Higher-twist Processes Research Articles

Multiple scattering and parton energy loss in nuclei

When a fast parton propagates in nuclear medium it may suffer multiple scattering with other partons, which leads to two effects: transverse momentum broadening and jet quenching. In this article, we review the theoretical development of the transverse momentum broadening in the nuclear medium based on the pQCD factorization of higher twist processes, and discuss the implementation of the parton energy loss within higher twist expansion framework in the calculation of the leading hadron productions in relativistic heavy ion collisions at the RHIC and the LHC. We introduce the techniques of next-to-leading order (NLO) calculation for twist-4 process, and verify, for the first time, the universality of nuclear medium property, and we also identify the QCD evolution equation for the nuclear medium. We apply the high twist factorization formalism to calculate the hadron production at backward rapidity in proton-nucleus collisions at RHIC and the LHC, and find that the nuclear enhancement phenomena observed in experiments can be explained as incoherent double scattering. We further investigate parton energy loss effect in the framework of higher twist expansion on the leading hadron production in heavy ion collisions up to the accuracy of NLO pQCD, and calculate the yields and the nuclear modification factors of $\pi^0$, $\eta$, $\rho^0$, $\phi$, $K^0_{\rm~S}$ and $\omega$, and discuss the ratios of the identified hadron yields. By comparing our theoretical results with the latest experimental data on the leading hardon spectra in heavy ion collisions, we also extrapolate the jet transport coefficient. It is found the identified hadron yields in A+A collisions depend on three factors: the initial hard jet spectrum and the energy loss mechanism as well as parton fragmentation functions to the hadron in vacuum.

Studying possible higher twist contributions in the inclusive charged hadron cross sections

In the standard pQCD picture particles are produced via the parton jet fragmentation process. However, there are also other production mechanisms like higher twist (HT) processes. A usual example of a HT process is a direct production of an outgoing hadron, where the hadron is produced in the hard subprocess without fragmentation.We study the HT phenomena using a shape analysis (xT scaling) of the inclusive invariant cross sections of charged hadrons, measured by the ALICE collaboration at center-of-mass energies = 2.76 TeV and 7 TeV. The data is compared to PYTHIA8 event generator and to a phenomenological model for HT. Using PYTHIA8, we explore a possible enhancement of HT phenomena for isolated particles, by comparing the shapes of the isolated distributions to inclusive distributions. The results from the standard PYTHIA8, without HT, is compared to a PYTHIA8 where we had included a HT process.Finally, we found out that the effects observed in the xT spectra originate from kinematic biases posed by the isolation cuts, rather than from an enrichment of the HT hadrons at the observed cross sections. A more detailed data analysis is ongoing.

Direct hadron production in hadronic collisions

The scaling behavior of the invariant production cross section is a powerful tool in order to probe the dynamics of particle production. Here, we investigate the scaling properties of large- p ⊥ hadron, jet and prompt photon production in hadronic collisions by comparing systematically world data to NLO QCD predictions. In the hadron sector a significant discrepancy is reported, while prompt photon and jet production data prove in agreement with leadingtwist expectations. We interpret these results as coming from a non-negligible contribution of higher-twist processes, in which the hadron is produced directly in the hard subprocess. Predictions at RHIC are successfully compared to PHENIX preliminary measurements and LHC predictions are given.

Higher-Twist Dynamics in Large Transverse Momentum Hadron Production

A scaling law analysis of the world data on inclusive large-p(⊥) hadron production in hadronic collisions is carried out. Significant deviations from leading-twist perturbative QCD predictions at next-to-leading order are observed, particularly at high x(⊥)=2p(⊥)/sqrt[s]. In contrast, the production of prompt photons and jets exhibits near-conformal scaling behavior in agreement with leading-twist expectations. These results indicate a non-negligible contribution of higher-twist processes in large-p(⊥) hadron production, where the hadron is produced directly in the hard subprocess, rather than by quark and gluon fragmentation. Predictions for the scaling exponents at RHIC and LHC are given. Triggering on isolated large-p(⊥) hadron production will enhance the higher-twist processes. We also note that the use of isolated hadrons as a signal for new physics can be affected by the presence of direct hadron production.

Dynamic versus Static Hadronic Structure Functions

“Static” structure functions are the probabilistic distributions computed from the square of the light-front wavefunctions of the target hadron. In contrast, the “dynamic” structure functions measured in deep inelastic lepton-hadron scattering include the effects of rescattering associated with the Wilson line. Initial- and final-state rescattering, neglected in the parton model, can have a profound effect in QCD hard-scattering reactions, producing single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan reactions, nuclear shadowing, and non-universal nuclear antishadowing—novel leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also review how “direct” higher-twist processes – where a proton is produced in the hard subprocess itself – can explain the anomalous proton-to-pion ratio seen in high centrality heavy ion collisions.

The Conformal Template and New Perspectives for Quantum Chromodynamics

Conformal symmetry provides a systematic approximation to QCD in both its perturbative and nonperturbative domains. One can use the AdS/CFT correspondence between Anti-de Sitter space and conformal gauge theories to obtain an analytically tractable approximation to QCD in the regime where the QCD coupling is large and constant. For example, there is an exact correspondence between the fifth-dimensional coordinate of AdS space and a specific impact variable which measures the separation of the quark constituents within the hadron in ordinary space-time. This connection allows one to compute the analytic form of the frame-independent light-front wavefunctions of mesons and baryons, the fundamental entities which encode hadron properties and allow the computation of exclusive scattering amplitudes. One can also use conformal symmetry as a template for perturbative QCD predictions where the effects of the nonzero beta function can be systematically included in the scale of the QCD coupling. This leads to fixing of the renormalization scale and commensurate scale relations which relate observables without scale or scheme ambiguity. I also discuss a number of novel phenomenological features of QCD, including leading-twist single-spin asymmetries, diffractive deep inelastic scattering, hard diffractive hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan reactions, nuclear shadowing, non-universal antishadowing, and the unexpected effects of direct higher-twist processes.

