Café Sci March 2016

The second Monday of the month is Café Sci night at The Vat and Fiddle. For March, Peter Watkins from the School of Physics and Astronomy at the University of Birmingham came to talk about searching for new particles using the Large Hadron Collider at CERN.

There are four detectors at CERN – CMS, ALICE, LMCb and ATLAS. Of these, Peter is most interested in ATLAS, the largest of the four. Re-started in 2013 following an upgrade, it looks at collisions of particles. As protons travelling at the speed of light collide, new particles are created and these are what the experiment is searching for.

But first some background – what are things made of? Atoms. These are actually mainly empty space with electrons whizzing around in it. At the centre of the atom is the nucleus – neutrons and protons packed closely together.

We’ve known about electrons for around 100 years and there is no evidence that there is anything inside them. However, neutrons and protons are made up of something called quarks. There are the up, charm, down, strange and bottom quarks. Then 20 years ago, the top quark was discovered. Relatively speaking, the top quark is massive; around 200 times bigger than a proton.

We also know that there are four forces:

1) Gravity – 40 times weaker than the electromagnetic force
2) Electromagnetic – holds atoms together
3) Weak force – interacts with everything
4) Strong force – holds quarks together

These are all important in their own ways. For example, stars wouldn’t function without the weak force. The forces are all carried by particles. If we investigate the particles we can find out about the force. The strong force is carried by particles called gluons (these are particles that hold quarks together) and has energy but no waves. The weak force is carried by particles call W and Z bosons.

There are 6 quarks, 6 leptons (another elementary particle) and four forces. Is it possible to describe everything in terms of just one unified force?

CERN is the largest lab in the world for looking at particle collisions. The Large Hadron Collider itself is 27km long, with magnets round the edge that keep the particles travelling in a circle. As the two beams of particles collide, we’re interested in when quarks hit quarks or when gluons hit gluons. Unfortunately, this only happens once in 10,000,000,000,000 collisions.

These new, heavy particles decay very quickly so it’s difficult to detect them. You have to see what they decay into and try and work out what it originally was. The W, Z and Higgs bosons decay very close to the collision point. The Higgs boson can decay to a number of things, one of which is a pair of Z bosons. This only happens about 3% of the time but it is in fact how we discovered it. In fact the odds of finding the Higgs boson was 1 in 10 trillion.

There are 3000 people in 38 different countries involved in the ATLAS experiment. To analyse data, 25,000 CPUs are used in computers all over the world. Its recent shutdown added an extra layer of detection. Hopefully Peter will be back in a couple of years to talk about their next discovery.

Author’s Notes

Talk from 14/03/2016

Originally published on the On Nottingham website


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