The up quark or u quark (symbol: u) is the
lightest of all quarks, a type of elementary
particle, and a major constituent of matter.
It, along with the down quark, forms the neutrons
(one up quark, two down quarks) and protons
(two up quarks, one down quark) of atomic
nuclei.
It is part of the first generation of matter,
has an electric charge of +2/3 e and a bare
mass of 2.3+0.7−0.5 MeV/c2..
Like all quarks, the up quark is an elementary
fermion with spin 1/2, and experiences all
four fundamental interactions: gravitation,
electromagnetism, weak interactions, and strong
interactions.
The antiparticle of the up quark is the up
antiquark (sometimes called antiup quark or
simply antiup), which differs from it only
in that some of its properties, such as charge
have equal magnitude but opposite sign.
Its existence (along with that of the down
and strange quarks) was postulated in 1964
by Murray Gell-Mann and George Zweig to explain
the Eightfold Way classification scheme of
hadrons.
The up quark was first observed by experiments
at the Stanford Linear Accelerator Center
in 1968.
== History ==
In the beginnings of particle physics (first
half of the 20th century), hadrons such as
protons, neutrons and pions were thought to
be elementary particles.
However, as new hadrons were discovered, the
'particle zoo' grew from a few particles in
the early 1930s and 1940s to several dozens
of them in the 1950s.
The relationships between each of them were
unclear until 1961, when Murray Gell-Mann
and Yuval Ne'eman (independently of each other)
proposed a hadron classification scheme called
the Eightfold Way, or in more technical terms,
SU(3) flavor symmetry.
This classification scheme organized the hadrons
into isospin multiplets, but the physical
basis behind it was still unclear.
In 1964, Gell-Mann and George Zweig (independently
of each other) proposed the quark model, then
consisting only of up, down, and strange quarks.
However, while the quark model explained the
Eightfold Way, no direct evidence of the existence
of quarks was found until 1968 at the Stanford
Linear Accelerator Center.
Deep inelastic scattering experiments indicated
that protons had substructure, and that protons
made of three more-fundamental particles explained
the data (thus confirming the quark model).At
first people were reluctant to describe the
three bodies as quarks, instead preferring
Richard Feynman's parton description, but
over time the quark theory became accepted
(see November Revolution).
== Mass ==
Despite being extremely common, the bare mass
of the up quark is not well determined, but
probably lies between 1.8 and 3.0 MeV/c2.
Lattice QCD calculations give a more precise
value: 2.01±0.14 MeV/c2.When found in mesons
(particles made of one quark and one antiquark)
or baryons (particles made of three quarks),
the 'effective mass' (or 'dressed' mass) of
quarks becomes greater because of the binding
energy caused by the gluon field between each
quark (see mass–energy equivalence).
The bare mass of up quarks is so light, it
cannot be straightforwardly calculated because
relativistic effects have to be taken into
account.
Due to strong force mediated by gluons in
the gluon field, the quarks move at roughly
99.995% of the speed of light, leading to
Lorentz factor of roughly 100.
As a result, the combined rest mass of quarks
is barely 1% of proton or neutron mass.
== 
See also ==
Down quark
Isospin
Quark model
Quantum Mechanics
