The N-jettiness subtraction scheme at subleading power

Isgro, Andrea

doi:https://doi.org/10.21985/n2-dj6f-vh50

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The N-jettiness subtraction scheme at subleading power

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The recently increasing precision in experimental measurements of observables in collider physics has demanded for a matching precision in the theoretical expectations for such observables. In Quantum ChromoDynamics, the fundamental underlying theory for collider physics, theoretical predictions are expressed under the form of perturbative series, where the expansion parameter is the strong coupling constant $\alpha_s$. Although for some phenomenological applications the first order of the expansion (Leading Order or LO) and the second order (Next-to-Leading Order or NLO) are accurate enough, for most high-energy processes a NNLO description is required. In particular, a process of great interest due to its abundance at the Large Hadron Collider is the production of a boson in association with one or more jets of energetic highly collimated particles. Among the factors which complicate the evaluation of NNLO jet cross sections, perhaps the hardest obstacle to overcome is the cancellation of infrared poles that appear due to soft and collinear emissions. Subtraction schemes are designed precisely to solve this problem, and one of them, which goes under the name of $N$-jettiness subtraction scheme, is the main subject of this dissertation. $N$-jettiness subtraction makes use of the Soft-Collinear Effective Theory (SCET) formalism to regularize infrared divergences. The main idea is to define a jet veto event-shape $\Tau$ to distinguish between the case where the emitted radiation can be soft or collinear (small-$\Tau$ limit) and the case where the emitted radiation is energetic and therefore produces a jet (large-$\Tau$ limit). The small-$\Tau$ limit is treated with an appropriate SCET factorization formula, while the large-$\Tau$ limit corresponds to an equivalent process with one more jet in the final state and one less order in the perturbative expansion. The value of the parametric cutoff $\Tauc$ which separates the small-$\Tau$ and large-$\Tau$ limit affects the final result with logarithmically enhanced power corrections, due to the fact that the SCET factorization formula only accounts for leading power operators. Such corrections are generally negligible for infinitely small values of $\Tauc$, but such small values of $\Tauc$ can sometimes compromise the numerical stability of the code. Therefore, the focus of this dissertation is to explore the Next-to-Leading Power (NLP) structure of $N$-jettiness subtraction and include power corrections into the subtraction scheme, hence greatly improving its performance.

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