Round Table Talk : Conversation with

Takaaki Kajita Kavli IPMU Principal Investigator Hitoshi Murayama Kavli IPMU Director Masataka Fukugita Kavli IPMU Professor Tsutomu Yanagida Kavli IPMU Principal Investigator

Participated in Proton Decay Hitoshi Murayama Takaaki Kajita Search Conducted by Kamiokande Murayama: Congratulations on the ! I think you must be extremely busy. Kajita: Yes, the number of e-mails I receive has signicantly increased, for instance… Murayama: You will feel things are tough for a while. Thank you very much for attending this round- Tsutomu Yanagida Masataka Fukugita table talk even though you are so busy. I’d like to start with a question involved in the construction of graduate school? ̶ why did you decide to join the Kamiokande? Kajita: It was April in 1981. Koshiba group when you entered the Kajita: I had not visited the Murayama: At that time, was the Graduate School of Science at the construction site at all. It was just Koshiba group still involved in an ? when the rst batch of 20-inch experiment at DESY? Kajita: As I wanted to participate in PMTs (photomultipliers) were being Kajita: Yes. a particle experiment, I had produced. Murayama: Who decided a graduate a choice between the Fujii-Kamae Fukugita: Were you not involved with student would do which experiment? group and the Koshiba group. designing Kamiokande? Kajita: I have no idea. At the Honestly speaking, I didn’t have an Kajita: No, not at all. beginning of 1981, Katsushi Arisaka idea about which to choose. Murayama: Do you mean that was writing his master thesis by Murayama: Why did you want to when you entered graduate school, conducting a Monte Carlo simulation participate in a particle physics Kamiokande had been already study of the Kamiokande experiment. experiment? designed and Professor Koshiba was I had been encouraged by him to Kajita: As I was young, I hoped to do leading its construction and you just work on proton decay search before I research in fundamental science such joined it? realized it [laughs]. as elementary particle physics. Kajita: Exactly. Murayama: Was the IMB experiment Murayama: Were you immediately Fukugita: When did you enter already running at that time?

10 Kavli IPMU News No. 32 January 2016 Kajita: It was under construction. also put forward the idea of Super- Murayama: So, the Kamiokande team Murayama: It is usually said that Prof. Kamiokande, because Kamiokande realized that a larger detector was Koshiba worked on the development was too small for observing solar needed for a proton decay search of 20-inch PMTs in order to compete . I think the upgrading of and decided to switch Kamiokande with IMB. Was it really so? Kamiokande started some time in to solar observation. For Kajita: I heard that story only indirectly 1984. that, you constructed an anti-counter as you did. Fukugita: Though Prof. Koshiba and lowered the threshold. Did you Murayama: Then, the construction of was aiming to make then start a new phase aiming to Kamiokande started. I guess there observations, the primary reason make observations of solar neutrinos were only a few members in the for him to have proposed Super- and supernova explosions? team at that time ̶ , Kamiokande was his clear recognition Kajita: We did not consciously aim to Arisaka, you, and… that proton decay would never be observe supernova explosions. Kajita: Prof. Teruhiro Suda of the ICRR discovered unless we had a larger Fukugita: A supernova explosion (Institute for Research) detector than Kamiokande. happened by chance when the and a few more. Murayama: Did he think so because of threshold was successfully lowered. Murayama: was not there IMB? When was the rst p→eπ limit Murayama: And just a month before at that time? reported by IMB? it was not possible to observe it Kajita: Prof. Totsuka returned from Kajita: It was in 1982. because the radon level was too Germany in the spring, probably in Murayama: At that time, was there high. As Prof. Koshiba was retiring May, of the year I entered graduate a prevailing atmosphere that Grand in a month, there was only a two- school. He was doing R&D of the Unied Theories would not work? month window [laughs]. A supernova OPAL experiment, but helped us Fukugita: We did not conclude so, but had exploded exactly 160,000 years because the Kamiokande team … before the middle of this window. suffered from lack of manpower Yanagida: The prevailing atmosphere Impossible! [laughs]. was that we needed a larger detector. Yanagida: Incredibly tiny probability. Murayama: At that time, was the Fukugita: There were lots of Fukugita: Originally, the threshold was Kamiokande project being advanced arguments in favor of p→νK. near 100 MeV, wasn’t it? entirely by the Physics Department at Murayama: SUSY-GUT (Super- Kajita: No, it was 30 MeV. Probably the Hongo Campus? symmetric Grand Unied Theories). the analysis threshold was about Kajita: No, Prof. Suda of the ICRR was Kajita: Yanagida-san, you pointed out 100 MeV, but it was for electrons. a member, and Prof. Jiro Arafune, the importance of νK. Was it around Fukugita: Around that time, Rubakov who helped us with theoretical issues, 1981? pointed out that monopoles, if they was also at the ICRR. He then moved Yanagida: Yes, it was pointed out by existed, would catalyze proton decay. to the Tokyo Institute of Technology. Weinberg and by Sakai-Yanagida. It Murayama: Yes, Callan-Rubakov. Murayama: When was Kamiokande may be that Prof. Koshiba envisioned Fukugita: Then, a 30 MeV muon completed? νK around that time. Anyhow, he is neutrino would be emitted. So I told Kajita: In July 1983. very quick in identifying theoretical Prof. Totsuka many times to lower the issues. In 1981 or in the rst half of threshold so as to detect it. Round Kamiokande Observed Neutrino 1982, I was requested by him to give Murayama: That’s interesting. Table Burst Soon after Being Upgraded a seminar on the seesaw mechanism Fukugita: Yes, Prof. Totsuka said it Murayama: From then, how had it of neutrino mass because he was was possible to lower the threshold been running till the supernova interested in it. At that time, he was down to there, and he worked hard explosion in 1987? very impressed with my talk, but with Atsuto Suzuki to achieve it. So, Kajita: Prof. Koshiba was great. I other people weren’t at all [laughs]. before lowering the threshold down don’t remember the exact month, Murayama: He has a good sense. to 6 MeV, there was an intermediate but in the fall of 1983, he already Fukugita: His sense is extremely good. stage to detect 30 MeV neutrinos. proposed upgrading Kamiokande Yanagida: Yes it is. His air… How Yanagida: I didn’t know that story. I so as to observe solar neutrinos. He should I put it? didn’t understand why they aimed

