Abstract
Many extensions of the Standard Model predict the existence of charged heavy long-lived particles, such as R-hadrons or charginos. These particles, if produced at the Large Hadron Collider, should be moving non-relativistically and are therefore identifiable through the measurement of an anomalously large specific energy loss in the ATLAS pixel detector. Measuring heavy long-lived particles through their track parameters in the vicinity of the interaction vertex provides sensitivity to metastable particles with lifetimes from 0.6 ns to 30 ns. A search for such particles with the ATLAS detector at the Large Hadron Collider is presented, based on a data sample corresponding to an integrated luminosity of 18.4 fb^{-1} of pp collisions at sqrt{s} = 8 TeV. No significant deviation from the Standard Model background expectation is observed, and lifetime-dependent upper limits on R-hadrons and chargino production are set. Gluino R-hadrons with 10 ns lifetime and masses up to 1185 GeV are excluded at 95 % confidence level, and so are charginos with 15 ns lifetime and masses up to 482 GeV.
Highlights
The main motivation for heavy long-lived particle (LLP) searches at the Large Hadron Collider (LHC) arises from proposed solutions to the gauge hierarchy problem [1], which typically involve previously unseen particles at the TeV mass scale
The analysis described in this article has sensitivity to metastable particles if they have unit charge and their track length before decay is more than 45 cm in the radial direction, so that they can be measured in the first few layers of the ATLAS tracker
Background and data events are counted in a mass window of ±1.4σ around the signal peak, where the signal peak and the width are estimated by a Gaussian fit to the mass distribution in simulated signal Monte Carlo (MC) samples
Summary
The main motivation for heavy long-lived particle (LLP) searches at the Large Hadron Collider (LHC) arises from proposed solutions to the gauge hierarchy problem [1], which typically involve previously unseen particles at the TeV mass scale. C (2015) 75:407 a first approximation, on the LLP decay mode It can address many different models of New Physics, especially those predicting the production of metastable heavy particles with O(ns) lifetime at LHC energies, such as mini-split SUSY [25,26] or anomaly-mediated supersymmetry breaking (AMSB) models [27,28]. The ToT is proportional to the ionisation charge [32] and its maximum value corresponds to 8.5 times the average charge released by a MIP track normal to the silicon detectors and leaving all of its ionisation charge on a single pixel If this value is exceeded, the signal is lost. The minimum measurable βγ with the dE/dx method is ≈0.3 for particles with unit charge and is determined by the ToT overflow in any of the pixels in a cluster
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