TrackMomentumCalculator.cxx

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// \file  TrackMomentumCalculator.cxx
//
// \author sowjanyag@phys.ksu.edu

#include "larreco/RecoAlg/TrackMomentumCalculator.h"
#include "cetlib/pow.h"
#include "messagefacility/MessageLogger/MessageLogger.h"

#include <array>
#include <cassert>
#include <cmath>
#include <iostream>
#include <limits>
#include <string>
#include <tuple>

#include "Math/Functor.h"
#include "Math/GenVector/PositionVector3D.h"
#include "Minuit2/Minuit2Minimizer.h"
#include "Rtypes.h"
#include "TAxis.h"
#include "TGraphErrors.h"
#include "TMath.h"
#include "TMatrixDSymEigen.h"
#include "TMatrixDSymfwd.h"
#include "TMatrixDfwd.h"
#include "TMatrixT.h"
#include "TMatrixTSym.h"
#include "TPolyLine3D.h"
#include "TSpline.h"
#include "TVectorDfwd.h"
#include "TVectorT.h"
#include "canvas/Persistency/Common/Ptr.h"
#include "lardataobj/RecoBase/Track.h"

using std::cout;
using std::endl;

namespace {

  //TODO: Replace for some global variable present in the art file
  constexpr double LAr_density{1.396};
  constexpr double rad_length{14.0};
  constexpr double m_muon{0.1057}; // muon mass
  constexpr auto range_gramper_cm()
  {
    std::array<double, 73> Range_grampercm{
      {0.9833,   1.36,      1.786,    2.507,   3.321,   4.859,   6.598,   8.512,   10.58,   12.78,
       15.1,     17.52,     20.04,    25.31,   30.84,   36.59,   42.5,    54.73,   67.32,   86.66,
       106.3,    139.4,     172.5,    205.6,   238.5,   271.1,   303.5,   335.7,   367.7,   431.0,
       493.4,    555.2,     616.3,    736.8,   855.2,   1030.0,  1202.0,  1482.0,  1758.0,  2029.0,
       2297.0,   2562.0,    2825.0,   3085.0,  3343.0,  3854.0,  4359.0,  4859.0,  5354.0,  6333.0,
       7298.0,   8726.0,    10130.0,  12430.0, 14690.0, 16920.0, 19100.0, 21260.0, 23380.0, 25480.0,
       27550.0,  31610.0,   35580.0,  39460.0, 43260.0, 50620.0, 57680.0, 67780.0, 77340.0, 92220.0,
       1.06e+05, 1.188e+05, 1.307e+05}};
    for (double& value : Range_grampercm) {
      value /= LAr_density; // convert to cm
    }
    return Range_grampercm;
  }

  constexpr auto Range_grampercm = range_gramper_cm();
  constexpr std::array<double, 73> KE_MeV{
    {10.0,    12.0,    14.0,    17.0,    20.0,    25.0,    30.0,    35.0,    40.0,    45.0,
     50.0,    55.0,    60.0,    70.0,    80.0,    90.0,    100.0,   120.0,   140.0,   170.0,
     200.0,   250.0,   300.0,   350.0,   400.0,   450.0,   500.0,   550.0,   600.0,   700.0,
     800.0,   900.0,   1000.0,  1200.0,  1400.0,  1700.0,  2000.0,  2500.0,  3000.0,  3500.0,
     4000.0,  4500.0,  5000.0,  5500.0,  6000.0,  7000.0,  8000.0,  9000.0,  10000.0, 12000.0,
     14000.0, 17000.0, 20000.0, 25000.0, 30000.0, 35000.0, 40000.0, 45000.0, 50000.0, 55000.0,
     60000.0, 70000.0, 80000.0, 90000.0, 1e+05,   1.2e+05, 1.4e+05, 1.7e+05, 2e+05,   2.5e+05,
     3e+05,   3.5e+05, 4e+05}};
  TGraph const KEvsR{73, Range_grampercm.data(), KE_MeV.data()};
  TSpline3 const KEvsR_spline3{"KEvsRS", &KEvsR};

  // Stopping power data from pdg, to be used with MCS
  std::vector<double> dedx_GeV_per_cm()
  {
    // original table (in MeV cm2/g)
    std::vector<double> dEdx{
      {5.687, 4.979, 4.461, 3.902, 3.502, 3.042, 2.731, 2.508, 2.34,  2.21,  2.107, 2.023, 1.954,
       1.848, 1.771, 1.713, 1.67,  1.609, 1.57,  1.536, 1.519, 1.508, 1.51,  1.517, 1.526, 1.537,
       1.548, 1.559, 1.57,  1.591, 1.61,  1.628, 1.645, 1.675, 1.7,   1.733, 1.761, 1.799, 1.829,
       1.855, 1.877, 1.897, 1.914, 1.93,  1.944, 1.969, 1.991, 2.01,  2.028, 2.058, 2.084, 2.119,
       2.149, 2.193, 2.232, 2.269, 2.304, 2.337, 2.369, 2.4,   2.43,  2.49,  2.548, 2.606, 2.663,
       2.776, 2.888, 3.055, 3.224, 3.498, 3.774, 4.052, 4.332}};
    for (double& value : dEdx) {
      value *= LAr_density * 1e-3; // convert to GeV/cm
    }
    return dEdx;
  }

  const int ndedx = 73;
  const std::vector<double> dEdx_GeV_per_cm = dedx_GeV_per_cm();
  const std::vector<double> E_GeV{
    {0.115695, 0.117697, 0.119693, 0.122694, 0.125695, 0.13069,  0.135694, 0.14069,  0.145714,
     0.150689, 0.155682, 0.160666, 0.165693, 0.17566,  0.185714, 0.1957,   0.205644, 0.225683,
     0.245698, 0.275669, 0.305658, 0.355669, 0.405711, 0.45563,  0.505671, 0.555646, 0.605694,
     0.655676, 0.705661, 0.805664, 0.905689, 1.00557,  1.10606,  1.30529,  1.50571,  1.8061,
     2.10565,  2.60614,  3.1058,   3.60555,  4.10536,  4.60521,  5.10609,  5.606,    6.10591,
     7.10579,  8.10569,  9.10561,  10.1106,  12.1105,  14.1104,  17.1103,  20.1103,  25.1102,
     30.1102,  35.1102,  40.1101,  45.1101,  50.1101,  55.1101,  60.1101,  70.1101,  80.1101,
     90.1101,  100.1,    120.1,    140.1,    170.1,    200.1,    250.1,    300.1,    350.1,
     400.1}};
  TGraph const dEdx_vs_E{ndedx, E_GeV.data(), dEdx_GeV_per_cm.data()};
  TSpline3 const dEdx_vs_E_spline3{"dEdx_vs_E", &dEdx_vs_E};

