Small Field Region Research Articles

Global electroweak symmetric vacuum

Although the Higgs potential in the Standard Model (SM) contains only a simple electroweak symmetry breaking vacuum in the small field region, additional metastable or global vacua could exist in models beyond the SM. In this paper, we study one intriguing scenario with an additional electroweak symmetric vacuum that could be the global one. For the thermal universe ending at the current metastable vacuum, the electroweak symmetry should stay non-restored at high temperatures. We realize the scenario in a model with Higgs-portal couplings to SM singlet scalars with approximately global O(N) symmetries with a large N. For a large portion of model parameter space, both the quantum and thermal tunneling rates are suppressed such that our current metastable vacuum is long-lived enough. Our scenario predicts order-one changes for the Higgs self-couplings and a large contribution to the signal of the off-shell Higgs invisible decay. It can be partly probed at the LHC Run 3 and well tested at the high luminosity LHC. We also discuss the subcritical (anti-de Sitter) bubbles from the thermal tunneling that could have a large population and interesting cosmological implications.

Multiscale block averaging for QED in d = 3

We continue the study of the ultraviolet problem for quantum electrodynamics in d = 3. The model is defined on a fine toroidal lattice, and we seek control as the lattice spacing goes to zero. The problem is analyzed using Balaban’s formulation of the renormalization group. This involves a sequence of transformations consisting of a split into large and small field regions, then block averaging, and then scaling. The effective actions generated by this method depend strongly on certain multi-scale propagators and minimizers. The study of these objects both for fermions and for gauge fields is the content of this paper. Earlier work on the subject is reviewed. In addition for fermions, a polymer expansion is obtained for the determinants of the fermion propagators. For the gauge field, a detailed local regularity result is obtained for the minimizers.

Detector response in the buildup region of small MV fields.

The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen. We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface-near area of dose buildup for fields sized between 10×10 and 0.6×0.6cm2 . Depth dose curves were measured with 14 different detectors including plane-parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects. Depth-dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13cm3 scanning chamber CC13 and a plane-parallel chamber PPC05 agree very well in fields sized 4×4 and 10×10cm2 . For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used. A scanning chamber or a PPC40 chamber is suitable for fields larger than 4×4cm2 . Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half the inner radius of cylindrical ionization chambers for the effective point of measurement should be used. For small ionization chambers, averaging between both polarities is necessary for data obtained near the surface.

Non-Quadratic Transverse Magnetoresistance in the Nodal Line Dirac Semimetal InBi

The transverse magnetoresistance of a nodal line Dirac semi-metal InBi has been studied. It is found that the magnetoresistance is not quadratic. In the region of small magnetic fields $ B\lesssim 0.1$~T, it is characterized by high curvature, in the region of medium magnetic fields it is described by the sum of linear and quadratic contributions, and in the region of large magnetic fields $ B\gtrsim 1$~T, it approaches a quadratic law with a curvature several times smaller its zero field value. A phenomenological equation is proposed that allows to describe the entire dependence of the resistance on the magnetic field with an error not exceeding the measurement error of several percent.

How the structure of ferromagnetic cluster boundaries in an antiferromagnetic matrix impacts their magnetic properties in an external field

The question of how an external field impacts the magnetic moments of ferromagnetic clusters that are randomly located in a thin cylinder, is considered. The clusters have a magnetic dipole interaction. If there is sufficient spatial anisotropy, such a system can be described by a one-dimensional Ising model with a random exchange if an effective local field is present. A random effective field acting on the clusters reflects the inhomogeneity of the interface between the clusters and the antiferromagnet. In fields smaller than the saturation field, the ground state of such a model is a one-dimensional sequence of domains having different lengths. In contrast to the one-dimensional Ising model, at a constant field in the presence of a random effective field, there is a linear dependence of magnetization on the external field, in the small field region. The average of the random effective field determines the magnitude of the magnetization curve exchange bias, and the dispersion of the random effective field affects its slope. The results obtained in this study, together with experimental data, allow for a qualitative evaluation of the properties of the interface between the subsystems.

