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fix: fix scattering calculation in SeedFinderOrthogonal #1248

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May 10, 2022
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fix: fix scattering calculation in SeedFinderOrthogonal #1248

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42 changes: 18 additions & 24 deletions Core/include/Acts/Seeding/SeedFinderOrthogonal.ipp
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Original file line number Diff line number Diff line change
Expand Up @@ -284,24 +284,19 @@ void SeedFinderOrthogonal<external_spacepoint_t>::filterCandidates(

float deltaCotTheta = cotThetaB - lt.cotTheta;
float deltaCotTheta2 = deltaCotTheta * deltaCotTheta;
float error;
float dCotThetaMinusError2 = 0;
// if the error is larger than the difference in theta, no need to
// compare with scattering
if (deltaCotTheta2 - error2 > 0) {
deltaCotTheta = std::abs(deltaCotTheta);
// if deltaTheta larger than the scattering for the lower pT cut, skip
error = std::sqrt(error2);
dCotThetaMinusError2 =
deltaCotTheta2 + error2 - 2 * deltaCotTheta * error;
// avoid taking root of scatteringInRegion
// if left side of ">" is positive, both sides of unequality can be
// squared
// (scattering is always positive)

if (dCotThetaMinusError2 > scatteringInRegion2) {
continue;
}

// Apply a cut on the compatibility between the r-z slope of the two
// seed segments. This is done by comparing the squared difference
// between slopes, and comparing to the squared uncertainty in this
// difference - we keep a seed if the difference is compatible within
// the assumed uncertainties. The uncertainties get contribution from
// the space-point-related squared error (error2) and a scattering term
// calculated assuming the minimum pt we expect to reconstruct
// (scatteringInRegion2). This assumes gaussian error propagation which
// allows just adding the two errors if they are uncorrelated (which is
// fair for scattering and measurement uncertainties)
if (deltaCotTheta2 > (error2 + scatteringInRegion2)) {
continue;
}

float dU = lt.U - Ub;
Expand Down Expand Up @@ -331,13 +326,12 @@ void SeedFinderOrthogonal<external_spacepoint_t>::filterCandidates(
// convert p(T) to p scaling by sin^2(theta) AND scale by 1/sin^4(theta)
// from rad to deltaCotTheta
// if deltaTheta larger than allowed scattering for calculated pT, skip
if (deltaCotTheta2 - error2 > 0) {
if (dCotThetaMinusError2 > pT2scatter * iSinTheta2 *
m_config.sigmaScattering *
m_config.sigmaScattering) {
continue;
}
if (deltaCotTheta2 >
(error2 + (pT2scatter * iSinTheta2 * m_config.sigmaScattering *
m_config.sigmaScattering))) {
continue;
}

// A and B allow calculation of impact params in U/V plane with linear
// function
// (in contrast to having to solve a quadratic function in x/y plane)
Expand Down