[Paper Review] Explaining muon $g-2$ data in the $μν$SSM

Essodjolo Kpatcha, Iñaki Lara+3 more|arXiv (Cornell University)|Dec 6, 2019

Particle physics theoretical and experimental studies110 references4 citations

TL;DR

This paper proposes that the $μ\nu$SSM, an $R$-parity-violating supersymmetric model, explains the long-standing discrepancy in the muon $g-2$ by naturally generating light left-handed muon-sneutrinos and wino-like charginos through neutrino physics. Using a likelihood-based scan of the parameter space, it identifies viable regions—specifically $m_{\tilde{\nu}_\mu} \sim 120-620$ GeV, $M_1 \sim 120-2200$ GeV, and $M_2 \sim 200-900$ GeV—that simultaneously satisfy neutrino, Higgs, flavor, and LHC constraints, with multi-lepton + MET searches capable of probing these regions.

ABSTRACT

We analyze the anomalous magnetic moment of the muon $g-2$ in the $μν$SSM. This $R$-parity violating model solves the $μ$ problem reproducing simultaneously neutrino data, only with the addition of right-handed neutrinos. In the framework of the $μν$SSM, light left muon-sneutrino and wino masses can be naturally obtained driven by neutrino physics. This produces an increase of the dominant chargino-sneutrino loop contribution to muon $g-2$, solving the gap between the theoretical computation and the experimental data. To analyze the parameter space, we sample the $μν$SSM using a likelihood data-driven method, paying special attention to reproduce the current experimental data on neutrino and Higgs physics, as well as flavor observables such as $B$ and $μ$ decays. We then apply the constraints from LHC searches for events with multi-leptons + MET on the viable regions found. They can probe these regions through chargino-chargino, chargino-neutralino and neutralino-neutralino pair production. We conclude that significant regions of the parameter space of the $μν$SSM can explain muon $g-2$ data.

Motivation & Objective

• To explain the persistent $3.5\sigma$ discrepancy in the muon anomalous magnetic moment ($g-2$) beyond the Standard Model.
• To explore whether the $\mu\nu$SSM, a minimal $R$-parity-violating supersymmetric model, can naturally generate the required light sparticles to enhance the $g-2$ contribution.
• To identify viable parameter regions in the $\mu\nu$SSM that simultaneously satisfy constraints from neutrino physics, Higgs boson data, flavor observables (e.g., $B$ and $\mu$ decays), and LHC searches.
• To assess the collider viability of these solutions through LHC multi-lepton + missing energy ($E_T$) searches.

Proposed method

• A likelihood data-driven scanning method is employed to sample the $\mu\nu$SSM parameter space, incorporating constraints from neutrino oscillation data, Higgs boson measurements, and flavor observables such as $B$ and $\mu$ decays.
• The model's neutrino mass generation via the gaugino seesaw mechanism naturally leads to light left-handed muon-sneutrino and wino-like chargino masses.
• The SUSY contribution to $a_\mu$ is computed via chargino–sneutrino loop diagrams, which are enhanced by the light sneutrino mass.
• LHC constraints are applied using multi-lepton + MET search limits, focusing on chargino–chargino, chargino–neutralino, and neutralino–neutralino pair production channels.
• The analysis assumes fixed Higgs sector parameters and diagonal neutrino Yukawa couplings to reduce computational complexity, though generalizations are noted as future work.
• The parameter scan is validated by comparing predicted $a_\mu$ with the experimental value $\Delta a_\mu = (26.8 \pm 6.3 \pm 4.3) \times 10^{-10}$.

Experimental results

Research questions

• RQ1 Can the $\mu\nu$SSM naturally generate light left-handed muon-sneutrinos and charginos to enhance the $g-2$ contribution without fine-tuning?
• RQ2 What regions of the $\mu\nu$SSM parameter space simultaneously satisfy current neutrino, Higgs, flavor, and LHC constraints while explaining the $g-2$ anomaly?
• RQ3 How do LHC multi-lepton + MET searches constrain the viable parameter space for $g-2$-relevant scenarios in the $\mu\nu$SSM?
• RQ4 Can the $\mu\nu$SSM explain the $g-2$ discrepancy without requiring a stable LSP, thus avoiding dark matter tension?
• RQ5 What is the impact of varying sneutrino and smuon masses on the $g-2$ contribution and collider signatures?

Key findings

• Significant regions of the $\mu\nu$SSM parameter space—specifically $120 \lesssim m_{\tilde{\nu}_\mu} \lesssim 620$ GeV, $120 \lesssim M_1 \lesssim 2200$ GeV, and $200 \lesssim M_2 \lesssim 900$ GeV—can explain the observed $\Delta a_\mu$ discrepancy.
• The enhancement in the $g-2$ contribution arises primarily from chargino–sneutrino loops, driven by the natural lightness of the left muon-sneutrino due to neutrino physics in the $\mu\nu$SSM.
• LHC multi-lepton + MET searches can probe these viable regions through chargino–chargino, chargino–neutralino, and neutralino–neutralino pair production, making them testable in future data.
• The model remains consistent with current constraints from neutrino oscillations, Higgs boson properties, and flavor physics such as $B$ and $\mu$ decays.
• The results are robust to moderate variations in the right smuon mass, and even smaller values could further enhance the $g-2$ contribution.
• The study suggests that future $g-2$ measurements at $\sim 7\sigma$ significance could allow the $\mu\nu$SSM to be used to constrain the mass of the left muon-sneutrino and electroweak gauginos.