“Cautious excitement” among physicists: evidence of deviation from the Standard Model and rare observations at CERN
The LHCb experiment at the particle accelerator of European institute CERN, which also involves physicists from Maastricht University, today presented results that suggest a deviation from the Standard Model of physics. Although the results sparked great interest worldwide, a breakthrough cannot yet be claimed.
The Standard Model describes the smallest building blocks of matter and how they interact under the influence of the four known forces of physics. The Standard Model is around 50 years old and has thus far been able to predict and explain all measurements in particle physics.
CERN, LHCb, Nikhef and Maastricht University
CERN is the European organisation for elementary particle research. The world’s largest and most powerful particle accelerator, the Large Hadron Collider (LHC), is in use by thousands of international researchers affiliated with CERN. Dutch participation is coordinated by the Nikhef collaboration, of which Maastricht University a member. The LHCb experiment is one of the four experiments at the LHC in Geneva, Switzerland, and involves 1475 researchers from 89 different institutes.
Evidence of a deviation
The results presented on March 23 may mark the beginning of a new phase for the Standard Model: researchers from the LHCb experiment found evidence that a presently undescribed fifth force of physics may be at play. The new measurements spark “cautious excitement” among physicists.
Marcel Merk, national programme leader of the LHCb project and professor of particle physics at UM, says the developments are super exciting: “On January 21st this year, the analysts saw the data for the first time. ‘More than 3 sigma.’ My heart skipped a beat. Wow, I thought, this is it: this is the reason I started research with LHCb 25 years ago.” Statistically speaking, however, a new discovery cannot yet be claimed.
A potential fifth force of physics
So what can be claimed? The reported deviation has a significance of ‘3 sigma’: odds of around 1 in 333 that it occurred by chance. Experimental particle physicists refer to such a statistic outcome as ‘evidence’. In order to be considered a discovery, they maintain a limit of ‘5 sigma’ – odds of 1 in 3.3 million for a fluke – and that requires more research.
LHCb physics coordinator Niels Tuning of Nikhef: “The exciting thing about such a deviation is that it can still go anywhere: will the result move towards the Standard Model or towards a discovery, when a second measurement is added later this year?”
What was actually found?
The deviation that was measured centers around the way forces are experienced by distinct types of elementary particles. The Standard Model predicts that these should be identical (“universal”). The LHCb researchers’ observation instead suggests a difference. That difference may be explained by the influence of an undiscovered fifth force of physics.
More good (and rare) news
CERN shared additional exciting results on March 23. LHCb researchers also succeeded in measuring a very rare particle decay. Only 1 in 3 billion so-called ‘Bs mesons’ decay into two ‘muons’, a different type of particles: that makes it the rarest decay event ever measured at the Large Hadron Collider.
Measurements like this one are a test for the Standard Model, and they contain possible clues about the nature of the speculated fifth force of physics. LHCb researchers therefore enthusiastically continue the hunt for even rarer decays. The work contains important contributions by Nikhef/UM researchers Silvia Ferreres and Jacco de Vries, and by CERN fellow Mick Mulder.
Assistant professor Jacco de Vries: "To measure a rare decay like that, all analysis steps need to be calibrated carefully – we’ve been working hard on that during the past years.” PhD candidate Silvia Ferreres adds: “Working hard on calibrations for two years was entirely worth it! To bring all pieces of the puzzle together and see a beautiful result like this, it’s a dream come true.”
What’s next - and so what?
Researchers expect that more will become clear over the course of 2021: whether the deviation from the Standard Model will get the status of ‘discovery’, and if so, what the nature of it is. The LHCb experiment is conducting multiple analyses for this purpose, but it is also important that other experiments confirm the observations. Two other experiments at the Large Hadron Collider, CMS and ATLAS, are currently involved. The Japanese Belle II experiment will soon join.
If the existence of a fifth force of physics is confirmed, the discovery brings us one step closer to understanding the origins of the universe. This new force may have played an important role in the early universe.
Relevant links
Gravitational Waves and Fundamental Physics
Particle Physics research at UM
With thanks to Nikhef for the quotes used in this article.
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