Manual BOX Zjj
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Preprint typeset in JHEP style - PAPER VERSION March 23, 2013
POWHEG BOX 1.0
The POWHEG BOX user manual:
Z+ 2 jets production via strong interactions
Emanuele Re
Institute for Particle Physics Phenomenology, Department of Physics
University of Durham, Durham, DH1 3LE, UK
E-mail: emanuele.re@durham.ac.uk
E-mail: emanuele.re@physics.ox.ac.uk
Abstract: This note documents the use of the package POWHEG BOX for the simulation of
the Z+ 2 jets production process. Results can be easily interfaced to shower Monte Carlo
programs, in such a way that both NLO and shower accuracy are maintained. Please read
carefully the manual before using this code.
Keywords: POWHEG, Shower Monte Carlo, NLO.
Contents
1. Introduction 1
2. Installation 1
2.1 Installation of BlackHat 2
2.2 Compiling POWHEG and linking with BlackHat 2
3. Generation of events 3
3.1 MC-OLP Negotiation stage 3
3.2 Sanity Checks 4
3.3 Other useful informations 5
4. Process specific input parameters 5
1. Introduction
The POWHEG BOX program is a framework for implementing NLO calculations in Shower
Monte Carlo programs according to the POWHEG method. An explanation of the method
and a discussion of how the code is organized can be found in refs. [1, 2, 3]. The code is
distributed according to the “MCNET GUIDELINES for Event Generator Authors and
Users” and can be found at the web page
http://powhegbox.mib.infn.it
In this manual, we will focus on the POWHEG implementation of the NLO corrections to Z+
2 jets hadroproduction, where the Z→`+`−decay is taken into account fully. Source files
can be found in the POWHEG-BOX/Zjj subdirectory, and some details of the implementation
are described in ref. [4], where a comparison with LHC data was also carried out.
2. Installation
In order to run the POWHEG BOX program, we recommend the reader to start from the
POWHEG BOX user manual, which contains all the information and settings that are common
between all subprocesses. In this note we focus on the settings and parameters specific for
the process at hand.
The program needs to be linked with an external code able to provide the one loop
corrections, first computed in ref. [5]. There are currently several tools available to achieve
– 1 –
this task, such as BlackHat,GoSam and MadLoop. The needed loop amplitudes are also
contained in the MCFM code, against which the POWHEG implementation has been checked.
In the following we describe how to link BlackHat (BH) [6] to the POWHEG BOX program
for the Z+ 2 jets production process. Notice, however, that this is achieved using the
Binoth-LH Accord (BLHA) [7] that one-loop programs (OLP) generically implement (or
will implement): therefore, linking to another code other than BH should be relatively
straightforward.
2.1 Installation of BlackHat
The BlackHat library can be downloaded from
https://blackhat.hepforge.org/trac/wiki
It has to be installed following the instructions available at the above web address.
Notice in particular that the qd package has to be installed as well.
In order to link BH with POWHEG, BH has to be installed with the BLHA interface
enabled: hence the configuration command --enable-LHpythoninterface has to be used
at the configuration stage. After BH has been installed, it is useful to explicitly check that
this interface has been created: an executable named LH reader should be present in the
bin directory of the BH installation folder. Standard checks to test the correct installation
of BH can be also performed, as detailed in the BH manual.
As usual, at this point the user should also remember to (re)define the environmental
variable LD LIBRARY PATH in order to include the absolute path of the BH lib directory.
It could be useful to also update the PATH variable to include the bin directory where the
executables blackhat-config and LH reader are located.
2.2 Compiling POWHEG and linking with BlackHat
Before compiling the POWHEG code in the Zjj subdirectory, the user should edit the Makefile
as follows:
1. Set the variables BHPATH and QDPATH to the absolute paths of the directories where
qd and BH have been installed.
2. Set the variable VIRTUAL equal to BH.
3. Set the variable BH equal to true.
If other libraries as LHAPDF [8] or FastJet [9, 10] need to be used, as usual the other
corresponding settings in the Makefile (and possibly the LD LIBRARY PATH variable) should
be edited. Afterwards, it should be possible to compile the POWHEG code with the usual
command
$ make pwhg main.
The two fortran routines invoking the BH library are OLP start and OLP EvalSubprocess,
– 2 –
and they are called in virtual.f. Their source code is part of the BH library. Therefore, if
something went wrong with the BH installation or in the linking procedure between POWHEG
BOX and BH, likely at this stage some error message regarding these two routines will be
issued by the compiler and/or the linker.
