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Deep Inelastic Scattering in the Valence Regime

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Deep Inelastic Scattering in the Valence Regime Deep Inelastic Scattering in the Valence Regime Thia Keppel ICNFP 2019 Thomas Jefferson National Accelerator Facility Kolympari, Crete, Greece Newport News, VA How to probe the nucleon structure? • Electron scattering experiments employ high momentum point-like leptons, + electromagnetic interactions, which are well understood, to probe hadronic structure (which isn’t). High energy electrons are a great tool for the job! short distance -> large momentum Deep Inelastic Electron Scattering Beginnings: e-p and e-d at SLAC J. Friedman, H.Kendall, R.Taylor Nobel Prize 1990 • Q2 = four-momentum transfer in electron scattering process • First SLAC experiment (‘69): – expected from proton form factor: 2 ds / dE'dW æ 1 ö -8 = ç ÷ µ Q ç 2 2 ÷ (ds / dW)Mott è (1+ Q / 0.71) ø • First data show big surprise: – very weak Q2-dependence – scattering off point-like objects? – quark structure of the proton! Structure Functions in Deep Inelastic Electron-Nucleon Scattering Probability of inelastic interaction: 푑2휎 훼2 휃 1 2 휃 = cos2 퐹 푥, 푄2 + 퐹 푥, 푄2 tan2 ′ 휃 2 1 푑Ω푑퐸 4퐸2 sin4 2 휈 푀 2 0 2 Unpolarized “Structure 2 2 Functions” F1(x,Q ) and F2(x,Q ): - Account for the sub-structure of the protons and neutrons - Give access to partonic structure of the nucleon, i.e. x : Bjorken “scaling” variable (= Q2/2Mn), momentum fraction of struck quark Fast forward…. 50 years of charged lepton Deep Inelastic Scattering at multiple laboratories including SLAC (to ~2000), CERN 80-90s EMC, NMC, BCDMS..), DESY (90s – 21st century H1, ZEUS,...), and more! H1 HERMES HERMES BCDMS 2 Q Evolution of the F2 Proton Structure Function Allows extraction of “Parton Distribution Functions” f(x,Q2) - think momentum distribution of quarks Some 50% things of momentumwe now know carriedreally bywell…. gluons …and some things we don’t. n p F2 /F2 • The neutron structure function is not well known at large x • Large x is the valence regime Some 50% things of momentumwe now know of nucleon structure carriedreally bywell…. gluons Deep Inelastic Scattering in the Valence Regime Thia Keppel ICNFP 2019 Thomas Jefferson National Accelerator Facility Kolympari, Crete, Greece Newport News, VA Why is the valence regime interesting? • Partonic structure in the valence region defines a hadron - Baryon number, charge, flavor content, total spin, … • “Valence regime” at large x, low Q2 evolves to low x, high Q2 - Intersection of nuclear and particle physics • Keen discriminator of nucleon structure models • New generation of experiments at JLab focused on high x 1 0 Neutron Structure Function sea quark dominance F n/F p 1 Parton model: 2 2 u quark dominance, d/u 0 n p F2 /F2 Neutron Structure Function sea quark dominance F n/F p 1 Parton model: 2 2 u quark dominance, d/u 0 n p F2 /F2 The deuteron is a nucleus! Neutron structure is typically derived from deuterium target data by subtracting proton data …..but….. Large uncertainty in unfolding nuclear effects (Fermi motion, off-shell effects, deuteron wave function, coherent scattering, final state interactions, nucleon structure modification (“EMC”-effect),……….. d p F2 /F2 The moon at nuclear density n Deuteron nuclear effects are an obstacle to extracting F2 n p No free neutrons, so F2 /F2 use the deuteron SAME data set Different deuteron nuclear effect