Flavour Physics Exercise Sheet 5

Flavour Physics Exercise Sheet 5

FS 17 G. Isidori, R. Coutinho A. Puig, D. van Dyk

www.physik.uzh.ch/de/lehre/PHY568.html

Exercise 1: EvtGen: b-physics event generator within Pythia (5 Pts.)

In order to perform a more detailed description of b-hadron decays than those provided by default in Pythia, many useful packages can be utilised, e.g. QQ, JETSET and EvtGen. In this exercise the EvtGen package is examined as either a plugin for Pythia or a “standalone” simulation.

Consider the latter case in which the EvtGen interface is provided by the plugin class EvtGenDecays.

In addition to the setup discussed in exercise Sheet04, the external package HepMC is required in the Pythia installation. Furthermore, the packages EvtGen, PHOTOS and TAUOLA are required for the others steps of this list. Some instructions about the installation are given in a separate bash script added to the course webpage, which can be slightly easier if you have access to the uzh machines ¹ . From the baseline script obtained in the previous class, let’s investigate how to integrate EvtGen by a series of simple modifications:

a) Add to the Makefile script the libraries for HepMC and EvtGen. This can be validated by adding in the header of your ^∗ .cc script the line #include “Pythia8Plugins/EvtGen.h” and checking the compilation outcome.

b) Initialise a constructor for the EvtGenDecays class. This can be initialised using the inputs found in the EvtGen package corresponding to the EvtGen decay file (e.g. DECAY_2010.DEC) and the EvtGen particle data (e.g. evt.pdl).

c) The properties of any B decay are configured using what is referred to as decfile. A simple example for the B ⁰ → K ^± π ^∓ channel is shown below:

# D e s c r i p t o r : [ B0 - > K + pi -] cc D e c a y B0

1 . 0 0 0 K + pi - P H S P ; E n d d e c a y

C D e c a y anti - B0

# End

where the PHSP model stands for a generic phase space of n-bodies. Create this relevant ^∗ .dec and import this to your script.

– please turn over –

1

All uzh machines have been already configured with HepMC-2.06.09, Pythia-8215 and ROOT-v6.04.14. The addi-

tional packages need to be configured locally for each user.

Exercises for FP Sheet 5

d) Investigate the properties of the produced daughter particles by storing their kinematic variables into a root file.

e) Try to produce more complicate decay modes, e.g. B ⁰ → K ⁰ (892)J/ψ(1S), where K ⁰ (892) → K ^± π ⁻ and J/ψ(1S) → µ ⁺ µ ⁻ . Can you plot the phase space distribution of this decay?

Hint: use the example of Pythia as reference (named “main48.cc”) and note that for EvtGen it is necessary to consider both EvtGenExternal and EvtGen in the Makefile.

Exercise 2: EvtGen standalone use (5 Pts.)

Although within the EvtGen configuration the Pythia setup can be called, in this exercise we will focus only in the attributtes of its standalone use. Considering the examples shown in the “validation”

directory inside EvtGen, let’s try to write our first Dalitz-plot model (there will be a special lecture about this in the course).

a) As initial task consider the script genExampleRootFiles.cc, which provides a generic framework to generate simple examples. After compiling this script verify that EvtGen is properly configured by running :

./ g e n E x a m p l e R o o t F i l e s D a l i t z F i l e s / D a l i t z D e c a y 1 . dec r o o t F i l e s / D a l i t z D e c a y 1 . r o o t D + 1 0 0 0 0

This should generate 10K events of D ⁺ → K ⁻ π ⁺ π ⁺ decays. The Dalitz plot is defined as the bi- dimensional phase-space constructed from two of the invariant masses. What is the phase-space distribution obtained from this ?

b) Generalize this implementation by generating events for B ^± → K ^± π ^∓ π ^± using the PHSP model.

What are the differences with respect to the previous phase space distribution?

c) An useful feature of EvtGen is the support to XML decay files. In your decfile configuration, replace the PHSP by “GENERIC_DALITZ Bd2Kpipi.xml”. This allows you to provide a more detailed description of the dynamics of the process. Check that the previous simulation can be obtained by adding to your xml file the following lines:

< data >

< d a l i t z D e c a y p a r t i c l e = " B + " d a u g h t e r s =" K + pi - pi + " >

< r e s o n a n c e r e a l = " 1.0 " i m a g = " 0.0 " s h a p e = " N o n R e s "/ >

</ d a l i t z D e c a y >

</ data >

d) Let’s add some dynamics to the process by including a resonance state, e.g. K ^∗0 (892). The implementation uses the same scheme:

< r e s o n a n c e mag =" 1.0 " p h a s e = " 0.0 " w i d t h =" 0 . 0 5 0 3 " m a s s = " 0 . 8 9 2 "

s p i n = " 1 " r e s D a u g h t e r s =" K + pi - " s h a p e =" RBW " / >

The difference is that now you need to provide the information about the resonance, such as mass, spin and distribution (in this case “RBW” correspond to a relativistic Breit-Wigner).

What is the effect of adding this resonance to the phase space? Can you identify the main

characteristic of this resonance visually?