DOCSLIB.ORG

What We Know and What We Don't Know About the Proton Spin After 30 Years

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
What We Know and What We Don't Know About the Proton Spin After 30 Years REVIEWS What we know and what we don’t know about the proton spin after 30 years Xiangdong Ji 1,2 ✉ , Feng Yuan 3 ✉ and Yong Zhao4 ✉ Abstract | More than three decades ago, the European Muon Collaboration published a surprising result on the spin structure of the proton: the spins of its three quark components account for only a small part of the spin of the proton. Ever since, theoretical and experimental progress has been made in understanding the origins of the proton spin. In this Review, we discuss what has been learned so far, what is still missing and what could be learned from the upcoming experiments, including the Jefferson Lab 12 GeV upgrade and the proposed Electron-Ion Collider. In particular, we focus on first- principles calculations and experimental measurements of the total gluon helicity ΔG, and the quark and gluon orbital angular momenta. The proton is a spin-1/2 particle, thought to be funda- the polarized structure function based on the quark mental when discovered in 1917 by Ernest Rutherford1. picture16. The EMC result shocked the physics com- However, the subsequent measurement of its magnetic munity, and created the so- called proton spin ‘crisis’ or moment2 showed a significant deviation from the value proton spin problem. The discrepancy has since inspired expected for a point- like object3. The proton substruc- a large number of experimental and theoretical studies, ture, along with the origin of its spin and magnetic which have been reviewed in a number of papers17–22. moment, has intrigued nuclear and particle physicists The most important lesson learned so far is that the ever since. underlying theory for the proton structure, QCD, has Every model of the proton ought to give an explana- a much more sophisticated way to build up the proton tion for its spin: from the Skyrme model4, to Gell–Mann spin. and Zweig’s quark model5,6, and to many other models QCD is fundamental and beautiful on the one hand, proposed in the 1970s and 1980s (REFS7,8). The simplest and sophisticated and defying simple ways to under- and most successful one has been the quark model, which stand it on the other. For example, in a QCD frame- inspired, among other things, the discovery of quantum work, it is no longer feasible, or we have failed so far, to chromodynamics (QCD)9 — the fundamental theory of come up with an entire quark and gluon wave function strong interactions. The non- relativistic quark model for the proton and to check the content of its various has an exceedingly simple explanation for the proton/ components. Therefore, we will consider instead the 1Center for Nuclear neutron spin and the associated magnetic moments10, as so- called sum rules or decompositions of the spin into Femtography, SURA, well as their excitations11. The three constituent quarks various physical parts. This has been the main approach Washington DC, USA. are all in the s- wave orbit, and their spins couple to 1/2 to understand the origins of the proton spin so far. 2 Department of Physics, in a way that is consistent with the SU(2) × SU(3) This is not a comprehensive review of hadron University of Maryland, spin flavour 12 College Park, MD, USA. combined spin–flavour symmetry . spin physics. Nor is it meant to be an update on other 19,22 3Nuclear Science Division, The quark model picture was directly tested through reviews , which have done an excellent job. Rather, Lawrence Berkeley National measurements of the polarized deep inelastic scattering of this Review focuses on the questions related to the ori- Laboratory, Berkeley, leptons (electrons or muons) off a polarized proton tar- gins of the proton spin. We mainly discuss issues such CA, USA. get13. In 1987, the European Muon Collaboration (EMC) as: does it make sense to talk about different parts of 4Physics Department, reported that the fraction of the proton spin carried by the proton spin? What will be an interesting and phys- Brookhaven National the spins of three quarks at the measured scale Q2 is14,15 ically meaningful decomposition for the spin? To what Laboratory, Upton, NY, USA. ✉ extent do we believe that we can measure each part e- mail: [email protected]; 22 (1) [email protected]; yzhao@ ΔΣ(=Q 107G. eV )=00...60±0 047±0 069, experimentally? How can one calculate these parts in bnl.gov fundamental theory and put the results to experimental https://doi.org/10.1038/ which is practically nothing. The EMC data also showed tests? We hope that, after more than 30 years after the s42254-020-00248-4 a significant deviation from the Ellis–Jaffe sum rule for EMC result, this Review can help the broader physics NATURE REVIEWS | PHYSICS VOLUME 3 | JANUARY 2021 | 27 REVIEWS Key points A