Small-xx phenomenology at the LHC and beyond: HELL 3.0 and the case of the Higgs cross section

Small-$x$ resummation has been proven recently to be a crucial ingredient for describing small-$x$ HERA data, and the inclusion of small-$x$ resummation in parton distribution function (PDF) determination has a sizeable effect on the PDFs even at the electroweak scale. In this work we explore the implications of small-$x$ resummation at the Large Hadron Collider (LHC) and at a Future Circular Collider (FCC). We construct the theoretical machinery for resumming physical inclusive observables at hadron colliders, and describe its implementation in the public code HELL 3.0. We focus on Higgs production in gluon fusion as a prototypical example, both because it is sensitive to small-$x$ gluons and because of its importance for the LHC physics programme. We find that adding small-$x$ resummation to the N$^3$LO Higgs production cross section can lead to an increase of up to 10% at FCC, while the effect is smaller (+1%) at LHC but still important to achieve a high level of precision.Comment: 42 pages, 8 figures. Added new Fig 6 and some discussions. Final version published in EPJ

Towards small-xx resummed parton distribution functions

We present preliminary results for fits of parton distribution functions (PDFs) which include the resummation of small-$x$ logarithms at NLLx accuracy, performed in the NNPDF framework. We observe an improvement in the description of DIS data at small values of Bjorken $x$ when resummation effects are included. The improvement is more marked when comparing NNLO+NLLx fits to NNLO ones, and is particularly noticeable for small-$x$ and small-$Q^2$ HERA inclusive structure functions. The main effect of the resummation is an enhancement of the gluon and singlet PDFs at small-$x$, which persists at high scales.Comment: 6 pages, 2 figures. To appear in the proceedings of the XXV International Workshop on Deep-Inelastic Scattering and Related Subjects, 3-7 April 2017, University of Birmingham, U

The threshold region for Higgs production in gluon fusion

We provide a quantitative determination of the effective partonic kinematics for Higgs production in gluon fusion in terms of the collider energy at the LHC. We use the result to assess, as a function of the Higgs mass, whether the large top mass approximation is adequate and whether Sudakov resummation is advantageous. We argue that our results hold to all perturbative orders. Based on it, we conclude that the full inclusion of finite top mass corrections is likely to be important for accurate phenomenology for a light Higgs with m_H ~ 125 GeV at the LHC with sqrt{s} = 14 TeV.Comment: 5 pages, 3 figures. Refs 9 and 15 added, several small textual improvements. Final version, to be published in Physical Review Letter

Small-xx resummation from HELL

Small-$x$ logarithmic enhancements arising from high-energy gluon emissions affect both the evolution of collinearly-factorized parton densities and partonic coefficient functions. With the higher collider energy reached by the LHC, the prospect of a future high-energy collider, and the recent deep-inelastic scattering (DIS) results at small-$x$ from HERA, providing phenomenological tools for performing small-$x$ resummation has become of great relevance. In this paper we discuss a framework to perform small-$x$ resummation for both parton evolution and partonic coefficient functions and we describe its implementation in a computer code named High-Energy Large Logarithms (HELL). We present resummed and matched results for the DGLAP splitting functions and, as a proof of principle, for the massless structure functions in DIS.Comment: Version accepted by EPJ C. 26 pages, 7 figures. Section 2.4 largely re-written. Added estimate of theoretical uncertainty and comparison to CCS

Resummation prescriptions and ambiguities in SCET vs. direct QCD: Higgs production as a case study

We perform a comparison of soft-gluon resummation in SCET vs. direct QCD (dQCD), using Higgs boson production in gluon fusion as a case study, with the goal of tracing the quantitative impact of each source of difference between the two approaches. We show that saddle-point methods enable a direct quantitative comparison despite the fact that the scale which is resummed in the two approaches is not the same. As a byproduct, we put in one-to-one analytic correspondence various features of either approach: specifically, we show how the SCET method for treating the Landau pole can be implemented in dQCD, and how the resummation of the optimal partonic scale of dQCD can be implemented in SCET. We conclude that the main quantitative difference comes from power-suppressed subleading contributions, which could in fact be freely tuned in either approach, and not really characteristic of either. This conclusion holds for Higgs production in gluon fusion, but it is in fact generic for processes with similar kinematics. For Higgs production, everything else being equal, SCET resummation at NNLL in the Becher-Neubert implementation leads to essentially no enhancement of the NNLO cross-section, unlike dQCD in the standard implementation of Catani et al..Comment: 21 pages, 4 figures; final version, to be published in JHEP. Eq. 2.39 and subsequent discussion added, fig.1 and corresponding discussion added, discussion on sqrt{z} prefactor added on pag.1

On the Higgs cross section at N3^3LO+N3^3LL and its uncertainty

We consider the inclusive production of a Higgs boson in gluon-fusion and we study the impact of threshold resummation at next-to-next-to-next-to-leading logarithmic accuracy (N$^3$LL) on the recently computed fixed-order prediction at next-to-next-to-next-to-leading order (N$^3$LO). We propose a conservative, yet robust way of estimating the perturbative uncertainty from missing higher (fixed- or logarithmic-) orders. We compare our results with two other different methods of estimating the uncertainty from missing higher orders: the Cacciari-Houdeau Bayesian approach to theory errors, and the use of algorithms to accelerate the convergence of the perturbative series. We confirm that the best convergence happens at $\mu_R=\mu_F=m_H\,/\,2$, and we conclude that a reliable estimate of the uncertainty from missing higher orders on the Higgs cross section at 13 TeV is approximately $\pm4$%.Comment: 27 pages, 6 figures. Version to be published in JHE

Resummation and Matching of bb-quark Mass Effects in bbˉHb\bar{b}H Production

We use a systematic effective field theory setup to derive the $b\bar{b}H$ production cross section. Our result combines the merits of both fixed 4-flavor and 5-flavor schemes. It contains the full 4-flavor result, including the exact dependence on the $b$-quark mass, and improves it with a resummation of collinear logarithms of $m_b/m_H$. In the massless limit, it corresponds to a reorganized 5-flavor result. While we focus on $b\bar{b}H$ production, our method applies to generic heavy-quark initiated processes at hadron colliders. Our setup resembles the variable flavor number schemes known from heavy-flavor production in deep-inelastic scattering, but also differs in some key aspects. Most importantly, the effective $b$-quark PDF appears as part of the perturbative expansion of the final result where it effectively counts as an $O(\alpha_s)$ object. The transition between the fixed-order (4-flavor) and resummation (5-flavor) regimes is governed by the low matching scale at which the $b$-quark is integrated out. Varying this scale provides a systematic way to assess the perturbative uncertainties associated with the resummation and matching procedure and reduces by going to higher orders. We discuss the practical implementation and present numerical results for the $b\bar{b}H$ production cross section at NLO+NLL. We also provide a comparison to the corresponding predictions in the fixed 4-flavor and 5-flavor results and the Santander matching prescription. Compared to the latter, we find a slightly reduced uncertainty and a larger central value, with its central value lying at the lower edge of our uncertainty band.Comment: 54 pages, 16 figures. Final version to be published in JHEP (one ref added