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How a forgotten physicist’s discovery broke the symmetry of the Universe.html
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How a forgotten physicist’s discovery broke the symmetry of the Universe
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<li class="c-article-identifiers__item"><time datetime="2024-01-15">15 January 2024</time></li>
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<h1 class="c-article-magazine-title">How a forgotten physicist’s discovery broke the symmetry of the Universe</h1>
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When Rosemary Brown identified a strange particle
decay 75 years ago, it set events in motion that would rewrite the laws
of physics.
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Suzie Sheehy<svg width="16" height="16" focusable="false" role="img" aria-hidden="true" class="u-icon"><use xlink="http://www.w3.org/1999/xlink" xlink:href="#icon-email"></use></svg></a><sup class="u-js-hide"><a href="#Aff0">0</a></sup></li>
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<p class="c-article-author-affiliation__address">Suzie Sheehy is an associate professor in physics at the University of Melbourne, Australia.</p>
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<img class="figure__image" alt="An apple in a mirror reflecting cube shaped version of himself. Surreal cube shaped apple reflection of a red apple." loading="lazy" src="How%20a%20forgotten%20physicist%E2%80%99s%20discovery%20broke%20the%20symmetry%20of%20the%20Universe_files/d41586-024-00109-5_26621448.jpg">
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<p class="figure__caption u-sans-serif"><span class="mr10">Parity symmetry says that something viewed in a mirror should look the same.</span><span>Credit: Getty</span></p>
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</picture>
</figure><p>When a ‘scanner’ called Minnie van der Merwe handed Rosemary
Brown a photographic slide with an unusual configuration of particle
tracks, the physicist knew that she was on to something. “I looked very
carefully and thought: this is it,” she says.</p><p>That was in 1948,
when Brown — now 97 and known by her married name of Fowler — was a
22-year-old PhD student in Cecil Powell’s group at the University of
Bristol, UK. She was looking at particle tracks in photographic
emulsions that had been exposed to cosmic rays. Before the advent of
particle accelerators, such emulsions were the main source of data for
exotic high-energy particles. Fowler was in little doubt about what she
had found in what became labelled the ‘<i>k</i>-track’ plate — but
working out the ‘why’ of her discovery occupied particle physicists for
the best part of a decade. When they finally managed it, it blew apart
the idea that the laws of nature adhered to certain symmetrical ways of
working, with reverberations that continue to this day.</p><p>
</p><article class="recommended pull pull--left u-sans-serif" data-label="Related">
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</article><p></p><p>The decades either side of the Second World War
were a boom time for particle discovery. The 1930s had seen the list of
subatomic particles grow beyond the duo of the electron and proton, with
the discovery of the neutron, the muon (a heavier version of the
electron) and the first antimatter particle, the positron. In 1947,
Powell confirmed the existence of the pion<sup><a href="#ref-CR1" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">1</a></sup>,
the first of a new class of particles known as mesons. These were
predicted in 1934 by Japanese physicist Hideki Yukawa to be carriers of
the strong nuclear force — one of the four fundamental forces of nature.
(Mesons are now known to be made up of quarks, the interactions of
which, through the exchange of gluons, are the basis of this force.)</p><p>In
December 1947, George Rochester and Clifford Butler at the University
of Manchester, UK, took the meson discovery a stage further. They
detailed how, in 5,000 cloud-chamber photographs, they had found
evidence<sup><a href="#ref-CR2" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">2</a></sup> of what they called the theta zero (<i>ϴ</i><sup>0</sup>),
a neutrally charged meson that decayed into pions. Fowler’s discovery
just a few months later was both similar and strikingly different.
Powell’s laboratory had perfected the technique of using emulsion plates
to investigate the contents of cosmic rays entering Earth’s atmosphere.
The <i>k-</i>track plate came from a set exposed at the high-altitude
lab, at Jungfraujoch, Switzerland, located 3,571 metres above sea level.
It revealed a particle, identified as ‘tau’, with the same mass as a <i>ϴ</i><sup>0</sup>, but that decayed differently: to three pions, rather than two.</p><h2><b>A mirror crack’d</b></h2><p>An
intense period of work followed the discovery. “A lot of measurement
and calculation had to be done before the finding could be published. We
knew it was an important discovery so worked very hard to get
everything done quickly,” says Fowler. The team wrote three papers in
quick succession, including two that were published in <i>Nature</i> in January 1949. All three listed Fowler (then Brown) as the first author<sup><a href="#ref-CR3" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">3</a></sup><sup>–</sup><sup><a href="#ref-CR5" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">5</a></sup>.
