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Mind your Zs and Ws

By Hamish Johnston

On Wednesday I will be at CERN in Geneva to hear about the latest in the quest for the Higgs boson – and if the rumours are to be believed, I won’t be disappointed.

As the big day approaches, physicists have moved away from gossiping about whether a discovery will be announced (no, if you consider individual experiments, which are both expected to report evidence at the 4-sigma level…yes if you “unofficially” combine these results to get a statistical significance greater than the magical 5-sigma) and on to the nitty-gritty of what has been seen in the various detection channels.

Once the Higgs is created in the LHC, it can be detected in a number of different ways – or “channels” to use the jargon – several of which are being scrutinized by the LHC’s experiments. Two important channels involve the Higgs decaying to a pair of W bosons (WW) or a pair of Z bosons (ZZ). Conventional theories say that the physics behind these decays is similar, so the assumption is that evidence of both should be seen in the LHC data. However, rumours coming out of CERN suggest that this is not the case – the WW signal doesn’t appear to be there.

However, the Higgs can also decay creating two photons (what’s known as the diphoton channel) and the word on the street is that many more events than expected have been seen in this channel. This also contradicts the absence of WW events because conventional theory predicts that a large diphoton signal should also be accompanied by a large WW signal.

Meanwhile, researchers at Fermilab in the US have been rifling through their data from the now switched-off Tevatron collider and have announced further analysis of their own Higgs search today. The Tevatron results seem to suggest that the LHC should be seeing WW events.

So why isn’t the LHC seeing WW decays? One possibility is that there’s something wrong with how the LHC is looking for these events – a rather boring situation that can be fixed. More tantalizing is that the LHC is right about the WW deficit – which could mean that the particle glimpsed so far is not the “real Higgs”, but rather an “imposter”!

The unexpectedly high diphoton signal is also interesting in itself. It could point to the existence of a new charged particle not described by the Standard Model of particle physics, or it could mean that there are a multitude of Higgs particles – or something completely different.

Indeed, the only certainty is that much more work will be needed before physicists get a handle on the Higgs. So Wednesday will likely be remembered as the beginning of a new era in particle physics – as well as the end of a long search for the Higgs boson.

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