Search for Compositeness, Leptoquarks and Large Extra Dimensions in eq Contact Interactions at HERA

TL;DR

This study analyzes high-Q^2 neutral-current e+p scattering at HERA with the H1 detector to search for indirect signatures of new physics via four-fermion contact interactions. Using a SM-based baseline and a chi-squared framework across multiple parton-distribution sets, the analysis constrains a range of new-physics scenarios, including fermion compositeness, quark form factors, leptoquarks, and gravity in large extra dimensions. The results yield 95% CL limits on compositeness scales (1.3–5.5 TeV), quark radii (R_q < 1.7×10^-16 cm), leptoquark mass/coupling ratios (M_LQ/λ ~ TeV), and the gravity scale M_S (~0.5–0.7 TeV, depending on coupling), with no significant deviation from the Standard Model. These findings strengthen existing bounds and provide complementary constraints to LEP and Tevatron searches, extending TeV-scale limits from deep inelastic scattering at HERA.

Abstract

The reaction e+p --> e+X is studied with the H1 detector at Hera. The data cover momentum transfers Q^2 between 200 GeV^2 and 30,000 GeV^2 and correspond to an integrated luminosity of 35.6 pb^(-1). The differential cross section dsigma/dQ^2 is compared to the Standard Model expectation for neutral current scattering and analysed to search for bar(e)e bar(q)q contact interactions. No evidence for new phenomena is observed. The results are used to set limits on scales within models of electron--quark compositeness, quark form factors and the exchange of virtual heavy leptoquarks. A search for gravitational effects mediated through the exchange of virtual gravitons which propagate into large extra dimensions is presented.

Figures (6)

• Figure 1: Differential NC cross section ${\rm d}\sigma (e^+p \rightarrow e^+X) / {\rm d}Q^2$. H1 data ($\bullet$) are compared with the Standard Model expectation (---) using CTEQ5D parton distributions. The errors represent statistics and uncorrelated experimental systematics. The overall normalisation uncertainty is 1.5%.
• Figure 2: Distributions of $\chi^2- \chi^2_{min}$ versus $\epsilon/\Lambda^2$ from fits to various compositeness models using CTEQ5D, MRST 99 and GRV 94 parton distributions and including full error propagation.
• Figure 3: Analysis results of the parameter $\epsilon/\Lambda^2$ for various compositeness models. The thick hori-zontal bars indicate the limits on $\Lambda^+$ and $\Lambda^-$ including parton distribution uncertainties; values outside these regions are excluded at 95% confidence level. The corresponding thin horizontal bars show the fit results for $\epsilon/\Lambda^2$ using CTEQ5D parton distributions; inner and outer error bars represent one and two standard deviations respectively. The scale for $\epsilon\,\Lambda$ is shown for convenience.
• Figure 4: NC cross section ${\rm d}\sigma / {\rm d}Q^2$ normalised to the Standard Model expectation using CTEQ5D parton distributions. H1 data ($\bullet$) are compared with fits to the VV model corresponding to 95% CL exclusion limits of $\Lambda^+$$(- -)$ and $\Lambda^-$ (---). The errors represent statistics and uncorrelated experimental systematics. The overall normalisation uncertainty is 1.5%.
• Figure 5: NC cross section ${\rm d}\sigma / {\rm d}Q^2$ normalised to the Standard Model expectation using CTEQ5D parton distributions. H1 data ($\bullet$) are compared with 95% CL exclusion limits of the leptoquarks $S^R_{1/2}$ (---) and $V^R_{1/2}$$(- -)$. The errors represent statistics and uncorrelated experimental systematics. The overall normalisation uncertainty is 1.5%.