[Paper Review] Probing New Physics with Isotope Shift Spectroscopy

Cédric Delaunay, Yotam Soreq|arXiv (Cornell University)|Feb 15, 2016

Advanced Frequency and Time Standards1 references17 citations

TL;DR

This paper proposes using precision isotope shift (IS) measurements in optical atomic clocks—specifically in singly ionized ytterbium—to probe physics beyond the Standard Model. By analyzing King plots of IS data, it demonstrates sensitivity to new physics at the TeV scale, including constraints on 750 GeV scalar resonances, sub-GeV forces, and Z⁰ couplings, with reach surpassing LHC limits in some cases, especially when combined with $g_e-2$ and diphoton data.

ABSTRACT

We investigate the potential to probe physics beyond the Standard Model with isotope shift measurements of optical atomic clock transitions. We first derive the reach for generic new physics above the GeV scale at the effective field theory level, as well as estimate the limits on possible new spin-independent forces mediated by sub-GeV states coupled to electrons and neutrons. We also study the weak force and show that isotope shifts could provide strong constraints on the $Z^0$ couplings to valence quarks, which complement precision observables at LEP and atomic parity violation experiments. Finally, motivated by recent experimental hints of a new 750 GeV resonance in diphotons, we also consider the potential to probe its parity-preserving couplings to electrons, quarks and gluons with this method. In particular, combining the diphoton signal with indirect constraints from $g_e-2$ and isotope shifts in Ytterbium allows to probe the resonance coupling to electrons with unprecedented precision.

Motivation & Objective

• To explore the potential of isotope shift (IS) measurements in optical atomic clocks as a probe for new physics beyond the Standard Model (BSM).
• To assess the sensitivity of IS measurements to new physics at or above the GeV scale using effective field theory (EFT) techniques.
• To evaluate constraints on sub-GeV spin-independent forces mediated by new particles coupled to electrons and neutrons.
• To investigate the sensitivity of IS to deviations in Z⁰ boson couplings to valence quarks, complementing LEP and atomic parity violation data.
• To combine IS data with LHC diphoton signals and $g_e-2$ constraints to probe the electron coupling of a hypothetical 750 GeV resonance with unprecedented precision.

Proposed method

• Uses King plots constructed from isotope shift measurements in two narrow transitions (E2 and E3) in Yb⁺ to cancel theoretical uncertainties in mass shift (MS) and field shift (FS) terms.
• Applies effective field theory (EFT) to model new physics contributions to isotope shifts, parameterizing them via $X^{i}_{AA'}$ in the isotope shift formula $\delta\nu^{i}_{AA'} = K_i\mu_{AA'} + F_i\delta\langle r^2\rangle_{AA'} + X^{i}_{AA'}$.
• Estimates sensitivity to new physics by comparing experimental IS data with theoretical predictions, assuming non-linearities in King plots are suppressed.
• Combines IS constraints with existing constraints from $g_e-2$ and LHC diphoton searches to bound couplings of a 750 GeV scalar resonance to electrons and quarks.
• Uses naive dimensional analysis to estimate UV-sensitive contributions to $g_e-2$, accounting for logarithmic uncertainties in the effective coupling to photons.
• Projects future IS sensitivity using state-of-the-art experimental precision (Hz-level accuracy) and compares it with LHC and other indirect bounds.

Experimental results

Research questions

• RQ1 Can isotope shift measurements in Yb⁺ optical clocks probe new physics beyond the Standard Model at the TeV scale?
• RQ2 What is the sensitivity of IS measurements to sub-GeV spin-independent forces mediated by new particles coupled to electrons and neutrons?
• RQ3 How do IS measurements constrain the Z⁰ boson couplings to up and down quarks compared to LEP and atomic parity violation experiments?
• RQ4 Can IS data combined with LHC diphoton signals and $g_e-2$ measurements improve constraints on the electron coupling of a 750 GeV scalar resonance?
• RQ5 To what extent do King plots of IS data suppress theoretical uncertainties, enabling precise extraction of new physics contributions?

Key findings

• Isotope shift measurements in Yb⁺ can probe scalar mediators up to ~20 TeV if their neutron coupling is dominated by up or down quarks.
• For sub-GeV forces, IS measurements can constrain the interaction strength to ~10⁻¹¹ times the fine structure constant α ≈ 1/137, for mediator masses as low as 10 MeV.
• Projected IS constraints on Z⁰ couplings to down quarks are up to an order of magnitude stronger than those from LEP measurements at the Z⁰ pole.
• Combining IS data with LHC diphoton signals and $g_e-2$ constraints yields $|y_e| \lesssim 4.1 \times 10^{-3}$ for a 750 GeV resonance, improving over direct LHC limits in some production modes.
• For a 750 GeV resonance with $u\bar{u}$-dominated production, IS measurements can bound the electron-to-production coupling ratio to $|y_e| \lesssim (4.1, 3.2, 6.1, 14, 26, 130) \times 10^{-3}$ for up, down, strange, charm, bottom, and gluon couplings, respectively.
• The method provides a complementary, high-precision probe of new physics, especially for couplings to electrons and light quarks, where LHC sensitivity is limited.