The Next QCD Frontier - Skipper.physics.sunysb.edu
Electron Ion Collider:The Next QCD FrontierUnderstanding the glue that binds us all
White Paper Writing CommitteeElke C. AschenauerBrookhaven National LaboratoryWilliam BrooksUniversidad Técnica Federico Santa MariaAbhay Deshpande1Stony Brook UniversityMarkus DiehlDeutsches Elektronen-Synchrotron DESYHaiyan GaoDuke UniversityRoy HoltArgonne National LaboratoryTanja HornThe Catholic University of AmericaAndrew HuttonThomas Jefferson National Accelerator FacilityYuri KovchegovThe Ohio State UniversityKrishna KumarUniversity of Massachusetts, AmherstZein-Eddine Meziani1Temple UniversityAlfred MuellerColumbia UniversityJianwei Qiu1Brookhaven National LaboratoryMichael Ramsey-MusolfUniversity of WisconsinThomas RoserBrookhaven National Laboratory1Co-Editor1
Franck SabatiéCommissariat à l’ Énergie Atomique-SaclayErnst SichtermannLawrence Berkeley National LaboratoryThomas UllrichBrookhaven National LaboratoryWerner VogelsangUniversity of TübingenFeng YuanLawrence Berkeley National LaboratoryLaboratories Management RepresentativesRolf EntThomas Jefferson National Accelerator FacilityRobert McKeownThomas Jefferson National Accelerator FacilityThomas LudlamBrookhaven National LaboratorySteven VigdorBrookhaven National Laboratory2
Contents1 Executive Summary: Exploring the Glue that Binds Us All1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2 Science Highlights of the Electron Ion Collider . . . . . . . . .1.2.1 Nucleon Spin and its 3D Structure and Tomography . .1.2.2 The Nucleus, a QCD Laboratory . . . . . . . . . . . . .1.2.3 Physics Possibilities at the Intensity Frontier . . . . . .1.3 The Electron Ion Collider and its Realization . . . . . . . . . .1.4 Physics Deliverables of the Stage I of EIC . . . . . . . . . . . .2 Spin and Three-Dimensional Structure of the Nucleon2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2 Longitudinal Spin of the Nucleon . . . . . . . . . . . . . . .2.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . .2.2.2 Status and Near Term Prospects . . . . . . . . . . .2.2.3 Open Questions and the Role of an EIC . . . . . . .2.3 Confined Motion of Partons in Nucleons: TMDs . . . . . .2.3.1 Introcution . . . . . . . . . . . . . . . . . . . . . . .2.3.2 Opportunites for Measurements of TMDs at the EICSemi-inclusive Deep Inelastic Scattering (SIDIS) . .Access to the Gluon TMDs . . . . . . . . . . . . . .2.3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . .2.4 Spatial Imaging of Quarks and Gluons . . . . . . . . . . . .2.4.1 Physics Motivations and Measurement Principle . .2.4.2 Processes and Observables . . . . . . . . . . . . . . .2.4.3 Parton Imaging Now and in the Next Decade . . . .2.4.4 Accelerator and Detector Requirements . . . . . . .2.4.5 Parton Imaging with the EIC . . . . . . . . . . . . .2.4.6 Opportunities with Nuclei . . . . . . . . . . . . . . .3 The Nucleus: A Laboratory for QCD3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2 Physics of High Gluon Densities in Nuclei . . . . . . . . . . . .3.2.1 Gluon Saturation: a New Regime of QCD . . . . . . . .Nonlinear Evolution . . . . . . . . . . . . . . . . . . . .Classical Gluon Fields and the Nuclear “Oomph” FactorMap of High Energy QCD and the Saturation Scale . .Nuclear Structure Functions . . . . . . . . . . . . . . . 256.5858636363666871
