The first experimental evidence of the breaking of charge–parity (CP) symmetry in baryons has been obtained by CERN’s LHCb Collaboration. The result is consistent with the Standard Model of particle physics and could lead to constraints on theoretical attempts to extend the Standard Model to explain the excess of matter over antimatter in the universe.

Current models of cosmology say that the Big Bang produced a giant burst of matter and antimatter, the vast majority of which recombined and annihilated shortly afterwards. Today however, the universe appears to be made almost exclusively of matter with very little antimatter in evidence. This excess of matter is not explained by the Standard Model and it existence is an important mystery in physics.

In 1964, James Cronin, Valentine Fitch and colleagues at Princeton University in the US conducted an experiment on the decay of neutral K mesons. This showed that the weak interaction violated CP symmetry, indicating that matter and antimatter could behave differently. Fitch and Cronin bagged the 1980 Nobel Prize for Physics and the Soviet physicist Andrei Sakharov subsequently suggested that, if amplified at very high mass scales in the early universe, CP violation could have induced the matter–antimatter asymmetry shortly after the Big Bang.

Numerous observations of CP violation have subsequently been made in other mesonic systems. The phenomenon is now an accepted part of the Standard Model is parametrized by the Cabibbo–Kobayashi–Maskawa (CKM) matrix. This describes the various probabilities of quarks of different generations changing into each other through the weak interaction – a process called mixing.

Tiny effect

However, the CP violation produced through the CKM mechanism is much smaller effect than would have been required to create the matter left over by the Big Bang, as Xueting Yang of China’s Peking University explains.

“The number of baryons remaining divided by the number of photons produced when the baryons and antibaryons met and produced two photons is required to be about 10^-10 in Big Bang theory…whereas this kind of quantity is only 10^-18 in the Standard Model prediction.”

What is more, CP violation had never been observed in baryons. “Theoretically the prediction for baryon decay is very imprecise,” says Yang, who is a member of the LHCb collaboration. “It’s much more difficult to calculate it than the meson decays because there’s some interaction with the strong force.” Baryons (mostly protons and neutrons) make up almost all the hadronic matter in the universe, so this left open the slight possibility that the explanation might lie in some inconsistency between baryonic CP violation and the Standard Model prediction.

In the new work, Yang and colleagues at LHCb looked at the decays of beauty (or bottom) baryons and antibaryons. These heavy cousins of neutrons contain an up quark, a down quark and a beauty quark and were produced in proton–proton collisions at the Large Hadron Collider in 2011–2018. These baryons and antibaryons can decay via multiple channels. In one, a baryon decays to a proton, a positive K-meson and a pair of pions – or, conversely, an antibaryon decays to an antiproton, a negative K-meson and a pair of pions. CP violation should create an asymmetry between these processes, and the researchers looked for evidence of this asymmetry in the numbers of particles detected at different energies from all the collisions.

Standard Model prevails

The team found that the CP violation seen was consistent with the Standard Model and inconsistent with zero by 5.2σ. “The experimental result is more precise than what we can get from theory,” says Yang. Other LHCb researchers scrutinized alternative decay channels of the beauty baryon: “Their measurement results are still consistent with CP symmetry…There should be CP violation also in their decay channels, but we don’t have enough statistics to claim that the deviation is more than 5σ.”

The current data do not rule any extensions to the current Standard Model out, says Yang, simply because none of those extensions make precise predictions about the overall degree of CP violation expected in baryons. However, the LHC is now in its third run, and the researchers hope to acquire information on, for example, the intermediate particles involved in the decay: “We may be able to provide some measurements that are more comparable for theories and which can provide some constraints on the Standard Model predictions for CP violation,” says Yang.

The research is described in a paper in Nature.

“It’s an important paper – an old type of CP violation in a new system,” says Tom Browder of  the University of Hawaii. “Theorists will try to interpret this within the context of the Standard Model, and there have already been some attempts, but there are some uncertainties due to the strong interaction that preclude making a precise test.” He says the results could nevertheless potentially help to constrain extensions of the Standard Model, such as CP violating decays involving dark matter proposed by the late Ann Nelson at the University of Washington in Seattle and her colleagues.

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This episode of the Physics World Weekly podcast features Travis Humble, who is director of the Quantum Science Center at Oak Ridge National Laboratory.

Located in the US state of Tennessee, Oak Ridge is run by the US Department of Energy (DOE). The Quantum Science Center links Oak Ridge with other US national labs, universities and companies.

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Physics metaphors don’t work, or so I recently claimed. Metaphors always fail; they cut corners in reshaping our perception. But are certain physics metaphors defective simply because they cannot be experimentally confirmed? To illustrate this idea, I mentioned the famous metaphor for how the Higgs field gives particles mass, which is likened to fans mobbing – and slowing – celebrities as they walk across a room.

I know from actual experience that this is false. Having been within metres of filmmaker Spike Lee, composer Stephen Sondheim, and actors Mia Farrow and Denzel Washington, I’ve seen fans have many different reactions to the presence of nearby celebrities in motion. If the image were strictly true, I’d have to check which celebrities were about each morning to know what the hadronic mass would be that day.

I therefore invited Physics World readers to propose other potentially empirically defective physics metaphors, and received dozens of candidates. Technically, many are similes rather than metaphors, but most readers, and myself, use the two terms interchangeably. Some of these metaphors/similes were empirically confirmable and others not.

Shoes and socks

Michael Elliott, a retired physics lecturer from Oxford Polytechnic, mentioned a metaphor from Jakob Schwichtenberg’s book No-Nonsense Quantum Mechanics that used shoes and socks to explain the meaning of “commutation”. It makes no difference, Schwichtenberg wrote, if you put your left sock on first and then your right sock; in technical language the two operations are said to commute. However, it does make a difference which order you put your sock and shoe on.

