Left-Right-Symmetry Processes

At current energies, the world is left-handed, i.e. the Standard Model contains an SU(2)_L group. Left-right symmetry at some larger scale implies the need for an SU(2)_R group. Thus the particle content is expanded by right-handed Z_R^0 and W_R^+- and right-handed neutrinos. The Higgs fields have to be in a triplet representation, leading to doubly-charged Higgs particles, one set for each of the two SU(2) groups. Also the number of neutral and singly-charged Higgs states is increased relative to the Standard Model, but a search for the lowest-lying states of this kind is no different from e.g. the freedom already accorded by the MSSM Higgs scenarios.

PYTHIA implements the scenario of [Hui97].

The W_R^+- has been implemented as a simple copy of the ordinary W^+-, with the exception that it couples to right-handed neutrinos instead of the ordinary left-handed ones. Thus the standard CKM matrix is used in the quark sector, and the same vector and axial coupling strengths, leaving only the mass as free parameter. The Z_R^0 implementation (without interference with the photon or the ordinary Z^0) allows decays both to left- and right-handed neutrinos, as well as other fermions, according to one specific model ansatz. Obviously both the W_R^+- and the Z_R^0 descriptions are likely to be simplifications, but provide a starting point.

For the doubly-charged Higgs bosons, the main decay modes implemented are H_L^++ → W_L^+ W_L^+, l_i^+ l_j^+ (i, j generation indices) and H_R^++ → W_R^+ W_R^+, l_i^+ l_j^+.

The right-handed neutrinos can be allowed to decay further. Assuming them to have a mass below that of W_R^+-, they decay to three-body states via a virtual W_R^+-, nu_Rl → l+- f fbar', where both lepton charges are allowed owing to the Majorana character of the neutrinos. If there is a significant mass splitting, also sequential decays nu_Rl → l+- l'-+ nu'_Rl are allowed. Currently the decays are isotropic in phase space. If the neutrino masses are close to or above the W_R^ ones, this description has to be substituted by a sequential decay via a real W_R^ (not implemented, but actually simpler to do than the one here).

Production processes

A few different production processes have been implemented, which normally would not overlap and therefore could be run together.

flag  LeftRightSymmmetry:all   (default = off)
Common switch for the group of implemented processes within a left-right-symmetric scenario.

flag  LeftRightSymmmetry:ffbar2ZR   (default = off)
Scatterings f fbar → Z_R^0. Code 3101.

flag  LeftRightSymmmetry:ffbar2WR   (default = off)
Scatterings . Code 3102.

flag  LeftRightSymmmetry:ll2HL   (default = off)
Scatterings l_i l_j → H_L^--. Code 3121.

flag  LeftRightSymmmetry:lgm2HLe   (default = off)
Scatterings l_i gamma → H_L^-- e^+. Code 3122.

flag  LeftRightSymmmetry:lgm2HLmu   (default = off)
Scatterings l_i gamma → H_L^-- mu^+. Code 3123.

flag  LeftRightSymmmetry:lgm2HLtau   (default = off)
Scatterings l_i gamma → H_L^-- tau^+. Code 3124.

flag  LeftRightSymmmetry:ff2HLff   (default = off)
Scatterings f_1 f_2 → H_L^-- f_3 f_4 via WW fusion. Code 3125.

flag  LeftRightSymmmetry:ffbar2HLHL   (default = off)
Scatterings f fbar → H_L^++ H_L^--. Code 3126.

flag  LeftRightSymmmetry:ll2HR   (default = off)
Scatterings l_i l_j → H_R^--. Code 3141.

flag  LeftRightSymmmetry:lgm2HRe   (default = off)
Scatterings l_i gamma → H_R^-- e^+. Code 3142.

flag  LeftRightSymmmetry:lgm2HRmu   (default = off)
Scatterings l_i gamma → H_R^-- mu^+. Code 3143.

flag  LeftRightSymmmetry:lgm2HRtau   (default = off)
Scatterings l_i gamma → H_R^-- tau^+. Code 3144.

flag  LeftRightSymmmetry:ff2HRff   (default = off)
Scatterings f_1 f_2 → H_R^-- f_3 f_4 via WW fusion. Code 3145.

flag  LeftRightSymmmetry:ffbar2HRHR   (default = off)
Scatterings f fbar → H_R^++ H_R^--. Code 3146.

Parameters

The basic couplings of the model are

parm  LeftRightSymmmetry:gL   (default = 0.64; minimum = 0.0)
lefthanded coupling g_L = e / sin(theta).

parm  LeftRightSymmmetry:gR   (default = 0.64; minimum = 0.0)
righthanded coupling g_R, assumed the same as g_L.

parm  LeftRightSymmmetry:vL   (default = 5.; minimum = 0.0)
vacuum expectation value v_L (in GeV) for the left-triplet.

The corresponding vacuum expectation value v_R is assumed given by v_R = sqrt(2) M_WR / g_R and is not stored explicitly.

The Yukawa couplings of a lepton pair to a H^--, assumed the same for H_L^-- and H_R^--, is described by a symmetric 3-by-3 matrix. The default matrix is dominated by the diagonal elements and especially by the tau tau one.

parm  LeftRightSymmmetry:coupHee   (default = 0.1; minimum = 0.0)
Yukawa coupling for H^-- → e- e-.

parm  LeftRightSymmmetry:coupHmue   (default = 0.01; minimum = 0.0)
Yukawa coupling for H^-- → mu- e-.

parm  LeftRightSymmmetry:coupHmumu   (default = 0.1; minimum = 0.0)
Yukawa coupling for H^-- → mu- mu-.

parm  LeftRightSymmmetry:coupHtaue   (default = 0.01; minimum = 0.0)
Yukawa coupling for H^-- → tau- e-.

parm  LeftRightSymmmetry:coupHtaumu   (default = 0.01; minimum = 0.0)
Yukawa coupling for H^-- → tau- mu-.

parm  LeftRightSymmmetry:coupHtautau   (default = 0.3; minimum = 0.0)
Yukawa coupling for H^-- → tau- tau-.