Wednesday, July 4, 2012

On the discovery of a new particle (Higgs?)

And so it happened today and we have announced the observation of a new particle at the LHC. CMS and ATLAS both  showed evidence on the level of "five sigma", which means that the probability that what they're seeing is just a result of pure chance is more or less the probability of rolling the dice eight times and getting a six each time.

This particle might be the Higgs boson - something that was postulated for the first time in 1964 and had since then escaped detection. The significance of this discovery is bigger than many people think. Some are calling it the biggest scientific achievement of the 21st century, and something bigger than the landing on the Moon - and I think they're right.

Let me try to put this into perspective, from the point of view of a physicist working on the same experiment but searching for different, yet undiscovered, particles. This will be a bit long, so bear with me.

First of all - particle physics is what we call "fundamental research". We don't ask "what can I use this for?" or "whom can I sell it to and for how much?". We ask "why?". And we're trying to find answers to this question studying Nature at the deepest and most fundamental level - at tle level of interactions of particles that everything Science can describe is made of.

Its a bit like trying to solve a jigsaw puzzle. Blindfolded.

You reach into the box, hoping that your hand finds a new piece, and then try to arrange the pieces into something that makes sense.

And in fact in the 2nd half of 20th century we managed to arrange them pretty well, into something that we call the standard model of particle interactions. It's the most precisely measured theory to date. In the early 1990's it looked more or less like this:

You can see it looks pretty good, there are just two missing pieces. The one on the right was pretty obvious and it was found in 1995 by the experiments running on the Tevatron collider in Fermilab near Chicago. That's the top quark, the sixth particle in a family of particles making up the protons and neutrons in the atomic nuclei. Something that just had to be there.

The missing piece in the corner is something different. That's the Higgs boson. We need it to complete the puzzle, without it our theory is incomplete in an obvious way... but that's not the end of the story. You can see that the puzzle should extend in other directions, we believe that this is also the case with the standard model. We have many reasons to believe that the standard model is not the final, ultimate "theory of everything". In fact it doesn't even describe "everything" (it cannot describe gravity for example). But we have yet to see something new, some piece that would fit somewhere along the borders.

How are we looking for new pieces? We're reaching into the box where the pieces should be. Building stronger and stronger accelerators, we're able to reach further and further into the box. Remember, we can't see, we can just reach out and hope we find something.

And today we finally announced that our hand searching the box found a new piece:

At first glance it looks like it could be the missing one there in the corner. But checking whether it fits exactly will still take some time. This is the situation we're in at this moment.
What are the possible outcomes?

Option one
It is the standard Higgs boson. Everything fits. We have solved the puzzle... and unless we find some other new pieces in the meantime, we are starting to be in trouble:

we sort of expect there to be more to it, but we're not really sure where to start looking... game over?

Option two
It is some more exotic variant of the Higgs particle - it fits, but not exactly in the spot we thought initially:

this is much a better option as we would finally have a clear indication where to look for new things. Look: there's a whole new unpopulated row now!

Option three
It is something that ends up not fitting at all. I'd say its very unlikely, but possible - and would for sure be fun.

And what is my part in the game? I, along with many other physicists, still have my hand in the box. We're still reaching out, way beyond the spot where the piece found today was lying. We're doing this in parallel to the Higgs searches, hoping that we will eventually find something like this:

A completely new particle, in a completely different place. Hinting at a lot more to come...

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