  TVector3 const basex{1, 0, 0};
  TVector3 const basey{0, 1, 0};
  TVector3 const basez{0, 0, 1};
  constexpr double kcal{0.0024}; // Approximation of dE/dx for mip muon in LAr

  constexpr double MomentumDependentConstant(const double p)
  {
    // values measured with MC: https://indico.fnal.gov/event/62283/contributions/299163/attachments/181289/248600/20240910_SimReco_Update_CM.pdf
    double a = 0.079;
    double c = 10.435;
    return (a / (p * p)) + c;
  }
  double ComputeExpetecteRMS(const double p, const double red_length)
  {
    double beta = std::sqrt(1 - ((m_muon * m_muon) / (p * p + m_muon * m_muon)));
    return (MomentumDependentConstant(p) / (p * beta)) *
           (1.0 + 0.038 * TMath::Log(red_length / cet::square(beta))) * std::sqrt(red_length);
  }
  class FcnWrapper {
  public:
    explicit FcnWrapper(std::vector<double>&& xmeas,
                        std::vector<double>&& ymeas,
                        std::vector<double>&& eymeas,
                        double correction)
      : xmeas_{std::move(xmeas)}
      , ymeas_{std::move(ymeas)}
      , eymeas_{std::move(eymeas)}
      , correction_{correction}
    {}

    double my_mcs_chi2(double const* x) const
    {
      double result = 0.0;

      double p = x[0];
      double theta0 = x[1];

      auto const n = xmeas_.size();
      assert(n == ymeas_.size());
      assert(n == eymeas_.size());

      for (std::size_t i = 0; i < n; ++i) {
        double const xx = xmeas_[i];
        double const yy = ymeas_[i];
        double const ey = eymeas_[i];

        if (std::abs(ey) < std::numeric_limits<double>::epsilon()) {
          std::cout << " Zero denominator in my_mcs_chi2 ! " << std::endl;
          return -1;
        }

        double const l0 = xx / rad_length;
        double res1 = 0.0;
        // Highland formula
        // Parameters given at Particle Data Group https://pdg.lbl.gov/2023/web/viewer.html?file=../reviews/rpp2022-rev-passage-particles-matter.pdf
        double beta = std::sqrt(1 - ((m_muon * m_muon) / (p * p + m_muon * m_muon)));
        if (xx > 0 && p > 0)
          res1 = (13.6 / (p * beta)) * (1.0 + 0.038 * TMath::Log(l0 / cet::square(beta))) *
                 std::sqrt(l0);
        res1 *= correction_;

        res1 = std::hypot(res1, theta0);

        double const diff = yy - res1;
        result += cet::square(diff / ey);
      }

      if (std::isnan(result) || std::isinf(result)) {
        MF_LOG_DEBUG("TrackMomentumCalculator") << " Is nan in my_mcs_chi2 ! ";
        return -1;
      }

      return result;
    }

  private:
    std::vector<double> const xmeas_;
    std::vector<double> const ymeas_;
    std::vector<double> const eymeas_;
    double const correction_;
  };

  class FcnWrapperLLHD {
  public:
    explicit FcnWrapperLLHD(std::vector<double>&& dEi,
                            std::vector<double>&& dEj,
                            std::vector<double>&& dthij,
                            std::vector<double>&& ind,
                            std::vector<bool>&& dthij_valid,
                            double stepsize,
                            double correction)
      : dEi_{std::move(dEi)}
      , dEj_{std::move(dEj)}
      , dthij_{std::move(dthij)}
      , ind_{std::move(ind)}
      , dthij_valid_{std::move(dthij_valid)}
      , stepsize_{stepsize}
      , correction_{correction}
    {}

    double my_mcs_llhd(double const* x) const
    {
      double result = 0.0;

      double red_length = stepsize_ / rad_length;

      double p = x[0];
      double theta0 = x[1];

      // Total initial energy of the muon (converting the input "p" into energy with muon mass)
      double Etot = std::hypot(p, m_muon);

      double Ei{Etot};

      double dEi{0};
      auto const n = dEi_.size(); // number of segments of energy

      bool addpenality = false;
      for (std::size_t i = 0; i < n; ++i) {
        Ei -= dEi;
        if (Ei >= E_GeV[0]) { //otherwise keep dEi the same (In any case, this is close to p = 0)
          dEi = dEdx_vs_E_spline3.Eval(Ei) * stepsize_;
        }
        else {
          dEi = dEdx_GeV_per_cm[0] * stepsize_;
        }
        if (Ei <= m_muon) {
          Ei =
            m_muon +
            0.010; // Reached zero energy, keep this value constant to not evaluate `nan` in pij nor tH0
          addpenality = true;
        }

        if (dthij_valid_.at(i) == false) continue;

        // Total momentum of the muon including momentum lost upstream of this segment (converting Eij to momentum)
        double pij = std::sqrt(Ei * Ei - m_muon * m_muon);

        // Highland formula
        // Parameters given at Particle Data Group https://pdg.lbl.gov/2023/web/viewer.html?file=../reviews/rpp2022-rev-passage-particles-matter.pdf
        // Modified with uboone studies: https://iopscience.iop.org/article/10.1088/1748-0221/12/10/P10010
        double tH0 = ComputeExpetecteRMS(pij, red_length);

        // The tH0 (theta rms) is calculated for projected angles
        // If space angles are used instead, tH0 needs to be multiplied by sqrt(2)
        tH0 *= correction_;

        double rms_square = -1.0;

        double prob = 0;
        double DT = 0;
        // Computes the rms of angle (no need to evaluate sqrt)
        rms_square = cet::sum_of_squares(tH0, theta0);

        DT = dthij_.at(i);

        // Formula is modified so we don't compute sqrt(rms), use factor in log instead
        prob = -0.5 * std::log(2.0 * TMath::Pi()) - 0.5 * std::log(rms_square) -
               0.5 * DT * DT / rms_square;

        if (addpenality) { prob -= 2 * rms_square; }

        result = result - 2.0 * prob; // Adds for each segment
      }

      return result;
    }

  private:
    std::vector<double> const dEi_;
    std::vector<double> const dEj_;
    std::vector<double> const dthij_;
    std::vector<double> const ind_;
    std::vector<bool> const dthij_valid_;
    double const stepsize_;
    double const correction_;
  };

}

namespace trkf {

  TrackMomentumCalculator::TrackMomentumCalculator(double const min,
                                                   double const max,
                                                   double const stepsize,
                                                   int const angleMethod,
                                                   int const nsteps)
    : minLength{min}
    , maxLength{max}
    , steps_size{stepsize}
    , fMCSAngleMethod{static_cast<ScatterAngleMethods>(angleMethod)}
    , n_steps{nsteps}
  {
    for (int i = 1; i <= n_steps; i++) {
      steps.push_back(steps_size * i);
    }
  }

  double TrackMomentumCalculator::GetTrackMomentum(double trkrange, int pdg) const
  {
    /* Muon range-momentum tables from CSDA (Argon density = 1.4 g/cm^3)
       website:
       http://pdg.lbl.gov/2012/AtomicNuclearProperties/MUON_ELOSS_TABLES/muonloss_289.pdf

