GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s51> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s50> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s49> GLYPH<s48>GLYPH<s46>GLYPH<s48> GLYPH<s48>GLYPH<s46>GLYPH<s49> GLYPH<s48>GLYPH<s46>GLYPH<s50> GLYPH<s48>GLYPH<s46>GLYPH<s51> GLYPH<s48>GLYPH<s46>GLYPH<s51> GLYPH<s48>GLYPH<s46>GLYPH<s50> GLYPH<s48>GLYPH<s46>GLYPH<s49> GLYPH<s48>GLYPH<s46>GLYPH<s48> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s49> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s50> GLYPH<s32> GLYPH<s32> GLYPH<s32> -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.3 0.2 0.1 0.0 -0.1 -0.2 400 350 300 250 200 150 x10 3 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.2 0.0 0.2 SpinGLYPH<.notdef>polarization IntensityGLYPH<.notdef>(arb.GLYPH<.notdef>units) k (ÅGLYPH<.notdef>GLYPH<.notdef>) x -1 k (ÅGLYPH<.notdef>GLYPH<.notdef>) x -1 -0.2 -0.1 0.0 0.1 0.2 0.3 k (ÅGLYPH<.notdef>GLYPH<.notdef>) x -1 2 3 4 P y P z P x (e) (f) EGLYPH<.notdef>GLYPH<.notdef>=GLYPH<.notdef>-20GLYPH<.notdef>meV B EGLYPH<.notdef>GLYPH<.notdef>=GLYPH<.notdef>-20GLYPH<.notdef>meV B I I$^{y}$ y -0.2 -0.1 0.0 0.1 0.2 -0.2 -0.1 0.0 0.1 0.2 EGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>(eV) B -0.3 M GLYPH<c71> M M Low High -0.1 0.0 0.1 -0.1 0.0 0.1 Bi Te 2 3 (g) -0.2 -0.1 0.0 -0.2 -0.1 0.0 0.1 0.2 -0.1 0.0 0.1 -0.4 -0.2 0.0 k (ÅGLYPH<.notdef>GLYPH<.notdef>) x -1 k (ÅGLYPH<.notdef>GLYPH<.notdef>) x -1 k (ÅGLYPH<.notdef>GLYPH<.notdef>) y -1 (d) TunedGLYPH<.notdef>BiGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>CaGLYPH<.notdef>GLYPH<.notdef>Se$_{3}$ k x k y M (h) K 5 86 90 5 2-$_{GLYPH<c100>}$ GLYPH<c100> (a) (b) (c) MomentumGLYPH<.notdef>distributionGLYPH<.notdef>ofGLYPH<.notdef>spin MomentumGLYPH<.notdef>distributionGLYPH<.notdef>ofGLYPH<.notdef>spin Spin-helicalGLYPH<.notdef>DiracGLYPH<.notdef>cones FIG. 1: Hsieh, Xia, Qian et al. , NATURE 460, 1101 (2009). 5, 12], which are predicted to manifest in several ways, provided the system can be tuned to the topological transport regime where the charge density vanishes (analogous to the charge neutrality point in graphene [19]). These manifestations include an anomalous half-integer quantization of Hall conductance [3–6], a realization of Majorana fermions (particles with anyon exchange statistics that differs from the conventional Bose or Fermi-Dirac statistics) [7, 8], and a generation of fractionally charged particles [9]. Helical fermions are believed to exist on the edges of certain types of three-dimensional (3D) topological insulators [3, 4, 12], with material candidates Bi$_{2}$X$_{3}$ (X = Se, Te) recently proposed based on observations [15, 17] and models [4, 20]. However, these materials cannot be used to detect helical Dirac fermion physics for three reasons: First, the helical properties of the surface electrons are unknown and depend on the materials class. Second, their electronic structure is not in the topological transport regime, thus not allowing any of the interesting topological insulator experiments to be performed to this date. And third, unlike 2D quantum Hall Dirac systems such as graphene [10, 19], 3D topological insulators cannot be tuned to this zero carrier density regime through electrical gating, which has prevented a revolution like that witnessed for graphene [10] from taking place for topological insulators. To determine the key helical properties of the surface electrons near the Fermi level ( E$_{F}$ ) in the candidate Bi$_{2}$X$_{3}$ class, we performed spin- and angle-resolved photoemission 3