Higgs boson

What is it?	A tiny particle that helps explain how other things have weight
Discovered	In experiments at the Large Hadron Collider between 2011 and 2013
Mass	About 125 times the weight of a proton
Special feature	It has no electric charge and spins slowly

The Higgs boson, sometimes called the Higgs particle, is a tiny piece of matter called an elementary particle that scientists study. It is part of the Standard Model of particle physics, which is our best way to understand how very small parts of the universe behave. The Higgs boson comes from something called the Higgs field, which is everywhere in space, even in places that seem empty.

The Higgs field helps give mass, or weight, to other particles. Without it, many particles would not have mass at all. This idea was first suggested in 1964 by a scientist named Peter Higgs and five other researchers. They thought that if the Higgs field was real, there should also be a special particle, which we now call the Higgs boson.

For many years, scientists searched for this particle. In 2012, they finally found it using a huge machine called the Large Hadron Collider at CERN near Geneva, Switzerland. The particle they found matched what they expected for the Higgs boson. In 2013, Peter Higgs and another scientist, François Englert, won a Nobel Prize in Physics for their important work in predicting this discovery.

Introduction

Standard Model

Physicists explain the tiny parts and forces of the universe using the Standard Model. This is a well-known idea based on quantum field theory that can predict almost all known particles and forces, except for gravity. (We use a different theory, called general relativity, to understand gravity.) In the Standard Model, particles and forces (except gravity) come from special patterns in quantum fields.

Gauge-invariant theories and symmetries

Gauge-invariant theories are a useful way to describe how forces work. These theories say that changing certain things does not change what we see in experiments. For example, changing the electric potential of a magnet by 100 volts does not change the magnetic field it creates. These unchanging results show symmetries in nature. These symmetries help us understand the basic forces and particles.

Gauge boson rest mass problem

For many years, scientists tried to make a theory for the weak force using gauge invariance. But they faced a problem: the theory said the weak force's particles, called W and Z bosons, should have no mass. But experiments showed they did have mass. This created a big puzzle for scientists.

Symmetry breaking

In the late 1950s, a scientist named Yoichiro Nambu idea that symmetry could break under certain conditions. This means a system that looks balanced can become unbalanced. Another scientist, Philip Anderson, suggested this breaking could help solve the problem with the weak force.

Higgs mechanism

Main articles: Higgs mechanism and Standard Model

Later, three groups of scientists showed that if a special kind of field existed everywhere in space, it could break the symmetry and give mass to the W and Z bosons. This field was called the Higgs field, named after Peter Higgs. This idea became known as the Higgs mechanism.

The sombrero potential of the Higgs field is responsible for some particles gaining mass.

Higgs field

The Standard Model includes the Higgs field to break the symmetry of the electroweak interaction and give particles their mass. This field is unlike other fields because it has a special shape that makes it have a value that is not zero, even in empty space. This nonzero value helps explain why some particles have mass.

The "central problem"

Before finding the Higgs Boson, scientists did not have direct proof that the Higgs field exists. But the Standard Model's success in predicting particles led many to believe it was correct. The Higgs field was the last piece of the Standard Model that needed proof. Scientists needed to find the Higgs boson, a particle linked to the Higgs field, to confirm its existence.

Search and discovery

Proving the Higgs field exists was very hard because creating and detecting Higgs bosons needs a lot of energy. It took over 30 years to build machines like the Large Hadron Collider at CERN to search for Higgs bosons.

On 4 July 2012, scientists announced finding a new particle with a mass between 125 and 127 GeV/c². They thought it was the Higgs boson. By March 2013, they confirmed it was the Higgs boson. This discovery helped explain why some particles have mass and solved other problems in particle physics.

Interpretation

Scientists use different comparisons to explain the Higgs field and boson, like rainbows or ripples on water. But some common comparisons, like moving through syrup, can be misleading.

Overview of Higgs boson and field properties

In the Standard Model, the Higgs boson is a heavy particle. Its mass has been measured to be about 125 GeV/c². It has no spin, no electric charge, and no colour charge. It interacts with particles that have mass and decays very quickly into other particles.

The Higgs field is a special field with a shape that makes it have a nonzero value in empty space. This nonzero value breaks a symmetry in the electroweak interaction and helps give mass to particles like the W and Z bosons and other particles called fermions.

Significance

Evidence for the Higgs field and its properties is very important. The Higgs boson helps explain how certain particles get their mass. It was lucky that its mass could be studied using current technology, which allowed scientists to test and learn more about the whole Higgs field theory. If the Higgs field and boson did not exist in the expected mass range, it would also have been very important for science.

