arXiv subjects

Jennifer M. Smillie

Publications and source records attributed to Jennifer M. Smillie.

32 records · Page 2Linked to original sources

arXiv2012

Probing higher-order corrections in dijet production at the LHC

Both the ATLAS and CMS Collaborations have sought for effects beyond pure next-to-leading order in dijet observables, with the goal to distinguish between the perturbative descriptions provided by a next-to-leading order plus collinear-resummation calculation and by the resummation of wide-angle, hard emissions. In this paper we identify regions of phase space in dijet production where some observables receive large corrections beyond next-to-leading order and study their theoretical description with two tools that perform these two different resummations: the POWHEG BOX and HEJ. Furthermore, we suggest analyses where the predictions from POWHEG and HEJ can be clearly distinguished experimentally.

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arXiv2011

On the renormalization of multiparton webs

We consider the recently developed diagrammatic approach to soft-gluon exponentiation in multiparton scattering amplitudes, where the exponent is written as a sum of webs - closed sets of diagrams whose colour and kinematic parts are entangled via mixing matrices. A complementary approach to exponentiation is based on the multiplicative renormalizability of intersecting Wilson lines, and their subsequent finite anomalous dimension. Relating this framework to that of webs, we derive renormalization constraints expressing all multiple poles of any given web in terms of lower-order webs. We examine these constraints explicitly up to four loops, and find that they are realised through the action of the web mixing matrices in conjunction with the fact that multiple pole terms in each diagram reduce to sums of products of lower-loop integrals. Relevant singularities of multi-eikonal amplitudes up to three loops are calculated in dimensional regularization using an exponential infrared regulator. Finally, we formulate a new conjecture for web mixing matrices, involving a weighted sum over column entries. Our results form an important step in understanding non-Abelian exponentiation in multiparton amplitudes, and pave the way for higher-loop computations of the soft anomalous dimension.

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arXiv2011

A Parton Shower for High Energy Jets

We present a method to match the multi-parton states generated by the High Energy Jets Monte Carlo with parton showers generated by the Ariadne program using the colour dipole model. The High Energy Jets program already includes a full resummation of soft divergences. Hence, in the matching it is important that the corresponding divergences in the parton shower are subtracted, keeping only the collinear parts. We present a novel, shower-independent method for achieving this, enabling us to generate fully exclusive and hadronized events with multiple hard jets, in hadronic collisions. We discuss in detail the arising description of the soft, collinear and hard regions by examples in pure QCD jet-production.

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arXiv2011

Multiple Jets at the LHC with High Energy Jets

We present a flexible Monte Carlo implementation of the perturbative framework of High Energy Jets, describing multi-jet events at hadron colliders. The description includes a resummation which ensures leading logarithmic accuracy for large invariant mass between jets, and is matched to tree-level accuracy for multiplicities up to 4 jets. The resummation includes all-order hard corrections, which become important for increasing centre-of-mass energy of the hadronic collision. We discuss observables relevant for confronting the perturbative framework with 7 TeV data from the LHC, and the impact of the perturbative corrections on several dijet and trijet observables which are relevant in the search for new physics.

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arXiv2010

High Energy Description of Processes with Multiple Hard Jets

High Energy Jets (HEJ) is a new framework for approximating the all-order perturbative corrections to multi-jet processes, with a focus on the hard, wide-angle QCD emissions, which underpins the perturbative description of hard jets. In this contribution we review the basic concepts of HEJ, and present some new predictions for observables in dijet-production, and for W-boson production in association with at least 3 jets.

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arXiv2010

Resummation of transverse energy in vector boson and Higgs boson production at hadron colliders

We compute the resummed hadronic transverse energy (E_T) distribution due to initial-state QCD radiation in vector boson and Higgs boson production at hadron colliders. The resummed exponent, parton distributions and coefficient functions are treated consistently to next-to-leading order. The results are matched to fixed-order calculations at large E_T and compared with parton-shower Monte Carlo predictions at Tevatron and LHC energies.

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arXiv2010

All-Order Corrections and Multi-Jet Rates

We discuss results from a recently proposed all-order description of hard, radiative corrections to certain multi-jet processes at hadron colliders. The description is based on obtaining an all-order estimate of the t-channel singularities of scattering amplitudes. As a simple example, we illustrate the similarities between qQ and qg-scattering. In particular, we discuss how at tree-level, all non-suppressed helicity-amplitudes for these processes consist of a pure t-channel pole. This structure is used in the construction of all-order approximations.

