Background Amplitudes Research Articles

Design and characterization of a new high-count wide-range fission chamber based on GEM

As one of the main detectors for monitoring neutron flux rate in a nuclear reactor, the fission chamber (FC) suffers from issues such as low sensitivity, limited counting rate dynamic range, and system mode switching is cumbersome. This study utilizes the advantages of gas electron multipliers (GEM), which are easy to fabricate in large areas and have high counting rates, to design a novel fission chamber. By conducting Monte Carlo simulations on parameters such as the thickness of the 235U coating, the drift distance, and the operating electric field strength, a wide-range fission chamber design that combines high sensitivity and a counting rate range based on GEM has been obtained. The simulation results demonstrated that using a GEM detector to build a fission chamber can significantly improve sensitivity and extend the counting rate range. Subsequently we built a proof-of-concept GEM-based fission chamber and tested it with a 55Fe low-energy X-ray source and an Am-Be neutron source. The results showed that the proof-of-concept detector had a good signal-to-noise ratio and energy linearity, as well as clear discrimination between alpha background and neutron pulse amplitudes.

The environments and hosts of metal absorption at z > 5

ABSTRACT A growing population of metal absorbers are observed at z > 5, many showing strong evolution in incidence approaching the epoch of hydrogen reionization. Follow-up surveys examining fields around these metals have resulted in galaxy detections but the direct physical relationship between the detected galaxies and absorbers is unclear. Upcoming observations will illuminate this galaxy–absorber relationship, but the theoretical framework for interpreting these observations is lacking. To inform future z > 5 studies, we define the expected relationship between metals and galaxies using the Technicolor Dawn simulation to model metal absorption from z = 5 to 7, encompassing the end of reionization. We find that metal absorber types and strengths are slightly better associated with their environment than with the traits of their host galaxies, as absorption system strengths are more strongly correlated with the local galaxy overdensity than the stellar mass of their host galaxy. For redshifts prior to the end of the epoch of reionization, strong high-ionization transitions like C iv are more spatially correlated with brighter galaxies on scales of a few hundred proper kpc than are low-ionization systems, due to the former’s preference for environments with higher ultraviolet background amplitudes and those ions’ relative rarity at z > 6. Post-reionization, the galaxy counts near these high-ionization ions are reduced, and increase surrounding certain low-ionization ions due to a combination of their relative abundances and preferred denser gas phase. We conclude that galaxy–absorber relationships are expected to evolve rapidly such that high-ionization absorbers are better tracers of galaxies pre-reionization, while low-ionization absorbers are better post-reionization.

Dynamics of B cell repertoires and emergence of cross-reactive responses in patients with different severities of COVID-19.

SummaryIndividuals with the 2019 coronavirus disease (COVID-19) show varying severity of the disease, ranging from asymptomatic to requiring intensive care. Although monoclonal antibodies specific to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been identified, we still lack an understanding of the overall landscape of B cell receptor (BCR) repertoires in individuals with COVID-19. We use high-throughput sequencing of bulk and plasma B cells collected at multiple time points during infection to characterize signatures of the B cell response to SARS-CoV-2 in 19 individuals. Using principled statistical approaches, we associate differential features of BCRs with different disease severity. We identify 38 significantly expanded clonal lineages shared among individuals as candidates for responses specific to SARS-CoV-2. Using single-cell sequencing, we verify the reactivity of BCRs shared among individuals to SARS-CoV-2 epitopes. Moreover, we identify the natural emergence of a BCR with cross-reactivity to SARS-CoV-1 and SARS-CoV-2 in some individuals. Our results provide insights important for development of rational therapies and vaccines against COVID-19.

Low Energy Theorem for Kaon Photoproduction

We study the Low Energy Theorem (LET) for kaon photoproduction at threshold region. To this end we expand the transition amplitude in terms of the kaon mass and the averaged mass of nucleon and Λ-hyperon. The background amplitudes are obtained from the appropriate Feynman diagrams. The amplitudes have been so far expanded up to the 3th order.