11 was the actual relation? Fukugita: Kamiokande was asked by IMB about the burst timing. As IMB had a higher noise level, they searched for and found events at around the timing which they had been informed of. Murayama: Without knowing Kamiokande’s timing, IMB would not at monopole-catalyzed proton decay was sufciently clean. have been able to nd the events? because I couldn’t imagine that Murayama: I see. At rst, you were not Fukugita: I think it would have been experimentalists knew Callan-Rubakov. using the water recirculation system. difcult. It may be that they would Fukugita-san, you suggested… Kajita: That’s true. At the level of eventually have found them, but not Fukugita: So, they succeeded in Kamiokande, we only needed to so quickly like that. lowering the threshold… recirculate water while keeping its Murayama: It sounds like it was Murayama: But, it must have been purity. But, as our pure water system important. very hard to further lower the was not very good, it was very Fukugita: Yes, it was extremely threshold below 10 MeV. difcult to do this. important information. Fukugita: It was really hard, in Murayama: So, you managed to lower particular, to lower the radon level. the threshold down to about 10 MeV De cit of the Muon Neutrino Flux Murayama: What was the radon only about a month before the removing process? supernova. Murayama: After the supernova Kajita: First of all, we didn’t know Fukugita: Probably it was lowered excitement, Kamiokande nally about the existence of radon. We to that level around the end of entered an era of solar neutrino thought that the new hardware December, because it was 6 to 7 MeV observation. Were you involved with would allow us to lower the threshold in January. the solar neutrino analysis? down to 5–7 MeV as far as we took Kajita: I don’t remember any more, Kajita: No, I was not. the accidental rate into account. But but probably you are right. Murayama: Have you been working actually we found a trigger rate of Murayama: Then, a month or two on atmospheric neutrinos since then? more than 1,000 Hz. We started by after the threshold had been lowered, Kajita: Yes. wondering what the reason was. a neutrino burst suddenly happened. Fukugita: Already around that time Murayama: Do you mean that it was What did you feel at that time? Kamiokande people were speaking difcult to understand it? Kajita: Then, I was staying at CERN. about the atmospheric neutrino Kajita: Prof. Atsuto Suzuki was great. So, I didn’t know about it. As I was problem. As the trigger rate was 1,000 Hz, he formally afliated with the ICEPP Kajita: No, in the spring of 1987 stopped the pure water system. Then, at the University of Tokyo, it was we never spoke about it to people the trigger rate rapidly dropped, and my duty to help with the OPAL outside the Kamiokande group. from its decay rate he found that experiment from time to time. Fukugita: I heard about a decit in radon might be the reason. Murayama: By chance, I heard Maurice 1984 or 85, well before 1987. Murayama: How did you move Goldhaber’s speech at a dinner party Kajita: That must be a decit of muon forward with removing impurities before his retirement. He rst showed decays. from water? a slide of the Kamiokande’s neutrino Fukugita: In 1984 or 85, I heard from Kajita: First of all, radon decays. So we burst events, and then another slide Prof. Totsuka many times about a had to prevent Rn from leaking into of the IMB’s events. He said“, The decit of muon decays…, no, muon the Kamiokande detector. At rst, IMB events have higher energies. This ux. A muon neutrino produces a we continuously supplied fresh mine means that IMB observed the burst muon, and an electron is produced water, that passed through lters, slightly after Kamiokande. So, Koshiba from muon decay. When he was into Kamiokande because mine water got it, but that’s life” [laughs]. What working hard on proton decay, he was

