[Paper Review] Physics case for an LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era

LHCb Collaboration, B. Adeva+832 more|arXiv (Cornell University)|Jan 1, 2018

Particle physics theoretical and experimental studiesPhysics and Astronomy525 references234 citations

TL;DR

This paper proposes LHCb Upgrade II, a major detector enhancement for the High-Luminosity LHC (HL-LHC), enabling precision measurements in flavour physics and new physics searches. Operating at 10× higher luminosity (2×10³⁴ cm⁻²s⁻¹), it will probe rare B-meson decays, test minimal flavour violation via μ+μ− modes, and achieve 10⁻⁵ sensitivity to charm CP violation—extending new physics reach by nearly a factor of two compared to pre-HL-LHC era experiments.

ABSTRACT

The LHCb Upgrade II will fully exploit the flavour-physics opportunities of the HL-LHC, and study additional physics topics that take advantage of the forward acceptance of the LHCb spectrometer. The LHCb Upgrade I will begin operation in 2020. Consolidation will occur, and modest enhancements of the Upgrade I detector will be installed, in Long Shutdown 3 of the LHC (2025) and these are discussed here. The main Upgrade II detector will be installed in long shutdown 4 of the LHC (2030) and will build on the strengths of the current LHCb experiment and the Upgrade I. It will operate at a luminosity up to $ 2 imes 10^{34} m cm^{-2}s^{-1}$, ten times that of the Upgrade I detector. New detector components will improve the intrinsic performance of the experiment in certain key areas. An Expression Of Interest proposing Upgrade II was submitted in February 2017. The physics case for the Upgrade II is presented here in more depth. $CP$-violating phases will be measured with precisions unattainable at any other envisaged facility. The experiment will probe $b o s \ell^+\ell^-$ and $b o d \ell^+\ell^-$ transitions in both muon and electron decays in modes not accessible at Upgrade I. Minimal flavour violation will be tested with a precision measurement of the ratio of $B(B^0 oμ^+μ^-)/B(B_s^0 o μ^+μ^-)$. Probing charm $CP$ violation at the $10^{-5}$ level may result in its long sought discovery. Major advances in hadron spectroscopy will be possible, which will be powerful probes of low energy QCD. Upgrade II potentially will have the highest sensitivity of all the LHC experiments on the Higgs to charm-quark couplings. Generically, the new physics mass scale probed, for fixed couplings, will almost double compared with the pre-HL-LHC era; this extended reach for flavour physics is similar to that which would be achieved by the HE-LHC proposal for the energy frontier.

Motivation & Objective

• To extend the reach of flavour physics at the HL-LHC by enabling precision measurements of rare B-meson decays, particularly b → sℓ⁺ℓ⁻ and b → dℓ⁺ℓ⁻ transitions.
• To test the Standard Model's prediction of minimal flavour violation through a precision measurement of the ratio B(B⁰ → μ⁺μ⁻)/B(Bₛ⁰ → μ⁺μ⁻).
• To search for CP violation in charm decays at the 10⁻⁵ level, potentially leading to its long-sought discovery.
• To enhance hadron spectroscopy and probe low-energy QCD with improved detector performance.
• To achieve the highest sensitivity among LHC experiments for Higgs boson couplings to charm quarks.

Proposed method

• The Upgrade II detector will operate at a luminosity of 2×10³⁴ cm⁻²s⁻¹, ten times higher than Upgrade I, enabling increased statistics and improved precision.
• New detector components will be installed in Long Shutdown 4 (2030), including full instrumentation of the decay volume with tracking layers on all six sides to suppress backgrounds and improve vertex reconstruction.
• The design incorporates a compact, low-background environment to enable detection of long-lived particles (LLPs), with potential for timing layers to reconstruct LLP momentum and mass via time-of-flight.
• Theoretical uncertainties in form factors are expected to reduce by a factor of 0.25 by the Upgrade-II era, improving the precision of observables like RK and RK*.
• New physics reach is determined by translating experimental uncertainties in angular observables and RK into constraints on Wilson coefficients, using the flavio software for effective field theory analysis.
• Exclusion limits on new physics scales (ΛNP) are computed at 90% confidence level using the relation ΛNP ∝ 1/σ(C′), with κ values depending on the New Physics model (e.g., κ = 1 for tree-level, κ = VtbV*ts/(16π²) for loop-level MFV).

Experimental results

Research questions

• RQ1 Can LHCb Upgrade II achieve 10⁻⁵ sensitivity to CP violation in charm decays, potentially leading to its discovery?
• RQ2 What is the projected precision on the ratio B(B⁰ → μ⁺μ⁻)/B(Bₛ⁰ → μ⁺μ⁻), and how does it test minimal flavour violation?
• RQ3 How much will the new physics reach for flavour-sensitive processes increase compared to the pre-HL-LHC era?
• RQ4 What is the expected sensitivity of Upgrade II to Higgs boson couplings to charm quarks compared to other LHC experiments?
• RQ5 To what extent can the detector’s full-volume tracking and timing capabilities extend the reach for long-lived particles and exotic Higgs decays?

Key findings

• LHCb Upgrade II will probe new physics at a scale nearly double that accessible in the pre-HL-LHC era, for fixed couplings, matching the reach expected from the HE-LHC energy frontier proposal.
• The experiment will measure CP-violating phases with precisions unattainable at any other current or planned facility.
• The ratio B(B⁰ → μ⁺μ⁻)/B(Bₛ⁰ → μ⁺μ⁻) will be measured with high precision, providing a stringent test of minimal flavour violation.
• The detector’s full-volume tracking and timing capabilities will allow reconstruction of LLP boost and mass with sub-50 ps timing resolution, extending reach to light, long-lived particles.
• Theoretical uncertainties on form factors are expected to be reduced by 75% by the Upgrade-II era, significantly improving the precision of observables like RK and RK*.
• The CODEX-b extension, integrated into the LHCb cavern, will extend the reach for long-lived particles to mϕ < 1 GeV and cτ up to 10 m, outperforming other experiments in this region.