[Paper Review] Anomalous Triple Gauge Vertices at the Large Hadron-Electron Collider

Sudhansu S. Biswal, Monalisa Patra+1 more|arXiv (Cornell University)|May 23, 2014

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper proposes using the Large Hadron-Electron Collider (LHeC) to probe anomalous $WW\gamma$ triple gauge couplings via charged current $ep \to W\gamma j$ events, leveraging the azimuthal angle difference between the jet and missing transverse momentum as a sensitive observable. With integrated luminosities of 100–1000 fb$^{-1}$, the LHeC can surpass all existing experimental bounds, offering a clean, $WWZ$-vertex-free probe of $\Delta\kappa_\gamma$ and $\lambda_\gamma$ beyond the LEP constraints.

ABSTRACT

At a high energy $ep$ collider, such as the Large Hadron-Electron Collider (LHeC) which is being planned at CERN, one can access the $WWγ$ vertex exclusively in charged current events with a radiated photon, with no interference from the $WWZ$ vertex. We find that the azimuthal angle between the jet and the missing momentum in each charged current event is a sensitive probe of anomalous $WWγ$ couplings, and show that for quite reasonable values of integrated luminosity, the LHeC can extend the discovery reach for these couplings beyond all present experimental bounds.

Motivation & Objective

• To explore the potential of the LHeC to probe anomalous $WW\gamma$ triple gauge boson vertices (TGVs) with high sensitivity.
• To identify a clean, interference-free observable sensitive to $WW\gamma$ couplings, avoiding contamination from $WWZ$ vertex effects.
• To determine the discovery reach of the LHeC for $\Delta\kappa_\gamma$ and $\lambda_\gamma$ couplings using realistic luminosity scenarios.
• To compare the LHeC's projected sensitivity with existing bounds from LEP, Tevatron, and LHC experiments.

Proposed method

• The study uses the $ep \to W^+W^-\gamma$ process at the LHeC, focusing on charged current events with a radiated photon to isolate the $WW\gamma$ vertex.
• The azimuthal angle difference $\Delta\phi$(jet, $\not{p}_T$) between the jet and missing transverse momentum is used as a key observable to probe anomalous couplings.
• A $\chi^2$-based statistical technique is applied to quantify deviations from the Standard Model, with the test statistic defined in Eq. (7) and (8).
• The analysis assumes a fixed electron beam energy of 140 GeV and varies $\Delta\kappa_\gamma$ and $\lambda_\gamma$ over their allowed ranges to map discovery contours.
• The method explicitly separates the $WW\gamma$ vertex from the $WWZ$ vertex, avoiding interference that plagues other colliders.
• Discovery limits are derived at 95% confidence level (C.L.) and compared with existing experimental bounds from LEP, CDF, D0, ATLAS, and CMS.

Experimental results

Research questions

• RQ1 Can the LHeC provide a more sensitive probe of anomalous $WW\gamma$ couplings than existing experiments?
• RQ2 Is the azimuthal angle difference $\Delta\phi$(jet, $\not{p}_T$) a viable and sensitive observable for detecting $WW\gamma$ anomalous couplings at the LHeC?
• RQ3 To what extent can the LHeC surpass the current most stringent bounds from LEP, especially in the presence of non-zero $\lambda_\gamma$?
• RQ4 How does the discovery reach depend on integrated luminosity, and at what luminosity does the LHeC exceed LEP's sensitivity?
• RQ5 Can the LHeC provide a clean, $WWZ$-vertex-free measurement of $WW\gamma$ couplings, avoiding interference from other anomalous vertices?

Key findings

• With an integrated luminosity of 100 fb$^{-1}$, the LHeC can probe $\Delta\kappa_\gamma$ values below the current LEP bounds, especially for $\Delta\kappa_\gamma > 0$.
• For $\Delta\kappa_\gamma < 0$, a luminosity of 25–50 fb$^{-1}$ is sufficient to surpass LEP constraints, depending on electron beam energy.
• At 200 fb$^{-1}$, the LHeC's discovery reach exceeds all existing experimental bounds, including those from LHC and Tevatron.
• With 1000 fb$^{-1}$, the LHeC can probe a significantly reduced inaccessible region in the $\Delta\kappa_\gamma$–$\lambda_\gamma$ plane, approaching the full parameter space.
• The LHeC's sensitivity is robust even when both $\Delta\kappa_\gamma$ and $\lambda_\gamma$ are non-zero, as shown by joint discovery contours in Fig. 5.
• The LHeC provides a clean probe of $WW\gamma$ couplings, free from contamination by $WWZ$ anomalous vertex effects, a key advantage over hadron colliders.