Editing Higgs mechanism (section)

== History of research ==

=== Background ===
[[Spontaneous symmetry breaking]] offered a framework to introduce bosons into relativistic quantum field theories. However, according to [[Goldstone's theorem]], these bosons should be massless.<ref>{{cite journal |author1=Guralnik |first=G. S. |author2=Hagen |first2=C. R. |author3=Kibble |first3=T. W. B. |year=1967 |title=Broken symmetries and the Goldstone theorem |url=http://www.physics.princeton.edu/~mcdonald/examples/EP/guralnik_ap_2_567_67.pdf |url-status=dead |journal=Advances in Physics |volume=2 |archive-url=https://web.archive.org/web/20150924072804/http://www.physics.princeton.edu/~mcdonald/examples/EP/guralnik_ap_2_567_67.pdf |archive-date=2015-09-24 |access-date=2014-09-16}}</ref> The only observed particles which could be approximately interpreted as Goldstone bosons were the [[pion]]s, which [[Yoichiro Nambu]] related to [[chiral symmetry]] breaking.

A similar problem arises with [[Yang–Mills theory]] (also known as [[non-abelian gauge theory]]), which predicts massless [[Spin (physics)|spin]]-1 [[gauge boson]]s. Massless weakly-interacting gauge bosons lead to long-range forces, which are only observed for electromagnetism and the corresponding massless [[photon]]. Gauge theories of the [[weak force]] needed a way to describe massive gauge bosons in order to be consistent.

=== Discovery ===
[[File:Andersonphoto.jpg|thumb|Philip W. Anderson, the first to implement the mechanism in 1962.]]
[[File:AIP-Sakurai-best.JPG|thumb|Five of the six 2010 APS [[Sakurai Prize]] Winners – (L to R) Tom Kibble, Gerald Guralnik, Carl Richard Hagen, François Englert, and Robert Brout]]
[[File:Higgs, Peter (1929).jpg|thumb|Peter Higgs in 2009]]

That breaking gauge symmetries did not lead to massless particles was observed in 1961 by [[Julian Schwinger]],<ref>{{cite journal |last=Schwinger |first=Julian |year=1961 |title=Gauge invariance and mass |journal=Phys. Rev. |volume=125 |issue=1 |pages=397–398 |bibcode=1962PhRv..125..397S |doi=10.1103/PhysRev.125.397}}</ref> but he did not demonstrate massive particles would eventuate. This was done in [[Philip Warren Anderson]]'s 1962 paper<ref name="Anderson" /> but only in non-relativistic field theory; it also discussed consequences for particle physics but did not work out an explicit relativistic model. The relativistic model was developed in 1964 by three independent groups:
* [[Robert Brout]] and [[François Englert]]<ref name="Englert" />
* [[Peter Higgs]]<ref name="Higgs" />
* [[Gerald Guralnik]], [[C. R. Hagen|Carl Richard Hagen]], and [[Tom Kibble]].<ref name="GHK" /><ref name="Guralnik" /><ref name="Scholarpedia_history" />
Slightly later, in 1965, but independently from the other publications<ref name="Polyakov">{{cite arXiv |eprint=hep-th/9211140 |first=A. M. |last=Polyakov |title=A view from the island |year=1992}}</ref><ref>{{cite book |author1=Farhi |first=E. |title=Dynamical Gauge Symmetry Breaking: A collection of reprints |author2=Jackiw |first2=R. W. |publisher=World Scientific |year=1982 |location=Singapore}}</ref><ref>{{cite book |first=Frank |last=Close |url=http://blog.oup.com/2012/07/frank-close-new-boson-particle-higgs-find/ |title=The Infinity Puzzle |year=2011 |page=158}}</ref><ref>{{cite news |first=Norman |last=Dombey |url=https://www.theguardian.com/science/2012/jul/06/higgs-boson-credit |title=Higgs Boson: Credit where it's due |newspaper=The Guardian |date=6 July 2012}}</ref><ref name="CernCourier">{{cite news |url=http://cerncourier.com/cws/article/cern/29554/2 |title=article 29554 |newspaper=Cern Courier |date=1 March 2006 |access-date=25 April 2015 |archive-date=9 July 2011 |archive-url=https://web.archive.org/web/20110709210059/http://cerncourier.com/cws/article/cern/29554/2 |url-status=dead }}</ref><ref>{{cite book |first=Sean |last=Carrol |title=The Particle at the End of the Universe: The hunt for the Higgs and the discovery of a new world |year=2012 |page=228 |url=http://www.goodreads.com/book/show/15744013-the-particle-at-the-end-of-the-universe}}</ref> the mechanism was also proposed by [[Alexander Migdal]] and [[Alexander Markovich Polyakov|Alexander Polyakov]],<ref>{{cite journal |author1=Migdal |first=A. A. |author2=Polyakov |first2=A. M. |date=July 1966 |title=Spontaneous breakdown of strong interaction symmetry and absence of massless particles |url=http://www.jetp.ac.ru/cgi-bin/dn/e_024_01_0091.pdf |journal=Journal of Experimental and Theoretical Physics |volume=51 |page=135 |bibcode=1967JETP...24...91M}} English translation: ''Soviet Physics Journal of Experimental and Theoretical Physics'', '''24''', 1, January&nbsp;1967).</ref> at that time Soviet undergraduate students. However, their paper was delayed by the editorial office of [[Journal of Experimental and Theoretical Physics|JETP]], and was published late, in 1966.

