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Alberto Accardi

Publications and source records attributed to Alberto Accardi.

At least 37 records · Page 2Linked to original sources
arXiv2018

Transversity in inclusive DIS and novel TMD sum rules

A reanalysis of collinear factorization for inclusive Deep Inelastic Scattering shows that a novel, non perturbative spin-flip term associated with the invariant mass of the produced hadrons couples, at large enough Bjorken $x_B$, to the target's transversity distribution function. The resulting new contribution to the $g_2$ structure function can potentially explain the discrepancy between recent calculations and fits of this quantity. The new term also breaks the Burkhardt-Cottingham sum rule, now featuring an interplay between the $g_2$ and $h_1$ functions that calls for a re-examination of their small-$x$ behavior. As part of the calculation leading to these results, a new set of TMD sum rules is derived by relating the single-hadron quark fragmentation correlator to the fully dressed quark propagator by means of integration over the hadronic momenta and spins. A complete set of momentum sum rules is obtained for transverse-momentum-dependent quark fragmentation functions up to next-to-leading twist.

hep-ph↗
arXiv2018

Parton distributions and lattice QCD calculations: a community white paper

In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs.

hep-ph↗
arXiv2017

Hadron Mass Effects: Kaons at HERMES vs. COMPASS

Experimental data for integrated kaon multiplicities taken at HERMES and COMPASS measurements look incompatible with each other. In this talk, we investigate the effects of hadron masses calculated at leading-order and leading twist at the kinematics of these two experiments. We present evidence that Hadron Mass Corrections can fully reconcile the data for the $K^+/K^-$ multiplicity ratio, and can also sizeably reduce the apparent large discrepancy in the case of $K^+ + K^-$ data. Residual differences in the shape of the latter one remains to be understood.

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arXiv2017

Accessing the nucleon transverse structure in inclusive deep inelastic scattering

We revisit the standard analysis of inclusive Deep Inelastic Scattering off nucleons taking into account the fact that on-shell quarks cannot be present in the final state, but they rather decay into hadrons - a process that can be described in terms of suitable "jet" correlators. As a consequence, a spin-flip term associated with the invariant mass of the produced hadrons is generated non perturbatively and couples to the target's transversity distribution function. In inclusive cross sections, this provides an hitherto neglected and large contribution to the twist-3 part of the g2 structure function, that can explain the discrepancy between recent calculations and fits of this quantity. It also provides an extension of the Burkhardt-Cottingham sum rule, putting constraints on the small-x behavior of the transversity function, as well as an extension of the Efremov-Teryaev-Leader sum rule, suggesting a novel way to measure the tensor charge of the proton.

hep-ph↗
arXiv2017

Spectator-Tagged Deeply Virtual Compton Scattering on Light Nuclei

The three-dimensional picture of quarks and gluons in the proton is set to be revealed through Deeply virtual Compton scattering while a critically important puzzle in the one-dimensional picture remains, namely, the origins of the EMC effect. Incoherent nuclear DVCS, i.e. DVCS on a nucleon inside a nucleus, can reveal the 3D partonic structure of the bound nucleon and shed a new light on the EMC effect. However, the Fermi motion of the struck nucleon, off-shell effects and final-state interactions (FSIs) complicate this parton level interpretation. We propose here a measurement of incoherent DVCS with a tagging of the recoiling spectator system (nucleus A-1) to systematically control nuclear effects. Through spectator-tagged DVCS, a fully detected final state presents a unique opportunity to systematically study these nuclear effects and cleanly observe possible modification of the nucleon's quark distributions. We propose to measure the DVCS beam-spin asymmetries (BSAs) on $^4$He and deuterium targets. The reaction $^4$He$(e,e^{\prime}γ\,p\,^3$H$)$ with a fully detected final state has the rare ability to simultaneously quantify FSIs, measure initial nucleon momentum, and provide a sensitive probe to other nuclear effects at the parton level. The DVCS BSA on a (quasi-free) neutron will be measured by tagging a spectator proton with a deuteron target. Similarly, a bound neutron measurement detects a spectator $^3$He off a $^4$He target. These two observables will allow for a self-contained measurement of the neutron off-forward EMC Effect.