Diffractive Physics . . . . . . . . . . . . . . . . . . . . . .Key Measurements . . . . . . . . . . . . . . . . . . . . . .Structure Functions . . . . . . . . . . . . . . . . . . . . .Di-Hadron Correlations . . . . . . . . . . . . . . . . . . .Measurements of Diffractive Events . . . . . . . . . . . . .Quarks and gluons in the nucleus . . . . . . . . . . . . . . . . . .3.3.1 The distributions of quarks and gluons in a nucleus . . . .3.3.2 Propagation of a fast moving color charge in QCD matter3.3.3 Strong color fluctuations inside a large nucleus . . . . . .Connections to pA, AA and Cosmic Ray Physics . . . . . . . . .3.4.1 Connections to pA Physics . . . . . . . . . . . . . . . . .3.4.2 Connections to Ultrarelativistic Heavy-Ion Physics . . . .Initial Conditions in AA collisions . . . . . . . . . . . . .Energy Loss and Hadronization . . . . . . . . . . . . . . .3.4.3 Connections to Cosmic Ray Physics . . . . . . . . . . . .3.2.23.33.4.747677798287879093959598991031054 Possibilities at the Luminosity Frontier: Physics Beyond the StandardModel1074.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074.2 Specific Opportunities in Electroweak Physics . . . . . . . . . . . . . . . . . 1084.2.1 Charged Lepton Flavor Violation . . . . . . . . . . . . . . . . . . . . 1084.2.2 Precision Measurements of Weak Neutral Current Couplings . . . . 1094.3 EIC Requirements for Electroweak Physics Measurements . . . . . . . . . . 1125 The Accelerator Designs and Challenges5.1 eRHIC . . . . . . . . . . . . . . . . . . . .5.1.1 eRHIC Design . . . . . . . . . . .5.1.2 eRHIC Interaction Region . . . . .5.1.3 eRHIC R&D . . . . . . . . . . . .5.2 MEIC: The Jefferson Lab Implementation5.2.1 Jefferson Lab Staged Approach . .5.2.2 Baseline Design . . . . . . . . . . .Ion Complex . . . . . . . . . . . .Collider Rings . . . . . . . . . . .Interaction Regions . . . . . . . . .Electron Cooling . . . . . . . . . .1131131151171171181181191221221231236 The EIC Detector Requirements and Design Ideas6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.2 Kinematic Coverage . . . . . . . . . . . . . . . . . . . . . . . .6.2.1 y Coverage . . . . . . . . . . . . . . . . . . . . . . . . .6.2.2 Angle and Momentum Distributions . . . . . . . . . . .6.2.3 Recoil Baryon Angles and t Resolution . . . . . . . . . .6.2.4 Luminosity Measurement . . . . . . . . . . . . . . . . .6.2.5 Hadron and Lepton Polarimetry . . . . . . . . . . . . .6.3 Detector and Interaction Region (IR) Layout . . . . . . . . . .6.3.1 eRHIC Detector & IR Considerations and Technologies6.3.2 Detector Design for MEIC/ELIC . . . . . . . . . . . . .1251251251251261291291311321321344.
