[Paper Review] Anomalously interacting extra neutral bosons

M. V. Chizhov, V. A. Bednyakov+1 more|arXiv (Cornell University)|May 16, 2010

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

This paper proposes that anomalous, spin-1 chiral gauge bosons—arising from extensions of the Standard Model such as Gauge-Higgs unification or technicolor—could be produced as resonances at the LHC. These $Z^*$ bosons couple via derivative (tensor) interactions to fermions, leading to distinctive angular and transverse momentum distributions in dilepton decays that differ sharply from standard $Z'$ bosons, enabling their unique identification in early LHC data.

ABSTRACT

We study phenomenological consequences of the Standard Model extension by the new spin-1 chiral fields with the internal quantum numbers of the electroweak Higgs doublets. There are at least three different classes of theories, all motivated by the hierarchy problem, which predict new vector weak-doublets with masses not far from the electro-weak scale. We discuss resonance production of these neutral chiral Z* bosons at hadron colliders. The bosons can be observed as a Breit-Wigner resonance peak in the invariant dilepton mass distributions in the same way as the well-known extra gauge Z' bosons. This includes them into a list of very interesting objects for early searches with the first LHC data. Moreover, the Z* bosons have unique signatures in transverse momentum, angular and pseudorapidity distributions of the final leptons, which allow to distinguish them from the other heavy neutral resonances.

Motivation & Objective

• To explore phenomenological signatures of new spin-1 chiral bosons that arise in extensions of the Standard Model motivated by the hierarchy problem.
• To identify unique experimental signatures—specifically angular and transverse momentum distributions—of these $Z^*$ bosons in dilepton final states at hadron colliders.
• To distinguish $Z^*$ bosons from conventional $Z'$ resonances based on their decay kinematics, particularly the absence of a Jacobian peak in $p_T$ distributions.
• To assess the potential for early discovery of such resonances using invariant mass distributions and kinematic distributions in the first LHC data.

Proposed method

• Formalize a minimal extension of the SM with a new weak doublet of spin-1 chiral bosons ($W^{*}_{ u}$) that couple to fermions via derivative (tensor) currents, rather than minimal vector currents.
• Derive the effective Lagrangian for neutral $Z^*$ and $\widetilde{Z}^*$ bosons, given by $\mathcal{L}^{*}_{NC} \propto \frac{g}{M} \left( \bar{D} \sigma^{\mu\nu} D \cdot \partial_\mu Z^*_\nu + i \bar{D} \sigma^{\mu\nu} \gamma^5 D \cdot \partial_\mu \widetilde{Z}^*_\nu \right)$, encoding anomalous couplings.
• Compute the differential cross-section for $q\bar{q} \to Z^* \to \ell^+\ell^-$, showing $\frac{d\sigma}{d\cos\theta} \propto \cos^2\theta$, contrasting with the $1 + \cos^2\theta$ dependence of $Z'$ bosons.
• Analyze the transverse momentum ($p_T$) distribution of final-state leptons, demonstrating that $Z^*$ decays produce a broad, smooth hump peaking below $M/2$, unlike the sharp Jacobian peak of $Z'$ bosons.
• Use kinematic distributions (angular, $p_T$, pseudorapidity) to show that $Z^*$ and $Z'$ bosons can be disentangled up to 1.15 TeV mass, with improved separation at higher masses.
• Estimate exclusion limits via statistical reasoning: a 95% confidence level exclusion for $Z^*$ resonances up to 1.65 TeV is possible if no events are observed above 1 TeV in the dilepton invariant mass window.

Experimental results

Research questions

• RQ1 Can anomalous, spin-1 chiral bosons with tensor couplings be produced as resonances at the LHC, and what are their distinctive decay signatures?
• RQ2 How do the angular and transverse momentum distributions of dilepton final states from $Z^*$ bosons differ from those of standard $Z'$ bosons?
• RQ3 To what extent can the absence of a Jacobian peak in the $p_T$ distribution of $Z^*$ decay products be used to distinguish them from $Z'$ bosons?
• RQ4 What is the reach of early LHC data in excluding $Z^*$ resonances based on invariant mass and kinematic distributions?
• RQ5 Can the unique kinematic features of $Z^*$ bosons serve as a signature for new physics such as compositeness, new symmetries, or extra dimensions?

Key findings

• The $Z^*$ boson decay distribution exhibits $\frac{d\sigma}{d\cos\theta} \propto \cos^2\theta$, in contrast to the $1 + \cos^2\theta$ dependence of $Z'$ bosons, leading to a fundamentally different angular distribution.
• The $p_T$ distribution of leptons from $Z^*$ decays reaches zero at the kinematic endpoint $p_T = M/2$, unlike the sharp Jacobian peak seen in $Z'$ decays, due to cancellation of the pole in the amplitude.
• The absence of a Jacobian peak in $Z^*$ decays results in a broad, smooth hump in the $p_T$ spectrum, making mass reconstruction via peak position unreliable and complicating resonance identification.
• Kinematic distributions in $p_T$, angular, and pseudorapidity variables allow for the disentanglement of $Z^*$ and $Z'$ resonances up to a mass of 1.15 TeV.
• Statistical analysis shows that a 95% confidence level exclusion limit for $Z^*$ resonances extends up to 1.65 TeV if no events are observed in the invariant dilepton mass window above 1 TeV.
• The unique kinematic signatures of $Z^*$ bosons provide a clear experimental handle to probe new physics, including compositeness, new symmetries, or extra dimensions, should such resonances be discovered.