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Unitarity cuts of the worldsheet. (English) Zbl 07901055

Summary: We compute the imaginary parts of genus-one string scattering amplitudes. Following Witten’s \(i\varepsilon\) prescription for the integration contour on the moduli space of worldsheets, we give a general algorithm for computing unitarity cuts of the annulus, Möbius strip, and torus topologies exactly in \(\alpha^\prime\). With the help of tropical analysis, we show how the intricate pattern of thresholds (normal and anomalous) opening up arises from the worldsheet computation. The result is a manifestly-convergent representation of the imaginary parts of amplitudes, which has the analytic form expected from Cutkosky rules in field theory, but bypasses the need for performing laborious sums over the intermediate states. We use this representation to study various physical aspects of string amplitudes, including their behavior in the \((s, t)\) plane, exponential suppression, decay widths of massive strings, total cross section, and low-energy expansions. We find that planar annulus amplitudes exhibit a version of low-spin dominance: at any finite energy, only a finite number of low partial-wave spins give an appreciable contribution to the imaginary part.

MSC:

81Txx Quantum field theory; related classical field theories
81Uxx Quantum scattering theory
83Exx Unified, higher-dimensional and super field theories

Software:

feyntrop

References:

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