Little Hierarchy, Little Higgses, and a Little Symmetry

Abstract

Little Higgs theories are an attempt to address the "little hierarchy problem," i.e., the tension between the naturalness of the electroweak scale and the precision electroweak measurements showing no evidence for new physics up to 5 -10 TeV. In little Higgs theories, the Higgs mass-squareds are protected at one-loop order from the quadratic divergences. This allows the cutoff of the theory to be raised up to ∼ 10 TeV, beyond the scales probed by the current precision data. However, strong constraints can still arise from the contributions of the new TeV scale particles which cancel the one-loop quadratic divergences from the standard model fields, and hence re-introduces the fine-tuning problem. In this paper we show that a new symmetry, denoted as T -parity, under which all heavy gauge bosons and scalar triplets are odd, can remove all the tree-level contributions to the electroweak observables and therefore makes the little Higgs theories completely natural. The T -parity can be manifestly implemented in a majority of little Higgs models by following the most general construction of the low energy effective theoryà la Callan, Coleman, Wess and Zumino. In particular, we discuss in detail how to implement the T -parity in the littlest Higgs model based on SU (5)/SO(5). The symmetry breaking scale f can be even lower than 500 GeV if the contributions from the higher dimensional operators due to the unknown UV physics at the cutoff are somewhat small. The existence of T -parity has drastic impacts on the phenomenology of the little Higgs theories. The T -odd particles need to be pair-produced and will cascade down to the lightest T -odd particle (LTP) which is stable. A neutral LTP gives rise to missing energy signals at the colliders which can mimic supersymmetry. It can also serve as a good dark matter candidate.

Signatures of the Minimal Moose Model With Exact Parity

“…[4]. As a result, the original T-parity proposed by Cheng and Low [4] is also respected by our model on the classical level, although it is broken by the WZW term as explained above.…”

“…[4]. As a result, the original T-parity proposed by Cheng and Low [4] is also respected by our model on the classical level, although it is broken by the WZW term as explained above. However, since the WZW term is the only source of T-parity breaking, T-parity is still a good quantum number for most production and decay processes, and therefore it is also listed in Tab.…”

“…These new particles lead to large tree-level corrections to electroweak precision observables [2], which rule out values of f below several TeV. However, the introduction of a discrete symmetry called T-parity [3,4] forbids the dangerous tree-level diagrams. As a result, models with T-parity are in agreement with precision data for values of f even below 1 TeV [5].…”

Study ofq˜Lq˜Lproduction at CERN LHC in thel±lNojiri1, Takeuchi2 2007Phys. Rev. D22080View full textAdd to dashboardCite

“…A similar mass spectrum and decay pattern might be obtained for the Universal Extra Dimension model (UED) [4] and the Littlest Higgs model with T parity (LHT) [5,6,7]. To study the origin of the E / T signature, therefore, it is important to measure the other features that are characteristic of the MSSM.…”

Measuring the shape of the extra dimension

“…Another important feature is the existence a stable particle, that is the lightest particle charged under the parity, which is a good candidate for dark matter if it is electrically neutral. Examples of such a parity, other than the KK parity, are the R parity in supersymmetry and the T parity in little Higgs models [15,16,17].…”