       CSDA table values:
       double Range_grampercm[30] = {9.833E-1, 1.786E0, 3.321E0,
       6.598E0, 1.058E1, 3.084E1, 4.250E1, 6.732E1, 1.063E2, 1.725E2,
       2.385E2, 4.934E2, 6.163E2, 8.552E2, 1.202E3, 1.758E3, 2.297E3,
       4.359E3, 5.354E3, 7.298E3, 1.013E4, 1.469E4, 1.910E4, 3.558E4,
       4.326E4, 5.768E4, 7.734E4, 1.060E5, 1.307E5}; double KE_MeV[30] = {10, 14,
       20, 30, 40, 80, 100, 140, 200, 300, 400, 800, 1000, 1400, 2000, 3000,
       4000, 8000, 10000, 14000, 20000, 30000, 40000, 80000, 100000, 140000,
       200000, 300000, 400000};

       Functions below are obtained by fitting polynomial fits to KE_MeV vs
       Range (cm) graph. A better fit was obtained by splitting the graph into
       two: Below range<=200cm,a polynomial of power 4 was a good fit; above
       200cm, a polynomial of power 6 was a good fit

       Fit errors for future purposes:
       Below 200cm, Forpoly4 fit: p0 err=1.38533;p1 err=0.209626; p2
       err=0.00650077; p3 err=6.42207E-5; p4 err=1.94893E-7; Above 200cm,
       Forpoly6 fit: p0 err=5.24743;p1 err=0.0176229; p2 err=1.6263E-5; p3
       err=5.9155E-9; p4 err=9.71709E-13; p5 err=7.22381E-17;p6
       err=1.9709E-21;*/

    //*********For muon, the calculations are valid up to 1.91E4 cm range
    //corresponding to a Muon KE of 40 GeV**********//

    /*Proton range-momentum tables from CSDA (Argon density = 1.4 g/cm^3):
      website: https://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html

      CSDA values:
      double KE_MeV_P_Nist[31]={10, 15, 20, 30, 40, 80, 100, 150, 200, 250, 300,
      350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000,
      1500, 2000, 2500, 3000, 4000, 5000};

      double Range_gpercm_P_Nist[31]={1.887E-1,3.823E-1, 6.335E-1, 1.296,
      2.159, 7.375, 1.092E1, 2.215E1, 3.627E1, 5.282E1, 7.144E1,
      9.184E1, 1.138E2, 1.370E2, 1.614E2, 1.869E2, 2.132E2, 2.403E2,
      2.681E2, 2.965E2, 3.254E2, 3.548E2, 3.846E2, 4.148E2, 4.454E2,
      7.626E2, 1.090E3, 1.418E3, 1.745E3, 2.391E3, 3.022E3};

      Functions below are obtained by fitting power and polynomial fits to
      KE_MeV vs Range (cm) graph. A better fit was obtained by splitting the
      graph into two: Below range<=80cm,a a*(x^b) was a good fit; above 80cm, a
      polynomial of power 6 was a good fit

      Fit errors for future purposes:
      For power function fit: a=0.388873; and b=0.00347075
      Forpoly6 fit: p0 err=3.49729;p1 err=0.0487859; p2 err=0.000225834; p3
      err=4.45542E-7; p4 err=4.16428E-10; p5 err=1.81679E-13;p6
      err=2.96958E-17;*/

    //*********For proton, the calculations are valid up to 3.022E3 cm range
    //corresponding to a Muon KE of 5 GeV**********//

    if (trkrange < 0 || std::isnan(trkrange)) {
      mf::LogError("TrackMomentumCalculator") << "Invalid track range " << trkrange << " return -1";
      return -1.;
    }

    double KE, Momentum, M;
    constexpr double Muon_M = m_muon * 1e3, Proton_M = 938.272;

    if (abs(pdg) == 13) {
      M = Muon_M;
      KE = KEvsR_spline3.Eval(trkrange);
    }
    else if (abs(pdg) == 2212) {
      M = Proton_M;
      if (trkrange > 0 && trkrange <= 80)
        KE = 29.9317 * std::pow(trkrange, 0.586304);
      else if (trkrange > 80 && trkrange <= 3.022E3)
        KE = 149.904 + (3.34146 * trkrange) + (-0.00318856 * trkrange * trkrange) +
             (4.34587E-6 * trkrange * trkrange * trkrange) +
             (-3.18146E-9 * trkrange * trkrange * trkrange * trkrange) +
             (1.17854E-12 * trkrange * trkrange * trkrange * trkrange * trkrange) +
             (-1.71763E-16 * trkrange * trkrange * trkrange * trkrange * trkrange * trkrange);
      else
        KE = -999;
    }
    else
      KE = -999;

    if (KE < 0)
      Momentum = -999;
    else
      Momentum = std::sqrt((KE * KE) + (2 * M * KE));

    Momentum = Momentum / 1000;

    return Momentum;
  }

  // Momentum measurement via Multiple Coulomb Scattering (MCS)

  // author: Leonidas N. Kalousis (July 2015)

  // email: kalousis@vt.edu

  // Updated by: Henrique Vieira de Souza (June 2024)
  // email: hvsouza@apc.in2p3.fr

  double TrackMomentumCalculator::GetMomentumMultiScatterLLHD(const art::Ptr<recob::Track>& trk,
                                                              const bool checkValidPoints,
                                                              const int maxMomentum_MeV,
                                                              const double min_resolution,
                                                              const double max_resolution,
                                                              const bool check_valid_scattered,
                                                              const double angle_correction)
  {

    std::vector<double> recoX;
    std::vector<double> recoY;
    std::vector<double> recoZ;

    int n_points = trk->NumberTrajectoryPoints();

    for (int i = 0; i < n_points; i++) {
      if (checkValidPoints && !trk->HasValidPoint(i)) continue;
      auto const& pos = trk->LocationAtPoint(i);
      recoX.push_back(pos.X());
      recoY.push_back(pos.Y());
      recoZ.push_back(pos.Z());
    }

    if (recoX.size() < 2) return -1.0;

    if (!plotRecoTracks_(recoX, recoY, recoZ)) return -1.0;

    double const seg_size{steps_size};

    auto const segments = getSegTracks_(recoX, recoY, recoZ, seg_size, check_valid_scattered);
    if (!segments.has_value()) return -1.0;

    auto const seg_steps = segments->x.size();
    if (seg_steps < 2) return -1;

    double const recoSegmentLength = segments->L.at(seg_steps - 1);
    if (recoSegmentLength < minLength || recoSegmentLength > maxLength) return -1;

    std::vector<double> dEi;
    std::vector<double> dEj;
    std::vector<double> dthij;
    std::vector<double> ind;
    std::vector<bool> dthij_valid = segments->nvalid;
    if (getDeltaThetaij_(dEi, dEj, dthij, ind, *segments, seg_size, angle_correction) != 0)
      return -1;

    auto const ndEi = dEi.size();
    if (ndEi < 1) return -1;

    double correction = 1.;
    if (fMCSAngleMethod == kAngleCombined) { correction = std::sqrt(2.); }

