Penguin Hunting

When I am asked about my research interests I sometimes claim I am a penguin hunter. But don't picture me spending my life on Antarctic's ice fields. I am a physicist and I prefer the offices with central heating you can find in research labs.

More accurately I am a particle physicist. I am working in huge research lab trying to understand the intimate life of matter and energy. We boost well-known particles like electrons and protons to speeds close to the speed of light and collide them to create other particles.

Our goal is to be able to answer fundamental questions like "what are we made of?" or "how does it stick together?". More precisely we would like to design a theory that explains how matter is built. This theory and its consequences would then be taught in universities to future generations of physicists, but also engineers, chemists, biologists, etc.

The latest gear advertised in geek's magazines in still based on quantum mechanics and relativity, fundamental research made 100 years ago. What's coming next?

Back to our theory. Actually it already exists for more than 30 years. It's called the "Standard Model" and it allows to make extremely accurate predictions that have never been challenged by experiment. A very good model indeed!

But yet it's wrong.

It's wrong because it contradicts Einstein's general relativity. It actually describes all fundamental interactions all all known particles (plus one yet to be discovered) except gravitation. The very gravitation that binds the earth to the sun, that makes us life difficult when we bring back loads of water bottles from the supermarket. Just imagine a world without gravitation...

There are many other things to criticise in the Standard Model but I don't want to go into too much details. In any case there seems to be a consensus among physicists saying that the Standard Model is fine as long as you don't go to too high energies. The Standard Model is a small part of a more general theory. But this theory, this extension of the Standard Model, is still awaited for.

There are many candidate extensions available on the market. They are models extending the Standard Model that are neither confirmed nor disprove by experiment. Most of them predict the existence of new particles that are not yet observed. The future LHC collider at CERN near Geneva will try to find these new particles. The only thing we know for sure is that it will be very difficult.

There's an indirect way of finding these new particles: penguins.

In this case penguins are not birds but a category of particles decays. It's an almost magic process. Imagine a sheep. By some magic process, transform it into a whale and an elephant. Then collide the elephant and the whale until they transform into a rabbit and a mouse. What you get is sheep gets mouse plus rabbit. You think it's impossible? For sure it is for sheep, but not for particles. They do that all the time. The problem is that the process is so fast you never see the whale and the elephant.

The quark b (for "beauty"), one of the basic particles of the Standard Model does this. it decays to a W⁺ (the elephant) and another quark, called t (for "top", here our whale). The t quark then emits a photon "γ" (the mouse) and swallows the W⁺ to form an s ("strange") quark (the rabbit). It's the decay b → s γ illustrated here. You will never see any of these particles outside a physics lab; they do not exist in nature. Except for the photon that is nothing more than a grain of light, like the like that transmits this text from the screen to your eye.

What would happen if one would replace the whale and the elephant by some other more exotic animals? Usually it doesn't work because it violates some well established rule. But a mad sorcerer could predict that the sheep to rabbit and mouse process could also occur mediated by a unicorn and a dahu⁽²⁾ instead of the whale and the elephant. Imagine the shepherd notices that a sheep has decayed to a a rabbit and a mouse in an atypical way (the mouse is upside down for instance) or that there are more sheep replaced by rabbits and mice than expected according to theory. Then the mad sorcerer would interpret this as the proof of the existence of dahus or unicorns.

Sounds strange, but that's what physicist do. There are many scientific papers that predict that the rate of the b → s γ decay should be larger than what the Standard Model predicts. My modest contribution to this debate was to measure this rate as accurately as possible. I went to Japan to join the Belle experiment, a collaboration of 300 physicists from around the globe. There we measured a probability for this decay of 0.0355 ± 0.0046 %, which is not much. It's actually very close to the value predicted by the Standard Model (0.038%), which upsets all the physicists who would like to see the Standard Model challenged.

This graph is one of the results of this study. It shows the distribution of the photon's energy (the points are number of observed decays and the bars are the measurement errors). One can see that most photons have an energy between 1.8 and 2.8 GeV ("Giga-electron-volt", which is about a third of a billionth of a Joule... not much, really). Above these energies the probability is 0 and below it's small but we can't tell anything because the errors get too large. All this does not tell much about extensions of the Standard Model, but a lot about the b quark. For instance the value at the peak around 2.2 GeV is half of the b quark mass.

Today I am working in the LHCb collaboration that is operating a detector on the new LHC collider at CERN. At LHC we can for instance measure semileptonic penguins, where the photon is then again decayed into two particles called leptons, like electrons for instance. In our zoological analogy that would mean decaying the mouse into two ducks, or b → s l l with l for lepton as shown on the diagram. There are actually two different diagrams that lead to the same effect.

The big disadvantage is that this decay is a 100 times more rare that the radiative counterpart, that already had a probability of 0.04%. It is therefore very difficult to observe. Belle has collected a few hundred cases so far, which is not enough for statistic treatments. But with the huge power of the LHC we are able to measure the angle between the trajectory of the leptons (the ducks) among others. This will tell a lot more about whales and elephants or dahus and unicorns than the bare measurement of the decay probability, as we did in the radiative case.

It's John Ellis, a CERN theorist who "invented" the name. One day he lost at a darts game, which was a very rare event. He got then challenged to insert the word "penguin" in his next paper. This paper was about the possibility of such decays. See the wikipedia article.

Since then many physicists have distorted these poor diagrams in all possible ways to make them look like penguins. I like the one shown here, which was given to me by Tobias Hurth, another CERN theorist.

A few papers:

• The paper on b → s γ: ArXiv:hep-ex/0403004, published in Physical Review Letters 93 (2004) 061803.
• A Belle paper on semileptonic decays: ArXiv:hep-ex/0308044, published in Physical Review Letters 91, 261601 (2003).
• The first LHCb paper on the subject: ArXiv:1112.3515 (hep-ex).
• My thesis.

For the general public:

• CERN's educational site.

(1) Never heard of the dahu? Check it out on Wikipedia. It's somehow similar to the Scottish wild haggis.