Higgs boson
Composition :
Statistics :
Family :
Symbol :
Mass :
Decay :
Electric charge :
Color charge
Spin :
Weak isospin
Weak hypercharge :
Elementary particle
Bosonic
Scalair boson
H
125.10 ± 0.14 GeV/c² [1]
see [0]
0 e
none
0 [2] [3]
±1
0
General
The Higgs boson, also known as the Higgs particle, stands as a cornerstone in the edifice of modern particle physics. Within the intricate framework of the Standard Model, it emerges as an elementary particle borne from the quantum fluctuations of the Higgs field, a fundamental component in the tapestry of particle physics theory.
Defined within the Standard Model, the Higgs boson embodies the characteristics of a massive scalar boson, devoid of spin, possessing positive parity, and devoid of electric or color charge. Its existence intertwines with the concept of mass, as it couples with particles endowed with mass, albeit fleetingly, as it swiftly decays into other particles upon its creation.
The Higgs field, meanwhile, manifests as a scalar field comprising two neutral and two electrically charged components, forming a complex doublet within the symmetrical space of weak isospin SU(2). Its omnipresent value, dictated by the so-called "Sombrero potential," pervades all space, disrupting the once-balanced symmetry of weak isospin interactions through the Higgs mechanism. This mechanism imparts mass upon elementary particles within the Standard Model, including the Higgs boson itself.
This cosmic dance of particles and fields finds its roots in the work of physicist Peter Higgs and his contemporaries in 1964. Their groundbreaking proposal, the Higgs mechanism, illuminated a path for particles to acquire mass, elucidating a conundrum that had confounded physicists for decades. The anticipation of a corresponding scalar boson, christened the Higgs boson, emerged as a pivotal aspect of this theory, promising to validate the elusive Higgs field.
After an arduous quest spanning four decades, the elusive Higgs boson materialized in 2012, unveiled by the collaborative efforts of the ATLAS and CMS experiments at the Large Hadron Collider (LHC) nestled in CERN, Switzerland. Its properties aligned seamlessly with theoretical expectations, ushering in a new era of understanding in particle physics. The accolades followed suit, with Peter Higgs and François Englert receiving the Nobel Prize in Physics in 2013 for their prophetic contributions.
Particle Physics
Validation of the Standard Model
The discovery of the Higgs boson validates the Standard Model's explanation for mass generation. As we refine our understanding through precise measurements, we can either extend or rule out theoretical possibilities. Ongoing experiments investigating the behaviors of the Higgs field promise deeper insights. Without the Higgs, the Standard Model would face significant challenges and likely require revisions. Physicists anticipate the existence of "new" physics beyond the model, using the Higgs discovery and data from the LHC to guide future theoretical developments.
Symmetry breaking of the electroweak interaction
The Higgs field plays a crucial role in imparting mass to particles like quarks, charged leptons, and the W and Z gauge bosons through mechanisms such as Yukawa coupling and the Higgs mechanism.
It's important to clarify that the Higgs field doesn't create mass from nothing, as this would violate the law of conservation of energy. Additionally, not all particles derive their mass from the Higgs field. For example, the majority of the mass of baryons, such as the proton and neutron, arises from quantum chromodynamic binding energy, which includes the kinetic energies of quarks and the energies of massless gluons mediating the strong interaction within these particles.
In theories incorporating the Higgs field, mass is considered a result of potential energy transferred to fundamental particles when they interact, or "couple," with the Higgs field. This energy was initially stored within the Higgs field in the form of potential energy.
Figure 320 - Higgs Boson - Symmetry breaking
The Mexican-hat potential energy density considered by Jeffrey Goldstone in his seminal 1961 paper. 2 The energy density is a function of the real (Re) and imaginary (Im) values of a spinless field ϕ. In the context of the electroweak theory developed later in the decade, the yellow ball at the top of the hat would represent the symmetric solution for the potential, in which the photon, W bosons, and Z boson are all massless. The blue ball in the trough represents the solution after symmetry breaking. In that solution the W and Z bosons are massive and the photon remains massless. The steepness of the trough is related to the mass of the Higgs boson [1]
Particle mass acquisition
The Higgs field plays a central role in bestowing mass upon particles such as quarks, charged leptons, and the W and Z gauge bosons through mechanisms like Yukawa coupling and the Higgs mechanism.
It's important to clarify that the Higgs field doesn't spontaneously generate mass from nothing, as this would violate the law of conservation of energy. Furthermore, not all particles' masses are attributable to the Higgs field. For instance, the mass of baryons like the proton and neutron is primarily a result of quantum chromodynamic binding energy, which encompasses the kinetic energies of quarks and the energies of massless gluons mediating the strong interaction within these particles.
In theories incorporating the Higgs field, mass is understood as a manifestation of potential energy transferred to fundamental particles when they interact, or "couple," with the Higgs field. This energy was initially contained within the Higgs field in the form of potential energy.
