Enhancement of “CP-odd” Higgs Boson Production in the Minimal Supersymmetric Standard Model with Explicit CP Violation

We calculate the production cross section of the “CP-odd” Higgs boson via gluon fusion in the minimal supersymmetric standard model with explicit CP violation in the stop sector. We show that there is a parameter region in which the cross section is enhanced by a factor of about 1000, as compared to the case without CP violation in the stop sector. In the parameter region where the “CP-odd” Higgs boson can decay into a stop pair, the stop pair events will be the important signature of the enhanced “CP-odd” Higgs boson. In the case where the “CP-odd” Higgs boson cannot decay into any superparticles, the γγ and ττ decay channels could become important for discovering the “CP-odd” Higgs boson. We also discuss the constraints from electric dipole moments of electron, neutron and mercury on the viable parameter space mentioned above. Low energy supersymmetry (SUSY) is one of the most promising candidates of physics beyond the Standard Model (SM). SUSY gives an elegant solution to the naturalness problem of the stability of the weak scale by canceling quadratically divergent radiative corrections. One of the most important predictions of the minimal supersymmetric standard model (MSSM) is the upper bound of the lightest Higgs boson mass. At tree level, the MSSM predicts the lightest Higgs boson mass to be less than the Z boson mass. However, after including loop corrections, the contributions from top and stop loops are so important that the upper bound of the lightest Higgs boson mass can be increased to around 130 GeV [1]. This upper bound should be compared with the current lower limit of 89.8 GeV from the MSSM Higgs search at LEP [2]. If the lightest Higgs boson is discovered and its mass turns out to be less than 130 GeV, it is a strong hint for the MSSM. If the MSSM is true, the CERN Large Hadron Collider (LHC) is expected to probe the Higgs sector by copiously producing the Higgs bosons. The Higgs sector in the MSSM has a rich structure; there are two CP-even Higgs bosons, one CP-odd Higgs boson and one (complex) charged Higgs boson. Their production and decay properties depend on various parameters in the MSSM including the SUSY breaking parameters. Therefore, to study the properties of the Higgs bosons at the LHC, a precise knowledge of the production cross section of the Higgs bosons is extremely important. It has been shown that CP-violation in the Higgs sector could significantly affect the production and decay properties of the Higgs bosons [3, 4, 5]. In order to prepare for the discoveries of the MSSM Higgs bosons at the LHC in any case, further detailed studies on the MSSM with CP-violation would be important. The aim of this letter is to present our findings on the production cross section of the “CP-odd” Higgs boson in the MSSM with CP-violation. We show that the production cross section of the “CP-odd” Higgs boson can be enhanced by a factor of about 1000 compared to the case without CP-violation, and discuss some important decay signatures of the “CP-odd” Higgs boson. We also discuss some constraints on our CP-violating scenarios. The strongest constraint comes from the electric dipole moments (EDMs) of electron and neutron. Since there are possibilities that cancellations among many contributions to EDMs could happen, the searches for the “CP-odd” Higgs boson at the current and future colliders could provide important information on the CP-violation mechanism in the MSSM, which is generally independent of those from the EDM searches. The MSSM has two Higgs doublets, H1 and H2. The neutral components H 0 1 and H 0 2 of the Higgs bosons develop vacuum expectation values (VEVs), which trigger the electroweak symmetry breaking (EWSB). After EWSB, there are three neutral Higgs bosons and a pair of charged Higgs bosons. If CP is a good symmetry in the Higgs sector, we can label the neutral Higgs bosons in Strictly speaking, when CP is violated, we cannot define a “CP-odd” Higgs boson because all three neutral Higgs bosons are mixed with each other. As we will discuss later, however, in the parameter sets we consider, CP-violating Higgs boson mixing is small. Therefore, we still use the terminology “CP-odd” Higgs boson even in the CP-violating case. Although in this letter we concentrate on the “CP-odd” Higgs production via gluon fusion at hadron colliders, we note that the same enhancement of the “CP-odd” Higgs production is also possible at a γγ collider.