CMS-BPH-25-001

CMS-BPH-25-001 ; ATL-PHYS-PUB-2025-020 ; LHCb-PUB-2025-008 ; Belle II Preprint 2025-007 ; KEK Preprint 2025-5
Projections for Key Measurements in Heavy Flavour Physics
ATLAS, LHCb, Belle-2 and CMS Collaborations
31 March 2025
Submitted to EPPSU
Abstract: Precision studies of flavour-changing processes involving quarks and leptons provide a number of ways to improve knowledge of the Standard Model and search for physics beyond it. There are excellent short- and mid-term prospects for significantly improved measurements in heavy flavour physics (involving b and c hadrons and τ leptons), with upgrades in progress or planned for the ATLAS, CMS and LHCb experiments exploiting proton-proton collisions at CERN’s Large Hadron Collider, and for the Belle II experiment operating with electron-positron collisions from the SuperKEKB accelerator in KEK. The expected sensitivities that can be achieved from these experiments for a number of key observables are presented, highlighting the complementarity of the different experiments and showing how the precision will improve with time. This international programme in heavy flavour physics will result in unprecedented capability to probe this sector of the Standard Model and, potentially, observe imprints of physics at higher energy scales than can be accessed directly.
Links: CDS record ; CADI line (restricted) ;

Summary	References	CMS Publications

Projections for Key Measurements in Heavy Flavour Physics: File submitted to the European Strategy Update.

Tables
png pdf	Table 1: Measured uncertainties on selected key observables from the most recent analyses performed by the different experiments. Uncertainties quoted as % are relative to the measured or SM central value. Upper limits, indicated by $<$ , are at 90% confidence level, except where indicated by $^{*}$ in which case the limits are at 95% confidence level. The size of the data samples used varies between the analyses as indicated by luminosity range in the top row. Some Belle II results include analysis of Belle data.
png pdf	Table 2: Expected sensitivity to the selected key observables from LHCb and Belle II with data samples that will be recorded by the early 2030s. Uncertainties quoted as % are relative to the measured or SM central value. Upper limits, indicated by $<$ , are at 90% confidence level assuming negligible signal yield.
png pdf	Table 3: Sensitivity to the selected key observables from the different experiments with their final data samples, recorded by the early 2040s. Uncertainties quoted as % are relative to the measured or SM central value. Upper limits, indicated by $<$ , are at 90% confidence level assuming negligible signal yield.

Summary

Heavy flavour physics remains a crucial component of the international particle physics programme. The ATLAS, CMS, LHCb and Belle II experiments, and their planned upgrades, have complementary strengths but will also compete to achieve the best precision for certain observables, allowing for important consistency checks and even better precision in world average combinations. Collectively, these experiments have the capability to significantly advance the precision on essentially all the key observables in

b

$b$ ,

c

$c$ and

τ

$\tau$ physics. The expected improvement in accuracy that will be achieved by the completion of these experiments in the early 2040s is typically around an order of magnitude from what is available today. Nevertheless, this is only a partial assessment of the actual physics reach: experimental progress and further analysis optimization could in many cases push the physics reach beyond the values projected here, and more measurements than have been presented will be performed. In particular, measurements of many suppressed processes which have not yet been studied and more detailed investigations of differential distributions will be possible with much larger samples that will become available. The precision that can be achieved with these experiments provides exciting and unprecedented capability to probe the flavour sector of the Standard Model. The results can be used to constrain or rule out specific models beyond the SM and to assess potential contributions from a wide range of operators, associated with flavour-changing interactions, in effective field theory approaches. The vastly improved knowledge that will be obtained in this sector could allow the observation of imprints of physics at higher energy scales than can be accessed directly.

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