Discovery of VHE $\gamma$-ray emission from the SNR G54.1+0.3

We report the discovery of very high energy (VHE) gamma-ray emission from the direction of the SNR G54.1+ 0.3 using the VERITAS ground-based gamma-ray observatory. The TeV signal has an overall significance of 6.8s and appears pointlike given the resolution of the instrument. The integral flux above 1 TeV is 2.5% of the Crab Nebula flux and significant emission is measured between 250 GeV and 4 TeV, well described by a power-law energy spectrum dN/dE similar to E-Gamma with a photon index Gamma = 2.39 +/- 0.23(stat) +/- 0.30sys. We find no evidence of time variability among observations spanning almost two years. Based on the location, the morphology, the measured spectrum, the lack of variability, and a comparison with similar systems previously detected in the TeV band, the most likely counterpart of this new VHE gamma-ray source is the pulsar wind nebula (PWN) in the SNR G54.1+0.3. The measured X-ray to VHE gamma-ray luminosity ratio is the lowest among all the nebulae supposedly driven by young rotation-powered pulsars, which could indicate a particle-dominated PWN.