Factorization Breaking Effects Research Articles

Photoproduction of diffractive dijets in Pythia 8

The new framework for the simulations of hard diffractive events in photoproduction within Pythia 8 is presented. The model, originally introduced for proton-proton collisions, applies the dynamical rapidity gap survival probability based on the multiparton interaction model in Pythia. These additional interactions provide a natural explanation for the observed factorization-breaking effects in hard diffraction by filling up the rapidity gaps used to classify the events of diffractive origin. The generated cross sections are well in line with the existing data from HERA experiments and predictions for the future electron-ion collider are presented. In addition, also predictions for ultra-peripheral collisions at the LHC have been calculated where the factorization-breaking effects are found to be more pronounced.

Factorization and its breaking in dijet single transverse spin asymmetries in pp collisions

We study factorization in single transverse spin asymmetries for dijet production in proton-proton collisions, by considering soft gluon radiation at one-loop order. We show that the associated transverse momentum dependent (TMD) factorization is valid at the leading logarithmic level. At next-to-leading-logarithmic (NLL) accuracy, however, we find that soft gluon radiation generates terms in the single transverse spin dependent cross section that differ from those known for the unpolarized case. As a consequence, these terms cannot be organized in terms of a spin independent soft factor in the factorization formula. We present leading logarithmic predictions for the single transverse spin dijet asymmetry for $pp$ collisions at RHIC, based on quark Sivers functions constrained by semi-inclusive deep inelastic scattering data. We hope that our results will contribute to a better understanding of TMD factorization breaking effects at NLL accuracy and beyond.

γ*γ*→ηc(1S,2S) transition form factors for spacelike photons

We derive the light-front wave function (LFWF) representation of the $\gamma^* \gamma^* \to \eta_c(1S)\,,\eta_c(2S)$ transition form factor $F(Q_1^2, Q_2^2)$ for two virtual photons in the initial state. For the LFWF, we use different models obtained from the solution of the Schr\"odinger equation for a variety of $c \bar c$ potentials. We compare our results to the BaBar experimental data for the $\eta_c(1S)$ transition form factor, for one real and one virtual photon. We observe that the onset of the asymptotic behaviour is strongly delayed and discuss applicability of the collinear and/or massless limit. We present some examples of two-dimensional distributions for $F (Q_1^2,Q_2^2)$. A factorization breaking measure is proposed and factorization breaking effects are quantified and shown to be almost model independent. Factorization is shown to be strongly broken, and a scaling of the form factor as a function of $\bar Q^2 = (Q_1^2 + Q_2^2)/2$ is obtained.

Nonperturbative transverse-momentum-dependent effects in dihadron and direct photon-hadron angular correlations in p+p collisions at s=200 GeV

Dihadron and isolated direct photon-hadron angular correlations are measured in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV. The correlations are sensitive to nonperturbative initial-state and final-state transverse momentum $k_T$ and $j_T$ in the azimuthal nearly back-to-back region $\Delta\phi\sim\pi$. In this region, transverse-momentum-dependent evolution can be studied when several different hard scales are measured. To have sensitivity to small transverse momentum scales, nonperturbative momentum widths of $p_{\rm out}$, the out-of-plane transverse momentum component perpendicular to the trigger particle, are measured. These widths are used to investigate possible effects from transverse-momentum-dependent factorization breaking. When accounting for the longitudinal momentum fraction of the away-side hadron with respect to the near-side trigger particle, the widths are found to increase with the hard scale; this is qualitatively similar to the observed behavior in Drell-Yan and semi-inclusive deep-inelastic scattering interactions. The momentum widths are also studied as a function of center-of-mass energy by comparing to previous measurements at $\sqrt{s}=510$ GeV. The nonperturbative jet widths also appear to increase with $\sqrt{s}$ at a similar $x_T$, which is qualitatively consistent to similar measurements in Drell-Yan interactions. To quantify the magnitude of any transverse-momentum-dependent factorization breaking effects, calculations will need to be performed to compare to these measurements.