Non-equilibrium Spin-Hall effect in irregularly shaped aluminum and tungsten samples

The method proposed by us in Ref. [1], which eliminates obstacles in the application of electrical methods for studying the spin-Hall effect (SHE) by creating a spin unbalance, which generates a charge unbalance, using the form effect without using polarized injected current, is used in this work to study and compare SHE of different origin - internal (band) and external (structural and impurity). The internal SHE (ISHE) was studied on ultrapure single crystal samples of uncompensated (aluminum) and compensated (tungsten) metals. The influence of external factors on SHE was studied on polycrystalline and impurity aluminum samples. The investigations were carried out in the region of small magnetic fields, which ensure the symmetry of the conventional Hall effect (CHE), which makes it possible to study the chiral behavior of the spin Hall effect.

The Scaling Limit of the KPZ Equation in Space Dimension 3 and Higher

We study in the present article the Kardar–Parisi–Zhang (KPZ) equation $$\begin{aligned} \partial _t h(t,x)=\nu \Delta h(t,x)+\lambda |\nabla h(t,x)|^2 +\sqrt{D}\, \eta (t,x), \qquad (t,x)\in \mathbb {R}_+\times \mathbb {R}^d \end{aligned}$$ in $$d\ge 3$$ dimensions in the perturbative regime, i.e. for $$\lambda >0$$ small enough and a smooth, bounded, integrable initial condition $$h_0=h(t=0,\cdot )$$ . The forcing term $$\eta $$ in the right-hand side is a regularized space-time white noise. The exponential of h—its so-called Cole-Hopf transform—is known to satisfy a linear PDE with multiplicative noise. We prove a large-scale diffusive limit for the solution, in particular a time-integrated heat-kernel behavior for the covariance in a parabolic scaling. The proof is based on a rigorous implementation of K. Wilson’s renormalization group scheme. A double cluster/momentum-decoupling expansion allows for perturbative estimates of the bare resolvent of the Cole-Hopf linear PDE in the small-field region where the noise is not too large, following the broad lines of Iagolnitzer and Magnen (Commun Math Phys 162(1):85–121, 1994). Standard large deviation estimates for $$\eta $$ make it possible to extend the above estimates to the large-field region. Finally, we show, by resumming all the by-products of the expansion, that the solution h may be written in the large-scale limit (after a suitable Galilei transformation) as a small perturbation of the solution of the underlying linear Edwards–Wilkinson model ( $$\lambda =0$$ ) with renormalized coefficients $$\nu _{eff}=\nu +O(\lambda ^2),D_{eff}=D+O(\lambda ^2)$$ .

Instantaneous PIV/PTV-based pressure gradient estimation: a framework for error analysis and correction

A framework for the exact determination of the pressure gradient estimation error in incompressible flows given erroneous velocimetry data is derived which relies on the calculation of the curl and divergence of the pressure gradient error over the domain and then the solution of a div–curl system to reconstruct the pressure gradient error field. In practice, boundary conditions for the div–curl system are unknown, and the divergence of the pressure gradient error requires approximation. The effect of zero pressure gradient error boundary conditions and approximating the divergence are evaluated using three flow cases: (1) a stationary Taylor vortex; (2) an advecting Lamb–Oseen vortex near a boundary; and (3) direct numerical simulation of the turbulent wake of a circular cylinder. The results show that the exact form of the pressure gradient error field reconstruction converges onto the exact values, within truncation and round-off errors, except for a small flow field region near the domain boundaries. It is also shown that the approximation for the divergence of the pressure gradient error field retains the fidelity of the reconstruction, even when velocity field errors are generated with substantial spatial variation. In addition to the utility of the proposed technique to improve the accuracy of pressure estimates, the reconstructed error fields provide spatially resolved estimates for instantaneous PIV/PTV-based pressure error.

Inflation with a graceful exit in a random landscape

We develop a stochastic description of small-field inflationary histories with a graceful exit in a random potential whose Hessian is a Gaussian random matrix as a model of the unstructured part of the string landscape. The dynamical evolution in such a random potential from a small-field inflation region towards a viable late-time de Sitter (dS) minimum maps to the dynamics of Dyson Brownian motion describing the relaxation of non-equilibrium eigenvalue spectra in random matrix theory. We analytically compute the relaxation probability in a saddle point approximation of the partition function of the eigenvalue distribution of the Wigner ensemble describing the mass matrices of the critical points. When applied to small-field inflation in the landscape, this leads to an exponentially strong bias against small-field ranges and an upper bound N ≪ 10 on the number of light fields N participating during inflation from the non-observation of negative spatial curvature.

