Carlo Flore, Jean-Philippe Lansberg, Hua-Sheng Shao, Yelyzaveta Yedelkina
We study the inclusive J/psi production at large transverse momenta at lepton-hadron colliders in the limit when the exchange photon is quasi real, also referred to as photoproduction. Our computation includes the leading-P_T leading-v next-to-leading alpha_s corrections. In particular, we consider the contribution from J/psi plus another charm quark, by employing for the first time in quarkonium photoproduction the variable-flavour-number scheme. We also include a QED-induced contribution via an off-shell photon which remained ignored in the literature and which we show to be the leading contribution at high P_T within the reach of the EIC. In turn, we use our computation of J/psi+charm to demonstrate its observability at the future EIC and the EIC sensitivity to probe the non-perturbative charm content of the proton at high x.
Nodoka Yamanaka, Jean-Philippe Lansberg, Hua-Sheng Shao, Yu-Jie Zhang
Quarkonium production in proton-proton (pp) collision provides interesting means to study the parton content and their correlations in the proton. Recent LHC and Tevatron data of J/psi + Z, J/psi + W and J/psi +J/psi production suggest the relevance of double parton scatterings (DPSs) as opposed to single parton scatterings (SPSs). In this proceedings contribution, we review the corresponding SPS contributions and discuss their upper limits set up by quark-hadron duality. These allow us to perform an improved extraction of the DPS yields and of their implications.
Stefano Frixione, Benjamin Fuks, Valentin Hirschi, Kentarou Mawatari, Hua-Sheng Shao, Marthijn P. A. Sunder, Marco Zaro
The MadGraph5 aMC@NLO framework aims to automate all types of leading- and next-to-leading-order-accurate simulations for any user-defined model that stems from a renormalisable Lagrangian. In this paper, we present all of the key ingredients of such models in the context of supersymmetric theories. In order to do so, we extend the FeynRules package by giving it the possibility of dealing with different renormalisation options that are relevant to supersymmetric models. We also show how to deal with the problem posed by the presence of narrow resonances, thus generalising the so-called on-shell subtraction approaches. We extensively compare our total rate results with those of both Prospino2 and Resummino, and present illustrative applications relevant to the 13 TeV LHC, both at the total-rate and differential levels. The computer programs that we have used to obtain the predictions presented here are all publicly available.
Ajjath A H, Hua-Sheng Shao, Lukas Simon
We extend the local infrared-divergence subtraction formalism, originally proposed by Frixione, Kunszt and Signer (FKS), to calculate short-distance (differential) cross section for any inclusive process involving a quarkonium particle in non-relativistic QCD (NRQCD) factorisation at next-to-leading order (NLO) accuracy in the strong coupling constant $α_s$. The new formulas are generally applicable to the production of an S- or P-wave quarkonium state in association with any number of elementary particles. The main new ingredients derived in this paper are the local and integrated soft counterterms for the colour-singlet and colour-octet P-wave bound states. It, therefore, paves the way to the automation of the NLO calculations for heavy quarkonium inclusive and associated production processes.
Hua-Sheng Shao, Yu-Jie Zhang
The complete set of Feynman rules for the rational part R of QCD corrections in the MSSM are calculated at the one-loop level, which can be very useful in the next-to-leading order calculations in supersymmetric models. Our results are expressed in the 't Hooft-Veltman regularization scheme and in the Four Dimensional Helicity scheme with non-anticommutating and anticommutating $γ_5$ strategies.