Hard pion electroproduction at medium energies

Even at moderate energy machines, there is a regime where hard pion electroproduction proceeds by a perturbatively calculable process. The process, we claim, is not the leading twist fragmentation one but rather a higher twist process that produces kinematically isolated pions. Semiexclusive data may teach us more about parton distribution functions of the target and the pion distribution amplitude. In addition, there is a connection to generalized parton distribution calculations of exclusive processes in that the perturbative kernel is the same.

Coherent contributions of nuclear mesons to electroproduction and the HERMES effect

We show that nuclear σ, ω, and π mesons can contribute coherently to enhance the electroproduction cross section on nuclei for longitudinal virtual photons at low Q 2 while depleting the cross section for transverse photons. We are able to describe recent HERMES inelastic lepton–nucleus scattering data at low Q 2 and small x using photon–meson and meson–nucleus couplings which are consistent with (but not determined by) existing constraints from meson decay widths, nuclear structure, deep inelastic scattering, and lepton pair production data. We find that while nuclear-coherent pion currents are not important for the present data, they could be observed at different kinematics. Our model for coherent meson electroproduction requires the assumption of mesonic currents and couplings which can be verified in separate experiments. The observation of nuclear-coherent mesons in the final state would verify our theory and allow the identification of a specific dynamical mechanism for higher-twist processes.

Twist-4 nuclear parton distributions from photoproduction.

We analyze anomalous nuclear enhancement in the photoproduction of jets as a higher-twist process in perturbative QCD. We use the Fermilab E683 data on dijet momentum imbalance to estimate the size of the relevant twist-4 parton distributions. We find that twist-4 matrix elements are of the order of 0.05-0.1 ${\mathrm{GeV}}^{2}$ times typical twist-2 parton distributions. We discuss a physical interpretation of the nuclear enhancement as a measure of the average net transverse color force on an off-shell parton moving through nuclear matter, and give an order-of-magnitude estimate for the typical squared transverse field strengths encountered by a fast moving parton.

Evidence for higher twist mechanisms in highp ⊥π− p events with prompt ρ0 production at 38 GeV/c

The π−p interactions with at least one charged secondary produced at polar angle ∼90° in c.m.s. and having the transverse momentum above 1 GeV/c were investigated. The data were obtained using streamer chamber magnetic spectrometer RISK at 38 GeV/c π beam from Serpukhov accelerator. The analyzis of associated production in reconstructed events suggests, that if the transverse momentum of a pair of oppositely charged secondaries compensates the trigger particlep⊥ practically completely, this pair is the product of the ρ0 decay in marked fraction of such events. We observed a large spin-alignment for the ρ0-mesons selected as described above: the probability of zero spin projection onto the normal to the ρ0 production plane is equal to ρ00T=0.86±0.23. The enhancement of the number of events, in which the ρ0 picks up practically full momentum transfer carrying by the exchange, and also the enlarged tensor polarization for the ρ0-mesons in these events could be qualitatively explained as manifestation of direct ρ0-production via the QCD higher twist processes in the highp⊥π−p collisions. At the same time, the observed effects are markedly larger than the values predicted with QCD model in which the higher twist corrections were included.

Separation of minimum and higher twist in photoproduction of high-P T mesons

Photo- and hadroproduction data in the beam energy range 65–175 GeV have been studied with a view to isolating higher-twist processes in photoproduction from other point-like and hadron-like contributions. With selection of charged tracks havingp T >2 GeV/c and 0.28<x F <0.84 indications of a higher twist contribution have been found at a level that is consistent with QCD expectations.

Higher-twist effects in πp collisions at 200 GEV/C results from fermilab experiment E609

In a study of jet production in 200 GeV/c πp and pp collisions, we find evidence for pion-induced dijet events with little or no forward energy flow. The cross section and characteristics of these events are compatible with a predicted higher-twist process. No signal is seen in the pp data.

Evidence for higher twist mechanisms in prompt ϱ 0-meson production at PT > 2 GeV/ c in 300 GeV/ cΠ−N interactions

The production of ϱ 0 mesons in hadronic interactions is investigated at high transverse momentum ( p T > 2 GeV/ c). The data come from a sample of Π −N interactions at 300 GeV/ c obtained at the CERN OMEGA spectrometer, requiring a ϱ 0→Π +Π − with p T (pair) > 2 GeV/ c to recoil against an opposite particle with p T (opposite) > 1 GeV/ c . The signal/(signal + background) ratio of ϱ 0 production shows an increase of 38% as x E = p T (opposite)/ p T(pair) decreases from 0.5 < x E < 1 to x E ⩽ 0.5. This increase is not compatible with a QCD model of parton scattering and subsequent fragmentation. It is interpreted as evidence for direct ϱ 0 production via QCD higher twist processes, in terms of which a quantitative agreement with the data is found.

Evidence for higher-twist effects in hard pi p collisions at 200 GeV/c.

In a study of large-p/sub T/ jet production in 200-GeV/c ..pi..p and pp collisions, we find evidence for pion-induced dijet events with little or no forward energy flow. The rate and characteristics of these events are compatible with the predictions of Berger and Brodsky for higher-twist processes. No corresponding signal is seen in pp collisions.

Higher-twist contributions and hardon-jet correlations in photon-photon collisions

The QCD higher-twist process γq→ πq helps to understand the magnitude and the P T -dependence of the inclusive hadron cross section in γγ collisions. Triggering on both a fast hadron and the opposite-side jet can clarify the role of this contribution.