Acknowledgment139References1405
031323334Chapter 1Executive Summary: Exploring theGlue that Binds Us All1.1IntroductionNuclear science is concerned with the origin and structure of the core of the atom, thenucleus and the nucleons (protons and neutrons) within it, which account for essentiallyall of the mass of the visible universe. A half-century of investigations have revealed thatnucleons are themselves composed of more basic constituents called quarks, bound togetherby the exchange of gluons, and have led to the development of the fundamental theoryof strong interactions known as Quantum Chromo-Dynamics (QCD). Understanding theseconstituent interactions and the emergence of nucleons and nuclei from the properties anddynamics of quarks and gluons in QCD is a fundamental and compelling goal of nuclearscience.QCD attributes the forces among quarks and gluons to their color charge. In contrast tothe quantum electromagnetism, where the force carrying photons are electrically neutral,gluons carry color charge. This causes the gluons to interact with each other, generatingnearly all the mass of the nucleon and leading to a little-explored regime of matter, whereabundant gluons dominate its behavior. Hints of this regime become manifest when nucleonsor nuclei collide at nearly the speed of light, as they do in colliders such as HERA, RHICand LHC. The quantitative study of matter in this new regime requires a new experimentalfacility: an Electron Ion Collider (EIC).In the last decade, nuclear physicists have developed new phenomenological tools to enable remarkable tomographic images of the quarks and gluons inside protons and neutrons.These tools will be further developed and utilized to study the valence quark dominatedregion of the nucleon at the upgraded 12 GeV CEBAF at JLab and COMPASS at CERN.Applying these new tools to study the matter dominated by gluons and sea quarks originating from gluons will require the higher energy of an EIC.As one increases the energy of the electron-nucleon collision, the process probes regionsof progressively higher gluon density. However, the density of gluons inside a nucleon musteventually saturate to avoid untamed growth in the strength of the nucleon-nucleon interaction, which would violate the fundamental principle of unitarity. To date this saturatedgluon density regime has not been clearly observed, but an EIC could enable detailed studyof this remarkable aspect of matter. This pursuit will be facilitated by electron collisionswith heavy nuclei, where coherent contributions from many nucleons effectively amplify the1
60616263646566676869707172737475gluon density probed.The EIC was designated in the 2007 Nuclear Physics Long Range Plan as “embodyingthe vision for reaching the next QCD frontier” [1]. It would extend the QCD scienceprograms in the U.S. established at both the CEBAF accelerator at JLab and RHIC at BNLin dramatic and fundamentally important ways. The most intellectually pressing questionsthat an EIC will address that relate to our detailed and fundamental understanding of QCDin this frontier environment are: How are the sea quarks and gluons, and their spins distributed in spaceand momentum inside the nucleon? How are these quark and gluon distributionscorrelated with overall nucleon properties, such as its spin direction? What is the roleof orbital motion of sea quarks and gluons in building the nucleon spin? Where does the saturation of gluon densities set in? Is there a simple boundarythat separates this region from that of more dilute quark-gluon matter? If so, howdo the distributions of quarks and gluons change as one crosses the boundary? Doesthis saturation produce matter of universal properties in the nucleon and all nucleiviewed at nearly the speed of light? How does the nuclear environment affect the distribution of quarks andgluons and their interactions in nuclei? How does the transverse spatial distribution of gluons compare to that in the nucleon? How does nuclear matter respondto a fast moving color charge passing through it? Is this response different for lightand heavy quarks?Answers to these questions are essential for understanding the nature of visible matter.An EIC is the ultimate machine to provide answers to these questions for the followingreasons: A collider is needed to provide kinematic reach well into the gluon-dominated regime; Electron beams are needed to bring to bear the unmatched precision of the electromagnetic interaction as a probe; Polarized nucleon beams are needed to determine the