The Higgs boson shows that the current theory of particle physics, called the Standard Model, works for explaining how some particles get mass. As scientists measure its properties more precisely, they might find clues about new theories or rules that go beyond the Standard Model. The Higgs boson helps scientists look for signs that the Standard Model might not be complete.

The Higgs field played a key role when the universe was very young and extremely hot. It helped change the early universe so that atoms and stars could form. Without this change, the universe would look very different today.

The Higgs field gives mass to certain particles, like quarks and some other small particles, as well as to carriers of forces called W and Z bosons. However, most of the mass of larger particles like protons and neutrons comes from other sources, not the Higgs field.

The Higgs field is special because it is the only known field with no spin. This makes scientists think that other similar fields might exist.

Some theories suggest the Higgs field might also explain why the universe expanded quickly right after the Big Bang. However, these ideas are still being studied and have many challenges.

There are theories that suggest the universe might not be completely stable. If certain measurements of the Higgs boson and another particle called the top quark show specific values, it could mean the universe might change in the far future. More precise measurements are needed to understand this better.

The Higgs field has also been suggested as a possible source of the energy that fills empty space. However, there is a big mystery about why the amount of this energy is so small, which scientists are still trying to solve.

History

Theorisation

Particle physicists study matter made from fundamental particles whose interactions are mediated by exchange particles acting as force carriers. In the 1960s, many of these particles had been discovered or proposed, along with theories suggesting how they relate to each other. Some of these theories had been reformulated as field theories in which the objects of study are quantum fields and their symmetries. However, attempts to create quantum field models for two of the four known fundamental forces — the electromagnetic force and the weak nuclear force — and then to unify these interactions, were still unsuccessful.

One known problem was that gauge invariant approaches, including non-abelian models such as Yang–Mills theory, seemed to predict known massive particles as massless. Goldstone's theorem, relating to continuous symmetries within some theories, also appeared to rule out many obvious solutions, since it showed that zero-mass particles known as Goldstone bosons would also have to exist and were "not seen". According to Guralnik, physicists had "no understanding" how these problems could be overcome.

Nobel Prize Laureate Peter Higgs in Stockholm, December 2013

The Higgs mechanism is a process by which vector bosons can acquire rest mass without explicitly breaking gauge invariance, as a byproduct of spontaneous symmetry breaking. Initially, the mathematical theory behind spontaneous symmetry breaking was conceived and published within particle physics by Yoichiro Nambu in 1960, and the concept that such a mechanism could offer a possible solution for the "mass problem" was originally suggested in 1962 by Philip Anderson. These approaches were quickly developed into a full relativistic model, independently and almost simultaneously, by three groups of physicists: by François Englert and Robert Brout in August 1964; by Peter Higgs in October 1964; and by Gerald Guralnik, Carl Hagen, and Tom Kibble (GHK) in November 1964.

Experimental search

To produce Higgs bosons, two beams of particles are accelerated to very high energies and allowed to collide within a particle detector. Occasionally, although rarely, a Higgs boson will be created fleetingly as part of the collision byproducts. Because the Higgs boson decays very quickly, particle detectors cannot detect it directly. Instead the detectors register all the decay products and from the data the decay process is reconstructed. If the observed decay products match a possible decay process of a Higgs boson, this indicates that a Higgs boson may have been created.

Because Higgs boson production in a particle collision is likely to be very rare, and many other possible collision events can have similar decay signatures, the data of hundreds of trillions of collisions needs to be analysed. To conclude that a new particle has been found, particle physicists require that the statistical analysis of two independent particle detectors each indicate that there is less than a one-in-a-million chance that the observed decay signatures are due to just background random Standard Model events.

Coupling strength to Higgs boson in (top) and ratio to the standard model prediction (bottom) derived from cross section and branching ratio data. In the κ framework the couplings are κ V m V / v e v ( = κ V g V / 2 v e v ) {\displaystyle {\sqrt {{\kappa }_{V}}}{m}_{V}/{\rm {vev}}\quad (={\sqrt {{\kappa }_{V}{g}_{V}/2{\rm {vev}}}})} and κ F m V / v e v {\displaystyle {\kappa }_{F}{m}_{V}/{\rm {vev}}} for the vector bosons V (=Z,W) and for the fermions F ( = t, b, τ (μ not confirmed as 2022 but there is evidence)) respectively, where m V / F {\displaystyle {m}_{V/F}} the masses and v e v {\displaystyle vev} the vacuum expectation value ( g V {\displaystyle {g}_{V}} the absolute coupling strength).