Excited-states spectroscopies and its magnetic field effect of π-conjugated polymer-fullerene blends with below-gap excitation

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT) with below-gap excitation. Compared to above-gap excitation, the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too, but with narrower distribution profile in polaron states in millisecond time regime. However, magnetophoto-induced absorption (MPA) of polarons presented much weaker magnetic field effect in below-gap excitation (∼0.5mT) than in above-gap excitation (order of ∼10mT), although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.

Joint Interpolation of Multisensor Sea Surface Temperature Fields Using Nonlocal and Statistical Priors

This paper addresses the joint analysis of multisource and multiresolution remote sensing data for the interpolation of high-resolution sea surface geophysical fields. As case-study application, we consider the interpolation of sea surface temperature (SST) fields. We propose a novel statistical model that combines two key features: an exemplar-based prior and statistical priors. The exemplar-based prior, referred to as a nonlocal prior, exploits similarities between local patches (small field regions) to interpolate missing data areas from previously observed exemplars. This nonlocal prior also sets an explicit conditioning between the multisensor data. Two complementary statistical priors, namely a prior on the spatial covariance and a prior on the marginal distribution of the high-resolution details, are considered as sea surface geophysical fields that are expected to depict specific spectral and marginal features in relation to the underlying turbulent ocean dynamics. We report the experiments on both synthetic and real SST data. These experiments demonstrate the contributions of the proposed combination of nonlocal and statistical priors to interpolate visually consistent and geophysically sound SST fields from multisource satellite data. We further discuss the key features and parameterizations of this model as well as its relevance with respect to classical interpolation techniques.

Bose-Einstein condensation of bound pairs of relativistic fermions in a magnetic field

The Bose-Einstein condensation of bound pairs made of equally and oppositely charged fermions in a magnetic field is investigated using a relativistic model. The Gaussian fluctuations have been taken into account in order to study the spectrum of bound pairs in the strong coupling region. We found, in weak coupling reagion, the condensation temperature increases with an increasing magnetic field displaying the magnetic catalysis effect. In strong coupling region, the inverse magnetic catalysis appears when the magnetic field is low and is replaced by the usual magnetic catalysis effect when magnetic field is sufficiently high, in contrast to the nonrelativistic case where the inverse magnetic catalysis prevails in strong coupling region regardless of the strength of the magnetic field. The resulting response to the magnetic field is the consequence of the competition between the dimensional reduction by Landau orbitals in pairing dynamics and the anisotropy of the kinetic spectrum of the bound pairs. We thus conclude that dimensional reduction dominates in weak domain and strong coupling one except the small magnetic field region, where the enhanced fluctuations dominates.

SU‐E‐T‐496: A Study of Two Commercial Dose Calculation Algorithms in Low Density Phantom

Purpose:Some lung cancer patients have very low lung density due to comorbidities. We investigate calculation accuracy of Eclipse AAA and Acuros(AXB) using a phantom that simulates this situation.Methods:A 2.5 x 5.0 x 5 cm (long) solid water inhomogeneity positioned 10 cm deep in a Balsa lung phantom (density 0.099 gm/cc) was irradiated with an off‐center field such that the central axis was parallel to one side of the inhomogeneity. Radiochromic films were placed at 2.5cm(S1) and 5cm(S2) depths. After CT scanning, Hounsfield Units(HU) were converted to electron(ρe) and mass(ρm) density using in‐house(IH) and vendor‐supplied(V) calibration curves. IH electron densities were generated using a commercial electron density phantom. The phantom was exposed to 6 MV 3×3 and 20×20 fields. Dose distributions were calculated using the AAA and AXB algorithms.Results:The HU of BW is ‐910±40 which translates to ρe of 0.088±0.050(IH) and 0.090±0.050(V), and ρm of 0.101±0.045(IH) and 0.103±0.039(V). Both ρe(V) and ρm(V) are higher than ρe(IH) and ρm(IH) respectively by 1.4‐5.3% and 0.5‐12.3%. The average calculated dose inside the solid water ‘tumor’ are within 3.7% and 2.4% of measurements for both calibrations and field sizes using AAA and AXB. Within 10mm outside the ‘tumor’, AAA on average underestimates by 18.3% and 17.0% respectively for 3×3 using IH and V. AXB underestimates by 5.9%(S1)‐6.6%(S2) and 13.1%(S1)‐16.0%(S2) respectively using IH and V. For 20×20, AAA and AXB underestimate by 2.8%(S1)‐4.4%(S2) and 0.3%(S1)‐1.4%(S2) respectively with either calibration.Conclusion:The difference in the HU calibration between V and IH is not of clinical significance in normal field sizes. In the low density region of small fields, the calculations from both algorithms differ significantly from measurements. This may be attributed to the insufficient lateral electron transport modeled by two algorithms resulting in the over‐estimation in penumbra expansion.