The mechanism is closely analogous to phenomena previously discovered by [[Yoichiro Nambu]] involving the "vacuum structure" of quantum fields in [[superconductivity]].<ref>{{cite journal |last=Nambu |first=Y. |year=1960 |title=Quasi-particles and gauge invariance in the theory of superconductivity |journal=Physical Review |volume=117 |issue=3 |pages=648–63 |doi=10.1103/PhysRev.117.648 |bibcode=1960PhRv..117..648N}}</ref> A similar but distinct effect (involving an affine realization of what is now recognized as the Higgs field), known as the [[Stueckelberg action|Stueckelberg mechanism]], had previously been studied by [[Ernst Stueckelberg]].

These physicists discovered that when a gauge theory is combined with an additional field that spontaneously breaks the symmetry group, the gauge bosons can consistently acquire a nonzero mass. In spite of the large values involved (see below) this permits a gauge theory description of the weak force, which was independently developed by [[Steven Weinberg]] and [[Abdus Salam]] in 1967. Higgs's original article presenting the model was rejected by ''[[Physics Letters]]''. When revising the article before resubmitting it to ''[[Physical Review Letters]]'', he added a sentence at the end,<ref>{{cite journal |first=Peter |last=Higgs |year=2007 |title=Prehistory of the Higgs boson |journal=Comptes Rendus Physique |volume=8 |issue=9 |pages=970–72 |doi=10.1016/j.crhy.2006.12.006 |bibcode=2007CRPhy...8..970H}}</ref> mentioning that it implies the existence of one or more new, massive scalar bosons, which do not form complete [[group representation|representations]] of the symmetry group; these are the Higgs bosons.

The three papers by Brout and Englert; Higgs; and Guralnik, Hagen, and Kibble were each recognized as "milestone letters" by ''Physical Review Letters'' in 2008.<ref>{{cite journal|url=http://prl.aps.org/50years/milestones#1964 |journal=Physical Review Letters |title=50th anniversary milestone papers |access-date=2012-06-16 |df=dmy-all}}</ref> While each of these seminal papers took similar approaches, the contributions and differences among the [[1964 PRL symmetry breaking papers]] are noteworthy. All six physicists were jointly awarded the 2010 [[Sakurai Prize|J. J. Sakurai Prize for Theoretical Particle Physics]] for this work.<ref>{{cite web |url=http://www.aps.org/units/dpf/awards/sakurai.cfm |publisher=American Physical Society |title=J.J. Sakurai Prize Winners |website=aps.org |access-date=2012-06-16 |df=dmy-all}}</ref>

