3 <title>Couplings and Scales</title>
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9 <h2>Couplings and Scales</h2>
11 Here is collected some possibilities to modify the scale choices
12 of couplings and parton densities for all internally implemented
13 hard processes. This is based on them all being derived from the
14 <code>SigmaProcess</code> base class. The matrix-element coding is
15 also used by the multiparton-interactions machinery, but there with a
16 separate choice of <i>alpha_strong(M_Z^2)</i> value and running,
17 and separate PDF scale choices. Also, in <i>2 -> 2</i> and
18 <i>2 -> 3</i> processes where resonances are produced, their
19 couplings and thereby their Breit-Wigner shapes are always evaluated
20 with the resonance mass as scale, irrespective of the choices below.
22 <h3>Couplings and K factor</h3>
24 The size of QCD cross sections is mainly determined by
25 <p/><code>parm </code><strong> SigmaProcess:alphaSvalue </strong>
26 (<code>default = <strong>0.1265</strong></code>; <code>minimum = 0.06</code>; <code>maximum = 0.25</code>)<br/>
27 The <i>alpha_strong</i> value at scale <i>M_Z^2</i>.
31 The actual value is then regulated by the running to the <i>Q^2</i>
32 renormalization scale, at which <i>alpha_strong</i> is evaluated
33 <p/><code>mode </code><strong> SigmaProcess:alphaSorder </strong>
34 (<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
35 Order at which <i>alpha_strong</i> runs,
36 <br/><code>option </code><strong> 0</strong> : zeroth order, i.e. <i>alpha_strong</i> is kept
38 <br/><code>option </code><strong> 1</strong> : first order, which is the normal value.
39 <br/><code>option </code><strong> 2</strong> : second order. Since other parts of the code do
40 not go to second order there is no strong reason to use this option,
41 but there is also nothing wrong with it.
45 QED interactions are regulated by the <i>alpha_electromagnetic</i>
46 value at the <i>Q^2</i> renormalization scale of an interaction.
47 <p/><code>mode </code><strong> SigmaProcess:alphaEMorder </strong>
48 (<code>default = <strong>1</strong></code>; <code>minimum = -1</code>; <code>maximum = 1</code>)<br/>
49 The running of <i>alpha_em</i> used in hard processes.
50 <br/><code>option </code><strong> 1</strong> : first-order running, constrained to agree with
51 <code>StandardModel:alphaEMmZ</code> at the <i>Z^0</i> mass.
53 <br/><code>option </code><strong> 0</strong> : zeroth order, i.e. <i>alpha_em</i> is kept
54 fixed at its value at vanishing momentum transfer.
55 <br/><code>option </code><strong> -1</strong> : zeroth order, i.e. <i>alpha_em</i> is kept
56 fixed, but at <code>StandardModel:alphaEMmZ</code>, i.e. its value
57 at the <i>Z^0</i> mass.
62 In addition there is the possibility of a global rescaling of
63 cross sections (which could not easily be accommodated by a
64 changed <i>alpha_strong</i>, since <i>alpha_strong</i> runs)
65 <p/><code>parm </code><strong> SigmaProcess:Kfactor </strong>
66 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 4.0</code>)<br/>
67 Multiply almost all cross sections by this common fix factor. Excluded
68 are only unresolved processes, where cross sections are better
69 <a href="TotalCrossSections.html" target="page">set directly</a>, and
70 multiparton interactions, which have a separate <i>K</i> factor
71 <a href="MultipartonInteractions.html" target="page">of their own</a>.
72 This degree of freedom is primarily intended for hadron colliders, and
73 should not normally be used for <i>e^+e^-</i> annihilation processes.
76 <h3>Renormalization scales</h3>
78 The <i>Q^2</i> renormalization scale can be chosen among a few different
79 alternatives, separately for <i>2 -> 1</i>, <i>2 -> 2</i> and two
80 different kinds of <i>2 -> 3</i> processes. In addition a common
81 multiplicative factor may be imposed.
