Particle physics has flourished over the past 30 years but, as Christine Sutton points out, there are still few signs of any cracks in the Standard Model
“The Standard Model has survived intact for another year,” declared particle physicist Don Perkins from the University of Oxford three decades ago. “But is this a triumph or a frustration for physics?” Perkins’ remarks appeared in the October 1988 edition of CERN Courier magazine in a report about the 24th International Conference on High Energy Physics (ICHEP), which had been held in Munich a few months earlier.
Although Physics World did not report on the meeting, the Courier went on to say that the Standard Model was standing up to the closest inspection, revealing no cracks, while anomalous results seemed to be going away. Looking back on these words, they seem to me just as appropriate today, despite so much having happened in particle physics in the intervening 30 years.
Missing pieces
The key details of the Standard Model of particle physics, to which Perkins was referring, were well known three decades ago (see box). But there were missing pieces and unanswered questions. For example, both the sixth (“top”) quark and the sixth lepton – the tau neutrino – remained undiscovered. Perhaps more crucially, the Higgs boson was also missing.
“The Higgs is the most arbitrary part of the model,” said Paul Langacker, who was then at the DESY lab in Hamburg, when summarizing the Standard Model at that 1988 ICHEP meeting. “The only thing that can be said with complete certainty is that the mass of the Higgs particle, if it exists, must be between zero and infinity!”
Other questions were also highlighted at the conference. Do B0 mesons exhibit the subtle difference between matter and antimatter, known as charge–parity (CP) violation, which had already been seen in the lighter K0 mesons? Do neutrinos have mass and can they oscillate from one type to another? Can quarks roam freely at extreme temperatures and densities, in a “quark–gluon plasma”? And why have fewer solar neutrinos than expected been detected here on Earth?
The answers to some of these questions were soon revealed as new, powerful particle colliders came on the scene. These machines took studies of the Standard Model to a new level, probing it ever more deeply and with increasing precision over the next decade and more. Key to these developments were the SLAC Linear Collider in the US and the Large Electron–Positron (LEP) collider at CERN, which began to fulfil their missions as “Z factories”, with the first results on the boson’s “width” in 1989. This measurement proved that there could be only three types of lightweight neutrino, and hence only three “generations” in the families of quarks and leptons.
Standard thinking
The Standard Model of particle physics, which rose to prominence during the 1970s, unites the electromagnetic, weak and strong forces in a single theoretical framework. Together, the model describes all non-gravitational interactions between particles of matter, which comprise six charged quarks and six leptons, three of which are charged (the electron, muon and tau) and three uncharged (the neutrinos).
The interactions between these matter particles are propagated by field particles: the photon plus the charged W bosons and neutral Z boson for electroweak interactions, and eight gluons for the strong interactions described by quantum chromodynamics. In the basic theory, all particles are massless. The Standard Model therefore requires an additional element to allow the W and Z particles, for example, to differ from the photon (and the gluons) by being massive.
This extra component takes the form of an additional field, with the particles gaining mass by interacting with it. Associated with the field there is (at least) one spin-zero boson, known as the Higgs boson, after the British physicist Peter Higgs.
The top quark was duly found at the Tevatron proton–antiproton collider at Fermilab in the US in 1995, and the tau neutrino was seen at last in 2000 in an experiment that used protons from the Tevatron to generate a neutrino beam. A year later, the BaBar and Belle experiments, which produced lots of B mesons at small, high-intensity electron–positron colliders in the US and Japan, respectively, found CP violation in the decay of these B particles.
And, after heavy-ion collisions at CERN had revealed tantalizing glimpses of quark–gluon plasma in 2000, the Relativistic Heavy Ion Collider in the US went on to yield definitive and surprising results on this new state of matter. By 2005 these had shown that, rather than being the expected gas, the plasma behaves like an almost perfect liquid.
