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Donovan Young

Publications and source records attributed to Donovan Young.

At least 19 recordsLinked to original sources
arXiv2024

Bubbles in Linear Chord Diagrams: Bridges and Crystallized Diagrams

In a linear chord diagram a short chord joins adjacent vertices while a bubble is a region devoid of short chords. We define a bridge to be a chord joining a vertex interior to a bubble to one exterior to it. Building on earlier work, we investigate the distribution of bridges in the limit of large bubbles and diagrams, and show that the number of bridges is asymptotically normal, obtaining expressions for the associated mean and variance as a function of bubble size. We introduce the notion of a crystallized diagram, defined by the criteria that all its chords are either short or are bridges. We count the number of crystallized diagrams by the number of short chords they contain, and provide the asymptotic distribution in the limit of large crystallized diagrams. We show that for very large diagrams, the number of short chords is normal, and sharply peaked at $\sqrt{2n/\log n}$, where $n$ is the total number of chords in the diagram.

math.CO↗
arXiv2023

Counting Bubbles in Linear Chord Diagrams

In a linear chord diagram a short chord is one which joins adjacent vertices. We define a bubble to be a region in a linear chord diagram devoid of short chords. We derive a formal generating function counting bubbles by their size and find an exact result for the mean bubble size. We find that once one discards diagrams which have no short chords at all, the distribution of bubble sizes is given by a smooth function in the limit of long diagrams. Using a summation over short chords, the exact form of this asymptotic distribution is found.

math.CO↗
arXiv2020

Linear $k$-Chord Diagrams

We generalize the notion of linear chord diagrams to the case of matched sets of size $k$, which we call $k$-chord diagrams. We provide formal generating functions and recurrence relations enumerating these $k$-chord diagrams by the number of short chords, where the latter is defined as all members of the matched set being adjacent, and is the generalization of a short chord or loop in a linear chord diagram. We also enumerate $k$-chord diagrams by the number of connected components built from short chords and provide the associated generating functions in this case. We show that the distributions of short chords and connected components are asymptotically Poisson, and provide the associated means. Finally, we provide recurrence relations enumerating non-crossing $k$-chord diagrams by the number of short chords, generalising the Narayana numbers, and establish asymptotic normality, providing the associated means and variances. Applications to generalized games of memory are also discussed.

math.CO↗
arXiv2020

A critical quartet for queuing couples

We enumerate arrangements of $n$ couples, i.e. pairs of people, placed in a single-file queue, and consider four statistics from the vantage point of a distinguished given couple. In how many arrangements are exactly $p$ of the $n-1$ other couples i) interlaced with the given couple, ii) contained within them, iii) containing the given couple, and iv) lying outside the given couple? We provide generating functions which enumerate these arrangements and obtain the associated continuous asymptotic distributions in the $n\to\infty$ limit. The asymptotic distributions corresponding to cases i), iii), and iv) evince critical phenomena around the value $p_c=(n-1)/2$, such that the probability that 1) the couple is interlaced with more than half of the other couples, and 2) the couple is contained by more than half of the other couples, are both zero in the strict $n\to\infty$ limit. We further show that the cumulative probability that less than half of the other couples lie outside the given couple is $π/4$ in the limit, and that the associated distribution is uniform for $p<p_c$.

math.CO↗
arXiv2019

Generating Functions for Domino Matchings in the $2\times k$ Game of Memory

When all the elements of the multiset $\{1,1,2,2,3,3,\ldots,k,k\}$ are placed in the cells of a $2\times k$ rectangular array, in how many configurations are exactly $v$ of the pairs directly over top one another, and exactly $h$ directly beside one another --- thus forming $2\times 1$ or $1\times 2$ dominoes? We consider the sum of matching numbers over the graphs obtained by deleting $h$ horizontal and $v$ vertical vertex pairs from the $2\times k$ grid graph in all possible ways, providing a generating function for these aggregate matching polynomials. We use this result to derive a formal generating function enumerating the domino matchings, making connections with linear chord diagrams.