    // Gets energy evaluated by range
    auto recoL = trk->Length();
    double const minP = this->GetTrackMomentum(recoL, 13);

    ROOT::Minuit2::Minuit2Minimizer mP{};
    FcnWrapperLLHD const wrapper{std::move(dEi),
                                 std::move(dEj),
                                 std::move(dthij),
                                 std::move(ind),
                                 std::move(dthij_valid),
                                 seg_size,
                                 correction};
    ROOT::Math::Functor FCA([&wrapper](double const* xs) { return wrapper.my_mcs_llhd(xs); }, 2);

    mP.SetFunction(FCA);

    // Start point for resolution
    // If max_resolution is zero, startpoint is equal min_resolution
    double startpoint = 2;
    if (startpoint < min_resolution)
      startpoint = (max_resolution - min_resolution) / 2.; // prevent wrong values
    if (max_resolution == 0) startpoint = min_resolution;

    // Starting energy as double of the energy by range
    // Assumes that the smallet possible energy is given by 60%  with CSDA
    // Step as 10 %
    mP.SetLimitedVariable(0, "p_{MCS}", minP * 2, minP * 0.1, minP * 0.6, maxMomentum_MeV / 1.e3);
    mP.SetLimitedVariable(
      1, "#delta#theta", startpoint, startpoint / 2., min_resolution, max_resolution);
    if (max_resolution == 0) { mP.FixVariable(1); }
    mP.SetMaxFunctionCalls(1.E9);
    mP.SetMaxIterations(1.E9);
    mP.SetTolerance(0.01);
    mP.SetStrategy(2);
    mP.SetErrorDef(1);

    bool const mstatus = mP.Minimize();

    mP.Hesse();

    const double* pars = mP.X();

    double const p_mcs = pars[0];

    return mstatus ? p_mcs : -1.0;
  }

  TVector3 TrackMomentumCalculator::GetMultiScatterStartingPoint(const art::Ptr<recob::Track>& trk)
  {
    double const LLHDp = GetMuMultiScatterLLHD3(trk, true);
    double const LLHDm = GetMuMultiScatterLLHD3(trk, false);

    if (LLHDp != -1 && LLHDm != -1 && LLHDp > LLHDm) {
      int const n_points = trk->NumberTrajectoryPoints();
      return trk->LocationAtPoint<TVector3>(n_points - 1);
    }
    else {
      return trk->LocationAtPoint<TVector3>(0);
    }

    // Should never get here
    return TVector3{};
  }

  double TrackMomentumCalculator::GetMuMultiScatterLLHD3(art::Ptr<recob::Track> const& trk,
                                                         bool const dir)
  {
    std::vector<double> recoX;
    std::vector<double> recoY;
    std::vector<double> recoZ;

    int const n_points = trk->NumberTrajectoryPoints();
    for (int i = 0; i < n_points; ++i) {
      auto const index = dir ? i : n_points - 1 - i;
      auto const& pos = trk->LocationAtPoint(index);
      recoX.push_back(pos.X());
      recoY.push_back(pos.Y());
      recoZ.push_back(pos.Z());
    }

    if (recoX.size() < 2) return -1.0;

    if (!plotRecoTracks_(recoX, recoY, recoZ)) return -1.0;

    constexpr double seg_size{5.0};
    auto const segments = getSegTracks_(recoX, recoY, recoZ, seg_size);
    if (!segments.has_value()) return -1.0;

    auto const seg_steps = segments->x.size();
    if (seg_steps < 2) return -1;

    double const recoSegmentLength = segments->L.at(seg_steps - 1);
    if (recoSegmentLength < 15.0 || recoSegmentLength > maxLength) return -1;

    std::vector<double> dEi;
    std::vector<double> dEj;
    std::vector<double> dthij;
    std::vector<double> ind;
    double angle_correction = 1; // never tested in this function
    if (getDeltaThetaij_(dEi, dEj, dthij, ind, *segments, seg_size, angle_correction) != 0)
      return -1;

    auto const recoL = trk->Length();
    double const p_range = recoL * kcal;
    double const logL = my_mcs_llhd(dEi, dEj, dthij, ind, p_range, 5.65);

    return logL;
  }

  int TrackMomentumCalculator::getDeltaThetaij_(std::vector<double>& ei,
                                                std::vector<double>& ej,
                                                std::vector<double>& th,
                                                std::vector<double>& ind,
                                                Segments const& segments,
                                                double const thick,
                                                double const angle_correction) const
  {
    auto const& segnx = segments.nx;
    auto const& segny = segments.ny;
    auto const& segnz = segments.nz;
    auto const& segL = segments.L;

    int const a1 = segnx.size();
    int const a2 = segny.size();
    int const a3 = segnz.size();

    if (a1 != a2 || a1 != a3) {
      std::cout << " ( Get thij ) Error ! " << std::endl;
      return -1.0;
    }

    int tot = a1;

    for (int i = 0; i < tot - 1; i++) {
      double const dx = segnx.at(i);
      double const dy = segny.at(i);
      double const dz = segnz.at(i);

      // Assumes z as propagation angle
      TVector3 const vec_z{dx, dy, dz};
      TVector3 vec_x;
      TVector3 vec_y;

      double const switcher = basex.Dot(vec_z);
      if (std::abs(switcher) <= 0.995) {
        vec_y = vec_z.Cross(basex).Unit();
        vec_x = vec_y.Cross(vec_z);
      }
      else {
        // cout << " It switched ! Isn't this lovely !!! " << endl; //
        vec_y = basez.Cross(vec_z).Unit();
        vec_x = vec_y.Cross(vec_z);
      }

      TVector3 const Rx{vec_x.Dot(basex), vec_x.Dot(basey), vec_x.Dot(basez)};
      TVector3 const Ry{vec_y.Dot(basex), vec_y.Dot(basey), vec_y.Dot(basez)};
      TVector3 const Rz{vec_z.Dot(basex), vec_z.Dot(basey), vec_z.Dot(basez)};

      double const here_dx = segnx.at(i + 1);
      double const here_dy = segny.at(i + 1);
      double const here_dz = segnz.at(i + 1);

