It's unlikely that most people reading this will, until recently, have heard of Rosemary Fowler. However, at the age of 98, she was finally awarded an honorary doctorate in recognition of having discovered a new particle in the late 1940s.

Whilst working towards her PhD, she spotted an unusual decay track in a stack of plates coated with a thick photographic emulsion, which had been exposed to energetic particles at a laboratory, high in the Swiss Alps. Her doctorate, however, didn't get completed as she gave up university life to marry fellow physicist, Peter Fowler (grandson of Ernest Rutherford), with whom she went on to have three children.

The particle in question was a type of K meson that decayed to three lighter particles called pions. Previous observations of K mesons, or kaons as they are usually called, showed them typically decaying to just two pions, so while kaons were already known to exist, the particle discovered by Rosemary Brown (as she was then) expanded the meson family, contributing hugely to the development of what was to become quark theory, whilst also helping to solve the mystery of so-called parity violation.

So what are mesons? Well, they are a type of hadron, particles composed of quarks, and while most 'normal' matter in the universe is formed of baryons, nucleons containing three quarks, mesons are composed of quark pairs. Pions were known previously from tracks in cloud chambers, produced by collisions between cosmic rays and atmospheric nuclei, resulting in showers of secondary particles. Cosmic rays (somewhat confusingly) are not continuous waves of electromagnetic radiation, but are, in fact, particles whose origins can range from as near as the Sun, to highly energetic sources in our galaxy and beyond in distant external galaxies. Previously, mesons, such as pions, were observed to have very short lifespans, in the order of 10^-23 seconds. However, in the early 1940s, a new type of particle was spotted which was heavier and with significantly longer lifespans of up to 10^-10 s; ten thousand billion times longer! Ultimately these characteristics were attributed to a property known as strangeness.

Strangeness is a quantifiable property, evident particularly in mesons composed of an up or down quark, paired with a so-called strange quark. It's worth noting that particles often have their rest mass measured in units of MeV/c². This reflects the part played by that most ubiquitous of equations, E = mc², in the formation of particles resulting from high-energy collisions. In all of these particle interactions, conservation of charge, energy and momentum is predictable, however, the discovery of the K⁺, K^- and K⁰ (and anti-K⁰) particles ultimately led to the understanding that strangeness is only conserved in fast interactions brought about by the strong nuclear force, but not the much slower individual particle decay, resulting from the weak force.

Around the time of Rosemary Fowler's discovery, theoretical physicists, like Murray Gell-Mann, hypothesised that the existence of previously undiscovered particles could be predicted by examining geometrical patterns, created by placing known particles on a grid, positioning them as a function of their charge, quark “flavour”, and so on. This strategy ultimately resulted in what we know of as the Standard Model; alongside further discoveries such as confirmation of the existence of the Higgs boson.

Sir Paul Nurse, upon presenting Rosemary Fowler with her honorary doctorate, in recognition of the discovery (which led to her team leader, at the University of Bristol, Cecil Powell, winning a Nobel Prize), described her as having “intellectual rigour and curiosity” which “paved the way for critical discoveries that continue to shape the work of today’s physicists, and our understanding of the universe”.

Rosemary Fowler has described her decision to change focus from physics to family life as being pragmatic. She is quoted as saying that, at the time of the discovery, “… I knew at once that it was new and would be very important. We were seeing things that hadn’t been seen before – that’s what research in particle physics was. It was very exciting.” And with regard to her receiving her doctorate, she said that she felt “very honoured" adding, however, that she hadn't “… done anything since to deserve special respect”. Well, one hopes that we would all agree that her charming modesty flies in the face of an early professional scientific achievement that most of us would be extremely happy to have secured!

Images

• Rosemary Fowler: PA/ University of Bristol
• Bosons - Hadrons - Fermions: Hugo Spinelli, CC0, via Wikimedia Commons