My Research Introduction

Recognition of the Current Particle Phenomenology

In past decades, people tried to develop the field of particle physics based on the so-called electroweak naturalness, namely how the electroweak scale (the energy scale creating the electromagnetic and weak forces from more fundamental ones; about the masses of the weak bosons, W and Z, which mediate the weak interaction, 10¹¹ eV = 100 GeV) should be naturally explained. Many new physics scenarios such as supersymmetry, extra-dimension and composite Higgs have been proposed in this context, and have been and are still being tested by various experiments and observations including the large hadron collider (LHC). However, people have recently started doubting this guiding principle, the electroweak naturalness, because new physics signals predicted by those scenarios have not been detected there at all. To be more precise, the doubt is that, although the electroweak scale should be naturally explained, it may be achieved by some other mechanisms (or ideas) which are different from what we have thought about thus far. People have very recently started seeking new mechanisms based on this consideration, but no one has yet succeeded in proposing a critical mechanism that many people agree with.

 

Uncovering the Nature of Dark Matter Attracts Attention

Under these circumstances, people start taking a new strategy: developing particle physics by resolving the cosmic dark matter problem. Dark matter was proposed by Fritz Zwicky in 1934 to explain the motion of galaxies inside the Coma Cluster, and its existence was established at the beginning of this century thanks to cosmological observations such as the fl­uctuation of the cosmic microwave background. Its detailed nature, however, is unknown. Moreover, it is known that no dark matter candidate exists in the standard model. As a result, once the nature of dark matter is clarified, it can be used as a bridgehead to launch out into an exploration of new physics. In other words, the strategy is that we first determine the nature of dark matter and then explore new physics beyond the standard model.

 

Various Dark Matter Studies at Kavli IPMU

Let me summarize what we know about dark matter. First, dark matter is electrically neutral. ^#1 Next, dark matter has, at least, a non-zero mass and moves non-relativistically in the present universe; namely its speed is much slower than that of light in the present universe. Third, its lifetime should be much longer than the age of the universe (13 billion years). Moreover, dark matter hardly interacts with ordinary matter like nuclei. Finally, as quantitative knowledge, it is known that the average mass density of dark matter in the universe, which is sometimes called dark matter abundance, is about 2 x 10^–30 g/cm³. Despite this knowledge, it is still difficult to say that we have enough information about dark matter. For instance, its mass is merely predicted to be in a range between 10^–55 g and 10³⁵ g; namely there is the uncertainty of about a hundred orders of magnitude. Since such huge uncertainty makes it impossible to perform a comprehensive search of dark matter experimentally, the present strategy of detecting dark matter relies on several influential hypotheses; those are classified into some regions as shown in Fig. 1. People develop a strategy of detection in each region.

 

Various dark matter candidates are now being deeply and comprehensively studied at Kavli IPMU. Here, let me very briefly summarize several studies of dark matter in our institute, which are spread over a wide range of dark matter candidates:

• Primordial Black Hole. Visit Masahiro Takada’s website for more details.
• Axion Dark Matter. Visit Masahiro Kawasaki’s website for more details.
• Ultralight Dark Matter. Visit Yevgeny Stadnik’s website for more details.
• Sterile Neutrino Dark Matter. Visit Alexander Kusenko’s website for more details.
• SIMP Dark Matter. Visit Hitoshi Murayama’s website for more details.
• …

Studies of WIMP-like Thermal Dark Matter

Among various dark matter candidates, the search based on the WIMP-like thermal dark matter hypothesis is more developed than the others, and my study is now mainly focusing on dark matter candidates of this kind. The thermal dark matter hypothesis is as follows: dark matter is an (undiscovered) elementary particle and its abundance observed today was produced by the so-called freeze-out mechanism. In the freeze-out mechanism, dark matter is assumed to be in chemical and kinematical equilibrium with the thermal bath composed of standard model particles during a very early epoch of the universe. However, it eventually decoupled from the bath because the reaction rate which was maintaining the equilibrium becomes smaller than the expansion rate of the universe. After the decoupling, the amount of dark matter in the universe becomes constant, determining dark matter abundance today. This mechanism is also applied to other phenomena of cosmology (big-bang nucleosynthesis and recombination) and explains observational results very successfully. Because of this fact, the thermal dark matter hypothesis is regarded as one of the natural hypotheses about dark matter. When dark matter mass is around the electroweak scale, the thermal dark matter of this kind is called WIMP (Weakly Interacting Massive Particle), and it is intensively discussed in various new physics scenarios beyond the standard model.

 

Classification of WIMP-like Thermal Dark Matter

Since the thermal dark matter is assumed to be in equilibrium in the early universe, it inevitably interacts with standard model particles. The detection of the thermal dark matter relies on this fact, and the important question here is which standard model particle the dark matter interacts with. The strategy of dark matter detection depends strongly on the answer to this question. Hence, the WIMP-like thermal dark matter is further classified based on its quantum numbers to search for it systematically and comprehensively. Here, the quantum number means a charge associated with a force, for instance, the electric charge of the electromagnetic force. Dark matter is electrically neutral as mentioned above, and it should not carry a charge of the strong interaction, a color charge; otherwise it would already have been discovered by various experiments. As a result, the quantum numbers we should focus on are those of the weak and gravitational forces. The quantum number of the weak force is called a weak charge, while one of those of the gravitational force is a spin. Another quantum number of the gravitational force is nothing but mass, and it is determined by requiring that the abundance of dark matter, which is computed after fixing its weak charge and spin, coincides with the result of cosmological observation. In other words, the mass becomes a derived quantum number for the thermal dark matter.

