5

Comparison of a U.S.-Based Electron-Ion Collider to Current and Future Facilities

INTRODUCTION

The physics program of an electron-ion collider (EIC) will be part of the worldwide activity in nuclear and elementary particle physics, although the EIC is proposed for construction in the United States. Furthermore, the EIC will serve the international physics community, just as other facilities do elsewhere. This chapter sets it in its international context by reviewing the capabilities and physics programs of other accelerators and colliders, starting with the Hadron-Electron Ring Accelerator (HERA), the only lepton-hadron collider that has operated to date, and moving on to survey other types of accelerators and colliders that are presently operating and whose physics programs are related to that of the EIC. Finally, other proposals for possible future machines are discussed. This will serve to highlight the unique capabilities and scientific value of an EIC and how that value will be preserved into the future.

HERA AT DESY

Following its proposal in 1981,¹ the HERA collider^2,3 operated from 1991 to 2007 at the Deutsches Elektronen-Synchrotron (DESY) laboratory in Hamburg,

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¹ G.A. Voss et al., 1981, “HERA—a proposal for a large electron-proton colliding beam facility at DESY, DESY report HERA 81/10.

² See HERA page at DESY website, http://www.desy.de/research/facilities_projects/hera/, accessed August 13, 2018.

³ F. Willeke, 2016, The HERA lepton-proton collider, in Challenges and Goals for Accelerators in the XXI Century, World Scientific, Singapore.

Germany, colliding 27.5 GeV electron (or positron) beams with up to 920 GeV proton beams in its 6.3 km circumference ring, attaining a center-of-mass energy up to GeV. While its energy was higher than that of the EICs described in Chapter 4, its peak luminosity reached 5 × 10³¹ cm⁻²s⁻¹, a few hundred times less than the ultimate goals of the present EIC proposals. The total integrated e-p luminosity delivered to the H1 and ZEUS experiments was about 0.5 × 10³⁹ cm⁻², conventionally denoted as 0.5 fb⁻¹ each. Moreover, HERA only collided electrons with protons, never any other ions.

HERA pioneered the use of polarized stored electron beams in collisions. Unlike the EICs, transverse beam spin-polarization could be allowed to build up on a timescale of an hour by radiative self-polarization or Sokolov-Ternov polarization (see Chapter 4) and was maintained by the implementation of correction procedures in the ring optics to cancel small effects that tend to destroy the polarization. Movable spin rotator magnets were deployed to rotate the transverse polarization into the longitudinal direction at the collision point. Moreover, the hadron ring was one of the first to use superconducting magnets.

HERA was designed for the needs of the high-energy particle physics community, primarily to search for new physics beyond the Standard Model. However, given that it discovered no new physics, HERA is remembered mainly for the wealth of electroweak and quantum chromodynamics (QCD) measurements it performed (Figure 5.1).

As the first high-energy electron-proton collider, reaching beyond fixed-target experiments, HERA provided data in the H1 and ZEUS experiments on proton structure in unprecedented energy regimes, reaching GeV. With ample data well into the regime where theoretical techniques could be applied, HERA provided tremendously better constraints on parton distribution functions (PDFs) in the proton than were previously available. In turn, the availability from HERA of higher-precision data over a wider kinematic range helped to spur a number of the theoretical advances in QCD that took place throughout the 1990s and early 2000s.

While an EIC will revisit many of the QCD measurements that HERA already performed, it will be able to radically improve upon many of them, taking advantage of, among other factors, instantaneous luminosities at least two orders of magnitude higher than HERA, as discussed in Chapter 4. Further improvements will come, for example, from detector technology and design as well as the ability and intent to run an EIC at a wide range of center-of-mass energies in order to optimize QCD measurements. These are important for measurements sensitive to the gluon distribution. Other measurements performed by HERA that will be pursued further at an EIC include those of the inclusive neutral- and charged-current cross sections for electron-proton scattering at a range of energies, heavy flavor production in electron-proton scattering, and inclusive jet and dijet production.

Of particular relevance for an EIC are the measurements HERA performed of

proton structure at low x values, given their potential connection to a predicted regime where the density of gluons is so high that the gluons typically interact with each other rather than with quarks, which an EIC is being designed to explore (see the section “Gluons in Nuclei”). HERA data reached x values less than 10⁻⁴ in the proton, a region where such nonlinear effects have been predicted. While the necessity of such effects to describe the HERA measurements has been hotly debated and remains inconclusive, the HERA data have prompted extensive theoretical advances of the field. An EIC will reach higher gluon densities than HERA by using beams of heavy nuclei as opposed to protons. Based on what has been learned from the relevant HERA measurements and the phenomenological efforts invested in interpreting them, a comprehensive suite of measurements is being planned for an EIC to study the gluon-dense regime of nuclear matter in depth.

