Laser pulse-length effects in trident pair production

Laser pulses facilitate multiphoton contributions to the trident pair production $e_L^- \to e_L^- + e_L^- + e_L^+$ , where the label $L$ indicates a laser field dressed electron ($e^-$) or positron ($e^+$). We isolate the impact of the pulse envelope in the trident S matrix element, formulated within the Furry picture, in leading order of a series expansion in the classical non-linearity parameter $a_0$. Generally, the Fourier transform of the envelope carries the information on the pulse length, which becomes an easily tractable function in the case of a $\cos^2$ pulse envelope. The transition to a monochromatic laser wave can be handled in a transparent manner, as also the onset of multiphoton effects for short pulses can be factorized out and studied separately.

e e e e L L L L , where the label L indicates a laser field dressed electron (e − ) or positron (e + ). We isolate the impact of the pulse envelope in the trident S matrix element, formulated within the Furry picture, in leading order of a series expansion in the classical nonlinearity parameter a 0 . Generally, the Fourier transform of the envelope carries the information on the pulse length, which becomes an easily tractable function in the case of a cos 2 pulse envelope. The transition to a monochromatic laser wave can be handled in a transparent manner, as also the onset of bandwidth effects for short pulses can be factorized out and studied separately.
Keywords: trident process, pair production, weak-field expansion, strong-field QED (Some figures may appear in colour only in the online journal)

Introduction
High-intensity laser beams in the optical regime are customarily generated by the chirped pulse amplification (see [1]). Intensities up to 10 22 W cm −2 are achievable nowadays in several laboratories [2], yielding a classical nonlinearity parameter of =  a 10 50 0 ( -) in the focal spot 3 . Ongoing projects [4][5][6] of 10 PW class lasers envisage even larger values of a 0 . Due to higher frequencies in XFEL beams, w =  10 keV ( ), the parameter a 0 stays significantly below unity, despite similar intensities of - 10 W cm 22 2 ( )when tight focusing is attained [7]. Given such a variety of laser facilities, the experimental exploration of nonlinear QED effects became feasible and is currently further promoted. Elementary processes are under consideration with the goal of testing QED in the strong-field regime. Most notable is the nonlinear Compton process , also w.r.t. the subsequent use of the high energy photons (γ), up to prospects of industrial applications. While in the pioneering theoretical studies [8,9] the higher harmonics, related to multi-photon effects, i.e. the simultaneous interaction of the electron with a multitude of photons, in monochromatic laser beams have been considered, the study of laser pulses revealed a multitude of novel structures in the γ spectrum [3,[10][11][12][13][14][15][16][17][18]. The nonlinear Breit-Wheeler process, g  + -+ e e L L [3,13,[19][20][21][22][23][24][25][26][27][28][29], as cross channel of the nonlinear Compton process, is in contrast a threshold process-sometimes termed a genuine quantum process-since the probe photon γ energy in combination with the laser must supply sufficient energy to produce a e + e − pair. When considering the seminal SLAC experiment E-144 [30,31] as a two-step process (first step: generation of a high-energy photon γ by Compton backscattering [32], second step: Breit-Wheeler process γ+ L→e + e − ), also the nonlinear Breit-Wheeler process has been identified with the simultaneous interaction of up to five photons in the elementary subprocess. Strictly speaking, the mentioned two-step process is only a part of trident pair production  + + --+ e e e e L L L L , as stressed in [33,34]. Since the trident process is the starting point of seeded QED avalanches, expected to set in at high-intensities, Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 3 The relation of a 0 versus the laser peak intensity I L and frequency ω reads   , see [3].
it is currently a subject of throughout analyses [35][36][37], also for benchmarking PIC codes [38]. Given the high repetition rate of the European XFEL [39] a potentially interesting option is to combine it with a synchronized electron beam of about 50 MeV (to operate slightly above the threshold) in order to facilitate a high-statistics search for the dark photon. Such a dark photon (also dubbed U boson or hidden photon) is a candidate for Dark Matter beyond the standard model of particle physics; it is a possible extension which enjoys intense theoretical [40][41][42] and experimental [43][44][45][46] considerations. A corresponding theoretical analysis of the trident process can be found in [47]. In fact, the trident process-in a perturbative QED (pQED) language-includes sub-diagrams of the type γ * → e + e − , i.e. an intermediate (virtual) photon which decays into a e + e − pair. Via kinetic mixing, that virtual photon may 'temporarily' couple to a dark photon ¢ A , e.g. g g  ¢  A * * , thus signalizing its presence as a peak of the invariant mass distribution of e + e − . The peak would be at the mass of the dark photon and its width is related to the kinetic mixing strength.
We briefly mention the trident option of the LUXE project [48,49] at DESY/Hamburg, which however is primarily dedicated to explore the 'boiling of the vacuum' by means of the nonlinear Breit-Wheeler process in the Ritus corner, i.e. a kinematical region with a nonperturbative field strength dependence and coupling constant e | | dependence analog to the Schwinger pair creation probability.
While most of the above quoted papers focus on nonlinear effects in strong laser pulses, that is the impact of multiphoton contributions, we aim here at the study of apparent multiphoton effects due to bandwidth effects in weak and moderately strong laser pulses with a 0 <1. The analysis of the Breit-Wheeler pair production in [22,50] revealed that in such a regime interesting features appear for short and ultra-short pulses. For instance, despite of a 0 <1 a significant subthreshold pair production is enabled. Roughly speaking, in short pulses the frequency spectrum contains high Fourier components, thus enabling the subthreshold pair creation. In that respect, we are going to study the relevance of the pulse duration for the trident process. In contrast to the elementary one-vertex processes, the trident process as a twovertex process obeys a higher complexity, similar to the twophoton Compton scattering [51][52][53].
Our paper is organized as follows. In section 2, the matrix element is evaluated with emphasis on a certain regularization required to uncover the perturbative limit. The weak-field expansion is presented in section 3, where the Fourier transform of the background field amplitude is highlighted as a central quantity. The case of a cos 2 envelope is elaborated in dome detail in section 4, where also numerical examples are exhibited. The conclusion can be found in section 5.

