arXiv subjects

Li Lin Yang

Publications and source records attributed to Li Lin Yang.

At least 37 records · Page 2Linked to original sources

arXiv2022

Two-loop infrared singularities in the production of a Higgs boson associated with a top-quark pair

The associated production of a Higgs boson and a top-quark pair is important for probing the Yukawa coupling of the top quark, and calls for better theoretical modeling. In this paper, we calculate the two-loop infrared divergences in $t\bar{t}H$ production at hadron colliders. To do that we compute the one-loop amplitudes to higher orders in the dimensional regulator $ε$. Numeric results for the infrared poles are given as a reference at several representative phase-space points. The result in this work servers as a part of the ongoing efforts towards the $t\bar{t}H$ cross sections at the next-to-next-to-leading order.

hep-ph↗

arXiv2022

Next-to-leading order corrections for $gg \to ZH$ with top quark mass dependence

In this Letter, we present for the first time a calculation of the complete next-to-leading order corrections to the $gg \to ZH$ process. We use the method of small mass expansion to tackle the most challenging two-loop virtual amplitude, in which the top quark mass dependence is retained throughout the calculations. We show that our method provides reliable numeric results in all kinematic regions, and present phenomenological predictions for the total and differential cross sections at the Large Hadron Collider and its future upgrades. Our results are necessary ingredients towards reducing the theoretical uncertainties of the $pp \to ZH$ cross sections down to the percent-level, and provide important theoretical inputs for future precision experimental collider programs.

hep-ph↗

arXiv2022

Baikov representations, intersection theory, and canonical Feynman integrals

The method of canonical differential equations is an important tool in the calculation of Feynman integrals in quantum field theories. It has been realized that the canonical bases are closely related to $d$-dimensional $d\log$-form integrands. In this work, we introduce the concept of generalized loop-by-loop Baikov representation, and clarify its relation and difference with Feynman integrals using the language of intersection theory. We then utilize the generalized Baikov representation to construct $d$-dimensional $d\log$-form integrands, and discuss how to convert them to Feynman integrals. We describe the technical details of our method, in particular how to deal with the difficulties encountered in the construction procedure. Our method provides a constructive approach to the problem of finding canonical bases of Feynman integrals, and we demonstrate its applicability to complicated scattering amplitudes involving multiple physical scales.

hep-th↗

arXiv2022

Alphabet of one-loop Feynman integrals

In this paper, we present the universal structure of the alphabet of one-loop Feynman integrals. The letters in the alphabet are calculated using the Baikov representation with cuts. We consider both convergent and divergent cut integrals and observe that letters in the divergent cases can be easily obtained from convergent cases by applying certain limits. The letters are written as simple expressions in terms of various Gram determinants. The knowledge of the alphabet enables us to easily construct the canonical differential equations of the $ d\log $ form and aids in bootstrapping the symbols of the solutions.

hep-th↗

arXiv2022

Thrust distribution in Higgs decays at the next-to-leading order and beyond

We present predictions for the thrust distribution in hadronic decays of the Higgs boson at the next-to-leading order and the approximate next-to-next-to-leading order. The approximate NNLO corrections are derived from a factorization formula in the soft/collinear phase-space regions. We find large corrections, especially for the gluon channel. The scale variations at the lowest orders tend to underestimate the genuine higher order contributions. The results of this paper is therefore necessary to control the perturbative uncertainties of the theoretical predictions. We also discuss on possible improvements to our results, such as a soft-gluon resummation for the 2-jets limit, and an exact next-to-next-to-leading order calculation for the multi-jets region.

hep-ph↗

arXiv2022

Calculation of the transverse parton distribution functions at next-to-next-to-leading order

We describe the perturbative calculation of the transverse parton distribution functions in all partonic channels up to next-to-next-to-leading order based on a gauge invariant operator definition. We demonstrate the cancellation of light-cone divergences and show that universal process-independent transverse parton distribution functions can be obtained through a refactorization. Our results serve as the first explicit higher-order calculation of these functions starting from first principles, and can be used to perform next-to-next-to-next-to-leading logarithmic $q_T$ resummation for a large class of processes at hadron colliders.

