1 <chapter name="Fourth-Generation Processes">
3 <h2>Fourth-Generation Processes</h2>
5 A fourth generation can be accommodated within the Standard Model,
6 without the introduction of any new concepts. Many experimental
7 constraints exist, but it has not been fully excluded. Therefore
8 we offer a simple implementation, along the lines of the top.
9 It could also be useful as a template for studies of other
10 new particles with similar characteristics.
13 The fourth generation are given names as in the third, but with a prime,
14 i.e. <ei>b'</ei> with PDG code 7, <ei>t'</ei> with code 8,
15 <ei>tau'</ei> with code 17, and <ei>nu'_tau</ei> with code 18.
16 Most important for you is to assign a mass hierarchy, to decide which
17 fermions can decay into which. The current implementation assumes that
18 mass splittings are big enough that fourth-generation fermions can
19 decay to third-generation ones by the emission of an on-shell <ei>W</ei>.
20 To this end, the standard three-generation CKM mixing matrix has been
21 extended to include a fourth generation, see below. Since no mixing has
22 been implemented in the neutrino sector it would be assumed that the
23 lighter of <ei>tau'</ei> and <ei>nu'_tau</ei> is stable. No decay modes
24 have been implemented that go beyond the Standard Model, so
25 modifications would be needed if e.g. also SUSY is included in the game.
27 <h3>Production processes</h3>
29 <h4>1) <ei>b'</ei> processes</h4>
31 Different ways to produce <ei>b'</ei> quarks, singly or in pairs.
32 For a <ei>b' t'</ei> pair see section 3 below.
34 <flag name="FourthBottom:all" default="off">
35 Common switch for the group of <ei>b'</ei> production.
36 Also includes the process <ei>f fbar' -> t' b'bar</ei> in section 3 below.
39 <flag name="FourthBottom:gg2bPrimebPrimebar" default="off">
40 Scatterings <ei>g g -> b' b'bar</ei>.
44 <flag name="FourthBottom:qqbar2bPrimebPrimebar" default="off">
45 Scatterings <ei>q qbar -> b' b'bar</ei> by gluon exchange.
49 <flag name="FourthBottom:qq2bPrimeq(t:W)" default="off">
50 Scatterings <ei>q q' -> b' q''</ei> by <ei>t</ei>-channel exchange
51 of a <ei>W^+-</ei> boson.
55 <flag name="FourthBottom:ffbar2bPrimebPrimebar(s:gmZ)" default="off">
56 Scatterings <ei>f fbar -> b' b'bar</ei> by <ei>s</ei>-channel exchange
57 of a <ei>gamma^*/Z^0</ei> boson.
61 <flag name="FourthBottom:ffbar2bPrimeqbar(s:W)" default="off">
62 Scatterings <ei>f fbar' -> b' qbar''</ei> by <ei>s</ei>-channel exchange
63 of a <ei>W^+-</ei> boson. Here <ei>q''</ei> is either <ei>u</ei> or
68 <flag name="FourthBottom:ffbar2bPrimetbar(s:W)" default="off">
69 Scatterings <ei>f fbar' -> b' tbar</ei> by <ei>s</ei>-channel exchange
70 of a <ei>W^+-</ei> boson.
74 <h4>2) <ei>t'</ei> processes</h4>
76 Different ways to produce <ei>t'</ei> quarks, singly or in pairs.
77 For a <ei>b' t'</ei> pair see section 3 below.
79 <flag name="FourthTop:all" default="off">
80 Common switch for the group of <ei>t'</ei> production.
81 Also includes the process <ei>f fbar' -> t' b'bar</ei> in section 3 below.
84 <flag name="FourthTop:gg2tPrimetPrimebar" default="off">
85 Scatterings <ei>g g -> t' t'bar</ei>.
89 <flag name="FourthTop:qqbar2tPrimetPrimebar" default="off">
90 Scatterings <ei>q qbar -> t' t'bar</ei> by gluon exchange.
94 <flag name="FourthTop:qq2tPrimeq(t:W)" default="off">
95 Scatterings <ei>q q' -> t' q''</ei> by <ei>t</ei>-channel exchange
96 of a <ei>W^+-</ei> boson.
100 <flag name="FourthTop:ffbar2tPrimetPrimebar(s:gmZ)" default="off">
101 Scatterings <ei>f fbar -> t' t'bar</ei> by <ei>s</ei>-channel exchange
102 of a <ei>gamma^*/Z^0</ei> boson.
106 <flag name="FourthTop:ffbar2tPrimeqbar(s:W)" default="off">
107 Scatterings <ei>f fbar' -> t' qbar''</ei> by <ei>s</ei>-channel exchange
108 of a <ei>W^+-</ei> boson.
112 <h4>3) Pair-processes with different flavours</h4>
114 Different ways to produce two different fourth-generation fermions.
116 <flag name="FourthPair:ffbar2tPrimebPrimebar(s:W)" default="off">
117 Scatterings <ei>f fbar' -> t' b'bar</ei> by <ei>s</ei>-channel exchange
118 of a <ei>W^+-</ei> boson.
122 <flag name="FourthPair:ffbar2tauPrimenuPrimebar(s:W)" default="off">
123 Scatterings <ei>f fbar' -> tau' nu'_taubar</ei> by <ei>s</ei>-channel
124 exchange of a <ei>W^+-</ei> boson.
129 Missing in this list is scatterings <ei>q q' -> t' b'</ei> by
130 <ei>t</ei>-channel exchange of a <ei>W^+-</ei> boson, since currently
131 the matrix element for such processes have not been implemented for
132 two massive particles in the final state. Since this process would
133 involve two CKM-suppressed vertices it ought to be small.
137 The Cabibbo-Kobayashi-Maskawa matrix is extended by seven further values.
138 So as not to mess up the Standard Model, the normal 3 * 3 matrix is
139 kept unitary, and so the new off-diagonal elements lead to a slight
140 breaking of this. For exploratory studies this should be good enough;
141 more detailed 4 * 4 tunes to data would only make sense the day there
142 are evidence for the existence of a fourth generation.
144 <parm name="FourthGeneration:VubPrime" default="0.001" min="0.0" max="1.0">
145 The <ei>V_ub'</ei> matrix element in the 4 * 4 CKM matrix.
148 <parm name="FourthGeneration:VcbPrime" default="0.01" min="0.0" max="1.0">
149 The <ei>V_cb'</ei> matrix element in the 4 * 4 CKM matrix.
152 <parm name="FourthGeneration:VtbPrime" default="0.1" min="0.0" max="1.0">
153 The <ei>V_tb'</ei> matrix element in the 4 * 4 CKM matrix.
156 <parm name="FourthGeneration:VtPrimed" default="0.001" min="0.0" max="1.0">
157 The <ei>V_t'd</ei> matrix element in the 4 * 4 CKM matrix.
160 <parm name="FourthGeneration:VtPrimes" default="0.01" min="0.0" max="1.0">
161 The <ei>V_t's</ei> matrix element in the 4 * 4 CKM matrix.
164 <parm name="FourthGeneration:VtPrimeb" default="0.1" min="0.0" max="1.0">
165 The <ei>V_t'b</ei> matrix element in the 4 * 4 CKM matrix.
168 <parm name="FourthGeneration:VtPrimebPrime" default="0.99" min="0.0" max="1.0">
169 The <ei>V_t'b'</ei> matrix element in the 4 * 4 CKM matrix.
174 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->