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Lepton Beams

If no initial-state radiation is assumed, an electron (or, in general, a lepton or a neutrino) leaves behind no beam remnant. Also when radiation is included, one would expect to recover a single electron with the full beam energy when the shower initiator is reconstructed. This does not have to happen, e.g. if the initial-state shower is cut off at a non-vanishing scale, such that some of the emission at low $Q^2$ values is not simulated. Further, for purely technical reasons, the distribution of an electron inside an electron, $f_{\mathrm{e}}^{\mathrm{e}}(x,Q^2)$, is cut off at $x = 1 - 10^{-10}$. This means that always, when initial-state radiation is included, a fraction of at least $10^{-10}$ of the beam energy has to be put into one single photon along the beam direction, to represent this not simulated radiation. The physics is here slightly different from the standard beam-remnant concept, but it is handled with the same machinery. Beam remnants can also appear when the electron is resolved with the use of parton distributions, but initial-state radiation is switched off. Conceptually, this is a contradiction, since it is the initial-state radiation that builds up the parton distributions, but sometimes the combination is still useful. Finally, since QED radiation has not yet been included in events with resolved photons inside electrons, also in this case effective beam remnants have to be assigned by the program.

The beam-remnant assignments inside an electron, in either of the cases above, is as follows.

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An $\mathrm{e}^-$ initiator leaves behind a $\gamma$ remnant.
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A $\gamma$ initiator leaves behind an $\mathrm{e}^-$ remnant.
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An $\mathrm{e}^+$ initiator leaves behind an $\mathrm{e}^- + \mathrm{e}^-$ remnant.
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A $\mathrm{q}$ ( $\overline{\mathrm{q}}$) initiator leaves behind a $\overline{\mathrm{q}}+ \mathrm{e}^-$ ( $\mathrm{q}+ \mathrm{e}^-$) remnant.
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A $\mathrm{g}$ initiator leaves behind a $\mathrm{g}+ \mathrm{e}^-$ remnant. One could argue that, in agreement with the treatment of photon beams above, the remnant should be $\mathrm{q}+ \overline{\mathrm{q}}+ \mathrm{e}^-$. The program currently does not allow for three beam-remnant objects, however.


next up previous contents
Next: Primordial Up: Beam Remnants Previous: Photon Beams   Contents
Stephen_Mrenna 2012-10-24