1 <chapter name="R-hadrons">
5 When a coloured SUSY particle is longer-lived than typical
6 hadronization scales, i.e. around c*tau > 1 fm, or equivalently
7 width Gamma < 0.2 GeV, it will have time to hadronize into a colour
8 singlet hadronic state, a R-hadron. Currently a set of such
9 R-hadrons have been implemented for the case of a long-lived
10 gluino, stop or sbottom. Needless to say, the normal case would be
11 that only one of them will be long-lived enough to form R-hadrons.
14 For simplicity all gluino-mesons are assumed to have light-flavour
15 spin 1, since those are the lightest and favoured by spin-state
16 counting. Further, all gluino-baryons are bookkept as having
17 light-flavour spin 3/2, and flavours are listed in descending order.
18 This is more for convenience of notation, however, since the normal
19 baryon octet e.g. has no uuu = "p++" state. When a diquark is
20 extracted, a mixture of spin 0 and spin 1 is allowed. Names and codes
21 are essentially in agreement with the PDG conventions, e.g.
22 <br/>1000993 <code>R0(~g g)</code> (or gluinoball)
23 <br/>1009213 <code>R+(~g u dbar)</code> (or gluino-rho+)
24 <br/>1092214 <code>R+(~g uud)</code> (or gluino-Delta+)
25 <br/>For internal bookkeeping of momenta, the code 1009002,
26 <code>Rtemp(~g q)</code>, is used to denote the intermediate
27 state formed when only one of the two string pieces attached to
28 the gluino has broken.
31 For the stop- and sbottom-hadrons the spin counting is simpler,
32 since it is entirely given by the constituent quark or diquark spin.
33 Again names and codes follow PDG conventions, e.g.
34 <br/>1000612 <code>R+(~t dbar)</code>
35 <br/>1006211 <code>R+(~t ud0)</code>
38 The spin and electromagnetic charge of the new particle plays only
39 a minor role in the hadronization process, that can be neglected
40 to first approximation. Therefore it is possible to use the same
41 R-hadrons framework instead for other BSM scenarios with long-lived
42 coloured particles, e.g. with massive extra-dimensions copies
43 of gluons and quarks, or with leptoquarks. This can be regulated by
44 the switches below. Note that the codes and names of the R-hadrons
45 is not changed when the heavy particle involved is switched, for
46 reasons of administrative simplicity. R-hadron mass spectra and
47 other relevant particle data is automatically updated to reflect
50 <flag name="RHadrons:allow" default="off">
51 Allows the gluino, stop and sbottom to hadronize if their respective
52 widths are below the limit <code>RHadrons:maxWidth</code>.
55 <parm name="RHadrons:maxWidth" default="0.2" min="0.0" max="1.0">
56 The maximum width of the gluino for which it is possible to form
57 R-hadrons, provided that <code>RHadrons:allow</code> is on.
60 <mode name="RHadrons:idGluino" default="1000021">
61 The gluino identity code. For other scenarios than SUSY this code
62 could be changed to represent another long-lived uncharged colour
63 octet particle, that then would be treated in the same spirit.
64 Could be set to 0 to forbid any gluino R-hadron formation even when
65 the above two criteria, <code>RHadrons:allow</code>
66 and <code>RHadrons:maxWidth</code>, are met.
69 <mode name="RHadrons:idStop" default="1000006">
70 The lightest stop identity code. For other scenarios than SUSY this
71 code could be changed to represent another long-lived charge 2/3
72 colour triplet particle, that then would be treated in the same
73 spirit. As above it could be set to 0 to forbid any stop R-hadron
77 <mode name="RHadrons:idSbottom" default="1000005">
78 The lightest sbottom identity code. For other scenarios than SUSY this
79 code could be changed to represent another long-lived charge -1/3
80 colour triplet particle, that then would be treated in the same
81 spirit. As above it could be set to 0 to forbid any sbottom R-hadron
85 <flag name="RHadrons:allowDecay" default="on">
86 Allows the R-hadrons to decay or not. If the gluino/stop/sbottom is
87 stable or too long-lived to decay inside the detector this switch
88 has no real function, since then no decays will be performed anyway.
89 If the sparticle is so short-lived that it decays before reaching
90 the beam pipe then having the decay on is the logical choice.
91 So the interesting region is when the decays happens after the
92 R-hadron has passed through part of the detector, and changed its
93 momentum and quite possibly its flavour content before it is to
94 decay. Then normal decays should be switched off, and the R-hadron
95 tracked through matter by a program like GEANT
96 <ref>Kra04,Mac07</ref>. After that, the new R-hadron info can be
97 overwritten into the event record and the
98 <code>Pythia::forceRHadronDecay()</code> method can be called
99 to force this modified R-hadron to decay.
102 <flag name="RHadrons:setMasses" default="on">
103 Use simple mass formulae to construct all available R-hadron masses
104 based on the currently initialized gluino/squark masses and the
105 constituent masses of the other partons in the hadron. If you switch
106 this off, it is your responsibility to set each of the R-hadron masses
107 on your own, and set them in an internally consistent way. If you
108 mess up on this you may generate accordingly crazy results.
109 Specifically, it is to be assumed that none of the R-hadrons has a
110 mass below its constituent sparticle, i.e. that the light degrees
111 of freedom and the additional confinement gluon field gives a net
112 positive contribution to the R-hadron mass.
115 <parm name="RHadrons:probGluinoball" default="0.1" min="0.0" max="1.0">
116 The fraction of produced gluino R-hadrons that are contain a "valence"
117 gluon, with the rest containing a meson or baryon quark flavour content.
120 <parm name="RHadrons:mOffsetCloud" default="0.2" min="0.0">
121 Extra mass (in GeV) added to each of the one or two extra constituent
122 masses in an R-hadron, to calculate the mass of a R-hadron. The same
123 offset is also used when the R-hadron momentum and mass is split
124 between the squark or gluino and the one or two light (di)quarks,
125 one for a squark and two for a gluino. Thus once or twice this amount
126 represents a part of the nominal squark or gluino mass that will not
127 decay weakly, since it is taken to correspond to the cloud of gluons
128 that surround the squark or gluino.
131 <parm name="RHadrons:mCollapse" default="1.0" min="0.0">
132 A colour singlet system with an invariant mass less than this amount,
133 above the R-hadron mass with the given flavour content, is assumed to
134 collapse to this single R-hadron, whereas a full fragmentation handling
135 is applied above this mass.
138 <parm name="RHadrons:diquarkSpin1" default="0.5" min="0.0" max="1.0">
139 Probability that a diquark extracted from the flavour code of a gluino
140 R-hadron should be assigned spin 1, with the rest being spin 0. Does
141 not apply for two identical quarks, where spin 1 is only possibility.
142 Note that gluino R-hadron codes for simplicity are assigned as if spin
143 is 1 always, and so give no guidance. For stop and sbottom the diquark
144 spin is preserved in the particle code, so there is no corresponding
150 <!-- Copyright (C) 2013 Torbjorn Sjostrand -->