arXiv subjects

K. S. Kumar

Publications and source records attributed to K. S. Kumar.

At least 73 records · Page 4Linked to original sources

arXiv2017

Measurement of the Drift Velocity and Transverse Diffusion of Electrons in Liquid Xenon with the EXO-200 Detector

The EXO-200 Collaboration is searching for neutrinoless double beta decay using a liquid xenon (LXe) time projection chamber. This measurement relies on modeling the transport of charge deposits produced by interactions in the LXe to allow discrimination between signal and background events. Here we present measurements of the transverse diffusion constant and drift velocity of electrons at drift fields between 20~V/cm and 615~V/cm using EXO-200 data. At the operating field of 380~V/cm EXO-200 measures a drift velocity of 1.705$_{-0.010}^{+0.014}$~mm/$μ$s and a transverse diffusion coefficient of 55$\pm$4~cm$^2$/s.

physics.ins-det↗

arXiv2017

Neutral-Current Weak Interactions at an EIC

A simulation study of measurements of neutral current structure functions of the nucleon at the future high-energy and high-luminosity polarized electron-ion collider (EIC) is presented. A new series of $γ-Z$ interference structure functions, $F_1^{γZ}$, $F_3^{γZ}$, $g_1^{γZ}$, $g_5^{γZ}$ become accessible via parity-violating asymmetries in polarized electron-nucleon deep inelastic scattering (DIS). Within the context of the quark-parton model, they provide a unique and, in some cases, yet-unmeasured combination of unpolarized and polarized parton distribution functions. The uncertainty projections for these structure functions using electron-proton collisions are considered for various EIC beam energy configurations. Also presented are uncertainty projections for measurements of the weak mixing angle $\sin^2 θ_W$ using electron-deuteron collisions which cover a much higher $Q^2$ than that is accessible in fixed target measurements. QED and QCD radiative corrections and effects of detector smearing are included with the calculations.

nucl-ex↗

arXiv2016

Cosmogenic Backgrounds to 0νββ in EXO-200

As neutrinoless double-beta decay experiments become more sensitive and intrinsic radioactivity in detector materials is reduced, previously minor contributions to the background must be understood and eliminated. With this in mind, cosmogenic backgrounds have been studied with the EXO-200 experiment. Using the EXO-200 TPC, the muon flux (through a flat horizontal surface) underground at the Waste Isolation Pilot Plant (WIPP) has been measured to be Φ = 4.07 $\pm$ 0.14 (sys) $\pm$ 0.03 (stat) $\times$ $10^{-7}$cm$^{-2}$ s$^{-1}$, with a vertical intensity of $I_{v}$ = 2.97$^{+0.14}_{-0.13}$ (sys) $\pm$ 0.02 (stat) $\times$ $10^{-7}$cm$^{-2}$ s$^{-1}$ sr$^{-1}$. Simulations of muon-induced backgrounds identified several potential cosmogenic radionuclides, though only 137Xe is a significant background for the 136Xe 0νββ search with EXO-200. Muon-induced neutron backgrounds were measured using γ-rays from neutron capture on the detector materials. This provided a measurement of 137Xe yield, and a test of the accuracy of the neutron production and transport simulations. The independently measured rates of 136Xe neutron capture and of 137Xe decay agree within uncertainties. Geant4 and FLUKA simulations were performed to estimate neutron capture rates, and these estimates agreed to within ~40% or better with measurements. The ability to identify 136Xe(n,γ) events will allow for rejection of 137Xe backgrounds in future 0νββ analyses.

nucl-ex↗

arXiv2016

First Search for Lorentz and CPT Violation in Double Beta Decay with EXO-200

A search for Lorentz- and CPT-violating signals in the double beta decay spectrum of $^{136}$Xe has been performed using an exposure of 100 kg$\cdot$yr with the EXO-200 detector. No significant evidence of the spectral modification due to isotropic Lorentz-violation was found, and a two-sided limit of $-2.65 \times 10^{-5 } \; \textrm{GeV} < \mathring{a}^{(3)}_{\text{of}} < 7.60 \times 10^{-6} \; \textrm{GeV}$ (90% C.L.) is placed on the relevant coefficient within the Standard-Model Extension (SME). This is the first experimental study of the effect of the SME-defined oscillation-free and momentum-independent neutrino coupling operator on the double beta decay process.

