Physicists explain the properties of forces between elementary particles in terms of the Standard Model – a widely accepted framework for understanding almost everything in fundamental physics, other than gravity. (A separate theory, general relativity, is used for gravity.) In this model, the fundamental forces in nature arise from properties of our universe called gauge invariance and symmetries. The forces are transmitted by particles known as gauge bosons.[13][14]
The problem of gauge boson mass
Field theories had been used with great success in understanding the electromagnetic field and the strong force, but by around 1960 all attempts to create a gauge invariant theory for the weak force (and its combination with fundamental force electromagnetism, the electroweak interaction) had consistently failed, with gauge theories thereby starting to fall into disrepute as a result. The problem was that gauge invariant theory contains symmetry requirements, and these incorrectly predicted that the weak force's gauge bosons (W and Z) should have zero mass. It is known from experiments that they have non-zero mass.[15] This meant that either gauge invariance was an incorrect approach, or something else – unknown – was giving these particles their mass. By the late 1950s, physicists had not resolved these issues and were still unable to create a comprehensive theory for particle physics, because all attempts to solve this problem just created more theoretical problems.
Symmetry breaking
In the late 1950s, Yoichiro Nambu recognised that spontaneous symmetry breaking, a process where a symmetric system ends up in an asymmetric state, could occur under certain conditions.[c] In 1962 physicist Philip Anderson, working in the field of condensed matter physics, observed that symmetry breaking played a role in superconductivity, and may have relevance to the problem of gauge invariance in particle physics. In 1963, this was shown to be theoretically possible, at least for some limited (non-relativistic) cases.
Higgs mechanism
Following the 1962 and 1963 papers, three groups of researchers independently published the 1964 PRL symmetry breaking papers with 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, some fundamental particles would acquire mass. The field required for this to happen (which was purely hypothetical at the time) became known as the Higgs field (after Peter Higgs, one of the researchers) 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 (or vacuum expectation) 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.[d] During the 1970s these theories rapidly became the Standard Model of particle physics.
Higgs field
The Standard Model includes a field of the kind needed to "break" electroweak symmetry and give particles their correct mass. This field, called the "Higgs Field", exists throughout space, and it breaks some symmetry laws of the electroweak interaction, triggering the Higgs mechanism. It therefore causes the W and Z gauge bosons of the weak force to be massive at all temperatures below an extreme high value.[e] When the weak force bosons acquire mass, this affects the distance they can freely travel, which becomes very small, also matching experimental findings.[f] 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 constant value in vacuum.
The "central problem"
There was not yet any direct evidence that the Higgs field existed, but even without proof of the field, 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.[g]
The hypothesised Higgs mechanism made several accurate predictions.[d][17]:22 One crucial prediction was that a matching particle called the "Higgs boson" should also exist. Proving the existence of the Higgs boson could 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.[8][9]
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".[18][19]
Search and discovery
Although the Higgs field exists everywhere, proving its existence was far from easy. In principle, it can be proved to exist by detecting its excitations, which manifest as Higgs particles (the Higgs boson), but these are extremely difficult to produce and detect, due to the energy required to produce them and their very rare production even if the energy is sufficient. It was therefore several decades before the first evidence of the Higgs boson was found. Particle colliders, detectors, and computers capable of looking for Higgs bosons took more than 30 years (c. 1980–2010) to develop.
The importance of this fundamental question led to a 40-year search, and the construction of one of the world's most expensive and complex experimental facilities to date, CERN's Large Hadron Collider,[20] in an attempt to create Higgs bosons and other particles for observation and study. On 4 July 2012, the discovery of a new particle with a mass between 125 and 127 GeV/c2 was announced; physicists suspected that it was the Higgs boson.[21][22][23] Since then, the particle has been shown to behave, interact, and decay in many of the ways predicted for Higgs particles by the Standard Model, as well as having even parity and zero spin,[6][7] two fundamental attributes of a Higgs boson. This also means it is the first elementary scalar particle discovered in nature.[24]
By March 2013, the existence of the Higgs boson was confirmed, and therefore, the concept of some type of Higgs field throughout space is strongly supported.[21][23][6]
The presence of the field, now confirmed by experimental investigation, explains why some fundamental particles have mass, despite the symmetries controlling their interactions implying that they should be massless. It also resolves several other long-standing puzzles, such as the reason for the extremely short distance travelled by the weak force bosons, and therefore the weak force's extremely short range.
As of 2018, in-depth research shows the particle continuing to behave in line with predictions for the Standard Model Higgs boson. More studies are needed to verify with higher precision that the discovered particle has all of the properties predicted, or whether, as described by some theories, multiple Higgs bosons exist.[25]
The nature and properties of this field are now being investigated further, using more data collected at the LHC.[1]
Interpretation
Various analogies have been used to describe the Higgs field and boson, including analogies with well-known symmetry-breaking effects such as the rainbow and prism, electric fields, and ripples on the surface of water.
Other analogies based on resistance of macro objects moving through media (such as people moving through crowds, or some objects moving through syrup or molasses) are commonly used but misleading, since the Higgs field does not actually resist particles, and the effect of mass is not caused by resistance.
SOURCE : WIKIPEDIA
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