alpha_T.h

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#ifndef ALPHA_T_H
#define ALPHA_T_H

#include <cmath>
#include <vector>
#include <numeric>
#include <functional>
#include <algorithm>

struct alpha_T {
  template <class LVs>
  double operator()(const LVs& p4s) const {
    typedef typename LVs::value_type LorentzV;
    if (p4s.size() < 2)
      return 0;

    std::vector<double> pTs;
    transform(p4s.begin(), p4s.end(), back_inserter(pTs), std::mem_fn(&LorentzV::Pt));

    const double DsumPT = minimum_deltaSumPT(pTs);
    const double sumPT = accumulate(pTs.begin(), pTs.end(), double(0));
    const LorentzV sumP4 = accumulate(p4s.begin(), p4s.end(), LorentzV());

    return 0.5 * (sumPT - DsumPT) / sqrt(sumPT * sumPT - sumP4.Perp2());
  }

  static double minimum_deltaSumPT(const std::vector<double>& pTs) {
    std::vector<double> diff(1 << (pTs.size() - 1), 0.);
    for (unsigned i = 0; i < diff.size(); i++)
      for (unsigned j = 0; j < pTs.size(); j++)
        diff[i] += pTs[j] * (1 - 2 * (int(i >> j) & 1));

    return fabs(*min_element(diff.begin(), diff.end(), [](auto x, auto y) { return fabs(x) < fabs(y); }));
  }
};

#endif