Parton showers with more exact color evolution

Abstract

Parton shower event generators typically approximate evolution of QCD color so that only contributions that are leading in the limit of an infinite number of colors are retained. Our parton shower generator, Deductor, has used an "LC+" approximation that is better, but still quite limited. In this paper, we introduce a new scheme for color in which the approximations can be systematically improved. That is, one can choose the theoretical accuracy level, but the accuracy level that is practical is limited by the computer resources available. 1 In the implementation of the parton shower in Deductor, we average over spins at each stage. This is evidently an approximation. We analyzed what to do with spin in Ref. [6], but this strategy for spin is not implemented in Deductor.

Safe use of jet pull

“…Soft radiation exhibit strong sensitivity to the pattern of colour correlations and therefore these observables can provide a valuable testing ground for Monte Carlo parton showers that attempt to go beyond the large-N c limit, e.g. [37,38].…”

Exponentiating virtual imaginary contributions in a parton shower

“…The accuracy of leading order parton shower programs could be enhanced by improving the treatment of QCD color [2,[4][5][6][7]. We have worked to make our program, Deductor, more accurate with respect to color [1,3,8,9]. In the organization of Deductor, there are three operators that have a color structure that cannot be incorporated into the LC+ approximation that Deductor uses: ∆H, ∆V Re , and V iπ .…”

“…The operator a lc+ soft (t, {p, f } m ) contains a soft singularity only, so it does not have a contribution proportional to t. From its functional form [8], one sees that this operator is independent of the shower time except for shower times close to 0. It has the expansion…”

“…There have been efforts to improve the accuracy of leading order parton shower programs by moving away from the leading color (LC) approximation in the treatment of QCD color [1][2][3][4][5][6][7][8][9]. For this effort to be consistent with quantum mechanics, one needs to work with bra and ket color amplitudes and to implement the color content of virtual graphs acting on these amplitudes [1,3,4,[7][8][9].…”

Resummed predictions for jet-resolution scales in multijet production in e+e− annihilation

“…This is of particular interest for example in the context of recent and ongoing developments to systematically include such corrections into parton-shower event generators, see e.g. [48,49,50,51,52,53,54]. To assess the effect of subleading colour contributions, we redo our calculation in the t'Hooft large-N C limit, defined by taking N C → ∞ while keeping α s N C fixed [55].…”