Imperial College London

Professor Leader

Faculty of Natural SciencesDepartment of Physics

Senior Research Investigator
 
 
 
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Contact

 

+44 (0)20 7594 7792e.leader Website

 
 
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Assistant

 

Ms Paula Brown +44 (0)20 7594 7823

 
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Location

 

504Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

125 results found

Khorramian A, Leader E, Stamenov DB, Shabanpour Aet al., 2021, Sensitivity of strange quark polarization to flavor SU(3) symmetry breaking, PHYSICAL REVIEW D, Vol: 103, ISSN: 2470-0010

Journal article

Christova E, Kotlorz D, Leader E, 2020, New study of the Boer-Mulders function: Implications for the quark and hadron transverse momenta, Physical Review D: Particles, Fields, Gravitation and Cosmology, Vol: 102, Pages: 014035 – 1-014035 – 12, ISSN: 1550-2368

In series of papers, the Boer-Mulders function for a given quark flavor has been extracted: (i) from data on semi-inclusive deep inelastic scattering, using the simplifying, but theoretically inconsistent, assumption that it is proportional to the Sivers function for each quark flavor and (ii) from data on Drell-Yan reactions. In earlier papers, using the semi-inclusive deep inelastic COMPASS deuteron data on the ⟨cosϕh⟩ and ⟨cos2ϕh⟩ asymmetries, we extracted the collinear xB dependence of the Boer-Mulders function for the sum of the valence quarks QV=uV+dV using a small number of model dependent assumptions and found a significant disagreement with the analysis in (i). In the present paper, we provide a more complete analysis of the semi-inclusive deep inelastic scattering reaction, including a discussion of higher twist and interaction-dependent terms, and also a comparison with the Boer-Mulders function extracted from data on the Drell-Yan reaction. We confirm that the proportionality relation of the BM function to the Sivers function, for each quark flavor, fails badly, but find that it holds rather well if applied to the nonsinglet valence-quark combination, QV. We also find good agreement with the results of the Drell-Yan analysis. Furthermore, we obtain interesting information on the quark transverse momentum densities in the nucleon and on the hadron transverse momentum dependence in quark fragmentation.

Journal article

Christova E, Leader E, Stoilov M, 2018, Consistency tests for the extraction of the Boer-Mulders and Sivers functions, Physical Review D - Particles, Fields, Gravitation and Cosmology, Vol: 97, ISSN: 1550-2368

At present, the Boer-Mulders (BM) function for a given quark flavor is extracted from data on semi-inclusive deep inelastic scattering (SIDIS) using the simplifying assumption that it is proportional to the Sivers function for that flavor. In a recent paper, we suggested that the consistency of this assumption could be tested using information on so-called difference asymmetries i.e. the difference between the asymmetries in the production of particles and their antiparticles. In this paper, using the SIDIS COMPASS deuteron data on the ⟨cosϕh⟩, ⟨cos2ϕh⟩ and Sivers difference asymmetries, we carry out two independent consistency tests of the assumption of proportionality, but here applied to the sum of the valence-quark contributions. We find that such an assumption is compatible with the data. We also show that the proportionality assumptions made in the existing parametrizations of the BM functions are not compatible with our analysis, which suggests that the published results for the Boer-Mulders functions for individual flavors are unreliable. The ⟨cosϕh⟩ and ⟨cos2ϕh⟩ asymmetries receive contributions also from the, in principle, calculable Cahn effect. We succeed in extracting the Cahn contributions from experiment (we believe for the first time) and compare with their calculated values, with interesting implications.

Journal article

Leader E, 2018, A proposed measurement of optical orbital and spin angular momentum and its implications for photon angular momentum, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, Vol: 779, Pages: 385-387, ISSN: 0370-2693

The expression for the total angular momentum carried by a laser optical vortex beam, splits, in the paraxial approximation, into two terms which seem to represent orbital and spin angular momentum respectively. There are, however, two very different competing versions of the formula for the spin angular momentum, one based on the use of the Poynting vector, as in classical electrodynamics, the other related to the canonical expression for the angular momentum which occurs in Quantum Electrodynamics. I analyze the possibility that a sufficiently sensitive optical measurement could decide which of these corresponds to the actual physical angular momentum carried by the beam.

Journal article

Christova E, Leader E, 2016, Problem of kinematic mass corrections for unpolarized semi-inclusive deep inelastic scattering, PHYSICAL REVIEW D, Vol: 94, ISSN: 2470-0010

Miraculously, target-mass corrections for inclusive deep inelastic scattering can be calculated exactly. On the contrary, there does not exist a consistent derivation of kinematic hadron mass corrections for semi-inclusive deep inelastic scattering. Recently this has become of topical interest, since there is a significant difference between the measured HERMES and COMPASS pion and kaon multiplicities, which cannot be explained as a consequence of evolution in Q2, and it has been suggested that the difference can be understood if kinematic hadron mass corrections are taken into account. We explain why this argument is incorrect.