Matrix element in the Furry picture
The leading-order tree level Feynman diagram of the trident process is exhibited in figure 1(a) , y its adjoint, and D μν is the photon propagator. The « p p 1 2 term ensures the antisymmetrization of two identical fermions (mass m, charge e | |) in the final state. The laser field A μ and its polarisation four-vector ε μ and phase f = k x · is specialized further on below. The momenta p, p 1,2,3 and k are four-vectors as well, and γ μ stands for Dirac's gamma matrices. Transforming the photon propagator into momentum space, and employing the Feynman gauge, , and a suitable splitting of phase factors of the Volkov solution, e.g.
· , one can cast the above matrix element in the form A key for that is as well as as the nonlinear Volkov phase part. The quantities u(p) and v(p) are a free-field Dirac bispinors, with spin indices suppressed for brevity. In intermediate steps, one meets the local energymomentum balance -+ ¢ -= p p k sk 0 1 and +p p 2 3 ¢ -= k rk 0, which combine to the overall conservation in δ (4) . The corresponding representation of the matrix element in momentum space is exhibited in figure 1(b)-left, where we exposed the interaction with s and r laser photons marked by the crosses. In figure 1(b)-right, we suppress these explicit representations of the laser background field. Note that the momentum space diagrammatics differs from the notation in [54,55]. We introduce the short-hand notations C and BW to mean momentum dependences on (p 1 , −p) and (p 2 , (2), we note the decomposition, emerging from inserting the Volkov solution, G * = m r, ) with * meaning C or BW and, by using a generic momentum pair q q , which can be combined to arrive at the notation such as those of [34,36]: where Ψ(q)=u(q) for C and Ψ(q)=v(q) for BW, respectively. The phase integrals in equation (4) read Note that in (6), l is an index (label) on the lhs, while it is a power on the rhs, as n too. An important step is the isolation of the divergent part in Γ 0 μ or B 0 . We note and regularize G m 0 by inserting a damping factor f - e | | and performing the limit ò → 0, similar to the method in [56], which results in G m s q q , , where  means the principal value in the variable s. One can exploit in reading (4) the crossing symmetry Δ μ (r, BW= (p 2 , p 3 ))=Δ μ (r→s, BW → C=(p 1 , −p)). Employing (8) with the short-hand notation

