What’s the matter with antimatter? particle physics

10 February 2017

Built by the European Organisation for Nuclear Research (CERN), the Large Hadron Collider (LHC) in Geneva, Switzerland is the largest and most powerful particle accelerator in the world. Situated 100 metres underground in a 27km long circular tunnel, the LHC conducted its first particle collision in 2009. Inside the accelerator, protons collide at extremely high energies. Particle detectors are placed at the collision points, allowing scientists to study the particles created during the collision process.

Ms Shu Faye Cheung completed her DPhil in particle physics, the study of the fundamental constituents of matter and the forces by which they interact, at the University of Oxford. While at Oxford, she studied data collected from one of the particle detectors at the LHC. As part of her DPhil programme, she was based at CERN for over a year. 

Matter and antimatter 

Cheung’s research interest is a group of particles called antimatter. “The Standard Model of particle physics is a framework that summarises our current knowledge of the properties of subatomic particles and how they interact with each other. However, it only accounts for about 5% of the content of the Universe,” explained Cheung. “The objective of particle physics research is to fill in the missing pieces of the puzzle.”

She explains that every particle comes in two forms: matter and antimatter. These particles have opposite electric charges. The antimatter version of a negatively-charged electron is a positively-charged particle called the positron, which has the same mass as the electron. If a matter particle meets its antimatter partner, annihilation will occur. 

Although first observed in the 1930s, antimatter is still a topic of huge interest because it is key to understanding the formation of our matter-filled Universe. “Antimatter explains why the Universe looks the way it does today. We believe that nearly equal amounts of matter and antimatter were created in the early Universe,” said Cheung. “In a scenario with slightly more matter than antimatter, matter-antimatter annihilation processes would only leave matter particles behind to form the Universe as we observe it. But understanding how there could initially be more matter than antimatter is still unknown.”

The LHC has a relatively large price tag but the knowledge that it generates, not only in terms of particle physics but in terms of transferable technology, is invaluable.

Four fundamental forces of nature

Essential to solving this mystery is studying how matter and antimatter particles interact with the four fundamental forces of nature: electromagnetism, the strong force, the weak force and gravity. Of these forces, the weak force (the force that governs radioactive decay) is known to behave differently for matter and antimatter particles and influences the way they decay.

Cheung performed a precision measurement of a parameter called the Cabibbo-Kobayashi-Maskawa (CKM) matrix. The CKM matrix quantifies how differently matter and antimatter behave within the Standard Model paradigm. It is measured when the particle has decayed though quantum interference. “In quantum mechanics, there is a curious property where, if a particle can decay via two paths to the same final state, it will decay through both paths simultaneously,” explains Cheung, “This is known as quantum interference.”

This interference pattern can be observed using a two-dimensional representation of a multibody particle decay called a Dalitz plot. The Dalitz plot is used to visually represent the relative frequency a three-body decay. The kinematics of a three-body decay can be completely described using two variables. By comparing the Dalitz plots from the decay of matter and antimatter particles, the parameter gamma can be measured.

Applications in other fields on science

Critics argue that spending billions on supersize particle accelerators is a waste of money, but scientists say fundamental research has made a significant contribution in developing technologies with applications in other fields of science.

“Despite the fact that the research isn’t application driven, particle physics experiments give rise to new technologies that have applications in other fields as well,” said Cheung. “The LHC has a relatively large price tag but the knowledge that it generates, not only in terms of particle physics but in terms of transferable technology, is invaluable.”

To date, there are thousands of particle accelerators around the world. Their primary use is in hospitals, where cancers are treated with radiotherapy or hadron therapy. Additionally, PET (Positron Emission Tomography) scanners, important in imaging organ activity, use matter-antimatter annihilation and the sensor technologies and image reconstruction methods originally developed for particle detectors.

Future challenges

After the first period of data collection from 2009 to 2012, the LHC was shutdown in preparation for a second run of data involving higher collision energies. In 2015, the LHC started up again and researchers will continue to collect data until 2018.

“As we move to higher energies, one of the challenges we face is that particle physicists have to become increasingly creative with the way data is recorded and analysed in order to cope with the amount of data generated,” notes Cheung.

“Trying to work out what theories could account for the unexplained 95% of the Universe is a priority for the field,” said Cheung. “This could come in the form of new particles or new forces. However, we have not yet conclusively observed anything that deviates from the predictions of the Standard Model.”

Ms Shu Faye Cheung  graduated in 2012 with a BA (Hons) MSci degree in Natural Sciences (Experimental and Theoretical Physics) from the University of Cambridge. Cheung completed DPhil in Particle Physics under the supervision of Prof Neville Harnew and Dr Sneha Malde at the University of Oxford. She was awarded a University of Oxford Croucher Scholarship in 2012. 

To view Ms Shu Faye Cheung’s Croucher profile, please click here.