Rosemary Fowler (née Brown; born 1926) is a British physicist who in 1948 as a 22-year-old doctoral researcher discovered the kaon (or K meson particle). While studying photographic plates that had been left exposed to cosmic rays, she identified a new configuration of tracks within the photographic emulsion that she recognised as being the decay of an unknown charged particle. The discovery of the kaon led to the identification of CP symmetry violation, now a fundamental part of the standard model of physics.

Born in Suffolk in 1926, Brown grew up in Malta, Portsmouth and finally Bath, as her family moved to follow the postings of her father, a Royal Naval engineer. ^[1] At school in wartime Bath, she excelled in maths and science, and was the only girl in her year to go to university. ^[2] In 1947, she became one of the first women to gain first class honours in physics at the University of Bristol. ^[3]

After graduating from Bristol, Brown became a doctoral researcher in the group of Cecil Powell, a British physicist and pioneer in the use of nuclear emulsion coated plates to investigate cosmic rays entering the Earth's atmosphere. ^[4] Powell and his team had achieved success with these techniques, and had already discovered a theorised particle, the pi meson or pion, work for which Powell would be awarded the Nobel prize in physics. ^[5]

Working alongside her fellow PhD student and future husband Peter Fowler, Brown studied the tracks left on stacks of photographic plates that were exposed to cosmic rays at the Sphinx Observatory, a high-altitude laboratory at Jungfraujoch, Switzerland. When Minnie van der Merwe, one of the team of "scanners" (non-physicist assistants), passed her a plate with an unusual configuration of tracks, Brown recognised it as a candidate for the decay of a new particle, called at that time the 'tau meson' (not to be confused with the entirely different elementary particle currently called the tau). ^[4] The group published their findings in a 1949 paper in Nature ^[6] which included a photograph of what became known as the "k track" plate.

The particle she named "tau" appeared similar or identical to the "theta" particle discovered earlier by G.D. Rochester and C.C. Butler of the University of Manchester, ^[7] except that it decayed by a different path, a three-pion decay mode instead of the two-pion decay of the "theta". ^[4] Since the two decay paths have different parities, the physics of the time suggested that they could not be the same. In fact, later work ^[4] determined that they were different decay paths of the same particle (later named K), and that parity was not conserved by the weak interaction.

Powell was awarded the Nobel Prize for Physics in 1950 for his development of the method Fowler used to discover the particle, awarded "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method." ^[8]

Rosemary Brown and Peter Fowler married in 1949. ^[9] She never completed her doctorate, but continued to assist her husband while raising their three daughters ^[4] – one of whom, the geophysicist Mary Fowler, became Master of Darwin College, Cambridge.

In 2004, Rosemary Fowler supported the Royal Astronomical Society to establish the Fowler award for early achievement in astronomy, in memory of her husband Peter and her father-in-law Ralph H. Fowler. ^[3]

In July 2024, at the age of 98, Fowler was awarded an honorary doctorate by the University of Bristol. ^{[2] [1]} The university stated "Rosemary’s discovery of particles and contribution to our understanding of fundamental interactions in physics has often been attributed to Powell and her husband Peter Fowler, and this honorary degree acknowledges the vital role she has played in science." ^[10] Presenting the award, Sir Paul Nurse, Chancellor of the University of Bristol, said "Rosemary’s work in particle discovery in the 1940s, as a physicist at Bristol, paved the way for critical discoveries that continue to shape the work of today’s physicists, and our understanding of the universe." ^[11]

• R. Brown; U. Camerini; P. H. Fowler; H. Muirhead; C. F. Powell; D. M. Ritson (January 1949). "Observations with Electron-Sensitive Plates Exposed to Cosmic Radiation". Nature. 163 (4132): 47–51. Bibcode: 1949Natur.163...47B. doi: 10.1038/163047A0. ISSN  1476-4687. Wikidata  Q60256948.
• R. Brown; U. Camerini; P. H. Fowler; H. Muirhead; C. F. Powell; D. M. Ritson (January 1949). "Observations with Electron-Sensitive Plates Exposed to Cosmic Radiation*: PART 2. FURTHER EVIDENCE FOR THE EXISTENCE OF UNSTABLE CHARGED PARTICLES, OF MASS ∼ 1,000 me, AND OBSERVATIONS ON THEIR MODE OF DECAY". Nature. 163 (4133): 82–87. Bibcode: 1949Natur.163...82B. doi: 10.1038/163082A0. ISSN  1476-4687. Wikidata  Q60246802.
• Brown, R.H.; Camerini, U.; Fowler, P.H.; Heitler, H.; King, D.T.; Powell, C.F. (15 July 1949). "LXXIX. Nuclear transmutations produced by cosmic-ray particles of great energy .-Part I. Observations with photographic plates exposed at an altitude of 11,000 feet". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 40 (307): 862–881. doi: 10.1080/14786444908561408. ISSN  1941-5982.