The technique used for combining different particleIDs

The technique used for combining the information from the different
subdetectors was presented during the Tracking-PID-physics meeting of
the last LHCb week, which you can find at http://agenda.cern.ch/age?a03997 
The product is made of the likelihoods for each particle type from the
different detectors.  Particles are then selected by cutting on the
ratio of likelihoods between different hypotheses, or equivalently on the
difference of log-likelihood (DLL), eg:
   DLL(e-pi) = ln L(e) - ln L(pi) = ln [ L(e) / L(pi) ]
for the electron and pion hypotheses.  The output of the combination is,
for each track, a list of the detector information that is available
(RICH/CALO/MUON) and the 4 log-likelihood differences for the (e, mu, K, p)
hypotheses compared to the pion hypothesis.  Any other DLL can be simply
determined from these, eg. DLL(K-p) = DLL(K-pi) - DLL(p-pi).

To select particles of a given type, one then specifies which detector
information is required to be available, and which cut (or cuts) on DLL
values are required.  For example, to select electrons, one would normally
require the Calorimeter information to be available, as this is the most
important detector element for electron ID.  The most significant
background is from pions, so one cuts on the DLL between the electron and
pion hypotheses.  This DLL is distributed mostly to positive values for
electrons, and negative for pions, so a reasonable choice for the cut is
DLL(e-pi) > 0.  As shown in the presentation mentioned above, this gives an
efficiency of 95% for electrons that are within the Calorimeter acceptance,
with a pion misid rate of about 0.7%.  The pions could be even more
strongly suppressed by increasing the cut on DLL(e-pi), at the cost of a
loss in electron efficiency.  Reasonable values for the cut on DLL values
are usually in the range -10 to +10, according to the compromise between
efficiency and purity.

The combined particle ID is implemented in a DaVinci "Particle Maker" that
takes tracks (protoparticles) and assigns a particle ID to them to form
particles which will be used in the physics analysis.  The particle maker
is called "CombinedParticleMaker", and does the job previously done by a
number of different particle makers (ElectronParticleMaker,
RichParticleMaker etc).  Reasonable values are set by default for the
particle ID selection cuts, but they can be modified using options,
according the efficiency/purity requirements of your analysis.  The
particle types to be selected are defined by a "Particles" option, with the
names of the 5 common charged particle types as arguments.  The default is
Particles = { "muon", "electron", "kaon", "proton", "pion" } 
If you wish only to select kaons and pions then you would put: 
Particles = { "kaon", "pion" }

The selection can be exclusive, or not, as set by an ExclusiveSelection
option that is set either true or false.  If true, as is the case by
default, then the selection criteria are applied in the order that the
particle types were listed in the Particles option, and if the selection is
satisfied for one particle type the track is not considered for subsequent
particle types.  So for the default Particles option given above, muons are
first searched for, then electrons, and so on.  In this way each track will
have at most one selected particle type.  Conversely, if ExclusiveSelection
is set false then the selection criteria are applied independently for the
different particle types.  Tracks can then be selected as more than one
particle type, and care should be taken to avoid double counting in the
physics algorithm.

The selections themselves have the form:
ElectronSelection = { "det='CALO' e-pi='0.0'" }
This defines that for selecting electrons, the Calorimeter detector
information should be available, and cut is applied on DLL(e-pi) > 0.  
To require that both Calorimeter and RICH are available, one would put
det='CALO,RICH'. e-pi is a symbol that has been defined to represent the
lower limit on the difference in log-likelihood between electron and pion
hypotheses.  All other DLLs have similarly defined symbols, constructed
using e,mu,pi,k,p to represent the particle types.  More than one cut can
be applied if desired.  For example, to select kaons one may wish to reject
pions (and hence cut on DLL(k-pi)) but also reject protons (and hence also
cut on DLL(k-p)).  This is done in the default selection as follows:
KaonSelection = { "det='RICH' k-pi='2.0' k-p='-2.0'" } 
If your algorithm is interested in selecting kaons and doesn't suffer from
proton background you could remove the cut on DLL(k-p): this would emulate
what is currently done by the RichParticleMaker, identifying as kaons any
"heavy" particle (kaon or proton).

If you are really keen you can also apply different DLL cuts according
to which detectors are available.  For example:
KaonSelection = { "det='RICH,CALO' k-pi='0.0'",
                  "det='RICH'      k-pi='2.0'" }
would apply a looser cut when both RICH and Calorimeter are available, and
a tighter cut when only the RICH is available.  The criteria are applied in
the order they are listed.

In the default selection the pion criteria are left empty, so that all
tracks that have not been selected as other particle types are called
pions.  Obviously this relies on the fact that ExclusiveSelection is true,
and the pion hypothesis is listed last in the Particles option.

As an example, the default DaVinci options for the combined particle ID are
given below. ("PreLoadParticles" should be replaced by the name of your
algorithm).  The values chosen should be understandable once you have taken
a look at the distributions shown in the presentation mentioned above.  An
example of how to control an algorithm using the CombinedParticleMaker is
provided with DaVinci v8r1 in:
  $DAVINCIROOT/options/PreLoadCombinedParticles.opts
Note that DaVinci.opts provides the possibility of executing such an
example algorithm.

PreLoadParticles.PhysDesktop.ParticleMakerType = "CombinedParticleMaker";
PreLoadParticles.PhysDesktop.CombinedParticleMaker.Particles = {
  "muon", "electron", "kaon", "proton", "pion" };
PreLoadParticles.PhysDesktop.CombinedParticleMaker.ExclusiveSelection =
true;
PreLoadParticles.PhysDesktop.CombinedParticleMaker.MuonSelection = {
  "det='MUON' mu-pi='-8.0'" };
PreLoadParticles.PhysDesktop.CombinedParticleMaker.ElectronSelection = {
  "det='CALO' e-pi='0.0'" };
PreLoadParticles.PhysDesktop.CombinedParticleMaker.KaonSelection = {
  "det='RICH' k-pi='2.0' k-p='-2.0'" };
PreLoadParticles.PhysDesktop.CombinedParticleMaker.ProtonSelection = {
  "det='RICH' p-pi='3.0'" };