In LHC collisions, scientists observe events in which a lepton matter-antimatter pair is produced. These leptons are produced back to back, and scientists measure the fraction of times that the lepton is produced on the pink side. This allows researchers to make precise measurements of the electroweak force. These measurements are very sensitive to seeing new physics.
Scientists use LHC data to make many measurements. Some of them are serious discoveries. Some are careful measurements of things we already know about taken at higher energies to verify that our theory still works. And a few of them are super-precise measurements that straddle the two. This means that a precisely known variable is measured with equal precision to see if the prediction and measurement agree. If they don’t, we could be on to something.
One such measurement is when an electron-antielectron pair or, equivalently, a muon-antimuon pair is created from a particle collision. These two particles are of the class of particles called leptons. This process is unusual. Normally when two protons collide in the LHC, either the quarks or gluons that make up the protons interact via the strong force.
However, very rarely, a quark from one proton will interact with an antimatter quark from the other proton via either the electromagnetic force or weak force. In fact, one of the achievements in theoretical physics that led to the Higgs boson discovery was the demonstration that the weak and electromagnetic forces were two components of a single force, called the electroweak force.
When quark-antiquark pairs interact via the electroweak force, they make a particle that is a mix of the photon and the Z boson that is governed by the rules of quantum mechanics. Because of the quantum mechanical mix, the newly created particle has properties of both the photon and Z boson.
These physicists contributed to this analysis.
Electroweak theory makes very precise predictions of the angle between the colliding quark direction (which is in the same direction as the LHC beams) and the direction that the postcollision lepton is scattered. This angle could be at 90 degrees with respect to the beam, or the angle could be very small, meaning that the lepton is produced near the beam, either nearly parallel to the quark or antiquark. By calculating the ratio of how often the lepton is on the quark side versus the antiquark side, scientists can precisely probe electroweak theory.
If the data disagrees with the prediction, it would mean that new and unanticipated particles were being made. This measurement is a very sensitive way to search for new physics. Unfortunately, the measurement showed no indication of new physics but, on a positive note, the measurement was an impressive endorsement of electroweak theory.
This measurement used LHC data recorded during 2012 at a collision energy of 8 trillion electronvolts (TeV). Since 2015, the LHC has been recording data at a collision energy of 13 TeV, which is 60 percent higher than before. This analysis is being repeated at the higher energy, and we hope the higher energy will reveal something new.