physically interesting spin sum rule should have the following properties: • There are two established approaches to look at the composition of the proton spin: • Experimental measurability. The interest in the the frame- independent spin structure (or the Ji sum rule) and the infinite-momentum- proton spin began with the EMC data. Many of frame or parton spin structure (or the Jaffe–Manohar sum rule). the follow- up experiments, including HERMES and • In the frame- independent approach, the quark orbital and gluon angular momentum COMPASS, the polarized Relativistic Heavy Ion contributions can be extracted from the moments of the generalized parton Collider (RHIC)28, the Jefferson Lab (JLab) 12 GeV distributions. Results from the Jefferson Lab 6 GeV and HERMES experiments suggest upgrade29 and the EIC23,24, have been partially moti- that there is a substantial quark orbital contribution. vated to search for a full understanding of the proton • In terms of partons, the quark and gluon helicity contributions have a simple physical spin. interpretation, and the result from Relativistic Heavy Ion Collider spin experiments • Frame independence. As spin is an intrinsic prop- has provided a first important constraint on the total gluon helicity. erty of a particle, it is natural to look for a descrip- • The development of a large- momentum effective theory along with lattice quantum tion of its structure independent of its momentum. chromodynamics simulations provide first- principles calculations of the spin structure. The results on the quark and gluon helicity contributions, and the quark To know how the individual contributions to the orbital and gluon angular momentum contributions have provided the first complete total spin depend on the reference frame requires theoretical picture. an understanding of the properties of the Lorentz • The Jefferson Lab 12 GeV programme will provide better information on the quark transformations of Jα. As the proton structure probed orbital angular momentum and gluon angular momentum. The future Electron- Ion in high- energy scattering is best described in the Collider will provide high-precision measurements on the gluon helicity and gluon infinite- momentum frame (IMF), a partonic picture angular momentum. of the spin is interesting in this special frame of reference. • According to the above remarks, two sum rules have community to understand what we know now, what we been well established in the literature (TABLE 1): the don’t and what we can expect in the future. In particular, frame- independent one30 and the IMF one31, as we how will the Electron-Ion Collider (EIC) help to answer explain below. the fundamental questions about the origins of the proton spin23,24. QCD sources of angular momentum. To obtain a spin sum rule, an expression for the QCD AM opera- Spin structure in sum rules tor is needed. This can be derived through Noether’s Without knowing the wave function of the composite theorem32 based on the space- time symmetry of the system, the angular momentum (AM), or spin structure, QCD Lagrangian density can be studied through the various contributions to the 1 total. Thus, to explore origins of the proton spin, one L μν / QCD = − FFa μνa +(f ψf iDm− f )ψf , (3) can start from the QCD AM operator JQCD in terms of 4 ∑ individual sources Jα, μν where Fa (μ and ν = 0, 1, 2, 3 are Lorentz indices) is a gluon JJ=, QCD ∑α α (2) field strength tensor or simply gluon field with colour indices a = 1, . ., 8 and ψf is a quark spinor field of flavour through which, the spin projection ħ/2 can be expressed f = u, d, s, . (up, down, strange, . .), and mf is the quark as a sum of different contributions. mass. The relation between the gauge field and gauge μ μν μν ν μ abc μ ν This approach has some limitations. Since the proton potential Aa is Fa = ∂ AA−∂ a − gfs AAb c with 25 is an eigenstate of the relativistic Pauli–Lubanski spin , gs as the strong coupling constant. The covariant deriv- μ μ μ μ μ the individual contributions can only be the quantum ative is D = ∂ + igsA and D/ = D γμ with γ as Dirac μ μ a a mechanical expectation values of the AM sources from matrices. A = Ata and t are the generators of the the entire bound- state wave function. Moreover, they SU(3) colour group and f abc is the structure constant. are ‘renormalization- scale dependent’, because indi- A straightforward calculation yields the canonical AM vidual operators are not separately conserved, and the expression31 resulting ultraviolet divergences must be renormalized meaning that the short- distance physics is included in 3 ⎡ ††Σ 26 Jx=d⎢ψψ+ ψ xE× (−i∇)+ψ × A the effective AM operators . In non- relativistic sys- QCD ∫ ⎣⎢ ff2 ffaa tems, with the exception of particles moving in a mag- (4) i i ⎤ netic field, the AM sources corresponding to different +(EAx × ∇) ⎥, a a⎥ physical degrees of freedom obey the separate AM ⎦ commutation relations.
