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5ad4eb21 | 1 | <chapter name="Leptoquark Processes"> |
2 | ||
3 | <h2>Leptoquark Processes</h2> | |
4 | ||
5 | Leptoquarks arise in many scenarios, and can have widely different | |
6 | characteristics, with respect to spin, isospin amd flavour. | |
7 | The current implentation in no sense attempts to exhaust these | |
8 | possibilities, but only to encode one of the simplest possibilities, | |
9 | with a single scalar leptoquark, denoted <ei>LQ</ei> and assigned PDG | |
10 | code 42. The leptoquark is assumed to carry specific quark | |
11 | and lepton quantum numbers, by default <ei>u</ei> quark plus electron. | |
12 | These flavour numbers are conserved, i.e. a process such as | |
13 | <ei>u e^- -> LQ -> d nu_e</ei> is not allowed. | |
14 | ||
15 | <p/> | |
16 | Although only one leptoquark is implemented, its flavours may be | |
17 | changed arbitrarily to study the different possibilities. The | |
18 | flavours of the leptoquark are defined by the quark and lepton | |
19 | flavours in the decay mode list. Therefore, to change from the | |
20 | current <ei>u e^-</ei> to <ei>c mu^+</ei>, say, you only need | |
21 | a line | |
22 | <br/><code>pythia.readString("42:0:products = 4 -13");</code> | |
23 | <br/>in your main program, or the equivalent in a command file. | |
24 | The former must always be a quark, while the latter could be a lepton | |
25 | or an antilepton; a charge-conjugate partner is automatically defined | |
26 | by the program. At initialization, the charge is recalculated as a | |
27 | function of the flavours defined; also the leptoquark name is redefined | |
28 | to be of the type <code>LQ_q,l</code>, where actual quark and lepton | |
29 | flavours are displayed. | |
30 | ||
31 | <p/> | |
32 | The leptoquark is likely to be fairly long-lived, in which case it | |
33 | could have time to fragment into a mesonic- or baryonic-type state, which | |
34 | would decay later on. Currently this posibility is not handled; therefore | |
35 | the leptoquark is always assumed to decay before fragmentation. | |
36 | For that reason the leptoquark can also not be put stable. | |
37 | ||
38 | <h3>Production processes</h3> | |
39 | ||
40 | Four production processes have been implemented, which normally would | |
41 | not overlap and therefore could be run together. | |
42 | ||
43 | <flag name="LeptoQuark:all" default="off"> | |
44 | Common switch for the group of lowest-order <ei>LQ</ei> production | |
45 | processes, i.e. the four ones below. | |
46 | </flag> | |
47 | ||
48 | <flag name="LeptoQuark:ql2LQ" default="off"> | |
49 | Scatterings <ei>q l -> LQ</ei>. | |
50 | Code 3201. | |
51 | </flag> | |
52 | ||
53 | <flag name="LeptoQuark:qg2LQl" default="off"> | |
54 | Scatterings <ei>q g -> LQ l</ei>. | |
55 | Code 3202. | |
56 | </flag> | |
57 | ||
58 | <flag name="LeptoQuark:gg2LQLQbar" default="off"> | |
59 | Scatterings <ei>g g -> LQ LQbar</ei>. | |
60 | Code 3203. | |
61 | </flag> | |
62 | ||
63 | <flag name="LeptoQuark:qqbar2LQLQbar" default="off"> | |
64 | Scatterings <ei>q qbar -> LQ LQbar</ei>. | |
65 | Code 3204. | |
66 | </flag> | |
67 | ||
68 | <h3>Parameters</h3> | |
69 | ||
70 | In the above scenario the main free parameters are the leptoquark flavour | |
71 | content, set as already described, and the <ei>LQ</ei> mass, set as usual. | |
72 | In addition there is one further parameter. | |
73 | ||
74 | <parm name="LeptoQuark:kCoup" default="1.0" min="0.0"> | |
75 | multiplicative factor in the <ei>LQ -> q l</ei> squared Yukawa coupling, | |
76 | and thereby in the <ei>LQ</ei> width and the <ei>q l -> LQ</ei> and | |
77 | other cross sections. Specifically, <ei>lambda^2/(4 pi) = k alpha_em</ei>, | |
78 | i.e. it corresponds to the $k$ factor of <ref>Hew88</ref>. | |
79 | </parm> | |
80 | ||
81 | </chapter> | |
82 | ||
83 | <!-- Copyright (C) 2008 Torbjorn Sjostrand --> | |
84 |