Ultrahigh energy cosmic ray air showers probe particle physics at energies beyond the reach of accelerators. Here we introduce a new method to test hadronic interaction models without relying on the absolute energy calibration, and apply it to events with primary energy 6–16 EeV (ECM=110–170 TeV), whose longitudinal development and lateral distribution were simultaneously measured by the Pierre Auger Observatory. The average hadronic shower is 1.33±0.16 (1.61±0.21) times larger than predicted using the leading LHC-tuned models EPOS-LHC (QGSJetII-04), with a corresponding excess of muons.Received 23 April 2016DOI:https://doi.org/10.1103/PhysRevLett.117.192001© 2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle interactionsParticle productionStrong interactionTechniquesCosmic ray & astroparticle detectorsParticle data analysisParticles & Fields