1 | <chapter name="R-hadrons"> |
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2 | |
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3 | <h2>R-hadrons</h2> |
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4 | |
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5 | When a coloured SUSY particle is longer-lived than typical |
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6 | hadronization scales, i.e. around c*tau > 1 fm, or equivalently |
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7 | width Gamma < 0.2 GeV, it will have time to hadronize into a colour |
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8 | singlet hadronic state, a R-hadron. Currently a set of such |
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9 | R-hadrons have been implemented for the case of a long-lived |
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10 | gluino, stop or sbottom. Needless to say, the normal case would be |
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11 | that only one of them will be long-lived enough to form R-hadrons. |
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12 | |
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13 | <p/> |
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14 | For simplicity all gluino-mesons are assumed to have light-flavour |
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15 | spin 1, since those are the lightest and favoured by spin-state |
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16 | counting. Further, all gluino-baryons are bookkept as having |
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17 | light-flavour spin 3/2, and flavours are listed in descending order. |
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18 | This is more for convenience of notation, however, since the normal |
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19 | baryon octet e.g. has no uuu = "p++" state. When a diquark is |
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20 | extracted, a mixture of spin 0 and spin 1 is allowed. Names and codes |
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21 | are essentially in agreement with the PDG conventions, e.g. |
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22 | <br/>1000993 <code>R0(~g g)</code> (or gluinoball) |
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23 | <br/>1009213 <code>R+(~g u dbar)</code> (or gluino-rho+) |
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24 | <br/>1092214 <code>R+(~g uud)</code> (or gluino-Delta+) |
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25 | <br/>For internal bookkeeping of momenta, the code 1009002, |
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26 | <code>Rtemp(~g q)</code>, is used to denote the intermediate |
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27 | state formed when only one of the two string peices attached to |
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28 | the gluino has broken. |
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29 | |
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30 | <p/> |
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31 | For the stop- and sbottom-hadrons the spin counting is simpler, |
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32 | since it is entirely given by the constituent quark or diquark spin. |
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33 | Again names and codes follow PDG conventions, e.g. |
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34 | <br/>1000612 <code>R+(~t dbar)</code> |
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35 | <br/>1006211 <code>R+(~t ud0)</code> |
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36 | |
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37 | <p/> |
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38 | The spin and electromagnetic charge of the new particle plays only |
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39 | a minor role in the hadronization process, that can be neglected |
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40 | to first approximation. Therefore it is possible to use the same |
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41 | R-hadrons framework instead for other BSM scenarios with long-lived |
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42 | coloured particles, e.g. with massive extra-dimensions copies |
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43 | of gluons and quarks, or with leptoquarks. This can be regulated by |
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44 | the switches below. Note that the codes and names of the R-hadrons |
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45 | is not changed when the heavy particle involved is switched, for |
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46 | reasons of administrative simplicity. R-hadron mass spectra and |
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47 | other relevant particle data is automatically updated to reflect |
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48 | the change, however. |
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49 | |
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50 | <flag name="RHadrons:allow" default="off"> |
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51 | Allows the gluino, stop and sbottom to hadronize if their respective |
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52 | widths are below the limit <code>RHadrons:maxWidth</code>. |
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53 | </flag> |
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54 | |
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55 | <parm name="RHadrons:maxWidth" default="0.2" min="0.0" max="1.0"> |
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56 | The maximum width of the gluino for which it is possible to form |
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57 | R-hadrons, provided that <code>RHadrons:allow</code> is on. |
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58 | </parm> |
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59 | |
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60 | <mode name="RHadrons:idGluino" default="1000021"> |
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61 | The gluino identity code. For other scenarios than SUSY this code |
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62 | could be changed to represent another long-lived uncharged colour |
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63 | octet particle, that then would be treated in the same spirit. |
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64 | Could be set to 0 to forbid any gluino R-hadron formation even when |
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65 | the above two criteria, <code>RHadrons:allow</code> |
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66 | and <code>RHadrons:maxWidth</code>, are met. |
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67 | </flag> |
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68 | |
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69 | <mode name="RHadrons:idStop" default="1000006"> |
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70 | The lightest stop identity code. For other scenarios than SUSY this |
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71 | code could be changed to represent another long-lived charge 2/3 |
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72 | colour triplet particle, that then would be treated in the same |
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73 | spirit. As above it could be set to 0 to forbid any stop R-hadron |
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74 | formation. |
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75 | </flag> |
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76 | |
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77 | <mode name="RHadrons:idSbottom" default="1000005"> |