analyses n p 0.2 < F2 /F2 < 0.8 ?!?! n p Predictions for F2 /F2 (neutron/proton) ratio at Large x • SU(6)-symmetric wave function of the proton in the quark model: 1 p 3uud uud 2uud 2duu 2duu 18 S 0 S1 S1 S1 S1 – SU(6) spin/flavor symmetry in u,d n p – In this model: d/u = 1/2, F2 /F2 = 2/3 for x -> 1 – But, N and D would be degenerate in mass…. • SU(6) symmetry is broken: N-D Mass Splitting – Mechanism produces mass splitting between S=1 and S=0 diquark spectator. – symmetric states are raised, antisymmetric states are lowered (~300 MeV). n p – S=1 suppressed => d/u = 0, F2 /F2 = 1/4, for x -> 1 n p • pQCD: helicity conservation (qp) => d/u =2/(9+1) = 1/5, F2 /F2 = 3/7 for x -> 1 There are • Dyson-Schwinger Eq.: Contains finite size S=0 and S=1 diquarks n p more! – d/u = 0.28, F2 /F2 = 0.49 for x -> 1 Multiple predictions for large x 1 1 1 p u (ud) u (ud) u (ud) 2 S 0 18 S 1 3 S 1 1 2 d (uu) d (uu) 3 S1 3 S1 n p A Longstanding Problem! Nucleon Model F2 /F2 d/u Numerous Review Articles: SU(6) 2/3 1/2 • N. Isgur, PRD 59 (1999) • S Brodsky et al NP B441 (1995) Valence Quark 1/4 0 • W. Melnitchouk and A. Thomas PL B377 (1996) DSE contact 0.41 0.18 • R.J. Holt and C. D. Roberts, Rev. interaction Mod. Phys. 82 (2010) DSE realistic 0.49 0.28 • I. Cloet et al, Few Body Syst. 46 interaction (2009) 1. pQCD 3/7 1/5 A measurement is needed… 12 n p F2 /F2 (and, hence, d/u) essentially unknown at large x: - Conflicting fundamental theory pictures n - F2 data inconclusive due to uncertainties in deuterium nuclear corrections - Translates directly to large uncertainties on d(x), g(x) parton distribution functions SU(6) symmetry pQCD DSE: 0+ & 1+ qq 0+ qq only 17 Present status: large uncertainties on PDFs at large x 18 Why is the valence regime interesting? • Partonic structure in the valence region defines a hadron - Baryon number, charge, flavor content, total spin, … • “Valence regime” at large x, low Q2 evolves to low x, high Q2 - Intersection of nuclear and particle physics • Keen discriminator of nucleon structure models • New generation of experiments at JLab focused on high x Jefferson Lab Overview • DOE Office of Science Laboratory with a single program focused on Nuclear Physics. o In operation since 1995 • Created to build and operate the Continuous Electron Beam Accelerator Facility (CEBAF), unique user facility for Nuclear Physics. • Mission is to gain a deeper understanding of the structure of Jefferson Lab by the numbers: matter: • 700 employees, 27 Joint faculty o Through advances in • 169 acre site fundamental research in nuclear physics • 1,630 Active Users • 630 PhDs granted to-date (212 in progress) o Through advances in accelerator • K-12 programs serve more than 12,000 and nuclear science and students and 950 teachers annually technology • Scientific users from 39 countries, 278 institutes (124 institutions in 34 states) 20 Jefferson Lab’s user community continues to grow 1800 1600 1400 1200 1000 US University Users International Users 800 Other Users Total Users 600 400 200 0 FY06 FY07 FY08 FY09 FY10 FY11 FY12 FY13 FY14 FY15 FY16 FY17 FY18 21 Diagram representational of below ground structure 22 RECENTLY COMPLETED UPGRADE AT JEFFERSON LAB (as of September 27, 2017) Secures a forefront scientific program for the next 10 or more years 23 TDIS to access nucleon valence structure ”BONuS” Experiment at Jefferson Lab – use fixed target Tagged DIS to create an effective free neutron target Measure DIS electron in coincidence with proton tag n p • “Hard” scattering inelastic event (high Q, W) p Deuteron Target • Proton remains intact • Low momentum proton = nucleons barely off shell Neutron target! The BONuS approach: use low mass radial TPC detector / target in magnetic field to TAG “spectator” proton at (very) low momenta (~65 MeV/c) and large angles (> 90o in lab) ….difficult but doable 2.0 1.4 ps = 0 2 2 A Q = 4 (GeV/c) 1.2 p = 0.1 I Bound / free neutron s W A p = 0.2 I P s 1.5 1.0 / x = 0.6 structure W P ) . S / F I . 