      TVector3 const here_vec{here_dx, here_dy, here_dz};
      TVector3 const rot_here{Rx.Dot(here_vec), Ry.Dot(here_vec), Rz.Dot(here_vec)};

      double const scx = rot_here.X();
      double const scy = rot_here.Y();
      double const scz = rot_here.Z();

      double const azy = find_angle(scz, scy);
      double const azx = find_angle(scz, scx);

      constexpr double ULim = 10000.0; // Avoid huge (wrong) angles
      constexpr double LLim = -10000.0;

      double const Li = segL.at(i);
      double const Lj = segL.at(i + 1);

      if (azy <= ULim && azy >= LLim) { // safe scatter in the yz plane

        if (azx <= ULim && azx >= LLim) { // safe scatter in the za plane
          ei.push_back(Li);               // Energy deposited at i
          ej.push_back(Lj);               // Energy deposited at i+1
          if (fMCSAngleMethod == kAnglezx) {
            th.push_back(azx); // scattered angle z-x
          }
          else if (fMCSAngleMethod == kAnglezy) {
            th.push_back(azy);
          }
          else if (fMCSAngleMethod == kAngleCombined) {
            th.push_back(std::hypot(azx, azy) /
                         angle_correction); // space angle (applying correction)
          }
        }
      }
    }

    return 0;
  }

  double TrackMomentumCalculator::GetMomentumMultiScatterChi2(const art::Ptr<recob::Track>& trk,
                                                              const bool checkValidPoints,
                                                              const int maxMomentum_MeV,
                                                              const double min_resolution,
                                                              const double max_resolution)
  {
    std::vector<double> recoX;
    std::vector<double> recoY;
    std::vector<double> recoZ;

    int n_points = trk->NumberTrajectoryPoints();

    for (int i = 0; i < n_points; i++) {
      if (checkValidPoints && !trk->HasValidPoint(i)) continue;
      auto const& pos = trk->LocationAtPoint(i);
      recoX.push_back(pos.X());
      recoY.push_back(pos.Y());
      recoZ.push_back(pos.Z());
    }

    if (recoX.size() < 2) return -1.0;

    if (!plotRecoTracks_(recoX, recoY, recoZ)) return -1.0;

    double const seg_size{steps_size};
    auto const segments = getSegTracks_(recoX, recoY, recoZ, seg_size);
    if (!segments.has_value()) return -1.0;

    auto const seg_steps = segments->x.size();
    if (seg_steps < 2) return -1;

    double const recoSegmentLength = segments->L.at(seg_steps - 1);
    if (recoSegmentLength < minLength || recoSegmentLength > maxLength) return -1;

    double ymax = -999.0;
    double ymin = +999.0;

    std::vector<double> xmeas;
    std::vector<double> ymeas;
    std::vector<double> eymeas;
    xmeas.reserve(n_steps);
    ymeas.reserve(n_steps);
    eymeas.reserve(n_steps);
    for (int j = 0; j < n_steps; j++) {
      double const trial = steps.at(j);
      // computes the rms by groups of trial, if seg_size was chosen as 10, trials will be 10, 20, etc.. until 10 * n_steps
      auto const [mean, rms, rmse] = getDeltaThetaRMS_(*segments, trial);

      if (std::isnan(mean) || std::isinf(mean)) {
        mf::LogDebug("TrackMomentumCalculator") << "Returned mean is either nan or infinity.";
        continue;
      }
      if (std::isnan(rms) || std::isinf(rms)) {
        mf::LogDebug("TrackMomentumCalculator") << "Returned rms is either nan or infinity.";
        continue;
      }
      if (std::isnan(rmse) || std::isinf(rmse)) {
        mf::LogDebug("TrackMomentumCalculator") << "Returned rmse is either nan or infinity.";
        continue;
      }

      if (mean == -1 && rms == -1 && rmse == -1) continue;
      xmeas.push_back(trial); // x values are different steps length, ex: 10, 20, 30 cm
      ymeas.push_back(rms);   // y values are the RMS of the scattered angle for each step defined
      eymeas.push_back(
        std::hypot(rmse, 0.05 * rms)); // <--- conservative syst. error to fix chi^{2} behaviour !!!

      if (ymin > rms) ymin = rms;
      if (ymax < rms) ymax = rms;
    }

    assert(xmeas.size() == ymeas.size());
    assert(xmeas.size() == eymeas.size());
    if (xmeas.empty()) { return -1.0; }

    TGraphErrors gr_meas{n_steps, xmeas.data(), ymeas.data(), nullptr, eymeas.data()};

    gr_meas.SetTitle("(#Delta#theta)_{rms} versus material thickness; Material "
                     "thickness in cm; (#Delta#theta)_{rms} in mrad");

    gr_meas.SetLineColor(kBlack);
    gr_meas.SetMarkerColor(kBlack);
    gr_meas.SetMarkerStyle(20);
    gr_meas.SetMarkerSize(1.2);

    gr_meas.GetXaxis()->SetLimits(steps.at(0) - steps.at(0), steps.at(n_steps - 1) + steps.at(0));
    gr_meas.SetMinimum(0.0);
    gr_meas.SetMaximum(1.80 * ymax);

    double correction = 1.;
    if (fMCSAngleMethod == kAngleCombined) { correction = std::sqrt(2.); }

    ROOT::Minuit2::Minuit2Minimizer mP{};
    FcnWrapper const wrapper{std::move(xmeas), std::move(ymeas), std::move(eymeas), correction};
    ROOT::Math::Functor FCA([&wrapper](double const* xs) { return wrapper.my_mcs_chi2(xs); }, 2);

    // Start point for resolution
    double startpoint = 2;
    if (startpoint < min_resolution) startpoint = (max_resolution - min_resolution) / 2.;
    if (max_resolution == 0) startpoint = min_resolution;

    mP.SetFunction(FCA);
    mP.SetLimitedVariable(0, "p_{MCS}", 1.0, 0.01, 0.001, maxMomentum_MeV / 1.e3);
    mP.SetLimitedVariable(
      1, "#delta#theta", startpoint, startpoint / 2., min_resolution, max_resolution);
    if (max_resolution == 0) { mP.FixVariable(1); }
    mP.SetMaxFunctionCalls(1.E9);
    mP.SetMaxIterations(1.E9);
    mP.SetTolerance(0.01);
    mP.SetStrategy(2);
    mP.SetErrorDef(1);

    bool const mstatus = mP.Minimize();

    mP.Hesse();

    const double* pars = mP.X();

    auto const recoL = trk->Length();
    double const deltap = (recoL * kcal) / 2.0;

    double const p_mcs = pars[0] + deltap;

    return mstatus ? p_mcs : -1.0;
  }

  bool TrackMomentumCalculator::plotRecoTracks_(std::vector<double> const& xxx,
                                                std::vector<double> const& yyy,
                                                std::vector<double> const& zzz)
  {
    auto const n = xxx.size();
    auto const y_size = yyy.size();
    auto const z_size = zzz.size();

    if (n != y_size || n != z_size) {
      cout << " ( Get reco tacks ) Error ! " << endl;
      return false;
    }

