# HE.5.1.02 

# Detection of Galactic Dark Matter by GLAST

**Alexander Moiseev**^1,2^**, Jonathan Ormes**^1^**, Heather
Arrighi**^1^**, Elliott Bloom**^3^**, Chris Chaput**^3^**, Seth
Digel**^1^**, Daniel Engovatov**^3^**, Jay Norris**^1^**, and Jeff
Silvis**^1^

^1^NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

^2^University Space Research Association, Seabrook, MD 20706, USA

^3^Stanford Linear Accelerator Center, Stanford, CA 94309, USA

##  Abstract

The mysterious dark matter has been a subject of special interest to
high energy physicists, astrophysicists and cosmologists for many years.
According to theoretical models, it can make up a significant fraction
of the mass of the Universe. One possible form of galactic dark matter,
Weakly Interacting Massive Particles (WIMPs), could be detected by their
annihilation into monoenergetic gamma-ray line(s). This paper will
demonstrate that the Gamma-ray Large Area Space Telescope (GLAST),
scheduled for launch in 2005 by NASA, will be capable of searching for
these gamma-ray lines in the energy range from 20 GeV to \~500 GeV and
will be sufficiently sensitive to test a number of models. The required
instrument performance and its capability to reject backgrounds to the
required levels are explicitly discussed.

1.  **Introduction**

On many scales, from galaxies to the largest structures in the Universe,
there is a discrepancy between observed (luminous) matter in the
Universe and that inferred from dynamical considerations. This is seen
on the scale of our own galaxy. Galactic dark matter was suggested to
solve this discrepancy (Trimble, 1987; Sikivie, 1995); one possible form
could be the proposed, but undiscovered, SUSY particles known as WIMPs
(Weakly Interacting Massive Particles). WIMPs can be detected through
stable products of their annihilations: energetic neutrinos,
antiprotons, positrons, gamma-quanta etc. (Jungman, Kamionkowski &
Griest, 1996; and references therein). It is very important to search
for a signature of WIMPS which could not be misinterpreted. In
principle, WIMPs cannot annihilate directly to photons, but there should
be a small cross section into photons through intermediate one-loop
processes. In this case there should be high energy (10-1000 GeV)
monochromatic gamma-lines; the lines should be very narrow because of
the low velocity of WIMPs in the galaxy. Estimations of the possible
intensity of these lines depend upon a number of assumptions. The
highest gamma-line intensities are predicted assuming WIMPs have
condensed into the Galactic Center or into clumps in the galactic plane.
For example, Bergstrom, Ulio and Buckley (1998) show that some models
might produce a flux as large as \~2×10^-11^ cm^-2^s^-1^ at 100 GeV from
the 10^-5^ sr cone around the Galactic Center.

2.  **Conditions of the experiment**

The requirements for an experiment to search for possible lines are that
the lines should be seen above a background which is a continuum of
galactic gamma rays. Optimally there should be negligible residual
contamination from cosmic rays misidentified in the detector. Thus,
energy resolution and geometry factor/sensitive area of the detector,
background rejection, and the observation time are the critical factors
to be optimized.

GLAST is a mission scheduled for launch in 2005 to continue the detailed
exploration of the Universe in \>100 MeV gamma-rays began by EGRET
(Atwood et al., 1994). GLAST is sensitive to gamma radiation in the
range of 30 --300 GeV (fig.1). The energy of the detected photon is
measured by a CsI calorimeter, which is situated below the tracker, and
has a size of 170 cm × 170 cm × 20 cm. The effective calorimeter
thickness for the normally incident particles is \~10 radiation lengths
which provides \~10% energy resolution at 300 GeV. Much better energy
resolution can be achieved for the off-angle events with longer paths in
the calorimeter (shown in fig.1).

