[Paper Review] Search for excited fermions in ep collisions at HERA

Ainas Weber|ArXiv.org|Jul 15, 2002

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a search for excited fermions (e* and ν*) in ep collisions at HERA using 120 pb⁻¹ (H1) and 82 pb⁻¹ (ZEUS) of data. No evidence for excited fermions was found, and the study sets the most stringent 95% confidence level limits on the cross-section times branching ratio and the coupling-to-compositeness scale f/Λ, extending to higher masses than previous experiments, particularly for ν* in the high-mass region.

ABSTRACT

Heavy excited electrons and neutrinos have been sought by the H1 and ZEUS experiments at HERA. For the e* (nu*) searches, 120 pb^-1 (16 pb^-1) of ep collision data have been analysed. No evidence for any excited lepton has been found, and limits on the characteristic couplings have been derived.

Motivation & Objective

• To search for excited electrons (e*) and neutrinos (ν*) produced in ep collisions at HERA’s center-of-mass energy of 318 GeV.
• To test the hypothesis that fermions are composite by searching for resonant production of excited states via the Hagiwara phenomenological compositeness model.
• To set improved experimental limits on the coupling scale f/Λ and the product of cross-section and branching ratio (σ×BR) for excited fermions.
• To extend the sensitivity to high masses, especially for ν*, where no prior indirect limits existed from LEP.
• To derive model-independent limits on f/Λ for ν* by varying the ratio f′/f over a wide range.

Proposed method

• Analyzed 120 pb⁻¹ (H1) and 82 pb⁻¹ (ZEUS) of ep collision data from HERA, focusing on final states from e* and ν* decays: e* → eγ, e* → νW → νq̄q′, e* → eZ → eq̄q; ν* → νγ, ν* → eW → eq̄q′, ν* → νZ → νq̄q.
• Used Monte Carlo simulations based on the Hagiwara et al. compositeness model to generate signal events for e* and ν*.
• Modelled Standard Model backgrounds including neutral-current and charged-current deep inelastic scattering (NC/DIS, CC/DIS), photoproduction (PHP), and QED-Compton processes.
• Applied distinct event selection strategies: H1 used jet reconstruction and electron vetoes; ZEUS used global event variables like transverse hadronic energy and hadronic mass.
• Calculated 95% confidence level upper limits on σ×BR and f/Λ, with ZEUS using only e* → eγ for f/Λ limits and H1 combining all channels under the f = f′ assumption.
• For ν*, derived model-independent f/Λ limits by scanning f′/f over −5 to 5 and selecting the most conservative (worst-case) limit at each mass point.

Experimental results

Research questions

• RQ1 What are the most stringent limits on the production cross-section times branching ratio (σ×BR) for excited electrons and neutrinos in ep collisions at HERA?
• RQ2 How do the coupling-to-compositeness scale f/Λ limits for e* and ν* compare to previous experiments, especially at high masses?
• RQ3 Can model-independent f/Λ limits be derived for ν* by varying the ratio f′/f over a broad range?
• RQ4 Why is the e⁻p data set more sensitive to ν* production than e⁺p, and how does this affect the limits?
• RQ5 What are the dominant backgrounds in each decay channel, and how were they effectively suppressed or estimated?

Key findings

• No significant excess of events over the expected Standard Model background was observed in any of the six decay channels for e* or ν*.
• The H1 and ZEUS experiments set the most stringent 95% confidence level limits on σ×BR for e* and ν* to date, extending to higher masses than previous LEP and HERA results.
• For e*, the H1 limit on f/Λ, derived from combining all three decay channels under the f = f′ assumption, is more stringent than previous limits and extends to higher masses.
• For ν*, the H1 experiment derived model-independent f/Λ limits by scanning f′/f over the range −5 to 5, selecting the worst-case limit at each mass point, resulting in a conservative and robust constraint.
• The ZEUS limit on f/Λ for e* → eγ, based on 82 pb⁻¹ of data, is competitive with and comparable to the LEP limits, but with improved sensitivity in the high-mass region.
• For ν*, with no indirect limits from LEP, the H1 and ZEUS results provide the most stringent direct limits in the high-mass region, particularly for the f = −f′ case enabling ν* → νγ decay.