correlations of sea quark andgluon distributions with the nucleon spin; Heavy ion beams are needed to provide precocious access to the regime of saturatedgluon densities and offer a precise dial in the study of propagation-length for colorcharges in nuclear matter.The EIC would be distinguished from all past, current, and contemplated facilitiesaround the world by being at the intensity frontier with a versatile range of kinematics andbeam polarizations, as well as beam species, allowing the above questions to be tackledat one facility. In particular, the EIC design exceeds the capabilities of HERA, the onlyelectron-proton collider to date, by adding a) polarized proton and light-ion beams; b) awide variety of heavy-ion beams; c) two to three orders of magnitude increase in luminosityto facilitate tomographic imaging; d) wide energy variability to enhance the sensitivity togluon distributions. Realizing these challenging technical improvements will extend U.S.leadership in accelerator science and in nuclear science.2
82The scientific goals and the machine parameters of the EIC were delineated in deliberations at a community-wide program held at the Institute for Nuclear Theory (INT) [2].The physics goals were set by identifying critical questions in QCD that remain unanswereddespite the significant experimental and theoretical progress made over the past decade.This White Paper is prepared for the broader nuclear science community, and presents asummary of those scientific goals with a brief description of the golden measurements andaccelerator and detector technology advances required to achieve cience Highlights of the Electron Ion ColliderNucleon Spin and its 3D Structure and TomographySeveral decades of experiments on deep inelastic scattering (DIS) of electron or muon beamsoff nucleons have taught us about how quarks and gluons (collectively called partons) sharethe momentum of a fast-moving nucleon. They have not, however, resolved the question ofhow partons share the nucleon’s spin, and build up other nucleon intrinsic properties, suchas its mass and magnetic moment. The earlier studies were limited to providing the longitudinal momentum distribution of quarks and gluons, a one-dimensional view of nucleonstructure. The EIC is designed to yield a much greater insight into the nucleon structure(Fig. 1.1, from left to right), by facilitating multi-dimensional maps of the distributions ofpartons in space, momentum (including momentum components transverse to the nucleonmomentum), spin, and flavor.Figure 1.1: Evolution of our understanding of the nucleon spin structure. Left: in the 1980s,it was naively explained by the alignment of the spins of its constituent quarks. Right: currentpicture where valence quarks, sea quarks and gluons, and their possible orbital motion areexpected to contribute.9596979899100101The 12 GeV upgrade of CEBAF at JLab will start on such studies in the kinematicregion of the valence quarks, and a similar program will be carried out by COMPASS atCERN. However, these programs will be dramatically extended at the EIC to explore therole of the gluons and sea quarks in determining the hadron structure and properties. Thiswill resolve crucial questions, such as whether a substantial “missing” portion of nucleonspin resides in the gluons. By providing high-energy probes of partons transverse momenta,the EIC should also illuminate the role of their orbital motion contributing to nucleon spin.3
102103104105106107108109110111112113114The Spin and Flavor Structure of the Nucleon:An intensive and worldwide experimental program over the past two decades has shown thatthe spin of quarks and antiquarks is only responsible for 30% of the proton spin, whilerecent RHIC results indicate that the gluons’ spin contribution in the currently exploredkinematic region is non-zero, but not yet sufficient to account for the missing 70%. Thepartons total helicity contribution to the proton spin is very sensitive to their minimummomentum fraction x accessible by the experiments. With the unique capability to reachtwo orders of magnitude lower in x and to span a wider range of momentum transfer Qthan previously achieved, the EIC would offer the most powerful tool to precisely quantifyhow the spin of gluons and that of quarks of various flavors contribute to the protons spin.The EIC would realize this by colliding longitudinally polarized electrons and nucleons,with both inclusive and semi-inclusive DIS measurements. In the former, only the scatteredelectron is detected, while in the