Load more

Recommended publications
  • Asymptotic Freedom, Quark Confinement, Proton Spin Crisis, Neutron Structure, Dark Matters, and Relative Force Strengths
    Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 17 February 2021 doi:10.20944/preprints202102.0395.v1 Asymptotic freedom, quark confinement, proton spin crisis, neutron structure, dark matters, and relative force strengths Jae-Kwang Hwang JJJ Physics Laboratory, Brentwood, TN 37027 USA Abstract: The relative force strengths of the Coulomb forces, gravitational forces, dark matter forces, weak forces and strong forces are compared for the dark matters, leptons, quarks, and normal matters (p and n baryons) in terms of the 3-D quantized space model. The quark confinement and asymptotic freedom are explained by the CC merging to the A(CC=-5)3 state. The proton with the (EC,LC,CC) charge configuration of p(1,0,-5) is p(1,0) + A(CC=-5)3. The A(CC=-5)3 state has the 99.6% of the proton mass. The three quarks in p(1,0,-5) are asymptotically free in the EC and LC space of p(1,0) and are strongly confined in the CC space of A(CC=-5)3. This means that the lepton beams in the deep inelastic scattering interact with three quarks in p(1,0) by the EC interaction and weak interaction. Then, the observed spin is the partial spin of p(1,0) which is 32.6 % of the total spin (1/2) of the proton. The A(CC=-5)3 state has the 67.4 % of the proton spin. This explains the proton spin crisis. The EC charge distribution of the proton is the same to the EC charge distribution of p(1,0) which indicates that three quarks in p(1,0) are mostly near the proton surface.
    [Show full text]
  • Hadron Structure in Deep Inelastic Scattering
    Hadron Structure in Deep Inelastic Scattering Andrew Casey School of Chemistry & Physics University of Adelaide A thesis submitted as a portfolio of publications for the degree of Doctor of Philosophy August 2013 Contents Contentsi Dedication iv Abstractv Statement of Originality vii Acknowledgement ix List of Figuresx 1 Contextual Statement1 1.1 Contextual Statement . .1 1.1.1 Calculating Dihadron Fragmentation Function in the NJL- jet model . .1 1.1.2 Dihadron Fragmentation Functions from the NJL-jet model and their QCD Evolution . .2 1.1.3 Gluon Polarization in the Proton . .2 2 QCD and the Parton Model3 2.1 History . .3 2.2 Quantum Chromodynamics . .4 2.2.1 QCD Lagrangian . .5 2.2.2 Running of αs and Asymptotic Freedom . .8 2.3 Experimental Processes . 10 i CONTENTS 2.3.1 Overview . 10 2.3.2 QCD Factorization Theorem . 10 2.3.3 Deep Inelastic Scattering . 12 2.3.4 Semi-Inclusive Deep-Inelastic Scattering . 16 2.4 Parton Model . 18 2.4.1 Infinite Momentum Frame . 18 2.4.2 Bjorken Scaling . 20 2.4.3 DGLAP Evolution Equations . 20 2.4.3.1 Parton Distribution Function Evolution Equations 22 2.4.3.2 Single Hadron Fragmentation Function Evolution Equations . 25 2.4.3.3 Dihadron Fragmentation Function Evolution Equa- tions . 27 2.5 Proton Spin Crisis . 30 2.5.1 Polarization of the Gluon in the Proton . 31 3 Nambu{Jona-Lasinio model 34 3.1 Concepts and Properties of the Nambu{Jona-Lasinio model . 34 3.1.1 Nambu{Jona-Lasinio Model Lagrangian . 35 3.1.2 Mass Gap Equation .