Spatial Segregation of Adaptation and Predictive Sensitization in Retinal Ganglion Cells

Sensory systems change their sensitivity based on recent stimuli to adjust their response range to the range of inputs and to predict future sensory input. Here, we report the presence of retinal ganglion cells that have antagonistic plasticity, showing central adaptation and peripheral sensitization. Ganglion cell responses were captured by a spatiotemporal model with independently adapting excitatory and inhibitory subunits, and sensitization requires GABAergic inhibition. Using a simple theory of signal detection, we show that the sensitizing surround conforms to an optimal inference model that continually updates the prior signal probability. This indicates that small receptive field regions have dual functionality--to adapt to the local range of signals but sensitize based upon the probability of the presence of that signal. Within this framework, we show that sensitization predicts the location of a nearby object, revealing prediction as a functional role for adapting inhibition in the nervous system.

SU-E-T-558: Photon Fluence Model for Distributed Radiation Sources Using the Convolution Method

Purpose: Sophisticated dose calculation algorithms based on principles of convolution theory are commonly used in commercial treatment planning systems for calculating photon dose from point‐like radiation sources. However, not much has been studied regarding the modeling of distributed sources such as a Cobalt‐60 (Co‐60) source of 2 cm diameter. In this work, we present a convolution based photon fluence model that can be used for dose calculations for any finite size source. Methods: The photon fluence model proposed in this work is based on two key functions: the source distribution function and the aperture function. The source distribution function, which models primary and scatter radiation independently, was determined using a one‐time Monte Carlo (MC) simulation. Since the focus of this work is on Co‐60 delivery, the MC modeling was based on a Theratronics T780 Co‐60 unit (Best Theratronics). The validity of the MC model was verified with the ion chamber measurements made in a water phantom. The aperture function is a rectangular window function representing the field size. The fluence model was evaluated by comparing the calculated photon fluence to the MC simulated fluence. Results: The comparisons between the photon fluence calculated using the proposed model and the MC simulation fluences show good agreement, better than 2% in the in‐field region of all large and small fields. In the tail and sharp dose gradient regions, this agreement is better than 5% for the large field sizes and 1.5% for the small field sizes. The primary and scatter fluence output factors are within 2% of those obtained with the MC simulations. Conclusion: The results of our investigations indicate that the proposed photon fluence model can accurately determine fluence for large and small radiation beams from finite size sources. This model has the potential of playing an integral role in Co‐60 based IMRT treatment planning.

Field emission from ZnO whiskers under intervalley electron redistribution

ZnO field-emitter whiskers with nanometer diameter were fabricated by metal-organic chemical vapor deposition (MOCVD) growth on Si substrates. Their electron field emission properties and electron transfer effect between the valleys were investigated in a high vacuum chamber. The Fowler–Nordheim (F–N) plots of the emission current show different slopes for the small and high electric field regions. A model based on the electron-emission from valleys having different specific electron affinities is proposed to explain the experimental results. The paper presents a study of the conduction band of nano-structured ZnO with the help of field emission experiments. The energy difference between the lower and upper valleys was determined to be between 3.02eV and 3.3eV. The effective work function from the satellite valley is much lower than from the Γ-valley. These results can explain the usually obtained large discrepancies between extremely high field enhancement factors by fitting using F–N equation with known work function Φ from the Γ-valley and the geometrical estimated field enhancement factors for ZnO emitter. These functional field emitters based on ZnO materials and their ternaries can also be used as ultraviolet photodetector and find new applications for miniaturized photo-field assisted vacuum devices.

