ABSTRACTS VCES 2017

*Christof Wetterich *

Nov. 30^{th}- 09:30

__Scalar force in cosmology__

An almost massless scalar field can be observable in cosmology, tests of the equivalence principle and time varying fundamental constants. Strict observational bounds limit its coupling to atoms to be several orders of magnitude weaker than gravity. Approximate spontaneously broken scale symmetry gives a reason for an almost massless scalar “cosmon” field as a Goldstone boson, and explains why its coupling to atoms is tiny. A scalar with these properties can be responsible for dynamical dark energy. Quantum gravity computations reveal the presence of ultraviolet and infrared fixed points and the associated quantum scale symmetry. Realistic cosmology for variable gravity models interpolates from the ultraviolet fixed point in the infinite past to the infrared fixed point in the infinite future. The why now problem of dark energy may be solved by a substantial coupling of the cosmon to neutrinos, rendering the cosmic neutrino background potentially observable by the formation of huge neutrino lumps.

*Philippe Brax *

Nov. 30^{th}- 10:15

__R ^{2} dark energy in the laboratory__

I will describe the consequences of seeing $R^2$ gravity as the dominant contribution in the gravity effective action, its possible link with dark energy and the prospects for laboratory experiments. I will also describe a string inspired supergravity embedding.

*Steven D. Bass *

Nov. 30^{th}- 11:30

__The Cosmological Constant with Emergent Particle Physics__

Hadron physics is emergent from more fundamental QCD. At a deeper level, a key question is whether the local gauge symmetries which determine particle interactions might also be emergent, perhaps associated with new critical phenomena in the ultraviolet. Possible support for this scenario comes from the observation that LHC results extrapolated into the ultraviolet are consistent with a Higgs vacuum close to the border of stable and metastable. In the emergence scenario the local gauge symmetries of particle physics would “dissolve” in the extreme UV, in contrast to GUT models which assume extra symmetry at the highest scales. We discuss this scenario and its interface with the cosmological constant puzzle.

*Alexander Westphal *

Nov. 30^{th}- 12:00

__The Flattened Road to Acceleration in String Theory__

Stages of accelerated expansion during the history of the universe - both a very late and very early variant - have very strong support from cosmological data. A phase of (almost) de Sitter space-time can describe both stages of acceleration, both the extremely tiny dark energy of today and the stage of cosmological inflation early on. As both forms of dark energy are radiatively unstable, there is a strong motivation to understand positive vacuum energy in string theory. Constructing models of de Sitter space or inflation in string theory does not come easy due to the presence of extra dimensions and moduli. We will discuss a general phenomenon in string theory that the build-up of positive vacuum energy tends to ‘flatten out’ with increasing amount of supersymmetry breaking due to the backreaction effects. We will show how this effect occurs in string theory models of inflation, flattening the inflaton potential and suppressing the amount of primordial gravitational waves generated. We will then argue that the same flattening effects occur in the construction of static de Sitter minima as well. These backreaction effects render the construction of a positive cosmological constant in string theory more difficult.

*Amol Upadhye *

Nov. 30^{th}- 12:30

__Laboratory fifth force experiments probe dark energy__

The discovery of the accelerated expansion of the Universe has motivated searches for new forces near two characteristic length scales: the Hubble scale at which the acceleration set in, and the sub-millimeter scale corresponding to the energy density responsible for the acceleration. Previous astrophysical searches have tightly constrained such physics. However, the possibility that this force is associated with dark energy introduces new mechanisms that dynamically suppress the fifth force at long ranges. I will describe two examples, the chameleon and symmetron mechanisms, which screen fifth forces in dense regions. Since a screened fifth force due to the Sun is sourced only by a thin shell of matter near the Sun's surface, and acts only on a thin outer shell of each planet, deviations from Kepler's Law in the Solar System are negligible. Residual fifth forces are detectable at millimeter and sub-millimeter ranges using a properly-designed experiment. I will briefly survey such experiments, from a tabletop torsion pendulum to atom and neutron interferometers, and show that many orders of magnitude in parameter space are accessible to near-future experiments.

*Pedro Ferreira *

Nov. 30^{th}- 14:30

__The ubiquity of a fifth force__

I discuss why we should expect gravitational fifth forces, how we should look for them on astrophysical and cosmological scales, how they indirectly affect the propagation of gravitational waves and how they might be hidden by a fundamental new symmetry of nature.

*Ferruccio Feruglio *

Nov. 30^{th}- 15:15

__Are neutrino masses modular forms?__

The hope of connecting fermion masses and mixing angles to some fundamental underlying principle has fostered an intense activity aimed at identifying possible symmetry patterns in the data.

