What is the Higgs Boson?

| By Gerald Cleaver

What is the Higgs Boson?
The image above shows three virtual views of an event recorded by CERN’s ATLAS detector on 18-Jun-2012. Muon tracks are colored red, electron tracks and clusters in the LAr calorimeter are colored green. The larger inset in the upper right shows a zoom into the tracking detector. The smaller inset lower right shows a zoom into the vertex region, indicating that the 4 leptons originate from the same primary vertex. ATLAS Experiment © 2012 CERN

At a press conference on July 4, 2012, and with 99.99994% confidence (5 sigma), CERN announced the discovery of a particle consistent with that of a Higgs boson (a.k.a. “the God particle”). This is very exciting for elementary particle physicists. It is also getting the attention of press and general public. But what is the Higgs particle, and what is its meaning?

It has been widely reported that the moniker “God particle” was not its originator’s first choice. Still, Leon Lederman, director emeritus of Fermilab and Nobel laureate for neutrino research, did accept the nickname “God particle” because the particle is “so central to the state of physics today, so crucial to our final understanding of the structure of matter, yet so elusive.” “God particle” was quickly accepted by the press and general public because it seemed an appropriate title for a particle theorized to give mass to all elementary matter particles and the force carrying W and Z bosons. Serving this mass-giving function since near the beginning of the universe, a Higgs field(more fundamental than the actual Higgs boson ) must necessarily exist everywhere in the universe and be unchanging. With an omnipresent and immutable field, analogies between the Higgs boson and God naturally developed within the press and the public—“God particle” became deeply rooted. Relatedly, the Higgs boson become an excellent source for theological analogies. (See for example this article.)

Nevertheless, as physicists seek to emphasize, neither the Higgs boson particle nor its field have religious properties. Thus, elementary particle physicists are not fond of the “God particle” appellation. In the opinion of Oliver Buchmueller, of CERN’s CMS group, calling the Higgs boson the “God particle is completely inappropriate. It’s not doing justice to the Higgs and what we think its role in the universe is. It has nothing to do with God“. As Pippa Wells, another CERN scientist expressed, “Calling [it] the God particle … confuses people about what we are trying to do at CERN”. (Source: Reuters)

One alternate name for the Higgs particle that is used within the physics community is the “BEH” particle. “BEH” stands for Brout–Englert–Higgs, three of the six authors of 1964 papers that first proposed a mechanism for giving mass to elementary particles. In addition to Peter Higgs, the five other authors are Robert Brout and Francois Englert, and Tom Kibble, C.R. Hagen, and Gerald Guralnik. The process for giving mass to particles is thus sometimes referred to not just as the Higgs mechanism, but as the Brout–Englert–Higgs–Hagen–Guralnik–Kibble (BEHHGK) mechanism. (Saying all six names a couple of times makes it obvious why we most often only call it the Higgs.)

But issues of naming aside, what is the Higgs and why is it so elusive? According to the Standard Model, the particles that compose matter (the quarks and leptons) are in a category called spin-1/2 particles. The force carrying particles (the photon, the W's, the Z, and the gluons) are spin-1 particles. What the physicists above proposed was the existence of a type of spinless, or spin-0 particle. Not only does the Higgs boson form its own class of particles, it also gives mass to itself and to all the other particles that have mass: to all of the leptons and quarks, and to the W's and Z bosons, but not photons or gluons. This set of relationships is shown in the image below, indicated by the lines connecting the Higgs to these other particles. There are no lines directly connecting the Higgs boson to photons and gluons because the Higgs boson does not interact with these force carrying particles and, thus, photons and gluons remain massless.

But the story of the Higgs particle actually begins with the associated Higgs field, an invisible field (something like a generalization of an electric field) that has a non-zero, constant value everywhere throughout the universe. This Higgs field continuously interacts with all matter particles and the W and Z force carrying particles. Matter and massive force particles are slowed down as they move through the Higgs field, just as are balls rolling through thick mud. The Higgs field is sometimes described as a “cosmic molasses”. Different particles interact with the Higgs field to varying degrees—those interacting more, are slowed down more, those interacting less are slowed down less. Slowing down more equates to acquiring more mass. If not for the Higgs field, all particles would be massless, zipping through the universe at the speed of light. The universe would be without structure—no galaxies, no plants, no life. Without the Higgs field, not even atoms could have formed.

