Detecting dark matter annihilation at the ground EAS detectors X.J. Bi (IHEP)

Candidates of the cold dark matter There are dozens of theoretical models in the literature Weakly Interacting Massive Particles (WIMPs) as thermal relics of Big Bang is a natural candidate of CDM. such as neutralinos, KK states, Mirror particles … The WIMP miracle: for typical gauge couplings and masses of order the electroweak scale,  wimp h 2  0.1 (within factor of 10 or so)

Thermal history of the WIMP (thermal production) At T >> m, At T < m, At T ~ m/20, decoupled, relic density is inversely proportional to the interaction strength The relic density of dark matter is deter- mined by solving the Boltzmann equation. For the weak scale interaction and mass scale (non-relativistic dark matter particles), if and WIMP is a natural dark matter candidate giving correct relic density. Thermal equilibrium abundance

Detection of WIMP Direct detection of WIMP at terrestrial detectors via scattering of WIMP of the detector material. Indirect detection looks for the annihilation products of WIMPs, such as the neutrinos, gamma rays, positrons at the ground/space-based experiments   Direct detection  p e+e+  _ indirect detection

Indirect detection Flux is determined by the products of two factors The first factor is the strength of the interaction, determined completely by particle physics The second factor is by the distribution of DM The factor is enhanced at the clumps of DM, such as at the GC, subhalos, or at the core of Sun and Earth. The flux depends on both the astrophysics and the particle aspects.

Effects of non-thermal production Large annihilation cross section Help to solve the HEAT, EGRET exotic signal by DM annihilation, while the annihilation signal is too small if they are produced thermally. Large tan  : m  ~ m A,H /2 ~ 10101010 ~ 10101010 A,H l/q l/q  ~ 1/  ~ 1/ ~ 1/(4m  2 – m A,H 2 )2 too big ~ 1/(4m  2 – m A,H 2 )2 too big   too small Region for non- thermal prod Lin et al., PRL86, 954 (2001)

Enhancement by clumpy dark matter The fluxes of the annihilation products are proportional to the annihilation cross section and the DM density square. Fluxes are greatly enhanced by clumps of DM. The Galactic center and center of subhalos have high density.  There are 5%~10% DM of the total halo mass are enclosed in the clumps.  The following characters make subhalos more suitable for DM detection: GC is heavily contaminated by baryonic processes. Structures in CDM from hierarchically, i.e., the smaller objects form earlier and have high density. Subhalos may be more cuspy profile than the GC. Mass is more centrally concentrated when an object is in an environment with high density.

Distribution of the subhalos N-body simulation (MNRAS352,535 (2004) ) gives the probability for a subhalo of the mass m and at the position r with M, host mass, r cl =0.14r virial andα ＝－ 1.9 The tidal effect will strip the particles beyond a tidal radius, We get the distribution as

Profiles of the subhalos Two generally adopted DM profiles are the Moore and NFW profiles They have same density at large radius, while different slope as r->0 NFW: Moore:

Concentration parameter of subhalos The are determined by the virial mass and concentration parameter.For larger C, the DM is more centrally concentrated. A semi-analytic model: the collapse epoch is determined by the collapsing time of a fraction of the object mass, σ(M * =FM)=δ sc ; The concentration parameter is determined by another free parameter c(M,z)=K(1+z c )/(1+z). We have taken a standard scale invariant spectrum and the cosmological parameter as in the figure. From the figure, the concentration parameter decreases with the virial mass.

 -rays from the subhalos Reed et al, MNRAS35 7,82(2004)  -rays from subhalos  -rays from smooth bkg source sunGC 

 -ray sources from the subhalos Bullock et al., MNRAS32 1,559(2001)  -rays from smooth bkg  -rays from subhalos

Statistical result The curves are due to different author’s simulations. The threshold is taken as 100 GeV. The susy factor is taken an optimistic value for neutralino mass between 500 GeV and 1TeV. Results are within the field of view of ARGO. X.J. Bi, Nucl. Phys. B741, 83 (2006)

Complementary capabilities ground-based space-based ACTEAS Pair angular resolutiongoodfair good duty cyclelowhigh high arealargelarge small field of viewsmalllarge large + can reorient energy resolutiongoodfair good, with smaller systematic uncertainties Gamma ray detection from DM annihilation my estimate HAWC~0.04I CRAB

Search the subhalos at different detectors Simulation can not predict the position of subhalos we can only look for subhalos with high sensitivity and large field of view detectors. Satellite-based experiments, GLAST ， AMS02, have large field of view, high identification efficiency of  /P, while small effective area ~1 m 2, low threshold energy. EAS ARGO/MILAGRO/HAWC observatories, have large field of view, (low identification efficiency of  /P), while large effective area ~ m 2, high threshold energy and high sensitivity. Cerenkov telescopes have high angular resolution, high identification efficiency of  /P, large effective area ~10 4 m 2, small filed of view.

中意合作 ARGO 实验 RPC 大厅 中日合作 AS γ 实验区闪烁体探测器阵列 AS  and ARGO ： (High Duty cycle,Large F.O.V) ~TeV ~100GeV ARGO hall, floored by RPC. Half installed. Here comes the two experiments hosted by YBJ observatory. One is call AS , a sampling detector covering 1% of the area and have been operated for 15 years. The other full coverage one is called ARGO, still under installation. AS  use scintillation counter and ARGO use RPC to detector the arrival time and the number of secondary particles, with which the original direction and energy of CR particle can be restored. AS  has a threshold energy at a few TeV while ARGO down to about 100GeV. Both experiment have the advantages in high duty cycle and large field of view. Because for both of the experiments there is only one layer of detector, it is very difficult to separate the  ray shower from CR nuclei showers. Working in the similar energy range on mountain Jemez near Los Alamos, by using water cherenkov technique, MILAGRO has two layer of PMT, which enable it a rather good capability to separate  ray from background. Though it locates in a low altitude, has a smaller effective area, it has similar sensitivity to AS  experiment. To combine this technique with high altitude would greatly improve the sensitivity of our current EAS experiments.

Sensitivity study of ARGO We adopt the simulated effective area of ARGO, assuming a constant angular resolution of 1°and energy threshold of 100 GeV. X.X. Zhou et al., ICRC 29 th

Sensitivity at ARGO （ 95 ％ C.L. ）

Sensitivity study of HAWC We adopt the simulated effective area of HAWC, assuming a constant angular resolution of 1°and taking energy threshold of 100 GeV. G. Sinnis et al., astro- ph/

Sensitivity at HAWC （ 95 ％ C.L. ）

Summary Flux of gamma rays from the subhalos of the Milky Way halo is calculated. Sensitivity of the ground EAS detectors, ARGO/HAWC, is studied. We find it is possible to detect the DMA signals (or put constraint on the SUSY parameter space) by these detectors. Non-thermal production and steep central cusp of the subhalos can help to enhance the DMA signals.