1 <chapter name="Spacelike Showers">
3 <h2>Spacelike Showers</h2>
5 The PYTHIA algorithm for spacelike initial-state showers is
6 based on the article <ref>Sjo05</ref>, where a
7 transverse-momentum-ordered backwards evolution scheme is introduced,
8 with the extension to fully interleaved evolution covered in
10 This algorithm is a further development of the virtuality-ordered one
11 presented in <ref>Sj085</ref>, with matching to first-order matrix
12 element for <ei>Z^0</ei>, <ei>W^+-</ei> and Higgs (in the
13 <ei>m_t -> infinity</ei> limit) production as introduced in
17 The normal user is not expected to call <code>SpaceShower</code>
18 directly, but only have it called from <code>Pythia</code>,
19 via <code>PartonLevel</code>. Some of the parameters below,
20 in particular <code>SpaceShower:alphaSvalue</code>,
21 would be of interest for a tuning exercise, however.
23 <h3>Main variables</h3>
25 The maximum <ei>pT</ei> to be allowed in the shower evolution is
26 related to the nature of the hard process itself. It involves a
27 delicate balance between not doublecounting and not leaving any
28 gaps in the coverage. The best procedure may depend on information
29 only the user has: how the events were generated and mixed (e.g. with
30 Les Houches Accord external input), and how they are intended to be
31 used. Therefore a few options are available, with a sensible default
34 <modepick name="SpaceShower:pTmaxMatch" default="0" min="0" max="2">
35 Way in which the maximum shower evolution scale is set to match the
36 scale of the hard process itself.
37 <option value="0"><b>(i)</b> if the final state of the hard process
38 (not counting subsequent resonance decays) contains at least one quark
39 (<ei>u, d, s, c ,b</ei>), gluon or photon then <ei>pT_max</ei>
40 is chosen to be the factorization scale for internal processes
41 and the <code>scale</code> value for Les Houches input;
42 <b>(ii)</b> if not, emissions are allowed to go all the way up to
43 the kinematical limit.
44 The reasoning is that in the former set of processes the ISR
45 emission of yet another quark, gluon or photon could lead to
46 doublecounting, while no such danger exists in the latter case.
48 <option value="1">always use the factorization scale for an internal
49 process and the <code>scale</code> value for Les Houches input,
50 i.e. the lower value. This should avoid doublecounting, but
51 may leave out some emissions that ought to have been simulated.
52 (Also known as wimpy showers.)
54 <option value="2">always allow emissions up to the kinematical limit.
55 This will simulate all possible event topologies, but may lead to
57 (Also known as power showers.)
59 <note>Note 1:</note> These options only apply to the hard interaction.
60 Emissions off subsequent multiparton interactions are always constrainted
61 to be below the factorization scale of the process itself.
62 <note>Note 2:</note> Some processes contain matrix-element matching
63 to the first emission; this is the case notably for single
64 <ei>gamma^*/Z^0, W^+-</ei> and <ei>H^0</ei> production. Then default
65 and option 2 give the correct result, while option 1 should never
69 <parm name="SpaceShower:pTmaxFudge" default="1.0" min="0.25" max="2.0">
70 In cases where the above <code>pTmaxMatch</code> rules would imply
71 that <ei>pT_max = pT_factorization</ei>, <code>pTmaxFudge</code>
72 introduces a multiplicative factor <ei>f</ei> such that instead
73 <ei>pT_max = f * pT_factorization</ei>. Only applies to the hardest
74 interaction in an event, cf. below. It is strongly suggested that
75 <ei>f = 1</ei>, but variations around this default can be useful to
79 <parm name="SpaceShower:pTmaxFudgeMPI" default="1.0" min="0.25" max="2.0">
80 A multiplicative factor <ei>f</ei> such that
81 <ei>pT_max = f * pT_factorization</ei>, as above, but here for the
82 non-hardest interactions (when multiparton interactions are allowed).
85 <modepick name="SpaceShower:pTdampMatch" default="0" min="0" max="2">
86 These options only take effect when a process is allowed to radiate up
87 to the kinematical limit by the above <code>pTmaxMatch</code> choice,
88 and no matrix-element corrections are available. Then, in many processes,
89 the fall-off in <ei>pT</ei> will be too slow by one factor of <ei>pT^2</ei>.
