[Paper Review] Disentangling between Z' and Z* with first LHC data

M. V. Chizhov|ArXiv.org|Jul 31, 2008

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper proposes distinguishing chiral spin-1 bosons ($Z^*$) from $Z'$ gauge bosons using angular and kinematic distributions in dilepton final states at the LHC. It identifies a unique 'swallowtail' shape in the Collins–Soper angular distribution and absence of a Jacobian peak in transverse momentum as key signatures, with a center-edge asymmetry $A_{CE} = -0.69 \pm 0.10$ for $Z^*$, distinct from $Z'$ at $-0.11 \pm 0.06$, enabling early discrimination in first LHC data.

ABSTRACT

The resonance production of new chiral spin-1 excited bosons, Z*, and their detection through the Drell-Yan process in the first physical runs at the CERN LHC are considered. The new neutral chiral bosons can be observed as a Breit-Wigner resonance peaks in the invariant dilepton mass distribution in the same way as the well-known hypothetical gauge bosons, Z'. However, unique new signatures of the chiral bosons exist. These signatures could be very important for the interpretation of the first LHC data. First, there is no Jacobian peak in the lepton transverse momentum distribution at the kinematical endpoint of the new resonance. Second, the lepton angular distribution in the Collins-Soper frame for the high on-peak invariant masses of the lepton pairs has a peculiar "swallowtail" shape.

Motivation & Objective

• To identify experimentally accessible signatures that distinguish chiral spin-1 bosons ($Z^*$) from $Z'$ gauge bosons in the first LHC data.
• To address the challenge of misidentifying $Z^*$ as a $Z'$ due to similar resonance peaks in the invariant dilepton mass distribution.
• To provide a method for early discrimination using kinematic and angular distributions sensitive to spin and chiral structure.
• To demonstrate that the Collins–Soper frame angular distribution of dileptons reveals a unique 'swallowtail' shape for $Z^*$ bosons.
• To quantify the center-edge asymmetry $A_{CE}$ as a robust observable to distinguish $Z^*$ from $Z'$ at the 5σ level.

Proposed method

• Analyzing the Drell–Yan process $q\bar{q} \to \ell^+\ell^-$ for resonance production of $Z^*$ and $Z'$ bosons at $\sqrt{s} = 10$ TeV.
• Using the effective Lagrangian $\mathcal{L}_{\text{excited}} = \frac{g}{2\sqrt{2}M}(\bar{\ell}\sigma^{\mu\nu}\ell + \bar{d}\sigma^{\mu\nu}d)(\partial_\mu Z^*_\nu - \partial_\nu Z^*_\mu)$ to model chiral boson couplings via tensor currents.
• Computing differential cross sections as functions of lepton transverse momentum, pseudorapidity, and Collins–Soper angle $\cos\theta^*_{\text{CS}}$.
• Applying the center-edge asymmetry $A_{CE}$ defined as the difference between central and edge regions of the $\cos\theta^*_{\text{CS}}$ distribution.
• Comparing distributions for $Z^*$ and $Z'$ using parton-level $d\bar{d}$ and $\bar{d}d$ contributions to account for parton momentum asymmetries.
• Evaluating the impact of longitudinal boosts on the angular distribution, showing that the 'swallowtail' dip at $\cos\theta^*_{\text{CS}} = 0$ persists at $\sqrt{s} = 10$ TeV but is filled at $14$ TeV.

Experimental results

Research questions

• RQ1 Can the 'swallowtail' shape in the Collins–Soper angular distribution distinguish chiral $Z^*$ bosons from $Z'$ gauge bosons in early LHC data?
• RQ2 Does the absence of a Jacobian peak in the lepton transverse momentum distribution serve as a unique signature for $Z^*$ bosons?
• RQ3 To what extent does the pseudorapidity distribution of dileptons reveal a dip at $\eta_\ell = 0$ for $Z^*$ bosons due to a forbidden emission plane?
• RQ4 How does the center-edge asymmetry $A_{CE}$ differ between $Z^*$ and $Z'$ bosons, and can it be used to exclude $Z'$ at high significance?
• RQ5 What role do parton momentum asymmetries ($x > \bar{x}$) play in shaping the lepton pseudorapidity distribution for $Z^*$ and $Z'$?

Key findings

• The Collins–Soper angular distribution for $Z^*$ bosons exhibits a distinctive 'swallowtail' shape with a deep dip at $\cos\theta^*_{\text{CS}} = 0$, absent in $Z'$ distributions.
• The center-edge asymmetry $A_{CE}$ for $Z^*$ is $-0.69 \pm 0.10$, significantly more negative than for $Z'$ ($-0.11 \pm 0.06$), differing at the 5σ level.
• The transverse momentum distribution of leptons shows no Jacobian peak at the kinematic endpoint for $Z^*$ bosons, unlike $Z'$, due to the absence of a $1/\sqrt{1 - \beta^2}$ singularity.
• The pseudorapidity distribution for $Z^*$ bosons shows a dip at $\eta_\ell = 0$ due to a forbidden emission plane in the parton rest frame, visible at $\sqrt{s} = 10$ TeV but suppressed at $14$ TeV due to higher boosts.
• The $Z^*$ angular distribution is insensitive to parton momentum asymmetries, leading to a symmetric shape despite $\bar{x} < x$, due to opposite shifts in $d\bar{d}$ and $\bar{d}d$ contributions.
• The $A_{CE}$ asymmetry is robust and can be measured with high significance in early LHC data, providing a clean signature to exclude $Z'$ and confirm $Z^*$ if observed.