100 incredible years of physics – particle physics

“This is a common question, but the problem first of all is with the Higgs boson itself – we really don’t understand it,” says IOP Fellow Valerie Gibson, a particle physicist from the University of Cambridge. “It has got a mass that is approximately 125 times the proton mass, which requires new physics in order to make it that low.” In fact, the Higgs boson is merely a ripple in the Higgs field that permeates all space and is thought to be the origin of mass in the Universe. “The Higgs field interacts with other particles and is supposed to interact with itself as well. So, this is something else we want to look at, to really understand the Higgs boson.”

Moreover, just because finding the Higgs boson completes the Standard Model does not mean that the Standard Model completely describes particle physics. “One of the things that we certainly do know, and which doesn’t fit in the Standard Model, is that neutrinos have mass  – that is not part of the Standard Model.” Gibson continues: “There are a lot of unsolved problems to work on for the next century and beyond.” These include dark matter, dark energy and the question of why matter dominates over antimatter in the Universe. 

A cascade of particles

Gibson herself is researching the latter mystery right now. She is part of the Large Hadron Collider beauty (LHCb) collaboration, one of four experiments running at the LHC. The LHCb experiment is actively searching for new physics that may be able to explain why we live in a matter-dominated Universe. 

Though solving the matter–antimatter asymmetry problem seems like an extremely ambitious aim, Gibson would be forgiven for being optimistic: her earliest experience at CERN was as a summer student in 1983 from which she remembers all of the celebrations surrounding the announcement of the discovery of a heavy fundamental particle that mediates the weak force: the W boson. And if we look back further, there is even more reason for optimism. “In 100 years we have gone from knowing that the hydrogen atom consists of a proton and an electron to where we are now; it is incredible, isn’t it?”

In 1920, the same year the IOP was founded, Ernest Rutherford (IOP President 1931 – 33) coined the term ‘proton’ for the particle he had discovered the year before. Then in 1932, IOP Honorary Fellow James Chadwick found the neutron around the same time that the first antimatter particle, the positron, predicted by another IOP Honorary Fellow Paul Dirac, was unearthed by Carl Anderson. “That was big breakthrough; doubling the number of particles immediately,” adds Gibson. 

Much like the particle cascades these researchers observed in their tabletop cloud chamber and mountaintop cosmic ray experiments, what followed in the ensuing decades was a torrent of new particle discoveries. And the construction of the first powerful particle accelerators after World War II in the 1950s and 60s accelerated discoveries even further. 

This zoo of new particles was fascinating. Physicists began to see there was a whole microcosmos involving hundreds of particles to explore – and more to find. But the complex connections between the particles made it hard to make sense of what was going on.

Things began to become clearer when in 1961 Murray Gell-Mann and Yuval Ne’eman independently came up with a scheme that brought some order to the chaos of the particle zoo. Dubbed the ‘eightfold way’, Gell-Mann and George Zweig independently used this scheme to propose the existence of a new type of particle that makes up bigger particles such as neutrons and protons in 1964. Naming them ‘quarks’, their existence was confirmed at the Stanford Linear Accelerator Center (SLAC) in the US just four years later. 

The discovery of quarks ultimately led to quantum chromodynamics (QCD) – the quantum theory of the atomic nucleus, and the particles within it. But QCD is only concerned with the interaction that mediates the strong force, which binds neutrons and protons in the nucleus. What about the other four fundamental forces – the weak force, electromagnetism and gravity – that govern particle physics? 

It took a collective worldwide effort combining theory and experiment for a more complete picture to form. This effort culminated in the theory of the electromagnetic and weak forces (electroweak theory) being combined with the theory of the strong force (QCD) by, among others, Physical Society Fellow Abdus Salam in what became known as the Standard Model, a term first coined in 1975. The Standard Model reduced all of the known particles down to just a few elementary ones, placing them into lists and groups, much like the periodic table of elements. 

From theory to experiment

If the late 1960s and early 70s can be defined as the birth of the Standard Model, the late 1970s and early 80s is the era in which many of its predictions were verified experimentally. In particular, the discovery of “the W and Z bosons really cemented the Standard Model,” adds Gibson. By using CERN’s Large Electron Positron (LEP) Collider as a W and Z factory, particle physicists could then thoroughly test the Standard Model, leading them to determine that there were only three families of quarks (up and down; charm and  strange; top and bottom) and leptons (electrons, muons and taus, and their neutrinos).

By 1990, only three of these fundamental particles remained out of reach: the top quark, tau neutrino and Higgs boson. Fermilab in the US found the first two, and it was expected that the Superconducting Super Collider (SSC) being built in Texas in the early 1990s would find the last piece of the puzzle: the Higgs boson. 

“But given the US’s space dominance, the breakup of the Soviet Union and budget concerns, they cancelled the SSC in 1993,” explains Gibson. “This had a huge effect on particle physics, with a substantial number of US physicists coming to CERN. At that point, CERN was no longer a European laboratory; it became a world laboratory for particle physics.” 

Though CERN’s LHC would not start operations until 2010 and had roughly a third of the power of the cancelled SSC, it was the world’s best hope of finding the Higgs boson. Two years after the LHC’s first particle collisions, CERN tentatively announced they had found the Higgs boson. 

Joe Incandela and IOP Honorary Fellow Fabiola Gianotti, spokespersons for the two teams hunting for the Higgs particle, made the announcement at CERN on 4 July 2012.  Gibson, former Chair of the IOP Juno team that takes action on gender equality in physics, speaks of Gianotti as an inspirational role model. Gianotti is now CERN Director-General, the first woman to hold this position. “She represents to me how we do particle physics today in a collaborative way, with nobody being put on a pedestal – but just bringing people together to do the science that we enjoy,” says Gibson.

“I don’t think it is just individuals driving the science anymore, it really is collaborations of people, who really work by consensus and get the job done,” she continues. “I admire all the thousands of accelerator physicists, engineers, technicians and physicists who make these large projects work peacefully and without any bias.”   

Gibson’s only concern for the future of particle physics is the sheer size of these collaborations. She cites the LHC’s ATLAS experiment – designed to search for improved measurements of the Standard Model and evidence of theories of particle physics beyond it – as a huge endeavour involving thousands of people in which it is “difficult to get a feel for the whole experiment and the whole project”. In contrast, “the LHCb experiment, has only around 900 people; it is just about manageable to know everybody, to understand the experiment and for each person to play a major impact.” 

“I think it is important when you are training the next generation that they feel that they can have an impact on the research, because the next generation are the people who are going to deliver the next generation of colliders – for example, the future linear or circular colliders – that will drive the next 100 years of particle physics.”