1 <chapter name="Couplings and Scales">
3 <h2>Couplings and Scales</h2>
5 Here is collected some possibilities to modify the scale choices
6 of couplings and parton densities for all internally implemented
7 hard processes. This is based on them all being derived from the
8 <code>SigmaProcess</code> base class. The matrix-element coding is
9 also used by the multiparton-interactions machinery, but there with a
10 separate choice of <ei>alpha_strong(M_Z^2)</ei> value and running,
11 and separate PDF scale choices. Also, in <ei>2 -> 2</ei> and
12 <ei>2 -> 3</ei> processes where resonances are produced, their
13 couplings and thereby their Breit-Wigner shapes are always evaluated
14 with the resonance mass as scale, irrespective of the choices below.
16 <h3>Couplings and K factor</h3>
18 The size of QCD cross sections is mainly determined by
19 <parm name="SigmaProcess:alphaSvalue" default="0.1265"
20 min="0.06" max="0.25">
21 The <ei>alpha_strong</ei> value at scale <ei>M_Z^2</ei>.
25 The actual value is then regulated by the running to the <ei>Q^2</ei>
26 renormalization scale, at which <ei>alpha_strong</ei> is evaluated
27 <modepick name="SigmaProcess:alphaSorder" default="1" min="0" max="2">
28 Order at which <ei>alpha_strong</ei> runs,
29 <option value="0">zeroth order, i.e. <ei>alpha_strong</ei> is kept
31 <option value="1">first order, which is the normal value.</option>
32 <option value="2">second order. Since other parts of the code do
33 not go to second order there is no strong reason to use this option,
34 but there is also nothing wrong with it.</option>
38 QED interactions are regulated by the <ei>alpha_electromagnetic</ei>
39 value at the <ei>Q^2</ei> renormalization scale of an interaction.
40 <modepick name="SigmaProcess:alphaEMorder" default="1" min="-1" max="1">
41 The running of <ei>alpha_em</ei> used in hard processes.
42 <option value="1">first-order running, constrained to agree with
43 <code>StandardModel:alphaEMmZ</code> at the <ei>Z^0</ei> mass.
45 <option value="0">zeroth order, i.e. <ei>alpha_em</ei> is kept
46 fixed at its value at vanishing momentum transfer.</option>
47 <option value="-1">zeroth order, i.e. <ei>alpha_em</ei> is kept
48 fixed, but at <code>StandardModel:alphaEMmZ</code>, i.e. its value
49 at the <ei>Z^0</ei> mass.
54 In addition there is the possibility of a global rescaling of
55 cross sections (which could not easily be accommodated by a
56 changed <ei>alpha_strong</ei>, since <ei>alpha_strong</ei> runs)
57 <parm name="SigmaProcess:Kfactor" default="1.0" min="0.5" max="4.0">
58 Multiply almost all cross sections by this common fix factor. Excluded
59 are only unresolved processes, where cross sections are better
60 <aloc href="TotalCrossSections">set directly</aloc>, and
61 multiparton interactions, which have a separate <ei>K</ei> factor
62 <aloc href="MultipartonInteractions">of their own</aloc>.
63 This degree of freedom is primarily intended for hadron colliders, and
64 should not normally be used for <ei>e^+e^-</ei> annihilation processes.
67 <h3>Renormalization scales</h3>
69 The <ei>Q^2</ei> renormalization scale can be chosen among a few different
70 alternatives, separately for <ei>2 -> 1</ei>, <ei>2 -> 2</ei> and two
71 different kinds of <ei>2 -> 3</ei> processes. In addition a common
72 multiplicative factor may be imposed.
74 <modepick name="SigmaProcess:renormScale1" default="1" min="1" max="2">
75 The <ei>Q^2</ei> renormalization scale for <ei>2 -> 1</ei> processes.
76 The same options also apply for those <ei>2 -> 2</ei> and <ei>2 -> 3</ei>
77 processes that have been specially marked as proceeding only through
78 an <ei>s</ei>-channel resonance, by the <code>isSChannel()</code> virtual
79 method of <code>SigmaProcess</code>.
80 <option value="1">the squared invariant mass, i.e. <ei>sHat</ei>.
82 <option value="2">fix scale set in <code>SigmaProcess:renormFixScale</code>
87 <modepick name="SigmaProcess:renormScale2" default="2" min="1" max="5">
88 The <ei>Q^2</ei> renormalization scale for <ei>2 -> 2</ei> processes.
89 <option value="1">the smaller of the squared transverse masses of the two
90 outgoing particles, i.e. <ei>min(mT_3^2, mT_4^2) =
91 pT^2 + min(m_3^2, m_4^2)</ei>.
