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## Four Lectures on Web Formalism and Categorical Wall-Crossing

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**Four Lectures on Web Formalism and Categorical Wall-Crossing**Lyon, September 3-5, 2014 Gregory Moore, Rutgers University collaboration with DavideGaiotto & Edward Witten draft is ``nearly finished’’…**Plan for four lectures**Lecture 1: Landau-Ginzburg models; Morse theory and SQM; Motivation from spectral networks; Motivation from knot homology Lecture 2: Webology part 1: Plane webs. Definition of a Theory. Half-plane webs. Lecture 3: Webology part 2: Vacuum and Brane A categories; Examples. Lecture 4: Webology part 3: Domain walls and Interfaces; Composition of Interfaces; Parallel transport of Brane Categories; Categorified wall-crossing.**Three Motivations**IR sector of massive 1+1 QFT with N =(2,2) SUSY 2. Knot homology. 3. Spectral networks & categorification of 2d/4d wall-crossing formula [Gaiotto-Moore-Neitzke]. (A unification of the Cecotti-Vafa and Kontsevich-Soibelman formulae.)**d=2, N=(2,2) SUSY**We will be interested in situations where two supersymmetries are unbroken:**Outline**Introduction & Motivations Some Review of LG Theory Overview of Results; Some Questions Old & New LG Theory as SQM Boosted Solitons & Soliton Fans More about motivation from knot homology More about motivation from spectral networks**Example: LG Models - 1**Chiral superfield Holomorphic superpotential Massive vacua are Morse critical points: Label set of vacua:**Example: LG Models -2**More generally,… (X,): Kähler manifold. W: X C Superpotential(A holomorphicMorse function)**Boundary conditions for **Boundaries at infinity: Boundaries at finite distance: Preserve -susy: (Simplify: =d)**Fields Preserving -SUSY**U()[Fermi] =0 implies the -instanton equation: Time-independent: -soliton equation:**Projection to W-plane**The projection of solutions to the complex W plane are contained in straight lines of slope **Lefshetz Thimbles**If D contains x - If D contains x + Inverse image in X of all solutions defines left and right Lefshetz thimbles They are Lagrangiansubvarieties of X**Solitons For D=R**Scale set by W For general there is no solution. But for a suitable phase there is a solution This is the classical soliton. There is one for each intersection (Cecotti & Vafa) (in the fiber of a regular value)**Witten Index**Some classical solitons are lifted by instanton effects, but the Witten index: can be computed with a signed sum over classical solitons:**These BPS indices were studied by [Cecotti, Fendley,**Intriligator, Vafa and by Cecotti & Vafa]. They found the wall-crossing phenomena: Given a one-parameter family of W’s:**One of our goals will be to categorify this wall-crossing**formula.**Outline**Introduction & Motivations Some Review of LG Theory Overview of Results; Some Questions Old & New LG Theory as SQM Boosted Solitons & Soliton Fans More about motivation from knot homology More about motivation from spectral networks**Goals & Results - 1**Goal: Say everything we can about the theory in the far IR. Since the theory is massive this would appear to be trivial. Result: When we take into account the BPS states there is an extremely rich mathematical structure. We develop a formalism – which we call the ``web-based formalism’’ -- which shows that:**Goals & Results - 2**BPS states have ``interaction amplitudes’’ governed by an L algebra There is an A category of branes/boundary conditions, with amplitudes for emission of BPS particles from the boundary governed by an A algebra. (A and L are mathematical structures which play an important role in open and closed string field theory, respectively. Strangely, they show up here. )**Goals & Results - 3**If we have continuous families of theories (e.g. a continuous family of LG superpotentials) then we can construct half-supersymmetric interfaces between the theories. These interfaces can be used to ``implement’’ wall-crossing. Half-susy interfaces form an A 2-category, and to a continuous family of theories we associate a flat parallel transport of brane categories. The flatness of this connection implies, and is a categorification of, the 2d wall-crossing formula.**Some Old Questions**What are the BPS states on R in sector ij ? Fendley & Intriligator; Cecotti, Fendley, Intriligator, Vafa; Cecotti & Vafa c. 1991 Some refinements. Main new point: L structure What are the branes/half-BPS boundary conditions ? Hori, Iqbal, Vafa c. 2000 & Much mathematical work on A-branes and Fukaya-Seidel categories. We clarify the relation to the Fukaya-Seidel category & construct category of branes from IR.**Some New Questions -1**What are the BPS states on the half-line ?**Some New Questions - 2**Given a pair of theories T1, T2 what are the supersymmetric interfaces? Is there an (associative) way of ``multiplying’’ interfaces to produce new ones? And how do you compute it?**Some New Questions - 3**We give a method to compute the product. It can be considered associative, once one introduces a suitable notion of ``homotopy equivalence’’ of interfaces.**Some New Questions - 4**Using interfaces we can ``map’’ branes in theory T1, to branes in theory T2.**This will be the key idea in defining a ``parallel**transport’’ of Brane categories.**Example of a surprise:**The theory is massive: For a susy state, the field in the middle of a large interval is close to a vacuum: What is the space of BPS states on an interval ?**Does the Problem Factorize?**For the Witten index: Yes Naïve categorification? No!**Enough with vague generalities!**Now I will start to be more systematic. The key ideas behind everything we do come from Morse theory.**Outline**Introduction & Motivations Some Review of LG Theory Overview of Results; Some Questions Old & New LG Theory as SQM Boosted Solitons & Soliton Fans More about motivation from knot homology More about motivation from spectral networks**SQM & Morse Theory(Witten: 1982)**M: Riemannian; h: MR, Morse function SQM: Perturbativevacua:**Instantons & MSW Complex**Instanton equation: ``Rigid instantons’’ - with zero reduced moduli – will lift some perturbativevacua. To compute exact vacua: MSW complex: Space of groundstates (BPS states) is the cohomology.**Why d2 = 0**Ends of the moduli space correspond to broken flows which cancel each other in computing d2 = 0. A similar argument shows independence of the cohomology from h and gIJ.**1+1 LG Model as SQM**Target space for SQM: Recover the standard 1+1 LG model with superpotential: Two –dimensional -susy algebra is manifest.**Families of Theories**This presentation makes construction of half-susy interfaces easy: Consider a family of Morse functions Let be a path in C connecting z1 to z2. View it as a map z: [xl, xr] C with z(xl) = z1 and z(xr) = z2 C**Domain Wall/Interface**Using z(x) we can still formulate our SQM! From this construction it manifestly preserves two supersymmetries.**MSW Complex**Now return to a single W. Another good thing about this presentation is that we can discuss ijsolitons in the framework of Morse theory: Equivalent to the -soliton equation (Taking some shortcuts here….)**Instantons**Instanton equation At short distance scales W is irrelevant and we have the usual holomorphic map equation. At long distances the theory is almost trivial since it has a mass scale, and it is dominated by the vacua of W.**BPS Solitons on half-line D:**Semiclassically: Q-preserving BPS states must be solutions of differential equation Classical solitons on the positive half-line are labeled by:**Quantum Half-Line Solitons**MSW complex: Grading the complex: Assume X is CY and that we can find a logarithm: Then the grading is by**Half-Plane Instantons**Scale set by W**Solitons On The Interval**Now return to the puzzle about the finite interval [xl, xr] with boundary conditions Ll, Lr When the interval is much longer than the scale set by W the MSW complex is The Witten index factorizes nicely: But the differential is too naïve !**Outline**Introduction & Motivations Some Review of LG Theory Overview of Results; Some Questions Old & New LG Theory as SQM Boosted Solitons & Soliton Fans More about motivation from knot homology More about motivation from spectral networks