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

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

Dissolving N=4 loop amplitudes into QCD tree amplitudes

We use the infrared consistency of one-loop amplitudes in N=4 Yang-Mills theory to derive a compact analytic formula for a tree-level NNMHV gluon scattering amplitude in QCD, the first such formula known. We argue that the IR conditions, coupled with recent advances in calculating one-loop box coefficients, can give a new tool for computing tree-level amplitudes in general. Our calculation suggests that many amplitudes have a structure which is even simpler than that revealed so far by current twistor-space constructions.Comment: 9 pages, revtex4. v2: minor changes, typo in (1) correcte

String Theory in Beta Deformed Spacetimes

Fluxbrane-like backgrounds obtained from flat space by a sequence of T-dualities and shifts of polar coordinates (beta deformations) provide an interesting class of exactly solvable string theories. We compute the one-loop partition function for various such deformed spaces and study their spectrum of D-branes. For rational values of the B-field these models are equivalent to Z_N \times Z_N orbifolds with discrete torsion. We also obtain an interesting new class of time-dependent backgrounds which resemble localized closed string tachyon condensation.Comment: 26 pages, harvmac, v2: statements about tachyons corrected, v3: very minor change