Probing annihilations and decays of low-mass galactic dark matter in
IceCube DeepCore array: Track events
Fei-Fan Lee and Guey-Lin Lin
Institute of Physics, National Chiao-Tung University, Hsinchu 30010, Taiwan
(Received 5 July 2011; revised manuscript received 7 December 2011; published 25 January 2012) The deployment of DeepCore array significantly lowers IceCube’s energy threshold to about 10 GeV and enhances the sensitivity of detecting neutrinos from annihilations and decays of light dark matter. To match this experimental development, we calculate the track event rate in DeepCore array due to neutrino flux produced by annihilations and decays of galactic dark matter. We also calculate the background event rate due to the atmospheric neutrino flux for evaluating the sensitivity of DeepCore array to galactic dark matter signatures. Unlike previous approaches, which set the energy threshold for track events at around 50 GeV (this choice avoids the necessity of including the oscillation effect in the estimation of atmospheric background event rate), we have set the energy threshold at 10 GeV to take full advantage of DeepCore array. We compare our calculated sensitivity with those obtained by setting the threshold energy at 50 GeV. We conclude that our proposed threshold energy significantly improves the sensitivity of DeepCore array to the dark matter signature for m< 100 GeV in the annihilation scenario and m<
300 GeV in the decay scenario.
DOI:10.1103/PhysRevD.85.023529 PACS numbers: 95.35.+d, 14.60.Ef, 14.60.Pq
I. INTRODUCTION
Many astrophysical observations have confirmed the existence of dark matter (DM), which contributes to roughly 23% of the energy density of the Universe. Among many proposed DM candidates, weakly interacting massive particles (WIMPs) [1,2] are popular proposals since they are theoretically well motivated and also capable of producing the correct relic density. WIMPs could anni-hilate or decay into particles such as electrons, positrons, protons, antiprotons, photons, and neutrinos. It is possible to establish the WIMP signature through detecting these particles [3–15].
Recently, research activities on WIMPs have been boosted in the efforts of explaining the observed anoma-lous positron excess in the data of PAMELA [8] and positron plus electron excess in the data of FERMI [12]. To account for spectral shapes observed by these experi-ments, WIMPs must annihilate or decay mostly into lep-tons in order to avoid the overproduction of antiprolep-tons. This could indicate that DM particles are leptophilic in their annihilations or decays [16,17]. It has been pointed out that the observation of neutrinos can give stringent constraints on the above scenario. Measurements of up-ward going muons by the Super-Kamiokande observatory place a limit on the galactic muon neutrino flux, which in turn rules out the possibility of WIMP annihilations to þ as a source of e anomalies [18–20]. Furthermore, one expects that the possibilities of WIMP annihilations into , and WIMP decays into and , will all be stringently constrained [21–23] (see also discussions in Ref. [24]) by the data from IceCube detector augmented with DeepCore array.
The DeepCore array [25,26] is located in the deep center region of IceCube detector. This array consists of six densely instrumented strings plus seven nearest standard IceCube strings. The installation of DeepCore array sig-nificantly improves the rejection of downward going at-mospheric muons in IceCube and lowers the threshold energy for detecting muon track or cascade events to about 5 GeV. As summarized in Ref. [26], the low detection threshold of DeepCore array is achieved by three improve-ments over the IceCube detector. First, the photo-sensors in the DeepCore are more densely instrumented than those of IceCube, as just mentioned. Second, the ice surrounding the DeepCore array is on average twice as clear as the average ice above 2000 m [27]. Such a property is useful for reconstructing lower-energy neutrino events. Finally, the DeepCore array uses new type of phototube which has a higher quantum efficiency.