93 <option value="2">the geometric mean of the squared transverse masses of
94 the two outgoing particles, i.e. <ei>mT_3 * mT_4 =
95 sqrt((pT^2 + m_3^2) * (pT^2 + m_4^2))</ei>.
97 <option value="3">the arithmetic mean of the squared transverse masses of
98 the two outgoing particles, i.e. <ei>(mT_3^2 + mT_4^2) / 2 =
99 pT^2 + 0.5 * (m_3^2 + m_4^2)</ei>. Useful for comparisons
100 with PYTHIA 6, where this is the default.
102 <option value="4">squared invariant mass of the system,
103 i.e. <ei>sHat</ei>. Useful for processes dominated by
104 <ei>s</ei>-channel exchange.
106 <option value="5">fix scale set in <code>SigmaProcess:renormFixScale</code>
111 <modepick name="SigmaProcess:renormScale3" default="3" min="1" max="6">
112 The <ei>Q^2</ei> renormalization scale for "normal" <ei>2 -> 3</ei>
113 processes, i.e excepting the vector-boson-fusion processes below.
114 Here it is assumed that particle masses in the final state either match
115 or are heavier than that of any <ei>t</ei>-channel propagator particle.
116 (Currently only <ei>g g / q qbar -> H^0 Q Qbar</ei> processes are
117 implemented, where the "match" criterion holds.)
118 <option value="1">the smaller of the squared transverse masses of the three
119 outgoing particles, i.e. min(mT_3^2, mT_4^2, mT_5^2).
121 <option value="2">the geometric mean of the two smallest squared transverse
122 masses of the three outgoing particles, i.e.
123 <ei>sqrt( mT_3^2 * mT_4^2 * mT_5^2 / max(mT_3^2, mT_4^2, mT_5^2) )</ei>.
125 <option value="3">the geometric mean of the squared transverse masses of the
126 three outgoing particles, i.e. <ei>(mT_3^2 * mT_4^2 * mT_5^2)^(1/3)</ei>.
128 <option value="4">the arithmetic mean of the squared transverse masses of
129 the three outgoing particles, i.e. <ei>(mT_3^2 + mT_4^2 + mT_5^2)/3</ei>.
131 <option value="5">squared invariant mass of the system,
134 <option value="6">fix scale set in <code>SigmaProcess:renormFixScale</code>
139 <modepick name="SigmaProcess:renormScale3VV" default="3" min="1" max="6">
140 The <ei>Q^2</ei> renormalization scale for <ei>2 -> 3</ei>
141 vector-boson-fusion processes, i.e. <ei>f_1 f_2 -> H^0 f_3 f_4</ei>
142 with <ei>Z^0</ei> or <ei>W^+-</ei> <ei>t</ei>-channel propagators.
143 Here the transverse masses of the outgoing fermions do not reflect the
144 virtualities of the exchanged bosons. A better estimate is obtained
145 by replacing the final-state fermion masses by the vector-boson ones
146 in the definition of transverse masses. We denote these combinations
147 <ei>mT_Vi^2 = m_V^2 + pT_i^2</ei>.
148 <option value="1">the squared mass <ei>m_V^2</ei> of the exchanged
151 <option value="2">the geometric mean of the two propagator virtuality
152 estimates, i.e. <ei>sqrt(mT_V3^2 * mT_V4^2)</ei>.
154 <option value="3">the geometric mean of the three relevant squared
155 transverse masses, i.e. <ei>(mT_V3^2 * mT_V4^2 * mT_H^2)^(1/3)</ei>.
157 <option value="4">the arithmetic mean of the three relevant squared
158 transverse masses, i.e. <ei>(mT_V3^2 + mT_V4^2 + mT_H^2)/3</ei>.
160 <option value="5">squared invariant mass of the system,
163 <option value="6">fix scale set in <code>SigmaProcess:renormFixScale</code>
168 <parm name="SigmaProcess:renormMultFac" default="1." min="0.1" max="10.">
169 The <ei>Q^2</ei> renormalization scale for <ei>2 -> 1</ei>,
170 <ei>2 -> 2</ei> and <ei>2 -> 3</ei> processes is multiplied by
171 this factor relative to the scale described above (except for the options
172 with a fix scale). Should be use sparingly for <ei>2 -> 1</ei> processes.
