Protos - PROgram for TOp Simulations

Protos is a generator for some new physics processes involving the top quark.

This Twiki is based on the Protos manual.

Compiling Protos

Download Protos from the author website.

For last version Protos2.2, you can wget it with:

wget http://jaguilar.web.cern.ch/jaguilar/protos/Protos22.tar.gz && tar -xf Protos22.tar.gz

Edit extlib.mk to link with the LHAPDF libraries that are needed, on lxplus machines:

EXTLIB = -L/afs/cern.ch/sw/lcg/external/MCGenerators/lhapdf/5.8.5/x86_64-slc5-gcc43-opt/lib/ -lLHAPDF

Furthermore, one needs to set the PATH and LD_LIBRARY_PATH environment variables, with: export.sh

Now, you can compile Protos with make.

Running Protos

Subdirectory Structure

• Red: Generators for single top and top pair production with anomalous Wtb couplings.
  • Available processes: tj, tbj, tb, tW, tWb and tt.
• White: Generators for single top production through top flavour-changing neutral (FCN) couplings and top pair production with top FCN decay.
  • Available processes: Zqt, Zt, Aqt, At, gqt, t, Hqt and Ht.
• Green: Top processes with four-fermion effective operators.
  • Available processes: llqt and ttlike.
• Rose: Heavy vector-like quark production.
  • Available processes: BB, Bbj, Bj, Tbj, Tj, TT, Ttj, Xtj, XX, Ybj, Yj and YY.

In each Protos subdirectory of the form xx (tj, tbj, tt, etc.) one can generate events of the different available processes.

The different running settings are:

• Running parameters selected in input/run.dat.
• Standard Model parameters selected in input/smpar.dat.
• For some processes a file input/tweak.dat is available for special features.
• Couplings values are stored in input/template.ini.

After running the program with ./Protos-xx, results are found in:

• If plots are generated (see Running parameters), they are found in the main Protos directory Protos22/plots.
• If log files are generated (see Running parameters), they are found in the main Protos directory Protos22/output.
• Generated events are found in the main Protos directory Protos22/events.

A quick example - single top production

1. Go to Red/tj directory

2. Set common ATLAS Standard Model parameters in input/smpar.dat: smpar.dat

3. One has to provide to Protos the right values of the following couplings: VL, VR, gR and gL.

These values (which yield the W helicities) depend on the top quark mass. For a top-quark mass of 172.5, one has to modify the input/template.ini file:

cp ../../DATA/template-172.5.ini input/template.ini

4. Also, one can edit =input/run.dat=with the appropiate run parameters: run.dat

• In addition, one can choose the decay of final state W boson, i.e. the electron or muon channel, with the ILEP variable:
  • ILEP - 1, 2, 3 selects decay of the W boson to enu, munu and taunu, respectively.

5. To generate the weighted events, do: ./Protos-tj

6. When it is finished, the results will be found on the events directory (in the Protos main directory).

Two files are created: tj.wgt containing all weighted events and tj.par containing the process parameters.

If one wants to cover the whole phase space, has to simulate millions of events in order to cover the less probable regions of the phase space.

7. To unweight the events using the unw program which can be found in the unw directory.

Just edit unw/input/run.dat and set PROCNAME to the correct value and do: ./Protos-unw

This creates two new files in the events directory: tj.unw with the unweighted events and tj_unw.par with the corresponding parameters.

Running parameters

In input/run.dat one can set the following entries:

• PROCNAME - This is the name that will label all generated files (events, plots and log files)
• CM energy
• IPPBAR - Set to 0 for pp collisions and to 1 for ppbar collisions.
• NRUNS - In order to enable iterative runs this must be set to =NRUNS=>1. Maximum is 99.

Phase space integration is done using VEGAS in two steps.

First, a "warm-up" integration is done to build adequate grids for the integration of the different subprocesses. In this phase, the relative weight of these subprocesses (determined by the cross sections) are also calculated. Second, the final integration is performed.

