source: HiSusy/trunk/Pythia8/pythia8170/xmldoc/RHadrons.xml @ 1

Last change on this file since 1 was 1, checked in by zerwas, 11 years ago

first import of structure, PYTHIA8 and DELPHES

File size: 7.0 KB
Line 
1<chapter name="R-hadrons">
2
3<h2>R-hadrons</h2>
4
5When a coloured SUSY particle is longer-lived than typical
6hadronization scales, i.e. around c*tau > 1 fm, or equivalently
7width Gamma < 0.2 GeV, it will have time to hadronize into a colour 
8singlet hadronic state, a R-hadron. Currently a set of such 
9R-hadrons have been implemented for the case of a long-lived 
10gluino, stop or sbottom. Needless to say, the normal case would be 
11that only one of them will be long-lived enough to form R-hadrons.
12
13<p/>
14For simplicity all gluino-mesons are assumed to have light-flavour
15spin 1, since those are the lightest and favoured by spin-state
16counting. Further, all gluino-baryons are bookkept as having
17light-flavour spin 3/2, and flavours are listed in descending order.
18This is more for convenience of notation, however, since the normal
19baryon octet e.g. has no uuu = "p++" state. When a diquark is
20extracted, a mixture of spin 0 and spin 1 is allowed. Names and codes
21are 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
27state formed when only one of the two string peices attached to
28the gluino has broken.
29
30<p/>
31For the stop- and sbottom-hadrons the spin counting is simpler,
32since it is entirely given by the constituent quark or diquark spin.
33Again names and codes follow PDG conventions, e.g.
34<br/>1000612 <code>R+(~t dbar)</code> 
35<br/>1006211 <code>R+(~t ud0)</code> 
36
37<p/>
38The spin and electromagnetic charge of the new particle plays only
39a minor role in the hadronization process, that can be neglected
40to first approximation. Therefore it is possible to use the same
41R-hadrons framework instead for other BSM scenarios with long-lived
42coloured particles, e.g. with massive extra-dimensions copies
43of gluons and quarks, or with leptoquarks. This can be regulated by
44the switches below. Note that the codes and names of the R-hadrons
45is not changed when the heavy particle involved is switched, for
46reasons of administrative simplicity. R-hadron mass spectra and
47other relevant particle data is automatically updated to reflect
48the change, however.
49
50<flag name="RHadrons:allow" default="off">
51Allows the gluino, stop and sbottom to hadronize if their respective
52widths are below the limit <code>RHadrons:maxWidth</code>.
53</flag>
54
55<parm name="RHadrons:maxWidth" default="0.2" min="0.0" max="1.0">
56The maximum width of the gluino for which it is possible to form
57R-hadrons, provided that <code>RHadrons:allow</code> is on.
58</parm>
59
60<mode name="RHadrons:idGluino" default="1000021">
61The gluino identity code. For other scenarios than SUSY this code
62could be changed to represent another long-lived uncharged colour
63octet particle, that then would be treated in the same spirit.
64Could be set to 0 to forbid any gluino R-hadron formation even when
65the above two criteria, <code>RHadrons:allow</code> 
66and <code>RHadrons:maxWidth</code>, are met.
67</flag>
68
69<mode name="RHadrons:idStop" default="1000006">
70The lightest stop identity code. For other scenarios than SUSY this
71code could be changed to represent another long-lived charge 2/3
72colour triplet particle, that then would be treated in the same
73spirit. As above it could be set to 0 to forbid any stop R-hadron
74formation.
75</flag>
76
77<mode name="RHadrons:idSbottom" default="1000005">
78The lightest sbottom identity code. For other scenarios than SUSY this
79code could be changed to represent another long-lived charge -1/3
80colour triplet particle, that then would be treated in the same
81spirit. As above it could be set to 0 to forbid any sbottom R-hadron
82formation.
83</flag>
84
85<flag name="RHadrons:allowDecay" default="on">
86Allows the R-hadrons to decay or not. If the gluino/stop/sbottom is
87stable or too long-lived to decay inside the detector this switch
88has no real function, since then no decays will be performed anyway.
89If the sparticle is so short-lived that it decays before reaching
90the beam pipe then having the decay on is the logical choice.
91So the interesting region is when the decays happens after the
92R-hadron has passed through part of the detector, and changed its
93momentum and quite possibly its flavour content before it is to
94decay. Then normal decays should be switched off, and the R-hadron
95tracked through matter by a program like GEANT
96<ref>Kra04,Mac07</ref>. After that, the new R-hadron info can be
97overwritten into the event record and the
98<code>Pythia::forceRHadronDecay()</code> method can be called
99to force this modified R-hadron to decay.
100</flag>
101
102<flag name="RHadrons:setMasses" default="on">
103Use simple mass formulae to construct all available R-hadron masses
104based on the currently initialized gluino/squark masses and the
105constituent masses of the other partons in the hadron. If you switch
106this off, it is your responsibility to set each of the R-hadron masses
107on your own, and set them in an internally consistent way. If you
108mess up on this you may generate accordingly crazy results.
109Specifically, it is to be assumed that none of the R-hadrons has a
110mass below its constituent sparticle, i.e. that the light degrees
111of freedom and the additional confinement gluon field gives a net
112positive contribution to the R-hadron mass.
113</flag>
114
115<parm name="RHadrons:probGluinoball" default="0.1" min="0.0" max="1.0">
116The fraction of produced gluino R-hadrons that are contain a "valence"
117gluon, with the rest containing a meson or baryon quark flavour content.
118</parm>
119
120<parm name="RHadrons:mOffsetCloud" default="0.2" min="0.0">
121Extra mass (in GeV) added to each of the one or two extra constituent
122masses in an R-hadron, to calculate the mass of a R-hadron. The same
123offset is also used when the R-hadron momentum and mass is split
124between the squark or gluino and the one or two light (di)quarks,
125one for a squark and two for a gluino. Thus once or twice this amount
126represents a part of the nominal squark or gluino mass that will not
127decay weakly, since it is taken to correspond to the cloud of gluons
128that surround the squark or gluino.
129</parm>
130
131<parm name="RHadrons:mCollapse" default="1.0" min="0.0">
132A colour singlet system with an invariant mass less than this amount,
133above the R-hadron mass with the given flavour content, is assumed to
134collapse to this single R-hadron, whereas a full fragmentation handling
135is applied above this mass.
136</parm>
137
138<parm name="RHadrons:diquarkSpin1" default="0.5" min="0.0" max="1.0">
139Probability that a diquark extracted from the flavour code of a gluino
140R-hadron should be assigned spin 1, with the rest being spin 0. Does
141not apply for two identical quarks, where spin 1 is only possibility.
142Note that gluino R-hadron codes for simplicity are assigned as if spin
143is 1 always, and so give no guidance. For stop and sbottom the diquark
144spin is preserved in the particle code, so there is no corresponding
145issue.
146</parm>
147
148</chapter>
149
150<!-- Copyright (C) 2012 Torbjorn Sjostrand -->
151
Note: See TracBrowser for help on using the repository browser.