Wednesday, February 6, 2013

(Almost) Nothing is forever: Decays

A truly remarkable fact is that most elementary particles do not live forever. Most of them exist only for a very brief moment in time. The only particles, for which we are reasonable sure that they live forever are photons. Then there are a number of particles, which live for almost forever. This means their life expectancy is much, much longer than the current age of the universe. The electron, the proton, and at least one neutrino belong to this group. And so do many nuclei. That is good, because almost all matter around us is made of these particles. It is rather reassuring that it will not disintegrate spontaneously anytime soon.

However, the vast majority of elementary particles has not a very long life expectancy. For some, their lifetime is still such that we can observe them directly. But for particles like the infamous Higgs, this is not the case.

If we want to learn something about such particles, we have to cope with this problem. A very important help is that a particle cannot just vanish without a trace. If a particle's life ends, it decays into other, lighter particles. For example, the Higgs can decay into two photons. Or into two light quarks. Or in some other particles, as long as the sum of their masses is lighter than the Higgs' mass. And these decays follow very specific rules. Take again the Higgs. If it is lying somewhere around, and then decays into an electron and a positron, these two particles are not completely free in their actions. Very basic rules of physics require them then to move away from the position of the Higgs in opposite directions, with the same speed. And this speed is also fixed.

These laws are what helps us. If we detect the electron and the positron, and measure their speed, we can reconstruct that they come from a Higgs. This is a bit indirect, and one has to measure things rather precisely. But it is possible. And it was exactly this approach with which we have detected (very likely) the Higgs last year.

Of course, there are a lot of subtleties involved. And not every decay can happen, which seems to be permitted at a first, superficial look. And so on. But all this follows very precise rules. And the experimental physicists have become very good at using these rules to their advantage.

And here enters my own research. One of my projects is about some particles which are build from Higgs particles and Ws and Zs. If I want to tell my experimental colleagues to check, whether my theory is right, I have to tell them for what to look. Since those complicated particles are not expected to live very long, they will decay. Hence, I should be able to tell how they decay, and into which particles with what speed. This will give what we call a signature: The traces a unstable particle leaves at the end of its life. Doing this is somewhat complicated, as you not only have to understand the structure of the particles, but also their dynamics. Fortunately, people have developed very sophisticated methods. I use them now in simulations. With them I start to obtain results how my complicated particles decay into two Ws. In addition, I learn how often and how effectively this will happen, and how long the original particle lived.

Of course, as always, my first results are not the final answer yet. But things look encouraging. And, what is best, I start to find hints that the complicated states may even live long enough that, just maybe, they could be seen by my experimental colleagues. This is important, because a particle which lives to short requires a precision to observe higher than what we have currently. But it is still a long road before anything is certain. As always.

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