Abstract
Despite extensive searches for an additional neutral massive gauge boson at the LHC, a Z′ at the weak scale could still be present if its couplings to the first two generations of quarks are suppressed, in which case the production in hadron colliders relies on tree-level processes in association with heavy flavors or one-loop processes in association with a jet. We consider the low-energy effective theory of a top-philic Z′ and present possible UV completions. We clarify theoretical subtleties in evaluating the production of a top-philic Z′ at the LHC and examine carefully the treatment of ananomalous Z′ current in the low-energy effective theory. Recipes for properly computing the production rate in the Z′ + j channel are given. We discuss constraints from colliders and low-energy probes of new physics. As an application, we apply these considerations to models that use a weak-scale Z′ to explain possible violations of lepton universality in B meson decays, and show that the future running of a high luminosity LHC can potentially cover much of the remaining parameter space favored by this particular interpretation of the B physics anomaly.
Highlights
To answer the question, it is useful to recall that searches at hadron colliders, the LHC in particular, lean heavily on new physics having significant couplings to partons inside the proton, and subsequently decaying into high pT objects, whether visible or invisible, in the detectors
Despite extensive searches for an additional neutral massive gauge boson at the LHC, a Z at the weak scale could still be present if its couplings to the first two generations of quarks are suppressed, in which case the production in hadron colliders relies on treelevel processes in association with heavy flavors or one-loop processes in association with a jet
While a theory with an anomalous U(1) current can be a consistent effective theory, there is a particular class of diagrams involving the “mixedanomaly” between one Z gauge boson and two gluons, which could have an important impact on the production mechanism of a top-philic Z at the LHC, because the Z does not couple to light flavor quarks and cannot be produced through the usual Drell-Yan process
Summary
The starting point of our discussion is the following effective Lagrangian, valid at the weak scale or below, Leff. While a theory with an anomalous U(1) current can be a consistent effective theory, there is a particular class of diagrams involving the “mixedanomaly” between one Z gauge boson and two gluons, which could have an important impact on the production mechanism of a top-philic Z at the LHC, because the Z does not couple to light flavor quarks and cannot be produced through the usual Drell-Yan process. Lies the second subtlety: the single production of an on-shell Z is forbidden, off-shell production is still possible In this channel, it is imperative to incorporate the width of the Z in a consistent fashion, for instance by adopting the complex mass scheme [35,36,37] and replacing MZ2 → MZ2 − iΓZ MZ everywhere in the calculation [38]. We see that the three-point coupling of one Z with two gluons features prominently, and must be dealt with carefully in the effective theory
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