HEP Theorie-Seminare WS 15/16
Do. 29.10.2015, 16.30 Uhr
F. Febres Cordero (Freiburg)
Divector Boson Production in Association with Jets at the LHC
Abstract: We present Next-to-Leading Order QCD corrections to W+W-
production in association with up to three jets at the LHC. We find a
considerable reduction in scale sensitivity for total rates and also
for distributions. Our results are of relevance for top and Higgs
physics as well as for searches of anomalous trilinear couplings and
BSM scenarios. We have extended functionalities of the BlackHat
library and we have used the SHERPA package in order to complete our
Do. 05.11.2015, 16.30 Uhr
G. Zaharijas (Nova Gorica)
The nature of GeV Galactic Center excess emission: options and their testability
The existence of an unaccounted for gamma-ray excess over the diffuse backgrounds at few GeV in the Fermi-LAT data in a region around the Galactic Center has been established by several independent groups over the past few years. This signal is consistent with putative signals of long sought for WIMP dark matter particles, however, several conventional astrophysical explanation of this emission appear viable and need to be understood and accounted for, before any robust inference can be made about dark matter signals. In particular, we show that the main features of this excess can be reproduced if they originate in the inverse Compton emission from high-energy electrons injected in a burst event of ~10^52-10^53 erg roughly O(10^6) years ago and we discuss the testability of this prediction. Another convincing astrophysical candidate for this emission is an unresolved population of MSP. Here we assess the MSP contribution by adopting a phenomenological approach and make clear predictions for the next generation experiments, which due to their improved angular resolution at few GeV should be able to test the point source origin of the excess.
Do. 19.11.2015, 16.30 Uhr
C. Burrage (Nottingham)
Detecting Dark Energy with Atom Interferometry
I will discuss the possibility that the nature of the dark energy driving the observed acceleration of the Universe on giga-parsec scales may be determined first through metre scale laboratory based atom interferometry experiments. I will begin by introducing the scalar fields that could be responsible for dark energy and show that in order to be compatible with fifth force constraints these fields must have a screening mechanism to hide their effects from us. Focusing in particular on one such screening mechanism, known as the chameleon effect, where the field's mass becomes dependent on the environment. I will show that atom-interferometry experiments are ideally suited to detect the acceleration due to the fifth force of the chameleon field - this will then allow us to either rule out large regions of the chameleon parameter space or maybe one day to detect the force due to the dark energy field in the laboratory.
Do. 03.12.2015, 16.30 Uhr
S. Kallweit (Mainz)
Hadronic vector-boson pair production at NNLO QCD
I report on the computation of vector-boson pair production at next-to-next-to-leading order in QCD perturbation theory by means of the transverse-momentum subtraction method. This method, which is applicable to achieve NNLO QCD accuracy in the production of any colourless final-state, is introduced, and the technical realization of the calculation within the MATRIX framework is briefly discussed. I present numerical results and comparisons to experimantal data on fiducial cross sections and selected distributions for the production of Zgamma/Wgamma and ZZ pairs with leptonic decays of the heavy vector bosons, as well as on inclusive cross sections for ZZ and WW production.
Do. 10.12.2015, 16.30 Uhr
J. Gaunt (Nikhef)
N-jettiness Subtractions for QCD Calculations at NNLO
I give an introduction to a subtraction method for arbitrary NNLO QCD calculations that makes use of the N-jettiness observable, Tau_N. The method utilizes soft-collinear effective theory (SCET) to determine the IR-singular contributions of N-jet cross sections as Tau_N->0, which then function as a global subtraction term in the NNLO calculation. As a demonstration of the method working in practice, NNLO results for the Drell-Yan and Higgs rapidity spectra are given. Possible extensions to the method are also discussed, including more-differential subtractions.
Do. 14.01.2016, 16.30 Uhr
S. Clesse (RWTH)
Massive Primordial Black Holes as Dark Matter and the Seeds of Galaxies
I will present a new scenario where massive primordial black holes (PBHs) have abundances comparable to those of dark matter today. These PBHs could have acquired large stellar masses today, via merging, and the model passes both the constraints from CMB distortions and microlensing events. This scenario is supported by Chandra observations of numerous BH candidates in the central region of Andromeda. Moreover, the tail of the PBH mass distribution could be responsible for the seeds of supermassive black holes at the center of galaxies, as well as for ultraluminous x-ray sources. In our model, primordial black holes are produced in the early Universe, due to the collapse of large density fluctuations generated during a phase of hybrid inflation. Finally, I will discuss how our inflation model can be easily embedded in high-energy frameworks such as supersymmetry.
Do. 21.01.2016, 16.30 Uhr
F. Kahlhoefer (DESY)
Towards discovering dark matter at the LHC
I will discuss the sensitivity of the LHC for dark matter in comparison with other dark matter detection experiments. A useful approach for this purpose is to assume that dark matter particles interact with Standard Model states via a new mediator and I will present a few examples for how this approach can be used to understand the complementarity of different search strategies. In the final part of my talk I will focus on various consistency conditions that should be imposed even on the most simplified models and how these conditions can imply the presence of new particles and interactions that may change the phenomenology of the model in important ways.
Do. 28.01.2016, 16.30 Uhr
C. Ringeval (Louvain)
Observing the inflationary reheating
Within inflationary cosmology, the thermal history of the universe starts with the so-called reheating era which smoothly connects the end of inflation to the radiation era. During this epoch the inflaton field decays into unknown states at an unknown rate and with unknown branching ratios; but ultimately produces a plasma of Standard Models particles in thermal equilibrium. In this talk, we show that within many models of inflation the Planck 2013 and 2015 CMB measurements put non-trivial constraints on various kinematic properties of the reheating era. Conversely, such a result shows that comparing theoretical inflationary models to cosmological observations can no longer be performed without specifying a minimal set of hypothesis on how the reheating proceeded.
Do. 04.02.2016, 16.30 Uhr
P. Scott (Imperial College London)
GAMBIT: The Global and Modular Beyond-the-Standard Model Inference Tool
Fully understanding the status of any theory for new physics is a big task. It requires predicting a wide range of potential observables, carefully comparing each of them to all available experimental data, and combining the results in a statistically meaningful way. Generalising beyond single parameter combinations to sections of parameter space or entire theories requires even more care. GAMBIT is a new global-fitting tool specifically designed to make it feasible to carry out such analyses for a wide range of theories. The primary design goals of GAMBIT are flexibility and modularity, making it as simple and automatic as possible to add new observables, likelihoods, scanners, models and external code interfaces to global fit analyses. GAMBIT has been in development by a team of 30 theorists and experimentalists across particle and astroparticle physics for the last 3 years. In this talk I will describe the GAMBIT code and first results, both of which will be released in the next few months.