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Proposal to Jefferson Lab PAC39 Exclusive Phi Meson Proposal to Jefferson Lab PAC39 Exclusive Phi Meson Electroproduction with CLAS12 H. Avakian,1 J. Ball,2 A. Biselli,3 V. Burkert,1 R. Dupr,2 L. Elouadrhiri,1 1 1, 4 5, 6 5, R. Ent, F.{X. Girod, ∗ S. Goloskokov, B. Guegan, M. Guidal, ∗ 5 7 8 5 2 6, H.{S. Jo, K. Joo, P. Kroll, A Marti, H. Moutarde, A. Kubarovsky, ∗ 1, 5 5 1 5 V. Kubarovsky, ∗ C. Munoz Camacho, S. Niccolai, K. Park, R. Paremuzyan, 2 2 6, 5 5 1 6, S. Procureur, F. Sabati´e, N. Saylor, D. Sokhan, S. Stepanyan, P. Stoler, y 7 9 1, 1 M. Ungaro, E. Voutier, C. Weiss, y D. Weygand, and the CLAS Collaboration 1Jefferson Lab, Newport News, VA 23606, USA 2IRFU/SPhN, Saclay, France 3Fairfield University 4Joint Institute for Nuclear Research, Dubna, Russia 5Institut de Physique Nucleaire Orsay, France 6Rensselaer Polytechnic Institute 7Department of Physics, University of Connecticut, Storrs, CT 06269, USA 8Wuppertal University, Wuppertal, Germany 9LPSC Grenoble, France ∗Spokespersons ySpokespersons,Contact persons 2 Summary We propose a measurement of exclusive φ meson electroproduction on the proton, ep ! e0 + φ + p, at 11 GeV beam energy with the CLAS12 detector. The kinematic range extends 2 2 2 in W from 2{5 GeV, Q from 1{12 GeV , and jt − tminj from near zero to ∼ 4 GeV , the precise limits depending on the specific values of the other variables. The φ will be detected + through the K K− and (for the first time) the KSKL mode, which allows for an independent test of the cross section extraction. Differential cross sections and beam spin asymmetries will be measured as functions of the φ ! KK decay angles, θ and φ, to extract the structure functions σT ; σL; σTT ; σLT and σLT 0 . Exclusive φ electroproduction at Q2 ∼ few GeV2 is of special significance as a probe of the nucleon's gluon generalized parton distribution (GPD), which represents the nucleon's \gluonic form factor" and reveals the transverse spatial distribution of gluons in the nucleon. Theoretical calculations including perturbative and non{perturbative QCD interactions de- scribe the available exclusive φ cross section data over a wide range of W and Q2, including the CLAS 6 GeV data, and permit a quantitative analysis in terms of the gluon GPD even at JLab energies. The nucleon's gluonic form factor and the spatial distribution of \valence{ like" gluons at x ∼ 0:2 − 0:5 will be extracted from the relative t{dependence of dσL=dt and is insensitive to finite–size corrections in the QCD process and the uncertainties of the abso- lute cross section measurements. Exclusive φ electroproduction at energies near threshold, W −Wth ∼ few 100 MeV, may also provide information on potential intrinsic strangeness in the nucleon (correlated ss¯ pairs), which is being discussed in connection with semi{inclusive measurements. The objectives of the proposed experiment are to (a) Quantify the approach to the regime of small{size configurations at high Q2 by testing model{independent features of the reaction mechanism, such as the Q2 scaling of cross sections and t{slopes, the change of W dependence with Q2, the L=T ratio obtained from φ ! KK decays and response functions, and other observables; (b) Extract the nucleon's gluonic size in the valence region from the relative t{dependence of dσL=dt, both model{independently (x{averaged size) and with information from GPD{based model calculations (x{dependent size); (c) Explore signatures of a possible intrinsic ss¯ component of the nucleon in exclusive φ production near threshold. FastMC simulations with a realistic cross section parametrization indicate that 60 days of beam time would provide sufficient statistics to address the stated physics objectives. This experiment would run in parallel with the approved CLAS12 DVCS (E12-06-119) and neutral 0 pseudoscalar meson production (π ; η; η0, E12-06-108) experiments. 3 Contents Summary 2 I. Introduction 4 II. Physics Motivation 7 A. Exclusive φ electroproduction and gluon GPD 7 B. Testing the approach to the small{size regime 10 C. Gluonic radius of nucleon 12 D. Helicity–flip vs. non–flip gluon GPD 18 E. Intrinsic strangeness 19 III. Kinematics and Cross Sections 23 A. Kinematics of the reaction ep ! e0p0φ 23 B. Cross section parametrization 24 IV. Exclusive φ Detection with CLAS12 28 A. CLAS12 detector 28 B. Particle identification. 29 1. Electron identification. 29 2. K± identification. 30 3. φ Detection through KSKL Decay Mode 32 C. Monte Carlo Simulations of Acceptance and Resolution 33 V. Projected Results 36 A. σL/σT separation 36 B. Differential Cross Sections and t{dependence 40 C. Slopes and t ! 