A jet is a narrow cone of hadrons and other
particles produced by the hadronization of
a quark or gluon in a particle physics or
heavy ion experiment. Particles carrying a
color charge, such as quarks, cannot exist
in free form because of QCD confinement which
only allows for colorless states. When an
object containing color charge fragments,
each fragment carries away some of the color
charge. In order to obey confinement, these
fragments create other colored objects around
them to form colorless objects. The ensemble
of these objects is called a jet, since the
fragments all tend to travel in the same direction,
forming a narrow "jet" of particles. Jets
are measured in particle detectors and studied
in order to determine the properties of the
original quarks.
In relativistic heavy ion physics, jets are
important because the originating hard scattering
is a natural probe for the QCD matter created
in the collision, and indicate its phase.
When the QCD matter undergoes a phase crossover
into quark gluon plasma, the energy loss in
the medium grows significantly, effectively
quenching (reducing the intensity of) the
outgoing jet.
Example of jet analysis techniques are:
jet reconstruction (e.g., anti-kT algorithm,
kT algorithm, cone algorithm)
jet correlation
flavor tagging (e.g., b-tagging).The Lund
string model is an example of a jet fragmentation
model.
== Jet production ==
Jets are produced in QCD hard scattering processes,
creating high transverse momentum quarks or
gluons, or collectively called partons in
the partonic picture.
The probability of creating a certain set
of jets is described by the jet production
cross section, which is an average of elementary
perturbative QCD quark, antiquark, and gluon
processes, weighted by the parton distribution
functions. For the most frequent jet pair
production process, the two particle scattering,
the jet production cross section in a hadronic
collision is given by
σ
i
j
→
k
=
∑
i
,
j
∫
d
x
1
d
x
2
d
t
^
f
i
1
(
x
1
,
Q
2
)
f
j
2
(
x
2
,
Q
2
)
d
σ
^
i
j
→
k
d
t
^
,
{\displaystyle \sigma _{ij\rightarrow k}=\sum
_{i,j}\int dx_{1}dx_{2}d{\hat {t}}f_{i}^{1}(x_{1},Q^{2})f_{j}^{2}(x_{2},Q^{2}){\frac
{d{\hat {\sigma }}_{ij\rightarrow k}}{d{\hat
{t}}}},}
with
x, Q2: longitudinal momentum fraction and
momentum transfer
σ
^
i
j
→
k
{\displaystyle {\hat {\sigma }}_{ij\rightarrow
k}}
: perturbative QCD cross section for the reaction
ij → k
f
i
a
(
x
,
Q
2
)
{\displaystyle f_{i}^{a}(x,Q^{2})}
: parton distribution function for finding
particle species i in beam a.Elementary cross
sections
σ
^
{\displaystyle {\hat {\sigma }}}
are e.g. calculated to the leading order of
perturbation theory in Peskin & Schroeder
(1995), section 17.4. A review of various
parameterizations of parton distribution functions
and the calculation in the context of Monte
Carlo event generators is discussed in T.
Sjöstrand et al. (2003), section 7.4.1.
== Jet fragmentation ==
Perturbative QCD calculations may have colored
partons in the final state, but only the colorless
hadrons that are ultimately produced are observed
experimentally. Thus, to describe what is
observed in a detector as a result of a given
process, all outgoing colored partons must
first undergo parton showering and then combination
of the produced partons into hadrons. The
terms fragmentation and hadronization are
often used interchangeably in the literature
to describe soft QCD radiation, formation
of hadrons, or both processes together.
As the parton which was produced in a hard
scatter exits the interaction, the strong
coupling constant will increase with its separation.
This increases the probability for QCD radiation,
which is predominantly shallow-angled with
respect to the originating parton. Thus, one
parton will radiate gluons, which will in
turn radiate qq pairs and so on, with each
new parton nearly collinear with its parent.
This can be described by convolving the spinors
with fragmentation functions
P
j
i
(
x
z
,
Q
2
)
{\displaystyle P_{ji}\!\left({\frac {x}{z}},Q^{2}\right)}
, in a similar manner to the evolution of
parton density functions. This is described
by a Dokshitzer-Gribov-Lipatov-Altarelli-Parisi
(DGLAP) type equation
∂
∂
ln
⁡
Q
2
D
i
h
(
x
,
Q
2
)
=
∑
j
∫
x
1
d
z
z
α
S
4
π
P
j
i
(
x
z
,
Q
2
)
D
j
h
(
z
,
Q
2
)
{\displaystyle {\frac {\partial }{\partial
\ln Q^{2}}}D_{i}^{h}(x,Q^{2})=\sum _{j}\int
_{x}^{1}{\frac {dz}{z}}{\frac {\alpha _{S}}{4\pi
}}P_{ji}\!\left({\frac {x}{z}},Q^{2}\right)D_{j}^{h}(z,Q^{2})}
Parton showering produces partons of successively
lower energy, and must therefore exit the
region of validity for perturbative QCD. Phenomenological
models must then be applied to describe the
length of time when showering occurs, and
then the combination of colored partons into
bound states of colorless hadrons, which is
inherently not-perturbative. One example is
the Lund String Model, which is implemented
in many modern event generators