Benjamin Fuks, Hua-Sheng Shao
Vector-like quarks are featured by a wealth of beyond the Standard Model theories and are consequently an important goal of many LHC searches for new physics. Those searches, as well as most related phenomenological studies, however rely on predictions evaluated at the leading-order accuracy in QCD and consider well-defined simplified benchmark scenarios. Adopting an effective bottom-up approach, we compute next-to-leading-order predictions for vector-like-quark pair-production and single production in association with jets, with a weak or with a Higgs boson in a general new physics setup. We additionally compute vector-like-quark contributions to the production of a pair of Standard Model bosons at the same level of accuracy. For all processes under consideration, we focus both on total cross sections and on differential distributions, most these calculations being performed for the first time in our field. As a result, our work paves the way to precise extraction of experimental limits on vector-like quarks thanks to an accurate control of the shapes of the relevant observables and emphasize the extra handles that could be provided by novel vector-like-quark probes never envisaged so far
Alessandro Ballestrero, Benedikt Biedermann, Simon Brass, Ansgar Denner, Stefan Dittmaier, Rikkert Frederix, Pietro Govoni, Michele Grossi, Barbara Jäger, Alexander Karlberg, Ezio Maina, Mathieu Pellen, Giovanni Pelliccioli, Simon Plätzer, Michael Rauch, Daniela Rebuzzi, Jürgen Reuter, Vincent Rothe, Christopher Schwan, Hua-Sheng Shao, Pascal Stienemeier, Giulia Zanderighi, Marco Zaro, Dieter Zeppenfeld
Vector-boson scattering processes are of great importance for the current run-II and future runs of the Large Hadron Collider. The presence of triple and quartic gauge couplings in the process gives access to the gauge sector of the Standard Model (SM) and possible new-physics contributions there. To test any new-physics hypothesis, sound knowledge of the SM contributions is necessary, with a precision which at least matches the experimental uncertainties of existing and forthcoming measurements. In this article we present a detailed study of the vector-boson scattering process with two positively-charged leptons and missing transverse momentum in the final state. In particular, we first carry out a systematic comparison of the various approximations that are usually performed for this kind of process against the complete calculation, at LO and NLO QCD accuracy. Such a study is performed both in the usual fiducial region used by experimental collaborations and in a more inclusive phase space, where the differences among the various approximations lead to more sizeable effects. Afterwards, we turn to predictions matched to parton showers, at LO and NLO: we show that on the one hand, the inclusion of NLO QCD corrections leads to more stable predictions, but on the other hand the details of the matching and of the parton-shower programs cause differences which are considerably larger than those observed at fixed order, even in the experimental fiducial region. We conclude with recommendations for experimental studies of vector-boson scattering processes.
Hua-Sheng Shao, Yu-Jie Zhang, Kuang-Ta Chao
We study Feynman rules for the rational part $R$ of the Standard Model amplitudes at one-loop level in the 't Hooft-Veltman $γ_5$ scheme. Comparing our results for quantum chromodynamics and electroweak 1-loop amplitudes with that obtained based on the Kreimer-Korner-Schilcher (KKS) $γ_5$ scheme, we find the latter result can be recovered when our $γ_5$ scheme becomes identical (by setting $g5s=1$ in our expressions) with the KKS scheme. As an independent check, we also calculate Feynman rules obtained in the KKS scheme, finding our results in complete agreement with formulae presented in the literature. Our results, which are studied in two different $γ_5$ schemes, may be useful for clarifying the $γ_5$ problem in dimensional regularization. They are helpful to eliminate or find ambiguities arising from different dimensional regularization schemes.
A. H. Ajjath, Hua-Sheng Shao
In this talk, we discuss the soft-gluon resummation for a pair of Higgs bosons in the dominant gluon fusion channel to next-to-next-to-next-to-leading logarithmic (N$^3$LL) accuracy. Through the study, we achieve sub-percent level accuracy in the uncertainties from the residual renormalisation and factorisation scales, for both inclusive and differential mass distribution in the infinite top quark mass limit. We also combine these results with the full top mass dependent next-to-leading order calculations.
