DIS: F2charm is non-zero at low Q2 in FFNS4 for uonly PDF
Opened this issue · 3 comments
- We find F2charm beeing non-zero when using the uonly PDF (which does what it name says uonly.tar.gz)
- We believe this to be wrong
F2charm with theory=99 using uonly
x Q2 APFEL yadism yadism_error rel_err[%]
0 0.01 2.000000 0.000260 0.0 0.0 -100.0
1 0.01 2.444444 0.000304 0.0 0.0 -100.0
2 0.01 2.888889 0.000336 0.0 0.0 -100.0
3 0.01 3.333333 0.000356 0.0 0.0 -100.0
4 0.01 3.777778 0.000366 0.0 0.0 -100.0
5 0.01 4.222222 0.000000 0.0 0.0 NaN
6 0.01 4.666667 0.000000 0.0 0.0 NaN
7 0.01 5.111111 0.000000 0.0 0.0 NaN
8 0.01 5.555556 0.000000 0.0 0.0 NaN
9 0.01 6.000000 0.000000 0.0 0.0 NaN
theory 99
{'ID': 22,
'PTO': 0,
'FNS': 'FFNS',
'DAMP': 0,
'IC': 0,
'ModEv': 'EXA',
'XIR': 1.0,
'XIF': 1.0,
'NfFF': 4,
'MaxNfAs': 6,
'MaxNfPdf': 6,
'Q0': 1,
'alphas': 0.11800000000000001,
'Qref': 91.2,
'QED': 0,
'alphaqed': 0.007496251999999999,
'Qedref': 1.777,
'SxRes': 0,
'SxOrd': 'LL',
'HQ': 'POLE',
'mc': 2,
'Qmc': 2,
'kcThr': 1.0,
'mb': 4,
'Qmb': 4,
'kbThr': 1.0,
'mt': 173.07,
'Qmt': 173.07,
'ktThr': 1.0,
'CKM': '0.97428 0.22530 0.003470 0.22520 0.97345 0.041000 0.00862 0.04030 0.999152',
'MZ': 91.1876,
'MW': 80.398,
'GF': 1.1663787e-05,
'SIN2TW': 0.23126,
'TMC': 0,
'MP': 0.938,
'Comments': 'LO baseline for small-x res',
'global_nx': 0,
'EScaleVar': 1,
'_modify_time': '2020-05-28 16:55:02.164765'}
might be related to #22
what is the uonly PDFs? Only up quark to all scales?
You use mc = 2 GeV, so mc2 = 4 GeV2 right? So F2c = 0 above the charm threshold, as it should be. The funny thing is that it is also different from zero below, but F2c should not be used there?
Dear @felixhekhorn, I also found your result worrisome.
Therefore, using a simple modification of the code that @juanrojochacon posted here #22 :
#include "APFEL/APFEL.h"
#include <utility>
#include <vector>
#include <math.h>
int main()
{
APFEL::SetPerturbativeOrder(0);
APFEL::SetPDFSet("uonly");
APFEL::InitializeAPFEL_DIS();
const std::vector<std::pair<double, double>> kin{
{0.01, 2.000000}, {0.01, 2.444444}, {0.01, 2.888889}, {0.01, 3.333333}, {0.01, 3.777778},
{0.01, 4.222222}, {0.01, 4.666667}, {0.01, 5.111111}, {0.01, 5.555556}, {0.01, 6.000000}};
for (auto k : kin)
{
APFEL::ComputeStructureFunctionsAPFEL(sqrt(k.second), sqrt(k.second));
std::cout << std::scientific << k.first << "\t" << k.second << "\t"
<< APFEL::F2charm(k.first) << "\t"
<< std::endl;
}
return 0;
}
and using the "uonly" PDF set, I find:
x Q2 F2c
1.000000e-02 2.000000e+00 0.000000e+00
1.000000e-02 2.444444e+00 0.000000e+00
1.000000e-02 2.888889e+00 0.000000e+00
1.000000e-02 3.333333e+00 0.000000e+00
1.000000e-02 3.777778e+00 0.000000e+00
1.000000e-02 4.222222e+00 0.000000e+00
1.000000e-02 4.666667e+00 0.000000e+00
1.000000e-02 5.111111e+00 0.000000e+00
1.000000e-02 5.555556e+00 0.000000e+00
1.000000e-02 6.000000e+00 0.000000e+00
that seems to be consistent with the expectation.
