The CERN Colloquium of T.D.Lee September 3, 2007Posted by dorigo in news, personal, physics, science.
The main auditorium hall at CERN (a larger one than the one which hosted Lisa Randall’s seminar the previous day, whose name I have not learned yet) was packed with maybe 400 people last Thursday afternoon. All had come to hear it from Tsung-Dao Lee, the co-discoverer of parity nonconservation in weak interactions, who gave a seminar in the fiftieth anniversary of his victory of the Nobel prize.
As the day before, I had not planned my attendance – I was expecting to leave CERN to Venice at about the same time. By car it is a 600 km trip, which I usually cover in a bit more than 5 hours, if all goes well. By leaving at four in the afternoon I would have a chance to be home at a very reasonable 9-ish PM, but the seminar was worth a delay.
T.D.Lee arrived fashionably late, when the buzz level in the auditorium had reached at least 70 or 80 decibels. He is still a handsome elderly figure, although he is well in his eighties (he was born in 1926). He wore a nice black jacket with golden buttons, a grey tie and brown trousers, and I would have given him 65 years of age, had I not known he is much older. He is clinging to much of his hair still, and most of what’s left is not even gray.
Funnily enough, despite his long absence from the arena (he noted at the start that CERN is timeless, by pointing to the two large round clocks hung on both sides of the vintage hall, both non-functioning: his last appearance here had been 18 years back), I had sort of interacted with him a couple of years ago, when I received a fax he wrote to support the research of a chinese physicist. But that is a story I already told.
Before I venture to summarize the contents of the colloquium, I wish to first briefly discuss their huge achievement, which is 50 years old but it still shines with pristine light as it shows how pure thought, and not even complex formulas, may be enough to reach the top. Let me quote from the presentation speech given in Stockholm in by prof. O.B.Klein:
“The starting-point of Lee and Yang in their revision of the whole question of right-left symmetry in elementary particle reactions were certain strange observations concerning a kind of new particles called K mesons, which looked as if they were in contrast with the assumption mentioned. Even if these observations puzzled greatly many physicists, it was only Lee and Yang who seriously took the consequences of them, in that they asked themselves what kind of experimental support there was for the assumption that all elementary particle processes are symmetric with respect to right and left. The result of their investigation was unexpected, namely that the validity of the symmetry assumption even in the best known processes had no experimental support whatsoever, the reason being that all experiments had been so arranged as to give the same result whether the assumption was valid or not. As if one had thought that Olav Tryggveson had his heart in the middle of the body because he was equally skilled with the left as with the right hand. Lee and Yang did not confine themselves to this negative statement but devised a number of experiments which would make it possible to test the right-left symmetry in different elementary particle transformations, and proposed them to their experimental colleagues.”
I intend to discuss in more detail elsewhere the whole history of this thrilling, revolutionary discovery, so here now I will just stress what Klein mentioned above: it was only Lee and Yang who took seriously the consequences of the “tau-theta puzzle” – as the riddle was named, after the two particles which looked the same but decayed to final states of opposite parity. And they won the house, because they were more disciplined, more skilled, and more imaginative. A lesson to be taught.
The presentation Lee gave was titled “Symmetry and Asymmetry in electroweak interaction”. It was by no means one of those commemorative seminars when one hears anecdotes and witty remarks on the atmosphere of a past discovery: not in the least! I do not know if I was the only one expecting a boring lecture, but for sure Tsung-Dao surprised me. After a very brief introduction centered on his hypothesis of parity violation in weak interactions and the confirmation by the experiment led by C.S.Wu, which allowed him to honor the memory of the chinese physicist, who died 10 years ago on February 17th, 1997, the seminar took a sharp turn.
Lee started by discussing how all known matter is made of 12 elementary particles. Of these, only electrons and muons were known 50 years ago. As for the neutrino, the one we knew back then was a mixture of those we know today. Science has made huge progress in these fifty years, but do we understand the two matrices that mix the leptons and the quarks, and ? They are the cornerstones of our particle physics. Each of them contains four relative phases, which are not measurable. So there are 12 masses and 8 phases in our model of matter. Do we understand them ? The answer, sadly, is still NO.
On the experimental side, the matrices have been measured quite well. For the neutrinos, we have not measured yet a complex phase (the measurement is not accurate enough to determine the imaginary part). So one has numbers available to test predictions. And Lee, together with R.Friedberg, used these numbers to try and understand if the matrices can be explained by a simple model. Their work is described in hep-ph/0705.4156.
I am not going to discuss in detail the description that Lee gave of his work, which you find in the link above; however, I will make an attempt at a summary. The starting observation for the work of Lee and Friedberg is that among the 12 fermions, which may be organized in four groups of three, each of those sets of three have one member of very small mass: they make up what is usually called “first generation”, . Then there is the observation that CP violation, or equivalently, T violation, arises only by virtue of a complex phase in the mixing matrix. So one can take a step back and make a zeroth order approximation: assume all the mixing matrix elements are real, and in addition, ensure that there is a zero-mass eigenstate in each sector by imposing a hidden symmetry. The underlying motivation for doing so is twofold:
1) in a spontaneous T-violation theory, the inertia (higgs) field responsible for T violation can be the same field that generates the small masses of “zero mass” fermions, .
2) T violation requires the existence of a reference frame that differentiates the sign of time flow. For a massive particle, this reference frame can simply be its rest frame.
Once one has set the stage with the initial unbroken theory, one can break the hidden symmetry and add a small T-invariance breaking term: both things can be achieved simultaneously by adding a phase factor in the interaction for each sector. One can then use the Jarlskog invariant to verify the match between measured values of the quark mixing matrix and the quark masses in this approximation, and one obtains a good match with experimental numbers. The same model fits the current knowledge of the neutrino sector very well too; however, further experimental input for neutrinos will clarify the matter more.
At the end of Lee’s talk, there were quite a few questions, but Lee was not intimidated ( wink). Here are a few:
Q: “You reminded us that higgs particle could be something like a cooper pair. The question is, do you have some suggestion on that ?” A: “Yes, I have a very good suggestion (for another talk)”. Laughs in the audience.
Q: “What value of the Wolfenstein parameter lambda can be extracted from your model ?” A: “In the expression for the J invariant, A and lambda are experimental values: the model is not predicting them, but using them as inputs.”
Q: “What about CPT violation ?” A: “Unlikely.” (Laughs in the audience)
Q: “(unintelligible mutterings from the back of the hall)”. A: “I do not understand your question, but the answer is no.” (More laughs).