The driving motivation behind particle physics experiments is the desire to uncover the true nature of fundamental forces and particles. Our current standard model is believed to be an effective theory, which has a deeper underlying theory reachable in the next generation of experiments. In the electroweak sector, where great successes of the past decades have predicted and verified the unification of the electromagnetic and weak nuclear forces, precision measurements at the CERN large electron positron collider (LEP) and the Fermilab proton-antiproton collider (Tevatron) demand that there be either a light Higgs particle with a mass less than about 200 GeV or a physical system mimicking its interactions. At the same time, the requirement that the theory be stable even with a Higgs, as well as the observation of cold dark matter in the universe, compellingly point to new physics at the Terascale.
In the flavour sector, a decade of increasingly precise measurements of the properties of heavy quarks has shown remarkable agreement with the standard model predictions, and we are now moving into an era of precise investigations of the neutrino sector. The demonstration by the Japanese Super-Kamiokande and Canadian SNO experiments that neutrinos flavours oscillate but that their masses are likely much smaller than those of the other elementary particles suggests that there are critical phenomena in particle physics which cannot be explained by the standard model. This physics likely had a very significant role in cosmology. There are some hints that this new physics might be accessible to upcoming experiments. In the strong sector, the theory of quantum chromodynamics (QCD) has been successfully used to predict the behaviour of quarks and gluons at high energies observed at the HERA collider at DESY Hamburg as well as LEP and the Tevatron, but the lower energy regime, where they are bound into particles such as protons and neutrons, remains theoretically and experimentally challenging and requires further investigation.
ATLAS at the CERN Large Hadron Collider
More than a hundred Canadian researchers from across the country, including the TRIUMF ATLAS group, are involved in the international ATLAS project based at CERN; the team is called ATLAS Canada. TRIUMF also supports the international ATLAS Canadian Tier-1 Data Centre.
The ATLAS experiment is one of the two general-purpose detectors at the LHC at CERN, the European Laboratory for Particle Physics. The LHC is designed to accelerate intense beams consisting of thousands of bunches, each containing up to 1011 protons, to an energy of 7 TeV—about 7 times more energy than the present world record. The protons will collide in the heart of the detectors, allowing their constituent partons to annihilate and liberate up to 14 TeV per collision for the creation of new particles. Bunches will collide 40 million times every second, giving a luminosity of 1034 cm-2s-1. This unprecedented combination of centre-of-mass energy and luminosity will allow the LHC to produce previously undiscovered particles with masses of a TeV (such as the Higgs) and more in sufficient numbers to ensure their discovery, probing the phase space crucial to understanding how electroweak symmetry is broken and mass is generated. This, in turn, is expected to contribute to an understanding of the connection of gravity to the three forces already described by the standard model. Among other outstanding issues, which the LHC is designed to address, are the nature of dark matter and the preponderance of matter over anti-matter in the composition of our universe.
Tokai to Kamioka: T2K
T2K is a next-generation neutrino experiment that will study flavour oscillations of neutrinos produced in a man-made beam. Neutrino oscillation is the first evidence for new physics beyond the standard model. Canada has been an international leader in this discovery through the experimental work done at SNO, and now the details of how neutrinos oscillate and their parameters is being studied. T2K will use accelerator-produced neutrinos, whose energy and composition can be directly controlled, to study oscillations of neutrinos as they travel hundreds of kilometres across Japan. Strong Canadian participation in this international experiment builds on, and maintains, Canada’s leadership role in neutrino research.
The Canadian team involved in T2K is called T2K Canada.
T2K will use the new Japan Proton Accelerator Research Complex (JPARC) proton synchrotron, located in Tokai, Japan, to produce an intense beam of muon neutrinos that will be directed towards the Super-Kamiokande neutrino detector in western Japan. By comparing the rates and types of neutrino interactions in Super-Kamiokande to those measured by a “near detector” in Tokai, T2K will measure neutrino oscillations across a 295-km baseline with unprecedented precision. The experiment hopes to be the first to measure the small neutrino mixing angle θ13, which can be determined by measuring the rate at which muon neutrinos oscillate into electron neutrinos over this distance.
SNOLAB’s ultra-low background places it centre stage in two quests: on the cosmic scale for interstellar dark matter and on the microscopic scale for neutrinoless double-beta decay. Astrophysical measurements indicate that 80% of the matter in the universe is “missing,” that is, we can see its gravitational effects but it does not emit any heat or light. This “dark matter” is hypothesized to be the stuff that shapes the destiny of the universe, and yet we have no idea what it really is.
Experiments at SNOLAB will search for hypothesized rare interactions between dark matter and normal matter. On the microscopic end of the spectrum, neutrinoless double beta decay probes the very nature of antimatter. Advanced theories of particle physics and the Big Bang suggest that the neutrino particle may have a special nature: it might be its own antiparticle. Answering this question about the neutrino could reveal new insights into why the modern universe is predominantly occupied by matter (including dark matter!) rather than anti-matter.
The initial program of SNOLAB will likely include experiments that focus on direct detection of dark matter (DEAP/CLEAN, PICASSO, Super-CDMS) and neutrinoless double-beta decay (SNO+, EXO).
Rare Kaon Decays
The standard model is the best theory that physicists currently have to describe the actions and interactions of fundamental particles, the building blocks of the universe. The standard model (SM) agrees with most of what has been observed, but is widely believed to be only an approximation of a more basic and fundamental model whose properties are not yet known.
The SM leaves open many questions. For example, the SM predicts only a subset of the possible particle interactions allowed by a more general theory that satisfies all aspects of a symmetry known as Lorentz invariance. Other interactions have been omitted based on empirical observations, but are they really completely absent? Experimental tests of the SM either place limits on the strength of these interactions or more optimistically find them and show the SM to be incomplete, requiring physics beyond the standard model.
The strong force is one of the four basic forces of nature, along with gravity, electromagnetism, and the less-familiar weak force. Inside the nucleus of the atom, the strong force binds the smallest particles of matter, quarks, together into protons and neutrons. In addition to protons and neutrons, it also binds together other, simpler particles, such as pions.
Like protons and neutrons, pions are built of quarks - two quarks (one quark and one anti-quark), to be exact. The quarks are bound together by the strong force. The field generated by the presence of the strong force also gives rise to additional, short-lived particles inside the pion: a bevy of quarks and gluons that constantly blink into and out of existence. This group of extra particles generated by the strong force is called the quark-gluon sea. By measuring the quark-gluon sea, scientists can study the strong force at its most basic level.
While a very good theoretical framework (called quantum chromodynamics, or QCD) is able to accurately describe how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), it has been very difficult to apply QCD to lower energy (larger distance) phenomena. The paradox is that the increasing complexity of the quark-gluon interaction as they get further apart is critically important to their observed confinement within nucleons and mesons (the nuclear building-blocks), but we cannot perform the QCD calculations necessary to confirm our understanding. This is because in the “confinement regime” the quark-gluon coupling strength is too large to allow perturbative theoretic methods to be reliably used. One of the central problems of modern physics remains the connection of the observed properties of the nuclear building blocks (protons, neutrons, mesons) to the underlying theoretical framework provided by QCD.