Lepton Angular Distributions Research Articles

Transverse momentum dependence of the T -even hadronic structure functions in the Drell-Yan process

We present detailed analysis of the T-even lepton angular distribution in the Drell-Yan process including γ/Z0 gauge boson exchange and using perturbative QCD based on the collinear factorization scheme at leading order in the αs expansion. We focus on the study of the transverse momentum QT dependence of the corresponding hadronic structure functions and angular coefficients up to next-to-next-to-leading order in the QT2/Q2 expansion. We analyze QT dependence numerically and compare T-even angular coefficients integrated over rapidity with available data of the ATLAS Collaboration at LHC. Additionally, we present our results for the forward-backward asymmetry and compare it with data. Published by the American Physical Society 2024

Binned top quark spin correlation and polarization observables for the LHC at 13.6 TeV

We consider top-antitop quark (tt¯) production at the Large Hadron Collider (LHC) with subsequent decays into dileptonic final states. We use and investigate a set of leptonic angular correlations and distributions with which all the independent coefficient functions of the top-spin-dependent parts of the tt¯ production spin density matrices can be experimentally probed. We compute these observables for the LHC center-of-mass energy 13.6 TeV within the Standard Model at next-to-leading order in the QCD coupling including the mixed QCD weak corrections. We determine also the tt¯ charge asymmetry where we take in addition also the mixed QCD-QED corrections into account. In addition we analyze and compute possible new physics (NP) effects on these observables within effective field theory in terms of a gauge-invariant effective Lagrangian that contains the operators up to mass dimension six that are relevant for hadronic tt¯ production. First, we compute our observables inclusive in phase space. Then, in order to investigate which region in phase space has, for a specific observable, a high NP sensitivity, we determine our observables also in two-dimensional (Mtt¯,cosθt*) bins, where Mtt¯ denotes the tt¯-invariant mass and θt* is the top quark scattering angle in the tt¯ zero-momentum frame. Published by the American Physical Society 2024

Perturbative T -odd asymmetries in the Drell-Yan process revisited

We calculate the perturbative T-odd contributions to the lepton angular distribution in the Drell-Yan process. Using collinear factorization, we work at the first order in QCD perturbation theory where these contributions appear, O(αs2), and address both W± and γ/Z0 boson exchange. A major focus of our calculation is on the regime where the boson’s transverse momentum QT is much smaller than its mass Q. We carefully expand our results up to next-to-next-to-leading power in QT/Q. Our calculation provides a benchmark for studies of T-odd contributions that employ transverse-momentum dependent parton distribution functions. In the neutral-current case we compare our results for the T-odd structure functions to available ATLAS data. Published by the American Physical Society 2024

Moments for positivity: using Drell-Yan data to test positivity bounds and reverse-engineer new physics

Moments of the leptonic angular distribution in the Drell-Yan process have recently been shown to be sensitive probes of a specific class of dimension-8, four-fermion operators in the Standard Model Effective Field Theory, involving a pair of quarks and leptons. The same operators are also subject to positivity bounds, when requiring the associated (unknown) UV completion to obey basic principles of quantum field theory. We perform a phenomenological study to quantify the sensitivity of the high-luminosity LHC to this set of operators and, by extension, the positivity bounds. We further extend the angular basis of moments and consider double differential information to improve the ability to disentangle the different operators, leading to a sensitivity to new physics scales up to 3 TeV. We use this information to explore the violation of positivity at the LHC as a way to test the underlying principles of quantum field theory. Finally, we present a case study which combines our results with information from other (current and prospective) experiments, as well as the positivity cone to infer the properties of possible tree-level UV completions. The data lead to robust, model-independent lower bounds on the M/sqrt{g} combination of the particle mass and coupling, for states that couple to right-handed leptons and/or up quarks.

