Next-to-leading Order QCD Corrections Research Articles

Drell-Yan production in third-generation gauge vector leptoquark models at NLO+PS in QCD

Motivated by the long-standing hints of lepton-flavour non-universality in the b → cℓν and b → sℓ+ℓ− channels, we study Drell-Yan ditau production at the Large Hadron Collider (LHC). In the context of models with third-generation gauge vector leptoquarks (LQs), we calculate the complete mathcal{O} (αs) corrections to the pp → τ+τ− process, achieving next-to-leading order (NLO) plus parton shower (NLO+PS) accuracy using the POWHEG method. We provide a dedicated Monte Carlo code that evaluates the NLO QCD corrections on-the-fly in the event generation and use it to study the numerical impact of NLO+PS corrections on the kinematic distributions that enter the existing experimental searches for non-resonant ditau final states. Based on our phenomenological analysis we derive NLO accurate constraints on the masses and couplings of third-generation gauge vector LQs using the latest LHC ditau search results corresponding to an integrated luminosity of around 140 fb−1 of proton-proton collisions at sqrt{s} = 13 TeV. The presented NLO+PS generator allows for an improved signal modelling, making it an essential tool for future ATLAS and CMS searches for vector LQs in τ+τ− final states at LHC Run III and beyond.

Tt¯bb¯ at the LHC: On the size of off-shell effects and prompt b -jet identification

We investigate full off-shell effects in $t\overline{t}b\overline{b}$ production in the dilepton channel at the LHC with the center-of-mass energy $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$. Specifically, we compute next-to-leading-order (NLO) QCD corrections to the $pp\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}b\overline{b}b\overline{b}+X$ process and provide a prescription for $b$-jet identification to distinguish prompt $b$ jets from $b$ jets originating from the decay of the top quarks. As an important irreducible background to $pp\ensuremath{\rightarrow}t\overline{t}H(H\ensuremath{\rightarrow}b\overline{b})$, $t\overline{t}$ production in association with two prompt $b$ jets is a primary source of uncertainty in the measurement of $t\overline{t}H(H\ensuremath{\rightarrow}b\overline{b})$. In quantifying full off-shell effects, we perform comparisons between the state-of-the-art full off-shell computation and the calculation in the narrow width approximation. The former includes all double-, single- and nonresonant Feynman diagrams, interferences as well as finite-width effects of the top quarks and $W$ gauge bosons. The latter restricts the unstable top quarks and $W$ gauge bosons to on-shell states and includes for the first time NLO QCD corrections to both production and decays. We observe that full off-shell effects are subdominant compared to the scale uncertainties for the integrated fiducial cross section and for the majority of differential observables in the phase-space regions that we investigated. However, for a number of observables related to beyond the Standard Model searches, full off-shell effects are significant. Furthermore, with our $b$-jet labeling prescription, the prompt $b$ jets and the $b$ jets from top-quark decays can be successfully disentangled.

Revisiting NLO QCD corrections to total inclusive J/ψ and ϒ photoproduction cross sections in lepton-proton collisions

We revisit inclusive J/ψ and ϒ photoproduction at lepton-hadron colliders, namely in the limit when the exchanged photon is quasi real. Our computation includes the next-to-leading-order (NLO) αs corrections to the leading-order contributions in v. Similarly to the case of NLO charmonium-hadroproduction processes, the resulting cross sections obtained in the MS‾ factorisation scheme are sometimes found to be negative. We show that the scale-fixing criterion which we derived in a previous study of ηc production successfully solves this problem from the EicC all the way up to the FCC-eh energies. We then elaborate on how to study a scale uncertainty akin to that derived by scale variations when one fixes a scale. In turn, we investigate where both J/ψ and ϒ photoproduction could be used to improve our knowledge of gluon content of the proton at scales as low as a couple of GeV.

NLO QCD corrections to pseudoscalar quarkonium production with two heavy flavors in photon-photon collision

We calculate the next-to-leading order (NLO) QCD corrections to $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\eta}}_{c}+c+\overline{c}$, $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\eta}}_{b}+b+\overline{b,}$ and $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{B}_{c}+b+\overline{c}$ processes in the framework of nonrelativistic QCD factorization formalism. The cross sections at the SuperKEKB electron-positron collider, as well as the future collider like the Circular Electron Positron Collider, are evaluated. Numerical results indicate that the NLO corrections are significant, and the uncertainties in theoretical predictions with NLO corrections are reduced as expected. Due to the high luminosity of the SuperKEKB collider, the ${\ensuremath{\eta}}_{c}+c+\overline{c}$ production is hopefully observable in the near future.

