Constraints on additional Z bosons

Hadronic Vacuum Polarization: (g-2)_{μ} versus Global Electroweak Fits

Hadronic vacuum polarization (HVP) is not only a critical part of the standard model (SM) prediction for the anomalous magnetic moment of the muon (g-2)_{μ}, but also a crucial ingredient for global fits to electroweak (EW) precision observables due to its contribution to the running of the fine-structure constant encoded in Δα_{had}^{(5)}. We find that with modern EW precision data, including the measurement of the Higgs mass, the global fit alone provides a competitive, independent determination of Δα_{had}^{(5)}|_{EW}=270.2(3.0)×10^{-4}. This value actually lies below the range derived from e^{+}e^{-}→hadrons cross section data, and thus goes into the opposite direction as would be required if a change in HVP were to bring the SM prediction for (g-2)_{μ} into agreement with the Brookhaven measurement. Depending on the energy where the bulk of the changes in the cross section occurs, reconciling experiment and SM predictions for (g-2)_{μ} by adjusting HVP would thus not necessarily weaken the case for physics beyond the SM (BSM), but to some extent shift it from (g-2)_{μ} to the EW fit. We briefly explore some options of BSM scenarios that could conceivably explain the ensuing tension.

Baryonic Z' explanation for the CDF Wjj excess

The latest CDF anomaly, the excess of dijet events in the invariant-mass window 120-160 GeV in associated production with a W boson, can be explained by a baryonic Z' model in which the Z' boson has negligible couplings to leptons. Although this Z' model is hardly subject to the Drell-Yan constraint from Tevatron, it is constrained by the dijet data from UA2 (√s=630 GeV), and the precision measurements at LEP through the mixing with the SM Z boson. We show that under these constraints this model can still explain the excess in the M(jj)∼120-160  GeV window, as well as the claimed cross section σ(WZ')∼4  pb. Implications at the Tevatron would be the associated production of γZ', ZZ', and Z'Z' with the Z'→jj. We show that with tightened jet cuts and improved systematic uncertainties both γZ'→γjj and ZZ'→ℓ(+)ℓ(-) jj channels could be useful to probe this model at the Tevatron.

Is the unitarity of the quark-mixing CKM matrix violated in neutron beta-decay?

We report on a new measurement of neutron beta-decay asymmetry. From the result A(0) = -0.1189(7), we derive the ratio of the axial vector to the vector coupling constant lambda = g(A)/g(V) = -1.2739(19). When included in the world average for the neutron lifetime tau = 885.7(7) s, this gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix V(ud). With this value and the Particle Data Group values for V(us) and V(ub), we find a deviation from the unitarity condition for the first row of the CKM matrix of Delta = 0.0083(28), which is 3.0 times the stated error.

Q-ball candidates for self-interacting dark matter

We show that nontopological solitons, known as Q-balls, are promising candidates for self-interacting dark matter. They can satisfy the cross-section requirements for a broad range of masses. Unlike previously considered examples, Q-balls can stick together after collision, reducing the effective self-interaction rate to a negligible value after a few collisions per particle. This feature modifies predictions for halo formation. We also discuss the possibility that Q-balls have large interaction cross sections with ordinary matter.

Indications for an extra neutral gauge boson in electroweak precision data

A new analysis of the hadronic peak cross section at LEP 1 implies a small amount of missing invisible width in Z decays, while the effective weak charge in atomic parity violation has been determined recently to 0.6% accuracy, indicating a significantly negative S parameter. As a consequence, the data are described well if the presence of an extra Z' boson, such as predicted in grand unified theories, is assumed. Moreover, the data are now rich enough to study an arbitrary extra Z' boson and to determine its couplings in a model independent way. An excellent fit to the data is obtained in this case, suggestive of a family nonuniversal Z' similar to those predicted in a class of superstring theories.