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The 4D Superconformal Index from q-deformed 2D Yang-Mills

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We identify the 2d topological theory underlying the N=2 4d superconformal index with an explicit model: q-deformed 2d Yang-Mills. By this route we are able to evaluate the index of some strongly-coupled 4d SCFTs, such as Gaiotto's T_N theories.
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... Coulomb branch operator Integrated current [19] Orbifold C 2 /Z M Change CFT to coset [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] 1d Change CFT by Drinfeld-Sokolov reduction [72][73][74][75][76][77][78][79][80][81][82][83][84][85] Symmetry-breaking wall Verlinde loop [43] 3d S-duality domain wall Modular kernel [86][87][88][89][90] Boundary Boundary CFT [91,92] 4d Coupling to a tinkertoy Vertex operator [5,[93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108][109] We discuss some offshoots of the AGT correspondence in section 9. Placing the 6d theory onto other product spaces M × C (with some twist) leads to interesting relations between theories on M and on C: the index/qYang-Mills (YM) correspondence [110], the 3d/3d correspondence [111], the 2d/4d correspondence [112]. In another direction, some class S theories (especially linear quiver gauge theories) can be realized as reductions of 5d N = 1 theories. ...
... The wave functions can be computed order by order in p, q, t, but are not known in closed form. In the Schur limit q = t correlators are functions of q only (p-dependent terms are Q-exact), wavefunctions are proportional to Schur polynomials, and the corresponding TQFT is q-deformed 2d Yang-Mills theory [110]. In the more general Macdonald limit p = 0, wavefunctions are essentially Macdonald polynomials in q, t and the TQFT must be deformed by changing the measure in the path integral of q-YM theory [667]. ...
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