Search for pair production of the scalar top quark in muon+tau final states

We present a search for the pair production of scalar top quarks ($\tilde{t}_{1}$), the lightest supersymmetric partners of the top quarks, in $p\bar{p}$ collisions at a center-of-mass energy of 1.96 TeV, using data corresponding to an integrated luminosity of {7.3 $fb^{-1}$} collected with the \dzero experiment at the Fermilab Tevatron Collider. Each scalar top quark is assumed to decay into a $b$ quark, a charged lepton, and a scalar neutrino ($\tilde{\nu}$). We investigate final states arising from $\tilde{t}_{1} \bar{\tilde{t}_{1}} \rightarrow b\bar{b}\mu\tau \tilde{\nu} \tilde{\nu}$ and $\tilde{t}_{1} \bar{\tilde{t}_{1}} \rightarrow b\bar{b}\tau\tau \tilde{\nu} \tilde{\nu}$. With no significant excess of events observed above the background expected from the standard model, we set exclusion limits on this production process in the ($m_{\tilde{t}_{1}}$,$m_{\tilde{\nu}}$) plane.Comment: Submitted to Phys. Lett.

Projected WIMP sensitivity of the LUX-ZEPLIN dark matter experiment

LUX-ZEPLIN (LZ) is a next-generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7 tonnes, LZ will search primarily for low-energy interactions with weakly interacting massive particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000 live day run using a 5.6-tonne fiducial mass, LZ is projected to exclude at 90% confidence level spin-independent WIMP-nucleon cross sections above 1.4 × 10-48cm2 for a 40 GeV/c2 mass WIMP. Additionally, a 5σ discovery potential is projected, reaching cross sections below the exclusion limits of recent experiments. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of 2.3 × 10−43 cm2 (7.1 × 10−42 cm2) for a 40 GeV/c2 mass WIMP is expected. With underground installation well underway, LZ is on track for commissioning at SURF in 2020

Energy transfer processes in solar energy conversion

The program involves the investigation of excitation transport and electron transfer in complex systems. In the area of electron transfer, we have been studying electron back transfer following donor-acceptor photoinduced electron transfer. We are addressing this problem both theoretically and experimentally. In the area of excitation transport, we have been examining transport in solid solutions, liquid solutions, and in clustered excitation transport systems. Again, we are pursuing both experimental and theoretical approaches. The problem of electron back transfer between photogenerated ions is of central importance in both artificial and biological solar energy conversion. Once an electron has been transferred from an optically excited donor to an acceptor, back transfer competes with the ability of the radical ions to go on to do useful chemistry. We are studying the back transfer process using picosecond transient grating experiments in conjunction with time resolved and steady state fluorescence quenching measurements. The transient grating experiments makes the back transfer process a direct experimental observable, while the fluorescence experiments allow the forward transfer to be examined. By combining the experiments, a complete picture emerges. 10 refs

Energy transfer processes in solar energy conversion

By combining picosecond optical experiments and detailed statistical mechanics theory we continue to increase our understanding of the complex interplay of structure and dynamics in important energy transfer situations. A number of different types of problems will be focused on experimentally and theoretically. They are excitation transport among chromophores attached to finite size polymer coils; excitation transport among chromophores in monolayers, bilayers, and finite and infinite stacks of layers; excitation transport in large vesicle systems; and photoinduced electron transfer in glasses and liquids, focusing particularly on the back transfer of the electron from the photogenerated radical anion to the radical cation. 33 refs., 13 figs

Energy transfer processes in solar energy conversion

We have made substantial progress in experimental and theoretical studies in two areas: Photoinduced donor to acceptor electron transfer followed by back transfer in random solutions; and electronic excitation transport in systems with complex inhomogeneous spatial geometries and inhomogeneous energy distributions. Through the development of accurate statistical mechanical theories, we have been able to relate dynamics in complex systems to experimental observables. We have then used the experimental observables, time resolved fluorescence depolarization and transient grating experiments, to examine well defined molecular systems. The agreement between theory and experiment is excellent. 11 refs

Energy transfer processes in solar energy conversion

This program involves the experimental and theoretical study of optically induced electron transfer and electronic excitation transport in systems with complex structures. The focus is to obtain an understanding of the intimate interplay among intermolecular interactions, structure, and dynamics. A combination of picosecond transient grating experiments, time resolved fluorescence depolarization experiments, conventional optical spectroscopy, and statistical mechanical theory is being employed to elucidated fundamental aspects of processes which are important in the conversion of solar energy to usable forms of energy. We are continuing to address the very important problem of electron back transfer following optically induced donor to acceptor electron transfer. In a system in which there are donors (low concentration) and acceptors (high concentration) randomly distributed in solution, optical excitation of a donor can be followed by transfer of an electron to an acceptor. One electron transfer has occurred. there exists a ground state radical cation (D{sup +}) near a ground state radical anion (A{sup {minus}}). Since the thermodynamically stable state is neutral ground state D and A, back transfer will occur. The electron will back transfer from A{sup {minus}} to D{sup +} to regenerate the neutral species. In liquid solution, back transfer competes with separation by diffusion. Separated ions are extremely reactive and can go on to do useful chemistry. 10 refs

Energy transfer processes in solar energy conversion

During the past year, we have been working in three general areas: electronic excitation transport in clustered chromophore systems and other complex systems, photo-induced electron transfer and back transfer in liquid solutions in which diffusion and charge interactions are important, and the construction of a new two color dye laser system to enhance our experimental capability