The (cid:12)rst ( ~e; e 0 ~p ) polarization transfer measurements on a nucleus heavier than deuterium have been carried out at Je(cid:11)erson Laboratory. Transverse and longitudinal components of the polarization of protons ejected in the reaction 16 O( ~e; e 0 ~p ) were measured in quasielastic perpendicular kinematics at a Q 2 of 0.8 (GeV/c) 2 . The data are in good agreement with state of the art calculations.

We measured with unprecedented precision the induced polarization P y in 4 He( e,e ′ ~p ) 3 H at Q 2 = 0 . 8 (GeV /c ) 2 and 1 . 3 (GeV /c ) 2 . The induced polarization is indicative of reaction-mechanism effects beyond the impulse approximation. Our results are in agreement with a relativistic distorted-wave impulse approximation calculation but are over-estimated by a calculation with strong charge-exchange effects. Our data are used to constrain the strength of the spin independent charge-exchange term in the latter calculation.

M. Paolone, S. P. Malace, S. Strauch, I. Albayrak, J. Arrington, B. L. Berman, E. J. Brash, B. Briscoe, A. Camsonne, J.-P. Chen, M. E. Christy, E. Chudakov, E. Cisbani, B. Craver, F. Cusanno, R. Ent, F. Garibaldi, R. Gilman, O. Glamazdin, J. Glister, D.W. Higinbotham, C. E. Hyde-Wright, Y. Ilieva, C.W. de Jager, X. Jiang, M.K. Jones, C. E. Keppel, E. Khrosinkova, E. Kuchina, G. Kumbartzki, B. Lee, R. Lindgren, D. J. Margaziotis, D. Meekins, R. Michaels, K. Park, L. Pentchev, C. F. Perdrisat, E. Piasetzky, V.A. Punjabi, A. J. R. Puckett, X. Qian, Y. Qiang, R. D. Ransome, A. Saha, A. J. Sarty, E. Schulte, P. Solvignon, R. R. Subedi, L. Tang, D. Tedeschi, V. Tvaskis, J.M. Udias, P. E. Ulmer, J. R. Vignote, F. R. Wesselmann, B. Wojtsekhowski, and X. Zhan

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Proton recoil polarization was measured in the quasielastic 4He(e,e'p)3H reaction at Q{2}=0.8 and 1.3  (GeV/c){2} with unprecedented precision. The polarization-transfer coefficients are found to differ from those of the 1H(e,e'p) reaction, contradicting a relativistic distorted-wave approximation and favoring either the inclusion of medium-modified proton form factors predicted by the quark-meson coupling model or a spin-dependent charge-exchange final-state interaction. For the first time, the polarization-transfer ratio is studied as a function of the virtuality of the proton.

M. Nozar, C. Salgado, D. P. Weygand, L. Guo,* G. Adams, Ji Li, P. Eugenio, M. J. Amaryan, M. Anghinolfi, G. Asryan, H. Avakian, H. Bagdasaryan, N. Baillie, J. P. Ball, N.A. Baltzell, S. Barrow, M. Battaglieri, I. Bedlinskiy, M. Bektasoglu, M. Bellis, N. Benmouna, B. L. Berman, A. S. Biselli, L. Blaszczyk, B. E. Bonner, S. Bouchigny, S. Boiarinov, R. Bradford, D. Branford, W. J. Briscoe, W.K. Brooks, S. Bültmann, V.D. Burkert, C. Butuceanu, J. R. Calarco, S. L. Careccia, D. S. Carman, B. Carnahan, L. Casey, A. Cazes, S. Chen, L. Cheng, P. L. Cole, P. Collins, P. Coltharp, D. Cords,1,kk P. Corvisiero, D. Crabb, H. Crannell, V. Crede, J. P. Cummings, D. Dale, N. Dashyan, R. De Masi, R. De Vita, E. De Sanctis, P. V. Degtyarenko, H. Denizli, L. Dennis, A. Deur, K. V. Dharmawardane, K. S. Dhuga, R. Dickson, C. Djalali, G. E. Dodge, D. Doughty, M. Dugger, S. Dytman, O. P. Dzyubak, H. Egiyan, K. S. Egiyan,41,kk