90 That is, while showers have an approximate <ei>dpT^2/pT^2</ei> shape, often
91 it should become more like <ei>dpT^2/pT^4</ei> at <ei>pT</ei> values above
92 the scale of the hard process. Whether this actually is the case
93 depends on the particular process studied, e.g. if <ei>t</ei>-channel
94 gluon exchange is likely to dominate. If so, the options below could
95 provide a reasonable high-<ei>pT</ei> behaviour without requiring
96 higher-order calculations.
97 <option value="0">emissions go up to the kinematical limit,
98 with no special dampening.
100 <option value="1">emissions go up to the kinematical limit,
101 but dampened by a factor <ei>k^2 Q^2_fac/(pT^2 + k^2 Q^2_fac)</ei>,
102 where <ei>Q_fac</ei> is the factorization scale and <ei>k</ei> is a
103 multiplicative fudge factor stored in <code>pTdampFudge</code> below.
105 <option value="2">emissions go up to the kinematical limit,
106 but dampened by a factor <ei>k^2 Q^2_ren/(pT^2 + k^2 Q^2_ren)</ei>,
107 where <ei>Q_ren</ei> is the renormalization scale and <ei>k</ei> is a
108 multiplicative fudge factor stored in <code>pTdampFudge</code> below.
110 <note>Note:</note> These options only apply to the hard interaction.
111 Emissions off subsequent multiparton interactions are always constrainted
112 to be below the factorization scale of the process itself.
115 <parm name="SpaceShower:pTdampFudge" default="1.0" min="0.25" max="4.0">
116 In cases 1 and 2 above, where a dampening is imposed at around the
117 factorization or renormalization scale, respectively, this allows the
118 <ei>pT</ei> scale of dampening of radiation by a half to be shifted
119 by this factor relative to the default <ei>Q_fac</ei> or <ei>Q_ren</ei>.
120 This number ought to be in the neighbourhood of unity, but variations
121 away from this value could do better in some processes.
125 The amount of QCD radiation in the shower is determined by
126 <parm name="SpaceShower:alphaSvalue" default="0.137" min="0.06" max="0.25">
127 The <ei>alpha_strong</ei> value at scale <code>M_Z^2</code>.
128 Default value is picked equal to the one used in CTEQ 5L.
132 The actual value is then regulated by the running to the scale
133 <ei>pT^2</ei>, at which it is evaluated
134 <modepick name="SpaceShower:alphaSorder" default="1" min="0" max="2">
135 Order at which <ei>alpha_strong</ei> runs,
136 <option value="0">zeroth order, i.e. <ei>alpha_strong</ei> is kept
138 <option value="1">first order, which is the normal value.</option>
139 <option value="2">second order. Since other parts of the code do
140 not go to second order there is no strong reason to use this option,
141 but there is also nothing wrong with it.</option>
145 QED radiation is regulated by the <ei>alpha_electromagnetic</ei>
146 value at the <ei>pT^2</ei> scale of a branching.
148 <modepick name="SpaceShower:alphaEMorder" default="1" min="-1" max="1">
149 The running of <ei>alpha_em</ei>.
150 <option value="1">first-order running, constrained to agree with
151 <code>StandardModel:alphaEMmZ</code> at the <ei>Z^0</ei> mass.
153 <option value="0">zeroth order, i.e. <ei>alpha_em</ei> is kept
154 fixed at its value at vanishing momentum transfer.</option>
155 <option value="-1">zeroth order, i.e. <ei>alpha_em</ei> is kept
156 fixed, but at <code>StandardModel:alphaEMmZ</code>, i.e. its value
157 at the <ei>Z^0</ei> mass.
162 The natural scale for couplings and PDFs is <ei>pT^2</ei>. To explore
163 uncertainties it is possibly to vary around this value, however, in
164 analogy with what can be done for
165 <aloc href="CouplingsAndScales">hard processes</aloc>.
167 <parm name="SpaceShower:renormMultFac" default="1." min="0.1" max="10.">
168 The default <ei>pT^2</ei> renormalization scale is multiplied by
169 this prefactor. For QCD this is equivalent to a change of
170 <ei>Lambda^2</ei> in the opposite direction, i.e. to a change of
171 <ei>alpha_strong(M_Z^2)</ei> (except that flavour thresholds
172 remain at fixed scales). Below, when <ei>pT^2 + pT_0^2</ei> is used
173 as scale, it is this whole expression that is multiplied by the prefactor.