 

This classification is summarized in Fig.2. First, the thermal dark matter has a weak charge in the unit of 1/2, namely, 0, 1/2, 1, etc. There is, however, an exception: dark matter may be described by the mixing (linear combination) of states which have different weak charges. Within the familiar electromagnetic dynamics, it corresponds to the case that we consider the particle whose electric charge is given by the mixing of, e.g., 0 and 1. Of course, such mixing occurs only for the weak force because of the electroweak symmetry breaking, and never occurs for other forces. As a result, dark matter can be classified into three categories: it has no weak charge (neutral under the weak force), has a non-zero weak charge, or has mixed weak charges. Next, if we restrict ourselves by considering a renormalizable theory to guarantee enough magnitude of interactions between dark matter and standard model particles, the spin of dark matter is restricted to be either 0, 1/2, or 1. Thanks to the nature of the relativistic quantum field theory, once the weak charge and the spin of dark matter are fixed, all interactions between dark matter and standard model particles are uniquely determined from the viewpoint of minimality and renormalizability, which allows us to discuss the physics of the WIMP-like thermal dark matter quantitatively.

 

How to detect WIMP-like Thermal Dark Matter

The above classification of the thermal dark matter has indeed been proposed by us (researchers studying particle phenomenology at the Kavli IPMU), detected in near future based on various collaborations with experimental and observational researchers. Some results of our study concerning the type of experiment, seeming to be the most efficient for detection in each category, are also shown in Fig. 2.

 

First, in the case where dark matter has mixed weak charges, the scattering cross-section between dark matter and a nucleus is always predicted to be large, for its origin is the same as those of the mixing [1]. As a result, direct dark matter detection in underground laboratories is very efficient for detecting such dark matter, and indeed many experiments are now being conducted around the world. Moreover, various future projects such as the XENONnT,^#2 (in which some of our colleagues in our institute are involved) have been proposed and approved. When the dark matter is in this category, it will be detected in near future.

 

Next, when the dark matter has a non-zero weak charge, its mass is generally predicted to be more than the electroweak scale, i.e. the TeV scale, which makes it difficult to detect it in the near future at collider experiments [2]. On the other hand, we have pointed out that its annihilation cross-section is significantly enhanced thanks to the Sommerfeld effect, ^#3 which makes indirect dark matter detection very efficient [3]. In particular, the observation of gamma-rays from Milky Way satellites^#4 by the CTA collaboration^#5 is expected to play an important role; however, to maximize its sensitivity, we also need to know precisely how dark matter is distributed in each satellite. Fortunately, this problem will be overcome by the PFS project at the Subaru telescope organized mainly by Kavli IPMU [4]. Moreover, when third-generation direct dark matter detection experiments such as the DARWIN project,^#6 the successor of XENONnT, become available, it will be possible to detect the TeV-scale dark matter. Here, it is important to address the fact that such TeV-scale dark matter is predicted by the so-called anomaly mediated SUSY breaking scenario^#7 (Pure Gravity Mediation model [5], etc.), the one attracting the most attention after the discovery of the Higgs boson, and thus detecting the TeV scale dark matter is now regarded as an urgent issue of particle physics. In addition, some new physics models predict dark matter having a non-zero weak charge but are produced not only by the freeze-out mechanism but also by some other non-thermal ones. In such cases, the mass region around the electroweak scale also becomes important to search for. The future LHC (HL-LHC) and lepton collider such as the international linear collider (ILC) experiments will play an important role in searching for such electroweak dark matters [6].

 

Finally, when the thermal dark matter is neutral under the weak force, i.e., when the dark matter is singlet, its mass is predicted to be in between O(1)MeV and the electroweak scale. Let us first think about the singlet dark matter with an electroweak scale mass. Though such a dark matter is the one most intensively searched for, there still be uncharted parameter regions, e.g., the leptophilic, Z-funnel, and CPV H-portal regions. The leptophilic dark matter is the one that interacts mainly with leptons, the Z-funnel dark matter interacts mainly with the Z boson with its mass being around half of the Z boson, and the CPV H-portal dark matter interacts mainly with the Higgs boson via the pseudo-scalar coupling. These candidates are found to be hardly detected at present collider and underground experiments, while those are easily done at future lepton colliders, i.e., ILC, via the mono-photon search, the invisible Higgs width measurement, etc. [7].

On the other hand, the singlet dark matter also allows us to think about the light dark matter region whose mass is much smaller than the electroweak scale. Moreover, such a light dark matter scenario often predicts the existence of a light mediator, and it becomes a target for new physics searches, too. To detect these light particles, we must develop new technologies on various dark matter detections. At collider detection, high luminosity collider experiments such as Belle II and various K meson experiments will be mandatory to detect such light but very weakly interacting particles. Recently, it has also been pointed out that the Higgs boson decaying into a pair of mediators will play an important role to detect the light particles [8]. In this case, the precision measurement of rare Higgs boson decays at high-energy colliders will become important. At direct detection, we must consider materials (detectors) that can detect less energetic signals caused by the light dark matter scattering. Various materials including chemical ones are now being intensively studied in collaborations with researchers in other fields [9]. At indirect detection, the light dark matter is expected to annihilate into photons with energy less than GeV, so that MeV gamma-ray detection, which is the topic that is being developed very recently, will play an important role [10].

 

Concluding Remarks

In conclusion, people are currently making great efforts to look for a new physics scale in particle physics after the completion of the standard model (discovery of the Higgs boson) and the non-observation of new physics signals. Clarifying the nature of dark matter is certainly expected to play an important role in this context. As mentioned in the latter half of this introductory essay, in recent decades, closer collaboration between theorists and experimentalists in various fields has never been more crucial than it is today. The Kavli IPMU provides an ideal environment under these circumstances and I will continue to vigorously develop such dark matter studies.