132 f(-p₁) + f(-p₂) → Z(e^-(p₃) + e⁺(p₄)) + Z(μ^-(p₅) + μ⁺(p₆)), ((gg → ZZ) squared)

This process calculable at leading order LO.

includes the effect of both the top quark mass and the bottom quark, while the first two generations are considered massless. For numerical stability, a small cut on the transverse momentum of the Z bosons is applied: p_T(Z) > 0.05 GeV. This typically removes less than 0.1% of the cross section. The values of these cutoffs can be changed by editing src∕ZZ∕getggZZamps.f and recompiling.

Process 130 includes all gg-intitiated diagrams that have a Higgs boson in the s-channel, namely the square of the s-channel Higgs boson production and the interference with the diagrams that do not contain a Higgs boson, (i.e. gg → Z∕γ^* + Z∕γ^*→ e^-e⁺μ^-μ⁺).

Process 131 calculates the full result for this process from gg-intitiated diagrams. This includes diagrams that have a Higgs boson in the s-channel, the continuum Z∕γ^* + Z∕γ^* diagrams described above and their interference.

Process 132 gives the result for the square of the box diagrams alone, i.e. the process gg → Z∕γ^* + Z∕γ^*→ e^-e⁺μ^-μ⁺.

Process 133 calculates the interference for the qg initiated process.

For those processes that include contributions from the Higgs boson, the form of the Higgs propagator may be changed by editing the file src/Need/sethparams.f. If the logical variable CPscheme is changed from the default value .false. to .true. then the Higgs propagator is computed using the “bar-scheme” that is implemented in the HTO code of G. Passarino [1, 2]. The value of the Higgs boson width has been computed with v1.1 of the HTO code, for Higgs masses in the interval 50 < m_H < 1500 GeV. These values are tabulated, in 0.5 GeV increments, in the file Bin/hto_output.dat. The widths for other masses in this range are obtained by linear interpolation. 

132.0.1 Specifying other final states

As described above, these processes refer to a final state e^-e⁺μ^-μ⁺. It is however possible to specify a final state that corresponds to a different set of Z boson decays. This is achieved by altering the value of NPROC in the input file by appending a period, followed by two 2-character strings that identify each of the decays. Possible values for the strings, and the corresponding decays, are shown in the table below.

Note that, for the case of neutrino decays, the sum over three flavours of neutrino is performed. The labelling of the particles in the output is best understood by example. Setting nproc=132.ELNU corresponds to the process gg → Z∕γ^* + Z∕γ^*→ e^-(p₃)e⁺(p₄)ν(p₅)ν(p₆). Note that the default process corresponds to the string ELMU so that, for instance nproc=132.ELMU is entirely equivalent to nproc=132. The effect of changing the lepton flavour is only seen in the output of LHE events, where the correct mass is then used when producing the event record.

132.1 H → ZZ → e^-e⁺μ^-μ⁺ production, (m_t = finite), processes 128-133

These processes represent the production of a Higgs boson that decays to ZZ, with subsequent decay into charged leptons. For process 128, the exact form of the triangle loop coupling a Higgs boson to two gluons is included, with both top and bottom quarks circulating in the loop. This is to be contrasted with process 116 in which only the top quark contribution is included in the effective coupling approach.

Process 129 includes only the effect of the interference of the Higgs and gg → ZZ amplitudes. The calculation is available at LO only. LO corresponds to O(α_s²) in this case. The calculation of loops containing the third quark generation includes the effect of both the top quark mass and the bottom quark, while the first two generations are considered massless. For numerical stability, a small cut on the transverse momentum of the Z bosons is applied: p_T(Z) > 0.05 GeV. This typically removes less than 0.1% of the cross section. The values of these cutoffs can be changed by editing src/ZZ/getggZZamps.f and recompiling.

Process 130 includes all gg-initiated diagrams that have a Higgs boson in the s-channel, namely the square of the s-channel Higgs boson production and the interference with the diagrams that do not contain a Higgs boson, (i.e. gg → Z∕γ^* + Z∕γ^*→ e^-e⁺μ^-μ⁺), i.e. |M_H|² + 2|M_H^*M_ZZ|.

Process 131 calculates the full result for this process from gg-intitiated diagrams. This includes diagrams that have a Higgs boson in the s-channel, the continuum Z∕γ^* + Z∕γ^* diagrams described above and their interference, i.e. |M_H + M_ZZ|².

Process 132 gives the result for the square of the box diagrams alone, i.e. the process gg → Z∕γ^* + Z∕γ^*→ e^-e⁺μ^-μ⁺, i.e. |M_ZZ|².

Process 133 calculates the interference for the qg initiated process.

For those processes that include contributions from the Higgs boson, the form of the Higgs propagator may be changed by editing the file src/Need/sethparams.f. If the logical variable CPscheme is changed from the default value .false. to .true. then the Higgs propagator is computed using the “bar-scheme” that is implemented in the HTO code of G. Passarino [1, 2]. The value of the Higgs boson width has been computed with v1.1 of the HTO code, for Higgs masses in the interval 50 < m_H < 1500 GeV. These values are tabulated, in 0.5 GeV increments, in the file Bin/hto_output.dat. The widths for other masses in this range are obtained by linear interpolation.

132.2 Plotter

nplotter_ZZlept.f is the default plotting routine.

132.3 Example input and output file(s)

input132.ini process132.out

References