# High Energy Theory Seminars.

These seminars take place every Wednesday at 15:00 in Room S.

High Energy Theory seminars are one-hour research seminars dedicated to high energy physics, cosmology and astrophysics, with a special emphasis on particle physics (PH, light gray) or string theory (TH, dark gray). Seminars intended for the whole group are marked in light yellow.

The organizers of the String Theory seminars (TH) are D. Cassani, L. Martucci and S. Giusto. The organizers of the Particle Physics Phenomenology seminars (PH) are M. Mandal, P. Mastrolia, P. Paradisi and A. Titov.

Sometimes we also host some special events, like workshops and conferences. These are highlighted in light blue.

You can click on the titles to see the slides of the talk.

### 2019/2020 Calendar PRINT

Date | Speaker (Affiliation) |
Title | Room &Time |

09-10-19 Wednesday | Stefano Di Vita INFN MIlan | Differential equations and iterated integrals for massive multi-loop Feynman integrals Abstract In this talk I will give an overview of the differential equations method for the calculation of multiloop Feynman diagrams, which we recently applied to several cases where massive legs and propagators are present. I will discuss how the systems of differential equations arise, how they are brought in canonical form with the help of the Magnus exponential, what functions can be used to give a closed-form representation of the solution, and how they are analytically continued to the kinematic regions of interest. Some examples will be given, such as the master integrals for the NNLO QED corrections to mu-e scattering, which are crucial for the physics program of MUonE, an experiment proposed with the goal of determining the leading hadronic contribution to the muon g-2 in a novel way. | Aula Voci 14:00 |

23-10-19 Wednesday | Kei Yamamoto Univ. Zurich | B-physics anomaly and SM flavor hierarchies Abstract The observed hierarchies in the Standard Model(SM) Yukawas represent one of the most interesting open problems. On the other hand, LHC, Belle and BABAR experiments have reported the test of lepton flavor universality in B-meson decays which seem to indicate a coherent pattern of deviations from the SM predictions. In this talk, I will review these hints and discuss the implications derived from effective field theory approach based on U(2) flavor symmetry, and leptoquark models. | Aula Voci 15:00 |

6-11-19 Wednesday | David Marzocca INFN, Trieste | From B-anomalies to Kaon and precision physics Abstract In this talk I will review some new physics interpretations for the B-physics anomalies, and their implications for other flavor and precision observables. Regarding the neutral-current anomalies, I will describe the Rank-One Flavor Violation hypothesis and how the setup can be constrained with other B-meson and Kaon decays. I will then discuss a combined interpretation of both neutral- and charged-current anomalies, with a complete one-loop matching to the SM Effective Field Theory, and some preliminary results for flavor as well as electroweak precision observables. | Aula Voci 15:00 |

13-11-19 Wednesday | Anthony Charles KU Leuven | Euclidean Black Saddles and AdS4 Black Holes Abstract The entropy of a class of asymptotically-AdS4 black holes can be reproduced by the partition function of the dual ABJM theory via localization. However, establishing this match requires a particular extremization over field theory parameters. This begs the question: what are the bulk dual geometries when we do not extremize in the field theory? In this talk, I will show that these bulk duals are smooth Euclidean geometries with finitely-capped throats. These geometries generically have no clear interpretation in Lorentzian signature, but when their throat becomes infinitely long they become black holes with an AdS2 near-horizon geometry. For any set of field theory parameters whose extremization is compatible with a black hole, we find a large family of Euclidean geometries whose on-shell action reproduces the ABJM partition function exactly, without the need to extremize, thus establishing a more complete understanding of AdS4/CFT3 holography. | Aula Voci 15:00 |

20-11-19 Wednesday | Itamar Shamir SISSA | Defect anomalies and marginal couplings Abstract Conformal anomalies play a prominent role in the study of conformal field theories with important applications to RG flows. In the presence of a boundary (or more generally defects) new types of conformal anomalies appear which are localised on the defect. I will review the basic aspects of defects in conformal field theory and the motivation to study them, focusing mainly on new types conformal anomalies and their properties. I will show that unlike the standard conformal anomalies in the bulk, the defect anomalies can have non-trivial dependence on exactly marginal couplings. | Aula Voci 15:00 |

