[Paper Review] Leptoquarks and Contact Interactions at LeHC

A. F. Żarnecki|ArXiv.org|Sep 17, 2008

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper evaluates the sensitivity of the proposed Large Hadron electron Collider (LeHC) to leptoquarks and contact interactions, demonstrating that it can probe leptoquark masses up to 2 TeV via resonance production and contact interaction scales up to 70 TeV. LeHC would significantly improve existing limits on new physics, including effective Planck scales up to 5.4 TeV and quark substructure down to 10⁻¹⁹ m.

ABSTRACT

The sensitivity of LeHC to different models of "new physics" has been studied, both for the resonance production and in the contact interaction approximation. Expected limits are compared for different running scenarios. Direct leptoquark production can be studied for masses up to about 2 TeV. For contact interaction models scales up to about 70 TeV can be explored. Significant improvement of existing limits is also expected for models with large extra dimensions. Effective Plank mass scales up to about 5.4 TeV can be probed. LeHC will be sensitive to the quark substructure of the order of 1E-19 m.

Motivation & Objective

• To assess the sensitivity of LeHC to leptoquark resonances and contact interactions as probes of new physics beyond the Standard Model.
• To compare expected exclusion limits across different LeHC running scenarios, including varying electron beam energies and integrated luminosities.
• To evaluate the potential of LeHC to constrain models with large extra dimensions and effective field theories via high-Q² deep inelastic scattering.
• To establish the reach of LeHC in probing quark substructure and Planck-scale physics through precision measurements of electron-proton scattering.

Proposed method

• Uses the narrow-width approximation (NWA) to model direct single leptoquark production in ep scattering, based on leptoquark mass and Yukawa coupling.
• Applies the contact interaction (CI) approach to model effective four-fermion interactions from leptoquark exchange, parameterized by coefficients ηᵢⱼ ∝ (λₗ𝚠/Mₗ𝚠)².
• Combines NWA and modified CI methods to extract limits on leptoquark couplings and masses, selecting the stronger constraint for each mass point.
• Performs Monte Carlo simulations to estimate statistical fluctuations and derive 95% confidence level (CL) exclusion limits on (λₗ𝚠, Mₗ𝚪) for different leptoquark types.
• Uses high-Q² neutral current deep inelastic scattering (NC DIS) data to constrain contact interaction scales Λ⁺ and effective Planck mass Mₛ in models with large extra dimensions.
• Applies classical form-factor approximation to derive limits on the effective quark charge radius R_q from high-Q² cross-section measurements.

Experimental results

Research questions

• RQ1 What is the maximum leptoquark mass that LeHC can probe via direct resonance production, and what are the corresponding limits on Yukawa coupling?
• RQ2 To what extent can LeHC improve on existing limits for contact interaction models, particularly for vector-like (VV) and lepton-number conserving (LL) types?
• RQ3 How sensitive is LeHC to models with large extra dimensions, and what effective Planck mass scale can be probed?
• RQ4 What is the sensitivity of LeHC to the quark substructure, and what lower bound can be set on the effective quark charge radius?
• RQ5 How do different LeHC running scenarios—varying electron beam energy and luminosity—affect the discovery and exclusion reach for new physics?

Key findings

• LeHC can probe leptoquark masses up to approximately 2 TeV through direct resonance production in ep collisions.
• For contact interaction models, LeHC can explore energy scales up to about 70 TeV, significantly improving on current limits.
• Effective Planck mass scales up to 5.4 TeV can be probed in models with large extra dimensions, depending on systematic uncertainties.
• LeHC will be sensitive to quark substructure at the level of 10⁻¹⁹ m, setting a lower bound on the effective quark charge radius.
• The modified contact interaction approach provides stronger constraints than NWA for low leptoquark masses, while NWA is more effective for higher masses.
• Expected limits from LeHC exceed those from HERA, the Tevatron, and the LHC for contact interactions and large extra dimensions, particularly at high luminosity.