As scientists announce new findings in the hunt for the Higgs boson, Victoria Martin offers a layman’s guide to the ‘God Particle’
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 said “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 with – or occasionally bumping into – another particle. That other 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 1624 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.
Yesterday, 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 this year, 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’s not enough data today to say for sure we did see the Higgs boson. The LHC is on its winter break right now and will start colliding protons again in the spring. 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 yesterday’s results as: “Be prudent. We have not found it yet; we have not excluded it yet. See you next year with a discovery!”
* Dr Victoria Martin is a lecturer and researcher in particle physics at the University of Edinburgh
Aborted camping trip prompted stunning discovery
PROFESSOR Peter Higgs’ place in history rests on a theory he came up with almost half a century ago.
His breakthrough came after returning to his New Town flat following an aborted weekend camping trip to the Highlands.
The question he had been pondering was one of the most basic problems of theoretical physics: what makes an object, like a brick, for instance, heavy when its atoms are weightless?
Higgs, pictured, came up with a theory suggesting the existence of a force field which all particles must pass through.
Some are slowed down more than others by the field, making objects heavier and lighter.
He also proposed that a particle exists, which he called a scalar boson – re-named the Higgs boson in the early 1970s – which clings to other particles as they pass through the field.
Higgs’ work is now considered a fundamental part of the standard model which scientists use to explain how the universe works.