nucl-ex↗

arXiv2015

Stimulated Raman adiabatic passage in a three-level superconducting circuit

The adiabatic manipulation of quantum states is a powerful technique that has opened up new directions in quantum engineering, enabling tests of fundamental concepts such as the Berry phase and its nonabelian generalization, the observation of topological transitions, and holds the promise of alternative models of quantum computation. Here we benchmark the stimulated Raman adiabatic passage process for circuit quantum electrodynamics, by using the first three levels of a transmon qubit. We demonstrate a population transfer efficiency above 80% between the ground state and the second excited state using two adiabatic Gaussian-shaped control microwave pulses coupled to the first and second transition. The advantage of this techniques is robustness against errors in the timing of the control pulses. By doing quantum tomography at successive moments during the Raman pulses, we investigate the transfer of the population in time-domain. We also show that this protocol can be reversed by applying a third adiabatic pulse on the first transition. Furthermore, we demonstrate a hybrid adiabatic-nonadiabatic gate using a fast pulse followed by the adiabatic Raman sequence, and we study experimentally the case of a quasi-degenerate intermediate level.

quant-ph↗

arXiv2015

Investigation of radioactivity-induced backgrounds in EXO-200

The search for neutrinoless double-beta decay (0νββ) requires extremely low background and a good understanding of their sources and their influence on the rate in the region of parameter space relevant to the 0νββ signal. We report on studies of various β- and γ-backgrounds in the liquid- xenon-based EXO-200 0νββ experiment. With this work we try to better understand the location and strength of specific background sources and compare the conclusions to radioassay results taken before and during detector construction. Finally, we discuss the implications of these studies for EXO-200 as well as for the next-generation, tonne-scale nEXO detector.

physics.ins-det↗

arXiv2015

Stückelberg interference in a superconducting qubit under periodic latching modulation

When the level separation of a qubit is modulated periodically across an avoided crossing, tunneling to the excited state - and consequently Landau-Zener-Stückelberg interference - can occur. The types of modulation studied so far correspond to a continuous change of the level separation. Here we study periodic latching modulation, in which the level separation is switched abruptly between two values and is kept constant otherwise. In this case, the conventional approach based on the asymptotic Landau-Zener (LZ) formula for transition probabilities is not applicable. We develop a novel adiabatic-impulse model for the evolution of the system and derive the resonance conditions. Additionally, we derive analytical results based on the rotating-wave approximation (RWA). The adiabatic-impulse model and the RWA results are compared with those of a full numerical simulation. These theoretical predictions are tested in an experimental setup consisting of a transmon whose flux bias is modulated with a square wave form. A rich spectrum is observed, with distinctive features correspoding to two regimes: slow-modulation and fast-modulation. These experimental results are shown to be in very good agreement with the theoretical models. Also, differences with respect to the well known case of sinusoidal modulation are discussed, both theoretically and experimentally.

quant-ph↗

arXiv2014

The MOLLER Experiment: An Ultra-Precise Measurement of the Weak Mixing Angle Using Møller Scattering

The physics case and an experimental overview of the MOLLER (Measurement Of a Lepton Lepton Electroweak Reaction) experiment at the 12 GeV upgraded Jefferson Lab are presented. A highlight of the Fundamental Symmetries subfield of the 2007 NSAC Long Range Plan was the SLAC E158 measurement of the parity-violating asymmetry $A_{PV}$ in polarized electron-electron (Møller) scattering. The proposed MOLLER experiment will improve on this result by a factor of five, yielding the most precise measurement of the weak mixing angle at low or high energy anticipated over the next decade. This new result would be sensitive to the interference of the electromagnetic amplitude with new neutral current amplitudes as weak as $\sim 10^{-3}\cdot G_F$ from as yet undiscovered dynamics beyond the Standard Model. The resulting discovery reach is unmatched by any proposed experiment measuring a flavor- and CP-conserving process over the next decade, and yields a unique window to new physics at MeV and multi-TeV scales, complementary to direct searches at high energy colliders such as the Large Hadron Collider (LHC). The experiment takes advantage of the unique opportunity provided by the upgraded electron beam energy, luminosity, and stability at Jefferson Laboratory and the extensive experience accumulated in the community after a round of recent successfully completed parity-violating electron scattering experiments

nucl-ex↗

arXiv2014

Search for Majoron-emitting modes of double-beta decay of $^{136}$Xe with EXO-200