L. El Fassi, L. Elouadrhiri, R. Fatemi, G. Fedotov, R. J. Feuerbach, T. A. Forest, A. Fradi, H. Funsten,40,kk M. Garçon, G. Gavalian, N. Gevorgyan, G. P. Gilfoyle, K. L. Giovanetti, F. X. Girod, J. T. Goetz, R.W. Gothe, K.A. Griffioen, M. Guidal, M. Guillo, N. Guler, V. Gyurjyan, C. Hadjidakis, K. Hafidi, H. Hakobyan, C. Hanretty, J. Hardie, N. Hassall, D. Heddle,1,x F.W. Hersman, K. Hicks, I. Hleiqawi, M. Holtrop, C. E. Hyde-Wright, Y. Ilieva, D.G. Ireland, B. S. Ishkhanov, E. L. Isupov, M.M. Ito, D. Jenkins, H. S. Jo, J. R. Johnstone, K. Joo, H.G. Juengst, N. Kalantarians, J. D. Kellie, M. Khandaker, W. Kim, A. Klein, F. J. Klein, M. Kossov, Z. Krahn, L. H. Kramer, V. Kubarovsky, J. Kuhn, S. E. Kuhn, S. V. Kuleshov, V. Kuznetsov, J. Lachniet, J.M. Laget, J. Langheinrich, D. Lawrence, K. Livingston, H.Y. Lu, M. MacCormick, N. Markov, P. Mattione, S. McAleer, B. McKinnon, J.W. C. McNabb, B. A. Mecking, S. Mehrabyan, M.D. Mestayer, C.A. Meyer, T. Mibe, K. Mikhailov, M. Mirazita, R. Miskimen, V. Mokeev, B. Moreno, K. Moriya, S. A. Morrow, M. Moteabbed, J. Mueller, E. Munevar, G. S. Mutchler, P. Nadel-Turonski, R. Nasseripour, S. Niccolai, G. Niculescu, I. Niculescu, B. B. Niczyporuk, M. R. Niroula, R. A. Niyazov, G. V. O’Rielly, M. Osipenko, A. I. Ostrovidov, K. Park,25,k E. Pasyuk, C. Paterson, S. Anefalos Pereira, S. A. Philips, J. Pierce, N. Pivnyuk, D. Pocanic, O. Pogorelko, E. Polli, I. Popa, S. Pozdniakov, B.M. Preedom, J.W. Price, Y. Prok,39,{ D. Protopopescu, L.M. Qin, B. A. Raue, G. Riccardi, G. Ricco, M. Ripani, B.G. Ritchie, F. Ronchetti, G. Rosner, P. Rossi, P. D. Rubin, F. Sabatié, J. Salamanca, J. P. Santoro, V. Sapunenko, R. A. Schumacher, V. S. Serov, Y.G. Sharabian, D. Sharov, N. V. Shvedunov, A.V. Skabelin, E. S. Smith, L. C. Smith, D. I. Sober, D. Sokhan, A. Stavinsky, S. S. Stepanyan, S. Stepanyan, B. E. Stokes, P. Stoler, I. I. Strakovsky, S. Strauch, M. Taiuti, D. J. Tedeschi, U. Thoma,** A. Tkabladze, S. Tkachenko, L. Todor, M. Ungaro, M. F. Vineyard, A.V. Vlassov, D. P. Watts, L. B. Weinstein, M. Williams, E. Wolin, M.H. Wood,35,xx A. Yegneswaran, L. Zana, J. Zhang, B. Zhao, and Z.W. Zhao

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The exclusive electroproduction process (cid:2) ep → e (cid:4) nπ + was measured in the range of the photon virtuality Q 2 = 1 . 7–4 . 5 GeV 2 , and the invariant mass range for the nπ + system of W = 1 . 15–1 . 7 GeV using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive π + production from protons with nearly full coverage in the azimuthal and polar angles of the nπ + center-of-mass system. The nπ + channel has particular sensitivity to the isospin 12 excited nucleon states, and together with the pπ 0 final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the σ LT (cid:4) structure function. In this experiment, 31,295 cross section and 4,184 asymmetry data points were measured. Because of the large volume of data, only a reduced set of structure functions and Legendre polynomial moments can be presented that are obtained in model-independent fits to the differential cross sections.

