Effects of longitudinal short-distance constraints on the hadronic light-by-light contribution to the muon g - 2.

We present a model-independent method to estimate the effects of short-distance constraints (SDCs) on the hadronic light-by-light contribution to the muon anomalous magnetic moment a µ HLbL . The relevant loop integral is evaluated using multi-parameter families of interpolation functions, which satisfy by construction all constraints derived from general principles and smoothly connect the low-energy region with those where either two or all three independent photon virtualities become large. In agreement with other recent model-based analyses, we find that the SDCs and thus the infinite towers of heavy intermediate states that are responsible for saturating them have a rather small effect on a µ HLbL . Taking as input the known ground-state pseudoscalar pole contributions, we obtain that the longitudinal SDCs increase a µ HLbL by ( 9.1 ± 5.0 ) × 10 - 11 , where the isovector channel is responsible for ( 2.6 ± 1.5 ) × 10 - 11 . More precise estimates can be obtained with our method as soon as further accurate, model-independent information about important low-energy contributions from hadronic states with masses up to 1-2 GeV become available.

Rescattering Effects in the Hadronic-Light-by-Light Contribution to the Anomalous Magnetic Moment of the Muon.

We present a first model-independent calculation of ππ intermediate states in the hadronic-light-by-light (HLBL) contribution to the anomalous magnetic moment of the muon (g-2)_{µ} that goes beyond the scalar QED pion loop. To this end, we combine a recently developed dispersive description of the HLBL tensor with a partial-wave expansion and demonstrate that the known scalar-QED result is recovered after partial-wave resummation. Using dispersive fits to high-statistics data for the pion vector form factor, we provide an evaluation of the full pion box a_{µ}^{π box}=-15.9(2)×10^{-11}. We then construct a suitable input for the γ^{*}γ^{*}→ππ helicity partial waves, based on a pion-pole left-hand cut and show that for the dominant charged-pion contribution, this representation is consistent with the two-loop chiral prediction and the COMPASS measurement for the pion polarizability. This allows us to reliably estimate S-wave rescattering effects to the full pion box and leads to our final estimate for the sum of these two contributions a_{µ}^{π box}+a_{µ,J=0}^{ππ,π-pole LHC}=-24(1)×10^{-11}.

New constraints on dark matter effective theories from standard model loops.

We consider an effective field theory for a gauge singlet Dirac dark matter particle interacting with the standard model fields via effective operators suppressed by the scale Λ â‰³ 1 TeV. We perform a systematic analysis of the leading loop contributions to spin-independent Dirac dark matter-nucleon scattering using renormalization group evolution between Λ and the low-energy scale probed by direct detection experiments. We find that electroweak interactions induce operator mixings such that operators that are naively velocity suppressed and spin dependent can actually contribute to spin-independent scattering. This allows us to put novel constraints on Wilson coefficients that were so far poorly bounded by direct detection. Constraints from current searches are already significantly stronger than LHC bounds, and will improve in the near future. Interestingly, the loop contribution we find is isospin violating even if the underlying theory is isospin conserving.

Calculating track-based observables for the LHC.

By using observables that only depend on charged particles (tracks), one can efficiently suppress pileup contamination at the LHC. Such measurements are not infrared safe in perturbation theory, so any calculation of track-based observables must account for hadronization effects. We develop a formalism to perform these calculations in QCD, by matching partonic cross sections onto new nonperturbative objects called track functions which absorb infrared divergences. The track function Ti(x) describes the energy fraction x of a hard parton i which is converted into charged hadrons. We give a field-theoretic definition of the track function and derive its renormalization group evolution, which is in excellent agreement with the pythia parton shower. We then perform a next-to-leading order calculation of the total energy fraction of charged particles in e+ e-→ hadrons. To demonstrate the implications of our framework for the LHC, we match the pythia parton shower onto a set of track functions to describe the track mass distribution in Higgs plus one jet events. We also show how to reduce smearing due to hadronization fluctuations by measuring dimensionless track-based ratios.