When the Large Hadron Collider was being planned and built, it was known – assuming the machine worked, and that no other experiment got there first – that it was guaranteed at least one result that would change physics for ever. It would either find the Higgs boson, or go beyond the Standard Model of particle physics.



This was the so-called “no lose” theorem, and the reasoning behind it is deeply tied up with a particular kind of particle scattering process which was the subject of a recent paper by the ATLAS experiment. The process is “Vector boson scattering” or, more specifically in this case, “WW scattering”.

The W boson is one of the particles which carries the weak nuclear force – that force which is so weak it is uncommon in everyday live, but lies behind Beta-decay in radioactivity, the way the Sun burns, and the interactions of neutrinos. The W boson comes in two versions, one with positive electric charge (W⁺) and one negative (W⁻).

Pairs of W bosons can be produced in several different ways when protons collide at the LHC – I discussed some of them here, way before we knew there was a Higgs boson. But in most cases the pairs have opposite electric charge. One W is a W⁺, the other a W⁻. This is because often they are produced by the decay of a neutral particle, so the charges have to cancel out because charge is conserved. A neutral particle has zero charge, so the sum of the charges of the particles it decays to must be zero too.

In WW scattering that is not the case any more. One way of thinking of it is that each W boson is emitted independently by an incoming quark, and then they scatter off each other. Since most of the quarks in a proton have positive charge, it is in fact rather likely that both Ws will be positive, W⁺ bosons, and will stay that way after they have scattered.

Another consequence of this “radiation off a quark” business is that the quark itself gets deflected and appears in the detector as a jet of hadrons - those are the yellowish wedges near to the beam-line in the picture at the top. Looking for those jets is one way of selecting WW scattering events.

The other features in that picture (apart from the detector itself, represented by the various green, red and blue rectangles) are: some lower-energy particles (orange lines), two muons (the red lines – one coming from each W decay. You can’t tell from the picture but both are measured to have a positive charge) and some missing energy, an energy imbalance, coming from the unseen neutrinos produced when the Ws decay. This is heading off directly downwards in this case, and is indicated by a thin blue line you might just be able to pick out.

All in all, while one can never be sure exactly what an individual event actually is (that’s quantum mechanics for you), this event looks very like a W⁺W⁺ scattering, and we have picked out enough of these in ATLAS to give strong evidence that W⁺W⁺ scattering is occurring at the LHC at roughly the rate expected in the Standard Model of particle physics.

Facebook Twitter Pinterest The distribution of WW scattering candidates as a function of the mass of the jet pair. The interesting contribution is the sort-of-puce-coloured “WW electroweak” bit, which gets more and more significant at the dijet mass grows. Photograph: ATLAS/CERN

And that is where we come back to the “no lose” theorem and the reason the LHC was bound to change physics.

There are lots of different ways a WW scattering can occur, and in the Standard Model, some of them involve the exchange of a Higgs boson. If there were no Higgs boson, those possibilities would be removed, of course, and that would change the predicted rate of WW scattering.



But something weird happens here. You might think that removing possible ways a scattering can occur would reduce the overall probability. However, in quantum mechanics, possibilities have to be added and then squared, to get the probability of a process actually occurring. And some possibilities come in with different relative signs. So sometimes they cancel out, in the same way that waves can cancel each other out if they arrive somewhere out of sychronisation – a phenomenon known as interference.

It turns out that removing the Higgs possibilities (before squaring the whole thing) in WW scattering actually dramatically increases the probability of such a scatter. In fact if you take the answer literally, it increases it so much that the probability would get bigger than one somewhere near the LHC collision energy. Probabilities bigger than one are “unphysical” – that is, nonsense. So the theory (without the Higgs) is wrong, and something else must occur in nature to stop in happening - something beyond the Standard Model. In this sense, WW scattering demands the Higgs, or it requires some other new physics instead.



I didn’t believe in the Higgs boson when I started work on the LHC, (as I describe near the start of Smashing Physics) so I worked on WW scattering out of a sort of pessimistic cynicism (my first LHC paper is here). But even now that we know there is a Higgs, this is still a vital process to measure, to see whether the Higgs contributions have the expected effect. This first ATLAS result is an important step forward. There will be a lot more when the LHC starts running again at higher energy next year.



Finally … I don’t know about you but quite apart from the somewhat tricky physics, I find “WW scattering” clumsy to say. It reminds me of “WWW”, the only abbreviation I know of which is much more cumbersome to say than the thing it stands for (“double-you double-you double-you”… ? Just say “World Wide Web”). Unfortunately “W” in the boson name doesn’t actually stand for anything, although it may have been inspired by “Weak”, or even “Weinberg”, I guess. So we’re stuck with it, unless we can just use “Vector boson scattering”.



Jon Butterworth’s book, Smashing Physics, is out now. A bunch of interesting events where you might be able to hear him talk about it etc are listed here. Also, Twitter.