FEATURE

Kavli IPMU Principal Investigator

Shigeki MatsumotoResearch Area： Theoretical Physics

Dark Matter Study as a Bridgehead for New
Physics Exploration

Where Can We Find New Physics ?
In past decades, people tried to develop

other mechanisms (or ideas) which are totallydifferent from what we have thought about thus
far. People have very recently started seeking new

the field of particle physics based on the so-mechanisms based on this consideration, but no one

called electroweak naturalness, namely how thehas yet succeeded in proposing a critical mechanism

electroweak scale (the energy scale creating thethat many people agree with.

electromagnetic and weak forces from more
fundamental ones; about the masses of the
weak bosons, W and Z, which mediate the weak

Uncovering the Nature of Dark Matter
Attracts Attention

interaction, 1011 eV = 100 GeV) should be naturally

Under these circumstances, people start taking

explained. In fact, many new physics scenarios sucha new strategy: developing particle physics byas supersymmetry, extra-dimension and composite

resolving the cosmic dark matter problem. Dark

Higgs have been proposed in this context, andmatter was proposed by Fritz Zwicky in 1934 inhave been and are still being tested by various

order to explain the motion of galaxies inside theexperiments and observations including the large

Coma Cluster, and its existence was established at

hadron collider (LHC). However, people havethe beginning of this century thanks to cosmologicalrecently started doubting this guiding principle,

observations such as the fluctuation of the cosmic

the electroweak naturalness, because new physics

microwave background. Its detailed nature, however,signals predicted by those scenarios have not beenis unknown. Moreover, it is known that no dark

detected there at all. To be more precise, the doubt

matter candidate exists in the standard model. As

is that, although the electroweak scale should bea result, once the nature of dark matter is clarified,naturally explained, it may be achieved by someit can be used as a bridgehead to launch out into

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Kavli IPMU News No. 45 March 2019an exploration of new physics. In other words, theis more developed than the others. I would therefore

strategy is that we first determine the nature oflike to focus on this hypothesis and describe its

dark matter and then explore new physics beyond

present status and prospects in this article.

the standard model.
Next, let me summarize what we know about
dark matter. First, dark matter is electrically neutral.
In fact, dark matter can have a very tiny electric

Thermal Dark Matter Hypothesis
The thermal dark matter hypothesis is as follows:

charge, or O(1) charge if it is vastly heavy; howeverdark matter is an (undiscovered) elementary particle

we do not consider such special cases in this article.

and its abundance observed today was produced by

Next, dark matter has, at least, a non-zero mass andthe so-called freeze-out mechanism. In the

moves non-relativistically in the present universe;

freeze-out mechanism, dark matter is assumed tonamely its speed is much slower than that of light

be in chemical and kinematical equilibrium with thein the present universe. Third, its lifetime should bethermal bath composed of standard model particles

much longer than the age of the universe (13 billion

during a very early epoch of the universe. However,years). Moreover, dark matter hardly interacts withit eventually decoupled from the bath because theordinary matter like nuclei. Finally, as quantitative

reaction rate which was maintaining the equilibrium

knowledge, it is known that the averaged mass

becomes smaller than the expansion rate of thedensity of dark matter in the universe, which isuniverse. After the decoupling, the amount ofsometimes called dark matter abundance, is about

dark matter in the universe becomes constant,–30

2 x 10g/cm .

determining dark matter abundance today. Fig. 1

3

In spite of this knowledge, it is still difficult to
say that we have enough information about darkshows typical behavior of the amount as a functionof m/T, with m and T being dark matter mass

matter. For instance, even if we assume that dark

and the temperature of the universe, respectively.

matter is an elementary particle, its mass is merely
g andDark matter abundance is seen to coincide withthat of equilibrium when m/T ~ O(1), while it

10 g; namely there is uncertainty of fifty orders

deviates from the equilibrium at m/T ~ O(10) andof magnitude. Since such huge uncertainty makes

eventually becomes constant. This mechanism is

it impossible to perform a comprehensive searchalso applied to other phenomena of cosmologyof dark matter experimentally, we are currently(big-bang nucleosynthesis and recombination) andtrying to detect dark matter based on several

explains observational results successfully. Because

influential hypotheses such as the thermal darkof this fact, the thermal dark matter hypothesis is–55

predicted to be in a range between 10–5matter, axion, sterile neutrino, and primordial black

regarded as one of the natural hypotheses abouthole hypotheses. Among these, the search for dark

dark matter. When dark matter mass is around thematter based on the thermal dark matter hypothesis

electroweak scale, the thermal dark matter is called

Feature5

Figure 1. Typical behavior of the dark matter amount (the ratio of the number density of dark matterto the number density of radiation). Here, m is the mass of the dark-matter particle and T is thetemperature of the universe, both expressed in units of energy.

