The PHY568 experimental exercices are designed to be done during the “hands-on” lecture and are expected to be handled at the end of the class.

Flavour Physics Exercise Sheet 4

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

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

The PHY568 experimental exercices are designed to be done during the “hands-on” lecture and are expected to be handled at the end of the class.

Exercise 1: PYTHIA generator for HEP (3 Pts.)

The aim of this exercise is to have a first contact with PYTHIA 8, a framework for Monte Carlo event generation for hadron and lepton colliders. There are two ways for running PYTHIA: using it as a plug-in of ROOT or directly. We will have a look at both approaches. Some useful documentation can be found in here:

Particle PDG definition scheme: http://pdg.lbl.gov/2007/reviews/montecarlorpp.pdf Pythia manual: http://home.thep.lu.se/~torbjorn/pythia81html/Welcome.html. In par-

ticular look at “beam parameter” and “process selection” sections.

Pythia tutorial: http://hep.ps.uci.edu/~arajaram/worksheet.pdf.

To provide a first interaction with PYTHIA standalone, please start from a simple script below:

# i n c l u d e " P y t h i a 8 / P y t h i a . h "

u s i n g n a m e s p a c e P y t h i a 8 ;

int m a i n () {

// Set up g e n e r a t i o n P y t h i a p y t h i a ;

p y t h i a . r e a d S t r i n g ( " Top : g g 2 t t b a r = on "); // S w i t c h on p r o c e s s p y t h i a . r e a d S t r i n g ( " B e a m s : eCM = 8 0 0 0 . "); // 8 TeV CM e n e r g y p y t h i a . i n i t () ; // I n i t i a l i z e ; i n c o m i n g pp b e a m is d e f a u l t // G e n e r a t e e v e n t ( s )

p y t h i a . n e x t (); // G e n e r a t e an ( o t h e r ) e v e n t . F i l l e v e n t r e c o r d p y t h i a . s t a t (); // E x t r a i n f o r m a t i o n

r e t u r n 0;

}

– please turn over –

Exercises for FP Sheet 4 Solutions to the exercise will be given during the lecture in the following directory and also an initial Makefile is already available:

/home/hep/rsilvaco/Lectures/FP_2017/exercises/Sheet3/

In addition, include the following paths to your .bashrc (alternatively see supplementary material) . /app/cern/root_v6.04.14/bin/thisroot.sh

export LD_LIBRARY_PATH=/app/cern/pythia_8215/lib:$LD_LIBRARY_PATH

Considering this simplified skeleton, a series of extensions can appended to the program:

a) Add different processes to the list, i.e. q q ¯ → t ¯ t and enable all Top processes.

b) Introduce an event loop around the pythia.next() and investigate the basic properties of these events. Provide an automated access to the event record by enclosing an additional loop.

c) Retrieve the transverse momentum of a given top decay (i.e. t → W b) and book its distribution using the simple PYTHIA histogramming class.

d) Provide input information using card files, i.e. Beams, seed and PartonLevel configurations.

e) Produce the histogram of the charge multiplicity of 1K events. Hint: the particle loop counter should be incremented whenever the particle isCharged() and isFinal().

Exercise 2: PYTHIA with ROOT plug-in (3 Pts.)

In order to enable the ROOT functionalities to these scripts, these libraries need to be added to the Makefile. For simplicity this has been already provided in the initial script. Please check the lines corresponding to ROOT and modify your script accordingly.

a) Try to modify your baseline script to properly initialise PYTHIA to generate some events in LHC enviroment (at √

s = 14 TeV and √

s = 7 TeV). The process to be considered are HardQCD:qqbar 2bbbar, HardQCD:gg2bbbar, HardQCD:gg2qqbarg, HardQCD:qqbar2qqbargDiff and HardQCD:

qqbar2qqbargSame. You should also turn on all quark flavours by HardQCD:nQuark New = 5.

b) How can we select only events containing b quarks? How can we select only B-mesons?

c) Try to save particle information into a tree.

Exercise 3: LHCb acceptance (4 Pts.) Using the baseline script obtained in the previous exercise, let’s try to examine the designed acceptance of LHCb in comparison with other general purpose detectors. This last part of the lecture is planned to be done individually and handled in the end of the class.

– please turn over –

Exercises for FP Sheet 4 a) Calculate the fraction of the events at the LHC environment that lies one of the b quarks in the

acceptance of LHCb. This is defined in the range of 2 < η < 5.

b) Are the b quarks the correct particles to select on? Which particles relevant for the experiment should lie within the acceptance?

c) Consider the polar angle (or pseudorapidity) of b and ¯ b events. Plot the distribution of θ

versus θ

(or pseudorapidity). What are the implications of the discussed pseudorapidity for b-physics in general purpose experiments such as CMS and ATLAS?

d) (Optional) Redo the exercise for Tevatron at √

s = 1.96 TeV. What is the difference?

Supplementary material: Pythia installation instructions

Download the file pythia8180.tgz to a suitable location from the link http://home.thep.lu.se/

~torbjorn/Pythia.html.

Unzip and expand it with tar xvfz pythia8180.tgz.

Move to the create pythia8180 dircetory.

Compile it as a shared library:

$ ./configure --enable-shared

$ make

Define the variable:

$ export PYTHIA8=path_to_PYTHIA8_installation

Magic stuff needed for success:

$ cp $PYTHIA8/lib/archive/libpythia8.a $PYTHIA8/lib/libpythia8.so

$ cp $PYTHIA8/lib/archive/libpythia8.a $PYTHIA8/lib/libpythia8.dylib

Re-compile ROOT with the given configuration options:

$ --enable-pythia8

--with-pythia8-incdir=$PYTHIA8/include --with-pythia8-libdir=$PYTHIA8/lib

$ make

Define the variables:

$ export PYTHIA8DATA=$PYTHIA8/xmldoc

$ export DYLD_LIBRARY_PATH=$PYTHIA8/lib:DYLD_LIBRARY_PATH