Study of Z boson plus jets events using variables sensitive to double-parton scattering in pp collisions at 13 TeV

Double-parton scattering is investigated using events with a Z boson and jets. The Z boson is reconstructed using only the dimuon channel. The measurements are performed with proton-proton collision data recorded by the CMS experiment at the LHC at $$ \sqrt{s} $$ s = 13 TeV, corresponding to an integrated luminosity of 35.9 fb−1 collected in the year 2016. Differential cross sections of Z+ ≥1 jet and Z+ ≥2 jets are measured with transverse momentum of the jets above 20 GeV and pseudorapidity |η| < 2.4. Several distributions with sensitivity to double-parton scattering effects are measured as functions of the angle and the transverse momentum imbalance between the Z boson and the jets. The measured distributions are compared with predictions from several event generators with different hadronization models and different parameter settings for multiparton interactions. The measured distributions show a dependence on the hadronization and multiparton interaction simulation parameters, and are important input for future improvements of the simulations.

The importance of kinematic twists and genuine saturation effects in dijet production at the Electron-Ion Collider

We compute the differential yield for quark anti-quark dijet production in high-energy electron-proton and electron-nucleus collisions at small x as a function of the relative momentum P⊥ and momentum imbalance k⊥ of the dijet system for different photon virtualities Q2, and study the elliptic and quadrangular anisotropies in the relative angle between P⊥ and k⊥. We review and extend the analysis in [1], which compared the results of the Color Glass Condensate (CGC) with those obtained using the transverse momentum dependent (TMD) framework. In particular, we include in our comparison the improved TMD (ITMD) framework, which resums kinematic power corrections of the ratio k⊥ over the hard scale Q⊥. By comparing ITMD and CGC results we are able to isolate genuine higher saturation contributions in the ratio Qs/Q⊥ which are resummed only in the CGC. These saturation contributions are in addition to those in the Weizsäcker-Williams gluon TMD that appear in powers of Qs/k⊥. We provide numerical estimates of these contributions for inclusive dijet production at the future Electron-Ion Collider, and identify kinematic windows where they can become relevant in the measurement of dijet and dihadron azimuthal correlations. We argue that such measurements will allow the detailed experimental study of both kinematic power corrections and genuine gluon saturation effects.

In-medium modification of dijets in PbPb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV

Modifications to the distribution of charged particles with respect to high transverse momentum (pT) jets passing through a quark-gluon plasma are explored using the CMS detector. Back-to-back dijets are analyzed in lead-lead and proton-proton collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 5.02 TeV via correlations of charged particles in bins of relative pseudorapidity and angular distance from the leading and subleading jet axes. In comparing the lead-lead and proton-proton collision results, modifications to the charged-particle relative distance distribution and to the momentum distributions around the jet axis are found to depend on the dijet momentum balance xj , which is the ratio between the subleading and leading jet pT. For events with xj ≈ 1, these modifications are observed for both the leading and subleading jets. However, while subleading jets show significant modifications for events with a larger dijet momentum imbalance, much smaller modifications are found for the leading jets in these events.

TMD factorization for dijet and heavy-meson pair in DIS

We study a transverse momentum dependent (TMD) factorization framework for the processes of dijet and heavy-meson pair production in deep-inelastic-scattering in an electron-proton collider, considering the measurement of the transverse momentum imbalance of the two hard probes in the Breit frame. For the factorization theorem we employ soft-collinear and boosted-heavy-quark effective field theories. The factorized cross-section for both processes is sensitive to gluon unpolarized and linearly polarized TMD distributions and requires the introduction of a new soft function. We calculate the new soft function here at one-loop, regulating rapidity divergences with the δ-regulator. In addition, using a factorization consistency relation and a universality argument regarding the heavy-quark jet function, we obtain the anomalous dimension of the new soft function at two and three loops.

The dijet mass distribution in heavy ion collisions

In these proceedings, we review the production of both light and heavy flavor dijets in heavy ion collisions and highlight a promising observable to expose their distinct signatures. We propose the modification of dijet invariant mass distributions in heavy ion collisions as a new observable that exhibits striking sensitivity to the heavy quark mass dependence of in-medium parton showers. This observable has the advantage of amplifying the effects of jet quenching in contrast to conventional observables, such as the dijet momentum imbalance shift, which involve cancellations of such effects and, hence, result in less pronounced signals. Predictions are presented for Au+Au collisions at √SNN = 200 GeV to guide the future sPHENIX program at the Relativistic Heavy Ion Collider.

Potential Vorticity Dynamics of Coastal Outflows

A simple quasigeostrophic model is used to examine the outflow from a river, estuary, or strait into a coastal ocean. As shown by Johnson et al., these quasigeostrophic outflows are accurately described by analytical long-wave solutions. This paper first uses these solutions and contour dynamics simulations to discuss the behavior of coastal outflows. Second, it extends the model and the long-wave theory to consider the effects of ambient currents, tides, winds, or a variable source flux. Third, consideration of the momentum flux at the source is used to understand the turning of the current, showing that steady solutions conserve momentum, hence resolving the momentum imbalance paradox of Pichevin and Nof. Finally, a new numerical scheme to compute steady outflow boundaries is developed. The model focuses on the key dynamics driven by the source velocity and the generation of vorticity as the buoyant fluid adjusts. The simplicity of the model, and insight given by the long-wave solutions, enables a full understanding of the dynamics. The outflows display a range of behaviors, including indefinitely growing near-source bulges, steady boundary profiles with varying offshore width, bidirectional currents, and rarefying or eddy-like leading heads, all of which can be understood with the long-wave theory. Despite the simplicity of the model, the results show good agreement in comparison with observations, experiments, and numerical models.