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Victoria Martin provides a layperson's guide to the Higgs Boson
A version of this article was first published in the Edinburgh Evening News on the 14th December 2011, following an announcement of results of Higgs bosons searches at CERN.
I believe I first heard about the Higgs boson as a third-year undergraduate student at the University of Edinburgh in the mid-1990s.
Walking through the corridors of the (then) Department of Physics between lectures, someone elbowed me and whispered covertly, “That’s Professor Higgs: he invented a particle!”
In my final undergraduate year in Edinburgh I took two classes with Professor Higgs: General Relativity and Groups & Symmetries. Although these subjects are both key to understanding the Higgs boson, I graduated without ever really understanding what Higgs’ particle was.
However something must have caught my imagination as for the past 15 years, since graduating, I’ve been doing research in particle physics. My current research is searching for the Higgs boson as a member of the Atlas experiment at the Large Hadron Collider (or LHC for short) at Cern, the European Organisation for Nuclear Research in Geneva.
So what is this particle Higgs invented? We particle physicists investigate the properties of fundamental subatomic particles. Just as everything is made of molecules and those molecules are made of atoms, it turns out those atoms themselves are made of more fundamental particles: electrons, protons and neutrons. We believe the electron is a truly fundamental particle: it isn’t built up of smaller building blocks. However we know that the protons and neutrons are not fundamental; they consist of three even smaller particles we call “quarks”. Quarks come in different “flavours” and always stick together in twos or threes. For example, the proton is made of two “up” quarks and one “down” quark.
This is where Higgs’ particle comes in. We can’t figure out why the electron and the quarks have a mass; unless, somehow, they obtain a mass by interacting in a special way with the so-called Higgs field. If this Higgs' explanation is correct and this Higgs field really exists and is present everywhere in the Universe, then one consequence is that the Higgs field can clump together and form a new kind of particle. This new particle is Higgs’ particle, which we call the Higgs boson. To see if Higgs’ theory is really true we will need to find some Higgs bosons and see if they really do interact with quarks, electrons and the other fundamental particles we know about.
So how do you find a Higgs boson? First you have to make one, and for that you need a particle collider, like the LHC. The numbers we use to describe the LHC are big. The LHC is a 27km circle, 100m beneath the Franco-Swiss border.
It consists of over 1200 magnets cooled to -271C, a temperature colder than outer space. These magnets are used to accelerate protons to 99.99998 per cent of the speed of light and then the protons are smashed together.
This collision recreates the conditions that existed just after the big bang when Higgs bosons may have first existed. However the Higgs boson is rare. Out of one billion of these proton collisions we expect to make just 10 Higgs bosons. Moreover, once created, the Higgs bosons will decay almost instantaneously. Therefore what we look for in our experiment are the particles left by the decay of the Higgs boson.
Particle physicists have been trying to make Higgs bosons at various colliders for the past 20 years. We know from those experiments if the Higgs boson exists, it must be heavy – at least 115 times as heavy as the proton – but not too heavy – less than 155 times as heavy as the proton.
On December 13th 2011, at Cern, the latest results on the search for the Higgs boson were announced, both from my experiment Atlas and our friendly competitor, the scientifically-named Compact Muon Solenoid experiment (CMS). Both Atlas and CMS detect the particles left by fleeting Higgs bosons.
After analysing most of the data collected in 2011, both experiments see tantalising hints consistent with making Higgs bosons with a mass of around 125 times as heavy as the proton. What’s really exciting is both experiments see something very similar, using more than one kind of particle left by the Higgs boson.
There wasn't enough data collected in 2011 to say for sure we did see the Higgs boson. We will need to take another three times as much data to be sure, but the LHC should easily deliver that before the end of 2012.
I’ll leave the last word to the Professor Rolf Heuer, the director general of Cern who summarised the results presented in December 2011 as: “Be prudent. We have not found it yet; we have not excluded it yet. See you [in 2012] with a discovery!”