176 <parm name="SpaceShower:factorMultFac" default="1." min="0.1" max="10.">
177 The default <ei>pT^2</ei> factorization scale is multiplied by
182 There are two complementary ways of regularizing the small-<ei>pT</ei>
183 divergence, a sharp cutoff and a smooth dampening. These can be
184 combined as desired but it makes sense to coordinate with how the
185 same issue is handled in multiparton interactions.
187 <flag name="SpaceShower:samePTasMPI" default="off">
188 Regularize the <ei>pT -> 0</ei> divergence using the same sharp cutoff
189 and smooth dampening parameters as used to describe multiparton interactions.
190 That is, the <code>MultipartonInteractions:pT0Ref</code>,
191 <code>MultipartonInteractions:ecmRef</code>,
192 <code>MultipartonInteractions:ecmPow</code> and
193 <code>MultipartonInteractions:pTmin</code> parameters are used to regularize
194 all ISR QCD radiation, rather than the corresponding parameters below.
195 This is a sensible physics ansatz, based on the assumption that colour
196 screening effects influence both MPI and ISR in the same way. Photon
197 radiation is regularized separately in either case.
198 <note>Warning:</note> if a large <code>pT0</code> is picked for multiparton
199 interactions, such that the integrated interaction cross section is
200 below the nondiffractive inelastic one, this <code>pT0</code> will
201 automatically be scaled down to cope. Information on such a rescaling
202 does NOT propagate to <code>SpaceShower</code>, however.
206 The actual <code>pT0</code> parameter used at a given CM energy scale,
207 <ei>ecmNow</ei>, is obtained as
209 pT0 = pT0(ecmNow) = pT0Ref * (ecmNow / ecmRef)^ecmPow
211 where <ei>pT0Ref</ei>, <ei>ecmRef</ei> and <ei>ecmPow</ei> are the
212 three parameters below.
214 <parm name="SpaceShower:pT0Ref" default="2.0"
215 min="0.5" max="10.0">
216 Regularization of the divergence of the QCD emission probability for
217 <ei>pT -> 0</ei> is obtained by a factor <ei>pT^2 / (pT0^2 + pT^2)</ei>,
218 and by using an <ei>alpha_s(pT0^2 + pT^2)</ei>. An energy dependence
219 of the <ei>pT0</ei> choice is introduced by the next two parameters,
220 so that <ei>pT0Ref</ei> is the <ei>pT0</ei> value for the reference
221 cm energy, <ei>pT0Ref = pT0(ecmRef)</ei>.
224 <parm name="SpaceShower:ecmRef" default="1800.0" min="1.">
225 The <ei>ecmRef</ei> reference energy scale introduced above.
228 <parm name="SpaceShower:ecmPow" default="0.0" min="0." max="0.5">
229 The <ei>ecmPow</ei> energy rescaling pace introduced above.
232 <parm name="SpaceShower:pTmin" default="0.2"
233 min="0.1" max="10.0">
234 Lower cutoff in <ei>pT</ei>, below which no further ISR branchings
235 are allowed. Normally the <ei>pT0</ei> above would be used to
236 provide the main regularization of the branching rate for
237 <ei>pT -> 0</ei>, in which case <ei>pTmin</ei> is used mainly for
238 technical reasons. It is possible, however, to set <ei>pT0Ref = 0</ei>
239 and use <ei>pTmin</ei> to provide a step-function regularization,
240 or to combine them in intermediate approaches. Currently <ei>pTmin</ei>
241 is taken to be energy-independent.
244 <parm name="SpaceShower:pTminChgQ" default="0.5" min="0.01">
245 Parton shower cut-off <ei>pT</ei> for photon coupling to a coloured
249 <parm name="SpaceShower:pTminChgL" default="0.0005" min="0.0001">
250 Parton shower cut-off mass for pure QED branchings.
251 Assumed smaller than (or equal to) <ei>pTminChgQ</ei>.
254 <flag name="SpaceShower:rapidityOrder" default="off">
255 Force emissions, after the first, to be ordered in rapidity,
256 i.e. in terms of decreasing angles in a backwards-evolution sense.
