1 | <chapter name="Leptoquark Processes"> |
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2 | |
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3 | <h2>Leptoquark Processes</h2> |
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4 | |
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5 | Leptoquarks arise in many scenarios, and can have widely different |
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6 | characteristics, with respect to spin, isospin amd flavour. |
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7 | The current implentation in no sense attempts to exhaust these |
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8 | possibilities, but only to encode one of the simplest possibilities, |
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9 | with a single scalar leptoquark, denoted <ei>LQ</ei> and assigned PDG |
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10 | code 42. The leptoquark is assumed to carry specific quark |
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11 | and lepton quantum numbers, by default <ei>u</ei> quark plus electron. |
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12 | These flavour numbers are conserved, i.e. a process such as |
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13 | <ei>u e^- -> LQ -> d nu_e</ei> is not allowed. |
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14 | |
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15 | <p/> |
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16 | Although only one leptoquark is implemented, its flavours may be |
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17 | changed arbitrarily to study the different possibilities. The |
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18 | flavours of the leptoquark are defined by the quark and lepton |
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19 | flavours in the decay mode list. Therefore, to change from the |
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20 | current <ei>u e^-</ei> to <ei>c mu^+</ei>, say, you only need |
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21 | a line |
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22 | <br/><code>pythia.readString("42:0:products = 4 -13");</code> |
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23 | <br/>in your main program, or the equivalent in a command file. |
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24 | The former must always be a quark, while the latter could be a lepton |
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25 | or an antilepton; a charge-conjugate partner is automatically defined |
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26 | by the program. At initialization, the charge is recalculated as a |
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27 | function of the flavours defined; also the leptoquark name is redefined |
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28 | to be of the type <code>LQ_q,l</code>, where actual quark and lepton |
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29 | flavours are displayed. |
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30 | |
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31 | <p/> |
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32 | The leptoquark is likely to be fairly long-lived, in which case it |
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33 | could have time to fragment into a mesonic- or baryonic-type state, which |
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34 | would decay later on. Currently this posibility is not handled; therefore |
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35 | the leptoquark is always assumed to decay before fragmentation. |
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36 | For that reason the leptoquark can also not be put stable. |
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37 | |
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38 | <h3>Production processes</h3> |
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39 | |
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40 | Four production processes have been implemented, which normally would |
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41 | not overlap and therefore could be run together. |
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42 | |
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43 | <flag name="LeptoQuark:all" default="off"> |
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44 | Common switch for the group of lowest-order <ei>LQ</ei> production |
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45 | processes, i.e. the four ones below. |
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46 | </flag> |
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47 | |
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48 | <flag name="LeptoQuark:ql2LQ" default="off"> |
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49 | Scatterings <ei>q l -> LQ</ei>. |
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50 | Code 3201. |
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51 | </flag> |
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52 | |
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53 | <flag name="LeptoQuark:qg2LQl" default="off"> |
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54 | Scatterings <ei>q g -> LQ l</ei>. |
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55 | Code 3202. |
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56 | </flag> |
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57 | |
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58 | <flag name="LeptoQuark:gg2LQLQbar" default="off"> |
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59 | Scatterings <ei>g g -> LQ LQbar</ei>. |
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60 | Code 3203. |
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61 | </flag> |
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62 | |
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63 | <flag name="LeptoQuark:qqbar2LQLQbar" default="off"> |
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64 | Scatterings <ei>q qbar -> LQ LQbar</ei>. |
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65 | Code 3204. |
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66 | </flag> |
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67 | |
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68 | <h3>Parameters</h3> |
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69 | |
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70 | In the above scenario the main free parameters are the leptoquark flavour |
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71 | content, set as already described, and the <ei>LQ</ei> mass, set as usual. |
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72 | In addition there is one further parameter. |
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73 | |
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74 | <parm name="LeptoQuark:kCoup" default="1.0" min="0.0"> |
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75 | multiplicative factor in the <ei>LQ -> q l</ei> squared Yukawa coupling, |
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76 | and thereby in the <ei>LQ</ei> width and the <ei>q l -> LQ</ei> and |
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77 | other cross sections. Specifically, <ei>lambda^2/(4 pi) = k alpha_em</ei>, |
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78 | i.e. it corresponds to the $k$ factor of <ref>Hew88</ref>. |
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79 | </parm> |
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80 | |
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81 | </chapter> |
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82 | |
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83 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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84 | |
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