and [17], and the realizationof the galactic emission named gll−iem−54−87Xexph7S.fit was used. During thelikelihood maximization only the normalization of the GALPROP model is varied,not its components;2) the Isotropic Background. This component should account for both the Extra-galactic gamma-ray emission and residual charged particles. It is modelled as anisotropic emission with a template spectrum.The diffuse gamma-ray backgrounds and discrete sources, as we know them today,can account for the large majority of the detected gamma-ray emission from the GalacticCenter. Nevertheless a residual emission is left, not accounted for by the above mod-els [18].Improved modelling of the Galactic diffuse model as well as the potential contributionfrom other astrophysical sources (for instance unresolved point sources) could provide abetter description of the data. Analyses are underway to investigate these possibilities.An excess in gamma-ray should also be seen in the Galactic diffuse spectrum. Figure 6(left) shows the LAT data averaged over all Galactic longitudes and the latitude range10◦ ≤ |b| ≤ 20◦. The hatched band surrounding the LAT data indicates the systematicuncertainty in the measurement due to the uncertainty in the effective area describedabove. Also shown on the right are the EGRET data for the same region of sky whereone can see that the LAT-measured spectrum is significantly softer than the EGRETmeasurement [19]. Figure 6 (right) compares the LAT spectrum with the spectra ofan a priori diffuse Galactic emission (DGE) model. While the LAT spectral shape isconsistent with the DGE model used in this paper, the overall model emission is toolow thus giving rise to a ∼ 10–15% excess over the energy range 100MeV to 10GeV.However, the DGE model is based on pre Fermi data and knowledge of the DGE. Thedifference between the model and data is of the same order as the uncertainty in themeasured CR nuclei spectra at the relevant energies. Overall, the agreement between54 A. MORSELLI10-310-2102 103 104E γ2 J γ (Eγ) (MeV cm-2 s-1  sr-1)Eγ (MeV)0° ≤ l ≤ 360°, 10° ≤ |b| ≤ 20°EGRETLAT10-410-310-2102 103 104E γ2 J γ (Eγ) (MeV cm-2 s-1  sr-1)Eγ (MeV)0° ≤ l ≤ 360°, 10° ≤ |b| ≤ 20°LATIsotropicSourcesπ0-decayICBremsstrahlungTotalFig. 6. – (Colour on-line) Left: preliminary diffuse emission intensity averaged over all Galacticlongitudes for latitude range 10◦ ≤ |b| ≤ 20◦. Data points: Fermi LAT, red dots; EGRET, bluecrosses. Systematic uncertainties: Fermi LAT, red; EGRET, blue. Right: preliminary FermiLAT data with model, source, and isotropic components for the same sky region.the LAT-measured spectrum and the model shows that the fundamental processes areconsistent with our data, thus providing a solid basis for future work understanding theDGE.Also at higher latitudes for the moment we did not observe any excess. In fig. 7 weshow the diffuse γ-rays in a mid-latitude region in the third quadrant (Galactic longitudeFig. 7. – Differential γ-ray emissivity from the local atomic hydrogen gas compared with thecalculated γ-ray production. The horizontal and vertical error bars indicate the energy rangesand 1 σ statistical errors, respectively. Estimated systematic errors of the LAT data are indicatedby the shaded area. A nucleus enhancement factor M of 1.84 is assumed for the calculation ofthe γ-rays from nucleon-nucleon interactions. Dotted lines indicate the emissivities for the caseof M = 1.45, the lowest values in the referenced literature.THE QUEST FOR DARK MATTER WITH THE FERMI EXPERIMENT 55Table I. – Flux (in 10−9 cm−2s−1), annihilation cross-section upper limits, and decay lifetimelower limits: γ-ray energies measured and corresponding 95% c.l. upper limits (CLUL) on fluxes,for |b| > 10◦ plus a 20◦×20◦ square around the Galactic center. For each energy and flux limit,〈σv〉γγ and 〈σv〉γZ upper limits, and τγγ and τγZ lower limits are given for three Galactic darkmatter distributions (see text). The systematic error in the absolute energy of the LAT discussedin the text propagates to a −20% + 10% systematic error on 〈σv〉γγ , while for the decay lowerlimits the systematic error in the absolute energy of the LAT discussed in the text propagates toa +10%− 5% systematic error on τγγ .Eγ 95%CLUL 〈σv〉γγ [γZ] (10−27 cm3s−1) τγγ [γZ] (1028 s)(GeV) NFW Einasto Isothermal NFW Einasto Isothermal30 3.5 0.3 [2.6] 0.2 [1.9] 0.5 [4.5] 17.6 [4.2] 17.8 [4.2] 17.5 [4.2]40 4.5 0.7 [4.2] 0.5 [3.0] 1.2 [7.2] 10.1 [2.9] 10.3 [2.9] 10.0 [2.9]50 2.4 0.6 [2.7] 0.4 [1.9] 1.0 [4.6] 15.5 [5.0] 15.7 [5.1] 15.4 [5.0]60 3.1 1.1 [4.2] 0.8 [3.0] 1.8 [7.3] 9.8 [3.5] 10.0 [3.5] 9.7 [3.5]70 1.2 0.6 [2.0] 0.4 [1.4] 1.0 [3.4] 21.6 [8.2] 21.9 [8.3] 21.5 [8.1]80 0.9 0.5 [1.7] 0.4 [1.2] 0.9 [2.9] 26.0 [10.4] 26.4 [10.5] 25.8 [10.3]90 2.6 2.0 [6.0] 1.5 [4.3] 3.5 [10.3] 7.7 [3.2] 7.8 [3.2] 7.6 [3.1]100 1.4 1.4 [3.8] 1.0 [2.8] 2.4 [6.6] 12.6 [5.4] 12.8 [5.4] 12.5 [5.3]110 0.9 1.0 [2.7] 0.7 [1.9] 1.7 [4.6] 18.9 [8.2] 19.2 [8.3] 18.8 [8.2]120 1.1 1.6 [4.0] 1.1 [2.9] 2.7 [6.9] 13.3 [5.9] 13.5 [6.0] 13.2 [5.9]130 1.8 3.0 [7.3] 2.1 [5.3] 5.1 [12.6] 7.6 [3.4] 7.8 [3.5] 7.6 [3.4]140 1.9 3.5 [8.4] 2.5 [6.0] 6.0 [14.3] 7.0 [3.2] 7.1 [3.3] 7.0 [3.2]150 1.6 3.5 [8.2] 2.5 [5.9] 6.0 [14.1] 7.5 [3.5] 7.6 [3.5] 7.4 [3.4]160 1.1 2.7 [6.3] 2.0 [4.5] 4.7 [10.9] 10.2 [4.8] 10.4 [4.8] 10.1 [4.7]170 0.6 1.7 [4.0] 1.3 [2.9] 3.0 [6.8] 17.0 [8.0] 17.2 [8.1] 16.9 [7.9]180 0.9 2.7 [6.1] 1.9 [4.4] 4.6 [10.4] 11.6 [5.5] 11.8 [5.6] 11.6 [5.4]190 0.9 3.2 [7.1] 2.3 [5.1] 5.5 [12.2] 10.4 [4.9] 10.5 [5.0] 10.3 [4.9]200 0.9 3.3 [7.3] 2.4 [5.2] 5.7 [12.5] 10.6 [5.1] 10.8 [5.1] 10.5 [5.0]l from 200◦ to 260◦ and latitude |b| from 22◦ to 60◦). The region contains no known largemolecular cloud and most of the atomic hydrogen is within 1 kpc of the solar system.The contributions of γ-ray point sources and inverse Compton scattering are estimatedand subtracted. The residual γ-ray intensity exhibits a linear correlation with the atomicgas column density in energy from 100MeV to 10GeV. The differential emissivity from100MeV to 10GeV agrees with calculations based on cosmic-ray spectra consistent withthose directly measured, at the 10% level. The results obtained indicate that cosmic raynuclei spectra within 1 kpc from the solar system in regions studied are close to the localinterstellar spectra inferred from direct measurements at the Earth within ∼ 10% [20].Finally a line at the WIMP mass, due to the 2γ production channel, could be observedas a feature in the astrophysical source spectrum [12]. Such an observation is a “smokinggun” for WIMP DM as it is difficult to explain by a process other than WIMP annihilationor decay and the presence of a feature due to annihilation into γZ in addition would beeven more convincing.Up to now however no lines was observed and we obtain γ-ray line flux upper limits inthe range 0.6−4.5×10−9 cm−2s−1, and give corresponding DM annihilation cross-sectionand decay lifetime limits shown in table I [21].56 A. MORSELLI2. – ConclusionRecent accurate measurements of cosmic-ray positrons and electrons by PAMELA,and Fermi have opened a new era in particle astrophysics. The CRE spectrum measuredby Fermi-LAT is significantly harder than previously thought on the basis of previousdata. Adopting the presence of an extra e± primary component with ∼ 2.4 spectral indexand Ecut ∼ 1TeV allows to consistently interpret Fermi-LAT CRE data (improving thefit), HESS and PAMELA. Such extra-component can be originated by pulsars for areasonable choice of relevant parameters or by annihilating dark matter for model withMDM ∼ 1TeV. Improved analysis and complementary observations (CRE anisotropy,spectrum and angular distribution of diffuse γ, DM sources search in γ) are requiredto possibly discriminate the right scenario. Their exotic origin has to be confirmed bycomplimentary findings in γ-rays by Fermi and atmospheric Cherenkov telescopes, andby LHC in the debris of high-energy proton destructions. A positive answer will be amajor breakthrough and will change our understanding of the universe forever. On theother hand, if it happens to be a conventional astrophysical source of cosmic rays, it willmean a direct detection of particles accelerated at an astronomical source, again a majorbreakthrough. In this case we will learn a whole lot about our local Galactic environment.However, independently of the origin of these excesses, exotic or conventional, we canexpect very exciting several years ahead of us.REFERENCES[1] Atwood W. B. et al. (Fermi Collaboration), Astrophys. J., 697 (2009) 1071[arXiv:0902.1089].[2] Abdo A. A. et al. (Fermi Collaboration), Astropart. Phys., 32 (2009) 193[arXiv:0904.2226][3] Abdo A. A. et al. (Fermi Collaboration), Phys. Rev. Lett., 102 (2009) 181101[arXiv:0905.0025].[4] Adriani O. et al. (PAMELA Collaboration), Nature, 458 (2009) 607 [arxiv:0810.4995].[5] Strong A. W. and Moskalenko I. V., Astrophys. J., 509 (1998) 212; 493 (1998) 694.[6] Lionetto A., Morselli A. and Zdravkovic V., JCAP, 09 (2005) 010 [astro-ph/0502406]; Ptuskin V. S. et al., Astrophys. J., 642 (2006) 902.[7] Morselli A. and Moskalenko I., PoS (idm2008)025 [arXiv:0811.3526].[8] Boulares A., Astrophys. J., 342 (1989) 807.[9] Aharonian F. A., Atoyan A. M. and Vo¨lk H. J., Astron. 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The quest for dark matter with the Fermi experiment

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The quest for dark matter with the Fermi experiment

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