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On the trail of long-lived particles

22 July 2015

When searching for new particles in ATLAS, it is often assumed that they will either decay to observable Standard Model particles at the centre of the detector, or escape undetected, in which case their presence can be inferred by measuring an imbalance of the total transverse momentum. This assumption was a guiding principle in designing the layout of the ATLAS detector.

However, another possibility exists: what if new particles are long lived? Many models of new physics include heavy particles with lifetimes large enough to allow them to travel measurable distances before decaying. Heavy particles typically decay quickly into lighter particles, unless the decay is suppressed by some mechanism. Suppression could occur if couplings are small, if the decaying particle is only slightly heavier than the only possible decay products, or if the decay is mediated by very heavy virtual exchange particles. Looking for signatures of these models in the LHC data implies exploiting the ATLAS detector in ways it was not necessarily designed for.

These models can give rise to a broad range of possible signatures, depending on the lifetime, charge, velocity and decay channels of the long-lived particle. Decays to charged particles within the ATLAS detector volume can be detected as “displaced vertices”. Heavy charged particles that traverse the detector will move more slowly than their Standard Model counterparts, and will leave a trail of large ionization-energy deposits. Particles with very long lifetimes could even stop in the dense material of the calorimeter and decay at a later time. The ATLAS collaboration has performed dedicated searches to explore all of these spectacular – and challenging – signatures.

Standard reconstruction algorithms are not optimal for such unconventional signatures, so the ATLAS collaboration has used detailed knowledge of the experiment’s sub-detectors to develop dedicated algorithms; for example, to reconstruct charged-particle tracks from displaced decays or to measure the ionization-charge deposited by long-lived charged particles. A class of specialized triggers for picking up these signatures has also been designed and deployed.

These searches generally have very low background, but it is nevertheless essential to estimate the level because some of the signatures could be faked by instrumental effects that are not well-modelled in the simulation. Sophisticated data-driven background estimation techniques have therefore been developed.

One postulated type of long-lived particle is the “R hadron” – a supersymmetric particle with colour-charge combined with Standard Model quarks and gluons. Several ATLAS searches are sensitive to R hadrons, and between them they cover a wide range of lifetimes, as the figure (top right) shows (ATLAS Collaboration 2013 and 2015a). Other analyses have searched for a long-lived hidden-sector pion (“v pion”) by looking for displaced vertices in different ATLAS sub-detectors (ATLAS Collaboration 2015b and 2015c). Exotic Higgs-boson decays to long-lived neutral particles that decay to jets were constrained to a branching ratio smaller than 1% at the 95% confidence level, for a range of lifetime values, as in the figure (right).

With 13-TeV collisions under way at the LHC, the probability of producing heavy new particles has increased enormously, revitalizing the searches for new physics. ATLAS experimentalists are rising to the challenge of exploring as many new physics signatures as possible, including those related to long-lived particles.

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