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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 with zero spin, even parity, no electric charge, and no colour charge that couples to mass. 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, 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.

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. If these ideas were correct, a particle known as a scalar boson 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 is sometimes called the "God particle" after the 1993 book The God Particle by Nobel Laureate Leon Lederman. The name has been criticised by physicists, including Higgs.

Introduction
Standard Model of particle physics


Physicists explain the fundamental particles and forces of our 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. In the Standard Model, the particles and forces in nature 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 which have a useful feature; some kinds of changes to the value of certain items do not make any difference to the outcomes or the measurements we make. An example: changing voltages in an electromagnet by +100 volts does not cause any change to the magnetic field it produces. Similarly, measuring the speed of light in vacuum seems to give the identical result, whatever the location in time and space, and whatever the local gravitational field.

In these kinds of theories, the gauge is an item whose value we can change. The fact that some changes leave the results we measure unchanged means it is a gauge invariant theory, and symmetries are the specific kinds of changes to the gauge which have the effect of leaving measurements unchanged. Symmetries of this kind are powerful tools for a deep understanding of the fundamental forces and particles of our physical world. Gauge invariance is therefore an important property within particle physics theory. They 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 they focus on properties of our universe, demonstrating this property of gauge invariance and the symmetries which are involved.

Gauge boson 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 had consistently failed. As a result of these failures, gauge theories began to fall into disrepute. The problem was symmetry requirements for these two forces incorrectly predicted the weak force's gauge bosons (W and Z) would have "zero mass". But experiments showed the W and Z gauge bosons had non-zero mass.

Further, many promising solutions seemed to require the existence of extra particles known as Goldstone bosons. But evidence suggested these did not exist either. This meant 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 create 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.

Symmetry breaking
In the late 1950s, Yoichiro Nambu recognised that spontaneous symmetry breaking, a process where a symmetric system becomes asymmetric, could occur under certain conditions. Symmetry breaking is when some variable that previously didn't affect the measured results now does affect the measured results. In 1962 physicist Philip Anderson, an expert in condensed matter physics, observed that symmetry breaking played a role in superconductivity, and suggested it could also be part of the answer to the problem of gauge invariance in particle physics.

Specifically, Anderson suggested that the Goldstone bosons that would result from symmetry breaking might instead, in some circumstances, be "absorbed" by the massless W and Z bosons. If so, perhaps the Goldstone bosons would not exist, and the W and Z bosons could gain mass, solving both problems at once. Similar behaviour was already theorised in superconductivity. In 1964, this was shown to be theoretically possible by physicists Abraham Klein and Benjamin Lee, at least for some limited cases.

Higgs mechanism
Main articles: Higgs mechanism and Standard Model
Following the 1963 and early 1964 papers, three groups of researchers independently developed these theories more completely, in what became known as the 1964 PRL symmetry breaking papers. All three groups reached similar conclusions and for all cases, not just some limited cases. They showed that the conditions for electroweak symmetry would be "broken" if an unusual type of field existed throughout the universe, and indeed, there would be no Goldstone bosons and some existing bosons would acquire mass.

The field required for this to happen became known as the Higgs field and the mechanism by which it led to symmetry breaking, known as the Higgs mechanism. A key feature of the necessary field is that it would take less energy for the field to have a non-zero value than a zero value, unlike all other known fields, therefore, the Higgs field has a non-zero value everywhere. This non-zero value could in theory break electroweak symmetry. It was the first proposal capable of showing how the weak force gauge bosons could have mass despite their governing symmetry, within a gauge invariant theory.

Although these ideas did not gain much initial support or attention, by 1972 they had been developed into a comprehensive theory and proved capable of giving "sensible" results that accurately described particles known at the time, and which, with exceptional accuracy, predicted several other particles discovered during the following years. During the 1970s these theories rapidly became the Standard Model of particle physics.

Higgs field
To allow symmetry breaking, the Standard Model includes a field of the kind needed to "break" electroweak symmetry and give particles their correct mass. This field, which became known as the "Higgs Field", was hypothesized to exist throughout space, and to break some symmetry laws of the electroweak interaction, triggering the Higgs mechanism. It, therefore, would cause the W and Z gauge bosons of the weak force to be massive at all temperatures below an extremely high value. When the weak force bosons acquire mass, this affects the distance they can freely travel, which becomes very small, also matching experimental findings. Furthermore, it was later realised that the same field would also explain, in a different way, why other fundamental constituents of matter (including electrons and quarks) have mass.

Unlike all other known fields, such as the electromagnetic field, the Higgs field is a scalar field, and has a non-zero average value in vacuum.

The "central problem"
There was not yet any direct evidence that the Higgs field existed, but even without direct proof, the accuracy of its predictions led scientists to believe the theory might be true. By the 1980s, the question of whether the Higgs field existed, and therefore whether the entire Standard Model was correct, had come to be regarded as one of the most important unanswered questions in particle physics.

For many decades, scientists had no way to determine whether the Higgs field existed because the technology needed for its detection did not exist at that time. If the Higgs field did exist, then it would be unlike any other known fundamental field, but it also was possible that these key ideas, or even the entire Standard Model, were somehow incorrect.

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".

The hypothesised Higgs theory made several key predictions. One crucial prediction was that a matching particle, called the "Higgs boson", should also exist. Proving the existence of the Higgs boson would prove whether the Higgs field existed, and therefore finally prove whether the

 Standard Model's explanation was correct. Therefore, there was an extensive search for the Higgs boson, as a way to prove the Higgs field itself existed.

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