[Paper Review] Report of Snowmass 2001 Working Group E2 : Electron-positron Colliders from the $ϕ$ to the Z

Z. Zhao, G. Eigen+41 more|arXiv (Cornell University)|Jan 30, 2002

Particle Accelerators and Free-Electron Lasers4 citations

TL;DR

This 2001 Snowmass report evaluates electron-positron colliders from the φ to Z energy range, assessing B and charm factories (KEKB/Belle, PEP-II/BaBar, CLEO-c), the φ factory DAΦNE, and future proposals like SuperKEK, SuperB, and a TESLA-based Z factory. It concludes that high-luminosity e⁺e⁻ colliders offer complementary precision in flavor physics, especially for rare B decays and CP violation, though competition with hadron colliders like LHCb is expected at high luminosities.

ABSTRACT

We report on the status and plans of experiments now running or proposed for electron-positron colliders at energies between the $ϕ$ and the Z. The $e^{+}e^{-}$ B and charm factories we considered were PEP-II/BABAR, KEKB/Belle, superKEK, SuperBABAR, and CESR-c/CLEO-c. We reviewed the programs at the $ϕ$ factory at Frascati and the proposed PEP-N facility at Stanford Linear Accelerator Center. We studied the prospects for B physics with a dedicated linear collider Z factory, associated with the TESLA high energy linear collider. In all cases, we compared the physics reach of these facilities with that of alternative experiments at hadron colliders or fixed target facilities.

Motivation & Objective

• Assess the physics potential of existing and proposed e⁺e⁻ colliders operating between φ and Z boson energies.
• Compare the physics reach of e⁺e⁻ colliders with hadron colliders (e.g., LHCb, BTeV) and fixed-target experiments for flavor physics and rare decays.
• Evaluate the feasibility and scientific impact of future high-luminosity e⁺e⁻ facilities, including SuperKEK, SuperB, and a dedicated Z factory.
• Determine the complementarity of e⁺e⁻ colliders to hadron colliders in probing the Standard Model's flavor sector and searching for new physics.
• Provide a roadmap for R&D in detector and machine design for future high-luminosity e⁺e⁻ facilities.

Proposed method

• Analyze the physics programs of existing facilities: DAΦNE (φ factory), PEP-II/BaBar, KEKB/Belle, and CESR-c/CLEO-c.
• Use luminosity scaling from B factories (e.g., BaBar) to project performance of upgraded facilities like SuperKEK and SuperB at 10³⁶ cm⁻²s⁻¹.
• Compare detector performance and background rates at high luminosity, assessing feasibility of detector upgrades.
• Model machine backgrounds, especially from continuous injection in SuperB, to evaluate tractability.
• Simulate physics reach for rare B decays, CP violation, and V_ub measurements using projected luminosity and detector efficiency.
• Assess the role of a future TESLA-based Z factory for precision B physics and comparison with hadron collider experiments.

Experimental results

Research questions

• RQ1 What is the projected physics reach of upgraded B factories (SuperKEK and SuperB) at 10³⁶ cm⁻²s⁻¹ luminosity for rare B decays and CP violation?
• RQ2 How does the physics potential of e⁺e⁻ colliders compare to that of hadron colliders (e.g., LHCb, BTeV) for measuring B physics observables?
• RQ3 What are the technical challenges and feasibility of operating a detector at 10³⁶ cm⁻²s⁻¹ luminosity, particularly for SuperB?
• RQ4 Can a dedicated Z factory at a linear collider provide complementary precision measurements to those from hadron colliders?
• RQ5 What is the role of e⁺e⁻ colliders in probing non-perturbative QCD, charm physics, and rare kaon decays compared to fixed-target experiments?

Key findings

• The φ factory DAΦNE at Frascati achieved 2.5×10³¹ cm⁻²s⁻¹ luminosity, approaching 5×10³² cm⁻²s⁻¹ design value, enabling meaningful physics with 200 pb⁻¹ integrated luminosity expected by end-2001.
• KLOE at DAΦNE aims to measure ε′/ε to 2×10⁻⁴ accuracy, requiring 5000 pb⁻¹, to probe direct CP violation, though theoretical uncertainties currently limit precision extraction.
• SuperKEK and SuperB are proposed to operate at 10³⁶ cm⁻²s⁻¹, offering a 300× luminosity increase over PEP-II/BaBar and KEKB/Belle, enabling high-statistics studies of rare B decays.
• At 10³⁵ cm⁻²s⁻¹, e⁺e⁻ B factories are no longer competitive with hadron colliders like LHCb/BTeV for most B physics observables, but remain superior for decays involving neutrinos.
• SuperB would be complementary to LHCb for rare B decays and competitive for π⁰ and γ final states, but limited in Bₛ and Λ_b physics due to poor time resolution and lack of resonance production.
• A dedicated Z factory at a future linear collider (e.g., TESLA) is proposed as a precision probe of the flavor sector, but requires further development and background modeling before full assessment.