3 <title>Standard-Model Parameters</title>
4 <link rel="stylesheet" type="text/css" href="pythia.css"/>
5 <link rel="shortcut icon" href="pythia32.gif"/>
9 <h2>Standard-Model Parameters</h2>
11 <h3>The strong coupling</h3>
13 The <code>AlphaStrong</code> class is used to provide a first- or
14 second-order running <i>alpha_strong</i> (or, trivially, a
15 zeroth-order fixed one). Formulae are the standard ones found in
16 [<a href="Bibliography.html" target="page">Yao06</a>]. The second-order expression used, eq. (9.5),
17 may be somewhat different in other approaches (with differences
18 formally of higher order), so do not necessarily expect perfect
19 agreement, especially not at small <i>Q^2</i> scales. The starting
20 <i>alpha_strong</i> value is defined at the <i>M_Z</i> mass scale.
21 The <i>Lambda</i> values are matched at the <i>b</i> and <i>c</i>
22 flavour thresholds, such that <i>alpha_strong</i> is continuous.
23 For second-order matching an approximate iterative method is used.
26 Since we allow <i>alpha_strong</i> to vary separately for
27 hard processes, timelike showers, spacelike showers and multiparton
28 interactions, the relevant values can be set in each of these classes.
29 The default behaviour is everywhere first-order running.
32 The <i>alpha_strong</i> calculation is initialized by
33 <code>init( value, order)</code>, where <code>value</code>
34 is the <i>alpha_strong</i> value at <i>M_Z</i> and <code>order</code>
35 is the order of the running, 0, 1 or 2. Thereafter the value can be
36 calculated by <code>alphaS(scale2)</code>, where
37 <code>scale2</code> is the <i>Q^2</i> scale in GeV^2.
40 For applications inside shower programs, a second-order <code>alpha_s</code>
41 value can be obtained as the product of the two functions
42 <code>alphaS1Ord(scale2)</code> and <code>alphaS2OrdCorr(scale2)</code>,
43 where the first gives a simple first-order running (but with the
44 second-order <i>Lambda</i>) and the second the correction factor,
45 below unity, for the second-order terms. This allows a compact handling
46 of evolution equations.
48 <h3>The electromagnetic coupling</h3>
50 The <code>AlphaEM</code> class is used to generate a running
51 <i>alpha_em</i>. The input <code>StandardModel:alphaEMmZ</code>
52 value at the <i>M_Z</i> mass is matched to a low-energy behaviour
53 with running starting at the electron mass threshold. The matching
54 is done by fitting an effective running coefficient in the region
55 betweeen the light-quark treshold and the charm/tau threshold. This
56 procedure is approximate, but good enough for our purposes.
59 Since we allow <i>alpha_em</i> to vary separately for
60 hard processes, timelike showers, spacelike showers and multiparton
61 interactions, the choice between using a fixed or a running
62 <i>alpha_em</i> can be made in each of these classes.
63 The default behaviour is everywhere first-order running.
64 The actual values assumed at zero momentum transfer and
65 at <i>M_Z</i> are only set here, however.
67 <p/><code>parm </code><strong> StandardModel:alphaEM0 </strong>
68 (<code>default = <strong>0.00729735</strong></code>; <code>minimum = 0.0072973</code>; <code>maximum = 0.0072974</code>)<br/>
69 The <i>alpha_em</i> value at vanishing momentum transfer
70 (and also below <i>m_e</i>).
73 <p/><code>parm </code><strong> StandardModel:alphaEMmZ </strong>
74 (<code>default = <strong>0.00781751</strong></code>; <code>minimum = 0.00780</code>; <code>maximum = 0.00783</code>)<br/>
75 The <i>alpha_em</i> value at the <i>M_Z</i> mass scale.
76 Default is taken from [<a href="Bibliography.html" target="page">Yao06</a>].
80 The <i>alpha_em</i> calculation is initialized by
81 <code>init(order)</code>, where <code>order</code> is the order of
82 the running, 0 or 1, with -1 a special option to use the fix value
83 provided at <i>M_Z</i>. Thereafter the value can be
84 calculated by <code>alphaEM(scale2)</code>, where
85 <code>scale2</code> is the <i>Q^2</i> scale in GeV^2.
