+++ /dev/null
-<chapter name="Parton Distributions">
-
-<h2>Parton Distributions</h2>
-
-The parton distributions file contains the <code>PDF</code> class.
-<code>PDF</code> is the base class, from which specific <code>PDF</code>
-classes are derived.
-
-<p/>
-The choice of which PDF to use is made by settings in the
-<code>Pythia</code> class, see <aloc href="PDFSelection">here</aloc>.
-These settings also allow to access all the proton PDF's available in the
-LHAPDF library <ref>Wha05</ref>. Thus there is no need for a normal user
-to study the <code>PDF</code> class. The structure must only be understood
-when interfacing new PDF's, e.g. ones not yet found in LHAPDF.
-
-<h3>The PDF base class</h3>
-
-<code>PDF</code> defines the interface that all PDF classes should respect.
-The constructor requires the incoming beam species to be given:
-even if used for a proton PDF, one needs to know whether the beam
-is actually an antiproton. This is one of the reasons why <code>Pythia</code>
-always defines two PDF objects in an event, one for each beam.
-
-<p/>
-Once a <code>PDF</code> object has been constructed, call it <code>pdf</code>,
-the main method is <code>pdf.xf( id, x, Q2)</code>, which returns
-<ei>x*f_id(x, Q2)</ei>, properly taking into account whether the beam
-is an antiparticle or not.
-
-<p/>
-Whenever the <code>xf</code> member is called with a new flavour, <ei>x</ei>
-or <ei>Q^2</ei>, the <code>xfUpdate</code> member is called to do the actual
-updating. This routine may either update that particular flavour or all
-flavours at this <ei>(x, Q^2)</ei> point. (In the latter case the saved
-<code>id</code> value <code>idSav</code> should be set to 9.) The choice is
-to be made by the producer of a given set, based on what he/she deems most
-effective, given that sometimes only one flavour need be evaluated, and
-about equally often all flavours are needed at the same <ei>x</ei> and
-<ei>Q^2</ei>. Anyway, the latest value is always kept in memory. This is
-the other reason why <code>Pythia</code> has one separate <code>PDF</code>
-object for each beam, so that values at different <ei>x</ei> can be kept
-in memory.
-
-<p/>
-Two further public methods are <code>xfVal( id, x, Q2)</code> and
-<code>xfSea( id, x, Q2)</code>. These are simple variants whereby
-the quark distributions can be subdivided into a valence and a sea part.
-If these are not directly accessible in the parametrization, onc can
-make the simplified choices <ei>u_sea = ubar_sea, u_val = u_tot - u_sea</ei>,
-and correspondingly for <ei>d</ei>. (Positivity will always be guaranteed
-at output.) The <code>xfUpdate</code> method should also take care of
-updating this information.
-
-<p/>
-A method <code>setExtrapolate(bool)</code> allows you to switch between
-freezing parametrizations at the <ei>x</ei> and <ei>Q^2</ei> boundaries
-(<code>false</code>) or extrapolating them outside the boundaries
-(<code>true</code>). This method is only implemented for the LHAPDF class
-below. If you implement a new PDF you are free to use this method, but it
-would be smarter to hardcode the desired limiting behaviour.
-
-<h3>Derived classes</h3>
-
-There is only one pure virtual method, <code>xfUpdate</code>, that therefore
-must be implemented in any derived class. Currently the list of such
-classes is tiny:
-
-<p/>
-For protons:
-<ul>
-<li><code>GRV94L</code> gives the GRV 94 L parametrization
-<ref>Glu95</ref>.</li>
-<li><code>CTEQ5L</code> gives the CTEQ 5 L parametrization
-<ref>Lai00</ref>.</li>
-<li><code>LHAPDFinterface</code> provides an interface to the
-LHAPDF library<ref>Wha05</ref>.</li>
-</ul>
-The default is CTEQ 5L, which is the most recent of the two hardcoded sets.
-
-<p/>
-For charged leptons (e, mu, tau):
-<ul>
-<li>Lepton gives a QED parametrization <ref>Kle89</ref>.
-In QED there are not so many ambiguities, so here one set should be
-enough. On the other hand, there is the problem that the
-lepton-inside-lepton pdf is integrably divergent for <ei>x -> 1</ei>,
-which gives numerical problems. Like in PYTHIA 6, the pdf is therefore
-made to vanish for <ei>x > 1 - 10^{-10}</ei>, and scaled up in the range
-<ei>1 - 10^{-7} < x < 1 - 10^{-10}</ei> in such a way that the
-total area under the pdf is preserved.</li>
-</ul>
-
-There is another method, <code>isSetup()</code>, that returns the
-base-class boolean variable <code>isSet</code>. This variable is
-initially <code>true</code>, but could be set <code>false</code> if the
-setup procedure of a PDF failed, e.g. if the user has chosen an unknown
-PDF set.
-
-</chapter>
-
-<!-- Copyright (C) 2008 Torbjorn Sjostrand -->