27-11-19 Wednesday | Luca Di Luzio Pisa U. | Rethinking the QCD axion Abstract Four decades after its prediction the axion remains the most compelling solution to the strong CP problem and a well motivated dark matter candidate, inspiring several ultrasensitive experiments based on axion-photon mixing. After reviewing the axion solution of the strong CP problem and the experimental landscape of axion searches, I will focus on some recent developments in axion model building which show that the QCD axion parameter space is much larger than what traditionally thought. The implications for astrophysical limits and future experiments will be discussed as well. | Aula Voci 15:00 |

04-12-19 Wednesday | Alexander Mitov Cambridge U. | Latest developments in NNLO calculations for 2-to-3 LHC processes Abstract There are two parallel lines of development in perturbative QCD: phenomenological predictions for high-precision LHC observables (typically in NNLO) and the calculation of multi-loop scattering amplitudes. These two research directions are not independent, however, since the corresponding two-loop amplitude is an essential ingredient to any NNLO calculation. I will review the recent progress in both scattering amplitudes and NNLO calculations and, as an example of their cross-talk, will present the first 2-to-3 NNLO calculation: 3 photon production at the LHC. The talk should be of interest to both formal and pheno-oriented theorists as well as experimentalists since it will lay out the deep interconnection between branches of theoretical particle physics and LHC measurements. | Aula Voci 15:00 |

16-12-19 Monday | Konstantinos Papadopoulos NCSR-DEMOKRITOS | Recent progress on two-loop massless pentabox integrals with one off-shell leg Abstract In the last decade we had the chance to witness some very important and long-awaited discoveries in physics such as the identification of the Higgs particle, the detection of the gravitational waves and the first image of a BH horizon. In all cases very sophisticated experimental instruments were used along with robust theoretical predictions in order to reveal underlying tiny signals that made these discoveries possible. In particle physics, higher-order perturbative corrections to scattering amplitudes proved important in exploiting the LHC data. Next-to-leading order accuracy is the nowadays theory standard. Nevertheless, in order to fully exploit the coming LHC high-luminosity data and even more so, the potential of the Future Circular Collider (FCC), we need to go beyond NLO. In this talk I will review the recent progress in NNLO perturbative calculations with emphasis to two-loop amplitude reduction and to the analytic computation of the Master Integrals involved. I will also report on the most recent results on non-planar five-point two-loop Master Integrals family with one off shell external particle (arXiv:1910.06275). | Aula Voci 15:00 |

18-12-19 Wednesday | Various speakers | MathemAmplitudes 2019 Abstract | Archivio Antico 9:00 |

19-12-19 Thursday | Various speakers | MathemAmplitudes 2019 Abstract | Aula Rosino 9:00 |

20-12-19 Friday | Various speakers | MathemAmplitudes 2019 Abstract | Aula Rosino 9:00 |

08-01-20 Wednesday | Federico Bonetti Johns Hopkins U. | Anomaly inflow, geometric engineering, and holography Abstract A large class of 4d SCFTs can be engineered by wrapping a stack of M5-branes on a compact space, possibly with defects. ‘t Hooft anomalies are crucial observables for such theories, which often do not admit any known Lagrangian description. Building on the seminal work of Freed, Harvey, Minasian, Moore, we develop systematic tools for extracting the ‘t Hooft anomalies of a geometrically engineered 4d theory using anomaly inflow from the M-theory bulk. We exemplify our tools by studying a class of setups with M5-branes probing a C^2/Z_2 singularity. We argue that these setups define 4d SCFTs which are dual to a class of AdS_5 solutions—first discussed by Gauntlett, Martelli, Sparks, Waldram—whose field theory interpretation has been a longstanding puzzle. | Aula Voci 15:00 |

15-01-20 Wednesday | Joachim Kopp CERN | "Hot" Topics in Astroparticle Physics - Finite Temperature Effects in Dark Matter Phenomenology Abstract We discuss the possible impact of thermal quantum field theory effects - in particular phase transitions - on the phenomenology of dark matter. After reviewing the physics of phase transitions in general, we discuss the electroweak phase transition in the early Universe, and we show how this transition can be modified in theories featuring more than one Higgs boson. We then argue that the observed abundance of dark matter today may have been determined by the dynamics of the electroweak phase transitions, rather than by conventional thermal freeze-out. We conclude by outlining possible tests of this hypothesis at the LHC and elsewhere. | Aula Voci 15:00 |

