vo007_PhysicsHelpers.C File Reference

Detailed Description

In this tutorial we demonstrate RVec helpers for physics computations such as angle differences \(\Delta \phi\), the distance in the \(\eta\)- \(\phi\) plane \(\Delta R\) and the invariant mass.

 
using namespace ROOT::VecOps;
 
void vo007_PhysicsHelpers()
{
   // The DeltaPhi helper computes the closest angle between angles.
   // This means that the resulting value is in the range [-pi, pi].
 
   // Note that the helper also supports to compute the angle difference of an
   // RVec and a scalar and two scalars. In addition, the computation of the
   // difference and the behaviour at the boundary can be adjusted to radian and
   // degrees.
   RVec<float> phis = {0.0, 1.0, -0.5, M_PI + 1.0};
   auto idx = Combinations(phis, 2);
 
   auto phi1 = Take(phis, idx[0]);
   auto phi2 = Take(phis, idx[1]);
   auto dphi = DeltaPhi(phi1, phi2);
 
   std::cout << "DeltaPhi(phi1 = " << phi1 << ",\n"
             << "         phi2 = " << phi2 << ")\n"
             << " = " << dphi << "\n";
 
   // The DeltaR helper is similar to the DeltaPhi helper and computes the distance
   // in the \f$\eta\f$-\f$\phi\f$ plane.
   RVec<float> etas = {2.4, -1.5, 1.0, 0.0};
 
   auto eta1 = Take(etas, idx[0]);
   auto eta2 = Take(etas, idx[1]);
   auto dr = DeltaR(eta1, eta2, phi1, phi2);
 
   std::cout << "\nDeltaR(eta1 = " << eta1 << ",\n"
             << "       eta2 = " << eta2 << ",\n"
             << "       phi1 = " << phi1 << ",\n"
             << "       phi2 = " << phi2 << ")\n"
             << " = " << dr << "\n";
 
   // The InvariantMasses helper computes the invariant mass of a two particle system
   // given the properties transverse momentum (pt), rapidity (eta), azimuth (phi)
   // and mass.
   RVec<float> pt3 = {40, 20, 30};
   RVec<float> eta3 = {2.5, 0.5, -1.0};
   RVec<float> phi3 = {-0.5, 0.0, 1.0};
   RVec<float> mass3 = {10, 10, 10};
 
   RVec<float> pt4 = {20, 10, 40};
   RVec<float> eta4 = {0.5, -0.5, 1.0};
   RVec<float> phi4 = {0.0, 1.0, -1.0};
   RVec<float> mass4 = {2, 2, 2};
 
   auto invMass = InvariantMasses(pt3, eta3, phi3, mass3, pt4, eta4, phi4, mass4);
 
   std::cout << "\nInvariantMass(pt1 = " << pt3 << ",\n"
             << "              eta1 = " << eta3 << ",\n"
             << "              phi1 = " << phi3 << ",\n"
             << "              mass1 = " << mass3 << ",\n"
             << "              pt2 = " << pt4 << ",\n"
             << "              eta2 = " << eta4 << ",\n"
             << "              phi2 = " << phi4 << ",\n"
             << "              mass2 = " << mass4 << ")\n"
             << " = " << invMass << "\n";
 
   // The InvariantMass helper also accepts a single set of (pt, eta, phi, mass) vectors. Then,
   // the invariant mass of all particles in the collection is computed.
 
   auto invMass2 = InvariantMass(pt3, eta3, phi3, mass3);
 
   std::cout << "\nInvariantMass(pt = " << pt3 << ",\n"
             << "              eta = " << eta3 << ",\n"
             << "              phi = " << phi3 << ",\n"
             << "              mass = " << mass3 << ")\n"
             << " = " << invMass2 << "\n";
}

 
DeltaPhi(phi1 = { 0, 0, 0, 1, 1, -0.5 },
         phi2 = { 1, -0.5, 4.14159, -0.5, 4.14159, 4.14159 })
 = { 1, -0.5, -2.14159, -1.5, 3.14159, -1.64159 }
 
DeltaR(eta1 = { 2.4, 2.4, 2.4, -1.5, -1.5, 1 },
       eta2 = { -1.5, 1, 0, 1, 0, 0 },
       phi1 = { 0, 0, 0, 1, 1, -0.5 },
       phi2 = { 1, -0.5, 4.14159, -0.5, 4.14159, 4.14159 })
 = { 4.02616, 1.48661, 3.21659, 2.91548, 3.48132, 1.92219 }
 
InvariantMass(pt1 = { 40, 20, 30 },
              eta1 = { 2.5, 0.5, -1 },
              phi1 = { -0.5, 0, 1 },
              mass1 = { 10, 10, 10 },
              pt2 = { 20, 10, 40 },
              eta2 = { 0.5, -0.5, 1 },
              phi2 = { 0, 1, -1 },
              mass2 = { 2, 2, 2 })
 = { 69.0799, 23.6971, 101.326 }
 
InvariantMass(pt = { 40, 20, 30 },
              eta = { 2.5, 0.5, -1 },
              phi = { -0.5, 0, 1 },
              mass = { 10, 10, 10 })
 = 220.308

Date

March 2019

Author

Stefan Wunsch

Definition in file vo007_PhysicsHelpers.C.