Figure 321 - How Higgs is giving mass to particles [2]
Scalar fields and extension of the Standard Model
The detection of the Higgs field marks a unique occurrence as it is the sole scalar (spin-0) field to have been observed within the framework of the Standard Model. In contrast, all other fundamental fields within this model manifest as either spin-1/2 fermions or spin-1 bosons. Rolf-Dieter Heuer, the director general of CERN during the discovery of the Higgs boson, emphasized the significance of this finding. He noted that beyond its role in determining particle mass, the existence of a scalar field serves as compelling evidence. This discovery suggests that other hypothetical scalar fields proposed by various theories, ranging from the inflaton to quintessence, could plausibly exist as well.
Cosmology
Inflation
Considerable scientific inquiry has focused on potential connections between the Higgs field and the inflaton, a theoretical field proposed to explain the rapid expansion of space during the early moments of the universe, known as the "inflationary epoch." Some theories propose that a fundamental scalar field could drive this expansion, and the Higgs field fits this description. Consequently, research has explored whether the Higgs field could also function as the inflaton responsible for the universe's exponential growth during the Big Bang. While these theories remain speculative and encounter challenges related to unitarity, they may hold promise when combined with additional elements such as significant non-minimal coupling, a Brans–Dicke scalar, or other "new" physics. Despite these hurdles, research indicates that Higgs inflation models remain theoretically intriguing and warrant further investigation.
Figure 322 - Higgs phase during cosmological inflation
Nature of the universe, and its possible fates
Within the Standard Model, there exists the notion that the universe's underlying state, referred to as the "vacuum," may be long-lived yet not entirely stable. In such a scenario, there's a possibility that the universe could undergo destruction by transitioning into a more stable vacuum state. This idea has occasionally been misrepresented as the Higgs boson being responsible for "ending" the universe. However, if we have more precise knowledge of the masses of the Higgs boson and top quark, and if the Standard Model accurately describes particle physics up to extreme energies at the Planck scale, it becomes feasible to determine whether the vacuum is stable or only long-lived. A Higgs boson mass within the range of 125–127 GeV/c² appears to be very close to the stability threshold, but a definitive conclusion necessitates more precise measurements of the top quark's pole mass. The introduction of new physics could alter this scenario significantly.
If measurements of the Higgs boson suggest that our universe resides within such a false vacuum, it could imply that, over the course of billions of years, the universe's forces, particles, and structures might cease to exist in their current form and be replaced by different ones if a true vacuum were to nucleate. Furthermore, it indicates that the Higgs self-coupling λ and its βλ function could approach zero at the Planck scale, with potentially intriguing implications for theories involving gravity and Higgs-based inflation. Precise measurements of the top quark from a future electron–positron collider would be instrumental in performing these calculations.
Vacuum energy and the cosmological constant
Furthermore, the Higgs field has been conjectured to embody the energy of the vacuum. At the extreme energies of the initial moments of the Big Bang, it is theorized to have rendered the universe into a state of featureless symmetry, characterized by undifferentiated and exceptionally high energy. This speculative notion proposes that the unified field of a Grand Unified Theory could be synonymous with, or modelled upon, the Higgs field. Through successive symmetry breakings of the Higgs field, or a comparable field, during phase transitions, the known forces and fields of the universe are postulated to have emerged.
Additionally, scientists have scrutinized the potential connection between the Higgs field and the observed vacuum energy density of the universe. Presently, the observed vacuum energy density is exceedingly close to zero. However, predictions stemming from the Higgs field, supersymmetry, and other existing theories typically yield energy densities many orders of magnitude larger. The reconciliation of these disparities poses a significant challenge. This unresolved issue, known as the cosmological constant problem, stands as a prominent enigma in physics.
Leading Higgs Boson interactions
Figure 323 - Leading Higgs Boson interactions
a–f, Higgs boson production in ggH (a) and VBF (b), associated production with a W or Z (V) boson (VH; c), associated production with a top or bottom quark pair (ttH or bbH; d) and associated production with a single top quark (tH; e,f). g–j, Higgs boson decays into heavy vector boson pairs (g), fermion–antifermion pairs (h) and photon pairs or Zγ (i,j). k–o, Higgs boson pair production through ggH (k,l) and through VBF (m,n,o). The different Higgs boson interactions are labelled with the coupling modifiers κ, and highlighted in different colours for Higgs–fermion interactions (red), Higgs–gauge-boson interactions (blue) and multiple Higgs boson interactions (green). The distinction between a particle and its antiparticle is dropped. [3]
Literature
References
[1] Citation: Phys. Today 66, 12, 28 (2013); http://dx.doi.org/10.1063/PT.3.2212
[2] Signs’ of Higgs boson particle found. Channel 4
[3] A portrait of the Higgs boson by the CMS experiment ten years after the discovery. The CMS Collaboration. CERN 2022