[[Benjamin W. Lee]] is often credited with first naming the "Higgs-like" mechanism, although there is debate around when this first occurred.<ref>{{cite web |publisher=Department of Physics and Astronomy, University of Rochester |url=http://www.pas.rochester.edu/urpas/news/Hagen_030708 |archive-url=https://web.archive.org/web/20080416064136/http://www.pas.rochester.edu/urpas/news/Hagen_030708 |url-status=dead |archive-date=2008-04-16 |title=Rochester's Hagen Sakurai Prize announcement |website=pas.rochester.edu |access-date=2012-06-16 |df=dmy-all}}</ref><ref>{{cite AV media |url=https://www.youtube.com/watch?v=QrCPrwRBi7E&feature=PlayList&p=BDA16F52CA3C9B1D&playnext_from=PL&index=9 |title=C.R. Hagen discusses naming of Higgs boson in 2010 Sakurai Prize talk |date=2010-02-15 |author=Fermi |first=Fred |medium=video |access-date=2012-06-16 |archive-url=https://ghostarchive.org/varchive/youtube/20211221/QrCPrwRBi7E |archive-date=2021-12-21 |url-status=live |via=YouTube |df=dmy-all}}{{cbignore}}</ref><ref>{{cite web |last=Sample |first=Ian |url=https://www.theguardian.com/science/blog/2009/may/29/why-call-it-the-god-particle-higgs-boson-cern-lhc |title=Anything but "the God particle" by Ian Sample |work=The Guardian |date=2009-05-29 |access-date=2012-06-16 |df=dmy-all}}</ref> One of the first times the ''Higgs'' name appeared in print was in 1972 when [[Gerardus 't Hooft]] and [[Martinus J. G. Veltman]] referred to it as the "Higgs–Kibble mechanism" in their Nobel winning paper.<ref>{{cite journal |author1='t Hooft |first=G. |author2=Veltman |first2=M. |year=1972 |title=Regularization and renormalization of gauge fields |url=https://repositorio.unal.edu.co/handle/unal/81144 |journal=Nuclear Physics B |volume=44 |issue=1 |pages=189–219 |bibcode=1972NuPhB..44..189T |doi=10.1016/0550-3213(72)90279-9 |hdl-access=free |hdl=1874/4845}}</ref><ref>{{cite web |url=http://igitur-archive.library.uu.nl/phys/2005-0622-155148/13877.pdf |title=Regularization and renormalization of gauge fields by t'Hooft and Veltman |access-date=2012-06-16 |url-status=dead |archive-url=https://web.archive.org/web/20120707134940/http://igitur-archive.library.uu.nl/phys/2005-0622-155148/13877.pdf |archive-date=2012-07-07 |df=dmy-all}}</ref>

=== Simple explanation of the theory, from its origins in superconductivity ===
The proposed Higgs mechanism arose as a result of theories proposed to explain observations in [[superconductivity]]. A superconductor does not allow penetration by external magnetic fields (the [[Meissner effect]]). This strange observation implies that the electromagnetic field somehow becomes short-ranged during this phenomenon. Successful theories arose to explain this during the 1950s, first for fermions ([[Ginzburg–Landau theory]], 1950), and then for bosons ([[BCS theory]], 1957).

In these theories, superconductivity is interpreted as arising from a [[Bose–Einstein condensate|charged condensate]]. Initially, the condensate value does not have any preferred direction. This implies that it is scalar, but its [[Phase (waves)|phase]] is capable of defining a gauge in gauge based field theories. To do this, the field must be charged. A charged scalar field must also be complex (or described another way, it contains at least two components, and a symmetry capable of rotating the compontents into each other). In naïve gauge theory, a gauge transformation of a condensate usually rotates the phase. However, in these circumstances, it instead fixes a preferred choice of phase. However it turns out that fixing the choice of gauge so that the condensate has the same phase everywhere, also causes the electromagnetic field to gain an extra term. This extra term causes the electromagnetic field to become short range.

[[Goldstone's theorem]] also plays a role in such theories. The connection is technically, when a condensate breaks a symmetry, then the state reached by acting with a symmetry generator on the condensate has the same energy as before. This means that some kinds of oscillation will not involve change of energy. Oscillations with unchanged energy imply that excitations (particles) associated with the oscillation are massless.

Once attention was drawn to this theory within particle physics, the parallels were clear. A change of the usually long range electromagnetic field to become short-ranged, within a gauge invariant theory, was exactly the needed effect sought for the bosons that mediate the weak interaction (because a long-range force has massless gauge bosons, and a short-ranged force implies massive gauge bosons, suggesting that a result of this interaction is that the field's gauge bosons acquired mass, or a similar and equivalent effect). The features of a field required to do this was also quite well-defined – it would have to be a charged scalar field, with at least two components, and complex in order to support a symmetry able to rotate these into each other.