hep-ph↗

arXiv2022

Mixed QCD-EW corrections for Higgs leptonic decay via $HW^+W^-$ vertex

We consider the two-loop corrections to the $HW^+W^-$ vertex at order $αα_s$. We construct a canonical basis for the two-loop integrals using the Baikov representation and the intersection theory. By solving the $ε$-form differential equations, we obtain fully analytic expressions for the master integrals in terms of multiple polylogarithms, which allow fast and accurate numeric evaluation for arbitrary configurations of external momenta. We apply our analytic results to the decay process $H \to ν_e e W$, and study both the integrated and differential decay rates. Our results can also be applied to the Higgs production process via $W$ boson fusion.

hep-ph↗

arXiv2021

FastGPL: a C++ library for fast evaluation of generalized polylogarithms

We present FastGPL, a C++ library for the fast evaluation of generalized polylogarithms which appear in many multi-loop Feynman integrals. We implement the iterative algorithm proposed by Vollinga and Weinzierl in a two-step approach, i.e., we generate concise expressions using an external program and hard-code them into the numeric library. This allows efficient and accurate numeric evaluations of generalized polylogarithms suitable for Monte Carlo integration and event generation. Floating-point arithmetics are carefully taken care of to avoid loss of accuracy. As an application and demonstration, we calculate the two-loop corrections for Higgs boson production in the vector boson fusion channel at electron-positron colliders. FastGPL is expected to be useful for event generators at the next-to-next-to-leading order accuracy.

hep-ph↗

arXiv2021

Top quark pair production near threshold: single/double distributions and mass determination

We investigate top quark pair production near the threshold where the pair invariant mass $M_{t\bar{t}}$ approaches $2m_t$, which provides sensitive observables to extract the top quark mass $m_t$. Using the effective field theory methods, we derive a factorization and resummation formula for kinematic distributions in the threshold limit up to the next-to-leading power, which resums higher order Coulomb corrections to all orders in the strong coupling constant. Our formula is similar to those in the literature but differs in several important aspects. We apply our formula to the $M_{t\bar{t}}$ distribution, as well as to the double differential cross section with respect to $M_{t\bar{t}}$ and the rapidity of the $t\bar{t}$ pair. We find that the resummation effects significantly increase the cross sections near the threshold, and lead to predictions better compatible with experimental data than the fixed-order ones. We demonstrate that incorporating resummation effects in the top quark mass determination can shift the extracted value of $m_t$ by as large as 1.4 GeV. The shift is much larger than the estimated uncertainties in previous experimental studies, and leads to a value of the top quark pole mass more consistent with the current world average.

hep-ph↗

arXiv2021

Invariant-mass distribution of top-quark pairs and top-quark mass determination

We investigate the invariant-mass distribution of top-quark pairs near the $2m_t$ threshold, which has strong impact on the determination of the top-quark mass $m_t$. We show that higher-order non-relativistic corrections lead to large contributions which are not included in the state-of-the-art theoretical predictions. We derive a factorization formula to resum such corrections to all orders in the strong-coupling, and calculate necessary ingredients to perform the resummation at next-to-leading power. We combine the resummation with fixed-order results and present phenomenologically relevant numeric results. We find that the resummation effect significantly enhances the differential cross section in the threshold region, and makes the theoretical prediction more compatible with experimental data. We estimate that using our prediction in the determination of $m_t$ will lead to a value closer to the result of direct measurement.

hep-ph↗

arXiv2021

Constructing Canonical Feynman Integrals with Intersection Theory

Canonical Feynman integrals are of great interest in the study of scattering amplitudes at the multi-loop level. We propose to construct $d\log$-form integrals of the hypergeometric type, treat them as a representation of Feynman integrals, and project them into master integrals using intersection theory. This provides a constructive way to build canonical master integrals whose differential equations can be solved easily. We use our method to investigate both the maximally cut integrals and the uncut ones at one and two loops, and demonstrate its applicability in problems with multiple scales.

hep-th↗

arXiv2020

Efficient computation of two-loop amplitudes for Higgs boson pair production

We present a precise and efficient computation of the two-loop amplitudes entering the Higgs boson pair production process via gluon fusion. Our approach is based on the small-Higgs-mass expansion while keeping the full dependence on the top quark mass and other kinematic invariants. We compare our results to the up-to-date predictions based on a combination of sector decomposition and high-energy expansion. We find that our method provides precision numeric predictions in the entire phase space, while at the same time is highly efficient as the computation can be easily performed on a normal desktop or laptop computer. Our method is valuable for practical phenomenological studies of the Higgs boson pair production process, and can also be applied to other similar processes.

hep-ph↗

arXiv2020

Top-quark pair production at complete-NLO accuracy with NNLO+NNLL$'$ corrections in QCD