ψ(2S) versus J/ψ suppression in proton-nucleus collisions from factorization violating soft color exchanges

We argue that the large suppression of the $\psi(2S)$ inclusive cross-section relative to the $J/\psi$ inclusive cross-section in proton-nucleus (p+A) collisions can be attributed to factorization breaking effects in the formation of quarkonium. These factorization breaking effects arise from soft color exchanges between charm-anticharm pairs undergoing hadronization and comoving partons that are long-lived on time scales of quarkonium formation. We compute the short distance pair production of heavy quarks in the Color Glass Condensate (CGC) effective field theory and employ an improved Color Evaporation model (ICEM) to describe their hadronization into quarkonium at large distances. The combined CGC+ICEM model provides a quantitative description of $J/\psi$ and $\psi(2S)$ data in proton-proton (p+p) collisions from both RHIC and the LHC. Factorization breaking effects in hadronization, due to additional parton comovers in the nucleus, are introduced heuristically by imposing a cutoff $\Lambda$, representing the momentum kick from soft color exchanges, in the ICEM model. Such soft exchanges have no perceptible effect on $J/\psi$ suppression in p+A collisions. In contrast, the interplay of the physics of these soft exchanges at large distances, with the physics of semi-hard rescattering at short distances, causes a significant additional suppression of $\psi(2S)$ yields relative to that of the $J/\psi$. A good fit of all RHIC and LHC $J/\psi$ and $\psi(2S)$ data, for transverse momenta $P_\perp\leq 5$ GeV in p+p and p+A collisions, is obtained for $\Lambda\sim 10$ MeV.

Hadronic decays of B→a1(1260)b1(1235) in the perturbative QCD approach

We calculate the branching ratios and polarization fractions of the $B\ensuremath{\rightarrow}{a}_{1}{b}_{1}$ decays in the perturbative QCD(pQCD) approach at leading order, where ${a}_{1}({b}_{1})$ stands for the axial-vector ${a}_{1}(1260)[{b}_{1}(1235)]$ state. By combining the phenomenological analyses with the perturbative calculations, we find the following results: (a) the large decay rates around ${10}^{\ensuremath{-}5}$ to ${10}^{\ensuremath{-}6}$ of the $B\ensuremath{\rightarrow}{a}_{1}{b}_{1}$ decays dominated by the longitudinal polarization(except for the ${B}^{+}\ensuremath{\rightarrow}{b}_{1}^{+}{a}_{1}^{0}$ mode) are predicted and basically consistent with those in the QCD factorization(QCDF) within errors, which are expected to be tested by the Large Hadron Collider and Belle-II experiments. The large ${B}^{0}\ensuremath{\rightarrow}{a}_{1}^{0}{b}_{1}^{0}$ branching ratio could provide hints to help explore the mechanism of the color-suppressed decays. (b) the rather different QCD behaviors between the ${a}_{1}$ and ${b}_{1}$ mesons result in the destructive(constructive) contributions in the nonfactorizable spectator diagrams with ${a}_{1}({b}_{1})$ emission. Therefore, an interesting pattern of the branching ratios appears for the color-suppressed ${B}^{0}\ensuremath{\rightarrow}{a}_{1}^{0}{a}_{1}^{0},{a}_{1}^{0}{b}_{1}^{0}$, and ${b}_{1}^{0}{b}_{1}^{0}$ modes in the pQCD approach, $\mathrm{BR}({B}^{0}\ensuremath{\rightarrow}{b}_{1}^{0}{b}_{1}^{0})>\mathrm{BR}({B}^{0}\ensuremath{\rightarrow}{a}_{1}^{0}{b}_{1}^{0})\ensuremath{\gtrsim}\phantom{\rule{0ex}{0ex}}\mathrm{BR}({B}^{0}\ensuremath{\rightarrow}{a}_{1}^{0}{a}_{1}^{0})$, which is different from $\mathrm{BR}({B}^{0}\ensuremath{\rightarrow}{b}_{1}^{0}{b}_{1}^{0})\ensuremath{\sim}\mathrm{BR}({B}^{0}\ensuremath{\rightarrow}{a}_{1}^{0}{b}_{1}^{0})\ensuremath{\gtrsim}\mathrm{BR}({B}^{0}\ensuremath{\rightarrow}{a}_{1}^{0}{a}_{1}^{0})$ in the QCDF and would be verified at future experiments. (c) the large naive factorization breaking effects are observed in these $B\ensuremath{\rightarrow}{a}_{1}{b}_{1}$ decays. Specifically, the large nonfactorizable spectator(weak annihilation) amplitudes contribute to the ${B}^{0}\ensuremath{\rightarrow}{b}_{1}^{+}{a}_{1}^{\ensuremath{-}}({B}^{+}\ensuremath{\rightarrow}{a}_{1}^{+}{b}_{1}^{0}\phantom{\rule[-0.0ex]{1em}{0.0ex}}\text{and}\phantom{\rule[-0.0ex]{1em}{0.0ex}}{B}^{+}\ensuremath{\rightarrow}{b}_{1}^{+}{a}_{1}^{0})$ mode(s), which demand confirmations via the precise measurements. Furthermore, the different phenomenologies shown among $B\ensuremath{\rightarrow}{a}_{1}{b}_{1}$, $B\ensuremath{\rightarrow}{a}_{1}{a}_{1}$, and $B\ensuremath{\rightarrow}{b}_{1}{b}_{1}$ decays are also expected to be tested stringently, which could shed light on the typical QCD dynamics involved in these modes, even further distinguish those two popular pQCD and QCDF approaches.

