First collisions at the LHC with unprecedented Energy! (Ask a particle physisicist)

[Alex K]

Why do they need to see multiple higgs bosons?  Last time they found it, I thought this time they would look for different particles

Ah, now we get to the proverbial meat of particle physics.

It's all a matter of statistics. A discovery of a particle is usually announced when the scattering events stick out from the so-called background events (those that look the same, but are from non-higgs events) by 5 standard deviations. For this, roughly a 100000 higgs bosons needed to be made. 

As soon as you have that, you can say with good confidence that you have found a new particle, but at the discovery threshold (i.e. the data they had analyzed in july 2012), you have only a very rough idea about its properties. In order to really tell how closely this guy rsembles the simplest version of the higgs boson that theorists have predicted 45 years ago, or whether it is part of a more elaborate scheme, you have to have precision measurements of its properties:

- what is its mass as measured from the different decay products
- how often does it decay to photons, to b quarks, to leptons and Z bosons, to W bosons, 
- how often is it produced in conjunction with a Z boson, with quark pairs and more, with top quarks, from gluons
- what are the angular and energy distributions of the particles coming out of these processes

For *all of these* except the higgs particle mass, the Standard Model gives a hard prediction. Every one of these measurements could be the place where one finds the crucial deviation to shoot down the theory and discover something new.

The more statistics you have, the more precise all these measurements get.

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What, do you think physicists sit around drinking wine all day mucking around on forums!

Well, now that you mention it...

Coffee is the key

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Maybe they want to make sure that they found what they think they found.

Yes, exactly. But they would love it even more if they found out thst they have discovered something, but *not* quite what they though they had.

It would be good to have another large particle accelerator to double-check things.  If they did, and they could not replicate the results, maybe they would dig into this one and find out that they had a speck of dirt somewhere in their equipment that is causing erroneous readings.

This gives me a question for Alex.  How do they know if the thing works properly, if there is only one of this size and they have nothing with which to compare things?

Excellent question.

While it is true that they only have one particle accelerator, that is not such a big problem. The accelerator only provides two high powered proton beams coming out of the wall for the experiments. As long as this beam is properly focused, that's good enough.

The crucial feature is that they have two experiments connected to this beam, ATLAS and CMS, which each have their own collisions at opposite ends of the ring, are constructed quite differently, but do exactly the same thing.

They are run be two different teams of 4000 people each, they have perpendicular magnetic geometry (one a Toroidal one where the magnetic field circles around the beam, and a solenoid where it goes in the direction of the beams), they use different detector technologies and independent analysis methods. Ideally they shouldn't even talk to each other about what they found before publishing, but of course you can't stop gossip. Either way, both have seen the new particle in their data with high significance, with compatible properties within the error bars.

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