Commit add3a091 authored by Felix Riehn's avatar Felix Riehn

comment on limited precision

parent 2aed5bcf
......@@ -117,7 +117,7 @@ TEST_CASE("SibyllInterface", "[processes]") {
SECTION("InteractionInterface") {
SECTION("InteractionInterface - low energy") {
setup::Stack stack;
const HEPEnergyType E0 = 60_GeV;
......@@ -138,19 +138,46 @@ TEST_CASE("SibyllInterface", "[processes]") {
[[maybe_unused]] const process::EProcessReturn ret = model.DoInteraction(projectile);
auto const pSum = sumMomentum(view, cs);
// for debugging!
std::cout << pSum.GetComponents(cs) << std::endl;
std::cout << plab.GetComponents(cs) << std::endl;
Interactions between hadrons (h) and nuclei (A) in Sibyll are treated in the
hadron-nucleon center-of-mass frame (hnCoM). In addition the incoming hadron (h) and
nucleon (n) are assumed massless, such that the energy and momentum in the hnCoM are
: E_i_cm = 0.5 * SQS and P_i_cm = +- 0.5 * SQS where i is either the projectile
hadron or the target nucleon and SQS is the hadron-nucleon center-of-mass energy.
CHECK(pSum.GetComponents(cs).GetX() / P0 ==
Any hadron-nucleus event can contain several nucleon interactions. In case of Nw
(number of wounded nucleons) in the hadron-nucleus interaction the total energy and
momentum in the hadron-nucleon center-of-mass frame are: momentum: p_projectile +
p_nucleon_1 + p_nucleon_2 + .... p_nucleon_Nw = -(Nw-1) Pcm with center-of-mass
momentum Pcm = p_projectile = - p_nucleon_i energy: E_projectile + E_nucleon_1 + ...
E_nucleon_Nw = (Nw+1) / 2 * SQS with SQS the hadron-nucleon center-of-mass energy.
In case of a single interaction (Nw=1), by definition, the total momentum is zero in
the hadron-nucleon frame. After the boost to the lab. frame the lab. momentum of the
projectile before the interaction is recoverred.
In case of multiple interactions (Nw>1), the momentum is not zero and the total
momentum in the lab. frame after the boost is different from the original projectile
(momentum violation).
The level of violation of momentum conservation is further enhanced due to the
approximation of massless hadrons. At low energies (~10GeV), where the masses can
not be neglected the violation is at the level of percent.
For this reason the numerical precision in these tests is limited to 5% at low
energies. see also:
Issue 272 / MR 204
CHECK(pSum.GetComponents(cs).GetX() / P0 == Approx(1).margin(0.05));
CHECK(pSum.GetComponents(cs).GetY() / 1_GeV == Approx(0).margin(1e-4));
CHECK(pSum.GetComponents(cs).GetZ() / 1_GeV == Approx(0).margin(1e-4));
(pSum - plab).norm() / 1_GeV ==
Approx(0).margin(plab.norm() * model.get_relative_precision_momentum() / 1_GeV));
CHECK(pSum.norm() / P0 == Approx(1).margin(model.get_relative_precision_momentum()));
CHECK((pSum - plab).norm() / 1_GeV == Approx(0).margin(plab.norm() * 0.05 / 1_GeV));
CHECK(pSum.norm() / P0 == Approx(1).margin(0.05));
[[maybe_unused]] const GrammageType length = model.GetInteractionLength(particle);
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