Neutrino masses and lepton mixing angles have played an important role in such attempts. In the first part of this talk I will review some of the existing proposals, discussing pros and cons on simple examples. In the second part I will illustrate a new class of models where the role of flavour symmetry is played by the modular invariance. Modular invariance is ubiquitous in string theory and in condense matter systems. I will illustrate its remarkable and unique properties and I will explain how it can be exploited to constrain neutrino masses and mixing angles.

*Thorsten Soldner*

Nov. 30^{th}- 16:30

__Search for eV sterile neutrinos -- the STEREO experiment__

Reactor oscillation experiments at short baselines observe less electron antineutrinos than predicted. This so-called reactor antineutrino anomaly RAA might be explained by the oscillation to a light (~ 1 eV) sterile neutrino. The RAA has triggered a series of experiments chasing these sterile neutrinos by specifically searching for oscillations at very short baselines from reactors or intense radioactive sources. I will introduce the motivation, discuss particular challenges of the reactor experiments and give an overview on the experimental approaches.

I will present the STEREO experiment in more detail.

*Levon Pogosian *

Nov. 30^{th}- 17:00

__What Can Cosmology Tell Us About Gravity?__

I will review the current status and future prospects of testing gravity using data from large scale structure surveys. I will then discuss certain aspects of the so-called model-independent tests of dark energy and modified gravity. One is the choice of priors when trying to constrain unknown functions of redshift. Another is the physical interpretation of such tests and their implications for particular types of modified gravity theories.

*Erik Verlinde – Public talk** *

Nov. 30^{th}- 18:30

__Gravity, Information and the Dark Side of the Cosmos__

In most circumstances gravity is well described by the theories of Newton and Einstein. However, there are important open questions regarding gravity at cosmological scales, especially on the nature of the observed dark matter and dark energy. At present we are witnessing a revolution in theoretical physics leading to a completely new view on space-time and gravity. Studies in string theory and black hole physics reveal a deep connection between the structure of space-time and the origin of gravity with key concepts of quantum information theory. Gravity appears as a consequence of a quantum version of the laws of thermodynamics. This means that gravity isn't a fundamental force of nature, but rather an emergent phenomenon - just like temperature is an emergent phenomenon that arises from the movement of microscopic particles. This new view on gravity and space-time has particularly important implications for cosmology, and naturally explains the observations in galaxies that are currently attributed to dark matter. Are we standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity?

*Eric Adelberger *

Dec. 1^{st} - 09:30

__Probing gravity with equivalence principle and inverse-square law tests__

I will discuss the motivations, principles and results of equivalence-principle and inverse-square law tests. The results will be interpreted as setting limits on new Yukawa interactions with arbitrary ranges and "charges" and strengths weaker than gravity.

*Ricardo Decca *

Dec. 1^{st} – 10:15

__Search for new interactions in the Casimir regime__

Results towards imposing stronger constraints on the existence of hypothetical new interactions (the fifth force) will be presented. The experiments make use of a high-Q microelectromechanical oscillator as the force sensitive element, with a separation between the small (but macroscopic) source and test masses in the submicrometer range. Combining the natural frequency selection of the oscillator, engineered samples and different experimental approaches it is possible to disentangle electrostatic background contributions from vacuum fluctuations ones. Furthermore, it is also feasible to further cancel the effect of vacuum fluctuations on the measurement to obtain limits on the existence of Yukawa-like hypothetical forces. New experimental directions to be pursued will be presented.

*Guillaume Pignol *

Dec. 1^{st }– 11:30

__Search for axionlike Dark Matter with nEDM apparatus__

I will report on a search for ultra-low mass axion-like Dark Matter through nuclear spin precession in electric and magnetic fields. The Dark matter could be formed by a light scalar field which undergo coherent oscillations with the frequency fixed by the inverse mass of the corresponding axion-like particle. Such a field is expected to have a CP violating coupling to gluons, hence inducing an oscillation electric dipole moment in nucleons and atoms. We analyse the data of two neutron EDM experiments: ILL, Grenoble, France (1998-2002) and PSI, Villigen, Switzerland (2015-16). We explicitly looked for an oscillating neutron EDM signal with periods ranging from minutes to years, covering the cosmologically interesting range of dark matter particle masses 1e-24 eV < m < 1e-18 eV. I will present the apparatus to search for the neutron electric dipole moment using ultracold neutrons, and I will present the data used for this analysis.

