[Paper Review] Search for contact interactions at HERA

A. Raval|ArXiv.org|Oct 8, 2008

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents a search for physics beyond the Standard Model at HERA using high-Q² deep inelastic scattering data from the H1 and ZEUS collaborations. By analyzing $e^\pm p$ collisions with high luminosity and polarized beams, it sets stringent limits on contact interactions, leptoquark couplings, large extra dimensions, and quark charge radii, with compositeness scales excluded up to 8.0 TeV at 95% CL.

ABSTRACT

The H1 and ZEUS collaborations at HERA have searched for signatures of physics beyond the Standard Model using high Q^2 neutral current deep inelastic electron-proton and positron-proton scattering events. No significant deviations from Standard Model predictions were observed. Various eeqq contact interaction models have been considered. Limits on the compositeness scale in general eeqq contact interaction models, mass to the Yukawa coupling ratio for heavy leptoquarks, the effective Planck mass scale in models with large extra dimensions and the effective quark charge radius are presented.

Motivation & Objective

• To search for deviations from the Standard Model in high-Q² neutral current deep inelastic scattering at HERA.
• To constrain effective field theory models of new physics, including contact interactions and large extra dimensions.
• To probe quark substructure via measurements of the effective quark charge radius.
• To set limits on leptoquark Yukawa couplings using virtual exchange effects.
• To improve sensitivity using high-luminosity HERA II data and polarized electron/positron beams.

Proposed method

• Analysis of $e^+p$ and $e^-p$ scattering events at center-of-mass energies up to 920 GeV using data from HERA I (1994–2000) and HERA II (2002–2007).
• Application of effective field theory to model contact interactions via the Lagrangian term $\mathcal{L}_{CI} = \sum_{i,j=L,R} \eta^{eq}_{ij}(\bar{e}_i\gamma^\mu e_i)(\bar{q}_j\gamma_\mu q_j)$.
• Use of CTEQ5D and CTEQ6D parton distribution functions to normalize data to Standard Model expectations.
• Comparison of observed $Q^2$ distributions with SM predictions to extract 95% confidence level (CL) exclusion limits.
• Incorporation of longitudinal electron/positron beam polarization (30–40%) to enhance sensitivity to chiral structures.
• Use of the classical form factor approximation to constrain the mean-square radius of the quark's electroweak charge.

Experimental results

Research questions

• RQ1 What are the limits on the compositeness scale $\Lambda$ for $eeqq$ contact interactions in $e^\pm p$ scattering?
• RQ2 How do the data constrain the effective Planck mass scale $M_S$ in models with large extra dimensions?
• RQ3 What are the upper bounds on the effective quark charge radius from deviations in the $Q^2$ distribution?
• RQ4 What are the 95% CL limits on the ratio of leptoquark mass to Yukawa coupling for different leptoquark types?
• RQ5 How do polarized beams improve sensitivity to chiral structures in contact interaction models?

Key findings

• For general $eeqq$ contact interaction models, the compositeness scale $\Lambda$ is excluded up to 8.0 TeV at 95% CL, based on ZEUS data from 1994–2006.
• Limits on the effective Planck mass scale $M_S$ in large extra dimension models exclude values below 0.90 TeV (ZEUS, negative coupling) and 0.88 TeV (ZEUS, positive coupling) at 95% CL.
• The H1 collaboration excludes quark charge radii larger than $0.74 \cdot 10^{-16}$ cm at 95% CL using combined HERA I and II data.
• ZEUS sets limits on the ratio of leptoquark mass to Yukawa coupling between 0.29 and 2.08 TeV, depending on the leptoquark type and mass.
• The H1 collaboration reports compositeness scale limits ranging from 1.6 to 5.5 TeV for HERA I data alone, consistent with the broader analysis.
• No significant deviations from Standard Model predictions were observed in any of the analyzed channels or models.