In addition to the H1 and ZEUS experiments at HERA, the HERA Measurement of Spin (HERMES) fixed-target experiment used HERA’s electron and positron beams on a variety of stationary targets, including longitudinally and transversely polarized proton targets and nuclear targets. The longitudinally polarized lepton beam in HERA permitted a variety of measurements on polarized targets in various configurations. In contrast to the collider experiments, HERMES was designed with a focus on QCD and the structure of the nucleon, particularly in

terms of its spin. HERMES not only improved constraints on the polarized PDFs, in particular for quarks, but, critically for a future EIC, laid much of the initial groundwork for transverse momentum-dependent distributions (TMDs) describing spin-momentum correlations in the proton and added to the sparse world knowledge of nuclear PDFs and hadronization from nuclei.

In summary, while HERA was not primarily designed to study QCD, without it, knowledge of QCD and proton structure in particular would not be anywhere close to where it is today. The Relativistic Heavy Ion Collider (RHIC) spin program, using much more complicated proton-proton collisions to study proton structure, would not be nearly as fruitful without the availability of both unpolarized and polarized PDF constraints from HERA as input to the analysis. The EIC is being proposed in the context of everything that was learned, directly and indirectly, from the HERA measurements and the theoretical progress they sparked. Being designed some three decades after HERA, the EIC will not only exploit all the accelerator and detector technological advances of the intervening years, but will also be optimized for a broad and powerful QCD program. In addition to the polarized proton and light ion capabilities and the ability to accelerate heavy ions that are discussed in Chapter 4, other factors such as the optimal beam-crossing angle in the experimental interaction regions are being considered. Compared to the fixed-target HERMES experiment at HERA, which had both polarized and nuclear targets, analogous measurements at the EIC will offer cleaner interpretation thanks to the possibility of larger momentum transfers. In studies of hadronization, the more favorable geometry of interactions in a collider will provide cleaner separation of target and current fragmentation regions. At HERA, the QCD community acquired practical experience and learned lessons in carrying out measurements in an electron-hadron collider configuration. This is being incorporated into the designs of accelerator and detectors for an EIC. Examples include performing diffractive measurements in electron-proton collisions, constraining the longitudinal structure function F_L by varying the center-of-mass energy, and accessing PDFs at high x by measuring small-angle jets.

CEBAF AT JLAB

The valence quark region, which will be a focus of the 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) science program, is an important bridge connecting to the gluon and sea quark region accessible by an EIC. Naturally, many of the some 1,600 physicists currently active at CEBAF form one of the communities driving the realization of an EIC. This section describes the past and present facilities and physics programs at the Thomas Jefferson National Accelerator Facility (JLab) and their relation to a future EIC.

CEBAF was built in the late 1980s to investigate the then largely unexplored

transition between the nucleon-meson and the quark-gluon descriptions of nuclear systems. For transition-region experiments, the originally envisioned machine required a combination of the following characteristics: multi-GeV energy for spatial resolution and kinematic flexibility, high intensity for precise measurement of relatively small electromagnetic cross sections, high duty factor to allow coincidence experiments, and beam quality sufficient for use with high-resolution spectrometers and detectors.

The original CEBAF accelerator was a five-pass recirculating linac capable of simultaneous delivery to three end stations of continuous wave (CW) beams of up to 200 μA with 75 percent polarization, geometric emittance less than 10⁻⁹ m rad, and relative momentum spread of a few 10⁻⁵. The original design beam energy was 4 GeV with the possibility of a later energy upgrade.

The CEBAF accelerator design introduced a number of innovations, the most important ones being the choice of superconducting radio frequency (SRF) technology and the use of multipass beam recirculation. Neither had been previously applied on such a scale, and CEBAF remained the world’s largest implementation of SRF technology until Large Electron Positron Collider 2 (LEP2) came into operation in the later 1990s. Beam recirculation was implemented with bend radii large enough to keep open the possibility of future energy upgrades. The CEBAF design included 42 cryomodules, each containing 8 SRF cavities, to achieve the design energy of 4 GeV. The cryomodules were evenly divided between two linacs, North and South, connected by magnetic spreaders, arcs, and recombiner sections. CEBAF reached its design energy of 4 GeV in 1995 and extended it to 6 GeV in 2000. It operated at energies up to 6 GeV until 2012. The beam parameters for the 6 GeV configuration can be found in Table 5.1. CEBAF supported simultaneous beam delivery to three experimental end stations, each receiving beams with a multiple of the one-pass energy, beam currents from below 1 nA to 190 μA, and beam polarization greater than 85 percent. During 6 GeV operations the users performed 178 experiments.