This followed the convention that authors be listed in alphabetical
order, but also recognized that she had been the one to make the
discovery.</p><p>Grasping the implications of Fowler’s discovery means
delving into what was thought to be a fundamental symmetry of nature,
known as parity. The statement ‘parity symmetry is conserved’ amounts to
saying that a mirror-reflected version of a physical process should
occur just as readily in nature as the original process does. In
particle physics, parity symmetry is expressed by a quantum number
describing how a particle acts if you flip it in one spatial coordinate.
Total parity is calculated by multiplying the parity numbers of all the
particles involved at the different stages of a particle process. If
parity symmetry is conserved, the total parity cannot change.</p><p>
</p><article class="recommended pull pull--left u-sans-serif" data-label="Related">
<a href="https://www.nature.com/articles/d41586-023-02296-z" class="u-link-inherit" data-track="click" data-track-label="recommended article"><img class="recommended__image" alt="" src="How%20a%20forgotten%20physicist%E2%80%99s%20discovery%20broke%20the%20symmetry%20of%20the%20Universe_files/d41586-024-00109-5_26621428_002.jpg"><p class="recommended__title u-serif">The particle-physics breakthrough that paved the way for the Higgs boson</p></a>
</article><p></p><p>A pion has a parity of −1, so the three-pion end
state of Fowler’s tau-meson decay also has an overall parity of −1. But
the two-pion end state of the <i>ϴ</i><sup>0</sup> decay has parity +1.
If parity is conserved, the two initial particles must have distinct
parities, too — and should therefore be different types of particle. But
no theoretical concept could explain why two particles of different
types could have exactly the same mass. This became known as the
tau–theta puzzle.</p><p>After Fowler’s first observation, many groups
followed in her tracks. They scoured cloud-chamber photographs and flew
stacks of emulsions high into the atmosphere in weather balloons to look
for signs of the tau-meson decay. By 1953, this activity had led to a
total of 11 events. By 1955, 35 more events had been produced using the
Bevatron, an enormous particle accelerator at Lawrence Berkeley National
Laboratory in Berkeley, California, that provided an alternative
source, beyond cosmic rays, of high-energy particles. Along the way, a
new naming convention was introduced: the initial particles became known
as <i>K</i> mesons or kaons, and theta and tau referred instead to the
decay modes that resulted in two and three pions, respectively. Given
that all the researchers involved would have been familiar with Fowler’s
<i>k</i>-track, it seems a highly likely source for this convention.</p><p>With
more-precise measurements, the masses of the two types of kaon remained
identical and the tau–theta puzzle only became more perplexing.
Finally, in April 1956, particle physicists gathered at a conference in
Rochester, New York, to thrash out exactly what was going on with kaons,
and several other confusing ‘strange’ particles that had been
discovered in the meantime. Neither Fowler nor Powell was there, but
luminaries such as Murray Gell-Mann and Richard Feynman were. In
Gell-Mann’s recollection, Feynman was sharing a room with
experimentalist Martin Block, who asked him: “What if parity isn’t
conserved? Then couldn’t the tau and theta be the same thing?” Feynman
proposed this at the meeting. It turned out that no one had ever
actually proved that parity was conserved, especially in the
weak-nuclear interaction, which governs kaon decays.</p><figure class="figure">
<picture class="embed intensity--high">
<source type="image/webp" srcset="How%20a%20forgotten%20physicist%E2%80%99s%20discovery%20broke%20the%20symmetry%20of%20the%20Universe_files/d41586-024-00109-5_26621446.webp 767w, How%20a%20forgotten%20physicist%E2%80%99s%20discovery%20broke%20the%20symmetry%20of%20the%20Universe_files/d41586-024-00109-5_26621446_002.webp 319w" sizes="(max-width: 319px) 319px, (min-width: 1023px) 100vw, 767px">
<img class="figure__image" alt="Staff, Research Staff and Research Students of the School of Physics, Bristol, 1948. Rosemary Brown (back left, next to pillar), one of few female physicists at Bristol." loading="lazy" src="How%20a%20forgotten%20physicist%E2%80%99s%20discovery%20broke%20the%20symmetry%20of%20the%20Universe_files/d41586-024-00109-5_26621446.jpg">
<figcaption>
<p class="figure__caption u-sans-serif"><span class="mr10">In 1948, Rosemary Brown (back left, next to pillar) was one of few female physicists at Bristol.</span><span>Credit: Archives of the School of Physics/University of Bristol</span></p>
</figcaption>
</picture>
</figure><p>Theorists Tsung-Dao Lee and Chen-Ning Yang were also at that meeting, and that October proposed that parity might be violated<sup><a href="#ref-CR6" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">6</a></sup>.
