Hyper-Kamiokande

Particles have their partners which have opposite “charge” called ‘anti-particles’. For example, protons and neutrons, which constitute usual matter called ‘Baryon’, have their counter-parts, anti-baryons.

In the early universe, it is a natural assumption that baryons and anti-baryons had been produced by equal amount, however, antibaryons are barely observed in the universe and there is a remarkable imbalance of the baryon and anti-baryons. This ‘Baryon Asymmetry’ is one of the unsolved problems. There are several hypotheses proposed to explain the observed imbalance of the matter and anti-matter. Among of them, one of the possible mechanisms to explain the asymmetry is ‘CP violation’.

C-symmetry is a symmetry under the exchange particle and anti-particle, which is called charge conjugation translation. If a phenomena in a system and corresponding phenomena in a charge conjugated system, the system has C-symmetry.

P-symmetry (Parity symmetry) is an symmetry under the mirror translation and CP-symmetry is a combination of C-symmetry and P-symmetry.

One interesting hypothesis explains that the baryon asymmetry is induced by the violation of the CP symmetry on neutrinos.

Partner of a neutrino under CP-symmetry is anti-neutrino and if CP-symmetry on neutrinos are violated, it is expected that the probabilities of some phenomena between neutrinos and anti-neutrinos are different.

Hyper-Kamiokande project is designed to investigate the difference of the neutrino oscillation probabilities between neutrinos and anti-neutrinos with intensive neutrino/anti-neutrino beam from J-PARC accelerator in Tokai village.

All neutrino mixing parameters are already measured and have finite values. Fortunately, the values are relatively large and it makes it possible to investigate the CP violation of neutrinos. CP violation is expressed by an angle δ. The effect of CP violation is maximumly observed at δ=±90 degree. For that case, Hyper-Kamiokande is expected to discover non-zero size of the violation of the CP-symmetry at 8 sigma significance in 10 years of operation, and for 75% of the δ space, we expect to discover non-zero CP violation with 3 sigma significance.

Though there are three neutrino types, until recently they were thought to be massless. With the discovery of “neutrino oscillations,” however, a phenomenon in which neutrinos change their type in flight, it is now known that not only do they have mass, but the masses of the three types (m₁,m₂,m₃) are different.

Experiments observing the oscillations of neutrinos produced in the sun have determined the mass difference between m₁ and m₂, and the squared difference between the masses m₂ and m₃ has been measured using the oscillations of neutrinos produced in the Earth’s atmosphere. However, since oscillation experiments can only probe the squared difference of the masses for m₂ and m₃, the question of whether or not m₂ is heavier than m₃ remain unknown. The latter question is known as the “neutrino mass hierarchy problem.” If m₂ is lighter than m₃, the hierarchy is said to be “normal,” but if it is heavier the hierarchy is called “inverted” (Figure 1).

Figure 1: Neutrino mass hierarchy. Though the value of the individual masses m₁,m₂,and m₃ are unknown, there are two possible orderings.

Why is the mass hierarchy important?

It turns out that neutrinos and their masses have deep connections with the other elementary particles and nuclei.

Though there are four forces that govern our world (the strong, weak, electromagnetic, and gravitational forces), it is thought that they were unified into a single force under the tremendous temperatures present at the birth of the universe. Theories that seek to explain this “unification” of the forces often predict that the neutrino mass hierarchy is normal. At the same time there are theories that explain the origins of the universe and its particles but which also predict an inverted mass hierarchy. Since it is not possible to recreate the conditions present at the beginning of the universe with modern technology, resolving the neutrino mass hierarchy problem is critical to understanding the early universe using these theories.

Resolving the mass hierarchy also plays a role in understanding the present-day universe. Indeed, it has deep connections to efforts to determine whether or not the neutrino is its own antiparticle. While the particles and antiparticles of all leptons other than neutrinos are known to be different from one another, if there is a lepton whose particle is indistinguishable from its antiparticle, the only possible candidate is the neutrino. For this reason if it could be determined that the neutrino is its own antiparticle it would be a discovery of profound significance. Further, the neutrino mass hierarchy is known to have a strong influence on the number and types of isotopes produced when a star ends its life with a supernova explosion.

