HEP Theory Seminars SS 18

 

Thu 12.04.2018, 16.30 h

J. Heeck (Université libre de Bruxelles)

Rare processes and the fate of baryon and lepton numbers

Abstract:
Neutrino oscillations are a definite proof of lepton flavor violation, but we have yet to see charged lepton flavor violation. We categorize lepton-flavor-violating processes by their quantum numbers and show how their discovery can be interpreted model independently, studying in particular which processes are required to establish that the entire flavor group is broken. Including baryon number in the discussion allows us to identify new proton decay modes that could dominate over the usually considered channels.

 

Thu 19.04.2018, 16.30 h

T. Tenkanen (Queen Mary University of London)

The Dawn of FIMP Dark Matter

Abstract:
I will present an overview of scenarios where the observed Dark Matter (DM) abundance consists of Feebly Interacting Massive Particles (FIMPs), produced non-thermally by the so-called "freeze-in" mechanism. In contrast to the usual freeze-out scenario, frozen-in FIMP DM interacts very weakly with particles in the visible sector and never attained thermal equilibrium with them in the early Universe. This makes frozen-in DM very difficult but not impossible to test. In this talk I will present the freeze-in mechanism and its variations previously considered in the literature, compare them to the standard DM freeze-out scenario, discuss several aspects of model building, and pay particular attention to observational properties of such feebly interacting DM.

 

Thu 26.04.2018, 16.30 h

H. Chawdhry (Cambridge)

PMC renormalisation scale-setting in top pair production

Abstract:
In perturbative QCD, uncertainties in theoretical predictions are estimated by varying the renormalisation scale. The Principle of Maximum Conformality (PMC) has been proposed as a way to choose values for this scale. In this talk I will review the PMC and will identify 3 ambiguities in the PMC scale-setting procedure. I will then apply the PMC to top pair production and study the numerical impact of these ambiguities.

 

Thu 03.05.2018, 16.30 h

L. Darme (Warschau)

Hunting light dark sectors in sub-GeV dark matter scenarios

Abstract:
Minimal scenarios with light (sub-GeV) dark matter are usually accompanied by a correspondingly light "dark sector". We will show that the presence of the latter usually leads to strong bounds from cosmology and
bright detection prospects at fixed target experiments and colliders. In particular, we will focus on the consequences of the presence of an often long-lived dark Higgs boson whose mass scale is the same as that of the typical vector mediator.

 

Thu 17.05.2018, 16.30 h

S. Ferrario (University of Milano-Bicocca)

Top-mass studies via new NLO+PS generators

Abstract:
The most precise measurements of the top mass heavily rely on the use of Monte Carlo event generators. Recently, many efforts have been made in order to implement NLO generators able to handle radiation in presence of intermediate resonances. In this talk I will show comparison among kinematic distributions obtained with three POWHEG BOX generators for top-pair production which describe the leptonic top decay with different level of accuracy. These NLO generators must be matched to a shower Monte Carlo program, like Pythia or Herwig. I will present the results obtained for both showering models.

 

Thu 24.05.2018, 16.30 h

M. Valli (INFN Rome)

Hints for New Physics from b to s transitions

 

Thu 07.06.2018, 16.30 h

M. Gerbino (Stockholm)

Parity-violating scenarios in the early Universe in light of Cosmic Microwave Background data

Abstract:
Parity symmetry is one of the essential properties of gravity and electromagnetism. It is preserved in the description provided by general relativity and standard Maxwell electromagnetism, while its breaking might represent a striking evidence of non-standard physics. Parity violation may occur within inflationary models through circularly polarised gravitational waves, which are referred as chiral gravity models, and also at later times through a new Chern-Simons-like coupling via the so called cosmological birefringence effect. In this talk, I will show how the polarisation anisotropies of the cosmic microwave background (CMB) can be used to test parity symmetry. I will present results from current cosmological data, as well as discuss the sensitivity of the next generation of CMB experiments to parity-violating models.