It is clear that DeepCore array improves the sensitivity as well as enlarges the energy window for observing neu-trinos from DM annihilations or decays in the galactic halo. Previous analyses on the detection of these neutrinos in the DeepCore [23,28] have set the threshold energy at 40–50 GeV for both track and cascade events. For neutrino events with energies higher than 50 GeV, the estimation of atmospheric background event rate is straightforward since oscillation effects can be neglected. However, to take the full advantage of DeepCore array, it is desirable to estimate the track and shower event rates due to atmospheric neu-trinos in the energy range 10 GeV E 50 GeV. In this energy range, the oscillations of atmospheric neutrinos cannot be neglected. In this article, we take into account this oscillation effect and calculate the track event rate with a threshold energy Eth¼ 10 GeV due to atmospheric
muon neutrinos from all zenith angles. Given such a back-ground event rate, we then evaluate the sensitivities of DeepCore array to the neutrino flux arising from DM annihilations and decays in the galactic halo. In the sub-sequent paper, we shall analyze the corresponding sensi-tivities associated with cascade events.
This paper will focus on neutrino signature induced by low-mass DM. Hence our interested DM mass range is far below TeV level implied by PAMELA and FERMI data. Therefore, we shall consider neutrino flux induced by DM annihilations/decays into both leptons and hadrons. Specifically, we consider the channels ! b b; þ, and þ for annihilations, and the channels ! b b; þ and þ for decays. Since we are only inter-ested in low-mass dark matter, we have neglected neutrino fluxes generated through DM annihilations or decays into tt, WþW, and ZZ final states. We also neglect neutrino fluxes arising from light meson decays, as the annihilation cross section for ! q q is likely to be suppressed by m2 q [2]. We shall compare the constraints on DM annihilation cross section and DM decay time for different values of threshold energy Eth
. For such a comparison, we employ the modes ! þand ! þfor illustrations. This paper is organized as follows. In Sec.II, we outline the calculation of muon neutrino flux from WIMP annihi-lations and decays in the galactic halo. In Sec. III, we calculate the atmospheric muon neutrino flux from all zenith angles with E 10 GeV. The oscillations between and are taken into account. In Sec.IV, we evaluate the sensitivity of DeepCore array to neutrino flux arising from WIMP annihilations or decays in the galactic halo. We compare our results with those obtained by setting Eth ¼ 50 GeV. We summarize in Sec.V.
II. NEUTRINO FLUX FROM ANNIHILATIONS AND DECAYS OF DARK MATTER
IN THE GALACTIC HALO
The differential neutrino flux from the galactic dark matter halo for neutrino flavor i can be written as [20]
di dEi ¼ 4 hi 2m2 X F BF dNF i dE R2 J2ðÞ (1) for the case of annihilating DM, and
di dEi ¼ 4 1 m X F BF dNF i dE R J1ðÞ (2) for the case of decaying DM, where R¼ 8:5 kpc is the distance from the galactic center (GC) to the solar system, ¼ 0:3 GeV=cm3 is the DM density in the solar neigh-borhood, mis the DM mass, is the DM decay time, and dNF
i=dE is the neutrino spectrum per annihilation or decay for a given annihilation or decay channel F with a corre-sponding branching fraction BF. The neutrino spectra dNiF=dE for different channels are summarized in
Refs. [28,29]. The quantityhi is the thermally averaged annihilation cross section, which can be written as
hi ¼ Bhi0; (3)
with a boost factor B [30,31]. We set hi0¼ 3 1026 cm3s1, which is the typical annihilation cross sec-tion for the present dark matter abundance under the stan-dard thermal relic scenario [1]. We treat the boost factor B as a phenomenological parameter. The dimensionless quantity JnðÞ is the DM distribution integrated over the line-of-sight (l.o.s.) and averaged over a solid angle ¼ 2ð1 coscmaxÞ, i.e.,
JnðÞ ¼ 1 Z dZ l:o:s: dl R ðrðl;cÞÞ n ; (4)
where is the DM density at a specific location described by the coordinateðl;cÞ, with l the distance from the Earth to DM andc the direction of DM viewed from the Earth with c ¼ 0 corresponding to the direction of GC. The distance r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR2
þ l2 2Rl cosc p
is the distance from GC to DM. The upper limit of the integration, lmax Rcosc þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R2
s R2sin2c p
, depends on Rs, the adopted size of the galactic halo. In this analysis, we take Rs¼ 20 kpc and use the Navarro-Frenk-White (NFW) DM den-sity profile [32] ðrÞ ¼ s Rs r Rs Rsþ r 2 ; (5)
with s¼ 0:26 GeV cm3 such that ¼ 0:3 GeV cm3.
