A Lattice Study of the Nucleon Excited States with Domain Wall Fermions

We present results of our numerical calculation of the mass spectrum for isospin one-half and spin one-half non-strange baryons, i.e. the ground and excited states of the nucleon, in quenched lattice QCD. We use a new lattice discretization scheme for fermions, domain wall fermions, which possess almost exact chiral symmetry at non-zero lattice spacing. We make a systematic investigation of the negative-parity $N^*$ spectrum by using two distinct interpolating operators at $\beta=6/g^2=6.0$ on a $16^3 \times 32 \times 16$ lattice. The mass estimates extracted from the two operators are consistent with each other. The observed large mass splitting between this state, $N^*(1535)$, and the positive-parity ground state, the nucleon N(939), is well reproduced by our calculations. We have also calculated the mass of the first positive-parity excited state and found that it is heavier than the negative-parity excited state for the quark masses studied.Comment: 46 pages, REVTeX, 11 figures included, revised version accepted for publication in Phys. Rev.

Form factors in lattice QCD

Lattice simulations of QCD have produced precise estimates for the masses of the lowest-lying hadrons which show excellent agreement with experiment. By contrast, lattice results for the vector and axial vector form factors of the nucleon show significant deviations from their experimental determination. We present results from our ongoing project to compute a variety of form factors with control over all systematic uncertainties. In the case of the pion electromagnetic form factor we employ partially twisted boundary conditions to extract the pion charge radius directly from the linear slope of the form factor near vanishing momentum transfer. In the nucleon sector we focus specifically on the possible contamination from contributions of higher excited states. We argue that summed correlation functions offer the possibility of eliminating this source of systematic error. As an illustration of the method we discuss our results for the axial charge, gA, of the nucleon.Comment: 16 pages, 11 figures, presented at Conclusive Symposium, CRC443, "Many-body structure of strongly interacting systems", 23-25 Feb 2011, Mainz, German

Simulations of electron cloud build-up and saturation in the APS

In studies with positron beams in the Advanced Photon Source, a dramatic amplification was observed in the electron cloud for certain bunch current and bunch spacings. In modeling presented previously, we found qualitative agreement with the observed beam-induced multipacting condition, provided reasonable values were chosen for the secondary electron yield parameters, including the energy distribution. In this paper, we model and discuss the build-up and saturation process observed over long bunch trains at the resonance condition. Understanding this saturation mechanism in more detail may have implications for predicting electron cloud amplification, multipacting, and instabilities in future rings

Simulations of electron cloud build-up and saturation in the APS

In studies with positron beams in the Advanced Photon Source, a dramatic amplification was observed in the electron cloud for certain bunch current and bunch spacings. In modeling presented previously, we found qualitative agreement with the observed beam-induced multipacting condition, provided reasonable values were chosen for the secondary electron yield parameters, including the energy distribution. In this paper, we model and discuss the build-up and saturation process observed over long bunch trains at the resonance condition. Understanding this saturation mechanism in more detail may have implications for predicting electron cloud amplification, multipacting, and instabilities in future rings

Moving Towards Self-Reliance: Living Conditions of Refugee Camps in Lebanon and Opportunities for Development

Refugee camps in Lebanon are harsh, continuously and rapidly deteriorating environments. In addition to poverty, numerous wars and the restrictions of civil rights, refugee camps that were not designed as a long-term settlement were made to accommodate their residents in addition to their descendents for a period that has lasted over 59 years. Since the establishment of the camps in 1948 the Palestinian refugees in Lebanon have fallen victim to multiple wars and as a result most camps have witnessed major destruction of homes and infrastructure, and a few were entirely destroyed. Today, the planning and development of the camps are highly restricted by the local government, building material is banned from entering the camps and horizontal as well as vertical expansion is prohibited by Lebanese law. According to the United Nations Relief and Work Agency (UNRWA) the hundreds of thousands of Palestinian refugees in Lebanon have the highest rate of people living in abject poverty in the Middle East. Meanwhile, the refugee community and the international aid agencies working in the camps are caught in the dilemma of investing in the development of a sustainable environment in a settlement with a temporary purpose and an uncertain future. This thesis explores the problems facing the built-environment in the camps within the political and socio-economic context, and takes the camp of Burj El Barajneh as a case study for deeper investigation. It then suggests three possible solution approaches that address the environmental problems within different future scenarios. The thesis also looks at the feasibility and requirements of an energy generation plant to provide part of the energy needs of the camp of Burj El Barjneh. Finally, a set of conclusions and recommendations are derived that address the refugee community, the international aid agencies and the host country. The significance of this study is to mitigate a possible humanitarian and environmental crisis in the most dire of refugee situations in the Middle East, with the hope that conclusions drawn from this study can be applied to refugee communities elsewhere in the region