In particle physics, a three-jet event is
an event with many particles in final state
that appear to be clustered in three jets.
A single jet consists of particles that fly
off in roughly the same direction.
One can draw three cones from the interaction
point, corresponding to the jets, and most
particles created in the reaction will appear
to belong to one of these cones.
These events are currently the most direct
available evidence for the existence of gluons,
and were first observed by the TASSO experiment
at the PETRA accelerator at the DESY laboratory.Since
jets are ordinarily produced when quarks hadronize,
and quarks are produced only in pairs, an
additional particle is required to explain
events containing an odd number of jets.
Quantum chromodynamics indicates that this
particle is a particularly energetic gluon,
radiated by one of the quarks, which hadronizes
much as a quark does.
A particularly striking feature of these events,
which were first observed at DESY and studied
in great detail by experiments at the LEP
collider, is their consistency with the Lund
string model.
The model indicates that "strings" of low-energy
gluons will form most strongly between the
quarks and the high-energy gluons, and that
the "breaking" of these strings into new quark–antiquark
pairs (part of the hadronization process)
will result in some "stray" hadrons between
the jets (and in the same plane).
Since the quark-gluon interaction is stronger
than the quark-quark interaction, such hadrons
will be observed much less frequently between
the two quark jets.
As a result, the model predicts that stray
hadrons will not appear between two of the
jets, but will appear between each of them
and the third.
This is precisely what is observed.
As a check, physicists have also considered
events with a photon produced in a similar
process.
In this case, the quark–quark interaction
is the only strong interaction, so a "string"
forms between the two quarks, and stray hadrons
now appear between the corresponding jets.
This difference between the three-jet events
and the two-jet events with a high-energy
photon, which indicates that the third jet
has unique properties under the strong interaction,
can only be explained by the original particle
in that jet being a gluon.
The line of reasoning is illustrated below.
The drawings are not Feynman diagrams; they
are "snapshots" in time and show two spatial
dimensions.
== Ellis–Karliner angle ==
The Ellis–Karliner angle is the kinematic
angle between the highest energy jets in a
three-jet event.
The angle is not measured in the lab frame,
but in a frame boosted along the energy of
the highest energy jet so that the second
and third jets are back-to-back.
By measuring the distribution of the Ellis–Karliner
angle at the PETRA electron–positron storage
ring at DESY, physicists determined that the
gluon has spin one rather than spin zero or
spin two.
Subsequent experiments at the LEP storage
ring at CERN confirmed this result
