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Home , Bhabha scattering, Feynman slash notation, R (cross section ratio)

Bhabha 1

Feynman diagrams

Annihilation

Scattering

Note: Time moves forward from the left side of the diagram to the right. The arrows are simply markers of particle motion, and are not the same as the arrows conventionally written into Feynman diagrams.

In , Bhabha scattering is the - scattering process:

There are two leading-order Feynman diagrams contributing to this interaction: an annihilation process and a scattering process. The Bhabha scattering rate is used as a luminosity monitor in electron-positron colliders. Bhabha scattering is named after the Indian physicist Homi J. Bhabha.

Differential cross section To leading order, the spin-averaged differential cross section for this process is

where s,t, and u are the Mandelstam variables, is the fine-structure constant, and is the scattering angle. This cross section is calculated neglecting the electron mass relative to the collision energy and including only the contribution from exchange. This is a valid approximation at collision energies small compared to the mass scale of the Z boson, about 91 GeV; for energies not too small compared to this mass, the contribution from Z boson exchange also becomes important. Bhabha scattering 2

Mandelstam variables In this article, the Mandelstam variables are defined by

Where the approximations are for the high-energy (relativistic) limit.

Deriving unpolarized cross section

Matrix elements Both diagrams contribute to the transition matrix element. By letting k and k' represent the four-momentum of the positron, while letting p and p' represent the four-momentum of the electron, and by using Feynman rules one can show the following diagrams give these matrix elements:

Where we use: are the , are the four-component spinors for fermions, while are the four-component spinors for anti-fermions (see Four spinors).

(scattering) (annihilation)

Notice that there is a relative sign difference between the two diagrams.

Square of matrix element To calculate the unpolarized cross section, one must average over the spins of the incoming particles (s and s e- e+ possible values) and sum over the spins of the outgoing particles. That is,

First, calculate : Bhabha scattering 3

= (scattering)

(interference)

(interference)

(annihilation)

Scattering term (t-channel)

Magnitude squared of M

(complex conjugate will flip order)

(move terms that depend on same momentum to be next to each other)

Sum over spins Next, we'd like to sum over spins of all four particles. Let s and s' be the spin of the electron and and r' be the spin of the positron.

(now use Completeness relations)

(now use Trace identities)

Now that is the exact form, in the case of one is usually interested in energy scales that far exceed the electron mass. Neglecting the electron mass yields the simplified form: Bhabha scattering 4

(use the Mandelstam variables in this relativistic limit)

Annihilation term (s-channel) The process for finding the annihilation term is similar to the above. Since the two diagrams are related by symmetry, and the initial and final state particles are the same, it is sufficient to permute the momenta, yielding

(This is proportional to where is the scattering angle in the center-of-mass frame.)

Solution Evaluating the interference term along the same lines and adding the three terms yields the final result

Simplifying steps

Completeness relations The completeness relations for the four-spinors u and v are

where (see ) Bhabha scattering 5

Trace identities Main article: Trace identities To simplify the trace of the Dirac gamma matrices, one must use trace identities. Three used in this article are: 1. The Trace of any product of an odd number of 's is zero 2. 3. Using these two one finds that, for example,

(the two middle terms are zero because of (1))

(use identity (2) for the term on the right)

(now use identity (3) for the term on the left)

Uses Bhabha scattering has been used as a luminosity monitor in a number of e+e- collider physics experiments. The accurate measurement of luminosity is necessary for accurate measurements of cross sections. • Small-angle Bhabha scattering was used to measure the luminosity of the 1993 run of the Stanford Large Detector (SLD), with a relative uncertainty of less than 0.5%.[1] • Electron-positron colliders operating in the region of the low-lying hadronic resonances (about 1 GeV to 10 GeV), such as the Beijing Electron Synchrotron (BES) and the Belle and BaBar "B-factory" experiments, use large-angle Bhabha scattering as a luminosity monitor. To achieve the desired precision at the 0.1% level, the experimental measurements must be compared to a theoretical calculation including next-to-leading-order radiative corrections.[2] The high-precision measurement of the total hadronic cross section at these low energies is a crucial input into the theoretical calculation of the anomalous magnetic dipole moment of the , which is used to constrain and other models of physics beyond the Standard Model.

References

[1] A Study of Small Angle Radiative Bhabha Scattering and Measurement of the Luminosity at SLD (http:/ / adsabs. harvard. edu/ abs/

1995PhDT...... 160W)

[2] http:/ / arxiv. org/ abs/ hep-ph/ 0003268 • Halzen, Francis; Martin, Alan (1984). & Leptons: An Introductory Course in Modern . John Wiley & Sons. ISBN 0-471-88741-2. • Peskin, Michael E.; Schroeder, Daniel V. (1994). An Introduction to . Perseus Publishing. ISBN 0-201-50397-2.

• Bhabha scattering on arxiv.org (http:/ / xstructure. inr. ac. ru/ x-bin/ theme3. py?level=1& index1=270563) Article Sources and Contributors 6 Article Sources and Contributors

Bhabha scattering Source: http://en.wikipedia.org/w/index.php?oldid=541687912 Contributors: Alexhunterlang, Bryan Derksen, Dark Matter Narcosis, Dauto, Difty, Download, Dreamer08, Forthommel, HEL, JabberWok, Maliz, Mmitchell10, Nick Mks, Sbyrnes321, Thiseye, TimothyRias, 30 anonymous edits Image Sources, Licenses and Contributors

Image:Electron-positron-annihilation.svg Source: http://en.wikipedia.org/w/index.php?title=File:Electron-positron-annihilation.svg License: GNU Free Documentation License Contributors: Original uploader was JabberWok at en.wikipedia Image:Electron-positron-scattering.svg Source: http://en.wikipedia.org/w/index.php?title=File:Electron-positron-scattering.svg License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: JabberWok Image:Mandelstam01.png Source: http://en.wikipedia.org/w/index.php?title=File:Mandelstam01.png License: GNU Free Documentation License Contributors: JabberWok Image:Feynman-scattering.svg Source: http://en.wikipedia.org/w/index.php?title=File:Feynman-scattering.svg License: GNU Free Documentation License Contributors: JabberWok Image:Feynman-annihilation.svg Source: http://en.wikipedia.org/w/index.php?title=File:Feynman-annihilation.svg License: GNU Free Documentation License Contributors: JabberWok License

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