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Proton decay searches

Beyond the Standard Model

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Figure1: The Standard Model [click to enlarge]

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.

 

GUTs to predict proton decay

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Figure2: Unification of three interactions [click to enlarge]

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.

Measure proton lifetime by huge water Cherenkov detector

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Figure3: Typical proton decay. A proton decays into a positron and a π0. [click to enlarge]

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 138x108 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.

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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. [click to enlarge]

Currently, the most sensitive detector in the world used to examine proton decay is Super-Kamiokande (SK), which contains 7.5x1033 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.

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.

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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. [click to enlarge]


Illustration of Proton Decay