Numerical Simulations of High‐Frequency Gravity Wave Propagation Through Fine Structures in the Mesosphere

AbstractAn anelastic numerical model is used to study the influences of fine structure (FS) in the wind and stability profiles on gravity wave (GW) propagation in the Mesosphere and Lower Thermosphere (MLT). Large amplitude GWs interacting with FS, that is, thin regions of enhanced wind and stability, evolve very differently depending on the precise vorticity source and sink terms for small‐scale motions induced by the FS gradients. The resulting small‐scale dynamics are deterministic, promoting local instabilities, dissipation, and momentum deposition at locations and orientations determined by the initial FS. The resulting momentum depositions yield significant changes to the background wind structure, having scales and amplitudes comparable to the effects of large‐scale features in the ambient atmosphere. The deterministic nature of the large‐scale impacts further suggests that they can be estimated without fully resolving the underlying instability dynamics. Given the significant amplitudes and ubiquitous occurrence of FS throughout the atmosphere, the influences of these important and diverse flow evolutions merit inclusion in broader modeling efforts.

Rogue wave solutions for a coupled nonlinear Schrödinger equation in the birefringent optical fiber

In this paper, we investigate a fourth-order coupled nonlinear Schrödinger equation, which describes the propagation of ultrashort optical pulses in the birefringent optical fiber. Lax pair is constructed through the symbolic computation, along with the corresponding Darboux transformation (DT) which is different from that given in the existing literatures. Rogue wave solutions are obtained by virtue of the DT. Three different types of the rogue waves based on the solutions are exhibited, i.e., the anti-eye-shaped, eye-shaped four-petaled ones. Moreover, we find that the wave structure is affected by the relative background amplitudes and certain parameter in the solutions, and the range of the first-order rogue wave along the x axis increases with the increase of the fourth order term γ, while the range of the rogue wave along the t axis decreases.

Search for a new scalar resonance decaying to a pair of Z bosons in proton-proton collisions at sqrt{s}=13 TeV

A search for a new scalar resonance decaying to a pair of Z bosons is performed in the mass range from 130 GeV to 3 TeV, and for various width scenarios. The analysis is based on proton-proton collisions recorded by the CMS experiment at the LHC in 2016, corresponding to an integrated luminosity of 35.9 fb−1 at a center-of-mass energy of 13 TeV. The Z boson pair decays are reconstructed using the 4ℓ, 2ℓ2q, and 2ℓ2ν final states, where ℓ = e or μ. Both gluon fusion and electroweak production of the scalar resonance are considered, with a free parameter describing their relative cross sections. A dedicated categorization of events, based on the kinematic properties of associated jets, and matrix element techniques are employed for an optimal signal and background separation. A description of the interference between signal and background amplitudes for a resonance of an arbitrary width is included. No significant excess of events with respect to the standard model expectation is observed and limits are set on the product of the cross section for a new scalar boson and the branching fraction for its decay to ZZ for a large range of masses and widths.

Predicting K0Λ photoproduction observables by using the multipole approach

We present an isobar model for kaon photoproduction on the proton $$\gamma p\to K^+\Lambda$$ that can nicely reproduce the available experimental data from threshold up to $$W=2.0$$ GeV. The background amplitude of the model is constructed from a covariant Feynman diagrammatic method, whereas the resonance one is formulated by using the multipole approach. All unknown parameters in both background and resonance amplitudes are extracted by adjusting the calculated observables to experimental data. With the help of SU(3) isospin symmetry and some information obtained from the Particle Data Group we estimate the cross section and polarization observables for the neutral kaon photoproduction on the neutron $$\gamma n\to K^0\Lambda$$. The result indicates no sharp peak in the $$K^0\Lambda$$ total cross section. The predicted differential cross section exhibits resonance structures only at $$\cos\theta=-1$$. To obtain sizable observables the present work recommends measurement of the $$K^0\Lambda$$ cross section with $$W\gtrsim 1.70$$ GeV, whereas for the recoiled $$\Lambda$$ polarization measurement with $$W\approx 1.65$$–1.90 GeV would be advised, since the predictions of existing models show a large variance at this kinematics. The predicted electric and magnetic multipoles are found to be mostly different from those obtained in previous works. For $$W=1.75$$ and 1.95 GeV it is found that most of the single and double polarization observables demonstrate large asymmetries.

