decay_H_to_ZZ.inc

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template <>
double CLASSNAME::get_partial_width<Higgs,ZBoson,ZBoson>(
   const context_base& context,
   typename field_indices<Higgs>::type const& indexIn,
   typename field_indices<ZBoson>::type const& indexOut1,
   typename field_indices<ZBoson>::type const& indexOut2)
{
 
   const double mHOS = context.physical_mass<Higgs>(indexIn);
   // There might be large differences between mZ from mass block
   // and one from slha input, especially in the decoupling limit
   // so we use the latter one. There might be a problem with
   // models where Z mixes with something else.
   // const double mZOS = context.physical_mass<Z>(indexOut1);
   const double mZOS = qedqcd.displayPoleMZ();
   const double x = Sqr(mZOS/mHOS);
   double res = 0;
 
   // mH < mZ
   const int offshell_VV_decays = flexibledecay_settings.get(FlexibleDecay_settings::offshell_VV_decays);
   if ((x > 1.0 && offshell_VV_decays != 0) ||
       (4.*x > 1.0 && offshell_VV_decays == 2)) {
 
      // integrand
      static constexpr double GammaZ = 2.4952;
      struct hVV_4body_params params = {mHOS, mZOS, GammaZ};
      gsl_monte_function G = {&hVV_4body, 2, &params};
 
      // setup integration
      gsl_rng_env_setup ();
      double xl[2] = {0, 0};
      double xu[2] = {Sqr(mHOS), Sqr(mHOS)};
      // this gives relative error < 0.05%
      static constexpr size_t calls = 1'000'000;
      double err;
      const gsl_rng_type *T = gsl_rng_default;
      gsl_rng *r = gsl_rng_alloc (T);
      gsl_monte_miser_state *s = gsl_monte_miser_alloc (2);
      gsl_monte_miser_integrate (&G, xl, xu, 2, calls, r, s,
                                 &res, &err);
 
      // clean-up
      gsl_monte_miser_free (s);
      gsl_rng_free (r);
 
      // prefactor
      const auto indices = concatenate(indexIn, indexOut2, indexOut1);
      const auto ghZZ =
         Vertex<Higgs, ZBoson, ZBoson>::evaluate(indices, context).value();
      const double normalization = 1./(64.*Cube(Pi)) * std::norm(ghZZ) * Cube(mHOS)/Power4(mZOS)/2.;
      res *= normalization;
      err *= normalization;
      const double rel_err = err/res;
      static constexpr double errorThresholdPerc = 0.1;
      if (rel_err*100 > errorThresholdPerc) {
         problems.add_warning(
            create_process_string<Higgs,ZBoson,ZBoson>(indexIn, indexOut1, indexOut2)
               + ": Relative integration error > " + std::to_string(errorThresholdPerc) + "%"
         );
      }
   // mZ < mH < 2*mZ
   // three-body decay
   }
   else if (4 * x > 1.0 && offshell_VV_decays != 0) {
 
      if (check_3body_Vff_decay<BSMForZdecay,ZBoson>(context, mHOS, indexOut1)) {
         const std::string index_as_string = indexIn.size() == 0 ? "" : "(" + std::to_string(indexIn.at(0)) + ")";
         WARNING("Warning in H" + index_as_string + "->ZZ decays: Single off-shell decays H->Zff' assume no possible BSM particles in the final state. Turning off.");
         return 0.;
      }
 
      const double sw2 = Sqr(std::sin(context.model.ThetaW()));
      const double deltaV = 7.0/12.0 - 10.0/9.0*sw2 + 40.0/27.0*Sqr(sw2);
 
      res = 3./(512.*Power3(Pi)) * 1./mHOS * deltaV * RT(x)/x;
 
      const auto indices = concatenate(indexIn, indexOut2, indexOut1);
      const auto ghZZ =
         Vertex<Higgs, ZBoson, ZBoson>::evaluate(indices, context).value();
 
      const double g2 = context.model.get_g2();
 
      res *= std::norm(ghZZ*g2)/(1-sw2);
   // mH > 2mZ
   // two-body decay
   }
   else if (4.*x < 1.0) {
 
      const double flux = 1. / (2 * mHOS);
      // phase space without symmetry factor
      const double ps = 1. / (8. * Pi) * std::sqrt(KallenLambda(1., x, x));
 
      // phase space symmetry factor
      const double ps_symmetry = 1./2.;
 
      // matrix element squared
      const auto mat_elem = calculate_amplitude<Higgs, ZBoson, ZBoson>(
         context, indexIn, indexOut1, indexOut2);
      const auto mat_elem_sq = mat_elem.square();
 
      // flux * phase space factor * matrix element squared
      res = flux * ps * ps_symmetry * mat_elem_sq;
   }
 
   return res;
}