Particles In Hadronic Collisions Research Articles

Recent Findings from Heavy-Flavor Angular Correlation Measurements in Hadronic Collisions

The study of angular correlations of heavy-flavor particles in hadronic collisions can provide crucial insight into the heavy quark production, showering, and hadronization processes. The comparison with model predictions allows us to discriminate among different approaches for heavy quark production and hadronization, as well as different treatments of the underlying event employed by the models to reproduce correlation observables. In ultra-relativistic heavy-ion collisions, where a deconfined state of matter, the quark–gluon plasma (QGP), is created, heavy-flavor correlations can shed light on the modification of the heavy quark fragmentation due to the interaction between charm and beauty quarks with the QGP constituents, as well as characterize their energy loss processes while traversing the medium. Insight into the possible emergence of collective-like mechanisms in smaller systems, resembling those observed in heavy-ion collisions, can also be obtained by performing correlation studies in high-multiplicity proton–proton and proton–nucleus collisions. In this review, the most recent and relevant measurements of heavy-flavor correlations performed in all collision systems at the LHC and RHIC will be presented, and the new understandings that they provide will be discussed.

Collectivity from interference

In hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant αs, it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics, e.g., in the analysis of azimuthal anisostropy coefficients vn at the LHC. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of m particles in hadronic collisions with N parton-parton interactions (“sources”). In an expansion in powers of 1/(Nc2 − 1) and to leading order in the number of sources N , we calculate interference effects in the m-particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies vn{2} and vn{4}. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for vn{2} and vn{4}, n even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured vn coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

A comprehensive framework for studying W\u2032 and Z\u2032 bosons at hadron colliders with automated jet veto resummation

The production of high-mass, color-singlet particles in hadron collisions is universally accompanied by initial state QCD radiation that is predominantly soft with respect to the hard process scale Q and/or collinear with respect to the beam axis. At TeV-scale colliders, this is in contrast to top quark and multijet processes, which are hard and central. Consequently, vetoing events with jets possessing transverse momenta above pTVeto in searches for new color-singlet states can efficiently reduce non-singlet backgrounds, thereby increasing experimental sensitivity. To quantify this generic observation, we in-vestigate the production and leptonic decay of a Sequential Standard Model W′ boson at the 13 TeV Large Hadron Collider. We systematically consider signal and background processes at next-to-leading-order (NLO) in QCD with parton shower (PS) matching. For color-singlet signal and background channels, we resum Sudakov logarithms of the form αsj(pTVeto) logk(Q/pTVeto) up to next-to-next-to-leading logarithmic accuracy (NNLL) with NLO matching. We obtain our results using the MadGraph5_aMC@NLO and MadGraph5_aMC@NLO-SCET frameworks, respectively. Associated Universal Feyn-Rules Output model files capable of handling NLO+PS- and NLO+NNLL-accurate computations are publicly available. We find that within their given uncertainties, both the NLO+PS and NLO+NNLL(veto) calculations give accurate and consistent predictions. Consequently, jet vetoes applied to color-singlet processes can be reliably modeled at the NLO+PS level. With respect to a b-jet veto of pTVeto = 30 GeV, flavor-agnostic jet vetoes of pTVeto = 30 − 40 GeV can further reduce single top and toverline{t} rates by a factor of 2-50 at a mild cost of the signal rate. Jet vetoes can increase the signal-to-noise ratios by roughly 10% for light W′ boson masses of 30 − 50 GeV and 25%-250% for masses of 300-800 GeV.

Non-relativistic high-energy physics: top production and dark matter annihilation

Non-relativistic physics is often associated with atomic physics and low-energy phenomena of the strong interactions between nuclei and quarks. In this review we cover three topics in contemporary high-energy physics at or close to the TeV scale, where non-relativistic dynamics plays an important if not defining role. We first discuss in detail the third-order corrections to top-quark pair production in electron-positron collisions in the threshold region, which plays a major role at a future high-energy e+e− collider. Threshold effects are also relevant in the production of heavy particles in hadronic collisions, where in addition to the Coulomb force soft gluon radiation contributes to enhanced quantum corrections. We review the joint resummation of non-relativistic and soft gluon effects for pair production of top quarks and supersymmetric particles to next-to-next-to-leading logarithmic accuracy. The third topic deals with pair annihilation of dark matter particles within the framework of the Minimal Supersymmetric Standard Model. Here the electroweak Yukawa force generated by the exchange of gauge and Higgs bosons can cause large “Sommerfeld” enhancements of the annihilation cross section in some parameter regions.