latter, an additional hadron created in the collisions is tobe detected and identified.110 3Current polarized DIS data:DESYJLabQ2 10 GeV2SLACCurrent polarized BNL-RHIC pp data:PHENIX π010 210CEI s140,eVG010. yCEI 950. s45,0eVGcurrentdata0.5STAR 1-jet1.0 y 950. GQ2 (GeV2)CERN0DSSV -0.5EIC 5 1005 250EIC 20 250all uncertainties for χ2 91-110 -410 -3x10 -210 -10.310.350.40.45 ΣFigure 1.2: Left: The range in parton momentum fraction x vs. the square of the transferredmomentum by the electron to the proton Q2 accessible with the EIC in e-p collisions at twodifferent center-of-mass energies, compared to existing data. Right: The projected reductionin the uncertainties of the gluon’s helicity contribution G vs. the quark helicity contribution Σ to the proton spin from the region of parton momentum fractions x 0.001, that wouldbe achieved by the EIC for different center-of-mass 8Figure 1.2 (Right) shows the reduction in uncertainties of the contributions to the nucleon spin from the spin of the gluons, quarks and antiquarks, evaluated in the x range from0.001 to 1.0. This would be achieved by the EIC in its early stage of operation. At the laterstage, the kinematic range could be further extended down to x 0.0001 reducing significantly the uncertainty on the contributions from the unmeasured small-x region. Whilethe central values of the helicity contributions in Fig. 1.2 are derived from existing data,they could change as new data become available in the low x region. The uncertaintiescalculated here are based on the state-of-the art theoretical treatment of all available worlddata related to the nucleon spin puzzle. Clearly, the EIC will make a huge impact on ourknowledge of these quantities, unmatched by any other existing or anticipated facility. Thereduced uncertainties would definitively resolve the question of whether parton spin preferences alone can account for the overall proton spin, or whether additional contributions areneeded from the orbital angular momentum of partons in the nucleon.4
6147148149150151152153154155The Confined Motion of Partons inside the Nucleon:The semi-inclusive DIS (SIDIS) measurements have two natural momentum scales: thelarge momentum transfer from the electron beam needed to achieve the desired spatial resolution, and the momentum of the produced hadrons perpendicular to the direction of themomentum transfer, which prefers a small value sensitive to the motion of confined partons.Remarkable theoretical advances over the past decade have led to a rigorous frameworkwhere information on the confined motion of the partons inside a fast-moving nucleon ismatched to transverse momentum dependent parton distributions (TMDs). In particular,TMDs are sensitive to correlations between the motion of partons and their spin, as well asthe spin of the parent nucleon. These correlations can arise from spin-orbit coupling amongthe partons, about which very little is known to date. TMDs thus allow us to investigatethe full three-dimensional dynamics of the proton, going well beyond the information aboutlongitudional momentum contained in conventional parton distributions. With both electron and nucleon beams polarized at collider energies, the EIC will dramatically advanceour knowledge of the motion of confined gluons and sea quarks in ways not achievable atany existing or proposed facility.Figure 1.3 (Left) shows the transverse-momentum distribution of up quarks inside aproton moving in the z direction (out of the page) with its spin polarized in the y direction. The color code indicates the probability of finding the up quarks. The anisotropy intransverse momentum is described by the Sivers distribution function, which is induced bythe correlation between the proton’s spin direction and the motion of its quarks and gluons.While the figure is based on a preliminary extraction of this distribution from current experimental data, nothing is known about the spin and momentum correlations of the gluonsand sea quarks. The achievable statistical precision of the quark Sivers function from theEIC kinematics is also shown in Fig. 1.3 (Right). Currently no data exist for extractingsuch a picture in the gluon-dominated region in the proton. The EIC would be crucial toinitiate and realize such a program.420Momentum along the y axis (GeV)129u quark25500.5150100020401530102051510321510 1x105010 2 0.5 0.500.5Momentum along the x axis (GeV)00.20.40.60.8110 3Quark transverse momentum (GeV)Figure 1.3: Left: Transverse-momentum distribution of up quark with longitudinal momentumfraction x 0.1 in a transversely polarized proton moving in the z-direction, while polarized inthe y-direction. The color code indicates the probability of finding the up quarks. Right: Thetransverse-momentum profile of the up quark Sivers function at five x values accessible to theEIC, and corresponding statistical uncertainties.5
