21.4.10. B2BII¶

The b2bii package in basf2 converts Belle MDST files (basf data format) to Belle II MDST (basf2 data format). This enables performing physics analysis using data collected with Belle detector with the analysis software and algorithms developed for the analysis of data collected with the Belle II detector. It allows for estimation and validation of performances of various advanced algorithms being developed for Belle II (such as Full Event Interpretation, Flavour Tagger, Tag Vertex, …).

B2BII converter¶

The b2bii convert reads and converts Belle MDST within basf2, and the converted data can be then analysed within the same job, without any intermediate steps.

The workflow is illustrated in this figure:

Read Belle MDST file.
Apply momentum or energy corrections in the objects.
Save objects in basf2 format
Time for basf2 analysis!

MC samples¶

As the Belle detector geometry is not implemented in basf2, one needs to either use the official Belle MC samples or generate own signal MC using basf. The information of generating Belle MC can be found here

Generic MC

Generic MC is the official MC samples at Belle, which was generated with run-dependent beam background. There are multiple streams of these samples, and each stream contains the same amount of events as present in the real Belle data.

But which types of MC samples are exactly there? Similar to Belle II MC, there are also different categories of Belle MC:

Generic \(B\) samples : charged (\(B^+ B^-\)) and mixed (\(B^{0}\overline{B}^{0}\))
Continuum samples : uds, charm
Y(5S) samples : bsbs, nonbsbs

Generic \(B\) samples only contain decay modes with \(b \to c\) quark transitions, and have been generated based on the decay tables at

/sw/belle/belle/b20090127_0910/share/data-files/evtgenutil/DECAY.DEC.

How to find Generic MC samples?

You can find the sample(s) you want through Belle File Search Engine

Warning

Belle File Search Engine is only accessible within KEK domain or via VPN. Or look in the SSH - Secure Shell tutorial for a way to access it via SSH forwarding.

Fig. 21.28 The Belle File Search Engine¶

By specifying Exp No, Event Type, Data Type, and Stream No, Event Type means different MC types (charged, mixed, uds, .. ). Data Type is for different energy runs (on-resonance, off-resonance, …). In total there are 10 streams of Generic \(B\) samples and 6 streams of continuum samples.

You can either use the file list (physical path) or URL as input file list for b2bii jobs.

Note

Belle File Search Engine is also for data files.

Analysis with b2bii¶

With Belle MDST in hand, you can use it for your first b2bii analysis. It is very simple, just replace inputMdst in your script with two simple lines:

from b2biiConversion import convertBelleMdstToBelleIIMdst
convertBelleMdstToBelleIIMdst(inputfile, path=mypath)

Now we can use basf2 and analysis tools in basf2 to perform analyses over Belle MDST files.

The relations between basf and basf2 objects are shown in this figure:

However, there are still many differences between the Belle detector and the Belle II detector, as well as basf and basf2. Therefore, we can’t simply use the same basf2 steering files, small modifications are needed.

Charged Final State Particles

basf and basf2 use different Helix parameterisations, however there exist a well defined transformation from one parameterisation to the other. Belle MDST stores in addition to the five helix parameters also the reference point (or pivot point), which is assumed to be always point (0,0,0) in the case of Belle II MDST.

Despite the different parameterisations, charged final state particles can still be reconstructed using fillParticleList function in basf2. But due to the different definition, as well as detector, it is not recommended to use Belle II style PID in b2bii.

basf provided three different packages for PID:

atc_pid (KID) to separate kaons and pions, but also used for proton id
eID (electron ID) to separate electrons from hadrons
muid (muon ID) to separate muons from hadrons

Each of them in its own way combined information collected from various subdetector systems (CDC, ACC, TOF, ECL, KLM). The combination of individual likelihoods from each sub detector system is in some cases (eID) combined with the usage of external information, such as a priori probabilities of each particle type that is read from the Belle DB. Due to this fact the Belle-like PID probabilities can not be reproduced in BASF2 from the raw likelihoods.

