arXiv subjects

Rhorry Gauld

Publications and source records attributed to Rhorry Gauld.

31 records · Page 2Linked to original sources

arXiv2016

One-loop corrections to $h\to b\bar b$ and $h\to τ\bar τ$ decays in the Standard Model Dimension-6 EFT: four-fermion operators and the large-$m_t$ limit

We calculate a set of one-loop corrections to $h\to b\bar b$ and $h\to τ\bar τ$ decays in the dimension-6 Standard Model effective field theory (SMEFT). In particular, working in the limit of vanishing gauge couplings, we calculate directly in the broken phase of the theory all large logarithmic corrections and in addition the finite corrections in the large-$m_t$ limit. Moreover, we give exact results for one-loop contributions from four-fermion operators. We obtain these corrections within an extension of the widely used on-shell renormalisation scheme appropriate for SMEFT calculations, and show explicitly how UV divergent bare amplitudes from a total of 21 different SMEFT operators are rendered finite within this scheme. As a by-product of the calculation, we also compute to one-loop order the logarithmically enhanced and finite large-$m_t$ corrections to muon decay in the limit of vanishing gauge couplings, which is necessary to implement the $G_F$ input parameter scheme within the SMEFT.

hep-ph↗

arXiv2016

QCD radiative corrections for $h\to b\bar b$ in the Standard Model Dimension-6 EFT

We calculate the $\mathcal{O}(α_s)$ QCD corrections to the inclusive $h\to b\bar b$ decay rate in the dimension-6 Standard Model Effective Field Theory (SMEFT). The QCD corrections multiplying the dimension-6 Wilson coefficients which alter the $hb\bar b$-vertex at tree-level are proportional to the Standard Model (SM) ones, so next-to-leading order results can be obtained through a simple rescaling of the tree-level decay rate. On the other hand, contributions from the operators $Q_{bG}$ and $Q_{HG}$, which alter the $gb\bar b$-vertex and introduce a $hgg$-vertex respectively, enter at $\mathcal{O}(α_s)$ and induce sizeable corrections which are unrelated to the SM ones and cannot be anticipated through a renormalisation-group analysis. We present compact analytic results for these contributions, which we recommend to be included in future phenomenological studies.

hep-ph↗

arXiv2016

The prompt atmospheric neutrino flux in the light of LHCb

The recent observation of very high energy cosmic neutrinos by IceCube heralds the beginning of neutrino astronomy. At these energies, the dominant background to the astrophysical signal is the flux of `prompt' neutrinos, arising from the decay of charmed mesons produced by cosmic ray collisions in the atmosphere. In this work we provide predictions for the prompt atmospheric neutrino flux in the framework of perturbative QCD, using state-of-the-art Monte Carlo event generators. Our calculation includes the constraints set by charm production measurements from the LHCb experiment at 7 TeV, and has been recently validated with the corresponding 13 TeV data. Our results for the prompt flux are a factor of about 2 below the previous benchmark calculation, in general agreement with two other recent estimates, and with an improved estimate of the uncertainty. This alleviates the existing tension between the theoretical prediction and IceCube limits, and suggests that a direct direction of the prompt flux is imminent.

hep-ph↗

arXiv2015

Leptonic top-quark asymmetry predictions at LHCb

The forward LHCb acceptance offers the possibility of measuring the top-quark pair asymmetry in a kinematic region that does not receive overwhelming dilution from the symmetric gluon-fusion channel. To investigate this possibility, two leptonic final states are identified, and analysis strategies are proposed for each channel with 14~TeV data. Leading fixed-order predictions are then provided for the relevant leptonic asymmetry variables in each channel. If backgrounds can be experimentally constrained, statistically, a non-zero asymmetry is estimated to be observable beyond 5$σ$ confidence level with the full LHCb 14~TeV data.

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arXiv2015

Forward $D$ predictions for $p\rm Pb$ collisions, and sensitivity to cold nuclear matter effects

Predictions are provided for double differential cross sections and forward-backward ratios of $D^{0}$ production in $p\rm Pb$ (forward) and $\rm{Pb}$$p$ (backward) collisions at 5.02~TeV. The effect of nuclear corrections on the ratio of differential cross sections ratios is estimated to be $\simeq$ (10-30)\% in the kinematically accessible region of LHCb, and interestingly this ratio is approximately flat with respect to $p_T(D^0)$ due to a compensation of shadowing and anti-shadowing effects arising from the input nuclear PDFs. In comparison to $J/ψ(\rightarrowμ^-μ^+)$ measurements which have already been performed with the available data, the cross section for $D^{0}(\rightarrow K^-π^+)$ production is expected to be two-orders of magnitude higher.

hep-ph↗

arXiv2015

The leptonic $W^{\pm}$ boson asymmetry in association with jets at LHCb, and PDF constraints at large-$x$

The unique forward LHCb acceptance opens the possibility for performing precision Standard Model (SM) processes in a kinematical regime beyond the reach of both ATLAS and CMS experiments. In this article we discuss the feasibility of performing $W^{\pm}j$ measurements with 14 TeV LHCb data, and find that the leptonic asymmetry at high pseudorapidity, with appropriate cuts on the associated jet, can provide a factor of three reduction on the $d$ quark PDF uncertainty in a region of Bjorken $x > 0.5$.

