3 <title>Diffraction</title>
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34 Diffraction is not well understood, and several alternative approaches
35 have been proposed. Here we follow a fairly conventional Pomeron-based
36 one, in the Ingelman-Schlein spirit [<a href="Bibliography.php" target="page">Ing85</a>],
37 but integrated to make full use of the standard PYTHIA machinery
38 for multiple interactions, parton showers and hadronization
39 [<a href="Bibliography.php" target="page">Nav10,Cor10a</a>]. This is the approach pioneered in the PomPyt
40 program by Ingelman and collaborators [<a href="Bibliography.php" target="page">Ing97</a>].
43 For ease of use (and of modelling), the Pomeron-specific parts of the
44 generation are subdivided into three sets of parameters that are rather
45 independent of each other:
46 <br/>(i) the total, elastic and diffractive cross sections are
47 parametrized as functions of the CM energy, or can be set by the user
48 to the desired values, see the
49 <?php $filepath = $_GET["filepath"];
50 echo "<a href='TotalCrossSections.php?filepath=".$filepath."' target='page'>";?>Total Cross Sections</a> page;
51 <br/>(ii) once it has been decided to have a diffractive process,
52 a Pomeron flux parametrization is used to pick the mass of the
53 diffractive system(s) and the <i>t</i> of the exchanged Pomeron,
55 <br/>(iii) a diffractive system of a given mass is classified either
56 as low-mass unresolved, which gives a simple low-<i>pT</i> string
57 topology, or as high-mass resolved, for which the full machinery of
58 multiple interactions and parton showers are applied, making use of
59 <?php $filepath = $_GET["filepath"];
60 echo "<a href='PDFSelection.php?filepath=".$filepath."' target='page'>";?>Pomeron PDFs</a>.
61 <br/>The parameters related to multiple interactions, parton showers
62 and hadronization are kept the same as for normal nondiffractive events,
63 with only one exception. This may be questioned, especially for the
64 multiple interactions, but we do not believe that there are currently
65 enough good diffractive data that would allow detailed separate tunes.
68 The above subdivision may not represent the way "physics comes about".
69 For instance, the total diffractive cross section can be viewed as a
70 convolution of a Pomeron flux with a Pomeron-proton total cross section.
71 Since neither of the two is known from first principles there will be
72 a significant amount of ambiguity in the flux factor. The picture is
73 further complicated by the fact that the possibility of simultaneous
74 further multiple interactions ("cut Pomerons") will screen the rate of
75 diffractive systems. In the end, our set of parameters refers to the
76 effective description that emerges out of these effects, rather than
77 to the underlying "bare" parameters.
81 As already mentioned above, the total diffractive cross section is fixed
82 by a default energy-dependent parametrization or by the user, see the
83 <?php $filepath = $_GET["filepath"];
84 echo "<a href='TotalCrossSections.php?filepath=".$filepath."' target='page'>";?>Total Cross Sections</a> page.
85 Therefore we do not attribute any significance to the absolute
86 normalization of the Pomeron flux. The choice of Pomeron flux model
87 still will decide on the mass spectrum of diffractive states and the
88 <i>t</i> spectrum of the Pomeron exchange.
90 <br/><br/><table><tr><td><strong>Diffraction:PomFlux </td><td> (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 4</code>)</td></tr></table>
