Data driving the top quark forward--backward asymmetry with a lepton-based handle

We propose that, within the standard model, the correlation between the $t\bar{t}$ forward--backward asymmetry $A_{t\bar t}$ and the corresponding lepton-based asymmetry $A_l$ -- at the differential level -- is strong and rather clean both theoretically and experimentally. Hence a combined measurement of the two distributions as a function of the lepton $p_T$, a direct and experimentally clean observable, would lead to a potentially unbiased and normalization-free test of the standard model prediction. To check the robustness of our proposal we study how the correlation is affected by mis-measurement of the $t\bar t$ system transverse momenta, acceptance cuts, scale dependence and compare the results of MCFM, POWHEG (with & without PYTHIA showering), and SHERPA's CSSHOWER in first-emission mode. We find that the shape of the relative differential distribution $A_{l} (p^{l}_{T}) [A_{t\bar{t}} (p^l_T)]$ is only moderately distorted hence supporting the usefulness of our proposal. Beyond the first emission, we find that the correlation is not accurately captured by lowest-order treatment. We also briefly consider other differential variables such as the system transverse mass and the canonical $t\bar t$ invariant mass. Finally, we study new physics scenarios where the correlation is significantly distorted and therefore can be more readily constrained or discovered using our method.Comment: 27 pages, 12 figure

Bikesharing and Bicycle Safety

The growth of bikesharing in the United States has had a transformative impact on urban transportation. Major cities have established large bikesharing systems, including Boston, Chicago, Denver, Minneapolis-Saint Paul, New York City, Salt Lake City, the San Francisco Bay Area, Seattle, Washington DC, and others. These systems began operating as early as 2010, and no fatalities have occurred within the US as of this writing. However, three have happened in North America—two in Canada and one in Mexico. Bikesharing has some qualities that appear inherently unsafe for bicyclists. Most prominently, helmet usage is documented to be quite low in most regions. Bikesharing is also used by irregular bicyclists who are less familiar with the local terrain. In this study, researchers take a closer look at bikesharing safety from qualitative and quantitative perspectives. Through a series of four focus groups, they discussed bikesharing usage and safety with bikesharing members and nonmembers in the Bay Area. They further engaged experts nationwide from a variety of fields to evaluate their opinions and perspectives on bikesharing and safety. Finally, researchers conducted an analysis of bicycle and bikesharing activity data, as well as bicycle and bikesharing collisions to evaluate injury rates associated with bikesharing when compared with benchmarks of personal bicycling. The data analysis found that collision and injury rates for bikesharing are lower than previously computed rates for personal bicycling. Experts and focus group participants independently pointed to bikesharing rider behavior and bikesharing bicycle design as possible factors. In particular, bikesharing bicycles are generally designed in ways that promote stability and limited speeds, which mitigate the conditions that contribute to collisions. Data analysis also explored whether there was evidence of a “safety in numbers benefit” that resulted from bikesharing activity. However, no significant impact from bikesharing activity on broader bicycle collisions could be found within the regions in which they operate. Discussion and recommendations are presented in the conclusion

Path Puzzles: Discrete Tomography with a Path Constraint is Hard

We prove that path puzzles with complete row and column information--or equivalently, 2D orthogonal discrete tomography with Hamiltonicity constraint--are strongly NP-complete, ASP-complete, and #P-complete. Along the way, we newly establish ASP-completeness and #P-completeness for 3-Dimensional Matching and Numerical 3-Dimensional Matching.Comment: 16 pages, 8 figures. Revised proof of Theorem 2.4. 2-page abstract appeared in Abstracts from the 20th Japan Conference on Discrete and Computational Geometry, Graphs, and Games (JCDCGGG 2017

Structure of Mandelate Racemase with Bound Intermediate Analogues Benzohydroxamate and Cupferron

Mandelate racemase (MR, EC 5.1.2.2) from Pseudomonas putida catalyzes the Mg2+-dependent interconversion of the enantiomers of mandelate, stabilizing the altered substrate in the transition state by 26 kcal/mol relative to the substrate in the ground state. To understand the origins of this binding discrimination, we determined the X-ray crystal structures of wild-type MR complexed with two analogues of the putative aci-carboxylate intermediate, benzohydroxamate and Cupferron, to 2.2-Å resolution. Benzohydroxamate is shown to be a reasonable mimic of the transition state and/or intermediate because its binding affinity for 21 MR variants correlates well with changes in the free energy of transition state stabilization afforded by these variants. Both benzohydroxamate and Cupferron chelate the active site divalent metal ion and are bound in a conformation with the phenyl ring coplanar with the hydroxamate and diazeniumdiolate moieties, respectively. Structural overlays of MR complexed with benzohydroxamate, Cupferron, and the ground state analogue (S)-atrolactate reveal that the para carbon of the substrate phenyl ring moves by 0.8−1.2 Å between the ground state and intermediate state, consistent with the proposal that the phenyl ring moves during MR catalysis while the polar groups remain relatively fixed. Although the overall protein structure of MR with bound intermediate analogues is very similar to that of MR with bound (S)-atrolactate, the intermediate−Mg2+ distance becomes shorter, suggesting a tighter complex with the catalytic Mg2+. In addition, Tyr 54 moves closer to the phenyl ring of the bound intermediate analogues, contributing to an overall constriction of the active site cavity. However, site-directed mutagenesis experiments revealed that the role of Tyr 54 in MR catalysis is relatively minor, suggesting that alterations in enzyme structure that contribute to discrimination between the altered substrate in the transition state and the ground state by this proficient enzyme are extremely subtle

