Dark Matter Search with the CALET Cosmic-ray Detector on the ISS
—and what it can contribute to society despite Dark Matter not being found yet—

Holger Martin MOTZ
Assistant Professor, Faculty of Science and Engineering, Waseda University

Newest Results from CALET and their Interpretation

The CALorimetric Electron Telescope (CALET) is a cosmic-ray detector installed on the International Space Station (ISS) in August 2015 [fig 1, fig 2], developed and operated by an international collaboration inducing the space Agencies JAXA (Japan), NASA (US) and ASI (Italy), as well as universities from the three countries. The lead institution is Waseda University, with professor Shoji Torii as the project's primary investigator. At the recent International Cosmic Ray Conference (ICRC2017) in Busan, Korea, the CALET collaboration showed its main result, the electron+positron cosmic ray energy spectrum up to 1 TeV of energy [fig 3] to the international research community [ref 1]. This result is the output of an effort by numerous physicists and engineers over a time-scale of two decades, so why is it of importance?

Figure 1: CALET was launched on August 19, 2015 from Tanegashima Space Center aboard the HTV-5 unmanned spaceship atop an H2B rocket. After arrival at the ISS on August 24, it was installed by the station's robotic arms on the external platform of the Japanese Experimental Module (Kibo) [picture: JAXA]

Figure 2: Schematic of the CALET detector, showing the location of the main instrument, the calorimeter, a particle detector optimized to measure the energy of cosmic rays, and the secondary Gamma-ray Burst Monitor (CGBM) instrument.

One of the main points in the physics case of CALET is the search for signatures of Dark Matter annihilation or decay. Dark Matter is a popular branch of research, aiming at the discovery of what is widely believed to be a new fundamental particle. It is the strongest sign at the moment, that there is more than the world of the Standard Model of Particle Physics, which allows to make very precise calculations for the interactions of all known particles, and was triumphantly corroborated by the discovery of the Higgs boson.

Dark Matter's existence and many of its properties are well confirmed from observations of its gravitational influence on several scales, e.g. in the velocity at which stars move in galaxies, to the bending of light around whole clusters of galaxies. It is about five times more prevalent than the ordinary matter comprising stars, planets, dust and gas clouds. The recent measurements of Gravitational Waves indicate that gravity is well described by the theory of General Relativity and further constrain alternative theories which propose a different law of gravity instead of additional mass to explain these observations. But since Dark Matter does not interact with light, and at most very weakly with the ordinary matter, its nature beyond its mass is largely unknown. However many theories describing the particle nature of Dark Matter predict that the particles can self-annihilate, i.e. the collision of two Dark Matter particles will destroy them, or decay with a very long life-time. Both processes yield energy from which high energetic visible particles are created. This might create additional cosmic rays with a distinct energy spectrum, basically a peak standing out above the smooth background spectrum of the usual cosmic rays from supernovae.

The measured spectrum shows several steps and peaks which could be interpreted as a signature from Dark Matter. To resolve such structures in the spectrum a good energy resolution, and since the flux of particles is small and drops steeply with energy, long term observation in space with a sufficiently large detector are necessary. CALET is the first experiment to meet both requirements for precise measurements of the spectrum in the TeV region.

Single astrophysical sources, supernovae or pulsars - fast rotating neutron stars - are a more mundane interpretation of these yet not very significant structures, and any claim that Dark Matter is their origin must be accompanied by a study that the visible universe can't create such a signature as well.

Figure 3: The energy spectrum of the electron+positron cosmic-rays measured by CALET. Deviations from a smooth curve like the blue model might be signatures of nearby astrophysical sources or Dark Matter (as shown by red example), but they are not - yet ? - statistically significant. Further observation will improve statistics.

Yet information about the nature of Dark Matter can be extracted from this spectrum, in the form of limits, excluding models which would be expected to create significantly higher peaks in the spectrum, or at different energies, than those observed [ref 2]. These negative results of Dark Matter search published by experiments applying various methods have already constrained the possible parameters of what Dark Matter could be.

Dark Matter search is just one of many goals of the CALET project though, others include the search for signatures of nearby supernovae remnants, to clarify the conditions for cosmic ray propagation in the galaxy and detection and study of gamma-ray bursts. CALET is also engaged in the search for gamma-ray signals in coincidence with Gravitational Wave events measured by LIGO and other detectors, caused by black-hole or neutron star mergers [ref 3].

Impact of Fundamental Science on Society and Education

As the cost of the CALET project is in the range of several billion yen (the launch cost of the H2B carrier rocket alone is about 15 billion, though not only CALET but also supplies for the ISS were carried on the same flight), the justification of such large scale fundamental science projects which have no direct monetary payback of the investment is a subject of constant debate. In this second part of the essay I describe what I see as important impacts from this research on society. There are obviously many other important benefits such as spin-off technologies which are not addressed.

