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Recently, the University of Bristol in the UK awarded an honorary doctorate to 98-year-old Rosemary Fowler. During her doctoral studies, she discovered a particle decay process that was considered strange at the time. This discovery is considered to be "an event that rewrites the laws of physics" and is also the reason why Tsung-Dao Lee and Chen-Ning Yang think about whether parity is conserved. However, Rosemary Fowler left the academic circle due to family reasons, and her outstanding work has rarely been mentioned since then. Now, her alma mater has recognized her contribution on behalf of the world.

Compiled by Liu Hang

75 years ago, a seemingly obscure discovery in particle physics caused physicists to rethink the most fundamental symmetries in nature. Its discoverer, Rosemary Fowler, now 98,（Rosemary Fowler，1926-）Ms., who was recently awarded an honorary doctorate of science by her alma mater, the University of Bristol, a degree she had given up for her family. Paul Nurse, president of the University of Bristol and Nobel Prize winner（Paul Nurse）It was at Bristol that her discovery of the decay of the K meson into three pions eventually led to a revolution in particle physics theory.NatureThe magazine described her discovery as an "event that could rewrite the laws of physics."

Rosemary Fowler was awarded an honorary doctorate | Source: davidjohnsonphotographic.co.uk

Discovery in cosmic rays

The decades before and after World War II were a heyday for particle discovery. In the 1930s, with the advent of neutrons and muons,(Second generation leptons)With the discovery of the first antimatter particle, the positron, the list of subatomic particles has expanded far beyond the electron and proton. At that time, the University of Bristol had a world-leading cosmic ray physics team under Cecil Powell.（Cecil Powell，1903-1969）Under the leadership of Professor Powell, the laboratory has perfected the technology of using photographic film to study cosmic rays. Before this, due to the low sensitivity of latex, only some tracks of particles with low energy and high ionization were recorded, and particles with high energy and low ionization were easily missed, which reduced the chance of discovering new particles. Powell and his collaborators improved the sensitivity of latex and increased the thickness of latex, so that charged particles will be ionized when passing through the latex, and black grains will appear after development, leaving tracks. Powell's experimental group is working hard to find new elementary particles from cosmic rays.

In 1947, Cecil Powell confirmedπThe existence of mesons, the lightest particles in the meson family. As early as 1934, Japanese physicist Hideki Yukawa（Hideki Yukawa，1907-1981）It was predictedπYukawa hypothesized that protons and neutrons attract each other through a field, which isπMuons, which act as carriers of the strong nuclear force - the residual interaction of the strong interaction.(A meson is made up of a pair of positive and negative quarks, and the strong interaction between quarks is transmitted through gluons.)

In December 1947, George Rochester of Manchester University in England（George Rochester，1908-2001）and Clifford Butler（Clifford Butler，1922-1999）They took muon research to a new level. They carefully analyzed 5,000 cloud chamber photos and discovered something calledθThe exotic particle is a neutral muon that decays into twoπA few months later, Rosemary discovered a particle very similar to the theta particle.

In 1948, 22-year-old Rosemary was a doctoral student in Cecil Powell's group. Her research was done in a high-altitude laboratory on the Jungfraujoch in Switzerland, observing photoemulsion photographs exposed to cosmic rays and studying high-energy particle reaction processes by analyzing the particle tracks in the emulsion photographs. She discovered something unusual - one decayed into threeπ"I immediately realized that this was something new and significant," she later recalled. "We were seeing results we had never seen before." The tracks she observed were later labeled "k-tracks," evidence for an unknown particle then known as the muon.τmeson.

The puzzling thing is that the tau meson should have been seen by the Manchester team before.θMirror images of particles, they appear identical in every way: same mass, same spin, etc. But they decay in completely different ways:τThe muon decays into threeπMeson, andθDecays into twoπRosemary's discovery seemed to break the "mirror symmetry" or "parity symmetry" that the two processes have opposite parities.

Rosemary Fowler discovered the cosmic ray track of the "tau" meson. The "tau" meson decays at point A toπ++π++π-，π-Then it breaks apart at point B.Image source: Nature, 163, 82 (1949).

In the early days of particle accelerator development, this method of taking cosmic ray emulsion photos was the main experimental method for studying high-energy particle physics. Rosemary was convinced of her discovery, and the research team carried out an intense period of analysis. "Before publishing this discovery, a lot of measurements and calculations had to be done. We knew it was an important discovery, so we worked very hard to complete everything as quickly as possible." Rosemary said.

