Reuse particles from a decay tree

Learning Objectives

  • Learn how to extract particles from a decay tree

  • Build a new particle from the extracted particles

Sometimes we want to extract a portion of the decay tree in order to build a different decay. To do that, we need to put the particles we’re interested in in a new container so they can afterwards be used as inputs to a CombineParticles instance (as we saw in the selection framework lesson). To achieve this we can use the FilterInTrees algorithm, a simple variation of FilterDesktop (doxygen).

Let’s start from the example in the selection framework lesson and let’s check that the \(` K^+ `\) child of the \(` D^0 `\) does not come from a \(` K^{*}(892)^{0} \to K^{+}\pi^{-} `\). To do that, we have to extract the \(` K^+ `\) from ([D0 -> K+ K-]CC) and combine it with all pions in Phys/StdAllNoPIDsPions/Particles.

Using FilterInTrees is done in the same way we would use FilterDesktop:

from Configurables import FilterInTrees
from PhysSelPython.Wrappers import Selection, DataOnDemand

stream = 'AllStreams'
line = 'D2hhPromptDst2D2KKLine'
tesLoc = '/Event/{0}/Phys/{1}/Particles'.format(stream, line)

kaons_from_d0 = FilterInTrees('kaons_from_d0_filter', Code="('K+' == ABSID)")
kaons_from_d0_sel = Selection("kaons_from_d0_sel",

The output of kaons_from_d0_sel is a container with all the kaons coming from the \(` D^0 `\).

The final step is easy, very similar to building your own decay:

from Configurables import CombineParticles
from PhysSelPython.Wrappers import Selection, DataOnDemand

Pions = DataOnDemand('Phys/StdAllNoPIDsPions/Particles')
kst = CombineParticles('kst_particles',
                       DecayDescriptor="[K*(892)0 -> K+ pi-]cc",
                       CombinationCut="ADAMASS('K*(892)0') < 300*MeV",
                       MotherCut='(VFASPF(VCHI2/VDOF)< 9)')
kst_sel = Selection('kst_sel',
                    RequiredSelections=[kaons_from_d0_sel, Pions])

Add the kst_sel to the example from building your own decay:

#Combine D and pions into Dstar                                                                                                              
dstar_sel = CombineSelection(
    [d0_sel, soft_pion_sel, kst_sel],
    DecayDescriptor='[D*(2010)+ -> D0 pi+]cc',

The full example can be found here.

An interesting detail

One can use FilterInTrees and FilterDecays to select several particles at once and obtain a flattened list. For example, if we had a Stripping line that builds [B-  -> (^D0 -> ^K- ^pi+) ^pi-]cc and we wanted to combine the D0 and pi- with an external pi0 to build [B- -> D0 pi- pi0]cc, we could do

flatlist = FilterInTrees ("FlatList", Code="('D0' == ABSID) | ('pi-' == ABSID)")
from Configurables import CombineParticles
add_pi0 = CombineParticles("MakeB",
                           DecayDescriptor = "[B- -> D0 pi- pi0]cc",
                           Inputs=[flatlist, resolvedPi0])

flatlist contains both D0 and pi-, which are then used to build the B.