HEP Theory Seminars SS 17

Thu 20.04.2017, 16.30 h

N. Saviano (Mainz)

The secret life of neutrinos in the cosmos

The Standard Model predicts neutrino-neutrino interactions mediated by the Z boson, with an effective strength given by the Fermi constant G_F. It is however interesting to investigate whether additional neutrino-neutrino interactions may exist, mediated by new particles. These interactions could in principle be probed by laboratory experiments, but also by cosmological observations (Big Bang Nucleosynthesis , Planck experiment) and astroparticle physics (neutrinos from supernovae, high-energy neutrinos in IceCube data). In my presentation, I will review the state-of-the-art of these searches, and the most recent developments in this field.


Thu 27.04.2017, 16.30 h

C. Duhr (CERN)

Precise predictions for Higgs production via gluon-fusion

Precise theoretical predictions for Higgs observables are of paramount importance for the Higgs-physics program at the upcoming Run II of the LHC. Such computations are notoriously difficult, and require the use of a whole variety of cutting-edge techniques in mathematics and physics. I will discuss the most precise predictions for the Higgs production cross section at hadron colliders, focusing on the recent determination of the inclusive cross section to N3LO in perturbative QCD and commenting on what are the remaining major sources of uncertainty that affect the cross section.


Thu 04.05.2017, 16.30 h

A. Mariotti (Brussels)

The SUSY Twin Higgs

I will first review the concept of naturalness and introduce the idea of neutral naturalness. I will then explain the Twin Higgs model and its effective Lagrangian, focusing on the mirror symmetry and on the fine tuning gain. Afterwards, I will discuss the supersymmetric UV completion of the Twin Higgs and the possible mechanisms to break the Twin Higgs mirror symmetry. Finally, I will present the phenomenology of SUSY Twin Higgs, concentrating on the most promising signatures at the LHC.


Thu 11.05.2017, 16.30 h

D. Baumann (GRAPPA, University of Amsterdam)

Relics from the Early Universe

New light particles arise in many attempts to address shortcomings of the Standard Model of particle physics, but their weak coupling to ordinary matter makes them hard to detect in terrestrial experiments. In the hot environment of the early universe, however, even extremely weakly coupled particles can be produced prolifically and their gravitational effects become detectable in cosmological observations.  In this seminar, I will show that future observations of the cosmic microwave background and of the large-scale structure of the universe have the sensitivity to detect new particles that were created just fractions of a second after the Big Bang. I will explain why these measurements are robust and how they are important as probes of fundamental physics.


Thu 01.06.2017, 16.30 h

C. Weniger (Amsterdam)

Indirect searches for dark matter: Current excesses and developments

The nature of dark matter in the Universe remains unknown. Indirect searches for possible annihilation or decay products of dark matter particles in the Universe probe an enormous range of dark matter masses. I will briefly review the basic concepts of indirect dark matter searches, current signal hints and typical constraints. I will then focus on the excess of GeV photons observed from the Galactic bulge by the Fermi Large Area Telescope, which is one of the best candidates for a WIMP annihilation signal to date. The most likely astrophysical explanation for the excess emission is the combined emission of thousands of millisecond pulsars in the bulge. I will discuss this scenario and how we plan to probe this interpretation in the near future with a dedicated radio survey of the bulge region. I will close with an extended outlook. In particular, I will discuss how new instruments, tools (GAMBIT) and concepts (Fisher information flux) can help to guide indirect dark matter searches in the years to come.


Thu 22.06.2017, 16.30 h

S. Iso (KEK)

Entanglement induced quantum radiation

It has been a long issue whether a uniformly accelerated object emits quantum radiation or not. A uniformly accelerated object sees the Minkowski vacuum as  thermally excited, whose temperature is  proportional to the acceleration of the object (Unruh effect). Then, base on an analogy with an object in a thermal bath, such an object cannot emit radiation. However, an explicit calculation shows that there is a net outgoing flux (quantum radiation). In my talk, I will explain what the issue is,  and show that such radiation has its origin in the quantum entanglement of the vacuum.


Thu 29.06.2017, 16.30 h

T. Tram (Aarhus)

How cosmology tells us about neutrinos

Cosmological observations can be used to test physics beyond the Standard Model, and they are especially suited for studying the neutrino sector. The reason is that neutrinos leave an imprint on many stages in the evolution of our Universe, from the Big Bang Nucleosynthesis (BBN) to the formation of the Cosmic Microwave Background (CMB) and the Large Scale Structure (LSS). In this talk I will show how different extensions of the neutrino sector affect cosmological observables, and I will discuss the surprising interplay between inflationary model selection and neutrino physics.


Thu 06.07.2017, 16.30 h

K. Schmidt-Hoberg (DESY)

Revisiting fine-tuning in the MSSM

We evaluate the amount of fine-tuning in constrained versions of the minimal supersymmetric standard model (MSSM), with different boundary conditions at the GUT scale. Specifically we study the fully constrained version as well as the cases of non-universal Higgs and gaugino masses. We allow for the presence of additional non-holomorphic soft-terms which we show further relax the fine-tuning. Of particular importance is the possibility of a Higgsino mass term and we discuss possible origins for such a term in UV complete models. We point out that loop corrections typically lead to a reduction in the fine-tuning by a factor of about two compared to the estimate at tree-level, which has been overlooked in many recent works. Taking these loop corrections into account, we discuss the impact of current limits from SUSY searches and dark matter on the fine-tuning. Contrary to common lore, we find that the MSSM fine-tuning can be as small as 10 while remaining consistent with all experimental constraints. If, in addition, the dark matter abundance is fully explained by the neutralino LSP, the fine-tuning can still be as low as ∼ 20 in the presence of additional non-holomorphic soft-terms. We also discuss future prospects of these models and find that the MSSM will remain natural even in the case of a non-discovery in the foreseeable future.


Thu 13.07.2017, 16.30 h

S. Pastor (Valencia)

The effective number of neutrinos: standard and non-standard calculations

The contribution of radiation, i.e. relativistic particles, to the total cosmological energy density is usually parameterized with the so-called effective number of neutrinos (N_eff). Present cosmological observations, in particular the CMB data from the Planck satellite, lead to a quite restricted allowed range for this parameter, that includes its standard value: N_eff=3. In this talk, I will review how the effective number of neutrinos is defined in the early Universe and describe some cosmological scenarios where its value is not 3, including the effect of non-instantaneous neutrino decoupling and flavour oscillations, as well as more exotic possibilities such as a very low-reheating case or the potential presence of neutrino-electron non-standard interactions. In these scenarios, the value of N_eff can be above or below 3, a situation that will be very constrained by data from cosmological observations in the near future.


Thu 20.07.2017, 16.30 h

J. Currie (Durham)

NNLO QCD Predictions for Single Jet Inclusive Production  at the LHC