On 8 October 2013, the Nobel prize for physics was awarded to Francois Englert and Peter Higgs. In one sense this was a long time coming: the theoretical work that won the prize took place in 1964 (Englert, and his late colleague Robert Brout, working independently of Higgs, published first; a few weeks later Higgs published a paper that explicitly predicted the existence of a scalar boson; another group of physicists – Gerald Guralnik, Carl Hagen and Tom Kibble – published related work later in the same year). In another sense the prize was awarded remarkably quickly: experimental proof of the existence of a fundamental boson was announced on 4 July 2012, and it wasn’t until 14 March 2013 that it was confirmed to be a scalar (spin-0) boson. (If you want to learn more about the Higgs mechanism, you can find a variety of explanations here.)
To my mind, the discovery of the Higgs is one the crowning achievements of human civilisation: it is the culmination of a process that began 2500 years ago with the Greeks. Physicists now have a standard model of fundamental particles: there exists a small number of spin-1/2 point particles (6 quarks; the electron, muon and tau each with their associated neutrino) which interact via the exchange of spin-1 particles that mediate the electroweak and strong (these exchange particles being the photon; W+, W– and Z0; 8 gluons). In the ‘pure’ theories underpinning this model the fundamental particles are massless; they acquire mass – and thus in a certain sense their very existence – by interacting with a spin-0 field that pervades the entire universe. This spin-0 field has an associated particle; the Higgs boson. And that’s it. End of story. Except…
We are really just at the beginning of the story. The theories underpinning the standard model are in conflict with the other central pillar of physics: general relativity. The standard model is based on quantum physics; general relativity is a classical theory. Physicists need to develop a quantum theory of gravity. Furthermore, we now know that the standard model applies to only 5% of the universe: 95% of the mass-energy content of the universe resides in the so-called ‘dark’ sector. We desperately need to understand the nature of dark matter and dark energy.
Now that the Large Hadron Collider has discovered the Higgs its next job, when it becomes operational again after its current upgrade, is to shed light on the dark sector.
Most newspaper reports of the recent discovery of a new fundamental boson (let’s agree to call it the Higgs, shall we?) mentioned the long time delay between Higgs postulating the particle and physicists detecting it. That got me to wondering, this morning, whether the time delay was particularly long.
Peter Higgs was the first to postulate the existence of a fundamental scalar particle that might be detectable. He did this in 1964. (Several physicists, at around the same time, argued that a fundamental scalar field was required to give other particles mass; you can read all about that elsewhere.) The point is, it took experimental physicists until 2012 – that’s 48 years – to find the particle and prove that it existed. Is 48 years a long time in this context?
Well, Wolfgang Pauli postulated the existence of the electron neutrino in 1930; it took until 1956 before it was discovered – a lag of 26 years between theory and experiment. There was a similar gap of about a quarter of a century between theorists postulating the existence of the top neutrino and experimenters finding it. (The muon neutrino was discovered a mere 14 years after it was postulated.) So it seems that neutrinos, which are notoriously difficult to study, were found much more quickly than the Higgs.
What about quarks? Well, the bottom and top quarks were postulated in 1973 by Kobayashi and Maskawa; the b quark was found in 1977 (a mere four year later) and the t quark was found in 1995 (a gap of 22 years). So the quark sector was cleared up fairly quickly too.
The W and Z bosons turned up in experiments in 1983, 15 years after Glashow, Salam and Weinberg told people to expect them.
So it would seem that the Higgs is indeed something of a standout amongst the elementary particles: it took almost twice as long to find the Higgs as it did to find any of the other fundamental particles that theorists posited. Personally I’m hoping that the LHC will turn up evidence for a supersymmetric particle. Although supersymmetry itself has a long history, going back to the 1970s, the first realistic supersymmetric version of the Standard Model didn’t arrive until 1981, with work by Georgi and Dimopoulos. If we take 1981 as the starting date, then, it won’t be until 2029 that the Higgs record for a delay between postulation and experiment is broken.
In December 2012 the ATLAS and CMS teams at the Large Hadron Collider announced that they had seen signals that were consistent with there being a Higgs boson with a mass somwhere in the region of about 124-126 GeV. Statistically, though, they were unable to claim a discovery.
Before Fermilab’s Tevatron collider ceased operations in September 2011 its two experiments – CDF and DZero – generated vast amounts of data that have only now been analysed. On 7 March 2012, scientists announced the results of that analysis at the Rencontres de Moriond conference. The data hint at a Higgs boson with a mass somewhere in the range 115-135 GeV. Again, the statistics fall far short of that required to claim a discovery.
The Tevatron collider at Fermilab, as seen from the air. The main ring and main injector are clearly visible. The ponds are there to dissipate waste heat from the machine.
Credit: Fermilab, Reider Hahn
This is tantalising! The ATLAS and CMS teams both make use of high-energy proton-proton collisions produced by the LHC, but they are quite different experiments focusing different things. The CDF and DZero experiments are different again: the Tevatron produced proton-antiproton collisions. So a variety of signals are pointing to a Higgs with a mass somewhere around 125 GeV. But there’s no certainty that it’s there: further data might cause the signal to vanish like the Cheshire Cat.
One thing is certain: by the end of 2012 we will know whether the Higgs exists and, if it does, what its mass is. The LHC is operating so well that there’s now nowhere left for Higgs to hide.
I’ve just spent the afternoon watching CERN’s live webcast of the latest CMS and ATLAS data (thank you, CERN, for inventing the Web!). After today there’s very little room for the Higgs still to hide.
ATLAS essentially rules out the existence of a Higgs boson, unless the Higgs mass is in the region 115 to 131 GeV. (I guess I should say that, for comparison, the proton mass is about 1 GeV (actually 0.938 GeV)). CMS seems to rule out a Higgs that is more massive than 127 GeV.
What is tantalising is that both experiments saw hints of a Higgs at around about 125 GeV. Unfortunately, the signal was not strong enough to claim a discovery: what they saw might have been a statistical fluke.
The only way to decide the matter is to take more data which is, of course, what the two experiments will do. In a few months time we will know one way or the other. Either the bumps that ATLAS and CMS saw will go away, and we can say that the Higgs doesn’t exist. Or the bumps will get larger and clearer, and we can say that the Higgs exists with a mass of around 125 GeV.
Either way, new physics will be required. Either way, it will be the discovery of the century.