Intriguing new results from the CERN Challenge Standard Model for Particle Physics | Physics

The Standard Model of particle physics currently provides our best description of fundamental particles and their interactions. New results from CERN’s LHCb (Large Hadron Collider beauty) collaboration suggest that the particles are not behaving as they should according to the Standard Model.

The disintegration of a B0 meson in 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 accurate predictions for the properties and interactions of fundamental particles, which have been confirmed by numerous experiments since the model’s inception 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 to explain observed patterns in particle interaction forces.

So particle physicists have been looking for the “new physics” — the new particles and interactions that can explain the shortcomings of the Standard Model.

“We were actually shaking when we first looked at the results, we were so excited. Our hearts beat a little faster,” said Dr Mitesh Patel, a physicist at Imperial College London and a member of the LHCb collaboration.

“It’s too early to tell if this is truly a departure 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’s been a long road to get here. »

The measurement made by the LHCb team compares two types of beauty quark decays.

The first decay involves the electron and the second the muon, another elementary particle similar to the electron but about 200 times heavier.

Electron and muon, along with a third particle called tau, are types of leptons and the difference between them is called flavors.

The Standard Model predicts that decays involving different lepton flavors should occur with the same probability, a characteristic known as lepton flavor universality which is usually measured by the ratio of decay probabilities. In the Standard Model of particle physics, the ratio should be very close to one.

The new results show signs of a deviation from one: the statistical significance of the result is 3.1 standard deviations, implying a probability of about 0.1% that the data is consistent with the predictions of the standard model.

“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 at CERN and spokesperson. of the LHCb Collaboration.

“Further studies on related processes are underway using existing LHCb data. We’ll be happy to see if they bolster the intriguing clues in the current results.

The discrepancy presented today is consistent with a pattern of anomalies measured in similar processes by LHCb and other experiments around the world over the past decade.

The new results determine the ratio of decay probabilities with greater precision than previous measurements and use for the first time all the data collected by the LHCb detector so far.

“These new results offer tantalizing clues to 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 the 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 of a series of LHCb results from the last decade that all seem to agree – and could all point to a common explanation.

“The results did not change, but their uncertainties decreased, increasing our ability to see possible differences with the Standard Model.”

“Discovering a new force in nature is the holy grail of particle physics,” added Dr Konstantinos Petridis, physicist at the University of Bristol and member of the LHCb collaboration.

“Our current understanding of the constituents of the Universe is remarkably insufficient – we don’t know what 95% of the Universe is made of or why there is such a great imbalance between matter and anti-matter.”

“The discovery of a new fundamental force or particle, as evidence for differences in these measurements suggests, could provide the breakthrough needed to begin answering these fundamental questions.”

“This result will certainly make the hearts of physicists beat a little faster today,” said Dr. Harry Cliff, physicist at the Cavendish Laboratory and member of the LHCb collaboration.

“We’re going to have a terribly exciting few years as we try to figure out if we’ve finally glimpsed something entirely new.”

“It is now up to the LHCb collaboration to further verify its results by gathering and analyzing more data, to see if there is any evidence left for some new phenomena.”

The results have been submitted for publication in the journal Natural Physics.


R.Aaij et al. (LHCb collaboration). 2021. Lepton Universality Test in Beauty Quark Decays. Natural Physics, submitted for publication; arXiv: 2103.11769

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