Jet (particle physics)

The event of the birth of two t-quarks in the CDF detector . There are 4 jets in this event. Since the t-quark decays into a b-quark and a W-boson , we have 4 quarks: 2 b-quarks and 2 quarks from the decay of one W-boson, the second W-boson decays into a lepton and a neutrino . Illustration according to the Tevatron accelerator .

The hadron jet is formed by several elementary particles flying in the same direction ^[1] in a narrow cone. The physical reason for the formation of a jet is the hadronization of a quark or gluon with high energy (much larger than the mass of a pion ). In nature, hadron jets are formed only artificially, in experiments in high-energy physics .

Content

1 Hadron jets in modern experiments
2 Jetting
3 Jet fragmentation
4 notes
5 Links
6 Computer simulation of jets

Hadron jets in modern experiments

Hadron jets are experimentally studied by analyzing the energy left by charged particles in the calorimeter of a particle detector. Usually, the calorimeter is divided into many small cells, in which the “highlighted” hadron energy is measured, that is, the interaction energy of charged particles or photons with the material of the calorimeter. The cells play the role of individual particles for the jet, and from them it is possible to reconstruct the jet and measure some of its characteristics.

Examples of important experimental techniques needed to study hadron jets:

Jet reconstruction (e.g., a simple cone reconstruction algorithm or k _T algorithm)
Compensation technique for the neutral component of the jet (energy carried away by neutral particles)
Quark flavor tagging (e.g. b-tagging ).

Jetting

Jets are formed in the processes of scattering of elementary particles, where colored objects of partons , quarks or gluons scatter or are born. Typical processes where jets are formed are the annihilation of an electron and a positron into a gamma-quantum / Z-boson state, during the decay of which 2 quarks are formed . Further, quarks are hadronized and form jets. Such events (they are called two-jet events) were first observed in experiments on the SPEAR electron-positron collider in the SLAC laboratory (USA) in 1975 .

The probability of obtaining a certain state with jets during proton scattering can be calculated using perturbative methods of quantum chromodynamics and the parton distribution function in a proton. More precisely, we can calculate the cross section for the production of two quarks, for example, in the tree approximation, then the momenta of the quarks will correspond to the direction of the jets in the event.

\sigma _{ij\rightarrow q_{1}q_{2}}=\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 q_{1}q_{2}}}{d{\hat {t}}}},

{\ displaystyle \ sigma _ {ij \ rightarrow q_ {1} q_ {2}} = \ 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 q_ {1} q_ {2}}} {d {\ hat {t}}}},}

Where $x$ ${\ displaystyle x}$ , $Q^{2}$ ${\ displaystyle Q ^ {2}}$ - Feynman variable (the fraction of the initial proton momentum carried by the parton) and the transmitted momentum in the process, respectively; ${\hat {\sigma }}_{ij\rightarrow q_{1}q_{2}}$ ${\ displaystyle {\ hat {\ sigma}} _ {ij \ rightarrow q_ {1} q_ {2}}}$ - cross section of the process of formation of two quarks $q_{1}$ ${\ displaystyle q_ {1}}$ and $q_{2}$ ${\ displaystyle q_ {2}}$ from the initial partons $i$ ${\ displaystyle i}$ and $j$ ${\ displaystyle j}$ ; $f_{i}^{a}(x,Q^{2})$ ${\ displaystyle f_ {i} ^ {a} (x, Q ^ {2})}$ - parton distribution for parton type $i$ ${\ displaystyle i}$ in a bundle $a$ ${\ displaystyle a}$ .

The top quark , the heaviest known particle, in most cases breaks up into three hadron jets, which are usually directed in different directions ^[2] .

Jet Fragmentation

Due to the hadronization effect, a quark or gluon emanating from the collision point (we will talk about parton later) emits gluons and quark-antiquark pairs. This phenomenon is akin to the inhibitory electromagnetic radiation of a charged particle flying in an electromagnetic field. The chromodynamic field is created both by other particles at the collision point and by particles emitted by the parton itself. A feature of jet formation is the discoloration of the initial parton. Since the initial parton has a color, and the jet should consist of colorless hadrons (or their decay products), it is impossible to construct an isolated mechanism of jet formation without taking into account interaction with other particles in the collision. The mechanism of the formation of a jet of colorless hadrons from several colored partons formed as a result of the evolution of the jet, taking into account color compensation, is called fragmentation of the jet.

Notes

↑ Experiments at hadron colliders (neopr.) . Hadron jets . Elements Date of treatment August 9, 2013. Archived on August 19, 2013.
↑ The detailed structure of hadron jets helps analyze new types of processes

Links

M. Peskin and D. Schroeder, “Introduction to quantum field theory” (Westview, Boulder, CO, 1995 (English) or “RHD”, 2001 (Russian) )
B. Andersson, The Lund Model (Cambridge University Press, 1998 )
Discovery of jets: G. Hanson et al., Confirmation of the jet structure of hadron production during e + e-annihilation , Phys. Rev. Lett. 35: 1609 (1975). (eng.)
String model for jets: B. Andersson et al., “Parton fragmentation and string dynamics” , Phys. Rep. 97 , 31-145 (1983). (eng.)
Algorithms for jet reconstruction: S. D. Ellis, D. E. Souper, “Sequential Combination Algorithm for Jets in Hadron Collisions,” Phys. Rev. D48 , 3160-3166 (1993). (eng.)
Jet quenching effect: M. Juliassi et al., “Jet quenching and radiation energy loss in dense nuclear matter” , in Quark-gluon plasma 3 , ed. R.S. Hwa and K.-N. Wang (World Scientific, Singapore, 2003). (eng.)
Lectures on QCD and jets: G. Sherman, “QCD and Jets” , preprint YITP-SB-04-59 (2004). (eng.)

Computer-aided Modeling

[1] Experiments at hadron colliders (neopr.) . Hadron jets . Elements Date of treatment August 9, 2013. Archived on August 19, 2013.

[2] The detailed structure of hadron jets helps analyze new types of processes