nucl-ex↗
arXiv2017

Tagged EMC Measurements on Light Nuclei

We propose to measure tagged deep inelastic scattering from light nuclei (deuterium and $^4$He) by detecting the low energy nuclear spectator recoil (p, $^3$H and $^3$He) in addition to the scattered electron. The proposed experiment will provide stringent tests leading to clear differentiation between the many models describing the EMC effect, by accessing the bound nucleon virtuality through its initial momentum at the point of interaction. Indeed, conventional nuclear physics explanations of the EMC effect mainly based on Fermi motion and binding effects yield very different predictions than more exotic scenarios, where bound nucleons basically loose their identity when embedded in the nuclear medium. By distinguishing events where the interacting nucleon was slow, as described by a mean field scenario, or fast, very likely belonging to a correlated pair, will clearly indicate which phenomenon is relevant to explain the EMC effect. An important challenge for such measurements using nuclear spectators is the control of the theoretical framework and, in particular, final state interactions. This experiment will directly provide the necessary data needed to test our understanding of spectator tagging and final state interactions in $^2$H and $^4$He and their impact on the semi-inclusive measurements of the EMC effect described above.

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arXiv2016

PDFs from nucleons to nuclei

I review recent progress in the extraction of unpolarized parton distributions in the proton and in nuclei from a unified point of view that highlights how the interplay between high energy particle physics and lower energy nuclear physics can be of mutual benefit to either field. Areas of overlap range from the search for physics beyond the standard model at the LHC, to the study of the non perturbative structure of nucleons and the emergence of nuclei from quark and gluon degrees of freedom, to the interaction of colored probes in a cold nuclear medium.

hep-ph↗
arXiv2015

The PDF4LHC report on PDFs and LHC data: Results from Run I and preparation for Run II

The accurate determination of the Parton Distribution Functions (PDFs) of the proton is an essential ingredient of the Large Hadron Collider (LHC) program. PDF uncertainties impact a wide range of processes, from Higgs boson characterisation and precision Standard Model measurements to New Physics searches. A major recent development in modern PDF analyses has been to exploit the wealth of new information contained in precision measurements from the LHC Run I, as well as progress in tools and methods to include these data in PDF fits. In this report we summarise the information that PDF-sensitive measurements at the LHC have provided so far, and review the prospects for further constraining PDFs with data from the recently started Run II. This document aims to provide useful input to the LHC collaborations to prioritise their PDF-sensitive measurements at Run II, as well as a comprehensive reference for the PDF-fitting collaborations.

hep-ph↗
arXiv2014

Interplay of Threshold Resummation and Hadron Mass Corrections in Deep Inelastic Processes

We discuss hadron mass corrections and threshold resummation for deep-inelastic scattering $\ell N\rightarrow\ell' X$ and semi-inclusive annihilation $e^+e^-\rightarrow h X$ processes, and provide a prescription how to consistently combine these two corrections respecting all kinematic thresholds. We find an interesting interplay between threshold resummation and target mass corrections for deep-inelastic scattering at large values of Bjorken $x_B$. In semi-inclusive annihilation, on the contrary, the two considered corrections are relevant in different kinematic regions and do not affect each other. A detailed analysis is nonetheless of interest in the light of recent high precision data from BaBar and Belle on pion and kaon production, with which we compare our calculations. For both deep inelastic scattering and single inclusive annihilation, the size of the combined corrections compared to the precision of world data is shown to be large. Therefore, we conclude that these theoretical corrections are relevant for global QCD fits in order to extract precise parton distributions at large Bjorken $x_B$, and fragmentation functions over the whole kinematic range.

hep-ph↗
arXiv2013

Large-x connections of nuclear and high-energy physics

I discuss how global QCD fits of parton distribution functions can make the somewhat separated fields of high-energy particle physics and lower energy hadronic and nuclear physics interact to the benefit of both. I review specific examples of this interplay from recent works of the CTEQ-Jefferson Lab collaboration, including hadron structure at large parton momentum and gauge boson production at colliders. I devote particular attention to quantifying theoretical uncertainties arising in the treatment of large partonic momentum contributions to deep inelastic scattering observables, and to discussing the experimental progress needed to reduce these.