Glauber gluons in pion-induced Drell-Yan processes revisited

We reanalyze the anomalous angular distribution of lepton pairs produced in a pion-induced Drell-Yan process by taking into account the Glauber gluon effect in the ${k}_{T}$ factorization theorem. Compared to the previous study, we adopt the realistic parton distribution functions (PDFs) for a proton from the Coordinated Theoretical-Experimental Project on QCD and for a pion from the xFitter, include the QCD evolutions of the strong coupling and the PDFs, and integrate the differential cross section over the kinematic regions for the NA10, E615 and COMPASS experiments. These improvements then allow rigorous confrontations of theoretical results with the data. It is shown that the lepton angular distribution and the violation of the Lam-Tung relation measured in all the above experiments can be well accommodated with a single Glauber phase. We illustrate the Glauber effect in the geometric picture for a Drell-Yan process, and its distinction from the conventional Boer-Mulders mechanism. The observables are pointed out, which can be used to discriminate the two proposals, when data become more precise.

Electromagnetic Dalitz decays of D(s)∗ mesons

Rare electromagnetic (EM) decays of charmed mesons are useful laboratories to explore the structure of hadronic states and the interactions between photons and charmed mesons, to test the chiral perturbation theory in flavor sector and to search for new physics including dark photons. In this paper, we calculate the relative branching ratios of EM Dalitz decays [Formula: see text] to their corresponding radiative decays [Formula: see text] with transition form factor in vector-meson dominance model, where [Formula: see text] represents [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. The corresponding dileptonic invariant mass spectra and the leptonic angular distributions are also presented.

Longitudinal Z-boson polarization and the Higgs boson production cross section at the Large Hadron Collider

Charged lepton pairs are produced copiously in high-energy hadron collisions via electroweak gauge boson exchange, and are one of the most precisely measured final states in proton-proton collisions at the Large Hadron Collider (LHC). We propose that measurements of lepton angular distributions can be used to improve the accuracy of theoretical predictions for Higgs boson production cross sections at the LHC. To this end, we exploit the sensitivity of the lepton angular coefficient associated with the longitudinal Z-boson polarization to the parton density function (PDF) for gluons resolved from the incoming protons, in order to constrain the Higgs boson cross section from gluon fusion processes. By a detailed numerical analysis using the open-source platform xFitter, we find that high-statistics determinations of the longitudinally polarized angular coefficient at the LHC Run III and high-luminosity HL-LHC improve the PDF systematic uncertainties of the Higgs boson cross section predictions by 50% over a broad range of Higgs boson rapidities.

Vector boson polarizations in the decay of the Standard Model Higgs

The kinematic distributions of the lepton pairs produced in the decay of the Standard Model Higgs to ZZ and WW are related to the polarization fractions of the virtual vector bosons. The full amplitude can be decomposed analytically into a sum of simple polarized terms. The vector polarization fractions in Higgs decay are completely determined as in the case of single boson inclusive production. Their measurement would provide a test of the SM. Several observables, in particular the invariant mass of two charged leptons, either from the same boson or from different ones, and the lepton angular distribution in the vector boson center of mass are shown to be sensitive to the boson polarizations.

Lepton angular distribution of W boson productions

The lepton angular distribution coefficients $A_i$ for $Z$ boson production in $pp$ and $\bar p p$ collisions have been measured at the LHC and the Tevatron. A recent study showed that many features of the measured angular distribution coefficients, including the transverse momentum ($q_T$) and rapidity dependencies and the violation of the Lam-Tung relation, can be well described using an intuitive geometric approach. In this paper, we extend this geometric approach to describe the angular distribution coefficients for $W$ boson produced in $\bar{p} p$ collisions at the Tevatron. We first compare the data with a perturbative QCD calculation at $\mathcal{O}(\alpha_s^2)$. We then show that the data and QCD calculations can be well described with the geometric approach. Implications for future studies at the LHC energy are also discussed.

Lepton angular distribution of Z boson production and jet discrimination

High precision data of lepton angular distributions in inclusive Z boson production, reported by the CMS and ATLAS Collaborations, showed pronounced transverse momentum (qT) dependencies of the A0 and A2 coefficients. Violation of the Lam-Tung relation, A0=A2, was also found. An intuitive understanding of these results can be obtained from a geometric approach. We predict that A0 and A2 for Z plus single gluon-jet events are very different from that of Z plus single quark-jet events, allowing a new experimental tool for checking various algorithms which attempt to discriminate quark jets from gluon jets. We also predict that the Lam-Tung relation would be more severely violated for the Z plus multiple-jet data than what has been observed so far for inclusive Z production data. These predictions can be readily tested using existing LHC data.

