SVD Approach to Data Unfolding

Distributions measured in high energy physics experiments are usually distorted and/or transformed by various detector effects. A regularization method for unfolding these distributions is re-formulated in terms of the Singular Value Decomposition (SVD) of the response matrix. A relatively simple, yet quite efficient unfolding procedure is explained in detail. The concise linear algorithm results in a straightforward implementation with full error propagation, including the complete covariance matrix and its inverse. Several improvements upon widely used procedures are proposed, and recommendations are given how to simplify the task by the proper choice of the matrix. Ways of determining the optimal value of the regularization parameter are suggested and discussed, and several examples illustrating the use of the method are presented.Comment: 22 page

On Instanton Contributions to the tau Hadronic Width

Comparing the results of finite energy and Borel transformed sum rules for the difference of vector and axial vector current correlators we evaluate the instanton contributions to the $\tau$ hadronic width. In contrast to an explicit theoretical calculation we find that the instanton contributions to the $\tau$ hadronic width are much smaller than the standard nonperturbative corrections.Comment: 6 pages, Latex fil

Terahertz magnonics in antiferromagnetic iron oxides

Antiferromagnets, magnetic materials with antiparallel spin ordering and therefore lacking magnetic moment have for a long time been studied for academic interest only. However, the realization that antiferromagnets possess intrinsic magnetic resonance frequencies in the Terahertz range, which is orders of magnitude higher than the Gigahertz ferromagnetic resonance frequencies, has recently renewed the interest for antiferromagnets for applications in energy efficient data storage and processing. Moreover waves of spin precession, or magnons, have been proposed as new methods for wave-based computing. The miniaturization of such potential technological devices requires the spin waves to have nanometer scale wavelengths, which has proven to be challenging to achieve in anitferromagnets. In this thesis, we will study the ultrafast spin dynamics and magnons in a specific class of antiferromagnetic iron oxides, the orthoferrites, RFeO3, where R is a rare-earth element. These antiferromagnets possess a weak ferromagnetic moment due to the canting of the antiparallel spins. After introducing the field of ultrafast magnetism and magnonics and associated concepts in Chapter 1, in the first part of this thesis, we will describe how the challenge of generating nanoscale spin waves can be overcome by exciting a confined region of spins near the sample face. We will show how strongly absorbed laser pulses will generate a propagating broad-band wavepacket of spin waves. In Chapter 2, we will introduce the basic concepts behind the experiments performed in this work. We will introduce the principle of ultrafast pump-probe spectroscopy experiments that can be used to measure such spin waves, and describe the design of the setup that allows us to drive the spin dynamics with intense Terahertz pulses. In Chapter 3, a thorough theoretical description of the technique to launch propagating broadband wavepackets of magnons will be given. Additionally, we will model the detection of these generated packets of spin waves acts in Magneto-Optical Kerr Effect experiments. We find that through the emergence of the Brillouin condition, by the appropriate choice of the wavelength of the probe pulse, we can select the detected wavenumber component of the wave packet, resulting in a probe wavelength dependent frequency observed in the experiment. In Chapter 4, we proceed to the experiment and search for the spin wave packets in HoFeO3. We will show that by exciting the spin dynamics with high energy photons above the bandgap energy, we can launch such propagating packets of spin waves. We find the theoretically predicted dependence of the detected magnon frequency on the wavelength of the probe light, and find that we excite a broad range of components of the spin wave packet. In Chapter 5, we build upon the experiment in Chapter 4, and study how the propagating spin waves can be controlled. We find that we can achieve a nonlinear control of the spin waves by introducing a second pump pulse. From theoretical calculations, we show that the coupling between the propagating magnon and photon acts as an additional nonlinear torque on the spins. We will see that this nonlinear torque allows for the conversion of the low frequency uniform precession mode of the spins into the higher frequency and higher wavenumber modes of the propagating spin wave packet. In Chapter 6, we will study the spin dynamics in ErFeO3 and TmFeO3 induced by intense THz pulses. Despite the magnetic similarities of these materials, the spin dynamics shows a very different trend at the Spin Reorientation Transition temperatures. In ErFeO3, we observe an unexpected giant enhancement of the amplitude, whereas in TmFeO3, this amplitude is suppressed. We will show that this difference in the dynamics can be attributed to the effect of the coupling between the iron spins and rare-earth ions. Finally, in Chapter 7, we will conclude our findings, and provide a concise outlook that shows that the intense THz setup is not only suitable for the study of antiferromagnetic oxides, but can also be used to study metallic thin films. We will demonstrate this with a short summary of experimental data measured in the FeRh, which is an antiferromagnet at room temperature and exhibits a phase transition to the ferromagnetic phase at high temperatures

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements