arXiv subjects

Tom Melia

Publications and source records attributed to Tom Melia.

At least 55 records · Page 3Linked to original sources

arXiv2018

Detecting Dark Blobs

Current dark matter detection strategies are based on the assumption that the dark matter is a gas of non-interacting particles with a reasonably large number density. This picture is dramatically altered if there are significant self interactions within the dark sector, potentially resulting in the coalescence of dark matter particles into large composite blobs. The low number density of these blobs necessitates new detector strategies. We study cosmological, astrophysical and direct detection bounds on this scenario and identify experimentally accessible parameter space. The enhanced interaction between large composite states and the standard model allows searches for such composite blobs using existing experimental techniques. This includes the detection of scintillation in MACRO, XENON and LUX, heat in calorimeters such as CDMS, acceleration and strain in gravitational wave detectors such as LIGO and AGIS, and spin precession in CASPEr. These searches leverage the fact that the transit of the dark matter occurs at a speed ~220 km/s, well separated from relativistic and terrestrial sources of noise. They can be searched for either through modifications to the data analysis protocol or relatively straightforward adjustments to the operating conditions of these experiments.

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arXiv2017

LHC limits on axion-like particles from heavy-ion collisions

In these proceedings we use recent LHC heavy-ion data to set a limit on axion-like particles coupling to electromagnetism with mass in the range 10-100 GeV. We recast ATLAS data as per the strategy proposed in 1607.06083, and find results in-line with the projections given there.

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arXiv2017

Operator bases, $S$-matrices, and their partition functions

Relativistic quantum systems that admit scattering experiments are quantitatively described by effective field theories, where $S$-matrix kinematics and symmetry considerations are encoded in the operator spectrum of the EFT. In this paper we use the $S$-matrix to derive the structure of the EFT operator basis, providing complementary descriptions in (i) position space utilizing the conformal algebra and cohomology and (ii) momentum space via an algebraic formulation in terms of a ring of momenta with kinematics implemented as an ideal. These frameworks systematically handle redundancies associated with equations of motion (on-shell) and integration by parts (momentum conservation). We introduce a partition function, termed the Hilbert series, to enumerate the operator basis--correspondingly, the $S$-matrix--and derive a matrix integral expression to compute the Hilbert series. The expression is general, easily applied in any spacetime dimension, with arbitrary field content and (linearly realized) symmetries. In addition to counting, we discuss construction of the basis. Simple algorithms follow from the algebraic formulation in momentum space. We explicitly compute the basis for operators involving up to $n=5$ scalar fields. This construction universally applies to fields with spin, since the operator basis for scalars encodes the momentum dependence of $n$-point amplitudes. We discuss in detail the operator basis for non-linearly realized symmetries. In the presence of massless particles, there is freedom to impose additional structure on the $S$-matrix in the form of soft limits. The most naïve implementation for massless scalars leads to the operator basis for pions, which we confirm using the standard CCWZ formulation for non-linear realizations.

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arXiv2017

Magnetic Bubble Chambers and Sub-GeV Dark Matter Direct Detection

We propose a new application of single molecule magnet crystals: their use as "magnetic bubble chambers" for the direct detection of sub-GeV dark matter. The spins in these macroscopic crystals effectively act as independent nano-scale magnets. When anti-aligned with an external magnetic field they form meta-stable states with a relaxation time that can be very long at sufficiently low temperatures. The Zeeman energy stored in this system can be released through localized heating, caused for example by the scattering or absorption of dark matter, resulting in a spin avalanche (or "magnetic deflagration") that amplifies the effects of the initial heat deposit, enabling detection. Much like the temperature and pressure in a conventional bubble chamber, the temperature and external magnetic field set the detection threshold for a single molecule magnet crystal. We discuss this detector concept for dark matter detection and propose ways to ameliorate backgrounds. If successfully developed, this detector concept can search for hidden photon dark matter in the meV - eV mass range with sensitivities exceeding current bounds by several orders of magnitude.

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arXiv2017

Searching for axion-like particles with ultra-peripheral heavy-ion collisions

We show that ultra-peripheral heavy-ion collisions at the LHC can be used to search for axion-like particles with mass below 100 GeV. The $Z^4$ enhanced photon-photon luminosity from the ions provides a large exclusive production rate, with a signature of a resonant pair of back-to-back photons and no other activity in the detector. In addition, we present both new and updated limits from recasting multi-photon searches at LEP II and the LHC, which are more stringent than those currently in the literature for the mass range 100 MeV to 100 GeV.

