PhD

I often get asked what it is I actually do for my PhD, so I decided to write a few paragraphs about it. I'm currently at SLAC, where the experiment is based, but my home institution is Brunel. The topic of my thesis will be "Search for the decay B→τν", but by then I'm hoping to change "Search for" to "An excellent measurement of".

• PhD links

First things first...

The standard model chart, showing all the known fundamental particles that account for our understanding of matter and energy in the universe.

Everything in the universe that we can touch and see is made up of quarks and leptons. An example of a lepton is an electron, which is a tiny particle that is most commonly found moving around inside atoms. Quarks can never been seen alone but are found in groups of two or three. If you put three quarks together you'll eventually end up with a proton. Neutrons are also made up of three quarks, but they're only stable when they sit next to a proton. So with quarks and leptons we have everything we need to make atoms!

That's all well and good, but it doesn't really explain what I'm doing. To have two quarks together we actually need a quark and an anti-quark. This is called a meson. Two quarks are never stable together and they decay (turn into other particles) either into leptons or into pure energy. By looking at these decays carefully we can discover something about the structure of the meson and get a better understanding of the universe.

So what how do you do it?

I look at the decay of the B meson to the τ and a neutrino. The τ is just a heavy electron and apart from the difference in mass the τ and electron are exactly the same. The neutrino is (almost) massless and has no charge. That means that we very rarely, if ever, see it in a decay. However, by looking at the rest of the particles I can reconstruct the τ and thereby indirectly observe the decay B→τν.

That sounds nice!

Hard at work collecting data at Interaction Point 2. Note that each one of those monitors has about 3 screens that you can move between. That's a lot of information!

Unfortunately things aren't as simple as that. To see the B meson we need to create the B meson, and that's what happens at SLAC. We create B mesons in pairs and then watch them decay. The main problem with the analysis is that the τ is so heavy that it decays very quickly (it can decay into more particles that it would be able to if it was lighter) and as it decays it produces another one or two neutrinoes. That means that every time I look at the decay I have either two or three particles that I have no chance of seeing. To make matters worse we have no way to guarantee that we get a pair of B mesons and in fact there are many other particles that are also produced. The B mesons themselves are so heavy that they can also decay into lots of other particles. On top of all this the B decays into τν about one time in ten thousand. That means that we're looking for a needle in a haystack! It's very difficult!

If the τ is no different to the electron, why not look at the decay B to electron ν instead?

The unitarity triangle. Looking at the decay B→τν can help place us to better understand the unitarity triangle and its influence on the interactions we see around us.

Well there's a peculiar conservation law called helicity conservation and when the B decays this way it has to break the conservation of helicity. The lighter the particles it decays into the more the law is broken, so the decay to the electron is very supressed, whereas the decay into the τ is only a bit supressed. It turns out that difficulties in observing the decay B→τν are not as big a problem as the helicity supression. (Although having said this there are groups working on the decay B to electron ν.)

Are there any other ways to get around helicity supression?

If, when the B decays, it also gives off some light (a photon) then there is no helicity supression. However, there is supression associated with the photon, and the two tend to balance out. Again, there are other groups looking at what are called "radiative decays" of the B meson.

What use is this measurement?

The measurement of B→τν gives us some very "clean" measurements of other parameters, including the structure function of the B meson and one of the elements of the CKM matrix. These are both complicated theoretical concepts which I'll get round to explaining at some other point, but they are both very important and we need good measurements of them if we are to discover new physics! In some instances we can get the same information from other decays, however when it comes to the structure function of the B meson we have no choice but to look at decays like B→τν.

How do you do the analysis?

A typical event at the BaBar detector. Click here to see more.

We collect data at the BaBar detector 24 hours a day. Lots of computing and voodoo magic happens, and eventually I end up with lots of files of data. At the same time we simulate the data using the most up to date knowledge of physics we can manage and generate what are known as Monte Carlo files. We can use the Monte Carlo files to analyse what we expect the data to look like. Then we compare this to some sample of the data that isn't what we're interested in, and see how well they compare. If the comparison is looking good then we can apply the methods to the data we're interseted in and get a measurement.

How do you do that?

I use two main pieces of software for the actual analysis. One of them is ROOT, which is a software framework used by just about every particle physicist to analyse data and monte carlo, and statistical pattern recognition tools to help with the "needle in a haystack problem". Writing and debugging the code can take a while!

You do all that yourself?

Oh no! It's a group effort and I'd be lost without the help and contributions of those around me. At the moment I only contribute a small amount of the effort needed to get this measurement and most of the work is done by other people. However, as I get more into my thesis the balance will shift and I'll take on the bulk of the analysis.

This is so true.