87 <h3>The electroweak couplings</h3>
89 There are two degrees of freedom that can be set, related to the
90 electroweak mixing angle:
92 <p/><code>parm </code><strong> StandardModel:sin2thetaW </strong>
93 (<code>default = <strong>0.2312</strong></code>; <code>minimum = 0.225</code>; <code>maximum = 0.240</code>)<br/>
94 The sine-squared of the weak mixing angle, as used in all <i>Z^0</i>
95 and <i>W^+-</i> masses and couplings, except for the vector couplings
96 of fermions to the <i>Z^0</i>, see below. Default is the MSbar value
97 from [<a href="Bibliography.html" target="page">Yao06</a>].
100 <p/><code>parm </code><strong> StandardModel:sin2thetaWbar </strong>
101 (<code>default = <strong>0.2315</strong></code>; <code>minimum = 0.225</code>; <code>maximum = 0.240</code>)<br/>
102 The sine-squared of the weak mixing angle, as used to derive the vector
103 couplings of fermions to the <i>Z^0</i>, in the relation
104 <i>v_f = a_f - 4 e_f sin^2(theta_W)bar</i>. Default is the
105 effective-angle value from [<a href="Bibliography.html" target="page">Yao06</a>].
109 The Fermi constant is not much used in the currently coded matrix elements,
110 since it is redundant, but it is available:
112 <p/><code>parm </code><strong> StandardModel:GF </strong>
113 (<code>default = <strong>1.16637e-5</strong></code>; <code>minimum = 1.0e-5</code>; <code>maximum = 1.3e-5</code>)<br/>
114 The Fermi coupling constant, in units of GeV<i>^-2</i>.
117 <h3>The quark weak-mixing matrix</h3>
119 The absolute values of the Cabibbo-Kobayashi-Maskawa matrix elements are
120 set by the following nine real values taken from [<a href="Bibliography.html" target="page">Yao06</a>] -
121 currently the CP-violating phase is not taken into account in this
122 parametrization. It is up to the user to pick a consistent unitary
123 set of new values whenever changes are made.
125 <p/><code>parm </code><strong> StandardModel:Vud </strong>
126 (<code>default = <strong>0.97383</strong></code>; <code>minimum = 0.973</code>; <code>maximum = 0.975</code>)<br/>
127 The <i>V_ud</i> CKM matrix element.
130 <p/><code>parm </code><strong> StandardModel:Vus </strong>
131 (<code>default = <strong>0.2272</strong></code>; <code>minimum = 0.224</code>; <code>maximum = 0.230</code>)<br/>
132 The <i>V_us</i> CKM matrix element.
135 <p/><code>parm </code><strong> StandardModel:Vub </strong>
136 (<code>default = <strong>0.00396</strong></code>; <code>minimum = 0.0037</code>; <code>maximum = 0.0042</code>)<br/>
137 The <i>V_ub</i> CKM matrix element.
140 <p/><code>parm </code><strong> StandardModel:Vcd </strong>
141 (<code>default = <strong>0.2271</strong></code>; <code>minimum = 0.224</code>; <code>maximum = 0.230</code>)<br/>
142 The <i>V_cd</i> CKM matrix element.
145 <p/><code>parm </code><strong> StandardModel:Vcs </strong>
146 (<code>default = <strong>0.97296</strong></code>; <code>minimum = 0.972</code>; <code>maximum = 0.974</code>)<br/>
147 The <i>V_cs</i> CKM matrix element.
150 <p/><code>parm </code><strong> StandardModel:Vcb </strong>
151 (<code>default = <strong>0.04221</strong></code>; <code>minimum = 0.0418</code>; <code>maximum = 0.0426</code>)<br/>
