Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics. W eyl semimetals have received significant attention in recent years because they extend the classification of topological phases beyond insulators, host exotic Fermi arc surface states, demonstrate unusual transport phenomena and provide an emergent condensed matter realization of Weyl fermions, which do not exist as fundamental particles in the standard model 1-21 . Such kind of topologically non-trivial semimetals are believed to open a new era in condensed matter physics. In contrast to topological insulators, where only the surface states are interesting, a Weyl semimetal features unusual band structure in the bulk and on the surface, leading to novel phenomena and potential applications. This opens up unparalleled research opportunities, where both bulk-and surface-sensitive experimental probes can measure the topological nature and detect quantum phenomena. In the bulk, a Weyl semimetal has a band structure with band crossings, Weyl nodes, which are associated with definite chiral charges. Unlike the two-dimensional Dirac points in graphene, the surface-state Dirac point of a threedimensional topological insulator or the three-dimensional Dirac points in the bulk of a Dirac semimetal, the degeneracy associated with a Weyl node does not require any symmetry for its protection, other than the translation symmetry of the crystal lattice. The low-energy quasiparticle excitations of a Weyl semimetal are chiral fermions described by the Weyl equation, well known in highenergy physics, which gives rise to a condensed matter analogue of the chiral anomaly associated with a negative magnetoresistance in transport [16][17][18][19][20][21] . On the surface, the non-trivial topology guarantees the existence of surface states in the f...

Observation and Properties of the X(3872) Decaying to J/ψ π + π − in pp Collisions at √ s= 1.96 TeV

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Using 1/N C expansion and dispersion theory techniques, without relying on any explicit resonance lagrangian, we generalize the KSRF relation beyond the leading chiral order. Two sum rules for the low energy constants L 2 , L 3 and a new relation between resonance couplings are derived. A rather detailed examination to the new relation is also given.