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arXiv2015

Charm production in the forward region: constraints on the small-x gluon and backgrounds for neutrino astronomy

The recent observation by the IceCube experiment of cosmic neutrinos at energies up to a few PeV heralds the beginning of neutrino astronomy. At such high energies, the conventional neutrino flux is suppressed and the prompt component from charm meson decays is expected to become the dominant background to astrophysical neutrinos. Charm production at high energies is however theoretically uncertain, both since the charm mass is at the boundary of applicability of perturbative QCD, and also because the calculations are sensitive to the poorly-known gluon PDF at small-x. In this work we provide detailed perturbative QCD predictions for charm and bottom production in the forward region, and validate them by comparing with recent data from the LHCb experiment at 7 TeV. Finding good agreement between data and theory, we use the LHCb measurements to constrain the small-x gluon PDF, achieving a substantial reduction in its uncertainties. Using these improved PDFs, we provide predictions for charm and bottom production at LHCb at 13 TeV, as well as for the ratio of cross-sections between 13 and 7 TeV. The same calculations are used to compute the energy distribution of neutrinos from charm decays in pA collisions, a key ingredient towards achieving a theoretically robust estimate of charm-induced backgrounds at neutrino telescopes.

hep-ph↗

arXiv2015

Beauty-quark and charm-quark pair production asymmetries at LHCb

The LHCb collaboration has recently performed a first measurement of the angular production asymmetry in the distribution of beauty quarks and anti-quarks at a hadron collider. We calculate the corresponding standard model prediction for this asymmetry at fixed-order in perturbation theory. Our results show good agreement with the data, which is provided differentially for three bins in the invariant mass of the $b \bar b$ system. We also present similar predictions for both beauty-quark and charm-quark final states within the LHCb acceptance for a collision energy of $\sqrt{s} = 13 \, {\rm TeV}$. We finally point out that a measurement of the ratio of the $b \bar b$ and $c \bar c$ cross sections may be useful for experimentally validating charm-tagging efficiencies.

hep-ph↗

arXiv2014

Feasibility of top quark measurements at LHCb and constraints on the large-$x$ gluon PDF

The forward LHCb acceptance opens interesting possibilities of studying precision Standard Model hard processes in a kinematical region beyond the reach of ATLAS and CMS. In this paper we perform a feasibility study for cross-section measurements of top quark pairs with the LHCb detector, with an analysis of signal and background rates for selected final states, and determine the potential precision achievable at $\sqrt{s} =$ 7 and 14~TeV. We then study the dependence of theoretical uncertainties on the pseudorapidity distribution of top quarks produced in pair production at NLO, and observe that a cross-section measurement at high pseudorapidity has enhanced sensitivity to probe the high-$x$ gluon PDF as compared to measurements in the central-region. Based on simulated pseudodata, the impact of a 14~TeV cross-section measurement on the gluon PDF and charge asymmetry is quantified.

hep-ph↗

arXiv2014

An explicit Z'-boson explanation of the B->K*mu+mu- anomaly

A global fit to the recent B->K*mu+mu- data shows indications for a large new-physics contribution to the Wilson coefficient of the semi-leptonic vector operator. In this article we consider a simple Z'-boson model of 3-3-1 type that can accommodate such an effect without violating any other constraint from quark-flavour physics. Implications for yet unobserved decay modes such as B->Xsnunubar and longstanding puzzles like B->piK are also discussed. The Z'-boson masses required to address the observed anomaly lie in the range of 7 TeV. Such heavy Z' bosons evade the existing bounds from precision data and direct searches, and will remain difficult to discover even at a high-luminosity LHC. The potential of an ILC as well as the next generation of low-energy parity-violation experiments in constraining the Z'-boson parameter space is also examined.

hep-ph↗

arXiv2013

On minimal Z' explanations of the B->K*mu+mu- anomaly

Recently LHCb has announced a discrepancy of 3.7 sigma in one of the theoretically clean observables accessible through studies of angular correlations in B->K*mu+mu-. We point out that in the most minimal Z' setup that can address this anomaly there is a model-independent triple-correlation between new physics (NP) in B->K*mu+mu-, B_s-Bbar_s mixing, and non-unitarity of the quark-mixing matrix. This triple-correlation can be cast into a simple analytic formula that relates the NP contribution DeltaC9 to the Wilson coefficient of the semileptonic vector operator to a shift in the mass difference DeltaM_{B_s} and a violation of |Vud|^2+ |Vus|^2+ |Vub|^2 = 1. In contrast to the individual observables the found relation depends only logarithmically on the Z' mass. We show that that our findings allow for useful future tests of the pattern of NP suggested by the B->K*mu+mu- anomaly.

hep-ph↗

arXiv2012

Development of Single- and Double-sided Ladders for the ILD Vertex Detectors

We discuss two projects exploring the integration of thin CMOS pixel sensors in order to prototype ladders matching the geometry needed for the ILD vertex detector. The PLUME project has designed and fabricated full-size and fully functional double- sided layers which currently reach 0.6 % X0 and aim for 0.3 % X0 in mid-2012. Another approach, SERNWIETE, consists in wrapping the sensors in a polyimide-based micro-cable to obtain a supportless single-sided ladder with a material budget around 0.15 % X0. First promising samples have been produced and the full-size prototype is expected in spring 2012.

physics.ins-det↗

arXiv2010

ISIS2: Pixel Sensor with Local Charge Storage for ILC Vertex Detector

ISIS (In-situ Storage Imaging Sensor) is a novel CMOS sensor with multiple charge storage capability developed for the ILC vertex detector by the Linear Collider Flavour Identification (LCFI) collaboration. This paper reports test results for ISIS2, the second generation of ISIS sensors implemented in a 0.18 micron CMOS process. The local charge storage and charge transfer were unambiguously demonstrated.

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