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SigmaLeptoquark.cc
1 // SigmaLeptoquark.cc is a part of the PYTHIA event generator.
2 // Copyright (C) 2018 Torbjorn Sjostrand.
3 // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
4 // Please respect the MCnet Guidelines, see GUIDELINES for details.
5 
6 // Function definitions (not found in the header) for the
7 // leptoquark simulation classes.
8 
9 #include "Pythia8/SigmaLeptoquark.h"
10 
11 namespace Pythia8 {
12 
13 //==========================================================================
14 
15 // Sigma1ql2LeptoQuark class.
16 // Cross section for q l -> LQ (leptoquark state).
17 
18 //--------------------------------------------------------------------------
19 
20 // Initialize process.
21 
22 void Sigma1ql2LeptoQuark::initProc() {
23 
24  // Store LQ mass and width for propagator.
25  mRes = particleDataPtr->m0(42);
26  GammaRes = particleDataPtr->mWidth(42);
27  m2Res = mRes*mRes;
28  GamMRat = GammaRes / mRes;
29 
30  // Yukawa coupling strength.
31  kCoup = settingsPtr->parm("LeptoQuark:kCoup");
32 
33  // Set pointer to particle properties and decay table.
34  LQPtr = particleDataPtr->particleDataEntryPtr(42);
35 
36  // Read out quark and lepton the LQ couples to.
37  idQuark = LQPtr->channel(0).product(0);
38  idLepton = LQPtr->channel(0).product(1);
39 
40 }
41 
42 //--------------------------------------------------------------------------
43 
44 // Evaluate sigmaHat(sHat), part independent of incoming flavour.
45 
46 void Sigma1ql2LeptoQuark::sigmaKin() {
47 
48  // Incoming width for correct quark-lepton pair.
49  widthIn = 0.25 * alpEM * kCoup * mH;
50 
51  // Set up Breit-Wigner.
52  sigBW = 4. * M_PI/ ( pow2(sH - m2Res) + pow2(sH * GamMRat) );
53 
54 }
55 
56 //--------------------------------------------------------------------------
57 
58 // Evaluate sigmaHat(sHat) for specific incoming flavours.
59 
60 double Sigma1ql2LeptoQuark::sigmaHat() {
61 
62  // Identify whether correct incoming flavours.
63  int idLQ = 0;
64  if (id1 == idQuark && id2 == idLepton) idLQ = 42;
65  else if (id2 == idQuark && id1 == idLepton) idLQ = 42;
66  else if (id1 == -idQuark && id2 == -idLepton) idLQ = -42;
67  else if (id2 == -idQuark && id1 == -idLepton) idLQ = -42;
68  if (idLQ == 0) return 0.;
69 
70  // Outgoing width and total sigma. Done.
71  return widthIn * sigBW * LQPtr->resWidthOpen(idLQ, mH);
72 
73 }
74 
75 //--------------------------------------------------------------------------
76 
77 // Select identity, colour and anticolour.
78 
79 void Sigma1ql2LeptoQuark::setIdColAcol() {
80 
81  // Flavours.
82  int idq = (abs(id1) < 9) ? id1 : id2;
83  int idLQ = (idq > 0) ? 42 : -42;
84  setId( id1, id2, idLQ);
85 
86  // Colour flow topology.
87  if (id1 == idq) setColAcol( 1, 0, 0, 0, 1, 0);
88  else setColAcol( 0, 0, 1, 0, 1, 0);
89  if (idq < 0) swapColAcol();
90 
91 }
92 
93 //==========================================================================
94 
95 // Sigma2qg2LeptoQuarkl class.
96 // Cross section for q g -> LQ l (leptoquark state).
97 
98 //--------------------------------------------------------------------------
99 
100 // Initialize process.
101 
102 void Sigma2qg2LeptoQuarkl::initProc() {
103 
104  // Store LQ mass and width for propagator.
105  mRes = particleDataPtr->m0(42);
106  GammaRes = particleDataPtr->mWidth(42);
107  m2Res = mRes*mRes;
108  GamMRat = GammaRes / mRes;
109 
110  // Yukawa coupling strength.