Rosemary and others wrote three papers in a short period of time, two of which were published in the January 1949 issue of Nature magazine. Rosemary was the first author, using her original surname, Brown.（R. Brown）This is in accordance with the alphabetical order of particle physics papers and also explains her main contribution in this work.θ-τ"It took particle physicists nearly a decade to figure out this paradox.

Violation of mirror symmetry

Before this, people generally believed that the laws of physics were left-right symmetric, that is, the mirror image of any physical process is also a possible physical process. Rosemary's discovery aroused the interest of scientists, who began to study "parity" in more depth, a symmetry that was previously considered to be a fundamental property of nature.

In particle physics, parity is expressed in terms of a quantum number that describes how a particle or field behaves when the axes of space are reversed. The total parity is calculated by multiplying the parity numbers of all particles involved at different stages of the process. If parity is conserved, the total parity cannot change.

πThe parity of the muon is -1, which Rosemary discovered.τThe meson decays into a three-meson final state, and its parity should also be -1.θThe parity of the final state of the decaying di-meson is +1. If parity is conserved, then the initial particles in the two processes must also have different parities, and therefore should be different types of particles. But no theory can explain why two different types of particles have exactly the same mass. This is the famousθ-τMystery.

Many collaborative groups followed in her footsteps, poring over the cloud chamber photographs and sending large quantities of photosensitive emulsion negatives into the atmosphere using weather balloons in search ofτSigns of muon decay. By 1953, physicists had observed 11 such events. Lawrence Berkeley National Laboratory（Lawrence Berkeley National Laboratory）The Bevatron, a large particle accelerator, began operating in 1954 and produced 35 events by 1955. Large particle accelerators provide another important research method for particle physics besides cosmic rays. In this process, scientists introduced a new naming convention: the first strange particles discovered were called K mesons, andθandτThe decay to two and threeπThe pattern of meson.

More precise measurements confirmed that the masses of the two types of kaons were indeed identical, which madeθ-τThe mystery became even more puzzling. In April 1956, particle physicists held a meeting in Rochester, New York, to discuss the kaon and several other puzzling and strange particles that had been discovered during this period. Although Rosemary and Powell did not attend the meeting, Gell-Mann（Murray Gell-Mann，1929-2019）Feynman（Richard Feynman，1918-1988）Several prominent scientists attended the meeting. In Gell-Mann's recollection, Feynman and experimentalist Martin Block（Martin Block，1925-2016）Living in a room together, Block asked him, “What if parity is not conserved? θandτCouldn't it be the same particle?" Feynman also raised this question at the conference.

A photo taken at the School of Physics, University of Bristol. Rosemary is the one leaning against the pillar on the left side of the back row. 丨Source: University of Bristol

It turns out that no one can really prove that parity is conserved, especially in the weak interaction process of decay. Tsung-Dao Lee and Chen-Ning Yang also attended that meeting. After careful research, they found that in fact, whether parity is conserved in weak interactions has not been tested. In October of the same year, they published a paper proposing several specific experiments to test whether parity is conserved. At first, their paper was questioned because parity conservation has long been the default view of most physicists; Feynman even bet 50 times the odds against parity non-conservation. In 1956, Tsung-Dao Lee discussed related issues with Chien-Shiung Wu, one of the most authoritative experts in the field of decay in the world at that time, and Chien-Shiung Wu decided to conduct experiments. Due to the importance of the experiment, Chien-Shiung Wu gave up her long-planned trip back to China to visit her relatives and organized an experimental team to start detailed experimental work. By observing the β decay of cobalt-60, they found that most of the final state electrons were emitted in the direction opposite to the polarization direction of cobalt-60. In Wu's experiment, the strong magnetic field polarized the direction of angular momentum, that is, the direction of spin, and in principle did not restrict the direction of motion of the final state electrons. So, if parity is conserved, then the final electron, the emittedβThe direction of the ray should be in the positive and negative directions of the nuclear polarization with equal probability. However, the experiment only monitored the rays in the opposite direction of the nuclear polarization, so they can conclude that the conservation of parity is indeed not valid in weak interactions.(Editor's note: Please refer to "Parity Violation and Wu's Experiment, which middle school students can understand")Subsequently, more experimental results followed until the violation of parity conservation in weak interactions became undeniable.

The answer to this puzzle is that the two kaons are the same particle and parity is not a fundamental symmetry of the weak interaction in nature.

Wu's experiment was so ingenious that she also managed to prove that nature breaks another symmetry - C(Charge conjugation)The discovery led physicists to realize that not only parity conservation, but also other hypothetical symmetries of nature needed to be tested precisely. "CP" - a combination of charge conservation and parity conservation - was thought to hold, but was later shown to be violated in a 1964 experiment with kaon decay. CP violation has even deeper implications than parity violation, and may be related to the fact that there is more matter than antimatter in the universe.