Nonperturbative-transverse-momentum effects and evolution in dihadron and direct photon-hadron angular correlations in p+p collisions at s=510 GeV

Dihadron and isolated direct photon-hadron angular correlations are measured in $p$$+$$p$ collisions at $\sqrt{s}=510$ GeV. Correlations of charged hadrons of $0.7<p_T<10$ GeV/$c$ with $\pi^0$ mesons of $4<p_T<15$ GeV/$c$ or isolated direct photons of $7<p_T<15$ GeV/$c$ are used to study nonperturbative effects generated by initial-state partonic transverse momentum and final-state transverse momentum from fragmentation. The nonperturbative behavior is characterized by measuring the out-of-plane transverse momentum component $p_{\rm out}$ perpendicular to the axis of the trigger particle, which is the high-$p_T$ direct photon or $\pi^0$. Nonperturbative evolution effects are extracted from Gaussian fits to the away-side inclusive-charged-hadron yields for different trigger-particle transverse momenta ($p_T^{\rm trig}$). The Gaussian widths and root mean square of $p_{\rm out}$ are reported as a function of the interaction hard scale $p_T^{\rm trig}$ to investigate possible transverse-momentum-dependent evolution differences between the $\pi^0$-h$^\pm$ and direct photon-h$^\pm$ correlations and factorization breaking effects. The widths are found to decrease with $p_T^{\rm trig}$, which indicates that the Collins-Soper-Sterman soft factor is not driving the evolution with the hard scale in nearly back-to-back dihadron and direct photon-hadron production in $p$$+$$p$ collisions. This behavior is in contrast to Drell-Yan and semi-inclusive deep-inelastic scattering measurements.

Single-diffractive Drell\u2013Yan pair production at the LHC

We present predictions for single-diffractive low-mass Drell–Yan pair production in pp collisions at the LHC at sqrt{s}=13 TeV. Predictions are obtained adopting a factorised form for the relevant cross sections and are based on a new set of diffractive parton distributions resulting from the QCD analysis of combined HERA leading proton data. We discuss a number of observables useful to characterise the expected factorisation breaking effects.

