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Ciaran Williams

Publications and source records attributed to Ciaran Williams.

At least 37 records · Page 2Linked to original sources
arXiv2016

Higher Order QCD predictions for Associated Higgs production with anomalous couplings to gauge bosons

We present predictions for the associated production of a Higgs boson at NLO+PS accuracy, including the effect of anomalous interactions between the Higgs and gauge bosons. We present our results in different frameworks, one in which the interaction vertex between the Higgs boson and Standard Model $W$ and $Z$ bosons is parameterized in terms of general Lorentz structures, and one in which Electroweak symmetry breaking is manifestly linear and the resulting operators arise through a six-dimensional effective field theory framework. We present analytic calculations of the Standard Model and Beyond the Standard Model contributions, and discuss the phenomenological impact of the higher order pieces. Our results are implemented in the NLO Monte Carlo program MCFM, and interfaced to shower Monte Carlos through the {\sc Powheg} box framework.

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arXiv2016

Associated production of a Higgs boson at NNLO

In this paper we present a Next-to-Next-to Leading Order (NNLO) calculation of the production of a Higgs boson in association with a massive vector boson. We include the decays of the unstable Higgs and vector bosons, resulting in a fully flexible parton-level Monte Carlo implementation. We also include all $\mathcal{O}(α_s^2)$ contributions that occur in production for these processes: those mediated by the exchange of a single off-shell vector boson in the $s$-channel, and those which arise from the coupling of the Higgs boson to a closed loop of fermions. We study final states of interest for Run II phenomenology, namely $H\rightarrow b\bar{b}$, $γγ$ and $WW^*$. The treatment of the $H\rightarrow b\bar{b}$ decay includes QCD corrections at NLO. We use the recently developed $N$-jettiness regularization procedure, and study its viability in the presence of a large final-state phase space by studying $pp\rightarrow V(H\rightarrow WW^*) \rightarrow$ leptons.

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arXiv2016

Color singlet production at NNLO in MCFM

We present the implementation of several color-singlet final-state processes at Next-to-Next-to Leading Order (NNLO) accuracy in QCD to the publicly available parton-level Monte Carlo program MCFM. Specifically we discuss the processes $pp\rightarrow H$, $pp\rightarrow Z$, $pp\rightarrow W$, $pp\rightarrow HZ$, $pp\rightarrow HW$ and $pp\rightarrowγγ$. Decays of the unstable bosons are fully included, resulting in a flexible fully differential Monte Carlo code. The NNLO corrections have been calculated using the non-local $N$-jettiness subtraction approach. Special attention is given to the numerical aspects of running MCFM for these processes at this order. We pay particular attention to the systematic uncertainties due to the power corrections induced by the $N$-jettiness regularization scheme and the evaluation time needed to run the hybrid openMP/MPI version of MCFM at NNLO on multi-processor systems.

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arXiv2015

Closing up on Dark Sectors at Colliders: from 14 to 100 TeV

We investigate the reach of the LHC Run 2 and that of a future circular hadron collider with up to 100 TeV centre of mass energy for the exploration of potential Dark Matter sectors. These dark sectors are conveniently and broadly described by simplified models. The simplified models we consider provide microscopic descriptions of interactions between the Standard Model partons and the dark sector particles mediated by the four basic types of messenger fields: scalar, pseudo-scalar, vector or axial-vector. Our analysis extends and updates the previously available results for the LHC at 8 and 14 TeV to 100 TeV for models with all four messenger types. We revisit and improve the analysis at 14 TeV, by studying a variety of analysis techniques, concluding that the most discriminating variables correspond to the missing transverse energy and the azimuthal angle between jets in the final state. Going to 100 TeV, the limits on simplified models of Dark Matter are enhanced significantly, in particular for heavier mediators and dark sector particles, for which the available phase space at the LHC is restricted. The possibility of a 100 TeV collider provides an unprecedented coverage of the dark sector basic parameters and a unique opportunity to pin down the particle nature of Dark Matter and its interactions with the Standard Model.

