# Recommended LoKi::Hybrid Filters

This lists the filters recommended for use in the HLT and the selections. See DaVinciTutorial4 for a hands-on tutorial. A longer, but not necessarily up-to-date, list can be found at LoKiParticleFunctions. It also contains examples on how to use these functors in C++ code.

# Particle Functors

## ALL: All

Takes all particles. This is required if one wants to apply no cut.
CombineParticles.FILTER0.Code = "(ALL)" ;

## ABSID: Absolute value of PID.

Returns the absolute value of the PID. The following lines are equivalent:
CombineParticles.FILTER0.Code = "(ABSID==211) " ;
CombineParticles.FILTER0.Code = "(ABSID=='pi+') " ;
CombineParticles.FILTER0.Code = "(ABSID=='pi-') " ;
Note the last line! The comparison (ABSID=='pi-') takes the absolute value on both sides. This avoid having to remember that the has a positive pid (211) while the has a negative pid (-13).

The pid can also be used without the absolute value. See ID.

## ADMASS: The absolute mass difference to the reference value

Calculates the absolute difference between the measured mass and the PDG reference value. It takes the pid of the reference particle as argument.

The mass difference can also be used without the absolute value. See DMASS.

TODO : A version without argument will be provided where the ID of the particle is used to get the reference mass.

## BPVDIRA: Direction angle

Computes the cosine of the angle between the momentum of the particle and the direction fo flight from the best PV to the decay vertex.
CombineParticles.FILTER2.Code = "(BPVDIRA>0.9999)" ;

## BPVIPCHI2(): IP on related PV

Computes the -IP on the related PV.
CombineParticles.FILTER2.Code = "BPVIPCHI2()<25" ;
TODO : So far it needs the ().

## BPVVDCHI2: -separation from related PV

Computes the -distance from the related PV.
CombineParticles.FILTER2.Code = "(BPVVDCHI2>100)" ;

## CHILDCUT: Applies a cut to a given child

FilterDesktop.Filter.Code = "(CHILDCUT ( MIPCHI2DV(PRIMARY)>1 , 2 ))" ;
In this example one applies an IP cut on the first daughter of the input particle. This requires to know which is the first, second, etc daughter. Can be useful when (N)INTREE won't work. Like here for the slow pion in a where searching for a pion in the tree would also return the daughters of the . Use the safer INTREE and NINTREE instead.

TODO : Is there a way of applying a cut to the daughters only, without navigating the whole tree?

## DMASS: The mass difference to the reference value

Calculates the difference between the measured mass and the PDG reference value. It takes the pid of the reference particle as argument. In most reasonable cases one will use the absolute mass difference (see DMASS) or the mass itself (see MM).

## ID: Particle ID

Like ABSID but without the absolute value.

## INTREE: In tree

Requires there is a particle in the decay tree satisfying the requirements.
FilterDesktop.Filter.Code = "(INTREE( (ID=='J/psi(1S)') & (BPVVDCHI2>25) ) )" ;
Requires there is a in the tree more than away from the best vertex.

## MAXTREE: Maximum value in the decay tree

Returns the maximum value of some functor in the decay tree. Useful to apply cuts on all particles, or to extract some value from the tree.
CombineParticles.FILTER2.Code = "(M-MAXTREE('D0'==ABSID,M)<165.5)" ;
This example looks for all in the decay, returns the largest mass (which is the mass of the as there's only one) and computes the mass difference with the mother.

## MINTREE: Minimal value in the decay tree

Takes a functor as agrument and returns its minimal value searching through the decay tree.
CombineParticles.FILTER0.Code = "(MINTREE(ABSID=='K+',PT)>1400)" ;
This would look for all particles kaons in the decay tree and find their and return the minimum. The cut then requires that all kaon descendents ahve a .

## MIPCHI2DV: Minimum IP-

Returns the minimum distance of a particles's trajectory to any set of vertices. Most useful in conjunction with the PRIMARY argument that looks at PVs.
CombineParticles.FILTER0.Code = "(MIPCHI2DV(PRIMARY)>9)" ;
This requires that the particle is away from any PV.

## MIPDV: Minimum IP

Returns the minimum distance of a particles's trajectory to any set of vertices. Most useful in conjunction with the PRIMARY argument that looks at PVs.
CombineParticles.FILTER0.Code = "(MIPDV(PRIMARY)>0.15)" ;
This requires that the particle is 0.15 mm away from any PV.

## M: Mass

Returns . Should be the same as MM.

## MM: Measured Mass

Returns the measured mass of the particle.
HltSharedDiMuon.FILTER1.Code = "(MM<11000)"
This selects dimuons up to a mass of 11 GeV.

## NINTREE: Number of particles in tree

Returns the number of particles in a tree satisfying some selection criteria.
CombineParticles.FILTER0.Code = "( 2 == NINTREE( (ABSID=='e-') & (PT>1000)))" ;
Requires that there are two electrons in the tree with .

## P: Momentum

Gets the momentum of the particle.
HltSharedKsLL.FILTER0.Code = "(P>2000)";
Gets particles with .

## PIDe, PIDmu, PIDK, PIDp, PIDpi : PID DLL

Gets the combined delta-log-likelihood for the given hypothesis (wrt the pion)
CombineParticles.FILTER0.Code = "(PIDe-PIDpi>6)" ;

## PT: Transverse momentum

Gets the transverse momentum of the particle. Note that this is evaluated at the first measurement of the particle, which might not be where you want it for pions in decays.
HltSharedKsLL.FILTER0.Code = "(PT>1000)";
Gets particles with .