    [Show full text]
  • The Spin Structure of the Proton from Lattice QCD
    19th International Spin Physics Symposium (SPIN2010) IOP Publishing Journal of Physics: Conference Series 295 (2011) 012009 doi:10.1088/1742-6596/295/1/012009 The spin structure of the proton from lattice QCD Philipp H¨agler Institut f¨urTheoretische Physik T39, Physik-Department der TU M¨unchen, James-Franck-Strasse, D-85747 Garching, Germany and Institut f¨urTheoretische Physik, Universit¨atRegensburg, D-93040 Regensburg, Germany E-mail: [email protected] Abstract. This proceeding discusses recent achievements in lattice QCD calculations of the longitudinal and transverse spin structure of the nucleon. Starting from the latest lattice results on moments of generalized parton distributions, we focus on the decomposition of the proton spin 1=2 in terms of quark (and gluon) spin and orbital angular momentum contributions. Recent results on the transverse nucleon spin structure obtained from lattice studies of transverse momentum distributions will be discussed and illustrated in the form of spin densities of quarks in the proton. We compare the results with expectations from phenomenological and model studies, and point out the most important systematic uncertainties and remaining challenges in the lattice approach. 1. Introduction In this contribution, we illustrate the advances that have been made in lattice QCD calculations of hadron structure by presenting and discussing a small number of results from recent dynamical lattice studies on the longitudinal and transverse spin structure of the nucleon. For more detailed accounts of the progress that has been made on the lattice, in particular with respect to the quark and gluon structure of the pion and the nucleon, we refer to [1, 2, 3, 4, 5, 6].
    [Show full text]
  • The Spin of the Proton
    The spin of the proton Bo-Qiang Ma (马伯强) Peking Univ (北京大学) ? The 7th Workshop on Hadron Physics in China and Opportunities Worldwide August 3-7, 2015, Duke Kunshan University Collaborators: Enzo Barone, Stan Brodsky, Jacques Soffer, Andreas Schafer, Ivan Schmidt, Jian-Jun Yang, Qi-Ren Zhang and students: Bowen Xiao, Zhun Lu, Bing Zhang, Jun She, Jiacai Zhu, Xinyu Zhang, Tianbo Liu 3 It has been 30 years of the proton “spin crisis” or “spin puzzle” • Spin Structure: experimentally u d s 0.020 u d s 0.3 spin “crisis” or “puzzle”: where is the proton’s missing spin? The Proton “Spin Crisis” In contradiction with the naïve quark model expectation: u d s 0.3 Why there is the proton spin puzzle/crisis? • The quark model is very successful for the classification of baryons and mesons • The quark model is good to explain the magnetic moments of octet baryons • The quark model gave the birth of QCD as a theory for strong interaction So why there is serious problem with spin of the proton in the quark model? The parton model (Feynman 1969) Infinite • photon scatters incoherently off Momentum massless, pointlike, spin-1/2 quarks Frame • probability that a quark carries fraction of parent proton’s momentum is q(), (0< < 1) 1 2 2 F2 (x) d eq q( ) (x ) eq x q(x) 0 q,q q,q 4 1 1 xu(x) x d(x) x s(x) ... 9 9 9 •the functions u(x), d(x), s(x), … are called parton distribution functions (pdfs) - they encode information about the proton’s deep structure •Parton model is established under the collinear approxiamtion: The transversal motion of partons is neglected or integrated over.