Challenges and opportunities for heavy scalar searches in the t t ¯ $$ t\overline{t} $$ channel at the LHC

Heavy scalar and pseudoscalar resonance searches through the $gg\rightarrow S\rightarrow t\bar t$ process are challenging due to the peculiar behavior of the large interference effects with the standard model $t\bar t$ background. Such effects generate non-trivial lineshapes from additional relative phases between the signal and background amplitudes. We provide the analytic expressions for the differential cross sections to understand the interference effects in the heavy scalar signal lineshapes. We extend our study to the case of CP-violation and further consider the effect of bottom quarks in the production and decay processes. We also evaluate the contributions from additional particles to the gluon fusion production process, such as stops and vector-like quarks, that could lead to significant changes in the behavior of the signal lineshapes. Taking into account the large interference effects, we perform lineshape searches at the LHC and discuss the importance of the systematic uncertainties and smearing effects. We present projected sensitivities for two LHC performance scenarios to probe the $gg\rightarrow S \rightarrow t\bar t$ channel in various models.

Signal-background interference for a singlet spin-0 digluon resonance at the LHC

Dijet mass distributions can be used to search for spin-0 resonances that couple to two gluons. I show that there is a substantial impact on such searches from the interference between the resonant signal and the continuum QCD background amplitudes. The signal dijet mass distribution is qualitatively modified by this interference, compared to the naive expectation from considering only the pure resonant contribution, even if the total width of the resonance is minimal and very small compared to the experimental dijet mass resolution. The impact becomes more drastic as the total width of the resonance increases. These considerations are illustrated using examples relevant to the 750 GeV diphoton excess recently observed at the LHC.

Interference effect on resonance studies in searches of heavy particles

The interference between resonance signal and continuum background can be either constructive or destructive, depending on the relative sign of couplings between the signal and background amplitudes. Different interference schemes lead to asymmetric distortions of the resonance line shape, which could be distinguished in experiments, when the internal resonance width is larger than the detector resolution. Interpreting the ATLAS diboson excesses by means of a toy [Formula: see text] model as an illustrative example (though it is disfavored by the 13 TeV data), we find that the signs of resonance couplings can only be revealed in the line shape measurements up to a high confidence level at a high luminosity, which could bring us further information on the underlying theory beyond resonance searches at future lepton and hadron colliders.

Two Binary Liquid Critical Mixtures Belong to Class of Universality

The dynamic shear viscosity of a binary liquid mixture phenol–water has been measured at different temperatures (50.0°C ≤ T ≤ 75.0°C) and different concentrations (0.00% up to 100.00% by weight of phenol). The critical temperature Tc and critical concentration xc are found to be 67.0°C and 33.90% by weight of phenol respectively, the critical mass density ρc is measured to be 0.8952 g/cm3. The critical and background amplitudes of specific heat at constant pressure are calculated to be 78.12 J/kg.K and 85.29 J/kg.K respectively. The pressure derivative of the critical temperature along the critical line ' cT T is calculated to be 9.722 ×10-6 K/Pa. In addition, dynamic shear viscosity of binary liquid mixture phenol–cyclohexane has been measured at different temperatures (14.0°C ≤ T ≤ 30.0°C) and different concentrations (2.00% up to 39.70% by weight of phenol). The critical temperature Tc and critical concentration xc are found to be 17.0°C and 2.70% by weight of phenol respectively; the critical mass density ρc is measured to be 0.7627 g/cm3. The critical and background amplitudes of isobaric thermal expansion coefficient αpc and αpb are calculated to be 8×10-6 K-1, 6×10-4 K-1 respectively. The pressure derivative of the critical temperature ' cT for the binary is calculated to be 2.8572 × 10-8 K/Pa. The universal quantity R+ ξ for the binary liquid critical mixture phenol–water is calculated to be 0.2716 ± 0.0005. In addition, the universal quantity R+ ξ for the binary liquid critical mixture phenol–cyclohexane is calculated to be 0.2699 ± 0.0001. The calculated values of the universal quantity R+ ξ are in a good agreement with the theoretical value of R+ ζ which is equal 0.2710. The two binary liquid critical mixture belong to the class of universality Two–Scale–Factor Universality.