12 Kavli IPMU News No. 32 January 2016 already aware of a decit of that rate. Murayama: But, the absolute uxes of atmospheric neutrinos were not very reliable at that time. Kajita: At that time, we had roughly separated single Cherenkov rings identifying them as being due to muon neutrinos and electron neutrinos. But the problem was that number of electrons from muon decays was region in the case, namely, single-ring events, I signicantly less than expected, though parameter space. found a signicantly smaller number the μ/e ratio looked OK. Fukugita: At that time, you reported of muons than expected. It was

Fukugita: In 1978, Frederic Reines et the double ratio, the observed νμ/νe around autumn in 1986. Then, I al. rst reported a muon ux decit to the expected Monte Carlo value. rst realized something strange in from their experiment in South Africa. It was not 2:1 as expected, but about atmospheric neutrinos.

They observed only 60% of what 1.2:1. That means that the νμ ux was Murayama: At that time, few people they had expected… about 60% of the expected value. believed it was possible to reliably Kajita: In their experiment, they used The ux uncertainties were cancelled distinguish between clear muon a detector that could count only out because you took the double single rings and fuzzy electron rings? penetrating muons. They reported the ratio. Kajita: I believed so, because cosmic- ratio of Monte Carlo prediction/Data Murayama: But, calorimeter ray muons were identied as muons was 1.6, but they did not explicitly experiments like Frejus did not with more than 98% probability, but mention the decit. observe the νμ ux decit at that … Fukugita: That means 60% of what time. Murayama: What did the people they had expected. So, the number Fukugita: That’s right. Both Nusex and around you think? I mean, as a became known then, but no one Frejus reported no νμ ux decit. community. believed it would be a reliable ux. Murayama: They said something like Kajita: I have no idea about that. Later, IMB also reported a slight “Particle identication conducted Probably people were confused decit earlier than Kamiokande. in a water Cherenkov detector may because soon after our paper, Frejus Kajita: Actually, the submission date be wrong,” because only water and Nusex published papers in which of our paper was a few days behind. Cherenkov detectors observed the νμ they reported no observation of a νμ Both Kamiokande and IMB reported ux decit. decit. data showing fewer muon-decay Fukugita: Yes. At that time, Prof. Murayama: Do you mean that there electrons than expected in single-ring Totsuka said“, If it’s wrong, we could were various opinions even inside the events. lose our credibility.” He was very Kamiokande group? Yanagida: When did we suspect the nervous. Kajita: Yes. Thanks to Professor possibility of neutrino oscillations? Koshiba we could publish our paper Fukugita: Kamiokande’s atmospheric Believed the Correctness of in 1992, I think. Round neutrino paper1 published in 1988 Kamiokande’s Particle ID Algorithm Murayama: Do you mean that he Table describes lots of details, and the Murayama: Kajita-san, did you thought it was very interesting? possibility of neutrino oscillations is rst devise Kamiokande’s particle Kajita: Rather than that, he advised suggested at the end of the paper. identication algorithm? me“, You must write the next version But, I believed neutrino oscillations Kajita: Yes, I devised an algorithm of the paper, because the current had been discovered when I read which, in the case of multi-ring paper remains unsatisfactory.” their paper2 published in 1992. events, identied particles by Fukugita: The paper published in Kajita: In our paper published in 1992, calculating electron-type and muon- 1992 was very important. we somehow doubled the amount of type probabilities for each Cherenkov Yanagida: We published our neutrino data used and reported an allowed ring. When I applied it to the simplest oscillation paper3 at a rather earlier

