Interference effects of neutral MSSM Higgs bosons with a generalised narrow-width approximation

Mixing effects in the MSSM Higgs sector can give rise to a sizeable interference between the neutral Higgs bosons. On the other hand, factorising a more complicated process into production and decay parts by means of the narrow-width approximation (NWA) simplifies the calculation. The standard NWA, however, does not account for interference terms. Therefore, we introduce a generalisation of the NWA (gNWA) which allows for a consistent treatment of interference effects between nearly mass-degenerate particles. Furthermore, we apply the gNWA at the tree and 1-loop level to an example process where the neutral Higgs bosons $h$ and $H$ are produced in the decay of a heavy neutralino and subsequently decay into a fermion pair. The $h-H$ propagator mixing is found to agree well with the approximation of Breit-Wigner propagators times finite wave-function normalisation factors, both leading to a significant interference contribution. The factorisation of the interference term based on on-shell matrix elements reproduces the full interference result within a precision of better than 1% for the considered process. The gNWA also enables the inclusion of contributions beyond the 1-loop order into the most precise prediction.Comment: 7 pages, 7 figures, Contribution to the proceedings of ICHEP 201

Impact of CP-violating interference effects on MSSM Higgs searches

Interference and mixing effects between neutral Higgs bosons in the MSSM with complex parameters are shown to have a significant impact on the interpretation of LHC searches for additional Higgs bosons. Complex MSSM parameters introduce mixing between the CP-even and CP-odd Higgs states and generate CP-violating interference terms. Both effects are enhanced in the case of almost degenerate states. Employing as an example an extension of a frequently used benchmark scenario by a non-zero phase $\phi_{A_t}$, the interference contributions are obtained for the production of neutral Higgs bosons in gluon-fusion and in association with b-quarks followed by the decay into a pair of $\tau$-leptons. While the resonant mixing increases the individual cross sections for the two heavy Higgs bosons $h_2$ and $h_3$, strongly destructive interference effects between the contributions involving $h_2$ and $h_3$ leave a considerable parameter region unexcluded that would appear to be ruled out if the interference effects were neglected.Comment: 30 pages, 7 figure

Interference effects in MSSM Higgs searches at the LHC

Complex parameters in the MSSM lead to mixing and interference between the two heavier neutral CP-even and CP-odd Higgs states. These effects can become very large in the case of almost degenerate states. In a CP-violating benchmark scenario, we investigate phenomenological implications of such interferences in view of the LHC searches for heavy Higgs bosons decaying to a pair of $\tau$-leptons and produced in gluon fusion and in association with b-quarks. Strongly destructive effects leave parameter regions unconstrained that would be regarded as excluded if no interference terms were taken into account.Comment: 5 pages, 2 figures; conference C16-08-03; references updated, some clarification

Interference effects of neutral MSSM Higgs bosons with a generalised narrow-width approximation

AbstractMixing effects in the MSSM Higgs sector can give rise to a sizeable interference between the neutral Higgs bosons. On the other hand, factorising a more complicated process into production and decay parts by means of the narrow-width approximation (NWA) simplifies the calculation. The standard NWA, however, does not account for interference terms. Therefore, we introduce a generalisation of the NWA (gNWA) which allows for a consistent treatment of interference effects between nearly mass-degenerate particles. Furthermore, we apply the gNWA at the tree and 1-loop level to an example process where the neutral Higgs bosons h and H are produced in the decay of a heavy neutralino and subsequently decay into a fermion pair. The h – H propagator mixing is found to agree well with the approximation of Breit-Wigner propagators times finite wave-function normalisation factors, both leading to a significant interference contribution. The factorisation of the interference term based on on-shell matrix elements reproduces the full interference result within a precision of better than 1% for the considered process. The gNWA also enables the inclusion of contributions beyond the 1-loop order into the most precise prediction

