publications

2026

1. Orbital angular momentum at small x in the large N_c&N_f limit

   G. Zardo Becker, Yuri Kovchegov, Ming Li, and 2 more authors

   Aug 2026

2025

1. 

   Elastic dijet production in electron scattering on a longitudinally polarized proton at small \(x\): A portal to orbital angular momentum distributions

   Yuri V. Kovchegov, and Brandon Manley

   Phys. Rev. D, Aug 2025

   Abstract arXiv

   We calculate the elastic production of dijets from electron collisions with a longitudinally polarized proton target at small values of the Bjorken \(x\,\)variable. Building on the pioneering proposals of Hatta et al. and Bhattacharya et al. for measuring the quark and gluon orbital angular momentum (OAM) distributions, our focus is on both the longitudinal double spin asymmetry (DSA) and longitudinal single spin asymmetry (SSA). We compute the numerators of these asymmetries in the small-\(x\,\)formalism of the light-cone operator treatment. Utilizing the small-\(x\,\)expressions for the OAM distributions derived in our earlier paper, we demonstrate that the DSA provides a robust probe for both the quark and gluon OAM distributions within the proton. In contrast, we find that while the SSA is also sensitive to the OAM distributions, extraction of the latter from the SSA would require new developments in small-\(x\,\)theory and phenomenology, and is probably not feasible at this point in time. These findings highlight the potential of DSA measurements in elastic dijet production at the future Electron-Ion Collider to provide the first-ever direct access to the quark and gluon OAM distributions at small \(x\,\), paving the way for new insights into the proton spin puzzle.

2024

1. 

   Orbital angular momentum small-\(x\,\)evolution: Exact results in the large- \(N_c\,\)limit

   Brandon Manley

   JHEP, Jan 2024

   Abstract arXiv

   We construct an exact solution to the revised small-x orbital angular momentum (OAM) evolution equations derived in an earlier work. These equations are derived in the double logarithmic approximation (summing powers of \(\alpha_s \ln^2(1/x) \)with \(\alpha_s\,\)the strong coupling constant and \(x\,\)the Bjorken \(x\,\)variable) and the large-\(N_c\,\)limit, with \(N_c\,\)the number of quark colors. From our solution, we extract the small-\(x\), large-\(N_c\,\)expressions of the quark and gluon OAM distributions. Additionally, we determine the large-\(N_c\,\)small-\(x\,\)asymptotics of the OAM distributions to be the same as obtained in the small-\(x\,\)helicity evolution, which can be approximated as \(\alpha_h ≈3.66074\,\). This result is in complete agreement with the literature. Additionally, we calculate the ratio of the quark and gluon OAM distributions to the flavor-singlet quark and gluon helicity parton distribution functions respectively in the small-x region

2023

1. 

   Orbital angular momentum at small \(x\,\)revisited

   Yuri V. Kovchegov, and Brandon Manley

   JHEP, Oct 2023

   Abstract arXiv

   We revisit the problem of the small Bjorken-\(x\,\)asymptotics of the quark and gluon orbital angular momentum (OAM) distributions in the proton utilizing the revised small-\(x\,\)helicity evolution derived recently. We relate the quark and gluon OAM distributions at small \(x\,\)to the polarized dipole amplitudes and their (first) impact-parameter moments. To obtain the OAM distributions, we derive novel small-\(x\,\)evolution equations for the impact-parameter moments of the polarized dipole amplitudes in the double-logarithmic approximation (summing powers of \(\alpha_s \ln^2(1/x) \)with \(\alpha_s\,\)the strong coupling constant). We solve these evolution equations numerically and extract the leading large-\(N_c\), small-\(x\,\)asymptotics of the quark and gluon OAM distributions, which we determine to be in agreement with the existing results in the literature within the precision of our numerical evaluation. (Here \(N_c\,\)is the number of quark colors.) We also investigate the ratios of the quark and gluon OAM distributions to their helicity distribution counterparts in the small-\(x\,\)region.

2021

1. 

   Sensitivity to millicharged particles in future proton-proton collisions at the LHC with the milliQan detector

   A. Ball, J. Brooke, C. Campagnari, and 26 more authors

   Phys. Rev. D, Aug 2021

   Abstract arXiv

   We report on the expected sensitivity of dedicated scintillator-based detectors at the LHC for elementary particles with charges much smaller than the electron charge. The dataset provided by a prototype scintillator-based detector is used to characterise the performance of the detector and provide an accurate background projection. Detector designs, including a novel slab detector configuration, are considered for the data taking period of the LHC to start in 2022 (Run 3) and for the high luminosity LHC. With the Run 3 dataset, the existence of new particles with masses between 10 MeV and 45 GeV could be excluded at 95% confidence level for charges between 0.003e and 0.3e, depending on their mass. With the high luminosity LHC dataset, the expected limits would reach between 10 MeV and 80 GeV for charges between 0.0018e and 0.3e, depending on their mass

2020

1. 

   Search for millicharged particles in proton-proton collisions at \( \sqrt s = 13\,\)TeV

   A. Ball, G. Beauregard, J. Brooke, and 35 more authors

   Phys. Rev. D, Aug 2020

   Abstract arXiv

   We report on a search for elementary particles with charges much smaller than the electron charge using a data sample of proton-proton collisions provided by the CERN Large Hadron Collider in 2018, corresponding to an integrated luminosity of 37.5 inverse femptobarns at a center-of-mass energy of 13 TeV. A prototype scintillator-based detector is deployed to conduct the first search at a hadron collider sensitive to particles with charges ≤0.1e. The existence of new particles with masses between 20 and 4700 MeV is excluded at 95% confidence level for charges between 0.006e and 0.3e, depending on their mass. New sensitivity is achieved for masses larger than 700 MeV.