Dynamically Assisted Schwinger Mechanism

Abstract

We study electron-positron pair creation from the Dirac vacuum induced by a strong and slowly varying electric field (Schwinger effect) which is superimposed by a weak and rapidly changing electromagnetic field (dynamical pair creation). In the sub-critical regime where both mechanisms separately are strongly suppressed, their combined impact yields a pair creation rate which is dramatically enhanced. Intuitively speaking, the strong electric field lowers the threshold for dynamical particle creation -or, alternatively, the fast electromagnetic field generates additional seeds for the Schwinger mechanism. These findings could be relevant for planned ultra-high intensity lasers.PACS numbers: 12.20. Ds, 11.15.Tk, 11.27.+d. As first realized by Dirac [1], a consistent relativistic quantum description of electrons necessarily involves negative energy levels, which -in the Dirac-sea pictureare filled up in the vacuum state. This entails the striking possibility of pulling an electron out of the vacuum by means of some external influence, such as a (classical) electromagnetic field [2], where the remaining hole in the Dirac sea is then associated with a positron. Of course, to create such an electron-positron pair out of the vacuum, one has to overcome the energy gap of 2mc 2 between the filled and the empty levels. There are basically two main mechanisms for doing so: In a strong electric field E over a sufficiently long distance L, "virtual" electron-positron pair fluctuations may gain this energy when qEL ≥ 2mc2 . This pair creation process is called the Schwinger mechanism [3,4] and can be understood as tunneling through the classically forbidden region (energy gap). Thus it is suppressed exponentially O(exp{−πE S /E}) for weak fields E, where E S = m 2 c 3 /( q) is the Schwinger critical field. For E ≃ E S , the work done by separating the electron-positron pair over a Compton wavelength is of the order of the energy gap 2mc2 . Alternatively, a classical time-dependent electromagnetic field will also create electron-positron pairs in general (dynamical pair creation). However, if the frequency ω of the external field is not large enough, ω < 2mc 2 , these non-adiabatic corrections correspond to higher-order (i.e., multi-photon) processes and are also suppressed exponentially exp{− O(1/ω)} for small ω [5]. These pair-production processes are fundamental predictions of quantum electrodynamics (QED), but only the multi-photon production process has so far been observed experimentally: the positron data taken at the SLAC E-144 experiment have convincingly been explained by nphoton production with n ≃ 5 [6]. However, a verification of the Schwinger mechanism has still remained an experimental challenge [7]. Since the Schwinger mechanism is non-perturbative in the field, its discovery would help exploring the non-perturbative realm of quantum field theory in a controlled fashion. Here, we propose a new mechanism which can help to overcome the strong exponential suppression. The basic idea is similar in spirit to ide...

Dynamically assisted Sauter-Schwinger effect in inhomogeneous electric fields

“…If the frequency exceeds this threshold value γ ω > γ crit ω , on the other hand, the instanton trajectory is reflected at the poles (i.e., b > 0) and thus the instanton action (4.2) is reduced by the weak temporal pulse ∝ E , leading to a significant enhancement of the pair creation probability. This enhancement is an example of the dynamically assisted Sauter-Schwinger effect, see [67][68][69][70][71][72], for an inhomogeneous electric field. In the homogeneous limit γ k ↓ 0, the threshold value (4.6) approaches γ crit ω = π/2 consistent with the results of [67].…”

Influence of a minimal length on the creation of scalar particles

“…The basic principle of these experiments is the enhancement of the Schwinger mechanism by the combination of a strong slow pulsed laser with a weak fast pulsed laser. It is shown in [4] that the faster pulse gives a multi-photon contribution, which reduces the barrier through which the particle tunnels and leads to an exponential enhancement. Then the Schwinger effect could be observed in the near future.…”

“…The field strength required to observe produced pairs is of order of the critical value E c = m 2 e = 10 16 Vcm −1 (for electron pairs), which seems to be beyond the current technological capabilities. However, in recent years, explicit experimental realizations have been proposed to see the Schwinger effect for the first time [4][5][6]. The basic principle of these experiments is the enhancement of the Schwinger mechanism by the combination of a strong slow pulsed laser with a weak fast pulsed laser.…”

Localisation in worldline pair production and lightfront zero-modes

“…To instead estimate the probability in a given field, describing e.g. focussed laser pulses [8][9][10][11][12][13], the locally constant approximation (LCA) is often used. In this approximation, the constant electric field E and a volume factor appearing in the famous expressions due to Heisenberg and Euler [2], and Schwinger [3], are replaced with E(x) and a integral over x µ , respectively.…”