22-01-20 Wednesday | Pedro F. Ramirez AEI, Potsdam | Stringy extremal black holes Abstract In the context of string theory, classical black hole solutions to the standard theories of supergravity receive corrections due to the presence of higher-derivative terms in the effective perturbative expansion. These corrections generically modify the physical properties of the solution and affect the allowed region of parameter space. I will review recent advances in the field for the case of extremal black holes (supersymmetric and not supersymmetric) of heterotic string theory, where the analytical resolution of the problem is feasible. In particular, I will describe the implications of our results in the context of the weak gravity conjecture, as the charge-to-mass ratio of extremal black holes is sometimes modified. Likewise, I will describe how the entropy, charges and mass are generically modified and emphasize the importance of properly taking into account such modification, discussing the implications for the so-called 'small black holes'. | Aula Voci 15:00 |

29-01-20 Wednesday | Alessandro Vicini Milan U. | Precision tests of the SM at the LHC and at future colliders Abstract I will review the high-precision determination of the EW parameters at the LHC and the perspectives offered by future colliders. Several recent technical developments allow a promising determination of the W boson mass and of the weak mixing angle, even in the complex environment of a high-luminosity hadron collider. The impact on the precision tests of the Standard Model will be discussed. I will eventually comment on the extreme phenomenological challenges emerging in the elaboration of the physics program of future electron-positron colliders. | Aula Voci 15:00 |

04-02-20 Tuesday | Ivano Basile Scuola Normale | On Vacuum Stability without Supersymmetry: Brane dynamics, bubbles and holography Abstract In this talk I will present some recent results on instabilities of anti-de Sitter flux compactifications in effective field theories arising from non-supersymmetric string models, namely the USp(32) and U(32) orientifold models and the SO(16) x SO(16) heterotic string. We discuss perturbative and non-perturbative instabilities and frame the vacua in terms of brane stacks, analyzing their back-reacted geometry and reproducing AdS its near-horizon limit. Then we describe the instabilities as branes separating from the stack, computing the associated decay rate matching a probe brane computation with the semi-classical Coleman-de Luccia result. We conclude briefly discussing possible implications regarding the fate of unstable vacua in string theory beyond the semi-classical limit, connecting this scenario to holographic renormalization group flows and to a "de Sitter on a brane" construction that was outlined in the recent literature. | Aula Voci 14:00 |

12-02-20 Wednesday | Severin Lüst Ecole Polytechnique | Uplifting Runaways and the Tadpole Problem Abstract I will discuss a mechanism which can lead to a possible instability of the KKLT construction for de Sitter vacua. The sphere at the tip of a warped deformed conifold throat can be destabilized by antibranes placed in the throat. Consequently, the stabilization of moduli should not be treated independently from the antibrane uplift in KKLT-like scenarios. A similar bound can be found for numerically constructed Klebanov-Strassler black holes. I will furthermore comment on the mass scales and the effects on supersymmetry breaking in this setup. This conifold destabilization mechanism can be avoided by turning on a large amount of flux on the sphere, but tadpole cancelation constraints the hierarchy of scales in a type IIB flux compactification. Even though sufficiently large tadpole bounds can be realized in string theory, they always come at the expense of a large number of moduli. The stabilization of these moduli by fluxes contributes to the tadpole condition as well, reducing the maximal flux on the KS-throat.I will discuss this problem for the example of M-theory on K3 x K3. | Aula Voci 15:00 |

20-05-20 Wednesday | Simon Badger Durham University | Simplifying multi-leg amplitude computations Abstract Scattering amplitudes are an essential ingredient in precision predictions for colliders. However, their computation can be so complicated that theoretical errors are lagging behind the experiments. I will describe some new approaches to multi-leg two-loop amplitudes in which the traditional bottleneck of algebraic complexity can be sidestepped using modular arithmetic. In an alternative direction, I will also explore how neural networks may be useful for approximating functions that are difficult to evaluate numerically. | Zoom 15:00 |