The parameters which control this are:

• ILOADGRID - If 0, it generates a new grid. If 1, it reads a previously stored grid (from input subdir.)
• ISAVEGRID - If 1, it saves the grid for later use. If 0, it does not.
• NCALL1 - Approximate number of points of warm-up integration.
• ITMX1 - Number of iterations of the warm-up integration.
• NCALL2 - Approximate number of points of final integration.
• ITMX2 - Number of iterations for the final integration.
• idum0 - Initial random seed used.

Usually, ILOADGRID is set to 1 and ISAVEGRID to 0.

• ILEP - Decay channel for one or two W (or ILEP1 and ILEP2 for one W and one Z). These values depend of process.
• ICUT - Allows to activate plase space cuts in the subroutine PSCUT of the file evtgen.f. By default, phase space cuts are not included but can be set by the user. The executable must be recompiled then.

Following parameters control generated outputs (activated if set to 1, deactivated if set to 0).

• EVTOUT - Output of weighted events.
• IPLOT - Plots in ASCII format.
• ILOG - Log files generation.
• IVERB - Verbose information from each VEGAS iteration.
• IPRSUB - Print the contribution to the cross section of the different subprocesses at the end of the run.
• IPRAS - Print the calue of selected asymmetries at the end of the program run.

After these flags, top anomalous couplings are set, dicussed for each process separately.

The following parameters are:

• ISCAN - If set to -1, it makes the initial random seed change by one unit in each run of NRUNS.
• SCANSTEP - If ISCAN different of -1, anomalous couplings are changed by adding a quantity SCANSTEP each run.
• IPDFSET - Allows to select a LHAPDF set.
• QFAC - or QFAC1 and QFAC2, are the numerical values multiplying the factorisation scale(s).

For each process, one can find special features to set.

Protos Documentation

Single Top Production Generators

These generators are described with more detail here.

Running parameters

Decay channel

ILEP - Chooses the decay channel of the final state W boson(s):

• For tj, tbj and tb: ILEP = 1, 2, 3 selects decay of the W boson to enu, munu and taunu, respectively. ILEP = 4 is used to include all three channels.
• For tW and tWb: ILEP = 1,2,3 selects decay of the W boson to enu, munu and taunu, respectively, and the other W decays hadronically. ILEP = 4 includes all three semileptonic channels. ILEP = 5 selects a fully hadronic channel for both W bosons, and ILEP = 6 selects the dilepton channel.

Vertex couplings

These generators include the most general Wtb vertex arising from dimension-six gauge-invariant effective operators, parameterised as here.

The four couplings VL, VR, gL and gR can be set in run.dat.

The first two parameters found in the file input/tweak.dat allow to switch on and off the anomalous couplings independently in top production and top decay.

A further option is to fix the helicity of W in the top decay, this can be done by selecting a scenario in tweak.dat: setting this parameter to 1, 2 or 3 ocerrides the values VL, VR, gL and gR in run.dat and replaces them by values which give a W boson with helicity of -1, 0 or 1. This option can be used in combination with the other tweaks to generate templates with a fixed single top cross section (and kinematical distributions) and different W helicities if anomalous couplings are deactivated in the production but not in the decay.

For tj and tW generators, the tweak.dat last option selects the flavour of the initial d/s/b quark.

WARNING: The values of the couplings depend on the top-quark mass. Values for masses of 175 and 172.5 are found in the Protos directory /data. The values for other top masses can be calculated by using the full analytic expressions for helicity fractions (see here ).

When running the program iteratively, the anomalous couplings can be varied in each run:

• ISCAN = 1, 2, 3, 4, 5, 6, 7 varies VL, Re(VR), Re(gL), Re(gR), Im(VR), Im(gL) or Im(gR), respectively, by a quantity SCANSTEP.

