arXiv subjects

Deepak Sah

Publications and source records attributed to Deepak Sah.

9 recordsLinked to original sources

arXiv2026

Pair creation via Amplitude-Modulated Periodic and Quasiperiodic Pulse Sequences

We study nonperturbative pair production driven by alternating-sign electric field pulse trains. Using a quantum kinetic approach, we analyze both the longitudinal momentum spectrum and the particle yield for pulse sequences with either strictly periodic temporal structure, in which the pulse amplitudes alternate in a regular and repeating (E_1, E_2) pattern, or quasiperiodic (Fibonacci-ordered) structure, where the amplitudes follow a deterministic but aperiodic sequence generated by the Fibonacci substitution rule, exhibiting long-range order without exact repetition. For N=12 pulses, periodic trains generate regularly modulated spectra characteristic of multi-slit (Ramsey-type) interference, whereas Fibonacci sequences produce fragmented structures and partial momentum-space localization. Increasing the pulse number to N=20 further enhances these effects: periodic driving yields sharper and higher-contrast interference fringes, while quasiperiodic ordering leads to stronger localization and increasingly irregular spectral features.The particle yield exhibits a strongly nonlinear dependence on the field-strength ratio. For weak modulation , both temporal orderings produce nearly identical yields. For stronger fields, a modest crossover behavior is observed, with quasiperiodic sequences yielding slightly larger values than the periodic case. Overall, temporal ordering primarily redistributes spectral weight in momentum space, while the integrated yield is governed predominantly by the effective field strength. These results establish long-range temporal ordering as an effective control parameter in multipulse Schwinger pair production and provide guidance for designing tailored pulse sequences in future high-intensity laser experiments.

hep-ph↗

arXiv2026

Unitarity constraints on 2HDM with higher dimensional operators

We study how the requirement of perturbative unitarity restricts the parameter space of the two-Higgs-doublet model (2HDM) when higher-dimensional operators up to dimension six are included. We demonstrate that such operators can enhance scalar production cross sections in vector boson fusion relative to 2HDM. Using S-matrix unitarity, we place bounds on several dimension-six bosonic operators. We also find that certain blind directions in the Wilson coefficients of T-parameter violating operators which are poorly constrained by electroweak precision data can be partially excluded when unitarity constraints are taken into account. These results demonstrate how high-energy consistency can complement experimental limits in defining the allowed parameter space of 2HDM effective field theory.

hep-ph↗

arXiv2025

Electron-positron pair creation induced by multi-pulse train of electric fields: effect of randomness in time-delay

We investigate the creation of electron-positron pairs (EPPs) in a sequence of alternating-sign, time-dependent electric field pulse trains by solving the quantum Vlasov equations. Specifically, we focus on Sauter-like pulse trains with random time delays between successive pulses, drawn from a Gaussian distribution wherein the extent of fluctuations is controlled by the standard deviation $\sigma_T$ of the distribution. We find that increasing $\sigma_T$ leads to a dramatic transformation in the longitudinal momentum spectrum. The well-known fringe pattern, akin to that in the multi-slit interference, gets significantly modified. The averaged spectra exhibit a robust Gaussian-like envelope with residual oscillations, which are much more prominent in the central momentum region. Notably, we find that in certain cases, stochastic time delays lead to a pronounced enhancement in the central peak of the distribution function for pulse train containing $N$ pulses. For example, for $N=20$ pulses, $\sigma_T \approx 31$ $[m^{-1}]$(about $17\%$ of the mean time delay) yields nearly a tenfold increase in the central peak, which for $\sigma_T \approx 50$ $[m^{-1}]$ (about $27\%$ of the mean time delay), scales up to $10^3.$ This may open up new possibilities for optimizing multi-pulse field configurations and guide future experimental designs aimed at maximizing EPPs creation.

quant-ph↗

arXiv2025

Vacuum polarization current in presence of intense Sauter field

The quantum vacuum becomes unstable under an external field, leading to spontaneous particle-antiparticle pair creation. In canonical quantization, the time-dependent particle number, defined via Bogoliubov transformations lacks physical meaning until the external field vanishes. To address this, we explore dynamical quantities that remain well-defined at both asymptotic and intermediate times, focusing on the vacuum polarization current. Investigating this observable provides insights into the system's intermediate-time behavior. We consider pair creation in a spatially homogeneous, time-dependent, intense Sauter field. Specifically, we analyze the real and imaginary parts of the correlation function, linking them to vacuum polarization effects. The vacuum polarization current in an intense laser pulse is computed numerically, revealing that it correlates with the real part of the correlation function. Initially, the current changes sign and gradually decreases, but unlike the particle number, it does not reach a constant asymptotic value. Instead, for large times, it exhibits nearly undamped oscillations, a distinctive feature of scalar particles, oscillating strongly around zero. Additionally, we explore the uniqueness of the vacuum polarization current in the adiabatic basis, comparing different reference mode function choices. Notably, we find that the current remains independent of the basis choice.