CEBAF was recently upgraded to deliver continuous electron beams to the experimental users at a maximum energy of 12 GeV. The 12 GeV upgrade design retained the same footprint as the original 4 GeV CEBAF, allowing the new accelerator to use the existing tunnel with the addition of a new extraction line transporting beam to a new end station named Hall D. To achieve the 12 GeV energy requirement, the design called for the following:

• An additional recirculating arc, Arc 10, to provide an additional pass of energy gain in the North Linac; and
• Additional cryomodules in each linac to increase the total energy gain in each linac from 600 to 1,100 MeV.

TABLE 5.1 Delivered Beam Parameters for 6 GeV and 12 GeV Continuous Electron Beam Accelerator Facility

SOURCE: A. Freyberger, “Commissioning and Operation of 12GeV CEBAF,” Proceedings of the 6th International Particle Accelerator Conference (IPAC2015), paper MOXGB2, http://accelconf.web.cern.ch/AccelConf/IPAC2015/papers/moxgb2.pdf.

The beam parameters for the 6 and 12 GeV designs are compared in Table 5.1. Apart from beam energy, the main difference is the increase in beam emittance and energy spread due to the copious synchrotron radiation in the high energy arcs.

JLab completed the accelerator upgrade and the associated experimental equipment upgrade in 2017, including a new experimental hall (Hall D). The energy-upgraded CEBAF accelerator is capable of delivering beams simultaneously to all four halls, up to 11 GeV electrons to Halls A, B, and C, and a 12 GeV electron beam to Hall D for producing 9 GeV tagged photons for meson spectroscopy, complementary to spectroscopy at the Large Hadron Collider (LHC) and possibly at the future EIC (Figure 5.2). The JLab Hall D program will investigate the role of gluonic excitations in the spectroscopy of light mesons by searching for states with exotic quantum numbers involving excitations of the gluonic field, states not taken into account in studying only quark and antiquark degrees of freedom. The discovery of such exotic states will elucidate the nature of quark confinement.

Owing to the limited kinematical reach of the upgraded JLab beam,⁴ 0.05 < x < 0.8 and Q² up to some 17 GeV², the upgraded JLab beam will study the valence

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⁴ J. Dudek et al., Physics opportunities with the 12 GeV upgrade at Jefferson Lab, Eur. Phys. J. A 48: 187(2012).

quark region at relatively low Q². By comparison, an EIC will be able to extend the study in the valence quark region up to Q² of order 1,000 GeV².

The original proton “spin crisis” discovered by the European Organization for Nuclear Research (CERN) European Muon Collaboration (EMC) experiment motivated experimental and theoretical activities worldwide in understanding the source of the proton spin. These studies also led to a more complete description of the partonic structure of the nucleon through three-dimensional (3D) distribution functions, generalized parton distributions (GPD) and TMD distributions, discussed in Chapter 2. The GPDs are accessed in exclusive scattering processes such as deeply virtual Compton scattering and deeply virtual meson production. The TMDs can be accessed in coincidence measurements in which the nucleon no longer remains intact, and one of the produced hadrons is detected together with the scattered electron. The resulting multidimensional distribution functions provide tomographic imaging of the nucleon and insight into the QCD dynamics inside the nucleon. Extensive programs on GPDs and TMDs are planned for Halls A, B, and C in the large x, also called “valence quark,” region with a 12 GeV CEBAF.

An ultimate goal of nuclear physics is to be able to predict and describe nuclear properties and reactions from the first principles of QCD. Understanding the structure of the nucleon from QCD is an important step toward this goal. Apart from the study of the structure of the nucleon, the 12 GeV CEBAF provides significant

opportunities to study QCD effects in nuclei. At the same time, nuclei also provide a unique laboratory to study QCD. Multinucleon correlations observed in nuclei at the 6 GeV CEBAF pave the way to addressing fundamental nuclear physics questions: Is there is a relation between short-range nucleon-nucleon correlations and the partonic structure of nuclei? What is the importance of the nucleon-nucleon wave function at short distances, the origin of the nucleon-nucleon force and effects of color transparency (the predicted vanishing of initial or final nuclear state rections)? Do hidden color configurations (not described by the usual color singlet nucleon states) exist in nuclei?

