4,917 research outputs found
Proposal for Higgs and Superpartner Searches at the LHCb Experiment
The spectrum of supersymmetric theories with R-parity violation are much more
weakly constrained than that of supersymmetric theories with a stable
neutralino. We investigate the signatures of supersymmetry at the LHCb
experiment in the region of parameter space where the neutralino decay leaves a
displaced vertex. We find sensitivity to squark production up to squark masses
of order 1 TeV. We note that if the Higgs decays to neutralinos in this
scenario, LHCb should see the lightest Higgs boson before ATLAS and CMS.Comment: 7 pages, 5 figure
Comparison of first pass bolus AIFs extracted from sequential 18F-FDG PET and DSC-MRI of mice.
Accurate kinetic modelling of in vivo physiological function using positron emission tomography (PET) requires determination of the tracer time-activity curve in plasma, known as the arterial input function (AIF). The AIF is usually determined by invasive blood sampling methods, which are prohibitive in murine studies due to low total blood volumes. Extracting AIFs from PET images is also challenging due to large partial volume effects (PVE). We hypothesise that in combined PET with magnetic resonance imaging (PET/MR), a co-injected bolus of MR contrast agent and PET ligand can be tracked using fast MR acquisitions. This protocol would allow extraction of a MR AIF from MR contrast agent concentration-time curves, at higher spatial and temporal resolution than an image-derived PET AIF. A conversion factor could then be applied to the MR AIF for use in PET kinetic analysis. This work has compared AIFs obtained from sequential DSC-MRI and PET with separate injections of gadolinium contrast agent and 18F-FDG respectively to ascertain the technique's validity. An automated voxel selection algorithm was employed to improve MR AIF reproducibility. We found that MR and PET AIFs displayed similar character in the first pass, confirmed by gamma variate fits (p<0.02). MR AIFs displayed reduced PVE compared to PET AIFs, indicating their potential use in PET/MR studies.This work was funded by an MRC studentship and travel to PSMR 2013 was funded by the EU COST action for PET/MR.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.nima.2013.08.07
Real-Time Imaging System using a 12-MHz Forward-Looking Catheter with Single Chip CMUT-on-CMOS Array
Forward looking (FL) imaging catheters would be an important tool for several intravascular ultrasound (IVUS) and intracardiac echocardiography (ICE) applications. Single chip capacitive micromachined ultrasonic transducer (CMUT) arrays fabricated on front-end CMOS electronics with simplified electrical interconnect have been previously developed for highly flexible and compact catheters. In this study, we present a custom built real time imaging system utilizing catheters with single chip CMUT-on-CMOS arrays and show initial imaging results. The fabricated array has a dual-ring structure with 64 transmit (Tx) and 56 receive (Rx) elements. The CMUT arrays fit on a 2.1 mm diameter circular region with all the required front-end electronics. The device operates at 12 MHz center frequency and has around 20 V collapse voltage. The single-chip system requires 13 external connections including 4 Rx channels and power lines. The electrical connections to micro cables in the catheter are made from the top side of the chip using polyimide flex tapes. The device is placed on a 6-Fr catheter shaft and secured with a medical grade silicon rubber. For real time data acquisition, we developed a custom design FPGA based imaging platform to generate digital control sequences for the chip and collect RF data from Rx outputs. We performed imaging experiments using wire phantoms immersed in water to test the real time imaging system. The system has the potential to generate images at 32 fps rate with the particular catheter. The overall system is fully functional and shows promising image performance
Variation with mass of \boldmath{B(E3; 0_1^+ \to 3_1^-)} transition rates in even-mass xenon nuclei
transition matrix elements have been measured for
even-mass Xe nuclei using sub-barrier Coulomb excitation in inverse
kinematics. The trends in energy and
excitation strengths are well reproduced using phenomenological models based on
a strong coupling picture with a soft quadrupole mode and an increasing
occupation of the intruder orbital.Comment: 5 pages, 4 figures, PRC in pres
Right Ventricular Dysfunction in the R6/2 Transgenic Mouse Model of Huntington's Disease is Unmasked by Dobutamine
Background: Increasingly, evidence from studies in both animal models and patients suggests that cardiovascular dysfunction is important in HD. Previous studies measuring function of the left ventricle (LV) in the R6/2 mouse model have found a clear cardiac abnormality, albeit with preserved LV systolic function. It was hypothesized that an impairment of RV function might play a role in this condition via mechanisms of ventricular interdependence.Objective: To investigate RV function in the R6/2 mouse model of Huntington's disease (HD).Methods: Cardiac cine- magnetic resonance imaging (MRI) was used to determine functional parameters in R6/2 mice. In a first experiment, these parameters were derived longitudinally to determine deterioration of cardiac function with disease progression. A second experiment compared the response to a stress test (using dobutamine) of wildtype and early-symptomatic R6/2 mice. Results: There was progressive deterioration of RV systolic function with age in R6/2 mice. Furthermore, beta-adrenergic stimulation with dobutamine revealed RV dysfunction in R6/2 mice before any overt symptoms of the disease were apparent.Conclusions: This work adds to accumulating evidence of cardiovascular dysfunction in R6/2 mice, describing for the first time the involvement of the right ventricle. Cardiovascular dysfunction should be considered, both when treatment strategies are being designed, and when searching for biomarkers for HD
