Introduction

An Indirect Dark Matter Detector

GAPS (General Antiparticle Spectrometer) is a proposed experiment to search for the anti-deuteron particle in the cosmic rays. Astrophysically produced anti-deuterons have never been detected and so the unambiguous detection of even a single event would be very significant. Anti-deuterons may also be a tell-tale signature of dark matter annihilations.

GAPS, initially configured as a long-duration balloon experiment, will detect anti-deuterons with an effectively background-free method. Anti-deuterons produced by the annihilation of weakly interacting massive particles (WIMPs), a well-motivated dark matter candidate, will be captured in the GAPS target material, resulting in an exotic atom in an excited state. This exotic atom will then quickly decay, producing X-rays of precisely defined energies and a correlated pion signature from nuclear annihilation.

The method of detection uses a time-of-flight (TOF) system, which tags candidate events and particle velocities and planes of pixillated Si(Li) detectors, which serve as the target material and tracking detector. The Si(Li) detectors provide both excellent X-ray energy resolution and good particle tracking. The GAPS method has already been successfully tested in an accelerator environment at KEK in 2004 and 2005. A prototype flight of a portion of the instrument is planned for 2011.

At low energies, antideuteron is rarely
produced by cosmic rays while dark
matter models present a high flux,
resulting in an effectively background
free experiment.

Science

Neutralino Self-annihilation

Approximately 90% of the mass in the universe is dark matter and discovering its nature is one of the most pressing goals in science. It is hypothesized to be a new particle which is both cold and non relativistic, a WIMP (weakly interacting massive particle). A leading WIMP dark matter candidate is the hypothetical neutralino. The neutralino is predicted by the supersymmetric (SUSY) extension to the standard model and is a heavy, stable particle with the WIMP energy scale.

In contrast to direct searches and accelerator experiments, GAPS will look for the antideuterons that result from neutralino self-annihilation. These resultant particles should be detectable from areas with high dark matter density. In addition, GAPS will also explore other antideuteron sources, such as evaporating primordial black holes.

Antideuteron detection is a novel approach to the dark matter hunt. While antiprotons are more copiously produced in neutralino-neutralino annihilation, they are difficult to distinguish from cosmic ray antiprotons. Cosmic rays, however, rarely produce antideuterons below 1 GeV. Furthermore, dark matter models predict a relatively high antideuteron flux in this energy range. Thus, at low energies, antideuteron detection is effectively a background free signture of dark matter.

Experiment

Previous Experiments

A GAPS prototype was tested at the KEK accelerator in 2004 and 2005. When antiprotons were dumped into the target, X-ray events with multiple signatures were detected. Furthermore, pion stars provided additional antiparticle identification and non antiparticle background was clearly identified and rejected. These experiments successfully demonstrated the multi X-ray and pion detection method to identify antiparticles. Furthermore, the absolute X-ray transition yields for antiprotons were measured for the first time in several target material.

The prototype instrument

Based on the accelerator testing and extensive design studies, we have designed a flight instrument capable of reaching deep into the parameter space of many CDM models. The heart of the instrument consists of layers of coarsely pixilated Si(Li) detectors that function as both a target in which exotic atoms are formed and a detector of the subsequent atomic transition X-rays. The Si(Li) detectors also serve as a particle tracker for the incoming antiparticle as well as the annihilation pions. An array of plastic scintillators surrounding the Si(Li) detectors serves as a time of flight (TOF) trigger, providing a measurement of the incoming antiparticle velocity.

Detection

Si(Li) detectors

GAPS provides a nearly background free technique of identifying antideuterons by using three different methods of detection for every event. Initially, a plastic time of flight (TOF) system tags the particle and records the velocity. This will distinguish antideuterons from lighter particles, such as antiprotons. The particle then slows down and stops in the target, forming an excited exotic atom. This atom then deexcites and releases both X-rays and a pion star.

Si(Li) wafers will be hexagonally packed into ten layers as the detectors. These have a timing of 50 ns and a 2 keV energy resolution while only requiring a 2 X-ray coincidence for detection. Because the X-ray energy only depends on the mass and charge of the particle, X-ray energy signatures precisely determine the type of antiparticle detected as it makes transitions to lower states. The pion star, the third layer of detection, provides greater background suppression.

Rendering of the 2009 prototype

2011 Experiment

A smaller, balloon based prototype flight is planned for 2011. We will flight test all of the critical features of the full experiment, and in particular we will: confirm proper operation of the Si(Li) detectors at float altitude; measure X-ray and particle backgrounds of relevance to determining the overall instrument sensitivity; confirm the thermal model for predicting the Si(Li) operating temperature and verify the concept for cooling the Si(Li) detectors. This prototype balloon flight will lead to a full balloon experiment that is expected to be ready to fly from Antarctica by 2014.

Papers and Talks

GAPS Team

	Columbia University T. Aramaki, C.J. Hailey (P.I.), J.E. Koglin, N. Madden, K. Mori, H.T. Yu
	University of California, Berkeley S.E. Boggs
	University of California, Los Angeles R.A. Ong, J. Zweerink
	Lawrence Livermore National Laboratory  W.W. Craig
	Institute of Space & Astronautical Science, Japan Aerospace Exploration Agency H. Fuke, T. Yoshida
	University of Latvia F.H.Gahbauer