Higgs boson

The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Standard Model, the Higgs particle is a massive scalar boson that couples to (interacts with) particles whose mass arises from their interactions with the Higgs field, has zero spin, even (positive) parity, no electric charge, and no color charge. It is also very unstable, decaying into other particles almost immediately upon generation. The Higgs field is a scalar field with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU(2) symmetry. Its "sombrero potential" leads it to take a nonzero value everywhere (including otherwise empty space), which breaks the weak isospin symmetry of the electroweak interaction and, via the Higgs mechanism, gives a rest mass to all massive elementary particles of the Standard Model, including the Higgs boson itself. The existence of the Higgs field became the last unverified part of the Standard Model of particle physics, and for several decades was considered "the central problem in particle physics". Both the field and the boson are named after physicist Peter Higgs, who in 1964, along with five other scientists in three teams, proposed the Higgs mechanism, a way for some particles to acquire mass. All fundamental particles known at the time should be massless at very high energies, but fully explaining how some particles gain mass at lower energies had been extremely difficult. If these ideas were correct, a particle known as a scalar boson (with certain properties) should also exist. This particle was called the Higgs boson and could be used to test whether the Higgs field was the correct explanation. After a 40-year search, a subatomic particle with the expected properties was discovered in 2012 by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland. The new particle was subsequently confirmed to match the expected properties of a Higgs boson. Physicists from two of the three teams, Peter Higgs and François Englert, were awarded the Nobel Prize in Physics in 2013 for their theoretical predictions. Although Higgs's name has come to be associated with this theory, several researchers between about 1960 and 1972 independently developed different parts of it. In the media, the Higgs boson has often been called the "God particle" after the 1993 book The God Particle by Nobel Laureate Leon M. Lederman. The name has been criticised by physicists, including Peter Higgs.

Introduction

Standard Model Physicists explain the fundamental particles and forces of the universe in terms of the Standard Model – a widely accepted framework based on quantum field theory that predicts almost all known particles and forces aside from gravity with great accuracy. (A separate theory, general relativity, is used for gravity.) In the Standard Model, the particles and forces in nature (aside from gravity) arise from properties of quantum fields known as gauge invariance and symmetries. Forces in the Standard Model are transmitted by particles known as gauge bosons.

Gauge-invariant theories and symmetries "It is only slightly overstating the case to say that physics is the study of symmetry" – Philip Anderson, Nobel Prize Physics Gauge-invariant theories are theories with a useful feature, namely that changes to certain quantities make no difference to experimental outcomes. For example, increasing the electric potential of an electromagnet by 100 volts does not itself cause any change to the magnetic field that it produces. Similarly, the measured speed of light in vacuum remains unchanged, whatever the location in time and space, and whatever the local gravitational field. In these theories, the gauge is a quantity that can be changed with no resultant effect. This independence of the results from some changes is called gauge invariance, and these changes reflect symmetries of the underlying physics. These symmetries provide constraints on the fundamental forces and particles of the physical world. Gauge invariance is therefore an important property within particle physics theory. The gauge symmetries are closely connected to conservation laws and are described mathematically using group theory. Quantum field theory and the Standard Model are both gauge-invariant theories – meaning that the gauge symmetries allow theoretical derivation of properties of the universe.

Gauge boson rest mass problem Quantum field theories based on gauge invariance had been used with great success in understanding the electromagnetic and strong forces, but by around 1960, all attempts to create a gauge invariant theory for the weak force (and its combination with the electromagnetic force, known together as the electroweak interaction) had consistently failed. As a result of these failures, gauge theories began to fall into disrepute. The problem was that symmetry requirements for these two forces incorrectly predicted that the weak force's gauge bosons (W and Z) would have zero mass (in the specialized terminology of particle physics, "mass" refers specifically to a particle's rest mass). But experiments showed the W and Z gauge bosons had non-zero (rest) mass. Further, many promising solutions seemed to require the existence of extra particles known as Goldstone bosons, but evidence suggested these did not exist. This meant that either gauge invariance was an incorrect approach, or something unknown was giving the weak force's W and Z bosons their mass, and doing it in a way that did not imply the existence of Goldstone bosons. By the late 1950s and early 1960s, physicists were at a loss as to how to resolve these issues, or how to create a comprehensive theory for particle physics.

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