Cautious excitement as hints of new physics grow at the Large Hadron Collider

Particle physicists announce results that potentially cannot be explained by the current laws of nature.

Beyond the Standard Model

The LHCb (Large Hadron Collider beauty) Collaboration at CERN has found particles not behaving the way they should according to the guiding theory of particle physics. The collaboration includes several academics, postdocs, and students from the School’s Institute for Particle and Nuclear Physics, who play crucial roles in maintaining and operating the LHCb detector and optimising its particle identification and reconstruction algorithms.

The new result, presented to the world today at the ‘Moriond’ Particle Physics Conference, could suggest the existence of new particles not explained by the Standard Model. The Standard Model is the current best theory of particle physics, describing all the known fundamental particles that make up our Universe and the forces that they interact with. However, the Standard Model cannot explain some of the deepest mysteries in modern physics, including what dark matter is made of and the imbalance of matter and antimatter in the Universe.

Building blocks of nature

Today’s results were produced by the LHCb experiment, one of four huge particle detectors at CERN’s Large Hadron Collider (LHC). The LHC is the world’s largest and most powerful particle collider – it accelerates subatomic particles to almost the speed of light, before smashing them into each other. These collisions produces a burst of new particles, which physicists then record and study in order to better understand the basic building blocks of nature.

The LHCb experiment is designed to study particles called ‘beauty quarks’, an exotic type of fundamental particle not usually found in nature but produced in huge numbers at the LHC. Once the beauty quarks are produced in the collision, they should then decay in a certain way, but the LHCb team now has evidence to suggest these quarks decay in a way not explained by the Standard Model.

Dr Silvia Gambetta, one of the University of Edinburgh LHCb collaborators, and the experiment’s Operations Coordinator, comments:

Collecting and calibrating the data needed to perform this measurement is an effort in itself. Performing measurements like this is what makes it worthwhile for the hundreds of scientists who work every day behind the scenes.

Questioning the laws of physics

The new result relates to an anomaly that was first hinted at in 2014 when LHCb physicists spotted that beauty quarks appeared to be decaying into electrons more often than they did to muons (the muon is in essence a carbon-copy of the electron, identical in every way except that it’s around 200 times heavier.) This means that muons and electrons interact with all the forces in the Standard Model (apart from the Higgs field) with precisely the same strength, and crucially, this implies that beauty quarks should decay into muons just as often as they do to electrons. The only way these decays could happen at different rates was if some never-before-seen particles were getting involved in the decay and tipping the scales in favour of electrons.

In 2019, LHCb performed the same measurement again but with extra data taken in 2015 and 2016. This time around the result moved closer towards the Standard Model prediction, but the uncertainty on the measurement got smaller and the upshot of which was that things weren’t much clearer than they’d been five years earlier.

Today’s result adds even more data, recorded in 2017 and 2018. To avoid accidentally introducing biases, the data was analysed ‘blind’, which means that all the procedures used in the measurement had to be tested, tested and tested again before being finalised after a lengthy internal review process carried out by the 1400-strong LHCb collaboration.

Not a foregone conclusion

In particle physics, the gold standard for discovery is five standard deviations – which means there is a 1 in 3.5 million chance of the result being a fluke. This result is three deviations – meaning there is still a 1 in 1000 chance that the measurement is a statistical coincidence.

It is therefore too soon to make any firm conclusions. However, while they are still cautious, the team members are nevertheless excited by this apparent deviation and its potentially far-reaching implications.

Professor Franz Muheim, who leads the University of Edinburgh LHCb research group, and as Chair of the LHCb Editorial Board was responsible for the editorial review of the corresponding paper, said:

If confirmed this result could unveil a new fundamental interaction and lead to a complete overhaul of the Standard Model of particle physics. However, extraordinary claims need extraordinary evidence, so more measurements, also on related quantities, are required.

Next steps

It is now for the LHCb collaboration to further verify their results by collating and analysing more data, to see if the evidence for some new phenomena remains. If this result is what scientists think it is, there may be a whole new area of physics to be explored. Only time will tell if we have finally seen the first glimmer of what lies beyond our current understanding of particle physics.