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\begin{document}
\title{Uncertainties in collider predictions}
\author{Sourendu Gupta +++}
\maketitle

QCD allows us to transfer information about one cross section to another
through the pdf's. These include interesting standard model (SM) processes
such as the Higgs production rate. They also include new physics signals
such as the production of superpartners of SM particles and the SM 
backgrounds to these processes such as W+jets.

There are two kinds of uncertainties that plague this
transfer---
\begin{enumerate}
\item since QCD predictions are computed in a power series in $\alpha_s$
  (to different orders for different processes) there may be some
  incompatibility in using these computations. Complications such as this
  should be termed irreducible theory uncertainties (IT?).
\item pdf's are extracted from data which have statistical and systematic
  uncertainties. As a result, pdf sets have inherent uncertainties which
  are also often called QCD uncertainties, but should really be termed
  theory-filtered experimental uncertainties (TFE?).
\end{enumerate}

Normally one takes a certain set of input cross sections measured in
experiments (DATA box), and fits common parton density sets to them
using QCD predictions at a certain available order (QCD box) using
certain statistical methods for parameter extraction (STAT box).
The output of this phase of the anaysis is a published PDF set (which
may or may not include statistical uncertainties). Users interested
in other processes then write codes for the processes they are interested
in (NEW-QCD box) and use the PDF sets as inputs to investigate cross
sections for these processes. The uncertainties in the results are
usually estimated in fairly crude ways.

This proposal is about building a toolset for colliders which does the
following:
\begin{enumerate}
\item allows users to systematically investigate either IT? or TFE?, or
  the two together.
\item reuse written code
\item add new code to the repertoire of available tools
\item smoothly join new data to the analysis as it becomes available
\end{enumerate}
The main physics job is to carefully define the interfaces between the
DATA, QCD, STAT and NEW-QCD boxes, and a replacement of the PDF sets
by a pipeline (which we might call, obviously, the PDF-pipe). 

It was pointed out that different groups use different methods of
statistical analysis, so the interface of the STAT box has to be
sufficiently general.

The PDF pipe does not have to be human readable, and hence can contain
much more information about statistical properties of the PDF's than
is customarily published (the specification should also allow for less
information).

One has to take care that the interfaces to the modules inside the QCD
box and the NEW-QCD box should be good enough to ``plug and play''. If
some one writes a NEW-QCD module (say for $pp\to Z+\bar bb$) now then it
should be possible to move it to the QCD box when data on this process
becomes available.

At the same time, the DATA box should have an interface general enough
that new data can be included and old data selectively removed in a
given computation.

It is proposed that a working group be set up for this and meet in March
to produce the next level of detail in the problem specification.
\end{document}