83 <p/><code>mode </code><strong> SigmaProcess:renormScale1 </strong>
84 (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 2</code>)<br/>
85 The <i>Q^2</i> renormalization scale for <i>2 -> 1</i> processes.
86 The same options also apply for those <i>2 -> 2</i> and <i>2 -> 3</i>
87 processes that have been specially marked as proceeding only through
88 an <i>s</i>-channel resonance, by the <code>isSChannel()</code> virtual
89 method of <code>SigmaProcess</code>.
90 <br/><code>option </code><strong> 1</strong> : the squared invariant mass, i.e. <i>sHat</i>.
92 <br/><code>option </code><strong> 2</strong> : fix scale set in <code>SigmaProcess:renormFixScale</code>
97 <p/><code>mode </code><strong> SigmaProcess:renormScale2 </strong>
98 (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 5</code>)<br/>
99 The <i>Q^2</i> renormalization scale for <i>2 -> 2</i> processes.
100 <br/><code>option </code><strong> 1</strong> : the smaller of the squared transverse masses of the two
101 outgoing particles, i.e. <i>min(mT_3^2, mT_4^2) =
102 pT^2 + min(m_3^2, m_4^2)</i>.
104 <br/><code>option </code><strong> 2</strong> : the geometric mean of the squared transverse masses of
105 the two outgoing particles, i.e. <i>mT_3 * mT_4 =
106 sqrt((pT^2 + m_3^2) * (pT^2 + m_4^2))</i>.
108 <br/><code>option </code><strong> 3</strong> : the arithmetic mean of the squared transverse masses of
109 the two outgoing particles, i.e. <i>(mT_3^2 + mT_4^2) / 2 =
110 pT^2 + 0.5 * (m_3^2 + m_4^2)</i>. Useful for comparisons
111 with PYTHIA 6, where this is the default.
113 <br/><code>option </code><strong> 4</strong> : squared invariant mass of the system,
114 i.e. <i>sHat</i>. Useful for processes dominated by
115 <i>s</i>-channel exchange.
117 <br/><code>option </code><strong> 5</strong> : fix scale set in <code>SigmaProcess:renormFixScale</code>
122 <p/><code>mode </code><strong> SigmaProcess:renormScale3 </strong>
123 (<code>default = <strong>3</strong></code>; <code>minimum = 1</code>; <code>maximum = 6</code>)<br/>
124 The <i>Q^2</i> renormalization scale for "normal" <i>2 -> 3</i>
125 processes, i.e excepting the vector-boson-fusion processes below.
126 Here it is assumed that particle masses in the final state either match
127 or are heavier than that of any <i>t</i>-channel propagator particle.
128 (Currently only <i>g g / q qbar -> H^0 Q Qbar</i> processes are
129 implemented, where the "match" criterion holds.)
130 <br/><code>option </code><strong> 1</strong> : the smaller of the squared transverse masses of the three
131 outgoing particles, i.e. min(mT_3^2, mT_4^2, mT_5^2).
133 <br/><code>option </code><strong> 2</strong> : the geometric mean of the two smallest squared transverse
134 masses of the three outgoing particles, i.e.
135 <i>sqrt( mT_3^2 * mT_4^2 * mT_5^2 / max(mT_3^2, mT_4^2, mT_5^2) )</i>.
137 <br/><code>option </code><strong> 3</strong> : the geometric mean of the squared transverse masses of the
138 three outgoing particles, i.e. <i>(mT_3^2 * mT_4^2 * mT_5^2)^(1/3)</i>.
140 <br/><code>option </code><strong> 4</strong> : the arithmetic mean of the squared transverse masses of
141 the three outgoing particles, i.e. <i>(mT_3^2 + mT_4^2 + mT_5^2)/3</i>.