math.CO↗
arXiv2018

The SU(2|3) dynamic two-loop form factors

We compute two-loop form factors of operators in the $SU(2|3)$ closed subsector of $N=4$ supersymmetric Yang-Mills. In particular, we focus on the non-protected, dimension-three operators $\mathrm{Tr} (X[Y,Z])$ and $\mathrm{Tr} ( ψψ)$ for which we compute the four possible two-loop form factors, and corresponding remainder functions, with external states $\langle \bar{X} \bar{Y} \bar{Z}|$ and $\langle \barψ \barψ|$. Interestingly, the maximally transcendental part of the two-loop remainder of $\langle\bar{X} \bar{Y} \bar{Z}| \mathrm{Tr} (X[Y,Z]) |0\rangle$ turns out to be identical to that of the corresponding known quantity for the half-BPS operator $\mathrm{Tr} (X^3)$. We also find a surprising connection between the terms subleading in transcendentality and certain a priori unrelated remainder densities introduced in the study of the spin chain Hamiltonian in the SU(2) sector. Next, we use our calculation to resolve the mixing, recovering anomalous dimensions and eigenstates of the dilatation operator in the SU(2|3) sector at two loops. We also speculate on potential connections between our calculations in $N=4$ super Yang-Mills and Higgs + multi-gluon amplitudes in QCD in an effective Lagrangian approach.

hep-th↗
arXiv2015

On the Regularization of Extremal Three-point Functions Involving Giant Gravitons

In the AdS_5/CFT_4 set-up, extremal three-point functions involving two giant 1/2 BPS gravitons and one point-like 1/2 BPS graviton, when calculated using semi-classical string theory methods, match the corresponding three-point functions obtained in the tree-level gauge theory. The string theory computation relies on a certain regularization procedure whose justification is based on the match between gauge and string theory. We revisit the regularization procedure and reformulate it in a way which allows a generalization to the ABJM set-up where three-point functions of 1/2 BPS operators are not protected and where a match between tree-level gauge theory and semi-classical string theory is hence not expected.

hep-th↗
arXiv2015

On Yangian symmetry of scattering amplitudes and the dilatation operator in N=4 super Yang-Mills

It is known that the Yangian of PSU(2,2|4) is a symmetry of the tree-level S-matrix of N=4 super Yang-Mills. On the other hand, the complete one-loop dilatation operator in the same theory commutes with the level-one Yangian generators only up to certain boundary terms found by Dolan, Nappi and Witten. Using a result by Zwiebel, we show how the Yangian symmetry of the tree-level S-matrix of N=4 super Yang-Mills implies precisely the Yangian invariance, up to boundary terms, of the one-loop dilatation operator.

hep-th↗
arXiv2015

Integrability and unitarity

We show how generalised unitarity can be used to determine the one-loop dilatation operator in N=4 super Yang-Mills. Our analysis focuses on two sectors, namely the bosonic SO(6) sector and the SU(2|3) sector. The calculation is performed on shell, with no off-shell information introduced at any stage. In this way, we establish a direct connection between scattering amplitudes and the dilatation operator of the N=4 theory.

hep-th↗
arXiv2015

Integrability and MHV diagrams in N=4 supersymmetric Yang-Mills theory

We apply MHV diagrams to the derivation of the one-loop dilatation operator of N=4 super Yang-Mills in the SO(6) sector. We find that in this approach the calculation reduces to the evaluation of a single MHV diagram in dimensional regularisation. This provides the first application of MHV diagrams to an off-shell quantity. We also discuss other applications of the method and future directions.

hep-th↗
arXiv2015

An Extremal Chiral Primary Three-Point Function at Two-loops in ABJ(M)

I compute the leading correction to the structure constant for the three-point function of two length-two and one length-four chiral primary operators in planar ABJ(M) theory at weak 't Hooft coupling. The computation is reduced to four-loop propagator type Feynman integrals via a manifestly finite integration over the position of the length-four operator.