Relaxion and light (pseudo)scalars at the HL-LHC and lepton colliders

We study the potential of future lepton colliders, running at the Z-pole and above, and the High-Luminosity LHC to search for the relaxion and other light scalars $\phi$. We investigate the interplay of direct searches and precision observables for both CP-even and -odd couplings. In particular, precision measurements of exotic Z-decays, Higgs couplings, the exotic Higgs decay into a relaxion pair and associated $Z\phi$ and $\gamma \phi$ production are promising channels to yield strong bounds.Comment: 27 pages + references, 5 figures, 2 table

CP\mathcal{CP}-violating effects on MSSM Higgs searches

We study the effects of $\mathcal{CP}$-violating phases on the phenomenology of the Higgs sector of the MSSM. Complex parameters in the MSSM lead to $\mathcal{CP}$-violating mixing between the tree-level $\mathcal{CP}$-even and $\mathcal{CP}$-odd neutral Higgs states, leading to three new loop-corrected mass eigenstates $h_a$, $a \in \lbrace 1,2,3\rbrace$. For scenarios where a light Higgs boson at about 125 GeV can be identified with the observed signal and where the other Higgs states are significantly heavier, a large admixture of the heavy neutral Higgs bosons occurs as a generic feature if $\mathcal{CP}$-violating effects are taken into account. Including interference contributions in the predictions for cross sections times branching ratios of the Higgs bosons is essential in this case. As a first step, we present the gluon-fusion and bottom-quark annihilation cross sections for $h_a$ for the general case of arbitrary complex parameters, and we demonstrate that squark effects strongly depend on the phases of the complex parameters. We then study the effects of interference between $h_2$ and $h_3$ for the example of the process $b\bar{b} \to \tau^+\tau^-$. We show that large destructive interference effects modify LHC exclusion bounds such that parts of the parameter space that would be excluded by MSSM Higgs searches under the assumption of $\mathcal{CP}$-conservation open up when the possibility of $\mathcal{CP}$-violation in the Higgs sector is accounted for.Comment: 11 pages, 4 figures, contribution to the HiggsTools Final Meeting, Durha

Phenomenology of relaxion-Higgs mixing

We show that the relaxion generically stops its rolling at a point that breaks CP leading to relaxion-Higgs mixing. This opens the door to a variety of observational probes since the possible relaxion mass spans a broad range from sub-eV to the GeV scale. We derive constraints from current experiments (fifth force, astrophysical and cosmological probes, beam dump, flavour, LEP and LHC) and present projections from future experiments such as NA62, SHiP and PIXIE. We find that a large region of the parameter space is already under the experimental scrutiny. All the experimental constraints we derive are equally applicable for general Higgs portal models. In addition, we show that simple multiaxion (clockwork) UV completions suffer from a mild fine tuning problem, which increases with the number of sites. These results favour a cut-off scale lower than the existing theoretical bounds.Comment: 46 pages, 6 figures, v3: typos fixed, references added, version matches the version published in JHE

Interference effects in BSM processes with a generalised narrow-width approximation

A generalisation of the narrow-width approximation (NWA) is formulated which allows for a consistent treatment of interference effects between nearly mass-degenerate particles in the factorisation of a more complicated process into production and decay parts. It is demonstrated that interference effects of this kind arising in BSM models can be very large, leading to drastic modifications of predictions based on the standard NWA. The application of the generalised NWA is demonstrated both at tree level and at one-loop order for an example process where the neutral Higgs bosons $h$ and $H$ of the MSSM are produced in the decay of a heavy neutralino and subsequently decay into a fermion pair. The generalised NWA, based on on-shell matrix elements or their approximations leading to simple weight factors, is shown to produce UV- and IR-finite results which are numerically close to the result of the full process at tree level and at one-loop order, where an agreement of better than $1\%$ is found for the considered process. The most accurate prediction for this process based on the generalised NWA, taking into account also corrections that are formally of higher orders, is briefly discussed.Comment: 44 pages, 17 figure