Higgs boson
Encyclopedia
The Higgs boson is a hypothetical massive elementary particle
that is predicted to exist by the Standard Model
(SM) of particle physics
. Its existence is postulated as a means of resolving inconsistencies in the Standard Model. Experiments attempting to find the particle are currently being performed using the Large Hadron Collider
(LHC) at CERN
, and were performed at Fermilab's
Tevatron
until Tevatron's closure in late 2011.
The Higgs boson is the only elementary particle predicted by the Standard Model that has not been observed in particle physics experiments
. It is a necessary requirement of the so-called Higgs mechanism
, the part of the SM which explains how most of the known elementary particles obtain their mass
.The masses of composite particles such as the proton and neutron would only be partly due to the Higgs mechanism, and are already understood as a consequence of the strong interaction
. For example, the Higgs mechanism would explain why the W and Z bosons
, which mediate weak interactions, are massive whereas the related photon
, which mediates electromagnetism, is massless. The Higgs boson is expected to be in a class of particles known as scalar boson
s. (Boson
s are particles with integer spin
, and scalar bosons have spin 0.)
Theories exist that do not need the Higgs boson, described as Higgsless model
s. Some theories suggest that any mechanism capable of generating the masses of the elementary particles must be visible at energies below 1.4 TeV
; therefore, the LHC is expected to be able to provide experimental evidence of the existence or non-existence of the Higgs boson. Experiments at Fermilab were also continuing previous attempts at detection, albeit hindered by the lower energy of the Tevatron accelerator. The Fermilab collider ceased operation 30th September 2011 although data analysis still continues.
is a process by which vector boson
s can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert
and Robert Brout
; by Peter Higgs
(inspired by ideas of Philip Anderson
); and by Gerald Guralnik
, C. R. Hagen
, and Tom Kibble.
The three papers written on this discovery were each recognized as milestone papers during Physical Review Letters
50th anniversary celebration. While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL symmetry breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics
for this work.
The 1964 PRL
papers by Higgs and by Guralnik, Hagen, and Kibble (GHK) both displayed equations for the field that would eventually become known as the Higgs boson. In the paper by Higgs the boson is massive, and in a closing sentence Higgs writes that "an essential feature" of the theory "is the prediction of incomplete multiplets of scalar and vector bosons". In the model described in the GHK paper the boson is massless and decoupled from the massive states. In recent reviews of the topic, Guralnik states that in the GHK model the boson is massless only in a lowest-order approximation, but it is not subject to any constraint and it acquires mass at higher orders. Additionally, he states that the GHK paper was the only one to show that there are no massless Nambu-Goldstone bosons in the model and to give a complete analysis of the general Higgs mechanism
. Following the publication of the 1964 PRL
papers, the properties of the model were further discussed by Guralnik in 1965 and by Higgs in 1966.
Steven Weinberg
and Abdus Salam
were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio between the W boson and Z boson masses as well as their coupling
s with each other and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.
The Higgs boson's existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, the Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model
yet is not expected to exist in Higgsless model
s, such as Technicolor
. A goal of the LHC and Tevatron experiments is to distinguish among these models and determine if the Higgs boson exists or not.
of the theoretical Higgs field. In empty space, the Higgs field has an amplitude different from zero; i.e. a non-zero vacuum expectation value
. The existence of this non-zero vacuum expectation plays a fundamental role; it gives mass to every elementary particle that couples to the Higgs field, including the Higgs boson itself. The acquisition of a non-zero vacuum expectation value spontaneously breaks
electroweak
gauge symmetry. This is the Higgs mechanism, which is the simplest process capable of giving mass to the gauge boson
s while remaining compatible with gauge theories. This field is analogous to a pool of molasses
that "sticks" to the otherwise massless fundamental particles that travel through the field, converting them into particles with mass that form (for example) the components of atoms.
In the Standard Model, the Higgs field consists of two neutral and two charged component fields
. Both of the charged components and one of the neutral fields are Goldstone boson
s, which act as the longitudinal third-polarization components of the massive W+, W–, and Z bosons
. The quantum of the remaining neutral component corresponds to the massive Higgs boson. Since the Higgs field is a scalar field
, the Higgs boson has no spin
, hence no intrinsic angular momentum
. The Higgs boson is also its own antiparticle
and is CP-even.