175 <parm name="SigmaProcess:renormFixScale" default="10000." min="1.">
176 A fix <ei>Q^2</ei> value used as renormalization scale for <ei>2 -> 1</ei>,
177 <ei>2 -> 2</ei> and <ei>2 -> 3</ei> processes in some of the options above.
180 <h3>Factorization scales</h3>
182 Corresponding options exist for the <ei>Q^2</ei> factorization scale
183 used as argument in PDF's. Again there is a choice of form for
184 <ei>2 -> 1</ei>, <ei>2 -> 2</ei> and <ei>2 -> 3</ei> processes separately.
185 For simplicity we have let the numbering of options agree, for each event
186 class separately, between normalization and factorization scales, and the
187 description has therefore been slightly shortened. The default values are
188 <b>not</b> necessarily the same, however.
190 <modepick name="SigmaProcess:factorScale1" default="1" min="1" max="2">
191 The <ei>Q^2</ei> factorization scale for <ei>2 -> 1</ei> processes.
192 The same options also apply for those <ei>2 -> 2</ei> and <ei>2 -> 3</ei>
193 processes that have been specially marked as proceeding only through
194 an <ei>s</ei>-channel resonance.
195 <option value="1">the squared invariant mass, i.e. <ei>sHat</ei>.
197 <option value="2">fix scale set in <code>SigmaProcess:factorFixScale</code>
202 <modepick name="SigmaProcess:factorScale2" default="1" min="1" max="5">
203 The <ei>Q^2</ei> factorization scale for <ei>2 -> 2</ei> processes.
204 <option value="1">the smaller of the squared transverse masses of the two
207 <option value="2">the geometric mean of the squared transverse masses of
208 the two outgoing particles.
210 <option value="3">the arithmetic mean of the squared transverse masses of
211 the two outgoing particles. Useful for comparisons with PYTHIA 6, where
214 <option value="4">squared invariant mass of the system,
215 i.e. <ei>sHat</ei>. Useful for processes dominated by
216 <ei>s</ei>-channel exchange.
218 <option value="5">fix scale set in <code>SigmaProcess:factorFixScale</code>
223 <modepick name="SigmaProcess:factorScale3" default="2" min="1" max="6">
224 The <ei>Q^2</ei> factorization scale for "normal" <ei>2 -> 3</ei>
225 processes, i.e excepting the vector-boson-fusion processes below.
226 <option value="1">the smaller of the squared transverse masses of the three
229 <option value="2">the geometric mean of the two smallest squared transverse
230 masses of the three outgoing particles.
232 <option value="3">the geometric mean of the squared transverse masses of the
233 three outgoing particles.
235 <option value="4">the arithmetic mean of the squared transverse masses of
236 the three outgoing particles.
238 <option value="5">squared invariant mass of the system,
241 <option value="6">fix scale set in <code>SigmaProcess:factorFixScale</code>
246 <modepick name="SigmaProcess:factorScale3VV" default="2" min="1" max="6">
247 The <ei>Q^2</ei> factorization scale for <ei>2 -> 3</ei>
248 vector-boson-fusion processes, i.e. <ei>f_1 f_2 -> H^0 f_3 f_4</ei>
249 with <ei>Z^0</ei> or <ei>W^+-</ei> <ei>t</ei>-channel propagators.
250 Here we again introduce the combinations <ei>mT_Vi^2 = m_V^2 + pT_i^2</ei>
251 as replacements for the normal squared transverse masses of the two
253 <option value="1">the squared mass <ei>m_V^2</ei> of the exchanged
256 <option value="2">the geometric mean of the two propagator virtuality
259 <option value="3">the geometric mean of the three relevant squared
262 <option value="4">the arithmetic mean of the three relevant squared
265 <option value="5">squared invariant mass of the system,
268 <option value="6">fix scale set in <code>SigmaProcess:factorFixScale</code>
273 <parm name="SigmaProcess:factorMultFac" default="1." min="0.1" max="10.">
274 The <ei>Q^2</ei> factorization scale for <ei>2 -> 1</ei>,
275 <ei>2 -> 2</ei> and <ei>2 -> 3</ei> processes is multiplied by
276 this factor relative to the scale described above (except for the options
277 with a fix scale). Should be use sparingly for <ei>2 -> 1</ei> processes.
280 <parm name="SigmaProcess:factorFixScale" default="10000." min="1.">
281 A fix <ei>Q^2</ei> value used as factorization scale for <ei>2 -> 1</ei>,
282 <ei>2 -> 2</ei> and <ei>2 -> 3</ei> processes in some of the options above.
287 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->