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Koji Hashimoto | Osaka univ. | Japan

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“Yonupa” Summer school.

Monday, August 31st, 2009

Annual summer school for graduate students majoring particle and nuclear physics is held in Nagano prefecture, in the middle of Japan. This time, after 10 years, I visited this school again, as a lecturer. It is nice to visit any places where I spent many days in my younger days. I have attended at the school every year when I was a graduate student. It was so joyful experience.

Before I got to the school venue, I was wondering if nature of young students may be very different from the one in my old days. But as soon as I started the lecture, this concern just went away. The summer school was really the same as before, nothing has changed, except for the names of the participants. It is very very good. This summer has nothing different from summers I experienced 10 years ago. Young graduates get to know each other and enjoy chatting, drinking, hiking and physics.

I in fact enjoyed giving lectures and a party after the lecture. I had opportunity to talk with fresh graduates. I hope this activity of the annual summer school will continue in the future. Since the organization of the summer school is completely by the graduate students themselves, I know it is kind of difficult to continue, but anyway this activity lasts at least for more than fourty years, and this fact encourages me to expect that this continues forever….


Strings, cosmology, hadrons and nuclear physics.

Thursday, August 13th, 2009

Almost every summer, a workshop called “summer institute” is held at Mt. Fuji. The venue is beautiful, confortable and relaxing, so it is a perfect place to stay for a while for physics discussions. In fact, this series of the annual workshops was initiated in around 1995 to have something like Aspen institute in Japan. I participated in this workshop from the second one, at that time I was a graduate student. What I remember about the workshop then was just that I gave one qestion in some seminar, other than that, I swam, I played table tennis, and that’s all — it was just great for discussions, communications and interactions. Now, after 15 years or so, from that viewpoint, nothing has changed. I was again in a relaxing mood, had discussions, took a walk on a grass, and such. I don’t care about any view of Mt. Fuji. Of course it is beautiful, but there is something more precious than the beautiful view.

Unfortunately the schedule of this summer institute session for string theory and cosmology completely overlaps with a workshop at KEK for hadrons and nuclear physics. Thanks to the KEK workshop organizer, my talk was arranged to the last session of the KEK workshop. I was just in time for my talk, which went well as opposed to my expectation. Discussions with hadron physicists and nuclear theorists were pleasant, which I truely appreciate. I have learned things which directly related to my study — In particular when I like to know something which is not about string thoery/particle physics, it is alwasy difficult to find the answer in archive papers, so it is the best and easiest to ask someone whose major is the subject of my concern. Workshops like this one are the place where I can do such things, and resultantly, have much influence on the direction of my research.

In Japan, now we are in a special period called “Obon”, and everyone is supposed to take a vacation. Do theoretical physicisits take any vacation? No, they have their ideas and physics problems in their brains, everyday, every moment. That’s why there are workshops in this Obon season. However, after two workshops, I got exhausted, may need some rest physically. Questions/problems in my head may allow me to take some rest, I hope — but it cannot be helped, enthusiasm for physics is often not under my control….


APCTP, Korea.

Saturday, August 1st, 2009


It was rather cold at Pohang, compared to the standard average temperature in summer seasons in Japan and Korea. “Cold” means “confortable temperature” for most of Japanese and Korean people, since typically in summer it is very hot and humid. The temperature at Pohang in Korea took non-typical values during the APCTP focus workshop on AdS/CFT and its application to QCD, for which I work as an organizer.

Thanks to the lecturers, the workshop is, I would say, successful. At least for me, it was very fruitful. Through discussions with many participants of the workshop, I gain much insights and also precious information relevant for my on-going work, a realization of color-flavor locking in holographic QCD. It was in fact precious momnet that I could share with my friends in Korea and Japan and in other countries. The workshop is the place to learn new things, of course, but in addition to that, I enjoyed really much the discussions with my friends.