Factorisation scales

For each process the factorization scales are:

• tj: Q^2 = -p(W)^2 for the light quark and Q^2 = -p(W)^2 + m(top)^2 for the b quark. This values are multiplied by QFAC1 and QFAC2 in run.dat.
• tbj: Q^2 = -p(W)^2 for the light quark and Q^2 = p(b)^2 + m(b)^2 for the gluon. This values are multiplied by QFAC1 and QFAC2 in run.dat.
• tb: Q = sqrt(s), multiplied by QFAC.
• tW and tWb: Q = m(top) + M(W), multiplied by QFAC.

These scales can only be changed by rewriting the code in evtgen.f, in the subroutine MSQ.

Double counting removal

For tj/tbj and tW/tWb, a matching can be done to remove double counting.

Matching options for tj/tbj are:

• IMATCH = 0, does not perform any matching.
• IMATCH = 1, applies subtraction to the b quark PDF for tj. It has no effect on tbj.
• IMATCH = 2, performs a matching based on the transverse momentum of the extra b quark. This procedure has no effect on tj at the generational level (the correction is applied later on the parton shower Monte Carlo). For tbj, the b quark transverse momentum is required to hace a minimun transverese momentum, whose value is determined by the parameter PTbmin, set by default to 10.

The matching options for tW are:

• IMATCH = 0, does not perform any matching.
• IMATCH = 1, applies subtraction to the b quark PDF, like the tj process.

For tWb the options are:

• IMATCH = 0, does not perform any matching.
• IMATCH = 1, subtracts the resonant tt contribution at the differential cross section level.
• IMATHC = 2, performs an invariant mass cut on the Wb invariant mass to remove the resonant tt contribution.

Program output

The program log provides the top and antitop cross sections in the channel specified, their ratio, and some observables at the generator level:

• The spin-independent asymmetries A_{FB}, A_{±} and W helicity fractions. The later are calculated from the former using analytical expressions (see in here), and for low statistics it may happen that the calculated F_{R} is negative.
• The spin asymmetries A_{l}, A_{nu} and A_{b}.
• Their ratios (which are independent of the top polarisation and only depend on Wtb anomalous couplings).

Definitions of these quantities are in Table 1 of Protos manual. See here and here, for details.

The program also provides several distributions, each has a suffix number that distiguishes them in Table 2 of the Protos manual. The format is raw text in two columns, ready to be plotted with gnuplot, for example.

Ttbar Generator

Decay Channel

The ILEP parameter chooses the decay channel:

• ILEP = 1, 2, 3 selects the semileptonic channel with one W decaying to enu, munu or taunu, respectively, and the other W decaying hadronically.
• ILEP = 4 includes the three semileptonic channels.
• ILEP = 5, selects the full hadronic channel.
• ILEP = 6, selects the dilepton channel.

Vertex couplings

Similar to the Single Top Production, but since this process is about two nearly on-shell top quarks and a Wtb vertes is not involved in the production, the options in tweak.dat are different.

The first parameter switches on and off these features:

• When off, the Wtb coupling is read from run.dat.
• When on, the Wtb coupling is overrided.

The other two parameters set the helicity for both the W bosons decaying leptonically and hadronically, respectively. In the full hadronic and dilepton channel, these variables set the helicities of the W+ and W-, respectively.

WARNING: The values of the couplings depend on the top-quark mass (see previous section and documentation in here).

When running the program iteratively, the anomalous couplings can be varied in each run:

• ISCAN = 1, 2, 3 varies VL, gL and gR, respectively, by a quantity SCANSTEP.

Factorisation scale

The default factorisation scale is Q = m(top), multiplied by QFAC. It only can be changed in the subroutine MSQ of evtgen.f.

Program output

The program log provides the tt cross section in the channel specified, as well as som observables at generator level, and also generates several distributions of interest (see Protos manual for further references).

• Set ALLOWTOPICVIEW = PatriciaFernandez