Precision intensity frontiers are complementary to energy frontiers in discovering new physics beyond the Standard Model of particle physics. The high-intensity polarized CEBAF beam is a powerful intensity frontier tool that offers discovery potential for physics beyond the Standard Model by utilizing precision measurements of mirror symmetry (parity) violation in electron scattering. Very precise measurements of parity violating asymmetries at the 6 GeV CEBAF have been performed to study the strangeness form factors and the weak charge of the proton in elastic electron-proton scattering. The energy upgraded CEBAF offers new opportunities using parity-violating electron scattering off atomic electrons and nuclei to probe new physics at energy scales of 10 to 20 TeV.

THE COMPASS EXPERIMENT AT CERN

The measurements performed by Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) have improved knowledge of nucleon structure and helped to drive theoretical work, in particular related to both momentum and spatial imaging, setting the stage for much of the nucleon imaging physics program to be executed at the EIC.

The COMPASS experiment at CERN began running in 2002 and has physics programs involving both a muon beam and a hadron beam on fixed targets, with beam energies ranging from 160 to 200 GeV, and a muon-hadron luminosity of a few times 10³¹ cm⁻²s⁻¹ (Figure 5.3). COMPASS builds upon a long legacy of fixed-target experiments at CERN using muon beams on polarized targets, following the EMC and Spin Muon Collaboration experiments. Continued running is currently planned through 2018, and there is a proposal to extend COMPASS data taking through 2021. The muon beam physics program includes lepton scattering and exclusive measurements on nuclear targets containing both longitudinally and transversely polarized nucleons. These measurements focus on the spin structure of the nucleon, as well as on momentum and spatial imaging of polarized and unpolarized nucleons. In the hadron-beam physics program, most of the data have been taken with pion beams. Negative pion beams are used to perform momentum imaging of transversely polarized protons, and to test understanding of color

interactions and how they differ in the related processes of lepton scattering and lepton production via quark-antiquark annihilation.

An EIC, with its variable energy and almost hermetic detectors, will greatly extend the kinematic coverage for lepton-nucleon scattering beyond that accessible by COMPASS, reaching larger center-of-mass energies, lower x and higher Q², and accumulating much larger data samples, with uniquely abundant statistics, especially in the gluon-dominated regime. The EIC will additionally perform a comprehensive program of lepton scattering and exclusive measurements on nuclei.

RHIC AT BROOKHAVEN NATIONAL LABORATORY

RHIC Collider

The RHIC, which has operated at Brookhaven National Laboratory (BNL) since 2000, was the first hadron collider to collide heavy nuclei and it also collides

polarized protons (Figure 5.4). It is expected to operate into the 2020s. Most of the RHIC runs have collided gold ions, but numerous other combinations including gold on deuterons or protons, as well as a range of nuclei from copper to uranium have been collided.⁵

RHIC built on the alternating gradient synchrotron (AGS; constructed in the late 1950s) and other machines at BNL, which now form its injector chain. The collider itself consists of two 3.8 km circumference rings of superconducting magnets, in which protons can be accelerated to energies of 255 GeV and heavy ions to 100 GeV/nucleon. BNLs long and distinguished record of innovation in accelerator physics and technology, with a strong orientation to the needs of physics programs, has continued with the RHIC machine.

Among numerous innovations enabling continual performance upgrades beyond design expectations, the heavy-ion luminosity was substantially upgraded, at low cost, with the implementation of bunched beam stochastic cooling. RHIC remains the only hadron collider that has succeeded in accelerating and colliding polarized proton beams. This experience is a crucial foundation for an EIC that must also accelerate and store polarized proton beams.

Projections for the RHIC collider extending to 2027, with a variety of nuclei and further polarized proton performance, have been given.⁶

The RHIC Physics Program Within the Context of the EIC

In operation since 2000, RHIC was designed to study QCD, with focus on high energy densities, the creation and study of a quark-gluon plasma, and the polarized structure of the proton. The current RHIC community is one of the principal communities interested in realizing an EIC. With a user community of approximately 1,000 scientists, RHIC has had two large, multipurpose experiments, the Solenoidal Tracker at RHIC (STAR) and the Pioneering High Energy Nuclear Interaction Experiment (PHENIX), involved in the full breadth of its physics program. PHENIX concluded operations in 2016, and the sPHENIX experiment is anticipated to start taking data in 2023. STAR will continue to run until at least the early 2020s. Two smaller experiments, BRAHMS and PHOBOS, finished operations in the mid-2000s.