K+Λelectroproduction above the resonance region

Background: In ${\ensuremath{\pi}}^{+}n$ and ${\ensuremath{\pi}}^{\ensuremath{-}}p$ electroproduction, conventional models cannot satisfactorily explain the data above the resonance region, in particular the transverse cross section. Although no high-energy l-t-separated cross-section data are available to date, a similar scenario can be inferred for ${K}^{+}\ensuremath{\Lambda}$ electroproduction.Purpose: Develop a phenomenological model for the $p({\ensuremath{\gamma}}^{*},{K}^{+})\ensuremath{\Lambda}$ reaction at forward angles and high-energies. Propose a universal framework for interpreting charged-kaon and charged-pion electroproduction above the resonance region.Method: Guided by the recent model for charged-pion electroproduction, developed by the authors, a framework for ${K}^{+}\ensuremath{\Lambda}$ electroproduction at high energies and forward angles is constructed. To this end, a Reggeized background model for ${K}^{+}\ensuremath{\Lambda}$ photoproduction is first developed. This model is used as a starting base to set up an electroproduction framework.Results: The few available data of the unseparated $p({\ensuremath{\gamma}}^{*},{K}^{+})\ensuremath{\Lambda}$ cross section are well explained by the model. Predictions for the l-t-separation experiment planned with the 12 GeV upgrade at Jefferson Lab are given. The newly proposed framework predicts an increased magnitude for the transverse structure function, similar to the situation in charged-pion electroproduction.Conclusions: Within a hadronic framework featuring Reggeized background amplitudes, $s$-channel resonance-parton effects can explain the observed magnitude of the unseparated $p({\ensuremath{\gamma}}^{*},{K}^{+})\ensuremath{\Lambda}$ cross section at high energies and forward angles. Thereby, no hardening of the kaon electromagnetic form factor is required.

Charged-pion electroproduction above the resonance region

[Background] Above the nucleon resonance region, the $N(e,e'\pi^\pm)N'$ data cannot be explained by conventional hadronic models. For example, the observed magnitude of the transverse cross section is significantly underestimated in a framework with Reggeized background amplitudes. [Purpose] Develop a phenomenological framework for the $N(e,e'\pi^\pm)N'$ reaction at high invariant mass $W$ and deep photon virtuality $Q^2$. [Method] Building on the work of Kaskulov and Mosel, a gauged pion-exchange current is introduced with a running cutoff energy for the proton electromagnetic transition form factor. A new transition form factor is proposed. It respects the correct on-shell limit, has a simple physical interpretation and reduces the number of free parameters by one. [Results] A study of the $W$ dependence of the $N(e,e'\pi^\pm)N'$ lends support for the newly proposed transition form factor. In addition, an improved description of the separated and unseparated cross sections at $-t \lesssim 0.5 \;\text{GeV}^2$ is obtained. The predictions overshoot the measured unseparated cross sections for $-t > 0.5 \;\text{GeV}^2$. Introducing a strong hadronic form factor in the Reggeized background amplitudes brings the calculations considerably closer to the high $-t$ data. [Conclusions] Hadronic models corrected for resonance/parton duality describe the separated pion electroproduction cross sections above the resonance region reasonably well at low $-t$. In order to validate the applicability of these models at high $-t$, separated cross sections are needed. These are expected to provide a more profound insight into the relevant reaction mechanisms.

Global suppression of electrocortical activity in unilateral perinatal thalamic stroke

We present an unusual case of persistent generalized electroencephalography (EEG) suppression and right-sided clonic seizures in a male infant born at 40+2 weeks' gestation, birthweight 3240g, with an isolated unilateral thalamic stroke. The EEG at 13 hours after birth showed a generalized very low amplitude background pattern, which progressed to frequent electrographic seizures over the left hemisphere. The interictal background EEG pattern remained grossly abnormal over the next 48 hours, showing very low background amplitudes (<10μV). Magnetic resonance imaging revealed an isolated acute left-sided thalamic infarction. This is the first description of severe global EEG suppression caused by an isolated unilateral thalamic stroke and supports the role of the thalamus as the control centre for cortical electrical activity.

The effect of fluctuations on the helium-ionizing background

Interpretation of He II Ly{\alpha} absorption spectra after the epoch of He II reionization requires knowledge of the He II ionizing background. While past work has modelled the evolution of the average background, the standard cosmological radiative transfer technique assumes a uniform radiation field despite the discrete nature of the (rare) bright quasars that dominate the background. We implement a cosmological radiative transfer model that includes the most recent constraints on the ionizing spectra and luminosity function of quasars and the distribution of IGM absorbers. We also estimate, for the first time, the effects of fluctuations on the evolving continuum opacity in two ways: by incorporating the complete distribution of ionizing background amplitudes into the standard approach, and by explicitly treating the quasars as discrete -- but isolated -- sources. Our model results in a He II ionization rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0 to z=2.5. This causes rapid evolution in the mean He II Ly{\alpha} optical depth -- as recently observed -- without appealing to the reionization of He II. The observed behaviour could instead result from rapid evolution in the mean free path of ionizing photons as the helium in higher H I column density absorbers becomes fully ionized.