There were also important findings at large, imaginative experiments looking at natural sources of particles. In 1998 the team using the Super-Kamiokande detector in Japan found that neutrinos created in the atmosphere oscillate from one type to another as they travel through the Earth. This phenomenon is possible only if the particles have mass – the first indication of physics beyond the Standard Model.
Half a world away, researchers at the Sudbury Neutrino Observatory in Canada had, by 2002, finally solved the mystery of the missing solar neutrinos. By detecting neutrinos of all types, they showed that previous experiments, which were sensitive only to electron neutrinos, had missed the fraction that change type as they travel from the heart of the Sun.
The Higgs and beyond
By the time of the 20th anniversary of Physics World in October 2008, many of the questions posed at the time of the magazine’s launch had been answered. But a crucial one remained: where is the Higgs boson? Fortunately, a new accelerator had just been completed at CERN – the Large Hadron Collider (LHC). Discovering the missing boson was high on the list of challenges for huge teams of researchers at the ground-breaking machine.
In 2012 the massive cost and human effort paid off, with the first observations of the long-sought particle. It weighed in at around 125 MeV/c2 – some 130 times heavier than the proton. Moreover, following the start of its experiments in earnest in 2010, the LHC began investigating the Standard Model, B-meson physics and the quark–gluon plasma to deeper levels than ever before.
Exciting discoveries elsewhere opened other new horizons too: studies of antihydrogen took off at CERN with the production of the first large quantities of this ephemeral stuff in 2002; the LIGO experiment in the US made its historic, first observation of gravitational waves in 2015; and this year the IceCube neutrino observatory at the South Pole has found the first evidence for a distant source of high-energy cosmic neutrinos. The latter two developments in particular reflect the strengthening links between particle physics, astronomy and cosmology, which have led to the birth of “astroparticle physics” as particle physicists have transferred their skills to detecting a variety of cosmic messengers.
Future challenges
It might seem that the items on the “shopping list” from that 24th ICHEP meeting 30 years have all been ticked off, but other questions from that time remain unresolved and new ones have joined the list. At this year’s ICHEP in Seoul – the 39th event in the series – Langacker, who is now at the Institute of Advanced Study in Princeton, was on hand once again to summarize progress.
While he drew attention to how well the Standard Model works in describing matter down to 10–16 cm – despite the incessant probing both at the LHC and elsewhere to prove otherwise – Langacker also highlighted current questions. What, for example, is the nature of dark matter and dark energy, which we now know form, respectively, 26.8% and 68.3% of the mass-energy of the universe? Is there a supersymmetry between the particles of matter and the particles that mediate their interactions – and can new particles required by this symmetry help to explain dark matter? What is the origin of the matter–antimatter imbalance that allows the existence of the matter universe? Is string theory, a leading contender for a quantum theory of gravity, verifiable?
The answers to these questions are certain to occupy particle physicists over the next 30 years. They will do this by pursuing higher intensities, for example at the High-Luminosity LHC scheduled to start up in 2025, as well as going to higher energies at potential new colliders, both linear and circular. There is also a vibrant programme of research in neutrino physics both at accelerators and at nuclear reactors.
Away from accelerators, ingenious experimental searches for physics beyond the Standard Model continue, including the increasingly sensitive hunts for weakly interacting massive particles, which could constitute dark matter. Meanwhile “multimessenger” studies in astroparticle physics, which combine measurements from cosmic rays, neutrinos, gravitational waves and other cosmic signals, seem bound to offer more exciting revelations about the observable universe.
All this and more may lead to a new Standard Model that describes the universe as far as we can see, as particle physics continues to probe the smallest scales and the highest energies
To paraphrase Langacker at ICHEP 2018, all this and more may lead to a new Standard Model that describes the universe as far as we can see, as particle physics continues to probe the smallest scales and the highest energies. Perhaps in 30 years’ time, Perkins’ remarks will no longer ring true and whoever summarizes ICHEP 2048 – should such a conference still exist – will be talking of an entirely new Standard Model of nature.