 

Thu 14.06.2018, 16.30 h

D. Schwarz (Bielefeld)

The cosmic dipole

Abstract:
The dipole of the cosmic microwave background defines a reference frame for cosmology. Is is assumed since its discovery that the CMB dipole is caused by the proper motion of the Solar system. This hypothesis leads to the prediction that the corresponding Doppler shifts and abberation effects are universal to all frequencies. Thus the CMB frame is assumed to be the comoving frame of freely falling Friedmann observers, which is essential in the analysis of many cosmological observables such as the Hubble diagram. As any fundamental hypothesis, also the proper motion hypothesis must be tested. We present results from a suite of cosmic radio dipole measurements based on public continuum catalogues across frequencies. We find that the cosmic radio dipole agrees with the direction of the CMB dipole within errors but shows an statistically significant excess in amplitude which increases with wavelength. The limitations and consequences of our finding are discussed. Finally, I'll present the prospects that SKA will offer confirm or reject the proper motion hypothesis at high significance.

 

Thu 21.06.2018, 16.30 h

F. Herzog (NIKHEF)

The R* method and five loop applications in QCD

Abstract:
The R*-operation is a generalisation of BPHZ renormalisation to the subtraction of euclidean infrared divergences. It can be used to efficiently compute anomalous dimensions of arbitrary QFT correlators at L loops from single scale propagator integrals of L-1 loops. I will explain the method and present several results which were obtained with it, such as the 5-loop beta function in QCD for an arbitrary simple compact gauge group, the hadronic R-ratio and hadronic Higgs decay rates at N4LO.

 

Thu 28.06.2018, 16.30 h

S. Sint (Dublin)

How strong are the strong interactions?

Abstract:
Quantum chromodynamics (QCD) constitutes one of the main pillars of the Standard Model of particle physics, and describes the strong nuclear force as interactions between quarks and gluons. On the other hand, experiments only observe their bound states, such as protons and pions. This makes it difficult to define and calculate basic properties of quarks, i.e. their masses, or the strength of their coupling to gluons, commonly parametrized by alpha_s. The strong coupling alpha_s is a decreasing function of energy, and thus becomes a useful perturbative expansion parameter at high energy scales. Perturbation theory provides some quantitative control in this high energy regime. However, its limitations are often unclear, and a genuinely non-perturbative approach is required to calculate properties of their bound states, for instance the life time or mass of a pion. In a long-term project, the ALPHA collaboration has deployed lattice QCD in combination with numerical simulations to calculate alpha_s taking low energy experimental observables as input. This has led to the recent result for alpha_s(m_Z) with sub-percent precision and full control of perturbative truncation errors. A remaining assumption concerns the  perturbative decoupling of charm quarks and current work aims to eliminate this uncertainty.  As a by-product of this project, we have used lattice QCD simulations at high energies as a testing ground for the applicability of perturbation theory. In this talk I will give an overview of the ALPHA collaboration strategy and its main results.

 

Thu 05.07.2018, 16.30 h

H. B. Hartanto (Durham)

Two-loop five-point amplitudes in massless QCD via integrand reduction

Abstract:
I will present calculations of the leading-colour five-point amplitude at two loop in massless QCD. The calculation is performed by means of D-dimensional integrand reduction method, complemented with the finite-field reconstruction techniques and momentum twistor parameterization. Numerical results for all helicity configurations are obtained by employing both sector decomposition methods and IBP reduction to master integrals.

 

Thu 12.07.2018, 16.30 h

E. Komatsu (MPI für Astrophysik, München)

Finding Cosmic Inflation

Abstract:
The cosmic microwave background (CMB) research told us a remarkable story: the structure we see in our Universe such as galaxies, stars, planets, and eventually ourselves originated from tiny quantum fluctuations generated in the early Universe. With the WMAP we have confirmed many of the key predictions of inflation including flatness and statistical homogeneity of our Universe, Gaussianity and adiabaticity of primordial density fluctuations, and a small but non-zero deviation from the scale-invariant spectrum of density fluctuations. Yet, the extraordinary claim requires extraordinary evidence. The last prediction of inflation that is yet to be confirmed is the existence of primordial gravitational waves whose wavelength can be as big as billions of light years. To this end we have proposed to JAXA a new satellite mission called "LiteBIRD", whose primary scientific goal is to find signatures of gravitational waves in the polarisation of the CMB. In this presentation we describe the current state of affairs regarding our understanding of the early Universe, physics of polarisation of CMB, a new source of gravitational waves from inflation, and the LiteBIRD proposal.