Moon shadow by cosmic rays under the influence of geomagnetic field and search for antiprotons at multi-TeV energies

Abstract We have observed the shadowingof galactic cosmic rayflux in the direction of the moon, the so-called moon shadow, using the Tibet-III air showerarray operating at Yangbajing (4300 m a.s.l.) in Tibet since 1999. Almost all cosmic raysare positively charged; for that reason, they are bent by the geomagnetic field, thereby shifting the moon shadowwestward. The cosmic rayswill also produce an additional shadowin the eastward direction of the moon if cosmic rayscontain negatively charged particles, such as antiprotons, with some fraction. We selected 1.5 × 10 10 air showerevents with energy beyond about 3 TeV from the dataset observed by the Tibet-III air showerarray and detected the moon shadowat ∼40 σ level. The center of the moon was detected in the direction away from the apparent center of the moon by 0.23° to the west. Based on these data and a full Monte Carlo simulation, we searched for the existence of the shadowproduced by antiprotonsat the multi-TeV energy region. No evidence of the existence of antiprotonswas found in this energy region. We obtained the 90% confidence level upper limit of the flux ratio of antiprotonsto protons as 7% at multi-TeV energies.