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arXiv2016

Merging WW and WW+jet with MINLO

We present a simulation program for the production of a pair of W bosons in association with a jet, that can be used in conjunction with general-purpose shower Monte Carlo generators, according to the POWHEG method. We have further adapted and implemented the MINLO' method on top of the NLO calculation underlying our WW+jet generator. Thus, the resulting simulation achieves NLO accuracy not only for inclusive distributions in WW+jet production but also WW production, i.e. when the associated jet is not resolved, without the introduction of any unphysical merging scale. This work represents the first extension of the MINLO' method, in its original form, to the case of a genuine underlying 2->2 process, with non-trivial virtual corrections.

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arXiv2016

Rays of light from the LHC

We consider models for the di-photon resonance observed at ATLAS (with 3.6 fb^{-1}) and CMS (with 2.6 fb^{-1}). We find there is no conflict between the signal reported at 13 TeV, and the constraints from both experiments at 8 TeV with 20.3 fb^{-1}. We make a simple argument for why adding only one new resonance to the standard model (SM) is not sufficient to explain the observation. We explore four viable options: (i): resonance production and decay through loops of messenger fermions or scalars; (ii): a resonant messenger which decays to the di-photon resonance + X; (iii): an edge configuration where A -> B gamma -> C gamma gamma, and (iv): Hidden Valley-like models where the resonance decays to a pair of very light (sub-GeV) states, each of which in turn decays to a pair of collimated photons that cannot be distinguished from a single photon. Since in each case multiple new states have been introduced, a wealth of signatures is expected to ensue at Run-2 of LHC.

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arXiv2015

Proof of a new colour decomposition for QCD amplitudes

Recently, Johansson and Ochirov conjectured the form of a new colour decomposition for QCD tree-level amplitudes. This note provides a proof of that conjecture. The proof is based on "Mario World" Feynman diagrams, which exhibit the hierarchical Dyck structure previously found to be very useful when dealing with multi-quark amplitudes.

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arXiv2015

Hilbert series and operator bases with derivatives in effective field theories

We introduce a systematic framework for counting and finding independent operators in effective field theories, taking into account the redundancies associated with use of the classical equations of motion and integration by parts. By working in momentum space, we show that the enumeration problem can be mapped onto that of understanding a polynomial ring in the field momenta. All-order information about the number of independent operators in an effective field theory is encoded in a geometrical object of the ring known as the Hilbert series. We obtain the Hilbert series for the theory of N real scalar fields in (0+1) dimensions--an example, free of space-time and internal symmetries, where aspects of our framework are most transparent. Although this is as simple a theory involving derivatives as one could imagine, it provides fruitful lessons to be carried into studies of more complicated theories: we find surprising and rich structure from an interplay between integration by parts and equations of motion and a connection with SL(2,C) representation theory which controls the structure of the operator basis.

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arXiv2014

Rare exclusive hadronic W decays in a t-tbar environment

The large cross section for t-tbar production at the LHC and at any future hadron collider provides a high-statistics and relatively clean environment for a study of W boson properties: after tagging on a leptonic decay of one of the Ws and the two b-jets, an additional W still remains in the event. We study the prospect of making the first exclusive hadronic decay of a fundamental boson of the standard model, using the decay modes W to pi gamma and W to pi pi pi, and other related decays. By using strong isolation criteria, which we impose by searching for jets with a single particle constituent, we show that the three particle hadronic W decays have potential to be measured at the LHC. The possibility of measuring an involved spectrum of decay products could considerably expand our knowledge of how the W decays, and experimental techniques acquired in making these measurements would be useful for application to future measurements of exclusive hadronic Higgs boson decays.

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arXiv2013

Getting more flavour out of one-flavour QCD

We argue that no notion of flavour is necessary when performing amplitude calculations in perturbative QCD with massless quarks. We show this explicitly at tree-level, using a flavour recursion relation to obtain multi-flavoured QCD from one-flavour QCD. The method relies on performing a colour decomposition, under which the one-flavour primitive amplitudes have a structure which is restricted by planarity and cyclic ordering. An understanding of SU(3)_c group theory relations between QCD primitive amplitudes and their organisation around the concept of a Dyck tree is also necessary. The one-flavour primitive amplitudes are effectively N=1 supersymmetric, and a simple consequence is that all of tree-level massless QCD can be obtained from Drummond and Henn's closed form solution to tree-level N=4 super Yang-Mills theory.