    // Here, we perform a const-cast to double* because, sadly,
    // TPolyLine3D requires a pointer to a non-const object.  We will
    // trust that ROOT does not mess around with the underlying data.
    auto xs = const_cast<double*>(xxx.data());
    auto ys = const_cast<double*>(yyy.data());
    auto zs = const_cast<double*>(zzz.data());

    auto const narrowed_size = static_cast<int>(n); // ROOT requires a signed integral type
    delete gr_reco_xyz;
    gr_reco_xyz = new TPolyLine3D{narrowed_size, zs, xs, ys};
    gr_reco_yz = TGraph{narrowed_size, zzz.data(), yyy.data()};
    gr_reco_xz = TGraph{narrowed_size, zzz.data(), xxx.data()};
    gr_reco_xy = TGraph{narrowed_size, xxx.data(), yyy.data()};

    return true;
  }

  // Compute the deviation in `segx, ...` of the segment stores at `segnx, ...`
  // vx, vy, vz are used and cleared afterwards
  // The `maximum fluctuation` is given by the direction of the give by the Principal Component Analysis (PCA)
  void TrackMomentumCalculator::compute_max_fluctuation_vector(const std::vector<double> segx,
                                                               const std::vector<double> segy,
                                                               const std::vector<double> segz,
                                                               std::vector<double>& segnx,
                                                               std::vector<double>& segny,
                                                               std::vector<double>& segnz,
                                                               std::vector<bool>& segn_isvalid,
                                                               std::vector<double>& vx,
                                                               std::vector<double>& vy,
                                                               std::vector<double>& vz,
                                                               bool check_valid_scattered)
  {
    auto const na = vx.size();

    double sumx = 0.0;
    double sumy = 0.0;
    double sumz = 0.0;

    bool isvalid = true;
    // In case vx, vy, etc have 3 points, probably two are just "linear"
    // interpolations. In this case, the angle of scattering will be zero.
    if (check_valid_scattered && na <= 2) { isvalid = false; }

    // computes the average in x, y, z
    for (std::size_t i = 0; i < na; ++i) {
      sumx += vx.at(i);
      sumy += vy.at(i);
      sumz += vz.at(i);
    }

    sumx /= na;
    sumy /= na;
    sumz /= na;

    std::vector<double> mx;
    std::vector<double> my;
    std::vector<double> mz;

    TMatrixDSym m(3);

    // Computes the Covariance matrix (Principal Component Analysis (PCA)
    for (std::size_t i = 0; i < na; ++i) {
      double const xxw1 = vx.at(i);
      double const yyw1 = vy.at(i);
      double const zzw1 = vz.at(i);

      mx.push_back(xxw1 - sumx);
      my.push_back(yyw1 - sumy);
      mz.push_back(zzw1 - sumz);

      double const xxw0 = mx.at(i);
      double const yyw0 = my.at(i);
      double const zzw0 = mz.at(i);

      m(0, 0) += xxw0 * xxw0 / na;
      m(0, 1) += xxw0 * yyw0 / na;
      m(0, 2) += xxw0 * zzw0 / na;

      m(1, 0) += yyw0 * xxw0 / na;
      m(1, 1) += yyw0 * yyw0 / na;
      m(1, 2) += yyw0 * zzw0 / na;

      m(2, 0) += zzw0 * xxw0 / na;
      m(2, 1) += zzw0 * yyw0 / na;
      m(2, 2) += zzw0 * zzw0 / na;
    }

    TMatrixDSymEigen me(m);

    // retrieve eigenvalues and vectors
    TVectorD eigenval = me.GetEigenValues();
    TMatrixD eigenvec = me.GetEigenVectors();

    double max1 = -666.0;

    double ind1 = 0;

    // get maximum eingevalue
    for (int i = 0; i < 3; ++i) {
      double const p1 = eigenval(i);

      if (p1 > max1) {
        max1 = p1;
        ind1 = i;
      }
    }

    // set the `direction` vector that points to the maximum fluctuation
    double ax = eigenvec(0, ind1);
    double ay = eigenvec(1, ind1);
    double az = eigenvec(2, ind1);

    if (n_seg > 1) {
      // for x, y and z, check if the last point is bigger then the previous point.
      // Ensures that the computed fluctation follows the trend of the track
      if (segx.at(n_seg - 1) - segx.at(n_seg - 2) > 0)
        ax = std::abs(ax);
      else
        ax = -1.0 * std::abs(ax);

      if (segy.at(n_seg - 1) - segy.at(n_seg - 2) > 0)
        ay = std::abs(ay);
      else
        ay = -1.0 * std::abs(ay);

      if (segz.at(n_seg - 1) - segz.at(n_seg - 2) > 0)
        az = std::abs(az);
      else
        az = -1.0 * std::abs(az);

      segnx.push_back(ax);
      segny.push_back(ay);
      segnz.push_back(az);
      segn_isvalid.push_back(isvalid);
    }

    // clear the vectors
    vx.clear();
    vy.clear();
    vz.clear();
  }

  /* This function will group each point of the track inside segments with
   * fixed size `seg_size`.
   * It returns computed new points `segx, segy, segz` separated by `seg_size`,
   * the deviation `segnx, ...` between this points and the distance `segL` in
   * steps of `seg_size` between these points
   */
  std::optional<TrackMomentumCalculator::Segments> TrackMomentumCalculator::getSegTracks_(
    std::vector<double> const& xxx,
    std::vector<double> const& yyy,
    std::vector<double> const& zzz,
    double const seg_size,
    bool const check_valid_scattered)
  {
    double stag = 0.0;

    int a1 = xxx.size();
    int a2 = yyy.size();
    int a3 = zzz.size();

    if ((a1 != a2) || (a1 != a3) || (a2 != a3)) {
      cout << " ( Digitize reco tacks ) Error ! " << endl;
      return std::nullopt;
    }

    int const stopper = seg_stop / seg_size;

    // values to be filled and returned
    std::vector<double> segx, segnx;
    std::vector<double> segy, segny;
    std::vector<double> segz, segnz;
    std::vector<bool> segn_isvalid;
    std::vector<double> segL;

    n_seg = 0;

    double x0{};
    double y0{};
    double z0{};

    double x00 = xxx.at(0);
    double y00 = yyy.at(0);
    double z00 = zzz.at(0);

    int indC = 0;

    // These vectors will keep the points inside reach segments
    // They are cleared inside the function `compute_max_fluctuation_vector`
    // each time it finishes with a segment
    std::vector<double> vx;
    std::vector<double> vy;
    std::vector<double> vz;

    for (int i = 0; i < a1; i++) {
      x0 = xxx.at(i);
      y0 = yyy.at(i);
      z0 = zzz.at(i);

      double const RR0 = std::hypot(x00 - x0, y00 - y0, z00 - z0);

      if (RR0 >= stag) { // stag is aways set to zero, this is always true

        segx.push_back(x0);
        segy.push_back(y0);
        segz.push_back(z0);

        segL.push_back(0);

