String Phenomenology 2021

12 Jul 2021, 08:50 → 16 Jul 2021, 13:00 US/Eastern

The annual String Phenomenology conference discusses recent progress in compactifications of string theory and their relation to particle physics and cosmology.

Due to the global pandemic, String Phenomenology 2021 will take place virtually. In order to allow for maximum participation across the globe, the conference scope has been reduced to a few hours each day to ameliorate global time zone constraints. To compensate for this reduced scope, there is a weekly seminar series on String Phenomenology.

---

Topics include

• Swampland and Quantum Gravity Conjectures 
• Machine learning and AI techniques in string theory
• Formal and mathematical aspects of string compactifications
• String model building in particle physics and cosmology

---

Plenary speakers

Lara Anderson (Virginia Tech) Anthony Ashmore (Chicago) Ralph Blumenhagen (MPI Munich) Xingang Chen (Harvard) Mirjam Cvetic (UPenn) Alon Faraggi (Liverpool) Anamaria Font (Caracas) Mariana Grana (Saclay) Thomas Grimm (Utrecht) Sergei Gukov (Caltech) Arthur Hebecker (Heidelberg) Jonathan Heckman (UPenn) Patrick Jefferson (MIT) Vishnu Jejjala (Witwatersrand) Sven Krippendorf (LMU Munich)

Magdalena Larfors (Uppsala) Andre Lukas (Oxford) Dieter Lust (LMU Munich) Liam McAllister (Cornell) Hans Peter Nilles (Bonn) Burt Ovrut (UPenn) Fernando Quevedo (Cambridge) Tom Rudelius (Berkeley) Sakura Schafer-Nameki (Oxford) Gary Shiu (Madison) Eva Silverstein (Stanford) David Tong (Cambridge) Angel Uranga (Madrid) Irene Valenzuela (Harvard) Timo Weigand (DESY)

---

Organizers

Local organizing committee: Jim Halverson, Brent Nelson, Matt Reece, Fabian Ruehle, Wati Taylor

---

Monday 12 July

1 Welcome

2 The gravitino and the swampland

In this talk we discuss a new swampland conjecture stating that the limit of vanishing gravitino mass corresponds to the massless limit of an infinite tower of states and to the consequent breakdown of the effective field theory. The proposal can be tested in large classes of models coming from compactification of string theory to four dimensions, where we identify the Kaluza-Klein nature of the tower of states becoming light. We point out a general relation between the gravitino mass and abelian gauge coupling in models with extended supersymmetry, which can survive also in examples with minimal supersymmetry. This allows us to connect our conjecture to the weak gravity conjecture or the absence of global symmetries in quantum gravity. We discuss phenomenological implications of our conjecture in (quasi-)de Sitter backgrounds and extract a lower bound for the gravitino mass in terms of the Hubble parameter.

Speaker: Dieter Lust (LMU-Muenchen/Max-Planck-Insitute Muenchen)

Slides

Video

3 Loop Algebras and Infinite Distance Limits

Speaker: Timo Weigand (Hamburg University)

Slides

Video

4 Swampland Conjectures in the High Temperature Phase of Quantum Gravity

The intuition for the swampland conjectures often derives from perturbative string theory. It is important to challenge their viability in other regimes like strong coupling or high temperature. Continuing previous work, we make an attempt to study swampland conjectures in the largely unknown high energy/temperature phase of string theory. Concretely, we analyze AdS and dS swampland conjectures in a three-dimensional higher spin theory with self-interacting matter, which contains conformal gravity and is almost topological. A theory of a similar type was proposed as the effective theory in the high energy phase of non-critical M-theory in 3D.

Speaker: Ralph Blumenhagen (Max-Planck-Institut fuer Physik, Munich)

Slides

Video

10:30 Coffee break

5 The singular-bulk problem of KKLT

After summarizing recent progress concerning the 10d description of gaugino condensation effects, I will focus on issues related to the parametric control of KKLT. Difficulties are expected since the radius of a throat providing an appropriate uplift is larger than the bulk 4-cycle supporting the non-perturbative effect. This may in principle be overcome at the price of strong warping in the bulk. However, according to our recent analysis, such warping generically leads to extended singularities in the bulk, threatening the construction. Possible escape routes will also be discussed.