Neutrinos are significantly mixed through oscillations when they travel a vast distance across the galaxy. We determine neutrino flavor oscillation probabilities in the tribimaximal limit [33] of neutrino mixing angles, i.e., sin2
23¼ 1=2, sin2 12¼ 1=3, and sin2 13¼ 0. The neu-trino fluxes on Earth are related to those at the source through [34–36] e ¼5 9 0 eþ29 0 þ29 0 ; (6) and ¼ ¼290eþ1870þ1870; (7) where 0
i is the neutrino flux of flavor i at the astrophys-ical source. It is understood that the recent T2K [37] and Double Chooz [38] experiments have indicated a nonzero value for 13. Taking the T2K best-fit value sin22 13¼ 0:11 at the CP phase ¼ 0 for the normal mass hierarchy, we have
e ¼ 0:530eþ 0:260þ 0:210; ¼ 0:260eþ 0:370þ 0:370; ¼ 0:210eþ 0:370þ 0:420:
To proceed our discussions, let us first take the neutrinos at the source to be those generated by B meson decays following the ! b b annihilation. In this special case, 0
e ¼ 0 ¼ 0 at the source, and consequently the relation e ¼ ¼ always holds due to the proba-bility conservation, irrespective of the form of oscillation probability matrix. Let us now consider neutrinos produced at the source by muon decays following the ! þ annihilation. In this case, one has 0
e ¼ 0 and 0 ¼ 0. Taking 0 e ¼ 0 0, one obtains e ¼ 0:780and ¼ ¼ 0:61
0for tribimaximal values of neutrino mixing parameters. On the other hand, with the T2K best-fit 13 value, one arrives at e ¼ 0:79
0, ¼ 0:630 and
¼ 0:580 for the normal mass hierarchy. Clearly e is almost unaffected while = 1 ¼ 9%. For the inverted mass hierarchy, one obtains the same e flux while = 1 ¼ 12%. Hence T2K result implies an Oð10%Þ difference between the arrival and fluxes for neutrinos produced by ! þ annihilations. Since this effect is not large and it is not possible to identify in our interested energy range, we shall still apply Eqs. (6) and (7) for determining the arrival neutrino fluxes.
III. ATMOSPHERIC NEUTRINO FLUXES Knowing atmospheric neutrino background is important for evaluating the sensitivity of DeepCore array to neutrino flux from DM annihilations or decays. We begin by com-puting the flux of intrinsic atmospheric muon neutrinos arising from pion and kaon decays, following the ap-proaches in Refs. [39,40]. The flux arising from decays can be written as
d2N ðE; ; XÞ dEdX ¼Z1 E dEN ZEN E dE ðE E 1 Þ dEð1 Þ ZX 0 dX0 N PðE; X; X0Þ 1 E FNðE; ENÞ exp X0 N NðENÞ; (9)
where E is the neutrino energy, X is the slant depth in units of g=cm2, is the zenith angle in the direction of incident cosmic-ray nucleons, r¼ m2=m2, d is the pion decay length in units of g=cm2,
N is the nucleon interac-tion length, and N is the corresponding nucleon attenu-ation length. The function PðE; X; X0Þ is the probability that a charged pion produced at the slant depth X0survives to the depth X ( > X0), which is given by [41]
PðE; X; X0Þ ¼ exp X X0 exp mc E ZX X0 dT ðTÞ ; (10)
where ¼ 160 g=cm2 is the pion attenuation length, is the pion lifetime at its rest frame, while ðTÞ is the atmosphere mass density at the slant depth T. Finally, FNðE; ENÞ is the normalized inclusive cross section for N þ air ! þ Y, which is given by [39]
FNðE; ENÞ E N dðE; ENÞ dE ¼ cþð1 xÞpþþ cð1 xÞp; (11) with x ¼ E=EN, cþ¼ 0:92, c¼ 0:81, pþ¼ 4:1, and p¼ 4:8.
