The LHCb prompt charm triggers

The LHCb experiment has fully reconstructed close to 10^9 charm hadron decays---by far the world's largest sample. During the 2011-2012 running periods, the effective proton-proton beam crossing rate was 11-15 MHz while the rate at which events were written to permanent storage was 3-5 kHz. Prompt charm candidates (produced at the primary interaction vertex) were selected using a combination of exclusive and inclusive high level (software) triggers in conjunction with low level hardware triggers. The efficiencies, background rates, and possible biases of the triggers as they were implemented will be discussed, along with plans for the running at 13 TeV in 2015 and subsequently in the upgrade era.Comment: To appear in the proceedings of The 6th International Workshop on Charm Physics (CHARM 2013

Charm physics results and prospects with LHCb

Precision measurements in charm physics offer a window into a unique sector of potential New Physics interactions. LHCb is poised to become a world leading experiment for charm physics, recording enormous samples with a detector tailored for flavor physics. This article presents recent charm production, CPV, and mixing studies from LHCb, including LHCb's first charm CP asymmetry measurement with 37 pb^-1 of data collected in 2010.Comment: 5 pages, 4 figures. Submitted to the proceedings of the 2011 Rencontres de Moriond QCD and High Energy Interaction

A Cluster Bootstrap for Two-Loop MHV Amplitudes

We apply a bootstrap procedure to two-loop MHV amplitudes in planar N=4 super-Yang-Mills theory. We argue that the mathematically most complicated part (the $\Lambda^2 B_2$ coproduct component) of the n-particle amplitude is uniquely determined by a simple cluster algebra property together with a few physical constraints (dihedral symmetry, analytic structure, supersymmetry, and well-defined collinear limits). We present a concise, closed-form expression which manifests these properties for all n.Comment: v2: a few comments adde

An analytic result for the two-loop seven-point MHV amplitude in N=4 SYM

We describe a general algorithm which builds on several pieces of data available in the literature to construct explicit analytic formulas for two-loop MHV amplitudes in N=4 super-Yang-Mills theory. The non-classical part of an amplitude is built from $A_3$ cluster polylogarithm functions; classical polylogarithms with (negative) cluster X-coordinate arguments are added to complete the symbol of the amplitude; beyond-the-symbol terms proportional to $\pi^2$ are determined by comparison with the differential of the amplitude; and the overall additive constant is fixed by the collinear limit. We present an explicit formula for the seven-point amplitude $R_7^{(2)}$ as a sample application.Comment: 17 pages, 1 figure; v2: minor changes; v3: added a section with some background materia

Four-Loop Collinear Anomalous Dimension in N = 4 Yang-Mills Theory

We report a calculation in N = 4 Yang-Mills of the four-loop term g^4 in the collinear anomalous dimension g(lambda) which governs the universal subleading infrared structure of gluon scattering amplitudes. Using the method of obstructions to extract this quantity from the 1/epsilon singularity in the four-gluon iterative relation at four loops, we find g^4 = -1240.9 with an estimated numerical uncertainty of 0.02%. We also analyze the implication of our result for the strong coupling behavior of g(lambda), finding support for the string theory prediction computed recently by Alday and Maldacena using AdS/CFT.Comment: 5 pages, 2 figure