3174175The Tomography of the Nucleon - Spatial Imaging of Gluons and Sea Quarks:By choosing particular final states in electron-proton scattering, the EIC would probe thetransverse spatial distribution of sea quarks and gluons in the fast-moving proton as afunction of the parton’s longitudinal momentum fraction x. This spatial distribution yieldsa picture of the proton that is complementary to the one obtained from the transversemomentum distribution of quarks and gluons, revealing aspects of proton structure that areintimately connected with the dynamics of QCD at large distances. With its broad range ofcollision energies, its high luminosity and nearly hermetic detectors, the EIC could imagethe proton with unprecedented detail and precision from small to large transverse distances.The accessible parton momentum fractions x extend from a region dominated by sea quarksand gluons to one where valence quarks become important, allowing a connection to theprecise images expected from the 12 GeV upgrade at JLab and COMPASS at CERN. Thisis exemplified in Fig. 1.4, which shows the precision expected for the spatial distribution ofgluons as measured in the exclusive process: electron proton electron J/Ψ proton.The tomographic images obtained from cross sections and polarization asymmetries forexclusive processes are encoded in generalized parton distributions (GPDs) that unify theconcepts of parton densities and of elastic form factors. They contain detailed informationabout spin-orbit correlations and the angular momentum carried by partons, including theirspin and their orbital motion. The combined kinematic coverage of EIC and of the upgradedCEBAF as well as COMPASS is essential for extracting quark and gluon angular momentumcontributions to the proton spin.e p e p J/ψDistribution of gluons156sta LdI,Iget f10-1bstage Ld-I,t f10-115.8 Q2 M2J/ψ 25.1 GeV2xVb20.120.10.080.060.040.020140.16 xV 0.250030.20.40.60.811.21.41.61.22651400.016 xV 0.02500.20.40.60.811.21.41.631.2210.0016 xV T (fm)Figure 1.4: Projected precision of the transverse spatial distribution of gluons as obtained fromthe cross section of exclusive J/Ψ production. It includes statistical uncertainty and systematicuncertainties due to extrapolation into the unmeasured region of momentum transfer to thescattered proton. The distance of the gluon from the center of the proton is bT in femtometers,2 /Q2 ) determines the gluon’s momentum fraction.and the kinematic quantity xV xB (1 MJ/ΨThe collision energies assumed for Stage-I and Stage-II are Ee 5, 20 GeV and Ep 100, 250GeV, respectively.6
1761771781791801811821831841851.2.2The Nucleus, a QCD LaboratoryThe nucleus is a QCD “molecule”, with a complex structure corresponding to bound statesof nucleons. Understanding the emergence of nuclei from QCD is an ultimate long-term goalof nuclear physics. With its wide kinematic reach, as shown in Fig. 1.5 (Left), the capabilityto probe a variety of nuclei in both inclusive and semi-inclusive DIS measurements, the EICwould be the first experimental facility capable of exploring the internal 3-dimensional seaquark and gluon structure of a fast-moving nucleus. Furthermore, the nucleus itself would bean unprecedented QCD laboratory for discovering the collective behavior of gluonic matterat an unprecedented occupation number of gluons, and for studying the propagation offast-moving color charge in a nuclear medium.2103Measurements with A 56 (Fe):DY (E772, E866)10CEI10.1perturbativenon-perturbative10-4 s90CEIV,Ge s010. y ,04510-3950.1.0 y 950.VGex10-210-1non-perturbative regionνA DIS (CCFR, CDHSW, CHORUS, NuTeV)102EnergyQ2 (GeV2)eA/μA DIS (E-139, E-665, EMC, NMC)saturationregionionsi tnatr gionreQsBK/JIMWLKBFKLdilute regionDGLAP1Probe resolutionFigure 1.5: Left: The range in parton momentum fraction x vs. the square of the transferredmomentum Q2 by the electron to the nucleus accessible to the EIC in e-A collisions at twodifferent center-of-mass energies, compared with the existing data. Right: The probe resolutionvs. energy landscape, indicating regions of non-perturbative and perturbative QCD, includingin the latter, low to high parton density, and the transition region between them.186187188189190191192193194195196197198199QCD