David d'Enterria, Hua-Sheng Shao
Rare two-body decays of the top quark into a neutral bottom-quark meson plus an up- or charm-quark: $t\to {\overline B}^0+ u, c$; $t\to {\overline B}^0_{s}+ c,u$; and $t \to Υ(nS)+ c,u$, are studied for the first time. The corresponding partials widths are computed at leading order in the non-relativistic QCD framework. The sums of all two-body branching ratios amount to $\mathcal{B}(t \to {\overline B}^0+ {\rm jet}) \approx \mathcal{B}(t \to {\overline B}^0_{s}+ {\rm jet}) \approx 4.2\cdot 10^{-5}$ and $\mathcal{B}(t \to Υ(nS)+ {\rm jet}) \approx 2\cdot 10^{-9}$, respectively. The feasibility to observe the $t\to {\overline B}^0_{(s)}+{\rm jet}$ decay is estimated in top-pair events produced in proton-proton collisions at $\sqrt{s} = 14, 100$ TeV at the LHC and FCC, respectively. Combining many exclusive hadronic ${\overline B}^0_{(s)}$ decays, with $J/ψ$ or $D^{0,\pm}$ final states, about 50 (16000) events are expected in 3 (20) ab$^{-1}$ of integrated luminosity at the LHC (FCC), after typical selection criteria, acceptance, and efficiency losses. An observation of the two-body top-quark decay can also be achieved in the interesting $t\to b(\rm{jet})+c(\rm{jet})$ dijet final state, where the ${\overline B}^0_{(s)}$ decay products are reconstructed as a jet, with 5300 and 1.4 million signal events above backgrounds expected after selection criteria at the LHC and FCC, respectively. Such unique final states provide a new direct method to precisely measure the top-quark mass via simple 2-body invariant mass analyses.
Hua-Sheng Shao
We propose a new method to determine the spatially or impact-parameter dependent nuclear parton distribution functions (nPDFs) using the double parton scattering (DPS) processes in high-energy heavy-ion (proton-nucleus and nucleus-nucleus) collisions. We derive a simple generic DPS formula in nuclear collisions by accommodating both the nuclear collision geometry and the spatially dependent nuclear modification effect, under the assumption that the impact-parameter dependence of nPDFs is only related to the nuclear thickness function. While the geometric effect is widely adopted, the impact of the spatially dependent nuclear modification on DPS cross sections has been overlooked so far, which can, however, be significant when the initial nuclear modification is large. In turn, the DPS cross sections in heavy-ion collisions can provide useful information on the spatial dependence of nPDFs. They can be, in general, obtained in minimum-bias nuclear collisions, featuring the virtue of independence of Glauber modeling.
Aldo Deandrea, Thomas Flacke, Benjamin Fuks, Luca Panizzi, Hua-Sheng Shao
We provide a comprehensive discussion, together with a complete setup for simulations, relevant for the production of a single vector-like quark at hadron colliders. Our predictions include finite width effects, signal-background interference effects and next-to-leading order QCD corrections. We explicitly apply the framework to study the single production of a vector-like quark $T$ with charge 2/3, but the same procedure can be used to analyse the single production of vector-like quarks with charge $-4/3$, $-1/3$, $2/3$ and $5/3$, when the vector-like quark interacts with the Standard Model quarks and electroweak bosons. Moreover, this procedure can be straightforwardly extended to include additional interactions with exotic particles. We provide quantitative results for representative benchmark scenarios characterised by the $T$ mass and width, and we determine the role of the interference terms for a range of masses and widths of phenomenological significance. We additionally describe in detail, both analytically and numerically, a striking feature in the invariant mass distribution appearing only in the $T \to th$ channel.
Davide Pagani, Hua-Sheng Shao, Ioannis Tsinikos, Marco Zaro
In this work we compute for the first time the so-called Complete-NLO predictions for top-quark pair hadroproduction in association with at least one isolated photon ($ t \bar t γ$). We also compute NLO QCD+EW predictions for the similar case with at least two isolated photons ($ t \bar t γγ$) and for single-top hadroproduction in association with at least one isolated photon. In addition, we complement our results with NLO QCD+EW predictions of the hadronic and leptonic decays of top-quark including an isolated photon. All these results have been obtained in a completely automated approach, by extending the capabilities of the MadGraph5_aMC@NLO framework and enabling the Complete-NLO predictions for processes with isolated photons in the final state. We discuss the technical details of the implementation, which involves a mixed EW renormalisation scheme for such processes.
