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{\Large\bf PYTHIA 8 Status}\\[5mm]
{\large\textit{Torbj\"orn Sj\"ostrand}}\\[3mm]
\textit{Department of Astronomy and Theoretical Physics,
Lund University}\\
\textit{S\"olvgatan 14A, SE-223 62 Lund, Sweden}
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The \textsc{Pythia~8} event generator [1] is the C++ successor 
to the Fortran-based \textsc{Pythia~6} [2], frequently used in 
the study of $pp/p\bar p$ and $e^+e^-$ physics.
 
One of the main developments is that \textsc{Pythia~8} now 
contains a complete interleaving of multiparton interactions,
initial-state radiation and final-state radiation, in one 
common sequence of decreasing $p_{\perp}$ scales. That is, 
the features at larger $p_{\perp}$ values set the stage for 
the subsequent dressing-up by softer emissions.

The approximate matching of showers to hard-scattering matrix 
elements has been improved for a large set of hard processes [3].
For showers in QCD processes the first emission is compared
with $2 \to 3$ matrix elements to confirm a reasonable rate [4].

The traditional multiparton interactions framework is largely 
retained, but some new possibilities are added. One is that it is 
now possible to preselect two separate hard processes in the
same event, to help simulate signals for double parton scattering.
Another is that rescattering, where one parton scatters twice 
(or more) against partons from the other hadron, can now be 
simulated [5]. Unfortunately it is not simple to find a good 
experimental signal for such events.

Other developments include an improved framework for the structure
of diffractive events, a richer mix  of underlying-event processes,
and an updated set of parton distributions. 

Early attempts to tune \textsc{Pythia~8} to minimum-bias data 
gave too much underlying-event activity. The problem has been 
traced to a doublecounting between some initial- and final-state
radiation. This has now been fixed, and tunes to Tevatron data
have been produced [4]. Unfortunately these tunes underestimate 
the activity observed in some of the early LHC data sets, which 
either may be owing to problems with the generator not reproducing
different cut conditions, or point to some tension in the data.
For now a slightly separate LHC tune has been made. 

\textsc{Pythia~8} does not reproduce the CMS ridge effect,
which thus shows that some physics mechanisms are still missing.
Similarly there are problems e.g. with the particle composition
and Bose-Einstein effects observed at LHC, that hints towards 
collective effects.\\[2mm]
\noindent\textbf{References}\\
{}[1] T. Sj\"ostrand, S. Mrenna and P. Skands, 
Comput. Phys. Comm. 178 (2008) 852\\
{}[2] T. Sj\"ostrand, S. Mrenna and P. Skands, JHEP05 (2006) 026\\
{}[3] R. Corke and T. Sj\"ostrand, Eur. Phys. J. C69 (2010) 1\\
{}[4] R. Corke and T. Sj\"ostrand, arXiv:1011.1759 [hep-ph]\\
{}[5] R. Corke and T. Sj\"ostrand, JHEP 01 (2010) 035

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