Magnetic Properties of the Molecular Nanomagnet Cr<SUB>7</SUB>Cd: Single Ion and Exchange Anisotropy Effects

In order to verify two microscopic models of the molecular nanomagnet Cr7Cd we analyze a number of thermodynamic quantities calculated for two sets of parameters. The first model, with only single ion anisotropy, was established on the basis of the thermodynamic properties (by fitting susceptibility and magnetization) whereas the second, with single ion and bond-dependent exchange anisotropies, was based on the inelastic neutron scattering and EPR spectra. The calculations are performed by means of non-perturbative, numerically exact quantum transfer matrix technique on large scale parallel computers. We demonstrate that the predictions of the models are consistent in the region of small magnetic fields which do not exceed 10 T and differ significantly in higher fields. Comparison with the experiment leads to a conclusion that better modeling of magnetic torque requires more complex microscopic model with single ion and bond-dependent exchange anisotropies.

Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magneticfields

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can alsoact as efficient magnetic shields. The magnetic screening properties under an axial DCmagnetic field are characterized at several temperatures below the liquid nitrogentemperature (77 K). Two main shielding properties are studied and compared with those ofBi-2223, a material that has been considered in the past for bulk magnetic shields. The firstproperty is related to the maximum magnetic flux density that can be screened,Blim; it is defined as the applied magnetic flux density below which the field attenuation measuredat the centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall thickness of5 mm and a large ratio of length over radius,Blim is evaluated to 1 Tat T = 10 K. This valuelargely exceeds the Blim value measured at the same temperature on similar tubes of Bi-2223.The second shielding property that is characterized is the dependence ofBlim with respect to variations of the sweep rate of the applied field,dBapp/dt. This dependence is interpreted in terms of the power lawE = Ec(J/Jc)n and allows us todetermine the exponent n of this E(J) characteristics for Bi-2212. The characterization of the magnetic field relaxation involves verysmall values of the electric field. This gives us the opportunity to experimentally determine theE(J) law in an unexplored region of small electric fields. Combining these resultswith transport and AC-shielding measurements, we construct a piecewiseE(J) law that spans over eight orders of magnitude of the electric field.

A study of undulator magnets characterization using the vibrating wire technique

The vibrating wire (VW) technique employs a stretched wire as a magnetic field sensor. Because of the wire’s small diameter (∼0.1 mm or smaller) and because the wire can be supported from outside the magnet, this technique is very appealing for field measurements in small gap/bore undulators with small good field regions and with limited access to the tested field. In addition, in the case of elliptical undulators, in which Hall probe (HP) measurements can be affected by the planar Hall Effect; VW technique can be used as an independent method to verify and supplement HP measurements. In this article, we studied the potential of the VW technique for measurement of magnetic field errors and for prediction of beam trajectories in undulator magnets, using a 3.8 m long LCLS undulator as a test bench. Introducing calibrated magnetic field distortion at various locations, we measured the sensitivity and spatial resolution of the method. The method demonstrated 0.9 mm precision in localizing of the field distortion at a distance up to a few meters as well as 0.37 G cm sensitivity to the variation of the local field and 2 G cm sensitivity to the total field integral. To compare Hall probe and Vibrating wire measurements side-by-side, we measured field errors in an LCLS undulator previously characterized by Hall probe measurements. The field errors found with the Vibrating Wire technique appeared to be in good agreement with errors measured with the Hall probe. Beam trajectory distortions calculated from both data sets are also in good agreement.

Power series representations for complex bosonic effective actions. I. A small field renormalization group step

We develop a power series representation and estimates for an effective action of the form: ln[∫ef(α1,…,αs;z*,z)dμ(z*,z)∕∫ef(0,…,0;z*,z)dμ(z*,z)]. Here, f(α1,…,αs;z*,z) is an analytic function of the complex fields α1(x),…,αs(x), z*(x), z(x) indexed by x in a finite set X, and dμ(z*,z) is a compactly supported product measure. Such effective actions occur in the small field region for a renormalization group analysis. Using methods similar to a polymer expansion, we estimate the power series of the effective action.