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Search Results: 1 - 10 of 9129 matches for " Timothy Cohen "
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Leptophilic Dark Matter from the Lepton Asymmetry
Timothy Cohen,Kathryn M. Zurek
Physics , 2009, DOI: 10.1103/PhysRevLett.104.101301
Abstract: We present a model of weak scale Dark Matter (DM) where the thermal DM density is set by the lepton asymmetry due to the presence of higher dimension lepton violating operators. In these models there is generically a separation between the annihilation cross-section responsible for the relic abundance (through lepton violating operators) and the annihilation cross-section that is relevant for the indirect detection of DM (through lepton preserving operators). Due to this separation, there is a perceived boost in the annihilation cross-section in the galaxy today relative to that derived for canonical thermal freeze-out. This results in a natural explanation for the observed cosmic ray electron and positron excesses, without resorting to a Sommerfeld enhancement. Generating the indirect signals also sets the magnitude of the direct detection cross-section which implies a signal for the next generation of experiments. More generically these models motivate continued searches for DM with apparently non-thermal annihilation cross-sections. The DM may also play a role in radiatively generating Majorana neutrino masses.
Magnetic resonance velocity imaging derived pressure differential using control volume analysis
Benjamin Cohen, Abram Voorhees, Timothy Wei
Fluids and Barriers of the CNS , 2011, DOI: 10.1186/2045-8118-8-16
Abstract: A flow phantom was constructed and water was the experimental fluid. The phantom was connected to a high-resolution differential pressure sensor and a computer controlled pump producing sinusoidal flow. Magnetic resonance velocity measurements were taken and subsequently analyzed to derive pressure differential waveforms using momentum conservation principles. Independent sensor measurements were obtained for comparison.Using magnetic resonance data the momentum balance in the phantom was computed. The measured differential pressure force had amplitude of 14.4 dynes (pressure gradient amplitude 0.30 Pa/cm). A 12.5% normalized root mean square deviation between derived and directly measured pressure differential was obtained. These experiments demonstrate one example of the potential utility of control volume analysis and the concepts involved in its application.This study validates a non-invasive measurement technique for relating velocity measurements to pressure differential. These methods may be applied to clinical measurements to estimate pressure differentials in vivo which could not be obtained with current clinical sensors.Hydrocephalus is a complex spectrum of neuropathophysiological disorders generally defined by increased cerebrospinal fluid (CSF) within the cerebral ventricles and elevated intracranial pressure (ICP) [1]. The diagnosis and treatment of hydrocephalus is hindered by a lack of systemic understanding of the interrelationships between pressures and flow of CSF in the brain. Advancements in interpreting and combining clinical measurements within a precise, direct, physics-based approach will improve quantification, understanding and diagnosis of hydrocephalus. Control volume analysis was proposed to incorporate clinical measurements within a simple, robust fluid dynamics analysis [2]. This method allows direct quantitative comparison between disparate data sets through the integral mass and momentum conservation equations, precluding the need f
Changes in Dark Matter Properties After Freeze-Out
Cohen, Timothy;Morrissey, David E.;Pierce, Aaron
High Energy Physics - Phenomenology , 2008, DOI: 10.1103/PhysRevD.78.111701
Abstract: The properties of the dark matter that determine its thermal relic abundance can be very different from the dark matter properties today. We investigate this possibility by coupling a dark matter sector to a scalar that undergoes a phase transition after the dark matter freezes out. If the value of Omega_DM h^2 calculated from parameters measured at colliders and by direct and indirect detection experiments does not match the astrophysically observed value, a novel cosmology of this type could provide the explanation. This mechanism also has the potential to account for the "boost factor" required to explain the PAMELA data.
Catch Rates, Composition and Fish Size from Reefs Managed with Periodically-Harvested Closures
Philippa Jane Cohen, Timothy J. Alexander
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0073383
Abstract: Periodically-harvested closures are commonly employed within co-management frameworks to help manage small-scale, multi-species fisheries in the Indo-Pacific. Despite their widespread use, the benefits of periodic harvesting strategies for multi-species fisheries have, to date, been largely untested. We examine catch and effort data from four periodically-harvested reef areas and 55 continuously-fished reefs in Solomon Islands. We test the hypothesis that fishing in periodically-harvested closures would yield: (a) higher catch rates, (b) proportionally more short lived, fast growing, sedentary taxa, and (c) larger finfish and invertebrates, compared to catches from reefs continuously open to fishing. Our study showed that catch rates were significantly higher from periodically-harvested closures for gleaning of invertebrates, but not for line and spear fishing. The family level composition of catches did not vary significantly between open reefs and periodically-harvested closures. Fish captured from periodically-harvested closures were slightly larger, but Trochus niloticus were significantly smaller than those from continuously open reefs. In one case of intense and prolonged harvesting, gleaning catch rates significantly declined, suggesting invertebrate stocks were substantially depleted in the early stages of the open period. Our study suggests periodically-harvested closures can have some short term benefits via increasing harvesting efficiency. However, we did not find evidence that the strategy had substantially benefited multi-species fin-fisheries.