**2.1. Background rejection**. The first task of GLAST is to remove the
abundant background of cosmic ray protons and helium nuclei whose
differential flux is 5 orders of magnitude higher than that of high
latitude diffuse gamma radiation at 30 GeV. We also must consider cosmic
ray electrons, which are 1000 times more abundant. In order to carry out
a sensitive search for gamma ray lines one should be able to reject
protons (electrons) with power better than 3×10^6^ and 3×10^4^
respectively. The main strategy for proton rejection is the following: a
track image in the tracker and lateral and longitudinal profile of the
shower in the calorimeter provide at least 10^3^ of the rejection
(Norris et al., 1997; Ormes et al., 1997), and an anticoincidence
detector (ACD) provides remaining 3×10^3^ (Moiseev et al. 1999). The
cosmic ray electrons are the more serious enemies. Their showers are
identical to those of photons in the calorimeter since both are
electromagnetic. Thus, the ACD is the main defense against electrons and
should have rejection power to minimum ionizing charged particles of
\~3000. An additional factor of 10 comes from the use of the tracker to
reach the 3×10^4^ requirement. We have measured the rejection power
(efficiency) of the scintillator paddles we plan to use for GLAST and
find there are sufficient photo-electrons to obtain \> 3×10^3^ rejection
for a threshold setting \< 0.3 × mip.

To find the ACD tile which was crossed by a detected off-angle particle,
which has a longer path in a calorimeter, we follow a two step process.
First we use the imaging capability of the calorimeter and reconstruct a
trajectory with a precision of 2-3 degrees (Norris et al., 1997). Then
we project that cone back into the tracker and look for the absence of a
track (for photons) in the past two layers before the ACD and the
absence of a hit in the ACD tile for electrons; the two tracker hits
(for electron) provide precise pointing to an ACD tile.

**2.2. Backsplash.** Use of an ACD creates the problem of backsplash.
High energy electromagnetic cascades produce soft radiation, mainly
minimum attenuation photons, that escape from the calorimeter. These
photons can produce a Compton electron in an ACD and create a self-veto,
making the instrument insensitive to gamma-radiation above 50-100 GeV.
For EGRET, built with a monolithic ACD dome, this effect reduced the
efficiency by a factor of 2 at 30 GeV (Thompson et al.,1993). To
minimize the effect of backsplash, GLAST has a segmented ACD, and only
the tile crossed by the projected event trajectory is used for vetoing
an event. The required ACD segmentation was studied in detail both using
Monte Carlo simulations and in SLAC beam test (Atwood et al., 1999;
Moiseev et al., 1999). On the top of GLAST, the ACD segmentation of
\~1000 cm^2^ is sufficient to maintain \>90% efficiency to the highest
energies. We wish to use events that enter GLAST at \>60^0^ incidence
angle to obtain a sample with few percent energy resolution. These
events enter through the sides of GLAST. For them the calorimeter, and
consequently the source of backsplash, are closer to the ACD. The closer
the ACD tiles to the "source" of backsplash, the smaller the tiles must
be. From our SLAC beam test and Monte Carlo study we have shown that the
backsplash into a given solid angle is almost uniform within a 60^0^
cone in the backward direction (Atwood et al., 1999). The required
segmentation can be given by A~90%~ = (R/60)^2^ ×1000 cm^2^ where
R\[cm\] is the distance from the ACD tile to the calorimeter. We find
that on the side of GLAST a tile size of \~200 cm^2^ limits backsplash
caused self-veto to be less than 10% at 300 GeV.

3.  **Capability of GLAST to detect gamma lines**

Energy resolution for 50-500 GeV photons should reach several percent
for path lengths of \>20 X~0~. The effective area for the GLAST
calorimeter for such trajectories is estimated to be 2 ×10^4^ cm^2^ .
The geometry factor of GLAST for isotropic flux through the top and
sides of the tracker is shown in fig.2 where the Earth obscuration is
accounted for by a factor of 0.74 (assuming zenith pointing) applied to
the events entering through the tracker sides. To take into account the
requirement that the trajectory passes through at least two tracker
trays, the geometry factor given in fig.2 is calculated for events which
cross ACD at least 6 cm above the top of
calorimeter.![](media/image1.wmf){width="3.075in" height="3.075in"}

The capability of GLAST's calorimeter to detect photons with high energy
resolution was simulated with the event generator Glastsim. A set of
event cuts was developed to select events with shower containment in the
calorimeter that provides the best energy resolution. The initial
selections were optimized to achieve the best energy resolution while
maximizing the fraction of retained photons. The geometry factor
strongly increases when we accept events with shorter path lengths, so
the minimization of the required pathlength was important. The results
of simulations can be summarized as follows: **2-3% energy resolution**
is achievable while retaining **\~50% of the photons** and requiring the
pathlength to be **more than 20X~0~**. The gain variation from CsI
crystal to crystal was assumed to be within 1%, a challenging problem
for a long duration space experiment.