David d'Enterria, Hua-Sheng Shao
The feasibility of observing true tauonium, the bound state of two tau leptons, $\mathcal{T}_0\equiv(τ^+τ^-)_0$, via photon-photon collisions at $e^+e^-$ colliders and at the LHC, is studied. The production cross sections of the process $γγ\to\mathcal{T}_0\toγγ$ -- as well as those of all relevant backgrounds: spin-0 and 2 charmonium resonances decaying to diphotons, and light-by-light scattering -- are computed in the equivalent photon approximation for $e^+e^-$ collisions at BES III ($\sqrt{s} = 3.8$ GeV), Belle II ($\sqrt{s} = 10.6$ GeV), and FCC-ee ($\sqrt{s} = 91.2$ GeV), as well as for ultraperipheral p-p, p-Pb, and Pb-Pb collisions at the LHC. Despite small $\mathcal{T}_0$ production cross sections and a final state swamped by decays from overlapping pseudoscalar and tensor charmonium states -- the $\mathrm{χ_{c2}}$, $\mathrm{η_{c}(2S)}$, and $\mathrm{χ_{c0}}$ states have masses only 2.5, 84, and 139 MeV away, respectively, from the $\mathcal{T}_0$ peak -- evidence and observation of the ground state of the heaviest leptonium appears feasible at Belle II and FCC-ee, respectively, with in-situ high-precision measurements of the irreducible backgrounds.
Aleksander Kusina, Jean-Philippe Lansberg, Ingo Schienbein, Hua-Sheng Shao
We study the relevance of experimental data on heavy-flavor [$D^0$, $J/ψ$, $B\rightarrow J/ψ$ and $Υ(1S)$ mesons] production in proton-lead collisions at the LHC to improve our knowledge of the gluon-momentum distribution inside heavy nuclei. We observe that the nuclear effects encoded in both most recent global fits of nuclear parton densities at next-to-leading order (nCTEQ15 and EPPS16) provide a good overall description of the LHC data. We interpret this as a hint that these are the dominant ones. In turn, we perform a Bayesian-reweighting analysis for each particle data sample which shows that each of the existing heavy-quark(onium) data set clearly points --with a minimal statistical significance of 7 $σ$-- to a shadowed gluon distribution at small $x$ in the lead. Moreover, our analysis corroborates the existence of gluon antishadowing. Overall, the inclusion of such heavy-flavor data in a global fit would significantly reduce the uncertainty on the gluon density down to $x\simeq 7\times 10^{-6}$ --where no other data exist-- while keeping an agreement with the other data of the global fits. Our study accounts for the factorization-scale uncertainties which dominate for the charm(onium) sector.
Jean-Philippe Lansberg, Hua-Sheng Shao
We demonstrate that the recent studies of J/psi-pair production by CMS at the LHC and by D0 at the Tevatron reveal the presence of different production mechanisms in different kinematical regions. We find out that next-to-leading-order single parton scattering contributions at alpha_s^5 dominate the yield at large transverse momenta of the pair. Our analysis further emphasises the importance of double parton scatterings --which are expected to dominate the yield at large J/psi-rapidity differences-- at large invariant masses of the pair in the CMS acceptance, and thereby solve a large discrepancy between the theory and the CMS data. In addition, we provide the first exact --gauge-invariant and infrared-safe-- evaluation of a class of leading-P_T (P_T^-4) next-to-next-to-leading-order contributions at alpha_s^6, which can be relevant in the region of large values of P_T,min=min(P_T1,P_T2). Finally, we derive simple relations for the feed-down fractions from the production of an excited charmonium state with a J/psi in the case of the dominance of the double parton scatterings, which significantly deviate from those for single parton scatterings. Such relations can be used to discriminate these extreme scenari, either DPS or SPS dominance.