3. Generation of events
As explained in the previous section, the executable is built with the command make
pwhg main. If the compilation was successful, all should be in place and ready to run
the code. However, according to the BLHA interface, before the actual running stage, an
initialization phase has to take place between POWHEG and the OLP program (BH in our
case). This phase involves the creation of an order file by the MC program, by means of
which the OLP code is queried about the availability of partonic subprocesses for which
the 1-loop amplitudes are needed. This negotiation stage also involves exchange of the
details of the physical parameters and the format relevant to return the 1-loop amplitudes.
Details of this procedure can be found in ref. [7]. In the following we describe schematically
how to go through this necessary stage. We also describe an important sanity check which
we strongly recommend to always perform when BH is used.
3.1 MC-OLP Negotiation stage
In practice, when linking POWHEG BOX with BH, these are the steps to follow:
1. Enter the run directory
$ cd rundir
and edit the POWHEG input card (named as usual <prefix>-powheg.input) to
enable the usage of the BLHA interface, by means of the flag use-OLP-interface:
use-OLP-interface 1
2. Run the POWHEG program by doing
$ ../pwhg main
and then insert the prefix of the input file when prompted.
3. The code should run for few seconds, and exit with the following message:
WARNING: contract.lh not found.
Run the OLP appropriate command to create it
(for BH, the command is LH reader)
– 3 –
At this stage a file named order.lh should be present in the run directory. Com-
mented lines in this file begin with the #character.
4. Now the actual negotiation phase should take place. This is done by calling the
BlackHat LH reader executable:
$ <path to BH>/bin/LH reader order.lh contract.lh
If this stage ends successfully, no warnings should be issued, and only a brief BH
output message should be printed on the terminal. Crucially, a contract.lh file
should have been created starting from the content of order.lh . Its format should
read like
# order file written by POWHEG-BOX
...
<parameter> <numerical value> | OK
...
# <in1 in2 ==> out3 out4 ... outM>
<[PDGcode1] [PDGcode2] -> [PDGcode3] ... [PDGcodeM]> | 1 n
...
# options
# ...
Lines starting with #in this file are are comments, and nshould be a progressive
integer.
Once this is done, by calling as usual the command pwhg main, the standard full POWHEG
run will start, according to the settings present in the input card. However, we suggest to
first go through a very quick sanity check, as described in the next subsection.
3.2 Sanity Checks
After the contract.lh file has bee created, edit the input card such that the flag check ref amp
is enabled:
check ref amp 1
Make also sure that the relevant physical parameters for this process are set exactly to
the following values:
Zmass 91.1876
Zwidth 2.49
sthw2 0.23
alphaem 0.007763854599
– 4 –
Then, run pwhg main.POWHEG BOX will check that the OLP results for the 1-loop
amplitudes agree with reference hard-coded values. After few seconds, the program will
stop, and an output message will be written on the terminal. Depending on its content,
you will know whether the sanity check was succesfull or not.
If all the aforementioned stages were succesfull, you can remove or comment the token
check ref amp from the input card, and you are ready for a normal run.
3.3 Other useful informations
It is possible to speed up the integration stage and/or the event-generation stage by splitting
these steps in several short runs. When this is done, the program will produce, respectively,
several grid files (ending *-grid.dat ) and/or several event files (ending *.lhe). The
full description of this procedure can be found in POWHEG-BOX/Docs/Manyseeds.pdf. In
summary, to parallelize these stages, the code has first to be run setting in the input card
“itmx2 0” and/or “numevts 0”, respectively.
When this run is finished, all the files for subsequent runs will be ready, but no grids
(or event files, respectively) will be present yet. At this point, the user should uncomment
the option manyseeds in the input card:
manyseeds 1
and decide the number of integration points (events) for each short run, setting itmx2
and ncall2 (or respectively numevts) as desired. In order to start the next step, a file
named <prefix>-seeds.dat has to be created, with the following format:
randomseed1
randomseed2
randomseed3
..
..
where each line has to be the integer that will be used to initialize the random num-
bers generator. At this point, by running the code with the usual command pwhg main,
the user will be asked not only the prefix of the input file, but also an integer. This integer
corresponds to the line in <prefix>-seeds.dat that the program will read to initialize
the random numbers generator. The program will use all the needed files already present
(combining them properly). If we are at the integration (event generation) stage, the grids
(event files) produced will contain in the name also the integer typed, so there is no risk
of overwriting files.
4. Process specific input parameters
•The physical parameters relevant for this process are the Z-boson mass and width,
the squared sine of the Weinberg angle (sin2θW) and αem. Their values are set with
– 5 –
the tokens Zmass, Zwidth, sthw2, alphaem.
