Jet (particle physics)

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. Because of QCD confinement, particles carrying a color charge, such as quarks, cannot exist in free form. Therefore they fragment into hadrons before they can be directly detected, becoming jets. These jets must be measured in a particle detector and studied in order to determine the properties of the original quark.

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 quenches the outgoing jet.

Example of jet analysis techniques are:

• jet reconstruction (e.g. k_T algorithm, cone algorithm)
• jet correlation
• flavor tagging (e.g. b-tagging).

Example of jet fragmentation models are:

• Lund string 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 to create a certain set of jets are 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

$\sigma _{ij\rightarrow k}=\sum _{i,j}\int dx_{1}dx_{2}d{\hat {t}}f_{i}^{1}(x_{1},Q^{2})f_{j}^{2}(x_{2},Q^{2}){\frac {d{\hat {\sigma }}_{ij\rightarrow k}}{d{\hat {t}}}},$

with

• x, Q²: longitudinal momentum fraction and momentum transfer
• ${\hat {\sigma }}_{ij\rightarrow k}$: perturbative QCD cross section for the reaction ij → k
• $f_{i}^{a}(x,Q^{2})$: parton distribution function for finding particle species i in beam a.

Elementary cross sections ${\hat {\sigma }}$ are e.g. calculated to the leading order of perturbation theory in Peskin & Schroeder (1995), section 17.4. A review of various parametrizations 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

Because of QCD confinement, the outgoing gluon or quark from the jet production will hadronize into colorless states.

Parton fragmentation functions observe Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) type evolution:

${\frac {\partial }{\partial \ln Q^{2}}}D_{i}^{h}(x,Q^{2})=\sum _{j}\int _{x}^{1}{\frac {dz}{z}}{\frac {\alpha _{S}}{4\pi }}P_{ji}\!\left({\frac {x}{z}},Q^{2}\right)D_{i}^{h}(z,Q^{2})$

References

• B. Andersson et al., "Parton Fragmentation and String Dynamics", Phys. Rep. 97(2–3), 31–145 (1983).
• S. D. Ellis, D. E. Soper, "Successive Combination Jet Algorithm For Hadron Collisions", Phys. Rev. D48, 3160–3166 (1993).
• M. Gyulassy et al., "Jet Quenching and Radiative Energy Loss in Dense Nuclear Matter", in R.C. Hwa & X.-N. Wang (eds.), Quark Gluon Plasma 3 (World Scientific, Singapore, 2003).
• J. E. Huth et al., in E. L. Berger (ed.), Proceedings of Research Directions For The Decade: Snowmass 1990, (World Scientific, Singapore, 1992), 134. (Preprint at Fermilab Library Server)
• M. E. Peskin, D. V. Schroeder, "An Introduction to Quantum Field Theory" (Westview, Boulder, CO, 1995).
• T. Sjöstrand et al., "Pythia 6.3 Physics and Manual", Report LU TP 03-38 (2003).
• G. Sterman, "QCD and Jets", Report YITP-SB-04-59 (2004).

External links

• The Pythia/Jetset Monte Carlo event generator