High Multiplicity Searches at the LHC Using Jet Masses

This article introduces a new class of searches for physics beyond the Standard Model that improves the sensitivity to signals with high jet multiplicity. The proposed searches gain access to high multiplicity signals by reclustering events into large-radius, or "fat," jets and by requiring that each event has multiple massive jets. This technique is applied to supersymmetric scenarios in which gluinos are pair-produced and then subsequently decay to final states with either moderate quantities of missing energy or final states without missing energy. In each of these scenarios, the use of jet mass improves the estimated reach in gluino mass by 20 % to 50 % over current LHC searches.Comment: 9 pages, 6 figures; v3 corrects a few small typo

Model-Independent Jets plus Missing Energy Searches

We present a proposal for performing model-independent jets plus missing energy searches. Currently, these searches are optimized for mSUGRA and are consequently not sensitive to all kinematically-accessible regions of parameter space. We show that the reach of these searches can be broadened by setting limits on the differential cross section as a function of the total visible energy and the missing energy. These measurements only require knowledge of the relevant Standard Model backgrounds and can be subsequently used to limit any theoretical model of new physics. We apply this approach to an example where gluinos are pair-produced and decay to the LSP through a single-step cascade, and show how sensitivity to different gluino masses is altered by the presence of the decay chain. The analysis is closely based upon the current searches done at the Tevatron and our proposal requires only small modifications to the existing techniques. We find that within the MSSM, the gluino can be as light as 125 GeV. The same techniques are applicable to jets and missing energy searches at the Large Hadron Collider.Comment: 22 pages, 6 figures, 3 tables, typos correcte

Nonequilibrium Green's function theory for transport and gain properties of quantum cascade structures

The transport and gain properties of quantum cascade (QC) structures are investigated using a nonequilibrium Green's function (NGF) theory which includes quantum effects beyond a Boltzmann transport description. In the NGF theory, we include interface roughness, impurity, and electron-phonon scattering processes within a self-consistent Born approximation, and electron-electron scattering in a mean-field approximation. With this theory we obtain a description of the nonequilibrium stationary state of QC structures under an applied bias, and hence we determine transport properties, such as the current-voltage characteristic of these structures. We define two contributions to the current, one contribution driven by the scattering-free part of the Hamiltonian, and the other driven by the scattering Hamiltonian. We find that the dominant part of the current in these structures, in contrast to simple superlattice structures, is governed mainly by the scattering Hamiltonian. In addition, by considering the linear response of the stationary state of the structure to an applied optical field, we determine the linear susceptibility, and hence the gain or absorption spectra of the structure. A comparison of the spectra obtained from the more rigorous NGF theory with simpler models shows that the spectra tend to be offset to higher values in the simpler theories.Comment: 44 pages, 16 figures, appearing in Physical Review B Dec 200

Coherent Transport through an interacting double quantum dot: Beyond sequential tunneling

Various causes for negative differential conductance in transport through an interacting double quantum dot are investigated. Particular focus is given to the interplay between the renormalization of the energy levels due to the coupling to the leads and the decoherence of the states. The calculations are performed within a basis of many-particle eigenstates and we consider the dynamics given by the von Neumann-equation taking into account also processes beyond sequential tunneling. A systematic comparison between the levels of approximation and also with different formalisms is performed. It is found that the current is qualitatively well described by sequential processes as long as the temperature is larger than the level broadening induced by the contacts.Comment: 11 pages, 5 figures included in tex

A "Littlest Higgs" Model with Custodial SU(2) Symmetry

In this note, a ``littlest higgs'' model is presented which has an approximate custodial SU(2) symmetry. The model is based on the coset space $SO(9)/(SO(5)\times SO(4))$. The light pseudo-goldstone bosons of the theory include a {\it single} higgs doublet below a TeV and a set of three $SU(2)_W$ triplets and an electroweak singlet in the TeV range. All of these scalars obtain approximately custodial SU(2) preserving vacuum expectation values. This model addresses a defect in the earlier $SO(5)\times SU(2)\times U(1)$ moose model, with the only extra complication being an extended top sector. Some of the precision electroweak observables are computed and do not deviate appreciably from Standard Model predictions. In an S-T oblique analysis, the dominant non-Standard Model contributions are the extended top sector and higgs doublet contributions. In conclusion, a wide range of higgs masses is allowed in a large region of parameter space consistent with naturalness, where large higgs masses requires some mild custodial SU(2) violation from the extended top sector.Comment: 22 pages + 8 figures; JHEP style, added references and extra discussion on size of T contributions, as well as some other minor clarifications. Version to appear in JHE

One Loop Predictions of the Finely Tuned SSM

We study the finely tuned SSM, recently proposed by Arkani-Hamed and Dimopoulos, at the one loop level. The runnings of the four gaugino Yukawa couplings, the mu term, the gaugino masses, and the Higgs quartic coupling are computed. The Higgs mass is found to be 130 - 170 GeV for M_s > 10^6 GeV. If the Yukawa coupling constants are measured at the 1% level, this can determine the SUSY breaking scale to within an order of magnitude. Measuring the relationships between the couplings to this accuracy provides a striking signal for this model.Comment: 5 pages, 4 figures; v2: Minor corrections to anomalous dimensions and beta functions. Numerical results are not significantly affected. v3: Minor changes to figures and references, as published in PR

Quantum-mechanical wavepacket transport in quantum cascade laser structures

We present a viewpoint of the transport process in quantum cascade laser structures in which spatial transport of charge through the structure is a property of coherent quantum-mechanical wavefunctions. In contrast, scattering processes redistribute particles in energy and momentum but do not directly cause spatial motion of charge.Comment: 6 pages, 5 figures included in tex, to appear in Physical Review