Helicity selection rules and noninterference for BSM amplitudes

Abstract

Precision studies of scattering processes at colliders provide powerful indirect constraints on new physics. We study the helicity structure of scattering amplitudes in the SM and in the context of an effective Lagrangian description of BSM dynamics. Our analysis reveals a novel set of helicity selection rules according to which, in the majority of 2 → 2 scattering processes at high energy, the SM and the leading BSM effects do not interfere. In such situations, the naive expectation that dimension-6 operators represent the leading BSM contribution is compromised, as corrections from dimension-8 operators can become equally (if not more) important well within the validity of the effective field theory approach.

Dimension-6 operator analysis of the CLIC sensitivity to new physics

“…The effects of operators with dimensions d > 6 are sub-dominant in such a decouplings scenario -with some exceptions [5] -justifying our focus on the dimension-6 terms in the SM EFT Lagrangian: for the purposes of our analysis, we express the dimension-6 operators O i in the basis of ref. [18].…”

“…When marginalising over the effects of all operators in our global fits this assumption holds less well for the TGCs [57], but is sufficient for our projections where we are mainly interested in a first estimate of the CLIC sensitivity to the scale of new physics. 5 Our fit constrains the coefficients at the respective centre-of-mass energy scales E at which they are measured: c i ≡ c i (E), which are related to their values at the matching scale, c i (Λ), by renormalisation-group equations (RGEs) that we do not consider here [59][60][61][62][63][64][65][66]. Also, we neglect dimension-8 and higher-order operators in our analysis, as well as the four-fermion operators that do not interfere with the SM amplitudes [13], 6 whose effects on Z-pole measurements are of the same order in Λ (or M ) as dimension-8 operators.…”

Same-sign WW scattering at the LHC: can we discover BSM effects before discovering new states?

“…For instance, the contribution of dimension-6 (D = 6) operators to a given process can be suppressed compared to dimension-8 (D = 8) operators contrary to a naive (E/ ) power counting [3][4][5][6] or, vice versa, the [SM × D = 8] interference contribution can be subleading with respect to the [D = 6] 2 one [2,4,7]. Clearly, the assumption about the choice of operators in the truncation in Eq.…”

“…The latter depends also on the relative magnitude of the couplings C, e.g., C (6) i versus C (8) i and/or within each of those sets of operators, separately, [1][2][3][4], as well as on the interference patterns in various amplitudes calculated from the Lagrangian Eq. (1.1) [5].…”

The Z decay width in the SMEFT: y t and λ corrections at one loop

“…[44], and then in refs. [45,46]. Due to the presence of the mixing angles of the SM rotating the vector fields to their mass eigenstates, and the SMEFT modifications of these mixing angles, defining the large y t and λ limit ofΓ Z at one loop in the SMEFT is subtle.…”