Difference between revisions of "CalcHEP reference"

Revision as of 15:51, 9 July 2014

Here is the CalcHEP batch mode reference. You should find description for every possible parameter the batch mode accepts except if it is redundant with some HEPMDB function.

This document is organised as follows: entries concerning similar or close information are gathered together. Each subsection is a parameter name. Sometimes only the main parameter is in the subsection title but the other sub-parameters are described in the same subsection.

Model information

Model

This line corresponds to the model used. When using HEPMDB, each batch file is associated with a specific model. As a consequence, the value of this line should not be changed.

Gauge

CalcHEP is able to use two different gauges for computation: Feynman gauge and unitary gauge. Some models require a specific gauge. CalcHEP is better suited to in Feynman gauge, as such it is the default setting. It is recommended that this should not be changed.

Model changed

Each time the batch interface is run, it checks initially whether or not the sub-process numerical code exists. If it does, CalcHEP reuses the code and skips the often long process of code generation. If the code does not already exist, the numerical code is then generated and added to the library. If the model is changed, the numerical codes are regenerated as appropriate.

Process description

Process

Using the process keyword, it is possible to specify which process to compute. Multiple processes can be required. The general syntax is:

Process: P1 [,P2] -> P3, P4 [,P5...]

This is in the form: incoming particles -> outgoing particles. Particles can be particles from the model you are using or composite particles. It is recommended that the number of outgoing particles does not exceed 6.

Decay

It is possible to specify decays for the outgoing particles. Each decay will be written as follow:

Decay: P -> P1, P2 [,P3...]

Decays will automatically be connected to outgoing particles if possible. Decays will even be connected with one another.

Important note: make sure the same decay does not appear twice.

Composite

When specifying processes and decays, you can use composite particles, i.e. labels that refer to various possible particles. This is particularly useful when colliding protons or when studying jets.

The syntax is:

Composite: label=P1 [,P2...]

Remove

Sometimes, only specific processes will be of interest. It is possible to tell CalcHEP which particle you do not want as virtual particles in the processes. The syntax is:

Remove: P1 [,P2...]

To remove a particular decay, the syntax is:

Remove Decay: P1 [,P2...]

Beam description

pdf1/pdf2

The parton density functions of incoming protons can be specified with pdf1 and pdf2. A list of the typical pdfs can be found in the comments of the template batch file, however the main pdfs and corresponding parameters are described here. For the LHC, the standard pdf setting is:

pdf1 : cteql(proton) pdf2 : cteql(proton)

The default setting is None.

It is also possible to set pdfs for electron-positron beams. The ISR & Beamstrahlung setting is exclusive to electron-positron collisions. For these processes, the beam parameters may be specified, the default values are:

Bunch x+y sizes (nm) : 560 Bunch length (mm) : 0.4 Number of particles : 2E+10

If the Equiv. Photon setting is chosen, then the photon particle and maximum Q value must be specified. The defaults are:

Photon Particle : e^+ |Q|max : 100

If Proton Photon is chosen, then the following parameters must be specified:

Incoming particle mass : 0.938 Incoming particle charge : -1 |Q^2|max : 2.0 Pt cut of outgoing proton : 0.15

p1/p2

These two parameters define the energies of the beams in GeV. When using pdf, it can also be used as an input parameter for the pdf. For instance, when using proton photon, this would corresponds to the energy of the initial proton.

Model parameters

Parameter

It is possible to change some parameters of a model using Parameter keyword. The parameters are model specific, any parameters unspecified here will be read from the default values of the model. Here is an example for selecting the elementary charge in the Standard Model:

Parameter: EE=0.31

Run parameter

It is usual for a model to have some free parameters not completely fixed by experiment. For that kind of situation, you have the possibility to run several times the same computation with a different value for a specific parameter.

In order to use this possibility, you have to write four consecutive lines. The first one specifies which parameter to study. To study the Higgs mass using the Standard Model, you would write:

Run parameter: Mh

Then you must specify the initial value and the increase of the value between each run:

Run begin: 100 Run step size: 10

Finally, the number of steps is defined by:

Run n steps: 6

In this specific example, you will get 6 runs using the following value for the Higgs mass: 100, 110, 120, 130, 140, 150.

It is possible to mark several parameters for running. But it is only possible to explore hyperrectangle in the parameter space.

QCD parameters

alpha Q

There are a number of QCD parameters that can be included in the batch file, however they need not all be included and most can be commented out. The main parameter to change is the QCD scale, specified in terms of the invariant mass of the final state particles. For example, in a process with two final state particles, the QCD scale would be defined as:

alpha Q : M34

It may also be specified by a formula in terms of a parameter of the model, for example, to make the QCD scale half the value of the Higgs mass in the Standard Model:

alpha Q : Mh/2

Cuts

Cut parameter

Cut invert

Cut min/max

Kinematics and regularization

Kinematics

In this part of the batch file the user needs to specify which kinematics are needed. The numbers in the kinematics refer to the particles in the process in order. For example, 12 relates to the first and second particles in the process. Here is an example of some kinematics:

Kinematics : 12 -> 34, 56 Kinematics : 34 -> 3 , 4 Kinematics : 56 -> 5 , 6

Regularization momentum

Applying regularization means that the phase space is adjusted so that resonances are properly accounted for. Applying regularization will mean that the result is far more accurate.

There are four lines needed in the batch file for each resonance. They are:

Regularization momentum: Regularization mass: Regularization width: Regularization power:

Events

Number of events (per run step)

This is totally up to the user and what he/she wishes to calculate. Please not that this number of events are being created for each run step. If events are not needed at all, please set this to 0.

Filename

This is self-explanatory.

Parallelization

Parallelization method

Max number of cpus

The maximum number of cpus depends on the computer/cluster you are about to run this batch file on. Typically, this value is 4, 8 or 12.

@@ Line 99: / Line 99: @@
 == QCD parameters ==
 === alpha Q ===
+There are a number of QCD parameters that can be included in the batch file, however they need not all be included and most can be commented out. The main parameter to change is the QCD scale, specified in terms of the invariant mass of the final state particles. For example, in a process with two final state particles, the QCD scale would be defined as:
+<center><tt>alpha Q : M34</tt></center>
+It may also be specified by a formula in terms of a parameter of the model, for example, to make the QCD scale half the value of the Higgs mass in the Standard Model:
+<center><tt>alpha Q : Mh/2</tt></center>
 == Cuts ==