Finally, by measuring the differences between the oscillations of neutrinos and antineutrinos it may be possible to solve the long standing mystery of why the present universe is filled with only particles and almost no antiparticles, even though both were thought to exist in equal numbers at its birth. However, if the neutrino mass hierarchy is not known it can obscure the differences in these oscillations and thereby hamper the measurement. Determining the neutrino mass hierarchy is essential to overcoming this difficulty.

Determining the mass hierarchy at Hyper-Kamiokande

Hyper-Kamiokande will observe large numbers of neutrinos produced by the collisions of cosmic rays with nuclei in the atmosphere. Those which are produced in the atmosphere on the opposite side of the Earth will be influenced by its matter on their way to the detector. Accordingly, the oscillations of such muon neutrinos into electron neutrinos as well as the oscillations of muon antineutrinos into electron antineutrinos will be affected. However, the extent of these effects depends upon the mass hierarchy such that for a normal hierarchy oscillations into electron neutrinos are enhanced, while for an inverted hierarchy oscillations into electron antineutrinos are enhanced.

For this reason, the number of events coming from the opposite side of the Earth that oscillate into electron neutrinos will be larger if the hierarchy is normal than if it is inverted (Figure 2). On the other hand, the number of events that oscillate into electron antineutrinos will be larger for the inverted hierarchy than for the normal hierarchy. The change in the event rate due to the mass hierarchy is about a few %. Hyper-Kamiokande is so large it will be able to detect even this small difference.

Figure 2: Determining the neutrino mass hierarchy at Hyper-Kamiokande. The difference in the expected number of events relative to the expectation assuming no oscillations is shown for the normal (red) and inverted (blue) mass hierarchy. The number of events from the opposite side of the Earth (“upward-going”) that oscillate into electron neutrinos is larger for the normal mass hierarchy.

Solar neutrinos

The sun produces light via nuclear fusion reactions in its core, reactions that also generate neutrinos. Since neutrinos arrive at Hyper-Kamiokande only 8 minutes after their generation inside the sun, the Schwabe cycle is the periodic change in the sun’s activity and appearance that has an average duration of about 11 years. Super-Kamiokande has been used to measure solar neutrinos since 1996, however this cycle has not been observed so far. Hyper-Kamiokande will study the source of the solar energy and reveal details as to the evolution of the sun.

Time variation of the solar neutrino flux observed by Super-Kamiokande. There is no correlation between flux and the number of sunspots.

Supernova burst neutrino

Massive stars which are more than 8 times the mass of the sun end their lives in supernova explosions, leaving neutron stars and black holes as their stellar remains. Supernova explosions are the most energetic phenomena involving stellar objects, with almost 99% of the energy carried out from the exploded stars in the form of neutrinos.

On February 23 1987, Kamiokande, the antecedent of Super-Kamiokande, observed neutrinos from a supernova explosion that occurred in the Large Magellanic Cloud (1987A). This observation was used in order to confirm the basic theory of supernova explosions. Unfortunately, no supernova explosions have yet occurred within the scope of the operation of Super-Kamiokande.

Hyper-Kamiokande will extend its reach and therefore its capability for observing supernova explosions to 2 Mpc. If a supernova occurs in our galaxy (10kpc), Hyper-Kamiokande will be able to detect approximately 50,000 neutrinos. Such promising statistics for neutrino observation would enable us to not only investigate the detailed mechanisms of supernova explosions, but also to elucidate the nature of neutrinos.

Supernova 1987A exploded in 1987. The right is before explosion and the left is after explosion.(copyright Australian Astronomical Observatory David Malin Images)

Dark Matter Search

Assuming the existence of dark matter is necessary in order to understand various astronomical observations. However, its nature as a form of elementary particle is not well understood, and many new particles have been proposed as dark matter candidates. One method of searching for a dark matter signal is an indirect search in which one searches for decay or an annihilation that produced neutrinos from the dark matter concentrated at the center of a large gravitational potential, such as the center of the galaxy, the sun or the earth. As Hyper-Kamiokande is a “neutrino telescope,” neutrinos originating from such objects may be discerned. If Hyper-Kamiokande is able to detect dark matter, the whole accepted picture of elementary particle physics and cosmology could be changed.