I have never been to Pohang where the APCTP, Asia Pacific Center for Theoretical Phyics, headquarter is located. It resides in a floor of a building in Postech, a new university. With their new offices which are beautiful, researchers, in particular young researchers, actively study theoretical physics. I was indeed surprised by the newly renovated part of the offices of the APCTP. Owing much to the secretaries, the workshop ran without any problem, it was amazingly smooth, I felt. This place, with this kind of workshop, is definitely the one which I like to visit again.

This time, I caught a cold right after I arrived at Pohang. Good health is very important praticularly when you are in foreign countiries. I took a rest for half a day, which was eventually sufficient for me to recover completely. Well, sometimes it is important to go slowly, in particular when you are in rush….. Icouldn’t do any calculation during the workshop, but the workshop is not the place to do calculations. Rather than that, discussions are important. In the coming week, I plan to write down equations for the ideas I came up with in the workshop. I thank all of my friends who shared time for the discussions.


From string theory to nuclear physics.

Sunday, July 19th, 2009

Of course, string theory has a high energy scale, and since it is so high that very little experiments can probe the theory itself. But nowadays it comes down to the energy scalr of a few MeV… that is the scale of nuclear physics. In between, there resides hadron physics and elementary particle physics, but, string theory could jump those scales and came down to the nuclear physics energy scale — that is what I am studying right now.

The basic ingredients of nuclear physics is nuclear force, the force between nucleons. The long-standing problem in nuclear physics is that there has been no direct derivation of the nuclear force from strongly coupled QCD. String theory now provides a new tool for resolving this issue, in a strange manner : holography. Together with my collaborators, I worked out the “derivation” of the nuclear force from QCD, in the large N_c limit and in the large ‘tHooft coupling (strong coupling) limit. Amazingly, string theory computes the repulsive core of nucleons, which has been known for more than 50, 60, 70 years. the computation revealed that the main source of the repulsive core is omega meson. This is indeed the case which Yoichiro Nambu predicted more than 50 years ago! As you may know, Nambu is one of the founders of string theory. So, we here see that two different findings of Nambu  meet each other, after 50 years of reccurence time.

If this is regarded as a success, one can say that string theory provides a new and powerful tool for nuclear physics. Among my works, my favorite is the one which provides a gravity dual of heavy nuclei. Some related works are in progress. You will see more on how string theory can be applied to nuclear physics. Let me keep posted on this.



Tuesday, July 14th, 2009

Last week I sepnt four days, or rather to say three nights, at Yukawa Institute, Kyoto university. This is the annual workshop on string theory and quantum field theory. Physicists from all over Japan gather here in every summer, and interact each other.

Since I spent almost ten years at this Kyoto university, North campus where Yukawa institute is located, to visit Kyoto reminds me of many many things. Although I am one of the organizers of the workshop, I actually enjoyed seeing my friends and having discussions with friends. On the last night of the workshop, there was a party to celebrate prof. Kugo’s 60th birthday. His students, including me, attended at the party and had a very nice evening together. It was such a happy moment to chat with Kugo-san and my friends there, and I felt as if I were a graduate student at that time. I was so happy to see Kugo-san’s happy face.

In such a way, everynight I attended at parties and drank till very late night (or rather to say, morning). Of course I woke up in the morning and went to the workshop venue at 9 am, since I am an organizer! It was tough days, but it was a lot of fun. I look forward to the next year to have such a time again.

Kyoto is one of the wonderful places in the world, for me. Everytime I went back there, I feel it. Physics is there, and friends are there.


Healthy life.

Sunday, July 5th, 2009

It is quite healthy for physicsts to have discussions and calculations everyday. I guess all theoretical physicists suffer from an “adictive custom” — that is to do calculations at a desk and to discuss physics with friends everywhere. I am also that kind of person, of course, though these years I guess I am not so addictive compared to my days in graduate school. I think I now know how to manage my potential “will” (to continue physics all day long forever) somehow, and cut my day into two parts; “on” and “off” physics. However, as a matter of fact, when I am deeply involved with a certain problem which I am really fascinated, I cannot manage myself and my daily life gradually gets broken. I miss time for sleeping, etc. I think I am getting better for managing myself — if I observe myself (which is the scientific standpoint for anything), these days I wake up at around 6:30 AM everyday, which is a clear indication that I am managing myself well. But the time to go to bed varies significantly. Yesterday I went to bed at 9:00 PM, that was fairly early, and on the day before yesterday it was at 12:00 PM, and on the day before that, it was 1:30 AM. So it differs. But the time to wake up is the same. Probably I have to say that I half-succeed in managing myself.