Discovery of candidate boson at CERN

On 4 July 2012 both of the CERN experiments announced they had independently made the same discovery: CMS of a previously unknown boson with mass 125.3±0.6 GeV/c² and ATLAS of a boson with mass 126.0±0.6 GeV/c². Using the combined analysis of two interaction types, both experiments independently reached a local significance of 5 sigma — implying that the probability of getting at least as strong a result by chance alone is less than one in three million.

Confirmation of existence and status

On 14 March 2013 CERN confirmed the following:

CMS and ATLAS have compared a number of options for the spin-parity of this particle, and these all prefer no spin and even parity — two fundamental criteria of a Higgs boson consistent with the Standard Model. This, coupled with the measured interactions of the new particle with other particles, strongly indicates that it is a Higgs boson.

This also makes the particle the first elementary scalar particle to be discovered in nature.

The six authors of the 1964 PRL papers, who received the 2010 J. J. Sakurai Prize for their work; from left to right: Kibble, Guralnik, Hagen, Englert, Brout; right image: Higgs.

Feynman diagrams showing the cleanest channels associated with the low-mass (~125 GeV/c²) Higgs boson candidate observed by ATLAS and CMS at the LHC. The dominant production mechanism at this mass involves two gluons from each proton fusing to a Top-quark Loop, which couples strongly to the Higgs field to produce a Higgs boson.

Left: Diphoton channel: Boson subsequently decays into two gamma ray photons by virtual interaction with a W boson loop or top quark loop.

Right: The four-lepton "golden channel": Boson emits two Z bosons, which each decay into two leptons (electrons, muons).

Experimental analysis of these channels reached a significance of more than five standard deviations (sigma) in both experiments.

Requirement	How tested / explanation	Status (as of July 2017)
Zero spin	Examining decay patterns. Spin-1 had been ruled out at the time of initial discovery by the observed decay to two photons (γ γ), leaving spin-0 and spin-2 as remaining candidates.	Spin-0 confirmed. The spin-2 hypothesis is excluded with a confidence level exceeding 99.9%.
Even (Positive) parity	Studying the angles at which decay products fly apart. Negative parity was also disfavoured if spin-0 was confirmed.	Even parity tentatively confirmed. The spin-0 negative parity hypothesis is excluded with a confidence level exceeding 99.9%.
Decay channels (outcomes of particle decaying) are as predicted	The Standard Model predicts the decay patterns of a 125 GeV/c² Higgs boson. Are these all being seen, and at the right rates? Particularly significant, we should observe decays into pairs of photons (γ γ), W and Z bosons (W⁻ W⁺ and Z Z), bottom quarks (b b), and tau leptons (τ⁻τ⁺), among the possible outcomes.	b b, γ γ, τ⁻ τ⁺, W⁻ W⁺ and Z Z observed. All observed signal strengths are consistent with the Standard Model prediction.
Couples to mass (i.e., strength of interaction with Standard Model particles proportional to their mass)	Particle physicist Adam Falkowski states that the essential qualities of a Higgs boson are that it is a spin-0 (scalar) particle which also couples to mass (W and Z bosons); proving spin-0 alone is insufficient.	Couplings to mass strongly evidenced ("At 95% confidence level c_V is within 15% of the standard model value c_V = 1").
Higher energy results remain consistent	After the LHC's 2015 restart at the higher energy of 13 TeV, searches for multiple Higgs particles (as predicted in some theories) and tests targeting other versions of particle theory continued. These higher energy results must continue to give results consistent with Higgs theories.	Analysis of collisions up to July 2017 do not show deviations from the Standard Model, with experimental precisions better than results at lower energies.

Theoretical issues

Main article: Higgs mechanism

Theoretical need for the Higgs

"Symmetry breaking illustrated": – At high energy levels (left) the ball settles in the centre, and the result is symmetrical. At lower energy levels (right), the overall "rules" remain symmetrical, but the "sombrero potential" comes into effect: "local" symmetry inevitably becomes broken since eventually the ball must at random roll one way or another.

One big challenge in particle physics is explaining how tiny particles called fermions and force carriers called W and Z bosons get their mass. Without special rules called gauge symmetry, these particles could not have mass. But keeping these rules while giving particles mass was very hard.

Scientists found a way around this using something called the Higgs mechanism. This idea says there is a special field everywhere in space, called the Higgs field. Unlike other fields, the Higgs field has a special shape that makes it easier for it to have a value that is not zero. When this happens, it changes how forces work and gives mass to particles that interact with it.