257 Could be used to probe sensitivity to unordered emissions.
258 Only affects QCD emissions.
261 <h3>Further variables</h3>
263 These should normally not be touched. Their only function is for
267 There are three flags you can use to switch on or off selected
268 branchings in the shower:
270 <flag name="SpaceShower:QCDshower" default="on">
271 Allow a QCD shower; on/off = true/false.
274 <flag name="SpaceShower:QEDshowerByQ" default="on">
275 Allow quarks to radiate photons; on/off = true/false.
278 <flag name="SpaceShower:QEDshowerByL" default="on">
279 Allow leptons to radiate photons; on/off = true/false.
283 There are some further possibilities to modify the shower:
285 <flag name="SpaceShower:MEcorrections" default="on">
286 Use of matrix element corrections; on/off = true/false.
289 <flag name="SpaceShower:MEafterFirst" default="on">
290 Use of matrix element corrections also after the first emission,
291 for dipole ends of the same system that did not yet radiate.
292 Only has a meaning if <code>MEcorrections</code> above is
296 <flag name="SpaceShower:phiPolAsym" default="on">
297 Azimuthal asymmetry induced by gluon polarization; on/off = true/false.
300 <flag name="SpaceShower:phiIntAsym" default="on">
301 Azimuthal asymmetry induced by interference; on/off = true/false.
304 <parm name="SpaceShower:strengthIntAsym" default="0.7"
306 Size of asymmetry induced by interference. Natural value of order 0.5;
307 expression would blow up for a value of 1.
310 <modeopen name="SpaceShower:nQuarkIn" default="5" min="0" max="5">
311 Number of allowed quark flavours in <ei>g -> q qbar</ei> branchings,
312 when kinematically allowed, and thereby also in incoming beams.
313 Changing it to 4 would forbid <ei>g -> b bbar</ei>, etc.
316 <h3>Technical notes</h3>
318 Almost everything is equivalent to the algorithm in [1]. Minor changes
322 It is now possible to have a second-order running <ei>alpha_s</ei>,
323 in addition to fixed or first-order running.
326 The description of heavy flavour production in the threshold region
327 has been modified, so as to be more forgiving about mismatches
328 between the <ei>c/b</ei> masses used in Pythia relative to those
329 used in a respective PDF parametrization. The basic idea is that,
330 in the threshold region of a heavy quark <ei>Q</ei>, <ei>Q = c/b</ei>,
331 the effect of subsequent <ei>Q -> Q g</ei> branchings is negligible.
334 f_Q(x, pT2) = integral_mQ2^pT2 dpT'2/pT'2 * alpha_s(pT'2)/2pi
335 * integral P(z) g(x', pT'2) delta(x - z x')
337 so use this to select the <ei>pT2</ei> of the <ei>g -> Q Qbar</ei>
338 branching. In the old formalism the same kind of behaviour should
339 be obtained, but by a cancellation of a <ei>1/f_Q</ei> that diverges
340 at the theshold and a Sudakov that vanishes.
342 The strategy therefore is that, once <ei>pT2 < f * mQ2</ei>, with
343 <ei>f</ei> a parameter of the order of 2, a <ei>pT2</ei> is chosen
344 like <ei>dpT2/pT2</ei> between <ei>mQ2</ei> and <ei>f * mQ2</ei>, a
345 nd a <ei>z</ei> flat in the allowed range. Thereafter acceptance
346 is based on the product of three factors, representing the running
347 of <ei>alpha_strong</ei>, the splitting kernel (including the mass term)
348 and the gluon density weight. At failure, a new <ei>pT2</ei> is chosen
349 in the same range, i.e. is not required to be lower since no Sudakov
353 The QED algorithm now allows for hadron beams with non-zero photon
354 content. The backwards-evolution of a photon in a hadron is identical
355 to that of a gluon, with <ei>CF -> eq^2</ei> and <ei>CA -> 0</ei>.
356 Note that this will only work in conjunction with
357 parton distribution that explicitly include photons as part of the
358 hadron structure (such as the MRST2004qed set). Since Pythia's
359 internal sets do not allow for photon content in hadrons, it is thus
360 necessary to use the LHAPDF interface to make use of this feature. The
361 possibility of a fermion backwards-evolving to a photon has not yet
362 been included, nor has photon backwards-evolution in lepton beams.
368 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->