[Paper Review] Super KEKB / Belle II Project

B. Golob|arXiv (Cornell University)|Jun 22, 2010

Particle Accelerators and Free-Electron Lasers3 citations

TL;DR

The paper presents the Super KEKB/ Belle II project, an upgraded $e^+e^-$ collider and detector system designed to achieve 8×10³⁴ cm⁻²s⁻¹ luminosity and collect 50 ab⁻¹ of data by 2020. The upgrade enables precision measurements of rare B-meson decays and $CP$ violation, with sensitivity to new physics beyond the Standard Model, including charged Higgs bosons and supersymmetric models, via improved vertexing, particle identification, and electromagnetic calorimetry.

ABSTRACT

We present the status of the KEKB collider and the Belle detector upgrade, along with several examples of physics measurements to be performed with Belle II at Super KEKB.

Motivation & Objective

• To address persistent hints of new physics in B-meson decays by significantly increasing the luminosity and data-taking capability of the B-factory.
• To enable precision measurements of rare decays and $CP$ violation observables sensitive to physics beyond the Standard Model.
• To upgrade the Belle detector and KEKB accelerator to handle 10–20× higher background rates and achieve sub-100 nm beam spot sizes.
• To perform complementary, model-independent tests of new physics through correlations of multiple observables, such as $CP$ asymmetries in $B^0 \to \phi K_S$ and $B^0 \to K^{*0} \gamma$.
• To achieve a sensitivity of better than 0.1 in the ratio $\mathcal{B}(B^+ \to \tau^+ \nu)/\mathcal{B}^{\text{SM}}$ with 50 ab⁻¹ of data, covering a large region of the $(m_{H^\pm}, \tan\beta)$ plane.

Proposed method

• Upgrade the KEKB collider to Super KEKB with reduced beam emittance and increased beam currents, achieving a luminosity of $8 \times 10^{34}$ cm⁻²s⁻¹ via shorter beta functions ($\beta^* \sim 0.27$ mm) and crab-waist collimation.
• Install new final focus quadrupoles, a new beam pipe, and a redesigned interaction region to reduce beam size and improve dynamic aperture.
• Implement a higher crossing angle (83 mrad) and slightly reduced beam energy asymmetry to mitigate Touschek losses and extend beam lifetime.
• Replace the Belle detector’s vertexing system with two layers of DEPFET pixel sensors for improved tracking and vertex reconstruction in high-rate environments.
• Upgrade the particle identification system with a new time-of-flight system and upgraded aerogel Cherenkov counters to enhance $K/\pi$ separation at high momentum.
• Replace the electromagnetic calorimeter electronics to enable amplitude-time measurements, reducing off-time background by a factor of 7, and consider replacing Tl-doped CsI crystals with radiation-hard pure CsI.

Experimental results

Research questions

• RQ1 Can the Belle II experiment achieve a five-sigma discovery reach for the charged Higgs boson in $B^+ \to \tau^+ \nu$ decays with 50 ab⁻¹ of data?
• RQ2 How do correlations between indirect $CP$ violation in $B^0 \to \phi K_S$ and $B^0 \to K^{*0} \gamma$ decays help distinguish between competing new physics models like mSUGRA and SUSY SU(5))?
• RQ3 To what extent can the lepton forward-backward asymmetry in $B \to K^* \ell^+ \ell^-$ decays resolve discrepancies between the Standard Model and current measurements, particularly in the $C_7$ Wilson coefficient?
• RQ4 What is the expected sensitivity of Belle II to new physics in rare $B$ decays, given the improved detector performance and $10^3$-fold increase in luminosity?
• RQ5 Can the upgraded Belle II detector maintain high photon detection efficiency (within 5–10% of current levels) while suppressing background from beam-gas and off-time interactions?

Key findings

• The Super KEKB collider is expected to achieve a peak luminosity of $8 \times 10^{34}$ cm⁻²s⁻¹, enabling the collection of 50 ab⁻¹ of data by 2020, with 5 ab⁻¹ achievable within two years of operation.
• The measurement of the lepton forward-backward asymmetry in $B \to K^* \ell^+ \ell^-$ decays will achieve sufficient precision to distinguish between the Standard Model and models with a reversed-sign $C_7$ Wilson coefficient, such as certain supersymmetric models.
• With 50 ab⁻¹ of data, the sensitivity to the ratio $\mathcal{B}(B^+ \to \tau^+ \nu)/\mathcal{B}^{\text{SM}}$ will reach below 0.1, enabling a five-sigma discovery of the charged Higgs boson in a large region of the $(m_{H^\pm}, \tan\beta)$ plane.
• The correlation between $CP$ violation in $B^0 \to \phi K_S$ and $B^0 \to K^{*0} \gamma$ decays can distinguish between mSUGRA and SUSY SU(5)) models with only 5 ab⁻¹ of data, despite similar mass spectra.
• The upgraded electromagnetic calorimeter will suppress off-time beam background by a factor of 7 through amplitude-time measurements, and maintain photon detection efficiency within 5–10% of current levels despite higher background rates.
• The first-level trigger rate is expected to increase to ~20 kHz, but the overall detector hermiticity and performance are expected to improve due to new electronics and radiation-hard components.