The standard model of particle physics and the search for the Higgs boson

Image created from tracking a lead-lead collision in the Large Hadron Collider.

Image credit: Pcharito/CC-SA.

First published on 12th June 2011. Last updated 11 August 2018 by Dr Helen Klus

In April 2011, Rolf-Dieter Heuer, Director General of CERN (the European Organisation for Nuclear Research), announced that if they've not discovered the Higgs boson by the end of 2012, then physicists should give up on finding it and reconsider the standard model of particle physics[1a].

The standard model was developed in the early 1970s in order to explain how all known particles interact. It divides elementary particles into fermions, which can combine to form atoms, and bosons, which carry forces[2].

Fermions are further divided into quarks, which can form protons and neutrons[3][4][5], and leptons, which include electrons and neutrinos[6].

Components of the standard model were theorised using quantum theories of fields. This concept, which combines quantum mechanics and special relativity, was first developed by British physicist Paul Dirac in 1927[7].

The quantum field theory of electromagnetism, known as quantum electrodynamics (QED), was developed in the 1940s[8]. QED explains how the electromagnetic force holds leptons, like electrons, to the nuclei of atoms.

Electroweak theory is a quantum field theory of the weak nuclear force, the force carried by +W, -W and Z bosons. Electroweak theory was developed in the 1960s, and explains radioactive decay[9][10a][11a].

The standard model was completed in 1973, with the development of quantum chromodynamics[12][13][14]. This is a quantum field theory of the strong nuclear force, the force carried by gluons, and explains how quarks stick together in order to form particles, like protons and neutrons.

Diagram showing the standard model of particle physics. Matter is composed of leptons and quarks. There are six quarks and six leptons. There are also four gauge bosons plus the Higgs boson.

The standard model of particle physics. Image credit: MissMJ/CC-A.


Quarks are fermions. There are six types, or flavours, of quarks, known as: up, down, charm, strange, top, and bottom, and every quark has an antimatter partner with an opposite spin and charge. The up and down quarks are the lightest, and this means they're the most stable. The top and bottom quarks are the most massive, they can only be created in high-energy collisions like those produced by particle accelerators, and soon decay into up and down quarks.

Quarks are never found in isolation, but combine to form particles called hadrons, which are held together by the strong nuclear force. Hadrons are split into two groups, baryons, which are made of three quarks, and mesons, which are made of one quark and one antiquark.

All hadrons are unstable except for protons and neutrons when they are inside atomic nuclei. A proton is composed of two up quarks and one down quark, which add up to have a charge of +1, and a neutron is made of one up quark and two down quarks, which have no overall charge.


Leptons are fermions. There are six flavours of leptons: electrons, electron neutrinos, muons, muon neutrinos, taus, and tau neutrinos. These all have antimatter partners. The electron, muon, and tau leptons are negatively charged, and have positively charged antimatter partners. The neutral leptons are known as neutrinos.

While the charged leptons can interact with hadrons via the electromagnetic force, neutrinos rarely interact with anything. As with quarks, the heavier muon and tau leptons can only be created in high-energy collisions, and soon decay. Electrons are the most stable leptons, and attach to atomic nuclei, neutralising atoms.


At least six bosons are needed in order to explain how fermions interact.

Bosons are particles that 'carry' force: the photon carries the electromagnetic force, the gluon carries the strong force, and the +W, -W, and Z bosons carry the weak force.

There are at least two remaining bosons that have yet to be observed, these are the Higgs boson, which allows the W and Z bosons to have mass, and the graviton, which is thought to carry the force of gravity.

The graviton is not part of the standard model but is predicted by theories of quantum gravity, which are needed to explain how quantum mechanics can be reconciled with general relativity.

By the time the standard model was formed, three types of quarks, the up, down, and strange quarks had already been discovered using particle accelerators[15][16], and all but two of the leptons had been discovered[17][18][19][20]. Since then, every elementary particle predicted by the standard model has been verified[21][22][23][24][25], except for the Higgs boson, which is thought explain why weak bosons - W and Z particles - have mass[10b][11b]. It's thought to 'carry' mass in a similar way to how the other bosons carry forces.

The Higgs boson is only produced in high-energy collisions, and physicists are currently using the two highest energy particle accelerators in the world to look for evidence of it. These are the Tevatron particle accelerator in Illinois and the Large Hadron Collider (LHC), which is run by CERN and situated beneath the Franco-Swiss border. The LHC produces the most energy, and the Tevatron is due to shut down in September as it has been made obsolete[26].

The standard model predicts that collisions in the LHC should produce a Higgs boson every few hours. At this rate, it should take two to three years to collect enough data to guarantee that one is detected, and another year to analyse the results[1b]. The LHC has been running successfully since late 2009, and this means that if the Higgs boson exists, then it should be discovered by the end of 2012. After this, the LHC is to be shut down so that it can be upgraded[27].

If the Higgs Boson is not found by 2013, then physicists will be faced with the problem of explaining how some particles acquire mass. However, this is not the only fundamental question that remains unanswered.

The discovery of the Higgs boson would not show whether the 12 elementary fermions are truly fundamental or if they can be further divided into smaller objects[28]. It would not provide a quantum field theory of gravity[29] and perhaps most importantly, it would not explain the origin of dark matter or dark energy, leaving 95% of the universe unaccounted for[30].

The discovery of the Higgs boson would show that we are on the right path, but the failure to find it might be even more exciting as this could lead to the discovery of completely new laws of physics.

UPDATE: The Higgs boson was discovered by physicists at CERN in July 2012.


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