Journal article

Leader E, Sidorov AV, Stamenov DB, 2016, Determination of the fragmenetation functions from an NLO QCD analysis of the HERMES data on pion multiplicities, Physical Review D, Vol: 93, ISSN: 1550-7998

An NLO QCD analysis of the final HERMES data on pion multiplicities is presented and a new setof pion fragmentation functions is extracted from the best fit to the data. We have studied the socalled[x, z] and [Q2, z] presentations of their data, as given by HERMES, which, in principle, shouldsimply be two different ways of presenting the experimental data. We have based our extraction onan excellent fit to the [Q2, z] presentation of the data.

Journal article

Leader E, 2016, The photon angular momentum contrroversy: Resolution of a conflict between laser optics and particle physics, Physics Letters B, Vol: 756, Pages: 303-308, ISSN: 0370-2693

The claim some years ago, contrary to all textbooks, that the angular momentum of a photon (andgluon) can be split in a gauge-invariant way into an orbital and spin term, sparked a major controversyin the Particle Physics community, exacerbated by the realization that many different forms of theangular momentum operators are, in principle, possible. A further cause of upset was the realizationthat the gluon polarization in a nucleon, a supposedly physically meaningful quantity, corresponds onlyto the gauge-variant gluon spin derived from Noether’s theorem, evaluated in a particular gauge. On thecontrary, Laser Physicists have, for decades, been happily measuring physical quantities which correspondto photon orbital and spin angular momentum evaluated in a particular gauge. This paper reconciles thetwo points of view, and shows that it is the gauge invariant version of the canonical angular momentumwhich agrees with the results of a host of laser optics experiments

Journal article

Christova E, Leader E, 2015, Tests of the extraction of the Sivers, Boer-Mulders, and transversity distributions in SIDIS reactions, Physical Review D, Vol: 92, ISSN: 1550-7998

A major experimental program is presently underway worldwide to determine the fundamentalnonperturbative functions, the Sivers, Boer-Mulders and transversity distributions, which are vital for anunderstanding of the internal structure of the nucleon. However, at present, many simplifying assumptionsare used in extracting these functions from the data. We demonstrate that if the binning of the data in Q2 issmall enough so that Q2 evolution can be neglected inside a bin, then one can obtain stringent tests of theseassumptions. Failure to satisfy these tests implies that the presently extracted nonperturbative functions areunreliable. To this end, we consider the measurement of the Sivers, Boer-Mulders and transversity differenceasymmetries for hadrons with opposite charges in SIDIS reactions with unpolarized and transverselypolarized deuteron and proton targets: l þ N → l0 þ h þ X, h ¼ π, K, h. Utilizing only charge andisospin invariance, and applying the above mentioned simplifying assumptions, we obtain several testablerelations amongst the measured asymmetries. If these tests are satisfied, then the measured asymmetriesdetermine two different combinations of the valence-quark transverse-momentum-dependent distributions,which can be determined separately without any contributions from the strange and other sea quarks.

Journal article

Leader E, Sidorov AV, Stamenov DB, 2015, New analysis concerning the strange quark polarization puzzle, PHYSICAL REVIEW D, Vol: 91, ISSN: 1550-7998

Journal article

Leader E, Sidorov AV, Stamenov DB, 2014, Problems with the perturbative QCD interpretation of HERMES data on semi-inclusive leptoproduction of pions, PHYSICAL REVIEW D, Vol: 90, ISSN: 1550-7998

Journal article

Leader E, Lorce C, 2014, The angular momentum controversy: what's it all about and does it matter?, Physics Reports, Vol: 541, Pages: 163-248, ISSN: 0370-1573

The general question, crucial to an understanding of the internal structure of the nucleon, of how to split the total angular momentum of a photon or gluon into spin and orbital contributions is one of the most important and interesting challenges faced by gauge theories like Quantum Electrodynamics and Quantum Chromodynamics. This is particularly challenging since all QED textbooks state that such a splitting cannot be done for a photon (and a fortiori for a gluon) in a gauge-invariant way, yet experimentalists around the world are engaged in measuring what they believe is the gluon spin! This question has been a subject of intense debate and controversy, ever since, in 2008, it was claimed that such a gauge-invariant split was, in fact, possible. We explain in what sense this claim is true and how it turns out that one of the main problems is that such a decomposition is not unique and therefore raises the question of what is the most natural or physical choice. The essential requirement of measurability does not solve the ambiguities and leads us to the conclusion that the choice of a particular decomposition is essentially a matter of taste and convenience. In this review, we provide a pedagogical introduction to the question of angular momentum decomposition in a gauge theory, present the main relevant decompositions and discuss in detail several aspects of the controversies regarding the question of gauge invariance, frame dependence, uniqueness and measurability. We stress the physical implications of the recent developments and collect into a separate section all the sum rules and relations which we think experimentally relevant. We hope that such a review will make the matter amenable to a broader community and will help to clarify the present situation.