111  kCoup = settingsPtr->parm("LeptoQuark:kCoup");
112 
113  // Read out quark and lepton the LQ couples to.
114  ParticleDataEntry* LQPtr = particleDataPtr->particleDataEntryPtr(42);
115  idQuark = LQPtr->channel(0).product(0);
116  idLepton = LQPtr->channel(0).product(1);
117 
118  // Secondary open width fraction.
119  openFracPos = LQPtr->resOpenFrac( 42);
120  openFracNeg = LQPtr->resOpenFrac(-42);
121 
122 }
123 
124 //--------------------------------------------------------------------------
125 
126 // Evaluate sigmaHat(sHat), part independent of incoming flavour.
127 
128 void Sigma2qg2LeptoQuarkl::sigmaKin() {
129 
130  // Evaluate cross section.
131  sigma0 = (M_PI / sH2) * kCoup * (alpS * alpEM / 6.) * (-tH / sH)
132  * (uH2 + s3 * s3) / pow2(uH - s3);
133 
134 }
135 
136 //--------------------------------------------------------------------------
137 
138 // Evaluate sigmaHat(sHat) for specific incoming flavours.
139 
140 double Sigma2qg2LeptoQuarkl::sigmaHat() {
141 
142  // Check that correct incoming flavour.
143  if (abs(id1) != idQuark && abs(id2) != idQuark) return 0.;
144 
145  // Answer, with secondary width correction.
146  double sigma = sigma0;
147  sigma *= (id1 == idQuark || id2 == idQuark) ? openFracPos : openFracNeg;
148  return sigma;
149 
150 }
151 
152 //--------------------------------------------------------------------------
153 
154 // Select identity, colour and anticolour.
155 
156 void Sigma2qg2LeptoQuarkl::setIdColAcol() {
157 
158  // Flavour set up for q g -> H q.
159  int idq = (id2 == 21) ? id1 : id2;
160  int idLQ = (idq > 0) ? 42 : -42;
161  int idlp = (idq > 0) ? -idLepton : idLepton;
162  setId( id1, id2, idLQ, idlp);
163 
164  // tH defined between f and LQ: must swap tHat <-> uHat if g q in.
165  swapTU = (id1 == 21);
166 
167  // Colour flow topologies. Swap when antiquarks.
168  if (id2 == 21) setColAcol( 1, 0, 2, 1, 2, 0, 0, 0);
169  else setColAcol( 2, 1, 1, 0, 2, 0, 0, 0);
170  if (idq < 0) swapColAcol();
171 
172 }
173 
174 //==========================================================================
175 
176 // Sigma2gg2LQLQbar class.
177 // Cross section for g g -> LQ LQbar (leptoquark state).
178 
179 //--------------------------------------------------------------------------
180 
181 // Initialize process.
182 
183 void Sigma2gg2LQLQbar::initProc() {
184 
185  // Store LQ mass and width for propagator.
186  mRes = particleDataPtr->m0(42);
187  GammaRes = particleDataPtr->mWidth(42);
188  m2Res = mRes*mRes;
189  GamMRat = GammaRes / mRes;
190 
191  // Secondary open width fraction.
192  openFrac = particleDataPtr->resOpenFrac(42, -42);
193 
194 }
195 
196 //--------------------------------------------------------------------------
197 
198 // Evaluate sigmaHat(sHat), part independent of incoming flavour.
199 
200 void Sigma2gg2LQLQbar::sigmaKin() {
201 
202  // Average outgoing masses and adjust kinematics accordingly.
203  double delta = 0.25 * pow2(s3 - s4) / sH;
204  double m2avg = 0.5 * (s3 + s4) - delta;
205  double tHavg = tH - delta;
206  double uHavg = uH - delta;
207 
208  // Evaluate cross section. Secondary width for G*.