The primary cosmic-ray spectrum NðENÞ in Eq. (9) includes contributions from cosmic ray protons and those from heavier nuclei. We have NðENÞ ¼
P
AAAðENÞ with A as the atomic number of each nucleus. The spec-trum of each cosmic-ray component is parametrized by [42,43]
AðENÞ ¼ K ðENþ b exp½c ffiffiffiffiffiffiffi EN p
Þ; (12) in units of m2 s1sr1GeV1. The fitting parameters ; K; b; c depend on the type of nucleus. They are tabulated in Table I[43]. The kaon contribution to the atmospheric flux has the same form as Eq. (9) with an inclusion of the branching ratio BðK ! Þ ¼ 0:635 and appropriate replacements in kinematic factors and the normalized in-clusive cross section. In particular, FNKðEK; ENÞ can be parametrized as Eq. (11) with cþ¼ 0:037, c¼ 0:045, pþ¼ 0:87, and p¼ 3:5.
Since our interested energy range is as low as 10 GeV, the three-body muon decay contribution to the atmospheric flux is not negligible, particularly in the near horizontal direction. To obtain this part of contribution, we first compute the atmospheric muon flux from pion and kaon decays. The muon flux induced by pion decays is given by [39,40]
TABLE I. Parameters for all five components in the fit of Eq. (12). Parameter/component K b c Hydrogen (A ¼ 1) ð 102 GeVÞ 2.74 14 900 2.15 0.21 Hydrogen (A ¼ 1) ð>102 GeVÞ 2.71 14 900 2.15 0.21 He (A ¼ 4) 2.64 600 1.25 0.14 CNO (A ¼ 14) 2.60 33.2 0.97 0.01 MgSi (A ¼ 25) 2.79 34.2 2.14 0.01 Iron (A ¼ 56) 2.68 4.45 3.07 0.41
dNðE; ; XÞ dE ¼ Z1 E0 dEN ZEN E0 dE ZX 0 dX 00P ðE; X; X00Þ ðE E0ÞðE 0 r EÞ dEð1 rÞ ZX00 0 dX0 N PðE; X00; X0Þ 1 E FNðE; ENÞ exp X0 N NðENÞ; (13)
where E0and E are muon energies at slant depths X00and X, respectively, while PðE; X; X00Þ is the muon survival probability. The muon flux induced by kaon decays can be calculated in a similar way. Since ðþÞ produced by ðþÞ decays are polarized, we classify muon flux into four different components such as dNþþ
R=dE, dN R=dE, dNþ þL=dE, and dN
L=dE. We also calculate additional four components of the muon flux arising from the kaon decays. Hence the flux arising from muon decays can be written as [40,41] d2N ðE; ; XÞ dEdX ¼ X s¼L;R Z1 E dE Fs!ðE=EÞ dðE; XÞE dNsðE; ; XÞ dE ; (14)
where dðE; XÞ is the muon decay length in units of g=cm2 at the slant depth X, and F
s!ðE=EÞ is the normalized decay spectrum of s ! . Summing the two-body and three-body decay contributions, we obtain the total intrinsic atmospheric muon neutrino flux. In Fig.1, we show the comparison of angle-averaged atmos-pheric muon neutrino flux obtained by our calculation and that obtained by Honda et al. [44]. At E ¼ 10 GeV, two calculations only differ by 3%. At E¼ 100 GeV, the difference is 10%. We also show in the same figure the atmospheric muon neutrino flux measured by AMANDA-II detector [45]. It is seen that both calculations agree well with AMANDA results.