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arXiv2015

Constraining Dark Sectors at Colliders: Beyond the Effective Theory Approach

We outline and investigate a set of benchmark simplified models with the aim of providing a minimal simple framework for an interpretation of the existing and forthcoming searches of dark matter particles at the LHC. The simplified models we consider provide microscopic QFT descriptions of interactions between the Standard Model partons and the dark sector particles mediated by the four basic types of messenger fields: scalar, pseudo-scalar, vector or axial-vector. Our benchmark models are characterised by four to five parameters, including the mediator mass and width, the dark matter mass and an effective coupling(s). In the gluon fusion production channel we resolve the top-quark in the loop and compute full top-mass effects for scalar and pseudo-scalar messengers. We show the LHC limits and reach at 8 and 14 TeV for models with all four messenger types. We also outline the complementarity of direct detection, indirect detection and LHC bounds for dark matter searches. Finally, we investigate the effects which arise from extending the simplified model to include potential new physics contributions in production. Using the scalar mediator as an example we study the impact of heavy new physics loops which interfere with the top mediated loops. Our computations are performed within the MCFM framework and we provide fully flexible public Monte Carlo implementation.

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arXiv2014

Four photon production at the LHC: an application of 2->4 analytic unitarity

We present Next-to-Leading Order (NLO) predictions for $γγγγ$ final states, including the effects of photon fragmentation. Our results are calculated fully analytically using the techniques of $D$-dimensional unitarity, and we discuss some refinements to existing methods, focusing particularly on the role of three-mass triangle coefficients. Using these techniques we are able to produce a numerically stable Monto Carlo code that runs fully in double precision. We investigate the $4γ$ signal at colliders, both for Run II of the LHC and at future machines. Our results are implemented in MCFM.

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arXiv2014

Bounding the Higgs width at the LHC

We present results for the Standard model description of the four-lepton production, mediated both by Higgs boson production and by other one-loop standard model processes. The description of four-lepton final states in MCFM v6.8 is reviewed, with special reference to the interference effects that can occur for identical species of leptons. We present results both for interference in the $l^+l^- l^- l^+$ and in the $l^-l^+ ν_l \barν_l$ final state. Prospects for further improvement in the theoretical description of four lepton production are also reviewed.

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arXiv2014

Triphoton production at hadron colliders

We present next-to-leading order predictions for the production of triphoton final states at the LHC and the Tevatron. Our results include the effect of photon fragmentation for the first time and we are able to quantify the impact of different isolation prescriptions. We find that calculations accounting for fragmentation effects at leading order, and those employing a smooth cone isolation where no fragmentation contribution is required, are in reasonable agreement with one another. However, larger differences in the predicted rates arise when higher order corrections to the fragmentation functions are included. In addition we present full analytic results for the $γγγ$ and $γγ+$jet one-loop amplitudes. These amplitudes, which are particularly compact, may be useful to future higher-order calculations. Our results are available in the Monte Carlo code MCFM.

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arXiv2013

Bounding the Higgs width at the LHC: complementary results from $H \to WW$

We investigate the potential of the process $gg \to H \to WW$ to provide bounds on the Higgs width. Recent studies using off-shell $H\rightarrow ZZ$ events have shown that Run 1 LHC data can constrain the Higgs width, $Γ_H < (25-45) Γ_{H}^{\rm SM}$. Using 20 fb-1 of 8 TeV ATLAS data, we estimate a bound on the Higgs boson width from the WW channel between $Γ_H < (100-500) Γ_H^{SM}$. The large spread in limits is due to the range of cuts applied in the existing experimental analysis. The stricter cuts designed to search for the on-shell Higgs boson limit the potential number of off-shell events, weakening the constraints. As some of the cuts are lifted the bounds improve. We show that there is potential in the high transverse mass region to produce upper bounds of the order of $(25-50) Γ_H^{SM}$, depending strongly on the level of systematic uncertainty that can be obtained. Thus, if these systematics can be controlled, a constraint on the Higgs boson width from the $H \to WW$ decay mode can complement a corresponding limit from $H \to ZZ$.

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arXiv2013

Event-by-event weighting at next-to-leading order

We present a general method of associating next-to-leading order weights to leading order phase space configurations at hadron colliders. The method relies on a re-organization of phase space for the real radiation contributions, defining a one-to-many map such that each point in the real phase space is associated with a distinct Born topology. As a result virtual and real singularities cancel at each Born phase space point. The new phase space maps can be used in a traditional approach for NLO calculations. However, interesting applications arise when one instead integrates out the real radiation up to a specified scale. This allows one to define NLO weights for individual phase space points that are present at LO. This method allows for the extension of matrix element methods to next-to-leading order, even in the presence of jets. We discuss some recent applications of the matrix element method to Higgs physics at the LHC.