Alternatively, we can use the following predefined Belle-style PID variables to reproduce them:

Separation

basf2

Kaon vs pion

atcPIDBelle(3,2)

electron vs hadron

eIDBelle

muon likelihood

muIDBelle

muon likelihood quality flag

muIDBelleQuality

Exercise

Try to create a list of \(K^{+}\) with \(\text{KID}>0.6\), and another list of \(\pi^{+}\) with \(\text{KID}<0.4\).

Solution

import basf2
import modularAnalysis as ma

mypath = basf2.create_path()
ma.fillParticleList('K+:sig', 'atcPIDBelle(3,2)>0.6', path=mypath)
ma.fillParticleList('pi+:sig', 'atcPIDBelle(3,2)<0.4', path=mypath)

Neutral Final State Particles

Belle MDST has two additional data types: mdst_gamma and mdst_pi0, for which there exist no equivalent data type in the Belle II MDST format. In other words, photons and \(\pi^0\) particles are already created in basf. During the conversion, b2bii converter by default creates gamma:mdst and pi0:mdst.

Warning

Don’t use fillParticleList to create photon candidates and don’t reconstruct \(\pi^0\) candidates from pairs of two photons by yourself.

Exercise

Reconstruct the decay \(D^0 \to K^{-} \pi^{+} \pi^{0}\) with mass between 1.7 to 2.0 GeV in a b2bii analysis.

Hint

Always use premade particle lists for neutrals!

Solution

ma.reconstructDecay('D0:Kpipi0 -> K-:sig pi+:sig pi0:mdst', '1.7 < M < 2.0', path=mypath)

V0 Particles

As mentioned in Charged Final State Particles, all charged tracks are parametrised with a helix with the reference point set to (0,0,0) in basf2. This is not optimal in the case of V0s whose decay vertices can be far away from the origin. Therefore, all V0 candidates from the Mdst_Vee2 table in basf are converted to Particles and saved in the particle lists K_S0:mdst, Lambda0:mdst, and gamma:v0mdst.

The created particles have momentum and decay vertex position set to values given in Belle’s Mdst_Vee2 table and their daughters particles with momentum and position at the pivot equal to V0 decay vertex. In addition, the quality indicators for \(K_{S}^{0}\) and \(\Lambda^{0}\) can be used by simply calling goodBelleKshort and goodBelleLambda, respectively.

Exercise

Select good Kshort from K_S0:mdst list.

Hint

Use cutAndCopyList to select candidates from an existing list.

Solution

ma.cutAndCopyList('K_S0:good', 'K_S0:mdst', cut='goodBelleKshort', path=mypath)

\(K_{L}^{0}\)

KLMClusters (Mdst_KLM_Cluster) and Klongs (Mdst_Klong) are converted. The Klongs are stored in the default K_L0:mdst.

Warning

Don’t use fillParticleList to create Klong candidates.

Task

Use the final task in First steering file lesson as a template, try to convert it to your first b2bii analysis script.

This time, let’s reconstruct \(B^{-} \to D^{0} \pi^{-}\) with \(D^{0} \to K^{-}\pi^{+}\pi^{0}\).

Apply the same PID selections for your \(K\) and \(\pi\) as in the earlier exercise.

In the end, save the PID (Belle-style electronID, muonID, and KID), with other variables for all particles in the decay chain to the ntuple.

You can use this line to get the example file:

basf2.find_file('b2bii_input_evtgen_exp_07_BptoD0pip-D0toKpipi0-0.mdst', 'examples', False)

Hint

First steering file lesson is your best friend!

Remember always using premade particle lists for neutrals, Don’t forget to use Belle-style PID for charged particles.