NLO electroweak and QCD corrections to the production of a photon with three charged lepton plus missing energy at the LHC

Electroweak (EW) triboson production processes with at least one heavy gauge boson are of increasing interest at the Large Hadron Collider (LHC) as direct precision probes of one of the least-tested sectors of the Standard Model (SM), the quartic couplings of the EW gauge bosons. These processes therefore offer promising opportunities for searches for indirect signals of Beyond-the-SM (BSM) physics. In this paper, we present results for fiducial cross sections at next-to-leading-order (NLO) EW and NLO QCD to $p\;p\to e^{+}\;\nu_{e}\;\mu^{+}\;\mu^{-}\;\gamma$ at the 13 TeV LHC. This signature includes the triboson production process $p\;p \to W^{+}\;Z\;\gamma$ with leptonic decays, $W^{+} \to e^{+}\;\nu_{e}$ and $Z \to \mu^{+}\;\mu^{-}$. The computation is based on the complete set of leading-order (LO) contributions of $\mathcal{O}(\alpha^5)$ and on NLO EW and NLO QCD cross sections of $\mathcal{O}(\alpha^6)$ and $\mathcal{O}(\alpha^5 \alpha_\text{s})$, respectively, and thus off-shell effects, spin correlations and non-resonance contributions are fully taken into account. We construct a Monte Carlo framework which provides total and differential cross sections for a chosen set of basic analysis cuts. We find that while NLO EW corrections enhance the fiducial LO total cross section by only $1\%$, they can significantly change some distributions in certain kinematic regions. For example, the relative NLO EW corrections to the muon transverse momentum distribution at 500 GeV amounts to $-20\%$. To illustrate how missing NLO EW corrections could masquerade as BSM physics, we show examples for the impact of dimension-8 operators in the SM Effective Field Theory framework on selected kinematic distributions.

J/ψ associated production with a bottom quark pair from the Higgs boson decay in next-to-leading order QCD

In this work, we investigate the next-to-leading order (NLO) QCD correction to $J/\ensuremath{\psi}$ associated production with a bottom quark pair from the Higgs boson decay within the nonrelativistic QCD framework. From numerical results, we find that the decay width of process $H\ensuremath{\rightarrow}b+J/\ensuremath{\psi}+\overline{b}$ at leading order (LO) mainly comes from the contribution of the Fock state ${^{3}S}_{1}^{(8)}$, and the NLO QCD corrections significantly enhance the decay width at LO accuracy by about two times. At NLO accuracy, the Fock states ${^{3}S}_{1}^{(8)}$ and ${^{3}P}_{J}^{(8)}$ channels give the main contribution, accounting for about 68% and 29% of the total decay width of $J/\ensuremath{\psi}$ associated production with a bottom quark pair at NLO accuracy from the Higgs boson decay, respectively. Considering the dominant contribution of color octet (CO) channels at NLO accuracy, the inclusive decay process $H\ensuremath{\rightarrow}b+J/\ensuremath{\psi}+\overline{b}+X$ has the potential to be found in future colliders with high energy/luminosity. The study of $J/\ensuremath{\psi}$ associated production with a bottom quark pair from the Higgs boson decay is not only useful to study the mechanism of color-octet, but also to assist in the investigation of the coupling for the Higgs boson with the bottom quark.

Master integrals for inclusive weak decays of heavy flavors at next-to-leading order

We present analytical results for master integrals emerging in the computation of differential rates for inclusive weak decays of heavy flavors at next-to-leading order (NLO) in QCD. As an immediate physical application, these master integrals allow for a calculation of the spectra of the leptonic invariant mass in inclusive semileptonic decays in the framework of the heavy quark expansion, including the NLO QCD corrections to power suppressed terms.

Next-to-leading-order QCD corrections to heavy quark fragmentation into {}^1S^{(1,8)}_0 quarkonia

Within NRQCD factorization framework, in this work we compute, at the lowest order in velocity expansion, the next-to-leading-order (NLO) perturbative corrections to the short-distance coefficients associated with heavy quark fragmentation into the {}^1S_0^{(1,8)} components of a heavy quarkonium. Starting from the Collins and Soper’s operator definition of the quark fragmentation function, we apply the sector decomposition method to facilitate the numerical manipulation. It is found that the NLO QCD corrections have a significant impact.

