[Paper Review] Polarization aspects in radiative neutralino production

Herbert K. Dreiner, O. Kittel+1 more|ArXiv.org|Jul 11, 2007

Particle physics theoretical and experimental studies9 references3 citations

TL;DR

This paper investigates radiative neutralino production $e^+e^- \to \tilde{\chi}_1^0\tilde{\chi}_1^0\gamma$ at the International Linear Collider (ILC), demonstrating that longitudinal beam polarization significantly enhances the signal cross section while suppressing the dominant Standard Model background from $e^+e^- \to \nu\bar{\nu}\gamma$. With optimized polarization, the signal significance reaches $S \approx 80$ and the signal-to-background ratio improves to $r \approx 20\%$, enabling observation even when other SUSY particles are too heavy to pair-produce at $\sqrt{s} = 500$ GeV.

ABSTRACT

We study the impact of beam polarization on radiative neutralino production e+e- o χ^0_1 χ^0_1 photon at the International Linear Collider. We show that longitudinal polarized beams significantly enhance the signal and simultaneously reduce the Standard Model background from radiative neutrino production e+e- o ν\barνphoton. We point out that the radiative production of neutralinos could be the only accessible SUSY particles, if neutralinos, charginos, sleptons, as well as squarks and gluinos are too heavy to be pair-produced in the first stage of the ILC at sqrt{s} = 500 GeV.

Motivation & Objective

• To assess the feasibility of detecting radiatively produced lightest neutralinos ($\tilde{\chi}_1^0\tilde{\chi}_1^0\gamma$) at the ILC when pair production is kinematically forbidden.
• To evaluate the impact of beam polarization on the signal-to-background ratio and statistical significance in radiative neutralino production.
• To identify scenarios where radiative neutralino production is the only accessible SUSY channel at $\sqrt{s} = 500$ GeV.
• To quantify the enhancement of signal significance and reduction of background via optimized electron and positron beam polarization.

Proposed method

• The study computes the tree-level cross sections for the signal ($e^+e^- \to \tilde{\chi}_1^0\tilde{\chi}_1^0\gamma$) and background ($e^+e^- \to \nu\bar{\nu}\gamma$) using amplitude squared formulas from Ref. [11].
• Infrared and collinear divergences are regularized by applying kinematical cuts: $|\cos\theta_\gamma| \leq 0.99$ and $0.02 \leq E_\gamma / E_{\text{beam}} \leq 1 - m_{\tilde{\chi}_1^0}^2 / E_{\text{beam}}^2$.
• Theoretical significance $S = \sigma\sqrt{\mathcal{L}} / \sqrt{\sigma + \sigma_{\text{B}}}$ and signal-to-background ratio $r = \sigma / \sigma_{\text{B}}$ are used as observability criteria.
• The dependence of cross sections on beam polarization ($P_{e^-}, P_{e^+}$), higgsino mass $\mu$, and gaugino mass $M_2$ is numerically evaluated in the context of SPS1a and mSUGRA scenarios.
• Monte Carlo studies are suggested to validate results, particularly for realistic detector-level analysis.
• The analysis includes kinematical limits for pair production of $\tilde{\chi}_1^0\tilde{\chi}_2^0$ and $\tilde{\chi}_1^+\tilde{\chi}_1^-$ to identify regions where only radiative production is possible.

Experimental results

Research questions

• RQ1 Can radiative neutralino production be observed at the ILC when pair production of neutralinos, charginos, sleptons, squarks, and gluinos is kinematically forbidden at $\sqrt{s} = 500$ GeV?
• RQ2 How does beam polarization affect the signal cross section and background suppression in radiative neutralino production?
• RQ3 What is the maximum achievable significance ($S$) and signal-to-background ratio ($r$) for radiative neutralino production under optimal beam polarization?
• RQ4 How do the parameters $\mu$ and $M_2$ influence the signal and background cross sections in the $\mu$–$M_2$ plane?
• RQ5 In which regions of the MSSM parameter space is radiative neutralino production the only accessible SUSY channel at $\sqrt{s} = 500$ GeV?

Key findings

• With beam polarization $(P_{e^-}, P_{e^+}) = (0.8, -0.6)$, the signal cross section reaches $\sigma = 70$ fb and the background is reduced to $\sigma_{\text{B}} = 330$ fb, yielding a significance of $S = 80$.
• The signal-to-background ratio increases to $r = 20\%$ under the same polarization, satisfying the $r > 1\%$ threshold for detectability.
• In the mSUGRA scenario with $M_0 = 200$ GeV, $M_{1/2} = 415$ GeV, and $A_0 = -200$ GeV, the signal cross section reaches $13$ fb with $P_{e^-} = 0.8$, $P_{e^+} = -0.6$, and significance $S = 17$.
• Without beam polarization, the signal significance is only $S = 1.8$, and the signal-to-background ratio is $0.1\%$, rendering detection impossible.
• For $P_{e^-} = 0.9$, the background is suppressed to $263$ fb, and the significance increases to $S = 15$, demonstrating the strong impact of high electron polarization.
• In regions where pair production of neutralinos and charginos is kinematically forbidden, radiative neutralino production remains observable with $S \approx 20$, confirming its role as the sole accessible SUSY channel.