The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and , then the Standard Model can be valid at energy scales all the way up to the Planck scale
(1016 TeV). Many theorists expect new physics beyond the Standard Model
to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity
is violated in certain scattering processes. There are over a hundred theoretical Higgs-mass predictions.
Extensions to the Standard Model including supersymmetry
(SUSY) predict the existence of families of Higgs bosons, rather than the one Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric Standard Model
(MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet
of scalar particles, two CP-even
neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±. Many supersymmetric models predict that the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around or less.
experiments at CERN and Fermilab.
Prior to the year 2000, the data gathered at the LEP collider at CERN allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of at the 95% confidence level. The same experiment has produced a small number of events that could be interpreted as resulting from Higgs bosons with a mass just above this cut off — around 115 GeV—but the number of events was insufficient to draw definite conclusions. The LEP was shut down in 2000 due to construction of its successor, the LHC, which is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the calibration and startup phase.
At the Fermilab
Tevatron
, there are ongoing experiments searching for the Higgs boson. , combined data from CDF
and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range - at the 95% confidence level. Preliminary results as of July 2011 have since extended the excluded region to the range - at the 90% confidence level. Data collection and analysis in search of Higgs are intensifying since March 30, 2010 when the LHC began operating at 3.5 TeV. Preliminary results from the ATLAS
and CMS
experiments at the LHC as of July 2011 exclude a Standard Model Higgs boson in the mass range - and - , respectively, at the 95% confidence level.
It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons
. Precision measurements of electroweak parameters, such as the Fermi constant
and masses of W/Z bosons, can be used to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion of a Standard Model Higgs boson having a mass greater than at 95% CL, and estimated its mass to be (the central value corresponding to approximately 138 proton
masses). As of August 2009, the Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at the 95% confidence level. However, it should be noted that these indirect constraints make the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above 186 GeV if it is accompanied by other particles between the Standard Model and GUT scales.
In a 2009 preprint, it was suggested that the Higgs boson might not only interact with the above-mentioned particles of the Standard model of particle physics, but also with the mysterious weakly interacting massive particles (or WIMPS) that may form dark matter
, and which play an important role in recent astrophysics.
Various reports of potential evidence for the existence of the Higgs boson have appeared in recent years but to date none have provided convincing evidence. In April 2011, there were suggestions in the media that evidence for the Higgs boson might have been discovered at the LHC in Geneva, Switzerland but these had been debunked by mid May. In regard to these rumors Jon Butterworth, a member of the High Energy Physics group on the Atlas experiment, stated they were not a hoax, but were based on unofficial, unreviewed results. The LHC detected possible signs of the particle, which were reported in July 2011, the ATLAS Note concluding: "In the low mass range (c 120−140 GeV) an excess of events with a significance of approximately 2.8 sigma
above the background expectation is observed" and the BBC
reporting that "interesting particle events at a mass of between 140 and 145 GeV" were found. These findings were repeated shortly thereafter by researchers at the Tevatron with a spokesman stating that: "There are some intriguing things going on around a mass of 140GeV." However, on 22 August it was reported that the anomalous results had become insignificant on the inclusion of more data from ATLAS and CMS and that the non-existence of the particle had been confirmed by LHC collisions to 95% certainty between 145–466 GeV (except for a few small islands around 250 GeV). A combined analysis of ATLAS and CMS data, published in November 2011, further narrowed the window for the allowed values of the Higgs boson mass to 114-141 GeV. .
that breaks electroweak symmetry. A partial list of these alternative mechanisms are:
Lederman initially wanted to call it the "goddamn particle," but his editor would not let him. While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider, many scientists dislike it, since it overstates the particle's importance, not least since its discovery would still leave unanswered questions about the unification of QCD
, the electroweak interaction and gravity, and the ultimate origin of the universe. A renaming competition conducted by the science correspondent for the British Guardian
newspaper chose the name "the champagne bottle boson" as the best from among their submissions: "The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it's not an embarrassingly grandiose name, it is memorable, and [it] has some physics connection too."