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arXiv2013

Dyck Words and Multi-Quark Primitive Amplitudes

I study group theory (Kleiss-Kuijf) relations between purely multi-quark primitive amplitudes at tree level, and prove that they reduce the number of independent primitives to (n-2)!/(n/2)!, where n is the number of quarks plus antiquarks, in the case where quark lines have different flavours. I give an explicit example of an independent basis of primitives for any n which is of the form A(1,2,sigma), where sigma is a permutation based on a Dyck word.

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arXiv2012

Gluon fusion contribution to W+W- + jet production

We describe the computation of the $gg \to W^+W^-g$ process that contributes to the production of two $W$-bosons and a jet at the CERN Large Hadron Collider (LHC). While formally of next-to-next-to-leading order (NNLO) in QCD, this process can be evaluated separately from the bulk of NNLO QCD corrections because it is finite and gauge-invariant. It is also enhanced by the large gluon flux and by selection cuts employed in the Higgs boson searches in the decay channel $ H \to W^+W^-$, as was first pointed out by Binoth {\it et al.} in the context of $gg \to W^+W^-$ production. For cuts employed by the ATLAS collaboration, we find that the gluon fusion contribution to $pp \to W^+W^-j$ enhances the background by about ten percent and can lead to moderate distortions of kinematic distributions which are instrumental for the ongoing Higgs boson searches at the LHC. We also release a public code to compute the NLO QCD corrections to this process, in the form of an add-on to the package {\tt MCFM}.

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arXiv2012

Spin before mass at the LHC

What can be said about the spin of new particles without knowing their mass during an initial discovery phase at CERN? We consider this question in a topology where mass measurement is particularly difficult, $pp\to Y\bar{Y}\to lX\bar{l}\,\bar{X}$, and introduce two new variables $\ctv$ and $\Av$ which we prove are both independent of the mass of $X$. The variable $\ctv$ approximates the polar production angle of $Y$, and we find that it possesses greater statistical power in determining the spin of this particle than a previous, related variable, $\ctll$ \cite{Barr:2005dz}. $\Av$ is an asymmetry which can provide information about the couplings of a spin half $Y$. Because these variables can be used from the outset, without any knowledge of the masses of the new particles, we find here that it is possible to reverse the usual `mass before spin' determination timeline.

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arXiv2012

Weak Bosons and Jets at the LHC

In this talk, I outline theoretical predictions for weak boson pair production in association with two jets at the LHC. I will discuss the next-to-leading order QCD corrections to the processes $pp\to W^+W^+jj$ and $pp\to W^+W^-jj$, and the interfacing of $pp\to W^+W^+jj$ with a parton shower using the {\tt POWHEG BOX} framework.

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arXiv2011

W+W-, WZ and ZZ production in the POWHEG BOX

We present an implementation of the vector boson pair production processes ZZ, W+W- and W Z within the POWHEG framework, which is a method that allows the interfacing of NLO calculations to shower Monte Carlo programs. The implementation is built within the POWHEG BOX package. The Z/γ^* interference, as well as singly resonant contributions, are properly included. We also considered interference terms arising from identical leptons in the final state. As a result, all contributions leading to the desired four-lepton system have been included in the calculation, with the sole exception of the interference between ZZ and W+W- in the production of a pair of same-flavour, oppositely charged fermions and a pair of neutrinos, which we show to be fully negligible. Anomalous trilinear couplings can be also set in the program, and we give some examples of their effect at the LHC. We have made the relevant code available at the POWHEG BOX web site.

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arXiv2011

W^+W^+jj at NLO in QCD: an exotic Standard Model signature at the LHC

The process $pp \to W^+W^+jj$ gives rise to an exotic Standard Model signature at the LHC, involving high-$p_{\perp}$ like-sign leptons, missing energy and jets. In this brief article the motivation for study, along with selected results from the computation of NLO QCD corrections to the QCD-mediated part of this process\cite{us} are presented. It is shown that the corrections reduce the dependence of the cross-section on renormalisation and factorisation scales, and produce a relatively hard third jet in a significant fraction of events.

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arXiv2011

Next-to-leading order QCD predictions for W+W+jj production at the LHC

Because the LHC is a proton-proton collider, sizable production of two positively charged W-bosons in association with two jets is possible. This process leads to a distinct signature of same sign high-pt leptons, missing energy and jets. We compute the NLO QCD corrections to the QCD-mediated part of pp -> W+W+jj. These corrections reduce the dependence of the production cross-section on the renormalization and factorization scale to about +- 10 percent. We find that a large number of W+W+jj events contain a relatively hard third jet. The presence of this jet should help to either pick up the W+W+jj signal or to reject it as an unwanted background.

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