27-05-20 Wednesday | Pietro Baratella Technical University of Munich | Renormalization of Higher-Dimensional Operators from On-shell Amplitudes Abstract On-shell amplitude methods allow to derive one-loop renormalization effects from just tree-level amplitudes, with no need of loop calculations. We derive a simple formula to obtain the anomalous dimensions of higher-dimensional operators from a product of tree-level amplitudes. We show how this works for dimension-6 operators of the Standard Model, providing explicit examples of the simplicity, elegance and efficiency of the method. Many anomalous dimensions can be calculated from the same Standard Model tree-level amplitude, displaying the attractive recycling aspect of the on-shell method. With this method, it is possible to relate anomalous dimensions that in the Feynman approach arise from very different diagrams, and obtain non-trivial checks of their relative coefficients. We compare our results to those in the literature, where ordinary methods have been applied. | Zoom 15:00 |

03-06-20 Wednesday | Alberto Bragagnolo University of Padova and INFN Padova | Measurement of the CP-violating phase phi_s with B_s mesons by CMS Abstract Since no direct evidence of new physics has been found at the CERN LHC so far, precision tests of the standard model (SM) of particle physics have become increasingly important. The study of CP violation (CPV) in decays of bottom hadrons provides a precision test of the predictions of the SM and represents a sensitive probe to search evidence for physics beyond the SM. The B_s -> J/psi phi(1020) decay is one of the most interesting process to study CP violation in the SM, since it offers the possibility to measure the weak phase phi_s. This CPV phase arises from the interference between B_s decays proceeding directly and through mixing to a CP final state. In the SM it is related to the elements of the Cabibbo-Kobayashi-Maskawa and its numerical value is predicted to be small but non-zero. In this seminar, the latest measurement of phi_s by the CMS Collaboration will be presented. This result is obtained using proton-proton collisions at center-of-mass energy of 13 TeV collected by CMS in 2017 and 2018 at the LHC. | Zoom 15:00 |

10-06-20 Wednesday | Pieter Bomans KU Leuven | Compact and wrapped branes: A holographic exploration into strongly coupled physics Abstract In this talk I will start with a brief overview of strongly coupled physics and the (theoretical) problems that arise when we want to describe them. Next, I will show how holographic methods can help us to better understand such systems. Specifically, I will introduce you to two setups, spherical -- or more generally compact -- branes on the one hand and wrapped branes on the other hand. By putting branes on a compact worldvolume we introduce a scale which acts as an IR cut-off in the dual field theory which in turn is reflected in the supergravity solutions as a regular cap-off of the geometry. This regularity allows us to formulate a detailed version of the holographic dictionary and perform a precision test of holography in a non-conformal setting. Secondly, by wrapping only part of the brane on a compact cycle we obtain an RG flow across dimensions starting in D dimensions and ending in D-2 dimensions, where we consider two-dimensional cycles, aka Riemann surfaces. I will first introduce you to the concept of punctures on the Riemann surface and next show how one can include such punctures in the dual supergravity description. | Zoom 15:00 |

17-06-20 Wednesday | Joao Penedo CFTP, IST Lisbon | The double cover of modular S_4 as a flavour symmetry I Abstract The interest in explaining fermion masses and mixing via a modular symmetry has been revived in the last couple of years. In such a scheme, finite quotients of the modular group take the role of discrete non-Abelian flavour symmetries, which can be consistently combined with a generalised CP (gCP) symmetry. In this talk, we will present the necessary tools for model building, focusing on the finite modular group S_4', a double cover of the modular permutation group S_4. We construct the lowest-weight modular forms in a minimal fashion, in terms of two Jacobi theta constants. These combine to form a triplet representation of S_4' not present in S_4. We further show that within S_4' there are two options to define the gCP transformation, and discuss the possible residual symmetries in theories based on modular and CP invariance. Finally, we provide examples of application of our results by constructing phenomenologically viable lepton flavour models. | Zoom 15:00 |

