[Paper Review] Probing New Physics with $μ^+ μ^- o bs$ at a Muon Collider

Wolfgang Altmannshofer, Sri Aditya Gadam+1 more|arXiv (Cornell University)|Jun 26, 2023

Particle physics theoretical and experimental studiesPhysics and Astronomy3 citations

TL;DR

This paper proposes using a high-energy muon collider to probe new physics via the process $\mu^+\mu^- \to bs$, analyzing all dimension-6 four-fermion contact interactions. It demonstrates that the forward-backward asymmetry of the b-jet can diagnose the chirality of new physics, and a 10 TeV muon collider can probe new physics scales up to 100 TeV even in the absence of a signal, with beam polarization enhancing sensitivity.

ABSTRACT

We show that bottom-strange production at a high-energy muon collider, $μ^+ μ^- o b s$, is a sensitive probe of new physics. We consider the full set of four-fermion contact interactions that contribute to this process at dimension 6, and discuss the complementarity of a muon collider and of the study of rare $B$ meson decays that also probe said new physics. If a signal were to be found at a muon collider, the forward-backward asymmetry of the $b$-jet provides diagnostics about the underlying chirality structure of the new physics. In the absence of a signal at a center of mass energy of $10$~TeV, $μ^+ μ^- o b s$ can indirectly probe new physics at scales close to $100$~TeV. We also discuss the impact that beam polarization has on the muon collider sensitivity performance.

Motivation & Objective

• To investigate the potential of a muon collider to probe new physics in $\mu^+\mu^- \to bs$ processes.
• To analyze the full set of dimension-6 four-fermion contact interactions contributing to $\mu^+\mu^- \to bs$.
• To compare the sensitivity of a muon collider to rare $B$ meson decays in probing lepton-flavor universal and muon-specific new physics.
• To assess the impact of beam polarization on sensitivity and background suppression.
• To estimate the reach of a 10 TeV muon collider in probing new physics scales up to 100 TeV in the absence of a signal.

Proposed method

• The study employs the effective Hamiltonian framework with dimension-6 four-fermion operators to describe new physics contributions to $\mu^+\mu^- \to bs$.
• It includes all leading-order four-fermion contact interactions involving $b$, $s$, and $\mu^+\mu^-$, including vector, scalar, and tensor currents with various chiralities.
• The analysis accounts for QCD and electroweak renormalization group running of Wilson coefficients from the high scale $\sqrt{s} = 10$ TeV down to the $b$-quark mass scale.
• It evaluates the differential cross-section for $\mu^+\mu^- \to bs$ using the full set of operators and includes the effects of polarized muon beams.
• Backgrounds are modeled as misidentified di-jets and irreducible SM processes, with a focus on distinguishing signal from background using kinematic and angular distributions.
• Sensitivity projections are computed using signal significance and expected limits, comparing muon collider reach to that of rare $B$ decays at the LHC.

Experimental results

Research questions

• RQ1 Can a muon collider probe new physics scales beyond the reach of the LHC through $\mu^+\mu^- \to bs$ processes?
• RQ2 How does the forward-backward asymmetry of the b-jet help identify the chirality structure of new physics couplings?
• RQ3 What is the sensitivity of a 10 TeV muon collider to new physics in the absence of a signal in $\mu^+\mu^- \to bs$?
• RQ4 How does beam polarization enhance the discovery potential and background discrimination at a muon collider?
• RQ5 How do the constraints from a muon collider compare to those from rare $B$ decays, especially after the latest LHCb updates on $R_K$ and $R_{K^*}$?

Key findings

• The forward-backward asymmetry of the b-jet provides a direct diagnostic tool to determine the chirality structure of new physics couplings in $\mu^+\mu^- \to bs$.
• In the absence of a signal at $\sqrt{s} = 10$ TeV, the muon collider can probe new physics scales up to approximately 100 TeV.
• Beam polarization significantly improves sensitivity, enhancing the ability to distinguish signal from background and to probe specific new physics models.
• The muon collider’s sensitivity to lepton-flavor universal new physics is competitive with and complementary to rare $B$ decay measurements.
• After the latest LHCb updates on $R_K$ and $R_{K^*}$, the global fit still supports a consistent explanation of $b\to s\mu\mu$ anomalies via a lepton-flavor universal 4-fermion contact interaction.
• The renormalization group running of Wilson coefficients induces small but non-negligible shifts (5–10%) between high- and low-energy scales, which are included for accuracy.