152 The <i>V_cb</i> CKM matrix element.
155 <p/><code>parm </code><strong> StandardModel:Vtd </strong>
156 (<code>default = <strong>0.00814</strong></code>; <code>minimum = 0.006</code>; <code>maximum = 0.010</code>)<br/>
157 The <i>V_td</i> CKM matrix element.
160 <p/><code>parm </code><strong> StandardModel:Vts </strong>
161 (<code>default = <strong>0.04161</strong></code>; <code>minimum = 0.039</code>; <code>maximum = 0.043</code>)<br/>
162 The <i>V_ts</i> CKM matrix element.
165 <p/><code>parm </code><strong> StandardModel:Vtb </strong>
166 (<code>default = <strong>0.9991</strong></code>; <code>minimum = 0.99907</code>; <code>maximum = 0.9992</code>)<br/>
167 The <i>V_tb</i> CKM matrix element.
170 <h3>The CoupSM class</h3>
172 The <code><a href="ProgramFlow.html" target="page">Pythia</a></code> class contains a
173 public instance <code>coupSM</code> of the <code>CoupSM</code> class.
174 This class contains one instance each of the <code>AlphaStrong</code>
175 and <code>AlphaEM</code> classes, and additionally stores the weak couplings
176 and the quark mixing matrix mentioned above. This class is used especially
177 in the calculation of cross sections and resonance widths, but could also
178 be used elsewhere. Specifically, as already mentioned, there are separate
179 <code>AlphaStrong</code> and <code>AlphaEM</code> instances for timelike
180 and spacelike showers and for multiparton interactions, while weak couplings
181 and the quark mixing matrix are only stored here. With the exception of the
182 first two methods below, which are for internal use, the subsequent ones
183 could also be used externally.
185 <a name="method1"></a>
186 <p/><strong>CoupSM::CoupSM() </strong> <br/>
187 the constructor does nothing. Internal.
190 <a name="method2"></a>
191 <p/><strong>void CoupSM::init(Settings& settings, Rndm* rndmPtr) </strong> <br/>
192 this is where the <code>AlphaStrong</code> and <code>AlphaEM</code>
193 instances are initialized, and weak couplings and the quark mixing matrix
194 are read in and set. This is based on the values stored on this page and
195 among the <a href="CouplingsAndScales.html" target="page">Couplings and Scales</a>.
199 <a name="method3"></a>
200 <p/><strong>double CoupSM::alphaS(double scale2) </strong> <br/>
201 the <i>alpha_strong</i> value at the quadratic scale <code>scale2</code>.
204 <a name="method4"></a>
205 <p/><strong>double CoupSM::alphaS1Ord(double scale2) </strong> <br/>
206 a first-order overestimate of the full second-order <i>alpha_strong</i>
207 value at the quadratic scale <code>scale2</code>.
210 <a name="method5"></a>
211 <p/><strong>double CoupSM::alphaS2OrdCorr(double scale2) </strong> <br/>
212 a multiplicative correction factor, below unity, that brings the
213 first-order overestimate above into agreement with the full second-order
214 <i>alpha_strong</i> value at the quadratic scale <code>scale2</code>.
217 <a name="method6"></a>
218 <p/><strong>double CoupSM::Lambda3() </strong> <br/>
220 <strong>double CoupSM::Lambda4() </strong> <br/>
222 <strong>double CoupSM::Lambda5() </strong> <br/>
223 the three-, four-, and five-flavour <i>Lambda</i> scale.
226 <a name="method7"></a>
227 <p/><strong>double CoupSM::alphaEM(double scale2) </strong> <br/>
228 the <i>alpha_em</i> value at the quadratic scale <code>scale2</code>.
231 <a name="method8"></a>
232 <p/><strong>double CoupSM::sin2thetaW() </strong> <br/>
234 <strong>double CoupSM::cos2thetaW() </strong> <br/>
235 the sine-squared and cosine-squared of the weak mixing angle, as used in
236 the gauge-boson sector.
239 <a name="method9"></a>
240 <p/><strong>double CoupSM::sin2thetaWbar() </strong> <br/>
241 the sine-squared of the weak mixing angle, as used to derive the vector
242 couplings of fermions to the <i>Z^0</i>.