13 time. I think it was because we had nally we wrote a paper. We felt, completed and the data-taking close contact with the Kamiokande “It wouldn’t help if we waited any started on April 1, 1996. group. We believed their results. longer.” Murayama: When did the main force Then, we thought if we could build a Murayama: Because Kamiokande’s of the IMB group join? model… ducial mass was only 1,000 tons, it Kajita: I think it was around 1992. Fukugita: We published it in 1993, was difcult. Moreover, looking at Murayama: Oh, immediately after the about a half year after Kamiokande’s the zenith-angle distribution, only construction had started. They joined publication in 1992. What most the rst bin was signicantly high, Super-Kamiokande, because they people did not believe was nearly but the other four bins could be could not x the water leak of the maximum mixing. They thought reasonably t to a at distribution. IMB tank. “How do you explain it?”. So, I felt uneasy like“, Is there really Kajita: Yes, it was due to their reason. Murayama: It was a prejudice. zenith-angle dependence?”. Murayama: Did the Japanese group Fukugita: Normally, we think the Kajita: The double ratio compares accept them immediately? mixing angle (sin θ) is about the the data and Monte Carlo of the µ/e Kajita: Basically, yes. square root of the mass ratio. Then ratio. Then, as the observed number it cannot be that large. However, of electron events was small, the Obtained Decisive Evidence from adopting Yanagida-san’s seesaw double ratio didn’t show a clear Super-Kamiokande mechanism, you can take the square zenith-angle dependence. Instead, if Murayama: Having started data- root once again. The 4th root of a you look at muons only, you can see taking in 1996, you very quickly small number is nearly 1. an up/down asymmetry with about accumulated the data. Soon the up/ Yanagida: That is peculiar to the 99% probability. But, 99% probability down asymmetry of muon neutrinos seesaw mechanism. The root of a is less than 3σ. showed up. Then were you very root is 1. Murayama: Was it not possible excited? Murayama: In 1991, I nished to see upward-going muons in Kajita: At that time, I was very glad at graduate school and went to Kamiokande? heart. Tohoku University. There, Yanagida- Kajita: We did publish it, but probably Murayama: Around the end of 1997, san immediately told me to see if later in 1998. Hank Sobel visited Berkeley as a atmospheric neutrino Monte Carlo Murayama: Oh, that late. colloquium speaker. He showed was really reliable. So, I made a Kajita: Yes. data indicating a large up/down

simulation to see if νμ:νe was really Murayama: IMB measured upward- asymmetry. So, I said to him“, It 2:1. going muons, and from the stop/ looks more than 5σ. Why don’t Yanagida: Did I tell you that? through ratio they reported an you announce it as a discovery?” Fukugita: Around that time, it was excluded region in the neutrino He answered“, No, I cannot say my

proposed that the νμ:νe ratio may oscillation parameter space. personal opinion, because this is the shift from 2:1 if you take muon Fukugita: It was around 1994. Collaboration’s result” [laughs]. How polarization into account. But, Murayama: Looking at their paper did it converge to a level that allowed calculations showed that it produced now, they very clearly excluded… a real announcement to be made? only a small effect. Kajita: Doubly excluded the perfectly You had been showing the data Murayama: I found that the ratio was right region. without concealing anything at all. really 2:1 even if I took every possible Murayama: What caused that? Was Kajita: Super-Kamiokande always effect into account. When did you that due to background? presents the data when they are see the zenith-angle dependence? Kajita: I do not remember precisely, summarized. So, we did not conceal Kajita: It was rst reported in our but I think it was caused by the data at all. paper published in 1994.4 something going slightly wrong. Murayama: What kind of discussion Murayama: It took a very long time. Murayama: In the meantime, was there in the group before you Kajita: Yes, it really took a long time construction of Super-Kamiokande nally announced the evidence at from 1988 when we started. We had started. the International Conference in accumulated data for 6 years, and Kajita: It started in 1991. It was Takayama?

