The Standard Model

The Standard Model of Particle Physics describes the fundamental particles of matter and the forces that govern them.

The Standard Model

The Standard Model is the current best theory that describes the interactions betwen fundamental particles. Originally formalised in the mid 70's, it has been the foundation of Particle Physics for over 30 years. It describes both the particles of matter - the Quarks and Leptons - and the particles responsible for the forces - the Bosons.

Particle Glossary


The leptons are matter particles. They do not feel the strong force. There are six of them, in three generations. Each generation contains one negatively charged particle together with its partner neutrino.


Electrons are fundamental particles, meaning they can not be divided into smaller parts. Electrons are negatively charged particles. In ATLAS they are detected in the Inner Detector and the Electromagnetic Calorimeter.


Muons are like electrons - they too have a negative charge, but they are much heavier. Because they are heavier they cannot be stopped by the calorimeters. This makes muons the only particles detected by the outer layer of ATLAS - the Muon Spectrometer.


Tau particles have negative charge. They are similar to the electron and muon, only heavier still. In fact, Taus are unstable and decay before we get chance to detect them in ATLAS. Instead, we indirectly see Tau particles by looking for their decay products, either lighter leptons, or pions.

Electron Neutrino

Electron neutrinos are the chargeless partners to the electron. Neutrinos are very light, and don't interact with the detector, so escape unseen.

Muon Neutrino

Muon neutrinos are the chargeless partners to the muons. Within ATLAS, it is impossible to distinguish one type of neutrino from another type, without looking at the rest of the event.

Tau Neutrino

Tau neutrinos are the chargeless partners to the taus. In ATLAS, neutrinos are not detected, but their presence can be inferred by looking at the amount of energy expected in an event. If there is some energy missing, a neutrino may have carried it.


There are six quarks, which come in three "generations". Each generation has one quark with charge +2/3 and one with charge -1/3.

Quarks are not seen in isolation in ATLAS – instead they share their energy among new particles and form jets of many composite particles.


The up quark has charge +2/3. It is in the first and lightest generation. Two up quarks and a down together make a proton, the simplest atomic nucleus.


The charm quark has charge +2/3. It is in the second generation. Together with “strange” forms the second pair of quarks, which are like "down" and "up", but heavier.


The top quark has charge +2/3. It is in the third and heaviest generation. It is the heaviest quark of all. This is because it interacts most with the Higgs field.


The down quark has charge -1/3. It is in the first generation. The combination of two down-quarks and an up-quark makes a neutron.


The strange quark has charge -1/3. It is in the second generation. It was proposed to explain why some particles have "strangely" long lifetimes.


The bottom quark has charge -1/3. It is in the third generation. It is sometimes called “beauty”. Measurements of this heavy quark help show how matter differs from anti-matter.


The Bosons are exchanged particles which transmit forces between the matter particles.


The gluon transmits the “strong” nuclear force between quarks. It is this force which is responsible for holding the nucleus together. Like quarks, gluons are not seen in isolation, but instead form jets of strongly interacting particles.


The heavy W boson transmits the “weak” nuclear force. It controls the amount of energy given out by the sun. W bosons can be most easily recognised when they decay into an electron (or a muon) together with a neutrino.


The Z boson can be most easily recognised when it decays into an electron and its anti-particle, or into a muon and its anti-particle.


The photon is the particle which transmits electromagnetism and light. Since it has no charge it leaves no track, but it can be spotted since it leaves an energy deposit in the electromagnetic calorimeter.

Higgs boson

The Higgs boson is a neutral particle. It rapidly decays into other lighter particles. A Higgs boson decays most often into a bottom quark together with its anti-particle, or into two W bosons, or into two Z bosons.


An anti-particle is a partner particle with the same mass as its corresponding particle but with the opposite charge.

Composite particles

Quarks cannot exist in isolation - instead they gather themselves into groups, known as Hadrons. Some examples are listed below.


A proton has positive charge. It is made out of two up quarks and one down quark.

Combinations of protons and neutron bind together to make up the nucleii of all atoms.


A neutron has no charge. It is made out of two down quarks and one up quark.


There are three types of pion. They have charges +1, 0 and -1 respectively.

The positive pion is made of the antiparticle of the down quark together with an up quark.

The negative pion is made of the antiparticle of the up quark together with a down quark.

The neutral pion is made of a mixture of the up and down quarks together with their anti-particles


A "jet" is the name for a large number of particles which are all travelling in roughly the same direction.

Jets are appear in ATLAS as many tracks in the Inner Detector, together with energy deposits in the Electromagnetic Calorimeter and in the Hadronic Calorimeter