Precise predictions for double-Higgs production via vector-boson fusion

Theoretical predictions with next-to-next-to-leading order (NNLO) QCD accuracy combined with the next-to-leading order (NLO) electroweak (EW) corrections are presented for differential observables of the double-Higgs production process via vector-boson fusion. While the QCD corrections were previously known, the EW ones are computed here for the first time. The numerical results are obtained for a realistic experimental set-up at the LHC and are presented in the form of fiducial cross sections and differential distributions. Within this setup we find that the VBF approximation employed in the NNLO QCD correction is accurate at the sub-percent level. We find that the NLO EW corrections within the fiducial volume are -,6.1%, making them of almost the same order as the NLO QCD corrections. In some kinematic regions they can grow as large as -,30% making them the dominant radiative corrections. When the EW corrections are combined with the NNLO QCD corrections we find a total correction of -,14.8%. The results presented here thus comprise the state-of-the-art theoretical predicition for the double-Higgs production via vector-boson fusion, which will be of value to the high-luminosity programme at the LHC.

Electroweak corrections to the angular coefficients in finite-p_TZ-boson production and dilepton decay

We present next-to-leading order (NLO) electroweak corrections to the dominant five angular coefficients parametrizing the Drell–Yan process in the Z-boson mass peak range for finite-p_T vector boson production. The results are presented differentially in the vector boson transverse momentum. The Lam–Tung violating difference A_0-A_2 is examined alongside the coefficients. A single lepton transverse momentum cut is needed in the case of electroweak corrections to avoid a double singularity in the photon induced diagrams, and the dependence on the value of this cut is examined. We compare the electroweak corrections to the angular coefficients to the NLO QCD corrections, including the single lepton cut. The size of the single lepton cut is found to affect the two coefficients A_0 and A_2 to largest extent. The relative size of the electroweak corrections to the coefficients is moderate for all single lepton cut values, and by extrapolation to the inclusive results, is moderate also for the full dilepton phase space case. However, for the Lam–Tung violation, there is a significant contribution from the electroweak corrections for low p_T of the lepton pair.

QCD and electro-weak corrections to the production of Higgs+2 jets at the LHC

We present complete next-to-leading-order (NLO) QCD correction and electro-weak (EW) correction to the production cross section of Higgs–Boson in association with two hard quark jets by using Monte-Carlo numerical calculation program, HAWK, for various photon parton distribution functions at the Large Hadron Collider with center of mass energy 7, 8, 13, and 14 TeV without and with typical vector-Boson-fusion cuts on the tagging jets. In our calculation we include complete contribution from the full set of t-channel, u-channel and s-channel Feynman diagrams, and corresponding interferences as well as NLO QCD and EW corrections.

Diphoton production in vector-boson scattering at the LHC at next-to-leading order QCD

In this paper, we present results at next-to-leading order (NLO) QCD for photon pair production in association with two jets via vector boson scattering within the Standard Model (SM), and also in an effective field theory framework with anomalous gauge coupling effects via bosonic dimension-6 and 8 operators. We observe that, com- pared to other processes in the class of two electroweak (EW) vector boson production in association with two jets, more exclusive cuts are needed in order to suppress the SM QCD-induced background channel. As expected, the NLO QCD corrections reduce the scale uncertainties considerably. Using a well-motivated dynamical scale choice, we find moderate K -factors for the EW-induced process while the QCD-induced channel receives much larger corrections. Furthermore, we observe that applying a cut of Delta {phi}_{j2gamma 1}^{mathrm{cut}}<2.5 for the second hardest jet and the hardest photon helps to increase the signal significance and reduces the impact of higher-order QCD corrections.