143 <br/><code>option </code><strong> 5</strong> : squared invariant mass of the system,
146 <br/><code>option </code><strong> 6</strong> : fix scale set in <code>SigmaProcess:renormFixScale</code>
151 <p/><code>mode </code><strong> SigmaProcess:renormScale3VV </strong>
152 (<code>default = <strong>3</strong></code>; <code>minimum = 1</code>; <code>maximum = 6</code>)<br/>
153 The <i>Q^2</i> renormalization scale for <i>2 -> 3</i>
154 vector-boson-fusion processes, i.e. <i>f_1 f_2 -> H^0 f_3 f_4</i>
155 with <i>Z^0</i> or <i>W^+-</i> <i>t</i>-channel propagators.
156 Here the transverse masses of the outgoing fermions do not reflect the
157 virtualities of the exchanged bosons. A better estimate is obtained
158 by replacing the final-state fermion masses by the vector-boson ones
159 in the definition of transverse masses. We denote these combinations
160 <i>mT_Vi^2 = m_V^2 + pT_i^2</i>.
161 <br/><code>option </code><strong> 1</strong> : the squared mass <i>m_V^2</i> of the exchanged
164 <br/><code>option </code><strong> 2</strong> : the geometric mean of the two propagator virtuality
165 estimates, i.e. <i>sqrt(mT_V3^2 * mT_V4^2)</i>.
167 <br/><code>option </code><strong> 3</strong> : the geometric mean of the three relevant squared
168 transverse masses, i.e. <i>(mT_V3^2 * mT_V4^2 * mT_H^2)^(1/3)</i>.
170 <br/><code>option </code><strong> 4</strong> : the arithmetic mean of the three relevant squared
171 transverse masses, i.e. <i>(mT_V3^2 + mT_V4^2 + mT_H^2)/3</i>.
173 <br/><code>option </code><strong> 5</strong> : squared invariant mass of the system,
176 <br/><code>option </code><strong> 6</strong> : fix scale set in <code>SigmaProcess:renormFixScale</code>
181 <p/><code>parm </code><strong> SigmaProcess:renormMultFac </strong>
182 (<code>default = <strong>1.</strong></code>; <code>minimum = 0.1</code>; <code>maximum = 10.</code>)<br/>
183 The <i>Q^2</i> renormalization scale for <i>2 -> 1</i>,
184 <i>2 -> 2</i> and <i>2 -> 3</i> processes is multiplied by
185 this factor relative to the scale described above (except for the options
186 with a fix scale). Should be use sparingly for <i>2 -> 1</i> processes.
189 <p/><code>parm </code><strong> SigmaProcess:renormFixScale </strong>
190 (<code>default = <strong>10000.</strong></code>; <code>minimum = 1.</code>)<br/>
191 A fix <i>Q^2</i> value used as renormalization scale for <i>2 -> 1</i>,
192 <i>2 -> 2</i> and <i>2 -> 3</i> processes in some of the options above.
195 <h3>Factorization scales</h3>
197 Corresponding options exist for the <i>Q^2</i> factorization scale
198 used as argument in PDF's. Again there is a choice of form for
199 <i>2 -> 1</i>, <i>2 -> 2</i> and <i>2 -> 3</i> processes separately.
200 For simplicity we have let the numbering of options agree, for each event
201 class separately, between normalization and factorization scales, and the
202 description has therefore been slightly shortened. The default values are
203 <b>not</b> necessarily the same, however.
205 <p/><code>mode </code><strong> SigmaProcess:factorScale1 </strong>
206 (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 2</code>)<br/>
207 The <i>Q^2</i> factorization scale for <i>2 -> 1</i> processes.
208 The same options also apply for those <i>2 -> 2</i> and <i>2 -> 3</i>
209 processes that have been specially marked as proceeding only through
210 an <i>s</i>-channel resonance.
211 <br/><code>option </code><strong> 1</strong> : the squared invariant mass, i.e. <i>sHat</i>.