These days, thanks to my collaborators, I am going through very healthy days. Almost for all day long, I have discussions and calculations, with collaborators here at RIKEN and also overseas. It is quite a fun. Hopefully we can publish a nice physics in the very near future.



Tuesday, June 23rd, 2009

Today is the last day of my three-day lecture at Osaka university. Since I was grown up in Osaka, this city has a lot of things which remind me of many many things. In particular, I am very happy to give lectures at my home town, in my Osaka dialect of Japanese.

It is obvious that when you speak in English (I am supposing that you are not native English speaker), your “nature” should be different from that when you speak in your native language. Your character depends on the language you use. This is in particular the case for me. When I talk in English, my communication “mode” changes. What I like to tell you is that this also applies to the dialogs. In fact, When I talk in Osaka dialect of Japanese, the way I express myself actually changes, compared to myself talking in so-called “standard Japanese” which I use in Tokyo. When I was young, my mother taught me how to talk in the standard Japanese, since she was not a person born in Osaka. Since Osaka dialect was thought to be a “dirty” dialect (This word “dirty” simply means that it is very different from the standard Japanese accent), my mother liked to tell us that there is an alternative in speaking in Japanese, seriously….. Of course, this standard Japanese can be available on TV. But it is in fact important to practive communication in the standard Japanese somehow. In this way, I tell my friends that I am a “bilingual.” But I am not sure how accurate my standard Japanese is. 

In any case, I could deliver my lectures in Osaka dialect, and I enjoyed it a lot. People in Osaka university, including graduate students, are very active and I enjoyed discussions with them. Hopefully they have got some sense of D-branes and their solitonic expressions, which are the main subject of the lectures.


Learning lattice.

Thursday, June 11th, 2009

Yesterday, I spent a whole day to learn a part of history of lattice QCD and brand-new results of baryon spectroscopy in lattice QCD simulations. My colleagues who are specialists of the lattice study kindly explained all of what I asked them.

The motivation came from two parts. The first one was the seminar of the day before yesterday, on a new result on a phase structure of QCD, with use of strong coupling expansion of lattice QCD. It was so interesting and I wanted to know how much this kind of strong coupling expansion makes sense in study, and my friend explained me also its histrical perspective. It was truely instructive for me, because I am not familiar with the lattice QCD and I am lack of the sense.

The second motivation just came eventually when I talked about my recent results on holographic QCD to my colleagues in RIKEN. What we computed in our paper is a baryon mass shift due to the change of the quark mass, in QCD. We used gauge/string duality to compute this, in the large Nc limit and large ‘t Hooft coupling limit. Anyway we got some number for the shift, our result is the shift delta M for nucleon is about 4 times pion mass squared. I told this story to my colleague, and he eventually answered that he has been involved with a Japanese lattice QCD collaboration and submitted a paper very recently, on this nucleon mass shift! And the result of this lattice QCD which has 158 MeV for the pion mass which is the closest ever in 2+1 dynamiical unquenched QCD, is in fact, 4 times pion mass squared! I was so surprised that this new technology of holographic QCD is really successful, although I don’t know why it is so successful, since we are working in large Nc and large ‘tHooft coupling expansion…..

He told me some references and I read those hep-lat papers, and I found another coincidence : We predicted that the mass shift for Delta excitation is the same as that of the nucleon, and in one paper of lattice QCD, they computed it and showed that it is again 4 times pion mass squared! So our prediction for the Delta is again confirmed. It was so exciting. Wow. Sring theory really helps real physics (in this case, hadron physics), in some sense.

I hope in the near future there appear some lattice results on the mass shift for the Roper and N(1440) S11 excitations fit with heavy baryon chiral perturbation theory. We, string theorists,  have predictions!


