The Standard Model of particle physics currently provides our best description of fundamental particles and their interactions. The new results from CERN’s LHCb (Large Hadron Collider beauty) Collaboration suggest particles are not behaving the way they should according to the Standard Model.
The decay of a B0 meson into a K0 and an electron-positron pair in the LHCb detector, which is used for a sensitive test of lepton universality in the Standard Model. Image credit: CERN.
The Standard Model of particle physics provides precise predictions for the properties and interactions of fundamental particles, which have been confirmed by numerous experiments since the inception of the model in the 1960s.
However, it is clear that the Standard Model is incomplete. The model is unable to explain cosmological observations of the dominance of matter over antimatter, the apparent dark-matter content of the Universe, or explain the patterns seen in the interaction strengths of the particles.
Particle physicists have therefore been searching for ‘new physics’ — the new particles and interactions that can explain the Standard Model’s shortcomings.
“We were actually shaking when we first looked at the results, we were that excited. Our hearts did beat a bit faster,” said Dr. Mitesh Patel, a physicist at Imperial College London and a member of the LHCb Collaboration.
“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.”
The measurement made by the LHCb team compares two types of decays of beauty quarks.
The first decay involves the electron and the second the muon, another elementary particle similar to the electron but approximately 200 times heavier.
The electron and the muon, together with a third particle called the tau, are types of leptons and the difference between them is referred to as flavors.
The Standard Model predicts that decays involving different flavors of leptons should occur with the same probability, a feature known as lepton flavor universality that is usually measured by the ratio between the decay probabilities. In the Standard Model of particle physics, the ratio should be very close to one.
The new results indicate hints of a deviation from one: the statistical significance of the result is 3.1 standard deviations, which implies a probability of around 0.1% that the data is compatible with the Standard Model predictions.
“If a violation of lepton flavor universality were to be confirmed, it would require a new physical process, such as the existence of new fundamental particles or interactions,” said Professor Chris Parkes, a physicist at the University of Manchester and CERN and a spokesperson of the LHCb Collaboration.
“More studies on related processes are under way using the existing LHCb data. We will be excited to see if they strengthen the intriguing hints in the current results.”
The deviation presented today is consistent with a pattern of anomalies measured in similar processes by LHCb and other experiments worldwide over the past decade.
The new results determine the ratio between the decay probabilities with greater precision than previous measurements and use all the data collected by the LHCb detector so far for the first time.
“These new results offer tantalizing hints of the presence of a new fundamental particle or force that interacts differently with these different types of particles,” said Dr. Paula Alvarez Cartelle, a physicist at Cavendish Laboratory, based at the University of Cambridge, and a member of the LHCb Collaboration.
“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.”
“The discovery of a new force in nature is the holy grail of particle physics,” added Dr. Konstantinos Petridis, a physicist at the University of Bristol and a member of the LHCb Collaboration.
“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.”
“This result is sure to set physicists’ hearts beating a little faster today,” said Dr. Harry Cliff, a physicist at Cavendish Laboratory and a member of the LHCb Collaboration.
“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 analyzing more data, to see if the evidence for some new phenomena remains.”
The results have been submitted for publication in the journal Nature Physics.
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R. Aaij et al. (LHCb Collaboration). 2021. Test of lepton universality in beauty-quark decays. Nature Physics, submitted for publication; arXiv: 2103.11769
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