Rivet quickstart guide
----------------------

Assuming you've got a copy of Rivet installed on your machine, you'll now want
to know how to use it. Here's an example session with the 'rivet' command, which
should be illustrative.


GENERATING AND ANALYSING

The main interface to Rivet is the rivet command. We will demonstrate how to use
this to analyse HepMC events from a text file in the "IO_GenEvent" HepMC format.
(This is the only supported HepMC text format from HepMC 2.5 onwards.)

Firstly, we recommend using a filesystem pipe (or 'FIFO') so that your events
don't create a huge file that takes all your disk space. The idea is that the
generator will push events into what looks like a file, and Rivet will read from
the same 'file'. In fact, though, the 'file' is a disguised pipe between the two
processes, so no slow filesystem access needs to take place, and the system will
automatically balance the data flow between the writing and reading processes:
the generator will only write more event data when Rivet has read that currently
available in the FIFO buffer. All this is completely transparent to the user:
good old Unix!  Here's how you do it:

> mkfifo hepmc.fifo
> my-generator --num-events=500000 --hepmc-output=hepmc.fifo &
> rivet --analysis=ANALYSIS_NAME hepmc.fifo

The backgrounding of the generator process is important: the generator will wait
until the hepmc.fifo pipe is being read by Rivet, so unless it is backgrounded
you will never get the terminal focus back to run rivet!

The generator used above can be anything which writes HepMC text output to the
specified FIFO. As a concrete example, the AGILe package
(http://projects.hepforge.org/agile/) provides an interface to several
generators with a command line tool called agile-rungen:

> agile-rungen Pythia6:418 --beams=TVT:1800 &
> rivet --analysis=CDF_2001_S4751469 hepmc.fifo

The agile-rungen and rivet commands both provide extensive command line
documentation when called with the -h/--help flag.


CHECKING THE DATA

By default, Rivet outputs its histograms in the AIDA XML format, which is not
particularly easy to read or plot. An easier format is available if you convert
the AIDA file to a flat text format (the long-term Rivet data format) with the
aida2flat command:

> aida2flat Rivet.aida | less

Again, check the aida2flat --help documentation for details on more options.

There is also an aida2root command which provides the data as ROOT TGraph
objects for those of a ROOTy disposition. Due to a ROOT bug, for TGraphs to
display properly you will have to specify the "AP" option in the top right-hand
box of the ROOT TBrowser or alternatively draw the TGraph programmatically using
'myTGraph->Draw("AP")'. This isn't a Rivet bug: it's ROOT forgetting to draw the
graph axes!


PLOTTING

Because we don't get on well with ROOT, Rivet comes with two commands ---
compare-histos and make-plots --- for comparing and plotting AIDA data. These
commands produce nice comparison plots of publication quality from the YODA
"dat" format text files, e.g.:

> compare-plots path/to/CDF_2001_S4751469.aida py.aida:'Pythia 6.418' hw.aida:'Herwig++ 2.3.0'

This command will have compared the three named data files (ending in .aida),
identified which plots are available in them, and combined the MC and reference
plots appropriately into a set of plot data files ending with .dat. The strings
after the ":" for the MC files are specifying ID strings to appear in the plot
legends. You can also run compare-plots to just compare MC-MC data files. More
options are described by running 'compare-histos --help'.

Incidentally, the reference files for each Rivet analysis are to be found in the
installed Rivet shared data directory. You can find the location of this by
using the rivet-config command:

> rivet-config --datadir

You can now plot the created data files using the make-plots command:

> make-plots --pdf *.dat

The --pdf flag makes the output plots in PDF format: by default the output is in
PostScript (.ps), and flags for conversion to EPS and PNG are also available.


THAT'S ALL!

We hope this was useful as a quick guide to getting started with running Rivet's
built-in analyses for Monte Carlo generator validation. Be aware that we will be
overhauling Rivet's histogramming system in version 1.3.0, from which point all
the conversions away from AIDA XML will no longer be necessary, and a fully
programmatic interface to the Rivet data files will be possible. Please bear
with us while we get this sorted out: we're as keen to have better histogramming
as you are! ;)