Speaker: Prof. Arthur Hebecker (Heidelberg University)

Slides

Video

6 Microscopic description of brane gauginos

Different aspects of explicit dS proposals in string theory have recently come under intense scrutiny. One key ingredient is D7-brane gaugino condensation, which can be straightforwardly treated using effective 4d supergravity. However, it is also desirable to derive the relevant scalar potential directly from a 10d Lagrangian which captures the interactions among the various localized sources and the background fields. While progress in this endeavour has recently been made, issues related to divergences and non-localities related to the quartic gaugino coupling have remained problematic in the available proposals. I will discuss an explicitly local and finite D7-brane quartic gaugino term which reproduces the relevant part of the 4d supergravity action upon dimensional reduction. This is both a step towards a more complete understanding of 10d type-IIB supergravity as well as specifically towards better control of dS constructions in string theory involving gaugino condensation.

Speaker: Prof. Gary Shiu (University of Wisconsin-Madison)

Slides

Video

7 A Field Trip to the Boundary of the Swampland

In this talk I will discuss recent developments of the Swampland Distance Conjecture (SDC) in the context of AdS/CFT and N=1 4d EFTs. First, inspired by this conjecture and supported by SCFT data, we propose that all theories at infinite distance in the conformal manifold possess an emergent higher-spin symmetry, generated by an infinite tower of currents whose anomalous dimensions vanish exponentially in the distance. Interpreted gravitationally, they imply that approaching infinite distance in moduli space at fixed AdS radius, a tower of higher-spin fields becomes massless at an exponential rate that is bounded from below in Planck units. This relates to recent developments of the SDC in N=1 4d supergravity theories, where higher spin fields also play an essential role to comply with the SDC. In particular, in this context the SDC follows from the universal presence of an axionic BPS string, dubbed EFT strings, at every infinite field distance limit that satisfies the Weak Gravity Conjecture. The backreaction of EFT strings can be interpreted as an RG flow of their couplings, such that the string becomes asymptotically tensionless and the EFT eventually breaks down. We propose that any 4d EFT infinite distance limit can be realised as an EFT string RG flow, providing this way a bottom-up derivation of the SDC and a lower bound on the exponential rate. We check our proposal for large classes of 4d N=1 string compactifications and find that, even if there are typically other towers of particles becoming light, the mass of the leading tower always scales with the string tension as m^2\sim T^w in Planck units with w=1,2,3. Finally, I will explain what happens in the presence of a scalar potential.

Speaker: Dr Irene Valenzuela (Harvard University)

Slides

Video

Tuesday 13 July

8 Reinforcement Learning and String Model Building

I will describe recent work which applies reinforcement learning to model building in string theory. After a brief review, I will start with a toy model which involves the index of line bundles to show that reinforcement systems can be used to engineer geometries with prescribed topological properties. Reinforcement learning is then used to explore the set of monad bundles on Calabi-Yau manifolds, in the context of heterotic string models. It turns out that reinforcement systems can learn properties of these models efficiently and are capable of finding new string standard models.

Speaker: Prof. Andre Lukas (University of Oxford)

Slides

Video

9 Exploring the toroidal heterotic landscape

The moduli space of heterotic strings compactified on a d-dimensional torus contains special points where the gauge symmetry is enhanced only to certain groups of rank (d+16). We state criteria, based on embeddings into the Narain lattice, to establish whether a gauge group occurs or not and describe how to obtain the corresponding moduli. Orbifold compactifications that realize the CHL strings with gauge groups of reduced rank (d+8) are also analyzed. For d=1,2 we obtain all the maximally enhanced gauge groups. Connections to the Swampland Program will be discussed.

