Resolve "ZDC benchmarks"

Closes #23 (closed)

benchmarks/zdc/scripts/zdc_neutrons.cxx

+//////////////////////////////////////////////////////////////
+// ZDC detector
+// Single Neutron dataset
+// J.KIM 05/07/2021
+//////////////////////////////////////////////////////////////
+#include "HepMC3/GenEvent.h"
+#include "HepMC3/Print.h"
+#include "HepMC3/ReaderAscii.h"
+#include "HepMC3/WriterAscii.h"
+
+#include <TMath.h>
+#include <cmath>
+#include <iostream>
+#include <math.h>
+#include <random>
+
+using namespace HepMC3;
+
+void zdc_neutrons(int n_events = 1e6, double e_start = 0.0, double e_end = 30.0, const char* out_fname = "./data/zdc_neutrons.hepmc") {
+  WriterAscii hepmc_output(out_fname);
+  int events_parsed = 0;
+  GenEvent evt(Units::GEV, Units::MM);
+
+  // Random number generator
+  TRandom* r1 = new TRandom();
+
+  // Constraining the solid angle, but larger than that subtended by the
+  // detector
+  // https://indico.bnl.gov/event/7449/contributions/35966/attachments/27177/41430/EIC-DWG-Calo-03192020.pdf
+  // See a figure on slide 26
+  double cos_theta_min = std::cos(M_PI * (0.0 / 180.0));
+  double cos_theta_max = std::cos(M_PI * (5.0 / 180.0));
+
+  for (events_parsed = 0; events_parsed < n_events; events_parsed++) {
+    // FourVector(px,py,pz,e,pdgid,status)
+    // type 4 is beam
+    // pdgid 11 - electron
+    // pdgid 111 - pi0
+    // pdgid 2212 - proton
+    // pdgid 2112 - neutron
+    GenParticlePtr p1 = std::make_shared<GenParticle>(FourVector(0.0, 0.0, 10.0, 10.0), 11, 4);
+    GenParticlePtr p2 = std::make_shared<GenParticle>(FourVector(0.0, 0.0, 0.0, 0.938), 2212, 4);
+
+    // Define momentum
+    Double_t p        = r1->Uniform(e_start, e_end);
+    Double_t phi      = r1->Uniform(0.0, 2.0 * M_PI);
+    Double_t costheta = r1->Uniform(cos_theta_min, cos_theta_max);
+    Double_t theta    = std::acos(costheta);
+    Double_t px       = p * std::cos(phi) * std::sin(theta);
+    Double_t py       = p * std::sin(phi) * std::sin(theta);
+    Double_t pz       = p * std::cos(theta);
+
+    // Generates random vectors, uniformly distributed over the surface of a
+    // sphere of given radius, in this case momentum.
+    // r1->Sphere(px, py, pz, p);
+
+    // type 1 is final state
+    // pdgid 11 - electron 0.510 MeV/c^2
+    GenParticlePtr p3 = std::make_shared<GenParticle>(FourVector(px, py, pz, sqrt(p * p + (0.939 * 0.939))), 2112, 1);
+
+    GenVertexPtr v1 = std::make_shared<GenVertex>();
+    v1->add_particle_in(p1);
+    v1->add_particle_in(p2);
+
+    v1->add_particle_out(p3);
+    evt.add_vertex(v1);
+
+    if (events_parsed == 0) {
+      std::cout << "First event: " << std::endl;
+      Print::listing(evt);
+    }
+
+    hepmc_output.write_event(evt);
+    if (events_parsed % 10000 == 0) {
+      std::cout << "Event: " << events_parsed << std::endl;
+    }
+    evt.clear();
+  }
+  hepmc_output.close();
+  std::cout << "Events parsed and written: " << events_parsed << std::endl;
+}