91 Parametrization of the Pomeron flux <ei>f_Pom/p( x_Pom, t)</ei>.
93 <input type="radio" name="1" value="1" checked="checked"><strong>1 </strong>: Schuler and Sjöstrand <ref>Sch94</ref>: based on a critical Pomeron, giving a mass spectrum roughly like <ei>dm^2/m^2</ei>; a mass-dependent exponential <ei>t</ei> slope that reduces the rate of low-mass states; partly compensated by a very-low-mass (resonance region) enhancement. Is currently the only one that contains a separate <ei>t</ei> spectrum for double diffraction and separate parameters for pion beams.<br/>
94 <input type="radio" name="1" value="2"><strong>2 </strong>: Bruni and Ingelman <ref>Bru93</ref>: also a critical Pomeron giving close to <ei>dm^2/m^2</ei>, with a <ei>t</ei> distribution the sum of two exponentials. <br/>
95 <input type="radio" name="1" value="3"><strong>3 </strong>: a conventional Pomeron description, in the RapGap manual <ref>Jun95</ref> attributed to Berger et al. and Streng <ref>Ber87a</ref>, but there (and here) with values updated to a supercritical Pomeron with <ei>epsilon > 0</ei> (see below), which gives a stronger peaking towards low-mass diffractive states, and with a mass-dependent (the <ei>alpha'</ei> below) exponential <ei>t</ei> slope.<br/>
96 <input type="radio" name="1" value="4"><strong>4 </strong>: a conventional Pomeron description, attributed to Donnachie and Landshoff <ref>Don84</ref>, again with supercritical Pomeron, with the same two parameters as option 3 above, but this time with a power-law <ei>t</ei> distribution.<br/>
99 In the last two options above, the Pomeron Regge trajectory is
102 alpha(t) = 1 + epsilon + alpha' t
104 The <i>epsilon</i> and <i>alpha'</i> parameters can be set
107 <br/><br/><table><tr><td><strong>Diffraction:PomFluxEpsilon </td><td></td><td> <input type="text" name="2" value="0.085" size="20"/> (<code>default = <strong>0.085</strong></code>; <code>minimum = 0.02</code>; <code>maximum = 0.15</code>)</td></tr></table>
108 The Pomeron trajectory intercept <i>epsilon</i> above. For technical
109 reasons <i>epsilon > 0</i> is necessary in the current implementation.
111 <br/><br/><table><tr><td><strong>Diffraction:PomFluxAlphaPrime </td><td></td><td> <input type="text" name="3" value="0.25" size="20"/> (<code>default = <strong>0.25</strong></code>; <code>minimum = 0.1</code>; <code>maximum = 0.4</code>)</td></tr></table>
112 The Pomeron trajectory slope <i>alpha'</i> above.
115 <h3>Separation into low and high masses</h3>
117 Preferably one would want to have a perturbative picture of the
118 dynamics of Pomeron-proton collisions, like multiple interactions
119 provide for proton-proton ones. However, while PYTHIA by default
120 will only allow collisions with a CM energy above 10 GeV, the
121 mass spectrum of diffractive systems will stretch to down to
122 the order of 1.2 GeV. It would not be feasible to attempt a
123 perturbative description there. Therefore we do offer a simpler
124 low-mass description, with only longitudinally stretched strings,
125 with a gradual switch-over to the perturbative picture for higher
126 masses. The probability for the latter picture is parametrized as
128 P_pert = 1 - exp( (m_diffr - m_min) / m_width )
130 which vanishes for the diffractive system mass
131 <i>m_diffr < m_min</i>, and is <i>1 - 1/e = 0.632</i> for
132 <i>m_diffr = m_min + m_width</i>.
134 <br/><br/><table><tr><td><strong>Diffraction:mMinPert </td><td></td><td> <input type="text" name="4" value="10." size="20"/> (<code>default = <strong>10.</strong></code>; <code>minimum = 5.</code>)</td></tr></table>
135 The abovementioned threshold mass <i>m_min</i> for phasing in a
136 perturbative treatment. If you put this parameter to be bigger than
137 the CM energy then there will be no perturbative description at all,
138 but only the older low-<i>pt</i> description.
141 <br/><br/><table><tr><td><strong>Diffraction:mWidthPert </td><td></td><td> <input type="text" name="5" value="10." size="20"/> (<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
142 The abovementioned threshold width <i>m_width.</i>
145 <h3>Low-mass diffraction</h3>
147 When an incoming hadron beam is diffractively excited, it is modeled
148 as if either a valence quark or a gluon is kicked out from the hadron.
149 In the former case this produces a simple string to the leftover
150 remnant, in the latter it gives a hairpin arrangement where a string
151 is stretched from one quark in the remnant, via the gluon, back to the
152 rest of the remnant. The latter ought to dominate at higher mass of
153 the diffractive system. Therefore an approximate behaviour like
159 <br/><br/><table><tr><td><strong>Diffraction:pickQuarkNorm </td><td></td><td> <input type="text" name="6" value="5.0" size="20"/> (<code>default = <strong>5.0</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
160 The abovementioned normalization <i>N</i> for the relative quark
161 rate in diffractive systems.
164 <br/><br/><table><tr><td><strong>Diffraction:pickQuarkPower </td><td></td><td> <input type="text" name="7" value="1.0" size="20"/> (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
165 The abovementioned mass-dependence power <i>p</i> for the relative
166 quark rate in diffractive systems.