Journal article

Leader E, Lorce C, 2013, Comment on "Proton Spin Structure from Measurable Parton Distributions", PHYSICAL REVIEW LETTERS, Vol: 111, ISSN: 0031-9007

Journal article

Leader E, 2013, A critical assessment of the angular momentum sum rules, Physics Letters B, Vol: 720, Pages: 120-124

Journal article

Anselmino M, Boglione M, D'Alesio U, Leader E, Melis S, Murgia F, Prokudin Aet al., 2012, Role of Collins effect in the single spin asymmetry A(N) in p up arrow p -> h X processes, PHYSICAL REVIEW D, Vol: 86, ISSN: 1550-7998

Journal article

Leader E, 2012, Reply to "Comment on 'Controversy concerning the definition of quark and gluon angular momentum'" [PRD 83, 096012 (2011)], PHYSICAL REVIEW D, Vol: 85, ISSN: 1550-7998

Journal article

Leader E, 2012, New relation between transverse angular momentum and generalized parton distributions, PHYSICAL REVIEW D, Vol: 85, ISSN: 1550-7998

Journal article

Leader E, Sidorov AV, Stamenov DB, 2011, Possible resolution of the strange quark polarization puzzle?, PHYSICAL REVIEW D, Vol: 84, ISSN: 1550-7998

Journal article

Leader E, 2011, Controversy concerning the definition of quark and gluon angular momentum, PHYSICAL REVIEW D, Vol: 83, ISSN: 1550-7998

Journal article

Leader E, Sidorov AV, Stamenov DB, 2011, The role of semi inclusive DIS data in determining polarized PDFs, 19th International Spin Physics Symposium (SPIN2010), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588

Conference paper

Leader E, Sidorov AV, Stamenov DB, 2010, Determination of polarized parton densities from a QCD analysis of inclusive and semi-inclusive deep inelastic scattering data, PHYSICAL REVIEW D, Vol: 82, ISSN: 1550-7998

Journal article

D'Alesio U, Leader E, Murgia F, 2010, Importance of Lorentz structure in the parton model: Target mass corrections, transverse momentum dependence, positivity bounds, PHYSICAL REVIEW D, Vol: 81, ISSN: 1550-7998

Journal article

Leader E, Sidorov AV, Stamenov DB, 2009, Some remarks on methods of QCD analysis of polarized DIS data, PHYSICAL REVIEW D, Vol: 80, ISSN: 1550-7998

Journal article

Leader E, 2009, The Power of Spin: A scalpel-like probe of theoretical ideas, 18th International Spin Physics Symposium, Publisher: AMER INST PHYSICS, Pages: 11-20, ISSN: 0094-243X

Conference paper

Christova E, Leader E, 2009, A Model Independent Approach for Determining the Fragmentation Functions, 18th International Spin Physics Symposium, Publisher: AMER INST PHYSICS, Pages: 381-+, ISSN: 0094-243X

Conference paper

Christova E, Leader E, 2009, Towards a model independent approach to fragmentation functions, PHYSICAL REVIEW D, Vol: 79, ISSN: 1550-7998

Journal article

Leader E, 2009, THE TRANSVERSE ANGULAR MOMENTUM SUM RULE, 2nd Workshop on Transverse Polarization Phenomena in Hard Processes, Publisher: WORLD SCIENTIFIC PUBL CO PTE LTD, Pages: 25-31

Conference paper

Anselmino M, Boglione M, D'Alesio U, Leader E, Melis S, Murgia Fet al., 2009, Intrinsic parton motion soft mechanisms and the longitudinal spin asymmetry A(LL) in high energy pp -> pi X, JOURNAL OF PHYSICS G-NUCLEAR AND PARTICLE PHYSICS, Vol: 36, ISSN: 0954-3899

Journal article

Kuhn SE, Chen J-P, Leader E, 2009, Spin structure of the nucleon-status and recent results, PROGRESS IN PARTICLE AND NUCLEAR PHYSICS, VOL 63, NO 1, Vol: 63, Pages: 1-50, ISSN: 0146-6410

Journal article

Leader E, Sidorov AV, Stamenov DB, 2009, Status of Longitudinal Polarized Parton Densities and Higher Twist, 18th International Spin Physics Symposium, Publisher: AMER INST PHYSICS, Pages: 343-+, ISSN: 0094-243X

Conference paper

Leader E, Sidorov AV, Stamenov DB, 2008, Recent results on polarized PDFs and higher twist, SPIN Praha 2007 Meeting, Publisher: SPRINGER HEIDELBERG, Pages: 19-24, ISSN: 1951-6355

Conference paper

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