Theory Seminars SS 20

Thu 16.04.2020, 16.30 h

T. Linden (Stockholm)

Thermal WIMPs on the Brink

Weakly Interacting Dark Matter Particles (WIMPs) are among the most well-motivated models for particle dark matter. While these particles can be detected through direct, indirect, and collider experiments, only their indirect annihilation cross-section is guaranteed within the confines of a thermal model. Intriguingly, experimental searches using both gamma-rays and cosmic-rays are beginning to close in on the coveted "thermal annihilation cross-section". More intriguingly, several excesses in Galactic center gamma-rays and cosmic-ray antiprotons have been discovered. Even more intriguingly, these excesses are potentially consistent with each other. In this talk, I will summarize the current state of the field, and argue that there is hope of resolving this puzzle within the next five years.

Host: P. Mertsch


Thu 23.04.2020, 16.30 h

M. Diehl (DESY Hamburg)

Double parton scattering

Double parton scattering can give important contributions to high-multiplicity final states in proton-proton collisions.  I review the status of the theory for this mechanism, with a focus on recent developments regarding higher-order calculations.

Host: M. Czakon


Thu 07.05.2020, 16.30 h

L. Lellouch (CNRS & Aix-Marseille U.)

New lattice QCD calculation of the hadronic vacuum polarization contribution to the muon magnetic moment

Charged-lepton magnetic moments play a special role in probing the standard model (SM) of particle physics, which is complementary to direct searches for new physics at the energy frontier. In particular, the
experimental value of the anomalous contribution to the muon magnetic moment, a_\mu=(g_\mu-2)/2, has exhibited a persistent discrepancy of over 3 standard deviations with the SM prediction, ever since the very precise measurement made at Brookhaven National Lab in the early 2000s. This is particularly enticing, because it could indicate the presence new, fundamental physics.  At present, theoretical and experimental uncertainties are comparable in size. However, a new experiment underway at Fermilab, and another one planned at J-PARC, are aiming to reduce the error on the measurement of a_\mu by a factor of 4. To fully leverage these future measurements, and possibly claim the presence of new fundamental physics, it is imperative to check the SM prediction with independent methods and to reduce its uncertainties. After an introduction and a discussion of the current experimental and theoretical status of a_\mu, I will present a new lattice QCD calculation of the contribution to this quantity that most limits the precision of its SM prediction.  Surprisingly, our result eliminates the need to invoke new physics to explain the current measurement of a_\mu.

Host: R. Harlander


Thu 14.05.2020, 16.30 h

C. Pitrou (Institut d’Astrophysique de Paris)

Global cosmological anisotropy as a long wavelength perturbation

The current concordance model of cosmology has been extremely successful in constraining the global properties of the universe. However, it is based on the assumption of statistical homogeneity and isotropy, and we are thus lead to test this global geometry. In this web seminar, I will review how a global anisotropy can imprint the various observables. In particular, an early anisotropic phase during inflation is imprinted in a direction-dependent initial power spectrum, whereas a late phase is imprinted in the evolution of cosmological perturbations and in the CMB radiative transfer. In practice, it is sufficient to constrain mild anisotropies, and I will show how anisotropy can be considered as a special long-wavelength perturbation around a Friedmann-Lemaître space-time. Within this framework, the same tools can be used to compute the anisotropic and non-stochastic perturbation, and the statistically isotropic and stochastic ones, allowing to test late anisotropy from all cosmological observables.

Host: J. Lesgourgues


Thu 04.06.2020, 16.30 h

O. Hahn (Observatoire de Nice)

Initial conditions for cosmological simulations: the next generation

Initial conditions for cosmological simulations have to bridge the gap between the multi-physics highly-accurate, but linear, modelling of the evolution of perturbations in the Universe after inflation, and the deeply non-linear but physically simpler regime of late-time cosmic structure formation. Inaccuracies in the initial conditions for cosmological N-body simulations could thus easily be the largest source of systematic error in predicting the non-linear large-scale structure, which is a sensitive cosmological probe. After giving an introduction on how cosmological simulations are set up, I will present our new results on high order very accurate ICs for N-body simulations, as well as a perspective on multi-fluid simulations including baryons and dark matter, and our recent work on field-level (propagator) perturbation theory.

Host: J. Lesgourgues


Thu 18.06.2020, 16.30 h

T. Hambye (Universite  Libre  de  Bruxelles)

Dark matter from dark photons: a taxonomy of dark matter production

We analyze how dark matter (DM) can be produced in the early Universe, working in the framework of a hidden sector charged under a U(1)′ gauge symmetry and interacting with the Standard Model through kinetic mixing. Depending on the masses of the dark matter particle and of the dark photon, as well as on the hidden U(1)′ gauge coupling and the kinetic mixing parameter, we classify all the distinct regimes along which the observed dark matter relic density can be accounted for.  Dark matter production in frameworks where the hidden sector basically doesn’t couple to the Standard Model particles will also be discussed briefly.

Host: F. Kahlhoefer


Thu 02.07.2020, 16.30 h

M. Zaro (Milano/Amsterdam)

A fragmentation-based study of heavy quark production

Processes involving heavy quarks are a crucial component of the LHC physics program, both by themselves and as backgrounds for Higgs physics and new physics searches. In this work, we critically reconsider the validity of the widely-adopted approximation in which heavy quarks are generated at the matrix-element level, with special emphasis on the impact of the collinear logarithms associated with final-state heavy quark and gluon splittings. Our study, based on a perturbative fragmentation-function approach validated by phenomenological predictions, explicitly shows that neglecting the resummation of collinear logarithms may yield inaccurate predictions, in particular when observables exclusive in the heavy quark degrees of freedom are considered.

Host: M. Worek


Thu 09.07.2020, 16.30 h

K. Petraki (LPTHE, Jussieu)

Dark matter goes nuclear: overhauling thermal decoupling at the TeV scale with bound states

The production of dark matter via thermal decoupling from the primordial plasma, and the direct, indirect and collider signals associated with this mechanism, have been the pillars of dark matter phenomenology in the past decades. In sharp contrast to the sub-TeV regime, the interactions of thermal-relic dark matter with multi-TeV or larger mass manifest as long-range. This is supported by unitarity arguments, and shown by explicit calculations in WIMP and other models. The long-range nature of the interactions gives rise to non-perturbative effects, with the most prominent being the existence of bound states. The formation of unstable bound states in the early universe depletes dark matter, thereby changing its predicted mass and/or couplings. This can have severe implications for all experimental probes, particularly for collider and indirect searches.

Host: F. Kahlhoefer