Lepton angular distributions of fixed-target Drell-Yan experiments in perturbative QCD and a geometric approach

The lepton angular distributions of the Drell-Yan process in fixed-target experiments are investigated by NLO and NNLO perturbative QCD. We present the calculated angular parameters $\lambda$, $\mu$, $\nu$ and the degree of violation of the Lam-Tung relation, $1-\lambda-2\nu$, for the NA10, E615 and E866 experiments. Predictions for the ongoing COMPASS and SeaQuest experiments are also presented. The transverse momentum ($q_T$) distributions of $\lambda$ and $\nu$ show a clear dependence on the dimuon mass ($Q$) while those of $\mu$ have a strong rapidity ($x_F$) dependence. Furthermore, $\lambda$ and $\nu$ are found to scale with $q_T/Q$. These salient features could be qualitatively understood by a geometric approach where the lepton angular distribution parameters are expressed in terms of the polar and azimuthal angles of the "natural axis" in the dilepton rest frame.

On the rotational invariance and non-invariance of lepton angular distributions in Drell–Yan and quarkonium production

Several rotational invariant quantities for the lepton angular distributions in Drell–Yan and quarkonium production were derived several years ago, allowing the comparison between different experiments adopting different reference frames. Using an intuitive picture for describing the lepton angular distribution in these processes, we show how the rotational invariance of these quantities can be obtained. This approach can also be used to determine the rotational invariance or non-invariance of various quantities specifying the amount of violation for the Lam–Tung relation. While the violation of the Lam–Tung relation is often expressed by frame-dependent quantities, we note that alternative frame-independent quantities are preferred.

Forward Drell-Yan and backward jet production as a probe of the BFKL dynamics

We propose a new process which probes the BFKL dynamics in the high energy proton-proton scattering, namely the forward Drell-Yan (DY) production accompanied by a backward jet, separated from the DY lepton pair by a large rapidity interval. The proposed process probes higher rapidity differences and smaller transverse momenta than in the Mueller-Navelet jet production. It also offers a possibility of measuring new observables like lepton angular distribution coefficients in the DY lepton pair plus jet production.

Probing the dark sector through mono-Z boson leptonic decays

Collider search for dark matter production has been performed over the years based on high pT standard model signatures balanced by large missing transverse energy. The mono-Z boson production with leptonic decay has a clean signature with the advantage that the decaying electrons and muons can be precisely measured. This signature not only enables reconstruction of the Z boson rest frame, but also makes possible recovery of the underlying production dynamics through the decaying lepton angular distribution. In this work, we exploit full information carried by the leptonic Z boson decays to set limits on coupling strength parameters of the dark sector. We study simplified dark sector models with scalar, vector, and tensor mediators and observe among them different signatures in the distribution of angular coefficients. Specifically, we show that angular coefficients can be used to distinguish different scenarios of the spin-0 and spin-1 models, including the ones with parity-odd and charge conjugation parity-odd operators. To maximize the statistical power, we perform a matrix element method study with a dynamic construction of event likelihood function. We parametrize the test statistic such that sensitivity from the matrix element is quantified through a term measuring the shape difference. Our results show that the shape differences provide significant improvements in the limits, especially for the scalar mediator models. We also present an example application of a matrix-element-kinematic-discriminator, an easier approach that is applicable for experimental data.

Dependencies of lepton angular distribution coefficients on the transverse momentum and rapidity of Z bosons produced in pp collisions at the LHC

High precision data of lepton angular distributions for $\gamma^*/Z$ production in $pp$ collisions at the LHC, covering broad ranges of dilepton transverse momenta ($q_T$) and rapidity ($y$), were recently reported. Strong $q_T$ dependencies were observed for several angular distribution coefficients, $A_i$, including $A_0 - A_4$. Significant $y$ dependencies were also found for the coefficients $A_1$, $A_3$ and $A_4$, while $A_0$ and $A_2$ exhibit very weak rapidity dependence. Using an intuitive geometric picture we show that the $q_T$ and $y$ dependencies of the angular distributions coefficients can be well described.

