[Paper Review] An equal-velocity scenario for hiding dark matter at the LHC

Kaoru Hagiwara, Toshifumi Yamada|arXiv (Cornell University)|Jul 5, 2013

Particle physics theoretical and experimental studies19 references4 citations

TL;DR

This paper proposes a novel search strategy for supersymmetric stop quarks in the 'equal-velocity scenario' at the LHC, where stop and neutralino masses are nearly degenerate with the top quark mass, suppressing missing transverse momentum. By exploiting the alignment and fixed ratio of the missing transverse momentum and top-quark pair transverse momentum vectors, the method enhances signal significance by 2x over standard techniques, achieving 8.4 significance for a 223 GeV stop at 300 fb⁻¹ luminosity.

ABSTRACT

We consider a light supersymmetric top quark (stop) scenario, where a stop and a neutralino are the lightest supersymmetric particles and the stop mass is close to the sum of the neutralino mass and the top quark mass. In this scenario, the top quark and the neutralino coming from the decay of a stop have almost equal velocity vectors, and hence the missing transverse momentum in stop pair production events at colliders is suppressed, which makes the stop search challenging, as in the degenerate scenario where the stop mass and the neutralino mass are nearby. In this paper, we propose a novel analysis technique aiming at discovering the stop pair production signal in such an `equal-velocity' scenario. The key is to look for events where the missing transverse momentum vector is proportional to the transverse momentum vector of the top quark pair with specific values for the proportionality coefficient, as this coefficient equals to the ratio of the neutralino mass over the top quark mass in the equal-velocity limit. We examine the possibility that the stop signal can be identified at the 14 TeV LHC by our analysis method.

Motivation & Objective

• To address the challenge of detecting stop quarks when their decay products (top quark and neutralino) have nearly equal velocities, suppressing missing transverse momentum and hiding the signal in SM backgrounds.
• To develop a new analysis technique that leverages the kinematic correlation between the missing transverse momentum vector and the top-quark pair transverse momentum vector in the equal-velocity limit.
• To improve signal-to-background discrimination in stop pair production searches at the 14 TeV LHC, particularly in mass regions where standard missing-E_T searches fail.
• To enable discovery of stops in the mass range 200–323 GeV with neutralino masses above 50 GeV, where conventional methods lose sensitivity.

Proposed method

• The method identifies events where the azimuthal angle Δϕ between the missing transverse momentum vector and the top-quark pair transverse momentum vector is small (Δϕ < 0.5), indicating alignment.
• It applies a cut on the ratio of missing transverse momentum to top-quark pair transverse momentum, requiring p_T / p_tt_T > 0.2, consistent with the expected ratio m_χ̃⁰₁ / m_t in the equal-velocity limit.
• The analysis uses scatter plots of (Δϕ, p_T / p_tt_T) to identify signal-rich regions where signal events cluster near (0, m_χ̃⁰₁ / m_t), while SM backgrounds are dispersed.
• Selection cuts are applied based on the scatter plot structure: Δϕ < 0.5 and p_T / p_tt_T > 0.2, which enrich the signal region and suppress background.
• Signal significance is evaluated via S / √(S + B) using full detector simulation, comparing standard counting with the new angular-momentum correlation method.
• The method allows for independent estimation of neutralino and stop masses under the equal-velocity assumption.

Experimental results

Research questions

• RQ1 Can stop quark signals be detected in the equal-velocity scenario where missing transverse momentum is suppressed due to near-degenerate stop and neutralino masses?
• RQ2 Does the alignment of the missing transverse momentum vector with the top-quark pair transverse momentum vector provide a robust signature to distinguish signal from SM background?
• RQ3 Can the ratio of missing transverse momentum to top-quark pair transverse momentum be used as a kinematic discriminator, especially when the neutralino mass exceeds 50 GeV?
• RQ4 How much does incorporating angular and momentum correlation information improve signal significance compared to standard missing-E_T-based searches?

Key findings

• For a stop mass of 223 GeV and neutralino mass of 48 GeV, the new method achieves a signal significance of 8.4 with 300 fb⁻¹ of data, compared to 4.1 in standard counting.
• For a heavier stop at 323 GeV and neutralino at 148 GeV, the significance improves from 0.54 to 1.6 using the new method, indicating detectability with sufficient luminosity.
• Signal events in the equal-velocity scenario form a distinct cluster in the (Δϕ, p_T / p_tt_T) plane centered at Δϕ ≈ 0 and p_T / p_tt_T ≈ m_χ̃⁰₁ / m_t, while SM backgrounds are uniformly distributed.
• The selection cuts Δϕ < 0.5 and p_T / p_tt_T > 0.2 effectively enrich the signal region and reduce background, especially for neutralino masses above 50 GeV.
• The method enables estimation of both stop and neutralino masses using the observed ratio p_T / p_tt_T under the equal-velocity assumption.
• The scatter plot analysis outperforms standard counting by leveraging geometric and kinematic correlations, demonstrating a clear advantage in signal discrimination.