        // TGraph
        x_seg[n_seg] = x0;
        y_seg[n_seg] = y0;
        z_seg[n_seg] = z0;

        n_seg++;

        vx.push_back(x0);
        vy.push_back(y0);
        vz.push_back(z0);

        indC = i + 1;

        break;
      }
    }

    for (int i = indC; i < a1 - 1; i++) { // starting at second point (i=1) if stag set to zero
      // current point
      double const x1 = xxx.at(i);
      double const y1 = yyy.at(i);
      double const z1 = zzz.at(i);

      // distante from previous point
      double const dr1 = std::hypot(x1 - x0, y1 - y0, z1 - z0);

      // next point
      double const x2 = xxx.at(i + 1);
      double const y2 = yyy.at(i + 1);
      double const z2 = zzz.at(i + 1);

      // distant of next point to previous point
      double const dr2 = std::hypot(x2 - x0, y2 - y0, z2 - z0);

      if (dr1 < seg_size) {
        vx.push_back(x1);
        vy.push_back(y1);
        vz.push_back(z1);
      }

      /* If current point is inside segment length w.r.t. the first point of
       * the segment (x0,y0,z0), but the next point is outsize: create a new
       * point in between (x1,y1,z1) and (x2,y2,z2) in which will be exacly at
       * the segment length (w.r.t. to the first point) This is done using the
       * cosines law for a given factor `t` times dr (x2-x1, ...), and so:
       *
       * (ds)^2 = (dr1)^2 + (t*dr)^2 + 2*dot_product(dr1, t*dr)
       * Using cos(180-theta) = -cos(theta))
       *
       * Solve for `t` the second degree equation: t^2 + beta*t + gamma = 0
       */
      if (dr1 <= seg_size && dr2 > seg_size) {
        double const dx = x2 - x1;
        double const dy = y2 - y1;
        double const dz = z2 - z1;
        double const dr = std::hypot(dx, dy, dz);

        if (dr == 0) {
          cout << " ( Zero ) Error ! " << endl;
          return std::nullopt;
        }

        double const beta = 2.0 * ((x1 - x0) * dx + (y1 - y0) * dy + (z1 - z0) * dz) / (dr * dr);

        double const gamma = (dr1 * dr1 - seg_size * seg_size) / (dr * dr);
        double const delta = beta * beta - 4.0 * gamma;

        if (delta < 0.0) {
          cout << " ( Discriminant ) Error ! " << endl;
          return std::nullopt;
        }

        // solves for t
        double const lysi1 = (-beta + std::sqrt(delta)) / 2.0;
        double const t = lysi1;

        // find next points in that will exactly at the segment length from x0
        double const xp = x1 + t * dx;
        double const yp = y1 + t * dy;
        double const zp = z1 + t * dz;

        // Add points to be returned
        segx.push_back(xp);
        segy.push_back(yp);
        segz.push_back(zp);

        segL.push_back(n_seg * seg_size);

        // for TGraph
        x_seg[n_seg] = xp;
        y_seg[n_seg] = yp;
        z_seg[n_seg] = zp;

        n_seg++;

        // This are the new `x0` points (for next segment)
        x0 = xp;
        y0 = yp;
        z0 = zp;

        // Add points to segment
        vx.push_back(x0);
        vy.push_back(y0);
        vz.push_back(z0);

        if (n_seg <= 1) // This should never happen
          return std::nullopt;

        // Now, compute the deviation in `segx, ...` of the segment
        // vx, vy, vz are used and cleared afterwards
        compute_max_fluctuation_vector(
          segx, segy, segz, segnx, segny, segnz, segn_isvalid, vx, vy, vz, check_valid_scattered);

        // Starting over
        vx.push_back(x0);
        vy.push_back(y0);
        vz.push_back(z0);
      }
      else if (dr1 > seg_size) { // in this case, just interpolate until reach `seg_size`

        // Rolling `i` back for the next iteration.
        // Because the current point does not belong to this segment
        i = (i - 1);

        double const dx = x1 - x0;
        double const dy = y1 - y0;
        double const dz = z1 - z0;
        double const dr = std::hypot(dx, dy, dz);

        if (dr == 0) {
          cout << " ( Zero ) Error ! " << endl;
          return std::nullopt;
        }

        // computes the point by simple interpolation
        double const t = seg_size / dr;
        double const xp = x0 + t * dx;
        double const yp = y0 + t * dy;
        double const zp = z0 + t * dz;

        segx.push_back(xp);
        segy.push_back(yp);
        segz.push_back(zp);
        segL.push_back(1.0 * n_seg * 1.0 * seg_size + stag);

        // for TGraph
        x_seg[n_seg] = xp;
        y_seg[n_seg] = yp;
        z_seg[n_seg] = zp;

        n_seg++;

        x0 = xp;
        y0 = yp;
        z0 = zp;

        vx.push_back(x0);
        vy.push_back(y0);
        vz.push_back(z0);

        if (n_seg <= 1) // This should never happen
          return std::nullopt;

        // Now, compute the deviation in `segx, ...` of the segment
        // vx, vy, vz are used and cleared afterwards
        compute_max_fluctuation_vector(
          segx, segy, segz, segnx, segny, segnz, segn_isvalid, vx, vy, vz, check_valid_scattered);

        // vectors are cleared in previous step
        vx.push_back(x0);
        vy.push_back(y0);
        vz.push_back(z0);
      }

      if (n_seg >= (stopper + 1.0) && seg_stop != -1) break;
    }

    delete gr_seg_xyz;
    gr_seg_xyz = new TPolyLine3D{n_seg, z_seg, x_seg, y_seg};
    gr_seg_yz = TGraph{n_seg, z_seg, y_seg};
    gr_seg_xz = TGraph{n_seg, z_seg, x_seg};
    gr_seg_xy = TGraph{n_seg, x_seg, y_seg};

    return std::make_optional<Segments>(
      Segments{segx, segnx, segy, segny, segz, segnz, segL, segn_isvalid});
  }

  /* Computes the rms by groups of `thick`
   *
   */
  std::tuple<double, double, double> TrackMomentumCalculator::getDeltaThetaRMS_(
    Segments const& segments,
    double const thick) const
  {
    auto const& segnx = segments.nx;
    auto const& segny = segments.ny;
    auto const& segnz = segments.nz;
    auto const& segL = segments.L;

    int const a1 = segnx.size();
    int const a2 = segny.size();
    int const a3 = segnz.size();

    if (a1 != a2 || a1 != a3) {
      cout << " ( Get RMS ) Error ! " << endl;
      return std::make_tuple(0., 0., 0.);
    }

    int const tot = a1;

    std::vector<double> buf0;

    for (int i = 0; i < tot; i++) {
      double const dx = segnx.at(i);
      double const dy = segny.at(i);
      double const dz = segnz.at(i);

      TVector3 const vec_z{dx, dy, dz};
      TVector3 vec_x;
      TVector3 vec_y;

      double const switcher = basex.Dot(vec_z);

      if (switcher <= 0.995) {
        vec_y = vec_z.Cross(basex).Unit();
        vec_x = vec_y.Cross(vec_z);
      }
      else {
        // cout << " It switched ! Isn't this lovely !!! " << endl;
        vec_y = basez.Cross(vec_z).Unit();
        vec_x = vec_y.Cross(vec_z);
      }