Measurement of polarization angular coefficients in Z boson leptonic decays with ATLAS at the LHC

This paper presents the complete set of polarization angular coefficients A 0−7 describing lepton angular distributions in Z boson decay, which were measured at the ATLAS experiment in proton–proton collisions with the energy √s = 8 TeV. Theoretical values for the difference A 0 − A 2 calculated in the fixed-order QCD perturbation theory O(α 2 ), demonstrate significant deviation from the measured data, which indicates the necessity of taking into account higher order corrections. The evidence of nonzero coefficients A 5,6,7 was obtained for the first time, in accordance with theoretical calculations in O(α 2 ) approximation. Measurement of the polarization angular coefficients A i is important for subsequent precision measurement of parameters of the electroweak model at the LHC, such as the sine of Weinberg electroweak mixing angle sin2 θ W and the W boson mass.

W production at LHC: lepton angular distributions and reference frames for probing hard QCD

Precision tests of the Standard Model in the Strong and Electroweak sectors play a crucial role, among the physics program of LHC experiments. Because of the nature of proton–proton processes, observables based on the measurement of the direction and energy of final state leptons provide the most precise probes of such processes. In the present paper, we concentrate on the angular distribution of leptons from W rightarrow ell nu decays in the lepton-pair rest-frame. The vector nature of the intermediate state imposes that distributions are to a good precision described by spherical harmonics of at most second order. We argue, that contrary to general belief often expressed in the literature, the full set of angular coefficients can be measured experimentally, despite the presence of escaping detection neutrino in the final state. There is thus no principle difference with respect to the phenomenology of the Z/gamma rightarrow ell ^+ ell ^- Drell–Yan process. We show also, that with the proper choice of the reference frames, only one coefficient in this polynomial decomposition remains sizable, even in the presence of one or more high p_T jets. The necessary stochastic choice of the frames relies on probabilities independent from any coupling constants. In this way, electroweak effects (dominated by the V-A nature of W couplings to fermions), can be better separated from the ones of strong interactions. The separation is convenient for the measurements interpretation.

Drell-Yan lepton angular distributions in perturbative QCD

We present a comprehensive comparison of the available experimental data for the Drell-Yan lepton angular coefficients $\lambda$ and $\nu$ to calculations at leading and next-to-leading order of perturbative QCD. To obtain the next-to-leading order corrections, we make use of publicly available numerical codes that allow us to compute the Drell-Yan cross section at second order in perturbation theory and from which the contributions we need can be extracted. Our comparisons reveal that perturbative QCD is able to describe the experimental data overall rather well, especially at colliders, but also in the fixed-target regime. On the basis of the angular coefficients alone, there appears to be little (if any) convincing evidence for effects that go beyond fixed-order collinear factorized perturbation theory, although the presence of such effects is not ruled out.

A set of top quark spin correlation and polarization observables for the LHC: Standard Model predictions and new physics contributions

We consider top-antitop quark ($t{\bar t}$) production at the Large Hadron Collider (LHC) with subsequent decay into dileptonic and lepton plus jets final states. We present a set of leptonic angular correlations and distributions with which experiments can probe all the independent coefficient functions of the top-spin dependent parts of the $t{\bar t}$ production spin density matrices. We compute these angular correlations and distributions for LHC center-of-mass energies 8, 13, and 14 TeV within the Standard Model at next-to-leading order in the QCD coupling including the mixed QCD-weak corrections and for the transverse top-quark polarization and the $t{\bar t}$ charge asymmetry also the mixed QCD-QED corrections. In addition we analyze and compute the effects of new interactions on these observables in terms of a gauge-invariant effective Lagrangian which contains all operators relevant for hadronic $t{\bar t}$ production up to mass dimension six.

Top-quark electroweak couplings at the FCC-ee

An optimal-observable analysis of the lepton angular and energy distributions from top-quark pair production with semi-leptonic decays in e+e− collisions is used to predict the potential sensitivity of the FCC-ee to the couplings of the top quark to the photon and the Z.