New result from LHCb experiment challenges leading theory in physics

Results from the LHCb Collaboration at CERN suggests particles are not behaving the way they should according to the guiding theory of particle physics – suggesting gaps in our understanding of the Universe.

Physicists from the Universities of Cambridge, Bristol, and Imperial College London led the analysis of the data to produce this result, with funding from the Science and Technology Facilities Council. The result - which has not yet been peer-reviewed - was announced today at the Moriond Electroweak Physics conference and published as a preprint.

Beyond the Standard Model

Scientists across the world will be paying close attention to this announcement as it hints at 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.

Dr Mitesh Patel of Imperial College London, and one of the leading physicists behind the measurement, said: “We were actually shaking when we first looked at the results, we were that excited. Our hearts did beat a bit faster.

“It’s too early to say if this genuinely is a deviation from the Standard Model but the potential implications are such that these results are the most exciting thing I’ve done in 20 years in the field. It has been a long journey to get here.”

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.

Questioning the laws of physics

The updated measurement could question the laws of nature that treat electrons and their heavier cousins, muons, identically, except for small differences due to their different masses. 

According to the Standard Model, muons and electrons interact with all forces in the same way, so beauty quarks created at LHCb should decay into muons just as often as they do to electrons.

But these new measurements suggest this is not happening.

One way these decays could be happening at different rates is if never-before-seen particles were involved in the decay and tipped the scales in favour of electrons.

Dr Paula Alvarez Cartelle from Cambridge’s Cavendish Laboratory, was one of the leaders of the team that found the result, said: “This new result offers tantalising hints of the presence of a new fundamental particle or force that interacts differently with these different types of particles.

“The more data we have, the stronger this result has become. This measurement is the most significant in a series of LHCb results from the past decade that all seem to line up – and could all point towards a common explanation.

“The results have not changed, but their uncertainties have shrunk, increasing our ability to see possible differences with the Standard Model.”

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.

The LHCb scientists say there has been a breadcrumb trail of clues leading up to this result – with a number of other, less significant results over the past seven years also challenging the Standard Model in a similar way, though with less certainty.

If this result is what scientists think it is – and hope it is – there may be a whole new area of physics to be explored.

Dr Konstantinos Petridis of the University of Bristol, who also played a lead role in the measurement, said: “The discovery of a new force in nature is the holy grail of particle physics. Our current understanding of the constituents of the Universe falls remarkably short – we do not know what 95% of the Universe is made of or why there is such a large imbalance between matter and anti-matter.

“The discovery of a new fundamental force or particle, as hinted at by the evidence of differences in these measurements could provide the breakthrough required to start to answer these fundamental questions.”

Dr Harry Cliff, LHCb Outreach Co-Convener, from Cambridge’s Cavendish Laboratory, said: “This result is sure to set physicists’ hearts beating a little faster today. We’re in for a terrifically exciting few years as we try to figure out whether we’ve finally caught a glimpse of something altogether new.”

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.

Additional information – about the result

The results compare the decay rates of Beauty mesons into final states with electrons with those into muons.

The LHCb experiment is one of the four large experiments at the Large Hadron Collider (LHC) at CERN in Geneva, and is designed to study decays of particles containing a beauty quark

This is the quark with the highest mass forming bound states. The resulting precision measurements of matter-antimatter differences and rare decays of particles containing a beauty quark allow sensitive tests of the Standard Model of particle physics.

Rather than flying out in all directions, beauty quarks that are created in the collisions of the proton beams at LHC stay close to the beam pipe.

The UK team studied a large number of beauty or b quarks decaying into a strange-quark and two oppositely charged leptons. By measuring how often the b-quark decays into a final state containing a pair of muons or a pair of electrons, they found evidence that the laws of physics might be different, depending on whether the final state contains electrons or muons. 

Since the b-quark is heavy compared to the masses of the electron and muon it is expected that the b-quark decays with the same probability into a final state with electrons and muons. The ratio between the two decay probabilities is hence predicted to be one.

However analysis of the UK team found evidence that the decay probability is less than one.