pythia8-data 8.1.80-1 / usr / share / pythia8-data / xmldoc / RHadrons.xml

<chapter name="R-hadrons">

<h2>R-hadrons</h2>

When a coloured SUSY particle is longer-lived than typical 
hadronization scales, i.e. around c*tau > 1 fm, or equivalently
width Gamma < 0.2 GeV, it will have time to hadronize into a colour 
singlet hadronic state, a R-hadron. Currently a set of such 
R-hadrons have been implemented for the case of a long-lived 
gluino, stop or sbottom. Needless to say, the normal case would be 
that only one of them will be long-lived enough to form R-hadrons.

<p/>
For simplicity all gluino-mesons are assumed to have light-flavour 
spin 1, since those are the lightest and favoured by spin-state 
counting. Further, all gluino-baryons are bookkept as having 
light-flavour spin 3/2, and flavours are listed in descending order. 
This is more for convenience of notation, however, since the normal 
baryon octet e.g. has no uuu = "p++" state. When a diquark is 
extracted, a mixture of spin 0 and spin 1 is allowed. Names and codes 
are essentially in agreement with the PDG conventions, e.g.  
<br/>1000993 <code>R0(~g g)</code> (or gluinoball) 
<br/>1009213 <code>R+(~g u dbar)</code> (or gluino-rho+)
<br/>1092214 <code>R+(~g uud)</code> (or gluino-Delta+)
<br/>For internal bookkeeping of momenta, the code 1009002,
<code>Rtemp(~g q)</code>, is used to denote the intermediate 
state formed when only one of the two string pieces attached to 
the gluino has broken.

<p/>
For the stop- and sbottom-hadrons the spin counting is simpler,
since it is entirely given by the constituent quark or diquark spin.
Again names and codes follow PDG conventions, e.g.
<br/>1000612 <code>R+(~t dbar)</code> 
<br/>1006211 <code>R+(~t ud0)</code> 

<p/>
The spin and electromagnetic charge of the new particle plays only 
a minor role in the hadronization process, that can be neglected 
to first approximation. Therefore it is possible to use the same
R-hadrons framework instead for other BSM scenarios with long-lived
coloured particles, e.g. with massive extra-dimensions copies 
of gluons and quarks, or with leptoquarks. This can be regulated by 
the switches below. Note that the codes and names of the R-hadrons
is not changed when the heavy particle involved is switched, for
reasons of administrative simplicity. R-hadron mass spectra and 
other relevant particle data is automatically updated to reflect 
the change, however.

<flag name="RHadrons:allow" default="off">
Allows the gluino, stop and sbottom to hadronize if their respective
widths are below the limit <code>RHadrons:maxWidth</code>.
</flag>

<parm name="RHadrons:maxWidth" default="0.2" min="0.0" max="1.0">
The maximum width of the gluino for which it is possible to form
R-hadrons, provided that <code>RHadrons:allow</code> is on.
</parm>

<mode name="RHadrons:idGluino" default="1000021">
The gluino identity code. For other scenarios than SUSY this code
could be changed to represent another long-lived uncharged colour 
octet particle, that then would be treated in the same spirit.
Could be set to 0 to forbid any gluino R-hadron formation even when 
the above two criteria, <code>RHadrons:allow</code> 
and <code>RHadrons:maxWidth</code>, are met. 
</flag>

<mode name="RHadrons:idStop" default="1000006">
The lightest stop identity code. For other scenarios than SUSY this 
code could be changed to represent another long-lived charge 2/3 
colour triplet particle, that then would be treated in the same 
spirit. As above it could be set to 0 to forbid any stop R-hadron 
formation.
</flag>

<mode name="RHadrons:idSbottom" default="1000005">
The lightest sbottom identity code. For other scenarios than SUSY this 
code could be changed to represent another long-lived charge -1/3 
colour triplet particle, that then would be treated in the same 
spirit. As above it could be set to 0 to forbid any sbottom R-hadron 
formation. 
</flag>

<flag name="RHadrons:allowDecay" default="on">
Allows the R-hadrons to decay or not. If the gluino/stop/sbottom is 
stable or too long-lived to decay inside the detector this switch 
has no real function, since then no decays will be performed anyway.
If the sparticle is so short-lived that it decays before reaching
the beam pipe then having the decay on is the logical choice.
So the interesting region is when the decays happens after the 
R-hadron has passed through part of the detector, and changed its
momentum and quite possibly its flavour content before it is to 
decay. Then normal decays should be switched off, and the R-hadron
tracked through matter by a program like GEANT 
<ref>Kra04,Mac07</ref>. After that, the new R-hadron info can be
overwritten into the event record and the 
<code>Pythia::forceRHadronDecay()</code> method can be called
to force this modified R-hadron to decay.
</flag>

<flag name="RHadrons:setMasses" default="on">
Use simple mass formulae to construct all available R-hadron masses
based on the currently initialized gluino/squark masses and the 
constituent masses of the other partons in the hadron. If you switch 
this off, it is your responsibility to set each of the R-hadron masses 
on your own, and set them in an internally consistent way. If you
mess up on this you may generate accordingly crazy results.
Specifically, it is to be assumed that none of the R-hadrons has a 
mass below its constituent sparticle, i.e. that the light degrees
of freedom and the additional confinement gluon field gives a net
positive contribution to the R-hadron mass.
</flag>

<parm name="RHadrons:probGluinoball" default="0.1" min="0.0" max="1.0">
The fraction of produced gluino R-hadrons that are contain a "valence"
gluon, with the rest containing a meson or baryon quark flavour content. 
</parm>

<parm name="RHadrons:mOffsetCloud" default="0.2" min="0.0">
Extra mass (in GeV) added to each of the one or two extra constituent 
masses in an R-hadron, to calculate the mass of a R-hadron. The same
offset is also used when the R-hadron momentum and mass is split 
between the squark or gluino and the one or two light (di)quarks,
one for a squark and two for a gluino. Thus once or twice this amount 
represents a part of the nominal squark or gluino mass that will not
decay weakly, since it is taken to correspond to the cloud of gluons 
that surround the squark or gluino.
</parm>

<parm name="RHadrons:mCollapse" default="1.0" min="0.0">
A colour singlet system with an invariant mass less than this amount, 
above the R-hadron mass with the given flavour content, is assumed to 
collapse to this single R-hadron, whereas a full fragmentation handling 
is applied above this mass.
</parm>

<parm name="RHadrons:diquarkSpin1" default="0.5" min="0.0" max="1.0">
Probability that a diquark extracted from the flavour code of a gluino
R-hadron should be assigned spin 1, with the rest being spin 0. Does 
not apply for two identical quarks, where spin 1 is only possibility. 
Note that gluino R-hadron codes for simplicity are assigned as if spin 
is 1 always, and so give no guidance. For stop and sbottom the diquark
spin is preserved in the particle code, so there is no corresponding
issue.
</parm>

</chapter>

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