P. E. Bosted,35,* R. Fersch,39 G. Adams,31 M. Amarian,29 S. Anefalos,17 M. Anghinolfi,18 G. Asryan,40 H. Avakian,17,35 H. Bagdasaryan,29,40 N. Baillie,39 J. P. Ball,2 N. A. Baltzell,34 S. Barrow,13 V. Batourine,35 M. Battaglieri,18 K. Beard,21 I. Bedlinskiy,20 M. Bektasoglu,29 M. Bellis,5,31 N. Benmouna,14 A. S. Biselli,11 B. E. Bonner,32 S. Bouchigny,19,35 S. Boiarinov,20,35 R. Bradford,5 D. Branford,10 W. K. Brooks,35 S. Bültmann,29 V. D. Burkert,35 C. Butuceanu,39 J. R. Calarco,26 S. L. Careccia,29 D. S. Carman,35 B. Carnahan,6 A. Cazes,34 S. Chen,13 P. L. Cole,16,35 P. Collins,2 P. Coltharp,13 D. Cords,35,† P. Corvisiero,18 D. Crabb,38 H. Crannell,6 V. Crede,13 J. P. Cummings,31 R. De Masi,7 R. De Vita,18 E. De Sanctis,17 P. V. Degtyarenko,35 H. Denizli,30 L. Dennis,13 A. Deur,35 C. Djalali,34 G. E. Dodge,29 J. Donnelly,15 D. Doughty,8,35 P. Dragovitsch,13 M. Dugger,2 K. V. Dharmawardane,29,‡ S. Dytman,30 O. P. Dzyubak,34 H. Egiyan,35,39,§ K. S. Egiyan,40,† L. Elouadrhiri,8,35 P. Eugenio,13 R. Fatemi,38 G. Fedotov,25 R. J. Feuerbach,5 T. A. Forest,29 A. Fradi,19 H. Funsten,39 M. Garçon,7 G. Gavalian,26,29 G. P. Gilfoyle,33 K. L. Giovanetti,21 F. X. Girod,7 J. T. Goetz,3 E. Golovatch,18,‖ R. W. Gothe,34 K. A. Griffioen,39 M. Guidal,19 M. Guillo,34 N. Guler,29 L. Guo,35 V. Gyurjyan,35 C. Hadjidakis,19 K. Hafidi,1 R. S. Hakobyan,6 J. Hardie,8,35 D. Heddle,8,35 F. W. Hersman,26 K. Hicks,28 I. Hleiqawi,28 M. Holtrop,26 M. Huertas,34 C. E. Hyde-Wright,29 Y. Ilieva,14 D. G. Ireland,15 B. S. Ishkhanov,25 E. L. Isupov,25 M. M. Ito,35 D. Jenkins,37 H. S. Jo,19 K. Joo,9 H. G. Juengst,29 N. Kalantarians,29 C. Keith,35 J. D. Kellie,15 M. Khandaker,27 K. Y. Kim,30 K. Kim,22 W. Kim,22 A. Klein,29,¶ F. J. Klein,6,12 M. Klusman,31 M. Kossov,20 L. H. Kramer,12,35 V. Kubarovsky,31,35 J. Kuhn,5,31 S. E. Kuhn,29 S. V. Kuleshov,20 J. Lachniet,5,29 J. M. Laget,7,35 J. Langheinrich,34 D. Lawrence,24 Ji Li ,31 A. C. S. Lima,14 K. Livingston,15 H. Lu,34 K. Lukashin,6 M. MacCormick,19 N. Markov,9 S. McAleer,13 B. McKinnon,15 J. W. C. McNabb,5 B. A. Mecking,35 M. D. Mestayer,35 C. A. Meyer,5 T. Mibe,28 K. Mikhailov,20 R. Minehart,38 M. Mirazita,17 R. Miskimen,24 V. Mokeev,25 L. Morand,7 S. A. Morrow,7,19 M. Moteabbed,12 J. Mueller,30 G. S. Mutchler,32 P. Nadel-Turonski,14 R. Nasseripour,12,34 S. Niccolai,14,19 G. Niculescu,21 I. Niculescu,14,21 B. B. Niczyporuk,35 M. R. Niroula,29 R. A. Niyazov,29,35 M. Nozar,35 G. V. O’Rielly,14 M. Osipenko,18,25 A. I. Ostrovidov,13 K. Park,22 E. Pasyuk,2 C. Paterson,15 S. A. Philips,14 J. Pierce,38 N. Pivnyuk,20 D. Pocanic,38 O. Pogorelko,20 E. Polli,17 S. Pozdniakov,20 B. M. Preedom,34 J. W. Price,4 Y. Prok,38,** D. Protopopescu,15,26 L. M. Qin,29 B. A. Raue,12,35 G. Riccardi,13 G. Ricco,18 M. Ripani,18 G. Rosner,15 P. Rossi,17 D. Rowntree,23 P. D. Rubin,33 F. Sabatié,7,29 C. Salgado,27 J. P. Santoro,35,37,†† V. Sapunenko,18,35 R. A. Schumacher,5 V. S. Serov,20 Y. G. Sharabian,35 J. Shaw,24 N. V. Shvedunov,25 A. V. Skabelin,23 E. S. Smith,35 L. C. Smith,38 D. I. Sober,6 A. Stavinsky,20 S. S. Stepanyan,22 S. Stepanyan,8,35,40 B. E. Stokes,13 P. Stoler,31 S. Strauch,34 R. Suleiman,23 M. Taiuti,18 S. Taylor,32 D. J. Tedeschi,34 U. Thoma,35,‡‡ A. Tkabladze,14 S. Tkachenko,29 L. Todor,5 M. Ungaro,9 M. F. Vineyard,33,36 