Physics Nobels

The Nobel prize for physics has been announced. This year half of it goes to Makoto Kobayashi, Toshihide Maskawa and the other half to Yoichiro Nambu; all particle physicists.

As always, there is a lot of controversy with this choice. When I first heard the recipients my first thought, and I suspect almost every particle physicist had the same thought, was: “Why Kobayashi and Maskawa and not Cabibbo?” The Kobayashi-Maskawa paper was, in many ways, a generalization of earlier work by Nicola Cabibbo. For some entertaining wild speculation about politics and motivation, see the comments on this post by Thomasio Dorigo.

My take is that this is one of the exceedingly difficult situations created by the rule that the prize can only be shared by three recipients. Nambu clearly deserves a Nobel prize, and good arguments can be made for all of Cabibbo, Kobayashi and Maskawa. On the other hand, the committee can’t give the prize to particle physics (or to any other branch of physics) every year. So, something weird and controversial has to happen.

Now, instead of directly explaining what the prizes were for, I’m going to slide into an Amusing Anecdote.

BaBar, the only particle physics experiment with a cartoon mascot

When my graduate adviser, JoAnne Hewett, was helping her first student, Frank, prepare for his defense, she told him one of the questions she would ask: how to measure something called “β”. At the time SLAC, The Stanford Linear Accelerator Center, was in the heyday of it’s B-physics program. The BaBar experiment collided electrons and positrons to produce particles called B mesons; these are particles made of one bottom quark and one other quark. By making millions of these and studying how they decayed they could determine the parameters of the so-called Cabibbo-Kobayashi-Maskawa (CKM) matrix. This matrix, and the theory behind it, tell us everything about how quarks interact with other particles; that theory is why Kobayashi and Maskawa received the Nobel Prize this year.

One of the most important parameters is called β; it’s the number that controls difference between matter and anti-matter. If it’s zero, they behave in exactly the same way; if it’s not the interactions of the two have slightly different strengths. The interesting feature of the Kobayashi-Maskawa theory is that this asymmetry between matter and anti-matter can only show up if there are 3 or more generations of quarks (at 2 per generation, that makes 6 or more total). At the time they wrote their paper only 2 generations were known, but there had also been an observation of matter-anti-matter asymmetry. They were therefore led to predict the existence of 6 quarks two years before the fourth quark was discovered. Over twenty years later we didn’t know the values of all the parameters in the theory, and measuring them was one of the primary goals of BaBar (along with it’s competing experiment, Belle).

So, just before his defense JoAnne tells Frank that, since he’s graduating from SLAC, he should know how to measure the parameter β. Apparently, Frank thought she was just making fun of him for not knowing it off the top of his head. Since professors almost never tell the students ahead of time which question they’ll ask, that was a pretty reasonable assumption. So, when the defense started and she asked, word for word, the same question, he was stunned; not to mention a little unprepared…

To measure beta: interfere trees and loops, making sure to properly account for penguins. Really.

Fast forward two years to my own defense. As I was beginning to prepare, JoAnne told me that, since the β story was so famous should would not be asking me that question, instead she’d ask me how to measure α, a different parameter in the CKM matrix, and one that BaBar was also measuring. So, I read up on how α was measured, and prepared a nice answer to the question. I was not going to be caught in the same trap. I think I was even more surprised than Frank when, near the end of my defense, JoAnne said, “So, how would you go about measuring β?”

(Both of us, incidentally, made it through the answer with a little help from the committee.)

I’m currently visiting my mother at her farm on Whidbey Island, so an Amusing Anecdote about Nambu’s work will have to wait.