To completely determine the atmospheric muon neu-trino flux, one also needs to calculate the intrinsic atmos-pheric tau neutrino flux, although this part of contribution is rather small. The intrinsic atmospheric flux arises from Ds decays. This flux can be obtained by solving cascade equations [40,46]. We obtain
d2N ðE; XÞ dEdX ¼ ZNDsZDs 1 ZNN expðX=NÞNðENÞ N ; (15)
where ZNN 1 N=Nand ZNDsis a special case of the generic expression ZijðEjÞ Z1 Ej dEi iðEiÞ iðEjÞ iðEjÞ iðEiÞ dniA!jYðEi; EjÞ dEj ; (16)
with dniA!jYðEi; EjÞ diA!jYðEi; EjÞ=iAðEiÞ and i the interaction length of particle i in units of g=cm2. The decay moment ZDs is given by
ZDsðEÞ Z1 EdEDs DsðEDsÞ DsðEÞ dDsðEÞ dDsðEDsÞ FDs!ðE=EDsÞ; (17) where dDs is the decay length of Dsand FDs!ðE=EDsÞ is the normalized decay distribution. In this work, we employ the next-to-leading order perturbative QCD [47] with CTEQ6 parton distribution functions to calculate the
differential cross section of NA ! c c and determine ZNDs. Finally, the atmospheric flux taking into account the neutrino oscillation effect is given by
d NðE; Þ dE ¼ Z dX d2N dEdX P! þd 2N dEdX ð1 P!Þ ; (18)
where P!ðE; LðX; ÞÞ ¼ P!ðE; LðX; ÞÞ sin22 23sin2ð1:27m231L=EÞ is the ! oscillation probability and LðX; Þ is the linear distance from the neutrino production point to the position of IceCube DeepCore array. The unit of m2
31 is eV2 while L and E are in units of km and GeV, respectively. The best-fit values for oscillation parameters obtained from a recent analysis [48] are m2
31¼ 2:47 103 eV2 and sin22 23¼ 1, respectively.
FIG. 1 (color online). The comparison of angle-averaged at-mospheric muon neutrino (þ ) flux obtained by our
cal-culation and that obtained by Honda et al. [44]. Angle-averaged þ flux fromAMANDA-IImeasurements [45] is also shown.
IV. RESULTS
In IceCube DeepCore, the event rate for contained muons is given by ¼ZEmax Eth dE ZEmax E dENAiceVtr d dE d CC NðE; EÞ dE þ ð ! Þ; (19)
where ice¼ 0:9 g cm3 is the density of ice, NA ¼ 6:022 1023 g1 is the Avogadro number, V
tr 0:04 km3 is the effective volume of IceCube DeepCore array for muon track events [25], d=dE is the muon neutrino flux arrived at IceCube, Emax is taken as m for annihilation and m=2 for decay, and Eth is the threshold energy for muon track events. In this work, we use differential cross sections dCC
NðE; EÞ=dE given by Ref. [49] withCTEQ6parton distribution functions. We also set Eth
¼ 10 GeV.
As stated before, we consider neutrino fluxes generated by the annihilation channels ! b b; þand þ, and the decay channels ! b b; þand þ. Given the atmospheric neutrino background, we present in Fig.2
the required DM annihilation cross section as a function of m for threshold energy Eth ¼ 10 GeV and a cone half-anglecmax¼ 1 such that the neutrino signature from DM annihilations can be detected at the 2 significance in five years. Nondetection of such a signature would then exclude the parameter region above the curve at the 2 level. We have presented results corresponding to different annihila-tion channels. One can see that the required annihilaannihila-tion cross section for 2 detection significance is smallest for the ! þ channel and largest for the channel ! b b. We also present the 3 constraint on ! þ annihilation cross section obtained from Super-Kamiokande data of upward going muons [18], which has been used to rule out WIMP annihilations into þ as a possible source of previously mentioned eanomalies [18–20]. Such a constraint can be compared with the expected 2 constraint on the same annihilation channel from the DeepCore detector.
