Nav: Home

Not so elementary

June 08, 2017

To anyone but a physicist, it sounds like something out of "Star Trek." But lepton universality is a real thing.

It has to do with the Standard Model of particle physics, which describes and predicts the behavior of all known particles and forces, except gravity. Among them are charged leptons: electrons, muons and taus.

A fundamental assumption of the Standard Model is that the interactions of these elementary particles are the same despite their different masses and lifetimes. That's lepton universality. Precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, but recent studies of the higher-mass tau lepton have produced observations that challenge the theory.

A new review of results from three experiments points to the strong possibility that lepton universality -- and perhaps ultimately the Standard Model itself -- may have to be revised. The findings by a team of international physicists, including UC Santa Barbara postdoctoral scholar Manuel Franco Sevilla, appear in the journal Nature.

"As part of my doctoral thesis at Stanford, which was based on earlier work carried out at UCSB by professors Jeff Richman and Michael Mazur, we saw the first significant observation of something beyond the Standard Model at the BaBaR experiment conducted at the SLAC National Accelerator Laboratory," Franco Sevilla said. This was significant but not definitive, he added, noting that similar results were seen in more recent experiments conducted in Japan (Belle) and in Switzerland (LHCb). According to Franco Sevilla, the three experiments, taken together, demonstrate a stronger result that challenges lepton universality at the level of four standard deviations, which indicates a 99.95 percent certainty.

BaBaR, which stands for B-Bbar (anti-B) detector, and Belle were carried out in B factories. These particle colliders are designed to produce and detect B mesons -- unstable particles that result when powerful particle beams collide -- so their properties and behavior can be measured with high precision in a clean environment. The LHCb (Large Hadron Collider b) provided a higher-energy environment that more readily produced B mesons and hundreds of other particles, making identification more difficult.

Nonetheless, the three experiments, which measured the relative ratios of B meson decays, posted remarkably similar results. The rates for some decays involving the heavy lepton tau, relative to those involving the light leptons -- electrons or muons -- were higher than the Standard Model predictions.

"The tau lepton is key because the electron and the muon have been well measured," Franco Sevilla explained. "Taus are much harder because they decay very quickly. Now that physicists are able to better study taus, we're seeing that perhaps lepton universality is not satisfied as the Standard Model claims."

While intriguing, the results are not considered sufficient to establish a violation of lepton universality. To overturn this long-held physics precept would require a significance of at least five standard deviations. However, Franco Sevilla noted, the fact that all three experiments observed a higher-than-expected tau decay rate while operating in different environments is noteworthy.

A confirmation of these results would point to new particles or interactions and could have profound implications for the understanding of particle physics. "We're not sure what confirmation of these results will mean in the long term," Franco Sevilla said. "First, we need to make sure that they're true and then we'll need ancillary experiments to determine the meaning."

University of California - Santa Barbara

Related Electrons Articles:

Cooling nanotube resonators with electrons
In a study in Nature Physics, ICFO researchers report on a technique that uses electron transport to cool a nanomechanical resonator near the quantum regime.
New method for detecting quantum states of electrons
Researchers in the Quantum Dynamics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) devised a new method -- called image charge detection -- to detect electrons' transitions to quantum states.
Slow electrons to combat cancer
Slow electons can be used to destroy cancer cells - but how exactly this happens has not been well understood.
How light steers electrons in metals
Researchers in the Department of Physics of ETH Zurich have measured how electrons in so-called transition metals get redistributed within a fraction of an optical oscillation cycle.
Twisting whirlpools of electrons
Using a novel approach, EPFL physicists have been able to create ultrafast electron vortex beams, with significant implications for fundamental physics, quantum computing, future data-storage and even certain medical treatments.
More Electrons News and Electrons Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...