17-06-20 Wednesday | Pavel Novichkov SISSA and INFN Trieste | The double cover of modular S_4 as a flavour symmetry II Abstract See the abstract above. | Zoom 15:30 |

24-06-20 Wednesday | Gary Shiu Wisconsin University | Weak Gravity Conjecture: Evidence and Applications Abstract String theory seems to offer an enormous number of possibilities for low energy physics. The huge set of solutions is often known as the String Theory Landscape. In recent years, however, it has become clear that not all quantum field theories can be consistently coupled to gravity. Theories that cannot be ultraviolet completed in quantum gravity are said to be in the Swampland. The weak gravity conjecture is arguably the most well-studied swampland criterion and it has wide-ranging phenomenological applications. In this talk, I’ll present evidence in support of this conjecture and discuss its phenomenological applications, e.g., in constraining certain models of inflation that gives detectable gravitational waves. | Zoom 15:00 |

01-07-20 Wednesday | Silvia Pascoli Durham University | Sterile neutrinos and dark sectors: theory and phenomenology Abstract Sterile neutrinos constitute a minimal extension of the Standard Model while offering an explanation for neutrino masses, the baryon asymmetry of the Universe and even dark matter. Barring theoretical prejudice, their lepton number violating masses can go from the eV to the GUT scale. In this talk, I will focus on the MeV-GeV range whose phenomenology is varied and interesting. I will review the searches for these particles, from peak searches to decays and lepton number violating signatures and highlight the potential that future experiments have in significantly improving the bounds thanks to more intense sources and larger detectors. I will then discuss a richer GeV scenario, in which sterile neutrinos are part of a gauge extension of the Standard Model: this will contain a new gauge boson, a dark scalar needed to break the U(1)' symmetry and some fermions charged under the new U(1)’. The sterile neutrinos allow the new fermion sector to communicate with the Standard Model, via the neutrino portal. I will show with a concrete anomaly-free example that the phenomenology of such model can be very different from the standard signatures of dark photons, sterile neutrinos and dark scalars. In particular, I will present a solution to several anomalies or excess which have been found: the anomalous magnetic moment of the muon via kinetic mixing, a mild unexplained excess of photon-like events in Babar, the MiniBooNE low energy excess, an old and still unexplained anomaly, namely the PS191 and E816 excess of electron-like events, and, with some further constraints on the model, also the KOTO recent anomaly. | Zoom 15:00 |

08-07-20 Wednesday | Alejandro Ruiperez IFT Madrid | Heterotic black holes with alpha' corrections Abstract I will show how the study of (supersymmetric) black holes with non-trivial Yang-Mills fields can naturally lead to the study of alpha' corrections in the context of the heterotic string. I will then discuss the construction of a general family of alpha’-corrected solutions preserving 1/4 of the supersymmetries of the theory and describing superpositions of solitonic 5-branes, fundamental strings, momentum and, optionally, Kaluza-Klein monopoles. After compactification, particular members of this family describe three- and four-charge extremal black holes in five and four dimensions, respectively. I will focus mainly on the four-dimensional ones, paying special attention to the so-called small black holes, which are two-charge black holes with a singular horizon in the zeroth-order (supergravity) approximation. I will argue that the alpha' corrections do not stretch the horizon and that the system remains singular even after the first-order alpha' corrections are taken into account. | Zoom 15:00 |

15-07-20 Wednesday | Clifford Cheung California Institute of Technology | From Gluon Scattering to Black Hole Orbits Abstract The study of scattering amplitudes has uncovered extraordinary dualities linking real-world particles such as gravitons, gluons, and pions. We discuss how these developments have been amalgamated with classic tools from effective field theory to derive new results relevant to the search for gravitational waves at LIGO. This approach has produced now state-of-the-art results on conservative orbital dynamics of binary black holes in the post-Minkowskian expansion. We also comment on recent work extending this framework to include tidal effects and spin. | Zoom 15:00 |

### Links to Past Calendars

High energy theory seminars for the a.y. 2017/2018, 2016/2017, 2015/2016, 2014/2015, 2013/2014, 2012/2013, 2011/2012, 2010/2011, 2009/2010, 2008/2009, 2007/2008, 2006/2007, 2005/2006