A. V. Vlassov,20 L. B. Weinstein,29 D. P. Weygand,35 M. Williams,5 E. Wolin,35 M. H. Wood,34,§§ A. Yegneswaran,35 J. Yun,29 L. Zana,26 J. Zhang,29 B. Zhao,9 and Z. Zhao34 (CLAS Collaboration) 1Argonne National Laboratory, Argonne, Illinois 60439, USA 2Arizona State University, Tempe, Arizona 85287-1504, USA 3University of California at Los Angeles, Los Angeles, California 90095-1547, USA 4California State University, Dominguez Hills, Carson, California 90747, USA 5Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA 6Catholic University of America, Washington, D.C. 20064, USA 7CEA-Saclay, Service de Physique Nucléaire, F-91191 Gif-sur-Yvette, France 8Christopher Newport University, Newport News, Virginia 23606, USA 9University of Connecticut, Storrs, Connecticut 06269, USA 10Edinburgh University, Edinburgh EH9 3JZ, United Kingdom 11Fairfield University, Fairfield, Connecticut 06824, USA 12Florida International University, Miami, Florida 33199, USA 13Florida State University, Tallahassee, Florida 32306, USA 14George Washington University, Washington, D.C. 20052, USA 15University of Glasgow, Glasgow G12 8QQ, United Kingdom 16Idaho State University, Pocatello, Idaho 83209, USA 17INFN, Laboratori Nazionali di Frascati, I-00044 Frascati, Italy 18INFN, Sezione di Genova, I-16146 Genova, Italy 19Institut de Physique Nucleaire ORSAY, Orsay, France 20Institute of Theoretical and Experimental Physics, RU-117259 Moscow, Russia 21James Madison University, Harrisonburg, Virginia 22807, USA 22Kyungpook National University, Daegu 702-701, South Korea 23Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA 24University of Massachusetts, Amherst, Massachusetts 01003, USA 25Moscow State University, General Nuclear Physics Institute, RU-119899 Moscow, Russia

Citation for published version: Nasseripour, R, Raue, BA, Carman, DS, Ambrozewicz, P, Amaryan, MJ, Anciant, E, Anghinolfi, M, Asavapibhop, B, Asryan, G, Audit, G, Auger, T, Avakian, H, Bagdasaryan, H, Baillie, N, Ball, JP, Baltzell, NA, Barrow, S, Battaglieri, M, Beard, K, Bedlinskiy, I, Bektasoglu, M, Bellis, M, Benmouna, N, Berman, BL, Biselli, AS, Blaszczyk, L, Bonner, BE, Bouchigny, S, Boiarinov, S, Bradford, R, Branford, D, Briscoe, WJ, Brooks, WK, Burkert, VD, Butuceanu, C, Calarco, JR, Careccia, SL, Casey, L, Cetina, C, Chen, S, Cheng, L, Cole, PL, Collins, P, Coltharp, P, Cords, D, Corvisiero, P, Crabb, D, Crede, V, Thompson, R, Watts, DP & CLAS Collaboration 2008, 'Polarized structure function sigma(')(LT) for H-1((e)over-rightarrow,e(')K(+))Lambda in the nucleon resonance region', Physical Review C, vol. 77, no. 6, 065208, pp. -. https://doi.org/10.1103/PhysRevC.77.065208

The cross section and decay angular distributions for the coherent φ meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t = ( p γ − p φ ) 2 = − 2 GeV 2 /c 2 , using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a re-scattering model. In a framework of vector meson dominance, the data are consistent with the total φ -N cross section σ φN at about 10 mb. If vector meson dominance is violated, a larger σ φN is possible by introducing larger t -slope for the φN → φN process than that for the γN → φN process. The decay angular distributions of the φ are consistent with helicity conservation.

Differential cross sections for the reaction γp→K*0Σ+ are presented in the photon energy range of 1.7 to 3.0 GeV. The K*0 was detected by its decay products, K+π−, in the Continuous Electron Beam Accelerator Facility's large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility. These data are the first K*0 photoproduction cross sections ever published over a broad range of angles. Comparison with a theoretical model based on the vector and tensor K∗-quark couplings shows good agreement with the data, except at forward angles, suggesting that the role of scalar κ meson exchange should be investigated.

We outline the compelling physics case for e+A collisions at an Electron Ion Collider (EIC). With its wide range in energy, nuclear beams, high luminosity and clean collider environment, the EIC offers an unprecedented opportunity for discovery and for the precision study of a novel universal regime of strong gluon fields in Quantum Chromodynamics (QCD). The EIC will measure, in a wide kinematic regime, the momentum and space-time distribution of gluons and sea-quarks in nuclei, the scattering of fast, compact probes in extended nuclear media and role of color neutral (Pomeron) excitations in scattering off nuclei. These measurements at the EIC will also deepen and corroborate our understanding of the formation and properties of the strongly interacting Quark Gluon Plasma (QGP) in high energy heavy ion collisions at RHIC and the LHC. Q uantum Chromodynamics (QCD), the theory of strong interactions, is a cornerstone of the standard model of physics. Approximately 99% of the mass of baryonic matter in the universe can be attributed to QCD. This mass derives from “emergent phenomena” of the QCD vacuum that are not evident from the Lagrangian. These phenomena include chiral symmetry breaking and confinement that are fundamental features of the strong interactions. Lattice gauge theory and effective field theories have taught us that the rich and complex structure of the QCD vacuum arises primarily from the dynamics of gluons with small contributions from the quark sea. Experiments probe the QCD vacuum in a variety of ways. In electron-positron annihilation, we observe the response of the vacuum to the deposition of enormous amounts of energy into minuscule space-time volumes. In hadron spectroscopy, we observe the way in which configurations of quarks and gluons carve out and inhabit bubbles in the vacuum. In relativistic heavy ion collisions, we observe the evolution of the vacuum after first heating a macroscopic ( 1 fm ) chunk of it to trillions of degrees Kelvin. Precision measurements of deep inelastic scattering (DIS) of leptons with hadrons study properties of the QCD vacuum that are manifest in the structure of matter at resolution scales of less than a femtometer. DIS experiments with nuclei can identify those features of the short distance structure of matter which are common to all strongly interacting states. The kinematic invariants in fully inclusive DIS are the Bjorken variable x, the momentum transfer squared Q2 > 0, the inelasticity y and s, the c.m. energy squared 1. For fixed y, x ∝ Q2/s; thus high energies allow us to probe small values of x. DIS experiments of electrons off protons at the HERA collider at DESY have shown that, for Q2 ΛQCD (where ΛQCD ∼ 200 MeV), the gluon density grows rapidly with decreasing x. For x < 0.01 the proton wave function is predominantly gluonic. DIS experiments with nuclei have established that quark and gluon distributions in nuclei exhibit shadowing; they are modified significantly relative to their distributions in the nucleon wave function. However, in sharp contrast to the proton, the gluonic structure of nuclei is not known for x < 0.01. The nature of gluon shadowing is terra incognita in QCD at high energies. We will discuss in the following how an Electron Ion Collider (EIC) can enhance our understanding of universal features of the dynamics of gluons in nuclei. At large x and at large Q2, the properties of quarks and gluons in the hadron are well described by the linear evolution equations of perturbative QCD (pQCD). The rapid