Constraints on DM annihilation cross section were also obtained from gamma ray observations and cosmology. The H.E.S.S. telescope performed a search for the very high-energy ( 100 GeV) -ray signal from DM annihi-lations over a circular region of radius 1 centered at the GC [50]. With DM particles assumed to annihilate into q q pairs, the limit on DM annihilation cross section as a function of m for NFW DM density profile is derived in Ref. [50]. We present this constraint in Fig.2as well. For m> 300 GeV, the parameter space with B > 100 (i.e., hvi > 3 1024 m3s1) in Fig.2could be excluded by the H.E.S.S. data. However, the H.E.S.S. constraint on ! q q becomes much weaker for m< 300 GeV. We
point out that this constraint is obtained with NFW profile normalized at ¼ 0:39 GeV cm3. The H.E.S.S. con-straint would be slightly less stringent if our adopted normalization ¼ 0:3 GeV cm3is used.
Cosmological constraints on DM annihilation cross sec-tion can be obtained from the data of big bang nucleosyn-thesis and cosmic microwave background (CMB). In such an analysis, DM annihilation cross section is assumed to be velocity dependent such that [51]
hvi ¼ hvi0 þ ðv=v0Þn
; (20)
where v0 is DM velocity at the freeze-out temperature, while the values for and n depend on specific models. For Sommerfeld enhancement [30] of the DM annihilation cross section induced by light-scalar exchange, one has n ¼ 1 and ’ m=mwith mthe light-scalar mass. The CMB anisotropy can be affected by the energy injection in the recombination epoch due to DM annihilation process such as ! eþe and ! WþW. In Fig. 2, we show the upper bound on hvi for ! eþe channel for n ¼ 1 and TKD¼ 1 MeV with TKD as the kinetic decoupling temperature. This upper bound is inferred from the upper bound on hvi0 such that the resulting CMB power spectrum remains consistent with observa-tions [51]. The above upper bound onhvi0 is shown to be sensitive to the parameter while the corresponding bound onhvi is insensitive to it. It will be interesting to FIG. 2 (color online). The dashed line, thin solid line, and dot-dashed lines are the expected constraints on DM annihilation cross section by the DeepCore detector for ! b b, ! þ, and ! þchannels, respectively. The thick solid line is the H.E.S.S constraint on the annihilation cross section of DM into the light quark pair ! q q [50]. The dot-dot-dashed line is the constraint on ! eþe annihilation cross section from the analysis of cosmic microwave background data [51]. The dotted line is the 3 constraint on the annihilation cross section of ! þ from Super-Kamiokande data [18].
convert the above bound on hð ! eþeÞvi into the one onhð ! þÞvi. However such a conversion is highly model dependent which is beyond the scope of the current work.
Having compared the expected sensitivities of the DeepCore detector with other experimental constraints on various DM annihilation channels, we discuss how the DeepCore constraint on DM annihilation cross section varies with the chosen cone half-angle and threshold en-ergy. We use the channel ! þ to illustrate these effects. Figure3shows the required DM annihilation cross section hð ! þÞvi for a 2 detection in five years for different cone half-angle cmax. One can see that the constraint on the DM annihilation cross section gets stronger ascmaxincreases from 1 to 2 . However, the constraint turns weaker as cmax increases further. This is due to the factor J2ðÞ which depends on the square of DM density [see Eq. (8)]. The constraint curve rises with an increasing cmax for cmax> 2 , since the signal in-creases slower than the background does for such a cmax range. In this figure, we also show the result for a higher threshold energy Eth
¼ 50 GeV with a cone half-angle cmax¼ 10 for comparison. This result is taken from Ref. [28] where cmax¼ 10 is identified as the most optimal cone half-angle for constraining DM annihilation cross section at that threshold energy. We note that, for large m, lowering Eth from 50 GeV to 10 GeV results in more enhancement on the event rate of atmospheric back-ground than that of DM annihilation. Hence, the constraint on DM annihilation cross section is weaker by choosing Eth
¼ 10 GeV. On the other hand, for small m, lowering Eth
enhances more on the event rate of DM annihilations
than that of atmospheric background. For m< 100 GeV, one can see that the constraint on DM annihilation cross section with Eth
¼ 10 GeV is always stronger than that with Eth
¼ 50 GeV. We note that DeepCore constraints on other annihilation channels have similar cone half-angle and threshold energy dependencies.
