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.
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.
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