[Paper Review] Neutralino polarization effect in the squark cascade decay at LHC

T. Goto|ArXiv.org|Nov 28, 2004

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

This paper investigates the neutralino polarization effect in squark cascade decays at the LHC, demonstrating that charge asymmetry in the quark-lepton invariant mass distribution arises due to chiral structure in sfermion-fermion-neutralino couplings. Using Monte Carlo simulations in the mSUGRA framework, it shows that the asymmetry is measurable and exhibits strong lepton flavor dependence, with distinct signatures for electrons, muons, and taus, enabling potential detection of $e$-$\mu$ non-universality at $\sqrt{\text{LHC}} = 14$ TeV with 300 fb$^{-1}$ luminosity.

ABSTRACT

We study the effect of the neutralino polarization in a squark cascade decay $ ilde{q} o q ildeχ^0_2 o q l^\pm ilde{l}^\mp o q l^\pm ilde{l}^\mp o q l^\pm l^\mp ildeχ^0_1$. Charge asymmetry in the lepton-jet invariant mass distribution appears depending on the chirality structure of the sfermion-fermion-neutralino coupling. With use of the Monte Carlo simulation, we show that the asymmetry is measurable in LHC. We also show that the distribution of the charge asymmetry is sensitive to the (s)lepton flavor.

Motivation & Objective

• To probe the chiral nature of sfermion-fermion-neutralino couplings via neutralino polarization in squark cascade decays at the LHC.
• To investigate whether the LHC can detect lepton flavor dependence in the $\tilde{\chi}^0_2 \to l\tilde{l}$ decay branching ratios and charge asymmetry.
• To assess the detectability of $e$-$\mu$ non-universality in the $m(ql^\pm)$ distribution using Monte Carlo simulations.
• To evaluate the impact of slepton left-right mixing and Yukawa couplings on the charge asymmetry, particularly for $\tilde{\tau}$ and $\tilde{\mu}$ modes.

Proposed method

• The study uses the effective Lagrangian for sfermion-fermion-neutralino interactions, parameterized by chiral coupling coefficients $L_{i\alpha}^f$ and $R_{i\alpha}^f$, which encode the chirality structure of the couplings.
• The angular distribution of the cascade decay $\tilde{q} \to q\tilde{\chi}^0_2 \to ql^\pm_1\tilde{l}^\mp \to ql^\pm_1l^\mp_2\tilde{\chi}^0_1$ is derived, with polarization-dependent asymmetry governed by $A(l) = \frac{|L|^2 - |R|^2}{|L|^2 + |R|^2}$.
• Monte Carlo simulations are performed using ISAJET for spectrum generation and HERWIG for event generation, interfaced via ISAWIG, with detector effects modeled using ATLFAST.
• The charge asymmetry is defined as $[N(ql^+) - N(ql^-)] / [N(ql^+) + N(ql^-)]$ and reconstructed from simulated events for SPS1a and SPS3 mSUGRA benchmark points.
• The analysis accounts for anti-squark contributions, estimating a dilution factor of 50–58% in the asymmetry measurement.

Experimental results

Research questions

• RQ1 Can the LHC detect charge asymmetry in the $m(ql^\pm)$ distribution arising from neutralino polarization in squark cascade decays?
• RQ2 How does the lepton flavor ($e$, $\mu$, $\tau$) affect the charge asymmetry and branching ratios in $\tilde{\chi}^0_2 \to l\tilde{l}_1$ decays?
• RQ3 To what extent does slepton left-right mixing, driven by Yukawa couplings and $\tan\beta$, induce $e$-$\mu$ non-universality in the asymmetry?
• RQ4 Is the $e$-$\mu$ non-universality in the charge asymmetry measurable at the LHC with $\int \mathcal{L} dt = 300$ fb$^{-1}$?

Key findings

• The charge asymmetry in the $m(ql^\pm)$ distribution is measurable at the LHC, with a clear negative asymmetry at low $m(ql)$ and positive asymmetry near the endpoint for SPS1a and SPS3.
• For SPS1a with $\tan\beta = 10$, the simulated charge asymmetry shows qualitative agreement with theoretical predictions, with a dilution factor of 50% due to anti-squark events.
• For SPS1a with $\tan\beta = 20$, the $e$-$\mu$ asymmetry difference reaches up to 30%, indicating strong flavor dependence due to enhanced left-right mixing in the stau sector.
• In SPS3, the asymmetry is flavor-dependent and dominated by $\tilde{\chi}^0_2 \to \tilde{l}_2$ decay, with $\Gamma(\tilde{\chi}^0_2 \to \mu\tilde{\mu}_1) > \Gamma(\tilde{\chi}^0_2 \to e\tilde{e}_1)$ possible at large $\tan\beta$.
• The $\tilde{\chi}^0_2 \to \tau\tilde{\tau}_1$ decay has a significantly larger width than $e$ and $\mu$ modes due to enhanced $L_{21}^\tau$ from Yukawa coupling, reversing the asymmetry sign compared to $e$ and $\mu$.
• The study concludes that $e$-$\mu$ non-universality in the charge asymmetry is detectable at the LHC with $300$ fb$^{-1}$ luminosity, provided $e$ and $\mu$ detection efficiencies are well understood.