[Paper Review] Two-fermion and Two-photon Final States at LEP2 and Search for Extra Dimensions

D. Bourilkov|ArXiv.org|Mar 22, 2001

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper presents a comprehensive analysis of fermion-pair and photon-pair final states in e⁺e⁻ collisions at LEP2, using the full data set from ALEPH, DELPHI, L3, and OPAL. It sets the most stringent model-independent limits to date on new physics, including contact interactions (Λ > 10–20 TeV) and low-scale gravity (Mₛ⁺ > 1.13 TeV, Mₛ⁻ > 1.39 TeV), with additional constraints on TeV-scale strings and D-brane models.

ABSTRACT

For the first time the experiments ALEPH, DELPHI, L3 and OPAL have presented preliminary results for fermion-pair and photon-pair production in e+e- collisions on the full LEP2 data set. The details of the experimental measurements and results from their LEP-wide combination are presented. No statistically significant deviations from the Standard Model expectations are observed and lower limits, some obtained in dedicated analyses, for new physics phenomena at 95 % confidence level are derived. The scales of contact interactions are constrained to lie above 10-20 TeV, depending on the helicity structure. The Standard Model has thus been tested at LEP2 down to distances 10^{-19} - 10^{-20} m. This has many implications, one example being the interpretation of the new result on the anomalous magnetic moment of the muon as coming from muon substructure. For gamma-gamma final states the QED cut-offs are Lambda_+ > 0.44 TeV and Lambda_- > 0.37 TeV. In a combined analysis, using e+e- and gamma-gamma final states, the most stringent lower limits to date, M_s^+ > 1.13 TeV (lambda = +1) and M_s^- > 1.39 TeV (lambda = -1), on the low gravity effective Planck scale are set. Constrains on the scales of string models like TeV strings and D-branes are derived. In the last case the lower limit is 1.5 - 4 TeV, depending on the coupling strength.

Motivation & Objective

• To analyze the full LEP2 data set (130–209 GeV) for deviations from Standard Model predictions in fermion-pair and photon-pair final states.
• To set stringent model-independent limits on new physics phenomena such as contact interactions and low-scale gravity.
• To improve constraints on TeV-scale string models and D-brane scenarios using combined e⁺e⁻ and γγ final state data.
• To test the Standard Model at sub-10⁻¹⁹ m distances, probing physics at the TeV scale.
• To provide updated limits on the effective Planck scale and coupling scales in models with extra dimensions or compositeness.

Proposed method

• Combines preliminary results from ALEPH, DELPHI, L3, and OPAL across the full LEP2 energy range (130–209 GeV), with statistical uncertainty reduced by a factor of two via combination.
• Uses differential cross sections and forward-backward asymmetries in e⁺e⁻ → f̄f and e⁺e⁻ → γγ processes to probe new physics via interference with Standard Model amplitudes.
• Applies the contact interaction framework with effective Lagrangians of dimension six (fermions) and eight (photons), parametrized by energy scale Λ or Mₛ.
• Performs a combined analysis of e⁺e⁻ and γγ final states to derive the most stringent limits on low-scale gravity (LSG) and D-brane models.
• Employs dedicated fits to total cross sections and forward-backward asymmetries in Bhabha scattering (e⁺e⁻ → e⁺e⁻) for D-brane model constraints.
• Uses 95% confidence level (CL) upper limits on signal excesses to derive lower bounds on new physics scales.

Experimental results

Research questions

• RQ1 Are there statistically significant deviations from Standard Model predictions in e⁺e⁻ → fermion pairs and γγ final states at LEP2 energies?
• RQ2 What are the most stringent lower limits on the scale of contact interactions in fermion and photon final states?
• RQ3 How do combined e⁺e⁻ and γγ final state analyses improve constraints on low-scale gravity models?
• RQ4 What are the updated limits on TeV-scale string models and D-brane scenarios from LEP2 data?
• RQ5 To what extent do the LEP2 results constrain models of compositeness or substructure in fermions, such as muon substructure?

Key findings

• No statistically significant deviations from the Standard Model are observed in the full LEP2 data set for fermion-pair or photon-pair final states.
• Contact interaction scales are constrained to Λ > 10–20 TeV, depending on helicity structure, testing the Standard Model down to distances of 10⁻¹⁹–10⁻²⁰ m.
• The most stringent lower limits to date on the low-scale gravity (LSG) effective Planck scale are Mₛ⁺ > 1.13 TeV (λ = +1) and Mₛ⁻ > 1.39 TeV (λ = –1), derived from combined e⁺e⁻ and γγ final states.
• Limits on TeV strings are set at M_S > 0.42 TeV from γγ final states, with the LEP2 combined analysis yielding M_S > 0.63 TeV for e⁺e⁻ final states.
• For D-brane models, the combined analysis yields a lower limit of 1.5–4 TeV on the string scale, depending on coupling strength, with improved constraints from inclusion of the electron channel and 2000 data.
• The results imply that the anomalous magnetic moment of the muon cannot be explained by muon substructure at scales probed by LEP2.