**4. Summary**

Here we present the sensitivity of GLAST based on two possible models
for the distribution of WIMPs in the galaxy. One model is a "dark matter
point source" in the Galactic Center assuming the WIMPs have fallen into
a small region within 10^-5^ sr (Bergstrom, Ullio and Buckley, 1998).
The second model assumes a broad distribution falling off like high
latitude diffuse radiation.

$I_{\gamma\ } = \ \frac{n_{\sigma}}{0\text{.}\text{68}}\sqrt{\frac{\ 2\ F_{b}\eta\ E_{\gamma}}{G\ T}}$
For high latitude model the sensitivity I~γ~ of GLAST for a gamma-line
of energy E~γ~ approximately is given by

$I_{\gamma}\  = \ \frac{n_{\sigma}}{0\text{.}\text{68}\ \sqrt{S\ f_{t}T}}\ \sqrt{\ 2{\eta E}_{\gamma}\left( F_{\text{GC}} + \ F_{b}\Delta\Omega \right)}$and
for Galactic Center model

where n~σ~ is the significance (in σ), F~b~ is the background flux,
F~GC~ is the differential gamma-radiation from the Galactic Center, G is
the instrument geometrical factor, S is the sensitive area, η is the
relative energy resolution (half width containing 68% of events), T is
the observation time, 2ηE~γ~ is the binning width, ∆Ω=10^-3^ sr is the
point-spread function for the calorimeter, and f~t~ (0.25) is the
fraction of time during which the Galactic Center lies in a direction
that provides a path length in a calorimeter of more than 20 X~0~ .

Table 1

+-----------------------+---------------------+-----------------------+
| Energy of the line    | High Latitude Model | Galactic Center Model |
|                       |                     |                       |
| I~γ~                  | Source \[cm^2^ s    | Source \[cm^2^        |
|                       | sr\]^-1^            | s\]^-1^               |
+-----------------------+---------------------+-----------------------+
| 50 GeV                | 1.8×10^-10^         | 1.2 ×10^-10^          |
+-----------------------+---------------------+-----------------------+
| 100 GeV               | 1.2×10^-10^         | 8 ×10^-11^            |
+-----------------------+---------------------+-----------------------+
| 500 GeV               | 5 ×10^-11^          | 3 ×10^-11^            |
+-----------------------+---------------------+-----------------------+

Table 1 contains our estimates of the sensitivity to WIMP lines in GLAST
for the case of the high latitude model, and the Galactic Center. To set
the scale of sensitivity, we have arbitrary calculated I~γ~ for a 3σ
signal. The GLAST observational parameters used were η=0.02, G=0.5 m^2^
sr (efficiency of the event selection is taken into account), S=6000
cm^2^ (effective area of the calorimeter to provide path length more
than 20 X~0~ ), and T = 3 years. More exact treatment raises the
estimated sensitivity by a factor of 1.1-1.6. We used the high latitude
gamma-radiation flux given in Sreekumar et al., 1998 and Galactic Center
radiation from Hunter et al., 1997; Mayer-Hasselwander et al., 1998. The
upper energy limit for this gamma ray line search is limited by the
dynamic range of the calorimeter readout electronics; also at higher
energies the low photon flux is the limiting factor. We note that our
sensitivity is a few times higher than the optimistic estimates for the
predicted flux (Bergstrom, Ullio & Buckley, 1998) but both the number of
assumptions in the expected flux calculations and inestimable importance
of a positive result motivated this work.

[Acknowledgements]{.underline}. The authors are grateful to David
Bertsch, Robert Hartman and David Thompson for their valuable comments.

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