    [Show full text]
  • The Electron Ion Collider (EIC)
    7/18/16 EIC Lecture 2 at NNPSS 2016 at MIT 1 The Electron Ion Collider (EIC) Abhay Deshpande Lecture 2: What do we need polarized beams? Nucleon Spin Structure: What’s the problem? 2. Quantum Chromodynamics: The Fundamental Description of the Heart of Visible Matter Sidebar 2.6: Nucleon Spin: So Simple and Yet So Complex The simple fact that the proton carries spin 1/2, quark spins to account for most of the proton’s spin measured in units of Planck’s famous constant, is but were quite surprised when experiments at CERN exploited daily in thousands of magnetic resonance and other laboratories showed that the spins of all imaging images worldwide. Because the proton is a quarks and antiquarks combine to account for no more composite system, its spin is generated from its quark than about 30% of the total. This result has led nuclear 7/18/16 EIC Lecture 2 at NNPSS 2016 at MIT and gluon constituents. Physicists’ evolving appreciation scientists2. Quantum to address Chromodynamics: the more daunting The Fundamental challenges2 Description of the Heart of Visible Matter of how the spin might be generated, and of how much involved in measuring the other possible contributions to we have yet to understand about it, is an illustrative the spin illustrated in Figure 1. The gluons also have an Thecase study nucleon of how seemingly simple spin properties of visible intrinsic spin (1 unit) and might be “polarized” (i.e., might matter emerge from complex QCD interactions. have a preferential orientation of their spins along or puzzle….
    [Show full text]
  • Pos(SPIN2018)123 ∗ Speaker
    The Non-existence of the Proton Spin Crisis PoS(SPIN2018)123 Elliot Leader∗ Imperial College London [email protected] The spin crisis arose some 30 years ago following results of a European Muon Collaboration ex- periment on polarized DIS. Since then many people have pointed out that the arguments involved are too naive, yet papers continue to appear devoted to “the spin crisis". We explain the subtleties involved in giving a precise statement of the spin sum rule and clarify why there is really no crisis. 23rd International Spin Physics Symposium - SPIN2018 - 10-14 September, 2018 Ferrara, Italy ∗Speaker. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ Spin Crisis Elliot Leader The controversy began some decades ago when the European Muon Collaboration (EMC) published results of their experiment on polarized fully inclusive DIS, which suggested that the constituents of the proton could not provide enough angular momentum to explain the fact that the proton had spin 1/2. And so 30 years ago Mauro Anselmino and E.L. published a paper in response to the results of the EMC experiment entitled A crisis in the parton model: Where, oh where is the proton’s spin?[1]. However, it has long been understood that there is no such crisis. Nonetheless papers keep appear- ing referring to “The proton spin crisis" . Why?????? Because people have forgotten that the belief in a spin-crisis emerged from an over naive interpre- PoS(SPIN2018)123 tation of the EMC experiment.
    [Show full text]
  • Curriculum Vitæ
    Curriculum vit½ Carl Johan Hansson BORN: November 16, 1966, in Luleºa,Sweden, Swedish citizen EXAMINA: High School Diploma (\gymnasieexamen") at Tornedalsskolan High School, Ha- paranda 1985 High School Diploma (\gymnasieingenjÄor",4-ºarigmaskinteknisk gren), fourth year at Midskogsskolan High School, Luleºa1986 Bachelor of Science (\¯loso¯e kandidat") in Physics at Uppsala University, Jan- uary 1993 Licentiate in Physics (\teknologie licentiat"), LuleºaUniversity of Technology, May 1996 Doctor of Philosophy in Physics, Ph.D. (\teknologie doktor"), LuleºaUniversity of Technology, March 1998. Faculty opponent: Professor Boris Kopeliovich, Max-Planck-Institut fÄurKernphysik, Heidelberg, Germany Associate Professor in Physics (\docent"), LuleºaUniversity of Technology, June 2005. THESES: Licentiate in Physics (\teknologie licentiat"), LuleºaUniversity of Technology, May 1996. Title: \The Proton as a Laboratory for the Fundamental Structure of Matter" 1996:05 L, ISSN 0280-8242 Doctor of Philosophy in Physics, Ph.D. (\teknologie doktor"), LuleºaUniversity of Technology, March 1998. Title: \The Fundamental Structure of Matter" 1998:04, ISSN 1402-1544, ISRN: LTU-DT{98/4{SE Thesis advisor: Professor Sverker Fredriksson EMPLOYMENTS: Teacher in ¯rst and secondary school (\lºag-,mellan- och hÄogstadiet"),on an hourly basis (\vikarie"), Haparanda, Sweden, 1985-88 Teacher (full time) in Mathematics and Physics in secondary school (\hÄogstadiet"), Bºalsta,Sweden, Aug - Dec 1991 PhD student employment (\doktorandtjÄanst"),Department of Physics, Luleºa
    [Show full text]
  • Nucleon Spin Structure Longitudinal Spin of the Proton
    Nucleon Spin Structure Longitudinal Spin of the Proton HUGS Summer School Jefferson National Laboratory June 1, 2011 Lecture 2 Abhay Deshpande Wednesday, June 1, 2011 Introduction & Overview (I) Abhay Deshpande, Nucleon Spin Lecture 2 of 6 at HUGS 2011 6/01/11 2 Wednesday, June 1, 2011 Introduction & Overview (I) • Lecture 1: Introduction & importance to “spin” – What is the spin crisis? Abhay Deshpande, Nucleon Spin Lecture 2 of 6 at HUGS 2011 6/01/11 2 Wednesday, June 1, 2011 Introduction & Overview (I) • Lecture 1: Introduction & importance to “spin” – What is the spin crisis? • Lecture 2 & Lecture 3 – Experimental method : Fixed Target Polarized Deep Inelastic Scattering (pDIS): early investigations – Spin Crisis, and the insights it enabled…. – Latest on pDIS experiments: result summaries – Principle limitations of fixed target experiments Abhay Deshpande, Nucleon Spin Lecture 2 of 6 at HUGS 2011 6/01/11 2 Wednesday, June 1, 2011 Introduction & Overview (II) Abhay Deshpande, Nucleon Spin Lecture 2 of 6 at HUGS 2011 6/01/11 3 Wednesday, June 1, 2011 Introduction & Overview (II) • Lecture 4 & Lecture 5 – Relativistic Heavy Ion Collider as a Polarized Collider – Comments experimental techniques – Review of results – Principle limitations Abhay Deshpande, Nucleon Spin Lecture 2 of 6 at HUGS 2011 6/01/11 3 Wednesday, June 1, 2011 Introduction & Overview (II) • Lecture 4 & Lecture 5 – Relativistic Heavy Ion Collider as a Polarized Collider – Comments experimental techniques – Review of results – Principle limitations • Lecture 6: Future
    [Show full text]
  • Lattice QCD Method to Study Proton Spin Crisis
    Lattice QCD Method To Study Proton Spin Crisis Gouranga C Nayak, ∗ (Dated: June 11, 2019) Abstract The proton spin crisis remains an unsolved problem in particle physics. The spin and angular momentum of the partons inside the proton are non-perturbative quantities in QCD which cannot be calculated by using the perturbative QCD (pQCD). In this paper we present the lattice QCD formulation to study the proton spin crisis. We derive the non-perturbative formula of the spin and angular momentum of the partons inside the proton from the first principle in QCD which can be calculated by using the lattice QCD method. PACS numbers: 12.38.-t, 11.30.-j, 14.20.Dh, 12.38.Gc arXiv:1906.03974v1 [physics.gen-ph] 3 Jun 2019 ∗E-Mail: [email protected] Typeset by REVTEX 1 I. INTRODUCTION 1 The spin of the proton is 2 . Since the proton consists of quarks and gluons it was expected 1 that the spin of the quarks and gluons inside the proton add up to give spin 2 of the proton. The proton in motion high energy consists of quarks, antiquarks and gluons because sea quark-antiquark pairs and gluons are produced from the QCD vacuum at high energy. 1 For the proton in motion along z-axis the spin/helicity 2 was expected to be 1 =< p,s|Sˆz|p,s > + < p,s|Sˆz|p,s > (1) 2 q g ˆz where < p,s|Sq |p,s > is the spin of the quarks plus antiquarks inside the proton and ˆz < p,s|Sg |p,s > is the spin of the gluons inside the proton.
    [Show full text]
  • The Proton-Spin Crisis: Another ABJ Anomaly?