WIMP (Weakly Interacting Massive Particle), andnumber is nothing but a charge associated with

it is intensively discussed in various new physics

a force, for instance, the electric charge of the

scenarios beyond the standard model.

electromagnetic force. Dark matter is electricallySince thermal dark matter is assumed to be

neutral as mentioned above, and it should not carryin equilibrium in the early universe, it inevitably

a charge of the strong interaction, a color charge;

interacts with standard model particles. The

otherwise it would already have been discovered

detection of the thermal dark matter relies on thisby various experiments. As a result, the quantum

fact, and the important question here is which

numbers we should focus on are those of the weakstandard model particle the dark matter interacts

and gravitational forces. The quantum number

with. The strategy of dark matter detectionof the weak force is called a weak charge, while

depends strongly on the answer to this question.

one of those of the gravitational force is a spin.

Hence, the thermal dark matter is classified based

Another quantum number of the gravitationalon its quantum numbers in order to search for it

force is nothing but mass, and it is determined

systematically and comprehensively. The quantum

by requiring that the abundance of dark matter,

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Kavli IPMU News No. 45 March 2019

Figure 2. Classification of the thermal dark matter and the most efficient detection method for each case.

which is computed after fixing its weak charge

charges. Next, if we restrict ourselves consideringand spin, coincides with the result of cosmologicala renormalizable theory to guarantee enoughobservation. In other words, the mass becomes amagnitude of interactions between dark matter andderived quantum number.

standard model particles, the spin of dark matteris restricted to be either 0, 1/2, or 1. Thanks to theClassification of Thermal Dark Matter
This classification is summarized in Fig. 2. First,

nature of the relativistic quantum field theory, once
the weak charge and the spin of dark matter are
fixed, all interactions between dark matter andthermal dark matter has a weak charge in thestandard model particles are uniquely determined,

unit of 1/2, namely, 0, 1/2, 1, …. There is, however,which allows us to discuss physics of the thermal

an exception: dark matter may be described bydark matter quantitatively.

the mixing (linear combination) of states whichhave different weak charges. Within the familiar
electromagnetic dynamics, it corresponds to the

How to Find Thermal Dark Matter
Based on Its Classificationcase that we consider the particle whose electric

The above classification of thermal dark mattercharge is given by the mixing of, e.g., 0 and 1. Of

has indeed been proposed by us (researchers

course, such a mixing occurs only for the weak force

studying particle phenomenology at the Kavli IPMU),

because of the electroweak symmetry breaking,and we are now comprehensively studying how

and never occurs for other forces. As a result, dark

dark matter in all the categories can be detected

matter can be classified into three categories: it hasin the near future based on various collaborations

no weak charge (neutral under the weak force),with experimental and observational researchers.

has a non-zero weak charge, or has mixed weakSome results of our study concerning the type of

7

Featureexperiment, seeming to be the most efficient for
detection in each category, are also shown in Fig. 2.

the most attention after the discovery of the Higgs
boson, and thus detecting the TeV scale dark matter

First, in the case where dark matter has mixed

is now regarded as an urgent issue of particle

weak charges, the scattering cross section between

physics. In addition, some new physics models

dark matter and a nucleus is always predicted to

predict dark matter having a non-zero weak charge

be large, for its origin is the same as those of theis produced not only by the freeze-out mechanism

mixing. As a result, direct dark matter detection

but also by some other non-thermal ones. In suchin underground laboratories is very efficient forcases, the mass region around the electroweak

detecting such dark matter, and indeed many

scale also becomes important to search for. The

experiments are now being conducted aroundfuture LHC (HL-LHC) and lepton collider such as

the world. Moreover, various future projects suchthe international linear collider (ILC) experiments

as the XENONnT, where it was decided that thewill play an important role in searching for such

XMASS group (in which some of our colleagues inelectroweak dark matter.