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
that is predicted to exist by the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
(SM) of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
. Its existence is postulated as a means of resolving inconsistencies in the Standard Model. Experiments attempting to find the particle are currently being performed using the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
(LHC) at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
, and were performed at Fermilab's
Fermilab
Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...
Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
until Tevatron's closure in late 2011.
The Higgs boson is the only elementary particle predicted by the Standard Model that has not been observed in particle physics experiments
Particle physics experiments
Particle physics experiments briefly discusses a number of past, present, and proposed experiments with particle accelerators, throughout the world. In addition, some important accelerator interactions are discussed...
. It is a necessary requirement of the so-called Higgs mechanism
Higgs mechanism
In particle physics, the Higgs mechanism is the process in which gauge bosons in a gauge theory can acquire non-vanishing masses through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking....
, the part of the SM which explains how most of the known elementary particles obtain their mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
.The masses of composite particles such as the proton and neutron would only be partly due to the Higgs mechanism, and are already understood as a consequence of the strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
. For example, the Higgs mechanism would explain why the W and Z bosons
W and Z bosons
The W and Z bosons are the elementary particles that mediate the weak interaction; their symbols are , and . The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own...
, which mediate weak interactions, are massive whereas the related photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
, which mediates electromagnetism, is massless. The Higgs boson is expected to be in a class of particles known as scalar boson
Scalar boson
-Explanation:The name "scalar boson" arises from quantum field theory. It refers to the particular transformation properties under Lorentz transformation. Boson means that it has an integer-valued spin, the scalar fixes this value to 0.- Examples :...
s. (Boson
Boson
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....
s are particles with integer spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
, and scalar bosons have spin 0.)
Theories exist that do not need the Higgs boson, described as Higgsless model
Higgsless model
In particle physics, a Higgsless model is a model that does not involve the Higgs boson or in which the Higgs field is not dynamic. Such models must employ a different mechanism of mass generation, electroweak symmetry breaking and unitarity....
s. Some theories suggest that any mechanism capable of generating the masses of the elementary particles must be visible at energies below 1.4 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
; therefore, the LHC is expected to be able to provide experimental evidence of the existence or non-existence of the Higgs boson. Experiments at Fermilab were also continuing previous attempts at detection, albeit hindered by the lower energy of the Tevatron accelerator. The Fermilab collider ceased operation 30th September 2011 although data analysis still continues.
Origin of the theory
The Higgs mechanismHiggs mechanism
In particle physics, the Higgs mechanism is the process in which gauge bosons in a gauge theory can acquire non-vanishing masses through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking....
is a process by which vector boson
Vector boson
In particle physics, a vector boson is a boson with the spin quantum number equal to 1.The vector bosons considered to be elementary particles in the Standard Model are the gauge bosons or, the force carriers of fundamental interactions: the photon of electromagnetism, the W and Z bosons of the...
s can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert
François Englert
François Englert is a Belgian theoretical physicist. He was awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics , the Wolf Prize in Physics in 2004 and the High Energy and Particle Prize of the European Physical Society François Englert (born 6 November 1932) is a...
and Robert Brout
Robert Brout
Robert Brout was an American-Belgian theoretical physicist who has made significant contributions in elementary particle physics...
; by Peter Higgs
Peter Higgs
Peter Ware Higgs, FRS, FRSE, FKC , is an English theoretical physicist and an emeritus professor at the University of Edinburgh....
(inspired by ideas of Philip Anderson
Philip Warren Anderson
Philip Warren Anderson is an American physicist and Nobel laureate. Anderson has made contributions to the theories of localization, antiferromagnetism and high-temperature superconductivity.- Biography :...
); and by Gerald Guralnik
Gerald Guralnik
Gerald Stanford Guralnik is the Chancellor’s Professor of Physics at Brown University. He is most famous for his co-discovery of the Higgs mechanism and Higgs Boson with C. R. Hagen and Tom Kibble...