at Extreme Parton Densities:In QCD, the large soft-gluon density enables the non-linear process of gluon-gluon recombination to limit the density growth. Such a QCD self-regulation mechanism necessarilygenerates a dynamic scale from the interaction of high density massless gluons, known asthe saturation scale, Qs , at which gluon splitting and recombination reach a balance. Atthis scale the density of gluons is expected to saturate, producing new and universal properties of hadronic matter. The saturation scale Qs separates the condensed and saturatedsoft gluonic matter from the dilute but confined quarks and gluons in a hadron, as shownin Fig. 1.5 (Right).The existence of such a saturated soft gluon matter, often referred to as Color GlassCondensate (CGC), is a direct consequence of gluon self-interactions in QCD. It has beenconjectured that the CGC of QCD has universal properties common to nucleons and allnuclei, which could be systematically computed if the dynamic saturation scale Qs is sufficiently large. However, such a semi-hard Qs is difficult to reach unambiguously in electron7
201202203204205206207208209proton scattering without a multi-TeV proton beam. Heavy ion beams at the EIC couldprovide precocious access to the saturation regime and the properties of the CGC becausethe virtual photon in forward lepton scattering probes matter coherently over a characteristic length proportional to 1/x, which can exceed the diameter of a Lorentz-contractednucleus. Then, all gluons at the same impact parameter of the nucleus, enhanced by thenuclear diameter proportional to A1/3 with the atomic weight A, contribute to the probeddensity, reaching saturation at far lower energies than would be needed in electron-protoncollisions. While HERA, RHIC and the LHC have only found hints of saturated gluonicmatter, the EIC would be in a position to seal the case, completing the process started atthose facilities.Ratio of diffractive-to-total crosssection for eAu over that in ep32.5Q2 5 GeV2x 3.3 10-3eAu stage-IRatio of coherent diffractive cross sectionsfor DIS on gold to DIS on proton200 Ldt 10 fb-1/A2saturation model1.51non-saturation model (LTS)0.5stat. errors & syst. uncertainties enlarged ( 10)02210Coherent events onlystage-II, Ldt 10 fb-1/Ax 0.012.0φnosaturation1.81.61.41.2J/ψ no saturation1J/ψ saturation (bSat)0.8bSat)φ saturation (0.60.4Experimental Cuts: η(Vdecay products) 4p(Vdecay products) 1 GeV/c0.201e p(Au) e’ p’(Au’) V2.212345678910Q2(GeV2)Mx (GeV )Figure 1.6: Left: The ratio of diffractive over total cross section for DIS on gold normalizedto DIS on proton plotted for different values, M2X , the mass square of hadrons produced inthe collisions for models assuming saturation and non-saturation. The grey bars are estimatedsystematic uncertainties. Right: The ratio of coherent diffractive cross section in e-Au toe-p collisions normalized by A4/3 plotted as a function of Q2 , plotted for saturation and nonsaturation models. The 1/Q is effectively the initial size of the quark-antiquark systems (φ andJ/Ψ) produced in the re 1.6 illustrates some of the dramatic predicted effects of gluon density saturation inelectron-nucleus vs. electron-proton collisions at an EIC. The left frame considers coherentdiffractive processes, defined to include all events in which the beam nucleus remains intactand there is a rapidity gap containing no produced particles. As shown in the figure, gluonsaturation greatly enhances the fraction of the total cross section accounted for by suchdiffractive events. An early measurement of coherent diffraction in e A collisions at theEIC would provide the first unambiguous evidence for gluon saturation.Figure 1.6 (Right) shows that gluon saturation is predicted to suppress vector mesonproduction in e A relative to e p collisions at the EIC. The vector mesons result fromquark-antiquark pair fluctuations of the virtual photon, which hadronize upon exchange ofgluons with the beam proton or nucleus. The magnitude of the suppression depends onthe size (or color dipole moment) of the quark-antiquark pair, being significantly larger forproduced φ (red points) than for J/Ψ (blue) mesons. An EIC measurement of the processes8
56257258259260261262263264265266267268269in Fig. 1.6 (Right
78 The physics goals were set by identifying critical questions in QCD that remain unanswered 79 despite the signi cant experimental and theoretical progress made over the past decade. 80 This White Paper is prepared
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