245 <a name="method10"></a>
246 <p/><strong>double CoupSM::GF() </strong> <br/>
247 the Fermi constant of weak decays, in GeV<i>^-2</i>.
250 <a name="method11"></a>
251 <p/><strong>double CoupSM::ef(int idAbs) </strong> <br/>
252 the electrical charge of a fermion, by the absolute sign of the PDF code,
253 i.e. <code>idAbs</code> must be in the range between 1 and 18.
256 <a name="method12"></a>
257 <p/><strong>double CoupSM::vf(int idAbs) </strong> <br/>
259 <strong>double CoupSM::af(int idAbs) </strong> <br/>
260 the vector and axial charges of a fermion, by the absolute sign of the PDF
261 code (<i>a_f = +-1, v_f = a_f - 4. * sin2thetaWbar * e_f</i>).
264 <a name="method13"></a>
265 <p/><strong>double CoupSM::t3f(int idAbs) </strong> <br/>
267 <strong>double CoupSM::lf(int idAbs) </strong> <br/>
269 <strong>double CoupSM::rf(int idAbs) </strong> <br/>
270 the weak isospin, left- and righthanded charges of a fermion, by the
271 absolute sign of the PDF code (<i>t^3_f = a_f/2, l_f = (v_f + a_f)/2,
272 r_f = (v_f - a_f)/2</i>; you may find other conventions in the literature
273 that differ by a factor of 2).
276 <a name="method14"></a>
277 <p/><strong>double CoupSM::ef2(int idAbs) </strong> <br/>
279 <strong>double CoupSM::vf2(int idAbs) </strong> <br/>
281 <strong>double CoupSM::af2(int idAbs) </strong> <br/>
283 <strong>double CoupSM::efvf(int idAbs) </strong> <br/>
285 <strong>double CoupSM::vf2af2(int idAbs) </strong> <br/>
286 common quadratic combinations of the above couplings:
287 <i>e_f^2, v_f^2, a_f^2, e_f * v_f, v_f^2 + a_f^2</i>.
290 <a name="method15"></a>
291 <p/><strong>double CoupSM::VCKMgen(int genU, int genD) </strong> <br/>
293 <strong>double CoupSM::V2CKMgen(int genU, int genD) </strong> <br/>
294 the CKM mixing element,or the square of it, for
295 up-type generation index <code>genU</code>
296 (<i>1 = u, 2 = c, 3 = t, 4 = t'</i>) and
297 down-type generation index <code>genD</code>
298 (<i>1 = d, 2 = s, 3 = b, 4 = b'</i>).
301 <a name="method16"></a>
302 <p/><strong>double CoupSM::VCKMid(int id1, int id2) </strong> <br/>
304 <strong>double CoupSM::V2CKMid(int id1, int id2) </strong> <br/>
305 the CKM mixing element,or the square of it, for
306 flavours <code>id1</code> and <code>id2</code>, both in the
307 range from <i>-18</i> to <i>+18</i>. The sign is here not
308 checked (so it can be used both for <i>u + dbar -> W+</i>
309 and <i>u -> d + W+</i>, say), but impossible flavour combinations
310 evaluate to zero. The neutrino sector is numbered by flavor
311 eigenstates, so there is no mixing in the lepton-neutrino system.
314 <a name="method17"></a>
315 <p/><strong>double CoupSM::V2CKMsum(int id) </strong> <br/>
316 the sum of squared CKM mixing element that a given flavour can couple to,
317 excluding the top quark and fourth generation. Is close to unity
318 for the first two generations. Returns unity for the lepton-neutrino
322 <a name="method18"></a>
323 <p/><strong>int CoupSM::V2CKMpick(int id) </strong> <br/>
324 picks a random CKM partner quark or lepton (with the same sign as
325 <code>id</code>) according to the respective squared elements, again
326 excluding the top quark and fourth generation from the list of
327 possibilities. Unambiguous choice for the lepton-neutrino sector.
333 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->