There are many connections between the RHIC program, the proton structure part in particular, and that envisioned at an EIC. RHIC’s high-energy polarized proton beams and collider configuration allow the spin structure of the proton

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⁵ W. Fischer and J.M. Jowett, 2014, Ion colliders, Reviews of Accelerator Science and Technology 7:49.

⁶ W. Fischer, M. Blaskiewicz, A. Fedotov, H. Huang, C. Liu, G. Marr, M. Minty, V. Ranjbar, and D. Raparia, 2017, RHIC Collider Projections (FY 2017-FY 2027), Brookhaven National Laboratory Note, May 2017.

to be studied at large Q² compared to polarized fixed-target experiments, over a relatively wide range of x, including significant overlap with the expected EIC kinematic coverage. The RHIC spin program was originally designed with a focus on determining the polarization of gluons in the polarized proton, and on delineating the polarizations of the up and down quark and antiquarks. The main probes sensitive to gluon spin at RHIC are inclusive hadron production, jets, and dijets. In 2014, RHIC announced the discovery of a moderate positive contribution from gluon spin to the spin of the proton; however, uncertainties remain relatively sizable and the measurements are sensitive only to a modest range of gluon momentum fractions x. The efforts to constrain the flavor-separated light sea quark helicity distributions at RHIC are based on the production of W-boson in polarized proton collisions and their subsequent decay into electrons, positrons, and muons, taking advantage of both the parity-violating nature and the flavor sensitivity of the weak interaction. RHIC has found evidence for a flavor-asymmetric polarized sea, which is presently stimulating further theoretical work. Measurements with transversely polarized beams at RHIC made the surprising discovery that the large spin-momentum correlations in forward hadron production initially observed with low-energy polarized hadronic collisions in the 1970s persist up to the maximum RHIC center-of-mass energy of 510 GeV and at hard scales of up to Q² ~ 50 GeV². The large size of these asymmetries and their intricate relationships to the typically smaller transverse spin effects observed in polarized deep-inelastic lepton-nucleon experiments have contributed considerably to the renewed interest in QCD phenomena with transverse spins and to advances in their understanding. The EIC will combine the strengths of the kinematic reach at a collider with the discriminating power of a lepton probe to elucidate the nucleon’s internal spin structure.

The study of the quark-gluon plasma and exploration of the QCD phase diagram are not part of the EIC physics program; however, improving knowledge of the partonic structure of nuclei is part of both the RHIC heavy ion and EIC physics programs. RHIC, in a similar spirit to an EIC, was designed to perform measurements with pp, p/dA, and AA collisions (where A denotes nuclei heavier than the deuteron) in matching kinematics, so that nuclear effects could be understood in relation to the proton. Deuteron-nucleus collision data from RHIC (as well as proton-nucleus data from the LHC, see later) have already been included in global fits of nuclear PDFs,⁷ with recent RHIC proton-nucleus measurements to be included in future fits. The kinematic reach of RHIC and its experiments is predicted to include a regime in which gluon distributions in nuclei saturate, and measurements for hadron production in dA, particularly in forward kinematics,

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⁷ K.J. Eskola, P. Paakkinen, H. Paukkunen et al., 2017, EPPS16: Nuclear parton distributions with LHC data, Eur. Phys. J. C 77:163, Table 1 and Figure 2, https://doi.org/10.1140/epjc/s10052-017-4725-9, accessed August 13, 2018.

have been cited as evidence for gluon saturation. However, with multiple effects potentially contributing to measurements in the complex environment of p/dA collisions, definitive interpretations of the data have proven elusive. An EIC, with a lepton beam on a variety of light and heavy nuclei at a range of center-of-mass energies, would make precision measurements of the flavor-separated partonic structure of nuclei through inclusive, semi-inclusive, and exclusive observables, comparable to similar measurements performed on protons. In the clean environment of lepton-nucleus collisions, and with critical kinematic coverage allowing calculations of observables sensitive to gluon saturation effects using both theoretical techniques specific to a saturation regime and traditional perturbative methods of calculation in QCD, definitive studies of gluon saturation will be possible at an EIC. RHIC has additionally performed a handful of diffractive measurements in ultraperipheral collisions and plans to make further measurements over the next several years. Such measurements are expected to offer insight on the magnitude of certain diffractive observables planned for an EIC.