      TVector3 const Rx{vec_x.Dot(basex), vec_x.Dot(basey), vec_x.Dot(basez)};
      TVector3 const Ry{vec_y.Dot(basex), vec_y.Dot(basey), vec_y.Dot(basez)};
      TVector3 const Rz{vec_z.Dot(basex), vec_z.Dot(basey), vec_z.Dot(basez)};

      double const refL = segL.at(i);

      for (int j = i; j < tot; j++) {
        double const L1 = segL.at(j);

        double const dz1 = L1 - refL;

        if (dz1 >= thick) {
          double const here_dx = segnx.at(j);
          double const here_dy = segny.at(j);
          double const here_dz = segnz.at(j);

          TVector3 const here_vec{here_dx, here_dy, here_dz};
          TVector3 const rot_here{Rx.Dot(here_vec), Ry.Dot(here_vec), Rz.Dot(here_vec)};

          double const scx = rot_here.X();
          double const scy = rot_here.Y();
          double const scz = rot_here.Z();

          double const azy = find_angle(scz, scy);
          double const azx = find_angle(scz, scx);

          constexpr double ULim = 10000.0; // Avoid huge (wrong) angles
          constexpr double LLim = -10000.0;

          if (azy <= ULim && azy >= LLim) { // safe scatter in the yz plane

            if (azx <= ULim && azx >= LLim) { // safe scatter in the za plane
              if (fMCSAngleMethod == kAnglezx) {
                buf0.push_back(azx); // scattered angle z-x
              }
              else if (fMCSAngleMethod == kAnglezy) {
                buf0.push_back(azy);
              }
              else if (fMCSAngleMethod == kAngleCombined) {
                buf0.push_back(std::hypot(azx, azy));
              }
            }
          }
          break; // of course !
        }
      }
    }

    int const nmeas = buf0.size();
    double nnn = 0.0;

    double mean = 0.0;
    double rms = 0.0;
    double rmse = 0.0;

    for (int i = 0; i < nmeas; i++) {
      mean += buf0.at(i);
      nnn++;
    }

    mean = mean / nnn;

    for (int i = 0; i < nmeas; i++)
      rms += ((buf0.at(i)) * (buf0.at(i)));

    rms = rms / (nnn);
    rms = std::sqrt(rms);
    rmse = rms / std::sqrt(2.0 * tot);

    double rms1 = rms;

    rms = 0.0;

    double ntot1 = 0.0;
    double const lev1 = 2.50;

    for (int i = 0; i < nmeas; i++) {
      double const amp = buf0.at(i);
      if (amp < (mean + lev1 * rms1) && amp > (mean - lev1 * rms1)) {
        ++ntot1;
        rms += amp * amp;
      }
    }

    rms = rms / (ntot1);
    rms = std::sqrt(rms);
    rmse = rms / std::sqrt(2.0 * ntot1);
    return std::make_tuple(mean, rms, rmse);
  }

  double TrackMomentumCalculator::find_angle(double vz, double vy) const
  {
    double thetayz = -999.0;

    if (vz > 0 && vy > 0) {
      double ratio = std::abs(vy / vz);
      thetayz = std::atan(ratio);
    }

    else if (vz < 0 && vy > 0) {
      double ratio = std::abs(vy / vz);
      thetayz = std::atan(ratio);
      thetayz = TMath::Pi() - thetayz;
    }

    else if (vz < 0 && vy < 0) {
      double ratio = std::abs(vy / vz);
      thetayz = std::atan(ratio);
      thetayz = thetayz + TMath::Pi();
    }

    else if (vz > 0 && vy < 0) {
      double ratio = std::abs(vy / vz);
      thetayz = std::atan(ratio);
      thetayz = 2.0 * TMath::Pi() - thetayz;
    }

    else if (vz == 0 && vy > 0) {
      thetayz = TMath::Pi() / 2.0;
    }

    else if (vz == 0 && vy < 0) {
      thetayz = 3.0 * TMath::Pi() / 2.0;
    }
    else if (vz > 0 && vy == 0) {
      thetayz = 0;
    }
    else if (vz < 0 && vy == 0) {
      thetayz = TMath::Pi();
    }

    if (thetayz > TMath::Pi()) { thetayz = thetayz - 2.0 * TMath::Pi(); }

    return 1000.0 * thetayz;
  }

  double TrackMomentumCalculator::my_g(double xx, double Q, double s) const
  {
    if (s == 0.) {
      cout << " Error : The code tries to divide by zero ! " << endl;
      return 0.;
    }

    double const arg = (xx - Q) / s;
    double const result = -0.5 * std::log(2.0 * TMath::Pi()) - std::log(s) - 0.5 * arg * arg;

    if (std::isnan(result) || std::isinf(result)) {
      cout << " Is nan ! my_g ! " << -std::log(s) << ", " << s << endl;
    }

    return result;
  }

  double TrackMomentumCalculator::my_mcs_llhd(std::vector<double> const& dEi,
                                              std::vector<double> const& dEj,
                                              std::vector<double> const& dthij,
                                              std::vector<double> const& ind,
                                              double const x0,
                                              double const x1) const
  {
    double p = x0;
    double theta0x = x1;

    double result = 0.0;
    double nnn1 = dEi.size(); // number of segments of energy

    double red_length = (steps_size) / rad_length;
    double addth = 0;

    for (int i = 0; i < nnn1; i++) {
      double Ei = p - dEi.at(i); // Estimated energy at point i
      double Ej = p - dEj.at(i); // Estimated enery at point j

      // If the momentum p choosen allows that the muon stopped inside, add 1 rad to the change in scatter angle (as the particle stops)
      if (Ei > 0 && Ej < 0) addth = 3.14 * 1000.0;

      Ei = std::abs(Ei);
      Ej = std::abs(Ej);

      // Highland formula
      // Parameters given at Particle Data Group https://pdg.lbl.gov/2023/web/viewer.html?file=../reviews/rpp2022-rev-passage-particles-matter.pdf
      double tH0 =
        (13.6 / std::sqrt(Ei * Ej)) * (1.0 + 0.038 * std::log(red_length)) * std::sqrt(red_length);

      double rms = -1.0;

      if (ind.at(i) == 1) {
        // Computes the rms of angle
        rms = std::hypot(tH0, theta0x);

        double const DT = dthij.at(i) + addth;
        double const prob = my_g(DT, 0.0, rms); // Computes log likelihood

        result = result - 2.0 * prob; // Adds for each segment
      }
    }

    if (std::isnan(result) || std::isinf(result)) {
      std::cout << " Is nan ! my_mcs_llhd ( 1 ) ! " << std::endl;
    }
    return result;
  }

} // namespace track