Hyper-Kamiokande detects the neutrinos generated by the interaction with darkmatters in the Sun or the earth.

Beyond the Standard Model

The Standard Model was completely established after the discovery of the Higgs boson at the LHC, a large accelerator in Europe. The Standard Model is a description involving quarks (which make up the composition of nucleons), leptons (such as electrons), and the strong, weak, and electromagnetic interactions between those particles. The Standard Model has been very successful in explaining various phenomena in elementary particle physics. However, there are several questions remaining, questions that the Standard Model has never been able to answer, such as “Why are there leptons and quarks?,” “Why do they have three generations?,” and “Why are there three interactions?”There must be larger theoretical framework beyond the Standard Model.

Figure 1: The Standard Model

GUTs to predict proton decay

In order to solve those fundamental questions, many theorists have proposed Grand Unified Theories (GUTs) that pass beyond the Standard Model. In these GUTs, strong, weak, and electromagnetic interactions can be unified in terms of a very high energy, around 10¹⁶ GeV, which corresponds to the birth of universe. It is not possible to reach such high energies using an accelerator, but GUTs contain another feature in order to remove the separation between quarks and leptons. GUTs predict that protons, a particle that all materials in the world contain, will decay at some point. Thus, proton decay is the key to unlocking the potential of these GUTs.

Figure 2: Unification of interactions

Measure proton lifetime by huge water Cherenkov detector

How long is proton lifetime?

There are several models for GUTs, but mostly they predict lifetimes longer than 10³⁰ years! This is unimaginably long, especially when compared to the age of the universe, which is estimated to be approximately 138×10⁸ years. Of course, we cannot keep on observing for such long period. However, the lifetime of particles is defined as the time in which the number of particles decreases to 1/2.72 of the initial number. So it may be possible to measure proton lifetime if we prepare a huge number of protons, even though the observation period is minute compared to the lifetime of a proton. This is the reason why we need a large detector in order to measure proton lifetime.

Currently, the most sensitive detector in the world used to examine proton decay is Super-Kamiokande (SK), which contains 7.5×10³³ protons. SK has been performing its observations for more than 12 years, but still proton decay has not been observed and 10³⁴ years has been obtained as the lower limit of proton lifetime.

Figure3: Typical proton decay. A proton decays into a positron and a π0.

Figure 4: Reconstructed proton mass distribution after 10 years run of Hyper-Kamiokande assuming the current lower limit as proton lifetime. Upper figure corresponds to lower reconstructed proton momentum case (< 100 MeV/c) and lower shows higher momentum case (100 ~ 250 MeV/c). Dots shows sum of signal and background, Hatched histogram shows only background.

Hyper-Kamiokande is about 10 times larger than SK and it can overtake the current reach by SK within two years. Fig. 3 shows typical decay mode in which a proton decays into positron and π0. Hyper-Kamiokande can detect all final particles and mass (938MeV/c²) and momentum of proton can be reconstructed from decayed particles. Fig.4 shows the reconstructed proton mass distribution expected after 10 years run of Hyper-Kamiokande. If proton lifetime is assumed as the current lower limit obtained by SK, we will see clear peak above background as seen in Fig.4. Especially, the lower proton momentum region (< 100 MeV/c, upper figure) is expected to be almost background free and only a few events in this region could be an evidence of the proton decay. Furthermore, Hyper-Kamiokande has sensitivity up to more than one order longer than the current lower lifetime of proton and most of GUT models can be examined (Fig. 5). Hyper-Kamiokande will discover proton decays and we will challenge the root of materials and mysteries in the genesis of the universe beyond the Standard Model.

Figure 5: Predictions of proton lifetime and sensitivity of Hyper-Kamiokande. The sensitivity can be jumped up more than 10 times after 10 years run of Hyper-Kamiokande and can cover most of predictions.