Rivet analyses

title: BESIII_2023_I2614192

$e^+e^-\to n \bar{n}$ between 2 and 2.95 GeV

Experiment: BESIII (BEPC)

Inspire ID: 2614192

Status: VALIDATED NOHEPDATA SINGLEWEIGHT

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 130 (2023) 15, 151905

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons

Measurement of the electric and magnetic form factors for $e^+e^-\to n \bar{n}$ between 2 and 2.95 GeV.

Source code:BESIII_2023_I2614192.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief e+ e- > n nbar class BESIII_2023_I2614192 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2614192);


/// @name Analysis methods
/// @{

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  declare(FinalState(), "FS");

  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_c[ix], "/TMP/c_" + toString(ix));
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  const FinalState& fs = apply<FinalState>(event, "FS");
  if (fs.particles().size() != 2) vetoEvent;
  double cTheta(0.);
  for (const Particle& p : fs.particles()) {
    if (abs(p.pid()) != PID::NEUTRON) vetoEvent;
    if (p.pid() == PID::NEUTRON) cTheta = p.mom().z() / p.mom().p3().mod();
  }
  _c[0]->fill();
  _c[1]->fill(2. - 5. * sqr(cTheta));
}


/// Normalise histograms etc., after the run
void finalize() {
  // storage of the values to fill histos
  const double mn = 0.93956542052;
  const double tau = 4. * sqr(mn / sqrtS());
  const double beta = sqrt(1. - tau);
  const double alpha = 7.2973525693e-3;
  const double GeV2nb = 0.3893793721e6;
  const double fact = 4. / 3. * M_PI * sqr(alpha) * beta / 2 / sqr(sqrtS()) * GeV2nb;
  scale(_c, crossSection() / sumOfWeights() / nanobarn / fact);

  double val[3], err[3];
  val[0] = sqrt(2. * _c[1]->val() / (_c[0]->val() - _c[1]->val()) / tau);
  err[0] = 0.5 / _c[1]->val() / tau
      * (sqr(_c[0]->err() * _c[1]->val()) + sqr(_c[1]->err() * _c[0]->val()))
      / pow(_c[0]->val() - _c[1]->val(), 3);
  val[1] = 1e2 * sqrt(0.5 * (_c[0]->val() - _c[1]->val()));
  err[1] = 1e2 * 0.125 * (sqr(_c[0]->err()) + sqr(_c[1]->err())) / (_c[0]->val() - _c[1]->val());
  val[2] = 1e2 * sqrt(_c[1]->val() / tau);
  err[2] = 1e2 * 0.25 * sqr(_c[1]->err()) / _c[1]->val() / tau;
  // fill histos
  for (unsigned int ix = 0; ix < 3; ++ix) {
    Estimate1DPtr mult;
    book(mult, 1, 1, 1 + ix);
    for (auto& b : mult->bins()) {
      if (inRange(sqrtS() / GeV, b.xMin(), b.xMax())) {
        b.set(val[ix], err[ix]);
      }
    }
  }
}

/// @}


/// @name Histograms
/// @{
CounterPtr _c[2];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2023_I2614192);

} ```