14 Kavli IPMU News No. 32 January 2016 Kajita: I don’t remember, but probably ease, like“ Yes, that’s it!” effective mass is 5 meV… we had been waiting until it was Murayama: I also felt that it gradually Murayama: Normal hierarchy is very conrmed that we could consistently converged. Though the Soudan difcult. The effective mass can be explain everything such as upward- experiment had a calorimeter even zero. going muons. detector, that group started to report Fukugita: The effective mass is zero if Murayama: Then, you thought that a low double ratio. At KEK, a water cancellation occurs due to opposite only the zenith-angle dependence tank was built to check the particle CP phases. Even if we exclude of the multi-GeV events was not identication capability. It is true that this possibility, the present limit is sufcient. It was not until you the uneasy factors were gradually 50 times greater than 5 meV. That obtained other pieces of supporting removed. But I remember around means a detector volume that is evidence… At that conference, I that time I heard many collider 2,500 times larger is needed. was deeply moved. Originally, the people saying things like“ As a water Kajita: The factor 2,500 is for the case conference had been scheduled to be Cherenkov detector is not very of no background. Background, if held in Sudbury, Canada, but because precise, it is unlikely that such a kind there is any, makes things worse. of a delay in SNO’s construction the of thing will be claried with it.” So, Fukugita: Even for the inverted venue was changed to Takayama at I found some PhD thesis on Super- hierarchy, if we assume the effective short notice. It was the right decision Kamiokande’s website and read mass to be 40 to 50 meV, it means for Japan to have hosted it. it thoroughly. This thesis carefully an improvement by a factor of ve, Fukugita: Around that time, neutrino studied various systematic errors. which in turn means a factor of 25 in oscillations were obvious to me. So, Then, when I was staying at CERN volume. It would be difcult to make I wasn’t very excited [laughs]. For I held a seminar. It was after the KamLAND bigger. me, Kamiokande’s paper in 1992 Takayama Conference. I mentioned Murayama: There is an idea of was extremely important. I rmly what I understood about atmospheric suspending a balloon in Super- recognized the reality of neutrino neutrinos, such as to what extent Kamiokande. oscillations. various factors were reliable. Many Fukugita: Even if you do it, it will Murayama: But, for the rst time in experimentalists staying at CERN be impossible to discover double 1998 it was shown experimentally came to this seminar. They were beta decay during my lifetime if the with more than 5σ that the Standard mostly skeptical at that time. normal hierarchy is the true one. Model is not perfect. Then, after Fukugita: I think John Bahcall was very Murayama: Do you think it is possible you had shown the data in 1998, skeptical even after the Takayama to verify leptogenesis? what reaction to you did the people Conference. He asked me many Fukugita: It’s difcult. We were around or the entire community times“, Is their experiment reliable?”. motivated by Rubakov et al.’s have? My impression was that argument5 that baryons created everybody believed it soon. Future Direction of Neutrino by normal baryogenesis will be Kajita: I had the impression that it Physics completely washed out. At rst, I was accepted more than expected. Murayama: Well, what is future suspected their argument was wrong, Murayama: Do you mean you direction of neutrino physics? but accepted it in due course. So, expected quite a bit of objections? Fukugita: Investigations of I thought we should try to nd a Round Kajita: I expected some, because I had (neutrinoless) double beta decay. solution. Table continuously witnessed those things Murayama: KamLAND-Zen is leading I have an interesting plot here. for 10 years [laughs]. the world now. Yanagida-san and I wrote this paper6 Fukugita: It might have been different Fukugita: The present effective in 1986. if there had not been a prehistory mass limit from KamLAND-Zen Murayama: You published it in 1986. before 1998. Because of that, people is somewhere between 120 and It was well before the discovery of thought“ Atmospheric neutrinos may 250 meV, considering rather large neutrino oscillations. possibly oscillate with a large mixing uncertainties in nuclear matrix Fukugita: This plot shows the angle.” So, by looking at the decisive elements. However, in the case of the yearly number of citations of the data presented in 1988 they felt at normal neutrino mass hierarchy, the leptogenesis paper. We published