Polarized gluon studies with charmonium and bottomonium at LHCb and AFTER

Recently it has been put forward that linearly polarized gluons inside unpolarized protons affect the transverse momentum distribution of final state particles in hadronic collisions. They lead to a characteristic modulation of the differential cross section in Higgs production and to azimuthal asymmetries in, for instance, heavy quark pair production. Here we study the effect on charmonium and bottomonium production in hadronic collisions, such as at LHCb and at the proposed fixed target experiment AFTER at LHC. We focus mainly on the scalar and pseudoscalar quarkonia, eta_c, chi_{c0}, eta_b, chi_{b0}, which allow for an angular independent investigation. Within the framework of transverse momentum dependent factorization in combination with the nonrelativistic QCD based color-singlet quarkonium model, we show for small transverse momentum (q_T^2 \ll 4 M_Q^2) that the scalar and pseudoscalar quarkonium production cross sections are modified in different ways by linearly polarized gluons, while their effects on the production of higher angular momentum quarkonium states are strongly suppressed. Comparisons to chi_{c2}, chi_{b2} production can help to cancel out uncertainties. Together with the analogous study in Higgs production at LHC, quarkonium production can moreover be used to test the scale dependence of the linearly polarized gluon distribution over a large energy range.

Threshold resummation for pair production of coloured heavy (s)particles at hadron colliders

We derive a factorization formula for the production of pairs of heavy coloured particles in hadronic collisions near the production threshold that establishes factorization of soft and Coulomb effects. This forms the basis for a combined resummation of Coulomb and soft corrections, including the non-trivial interference of the two effects. We develop a resummation formalism valid at NNLL accuracy using the momentum-space approach to soft gluon resummation. We present numerical results for the NLL resummed squark–antisquark production cross section at the LHC and Tevatron, including also the contribution of squark–antisquark bound states below threshold. The total correction on top of the next-to-leading order approximation is found to be sizeable, and amounts to (4–20)% in the squark mass region 200 GeV–3 TeV at the 14 TeV LHC. The scale dependence of the total cross section is also reduced.

Threshold expansion of massive coloured particle cross sections

Pair production of massive coloured particles in hadron collisions is accompanied by potentially large radiative corrections related to the suppression of soft gluon emission and enhanced Coulomb exchange near the production threshold. We recently developed a framework to sum both series of corrections for the partonic cross section using soft-collinear and non-relativistic effective theory. If it can be argued that the resummed cross section approximates the complete result over a significant kinematic range, an improvement of the hadronic cross section results, even when the production is not kinematically constrained to the threshold. This is discussed here for the case of top quark production.

Single-inclusive production of large-p T charged particles in hadronic collisions at TeV energies and perturbative QCD predictions

The single inclusive spectrum of charged particles with transverse momenta pT=3-150 GeV/c measured at midrapidity by the CDF experiment in proton-antiproton (p-pbar) collisions at sqrt(s)=1.96 TeV is compared to next-to-leading order (NLO) perturbative QCD calculations using the most recent parametrizations of the parton distributions and parton-to-hadron fragmentation functions. Above pT~20 GeV/c, there is a very sizeable disagreement of the Tevatron data compared to the NLO predictions and to xT-scaling expectations, suggesting a problem in the experimental data. We also present the predictions for the pT-differential charged hadron spectra and the associated theoretical uncertainties for proton-proton (p-p) collisions at LHC energies (sqrt(s)=0.9-14 TeV). Two procedures to estimate the charged hadron spectra at LHC heavy-ion collision energies (sqrt(s)=2.76,5.5 TeV) from p-p measurements are suggested.

Hadronic charm production by gluon fusion

Rates for production of charmed particles in hadronic collisions are calculated in the gluon-fusion model. Aspects studied include the following: a comparison of the results obtained for different gluon distributions, angular and energy distributions expected for the produced charmed quarks, and dependence of the results on the mass of the "charmed" quark (this allows comparison of the rates for particles containing $s$, $c$, or $b$ quarks).

Strong production mechanisms and total cross sections ofψparticles

New BNL and Fermilab data on the production of $\ensuremath{\psi}$ particles in hadron collisions are shown to imply that the intrinsic production cross section of a particle of mass $m$ varies as $\frac{1}{{m}^{2}}$ and scales in the variable $\frac{({s}^{\frac{1}{2}}\ensuremath{-}s_{\mathrm{threshold}}^{}{}_{}{}^{\frac{1}{2}})}{m}$. Vector-proton total cross sections are also observed to be proportional to $\frac{1}{{m}^{2}}$, and a connection between production mechanisms and total cross sections is pointed out. On the basis of the existence of high-mass diffraction dissociation in hadron collisions, explicit estimates for diffractive production of $\ensuremath{\psi}$ are also given. Our calculations are also directly applicable to the hypothetical charmed particles.