Weak-field expansion
With the argumentation given in the introduction we now attempt an expansion in powers of a 0 . We note µ m J a l l 0 and a µ a ; l l 0 again, l is a label (power) on the lhs (rhs). The leading-order terms of the phase integrals (6) thus become Denoting the Fourier transform of f (f) by The two delta distributions in the M 0 term in (10) enforce for the overall momentum conservation in (2) a factor δ (4) (p 1 +p 2 +p 3 −p) implying a zero contribution of the M 0 term (14a). In the spirit of the a 0 series expansion we neglect (14d) at all. (This term would give rise to on/off-shell contributions which require some care.) The remaining leading order terms in (14b) and (14c) generate the contributions  figure 1(a)) in the exit channel. Fixing q E , cos 3 2,3 and j 3 yields the contour plot s(E 2 , j 2 ). An example is exhibited in figure 3. The locus of the apparent one-(two-) photon contribution with s=1 (=2 ) is highlighted by fat curves.
The final result is the leading-order matrix element ( ) for the lightfront coordinates of a four-vector q μ =(q 0 , q 1 , q 2 , q 3 ).
where the tildes indicate here that the factor a 0 is scaled out. These structures are suggestive: M(BW) may be read as the coupling of the free Compton current m J C 0 ( ) to the modified Breit-Wheeler current (in parenthesis of (19a)) and a free Breit-Wheeler current m J BW 0 ( ) to a modified Compton current (in parenthesis of (19b)), both depicted in the middle panels of figure 1(c). The interaction with the external field is encoded in the modified currents. For practical purposes we replace the modified currents by the proper Volkov currents Δ μ (C) and Δ μ (BW) when evaluating numerically the matrix elements for a 1 0  . The differential probability is ( ) (see [16]). The matrix element  is given by (18) but without the pre-factor: We emphasize that the weak-field limit (18) in (20) corresponds to the standard perturbative tree level QED diagrams depicted in the figure 2, supposed s=1 selects then the admissible kinematics, and the phase space in (20) becomes five-dimensional. The decomposition (8) is essential for catching the proper weak-field perturbative limit. This is obvious when considering Møller or Bhabha scattering in an ambient background field as the cross channels of the trident process: for A → 0 the standard pQED must be recovered.

The case of a cos 2 envelope
We now specialize the linearly polarized external e.m. field   (25), but in general it aquires also an imaginary part. We therefore keep the notation F s 2 | ( )| . Given the properties of the sinc functions entering (26) we find for ¹ s 1, in particular for s>1, at finite values of Δf, we see that this signals bandwidth effects, despite a 1 0  . Such effects have been observed in [50] for the Breit-Wheeler pair production below the threshold and the Compton process as well [22,23]. Since s is a continuous variable one must not identify it with a 'photon number'; instead, s could be interpreted as fraction of energy or momentum in units of w = k | |  participating in creating a final state different from the initial state (see [9] for discussions of that issue). Even more, s>1 does not mean proper multiphoton effects due to our restriction on leading order in a 0 , rather one could speak an 'apparent multiphoton effects' caused by finite bandwidth of the pulse. The dependence of f D  envelopes are We use the absolute value of the analytic signal of F(s), i.e. the Fourier transform of the amplitude function (25) constrained to the positive half-line [57]. The first side maxima are located between  p f D