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arXiv2013

Bounding the Higgs width at the LHC using full analytic results for gg -> 2e 2μ

We revisit the hadronic production of the four-lepton final state, e^- e^+ μ^- μ^+, through the fusion of initial state gluons. This process is mediated by loops of quarks and we provide first full analytic results for helicity amplitudes that account for both the effects of the quark mass in the loop and off-shell vector bosons. The analytic results have been implemented in the Monte Carlo program MCFM and are both fast, and numerically stable in the region of low Z transverse momentum. We use our results to study the interference between Higgs-mediated and continuum production of four-lepton final states, which is necessary in order to obtain accurate theoretical predictions outside the Higgs resonance region. We have confirmed and extended a recent analysis of Caola and Melnikov that proposes to use a measurement of the off-shell region to constrain the total width of the Higgs boson. Using a simple cut-and-count method, existing LHC data should bound the width at the level of 25-45 times the Standard Model expectation. We investigate the power of using a matrix element method to construct a kinematic discriminant to sharpen the constraint. In our analysis the bound on the Higgs width is improved by a factor of about 1.6 using a simple cut on the MEM discriminant, compared to an invariant mass cut m_{4l} > 300 GeV.

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arXiv2013

The Matrix Element Method at Next-to-Leading Order

This paper presents an extension of the matrix element method to next-to-leading order in perturbation theory. To accomplish this we have developed a method to calculate next-to-leading order weights on an event-by-event basis. This allows for the definition of next-to-leading order likelihoods in exactly the same fashion as at leading order, thus extending the matrix element method to next-to-leading order. A welcome by-product of the method is the straightforward and efficient generation of unweighted next-to-leading order events. As examples of the application of our next-to-leading order matrix element method we consider the measurement of the mass of the Z boson and also the search for the Higgs boson in the four lepton channel.

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arXiv2013

Finding the Higgs boson in decays to Z gamma using the matrix element method at Next-to-Leading Order

We illustrate how the Matrix Element Method at Next-to-Leading Order (MEM@NLO) can be used to discriminate between events arising from the production of a Higgs boson, which subsequently decays to a final state consisting of ell^+ell^-gamma, and the background production of the same final state. We illustrate how the method could be used in an experimental analysis by devising cuts on the signal (P_S) and background (P_B) weights that are computed event-by-event in this approach. We find that we can increase the S/sqrt(B) ratio by around 50\% compared to an invariant mass fit on its own. Considering only statistical uncertainty, this is equivalent to recording a factor of around two times more integrated luminosity.

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arXiv2012

Next-to-Leading Order Predictions for Dark Matter Production at Hadron Colliders

We provide Next-to-Leading Order (NLO) predictions for Dark Matter (DM) production in association with either a jet or a photon at hadron colliders. In particular we study the production of a pair of fermionic DM particles through a mediator which couples to SM via either a vector, axial-vector, scalar, pseudo-scalar, or gluon-induced coupling. Experimental constraints on the scale of new physics associated with these operators are limited by systematics, highlighting the need for NLO signal modeling. We factorize the NLO QCD and the DM parts of the calculation, allowing the possibility of using the results presented here for a large variety of searches in monojet and monophoton final states. Our results are implemented into the Monte Carlo program MCFM.

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arXiv2012

Next-to-leading order predictions for Z gamma+jet and Z gamma gamma final states at the LHC

We present next-to-leading order predictions for final states containing leptons produced through the decay of a Z boson in association with either a photon and a jet, or a pair of photons. The effect of photon radiation from the final state leptons is included and we also allow for contributions arising from fragmentation processes. Phenomenological studies are presented for the LHC in the case of final states containing charged leptons and in the case of neutrinos. We also use the procedure introduced by Stewart and Tackmann to provide a reliable estimate of the scale uncertainty inherent in our theoretical calculations of jet-binned Z gamma cross sections. These computations have been implemented in the public code MCFM.

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arXiv2012

NLO Higgs boson production plus one and two jets using the POWHEG BOX, MadGraph4 and MCFM

We present a next-to-leading order calculation of Higgs boson production plus one and two jets via gluon fusion interfaced to shower Monte Carlo programs, implemented according to the POWHEG method. For this implementation we have used a new interface of the POWHEG BOX with MadGraph4, that generates the codes for generic Born and real processes automatically. The virtual corrections have been taken from the MCFM code. We carry out a simple phenomenological study of our generators, comparing them among each other and with fixed next-to-leading order results.

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