Professor Tara Shears, Physics
July 4th, 2012 – a landmark, red-letter day in a physicist’s calendar. It was the day that CERN webcast a special seminar of the latest results from the Large Hadron Collider. It was the day when the world went particle physics mad, celebrating the discovery of the long sought and long elusive Higgs boson.
Scientists here in the department of physics helped make that discovery. But, if you looked carefully, none of us claimed to have found the Higgs, just something new. And we’re still trying to understand what that something new is.
Special
The Higgs boson is special. We think that the universe is bathed in an invisible energy field – the Higgs field – that gives mass to subatomic particles like electrons and quarks. The only way to see if this field is there is to shake it up a bit – much like dropping a rock into a lake and seeing if you get splashed by one of the ripples. The Higgs boson is our ripple, the smoking gun that tells us that this field isn’t fantasy. And this field, although it might sound bizarre, is incredibly important because without it, without mass, we wouldn’t have stars, or atoms, just a very different universe full of light-speed bits whizzing round.
This field is so important to our understanding of the fabric of the universe, that the quest to find the Higgs boson has occupied almost every particle physics facility since the idea was devised almost 50 years ago.
Fast forward to last July, and our discovery of a new particle right where our theories predicted we might see a Higgs boson. Why didn’t we admit we’d seen it? Simply, and frustratingly, because we’re still not completely sure what it is. If this discovery is a Higgs boson it has to behave like one, and that means studying every signature that particle leaves behind in our detectors, every characteristic trace, and seeing if those features match our theory.
Not all of the studies are finished
Right now, at conferences deep in the Italian Alps, the latest results of these studies are being unveiled. Not all of the studies are finished. They are complex and involve the sifting of small signals from enormous backgrounds, and rely on a fantastically good understanding of the response of experimental equipment. In many of them we still don’t have enough data to be anything approaching conclusive – these analyses will need data taken once the Large Hadron Collider starts up again in 2015.
Admittedly not all the interesting results are in – there’s a big one due next week, the CMS study of two photon final states – but there have been many impressive updates. This search is incremental, without eureka moments. Certainty is still a waiting game, but our discovery is looking more and more like the Higgs we predicted.
Is this good news? Yes and no. Our theory might be good at predicting what we see in our experiments and mathematically beautiful, but it’s rubbish in some ways. There are swathes of the universe’s behaviour we just don’t understand – gravity, dark matter, why there is so little antimatter around to give just a few examples.
Explaining those is going to take a very different theory to the one we have now. I’d love this discovery not to be the Higgs after all, to be the first concrete sign that we need to revise our understanding; finding and taking that leap into the unknown is why we do particle physics. One of my friends hopes that these latest results show that we’re seeing `a’ Higgs, but not necessarily (hopefully…) `the’ Higgs”. Me too. Fingers crossed…
It is a totally new fundamental particle, we’re sure of that. Each type of fundamental particle has a characteristic mass which helps us identify and label it – the mass of this one is around 125 GeV/c2 (125 times the mass of a proton). No particle of that mass had ever been seen before last July. So yes, totally new.
If it turns out to be the Higgs (and this is looking more and more likely), then this particle was predicted by our theory and expected to exist. However, there’s no substitute for experimental test and observation, especially as we don’t know a priori that the theory is ultimately correct.
Making that final link between discovery and identification is what will let this all ultimately make sense. This discovery shows you science in the making; something new, something with promise to unite the unknowns in our theory together or blow it apart. And if you want to know why it takes so long, check this graphic from the ATLAS experiment – the discovery signal appears in blue, the red is what we expect, the points are our data – it’s a wonderful illustration of a waiting game in action.
https://twiki.cern.ch/twiki/pub/AtlasPublic/HiggsPublicResults//4l-FixedScale-NoMuProf2.gif
As an interested lay-man, I’m wondering if you can be more specific about what you may have found. Is it totally new, or just another expression of something we already know about?
I’m not one of the experimentalists but as a theorist
I can try and answer. What they have discovered is an unstable particle that is electically neutral and most likely of “intrinsic spin” zero, like the neutral Pi meson, or the Higgs boson that theory predicts. Their studies of the way it decays gives results so far consistent with the predicted properties of the Higgs. However, to be more confident that it is the Higgs that theory believes responsible for the quark, lepton and gauge boson masses, one would like to measure other things, like for example its self interactions; it would be nice to be able to collide two of the bosons, but that’s difficult!
I would take issue with one thing Tara writes: she implies that without the Higgs there would be no mass. That is wrong. The Higgs gives mass to quarks all right, but MOST of the PROTON mass comes from a different mechanism called colour confinement. In a universe without the Higgs, there would still be massive protons and neutrons; but electrons would be massless.