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Measuring the anisotropy of the arrival direction distribution of cosmic raysprovides important information on the propagation mechanisms and theidentification of their sources. In fact, the flux of cosmic rays is thought tobe dependent on the arrival direction only due to the presence of nearby cosmicray sources or particular magnetic-field structures. Recently, the observationof unexpected excesses at TeV energy down to angular scale as narrow as$\sim10\deg$ raised the possibility that the problem of the origin of galacticcosmic rays may be addressed by studying the anisotropy.  The ARGO-YBJ experiment is a full-coverage EAS array, sensitive to cosmicrays with energy threshold of a few hundred GeV. Searching for small-sizedeviations from the isotropy, the ARGO-YBJ collaboration explored thedeclination region $\delta\sim-20^{\circ}\div 80^{\circ}$, making use of about3.7$\cdot10^{11}$ events collected from November 2007 to May 2012.  In this paper the detection of different significant (up to 13 standarddeviations) medium-scale anisotropy regions in the arrival directions of CRs isreported. The observation was performed with unprecedented detail. The relativeexcess intensity with respect to the isotropic flux extends up to 10$^{-3}$.The maximum excess occurs for proton energies of 10-20 TeV, suggesting thepresence of unknown features of the magnetic fields the charged cosmic rayspropagate through, or some contribution of nearby sources never considered sofar.  The observation of new weaker few-degree excesses throughout the sky region$195^{\circ}\leq$ \ra $\leq 290^{\circ}$ is reported for the first time.

Medium scale anisotropy in the TeV cosmic ray flux observed by ARGO-YBJ