Simple explanation of the theory, from its origins in superconductivity

The idea for the Higgs mechanism came from studying materials that can conduct electricity without loss, called superconductors. In these materials, magnetic fields cannot enter, which happens because of a special field that changes how electromagnetic forces work. Scientists realized a similar process might explain how particles get mass.

Alternative models

Main article: Alternatives to the Standard Model Higgs

The simplest version of the Higgs idea uses just one field, but scientists have thought of more complex versions. Some ideas include extra fields or even no Higgs field at all, using different ways to give particles mass.

Further theoretical issues and hierarchy problem

Main articles: Hierarchy problem and Hierarchy problem § Higgs mass

One big puzzle is why the Higgs particle has the mass it does. Simple theories would predict it should be much heavier, but it is not. Solving this puzzle might need new ideas or very careful balancing of numbers, which scientists are still trying to understand.

Properties

The Higgs boson is a tiny particle that scientists study to understand how other particles have mass. It is very short-lived and breaks apart quickly into other particles.

The Higgs field is what gives other particles their mass. It has special properties that make it behave differently from other fields. When the Higgs field reaches a stable state, it creates the Higgs boson particle.

The Higgs boson has no spin and is very unstable. Its mass was found to be about 125 GeV/c², which fits well with current theories. Scientists study its properties to learn more about how particles get their mass and to search for new physics beyond current theories.

Feynman diagrams for Higgs production
Gluon fusion	Higgs Strahlung
Vector boson fusion	Top fusion

Public discussion

Naming

The particle linked to the field is called the Higgs boson, named after physicist Peter Higgs. For a while, it was known by different names based on the scientists who first talked about it. There was some debate about the best name to use, especially because of the chance of a shared Nobel Prize. Higgs himself suggested a few different names, like "the scalar boson."

Some people call the Higgs boson the "God particle" because of a popular science book with that title. The book's author wanted to call it "The Goddamn Particle" but changed it because it was too controversial. Many scientists dislike this nickname because it is too dramatic and makes it seem like the particle is very important in a way that it is not. Higgs himself did not like this name.

Educational explanations and analogies

Photograph of light passing through a dispersive prism: the rainbow effect arises because photons are not all affected to the same degree by the dispersive material of the prism.

There have been many efforts to explain the Higgs particle and field in simple terms. One way to think about it is like how light spreads out to make rainbows. Another way is to imagine an electric field: some particles are affected by it, and others are not, just like some particles are affected by the Higgs field.

One famous analogy is to imagine a room full of people: famous people move slower because everyone crowds around them, while others move easily. This helps explain how the Higgs field gives mass to particles. However, some analogies, like comparing it to syrup, can be misleading because they suggest the field just slows things down, which isn't quite right.

Recognition and awards

There has been discussion about who should get credit for the discovery of the Higgs boson, especially since a Nobel Prize was expected. The Nobel Prize can only be given to three people at a time, and some of the scientists involved had already won prizes or had passed away.

In 2013, the Nobel Prize in Physics was awarded to Peter Higgs and François Englert for their theoretical work that helped us understand how particles get mass. Other scientists have also been honored for their related work over the years.

Symmetry breaking in optics	In vacuum, light of all colours (or photons of all wavelengths) travels at the same velocity, a symmetrical situation. In some substances such as glass, water or air, this symmetry is broken (See: Photons in matter). The result is that light of different wavelengths have different velocities.
Symmetry breaking in particle physics	In "naive" gauge theories, gauge bosons and other fundamental particles are all massless – also a symmetrical situation. In the presence of the Higgs field this symmetry is broken. The result is that particles of different types will have different masses.

Technical aspects and mathematical formulation

The potential for the Higgs field, plotted as function of ϕ 0 {\displaystyle \phi ^{0}} and ϕ 3 {\displaystyle \phi ^{3}} . It has a sombrero or champagne-bottle profile at the ground.

In the Standard Model, the Higgs field is a special kind of field that helps give mass to other particles. This field forms a complex structure with four parts.

The Higgs field interacts with particles in a way that gives them mass. This happens because of how the field is set up in its lowest energy state. When the field is in this state, it gives mass to certain particles called W and Z bosons. These particles are important for carrying forces in the universe.

The Higgs boson itself is a tiny piece of this field that can be observed. It was discovered because scientists were looking for evidence of this field. The way the Higgs field works also affects other particles, like quarks and electrons, giving them their mass too.