209  sigma = (M_PI / sH2) * 0.5 * pow2(alpS)
210  * ( 7. / 48. + 3. * pow2(uHavg - tHavg) / (16. * sH2) )
211  * ( 1. + 2. * m2avg * tHavg / pow2(tHavg - m2avg)
212  + 2. * m2avg * uHavg / pow2(uHavg - m2avg)
213  + 4. * m2avg * m2avg / ((tHavg - m2avg) * (uHavg - m2avg)) );
214  sigma *= openFrac;
215 
216 }
217 
218 //--------------------------------------------------------------------------
219 
220 // Select identity, colour and anticolour.
221 
222 void Sigma2gg2LQLQbar::setIdColAcol() {
223 
224  // Flavours trivial.
225  setId( 21, 21, 42, -42);
226 
227  // Colour flow topologies: random choice between two mirrors.
228  if (rndmPtr->flat() < 0.5) setColAcol( 1, 2, 2, 3, 1, 0, 0, 3);
229  else setColAcol( 1, 2, 3, 1, 3, 0, 0, 2);
230 
231 }
232 
233 //==========================================================================
234 
235 // Sigma2qqbar2LQLQbar class.
236 // Cross section for q qbar -> LQ LQbar (leptoquark state).
237 
238 //--------------------------------------------------------------------------
239 
240 // Initialize process.
241 
242 void Sigma2qqbar2LQLQbar::initProc() {
243 
244  // Store LQ mass and width for propagator.
245  mRes = particleDataPtr->m0(42);
246  GammaRes = particleDataPtr->mWidth(42);
247  m2Res = mRes*mRes;
248  GamMRat = GammaRes / mRes;
249 
250  // Yukawa coupling strength.
251  kCoup = settingsPtr->parm("LeptoQuark:kCoup");
252 
253  // Read out quark and lepton the LQ couples to.
254  ParticleDataEntry* LQPtr = particleDataPtr->particleDataEntryPtr(42);
255  idQuark = LQPtr->channel(0).product(0);
256 
257  // Secondary open width fraction.
258  openFrac = particleDataPtr->resOpenFrac(42, -42);
259 
260 }
261 
262 //--------------------------------------------------------------------------
263 
264 // Evaluate sigmaHat(sHat), part independent of incoming flavour.
265 
266 void Sigma2qqbar2LQLQbar::sigmaKin() {
267 
268  // Average outgoing masses and adjust kinematics accordingly.
269  double delta = 0.25 * pow2(s3 - s4) / sH;
270  double m2avg = 0.5 * (s3 + s4) - delta;
271  double tHavg = tH - delta;
272  double uHavg = uH - delta;
273 
274  // Evaluate cross section for quark of different flavour than LQ.
275  sigmaDiff = (M_PI / sH2) * (pow2(alpS) / 9.)
276  * ( sH * (sH - 4. * m2avg) - pow2(uHavg - tHavg) ) / sH2;
277 
278  // Evaluate cross section for quark of same flavour as LQ.
279  sigmaSame = sigmaDiff + (M_PI / sH2) * (pow2(kCoup * alpEM) / 8.)
280  * (-sH * tHavg - pow2(m2avg-tHavg)) / pow2(tHavg)
281  + (M_PI / sH2) * (kCoup * alpEM * alpS / 18.) * ( (m2avg - tHavg)
282  * (uHavg - tHavg) + sH * (m2avg + tHavg) ) / (sH * tHavg);
283 
284  // Open fraction.
285  sigmaDiff *= openFrac;
286  sigmaSame *= openFrac;
287 
288 }
289 
290 //--------------------------------------------------------------------------
291 
292 // Select identity, colour and anticolour.
293 
294 void Sigma2qqbar2LQLQbar::setIdColAcol() {
295 
296  // Flavours trivial.
297  setId( id1, id2, 42, -42);
298 
299  // tH defined between f and LQ: must swap tHat <-> uHat if qbar q in.
300  swapTU = (id1 < 0);
301 
302  // Colour flow topologies.
303  if (id1 > 0) setColAcol( 1, 0, 0, 2, 1, 0, 0, 2);
304  else setColAcol( 0, 2, 1, 0, 1, 0, 0, 2);
305 
306 }
307 
308 //==========================================================================
309 
310 } // end namespace Pythia8
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