213 <br/><code>option </code><strong> 2</strong> : fix scale set in <code>SigmaProcess:factorFixScale</code>
218 <p/><code>mode </code><strong> SigmaProcess:factorScale2 </strong>
219 (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 5</code>)<br/>
220 The <i>Q^2</i> factorization scale for <i>2 -> 2</i> processes.
221 <br/><code>option </code><strong> 1</strong> : the smaller of the squared transverse masses of the two
224 <br/><code>option </code><strong> 2</strong> : the geometric mean of the squared transverse masses of
225 the two outgoing particles.
227 <br/><code>option </code><strong> 3</strong> : the arithmetic mean of the squared transverse masses of
228 the two outgoing particles. Useful for comparisons with PYTHIA 6, where
231 <br/><code>option </code><strong> 4</strong> : squared invariant mass of the system,
232 i.e. <i>sHat</i>. Useful for processes dominated by
233 <i>s</i>-channel exchange.
235 <br/><code>option </code><strong> 5</strong> : fix scale set in <code>SigmaProcess:factorFixScale</code>
240 <p/><code>mode </code><strong> SigmaProcess:factorScale3 </strong>
241 (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 6</code>)<br/>
242 The <i>Q^2</i> factorization scale for "normal" <i>2 -> 3</i>
243 processes, i.e excepting the vector-boson-fusion processes below.
244 <br/><code>option </code><strong> 1</strong> : the smaller of the squared transverse masses of the three
247 <br/><code>option </code><strong> 2</strong> : the geometric mean of the two smallest squared transverse
248 masses of the three outgoing particles.
250 <br/><code>option </code><strong> 3</strong> : the geometric mean of the squared transverse masses of the
251 three outgoing particles.
253 <br/><code>option </code><strong> 4</strong> : the arithmetic mean of the squared transverse masses of
254 the three outgoing particles.
256 <br/><code>option </code><strong> 5</strong> : squared invariant mass of the system,
259 <br/><code>option </code><strong> 6</strong> : fix scale set in <code>SigmaProcess:factorFixScale</code>
264 <p/><code>mode </code><strong> SigmaProcess:factorScale3VV </strong>
265 (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 6</code>)<br/>
266 The <i>Q^2</i> factorization scale for <i>2 -> 3</i>
267 vector-boson-fusion processes, i.e. <i>f_1 f_2 -> H^0 f_3 f_4</i>
268 with <i>Z^0</i> or <i>W^+-</i> <i>t</i>-channel propagators.
269 Here we again introduce the combinations <i>mT_Vi^2 = m_V^2 + pT_i^2</i>
270 as replacements for the normal squared transverse masses of the two
272 <br/><code>option </code><strong> 1</strong> : the squared mass <i>m_V^2</i> of the exchanged
275 <br/><code>option </code><strong> 2</strong> : the geometric mean of the two propagator virtuality
278 <br/><code>option </code><strong> 3</strong> : the geometric mean of the three relevant squared
281 <br/><code>option </code><strong> 4</strong> : the arithmetic mean of the three relevant squared
284 <br/><code>option </code><strong> 5</strong> : squared invariant mass of the system,
287 <br/><code>option </code><strong> 6</strong> : fix scale set in <code>SigmaProcess:factorFixScale</code>
292 <p/><code>parm </code><strong> SigmaProcess:factorMultFac </strong>
293 (<code>default = <strong>1.</strong></code>; <code>minimum = 0.1</code>; <code>maximum = 10.</code>)<br/>
294 The <i>Q^2</i> factorization scale for <i>2 -> 1</i>,
295 <i>2 -> 2</i> and <i>2 -> 3</i> processes is multiplied by
296 this factor relative to the scale described above (except for the options
297 with a fix scale). Should be use sparingly for <i>2 -> 1</i> processes.
300 <p/><code>parm </code><strong> SigmaProcess:factorFixScale </strong>
301 (<code>default = <strong>10000.</strong></code>; <code>minimum = 1.</code>)<br/>
302 A fix <i>Q^2</i> value used as factorization scale for <i>2 -> 1</i>,
303 <i>2 -> 2</i> and <i>2 -> 3</i> processes in some of the options above.
309 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->