Direct detection of a substellar companion to the young nearby star PZ Telescopii
Aims: We study the formation of substellar objects (exoplanets and brown
dwarfs) as companions to young nearby stars. Methods: With high contrast AO
imaging obtained with NACO at ESO's VLT we search for faint
companion-candidates around our targets, whose companionship can be confirmed
with astrometry. Results: In the course of our imaging campaign we found a
faint substellar companion of the nearby pre-main sequence star PZ Tel, a
member of the beta Pic moving group. The companion is 5-6 mag fainter than its
host star in JHK and is located at a separation of only 0.3 arcsec (or 15 AU of
projected separation) north-east of PZ Tel. Within three NACO observing epochs
we could confirm common proper motion (>39 sigma) and detected orbital motion
of PZ Tel B around its primary (>37 sigma). The photometry of the newly found
companion is consistent with a brown dwarf with a mass of 24 to 40 MJup, at the
distance (50 pc) and age (8-20 Myr) of PZ Tel. The effective temperature of the
companion, derived from its photometry, ranges between 2500 and 2700 K, which
corresponds to a spectral type between M6 and M8. After beta Pic b, PZ Tel B is
the second closest substellar companion imaged directly around a young star.Comment: accepted for publication in A&A Letter
The outburst of an embedded low-mass YSO in L1641
Strong outbursts in very young and embedded protostars are rare and not yet
fully understood. They are believed to originate from an increase of the mass
accretion rate onto the source. We report the discovery of a strong outburst in
a low-mass embedded young stellar object (YSO), namely 2MASS-J05424848-0816347
or [CTF93]216-2, as well as its photometric and spectroscopic follow-up. Using
near- to mid-IR photometry and NIR low-resolution spectroscopy, we monitor the
outburst, deriving its magnitude, duration, as well as the enhanced accretion
luminosity and mass accretion rate. [CTF93]216-2 increased in brightness by
~4.6, 4.0, 3.8, and 1.9 mag in the J, H, Ks bands and at 24 um, respectively,
corresponding to an L_bol increase of ~20 L_sun. Its early spectrum, probably
taken soon after the outburst, displays a steep almost featureless continuum,
with strong CO band heads and H_2O broad-band absorption features, and Br gamma
line in emission. A later spectrum reveals more absorption features, allowing
us to estimate T_eff~3200 K, M~0.25 M_sun, and mass accretion rate~1.2x10^{-6}
M_sun yr^{-1}. This makes it one of the lowest mass YSOs with a strong outburst
so far discovered.Comment: To be published in A&A letter; 5 pages, 4 figure
Incomplete Punishment Networks in Public Goods Games: Experimental Evidence
Abundant evidence suggests that high levels of contributions to public goods can be sustained through self-governed monitoring and sanctioning. This experimental study investigates the effectiveness of decentralized sanctioning institutions in alternative punishment networks. Our results show that the structure of punishment network significantly affects allocations to the public good. In addition, we observe that network configurations are more important than punishment capacities for the levels of public good provision, imposed sanctions and economic efficiency. Lastly, we show that targeted revenge is a major driver of anti-social punishment
A population Monte Carlo scheme with transformed weights and its application to stochastic kinetic models
This paper addresses the problem of Monte Carlo approximation of posterior
probability distributions. In particular, we have considered a recently
proposed technique known as population Monte Carlo (PMC), which is based on an
iterative importance sampling approach. An important drawback of this
methodology is the degeneracy of the importance weights when the dimension of
either the observations or the variables of interest is high. To alleviate this
difficulty, we propose a novel method that performs a nonlinear transformation
on the importance weights. This operation reduces the weight variation, hence
it avoids their degeneracy and increases the efficiency of the importance
sampling scheme, specially when drawing from a proposal functions which are
poorly adapted to the true posterior.
For the sake of illustration, we have applied the proposed algorithm to the
estimation of the parameters of a Gaussian mixture model. This is a very simple
problem that enables us to clearly show and discuss the main features of the
proposed technique. As a practical application, we have also considered the
popular (and challenging) problem of estimating the rate parameters of
stochastic kinetic models (SKM). SKMs are highly multivariate systems that
model molecular interactions in biological and chemical problems. We introduce
a particularization of the proposed algorithm to SKMs and present numerical
results.Comment: 35 pages, 8 figure
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