LHAPDF is hosted by Hepforge, IPPP Durham

Description of .LHpdf files

All text strings must be enclosed in single quotation marks.

The example shown here is from the MRST2002nlo.LHpdf file


The first line declares the earliest version of LHAPDF with which the files works, in the following way: 'Version' '5.0'
Next follows a section giving a description of the PDF set. It starts with the line 'Description:' followed by an arbitrary (max 20) number of lines. This is the text that is printed out with the call description() command. 'Description:'
'MRST fit: hep-ph/0211080'
'This set has 1 member PDFs. '
' mem=1 --> MRST2002nlo: Best fit (alpha_S=0.119)'
'Note that the best fit (mem=0) has been set equal'
'to MRST2002nlo'

Next is a section, starting with the line 'Evolution', which defines the evolution method to be used. It is followed by a number of other lines. The first of these lines has three parameters. (1) the order of the evolution, (2) the Q^2 of the PDF fit and (3) the renormalization factor, which for all the sets so far is 1.0. The next line declares the evolution code to be used, in this case QCDNUM_MRST. Other options are QCDNUM, QCDNUM_ZEUS_TR, QCDNUM_ZEUS_ZM and EVLCTEQ. For the varieties of QCDNUM evolution there is a final line in this section which defines the QCDNUM grid parameters, as file name, number of x bins, xmin, xmax, number of Q2 bins, Q2min and Q2max. 'Evolution:'
'nlo',1.0,1.0
'QCDNUM_MRST'
'MRST.large.grid',400,1d-6,1d0,112,1.d0,1d10

Next is a section, starting with the line 'Alphas:', describing how Alphas is to be calculated. It is followed by two lines. The three parameters of the first of these lines declare (1) either Variable or Fixed depending on whether or not Alphas is allowed to vary with Q**2, (2) the order of alphas lo, nlo or nnlo, and (3) the method used in calculating Alphas, either Internal, Evolcode, MRSTalfa or CTEQalfa. Internal uses code in the LHAPDF programme, Evolcode uses code specific to the evolution package, and the final two use the specific Alphas codes of MRST and CTEQ respectively. The third line sets various numerical values used in the Alphas calculation. The first parameter of this line defines Lambda_QCD either as the parameter number in the 'Parameterlist:' (see later) or for the case of 'Fixed' Alphas, the actual value of Lambda_QCD. The second parameter is mass(Z0)**2, and the remaining three are the masses of the charm, bottom and top quarks. 'Alphas:'
'Variable','nlo','MRSTalfa'
1,91.118,1.43,4.30,180.0

The next small section, starting with the line 'QCDparams:', gives LAMBDA_QCD4 and LAMBDA_QCD5 for each data set. These values are used for information only and not used explicitly in the PDF determination. The second line gives (1) the number of PDF sets and (2) either 1 or 0 depending on whether the same values or different ones are given for each set. The third line (and others if indivdual values are given for each set) gives LAMBDA_QCD4 and LAMBDA_QCD5. Note that as in the Parameterlist the default set 0 has to be included in the list. 'QCDparams:'
1,1
0.334,0.233

The next section defines the form of the parametrizations used for xf(x,Q2) at the fitted Q2. There are two forms defined in LHAPDF, namely those used by MRST and CTEQ. (The MRST form is also used by the ZEUS PDFs.) Following the initial line 'Parametrization:', the next line has three parameters which are (1) the evolution form to be used, (2) the weighting of the PDF (not implemented yet therefore set to -1), and (3) the number of functions. In the example below from MRST the number of functions is 6, ie uv,dv,g1,g2,S,Delta. This is then followed by a series of lines for each of these functions. In the MRST form each function is described by three lines comprising first the name of the function and the type of fit (in this case 'x-taylor'). The next line is the parameter n in the 'x-taylor' function, and the number of other parameters. The final line of the three gives the position of the parameters in the Paramterlist:. The form of 'x-taylor' is as follows:

'x-taylor' ==> P1xP2(1-x)P3(1+P4x(1/n)+P5x(2/n)+...) 'Parametrization:'
'MRST',-1,6
'uv','x-taylor'
2.0,5
2,3,4,5,6
'dv','x-taylor'
2.0,5
7,8,9,10,11
'g1','x-taylor'
2.0,5
12,13,14,15,16
'g2','x-taylor'
1.0,3
17,18,19
'S','x-taylor'
2.0,5
20,21,22,23,24
'Delta','x-taylor'
1.0,5
25,26,27,28,29
In the CTEQ6 fits other functional forms (log-pade and cteq6-ratio), defined as follows, are used:

'log-pade' ==> P1xP2(1-x)P3exP4(1+eP5x)P6

'cteq6-ratio' ==> P1xP2(1-x)P3 + (1+P4x)(1-x)P5


This next section, which must follow directly after the Parametrization section defines how the individual partons are made up from the functions defined in the Parametrization section. It starts with the line 'Compose: followed by the definitions for each parton. The format is as shown below in the example from MRST2002nlo.LHpdf. The options here are 'none', 'treshold' or 'composite'. The line following the latter defines the fraction of each of the functions make up that particular parton. 'treshold' is followed by the threshold Q values for the parton.'none' means there is none of that particular parton in the fit. 'Compose:'
'tbar','none'
'bbar','treshold'
4.3
'cbar','treshold'
1.43
'sbar','composite'
0.0,0.0,0.0,0.0,0.1,0.0
'ubar','composite'
0.0,0.0,0.0,0.0,0.2,-0.5
'dbar','composite'
0.0,0.0,0.0,0.0,0.2,0.5
'g','composite'
0.0,0.0,1.0,-1.0,0.0,0.0
'd','composite'
0.0,1.0,0.0,0.0,0.2,0.5
'u','composite'
1.0,0.0,0.0,0.0,0.2,-0.5
's','composite'
0.0,0.0,0.0,0.0,0.1,0.0
'c','treshold'
1.43
'b','treshold'
4.3
'tbar','none'
Note that for CTEQ6 there are other definitions used.
The parameters for each PDF set are given in the section starting with the line 'Parameterlist:'. This is followed by the line starting with 'list' followed by the number of PDF sets then the number of parameters in each set. 'Parameterlist:'
'list',1,29
0.119701 0.135127 0.233100 3.306000 5.807000 ....
0.119701 0.135127 0.233100 3.306000 5.807000 ....
Note that in the example given here the number of sets is 1 but there are in fact 2 rows of parameters. This is because set 0 is defined as the default set and in this case is exactly the same as set 1. Thus the number of parameter lines is always one more that the number of defined sets.
The end of the file it defined by the line 'End' 'End:'