Parton Distributions in Nucleons and Nuclei

We review the current status of parton distribution function (PDF) determinations for unpolarized and longitudinally polarized protons and for unpolarized nuclei, which are probed by high-energy hadronic scattering in perturbative quantum chromodynamics (QCD). We present the established theoretical framework, the experimental information, and the methodological aspects inherent to any modern PDF extraction. Furthermore, we summarize the present knowledge of PDFs and discuss their limitations in both accuracy and precision relevant to advancing our understanding of QCD proton substructure and pursuing our quest for precision in the Standard Model and beyond. In this respect, we highlight various achievements, discuss contemporary issues in PDF analyses, and outline future directions of progress.

Combination of the W boson polarization measurements in top quark decays using ATLAS and CMS data at $$ \sqrt{s} $$ = 8 TeV

The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb−1 for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal (F0), left-handed (FL), or right-handed (FR) polarizations. The resulting combined measurements of the polarization fractions are F0 = 0.693 ± 0.014 and FL = 0.315 ± 0.011. The fraction FR is calculated from the unitarity constraint to be FR = −0.008 ± 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for F0 (FL) with respect to the most precise single measurement. A limit on anomalous right-handed vector (VR), and left- and right-handed tensor (gL, gR) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [−0.11, 0.16] for VR, [−0.08, 0.05] for gL, and [−0.04, 0.02] for gR, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.

Inelastic meson-meson scattering in hadronic matter

We review studies of inelastic meson-meson scattering. In nonperturbative schemes with chiral-perturbation-theory Lagrangians and in models based on effective meson Lagrangians, inelastic meson-meson scattering leads to the successful identification of resonances in meson-meson reactions, adequate inclusion of final state interactions in particle decays, and so on. For mesons of which each consists of a quark and an antiquark, inelastic meson-meson scattering may be caused by quark-antiquark annihilation, quark-antiquark creation, quark interchange, and quark-antiquark annihilation and creation. In transition amplitudes for meson-meson scattering mesonic quark-antiquark relative-motion wave functions depend on hadronic matter, and transition potentials are given in perturbative quantum chromodynamics. Via transition amplitudes the cross sections for inelastic meson-meson scattering depend on the temperature of hadronic matter. Some prominent temperature dependence of the cross sections has been found. Inelastic meson-meson scattering becomes even more significant in proton-proton collisions and lead-lead collisions at the Large Hadron Collider.

Measurements of triple-differential cross sections for inclusive isolated-photon+jet events in $$\mathrm{p}\mathrm{p}$$ collisions at $$\sqrt{s} = 8\,\text {TeV} $$

Measurements are presented of the triple-differential cross section for inclusive isolated-photon+jet events in $$\mathrm{p}\mathrm{p}$$pp collisions at $$\sqrt{s} = 8$$s=8 TeV as a function of photon transverse momentum ($$p_{\mathrm {T}} ^{{\upgamma {}{}}}$$pTγ), photon pseudorapidity ($$\eta ^{{\upgamma {}{}}}$$ηγ), and jet pseudorapidity ($$\eta ^{\text {jet}}$$ηjet). The data correspond to an integrated luminosity of $$19.7{\,\text {fb}^{-1}} $$19.7fb-1 that probe a broad range of the available phase space, for $$|\eta ^{{\upgamma {}{}}} |<1.44$$|ηγ|<1.44 and $$1.57<|\eta ^{{\upgamma {}{}}} |<2.50$$1.57<|ηγ|<2.50, $$|\eta ^{\text {jet}} |<2.5$$|ηjet|<2.5, $$40< p_{\mathrm {T}} ^{{\upgamma {}{}}}<1000$$40<pTγ<1000$$\,\text {GeV}$$GeV, and jet transverse momentum, $$p_{\mathrm {T}} ^{\text {jet}}$$pTjet, > 25$$\,\text {GeV}$$GeV. The measurements are compared to next-to-leading order perturbative quantum chromodynamics calculations, which reproduce the data within uncertainties.

