1 <chapter name="Fourth-Generation Processes">
3 <h2>Fourth-Generation Processes</h2>
5 A fourth generation could have been accommodated within the
6 Standard Model, without the introduction of any new concepts.
7 Given the experimental constraints, however, its existence by now
8 is very unlikely. Nevertheless we offer a simple implementation,
9 along the lines of the top, since it could be useful also as a
10 template for studies of other new particles with similar
14 The fourth generation are given names as in the third, but with a prime,
15 i.e. <ei>b'</ei> with PDG code 7, <ei>t'</ei> with code 8,
16 <ei>tau'</ei> with code 17, and <ei>nu'_tau</ei> with code 18.
17 Most important for you is to assign a mass hierarchy, to decide which
18 fermions can decay into which. The current implementation assumes that
19 mass splittings are big enough that fourth-generation fermions can
20 decay to third-generation ones by the emission of an on-shell <ei>W</ei>.
21 To this end, the standard three-generation CKM mixing matrix has been
22 extended to include a fourth generation, see below. Since no mixing has
23 been implemented in the neutrino sector it would be assumed that the
24 lighter of <ei>tau'</ei> and <ei>nu'_tau</ei> is stable. No decay modes
25 have been implemented that go beyond the Standard Model, so
26 modifications would be needed if e.g. also SUSY is included in the game.
28 <h3>Production processes</h3>
30 <h4>1) <ei>b'</ei> processes</h4>
32 Different ways to produce <ei>b'</ei> quarks, singly or in pairs.
33 For a <ei>b' t'</ei> pair see section 3 below.
35 <flag name="FourthBottom:all" default="off">
36 Common switch for the group of <ei>b'</ei> production.
37 Also includes the process <ei>f fbar' -> t' b'bar</ei> in section 3 below.
40 <flag name="FourthBottom:gg2bPrimebPrimebar" default="off">
41 Scatterings <ei>g g -> b' b'bar</ei>.
45 <flag name="FourthBottom:qqbar2bPrimebPrimebar" default="off">
46 Scatterings <ei>q qbar -> b' b'bar</ei> by gluon exchange.
50 <flag name="FourthBottom:qq2bPrimeq(t:W)" default="off">
51 Scatterings <ei>q q' -> b' q''</ei> by <ei>t</ei>-channel exchange
52 of a <ei>W^+-</ei> boson.
56 <flag name="FourthBottom:ffbar2bPrimebPrimebar(s:gmZ)" default="off">
57 Scatterings <ei>f fbar -> b' b'bar</ei> by <ei>s</ei>-channel exchange
58 of a <ei>gamma^*/Z^0</ei> boson.
62 <flag name="FourthBottom:ffbar2bPrimeqbar(s:W)" default="off">
63 Scatterings <ei>f fbar' -> b' qbar''</ei> by <ei>s</ei>-channel exchange
64 of a <ei>W^+-</ei> boson. Here <ei>q''</ei> is either <ei>u</ei> or
69 <flag name="FourthBottom:ffbar2bPrimetbar(s:W)" default="off">
70 Scatterings <ei>f fbar' -> b' tbar</ei> by <ei>s</ei>-channel exchange
71 of a <ei>W^+-</ei> boson.
75 <h4>2) <ei>t'</ei> processes</h4>
77 Different ways to produce <ei>t'</ei> quarks, singly or in pairs.
78 For a <ei>b' t'</ei> pair see section 3 below.
80 <flag name="FourthTop:all" default="off">
81 Common switch for the group of <ei>t'</ei> production.
82 Also includes the process <ei>f fbar' -> t' b'bar</ei> in section 3 below.
85 <flag name="FourthTop:gg2tPrimetPrimebar" default="off">
86 Scatterings <ei>g g -> t' t'bar</ei>.
90 <flag name="FourthTop:qqbar2tPrimetPrimebar" default="off">
91 Scatterings <ei>q qbar -> t' t'bar</ei> by gluon exchange.
95 <flag name="FourthTop:qq2tPrimeq(t:W)" default="off">
96 Scatterings <ei>q q' -> t' q''</ei> by <ei>t</ei>-channel exchange
97 of a <ei>W^+-</ei> boson.
101 <flag name="FourthTop:ffbar2tPrimetPrimebar(s:gmZ)" default="off">
102 Scatterings <ei>f fbar -> t' t'bar</ei> by <ei>s</ei>-channel exchange
103 of a <ei>gamma^*/Z^0</ei> boson.
107 <flag name="FourthTop:ffbar2tPrimeqbar(s:W)" default="off">
108 Scatterings <ei>f fbar' -> t' qbar''</ei> by <ei>s</ei>-channel exchange
109 of a <ei>W^+-</ei> boson.
113 <h4>3) Pair-processes with different flavours</h4>
115 Different ways to produce two different fourth-generation fermions.
117 <flag name="FourthPair:ffbar2tPrimebPrimebar(s:W)" default="off">
118 Scatterings <ei>f fbar' -> t' b'bar</ei> by <ei>s</ei>-channel exchange
119 of a <ei>W^+-</ei> boson.
123 <flag name="FourthPair:ffbar2tauPrimenuPrimebar(s:W)" default="off">
124 Scatterings <ei>f fbar' -> tau' nu'_taubar</ei> by <ei>s</ei>-channel
125 exchange of a <ei>W^+-</ei> boson.
130 Missing in this list is scatterings <ei>q q' -> t' b'</ei> by
131 <ei>t</ei>-channel exchange of a <ei>W^+-</ei> boson, since currently
132 the matrix element for such processes have not been implemented for
133 two massive particles in the final state. Since this process would
134 involve two CKM-suppressed vertices it ought to be small.
138 The Cabibbo-Kobayashi-Maskawa matrix is extended by seven further values.
139 So as not to mess up the Standard Model, the normal 3 * 3 matrix is
140 kept unitary, and so the new off-diagonal elements lead to a slight
141 breaking of this. For exploratory studies this should be good enough;
142 more detailed 4 * 4 tunes to data would only make sense the day there
143 are evidence for the existence of a fourth generation.
145 <parm name="FourthGeneration:VubPrime" default="0.001" min="0.0" max="1.0">
146 The <ei>V_ub'</ei> matrix element in the 4 * 4 CKM matrix.
149 <parm name="FourthGeneration:VcbPrime" default="0.01" min="0.0" max="1.0">
150 The <ei>V_cb'</ei> matrix element in the 4 * 4 CKM matrix.
153 <parm name="FourthGeneration:VtbPrime" default="0.1" min="0.0" max="1.0">
154 The <ei>V_tb'</ei> matrix element in the 4 * 4 CKM matrix.
157 <parm name="FourthGeneration:VtPrimed" default="0.001" min="0.0" max="1.0">
158 The <ei>V_t'd</ei> matrix element in the 4 * 4 CKM matrix.
161 <parm name="FourthGeneration:VtPrimes" default="0.01" min="0.0" max="1.0">
162 The <ei>V_t's</ei> matrix element in the 4 * 4 CKM matrix.
165 <parm name="FourthGeneration:VtPrimeb" default="0.1" min="0.0" max="1.0">
166 The <ei>V_t'b</ei> matrix element in the 4 * 4 CKM matrix.
169 <parm name="FourthGeneration:VtPrimebPrime" default="0.99" min="0.0" max="1.0">
170 The <ei>V_t'b'</ei> matrix element in the 4 * 4 CKM matrix.
175 <!-- Copyright (C) 2008 Torbjorn Sjostrand -->