Microfabrication of photonic devices for mid-infrared optical applications

This thesis details research into the microfabrication of photonic devices for mid-infrared optical applications using the technique of ultrafast laser inscription. A diverse range of devices and materials is explored, including the first fabrication and development of an ultrafast laser inscribed mid-infrared waveguide laser source in thulium-doped sesquioxide ceramic gain media. The source produced 81 mW of output power at 1942 nm with a maximum slope efficiency of 9.5% demonstrating progress towards compact, low-threshold and efficient ultrafast laser written waveguide laser sources near 2 μm with the potential for high pulse repetition rate and ultrashort pulse operation. Also shown is the first demonstration of ultrafast laser inscription enabled selective chemical etching of chalcogenide glass. Investigations into the etching of modified regions in gallium lanthanum sulphide glass showed they could be etched at a rate ~13.3 times greater than the un-modified bulk. This result was explored further as a potential route to the production of optofluidic sensors for gas, liquid chemical or biomedical samples. The first demonstration and characterisation of ultrafast laser written waveguides in the chalcogenide glass GASIR-1 is also described. The waveguides were employed for chip scale supercontinuum generation producing the broadest and deepest Infrared supercontinuum from an ultrafast laser inscribed waveguide to-date, spanning ~4 μm from 2.5 to 6.5 μm, which has applications in remote sensing. Finally, the design, build and commissioning of an advanced laser processing setup suitable for ultrafast laser inscription is also detailed

Search for neutral MSSM Higgs bosons decaying to a pair of tau leptons in pp collisions

A search for neutral Higgs bosons in the minimal supersymmetric extension of the standard model (MSSM) decaying to tau-lepton pairs in pp collisions is performed, using events recorded by the CMS experiment at the LHC. The dataset corresponds to an integrated luminosity of 24.6 fb$^{−1}$, with 4.9 fb$^{−1}$ at 7 TeV and 19.7 fb$^{−1}$ at 8 TeV. To enhance the sensitivity to neutral MSSM Higgs bosons, the search includes the case where the Higgs boson is produced in association with a b-quark jet. No excess is observed in the tau-lepton-pair invariant mass spectrum. Exclusion limits are presented in the MSSM parameter space for different benchmark scenarios, m$_{h}^{max}$ , m$_{h}^{mod +}$ , m$_{h}^{mod −}$ , light-stop, light-stau, τ-phobic, and low-m$_{H}$. Upper limits on the cross section times branching fraction for gluon fusion and b-quark associated Higgs boson production are also given

Measurement of top quark-antiquark pair production in association with a W or Z boson in pp collisions at s\sqrt{s} = 8 TeV

A measurement of the cross section for the production of top quark–antiquark pairs ( ${\mathrm {t}}\overline{{\mathrm {t}}}$ ) in association with a vector boson V (W or Z) in proton-proton collisions at $\sqrt{s} = 8$ $\,\text {TeV}$ is presented. The results are based on a dataset corresponding to an integrated luminosity of 19.5 fb$^{-1}$ recorded with the CMS detector at the LHC. The measurement is performed in three leptonic (e and $\mu$) channels: a same-sign dilepton analysis targeting ${\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W}$ events, and trilepton and four-lepton analyses designed for ${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}}$ events. In the same-sign dilepton channel, the ${\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W}$ cross section is measured as $\sigma _{{\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W}} = 170 ^{+90-80}\,\text {(stat)} \pm 70\,\text {(syst)} \, \text {fb}$ , corresponding to a significance of 1.6 standard deviations over the background-only hypothesis. Combining the trilepton and four-lepton channels, a direct measurement of the ${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}}$ cross section, $\sigma _{{\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}}} = 200 ^{+80-70}\,\text {(stat)} ^{+40-30}\,\text {(syst)} \mathrm{fb}^{-1}$ , is obtained with a significance of 3.1 standard deviations. The measured cross sections are compatible with standard model predictions within their experimental uncertainties. The inclusive ${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {V}}$ process is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels

Search for Standard Model Production of Four Top Quarks in the Lepton + Jets Channel in pp Collisions at s\sqrt{s} = 8 TeV

A search is presented for standard model (SM) production of four top quarks ($t \bar{t} t \bar{t}$) in pp collisions in the lepton + jets channel. The data correspond to an integrated luminosity of 19.6 fb$^{-1}$ recorded at a centre-of-mass energy of 8 TeV with the CMS detector at the CERN LHC. The expected cross section for SM $t \bar{t} t \bar{t}$ production is $\sigma^{\mathrm{SM}}_{t \bar{t} t \bar{t}} \approx 1~\mathrm{fb}$. A combination of kinematic reconstruction and multivariate techniques is used to distinguish between the small signal and large background. The data are consistent with expectations of the SM, and an upper limit of 32 fb is set at a 95% confidence level on the cross section for producing four top quarks in the SM, where a limit of $32\pm{17}$ fb is expected.A search is presented for standard model (SM) production of four top quarks $\left(t\overline{t}t\overline{t}\right)$ in pp collisions in the lepton + jets channel. The data correspond to an integrated luminosity of 19.6 fb$^{−1}$ recorded at a centre-of-mass energy of 8 TeV with the CMS detector at the CERN LHC. The expected cross section for SM $t\overline{t}t\overline{t}$ production is ${\sigma}_{t\overline{t}t\overline{t}}^{\mathrm{SM}}\approx 1\,\mathrm{f}\mathrm{b}$ . A combination of kinematic reconstruction and multivariate techniques is used to distinguish between the small signal and large background. The data are consistent with expectations of the SM, and an upper limit of 32 fb is set at a 95% confidence level on the cross section for producing four top quarks in the SM, where a limit of 32 ± 17 fb is expected.A search is presented for standard model (SM) production of four top quarks (t t-bar t t-bar) in pp collisions in the lepton + jets channel. The data correspond to an integrated luminosity of 19.6 inverse femtobarns recorded at a centre-of-mass energy of 8 TeV with the CMS detector at the CERN LHC. The expected cross section for SM four top quark production is approximately 1 fb. A combination of kinematic reconstruction and multivariate techniques is used to distinguish between the small signal and large background. The data are consistent with expectations of the SM, and an upper limit of 32 fb is set at a 95% confidence level on the cross section for producing four top quarks in the SM, where a limit of 32 +/- 17 fb is expected

Performance of the missing transverse energy reconstruction by the CMS experiment in sqrt(s) = 8 TeV pp data

The performance of missing transverse energy reconstruction algorithms is presented using sqrt(s) = 8 TeV proton-proton (pp) data collected with the CMS detector. Events with anomalous missing transverse energy are studied, and the performance of algorithms used to identify and remove these events is presented. The scale and resolution for missing transverse energy, including the effects of multiple pp interactions (pileup), are measured using events with an identified Z boson or isolated photon, and are found to be well described by the simulation. Novel missing transverse energy reconstruction algorithms developed specifically to mitigate the effects of large numbers of pileup interactions on the missing transverse energy resolution are presented. These algorithms significantly reduce the dependence of the missing transverse energy resolution on pileup interactions. Finally, an algorithm that provides an estimate of the significance of the missing transverse energy is presented, which is used to estimate the compatibility of the reconstructed missing transverse energy with a zero nominal value