0 extrapolation 42 VI. Beam time request 44 VII. Exclusive φ production in the context of the JLab 12 GeV program 45 References 46 4 I. INTRODUCTION A central goal of the 12 GeV Upgrade of JLab is to explore the internal structure of the nucleon at resolution scales 1 fm, where it can be described in terms of the quark and gluon degrees of freedom of QCD [1]. Measurements of the inclusive eN structure functions will provide detailed information on the valence quarks' momentum distribution, including their spin and flavor dependence. Likewise, the form factors of elastic eN scattering reveal the spatial size of valence quark configurations in the nucleon and their response to polarization. The valence quarks, however, represent only part of the nucleon's structure at large light{ front momentum fractions x. It is known from fits to deep{inelastic scattering data that the nucleon contains a substantial density of gluons at x > 0:1, which carry more than 30% of its total momentum at low scales [2]. Recent results from semi{inclusive scattering [3] suggest that the nucleon may also include an \intrinsic" sea of quark{antiquark pairs at large x, as expected from theoretical considerations. These large{x gluons and sea quarks are thought to originate from non{perturbative correlations in the nucleon wave function and are physically distinct from the small{x gluons and sea quarks seen in high{energy scattering experiments, which are produced mostly by perturbative QCD radiation. Measuring the properties of the large{x gluons and sea quarks is essential to fully understand the nucleon in QCD as an interacting many{body system. Of particular interest is the spatial distribution of QCD quarks and gluons in the nucleon. For a relativistic system the proper way to quantify the spatial structure is in terms of the transverse densities of partons (quarks, gluons) in the infinite–momentum frame [4{ 7]. They are obtained as the Fourier transform of the Generalized Parton Distributions (or GPDs), which represent the nucleon form factors for partons with given light{front momentum fraction x; see Refs. [8{10] for a review. These concepts have enabled a rich program of \quark/gluon imaging" of the nucleon, involving a combination of experimental data, dynamical models, and even lattice QCD calculations. The transverse distribution of valence quarks is constrained by the nucleon elastic form factor data (transverse charge densities) and will be measured differentially in x in Deeply{virtual Compton scattering (DVCS) at JLab 12 GeV. Very little is known about the transverse spatial distribution of the \valence{like" gluons in the nucleon. One interesting question is how the nucleon's gluonic RMS radius relates to its quark radius, i.e., whether the gluons sit at smaller or larger transverse distances than the valence quarks. This question directly impacts on our understanding of the dynamical origin of large{x gluons in the nucleon. For instance, if the large{x gluons arise due to correlations of valence quarks one expects a smaller gluonic radius than quark radius, while a picture in which the large{x gluons are \packaged" inside constituent quarks would imply similar radii. Data from high{energy experiments at HERA [11, 12] and FNAL [13] suggest that the transverse spatial distribution of small{x gluons may be narrower than those of quarks; however, nothing is known about the spatial distribution of gluons with x > 0:1. Another interesting question is how the nucleon's gluonic form factor (or gluon GPD) behaves at large t ( 1 GeV2). This describes how the nucleon responds elastically when a large momentum is transferred to its gluon field and again reveals new information on its internal structure. 5 Here we propose to measure the nucleon's gluonic form factor and the spatial distribution of \valence{like" gluons through exclusive electroproduction of φ mesons on the proton, 2 γ∗(Q ) + p ! φ + p: (1) At Q2 ∼ few GeV2 the φ meson is produced in the form of a small{size ss¯ pair coupling to the gluon fields in the nucleon (\small{size configuration”). This reaction thus provides a clean probe of the nucleon's gluon GPD in the valence region (x ∼ 0:2−0:5). In recent years theorists made significant progress in developing the QCD{based description of exclusive φ electroproduction at Q2 ∼ few GeV2, implementing perturbative as well as non{perturbative QCD interactions [14, 15]. Quantitative predictions for absolute cross sections are available and describe the JLab 6 GeV data (details will be given below). Because of its almost pure ss¯ composition φ production is not affected by scattering from the nucleon's valence quarks or the light quark sea; the latter play a prominent role in the production of other light vector mesons (ρ, !) and make their GPD{based description more challenging. The interesting information on the t{dependence of the gluon GPD, which defines the spatial image of gluons in the nucleon, is contained in the relativet{dependence of the exclusive φ differential cross sections and thus insensitive to details of the QCD scattering process (coupling, absolute gluon density) and the φ meson distribution amplitude; this information can therefore be reliably extracted from the data at Q2 ∼ few GeV2.
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