Azimuthal separation in nearly back-to-back jet topologies in inclusive 2- and 3-jet events in $${\text {p}} {\text {p}} $$ collisions at $$\sqrt{s}=13\,\text {Te}\text {V} $$

A measurement for inclusive 2- and 3-jet events of the azimuthal correlation between the two jets with the largest transverse momenta, $$\varDelta \phi _{12}$$Δϕ12, is presented. The measurement considers events where the two leading jets are nearly collinear (“back-to-back”) in the transverse plane and is performed for several ranges of the leading jet transverse momentum. Proton-proton collision data collected with the CMS experiment at a center-of-mass energy of $$13\,\text {Te}\text {V} $$13Te and corresponding to an integrated luminosity of $$35.9{\,\text {fb}^{-1}} $$35.9fb-1 are used. Predictions based on calculations using matrix elements at leading-order and next-to-leading-order accuracy in perturbative quantum chromodynamics supplemented with leading-log parton showers and hadronization are generally in agreement with the measurements. Discrepancies between the measurement and theoretical predictions are as large as 15%, mainly in the region $$177^\circ< \varDelta \phi _{12} < 180^\circ $$177∘<Δϕ12<180∘. The 2- and 3-jet measurements are not simultaneously described by any of models.

Particle Production at High Energy: DGLAP, BFKL and Beyond

Particle production in high energy hadronic/nuclear collisions in the Bjorken limit Q 2 , s → ∞ can be described in the collinear factorization framework of perturbative Quantum ChromoDynamics (QCD). On the other hand in the Regge limit, at fixed and not too high Q 2 with s → ∞ , a k ⊥ factorization approach (or a generalization of it) is the appropriate framework. A new effective action approach to QCD in the Regge limit, known as the Color Glass Condensate (CGC) formalism, has been developed which allows one to investigate particle production in high energy collisions in the kinematics where collinear factorization breaks down. Here we give a brief overview of particle production in CGC framework and the evolution equation which governs energy dependence of the observables in this formalism. We show that the new evolution equation reduces to previously known evolution equations in the appropriate limits.

Prospects of Ξ(1530)0Ξ̄(1530)0pair production at BESIII

The study of the baryon–antibaryon pair production from the charmonium states provides a favorable test and an input for the perturbative quantum chromodynamics calculation. Based on a Monte Carlo study via a single tag baryon strategy, a clear observation of [Formula: see text](3686)[Formula: see text](1530)[Formula: see text](1530)0is predicted at BESIII with the current 3.08[Formula: see text]106[Formula: see text](3686) dataset. The averaged precision of the [Formula: see text] parameters in the decay angular distribution formula is estimated as 0.25, and is not effective to separate the theoretical predictions. It will be an opportunity to distinguish them with 3.0[Formula: see text]109[Formula: see text](3686) events taken by BESIII detector in the next few years.

Theoretical determination of the hadronic g − 2 of the muon

An approach is discussed on the determination of the leading order hadronic contribution to the muon anomaly, [Formula: see text], based entirely on theory. This method makes no use of [Formula: see text] annihilation data, a likely source of the current discrepancy between theory and experiment beyond the 3[Formula: see text] level. What this method requires is essentially knowledge of the first derivative of the vector current correlator at zero-momentum. In the heavy-quark sector, this is obtained from the well-known heavy-quark expansion in perturbative quantum chromodynamics (QCD), leading to values of [Formula: see text] in the charm- and bottom-quark region which were fully confirmed by later lattice QCD (LQCD) results. In the light-quark sector, using recent preliminary LQCD results for the first derivative of the vector current correlator at zero-momentum leads to the value [Formula: see text], which is significantly larger than values obtained from using [Formula: see text] data. A separate approach based on the operator product expansion (OPE), and designed to quench the contribution of these data, reduces the discrepancy by at least 40%. In addition, it exposes a tension between the OPE and [Formula: see text] data, thus suggesting the blame for the discrepancy on the latter.