The stochastic background: scaling laws and time to detection for pulsar timing arrays

We derive scaling laws for the signal-to-noise ratio of the optimal cross-correlation statistic, and show that the large power-law increase of the signal-to-noise ratio as a function of the observation time T that is usually assumed holds only at early times. After enough time has elapsed, pulsar timing arrays enter a new regime where the signal to noise only scales as . In addition, in this regime the quality of the pulsar timing data and the cadence become relatively unimportant. This occurs because the lowest frequencies of the pulsar timing residuals become gravitational-wave dominated. Pulsar timing arrays enter this regime more quickly than one might naively suspect. For T = 10 yr observations and typical stochastic background amplitudes, pulsars with residual root-mean-squares of less than about 1 μs are already in that regime. The best strategy to increase the detectability of the background in this regime is to increase the number of pulsars in the array. We also perform realistic simulations of the NANOGrav pulsar timing array, which through an aggressive pulsar survey campaign adds new millisecond pulsars regularly to its array, and show that a detection is possible within a decade, and could occur as early as 2016.

Detailed modeling of the statistical uncertainty of Thomson scattering measurements

The uncertainty of electron density and temperature fluctuation measurements is determined by statistical uncertainty introduced by multiple noise sources. In order to quantify these uncertainties precisely, a simple but comprehensive model was made of the noise sources in the MST Thomson scattering system and of the resulting variance in the integrated scattered signals. The model agrees well with experimental and simulated results. The signal uncertainties are then used by our existing Bayesian analysis routine to find the most likely electron temperature and density, with confidence intervals.In the model, photonic noise from scattered light and plasma background light is multiplied by the noise enhancement factor (F) of the avalanche photodiode (APD). Electronic noise from the amplifier and digitizer is added. The amplifier response function shapes the signal and induces correlation in the noise. The data analysis routine fits a characteristic pulse to the digitized signals from the amplifier, giving the integrated scattered signals. A finite digitization rate loses information and can cause numerical integration error.We find a formula for the variance of the scattered signals in terms of the background and pulse amplitudes, and three calibration constants. The constants are measured easily under operating conditions, resulting in accurate estimation of the scattered signals' uncertainty. We measure F ≈ 3 for our APDs, in agreement with other measurements for similar APDs. This value is wavelength-independent, simplifying analysis. The correlated noise we observe is reproduced well using a Gaussian response function. Numerical integration error can be made negligible by using an interpolated characteristic pulse, allowing digitization rates as low as the detector bandwidth. The effect of background noise is also determined.

Interference of Higgs diphoton signal and background in production with a jet at the LHC

The Higgs mass determination from diphoton events at the LHC can be affected by interference between the Higgs resonant and continuum background amplitudes with the same initial and final states. For the leading order gluon fusion process, this shift was previously found to exceed 100 MeV, with some dependence on the diphoton mass resolution and the methods used to extract and fit the peak from data. In this paper, I consider the mass shift for the process $pp\ensuremath{\rightarrow}j\ensuremath{\gamma}\ensuremath{\gamma}$ that includes an additional central jet in the final state. For cuts on the transverse momentum of the jet of 25 GeV or more, the diphoton Higgs peak mass shift due to interference is found to be very small, due in part to less interference for the gluon-gluon initiated subprocess, and in part to a cancellation between it and the quark-gluon initiated subprocess.

AN EFFICIENT APPROXIMATION TO THE LIKELIHOOD FOR GRAVITATIONAL WAVE STOCHASTIC BACKGROUND DETECTION USING PULSAR TIMING DATA

Direct detection of gravitational waves by pulsar timing arrays will become feasible over the next few years. In the low frequency regime ($10^{-7}$ Hz -- $10^{-9}$ Hz), we expect that a superposition of gravitational waves from many sources will manifest itself as an isotropic stochastic gravitational wave background. Currently, a number of techniques exist to detect such a signal; however, many detection methods are computationally challenging. Here we introduce an approximation to the full likelihood function for a pulsar timing array that results in computational savings proportional to the square of the number of pulsars in the array. Through a series of simulations we show that the approximate likelihood function reproduces results obtained from the full likelihood function. We further show, both analytically and through simulations, that, on average, this approximate likelihood function gives unbiased parameter estimates for astrophysically realistic stochastic background amplitudes.