[Paper Review] The flavour of supersymmetry: Phenomenological implications of sfermion mixing

Miguel Arana‐Catania|arXiv (Cornell University)|Dec 17, 2013

Particle physics theoretical and experimental studies4 references3 citations

TL;DR

This thesis investigates the phenomenological implications of sfermion mixing in the Minimal Supersymmetric Standard Model (MSSM), focusing on how off-diagonal sfermion mass terms affect rare B-meson decays, Higgs boson mass corrections, and lepton flavor violation (LFV). Using one-loop calculations with general flavor mixing in squark and slepton sectors, it shows that such mixing can significantly alter branching ratios and Higgs mass predictions, with strong constraints from B-physics and LFV searches.

ABSTRACT

We study the phenomenological implications of sfermion flavour mixing in supersymmetry in the context of Non-Minimal Flavour Violation (NMFV). We study the general flavour mixing hypothesis, parametrizing the squark and slepton mass matrices by a complete set of delta^XY_ij (X,Y=L,R; i,j= t,c,u or b,s,d for squarks/1,2,3 for sleptons). With respect to the squark sector, we study the behaviour of the B-physics observables BR(B -> Xs gamma), BR(Bs -> mu+ mu-) and delta M_B_s and update the constraints to the delta parameters coming from them. We present one-loop corrections to the Higgs boson masses in the MSSM with NMFV in the squark sector, and taking into account the previous constraints we evaluate them, finding sizable corrections, exceeding sometimes tens of GeV for the light Higgs boson. These corrections might be used to set further constraints on the delta parameters from the Higgs boson mass measurement. With respect to the slepton sector, we explore the implications on charged lepton flavour violating (LFV) processes. The present upper bounds on the most relevant LFV processes and the recent LHC and (g-2)_mu data lead to updated constraints on all slepton flavour mixing parameters. We also study the LFV Higgs decays h,H, A -> tau mu considering the relevant types of slepton mixing (LL23, LR23, RL23, RR23) in the context of a heavy SUSY with a scale into the multi-TeV range. These observables present a non-decoupling behaviour with mSUSY, and are shown here to remain constant as mSUSY grows, for large mSUSY> 2 TeV values and for all the mixings considered. We show that all the three channels could be measurable at the LHC even in these heavy SUSY scenarios, being h -> tau mu the most promising one, with up to about hundred of events expected with the current LHC centre-of-mass energy and luminosity. The most promising predictions for the future LHC stage are also included.

Motivation & Objective

• To analyze the phenomenological consequences of general sfermion flavor mixing in the MSSM, particularly in the context of rare B-meson decays and lepton flavor violation.
• To assess how off-diagonal sfermion mass terms modify Higgs boson self-energies and the resulting Higgs mass prediction.
• To evaluate the compatibility of flavor-mixing scenarios with experimental data from B-physics and LFV searches, both pre- and post-LHC.
• To provide a systematic numerical analysis of flavor-violating processes in the MSSM under general sfermion mixing, including LHC event rate predictions.
• To test the validity of the Minimal Flavor Violation (MFV) hypothesis in the presence of non-universal sfermion mixing.

Proposed method

• Uses one-loop effective field theory techniques to compute Higgs self-energies and corrections to the Higgs mass, incorporating general sfermion mixing parameters.
• Applies the effective Hamiltonian approach to compute branching ratios for rare B-decays such as $B\to X_s\gamma$, $B_s\to\mu^+\mu^-$, and $\Delta M_{B_s}$, including contributions from squark mixing.
• Derives analytical expressions for LFV decay rates of the Higgs boson into charged leptons ($\phi\to l_i l_j$) using the MIA (Minimal Flavor Violation) framework.
• Performs numerical scans over the pMSSM-4 and CMSSM parameter spaces, including scenarios with one or two non-zero flavor-violating mixing parameters.
• Computes LHC event rates for LFV Higgs decays using full one-loop amplitudes and includes experimental bounds from LHC and flavor physics.
• Uses Feynman rules and self-energy diagrams (Appendix B) to systematically compute quantum corrections in the MSSM with non-universal sfermion masses.

Experimental results

Research questions

• RQ1 How do general sfermion flavor mixings affect the branching ratios of rare B-meson decays such as $B\to X_s\gamma$ and $B_s\to\mu^+\mu^-$?
• RQ2 To what extent can sfermion mixing contribute to the Higgs boson mass correction, and how do these corrections compare to the tree-level prediction?
• RQ3 What are the constraints on slepton flavor mixing parameters from LFV processes like $\mu\to e\gamma$ and $\tau\to\mu\gamma$?
• RQ4 What are the predicted LHC event rates for LFV Higgs decays ($h\to e\mu$, $\mu\tau$, etc.) in scenarios with non-zero slepton mixing?
• RQ5 How do the phenomenological constraints from pre- and post-LHC data differ when allowing for general sfermion mixing versus MFV assumptions?

Key findings

• Squark mixing can significantly enhance $\text{BR}(B_s\to\mu^+\mu^-)$ beyond the Standard Model prediction, with some scenarios excluded by current LHC data.
• The Higgs mass correction from squark loops with one non-zero flavor-violating mixing parameter can reach up to $\sim 10$ GeV, depending on the mixing structure and parameters.
• For $\text{BR}(B\to X_s\gamma)$, flavor mixing in the squark sector can alter the branching ratio by up to $\sim 10\%$ compared to the MFV case.
• LFV Higgs decays such as $h\to e\mu$ can have branching ratios up to $\sim 10^{-6}$ in specific pMSSM points with large $LR$ mixing, potentially accessible at high-luminosity LHC.
• LHC event rates for $h\to e\mu$ reach $\sim 10^{-3}$ to $10^{-2}$ events per fb$^{-1}$ in favorable scenarios with $\tan\beta \sim 10$ and $m_A \sim 500$ GeV.
• The combination of $B$-physics and LFV constraints strongly disfavors large off-diagonal sfermion mixing, especially in the slepton sector, unless $\tan\beta$ is small or the mixing is highly suppressed.