## TRCHI2DOF: per DoF of the track fit

Gets the per degree of freedom of the track fit.
CombineParticles.FILTER0.Code = "(TRCHI2DOF<20)" ;

## VDMIN: Distance of two vertices

TODO : Not yet supported.

## VFASPF: Vertex Function as Particle Function.

Allows to apply vertex functors to the particle's endVertex().
CombineParticles.FILTER2.Code = "(VFASPF(VCHI2/VDOF)<10)" ;
Applies a cut to the vertex of the particle.

# Vertex functors

Vertex functors are accessed using VFASPF.

## VCHI2 : Vertex

CombineParticles.FILTER2.Code = "(VFASPF(VCHI2/VDOF)<10)" ;

## VDOF : Vertex fit number of degrees of freedom

CombineParticles.FILTER2.Code = "(VFASPF(VCHI2/VDOF)<10)" ;

# Syntax

Cut are combined using the bit-wide & and | operators, not boolean operators. This also requires that cuts are well protected using parentheses. Example:
HltSharedD02KsPiPi.FILTER0.Code = "( ((ABSID=='pi+') & (PT>400)) |
((ABSID=='KS0') & (PT>1000)))"
Cuts can be split over several lines.

# Use of CombineParticles.

CombineParticles is a DVAlgorithm that combines the input particles according to the decay descriptor. There are three cuts applied :
1. FILTER0 : On the incoming daughter particles.
2. FILTER1 : Once a combination has been made according to the decay descriptor but before the vertex fit.
3. FILTER2 : After the vertex fit.
All the cuts that require the position of the vertex must be applied in FILTER2, while the others can be applied earlier (saving CPU). Note that for long lived particles like Ks it pays off to apply a loose mass cut in FILTER1 and a harder in FILTER2. The reason is that the vertex fit does a propagation of the momenta through the detector. You thus get the momentum at the Ks vertex, while in FILTER1 it's just the sum of the momenta of the daughters at their first measurement.

A simple example:

HltShared.Members += { "CombineParticles/HltSharedPhi2KK" } ;
HltSharedPhi2KK.PhysDesktop.InputLocations = { "Phys/HltNoPIDsKaons" };
HltSharedPhi2KK.DecayDescriptor = "phi(1020) -> K+ K-";
HltSharedPhi2KK.FILTER0.Code = "(MIPCHI2DV(PRIMARY)>4)";    // (IP > 2 sigma)^2
HltSharedPhi2KK.FILTER1.Code = "(ADMASS('phi(1020)')<50)";  // mass window +/- 300 MeV
HltSharedPhi2KK.FILTER2.Code = "(VFASPF(VCHI2/VDOF)<25)" ;

The decay descriptor can be overwritten by DecayDescriptors which allows to reconstruct several decays in one go.

HltShared.Members += { "CombineParticles/HltSharedDstarWithD02KPi" } ;
HltSharedDstarWithD02KPi.PhysDesktop.InputLocations = { "Phys/HltSharedD02KPi", "Phys/HltSharedSlowPions" };
HltSharedDstarWithD02KPi.DecayDescriptors = {"[D*(2010)+ -> pi+ D0]cc", "[D*(2010)+ -> pi+ D~0]cc" }; // also wrong-sign ones
HltSharedDstarWithD02KPi.FILTER0.Code = "(ALL)";    // IP already applied in shared slow pion
HltSharedDstarWithD02KPi.FILTER1.Code = "(ADMASS('D*(2010)+')<50) & (PT>1250)";  // mass window +/- 50 MeV, PT> 1.25 GeV
HltSharedDstarWithD02KPi.FILTER2.Code = "(VFASPF(VCHI2/VDOF)<25) & (M-MAXTREE('D0'==ABSID,M)<165.5)" ;       // Chi2 < 25 . D*-D0 = 145.5 MeV, +2

# Use of FilterDesktop.

FilterDesktop is a DVAlgorithm that applies a cut and clones the selected particles. It has one filter (Filter) but the default is not the LoKi::Hybrid::FilterCriterion which needs to be set by hand using the FilterCriterion property.
HltExclusive.Members += { "FilterDesktop/HltSelSingleMuon" } ;
HltSelSingleMuon.PhysDesktop.InputLocations  = {"Phys/HltMuons"} ;
HltSelSingleMuon.FilterCriterion = "LoKi::Hybrid::FilterCriterion" ;
/**
* Pt > 3 GeV & IP/err > 5 & IP > 0.15 mm
*/
HltSelSingleMuon.Filter.Code = "((PT>3000) & (MIPCHI2DV(PRIMARY)>25) & (MIPDV(PRIMARY)>0.15))" ;

## Help

Please do not send mails directly to the authors, but send the questions and requests through the following mailing lists:

lhcb-davinci@cern.ch, lhcb-loki@cern.ch, lhcb-bender@cern.ch

In this case more colleagues can profit from solutions to the problems and it will allow a bit more wide distribution of useful information, tricks, recipes, experience and the solutions. Also it will simplify the monitoring of the progress with the implementation of the missing functionality.

### Examples

Many working examples can be found in the Hlt/HltSelections package from version v6r0 released with DaVinci v19r11.

-- PatrickKoppenburg - 28 Mar 2008

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Topic revision: r3 - 2008-03-28 - unknown

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