, C. R. Hagen
C. R. Hagen
Carl Richard Hagen is a professor of particle physics at the University of Rochester. He is most noted for his contributions to the Standard Model and Symmetry breaking as well as the co-discovery of the Higgs mechanism and Higgs boson with Gerald Guralnik and Tom Kibble...
, and Tom Kibble.
The three papers written on this discovery were each recognized as milestone papers during Physical Review Letters
Physical Review Letters
Physical Review Letters , established in 1958, is a peer reviewed, scientific journal that is published 52 times per year by the American Physical Society...
50th anniversary celebration. While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL symmetry breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics
Sakurai Prize
The J. J. Sakurai Prize for Theoretical Particle Physics, is presented by the American Physical Society at its annual "April Meeting", and honors outstanding achievement in particle physics theory...
for this work.
The 1964 PRL
Physical Review Letters
Physical Review Letters , established in 1958, is a peer reviewed, scientific journal that is published 52 times per year by the American Physical Society...
papers by Higgs and by Guralnik, Hagen, and Kibble (GHK) both displayed equations for the field that would eventually become known as the Higgs boson. In the paper by Higgs the boson is massive, and in a closing sentence Higgs writes that "an essential feature" of the theory "is the prediction of incomplete multiplets of scalar and vector bosons". In the model described in the GHK paper the boson is massless and decoupled from the massive states. In recent reviews of the topic, Guralnik states that in the GHK model the boson is massless only in a lowest-order approximation, but it is not subject to any constraint and it acquires mass at higher orders. Additionally, he states that the GHK paper was the only one to show that there are no massless Nambu-Goldstone bosons in the model and to give a complete analysis of the general Higgs mechanism
Higgs mechanism
In particle physics, the Higgs mechanism is the process in which gauge bosons in a gauge theory can acquire non-vanishing masses through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking....
. Following the publication of the 1964 PRL
Physical Review Letters
Physical Review Letters , established in 1958, is a peer reviewed, scientific journal that is published 52 times per year by the American Physical Society...
papers, the properties of the model were further discussed by Guralnik in 1965 and by Higgs in 1966.
Steven Weinberg
Steven Weinberg
Steven Weinberg is an American theoretical physicist and Nobel laureate in Physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interaction between elementary particles....
and Abdus Salam
Abdus Salam
Mohammad Abdus Salam, NI, SPk Mohammad Abdus Salam, NI, SPk Mohammad Abdus Salam, NI, SPk (Urdu: محمد عبد السلام, pronounced , (January 29, 1926– November 21, 1996) was a Pakistani theoretical physicist and Nobel laureate in Physics for his work on the electroweak unification of the...
were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio between the W boson and Z boson masses as well as their coupling
Coupling constant
In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian of a system can be separated into a kinetic part and an interaction part...
s with each other and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.
The Higgs boson's existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, the Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions; therefore every Standard Model particle has a partner that has yet to be discovered...
yet is not expected to exist in Higgsless model
Higgsless model
In particle physics, a Higgsless model is a model that does not involve the Higgs boson or in which the Higgs field is not dynamic. Such models must employ a different mechanism of mass generation, electroweak symmetry breaking and unitarity....
s, such as Technicolor
Technicolor (physics)
Technicolor theories are models of physics beyond the standard model that address electroweak symmetry breaking, the mechanism through which elementary particles acquire masses...
. A goal of the LHC and Tevatron experiments is to distinguish among these models and determine if the Higgs boson exists or not.
Theoretical overview
The Higgs boson particle is the quantumQuantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...
of the theoretical Higgs field. In empty space, the Higgs field has an amplitude different from zero; i.e. a non-zero vacuum expectation value
Vacuum expectation value
In quantum field theory the vacuum expectation value of an operator is its average, expected value in the vacuum. The vacuum expectation value of an operator O is usually denoted by \langle O\rangle...