15 100 meV. For a heavier neutrino mass, it’s not possible to create a baryon number. Murayama: Yes, the baryon number is washed out. Yanagida: It’s very important. Fukugita: It is a very important result of Buchmüller et al.’s calculation.7 This 100 meV is comparable to every limit. The upper limit from double beta decay is 120 meV. Should double beta decay be discovered it here, but at rst it was very Fukugita: At that time, Yanagida- there, leptogenesis would get into poorly accepted. In 1998, neutrino san was staying at DESY. So we difculties. Also, the present limit for oscillations were accepted by the submitted it to Physics Letters and it the sum of three neutrino masses community, and thereafter the was accepted soon. from cosmology is less than 200 meV. number of citations per year has Yanagida: Yes, it was. Murayama: Well, it is analysis- increased. Recently, this paper has Fukugita: So, its publication was dependent. been having about 120 or 130 delayed by three or four months. Fukugita: Analysis is difcult. What citations every year. These data are As I said, at rst it had a very poor I can trust is a limit of less than taken from ADS (Astrophysics Data reputation. So, a long-range view is 600 meV for the sum of three System). SPIRES (database of particle really needed. neutrino masses, because it uses physics literature) will give slightly Yanagida: It’s a good example of it. only CMB (cosmic microwave higher citation numbers for this But, this paper would not have been background). A limit of 200 meV uses paper. widely noticed if there had not been BAO (baryon acoustic oscillations). Murayama: Oh, I get it [laughs]. It’s neutrino oscillation results from That aside, 200 meV divided by 3 due to greater credibility since the experiments at Kamioka. I feel it is gives 60 meV. It is the present goal discovery of neutrino oscillations. very impressive. of double beta decay as well as Kajita: After the rst few years of Sakharov’s three conditions for cosmological constraint. So, the next the 2000s, we started to extensively creation of matter in the universe goal is about 50 meV. discuss how to explore CP violation are baryon number violation, CP Yanagida: In that sense, leptogenesis in the neutrino sector. In relation to violation, and departure from will be seriously checked from now it, the importance of leptogenesis has thermal equilibrium. To come up on, as to whether the upper limit of been highlighted. with leptogenesis, we changed only neutrino mass will be lowered from Yanagida: I am very happy with it. one factor in Sakharov’s framework, 100 meV or 50 meV. If neutrino It means that theoretical physics namely we changed the baryon mass turns out to be near these has greatly developed thanks to number to B–L. Then we came up nite values, leptogenesis would be experiments. This sort of thing does with the prediction that neutrinos strongly constrained. not happen so often. If double beta have Majorana masses. I think this Fukugita: For now, everything decay is discovered, it will be… prediction corresponds to Sakharov’s indicates neutrino mass to be less [laughs]. prediction of proton decay. Even than this upper limit. It is sort of like Fukugita: You can readily understand if it is not possible to really check searching for a sunken ship in the to what extent there is a delay leptogenesis, non-zero neutrino mass Pacic Ocean. in the increase in the number of has been experimentally veried. Yanagida: We should recognize that. citations. At rst, we submitted our Fukugita: Furthermore, it predicts It is very important if the upper limit leptogenesis paper to Physical Review a plausible neutrino mass. It is is really lowered. Letters, but it was rejected. interesting that the average neutrino Fukugita: Lowering this limit as much Murayama: Oh, was it?! mass should be less than about as possible is meaningful even if