At first their paper was viewed with scepticism, with Feynman even
placing a personal bet with odds of 50:1 against parity violation. An
experiment was needed to confirm or refute the idea. That experiment was
conducted, also in 1956, by Chien-Shiung Wu at the National Bureau of
Standards in Washington DC. She showed conclusively that parity was not
conserved in the β decay of cobalt-60, which also occurs through the
weak nuclear force<sup><a href="#ref-CR7" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">7</a></sup>.
Other experimental results soon added to the pile, until it was
undeniable. The solution to the tau–theta puzzle was that the two types
of kaon were one and the same, but parity was not a fundamental symmetry
of nature.</p><p>So neat was Wu’s experiment that she also managed to
prove that nature broke a second symmetry, called C for charge
conjugation. This expresses the idea that if you swap all the particles
in an interaction with their antiparticles, the interaction should still
happen in the same way. This finding set the stage for physicists to
revise their views on other assumed symmetries of nature. ‘CP’, the
combination of charge and parity conservation, was proposed to hold<i>,</i> but was then shown to be violated in 1964 — also in decays of kaons.</p><p>Also
in 1964 came the idea of ‘spontaneous’ symmetry breaking in particle
physics, followed in 1967 by the application of this idea to
‘electroweak’ symmetry breaking. Electroweak theory explains how the
weak nuclear and electromagnetic forces are unified at high energies,
such as those prevalent in the early Universe, but seem to us to be
distinct forces mediated by particles of very different masses.
Spontaneous symmetry breaking suggested the existence of the Higgs boson
— a particle eventually discovered in 2012 at the Large Hadron Collider
at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland.
Today, asymmetries in decays of kaons and other particles being
investigated at CERN and elsewhere might point the way towards new
effects beyond the standard model of particle physics.</p><h2><b>The ‘Matilda’ effect</b></h2><p>The
seminal nature of her discovery raises the question of why so few
people have heard of Fowler. In most physics departments of her time,
gender parity was maximally violated. Powell’s lab was something of an
exception. The confluence of war time and a new approach to science had
shifted its gender balance. The large amounts of photographic data being
gathered meant that Powell had employed teams of scanners, including
van der Merwe. These scanners, most of whom were women, painstakingly
trawled through the photographs, handing over anything unusual or
interesting to one of the physicists for further analysis.</p><p>
</p><article class="recommended pull pull--left u-sans-serif" data-label="Related">
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</article><p></p><p>Fowler was not a scanner. She was one of the few
women invited to do a physics PhD, after achieving a first-class degree —
an exceptional result for anyone especially in those days. Smart and
decisive, she took just two days’ holiday, and started work in June
1947. After making her discovery, the first person she told was fellow
PhD student Peter Fowler. “We spent a little while looking and thinking
and enjoying the moment of discovery. Then I told the others,” she says.
The grandson of nuclear pioneer Ernest Rutherford, Peter Fowler was
widely regarded as a brilliant young physicist. Three years older than
Rosemary<i>,</i> he was a year below her, because his studies had been interrupted by war service. The two married in July 1949.</p><p>When
I asked Rosemary why she left physics after that, without completing
her PhD, I expected a difficult conversation, but her response was
pragmatic. Living in a time of food rations, housing shortages and great
sacrifice, and with no time-saving appliances or childcare for their
three girls, she decided that it would be best for physics if Peter kept
working. She would assist him with his work from home, keep the house
and raise their children — and having made that decision, that is what
she did.</p><p>Rosemary’s contribution has, over time and in various
publications, often been attributed to her husband or to Powell. There
seems to be no maliciousness about this — Powell was meticulous in
acknowledging contributions. But it does seem to be a prime example of
the ‘Matilda’ effect, the phenomenon that female scientists’
contributions are often overlooked or attributed to their male
counterparts.</p><p>Rosemary is by no means the only one, even in this
story. Powell won the Nobel Prize in Physics in 1950 for the discovery
of the pion using the emulsion technique, while the contributions of the
technique’s inventor, Austrian physicist Marietta Blau, were
overlooked. Evidence for the pion also appeared in <i>Nature</i> papers by Indian physicist Bibha Choudhuri, published during the Second World War<sup><a href="#ref-CR8" data-track="click" data-action="anchor-link" data-track-label="go to reference" data-track-category="references">8</a></sup>;
her work is even less well known than Blau’s. Lee and Yang were awarded
the Nobel Prize in Physics in 1957 for their work on parity violation;
Wu received no such recognition. Now, 75 years after Rosemary’s
discovery and with the long view of its importance in physics, it seems
fitting to set her part of the record straight.</p>
</div>
<p><em>Nature</em> <strong>625</strong>, 448-449 (2024)</p>
<p><em>doi: https://doi.org/10.1038/d41586-024-00109-5</em></p>
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