. The existence of this non-zero vacuum expectation plays a fundamental role; it gives mass to every elementary particle that couples to the Higgs field, including the Higgs boson itself. The acquisition of a non-zero vacuum expectation value spontaneously breaks
Symmetry breaking
Symmetry breaking in physics describes a phenomenon where small fluctuations acting on a system which is crossing a critical point decide the system's fate, by determining which branch of a bifurcation is taken. To an outside observer unaware of the fluctuations , the choice will appear arbitrary...
electroweak
Electroweak interaction
In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different...
gauge symmetry. This is the Higgs mechanism, which is the simplest process capable of giving mass to the gauge boson
Gauge boson
In particle physics, gauge bosons are bosonic particles that act as carriers of the fundamental forces of nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles.-...
s while remaining compatible with gauge theories. This field is analogous to a pool of molasses
Molasses
Molasses is a viscous by-product of the processing of sugar cane, grapes or sugar beets into sugar. The word molasses comes from the Portuguese word melaço, which ultimately comes from mel, the Latin word for "honey". The quality of molasses depends on the maturity of the sugar cane or sugar beet,...
that "sticks" to the otherwise massless fundamental particles that travel through the field, converting them into particles with mass that form (for example) the components of atoms.
In the Standard Model, the Higgs field consists of two neutral and two charged component fields
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...
. Both of the charged components and one of the neutral fields are Goldstone boson
Goldstone boson
In particle and condensed matter physics, Goldstone bosons or Nambu–Goldstone bosons are bosons that appear necessarily in models exhibiting spontaneous breakdown of continuous symmetries...
s, which act as the longitudinal third-polarization components of the massive W+, W–, and Z bosons
W and Z bosons
The W and Z bosons are the elementary particles that mediate the weak interaction; their symbols are , and . The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own...
. The quantum of the remaining neutral component corresponds to the massive Higgs boson. Since the Higgs field is a scalar field
Scalar field
In mathematics and physics, a scalar field associates a scalar value to every point in a space. The scalar may either be a mathematical number, or a physical quantity. Scalar fields are required to be coordinate-independent, meaning that any two observers using the same units will agree on the...
, the Higgs boson has no spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
, hence no intrinsic angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
. The Higgs boson is also its own antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
and is CP-even.
The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and , then the Standard Model can be valid at energy scales all the way up to the Planck scale
Planck scale
In particle physics and physical cosmology, the Planck scale is an energy scale around 1.22 × 1019 GeV at which quantum effects of gravity become strong...
(1016 TeV). Many theorists expect new physics beyond the Standard Model
Beyond the Standard Model
Physics beyond the Standard Model refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the origin of mass, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy...
to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity
Unitarity (physics)
In quantum physics, unitarity is a restriction on the allowed evolution of quantum systems that insures the sum of probabilities of all possible outcomes of any event is always 1....
is violated in certain scattering processes. There are over a hundred theoretical Higgs-mass predictions.
Extensions to the Standard Model including supersymmetry
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...
(SUSY) predict the existence of families of Higgs bosons, rather than the one Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric Standard Model
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions; therefore every Standard Model particle has a partner that has yet to be discovered...
(MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet
Quintet
A quintet is a group containing five members.It is commonly associated with musical groups, such as a string quintet, or a group of five singers, but can be applied to any situation where five similar or related objects are considered a single unit....
of scalar particles, two CP-even
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...
neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±. Many supersymmetric models predict that the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around or less.
Experimental search
, the Higgs boson has yet to be confirmed experimentally, despite large efforts invested in acceleratorParticle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
experiments at CERN and Fermilab.
Prior to the year 2000, the data gathered at the LEP collider at CERN allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of at the 95% confidence level. The same experiment has produced a small number of events that could be interpreted as resulting from Higgs bosons with a mass just above this cut off — around 115 GeV—but the number of events was insufficient to draw definite conclusions. The LEP was shut down in 2000 due to construction of its successor, the LHC, which is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the calibration and startup phase.
At the Fermilab
Fermilab
Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...
Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
, there are ongoing experiments searching for the Higgs boson. , combined data from CDF
Collider Detector at Fermilab
The Collider Detector at Fermilab experimental collaboration studies high energy particle collisions at the Tevatron,the world's former highest-energy particle accelerator...
and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range - at the 95% confidence level. Preliminary results as of July 2011 have since extended the excluded region to the range - at the 90% confidence level. Data collection and analysis in search of Higgs are intensifying since March 30, 2010 when the LHC began operating at 3.5 TeV. Preliminary results from the ATLAS
Atlas
An atlas is a collection of maps; it is typically a map of Earth or a region of Earth, but there are atlases of the other planets in the Solar System. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats...
and CMS
Compact Muon Solenoid
The Compact Muon Solenoid experiment is one of two large general-purpose particle physics detectors built on the proton-proton Large Hadron Collider at CERN in Switzerland and France. Approximately 3,600 people from 183 scientific institutes, representing 38 countries form the CMS collaboration...
experiments at the LHC as of July 2011 exclude a Standard Model Higgs boson in the mass range - and - , respectively, at the 95% confidence level.
It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons
W and Z bosons
The W and Z bosons are the elementary particles that mediate the weak interaction; their symbols are , and . The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own...
. Precision measurements of electroweak parameters, such as the Fermi constant
Fermi's interaction
In particle physics, Fermi's interaction also known as Fermi coupling, is an old explanation of the weak force, proposed by Enrico Fermi, in which four fermions directly interact with one another at one vertex...
and masses of W/Z bosons, can be used to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion of a Standard Model Higgs boson having a mass greater than at 95% CL, and estimated its mass to be (the central value corresponding to approximately 138 proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
masses). As of August 2009, the Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at the 95% confidence level. However, it should be noted that these indirect constraints make the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above 186 GeV if it is accompanied by other particles between the Standard Model and GUT scales.
In a 2009 preprint, it was suggested that the Higgs boson might not only interact with the above-mentioned particles of the Standard model of particle physics, but also with the mysterious weakly interacting massive particles (or WIMPS) that may form dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
, and which play an important role in recent astrophysics.
Various reports of potential evidence for the existence of the Higgs boson have appeared in recent years but to date none have provided convincing evidence. In April 2011, there were suggestions in the media that evidence for the Higgs boson might have been discovered at the LHC in Geneva, Switzerland but these had been debunked by mid May. In regard to these rumors Jon Butterworth, a member of the High Energy Physics group on the Atlas experiment, stated they were not a hoax, but were based on unofficial, unreviewed results. The LHC detected possible signs of the particle, which were reported in July 2011, the ATLAS Note concluding: "In the low mass range (c 120−140 GeV) an excess of events with a significance of approximately 2.8 sigma
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
above the background expectation is observed" and the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
reporting that "interesting particle events at a mass of between 140 and 145 GeV" were found. These findings were repeated shortly thereafter by researchers at the Tevatron with a spokesman stating that: "There are some intriguing things going on around a mass of 140GeV." However, on 22 August it was reported that the anomalous results had become insignificant on the inclusion of more data from ATLAS and CMS and that the non-existence of the particle had been confirmed by LHC collisions to 95% certainty between 145–466 GeV (except for a few small islands around 250 GeV). A combined analysis of ATLAS and CMS data, published in November 2011, further narrowed the window for the allowed values of the Higgs boson mass to 114-141 GeV. .
Alternatives for electroweak symmetry breaking
In the years since the Higgs boson was proposed, several alternatives to the Higgs mechanism have been proposed. All of these proposed mechanisms use strongly interacting dynamics to produce a vacuum expectation valueVacuum expectation value
In quantum field theory the vacuum expectation value of an operator is its average, expected value in the vacuum. The vacuum expectation value of an operator O is usually denoted by \langle O\rangle...
that breaks electroweak symmetry. A partial list of these alternative mechanisms are:
- TechnicolorTechnicolor (physics)Technicolor theories are models of physics beyond the standard model that address electroweak symmetry breaking, the mechanism through which elementary particles acquire masses...
, a class of models that attempts to mimic the dynamics of the strong force as a way of breaking electroweak symmetry. - Extra dimensional Higgsless models where the role of the Higgs field is played by the fifth component of the gauge field.
- Abbott-Farhi models of composite W and Z vector bosons.