170 When a gluon is kicked out from the hadron, the longitudinal momentum
171 sharing between the the two remnant partons is determined by the
172 same parameters as above. It is plausible that the primordial
173 <i>kT</i> may be lower than in perturbative processes, however:
175 <br/><br/><table><tr><td><strong>Diffraction:primKTwidth </td><td></td><td> <input type="text" name="8" value="0.5" size="20"/> (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
176 The width of Gaussian distributions in <i>p_x</i> and <i>p_y</i>
177 separately that is assigned as a primordial <i>kT</i> to the two
178 beam remnants when a gluon is kicked out of a diffractive system.
181 <br/><br/><table><tr><td><strong>Diffraction:largeMassSuppress </td><td></td><td> <input type="text" name="9" value="2." size="20"/> (<code>default = <strong>2.</strong></code>; <code>minimum = 0.</code>)</td></tr></table>
182 The choice of longitudinal and transverse structure of a diffractive
183 beam remnant for a kicked-out gluon implies a remnant mass
184 <i>m_rem</i> distribution (i.e. quark plus diquark invariant mass
185 for a baryon beam) that knows no bounds. A suppression like
186 <i>(1 - m_rem^2 / m_diff^2)^p</i> is therefore introduced, where
187 <i>p</i> is the <code>diffLargeMassSuppress</code> parameter.
190 <h3>High-mass diffraction</h3>
192 The perturbative description need to use parton densities of the
193 Pomeron. The options are described in the page on
194 <?php $filepath = $_GET["filepath"];
195 echo "<a href='PDFSelection.php?filepath=".$filepath."' target='page'>";?>PDF Selection</a>. The standard
196 perturbative multiple interactions framework then provides
197 cross sections for parton-parton interactions. In order to
198 turn these cross section into probabilities one also needs an
199 ansatz for the Pomeron-proton total cross section. In the literature
200 one often finds low numbers for this, of the order of 2 mb.
201 These, if taken at face value, would give way too much activity
202 per event. There are ways to tame this, e.g. by a larger <i>pT0</i>
203 than in the normal pp framework. Actually, there are many reasons
204 to use a completely different set of parameters for MI in
205 diffraction than in pp collisions, e.g. with respect to the
206 impact-parameter picture. A lower number in some frameworks could
207 alternatively be regarded as a consequence of screening, with
208 a larger "bare" number.
211 For now, however, an attempt at the most general solution would
212 carry too far, and instead we patch up the problem by using a
213 larger Pomeron-proton total cross section, such that average
214 activity makes more sense. This should be viewed as the main
215 tunable parameter in the description of high-mass diffraction.
216 It is to be fitted to diffractive event-shape data such as the average
217 charged multiplicity. It would be very closely tied to the choice of
218 Pomeron PDF; we remind that some of these add up to less than unit
219 momentum sum in the Pomeron, a choice that also affect the value
222 <br/><br/><table><tr><td><strong>Diffraction:sigmaPomP </td><td></td><td> <input type="text" name="10" value="10." size="20"/> (<code>default = <strong>10.</strong></code>; <code>minimum = 2.</code>; <code>maximum = 40.</code>)</td></tr></table>
223 The assumed Pomeron-proton effective cross section, as used for
224 multiple interactions in diffractive systems. A larger value gives
225 less MI activity per event.
228 There is no point in making the cross section too big, however, since
229 then <i>pT0</i> will be adjusted downwards to ensure that the
230 integrated perturbative cross section stays above this assumed
231 total cross section. (The requirement of at least one perturbative
232 interaction per event.)
235 Also note that, even for a fixed CM energy of events, the diffractive
236 subsystem will range from the abovementioned threshold mass
237 <i>m_min</i> to the full CM energy, with a variation of parameters
238 such as <i>pT0</i> along this mass range. Therefore multiple
239 interactions are initialized for a few different diffractive masses,
240 currently five, and all relevant parameters are interpolated between
241 them to obtain the behaviour at a specific diffractive mass.
242 Furthermore, <i>A B ->X B</i> and <i>A B ->A X</i> are
243 initialized separately, to allow for different beams or PDF's on the
244 two sides. These two aspects mean that initialization of MI is
245 appreciably slower when perturbative high-mass diffraction is allowed.
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319 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->