    SWAT 98/212 CERN–TH/98–398 The Proton-Spin Crisis: another ABJ anomaly? G.M. Shore Department of Physics, University of Wales Swansea Singleton Park, Swansea, SA2 8PP, U.K. and Theory Division, CERN CH 1211 Geneva 23, Switzerland Contents: 1. Introduction p 2. The First Moment Sum Rule for g1 3. The Parton Model and the ‘Proton Spin’ 4. The CPV Method and Topological Charge Screening 5. Experiment 6. Semi-Inclusive Polarised DIS 1. Introduction For a decade, the ‘proton spin’ problem – the anomalous suppression observed in the flavour singlet component of the first moment of the polarised proton structure function p 2 g1 (x; Q ) – has puzzled and intrigued theorists and experimentalists alike. The consequent research effort has indeed been impressive: to date, the original EMC paper[1] alone has nearly one thousand citations. In this lecture, we first give a brief review of the ‘proton spin’ problem from the standard viewpoint of the parton model. We explain why the 1988 observation by the p EMC of a violation of the Ellis-Jaffe sum rule[2] for g1 was initially mis-interpreted in terms of quark spins and how the problem is resolved in the context of the full QCD parton model[3]. ∗ Lecture presented at the International School of Subnuclear Physics, `From the Planck Length to the Hubble Radius', Erice, 1998. SWAT 98/212 CERN–TH/98–398 December 1998 We then describe an alternative, complementary approach to the description of deep inelastic scattering (DIS), the ‘CPV’ method[4-6], which allows the problem to be viewed in a new light.
    [Show full text]
  • Physics Letters B a Possible Resolution of the Proton Spin Problem
    Physics Letters B 663 (2008) 302–305 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb A possible resolution of the proton spin problem ∗ F. Myhrer a, , A.W. Thomas b,c a Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208, USA b Jefferson Lab, 12000 Jefferson Ave., Newport News, VA 23606, USA c College of William and Mary, Williamsburg, VA 23187, USA article info abstract Article history: A number of lines of investigation into the structure of the nucleon have converged to the point where Received 24 September 2007 it seems possible to propose a consistent explanation of the well known proton spin problem. Received in revised form 14 April 2008 © 2008 Elsevier B.V. All rights reserved. Accepted 16 April 2008 Availableonline18April2008 Editor: J.-P. Blaizot There is no more fundamental challenge for strong interaction considerably higher than the initial EMC suggestion. Nevertheless, physics than mapping the distribution of energy, momentum, spin the modern value is still sufficiently small that it constitutes an and angular momentum onto the quarks and gluons that compose on-going “spin problem” of great interest. the nucleon. For the past two decades there has been a tremen- The apparent failure of polarized glue as an explanation for the dous level of activity associated with the latter two, sparked by spin problem leads us to focus again on suggestions made soon af- the discovery, almost 20 years ago, by the European Muon Col- ter the EMC announcement [14–16], which were based on physics laboration (EMC) of a proton “spin crisis” [1].Muchoftheearly that is more familiar to those modeling non-perturbative QCD.
    [Show full text]
  • RHIC Spin--To Study the Spin Structure of the Proton G
    34th SLAC Summer Institute On Particle Physics (SSI 2006), July 17-28, 2006 RHIC Spin--To Study the Spin Structure of the Proton G. Bunce BNL and RBRC, Upton, NY11973, USA The RHIC Spin program, colliding polarized protons at high energy, is about studying the internal structure of the proton. We probe this structure, the polarizations of the quarks and gluons in a polarized proton, using the quarks and gluons of the other colliding proton. With the collider, we achieve high energy where the collisions are understood by perturbative QCD. The program follows a series of important experiments using electrons and muons to probe the proton. These experiments, initiated at SLAC, have shown that the quarks and anti-quarks in the proton carry very little of the proton spin on average, a surprise. RHIC, with its strongly interacting probes, is directly sensitive to the gluons in the proton, and is in the process of measuring the gluon’s contribution to the proton spin. I describe why, how, and what we are learning, and our future program. 1. A HISTORY—FROM QUARKS TO SPIN STRUCTURE A short history of the strong interaction might begin in the mid 1960s, when “quarks” were introduced to begin to understand the zoo of strongly interacting particles that had been discovered from accelerator experiments. The “color” quantum number was needed to describe the Omega^-, a state of three strange quarks, each with the same spin ½ adding to a total spin of 3/2. In the later 1960s, an experiment at SLAC showed that these “quarks” were real.
    [Show full text]