We describe predictions for top-quark pair differential distributions at hadron colliders, which combine state-of-the-art NNLO QCD calculations and NLO electroweak corrections together with double resummation at NNLL$'$ accuracy of threshold logarithms and small-mass logarithms. This is the first time that such a combination has appeared in the literature. Numerical results are presented for the invariant-mass distribution, the transverse-momentum distribution as well as rapidity distributions.

hep-ph↗

arXiv2019

Transverse Parton Distribution and Fragmentation Functions at NNLO: the Quark Case

We revisit the calculation of perturbative quark transverse momentum dependent parton distribution functions and fragmentation functions using the exponential regulator for rapidity divergences. We show that the exponential regulator provides a consistent framework for the calculation of various ingredients in transverse momentum dependent factorization. Compared to existing regulators in the literature, the exponential regulator has a couple of advantages which we explain in detail. As a result, the calculation is greatly simplified and we are able to obtain the next-to-next-to-leading order results up to $\mathcal{O}(ε^2)$ in dimensional regularization. These terms are necessary for a higher order calculation which is made possible with the simplification brought by the new regulator. As a by-product, we have obtained the two-loop quark jet function for the Energy-Energy Correlator in the back-to-back limit, which is the last missing ingredient for its N$^3$LL resummation.

hep-ph↗

arXiv2019

Two-loop triangle integrals with 4 scales for the $HZV$ vertex

We calculate analytically the two-loop triangle integrals entering the $\mathcal{O}(αα_s)$ corrections to the $HZV$ vertex with $V=Z^*,γ^*$ using the method of differential equations. Our result provides a prototype to study the analytic properties of multi-loop multi-scale Feynman integrals, and also allows fast numeric evaluation for phenomenological studies. We apply our results to the leptonic decay of the Higgs boson and to $ZH$ production at electron-positron colliders. Besides the top quark loop, we include also the bottom quark loop contributions, whose evaluation takes a lot of time using purely numeric methods, but is very efficient with our analytic results.

hep-ph↗

arXiv2019

Resummation of soft and Coulomb corrections for $t\bar{t}h$ production at the LHC

In this paper, a combined resummation of soft and Coulomb corrections is performed for the associated production of the Higgs boson with a top quark pair at the LHC. We illustrate the similarities and critical differences between this process and the $t\bar{t}$ production process. We show that up to the next-to-leading power, the total cross section for $t\bar{t}h$ production admits a similar factorization formula in the threshold limit as that for $t\bar{t}$ production. This fact, however, is not expected to hold at higher powers. Based on the factorization formula, we perform the resummation at the improved next-to-leading logarithmic accuracy, and match to the next-to-leading order result. This allows us to give NLL$'$+NLO predictions for the total cross sections at the LHC. We find that the resummation effects enhance the NLO cross sections by about 6\%, and significantly reduce the scale dependence of the theoretical predictions.

hep-ph↗

arXiv2019

Towards a new approximation for pair-production and associated-production of the Higgs boson

We propose that loop integrals with internal heavy particles can be evaluated by expanding in the limit of small external masses. This provides a systematically improvable approximation to the integrals in the entire phase space, and works particularly well for the high energy tails of kinematic distributions (where the usual $1/M$ expansions cease to be valid). We demonstrate our method using Higgs boson pair production as an example. We find that at both one-loop and two-loop, our method provides good approximations to the integrals appearing in the scattering amplitudes. Comparing to existing expansion methods, our method are not restricted to a special phase space region. Combining our efficient method to compute the two-loop amplitude with an infrared subtraction method for the real emission corrections, we expect to have a fast and reliable tool to calculate the differential cross sections for Higgs boson pair production. This will be useful for phenomenological studies and for the extraction of the Higgs self-coupling from future experimental data. Our method can also be applied to other processes, such as the associated production of the Higgs boson with a jet or a $Z$ boson.

hep-ph↗

arXiv2018

Resummation for rapidity distributions in top-quark pair production

We extend our framework for the simultaneous resummation of soft and small-mass logarithms to rapidity distributions in top quark pair production. We give numerical results for the rapidity distribution of the top quark or the anti-top quark, as well as the rapidity distribution of the $t\bar{t}$ pair, finding that resummation effects stabilize the dependence of the differential cross sections on the choice of factorization scale. We compare our results with recent measurements at the Large Hadron Collider and find good agreement. Our results may be useful in the extraction of the gluon parton distribution function from $t\bar{t}$ production.

hep-ph↗