Development of a theoretical framework for analyzing cerebrospinal fluid dynamics
Benjamin Cohen, Abram Voorhees, S?ren Vedel, Timothy Wei
Fluids and Barriers of the CNS , 2009, DOI: 10.1186/1743-8454-6-12
Abstract: Control volume analysis is presented to introduce the reader to the theoretical background of this foundational fluid mechanics technique for application to general control volumes. This approach is able to directly incorporate the diverse measurements obtained by clinicians to better elucidate intracranial dynamics and progression to disorder.Several examples of meaningful intracranial control volumes and the particular measurement sets needed for the analysis are discussed.Control volume analysis provides a framework to guide the type and location of measurements and also a way to interpret the resulting data within a fundamental fluid physics analysis.The central nervous system consists of the brain and spinal cord which are bathed in a clear, nearly cell-free fluid termed cerebrospinal fluid (CSF). CSF resides in the subarachnoid spaces surrounding the brain and spinal cord, and in four fluid reservoirs within the brain known as the cerebral ventricles. The primary mechanical role of CSF is protecting the brain from abrupt movements and transmitting arterial volume displacement to venous blood which attenuates the intracranial pressure (ICP) pulse. Intracranial dynamics is defined as the complex physical coupling between the brain and the cerebral blood and CSF flows and pressures. A departure from equilibrium intracranial dynamics may lead to hydrocephalus, a complex spectrum of diseases, primarily defined by perturbation of the cranial contents (CSF volume) and ICP.The original theories of hydrocephalus, as a derangement of CSF bulk flow, an imbalance in production (by the choroid plexus) and absorption (or outflow resistance), led to the invention and ultimate adoption of the shunt as the primary treatment option. Indeed individuals experienced improvements after shunting, but shunts were not without complication (in fact have introduced new ones) and, as pointed out by Madsen et al., may have slowed progress, as hydrocephalus was viewed as a solved problem [
A synoptic climatology of heavy rain events in the Lake Eyre and Lake Frome catchments
Michael J. Pook,Caroline C. Ummenhofer,Timothy J. Cohen
Frontiers in Environmental Science , 2014, DOI: 10.3389/fenvs.2014.00054
Abstract: The rare occasions when Lake Eyre in central, southern Australia fills with water excite great interest and produce major ecological responses. The filling of other smaller lakes such as Lake Frome have less impact but can contribute important information about the current and past climates of these arid regions. Here, the dominant synoptic systems responsible for heavy rainfall over the catchments of Lake Eyre and Lake Frome since 1950 are identified and compared. Heavy rain events are defined as those where the mean catchment rainfall for 24 h reaches a prescribed threshold. There were 25 such daily events at Lake Eyre and 28 in the Lake Frome catchment. The combination of a monsoon trough at mean sea level and a geopotential trough in the mid-troposphere was found to be the synoptic system responsible for the majority of the heavy rain events affecting Lake Eyre and one in five of the events at Lake Frome. Complex fronts where subtropical interactions occurred with Southern Ocean fronts also contributed over 20% of the heavy rainfall events in the Frome catchment. Surface troughs without upper air support were found to be associated with 10% or fewer of events in each catchment, indicating that mean sea level pressure analyses alone do not adequately capture the complexity of the heavy rainfall events. At least 80% of the heavy rain events across both catchments occurred when the Southern Oscillation Index (SOI) was in its positive phase, and for Lake Frome, the SOI exceeded +10 on 60% of occasions, suggesting that the background atmospheric state in the Pacific Ocean was tilted toward La Ni?a. Hydrological modeling of the catchments suggests that the 12-month running mean of the soil moisture in a sub-surface layer provides a low frequency filter of the precipitation and matches measured lake levels relatively well.
Illuminating the 130 GeV Gamma Line with Continuum Photons
Timothy Cohen,Mariangela Lisanti,Tracy R. Slatyer,Jay G. Wacker
Physics , 2012, DOI: 10.1007/JHEP10(2012)134
Abstract: There is evidence for a 130 GeV gamma-ray line at the Galactic Center in the Fermi Large Area Telescope data. Dark matter candidates that explain this feature should also annihilate to Standard Model particles, resulting in a continuous spectrum of photons. To study this continuum, we analyze the Fermi data down to 5 GeV, restricted to the inner 3 degrees of the Galaxy. We place a strong bound on the ratio of continuum photons to monochromatic line photons that is independent of uncertainties in the dark matter density profile. The derived constraints exclude neutralino dark matter as an explanation for the line.
