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

TL;DR

This paper articulates a comprehensive physics case for LHCb Upgrade II to exploit HL-LHC luminosities with a forward-acceptance detector. It outlines detector enhancements (VELO timing, TORCH, MIghTy tracker, upgraded ECAL/μ systems) and a software-based trigger to enable precise time-dependent CP violation, lepton-universality tests, charm CPV, rare decays, and heavy-flavour spectroscopy across B, charm, and strange sectors. Anticipated measurements include φ_s and φ_d at the few 10^-3 rad and sub-degree levels, γ at ~0.35°, and numerous LFU and FCNC observables with sensitivity to NP scales well beyond direct searches. The document also emphasizes forward/high-p_T physics, exotic hadron spectroscopy, and dark-sector searches, showcasing Upgrade II as a pivotal step for probing beyond the Standard Model while complementing Belle II and HE-LHC initiatives. Overall, Upgrade II promises transformative precision and breadth in flavour physics, with broad implications for NP constraints, CKM tests, and hadron spectroscopy.

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 \times 10^{34} \rm 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 $b\to s \ell^+\ell^-$ and $b\to d \ell^+\ell^-$ 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 $B(B^0\toμ^+μ^-)/B(B_s^0\to μ^+μ^-)$. 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.

Figures (51)

• Figure 1: Timeline of accelerator and experiment operations over the decade 2021 to 2031. The periods of operations of the LHC and HL-LHC are indicated and the long shutdowns (LS). The LHCb operational periods are shown with gaps where the detector consolidation and upgrades discussed in this document occur. The running period of Belle II, the other major international flavour-physics facility, is also shown.
• Figure 2: Luminosity projections for the original LHCb, Upgrade I, and Upgrade II experiments as a function of time. The red points and the left scale indicate the anticipated instantaneous luminosity during each period, with the blue line and right scale indicating the integrated luminosity accumulated.
• Figure 3: Schematic side-view of the Upgrade II detector.
• Figure 4: Example event containing a $B^0 \rightarrow\xspace \pi^+ \pi^-$ candidate under Upgrade II conditions, illustrating the PV association challenge. Each PV is drawn as a 2D Gaussian distribution with the appropriate values and uncertainties for both spatial ($x$-axis) and temporal ($y$-axis) metrics used to associate the $B$ meson to a single origin PV. In this case, adding the temporal information allows the correct PV ['A', closest to $(0,0)$] to be identified where the spatial information alone would lead to the wrong choice ('B').
• Figure 5: Fraction of $B^0 \rightarrow\xspace \pi^+ \pi^-$ candidates which are associated with an incorrect primary vertex in Upgrade II conditions ($\mathcal{L}\xspace = 1.5 \times 10^{34}\rm \, cm^{-2}s^{-1}\xspace$), under a range of time precisions for the outer radial region ($20 < r < 35 \mathrm{ \,mm}\xspace$) of the VELO as described in the text. These performance numbers should be compared with the 20% PV mis-association fraction corresponding to a detector with no time information.