growth in gluon densities with decreasing x is understood to follow from a self similar Bremsstrahlung cascade where harder (large x) parent gluons successively shed softer daughter gluons. Gluon saturation is a simple mechanism for nature to tame this growth. When the density of gluons becomes large, softer gluons can recombine into harder gluons. The competition between linear QCD Bremsstrahlung and non-linear gluon recombination causes the gluon distributions to saturate at small x. The onset of saturation and the properties of the saturated phase are characterized by a dynamical scale Qs which grows with increasing energy (smaller x) and increasing nuclear size A. The nucleus is an efficient amplifier of the universal physics of high gluon densities. Simple considerations 2 suggest that Qs ∝ (A/x)1/3. Therefore DIS with large nuclei probes the same universal physics as seen in DIS with protons at x’s at least two orders of magnitude lower (or equivalently an order of magnitude larger √ s). Fig. 1 shows the saturation scale for protons and nuclei as a function of x in relation to the kinematic reach in x and Q2 of EIC. When Q2 Qs , one is in the well understood “linear” regime of QCD. For large nuclei there is a significant window at small x where Qs Q2 ΛQCD and where one is in the domain of strong non-linear gluon fields. The intensity of the chromo-electric and chromomagnetic fields in the strong gluon field regime is of order O(1/αS), where the asymptotic freedom of QCD dictates that the fine structure constant αS(Qs ) 1. These fields are therefore the strongest fields in nature! Remarkably, the weak coupling suggests that the onset and properties of this regime may be computed systematically in a QCD framework. The high occupation numbers of gluons ensures that their dynamics is classical and their piling up at a characteristic momentum scale (QA s ) is reminiscent of a Bose–Einstein condensate. Further, kinematic arguments suggest that the time scales of in1. For a brief primer on DIS kinematics, we refer the reader to the text box on page 4 2. For an expanded discussion, see the text box on page 9. Physics with an Electron Ion Collider 1 eRHIC e+Au, (20 GeV + 100 GeV/n) eRHIC e+Au, (10 GeV + 100 GeV/n) ELIC e+Ca, (7 GeV + 75 GeV/n) EIC 1 = y Q 2 s roton Q 2 s Ca (ntral) Q 2 s,q Au (ntral) ×9/4 Q 2 s,g 10 -1 1 10 10 2 10 3 10 4 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 x Q 2 V e G ( 2 ) Figure 1: Kinematic acceptance in the (Q2, x) plane for the EIC. Shown are lines for two complementary concepts to realize EIC, eRHIC and ELIC (see page 14 for details). Lines showing the quark saturation scale Qs for protons, Ca, and Au nuclei are superposed on the kinematic acceptance. As indicated, the gluon saturation scale in Au nuclei is larger by the color factor 9/4. teractions of gluons are in practice time dilated well beyond characteristic time scales for gluon interactions. This slowing down is analogous to what happens in spin glasses. These dynamical and kinematic considerations have led to a suggestion that the matter in nuclear wave functions at high energies is universal and can be described as a Color Glass Condensate (CGC) [1, 2]. Alternative candidates for the appropriate degrees of freedom in QCD at high energies include color neutral excitations with vacuum quantum numbers called Pomerons. These come in soft (non-perturbative) or hard (perturbative) varieties [3]. A wide range of measurements with EIC, which we shall discuss shortly, can distinguish between predictions in the CGC (or other “unconventional” frameworks) and those following from the linear evolution equations of pQCD. The nucleus is also a powerful analyzer of physics across the full range of x, Q2 and A. In e+A collisions at high energies, the virtual photon mediating the interaction splits into a compact qq̄ “dipole” which scatters off the nuclear medium. The interaction of these fast, compact dipoles with an extended gluon medium provides insight into how partons lose energy, are absorbed, and how hadron formation is modified in the presence of a colored medium. VaryR g ( x , Q 2 = 5 G e V 2 )