2
and  p f D

3
and their heights are 7×10 −4 of the respective main maximum, meaning that their contribution is not negligible, in particular for a kinematical situation where s>1 (see figure 3).
From the definition of s in equation (17), we note (i) the dependence s(E 2 , E 3 , θ 2,3 , j 2,3 ) and (ii) the U shape of s( Note the near-perfect match 5 . The selected kinematics implies s min ≈1.1, i.e. in the limit f D  ¥, this setting would be kinematically forbidden, but bandwidth effects for finite values of Δf enable the selected kinematics. In fact, increasing Δf causes (i) a rapid dropping of the differential cross section and (ii) make the oscillatory pattern more dense. At the heart of the behavior is essentially the quantity , meaning that with increasing values of Δf the differential cross section does not drop but gets concentrated at two values of E 2 (at given E 3 ) which are allowed for s = 1. In the limit f D  ¥, two delta peaks arise when E 3 is appropriately fixed. They are the result of cutting the strength distribution over the E 2 −E 3 plane at E 3 =const., i.e. there is a sharp ring given by the solution of E 3 (E 2 ). The tilt of the U shaped function s(E 2 ) at fixed other parameters to the right makes the region s≈1 larger at large but finite values of Δf. Correspondingly, the rhspeak structure is wider, as seen e.g. in the right bottom panel.
To compare with the standard pQED result, based on the evaluation of the Feynman diagrams depicted in figure 2, one has to perform the j 2 integration. In fact, making Δf larger and larger, the differential cross section s q W E E d d dcos d d It also demonstrates that for Δf<100 a significantly larger phase space beyond the perturbatively accessible (one-photon) region (indicated by ⟷) is occupied due to bandwidth effects in laser pulses. ) .

Summary
The length of laser pulses has a decisive impact on the pair production in the trident process. The rich phase space patterns, already found and analyzed in some detail for nonlinear one-vertex processes à la Compton and Breit-Wheeler, show up also in the two-vertex trident process. Even for weak laser fields, a region becomes accessible which would be kinematically forbidden in a strict perturbative, leading-order tree level QED approach. The key is the frequency distribution in a pulse which  , j 2 =π/2, j 3 =0 over the E 2 −E 3 plane (top) and as a function of E 2 for E 3 =1.76m (bottom, solid cyan curves). The initial electron is at rest and the laser frequency amounts to ω=k 0 =5.12m in this frame. The pulse length parameters are Δf=25, K, 500 as indicated. In the bottom panels, the function f f D D F s, 2 | ( )| is exhibited by dashed black curves, scaled up by a common factor of 5.2×10 −7 . differs significantly from a monochromatic laser beam, which would mean an 'infinitely long laser pulse'. By resorting to a special pulse model pf f f µ D cos 2 cos 2 ( ) ( ) , we quantify in a transparent manner the effect of the pulse duration Δf and identify the transition to monochromatic laser fields, f D  ¥, for small values of the classical laser nonlinearity parameter. To ensure the contact to a pQED approach the proper decomposition of a phase factor is mandatory. To be specific, the first term in (8) is essential for the two-vertex process; for one-vertex processes it does not contribute. The effect of short laser pulses manifests itself in bandwidth effects mimicking apparent multiphoton contributions even for weak fields, where the intermediate photon is off-shell and and the process is of one-step nature. The rich pattern of the differential phase space distribution is traced back to the Fourier transform of the external e.m. field. In obvious further extensions of our approach, more general pulse envelope shapes should be studied, e.g. within the slowly varying envelope approximation. Going to larger values of the classical laser nonlinearity parameter means checking whether an analog of the Fourier transform of the e.m. field can be isolated as crucial element of the phase space distribution of produced particles 6 . The final state phase distribution is important for planing corresponding experimental designs. and j 3 =0 for various values of Δf (solid curves, Δf=25: blue, 50: orange, 250: green, 500: red ) at a 0 =10 −4 . The initial electron is at rest and the laser frequency amounts to ω=k 0 =5.12m in this frame. The pQED result is depicted by the black dashed curve (we checked our pQED-software package by comparing with [58][59][60] and get confidence of our numerical evaluation and normalization of (22) by the smooth approach towards the pQED result for large values of Δf). 6 In fact, for a 0 <0.01 and large values of Δf we find numerical agreement of (22) and the pQED result.