High energy physics (also known as particle physics) is a branch of physics that studies the nature of the elementary constituents of matter and radiation, and the interactions between them. The name “high energy physics” originates from the natural conditions in which elementary particles are created; indeed they can be created and detected during energetic collisions of other particles, as performed in particle accelerators.
Since the 1970s, particle physicists have described the fundamental structure of matter using an elegant series of equations called the ‘Standard Model’. The model describes how everything that they observe in the universe is made from a few basic blocks called fundamental particles, governed by four forces (strong, weak, electromagnetic and gravitational).
The Standard Model has many species of elementary particles (fermions, vector and scalar bosons), which can combine to form composite particles, accounting for the hundreds of other species of particles discovered.
Over the years, it has explained many experimental results and precisely predicted a range of phenomena, such that today it is considered a well-tested physics theory. But the model only describes 4% of the known universe and questions remain, such as ‘why is the Higgs mass so light?’, ‘what is dark matter made of?’, ‘are all the forces unified into one force at high energy?’, ‘what happened to the antimatter in the early universe?’.
Today, the high energy physics community is trying to answer to these questions guided by intertwined science drivers:
The science drivers provide compelling lines of inquiry that show great promise for discovery. The HEP community executes its mission through a program that advances three frontiers of experimental scientific discovery and related efforts in theory and computing. Moreover, this program is integrated with the development of new accelerators, detectors and computational tools to enable the science.
The mission of the High Energy Physics (HEP) program is to understand how our universe works at its most fundamental level. It enables scientific discovery through a strategy organized along three interrelated frontiers of particle physics.
Energy Frontier researchers accelerate particles to the highest energies ever made by humanity and collides them to produce and study the fundamental constituents of matter and the architecture of the universe. They use the world’s largest and highest energy particle accelerator (Large Hadron Collider, LHC) to recreate the universe as it was a billionth of a second after the big bang and make huge discoveries about the smallest pieces of the universe.
The discovery of the Higgs boson by the ATLAS and CMS experiments at the Large Hadron Collider in July 2012 was the culmination of a global effort that began in 1964 when François Englert, Peter Higgs and other theorists proposed it as the final piece to the Standard Model of particle physics.
Now that the Higgs boson has been discovered, Energy Frontier researchers will use it as a new tool for discovery. The Higgs boson plays a central role in the Standard Model and affects many of its predictions.
By precisely measuring the Higgs boson’s properties, Energy Frontier researchers will be able to establish its exact character and discover if there are indications of new physics beyond the Standard Model.
High energy particle collisions also allow researchers to explore mechanisms of black hole production, search for extra dimensions of space, and pursue other exotic phenomena.
Intensity Frontier researchers use a combination of intense particle beams and highly sensitive detectors to make extremely precise measurements of particle properties, study some of the rarest particle interactions predicted by the Standard Model of particle physics and search for new physics.
Making precise measurements of known particles allows researchers to determine whether the Standard Model road map is complete (e.g. the experiment Belle II aims to precisely measure physical properties of quarks and leptons, search for the source of matter-antimatter asymmetry in the universe today, and discover new states of matter).
Researchers at the Intensity Frontier investigate some of the rarest processes in nature, including unusual interactions of fundamental particles and subtle effects that require large data sets to observe and measure (i.e. Long-Baseline Neutrino Facility, LBNF, and the Deep Underground Neutrino Experiment, DUNE, for the study of neutrino physics).
Intensity Frontier researchers also explore the unknown in search of new particles and forces by making extremely precise measurements of particle properties and studying some of the rarest particle interactions.
Some particle interactions, predicted to be extremely rare in the Standard Model of particle physics, would be significantly enhanced if new particles or forces exist. In some experiments, finding even a single one of these special interactions may be the first indication of new physics. Other experiments apply the same principle to extremely precise measurements of fundamental particle properties, where measuring a deviation from the Standard Model’s prediction would indicate that new particles exist.
Moreover, there are some models of dark matter that include new “dark” particles and forces which have ultra-weak couplings to normal matter. These models can be explored by using intense particle beams, highly capable, high rate detectors in a way that complements research efforts in the Cosmic and Energy Frontiers.
Cosmic Frontier researchers seek to reveal the nature of dark matter and dark energy by using particles from space to explore new phenomena.
They use diverse tools and technologies, from space-based observatories to ground-based telescopes and large detectors deep underground, to probe fundamental physics questions and offer new insight about the nature of dark matter, dark energy and other phenomena.
They exploit the observation of extra-terrestrial, ultra-high energy gamma rays and cosmic rays to explore for new particles and interactions beyond those in Standard Model.
Indeed, some sites exist in the universe (such as supermassive black holes, supernova remnants, active galactic nuclei, etc.) where extreme phenomena occur and shock mechanisms are generated. In these sites called cosmic accelerators, particles are accelerated at much higher energies than can be produced by particle accelerators on Earth. Studying these particles may give physicists a way of exploring energy scales far beyond what current or planned future particle accelerators can achieve.
As an example of space-borne experiments, the Fermi Gamma-ray Space Telescope uses two onboard instruments, the Large Area Telescope and the Gamma-ray Burst Monitor, in an effort to determine the cosmic origin of very high energy gamma-rays, and search for signs of new physics.
One of the biggest ground-based observatories for the cosmic frontier is the Pierre Auger Observatory which seeks to improve our understanding of the highest energy particles in the universe, which shower down on Earth in the form of cosmic rays.
As a complement to the three frontiers of particle physics theoretical and computational physics, as well as advanced technology R&D, are of great interest to the HEP community.
The former provides the framework to explain experimental observations and gain a deeper understanding of nature. Theoretical physicists take the lead in the interpretation of a broad range of experimental results and support current experiments by identifying new directions to follow. In the analysis of all the data produced by the experiments, advanced computing tools are necessary for designing, operating and interpreting experiments that enable discovery research in the three frontiers.
The latter, alternatively, fosters fundamental research into particle acceleration, detection techniques and instrumentation, providing the enabling technologies and new research methods which can advance scientific knowledge in high energy physics and a broad range of related fields.
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