Wino Dark Matter Under Siege
Timothy Cohen,Mariangela Lisanti,Aaron Pierce,Tracy R. Slatyer
Physics , 2013, DOI: 10.1088/1475-7516/2013/10/061
Abstract: A fermion triplet of SU(2)_L - a wino - is a well-motivated dark matter candidate. This work shows that present-day wino annihilations are constrained by indirect detection experiments, with the strongest limits coming from H.E.S.S. and Fermi. The bounds on wino dark matter are presented as a function of mass for two scenarios: thermal (winos constitute a subdominant component of the dark matter for masses less than 3.1 TeV) and non-thermal (winos comprise all the dark matter). Assuming the NFW halo model, the H.E.S.S. search for gamma-ray lines excludes the 3.1 TeV thermal wino; the combined H.E.S.S. and Fermi results completely exclude the non-thermal scenario. Uncertainties in the exclusions are explored. Indirect detection may provide the only probe for models of anomaly plus gravity mediation where the wino is the lightest superpartner and scalars reside at the 100 TeV scale.
FRAGMATIC: A randomised phase III clinical trial investigating the effect of fragmin? added to standard therapy in patients with lung cancer
Gareth O Griffiths, Sarah Burns, Simon I Noble, Fergus R Macbeth, David Cohen, Timothy S Maughan
BMC Cancer , 2009, DOI: 10.1186/1471-2407-9-355
Abstract: The study design is a randomised multicentre phase III trial comparing standard treatment and standard treatment plus daily LMWH for 24 weeks in patients with lung cancer. Patients eligible for this study must have histopathological or cytological diagnosis of primary bronchial carcinoma (small cell or non-small cell) within 6 weeks of randomisation, be 18 or older, and must be willing and able to self-administer 5000 IU dalteparin by daily subcutaneous injection or have it administered to themselves or by a carer for 24 weeks. A total of 2200 patients will be recruited from all over the UK over a 3 year period and followed up for a minimum of 1 year after randomisation. Patients will be randomised to one of the two treatment groups in a 1:1 ratio, standard treatment or standard treatment plus dalteparin. The primary outcome measure of the trial is overall survival. The secondary outcome measures include venous thrombotic event (VTE) free survival, serious adverse events (SAEs), metastasis-free survival, toxicity, quality of life (QoL), levels of breathlessness, anxiety and depression, cost effectiveness and cost utility.Current Controlled Trials ISRCTN80812769In the UK about 33,000 patients are diagnosed with lung cancer every year, with a median 1-year survival of 20% [1]. The death rate from lung cancer in the western world ranges from 5 to 27 per 100,000 per year in women and 25 to 77 per 100,000 per year in men [2,3]. Therapeutic advances over the last 30 years have had only a modest impact on overall survival and there is unlikely to be a marked improvement in survival rates in the coming years even in patients with resectable disease. There is therefore a clear need to investigate new approaches [4].Venous thromboembolism (VTE) is common in patients with any cancer and the incidence is increased by surgery, chemotherapy, radiotherapy and disease progression [5-8]. This results in a prevalence of clinically apparent VTE of up to 15% [9] across all cancer patie
Stellar Archaeology: a Keck Pilot Program on Extremely Metal-Poor Stars from the Hamburg/ESO Survey. II. Abundance Analysis
Eugenio Carretta,Raffaele Gratton,Judith G. Cohen,Timothy C. Beers,Norbert Christlieb
Physics , 2002, DOI: 10.1086/340955
Abstract: We present a detailed abundance analysis of 8 stars selected as extremely metal poor candidates from the Hamburg/ESO Survey (HES). For comparison, we have also analysed 3 extremely metal-poor candidates from the HK survey, and 3 additional bright metal-poor stars. With this work, we have doubled the number of extremely metal-poor stars ([Fe/H]$\le 3.0$) with high-precision abundance analyses. Our sample of extremely metal-poor candidates from the HES contains 3 stars with [Fe/H] $\le -3.0$, 3 more with [Fe/H]$\le -2.8$, and 2 stars that are only slightly more metal rich. Thus, the chain of procedures that led to the selection of these stars from the HES successfully provides a high fraction of extremely metal-poor stars. We verify that our stellar parameters, derived in Paper I, lead to acceptable ionization and excitation balances for Fe, ruling out substantial non-LTE effects in Fe. For the $\alpha-$elements Mg, Si, Ca, Ti, the light element Al, the iron-peak elements Sc, Cr, Mn, and the neutron capture elements Sr and Ba, we find trends in abundance ratios [X/Fe] similar to those found by previous studies. However,the scatter in most of these ratios, even at [Fe/H]$\le -3.0$ dex, is surprisingly small. Only Sr and Ba show scatter larger than the expected errors. Future work (the 0Z project) will provide much stronger constraints on the scatter (or lack thereof) in abundances for a greater number of stars. We discuss the implications of these results for the early chemical evolution of the Galaxy, including such issues as the number of contributing SN, and the sizes of typical fragments in which they were born. In addition, we have identified a very metal poor star that appears to be the result of the s-process chain, operating in a very metal-poor environment, with extremely enhanced C, Ba, and Pb, and somewhat enhanced Sr.
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