•Factorization and renormalization scale factors appearing here have to do with the
computation of the inclusive cross section (i.e. the ¯
Bfunction [1, 2, 3]), and can be
varied by a factor of order 1 to study scale dependence:
facscfact 1 ! factorization scale factor: mufact=muref*facscfact
renscfact 1 ! renormalization scale factor: muren=muref*renscfact
The default scale choices available are
µ=mZ,ˆ
HT/2, ET,Z ,
where ˆ
HT=qm2
Z+p2
T,Z +pT,1+pT,2,pT,Z is the transverse momentum of the
Z-boson, and all the quantities are computed using the underlying-Born kinematics.
They can be chosen by setting the token scalechoice equal to 1, 2 or 3 respectively.
Other scale choices are possible, and the experienced user can change this setting
modifying the set fac ren scales routine.
•This implementation needs a suppression factor, to make the integration of the ¯
B(Φn)
function numerically finite. We refer the reader to [4] for more details. In practise,
this is achieved multiplying the ¯
B(Φn) with the suppression factor
F(Φn) = p2
T,1
p2
T,1+ Λ2
pT!kIS p2
T,2
p2
T,2+ Λ2
pT!kIS s1,2
s1,2+ Λ2
mkFS
.
pT,1and pT,2are the transverse momenta of the 2 outgoing light partons in the
underlying-Born kinematics and s1,2is the invariant mass squared obtained from
their momenta.
The code will not work without this factor, and therefore the flag bornsuppfact
should always be present and set to 1 in the input card.
The function F(Φn) depends on the 4 parameters kIS,kFS, ΛpTand Λm. Their default
values are kIS =kFS = 2, ΛpT= 10 GeV and Λm= 5 GeV, and we suggest to use
them, unless corners of the phase-space (far from where the cross-section is large)
have to be populated with many events.
To set these parameters to values different from the default ones, the tokens supp pt2j
and supp m2inv56 can be used in the input card. The numerical values are inter-
preted as (GeV)2quantities, hence they will change the value of Λ2
pTand Λ2
mrespec-
tively.
As a consequence of the presence of a suppression factor, events will be weighted.
We also stress that for this process the fraction of negative-weighted events will
not be negligible. Therefore we strongly suggest to run the code with the flag
withnegweights set to 1.
– 6 –
•When generating events with this implementation, negative weights appear. The
fraction of negative weights in POWHEG can be reduced increasing foldcsi,foldy,
foldphi. Allowed values are 1, 2, 5, 10, 25, 50. The speed of the program is inversely
proportional to the product of these numbers, so that a reasonable compromise should
be found.
Since running this implementation is already quite computationally demanding, we
suggest to avoid using the folding procedure.
References
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JHEP 0411 (2004) 040 [arXiv:hep-ph/0409146].
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simulations: the POWHEG method,” JHEP 0711 (2007) 070 [arXiv:0709.2092 [hep-ph]].
[3] S. Alioli, P. Nason, C. Oleari and E. Re, “A general framework for implementing NLO
calculations in shower Monte Carlo programs: the POWHEG BOX,” JHEP 1006, 043 (2010)
[arXiv:1002.2581 [hep-ph]].
[4] E. Re, “NLO corrections merged with parton showers for Z+2 jets production using the
POWHEG method,” JHEP 1210, 031 (2012) [arXiv:1204.5433 [hep-ph]].
[5] Z. Bern, L. J. Dixon and D. A. Kosower, Nucl. Phys. B 513, 3 (1998) [hep-ph/9708239].
[6] C. F. Berger, Z. Bern, L. J. Dixon, F. Febres Cordero, D. Forde, H. Ita, D. A. Kosower and
D. Maitre, Phys. Rev. D 78, 036003 (2008) [arXiv:0803.4180 [hep-ph]].
[7] T. Binoth, F. Boudjema, G. Dissertori, A. Lazopoulos, A. Denner, S. Dittmaier, R. Frederix
and N. Greiner et al., Comput. Phys. Commun. 181, 1612 (2010) [arXiv:1001.1307 [hep-ph]].
[8] M. R. Whalley, D. Bourilkov and R. C. Group, “The Les Houches accord PDFs (LHAPDF)
and LHAGLUE,” [arXiv:hep-ph/0508110].
[9] M. Cacciari and G. P. Salam, Phys. Lett. B 641 (2006) 57 [hep-ph/0512210].
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– 7 –