hep-ph↗
arXiv2011

Subleading effects in QCD global fits

I discuss several corrections to leading twist calculations of nucleon structure functions which are needed to include experimental data at large parton fractional momentum x and at low scales Q^2 in global fits of parton distribution functions. In particular I discuss the results of the CTEQ6X global fit, and some work in progress. Topics covered include the interplay of target mass and higher-twist corrections, the importance of nuclear corrections for deuterium target data, and applications to the study of quark-hadron duality. Implications for collider physics are highlighted.

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arXiv2009

Parton Propagation and Fragmentation in QCD Matter

We review recent progress in the study of parton propagation, interaction and fragmentation in both cold and hot strongly interacting matter. Experimental highlights on high-energy hadron production in deep inelastic lepton-nucleus scattering, proton-nucleus and heavy-ion collisions, as well as Drell-Yan processes in hadron-nucleus collisions are presented. The existing theoretical frameworks for describing the in-medium interaction of energetic partons and the space-time evolution of their fragmentation into hadrons are discussed and confronted to experimental data. We conclude with a list of theoretical and experimental open issues, and a brief description of future relevant experiments and facilities.

nucl-th↗
arXiv2009

What can break the Wandzura--Wilczek relation?

We analyze the breaking of the Wandzura-Wilczek relation for the g_2 structure function, emphasizing its connection with transverse momentum dependent parton distribution functions. We find that the relation is broken by two distinct twist-3 terms, and clarify how these can be separated in measurements of double-spin asymmetries in semi-inclusive deep inelastic scattering. Through a quantitative analysis of available g_2 data we also show that the breaking of the Wandzura-Wilczek relation can be as large as 15-30% of the size of g_2.

hep-ph↗
arXiv2009

What can we learn from the breaking of the Wandzura-Wilczek relation?

We review the study the Wandzura-Wilczek relation for the structure function g_2, with a particular attention on the connection with the framework of Transverse Momentum Dependent factorization. We emphasize that the relation is broken by two distinct twist-3 terms. In the light of these findings, we clarify what can be deduced from the available experimental data on g_2, which indicate a breaking of the order 20-40%, and how to individually measure the twist-3 terms.

hep-ph↗
arXiv2009

Target mass corrections and beyond

I examine the uncertainty of perturbative QCD factorization for (un)polarized hadron structure functions in deep inelastic scattering at a large value of the Bjorken variable xB. The focus will be on Target Mass Corrections and Jet Mass Corrections in the collinear factorization framework.

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arXiv2008

An estimate of the prehadron production time

A semi-quantitative estimate of the prehadron production time based on recent preliminary HERMES data on hadron transverse momentum broadening in nuclear DIS is presented. The obtained production time can well explain the data except for their dependence on the photon virtuality, which remains a challenge to current theoretical models. A few mechanisms that may contribute to its explanation are suggested, along with possible experimental tests. The average time scale at HERMES is found to be of the order of 4-5 fm, comparable to heavy nuclei radii, so that by suitable kinematic cuts it will be possible to study both the case in which hadronization starts inside and the case in which it starts outside the nucleus. A comparison with recent CLAS preliminary data, which qualitatively but not quantitatively agree with HERMES, is performed.

nucl-th↗
arXiv2008

Collinear factorization for deep inelastic scattering structure functions at large Bjorken xB

We examine the uncertainty of perturbative QCD factorization for hadron structure functions in deep inelastic scattering at a large value of the Bjorken variable xB. We analyze the target mass correction to the structure functions by using the collinear factorization approach in the momentum space. We express the long distance physics of structure functions and the leading target mass corrections in terms of parton distribution functions with the standard operator definition. We compare our result with existing work on the target mass correction. We also discuss the impact of a final-state jet function on the extraction of parton distributions at large fractional momentum x.

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