W and double W

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The W boson carries the weak nuclear force. At the Large Hadron Collider pairs of them may be showing the first signs of the Higgs boson. But they are a decidedly mixed blessing.
The current data from the LHC show an effect which might, or might not, be the first indication of the presence of a Higgs boson. Most of this effect is due to the number of pairs of W bosons which are being produced in proton-proton collisions.

Diagram showing a Higgs boson decaying to a W+ and a W-, which then decay to leptons. This might be happening.

The W boson (along with the Z boson) is responsible for carrying the weak nuclear force, just as the photon carries the electromagnetic force. It is intimately connected with the Higgs boson. In the standard theory, the Higgs gives mass to the W and Z, and so breaks a symmetry which otherwise would mean the weak force was pretty much as strong as the electromagnetic one.

If the Higgs boson has enough mass (that is, enough energy if it isn’t moving, since E = mc2), it will decay very quickly into a pair of W bosons. So if the Higgs is there, extra pairs of W bosons are one way we might first see it.

Generally this is a common way of seeing new particles. It’s called resonant production. If you can measure the energy and momentum of the decay products (two W bosons in this case), you can reconstruct the mass of the original (in this case the Higgs boson). W  bosons can be produced in lots of different ways, but if one way is via the Higgs, you would expect a bump in the WW mass distribution at the mass of the Higgs….. (Just as the previous rumours were about a supposed bump in the photon-photon mass distribution, which is what the Higgs does if its mass is lower.)
Unfortunately, the W bosons in question decay to an electron, or muon, plus a neutrino (see my dodgy diagram above). The electrons and muons can be measured well, but the neutrinos can’t. Neutrinos do not interact much at all with anything, including our detectors. So we don’t see them. The missing momentum they carry away gives a clue, but it doesn’t allow us to reconstruct a Higgs mass bump.
This is one reason the current excess is so susceptible to theoretical uncertainty. You don’t need much of a theory to tell you whether or not there’s a bump in your distribution. But we can’t reconstruct a bump, so we just have to count WW pairs and see whether there are more than we expect. The theory tells us what to expect.
On the other hand, W bosons are really lovely things to measure, and they contain a lot of information. There are many, many collisions where only a single W is produced in an event. One thing we can use these for is to study the internal structure of the protons we are colliding. They are made up of quarks and gluons, and exactly how these are distributed is one of the big uncertainties in how many of anything (including Higgs bosons) is produced at the LHC. One of the things we presented at EPS was a very precise set of measurements of W and Z boson production, which will be used soon to pin down further how quarks and gluons are distributed in the proton. CMS have just published their results on this from the 2010 data. Amongst other things, this will reduce the theoretical uncertainty in the Higgs search.
In general, we are in a period of scoping out what physics really looks like at LHC energies. Whether the Higgs boson is part of the landscape or or not remains to be seen.
But we will see.
Jon Butterworth –

Written by physicsgg

August 8, 2011 at 1:34 pm

Posted in High Energy Physics

Tagged with , , ,

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