Abandoning the conservation of parity had a profound impact on physicists. Rosemary's discovery rewrote the development path of particle physics, and people's understanding of elementary particles and their interactions, especially the basic concept of symmetry, has undergone a radical change. Today, physicists are still using various experiments to study symmetry breaking in particle decay and looking for new physics beyond the standard model of particle physics.

The Matilda effect

Rosemary's story raises the question: Why have so few people heard of her? One reason may be that gender equality was difficult to achieve in most physics departments and the scientific community in general during her time. Powell's lab was an exception. During the war, men were required to serve in the military, and the new scientific method used in Powell's lab was being developed using emulsion technology to image cosmic rays, which was a labor-intensive job. Powell's lab collected a large number of cosmic ray photos, and he hired many scanners, who were very skilled in the field of cosmic ray imaging.(Most of them are women)They painstakingly search through the photos, handing off anything unusual or interesting to physicists for further analysis.

Rosemary Fowler was not a scanner. She was one of the few women invited to study for a doctorate in physics, and she received a first-class bachelor's degree - an extraordinary result for anyone, especially in those days. Rosemary showed an outstanding talent for science after school. She was good at mathematics but not very interested in writing, which may have something to do with her father being a British Royal Navy engineer. She became the only girl in her grade to enter university, and finally became Powell's graduate student with a beautiful transcript.

Smart and decisive, she took only two days off after joining the group and started working in June 1947. When she discovered the decay of the "tau meson", the first person she told was her doctoral student classmate Peter Fowler.（Peter H. Fowler）"We spent some time observing and thinking, enjoying the moment of discovery. Then I told the others about it," she said.（Ernest Rutherford，1871-1937）Grandson of quantum physics pioneer Ralph Fowler(Ralph H. Fowler, 1889-1944; Dirac's mentor)Peter Fowler, Rosemary's son, was a young physicist recognized for his talent. He was three years older than Rosemary, but had started school a year later, his studies interrupted by military service. The two married in 1949, at the same time that Rosemary decided to leave academia. With Rosemary's strong support, her husband Peter Fowler had a very distinguished career, making important achievements in experimental detection of cosmic rays.

When asked why she left academia after that, without completing her PhD, Rosemary's answer was pragmatic. Living in difficult times with food and housing shortages, and with three daughters who needed care and support, she decided that it was best for Peter Fowler to continue his work in physics. Rosemary's daughter Mary Fowler（Mary Fowler）She recalled: "When I was a child, I wanted to be a physicist. Both my parents were physicists. Physics and research were the talk of the kitchen table! Rosemary influenced all of us - we were all passionate about science and mathematics. No one thought that girls couldn't do it." She is now an outstanding geophysicist and served as the president of Darwin College, Cambridge University. Because Rosemary Fowler has difficulty in moving, the degree awarding ceremony was held at Darwin College, Cambridge.

Over time, in various publications, Rosemary's contributions were often attributed to either her husband or Powell. But Powell explicitly acknowledged Rosemary's major contribution in the discovery. But this does seem to be an example of the "Matilda" effect, whereby the contributions of female scientists are often overlooked or attributed to their male counterparts.[The Matilda effect is named after the American writer and activist Matilda Joslyn Gage. In 1870, she wrote a book called Woman as Inventor.Woman as Inventor) , denouncing the then-widespread view that women lacked creativity and scientific talent.]

Rosemary is not the only scientist whose contributions have been underestimated. Powell won the 1950 Nobel Prize in Physics for discovering the pion using a photosensitive emulsion technique, but the inventor of the technique, Austrian physicist Marietta Brau,（Marietta Blau，1894-1970）was ignored, even though she was later nominated for the Nobel Prize by Schrödinger several times; Indian physicist Biba Chaudhry（Bibha Choudhuri，1913-1991）Evidence for pions also appeared in a Nature paper published during World War II, and her work is even less well known than Blau’s…

Now, 75 years after her discovery, Rosemary has been awarded an honorary doctorate, proving that her important contribution will always be remembered.

References

[1] https://www.nature.com/articles/d41586-024-00109-5.

[2] https://www.bristol.ac.uk/news/2024/july/female-physics-pioneer-honoured.html.

[3] https://www.nature.com/articles/163082a0

[4]https://www.nature.com/articles/163047a0

[5] https://www.independent.co.uk/news/obituaries/obituary-professor-peter-fowler-1352277.html