The studies on Z rightarrow Upsilon (1S)+g+g at the next-to-leading-order QCD accuracy

In this paper, we carry out the next-to-leading-order (NLO) studies on Z rightarrow Upsilon (1S)+g+g via the color-singlet (CS) b{bar{b}} state. We find the newly calculated NLO QCD corrections to this process can significantly influence its leading-order (LO) results and greatly improve the dependence on the renormalization scale. By including the considerable feeddown contributions, the branching ratio {mathcal {B}}_{Z rightarrow Upsilon (1S)+g+g} is predicted to be (0.56–0.95) times 10^{-6}, which can reach up to 19–31% of the LO predictions given by the CS dominant process Z rightarrow Upsilon (1S)+b+{bar{b}}. Moreover, Z rightarrow Upsilon (1S)+g+g also seriously affects the CS predictions on the Upsilon (1S) energy distributions, especially when z is relatively small. In summary, for the inclusive Upsilon (1S) productions in Z decay, besides Z rightarrow Upsilon (1S)+b+{bar{b}}, the gluon radiation process Z rightarrow Upsilon (1S)+g+g can provide indispensable contributions as well.

Fiducial polarization observables in hadronic WZ production: a next-to-leading order QCD+EW study

We present a study at next-to-leading-order (NLO) of the process pp → W±Z → ℓνlℓ′+ℓ′−, where ℓ, ℓ′ = e, μ, at the Large Hadron Collider. We include the full NLO QCD corrections and the NLO electroweak (EW) corrections in the double-pole approximation. We define eight fiducial polarization coefficients directly constructed from the polar-azimuthal angular distribution of the decay leptons. These coefficients depend strongly on the kinematical cuts on the transverse momentum or rapidity of the individual leptons. Similarly, fiducial polarization fractions are also defined and they can be directly related to the fiducial coefficients. We perform a detailed analysis of the NLO QCD+EW fiducial polarization observables including theoretical uncertainties stemming from the scale variation and parton distribution function uncertainties, using the fiducial phase space defined by the ATLAS and CMS experiments. We provide results in the helicity coordinate system and in the Collins-Soper coordinate system, at a center-of-mass energy of 13 TeV. The EW corrections are found to be important in two of the angular coefficients related to the Z boson, irrespective of the kinematical cuts or the coordinate system. Meanwhile, those EW corrections are very small for the W± bosons.

Further studies on the exclusive productions of J/psi +chi _{cJ} (J=0,1,2) via e^+e^- annihilation at the B factories

By including the interference effect between the QCD and the QED diagrams, we carry out a complete analysis on the exclusive productions of e^+e^- rightarrow J/psi +chi _{cJ} (J=0,1,2) at the B factories with sqrt{s}=10.6 GeV at the next-to-leading-order (NLO) level in alpha _s, within the nonrelativistic QCD framework. It is found that the {mathcal {O}} (alpha ^3alpha _s)-order terms that represent the tree-level interference are comparable with the usual NLO QCD corrections, especially for the chi _{c1} and chi _{c2} cases. To explore the effect of the higher-order terms, namely {mathcal {O}} (alpha ^3alpha _s^2), we perform the QCD corrections to these {mathcal {O}} (alpha ^3alpha _s)-order terms for the first time, which are found to be able to significantly influence the {mathcal {O}} (alpha ^3alpha _s)-order results. In particular, in the case of chi _{c1} and chi _{c2}, the newly calculated {mathcal {O}} (alpha ^3alpha _s^2)-order terms can to a large extent counteract the {mathcal {O}} (alpha ^3alpha _s) contributions, evidently indicating the indispensability of the corrections. In addition, we find that, as the collision energy rises, the percentage of the interference effect in the total cross section will increase rapidly, especially for the chi _{c1} case.

NLO QCD corrections to triply heavy baryon fragmentation function considering the effect of nonperturbative dynamics of baryon bound states

Using a quark-diquark model of baryons and by working in the framework of perturbative quantum chromodynamics we present, for the first time, the next-to-leading-order (NLO) corrections to the fragmentation process of a heavy quark into a ground state triply heavy baryon. For a complete analysis, we also study the effect of nonperturbative dynamic of baryon bound state on the extracted fragmentation functions considering a typical wave function which is different from the Dirac-delta function that was applied in all previous works. Our results show that the NLO corrections are significant, while the effect of the baryonic wave function is of minor importance.

NLO QCD corrections to B c(B*c) production around the Z pole at an e + e - collider

The production of $B_c$ and $B_c^*$ mesons at a $Z$-factory (an $e^+e^-$ collider operating at energies around the $Z$ pole) is calculatedup to the next-to-leading order (NLO) QCD accuracy. The results show that the dependence of the total cross sections on the renormalization scale $\mu$ is suppressed by the corrections, and the NLO corrections enhance the total cross sections of $B_c$ by $52%$ and of $B_c^*$ by $33%$ when the renormalization scale is taken at $\mu=2m_b$. To observe the various behaviors of the production of the mesons $B_c$ and $B_c^*$, such as the differential cross section vs. the out-going angle, the forward-backward asymmetry, and the distribution vs. the energy fraction $z$ up to NLO QCD accuracy as well as the relevant $K$-factor (NLO to LO) for the production, are calculated, and it is pointed out that some of the observables obtained in the present work may be used as a specific precision test of the standard model.

