Combined preheating on the lattice with applications to Higgs inflation

We use classical lattice simulations in 3+1 dimensions to study the interplay between the resonant production of particles during preheating and the subsequent decay of these into a set of secondary species. We choose to work in a simplified version of Higgs inflation in which the Higgs field non-minimally coupled to gravity plays the role of the inflaton. Our numerical results extend the analytical estimates in the literature beyond the linear regime and shed some light on the limitations of the analytical techniques. The inclusion of fast and inefficient decays postpones the onset of parametric resonance by depleting the particles produced at the bottom of the potential. In spite of this delay, fermions are shown to play an important role on the destruction of the inflaton field. The limitations of our approach and its applications to a realistic Higgs inflation scenario are also discussed.

Brownian motion studied with an optical trap

Branimir Lukic

This thesis describes measurements of Brownian motion of a colloidal particle using optical trapping. Two aspects are investigated: (i) influence of inertial effects on Brownian motion, and (ii) effect of the optical trap on Brownian motion. The first part describes the experimental setup used: in short, a focused infrared laser beam is used to create an optical trap, and also provides the light source for the position detection. A single particle is trapped by the laser if it is brought near the focus, but it still moves due to Brownian motion. By analyzing the power spectral density of the position signal, the detection limit of the experimental setup is estimated to be 2 µs. The second part contains a study of inertial effects in Brownian motion of a colloidal particle at the microsecond time scale. Measurements are performed for three cases: a particle in a bulk fluid, a particle next to a wall and a particle in a confined fluid. For a particle in a bulk fluid, power law decay of t-3/2 is observed in the particle's velocity autocorrelation function. This power law decay has its origin in the non-negligible fluid inertia. A small effect caused by the particle's inertia is also observed. For the case of a particle placed next to a wall, a faster decay with power law of t-5/2 is observed in the velocity correlations parallel to the wall. Finally, for a particle in a confined fluid, absence of power law decay is observed, and the data instead agree with the model of an exponential decay of velocity correlations. These experimental observations are in accord with the theoretical predictions and show that the effect of the fluid's inertia is most significant in the case of a particle in a bulk fluid, less significant in the case of a particle next to a wall and completely absent in the case of a particle in a confined fluid. The third part is devoted to the characterization of the Brownian motion of a colloidal particle in an optical trap. Data for the mean-square displacement 〈[Δx(t)]2〉 and power spectral density are in excellent agreement with the theory for a Brownian particle in a harmonic potential, which also accounts for inertial effects. The motion of the particle is dominated by inertial effects at short times and by the trap potential at longer times. We find the time below which the particle's motion is not influenced by the potential to be approximately τκ/20, where τκ is the relaxation time of the restoring force of the potential. This allows us to exclude the existence of free diffusive motion, 〈[Δx(t)]2〉 ∝ t , even for a spherical particle with a radius as small as 0.27 µm in a potential with a spring constant as small as 1.5 µN/m. In the experiment with a micron-sized sphere in the weakest trap potential, estimation of the time below which the particle motion is not influenced by the potential gives τκ/20 ≈ 100 µs. In the time range 2-100 µs, the experimental setup then functions as a position detector probing the motion of a free Brownian particle. Moreover, it is shown that such a detector is achieved for any sphere size in the range between 0.53 µm to 4.16 µm. Hence, the results open up a possibility for using optically trapped Brownian particle as a local reporter its environment on microsecond time scales. This technique could be applied in more complex environments, like polymer networks, cell interiors or bacterial solutions.

EPFL2008

Evidence for the direct decay of the 125 GeV Higgs boson to fermions

Muhammad Ahmad, Georgios Anagnostou, Konstantin Androsov, Giovanni Bianchi, Roberto Castello, Jie Chen, Yixing Chen, Tian Cheng, Giuseppe Codispoti, Alessandro Degano, Charles Dietz, Dipanwita Dutta, Matthias Finger, Francesco Fiori, Yu Fu, Yifei Guo, Ruchi Gupta, Csaba Hajdu, Alexis Kalogeropoulos, Ji Hyun Kim, Donghyun Kim, Vineet Kumar, Ajay Kumar, Sanjeev Kumar, Ekaterina Kuznetsova, Ho Ling Li, Wei Li, Hao Liu, Shuai Liu, Werner Lustermann, Matteo Marone, Thomas Muller, Ioannis Papadopoulos, Vladimir Petrov, Quentin Python, Andrea Rizzi, Paolo Ronchese, Sourav Sen, Ashish Sharma, Varun Sharma, Lesya Shchutska, Muhammad Shoaib, Michal Simon, Gurpreet Singh, Jan Steggemann, Wei Sun, Andromachi Tsirou, Paul Turner, Joao Varela, Jian Wang, Xiao Wang, Mingkui Wang, Qian Wang, Zheng Wang, Matthias Weber, Matthias Wolf, Wenjing Wu, Fan Yang, Yong Yang, Kai Yi, Lei Zhang, Yu Zheng

The discovery of a new boson with a mass of approximately 125 GeV in 2012 at the LHC has heralded a new era in understanding the nature of electroweak symmetry breaking and possibly completing the standard model of particle physics. Since the first observation in decays to gamma gamma, WW, and ZZ boson pairs, an extensive set of measurements of the mass and couplings to W and Z bosons, as well as multiple tests of the spin-parity quantum numbers, have revealed that the properties of the new boson are consistent with those of the long-sought agent responsible for electroweak symmetry breaking. An important open question is whether the new particle also couples to fermions, and in particular to down-type fermions, since the current measurements mainly constrain the couplings to the up-type top quark. Determination of the couplings to down-type fermions requires direct measurement of the corresponding Higgs boson decays, as recently reported by the CMS experiment in the study of Higgs decays to bottom quarks and tau leptons. In this paper we report the combination of these two channels which results, for the first time, in strong evidence for the direct coupling of the 125 GeV Higgs boson to down-type fermions, with an observed significance of 3.8 standard deviations, when 4.4 are expected.

2014

Brownian motion studied with an optical trap

Branimir Lukic

EPFL2008