Discerning Noncommutative Extra Dimensions

Experimental limits on the violation of four-dimensional Lorentz invariance imply that noncommutativity among ordinary spacetime dimensions must be small. Noncommutativity among extra, compactified spatial dimensions, however, is far less constrained and may have discernable collider signatures. Here we study the experimental consequences of noncommutative QED in six dimensions, with noncommutativity restricted to a TeV-scale bulk. Assuming the orbifold T^2/Z_2, we construct the effective four-dimensional theory and study interactions unique to the noncommutative case. New vertices involving the Kaluza-Klein (KK) excitations of the photon yield order 100% corrections to the pair production and to the decays of some of the lighter modes. We show that these effects are difficult to resolve at the LHC, but are likely within the reach of a future Very Large Hadron Collider (VLHC).Comment: 20 pages LaTeX, 8 eps figures (minor revisions, version to appear in Phys. Rev. D

Persistent HIV-infected cells in cerebrospinal fluid are associated with poorer neurocognitive performance

BACKGROUND. Persistence of HIV in sanctuary sites despite antiretroviral therapy (ART) presents a barrier to HIV remission and may affect neurocognitive function. We assessed HIV persistence in cerebrospinal fluid (CSF) and associations with inflammation and neurocognitive performance during long-term ART. METHODS. Participants enrolled in the AIDS Clinical Trials Group (ACTG) HIV Reservoirs Cohort Study (A5321) underwent concurrent lumbar puncture, phlebotomy, and neurocognitive assessment. Cell-associated HIV DNA and HIV RNA (CA-DNA, CA-RNA) were measured by quantitative PCR (qPCR). in peripheral blood mononuclear cells (PBMCs) and in cell pellets from CSF. In CSF supernatant and blood plasma, cell-free HIV RNA was quantified by qPCR with single copy sensitivity, and inflammatory biomarkers were measured by enzyme immunoassay. RESULTS. Sixty-nine participants (97% male, median age 50 years, CD4 696 cells/mm3, plasma HIV RNA <100 copies/mL) were assessed after a median 8.6 years of ART. In CSF, cell-free RNA was detected in 4%, CA-RNA in 9%, and CA-DNA in 48% of participants (median level 2.1 copies/103 cells). Detection of cell-free CSF HIV RNA was associated with higher plasma HIV RNA (P = 0.007). CSF inflammatory biomarkers did not correlate with HIV persistence measures. Detection of CSF CA-DNA HIV was associated with worse neurocognitive outcomes including global deficit score (P = 0.005), even after adjusting for age and nadir CD4 count. CONCLUSION. HIV-infected cells persist in CSF in almost half of individuals on long-term ART, and their detection is associated with poorer neurocognitive performance

Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events

The $B^0$-$\bar B^0$ oscillation frequency has been measured with a sample of 23 million \B\bar B pairs collected with the BABAR detector at the PEP-II asymmetric B Factory at SLAC. In this sample, we select events in which both B mesons decay semileptonically and use the charge of the leptons to identify the flavor of each B meson. A simultaneous fit to the decay time difference distributions for opposite- and same-sign dilepton events gives $\Delta m_d = 0.493 \pm 0.012{(stat)}\pm 0.009{(syst)}$ ps$^{-1}$.Comment: 7 pages, 1 figure, submitted to Physical Review Letter

Model-based analyses: Promises, pitfalls, and example applications to the study of cognitive control

We discuss a recent approach to investigating cognitive control, which has the potential to deal with some of the challenges inherent in this endeavour. In a model-based approach, the researcher defines a formal, computational model that performs the task at hand and whose performance matches that of a research participant. The internal variables in such a model might then be taken as proxies for latent variables computed in the brain. We discuss the potential advantages of such an approach for the study of the neural underpinnings of cognitive control and its pitfalls, and we make explicit the assumptions underlying the interpretation of data obtained using this approach

Optimal Levels of Perturbation Signals for Nonlinear System Identification

A method is developed for determining the optimal levels of multilevel perturbation signals for nonlinear system identification, using condition numbers of submatrices of the Vandermonde matrix of the input levels vector. It is applicable when the perturbation signal is applied directly to a static nonlinearity. Optimal levels can be obtained for every order of nonlinearity less than the number of levels, and in most cases the optimal levels are not all distinct. The results show that there is no advantage in using signals with more than the minimum necessary number of distinct levels, although it may be advantageous if some of the distinct levels appear more than once in the input levels vector. The optimal levels are unchanged by multiple occurrences of every level of the input levels vector during a measurement period, and they are shown to be the global optima for pseudorandom perturbation signals derived from maximum-length sequences, in which the occurrence of the zero level is one less than the occurrences of the other levels during a period

Design of multilevel perturbation signals with harmonic properties suitable for nonlinear system identification

A method is described for determining the levels of multilevel pseudorandom perturbation signals generated from maximum-length sequences in Galois fields that give the signals the desirable characteristics that their even harmonics are suppressed and their odd harmonics are uniform. The method uses the fact that a short sequence comprised of the signal levels converted from the nonzero field elements taken in a particular order must have corresponding characteristics. Analytical expressions are obtained for the signal level conversions that are necessary for the signal to possess these characteristics. The results are tabulated in symbolic form for 3-level, 5-level and 7-level signals generated in the fields GF(3), GF(5), GF(7), GF(9), GF(l 1) and GF(13). The method is then extended to signals in which harmonic multiples of 2 and 3 are suppressed and nonzero harmonics are uniform, and similar results are tabulated for GF(7) and GF(13). An example is used to illustrate the application of the results

Design of Computer-Optimized Pseudorandom Maximum Length Signals for Linear Identification in the Presence of Nonlinear Distortions

The design of pseudorandom maximum length (PRML) signals for linear identification in the presence of nonlinear distortions is considered. For this application, it is advantageous for the signal to have harmonic multiples of two and three suppressed, in order to minimize the effect of nonlinearity, thus resulting in a better estimate of the underlying linear dynamics. Such signals may be designed through exhaustive search for the sequence-to-signal conversions. However, for signals generated from Galois fields GF(q) with q large, this method is computationally inefficient. An alternative technique is proposed where a primitive version of the signal, the period of which is considerably shorter than that of the required PRML signal, is first generated as a computer-optimized signal. The primitive signal is then used to define the conversions for the generation of the required PRML signal, which is a member of a new class of hybrid signal