“The ordering of the operations ‘putting shoes on’ and ‘putting socks on’ therefore matters,” Schwichtenberg had written, meaning that “the two operations do not commute.” Empirically verifiable, Elliott concluded triumphantly.

A metaphor that was used back in 1981 by CERN physicist John Bell in a paper addressed to colleagues requires more footgear and imagination. Bell’s friend and colleague Reinhold Bertlmann from the University of Vienna was a physicist who always wore mismatched socks, and in the essay “Bertlmann’s socks and the nature of reality” Bell explained the Einstein–Podolsky–Rosen (EPR) paradox and Bell’s theorem in terms of those socks.

If Bertlmann stepped into a room and an observer noticed that the sock on his first foot was pink, one could be sure the other was not-pink, illustrating the point of the EPR paper. Bell then suggested that, when put in the wash, pairs of socks and washing temperatures could behave analogously to particle pairs and magnet angles in a way that conveyed the significance of his theorem. Bell bolstered this conclusion with a scenario involving correlations between spikes of heart attacks in Lille and Lyon. I am fairly sure, however, that Bell never empirically tested this metaphor, and I wonder what the result would be.

Out in space, the favourite cosmology metaphor of astronomer and astrophysicist Michael Rowan-Robinson is the “standard candle” that’s used to describe astronomical objects of roughly fixed luminosity. Standard candles can be used to determine astronomical distances and are thus part of the “cosmological distance ladder” – Rowan-Robinson’s own metaphor – towards measuring the Hubble constant.

Retired computer programmer Ian Wadham, meanwhile, likes Einstein’s metaphor of being in a windowless spacecraft towed by an invisible being who gives the ship a constant acceleration. “It is impossible for you to tell whether you are standing in a gravitational field or being accelerated,” Wadham writes. Einstein used the metaphor effectively – even though, as an atheist, he was convinced that he would be unable to test it.

I was also intrigued by a comment from Dilwyn Jones, a consultant in materials science and engineering, who cited a metaphor from the 1939 book The Third Policeman by Irish novelist Flann O’Brien. Jones first came across O’Brien’s metaphor in Walter J Moore’s 1962 textbook Physical Chemistry. Atoms, says a character in O’Brien’s novel, are “never standing still or resting but spinning away and darting hither and thither and back again, all the time on the go”, adding that “they are as lively as twenty leprechauns doing a jig on top of a tombstone”.

But as Jones pointed out, that particular metaphor “can only be tested on the Emerald Isle”.

Often metaphors entertain as much as inform. Clare Byrne, who teaches at a high school in St Albans in the UK, tells her students that delocalized electrons are like stray dogs – “hanging around the atoms, but never belonging to any one in particular”. They could, however, she concedes “be easily persuaded to move fast in the direction of a nice steak”.

Giving metaphors legs

I ended my earlier column on metaphors by referring to poet Matthew Arnold’s fastidious correction of a description in his 1849 poem ”The Forsaken Merman”. After it was published, a friend pointed out to Arnold his mistaken use of the word “shuttle” rather than “spindle” when describing “the king of the sea’s forlorn wife at her spinning-wheel” as she lets the thing slip in her grief.

The next time the poem was published, Arnold went out of his way to correct this. Poets, evidently, find it imperative to be factual in metaphors, and I wondered, why shouldn’t scientists? The poet Kevin Pennington was outraged by my remark.

“Metaphors in poetry are not the same as metaphors used in science,” he insisted. “Science has one possible meaning for a metaphor. Poetry does not.” Poetic metaphors, he added are “modal”, having many possible interpretations at the same time – “kinda like particles can be in a superposition”.

I was dubious. “Superposition” suggests that poetic meanings are probabilistic, even arbitrary. But Arnold, I thought, was aiming at something specific when the king’s wife drops the spindle in “The Forsaken Merman”. After all, wouldn’t I be misreading the poem to imagine his wife thinking, “I’m having fun and in my excitement the thing slipped out of my hand!”

My Stony Brook colleague Elyse Graham, who is a professor of English, adapted a metaphor used by her former Yale professor Paul Fry. “A scientific image has four legs”, she said, “a poetic image three”. A scientific metaphor, in other words, is as stable as a four-legged table, structured to evoke a specific, shared understanding between author and reader.

A poetic metaphor, by contrast, is unstable, seeking to evoke a meaning that connects with the reader’s experiences and imagination, which can be different from the author’s within a certain domain of meaning. Graham pointed out, too, that the true metaphor in Arnold’s poem is not really the spinning wheel, the wife and the dropped spindle but the entirety of the poem itself, which is what Arnold used to evoke meaning in the reader.

That’s also the case with O’Brien’s atom-leprechaun metaphor. It shows up in the novel not to educate the reader about atomic theory but to invite a certain impression of the worldview of the science-happy character who speaks it.

The critical point

In his 2024 book Waves in an Impossible Sea: How Everyday Life Emerges from the Cosmic Ocean, physicist Matt Strassler coined the term “physics fib” or ”phib”. It refers to an attempted “short, memorable tale” that a physicist tells an interested non-physicist that amounts to “a compromise between giving no answer at all and giving a correct but incomprehensible one”.

The criterion for whether a metaphor succeeds or fails does not depend on whether it can pass empirical test, but on the interaction between speaker or author and audience; how much the former has to compromise depends on the audience’s interest and understanding of the subject. Metaphors are interactions. Byrne was addressing high-school students; Schwichtenberg was aiming at interested non-physicists; Bell was speaking to physics experts. Their effectiveness, to use one final metaphor, does not depend on empirical grounding but impedance matching; that is, they step down the “load” so that the “signal” will not be lost.

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