16 Kavli IPMU News No. 32 January 2016 the result is not drastic. Of course, cosmology. may be quite possible. It would be searching for double beta decay Fukugita: In cosmology, the most very interesting if KAGRA and Super- is the best way. But, if the normal reliable results are obtained by using Kamiokande detected signals at the hierarchy were true, it would be only CMB. But, the limit comes from same time at Kamioka, but telescopes hopeless. the fact that matter is nonrelativistic observe nothing. Kajita: How about if the inverted at the epoch of recombination. So, Fukugita: If that happens, it’s great. It hierarchy is true? using only CMB, we cannot lower the would be much more interesting if Fukugita: Still difcult. As the target is limit too much. This is a difcult point signals were detected only in Japan 40 meV, you have to lower the limit for cosmology. [laughs]. by a factor of 5. Murayama: Then, if we emphasize the Murayama: The third Nobel Prize from Kajita: My impression is that it may be surprise, our tentative goal should be Kamioka may not be a dream. possible to lower it by a factor of 5. showing a large neutrino mass and Fukugita: You have to have another It should be manageable one way or excluding leptogenesis [laughs]. detector another during our lifetime. somewhere for coincident signal Hoping for Coincident Supernova detection. With a gravitational What Will Be the Next Discovery Observation by KAGRA and Super-K detector, you have so many signals in Neutrino Physics? Murayama: Kajita-san, what is your with more than 10σ signicance. Murayama: What will be the next future plan with neutrino physics? Murayama: Coincidence is important. surprise, if there is any, in neutrino I think you must be very busy with Kajita: We expect coincidence with physics? the KAGRA gravitational wave LIGO in the U.S. LIGO is already Yanagida, Kajita: Inverted hierarchy. experiment for the moment. running. Murayama: How about sterile Kajita: As the neutrino community Murayama: Well, time’s up now. neutrinos? want to see Hyper-Kamiokande Kajita-san, good luck in your future. Fukugita: I don’t think they exist. achieved, I’d like to support it as I am really happy with your Nobel Yanagida: I don’t believe in them. much as I can. Prize. It means a lot to me. Murayama: Will you be astonished if Murayama: What is your view on the Kajita: Thank you very much. inverted hierarchy is proved true? prospects of KAGRA? Yanagida: If so, it will be astonishing. Fukugita: What is the prospect of 1 K.S. Hirata et al.“, Experimental Study of In some meeting, probably at DESY, detecting gravitational waves? Atmospheric Neutrino Flux,” Phys. Lett. B Ed Witten said“, Neutrinos’ large Kajita: I think we have good 205 (1988) 416. 2 mixing angle was a great surprise. possibilities. K.S. Hirata et al.“, Observation of a Small Atmospheric / e Ratio in Kamiokande,” We may have another surprise.” He Murayama: We can expect to conduct Phys. Lett. B 280 (1992) 146. stopped without saying anything astronomy with it. Some supernova 3 M. Fukugita, M. Tanimoto, and T. Yanagida, more. I wonder what the surprise is. explosions may not be optically “Phenomenoaogical Lepton Mass Matrix,” Prog. Theor. Phys. 89 (1993) 263. Is it inverted hierarchy? observed, but can be detected by 4 Y. Fukuda et al.“, Atmospheric / e Ratio in Fukugita: If inverted hierarchy should gravitational waves. the Multi-GeV Energy Range,” Phys. Lett. B be true, it would be a surprise. It Fukugita: I think supernova explosions 335 (1994) 237. 5 would be almost impossible to would not emit gravitational waves V.A. Kuzmin, V.A. Rubakov, and Round M.E. Shaposhnikov“, On Anomalous explain it. too much. So, should they be Table Electroweak Baryon-Number Non- Yanagida: It will be possible for me to detected by gravitational waves, Conservation in the Early Universe,” Phys. put forward a far-fetched argument clearly it would be a surprise. Lett. 155B (1985) 36. 6 M. Fukugita and T. Yanagida“, Baryogenesis [laughs]. Murayama: For instance, if the without Grand Uni cation,” Phys. Lett. B Fukugita: Yanagida-san is great. He gravitational collapse of a massive 174 (1986) 45. can always put forward a far-fetched star forms a black hole, no supernova 7 W. Buchmüller, P. Di Bari, and M. argument [laughs]. explosion occurs. Then, it can be Plümacher“, The Neutrino Mass Window for Baryogenesis,” Nucl. Phys. B 665 (2003) Yanagida: After all, double beta decay observed only by neutrinos and 445. is important, isn’t it? gravitational waves. It cannot be Murayama: Double beta decay and seen by telescopes. Such a scenario

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