Thursday, February 26, 2009

CP violation

While reading some stuffs related to neutrino oscillations, I realized that neutrino oscillations can happen only after Electroweak Symmetry Breaking (EWSB). Since before EWSB happens, neutrinos( in fact all leptons and quarks) are massless and hence there is no oscillation and thus there is no CP violations. I started wondering about the cosmological significance of the CP violation in the leptonic sector. According to Shakarov's criteria CP violation is a must for leptogenesis to start. Now if the CP violation we see in the leptonic sector has something to contribute to leptogenesis then EWSB must happen before leptogenesis started. But according to current understanding of the thermal history of the Universe the matter-antimatter asymmetry started long before the Gev scale (EWSB). Thus whatever CP violation is there in the leptonic sector has no contribution to the leptogenesis. Although I have very little knowledge in leptogenesis, I found this point interesting. I asked my supervisor regarding other sources of CP violations which can contribute to leptogenesis. Then he said right handed heavy neutrinos is one such source. I have seen many models with B-L extended gauge symmetries which can accomodate right handed neutrinos. Now there are certain things which are not clear to me:
1. Why right handed neutrinos should preferentially decay to leptons rather than anti-leptons. I want to know how the CP violations take place in this case. Its definitely not like the oscillation of leptons which gives rise to CP violation in the usual leptonic sector.
2. Is it true that total lepton number is violated in such decay of right handed neutrinos? If yes then how can we incorporate these lepton number violating interactions in B-L extended gauge models where B-L breaking scale is not as high as the scale of leptogenesis.
I think I need to do some more careful studies rather than blindly coming into any conclusions. If anyone can help me with some references or comments that will be highly applauded.

Monday, February 23, 2009

Mt Abu trip

After around two months my stay in Ahmedabad, India I could go outside the city for a small trip. Life in Ahmedabad is like in a prison for me since I have to be in the institute from 9 am(breakfast time) to 9 pm(dinner time). Anyway thanks to Dr Ramesh K Singh (Professor in ME, IITB) who made our dream trip a reality. Believe me if you are planning to go for some trip please include him in your team. He is such an enthusiastic and energetic person that he can make anything happen. Anyway, we went to Mt Abu, Rajasthan. We were six together in the trip; me, Brijesh, Ramesh, Vishnu(Professor in IEOR, IITB), Subrata( Lecturer in Chemistry,IITGN), Abhijeet(lecturer in Economics, IITGN). We started from Ahmedabad on 21st Feb(Saturday) and reached Mt Abu at around 2 pm on the same day. We hanged around the town that day and in the evening went to the sunset point. I had lots of junk foods there which fortunately didn't make me sick.
The next day morning we went for trekking which was the most interesting part of our trip. We chose the so called "Tiger Path" for trekking. Thank God we did not see any tiger there!!! According to the local people there, that wild-life sanctuary has some bears, tigers also. It was an 8 km long trek and was really awesome. I did not even feel tired. We went to some tourist spots after that (e.g. Guru Shikhar, Dilwara temple etc) and came back to Ahmedabad. We reached around 8 pm. It was an awesome trip. Ramesh was really amazing. Now we are planning a trip to Leh in May-June with Ramesh and Co. I hope he will make it happen once again. Leh is like heaven on earth...if this trip happens my dream of going to Kashmir will come true..

Sunday, February 15, 2009

Neutrinos: Yesterday, Today and Tomorrow

Recently I went to Physical Research Laboratory Ahmedabad, India to attend a colloquium titled "Neutrinos: Yesterday, Today and Tomorrow" by Sandip Pakvasa, University of Hawaii. The talk was really interesting. He presented almost everything within that one hour talk. The best thing was he talked about both experiments as well as theories regarding neutrinos. I was totally pissed off with the neutrino physics when my friend Sushant (working in neutrino physics) used to tell me the kinds of stuffs he was doing. Almost eighty percent of his work was some coding only without any touch of physics. According to him the people who work in neutrino oscillations phenomenology keep playing with the parameters involved and try to fit the models with observed data from neutrino experiments like SuperkamiokaNDE, kamLAND etc. But after Pakvasa's talk suddenly the neutrinos have become interesting to me again.
Sandip was talking about some constraints on neutrino magnetic moments. I have got confused at some point. What I want to know is can we measure neutrino magnetic moment? And if neutrino is a Majorana fermion how can it have magnetic moment? If we can measure neutrino magnetic moment can we safely rule out the possibility of neutrino being a majorana fermion? If neutrino is not a Majorana fermion it can not be its own anti-particle which means neutrinoless double beta decay experiments are not going to give any positive results. I do not know how profound these questions are but I would need to do some careful study to clear these doubts.
I was surprised when I was going through one of Bilenky's paper on neutrino where he talked about neutrino oscillations. He said that neutrino oscillations can not say anything about the neutrino nature that is, whether its a Dirac or Majorana fermion. It is because of the fact that the expression for probability of oscillation is same for Dirac as well as Majorana fermions. So oscillation data can not say anythng about the nature of neutrino. It can only give the mass squared differences of the neutrinos.
I welcome your comments regarding my confusions or anythng you like.