Search for long-lived charged particles using large specific ionisation loss and time of flight in 140 $fb^{-1}$ of $pp$ collisions at $\sqrt{s}\ = 13$ TeV with the ATLAS detector

TL;DR

This ATLAS study addresses the search for heavy, long-lived charged particles produced at the LHC, using 140 fb$^{-1}$ of 13 TeV data. It employs two complementary signal regions, a $\beta$-search using large $dE/dx$ together with a slow ToF measurement and a di-track region using two heavily ionising tracks, with each LLP mass inferred from two independent $\beta\gamma$ determinations. The analysis relies on data-driven background modeling via control regions and trapezoidal two-dimensional mass windows, delivering lifetime-dependent mass and cross-section limits for SUSY LLP scenarios (R-hadrons, charginos, and sta u). The results strengthen existing constraints, particularly for lifetimes above ~10 ns, and disfavor the excess seen in a prior pixel $dE/dx$–only search as arising from slow, highly ionising LLPs.

Abstract

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 $fb^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.

Figures (14)

• Figure 1: Representative production diagrams for (a) pair-produced gluinos which form $R$-hadrons decaying into neutralinos, (b) pair-produced charginos decaying into neutralinos, and (c) pair-produced staus decaying into gravitinos. The anti-particle labels are suppressed for simplicity.
• Figure 2: Schematic showing the TileCal cell layout and $|\eta|$ acceptance. The red dashed lines indicate where tracks from the origin with a given pseudorapidity will cross the calorimeter, the calorimeter cells A, (B, BC) and D belong to layers at increasing radius and all contribute to the ToF measurement. The special E-cells are not used in the analysis because their time resolution is poor. The calorimeter response is worse in the region $0.8<|\eta|<$1.0 (transition region between the barrel and the extended barrel).
• Figure 3: (a) Distribution of $\beta_{\mathrm{ToF}}\xspace$ obtained with isolated muons from $Z\rightarrow\mu\mu$ decays (2018 data) with the default calibration (Default) and with the calibration illustrated in Section \ref{['subsec:calibration']} (Calibrated). (b) Dependence of the resolution of $\beta_{\mathrm{ToF}}\xspace$ on the pseudorapidity, using isolated muons from $Z\rightarrow\mu\mu$ decays (2018 data) with all the calibrations applied.
• Figure 4: Calorimeter $\beta_{\mathrm{ToF}}\xspace$ distribution for $Z \rightarrow \mu \mu$ events in 2018 data and in Monte Carlo after the time smearing procedure. The ratio of the MC to data is shown in the bottom of the plot where a solid black line is drawn at unity for reference.
• Figure 5: Comparison of predicted (a) ${m_{\mathrm{ToF}}}\xspace$ and (b) ${m_{{\mathrm{d}}E/\mathrm{d}x\xspace}}\xspace$ background to data in the low-${\mathrm{d}}E/\mathrm{d}x$ validation region. The statistical and systematic uncertainty in the predicted background is calculated as indicated in Section \ref{['subsec:systematics']} and shown as a coloured band. The histogram overflow is added into the rightmost bin.