It should be noted, however, that the majority of the mass of protons and neutrons (and thus of atomic mass) does not come from interaction with the Higgs field. Each proton and neutron is composed of three quarks, which do receive their mass from their interaction with the Higgs field. However, the masses of protons and neutrons are much greater than the sum of their constituent quarks and are a result of the additional mass contribution from the binding energies of the “trapped” quarks.

It was theoretically possible for elementary particles to have mass without needing to acquire it through interaction with a Higgs-like field. However, as the standard model of elementary particles developed in the 1950’s and 1960’s, elementary particle theorists realized that if particles had their own innate mass, rather than acquiring it, many beautiful symmetries of particle interaction equations would be broken. To keep the beauty and symmetry in the theory was the essential reason the BEHHGK mechanism was developed, which immediately led to the prediction of Higgs bosons.

When there is enough external energy in a given volume, the Higgs field also produces Higgs bosons. But the Higgs bosons are very unstable and quickly decay. This is the process that enabled the discovery of the Higgs boson at CERN. At CERN, protons are accelerated to high energies via electric fields and directed in circular paths via magnetic fields. The protons then collide and release large amounts of energy. When sufficient energy is released in a collision, the Higgs field can use this energy to produce Higgs bosons. The Higgs bosons quickly decay leaving evidence of their existence through particular combinations of leftover particles that they have decayed into. Among those predicted by the mathematics of the Standard model are the muons and electrons identified by the CERN experimenters. The image at the top shows the identities and paths of particles produced in one of the CERN proton-proton collisions whose results fit with what would be expected from the decay of a Higgs boson.

For a proton-proton collision at the CERN LHC, the above diagrams show both the dominant modes for creation of a Higgs with a mass around 125 GeV, and the two dominant decay channels (modes). The creation mechanism (shown schematically in the left half of each diagram above) involves virtual gluons, the carriers of the strong nuclear force (represented by squiggly purple lines) from the protons. The gluons fuse into a virtual top quark loop (medium blue triangle), which then emits a Higgs boson (squiggly yellow line). The top quark couples more strongly to the Higgs than any of the five other quarks, so the top quark contributes the dominant loop.

The Higgs boson then dominantly decays into either (i) 2 gamma ray photons (the squiggly green lines) via another intermediate virtual top quark loop or a virtual W gauge particle loop (dark blue triangle), or (ii) two Z0 gauge particles (squiggly dark blue lines), which each then decay into a lepton (specifically an electron or a muon)/anti-lepton pair (light blue lines).

The likely discovery of the Higgs boson, and its implied existence of the associated Higgs field, is an amazing success for CERN. Past research and experience at Fermilab and by elementary particle physicists throughout the world also contributed to the discovery. The Higgs boson was the remaining particle in the Standard Model of Particle Physics to be found. With it, the Standard Model is in some sense complete. (Nevertheless, many questions about the Standard Model still remain—many inspired once again by beauty and symmetry. In particular, several numeric values associated with particle masses and interactions could only be experimentally measured, as with the Higgs, and not predicted from the Standard Model.)

With the apparent success of these experiments and seeming confirmation that the physical universe is, indeed, reflected by the complex and beautiful mathematics of the Standard Model, the international physics community is eager to keep delving deeper into the structure of creation. In addition to trying to verify that the 125 GeV particle is, indeed, the Higgs spinless particle and not some more exotic, new particle, CERN physicists are simultaneously seeking to discover an entire new class of particles, resulting from a theorized symmetry called supersymmetry. Discovery of the associated particles, if they exist, will likely take a few more years. For these discoveries we can only wait in anticipation.

Updated July 12, 2012.


About the Author

Gerald Cleaver

Gerald Cleaver is an Associate Professor of Physics at Baylor University. He is a member of the Physics Department's High Energy Physics group and also heads the Early Universe Cosmology and String Theory division of Baylor's Center for Astrophysics, Space Physics, and Engineering Research. Gerald earned his Ph.D. at Caltech in 1993, where he studied under John H. Schwarz, one of the founders of string theory. His research interests focus on elementary particles, fundamental forces, and superstring theory. His hobbies include radio-controlled model aviation, small-boat sailing, and tae kwon do.