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of $^{136}$Xe. Here we report on a search for various Majoron-emitting modes based on 100 kg$\cdot$yr exposure of $^{136}$Xe. A lower limit of $T^{^{136}Xe}_{1/2} >1.2 \cdot 10^{24}$ yr at 90% C.L. on the half-life of the spectral index = 1 Majoron decay was obtained, corresponding to a constraint on the Majoron-neutrino coupling constant of $|< g^{M}_{ee} >|<$ (0.8-1.7)$\cdot$10$^{-5}$.

hep-ex↗

arXiv2014

Measurement of Parity-Violating Asymmetry in Electron-Deuteron Inelastic Scattering

The parity-violating asymmetries between a longitudinally-polarized electron beam and an unpolarized deuterium target have been measured recently. The measurement covered two kinematic points in the deep inelastic scattering region and five in the nucleon resonance region. We provide here details of the experimental setup, data analysis, and results on all asymmetry measurements including parity-violating electron asymmetries and those of inclusive pion production and beam-normal asymmetries. The parity-violating deep-inelastic asymmetries were used to extract the electron-quark weak effective couplings, and the resonance asymmetries provided the first evidence for quark-hadron duality in electroweak observables. These electron asymmetries and their interpretation were published earlier, but are presented here in more detail.

nucl-ex↗

arXiv2014

Spectroscopy of Ba and Ba$^+$ deposits in solid xenon for barium tagging in nEXO

Progress on a method of barium tagging for the nEXO double beta decay experiment is reported. Absorption and emission spectra for deposits of barium atoms and ions in solid xenon matrices are presented. Excitation spectra for prominent emission lines, temperature dependence and bleaching of the fluorescence reveal the existence of different matrix sites. A regular series of sharp lines observed in Ba$^+$ deposits is identified with some type of barium hydride molecule. Lower limits for the fluorescence quantum efficiency of the principal Ba emission transition are reported. Under current conditions, an image of $\le10^4$ Ba atoms can be obtained. Prospects for imaging single Ba atoms in solid xenon are discussed.

physics.atom-ph↗

arXiv2014

Search for Majorana neutrinos with the first two years of EXO-200 data

Many extensions of the Standard Model of particle physics suggest that neutrinos should be Majorana-type fermions, but this assumption is difficult to confirm. Observation of neutrinoless double-beta decay ($0νββ$), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with $^{76}$Ge (GERDA experiment) and $^{136}$Xe (KamLAND-Zen and EXO-200 experiments) have established the lifetime of this decay to be longer than $10^{25}$ yr, corresponding to a limit on the neutrino mass of 0.2-0.4 eV. Here we report new results from EXO-200 based on 100 kg$\cdot$yr of $^{136}$Xe exposure, representing an almost fourfold increase from our earlier published datasets. We have improved the detector resolution at the $^{136}$Xe double-beta-decay Q-value to $σ$/E = 1.53% and revised the data analysis. The obtained half-life sensitivity is $1.9\cdot10^{25}$ yr, an improvement by a factor of 2.7 compared to previous EXO-200 results. We find no statistically significant evidence for $0νββ$ decay and set a half-life limit of $1.1\cdot10^{25}$ yr at 90% CL. The high sensitivity holds promise for further running of the EXO-200 detector and future $0νββ$ decay searches with nEXO.

nucl-ex↗

arXiv2014

Weak charge form factor and radius of 208Pb through parity violation in electron scattering

We use distorted wave electron scattering calculations to extract the weak charge form factor F_W(q), the weak charge radius R_W, and the point neutron radius R_n, of 208Pb from the PREX parity violating asymmetry measurement. The form factor is the Fourier transform of the weak charge density at the average momentum transfer q=0.475 fm$^{-1}$. We find F_W(q) =0.204 \pm 0.028 (exp) \pm 0.001 (model). We use the Helm model to infer the weak radius from F_W(q). We find R_W= 5.826 \pm 0.181 (exp) \pm 0.027 (model) fm. Here the exp error includes PREX statistical and systematic errors, while the model error describes the uncertainty in R_W from uncertainties in the surface thickness σof the weak charge density. The weak radius is larger than the charge radius, implying a "weak charge skin" where the surface region is relatively enriched in weak charges compared to (electromagnetic) charges. We extract the point neutron radius R_n=5.751 \pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm$, from R_W. Here there is only a very small error (strange) from possible strange quark contributions. We find R_n to be slightly smaller than R_W because of the nucleon's size. Finally, we find a neutron skin thickness of R_n-R_p=0.302\pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm, where R_p is the point proton radius.