Are you able to reproduce these results?
The differences with the last comment snippet and what we used in the original post are:
- the snippet is using the default mass scheme, i.e.
ZM-VFNS, while we are using explicitly (see the name of the issue)FFNS4 - the snippet is not setting the pole masses, so the Q2 chosen happen to be all on the same side of the threshold
mc2(that in the snippet is2, while for us it wasmc = 2and so the threshold it was4)
here there is the updated snippet, to make the correct comparison:
#include "APFEL/APFEL.h"
#include <utility>
#include <vector>
#include <math.h>
int main()
{
APFEL::SetPerturbativeOrder(0);
APFEL::SetPDFSet("uonly");
APFEL::SetPoleMasses(2, 4, 175);
APFEL::SetMassScheme("FFNS4");
APFEL::SetFFNS(4); // this statement is ignored by APFEL
APFEL::InitializeAPFEL_DIS();
const std::vector<std::pair<double, double>> kin{
{0.01, 2.000000}, {0.01, 2.444444}, {0.01, 2.888889}, {0.01, 3.333333}, {0.01, 3.777778},
{0.01, 4.222222}, {0.01, 4.666667}, {0.01, 5.111111}, {0.01, 5.555556}, {0.01, 6.000000}};
for (auto k : kin)
{
APFEL::ComputeStructureFunctionsAPFEL(sqrt(k.second), sqrt(k.second));
std::cout << std::scientific << k.first << "\t" << k.second << "\t"
<< APFEL::F2charm(k.first) << "\t"
<< std::endl;
}
return 0;
}and the results:
x Q2 F2c
1.000000e-02 2.000000e+00 2.603139e-04
1.000000e-02 2.444444e+00 3.037474e-04
1.000000e-02 2.888889e+00 3.357870e-04
1.000000e-02 3.333333e+00 3.562243e-04
1.000000e-02 3.777778e+00 3.656866e-04
1.000000e-02 4.222222e+00 0.000000e+00
1.000000e-02 4.666667e+00 0.000000e+00
1.000000e-02 5.111111e+00 0.000000e+00
1.000000e-02 5.555556e+00 0.000000e+00
1.000000e-02 6.000000e+00 0.000000e+00
APFEL parameters' report
WARNING: FFNS is a FFN scheme
... setting NF = 4 FFNS PDF evolution
Welcome to
/// //// //// ///_/ _/
_/ / / / / / /
//// //// /// /// _/
_/ _/ _/ _/ _/ _/
/ / / / //// ////
_____v3.0.4 A PDF Evolution Library, arXiv:1310.1394
Authors: V. Bertone, S. Carrazza, J. Rojo
Report of the evolution parameters:
QCD evolution
Space-like evolution (PDFs)
Unpolarized evolution
Evolution scheme: FFNS with 4 active flavours at N0LO
Solution of the DGLAP equation: 'exactalpha'
Solution of the coupling equations: 'exact'
Coupling reference value:
- AlphaQCD( 1.4142 GeV) = 0.350000
Pole heavy quark masses: - Mc = 2.0000 GeV
- Mb = 4.0000 GeV
- Mt = 175.0000 GeV
The matching thresholds coincide with the physical masses
muR / muF = 1.0000
Allowed evolution range [ 1.0000 : 10000.0000 ] GeV
The internal subgrids will be locked
Fast evolution enabled
Initialization of the evolution completed in 0.053 s
Report of the electroweak parameters:
Mass of the Z = 91.188 GeV
Mass of the W = 80.385 GeV
Mass of the proton = 0.9383 GeV
sin^2(thetaW) = 0.2313
GFermi = 1.16638E-05
| 0.9743 0.2254 0.0036 |
CKM = | 0.2252 0.9734 0.0414 |
| 0.0089 0.0405 0.9991 |
Z propagator correction = 0.00000
Report of the DIS parameters:
Computation in the FFNS4 mass scheme
Electromagnetic (EM) process
Scattering electron - proton
muR / Q = 1.0000
muF / Q = 1.0000
Target Mass corrections disabled
Intrinsic charm disabled
Initialization of the DIS module completed in 0.034 s