The Fermi-Lat CollaborationM. AjelloL. BaldiniG. BarbielliniD. BastieriK. BechtolR. BellazziniE. BissaldiR. D. BlandfordR. BoninoE. BottaciniT. J. BrandtP. BruelS. BusonR. A. CameronR. CaputoE. CavazzutiS. ChenG. ChiaroS. CipriniJ. Cohen-TanugiD. CostantinA. CuocoS. CutiniF. D'AmmandoP. De La Torre LuqueF. De PalmaA. DesaiS. W. DigelN. Di LallaL. Di VenereA. DomínguezS. J. FeganY. FukazawaS. FunkP. FuscoF. GarganoD. GasparriniN. GigliettoF. GiordanoM. GirolettiD. GreenI. A. GrenierS. GuiriecK. HayashiE. HaysJ. W. HewittD. HoranG. JóhannessonM. KussL. LatronicoJ. LiI. LiodakisF. LongoF. LoparcoP. LubranoS. MalderaA. ManfredaG. Martí-DevesaM. N. MazziottaM. MeehanI. MereuM. MeyerP. F. MichelsonN. MirabalW. MitthumsiriT. MizunoA. MorselliM. NegroE. NussN. OmodeiM. OrientiE. OrlandoV. S. PaliyaD. PanequeM. PersicM. Pesce-RollinsF. PironT. A. PorterG. PrincipeS. RainòR. RandoM. RazzanoS. RazzaqueA. ReimerO. ReimerD. SeriniC. SgròE. J. SiskindG. SpandreP. SpinelliD. J. SusonH. TajimaJ. B. ThayerD. F. TorresE. TrojaJ. VandenbrouckeM. YassineS. ZimmerIstituto Nazionale di Fisica Nucleare, Sezione di TriesteIstituto Nazionale di Fisica Nucleare, Sezione di PerugiaIstituto Nazionale di Fisica Nucleare, Sezione di BariIstituto Nazionale di Fisica Nucleare, Sezione di TorinoIstituto Nazionale di Fisica Nucleare, Sezione di PisaIstituto Nazionale Di Fisica Nucleare, Sezione di PadovaLaboratoire Univers et Particules de MontpellierUniversite Paris-SaclayLaboratoire Leprince-RinguetKavli Institute for Particle Astrophysics and CosmologyINAF Istituto di Astrofisica Spaziale e Fisica Cosmica, MilanNYCB Realtime Computing Inc.University Science Institute ReykjavikInstitució Catalana de Recerca i Estudis AvançatsUniversity of Wisconsin-MadisonAgenzia Spaziale ItalianaHiroshima UniversityDeutsches Elektronen-Synchrotron (DESY)Osservatorio Astronomico di TriesteStockholms universitetAlma Mater Studiorum Università di BolognaUniversidad Complutense de MadridClemson UniversityUniversity of Maryland, Baltimore CountyUniversitat Autònoma de BarcelonaUniversità degli Studi di BariIstituto Nazionale di Fisica Nucleare - INFNUniversity of MarylandPurdue University CalumetUniversità degli Studi di TriesteRheinisch-Westfälische Technische Hochschule AachenMahidol UniversityJulius-Maximilians-Universität WürzburgIstituto Di Radioastronomia, BolognaUniversità degli Studi di TorinoMax-Planck-Institut für Physik (Werner-Heisenberg-Institut)Medizinische Universitat InnsbruckNASA Goddard Space Flight CenterUniversité de GenèveThe George Washington UniversityUniversità degli Studi di PerugiaFriedrich-Alexander-Universität Erlangen-NürnbergUniversity of JohannesburgUniversità degli Studi di PadovaNagoya UniversityUniversity of North Florida2020-01-272020-01-272019-09-20Astrophysical Journal. Vol.883, No.1 (2019)153843570004637X2-s2.0-85073070536https://repository.li.mahidol.ac.th/handle/20.500.14594/50753© 2019. The American Astronomical Society. All rights reserved. The Fermi Large Area Telescope (LAT) has amassed a large data set of primary cosmic-ray protons throughout its mission. In fact, it is the largest set of identified cosmic-ray protons ever collected at this energy. The LAT's wide field of view and full-sky survey capabilities make it an excellent instrument for studying cosmic-ray anisotropy. As a space-based survey instrument, the LAT is sensitive to anisotropy in both R.A. and decl., while ground-based observations only measure the anisotropy in R.A. We present the results of the first-ever proton anisotropy search using Fermi LAT. The data set was collected over eight years and consists of approximately 179 million protons above 78 GeV, enabling it to probe dipole anisotropy below an amplitude of 10-3, resulting in the most stringent limits on the decl. dependence of the dipole to date. We measure a dipole amplitude δ = 3.9 ±1.5 ×10-4 with a p-value of 0.01 (pretrials) for protons with energy greater than 78 GeV. We discuss various systematic effects that could give rise to a dipole excess and calculate upper limits on the dipole amplitude as a function of minimum energy. The 95% confidence level upper limit on the dipole amplitude is δ UL = 1.3 ×10-3 for protons with energy greater than 78 GeV and δ UL = 1.2 ×10-3 for protons with energy greater than 251 GeV.Mahidol UniversityEarth and Planetary SciencesArticleSCOPUS10.3847/1538-4357/ab3a2e