[Paper Review] Detection of long-lived staus and gravitinos at the ILC

H.-U. Martyn|ArXiv.org|Sep 7, 2007

Cosmology and Gravitation Theories1 references5 citations

TL;DR

This paper demonstrates that the International Linear Collider (ILC) can detect long-lived stau particles ($\tilde{\tau}$) and precisely measure their mass, lifetime, and the gravitino ($\tilde{G}$) mass via the decay $\tilde{\tau} \to \tau \tilde{G}$, providing direct access to the gravitational coupling and Planck scale. Using kinematic reconstruction and decay time measurements, the ILC achieves sub-percent precision on the gravitino mass, enabling a unique test of supergravity and constraints on gravitino dark matter.

ABSTRACT

A study is presented illustrating the excellent potential of future International Linear Collider (ILC) experiments to detect metastable staus $\stau$, measure precisely their mass and lifetime, and to determine the mass of the gravitino $\sG$ from the decay $\stau oτ\sG$, thus providing direct access to the gravitational coupling, respectively Planck scale.

Motivation & Objective

• To demonstrate the ILC's capability to detect and measure long-lived stau particles ($\tilde{\tau}$) as metastable next-to-lightest supersymmetric particles (NLSPs).
• To precisely determine the gravitino mass ($m_{\tilde{G}}$) through measurement of the $\tau$ jet recoil energy in $\tilde{\tau} \to \tau \tilde{G}$ decays.
• To access the gravitational coupling and Planck scale via the measured $\tilde{\tau}$ mass, lifetime, and $\tilde{G}$ mass.
• To test supergravity models by measuring the gravitino mass with high precision, providing constraints on its role as a dark matter candidate.

Proposed method

• Use of a high-precision TPC with excellent dE/dx resolution to identify slow, heavy stau particles via ionization energy loss ($dE/dx \sim \beta^{-2}$).
• Employment of a highly segmented HCAL and instrumented iron yoke to trap and localize stopped staus within volumes of a few cm³.
• Reconstruction of $\tilde{\tau}$ mass from kinematic balance of final-state particles, with $\sum \vec{p}_i \approx 0$ and $\sum p_i < \sqrt{s}$.
• Measurement of the $\tilde{\tau}$ lifetime from the decay time distribution of stopped staus after beam collisions.
• Extraction of the gravitino mass from the $\tau$ jet recoil energy spectrum using the energy conservation relation $E_\tau = \frac{m_{\tilde{\tau}}}{2} \left(1 - \frac{m_{\tilde{G}}^2 - m_\tau^2}{m_{\tilde{\tau}}^2} \right)$.
• Use of event simulations including QED radiation, beamstrahlung, and detector resolutions to model signal and background, with efficient rejection of Standard Model backgrounds.

Experimental results

Research questions

• RQ1 Can the ILC detect long-lived stau particles ($\tilde{\tau}$) and measure their mass and lifetime with high precision?
• RQ2 Can the gravitino mass ($m_{\tilde{G}}$) be determined from the $\tau$ jet recoil energy in $\tilde{\tau} \to \tau \tilde{G}$ decays?
• RQ3 What is the sensitivity of the ILC to low gravitino masses, particularly in the eV–keV range expected in gauge-mediated SUSY breaking (GMSB) models?
• RQ4 How does the precision of gravitino mass measurement depend on the $m_{\tilde{G}}/m_{\tilde{\tau}}$ mass ratio, especially for $m_{\tilde{G}} \lesssim 0.1 \, m_{\tilde{\tau}}$?
• RQ5 Can radiative decays $\tilde{\tau} \to \tau \gamma \tilde{G}$ help distinguish the gravitino from other LSP candidates like neutralinos or axinos?

Key findings

• The ILC achieves a $\tilde{\tau}$ mass measurement of $m_{\tilde{\tau}_1} = 124.3 \pm 0.1\,\text{GeV}$ in the SPS 7 GMSB scenario at $\sqrt{s} = 410\,\text{GeV}$ and $\mathcal{L} = 100\,\text{fb}^{-1}$.
• The $\tilde{\tau}$ lifetime is measured with high precision as $\tau = 209.3 \pm 2.4\,\text{s}$, enabling a direct determination of the gravitino mass.
• The gravitino mass is determined to be $m_{\tilde{G}} = 100 \pm 1\,\text{MeV}$, demonstrating sub-percent precision in the $\tilde{\tau} \to \tau \tilde{G}$ decay channel.
• For gravitino masses below $1\,\text{GeV}$, the $\tau$ jet energy spectrum becomes insensitive to small mass splittings, setting an upper limit of $m_{\tilde{G}} < 9\,\text{GeV}$ at 95% CL.
• The method is sensitive to gravitino masses as low as $0.5\,\text{MeV}$ (corresponding to a $\tilde{\tau}$ lifetime of $5\,\text{ms}$), which are measurable with the ILC's timing and tracking capabilities.
• Radiative decays $\tilde{\tau} \to \tau \gamma \tilde{G}$ offer a potential pathway to distinguish the gravitino from neutralino or axino LSPs, though experimental challenges remain due to suppressed branching ratios and background discrimination.