Shape and flow fluctuations in ultracentral Pb + Pb collisions at the energies available at the CERN Large Hadron Collider

In ultracentral heavy-ion collisions, anisotropic hydrodynamic flow is generated by density fluctuations in the initial state rather than by geometric overlap effects. For a given centrality class, the initial fluctuation spectrum is sensitive to the method chosen for binning the events into centrality classes. We show that sorting events by total initial entropy or by total final multiplicity yields event classes with equivalent statistical fluctuation properties, in spite of viscous entropy production during the fireball evolution. With this initial entropy-based centrality definition we generate several classes of ultracentral Pb + Pb collisions at Cern Large Hadron Collider energies and evolve the events using viscous hydrodynamics with nonzero shear but vanishing bulk viscosity. Comparing the predicted anisotropic flow coefficients for charged hadrons with CMS data we find that both the Monte Carlo Glauber (MC-Glb) and Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) models produce initial fluctuation spectra that are incompatible with the measured final anisotropic flow power spectrum, for any choice of the specific shear viscosity. In spite of this failure, we show that the hydrodynamic model can qualitatively explain, in terms of event-by-event fluctuations of the anisotropic flow coefficients and flow angles, the breaking of flow factorization for elliptic, triangular, and quadrangular flow measured by the CMS experiment. For elliptic flow, this factorization breaking is large in ultracentral collisions. We conclude that the bulk of the experimentally observed flow factorization breaking effects are qualitatively explained by hydrodynamic evolution of initial-state fluctuations, but that their quantitative description requires a better understanding of the initial fluctuation spectrum.

Factorisation Breaking in Diffractive Dijets Photoproduction at HERA

Recent experimental data on dijet cross sections in diffractive photoproduction at HERA collider are analysed with an emphasis on QCD factorisation breaking effects. The possible sources of the different conclusions of H1 and ZEUS collaborations are studied.

Factorisation breaking in diffractive dijet photoproduction at HERA?

Recent experimental data on dijet cross sections in diffractive photoproduction at HERA collider are analysed with an emphasis on QCD factorisation breaking effects. The possible sources of the different conclusions of H1 and ZEUS collaborations are studied.

Factorisation in diffraction

Recent experimental data on dijet cross sections in diffractive photoproduction at HERA collider were analysed with emphasis on QCD factorisation breaking effects. Dijet photoproduction data of jets with low transverse energy E T1 > 5 GeV indicated a possible dependence on E T of the suppression of jet production with respect to the NLO QCD predictions. However, this suppression exhibited no significant dependence on the fraction x γ of the photon four-momentum entering the hard process. The ratio of diffractive to inclusive cross section for dijets in photoproduction was also measured for the first time.

REVIEW OF FACTORIZATION BREAKING IN DIFFRACTIVE PHOTOPRODUCTION OF DIJETS

After the final analyses of the H1 and ZEUS collaborations for the diffractive photoproduction of dijets have appeared, we have recalculated these cross sections in next-to-leading order (NLO) of perturbative QCD to see whether they can be interpreted consistently. The results of these calculations are compared to the data of both collaborations. We find that at NLO the cross sections disagree with the data, showing that factorization breaking occurs at this order. If direct and resolved contributions are both suppressed by the same amount, the global suppression factor depends on the transverse-energy cut and is 0.42 for the H1 and 0.71 for the ZEUS analysis. However, by suppressing only the resolved contribution by a factor of approximately three, also reasonably good agreement with all the data is found. The size of the factorization breaking effects for resolved photons agrees with absorptive-model predictions.

Factorization breaking in diffractive dijet photoproduction

We have calculated the diffractive dijet cross section in low-Q 2 ep scattering in the HERA regime. The results of the calculation in LO and NLO are compared to recent experimental data of the H1 Collaboration. We find that in LO the calculated cross sections are in reasonable agreement with the experimental results. In NLO, however, some of the cross sections disagree, showing that factorization breaking occurs in that order. By suppressing the resolved contribution by a factor of approximately three, good agreement with all the data is found. The size of the factorization breaking effects in diffractive dijet photoproduction agrees well with absorptive model predictions.

New ways to explore factorization in b decays

We propose to study factorization breaking effects in exclusive b decays where they are strongly enhanced over the factorizing contributions. This can be done by selecting final-state mesons with a small decay constant or with spin greater than one. We find a variety of decay modes which could help understand the dynamical origin of factorization and the mechanisms responsible for its breaking.