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[u/mrichter/AliRoot.git] / PYTHIA8 / pythia8130 / xmldoc / LeptoquarkProcesses.xml
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5ad4eb21 1<chapter name="Leptoquark Processes">
2
3<h2>Leptoquark Processes</h2>
4
5Leptoquarks arise in many scenarios, and can have widely different
6characteristics, with respect to spin, isospin amd flavour.
7The current implentation in no sense attempts to exhaust these
8possibilities, but only to encode one of the simplest possibilities,
9with a single scalar leptoquark, denoted <ei>LQ</ei> and assigned PDG
10code 42. The leptoquark is assumed to carry specific quark
11and lepton quantum numbers, by default <ei>u</ei> quark plus electron.
12These 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/>
16Although only one leptoquark is implemented, its flavours may be
17changed arbitrarily to study the different possibilities. The
18flavours of the leptoquark are defined by the quark and lepton
19flavours in the decay mode list. Therefore, to change from the
20current <ei>u e^-</ei> to <ei>c mu^+</ei>, say, you only need
21a 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.
24The former must always be a quark, while the latter could be a lepton
25or an antilepton; a charge-conjugate partner is automatically defined
26by the program. At initialization, the charge is recalculated as a
27function of the flavours defined; also the leptoquark name is redefined
28to be of the type <code>LQ_q,l</code>, where actual quark and lepton
29flavours are displayed.
30
31<p/>
32The leptoquark is likely to be fairly long-lived, in which case it
33could have time to fragment into a mesonic- or baryonic-type state, which
34would decay later on. Currently this posibility is not handled; therefore
35the leptoquark is always assumed to decay before fragmentation.
36For that reason the leptoquark can also not be put stable.
37
38<h3>Production processes</h3>
39
40Four production processes have been implemented, which normally would
41not overlap and therefore could be run together.
42
43<flag name="LeptoQuark:all" default="off">
44Common switch for the group of lowest-order <ei>LQ</ei> production
45processes, i.e. the four ones below.
46</flag>
47
48<flag name="LeptoQuark:ql2LQ" default="off">
49Scatterings <ei>q l -> LQ</ei>.
50Code 3201.
51</flag>
52
53<flag name="LeptoQuark:qg2LQl" default="off">
54Scatterings <ei>q g -> LQ l</ei>.
55Code 3202.
56</flag>
57
58<flag name="LeptoQuark:gg2LQLQbar" default="off">
59Scatterings <ei>g g -> LQ LQbar</ei>.
60Code 3203.
61</flag>
62
63<flag name="LeptoQuark:qqbar2LQLQbar" default="off">
64Scatterings <ei>q qbar -> LQ LQbar</ei>.
65Code 3204.
66</flag>
67
68<h3>Parameters</h3>
69
70In the above scenario the main free parameters are the leptoquark flavour
71content, set as already described, and the <ei>LQ</ei> mass, set as usual.
72In addition there is one further parameter.
73
74<parm name="LeptoQuark:kCoup" default="1.0" min="0.0">
75multiplicative factor in the <ei>LQ -> q l</ei> squared Yukawa coupling,
76and thereby in the <ei>LQ</ei> width and the <ei>q l -> LQ</ei> and
77other cross sections. Specifically, <ei>lambda^2/(4 pi) = k alpha_em</ei>,
78i.e. it corresponds to the $k$ factor of <ref>Hew88</ref>.
79</parm>
80
81</chapter>
82
83<!-- Copyright (C) 2008 Torbjorn Sjostrand -->
84