Outflows From Evolved Stars: The Rapidly Changing Fingers Of CRL 618

Our ultimate goal is to probe the nature of the collimator of the outflows in the pre-planetary nebula CRL 618. CRL 618 is uniquely suited for this purpose owing to its multiple, bright, and carefully studied finger-shaped outflows east and west of its nucleus. We compare new Hubble Space Telescope images to images in the same filters observed as much as 11 yr ago to uncover large proper motions and surface brightness changes in its multiple finger-shaped outflows. The expansion age of the ensemble of fingers is close to 100 yr. We find strong brightness variations at the fingertips during the past decade. Deep IR images reveal a multiple ring-like structure of the surrounding medium into which the outflows propagate and interact. Tightly constrained three-dimensional hydrodynamic models link the properties of the fingers to their possible formation histories. We incorporate previously published complementary information to discern whether each of the fingers of CRL 618 are the results of steady, collimated outflows or a brief ejection event that launched a set of bullets about a century ago. Finally, we argue on various physical grounds that fingers of CRL 618 are likely to be the result of a spray of clumps ejected at the nucleus of CRL 618 since any mechanism that form a sustained set of unaligned jets is unprecedented.HST GO 11580NASA through Space Telescope Science Institute GO11580NASA NAS5-26555Boeing ScholarshipOffice of Undergraduate Academic Affairs at the University of WashingtonSpanish MICINN CSD2009-00038NASA Office of Space Science NAG5-7584Astronom

Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant Planet Atmospheres

We calculate detailed chemical abundance profiles for a variety of brown dwarf and extrasolar giant planet atmosphere models, focusing in particular on Gliese 229B, and derive the systematics of the changes in the dominant reservoirs of the major elements with altitude and temperature. We assume an Anders and Grevesse (1989) solar composition of 27 chemical elements and track 330 gas--phase species, including the monatomic forms of the elements, as well as about 120 condensates. We address the issue of the formation and composition of clouds in the cool atmospheres of substellar objects and explore the rain out and depletion of refractories. We conclude that the opacity of clouds of low--temperature ($\le$900 K), small--radius condensibles (specific chlorides and sulfides), may be responsible for the steep spectrum of Gliese 229B observed in the near infrared below 1 \mic. Furthermore, we assemble a temperature sequence of chemical transitions in substellar atmospheres that may be used to anchor and define a sequence of spectral types for substellar objects with T$_{eff}$s from $\sim$2200 K to $\sim$100 K.Comment: 57 pages total, LaTeX, 14 figures, 5 tables, also available in uuencoded, gzipped, and tarred form via anonymous ftp at www.astrophysics.arizona.edu (cd to pub/burrows/chem), submitted to Ap.

Folding Polyominoes into (Poly)Cubes

We study the problem of folding a polyomino $P$ into a polycube $Q$, allowing faces of $Q$ to be covered multiple times. First, we define a variety of folding models according to whether the folds (a) must be along grid lines of $P$ or can divide squares in half (diagonally and/or orthogonally), (b) must be mountain or can be both mountain and valley, (c) can remain flat (forming an angle of $180^\circ$), and (d) must lie on just the polycube surface or can have interior faces as well. Second, we give all the inclusion relations among all models that fold on the grid lines of $P$. Third, we characterize all polyominoes that can fold into a unit cube, in some models. Fourth, we give a linear-time dynamic programming algorithm to fold a tree-shaped polyomino into a constant-size polycube, in some models. Finally, we consider the triangular version of the problem, characterizing which polyiamonds fold into a regular tetrahedron.Comment: 30 pages, 19 figures, full version of extended abstract that appeared in CCCG 2015. (Change over previous version: Fixed a missing reference.

Quirks at the Tevatron and Beyond

We consider the physics and collider phenomenology of quirks that transform nontrivially under QCD color, SU(2)_W as well as an SU(N)_{ic} infracolor group. Our main motivation is to show that the recent Wjj excess observed by CDF naturally arises in quirky models. The basic pattern is that several different quirky states can be produced, some of which beta-decay during or after spin-down, leaving the lightest electrically neutral quirks to hadronize into a meson that subsequently decays into gluon jets. We analyze LEP II, Tevatron, UA2, and electroweak precision constraints, identifying the simplest viable models: scalar quirks ("squirks") transforming as color triplets, SU(2)_W triplets and singlets, all with vanishing hypercharge. We calculate production cross sections, weak decay, spin-down, meson decay rates, and estimate efficiencies. The novel features of our quirky model includes: quirkonium decay proceeds into a pair of gluon jets, without a b-jet component; there is essentially no associated Zjj or gamma+jj signal; and there are potentially new (parameter-dependent) contributions to dijet production, multi-W production plus jets, W+gamma, gamma+gamma resonance signals, and monojet signals. There may be either underlying event from low energy QCD deposition resulting from quirky spin-down, and/or qualitatively modified event kinematics from infraglueball emission.Comment: 14 pages, 6 figure