*1

our institute are involved) would join, have been

Finally, we consider the case in which the thermal

proposed and approved. When dark matter isdark matter is neutral under the weak force. Whenactually in this category, it will be detected in thethe spin of dark matter is 0 or 1, its scattering crossnear future.

section off a nucleus is predicted to be large in

Next, when dark matter has a non-zero weakgeneral. Hence, the direct dark matter detection ischarge, its mass is generally predicted to be inagain efficient, as in the case of dark matter having

the TeV scale, which makes difficult to detect it

mixed weak charges. On the other hand, when thein the near future at collider experiments. On thespin is 1/2, the scattering cross section does notother hand, we pointed out that its annihilationnecessarily become large. In particular, when darkcross section is significantly enhanced thanks tomatter interacts mainly with leptons (leptophilic

the Sommerfeld effect,*2 which makes indirect

dark matter), mainly with the Higgs boson by thedark matter detection very efficient. In particular,

pseudo-scalar coupling (CP violating Higgs portal

the observation of gamma-rays from Milky Way

dark matter) or is lighter than the proton (light*3*4

satellites by the CTA collaboration is expected
to play an important role; however, in order tomaximize its sensitivity, we also need to know
precisely how dark matter is distributed in eachsatellite. Fortunately, this problem will be overcomeby the PFS project at the Subaru telescopeorganized mainly by Kavli IPMU. Moreover, whenthird generation direct dark matter detection
experiments such as the DARWIN project,*5 thesuccessor 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*6 (PureGravity Mediation model, etc.), the one attracting

8＊1

XENONnT is a direct dark-matter search experiment to be conductedat the Gran Sasso underground laboratory LNGS in Italy, using 8 tonsof liquid xenon. The experiment aims at detecting dark matter particles
through measurement of nuclear recoil events due to scattering of
dark-matter particles with xenon nuclei.
＊2
The Sommerfeld effect is the effect in which a long-range force
between the initial-state particles significantly changes the inelasticscattering cross section between them compared to that withouta long-range force. It is known that if the dark matter particle issufficiently heavier than the particles that mediate the weak force (Wand Z bosons), it will act as a long-range force for dark-matter particles
and significantly enhances their annihilation cross section.
*3
Among satellite galaxies, those called dwarf spheroidal galaxies
have particularly low luminosity and small mass. As their masses are
considered to be dark-matter dominated, they are included in the main
objectives of dark-matter search with gamma-ray observations.
*4
CTA（Cherenkov Telescope Array）is a gamma-ray observatory in the
20 GeV~100 GeV gamma-ray energy region.
*5
DARWIN (DARk matter WImp search with liquid xenoN) is a direct
dark-matter search experiment using 50 tons of liquid xenon.
*6
One of the supersymmetric models which has attracted particularattention since the discovery of the Higgs boson. It is known as thesimplest model that, while accounting for the Higgs mass and being
compatible with grand unified theories, is free from the problems
which often arise in supersymmetric models, i.e., the problems related
to flavor and cosmology.

Kavli IPMU News No. 45 March 2019

dark matter), most of their parameter regions are

precision measurements of rare Higgs boson decays

experimentally uncharted and new methods thatwill become important.

are different from those we have considered so
far are required. For former two cases, dark matterproduction (mono-photon process, etc.) and theprecise measurement of the Higgs boson in future

Conclusion
In conclusion, people are currently making great

lepton colliders such as ILC will play an importantefforts to look for a new physics scale in particle

role. On the other hand, for the light dark matter,physics after the completion of the standard modela new light mediator (a scalar or vector particle)(discovery of the Higgs boson) and non-observationis also predicted in addition to dark matter itself

of new physics signals. Clarifying the nature of dark

because of the nature (renormalizablility) of thematter is certainly expected to play an important

relativistic quantum field theory. High luminosity

role in this context. As mentioned in the latter halfcollider experiments such as Belle II and various Kof this article, in recent decades, closer collaboration

meson experiments are mandatory to detect such

between theorists and experimentalists in various

light but very weakly interacting particles. Recently,

fields has never been more crucial than it is today.

it has also been pointed out that the Higgs boson

The Kavli IPMU provides an ideal environment under

decay to a pair of mediators can be an importantthese circumstances and I will continue to vigorously

method for dark-matter search. In this case,develop such dark matter studies.

Tea Break :What IPMU Stands For?
There is an anecdote that the remarkable German mathematician and mathematical physicistof the late 19th and early 20th centuries David Hilbert once said, upon hearing that one of his
students had dropped out to study poetry: "Good, he did not have enough imagination to becomea mathematician.
We would like to appeal to your imagination and announce a competition for the best suggestion
on how someone unfamiliar with the meaning of the abbreviation IPMU could guess what it stands
for. For instance,

FeatureThe Institute for the Pretty Much Unknown,
The Infrared in Physics and the Mathematics of the Ultraviolet ...
We will post the best answers on the Kavli IPMU website and on a poster in the tea hall, making iteven more appropriate for the Tea Break rubric.
(Contributed by Alexander A . Voronov)9