Antimatter is one bit of particle physics that nearly all science fans have heard of, partly thanks to the science fiction authors and film makers who have made it the spacecraft fuel or weapon material of the future. Perhaps the most notorious example is Dan Brown’s thriller Angels & Demons, where a canister of antimatter is stolen from CERN and used to make a bomb. To particle physicists antimatter is both interesting—as it is central to some of the big questions we are working on today—and also boring old stuff, as it has been known about for over ninety years.
Antiparticles were first predicted by Paul Dirac around 1928. He was working on a theory to combine quantum mechanics, the theory which explains the behaviour of particles on very small scales, with that of relativity, which explains their behaviour at close to the speed of light. Quantum mechanics predicts that particles exist in states with a fixed energy. The maths says the energy is given by the square root of an expression involving its mass and momentum. As this root had both a positive and negative solution, the theory predicted the existence of negative energy states.
Dirac proposed that all the negative energy states were filled. Given enough energy, a particle can be raised from a negative energy state into a positive energy state, creating a particle and an antiparticle with the opposite electric charge. When a particle falls down into a negative energy state, it is annihilating with the antiparticle and releases the energy stored in both their masses. The antiparticle theory was verified in 1931 with the discovery of a particle with the mass of the electron, but curving in the opposite direction in a magnetic field, showing it had the opposite charge. This was an anti-electron or positron.
Positrons are emitted by some radioactive atoms. They are used in one of the best known real world applications of antimatter: Positron Emission Tomography (PET), a medical imaging technique where positrons emitted by a short-lived radioactive tracer annihilate with electrons inside the patient creating two gamma rays, emitted back-to-back. These are detected outside the body and their path leads straight back to the point of creation, allowing an image to be reconstructed.
Annihilating antiparticles is at the heart of the big particle physics experiments at accelerator laboratories. At the Large Electron Positron (LEP) collider, built in a 27km circular tunnel at the CERN laboratory in Switzerland in the 1980s, beams of electrons and positrons were steered to collision points, where the high energy annihilations recreated particles from the Big Bang. LEP ran until 2000 when it was shut down to make way for the Large Hadron Collider.
The LHC collisions are messier as the beam particles are protons. Each protons is made up of three quarks, but at high energies they also contain a sea of virtual quarks and antiquarks. Some of the best interactions to study are where a quark from one proton annihilates with an antiquark from the other. The collisions are studied at huge underground detectors, such as the ATLAS experiment—the biggest particle physics detector ever built, a cathedral-sized instrument built in a cylinder shape around the beamline, with layers of different detectors aiming to record all the particles flying out of the collision point. Oxford University was a founding member of the ATLAS collaboration and Oxford physicists have played a large part in its design, construction, and operation. The semiconductor tracker at the heart of ATLAS was built in our laboratory on Keble Road.
Antiparticles have been studied to death and the measurements match theoretical predictions very well, however there remains one big unsolved mystery about it: why is the universe we see made from matter, when we always see particles and antiparticles created together? The Big Bang created equal amounts of matter and antimatter, which should have then annihilated to leave a universe full of radiation, but devoid of stars, planets and physicists.
This mystery was investigated by cosmologist Andrei Sakharov (better known as the father of the Soviet hydrogen bomb and later human rights campaigner). In 1967 he showed the conditions which could explain this. A subtle asymmetry between matter and antimatter could give rise to a tiny excess of matter after all the primordial antimatter had annihilated. The excess was just one extra particle for every ten billion particle-antiparticle pairs, but this small left-over was enough to form the universe we see today.
Investigating this asymmetry, which has the formal name of CP (Charge Parity) violation, is one of the hot topics of current particle physics research. It has been observed in some particle interactions, but what we have seen is not sufficient to explain why the universe exists. We continue to probe new particles and their interactions to look for further asymmetries. Oxford researchers are involved with another big experiment at the Large Hadron Collider, with a particular focus on tackling this challenge. This is the LHCb project.
The LHCb detector (Photo: Brice, Maximilien: CERN)
LHCb probes the asymmetries of the decays of B-mesons, short-lived particles containing the heavy b-quark. Unlike the cylindrical ATLAS detector, LHCb sensors are arranged in a cone shape, as most of the B mesons shoot out only a small angle from the beamline. In addition to taking precision measurements of B mesons, LHCb has made further research breakthroughs. They recently announced the first discovery of CP violation in the D0 meson, and the first pentaquark particle.
Antimatter is a fascinating research topic, but could it be the rocket fuel of the future? There is a catch. With no antimatter left in the universe, it has to be made in accelerators and the process is very slow. It is said that all the antimatter ever made at CERN would only have the energy to light an electric bulb for a few minutes. Dan Brown got it wrong and antimatter drives are not going to appear any time soon.
Oxford is not working on developing an antimatter starship drive, or a bomb. This may disappoint some young sci-fi fans coming along to our open days. But we have built a colossal antimatter microscope to probe the nature of matter at the tiniest scale to look back in time to the moment of creation and understand how the universe we know actually exists at all. Isn’t that even more exciting?
To learn more about antimatter, check out this podcast by Donal Hill: What is Antimatter? or for the full story read “Antimatter” by Frank Close (OUP 2009). If you prefer a musical version, listen to this song by Jonny Berliner (and the people of Oxford):
Particle Physics Gadgeteering 