Speaker: Prof. Anamaria Font (Universidad Central de Venezuela)

Slides

Video

10 Gauged 2-form Symmetries in 6D SCFTs Coupled to Gravity

In this talk, I will discuss six-dimensional supergravity theories with superconformal sectors (SCFTs). Instances of such theories can be engineered using type IIB string theory, or more generally F theory. In the non-compact case, where gravity decouples, such constructions are known to provide examples of SCFTs with discrete 2-form global symmetries. Here, we will study the fate of these global symmetries in compact settings. As we will see, both for (2,0) and (1,0) theories, this fate is determined by the embedding of the SCFT charge lattices into the charge lattice of the supergravity theory. A primitive embedding implies that the 2-form symmetry is broken, while a non-primitive embedding allows a (gauged) subgroup to persist. This result will be illustrated with a few examples, and also connected to properties of 1-form symmetries in five dimensions.

Speaker: Dr Magdalena Larfors (Uppsala University)

Slides

Video

10:30 Coffee break

11 Hyperbolic compactification of M theory and accelerated expansion

Negative curvature manifolds are generic, including infinite sequences of hyperbolic manifolds with known metric. Their positive tree-level contribution to the potential energy and their rigidity properties, along with the stabilizing effects of the warp factor, make them well-suited for string/M theory compactification. This fits with empirical observations, which indicate positive potential energy generating accelerated expansion in the early and late universe, with significant constraints on low energy supersymmetry. In this talk we review recent work with De Luca and Torroba developing de Sitter and inflationary models in this setting. Along the way, we will describe ongoing work on repurposing cosmological models for machine learning optimization, and its application to solving PDEs (including those arising in string/M theory compactification).

Speaker: Prof. Eva Silverstein (Stanford)

Slides

Video

12 Hierarchies from Fluxes

I will present a construction of exponentially small superpotentials in type IIB compactifications and describe some applications to moduli stabilization.

Speaker: Prof. Liam McAllister (Cornell University)

Slides

Video

13 Hidden Sectors for Heterotic Theories and Gaugino Condensation

Detailed hidden sectors for a class of phenomenologically realistic B-L MSSM heterotic models will be presented and gaugino condensation - focusing on a wide range of condensation scales, the associated soft supersymmetry breaking terms and their low energy predictions - will be discussed.

Speaker: Prof. Burt Ovrut (University of Pennsylvania)

Slides

Video

12:30 Coffee Break

Panel Discussion: strINgCLUSION

on inclusivity and diversity in the string theory / string pheno community. The panelists are string theorists Silvia Penati and Marika Taylor, and sociologist Meytal Eran Jona.

Slides Penati

Slides Taylor

Video

Wednesday 14 July

14 A minimal structure for the string landscape

In the first part of the talk I will discuss some insights about moduli stabilization and the structure of the complex structure moduli space. I will then argue that a long conjectured and very non-trivial finiteness result about the flux vacua has now been proved. This will lead me to suggest that requiring ‘finiteness properties’ can be key to uncover the structure of the landscape in general. I will briefly introduce a framework that defines a general minimal structure with manifest finiteness properties.

Speaker: Mr Thomas Grimm (Utrecht University)

Slides

Video

15 Dynamical Cobordism and Swampland Distance Conjecture

I will discuss dynamical solutions in theories with non-trivial scalar potentials and their interplay with the Cobordism Conjecture and the Distance Conjecture.

Speaker: Prof. Angel Uranga (IFT, Madrid)

Slides

Video

16 Holography, 1-Form Symmetries, and Confinement

I will discuss 1-form symmetries in the context of holographic duals to confinement. Revising the Klebanov-Strassler solution, we determine the orgin of the 1-form symmetry in dual gravity solution. From the consistent truncation of the supergravity, we derive the chiral symmetry breaking as well as the mixed 't Hooft anomaly between the chiral symmetry and 1-form symmetry. In the far IR, the supergravity analysis furthermore allows deriving the TQFT that describes the gapped, confining phase.