Next-to-leading-order QCD corrections to e+e− → H + γ

The associated production of Higgs boson with a hard photon at lepton collider, i.e., e+e−→Hγ, is known to bear a rather small cross section in Standard Model, and can serve as a sensitive probe for the potential new physics signals. Similar to the loop-induced Higgs decay channels H→γγ,Zγ, the e+e−→Hγ process also starts at one-loop order provided that the tiny electron mass is neglected. In this work, we calculate the next-to-leading-order (NLO) QCD corrections to this associated H+γ production process, which mainly stem from the gluonic dressing to the top quark loop. The QCD corrections are found to be rather modest at lower center-of-mass energy range (s<300 GeV), thus of negligible impact on Higgs factory such as CEPC. Nevertheless, when the energy is boosted to the ILC energy range (s≈400 GeV), QCD corrections may enhance the leading-order cross section by 20%. In any event, the e+e−→Hγ process has a maximal production rate σmax≈0.08 fb around s=250 GeV, thus CEPC turns out to be the best place to look for this rare Higgs production process. In the high energy limit, the effect of NLO QCD corrections become completely negligible, which can be simply attributed to the different asymptotic scaling behaviors of the LO and NLO cross sections, where the former exhibits a milder decrement ∝1/s , but the latter undergoes a much faster decrease ∝1/s2.

Higgs pair production at NLO QCD for CP-violating Higgs sectors

Higgs pair production through gluon fusion is an important process at the LHC to test the dynamics underlying electroweak symmetry breaking. Higgs sectors beyond the Standard Model (SM) can substantially modify this cross section through novel couplings not present in the SM or the on-shell production of new heavy Higgs bosons that subsequently decay into Higgs pairs. CP violation in the Higgs sector is important for the explanation of the observed matter-antimatter asymmetry through electroweak baryogenesis. In this work we compute the next-to-leading order (NLO) QCD corrections in the heavy top quark limit, including the effects of CP violation in the Higgs sector. We choose the effective theory (EFT) approach, which provides a rather model-independent way to explore New Physics (NP) effects by adding dimension-6 operators, both CP-conserving and CP-violating ones, to the SM Lagrangian. Furthermore, we perform the computation within a specific UV-complete model and choose as benchmark model the general 2-Higgs-Doublet Model with CP violation, the C2HDM. Depending on the dimension-6 coefficients, the relative NLO QCD corrections are affected by several per cent through the new CP-violating operators. This is also the case for SM-like Higgs pair production in the C2HDM, while the relative QCD corrections in the production of heavier C2HDM Higgs boson pairs deviate more strongly from the SM case. The absolute cross sections both in the EFT and the C2HDM can be modified by more than an order of magnitude. In particular, in the C2HDM the resonant production of Higgs pairs can by far exceed the SM cross section.

Next-to-leading-order QCD and electroweak corrections to WWW production at proton-proton colliders

Triple-W-boson production in proton-proton collisions allows for a direct access to the triple and quartic gauge couplings and provides a window to the mechanism of electroweak symmetry breaking. It is an important process to test the Standard Model (SM) and might be background to physics beyond the SM. We present a calculation of the next-to-leading order (NLO) electroweak corrections to the production of WWW final states at proton-proton colliders with on-shell W bosons and combine the electroweak with the NLO QCD corrections. We study the impact of the corrections to the integrated cross sections and to kinematic distributions of the W bosons. The electroweak corrections are generically of the size of 5-10% for integrated cross sections and become more pronounced in specific phase-space regions. The real corrections induced by quark-photon scattering turn out to be as important as electroweak loops and photon bremsstrahlung corrections, but can be reduced by phase-space cuts. Considering that prior determinations of the photon parton distribution function (PDF) involve rather large uncertainties, we compare the results obtained with different photon PDFs and discuss the corresponding uncertainties in the NLO predictions. Moreover, we determine the scale and total PDF uncertainties at the LHC and a possible future 100 TeV pp collider.