New Leading Contribution to Neutrinoless Double-ß Decay.

Within the framework of chiral effective field theory, we discuss the leading contributions to the neutrinoless double-beta decay transition operator induced by light Majorana neutrinos. Based on renormalization arguments in both dimensional regularization with minimal subtraction and a coordinate-space cutoff scheme, we show the need to introduce a leading-order short-range operator, missing in all current calculations. We discuss strategies to determine the finite part of the short-range coupling by matching to lattice QCD or by relating it via chiral symmetry to isospin-breaking observables in the two-nucleon sector. Finally, we speculate on the impact of this new contribution on nuclear matrix elements of relevance to experiment.

Hunting mixed top squark decays.

We point out that, in the irreducible natural supersymmetric spectrum, top squarks have comparable branching fractions to chargino-bottom and neutralino-top final states in the vast bulk of parameter space, provided only that both decay modes are kinematically accessible. The total top squark pair branching fractions into tt + MET (MET=missing transverse energy) can therefore be reduced to O(50%), whereas bb + X branching fractions are typically much smaller, O(10%), thus limiting the reach of traditional top squark searches. We propose a new top squark search targeting the asymmetric final state t[over˜]t[over˜]* → t(χ)(0)b(χ)(-) +H.c., which can restore sensitivity to natural top squarks in the 7 and 8 TeV LHC runs. In addition, we present a new variable, topness, which efficiently suppresses the dominant top backgrounds to semileptonic top partner searches. We demonstrate the utility of topness in both our asymmetric search channel and traditional t[over˜]t[over ˜](*) →tt[over¯]+MET searches and show that it matches or outperforms existing variables.