LHC searches for new physics focus on combinations of hard physics objects. In this work we propose a qualitatively different soft signal for new physics at the LHC—the “anomalous underlying event.” Every hard LHC event will be accompanied by a soft underlying event due to QCD and pileup effects. Though it is often used for QCD and Monte Carlo studies, here we propose the incorporation of an underlying event analysis in some searches for new physics. An excess of anomalous underlying events may be a smoking-gun signal for particular new physics scenarios such as “quirks” or “hidden valleys” in which large amounts of energy may be emitted by a large multiplicity of soft particles. We discuss possible search strategies for such soft diffuse signals in the tracking system and calorimetry of the LHC experiments. We present a detailed study of the calorimetric signal in a concrete example, a simple quirk model motivated by folded supersymmetry. In these models the production and radiative decay of highly excited quirk bound states leads to an “antenna pattern” of soft unclustered energy. Using a dedicated simulation of a toy detector and a multipole analysis familiar in cosmic microwave background studies, we compare the signal to the expected backgrounds.

• Received 17 April 2009

DOI:https://doi.org/10.1103/PhysRevD.80.075015