hep-ph↗

arXiv2025

Dynamical Scaling in Pair Production for Scalar QED

We report on the dynamical scaling of momentum spectra for particle-antiparticle pairs at finite times within the framework of scalar Quantum Electrodynamics (QED). The analysis focuses on the momentum spectra in two different choices of adiabatic mode functions, which are related by a Wronskian normalization condition. Oscillations in the momentum spectra are attributed to quantum interference effects in the adiabatic number basis. A novel dynamical scaling behavior emerges when examining the oscillatory momentum spectra of pairs created by a Sauter pulsed field at intermediate times. While the oscillatory spectra are observed at distinct times in the two different choices, they overlap when time is rescaled by the point marking the initiation of the first occurrence of the Residual ParticleAntiparticle Plasma (RPAP) stage (or end of the transient stage) for the central momentum case. This scaling identifies the approximate time at which real particle-antiparticle pair formation becomes possible, shifting the focus from asymptotic times to finite-time dynamics. Additionally, in the multi-photon regime, we find that the momentum spectra exhibit a multi-modal profile structure at finite times, consistent across both choices and also follow the dynamical scaling in this case as well.

hep-ph↗

arXiv2024

Does the oscillatory behavior of the Momentum Spectrum depend on the basis in the Post-Transient Stage?

Pair creation by a spatially homogeneous, time-dependent electric field is studied within the framework of scalar quantum electrodynamics. We employ the standard Bogoliubov transformation approach to compute the single-particle distribution function in an adiabatic basis. We analyzed the distribution function of created particles in two different adiabatic bases (related by a unitary transformation). A novel dynamical scaling is observed while analyzing the oscillatory momentum spectrum of the pairs created by the Sauter pulsed field at intermediate times, calculated using the two adiabatic bases. In these bases, the same oscillatory momentum spectra are observed but at different times. However, when we scale the time by the point marking the end of the transient stage of dynamical evolution for each case of central momentum, the respective momentum spectra overlap. Furthermore, we study the time evolution of the momentum spectrum in the multi-photon regime and find that the spectra show a multi-modal profile structure at finite times for both choices of basis.

quant-ph↗

arXiv2024

Longitudinal Momentum Spectra of pair created in a pulsed field at finite times: Are Oscillations "Real"

We discuss the mechanism of production of electron-positron pairs from the vacuum in a time-varying, spatially uniform pulsed electric field. We analytically compute the probability of $(e^+ e^-) $pair production in momentum space using the exact solution of the one-particle time-dependent Dirac equation and compare the result with quantum kinetic theory (QKT). Both approaches allow us to study the particle momentum spectrum at any instant in time and can potentially unveil valuable information regarding quantum non-equilibrium physics. We analyze both approaches' momentum spectra of the created particles at any instant. We observe a multi-profile structure in the LMS. This multi-profile structure clearly illustrates the quantum interference effects associated with pair production. It is worth noting that both approaches exhibit quantum interference patterns at finite times, manifested as oscillations observed in the LMS. We highlight that this quantum signature is a universal behavior seen in the momentum spectra at finite times, where the electric field is nearly zero.

hep-ph↗

arXiv2023

Pair Production in time-dependent Electric field at Finite times

We investigate the finite-time behavior of pair production from the vacuum by a time-dependent Sauter pulsed electric field. By examining the temporal behavior of the single-particle distribution function, we observe oscillatory patterns in the longitudinal momentum spectrum of the particles at finite times. These oscillations arise due to quantum interference effects resulting from the various dynamical processes/channels leading to the creation of the (quasi-)particle of a given momentum. Furthermore, we derive an approximate and simplified analytical expression for the distribution function at finite times, allowing us to explain these oscillations' origin and behavior. The role of the vacuum polarization function and its counterterm are also discussed in this regard. The transverse momentum spectrum peaked at the nonzero value of the transverse momentum at finite times, which indicates the role of multiphoton transitions in the creation of quasiparticles.

hep-ph↗

arXiv2023

Pair Production in time-dependent Electric field at Finite times

We investigate the finite time behavior of pair production from the vacuum by time-dependent Sauter pulsed electric fields in spinor quantum electrodynamics (QED). Using the exact analytic solution of the mode function, we find the one-particle distribution function in momentum space. The longitudinal momentum spectrum of particles shows oscillatory behavior at a finite time in a small window of longitudinal momentum where the electric field diminishes to around one-hundredth of its maximum magnitude and its oscillation time is close to Compton time. This oscillation is asymmetric, i.e., the amplitude of oscillation is maximum for negative longitudinal momentum compared to positive rate. The change in the longitudinal momentum spectrum can occur due to the quantum interference effect, and this quantum interference effect comes from the result of dynamical tunneling. The transverse momentum spectrum shows the Gaussian structure with a peak at zero transverse momentum when $t = 0$. After $t \approx \tau/2$, the smooth Gaussian design becomes distorted, and we see inconstancy in spectrum structure, either a dip at the origin with an off-axis maximum or a peak at zero transverse momentum with small mountains up to $t \approx 2{\tau}$ observed. After that, the spectrum shows a maximum height at zero transverse momentum with weakly pronounced peaks.

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