Quark mixing from muon collider neutrinos

TL;DR

This paper explores using the high-energy neutrino flux from a muon collider to perform fixed-target DIS and SIDIS measurements with the aim of precisely determining the CKM quark-mixing matrix. By leveraging inclusive DIS and flavor-tagged SIDIS across multiple observables and building a joint CKM+PDF/FF fit, the study quantifies the irreducible parametric uncertainties from parton distributions and fragmentation functions as a floor to CKM sensitivity, while demonstrating substantial potential improvements over current PDG uncertainties. The approach relies on a dense set of cross-section ratios in carefully chosen kinematic bins, guided by perturbative QCD in the DIS regime and augmented by heavy-flavor tagging to access $V_{ud}$, $V_{us}$, $V_{cd}$, $V_{cs}$, $V_{cb}$ and $V_{ub}$. A baseline 10 TeV νMuC scenario with high luminosity yields dramatic gains, and a 3 TeV staging shows similarly strong CKM sensitivity, supporting a staged, parasitic neutrino program as a long-term extension of muon-collider physics. The work also highlights the byproduct potential to constrain PDFs and FFs with unprecedented precision, while acknowledging the need for future NNLO analyses, nuclear corrections, and detector studies to translate these projections into a concrete experimental program.

Abstract

A high energy muon collider naturally produces a collimated beam of neutrinos for a fixed-target experiment at a dedicated far-forward facility. The high intensity and energy of the beam makes it ideally suited for astonishingly precise measurements of neutrino scattering on nucleons in the deeply inelastic regime, enabling the determination of the Cabibbo--Kobayashi--Maskawa~(CKM) quark mixing matrix. We assess the floor to the attainable sensitivity set by irreducible sources of uncertainties from the imperfect knowledge of the parton distribution (PDF) and fragmentation functions, showing that a strong improvement is possible well above current standards. As a by-product, our analysis also outlines extraordinary perspectives for a combined determination of the PDF. The results demonstrate the potential of a parasitic neutrino experiment at the muon collider, motivating detailed future studies.

Figures (10)

• Figure 1: Schematic of the $\nu\text{MuC}$ experimental setup.
• Figure 2: Left: Energy distribution of the interacting neutrinos, compared with the neutrino beam energy spectrum. Right: Number of events distribution (with high luminosity (\ref{['eq:HighL']})) in $x$–$Q^2$ bins.
• Figure 3: Fragmentation functions of $B$ (top left), $D^{*+}$ (top right) and $K^-$ (bottom) mesons for for different partons, with 68% CL uncertainty bands. Quarks lighter than $b$ (top left) or all quarks except $b$, $\bar{b}$, and $c$ (top right) are indicated collectively as $q$. See the main text for details. The vertical dashed lines indicate the kinematic $z$ windows used in our analysis.
• Figure 4: 68% CL ellipses for CKM pairs, showing the PDG prior (red) and the baseline fit (blue). The bottom-right panel shows the correlation matrix of the complete fit and the ratio $\sigma/\sigma_{\rm{1p}}$ between the 1-parameter and the global precision.
• Figure 5: The gain---relative to current knowledge---in the CKM parameters precision as a function of the integrated luminosity at the 10 TeV $\nu\text{MuC}$ experiment.