0.8 S T F + S O (1%) A 0.6 I 1.0 W P ( p = 0.3 0.4 p = 0.4 p = 0.5 0.2 0.5 0 30 60 90 120 150 180 0 2 0406080 100 120 1.05 q (degrees) q [degr] pq x=0.3 1 0.4 0.95 0.5 x=0.6 n Final state Target fragmentation R 0.9 interactions 0.85 negligible O (5%) 0.8 0 100 200 300 400 |p| (MeV/c) The technique works! angle - momentum dependence W 2 = M 2 + 2Mn - Q2 78 MeV/c 93 MeV/c 110 MeV/c 135 MeV/c cospq cospq BONUS effective neutron target via TDIS achieved! Phys.Rev. C92 (2015) no.1, 015211 Phys.Rev. C91 (2015) no.5, 055206 Phys. Rev. C89 (2014) 045206 – editor’s suggestion Phys. Rev. Lett. 108 (2012) 199902 Nucl. Instrum. Meth. A592 (2008) 273-286 27 > 400 neutron data points! • Lower W, Q2 than desirable…. • Still powerful input for global PDF fits • Provides deuteron nuclear correction EMC effect in deuterium – correction for (nuclear) PDFs D n p n F2 /(F2 + F2 ) with F2 from BONUS S.I. Alekhin, S.A. Kulagin, R. Petti Phys. Rev. D 96, 054005 (2017) “The recent direct measurement of the deuteron nuclear correction by the BONuS experiment substantially reduces this uncertainty by constraining the normalization of the overall nuclear corrections.” 28 E12-06-113 “BONUS12”: Larger x and 2 higher Q CLAS12 Central High Impact Detector Hall B CLAS12 Spectrometer BONUS12 RTPC Adapter Down boards stream end plate Gas circulation slots Ground Foil Up stream Cathod field cage e Gas outlet Down Gas inlets Chamfe stream r field Downstream cage Rings Read Out GEM foil Read Out Connectors cylinders Up stream Pads end plate Up stream rings RTPC UNDER CONSTRUCTION Preparing for 2020 run! Self aligning puller assembly Removal of glued capton cylinder using puller mechanism Tagged Neutron Structure at an Electron Ion Collider The TDIS technique is better suited to colliders: no target material absorbing low-momentum nucleons, forward acceptance only! Need: • good momentum resolution (∆pT ~20 MeV, < Fermi momentum) • small intrinsic momentum spread in the ion beam for accurate reconstruction EIC being designed with this physics in mind e + D → e′ + p + X a la BONUS – neutron structure functions up to Q2 ~ 40 GeV2 JLEIC See Spectator Tagging Project at 32 https://www.jlab.org/theory/tag/ p d F2 & F2 Structure Functions in Hall C p JLab12 Hall C commissioning experiment aims to reduce uncertainties in F2 d and F2 structure functions at large x and high Q Goal @ 12 GeV: <2% total precision cross sections, (as achieved by E00-116 @ 6 GeV) E12-10-002: High Precision Measurement of the F2 Structure Function on p,D • 10.6 GeV beam • Targets: HMS SHMS LH2, LD2, Al Q2 • Fix energy and angle • Scan in momentum • Effective scan in x • Stringent check on spectrometer acceptance x 34 Preliminary Results: Structure Functions (and CJ fit) F d/F p p 2 2 F2 x x d F2 Still working on: • Some background subtractions • Systematic uncertainties • Looking towards F2d/F2p paper submission this Fall 35 Or, a nuclear physicists approach to the problem….
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