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arXiv2010

Constructing All-Order Corrections to Multi-Jet Rates

We discuss the universal behaviour of scattering cross sections in the limit of infinite rapidity separation between all produced particles, and illustrate the behaviour explicitly for the production of n jets, W+n jets, Z+n jets for n=2,3,4, and for H+2,3 jets. We give a set of rules for constructing scattering cross sections, which are exact in the given limit, and order-by-order reproduce well the full fixed order results when applied to LHC phenomenology. The approximation includes both real and virtual corrections, and is sufficiently simple to allow for the regulated all-order perturbative sum to be explicitly constructed. We test the expected accuracy by comparing results order-by-order with full fixed-order perturbation theory for the processes mentioned above.

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arXiv2009

The Factorisation of the t-channel Pole in Quark-Gluon Scattering

By exploring the scattering of specific helicity states in quark-gluon scattering at tree level we show explicitly that the t-channel pole can be described exactly as a contraction of two local currents. Furthermore, we demonstrate that out of eight non-zero helicity possibilities, only two suppressed channels have contributions that are not pure, factorised t-channel poles. We thereby extract a gauge-invariant definition for the t-channel current generated by the scattering of a gluon. This offers a slight improvement in the description of gluon scattering in the framework of arXiv:0908.2786 for the prediction of n-jet rates at hadron colliders.

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arXiv2008

Distinguishing Spins in Decay Chains at the Large Hadron Collider

If new particles are discovered at the LHC, it will be important to determine their spins in as model-independent a way as possible. We consider the case, commonly encountered in models of physics beyond the Standard Model, of a new scalar or fermion $D$ decaying sequentially into other new particles $C,B,A$ via the decay chain $D\to C q$, $C\to B l^{near}$, $B\to A l^{far}$, $l^{near}$ and $l^{far}$ being opposite-sign same-flavour charged leptons and $A$ being invisible. We compute the observable 2- and 3-particle invariant mass distributions for all possible spin assignments of the new particles, and discuss their distinguishability using a quantitative measure known as the Kullback-Leibler distance.

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arXiv2007

Spin Correlations in Decay Chains Involving W Bosons

We study the extent to which spin assignments of new particles produced at the LHC can be deduced in the decay of a scalar or fermion C into a new stable (or quasi-stable) particle A through the chain C \to B^\pm q, B^\pm \to A W^\pm, W^\pm \to \ell^\pm ν_\ell where \ell=e,μ. All possible spin assignments of the particles A and B are considered in order to remain as model-independent as possible. Explicit invariant mass distributions of the quark and lepton are given for each set of spins, valid for all masses. We also construct the asymmetry between the chains with a W- and those with a W+. The Kullback-Leibler distance between the distributions is then calculated to give a quantitative measure of our ability to distinguish the different spin assignments.

hep-ph↗

arXiv2007

QCD and Electroweak Interference in Higgs production by Gauge Boson Fusion

We explicitly calculate the contribution to Higgs production at the LHC from the interference between gluon-gluon and Weak Vector Boson fusion, and compare it to the pure QCD and pure Electro-weak result. While the effect is small at tree level, we speculate it will be significantly enhanced by loop effects.

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arXiv2007

Distinguishing Spins in Supersymmetric and Universal Extra Dimension Models at the Large Hadron Collider

An interesting alternative to supersymmetry (SUSY) for extending physics beyond the Standard Model is a model with universal extra dimensions (UED), in which the SUSY superpartners are replaced by Kaluza-Klein excitations of the Standard Model particles. If new particles are discovered at the LHC, even if their mass spectrum favours SUSY or UED, it will be vital to distinguish between their spin assignments in the two models as far as possible. We extend the method proposed by Barr [hep-ph/0405052] to the UED case and investigate the angular and charge asymmetries of decay distributions for sample mass spectra of both SUSY and UED types. For hierarchical (`SUSY-type') mass spectra there is a good chance of distinguishing the spin structures of the two models. However, a mass spectrum of the quasi-degenerate type expected in UED would make it difficult to observe spin correlations.

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arXiv2006

Addendum to "Distinguishing Spins in Decay Chains at the Large Hadron Collider"

We extend our earlier study of spin correlations in the decay chain $D\to C q$, $C\to B l^{near}$, $B\to A l^{far}$, where $A,B,C,D$ are new particles with known masses but undetermined spins, $l^{near}$ and $l^{far}$ are opposite-sign same-flavour charged leptons and $A$ is invisible. Instead of looking at the observable 2- and 3-particle invariant mass distributions separately, we compare the full three-dimensional phase space distributions for all possible spin assignments of the new particles, and show that this enhances their distinguishability using a quantitative measure known as the Kullback-Leibler distance.

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