- Top quark condensateTop quark condensateIn particle physics, the top quark condensate theory is an alternative to the Standard Model in which a fundamental scalar Higgs field is replaced by a composite field composed of the top quark and its antiquark. These are bound by a four-fermion interaction, analogous to Cooper pairs in a BCS...
theory in which a fundamental scalar Higgs field is replaced by a composite fieldComposite fieldIn quantum field theory, a composite field is a field defined in terms of other more "elementary" fields. It might describe a composite particle or it might not. It might be local, or it might be nonlocal...
composed of the top quarkTop quarkThe top quark, also known as the t quark or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and...
and its antiquark. - The braid modelBraid theoryIn topology, a branch of mathematics, braid theory is an abstract geometric theory studying the everyday braid concept, and some generalizations. The idea is that braids can be organized into groups, in which the group operation is 'do the first braid on a set of strings, and then follow it with a...
of Standard Model particles by Sundance Bilson-ThompsonSundance Bilson-ThompsonSundance Osland Bilson-Thompson is an Australian theoretical particle physicist. He has developed the idea that certain preon models may be represented topologically, rather than by treating preons as pointlike particles. His ideas have attracted interest in the field of loop quantum gravity, as...
, compatible with loop quantum gravityLoop quantum gravityLoop quantum gravity , also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the theories of quantum mechanics and general relativity...
and similar theories. - In theory of superfluid vacuumSuperfluid vacuumSuperfluid vacuum theory , sometimes dubbed as the theory of BEC vacuum, is an approach in theoretical physics and quantum mechanics where the physical vacuum is viewed as a superfluid or BEC....
masses of elementary particles can arise as a result of interaction with the physical vacuumVacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, similarly to the gap generation mechanism in superconductors.
"The God particle"
The Higgs boson is often referred to as "the God particle" by the media, after the title of Leon Lederman's book, The God Particle: If the Universe Is the Answer, What Is the Question?The God Particle: If the Universe Is the Answer, What Is the Question?
The God Particle: If the Universe Is the Answer, What is the Question? is a 1993 popular science book by Nobel Prize-winning physicist Leon M. Lederman and science writer Dick Teresi....
Lederman initially wanted to call it the "goddamn particle," but his editor would not let him. While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider, many scientists dislike it, since it overstates the particle's importance, not least since its discovery would still leave unanswered questions about the unification of QCD
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
, the electroweak interaction and gravity, and the ultimate origin of the universe. A renaming competition conducted by the science correspondent for the British Guardian
The Guardian
The Guardian, formerly known as The Manchester Guardian , is a British national daily newspaper in the Berliner format...
newspaper chose the name "the champagne bottle boson" as the best from among their submissions: "The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it's not an embarrassingly grandiose name, it is memorable, and [it] has some physics connection too."
See also
- Bose–Einstein statisticsBose–Einstein statisticsIn statistical mechanics, Bose–Einstein statistics determines the statistical distribution of identical indistinguishable bosons over the energy states in thermal equilibrium.-Concept:...
- BosonBosonIn particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....
- Higgs boson in fictionHiggs boson in fictionThe Higgs boson has appeared in several works of fiction in popular culture. These references rarely reflect the expected properties of the hypothetical elementary particle, or do so only vaguely, and often imbue it with fantastic properties...
- Higgs mechanismHiggs mechanismIn particle physics, the Higgs mechanism is the process in which gauge bosons in a gauge theory can acquire non-vanishing masses through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking....
- List of particles
- Quantum trivialityQuantum trivialityIn a quantum field theory, charge screening can restrict the value of the observable "renormalized" charge of a classical theory. Ifthe only allowed value of the renormalized charge is zero, the theory is said to be "trivial" or noninteracting...
- Yukawa interactionYukawa interactionIn particle physics, Yukawa's interaction, named after Hideki Yukawa, is an interaction between a scalar field \phi and a Dirac field \Psi of the typeV \approx g\bar\Psi \phi \Psi or g \bar \Psi \gamma^5 \phi \Psi ....
- ZZ dibosonZZ dibosonZZ dibosons are rare pairs of Z bosons that are extremely difficult to detect. They were first observed by the scientists of DØ collaboration at Fermilab. They are force-carrying particles produced in proton–antiproton collisions at the Tevatron, the world’s second highest-energy particle accelerator...