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78 | The lightest sbottom identity code. For other scenarios than SUSY this |
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79 | code could be changed to represent another long-lived charge -1/3 |
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80 | colour triplet particle, that then would be treated in the same |
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81 | spirit. As above it could be set to 0 to forbid any sbottom R-hadron |
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82 | formation. |
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83 | </flag> |
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84 | |
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85 | <flag name="RHadrons:allowDecay" default="on"> |
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86 | Allows the R-hadrons to decay or not. If the gluino/stop/sbottom is |
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87 | stable or too long-lived to decay inside the detector this switch |
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88 | has no real function, since then no decays will be performed anyway. |
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89 | If the sparticle is so short-lived that it decays before reaching |
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90 | the beam pipe then having the decay on is the logical choice. |
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91 | So the interesting region is when the decays happens after the |
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92 | R-hadron has passed through part of the detector, and changed its |
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93 | momentum and quite possibly its flavour content before it is to |
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94 | decay. Then normal decays should be switched off, and the R-hadron |
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95 | tracked through matter by a program like GEANT |
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96 | <ref>Kra04,Mac07</ref>. After that, the new R-hadron info can be |
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97 | overwritten into the event record and the |
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98 | <code>Pythia::forceRHadronDecay()</code> method can be called |
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99 | to force this modified R-hadron to decay. |
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100 | </flag> |
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101 | |
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102 | <flag name="RHadrons:setMasses" default="on"> |
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103 | Use simple mass formulae to construct all available R-hadron masses |
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104 | based on the currently initialized gluino/squark masses and the |
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105 | constituent masses of the other partons in the hadron. If you switch |
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106 | this off, it is your responsibility to set each of the R-hadron masses |
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107 | on your own, and set them in an internally consistent way. If you |
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108 | mess up on this you may generate accordingly crazy results. |
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109 | Specifically, it is to be assumed that none of the R-hadrons has a |
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110 | mass below its constituent sparticle, i.e. that the light degrees |
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111 | of freedom and the additional confinement gluon field gives a net |
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112 | positive contribution to the R-hadron mass. |
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113 | </flag> |
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114 | |
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115 | <parm name="RHadrons:probGluinoball" default="0.1" min="0.0" max="1.0"> |
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116 | The fraction of produced gluino R-hadrons that are contain a "valence" |
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117 | gluon, with the rest containing a meson or baryon quark flavour content. |
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118 | </parm> |
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119 | |
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120 | <parm name="RHadrons:mOffsetCloud" default="0.2" min="0.0"> |
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121 | Extra mass (in GeV) added to each of the one or two extra constituent |
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122 | masses in an R-hadron, to calculate the mass of a R-hadron. The same |
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123 | offset is also used when the R-hadron momentum and mass is split |
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124 | between the squark or gluino and the one or two light (di)quarks, |
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125 | one for a squark and two for a gluino. Thus once or twice this amount |
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126 | represents a part of the nominal squark or gluino mass that will not |
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127 | decay weakly, since it is taken to correspond to the cloud of gluons |
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128 | that surround the squark or gluino. |
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129 | </parm> |
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130 | |
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131 | <parm name="RHadrons:mCollapse" default="1.0" min="0.0"> |
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132 | A colour singlet system with an invariant mass less than this amount, |
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133 | above the R-hadron mass with the given flavour content, is assumed to |
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134 | collapse to this single R-hadron, whereas a full fragmentation handling |
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135 | is applied above this mass. |
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136 | </parm> |
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137 | |
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138 | <parm name="RHadrons:diquarkSpin1" default="0.5" min="0.0" max="1.0"> |
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139 | Probability that a diquark extracted from the flavour code of a gluino |
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140 | R-hadron should be assigned spin 1, with the rest being spin 0. Does |
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141 | not apply for two identical quarks, where spin 1 is only possibility. |
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142 | Note that gluino R-hadron codes for simplicity are assigned as if spin |
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143 | is 1 always, and so give no guidance. For stop and sbottom the diquark |
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144 | spin is preserved in the particle code, so there is no corresponding |
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145 | issue. |
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146 | </parm> |
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147 | |
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148 | </chapter> |
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149 | |
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150 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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151 | |
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