Besides studying DeepCore constraints on DM annihi-lation channels, we also present constraints on DM decay time for ! b b; þ and þ channels. Figure 4
shows the required DM decay time for a 2 detection of neutrino signature in five years for each channel. We have taken Eth¼ 10 GeV and cmax¼ 90 . Nondetection of such a signature would then exclude the parameter region below the curve at the 2 level. For comparison, we also show 3 limit on ! þfrom Super-Kamiokande data of upward going muons [18]. One can see that the channel ! þrequires the smallest decay width to reach the 2 detection significance in five years of DeepCore data taking.
Finally, we present how the DeepCore constraint on DM decay time varies with the chosen cone half-angle and threshold energy. We use the channel ! þto illus-trate these effects. Figure5shows the required DM decay timeð ! þÞ as a function of mfor different cone half-anglecmaxsuch that the neutrino signature from DM decays can be detected at the 2 significance in five years. For DM decays, the curve rises with increasingcmaxsince the event rate of DM signal increases faster than that of atmospheric background as cmax increases. For compari-son, we show the required DM decay time for a 2 detection in five years with Eth
¼ 50 GeV and cmax¼ 50 . It has been pointed out in Ref. [28] thatcmax¼ 50 gives the most stringent constraint on DM decay time for FIG. 3 (color online). The required DM annihilation cross
section ð ! þÞ as a function of m such that the
neutrino signature from DM annihilations can be detected at the 2 significance in five years. Results corresponding to different cmax are presented. For comparison, we also show
the result with Eth
¼ 50 GeV andcmax¼ 10 [28].
FIG. 4 (color online). The dot-dashed line, solid line, and dotted line are the required DM decay time for a 2 detection of neutrino signature in five years for ! b b; þand þ channels, respectively. The dashed line is the Super-Kamiokande constraint on ! þ [18].
Eth
¼ 50 GeV. One can see that the constraint on DM decay time is strengthen by lowering Eth
from 50 GeV to 10 GeV for m< 300 GeV.
V. SUMMARY
We have calculated the track event rate in IceCube DeepCore array resulting from muon neutrino flux pro-duced by annihilations and decays of dark matter in the galactic halo. In this calculation, we have employed NFW
profile for dark matter mass distribution and consider the channels ! b b; þ, and þ for annihilations, and the channels ! b b; þ and þ for decays. We also calculated the track event rate due to atmospheric background. We compare the signal event rate with that of the background for E 10 GeV.
We have presented sensitivities of IceCube DeepCore array to neutrino flux arising from dark matter annihila-tions and decays. For a given dark matter mass, we eval-uated the dark matter annihilation cross section and dark matter decay time such that a 2 detection significance for the above signatures can be achieved by DeepCore array for a five-year data taking. The DeepCore sensitivities on dark matter annihilation cross section were compared with the constraint obtained from H.E.S.S. gamma ray observa-tions and the constraint derived from the data of CMB power spectrum. Using ! þand ! þ as examples, we also presented how DeepCore constraints on dark matter annihilation cross section and dark matter decay time vary with the chosen cone half-angle and threshold energy. We like to point out that our calculated sensitivities based upon Eth
¼ 10 GeV are significantly more stringent than those obtained by taking Eth ¼ 50 GeV for m< 100 GeV in the annihilation channel and m< 300 GeV in the decay channel.
ACKNOWLEDGMENTS
This work is supported by the National Science Council of Taiwan under Grants No. 099-2811-M-009-055 and No. 99-2112-M-009-005-MY3, and Focus Group on Cosmology and Particle Astrophysics, National Center for Theoretical Sciences, Taiwan.
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