Solution

#!/usr/bin/env python3

import os

import basf2 as b2
import modularAnalysis as ma
import variables as va
import variables.collections as vc
import variables.utils as vu
from b2biiConversion import convertBelleMdstToBelleIIMdst

os.environ["USE_GRAND_REPROCESS_DATA"] = "1"
os.environ["PGUSER"] = "g0db"

# Convert
main = b2.create_path()
inputfile = b2.find_file(
    "b2bii_input_evtgen_exp_07_BptoD0pip-D0toKpipi0-0.mdst", "examples", False
)
convertBelleMdstToBelleIIMdst(inputfile, path=main)

# Only charged final state particles need to be loaded. The neutral particles
# gamma, pi0, K_S0, K_L0, and Lambda0 are already loaded to the 'gamma:mdst',
# 'pi0:mdst', 'K_S0:mdst', 'K_L0:mdst', and 'Lambda0:mdst' particle lists,
# respectively.
ma.fillParticleList("pi+:sig", "atcPIDBelle(3,2)<0.4", path=main)
ma.fillParticleList("K+:sig", "atcPIDBelle(3,2)>0.6", path=main)

# The Belle PID variables are: atcPIDBelle(sigHyp, bkgHyp), muIDBelle, and eIDBelle
va.variables.addAlias("Lkpi", "atcPIDBelle(3,2)")

# Now, let's really reconstruct a B decay with an intermediate D meson:
ma.reconstructDecay(
    "D0:Kpipi0 -> K-:sig pi+:sig pi0:mdst", cut="1.7 < M < 2.0", path=main
)
ma.reconstructDecay(
    "B+:D0pi -> anti-D0:Kpipi0 pi+:sig",
    cut="4.8 < Mbc >5.2 and abs(deltaE) < 0.15",
    path=main,
)

ma.matchMCTruth("B+:D0pi", path=main)

# create and fill flat Ntuple with MCTruth and kinematic information
# Create list of variables to save into the output file
b_vars = []

standard_vars = vc.kinematics + vc.mc_kinematics + vc.mc_truth + vc.inv_mass
b_vars += vc.deltae_mbc
b_vars += standard_vars

# Variables for D0 and all final state particles (kaons, pions, neutral pions)
b_vars += vu.create_aliases_for_selected(
    standard_vars,
    "B+ -> [ ^D0 -> ^K- ^pi+ ^pi0] ^pi+",
    prefix=["D0", "K", "pip1", "pi0", "pip2"],
)

# Variables for charged final states (kaons, pions)
belle1pid = (
    ["eIDBelle", "muIDBelleQuality", "muIDBelle", "Lkpi"]
    + vc.track
    + vc.track_hits
)

b_vars += vu.create_aliases_for_selected(
    belle1pid, "B+ -> [ D0 -> ^K- ^pi+ pi0] ^pi+", prefix=["K", "pip1", "pip2"]
)

ma.variablesToNtuple(
    "B+:D0pi", b_vars, filename="B2BII_B2D0pi_D2Kpipi0.root", path=main
)

# progress
main.add_module("Progress")

b2.process(main)

# Print call statistics
print(b2.statistics)

Key points

Making basf2 process Belle data is as easy as adding convertBelleMdstToBelle2Mdst() to the top of your steering file.
Be careful with particle lists and variables in your analysis.
Never use fillParticleList to create neutral final state particles!!

Stuck? We can help!

If you get stuck or have any questions to the online book material, the #starterkit-workshop channel in our chat is full of nice people who will provide fast help.

Refer to Collaborative Tools. for other places to get help if you have specific or detailed questions about your own analysis.

Improving things!

If you know how to do it, we recommend you to report bugs and other requests with JIRA. Make sure to use the documentation-training component of the Belle II Software project.

If you just want to give very quick feedback, use the last box “Quick feedback”.

Please make sure to be as precise as possible to make it easier for us to fix things! So for example:

typos (where?)
missing bits of information (what?)
bugs (what did you do? what goes wrong?)
too hard exercises (which one?)
etc.

If you are familiar with git and want to create your first pull request for the software, take a look at How to contribute. We’d be happy to have you on the team!

Quick feedback!

Author of this lesson

Chia-Ling Hsu