*Philipp Haslinger *

Dec. 1^{st} - 12:00

__Atom Interferometry: Gravity, Blackbody Radiation and Chameleons__

If dark energy, which drives the accelerated expansion of the universe, consists of a light scalar field it might be detectable as a “fifth force” between normal-matter objects. In order to be consistent with cosmological observations and laboratory experiments, some leading theories use a screening mechanism to suppress this interaction. However, atom-interferometry presents a tool to reduce this screening on so-called chameleon models. By sensing the gravitational acceleration of a 0.19 kg in vacuum source mass which is 10^{-9} times weaker than Earth´s gravity, we reached a natural bound for cosmological motivated scalar field theories and were able to place tight constraints. Blackbody (thermal) radiation is emitted by objects at finite temperature with an outward energy-momentum flow, which exerts an outward radiation pressure. At room temperature e. g. a cesium atom scatters on average less than one of these blackbody radiation photons every 10^{8} years. Thus, it is generally assumed that any scattering force exerted on atoms by such radiation is negligible. However, particles also interact coherently with the thermal electromagnetic field and this leads to a surprisingly strong force acting in the opposite direction of the radiation pressure.

*Suchita Kulkarni *

Dec. 1^{st} - 14:00

__Exploiting experimental data to constrain exotic dark matter scenarios__

In the search of dark matter (DM), understanding the implications for several theory DM scenarios is an important task. It therefore is interesting to ask how to maximally use the experimental information in order to best constrain/rule out DM properties. In this talk, I exemplify this philosophy with two different scenarios. I investigate the indirect detection signatures for DM annihilating to long-lived mediators. I detail the formalism to calculate the anisotropy of the decay of such long-lived mediator that can be utilised at indirect detection experiments. In the second part of my talk, I sketch the constraining power of future low threshold direct detection experiments for DM interactions involving light mediators. I understand the improvement in parameter reconstruction by combining data from several different experiments."

**Mikhail Shaposhnikov*** *

Dec. 1^{st} – 14:30

I will discuss how three sterile neutrinos alone can simultaneously explain neutrino oscillations, the observed dark matter, and the baryon asymmetry of the Universe without new physics above the Fermi scale. The experimental prospects to search for these particles will be outlined.

*Thomas Schwetz-Mangold** *

Dec. 1^{st} - 15:00

__Neutrino oscillations and indirect searches for new physics__

Abstract: Non-zero neutrino mass implies evidence for physics beyond the Standard Model, which is however, very indirect and does not point to a particular scale of new physics. In this talk I review the present status of neutrino oscillations in the standard three-flavour framework, discussing possible hints in favour of leptonic CP violation. Furthermore, I comment on the search for deviations from three-flavour oscillations in the context of beyond-Standard-Model interactions of neutrinos with matter.

*Carlo Giunti *

Dec. 1^{st} - 15:30

__Short-Baseline Neutrino Oscillation Anomalies and Light Sterile Neutrinos__

I review the experimental indications in favor of short-baseline neutrino oscillations. I discuss their interpretation in the framework of 3+1 neutrino mixing with a sterile neutrino at the eV scale. I present the results of the updated 3+1 global fit including the recent MINOS, IceCube and NEOS data. I also discuss the problem of the 5 MeV bump in the reactor antineutrino spectrum in connection with the reactor antineutrino anomaly and the interpretation of the Daya Bay fuel evolution data.

**Andreas Trautner*** *

Dec. 1^{st} - 16:30

__CP violation caused by another symmetry__

Understanding the origin of CP violation offers a new starting point to address the Standard Model flavor and strong CP puzzles. Group theoretically, the physical CP transformation of the Standard Model is a special outer automorphism ("symmetry of symmetry") of the theory and I will explain what that means in simple terms. Having this understood, it follows that certain classes of symmetries preclude the existence of CP transformations altogether, in which case CP is violated by calculable ("geometrical") phases.As an example, I will present a "Scalar-QCD" type of model in which the SU(3) gauge symmetry is spontaneously broken to the small discrete subgroup T7. In this case CP violation originates with quantized phases while the topological theta angle is protected at 0.

*Fred Jegerlehner *

Dec. 1^{st} - 17:00

The Higgs boson mass miraculously turns out to have a value which has been expected form vacuum stability up to the Planck scale. This opens the possibility that the Higgs boson not only provides masses to all SM particles, but very likely also supplied a huge dark energy which inflated the young universe just after the Big Bang. In case of a stable Higgs vacuum we can predict all Standard Model parameters up to the Planck scale. The predicted effective Higgs mass gets huge in the early universe (as ascertained by the hierarchy problem), such the the Higgs potential energy dominates the kinetic energy. The Higgs boson looks to be a natural candidate for the inflaton. Higgs decays are reheating the universe after inflation. I emphasize the role of the hierarchy problem and the cosmological constant problem.