Evidence of a Hardening in the Cosmic Ray Proton Spectrum at around 166 TeV Observed by the GRAPES-3 Experiment

We present the measurement of the cosmic ray proton spectrum from 50 TeV to 1.3 PeV using $7.81\times {10}^6$ extensive air shower events recorded by the ground-based GRAPES-3 experiment between 1 January 2014 and 26 October 2015 with a live time of 460 day. Our measurements provide an overlap with direct observations by satellite and balloon-based experiments. The electromagnetic and muon components in the shower were measured by a dense array of plastic scintillator detectors and a tracking muon telescope, respectively. The relative composition of the proton primary from the air shower data containing all primary particles was extracted using the multiplicity distribution of muons which is a sensitive observable for mass composition. The observed proton spectrum suggests a spectral hardening at $\sim 166\mathrm{TeV}$ and disfavors a single power law description of the spectrum up to the Knee energy ($\sim 3\mathrm{PeV}$).

Figure 1

A schematic of GRAPES-3 detector components showing plastic scintillator detectors (shaded triangle), muon telescope consisting of 16 modules (square). The red and black circles represent the selection cuts on the reconstructed EAS cores (see text for details). The red-filled area represents the fiducial area for this analysis.

Figure 2

Relative composition of proton primaries obtained using Gold’s unfolding for GRAPES-3 data as a function of shower size. The error bar represents statistical uncertainty, and the gray band represents the total systematic uncertainty.

Figure 3

Top: The proton energy spectrum measured with the GRAPES-3 data. The spectrum is scaled with ${\mathrm{E}}^{2.7}$ to show the hardening of the spectrum near 166 TeV. The red error bars and the gray band represent the statistical and total systematic uncertainty, respectively. Bottom: The statistical (thick blue line) and total systematic uncertainty (thick red line), calculated by adding the contribution of systematic uncertainty from different sources in quadrature, as a function of primary energy. Dashed lines with different colors represent the contribution from different sources of systematic uncertainty.

Figure 4

Cosmic ray proton energy spectrum measured with the GRAPES-3 data (red circles) compared with results from direct and indirect observations (see text for references). The statistical error bars are smaller than the marker size and the gray band represents systematic uncertainty.

Figure 5

The GRAPES-3 proton energy spectrum (red markers) fitted with the PL [Eq. (4)] and SBPL [Eq. (5)], represented by the blue dashed line and black line, respectively.