A quark is a member of one of the two families of fundamental particles – the other is leptons – from which all known matter is made. Quarks are the building blocks of hadrons. They come in six flavors (up, down, strange, charm, top, and bottom) and three colors (red, green, and blue).


Quarks take part in interactions through the strong force. They have an electric charge of either +2/3 (up, charm, and top) or -1/3 (down, strange, and bottom) in units where the charge on the proton is 1. A proton is made of two up quarks (each with an electric charge of +2/3) and one down quark (with a charge of -1/3), while a neutron is made of two downs and one up. Mesons consist of one quark and one antiquark.


Although quarks are believed to have existed in a free state in the first fraction of a second after the Big Bang, it is an open question whether they can still do so anywhere in the universe today. One possibility is inside quark stars.


Properties of quarks
generation weak isospin flavor name symbol charge mass
1 +1/2 Iz=+1/2 up u +2/3 1.5–4.0
1 -1/2 Iz=-1/2 down d -1/3 4–8
2 -1/2 S=-1 strange s -1/3 80–130
2 +1/2 C=1 charm c +2/3 1150–1350
3 -1/2 B'=-1 bottom b -1/3 4100–4400
3 +1/2 T=1 top t +2/3 170900±1800


A short history of quarks

Back in the eighteenth century, the Croatian philosopher Roger Boscovich came up with a curiously prescient fundamental theory of matter which described particles not unlike quarks. The modern story of quarks, however, began in 1964, when Murray Gell-Mann and George Zweig suggested that hundreds of the particles known at the time could be explained as combinations of just three fundamental particles. Gell-Mann chose the name "quarks" for these three particles, a nonsense word used by James Joyce in the novel Finnegan's Wake: "Three quarks for Muster Mark!"


In order to make their calculations work, the quarks had to be assigned fractional electrical charges of 2/3 and -1/3. Such charges had never been observed before, so that, initially, quarks were regarded as a mere mathematical contrivance. Subsequent experiments convinced physicists that not only do quarks exist, but there are six of them, not three.


The two lightest quarks are called up and down. The third quark is called strange. It was named after the "strangely" long lifetime of the K particle, the first composite particle found to contain this quark.


The fourth quark type, the charm quark, was named on a whim. It was discovered in 1974 almost simultaneously at both the Stanford Linear Accelerator Center (SLAC) and at Brookhaven National Laboratory. The fifth and sixth quarks were sometimes called truth and beauty in the past, but even physicists thought that was too cute. The bottom quark was first discovered at Fermi National Lab (Fermilab) in 1977, in a composite particle called upsilon. The top quark – the most massive quark – was discovered last, also at Fermilab, in 1995. It had been predicted for a long time but had never been observed successfully until then.