Jean-Philippe Lansberg, Hua-Sheng Shao
In view of the large discrepancy about the associated production of a prompt $J/ψ$ and a $Z$ boson between the ATLAS data at $\sqrt{s}=8$ TeV and theoretical predictions for Single Parton Scattering (SPS) contributions, we perform an evaluation of the corresponding cross section at one loop accuracy (Next-to-Leading Order, NLO) in a quark-hadron-duality approach, also known as the Colour-Evaporation Model (CEM). This work is motivated by (i) the extremely disparate predictions based on the existing NRQCD fits conjugated with the absence of a full NLO NRQCD computation and (ii) the fact that we believe that such an evaluation provides a likely upper limit of the SPS cross section. In addition to these theory improvements, we argue that the ATLAS estimation of the Double Parton Scattering (DPS) yield may be underestimated by a factor as large as 3 which then reduces the size of the SPS yield extracted from the ATLAS data. Our NLO SPS evaluation also allows us to set an upper limit on $σ_{\rm eff}$ driving the size of the DPS yield. Overall, the discrepancy between theory and experiment may be smaller than expected, which calls for further analyses by ATLAS and CMS, for which we provide predictions, and for full NLO computations in other models. As an interesting side product of our analysis, we have performed the first NLO computation of $dσ/ dP_T$ for prompt single-$J/ψ$ production in the CEM from which we have fit the CEM non-pertubative parameter at NLO using the most recent ATLAS data.
Jean-Philippe Lansberg, Hua-Sheng Shao
We proceed for the first time to the evaluation of the Born cross section for J/psi+eta(c) production, namely via g+g -> J/psi+eta(c)+g, and show that it has a harder P_T spectrum than the J/psi-pair yield at Born level. If one stuck to a comparison at Born level, one would conclude that J/psi+eta(c) production would surpass that of J/psi+J/psi at large P_T. This is nonetheless not the case since J/psi-pair production, as for single J/psi, receives leading-P_T contributions at higher orders in alpha_s. We also present the first evaluation of these leading-P_T next-to-leading order contributions. These are indeed significant for increasing P_T and are of essential relevance for comparison with forthcoming data. We also compute kinematic correlations relevant for double-parton-scattering studies. Finally, we evaluate the polarisation of a J/psi accompanied either by a eta(c) or a J/psi and another light parton. These results may be of great help to understand the polarisation of quarkonia produced at high energies.
Hua-Sheng Shao
This dissertation addresses a topic that I have worked on over the past decade: the automation of next-to-leading order electroweak corrections in the Standard Model of particle physics. After introducing the basic concepts and techniques of next-to-leading order QCD calculations that underpin the MadGraph5_aMC@NLO framework, I present a few key features relevant to the automated next-to-leading order electroweak contributions to short-distance cross sections, with an emphasis on the mixed QCD and electroweak coupling expansions. These include the FKS subtraction, the renormalization and electroweak input parameter schemes, and the complex mass scheme for dealing with unstable particles. Issues related to the initial or final photons and leptons are also discussed. Two remaining challenges are highlighted if one wishes to go beyond next-to-leading order computations. Some phenomenological applications at the LHC are given to demonstrate the relevance of electroweak corrections at colliders. Finally, an outlook on future studies concludes the dissertation.
Jean-Philippe Lansberg, Hua-Sheng Shao, Hong-Fei Zhang
We proceed to the first study of $η_c'$ prompt hadroproduction at next-to-leading order in $α_s$. Based on heavy-quark-spin symmetry, which is systematically used in quarkonium-production phenomenology, we demonstrate that prompt $η_c'$ can be studied at the LHC with the existing data. We emphasize its relevance to constrain $ψ'$ production, in the same way as the first prompt $η_c'$ data at the LHC lately strongly impacted the phenomenology of $J/ψ$ studies.