[Paper Review] Flavorful Supersymmetry

Yasunori Nomura, Michele Papucci+1 more|University of North Texas Digital Library (University of North Texas)|Dec 13, 2007

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper proposes 'flavorful supersymmetry,' a framework in which the same physics suppressing quark and lepton Yukawa couplings also suppresses flavor-violating supersymmetry breaking parameters. By linking flavor structure to supersymmetry breaking via high-scale operators, it naturally suppresses FCNCs and CP violation, allowing a broad region of parameter space to evade experimental constraints. The model predicts distinctive LHC signatures, including long-lived charged sleptons and monochromatic electrons or muons from decays of the NLSP, which could serve as smoking-gun signals for the mechanism.

ABSTRACT

Weak scale supersymmetry provides elegant solutions to many of the problems of the standard model, but it also generically gives rise to excessive flavor and CP violation. We show that if the mechanism that suppresses the Yukawa couplings also suppresses flavor changing interactions in the supersymmetry breaking parameters, essentially all the low energy flavor and CP constraints can be satisfied. The standard assumption of flavor universality in the supersymmetry breaking sector is not necessary. We study signatures of this framework at the LHC. The mass splitting among different generations of squarks and sleptons can be much larger than in conventional scenarios, and even the mass ordering can be changed. We find that there is a plausible scenario in which the NLSP is a long-lived right-handed selectron or smuon decaying into the LSP gravitino. This leads to the spectacular signature of monochromatic electrons or muons in a stopper detector, providing strong evidence for the framework.

Motivation & Objective

• To address the severe flavor and CP problem in weak-scale supersymmetry, where generic soft terms lead to excessive FCNCs and CP violation.
• To propose a mechanism in which the same dynamics suppressing Yukawa couplings also suppresses flavor-violating soft masses, avoiding the need for explicit flavor universality.
• To demonstrate that a large portion of parameter space remains viable under current experimental constraints, contrary to naive expectations.
• To identify distinctive collider signatures at the LHC, particularly in the context of a gravitino LSP and right-handed slepton NLSP.
• To provide a framework where flavor structure in superpartner masses can be probed through precision measurements of slepton production and decay.

Proposed method

• Assumes that the physics responsible for quark and lepton masses—implying suppression of non-gauge interactions—also suppresses the soft-breaking terms at the scale $M_*$, where $M_* \gtrsim M_U \approx 10^{16}$ GeV.
• Models the soft-breaking Lagrangian using suppressed operators involving a chiral superfield $X$ with $F$-term VEV, where suppression factors $\epsilon_{\Phi}$, $\epsilon_{H_u}$, $\epsilon_{H_d}$ control the size of soft terms.
• Imposes that the superpotential couplings $\zeta_u$, $\zeta_d$, $\zeta_e$ generating trilinear soft terms are suppressed, reducing flavor-violating contributions.
• Uses renormalization group evolution from $M_*$ to the weak scale to generate flavor-universal contributions to squark and slepton masses from gaugino masses.
• Analyzes the resulting superpartner spectrum, focusing on mass splittings among right-handed sleptons and the role of the gravitino as the lightest supersymmetric particle (LSP).
• Evaluates LHC signatures by considering the NLSP decaying into the gravitino, with particular attention to long-lived charged sleptons and monochromatic final states from flavor-violating decays.

Experimental results

Research questions

• RQ1 Can the same mechanism that suppresses quark and lepton Yukawa couplings also suppress flavor-violating soft terms in supersymmetry breaking?
• RQ2 What portion of the parameter space remains viable under current experimental constraints on flavor-changing and CP-violating processes?
• RQ3 What distinctive collider signatures arise in this framework, especially when the gravitino is the LSP?
• RQ4 How do mass splittings among right-handed sleptons differ from conventional scenarios, and what are their implications for detection?
• RQ5 Can precision measurements of slepton production and decay at the LHC or a linear collider probe the flavor structure of superpartner masses?

Key findings

• A large region of parameter space remains viable because the same suppression mechanism that controls Yukawa couplings also suppresses flavor-violating soft terms, avoiding the need for fine-tuning or explicit flavor universality.
• The model predicts significant mass splittings among right-handed sleptons, with the lightest one potentially being $\tilde{e}_R$, $\tilde{\mu}_R$, or $\tilde{\tau}_R$, depending on the flavor structure of the soft terms.
• If the lightest right-handed slepton is the NLSP and decays to the gravitino, its lifetime can exceed 100 seconds, enabling full reconstruction of decay chains at the LHC.
• The model leads to a dramatic signature of monochromatic electrons or muons in a stopper detector, arising from flavor-violating decays of the NLSP, which serves as a smoking-gun signal.
• The gravitino mass is typically $m_{3/2} \sim 10^{-2}$ to $10^{-3}$ times the typical superpartner mass, making it plausible that the gravitino is the LSP with a mass well below the TeV scale.
• Precision measurements of slepton production and decay—especially in topologies with long-lived charged particles—can probe the flavor content of the superpartner spectrum, offering a direct window into the origin of flavor.