nucl-ex↗

arXiv2014

Simulation of motional averaging using a superconducting circuit

The possibility of using a quantum system to simulate another one has been recognized for a long time as an important research direction in quantum information and quantum computing. In Ref. [J. Li et. al, Nat. Commun. 4, 1420 (2013)], a superconducting circuit (a transmon) was employed to simulate a NMR (nuclear magnetic resonance) effect known as motional averaging. In this paper we analyze the results of numerical integration of the time evolution of the density matrix of a qubit with random frequency fluctuations, and compare the results with those obtained by using the method of quantum trajectories. We show that both methods produce equivalent results, although some differences may appear in the range of intermediate modulation frequencies.

cond-mat.mes-hall↗

arXiv2014

An improved measurement of the 2νββ half-life of Xe-136 with EXO-200

We report on an improved measurement of the 2νββ half-life of Xe-136 performed by EXO-200. The use of a large and homogeneous time projection chamber allows for the precise estimate of the fiducial mass used for the measurement, resulting in a small systematic uncertainty. We also discuss in detail the data analysis methods used for double-beta decay searches with EXO-200, while emphasizing those directly related to the present measurement. The Xe-136 2νββ half-life is found to be 2.165 +- 0.016 (stat) +- 0.059 (sys) x 10^21 years. This is the most precisely measured half-life of any 2νββ decay to date.

nucl-ex↗

arXiv2013

Electroweak Measurements of Neutron Densities in CREX and PREX at JLab, USA

Measurement of the parity-violating electron scattering asymmetry is an established technique at Jefferson Lab and provides a new opportunity to measure the weak charge distribution and hence pin down the neutron radius in nuclei in a relatively clean and model-independent way. This is because the Z boson of the weak interaction couples primarily to neutrons. We will describe the PREX and CREX experiments on ${}^{208}$Pb and ${}^{48}$Ca respectively; these are both doubly-magic nuclei whose first excited state can be discriminated by the high resolution spectrometers at JLab. The heavier lead nucleus, with a neutron excess, provides an interpretation of the neutron skin thickness in terms of properties of bulk neutron matter. For the lighter ${}^{48}$Ca nucleus, which is also rich in neutrons, microscopic nuclear theory calculations are feasible and are sensitive to poorly constrained 3-neutron forces.

nucl-ex↗

arXiv2013

Motional Averaging in a Superconducting Qubit

Superconducting circuits with Josephson junctions are promising candidates for developing future quantum technologies. Of particular interest is to use these circuits to study effects that typically occur in complex condensed-matter systems. Here, we employ a superconducting quantum bit (qubit),a transmon, to carry out an analog simulation of motional averaging, a phenomenon initially observed in nuclear magnetic resonance (NMR) spectroscopy. To realize this effect, the flux bias of the transmon is modulated by a controllable pseudo-random telegraph noise, resulting in stochastic jumping of the energy separation between two discrete values. When the jumping is faster than a dynamical threshold set by the frequency displacement of the levels, the two separated spectral lines merge into a single narrow-width, motional-averaged line. With sinusoidal modulation a complex pattern of additional sidebands is observed. We demonstrate experimentally that the modulated system remains quantum coherent, with modified transition frequencies, Rabi couplings, and dephasing rates. These results represent the first steps towards more advanced quantum simulations using artificial atoms.

cond-mat.mes-hall↗

arXiv2013

Measurement of the Parity-Violating Asymmetry in Electron-Deuteron Scattering in the Nucleon Resonance Region

We report on parity-violating asymmetries in the nucleon resonance region measured using $5 - 6$ GeV longitudinally polarized electrons scattering off an unpolarized deuterium target. These results are the first parity-violating asymmetry data in the resonance region beyond the $Δ(1232)$, and provide a verification of quark-hadron duality in the nucleon electroweak $γZ$ interference structure functions at the (10-15)% level. The results are of particular interest to models relevant for calculating the $γZ$ box-diagram corrections to elastic parity-violating electron scattering measurements.

nucl-ex↗