Particle physics in general

All interactions between particles that have been directly observed can be explained by a beautiful theory called the standard model, which contains three of the four forces observed in nature: The electromagnetic force, which is responsible to well known effects such as electricity and magnetism, the weak force which is responsible for nuclear decays and for the burning of stars, as well as the strong force, which bounds particles together into the nucleus. Elementary matter particles that do not interact strongly are called leptons, while those that feel the strong force are called quarks. The particles mediating the electromagnetic, weak and strong force are called photon, W/Z boson and gluon.

All matter present under ordinary conditions is made up of two types of quarks, as well as electrons.Nuclear decays produce a neutrino which only interact very weakly with regular matter and are therefore have no everyday effect. However, the standard model model contains not one lepton, but three leptons, as well as three neutrinos. It also contains not two, but six different types of quarks. The extra particles that do not play a role in everyday life are heavier than their counterparts and have disappeared from the universe a short while after the big bang.

Experiments in particle physics typically collide particles with high enough energy to create the heavy particles in a laboratory environment and watch how they decay with large, complicated detectors. This gives insight into the interactions between the various particles, and has allowed us to theoretically understand the basic forces mentioned above. The theory describing all the interactions between particles is known as the standard model, and all experiments performed to date have confirmed this standard model.

However, there are several shortcomings of the standard model, two of which are the most important. First, one of the particles of the standard model, the Higgs boson, has not been observed so far. The Higgs boson is responsible for what is known as electroweak symmetry breaking, which gives all elementary particles its mass. Second, there is undeniable evidence that most of the matter that exists in the universe is not made from particles contained in the standard model and does not interact via the three forces mentioned above. Such matter is called dark matter, and is only observed through its gravitational pull on regular matter.

One of the main goals of modern particle physics is to understand the mechanism of electroweak symmetry breaking, as well as to produce dark matter. While there is nothing in principle that links dark matter and electroweak symmetry breaking, there is tantalizing evidence that the same physics might explain both phenomena.

Colliding particles at higher and higher energies has provided us with the best data on the fundamental interactions of elementary particles. Since energy and mass are equivalent, colliding particles at high energies allows to create heavy particles that existed only very early in the universe, but have long ceased to exist under normal conditions. However, studying these interactions allows us to gain insight into the conditions of the universe just a few seconds after the big bang.

Experiments in the past have confirmed the standard model of particle physics with unprecedented accuracy, and all particles except for the Higgs boson have been produced and experimentally and studied in great detail. In 2009 the large hadron collider (LHC) started operation after being designed and built for almost 20 years. The LCH is located at CERN in Geneva, Switzerland, and is the largest collider ever built. It will eventually test particle physics at energies 7 times higher than ever before. It is virtually certain that the LHC will discover the origin of electroweak symmetry breaking, and there is great hope that it will also shed light on the origin of dark matter in our universe.

Collider Physics

Contact information:

Dr. Christian W Bauer, Lawrence Berkeley National Laboratory, M/S 50A5104, Berkeley, CA 94720

Email:, Phone: (510) 486-7773, Fax: (510) 486-4608

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