Speaker: Prof. Sakura Schafer-Nameki (University of Oxford)

Slides

Video

10:30 Coffee break

17 Resolution independent structure in 4D F-theory flux vacua

I describe recent progress in exploring the landscape of 4D F-theory flux vacua with chiral massless spectra. The main result of this effort is a streamlined approach for computing vertical fluxes and associated chiral indices that combines tools in intersection theory with the low energy effective description of F-theory geometry. A key role is played by the vertical middle cohomology of smooth elliptic Calabi-Yau fourfolds resolving the singular F-theory limit, which we conjecture is independent of the choice of resolution and thus sheds light on geometric properties of the physics of the 4D vacuum​​​​. We analyze vertical flux backgrounds for models with simple gauge groups as well as models with a single abelian gauge factor; a notable example is the universal (SU(3) x SU(2) x U(1)) / Z_6 model constructed in 1912.10991. In all cases we study, we find that vertical flux backgrounds can produce nontrivial chiral indices for all anomaly-free chiral matter spectra, suggesting that F-theory geometry imposes no additional linear constraints beyond those implied by anomaly cancellation.​

Speaker: Patrick Jefferson (Massachusetts Institute of Technology)

Slides

Video

18 Constraints in smooth heterotic compactifications

I will describe a number of non-trivial constraints that arise in smooth heterotic compactifications and can lead to the resulting 4-dimensional, N=1 effective theories being highly restricted in form. These include topological vanishings of heterotic tri-linear couplings, as well other aspects of the potential and constraints on how gauge symmetry is broken and what discrete symmetries can appear.

Speaker: Prof. Lara Anderson (Virginia Tech)

Slides

Video

19 Life Without Holomorphy

Supersymmetry, in tandem with holomorphy, is often used as a tool in understanding the resulting low energy effective field theory generated by a string compactification. In this talk we discuss how to extract constraints on string compactifications as derived from robust topological structures associated with anomalies. We illustrate with two examples. The first is based on joint work with M. Cvetic, E. Torres and G. Zoccarato on a recent analysis of M-theory on a Spin(7) space given by a four-manifold of ADE singularities, in which we use parity anomalies to constraint the resulting effective field theory. The second is based on joint work with A. Debray, M. Dierigl and M. Montero on an analysis of duality anomalies in IIB string theory. This also leads us to the discovery of additional candidate topological sectors of IIB compactifications, with implications for the Swampland program.

Speaker: Prof. Jonathan Heckman (University of Pennsylvania)

Slides

Video

12:30 Coffee Break

Conference Panel Discussion

We will cover 2 topics:
* Climate, connectivity and conferences
* Future prospects and string pheno

Thursday 15 July

20 The Tadpole Problem

We conjecture that the charge induced by the fluxes needed to stabilise a large number of moduli grows linearly with the number of moduli. In this talk I will explain the conjecture, present its motivation, supporting evidence and consequences.

Speaker: Prof. Mariana Grana (CEA/Saclay)

Slides

Video

21 Exploring the string landscape with non-supervised learning

Speaker: Dr Sven Krippendorf (LMU Munich)

Slides

Video

22 Neural Network Approximations for Calabi-Yau Metrics

Ricci flat metrics for Calabi-Yau threefolds are not known analytically. We employ techniques from machine learning to determine numerical flat metrics for the Fermat and Dwork quintics. Measures that assess the Ricci flatness of the geometry decrease after training. Discrete symmetries of manifolds can be learned in the process of learning the metric.

Speaker: Prof. Vishnu Jejjala (University of the Witwatersrand)

Slides

Video

10:30 Coffee break

23 Numerical metrics and the swampland distance conjecture

Little is known about explicit metrics on Calabi-Yau manifolds, leaving us unable, for example, to compute particle masses or couplings in string models. I will review recent progress on using numerical methods to compute these metrics and the spectrum of the Laplacian on these spaces. I will finish with an example of what one can do with this new "data", namely probing the swampland distance conjecture in complex structure moduli space.

