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. Scientists study it to learn about the very small parts of the universe. It is part of the Standard Model of particle physics.

The Higgs boson comes from something called the Higgs field. This field is everywhere in space, even in places that look empty. It helps give mass, or weight, to other particles.

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 be a special particle, which we now call the Higgs boson.

For many years, scientists searched for this particle. In 2012, they 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.

Introduction

Standard Model

Physicists explain the tiny parts and forces of the universe using the Standard Model. This idea is based on quantum field theory and 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 help 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 puzzle for scientists.

Symmetry breaking

In the late 1950s, a scientist named Yoichiro Nambu had an 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

The Higgs boson helps explain how tiny parts of the universe get their weight. Scientists were lucky that they could study it with the tools they had. This helped them learn more about a big idea called the Higgs field theory.

The Higgs boson shows that the rules of particle physics, called the Standard Model, work for some particles. As scientists learn more about it, they might find new ideas that go beyond these rules.

When the universe was very young and hot, the Higgs field helped change it so that atoms and stars could form. Without this, the universe would look very different today.

The Higgs field gives weight to some small parts, like quarks and some force carriers. But most of the weight of bigger parts, like protons and neutrons, comes from other places.

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

Some ideas think the Higgs field might explain why the universe grew very fast after the Big Bang. But these ideas are still being studied.

There are ideas that the universe might not be completely stable. If certain measurements of the Higgs boson and another part called the top quark show special numbers, it could mean the universe might change far in the future. More study is needed to understand this.

The Higgs field might also be a source of energy in empty space. But why this energy is so small is still a mystery to scientists.

History

Theorisation

Particle physicists study matter made from tiny parts called particles. These particles interact with each other. In the 1960s, many of these particles were found or suggested in theories. Some theories described these particles as fields.

But there was a problem. Some theories said that certain particles should have no mass, but we know they do. A idea called Goldstone's theorem also made it hard to solve this problem.

The Higgs mechanism is a way for particles to get mass without breaking important rules. This idea was first published by Yoichiro Nambu in 1960. In 1964, three groups of scientists worked on this idea at the same time: François Englert and Robert Brout, Peter Higgs, and Gerald Guralnik, Carl Hagen, and Tom Kibble.

Nobel Prize Laureate Peter Higgs in Stockholm, December 2013

Experimental search

To find Higgs bosons, scientists crash two beams of particles together at very high speeds inside a particle detector. Sometimes, a Higgs boson is made for a very short time. Because it disappears so fast, detectors cannot see it directly. Instead, they look at the pieces left behind when it breaks apart. If these pieces match what we expect from a Higgs boson, it suggests one was made.

Finding Higgs bosons is very rare and hard. Scientists need to look at data from hundreds of trillions of crashes. They use statistical analysis to make sure what they see is real and not just random chance from 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, scientists at CERN announced they had found something new. Both of their experiments found a particle with a mass of about 125 GeV/c². They were very sure it was real.

Confirmation of existence and status

On 14 March 2013, CERN said they were sure this particle was a Higgs boson. Tests showed it matched the expected properties. It was the first ever discovery of this kind of particle.

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 question in particle physics is 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 giving particles mass while keeping these rules was very hard.

Scientists found a way using something called the Higgs mechanism. This idea says there is a special field everywhere in space, called the Higgs field. This field has a special shape that lets it 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 because of a special field that changes how electromagnetic forces work. Scientists thought 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 breaks apart quickly into other particles.

The Higgs field gives other particles their mass. It has special properties and behaves differently from other fields. When the Higgs field becomes stable, 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. Scientists study its properties to learn more about how particles get their mass and to find new physics.

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 had different names based on the scientists who first talked about it. Some people call it the "God particle" because of a popular science book. Many scientists do not like this name because it sounds too dramatic.

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.

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. 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.

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 field that helps give mass to other particles. This field has four parts.

The Higgs field gives mass to particles. This happens because of how the field works in its lowest energy state. In this state, it gives mass to particles called W and Z bosons. These particles help carry forces in the universe.

The Higgs boson is a tiny piece of this field that scientists can see. It was found because scientists were looking for proof of this field. The Higgs field also gives mass to other particles, like quarks and electrons.