Papers out.

Saturday, June 6th, 2009

I have submitted two papers on the same date, last week: one is about a gravitational dual of a tachyon condensation, and the other is about a baryon mass shift in QCD by quark masses. These two papers are completely unrelated with each other. Well, to be precise, both papers are on gauge/string duality, so in that sense they are related, but the motivation for these researches are very different. The former is (personally) for a possible reconcilation of two nonperturbative dafinitions of string theory, the gauge/string duality and the tachyon condensation. The latter is an application of the duality for seeing a hadron physics in strongly coupled QCD. The reason why I submitted the papers on the same date, is purely eventual. In fact, the former collaboration with Gary Gibbons and Shinji Hirano, started almost 4 years ago when I lived in Cambridge. The latter, with Taka Hirayama and Deog-Ki Hong,  started last year.

I tried to compute the “mathematical probability” of submission of papers of my projects on a same date, but I couldn’t get a reasonable answer. The period of research project varies, and it is not just a matter of 1/dates used for the research. And I actually prefer not to submit my paper on Tuesday (I mean, the papers appearing on Tuesday in Japanese Standard Time), becuase every Tuesday there appear too many papers and I don’t want my papers to be just one among too many.

Anyway, I like to thank my collaborators: the works are quite nice and I am very satisfactory with the results. If you are interested in the AdS/CFT correspondence, please take a look! I’ve got several e-mails just after the submission, and I am quite happy with them.

Last week was a silent week for me. I come to RIKEN every day and I sat on my desk, did calculations, chat with my colleagues, and I am now writing this blog. I had not so much progress on my research from the view point of the amount of the calculations, but this silence is very important for developping  research ideas. I’ve got several invitations to some meetings, and fixed dates of visiting schedules in the near future. The silence of the last week, and possibly the coming week, should be important, in view of the dense schedule coming by….


Visiting Nagoya university.

Thursday, May 28th, 2009

Last Tuesday, I have visited Nagoya university, to give a talk at GCOE colloquium. The talk was on cosmic superstrings, which I have been working on for more than 5 years. I hope I could deliver my feeling that in fact this fascinating possibility to find superstrings in the sky. The break-through work by Copeland, Myers and Polchinski revived the possibility from the 1985 Witten’s work — It would be wondeful if one can see the macroscopic gigantic superstring/D-brane in the sky! I am rather involved with purely theoretical aspects of this problem, the reconnection of colliding strings. The reconnection probability of the cosmic strings, whatever the identity of them is, is in fact relevant for any observation of them. Interestingly, this physics is related to tachyon condensation in Sen’s conjecture on D-brane annihilation.  Field theory strings appear not only in cosmology but also in condensed matter theory and even in low energy QCD and high density QCD : and they are all realized in a certain limit by a low energy limit of D-brane configurations, that is intriguing.

I wanted to ask a question to a collegue there, how GUTs and cosmic strings are seriously unified. In fact, there is an interesting paper which states that, under some assumptions, viable SUSY GUTs should support existence of cosmic strings. I liked to know how viable the assupmtions are. At the dinner after the talk, I could enjoy an opinion by the person who works on GUTs, and I am very satisfied. Well, long time ago, monopoles are one of the serious identifier of GUTs, and nowadays, after we have seen WMAP data coninciding with inflationary scenario of the universe,  I think we can say that cosmic strings are at the same stage, or rather to say, we need to add cosmic strings as serious ones for GUT study. I really hope that some gravitational wave bursts may be observed in the near future.

This morning, in a train to commute to my office, I found a tiny but new idea on my project. It was really a happy moment. I was about to miss my station to get off. Sometimes it is good to be confined in a crowded train, since you cannot read any paper, you cannot calculate on papers, you cannot do any other things, than just thinking. Every day I commute to my office by train for about one hour, and I feel that it is good for my physics. This morning was evidence for the statement —  or maybe just a justification for my statement which was made not to face my reality to spend two hours in trains everyday….