Physics, especially elementary particle physics, is the foundation of modern cosmology. Cosmology, as our understanding of the origin of the universe, strongly influences how we perceive our own role in it, influencing our behaviour. However science provides only a description of the universe, not a reason for its and thus our (human) existence, as it works on the principle of falsifiable theories which must be tested against observation, and no test of a theory on this ultimate reason seems conceivable. Nevertheless, modern cosmology can provide a rather complicated, yet viable theory for the development of matter and energy from a homogeneous hot plasma in the early universe to the current state of galaxies, star systems and planets, without any external ordering influence. Religion and other alternative approaches tend to provide seemingly easier and thus popular explanations for this development, which then are combined with a variety of other teachings with larger impact on daily life. While it is necessary to accept any form of believe for the fundamental right of freedom of thought and conscience should be respected, the actions taken by individuals based on their believes affects all of society. Due to this entanglement I believe that constant effort in fundamental science is necessary, as the only alternative to this ongoing search is conjured up lore.

What is taught to high-school and university undergraduate students in mechanics, electromagnetism and thermodynamics courses are mostly effective theories which provide a convenient way to make predictions and calculate effects within their validity range, confirmed by countless observations in everyday life or simple experiments, thus giving little reason to have doubt in them. It must be emphasized that in cosmology and related fields the process of proposing theories and checking them with observation is still vigorously going on, currently especially about the particle nature of Dark Matter. So while there is no answer yet about what Dark Matter is, the search for it can serve as an example to educate the general public and students alike in how science works.

The Humboldtian model of higher education promotes the unity of research and education, as well as the independence of study (the motto of Waseda University), as it aims at providing students with the ability to do independent research. Naturally this is the reason why university professors are recruited from the ranks of researchers, as the methods used in research can only be taught by someone who is proficient in it. Recently this principle is sometimes questioned, since the majority of graduates takes up careers outside academia, calling for an university education which focuses more on the perceived needs of prospective employers. In my opinion this change in the style of university education, which gradually already happens, is an unfortunate development. The ability to do independent research is not only needed in academia, but increasingly important in many professions, as many other occupational tasks could be taken over by artificial intelligence systems. While an existing solution for a problem may be good enough, rethinking the problem and challenging the established solution often is rewarded by discovering new methods, leading to advancement in technology. In this broader sense the tested theory is that the current method is the best one, which may be falsified by finding a new, better suited alternative.

These processes usually require to analyse and thoroughly understand the existing information on the given subject in order to add to it or revise it. This information has grown in most disciplines beyond the point where any individual can remain informed about all developments. While the Internet can quickly provide most relevant information about a specific problem, a solid basic knowledge and the ability to discern wrong or outdated information from useful and valid one is necessary and could be more valuable than in-depth knowledge.

Above skills to efficiently perform the "research process" and the scientific principles can not be taught from a textbook, but for what I think is good reason are traditionally transferred in working with mentors on a common project in a laboratory, with the results documented in a thesis. A discipline like Dark Matter search where the interplay of theory and experiment/observation can be experienced first-hand on a relatively short time-scale provides an excellent background for students to acquire these skills. So while the big price of discovering the nature of Dark Matter is maybe still far away, teaching of science certainly benefits from the search for it.

Figure 4: Doctoral student Saptashwa Bhattacharya (right) from India, who is supported by a JICA program, works with me in Torii-laboratory on the search for Dark Matter signatures in cosmic rays (one of his papers: [ref 4]).

  • ^ The CALorimetric Electron Telescope (CALET) on the ISS:
    Preliminary Results from the On-orbit Observations since October, 2015
    S. Torii for the CALET collaboration, presented at the ICRC 2017 conference
    Proceedings: PoS (ICRC2017) 1092
  • ^ CALET’s Sensitivity to Dark Matter Annihilation in the Galactic Halo
    H. Motz, Y. Asaoka, S. Torii and S. Bhattacharyya
    JCAP 1512 (2015) 0047
  • ^ CALET Upper Limits on X-ray and Gamma-ray Counterparts of GW 151226
    O. Adriani et al. [CALET collaboration]
    Astrophys. J. 829, no. 1, L20 (2016)
  • ^ Decaying Fermioninc Dark Matter Search with CALET
    S. Bhattacharyya, H. Motz, S. Torii, Y. Asaoka
    JCAP 1708 (2017) 0012

Holger Martin MOTZ
Assistant Professor, Faculty of Science and Engineering, Waseda University

Holger Martin MOTZ is Assistant Professor at the Faculty of Science and Engineering, Waseda University. He did his graduate work at Friedrich-Alexander Universität Erlangen-Nürnberg, Germany, completing his Ph.D. on Dark Matter Search with the ANTARES Neutrino Telescope in 2011. He came to Japan in 2012, working at the University of Tokyo's Institute for Cosmic Ray Research as a project researcher. He moved to Waseda University and joined Torii Laboratory and the CALET project in 2013.
Since 2014, he works on his current position in the International Center for Science and Engineering Programs, Waseda University. His academic interests are in Astroparticle Physics, Cosmic Ray Physics, and Dark Matter.