Melting LHC detectors: a novel search for stopped long-lived particles

TL;DR

The paper proposes a background-free search for stopped long-lived particles produced at the LHC by liquefying detector material, concentrating anomalously heavy atoms via iterative centrifugation, and detecting them with high-sensitivity mass spectrometry. Using the long-lived gluino in split SUSY as a benchmark, the authors simulate production, stopping, and distribution in silicon, liquid argon, intervening materials, and an external water pool, showing that HL-LHC could yield sensitivity up to about $m_{\tilde{g}}\approx 3$ TeV. The method leverages the macroscopic sample sizes and a background-free signal to achieve single-particle sensitivity, and could extend to stops and integer-charged particles; it also presents a practical pathway to repurpose retired detector components for post-collider precision searches. Processing entails liquefaction (for solids), iterative centrifugation to reduce volumes from $\sim 10^8$ L to $\sim 1~\mu$L, and mass spectrometry with potentially unity efficiency, making the approach a scalable, high-impact addition to the LHC program. Overall, this work introduces a novel, practical strategy for discovering or constraining heavy LLPs with lifetimes longer than years, independent of collider backgrounds and pile-up, by exploiting materials around the interaction point as a new detection channel.

Abstract

Particles at the TeV scale with lifetimes of a year or longer could have been abundantly produced at the LHC yet escaped detection because of backgrounds, and could still be trapped within detector materials. With gluinos in split-supersymmetry as a working example, we show that these trapped particles can be recovered from detector materials once prepared in liquid form, for example, by melting silicon detectors, extracting liquid argon from the electromagnetic calorimeter, or constructing a large water pool near ATLAS or CMS. These liquid samples can then be processed using iterative centrifugation followed by mass spectrometry, enabling single-particle sensitivity in macroscopic samples. This method can potentially discover gluinos up to 3 TeV in mass at the HL-LHC. It can also improve upon existing limits for other long-lived particles. For example, it can discover the stop up to 2 TeV, and will also be sensitive to integer-charged particles in the TeV range.

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