Speaker: Dr Anthony Ashmore (University of Chicago & Sorbonne Universite)

Slides

Video

24 Dimensional Reduction, Asymptotic Observables, and the Swampland

Dimensional reduction has proven to be a successful tool for sharpening Swampland conjectures. In this talk, we will see that dimensional reduction distinguishes a particular version of the de Sitter Conjecture, which holds that the strong energy condition must be satisfied in asymptotic regions of scalar field space. We will further connect this condition to older work on the difficulty of defining asymptotic observables in cosmological spacetimes. In the process, we will encounter a number of other swampland conjectures, such as the AdS Distance Conjecture, the Swampland Distance Conjecture, and the Weak Gravity Conjecture. This suggests that answers to fundamental questions about asymptotic observables in cosmology could help shed light on the Swampland program, and vice versa.

Speaker: Dr Tom Rudelius (UC Berkeley)

Slides

Video

25 Topology and early universe cosmology

Speaker: Prof. Sergei Gukov (California Institute of Technology)

Slides

Video

12:45 Virtual Conference Dinner

A casual meeting on gather town as a replacement for usual social interactions.

Friday 16 July

26 Unification of Flavor, CP and Modular Symmetries

Modular transformations of string theory are shown to play a crucial role in the discussion of discrete flavor symmetries in the Standard Model. They include CP transformations and provide a unification of CP with traditional flavor symmetries within the framework of the "eclectic flavor" scheme. The unified flavor group can be derived via the outer automorphisms of the "Narain Space Group". It is non-universal in moduli space and exhibits the phenomenon of "Local Flavor Unification", where different sectors of the theory (like quarks and leptons) can be subject to different flavor structures.

Speaker: Prof. Hans Peter Nilles (bctp, Univ. Bonn)

Slides

Video

27 On Realistic String Vacua and alpha' corrections

Speaker: Prof. Fernando Quevedo (University of Cambridge)

Slides

Video

28 Towards Exact matter spectra of F-theory MSSMs

Within the largest, currently known, class of one Quadrillion globally consistent F-theory StandardModels with gauge coupling unification and no chiral exotics, we present advances in the determination of the full matter spectra, which are governed by
fractional powers of line bundles, also known as root bundles on matter curves. We systematically determine root bundle constraints that result in the matter spectra without vector-like chiral exotics. On a technical level, we employ a diagrammatic description of root bundles on nodal matter curves and subsequently a deformation theory to smooth matter curves.

Speaker: Prof. Mirjam Cvetic (University of Pennsylvania)

Slides

Video

10:30 Coffee break

29 How to give chiral fermions a mass

I explain how to give chiral fermions a mass without breaking the chiral symmetry. As an example, I'll explain how to give one generation of fermions in the Standard Model a mass without breaking electroweak symmetry.

Speaker: Prof. David Tong (University of Cambridge)

Slides

Video

30 Primordial density perturbations as probes of high energy physics

The energy scale of the primordial universe can be much higher than those accessible by any terrestrial colliders. We show that particles excited during this epoch, either through a classical or quantum-mechanical process, left their signatures in the primordial density perturbations which seeded the large-scale structure of the universe today. The encoded information includes the mass and spin spectra of these particles, as well as the time dependence of the background scale factor. This provides a unique opportunity to study high energy physics, as well as to search for direct evidence for the inflation or an alternative scenario. We discuss the observational prospects of some of these signatures in the near future.

Speaker: Xingang Chen (Harvard University)

Slides

Video

31 "Modular maps" and BSM phenomenology

In the context of the free fermionic construction I define "Modular maps" as maps induced by groups of 4 periodic fermions that map between different sectors within vacua, or between different vacua. I will discuss three examples of such maps:
1. S-map and spacetime supersymmetry;
2. x-map, spinor-vector duality and light Z' phenomenology. Spinor-vector duality as a tool to explore the moduli spaces of (2,0) string compactifications will also be discussed.
3. {\tilde S}-map and non-tachyonic string vacua from tachyonic 10D vacua.
"modular maps" are ubiquitous in compactifications to 2 dimensions reflecting the large symmetry space of 24 dimensional lattices. Initial steps to explore this symmetry structure will be discussed.

Speaker: Prof. Alon Faraggi (University of Liverpool)

Slides

Video