hep-th↗
arXiv2014

Form Factors of Chiral Primary Operators at Two Loops in ABJ(M)

We calculate the colour-ordered form factor for chiral primary operators built from J scalar fields of ABJ(M) theory to J scalar final states. We work in the 't Hooft limit and show that the leading quantum correction is order lambda squared, where lambda is the 't Hooft coupling. We evaluate this leading correction using standard Feynman diagrams and dimensional regularization, and find that the leading divergence is 1/epsilon^2 where the spacetime dimension is d = 3 - 2 epsilon. We further find that the result respects maximal transcendentality.

hep-th↗
arXiv2014

Scattering Amplitudes of Massive N=2 Gauge Theories in Three Dimensions

We study the scattering amplitudes of mass-deformed Chern-Simons theories and Yang-Mills-Chern-Simons theories with N=2 supersymmetry in three dimensions. In particular, we derive the on-shell supersymmetry algebras which underlie the scattering matrices of these theories. We then compute various 3 and 4-point on-shell tree-level amplitudes in these theories. For the mass-deformed Chern-Simons theory, odd-point amplitudes vanish and we find that all of the 4-point amplitudes can be encoded elegantly in superamplitudes. For the Yang-Mills-Chern-Simons theory, we obtain all of the 4-point tree-level amplitudes using a combination of perturbative techniques and algebraic constraints and we comment on difficulties related to computing amplitudes with external gauge fields using Feynman diagrams. Finally, we propose a BCFW recursion relation for mass-deformed theories in three dimensions and discuss the applicability of this proposal to mass-deformed N=2 theories.

hep-th↗
arXiv2012

Chiral primary one-point functions in the D3-D7 defect conformal field theory

We compute the one-point functions of chiral primary operators in the non-supersymmetric defect conformal field theory that is dual to the IIB string theory on $AdS_5\times S^5$ background with a probe D7 brane with internal gauge field flux, both in perturbative Yang-Mills theory and in the string theory dual. The former is expected to be accurate at weak coupling whereas the latter should be accurate in the planar strong coupling limit of the gauge theory. We consider the distinct cases where the D7 brane has geometry $AdS_4\times S^4$ with an instanton bundle of the worldvolume gauge fields on $S^4$ and $AdS_4\times S^2\times S^2$ with Dirac monopole bundles on each $S^2$. The gauge theory computation and the string theory computation can be compared directly in the planar limit and then a subsequent limit where the worldvolume flux is large. We find that there is exact agreement between the two in the leading order.

hep-th↗
arXiv2012

Deconstructing Supersymmetric S-matrices in D <= 2 + 1

Global supersymmetries of the S-matrices of N = 2, 4, 8 supersymmetric Yang-Mills theories in three spacetime dimensions (without matter hypermultiplets) are shown to be SU(1|1), SU(2|2) and SU(2|2) X SU(2|2) respectively. These symmetries are not manifest in the off-shell Lagrangian formulations of these theories. A direct map between these symmetries and their representations in terms of the Yang-Mills degrees of freedom and the corresponding quantities in Chern-Simons-Matter theories with N >= 4 supersymmetry is also obtained. Dimensional reduction of the on-shell observables of the Yang-Mills theories to two spacetime dimensions is also discussed.

hep-th↗
arXiv2012

Giant Gravitons on AdS_4 x CP^3 and their Holographic Three-point Functions

We find a simple parametrization of the anti-symmetric giant graviton in AdS_4 x CP^3, first constructed in arXiv:1108.3084 [hep-th], dual to the anti-symmetric Schur polynomial involving two bi-fundamental complex scalar fields of ABJM theory. Using this parametrization we evaluate in a semi-classical approach the three-point function of two such giant gravitons and one point-like graviton considering both extremal and non-extremal configurations. We likewise discuss the case of the symmetric giant graviton in AdS_4 x CP^3. Finally, we provide an expression for the planar three-point function of chiral primary operators in ABJM at strong coupling and find that the results for the giant graviton three-point functions reduce to this expression in the point-like limit.

hep-th↗