LHC AT CERN

The LHC is the largest and highest energy particle collider in the world. It is operated by CERN at its laboratory near Geneva, Switzerland. CERN, is an international organization with 22 Member States but serves the global particle and nuclear physics community. The United States, along with Japan and Russia, has long been an Observer State, and physicists from U.S. universities and national laboratories are major participants in the LHC program. In 2016 the 1,925 U.S.based users were the largest national contingent at CERN.⁸

The first feasibility study of the LHC took place in 1983 and the machine was turned on in 2008, after many years of R&D on the technologies required for the accelerator and its experiments. The two rings of the LHC are composed of superconducting magnets in an approximately circular tunnel of 27 km circumference that previously housed the LEP electron-positron collider. Its injector complex includes several preexisting accelerators that have served many physics programs since the late 1950s. Decades of operation and continual improvement has allowed these machines to far exceed their initially foreseen capabilities.

The LHC spends most time colliding beams of protons for the elementary particle physics program. A major result of the pp program was the discovery of the Higgs boson in 2012. Proton beam operations have also led to a vast wealth of high-energy QCD results and the first exotic hadronic states have now been observed conclusively. In addition, a typical operating year includes 1 month devoted

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⁸ “CERN Users by Institute and Nationality 2016,” http://usersoffice.web.cern.ch/annual-statistics, accessed August 13, 2018.

to the nuclear (or “heavy-ion”) collision program.⁹ So far, lead nuclei have been collided with each other (PbPb) or with protons (pPb). A short pilot run colliding xenon nuclei (XeXe) took place recently, demonstrating the capability of the CERN complex to accelerate and collide other species if required in the future.

The total energy concentrated into nuclear volumes in the LHC’s Pb-Pb collisions, at over 1,000 TeV, is by far the highest achieved to date in any human-made particle collision. In nuclear physics the convention is to quote the center-of-mass energy per colliding nucleon-pair, which is at present 13, 8.16, and 5.02 TeV for pp, pPb, and PbPb collisions, respectively. After a total of about 11 weeks operation for Pb-Pb and 8 weeks for p-Pb collisions, solutions have been found to most of the expected performance limits and peak luminosity levels are already far beyond design.

The heavy-ion program of the LHC is largely driven by the specialized experiment A Large Ion Collider Experiment (ALICE), but all other large experiments, ATLAS, CMS, and LHCb (originally conceived for flavor physics), now participate in this program (Figure 5.5).

In addition to the nucleus-nucleus (AA) collisions, pp and pA collisions at

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⁹ W. Fischer and J.M. Jowett, 2014, Ion colliders, Reviews of Accelerator Science and Technology 7:49.

equivalent energies play a vital role in the experiments’ programs. The forward region of pA collisions may probe the very small x region of a nucleus and has implications for cosmic ray experiments. These pp, pA, and AA studies have stimulated much excitement and controversy concerning possible effects arising from effect of high gluon density, and studies at an EIC would do much to resolve various interpretations.

Although no electron-hadron collider is presently foreseen in Europe (see the discussion of the Large Hadron-Electron Collider later in this chapter), the continued running of the LHC above design luminosities in these modes in the coming years and decades will bring many results complementary to but relevant to an EIC. The general reason for this connection is that, at the very high (10 TeV) energy scale of the LHC, the bulk of all particles produced in all three collision combinations originate from processes involving gluons, a consequence of the dominance of gluons at low x in hadrons. Furthermore, because of the very high LHC energy, very low x values, down to the range of x = 10⁻⁶ are reachable, albeit not in the same clean and controlled conditions as at an EIC. This means, in particular, it is generally very difficult to determine precisely the important scale parameters Q² and x in hadronic collisions.

Recent studies at LHC energy of pp and pPb collisions as a function of charged particle multiplicity have revealed interesting and unexpected features. As the multiplicity increases to and beyond several times the average (inclusive) multiplicity, the collisions exhibit signs of apparent collective behavior. These include azimuthal anisotropies and azimuthal correlations that are smaller in magnitude but similar in shape to the hydrodynamic flow distributions observed dramatically in PbPb collisions. Some, but not all, of these results can be well described in models based on gluon saturation, where the hadronic wave functions are described in the framework of a color-glass condensate (see Chapter 2). At transverse momenta of the order of a few GeV, x values in the range of and below 10⁻⁴ can be reached, especially at forward rapidity (very close to the forward-going proton-beam direction).

Another area where low x gluon distributions can be probed is open charm (i.e., hadrons with nonzero net charm) production at forward rapidity. First results from the LHCb collaboration indicate that such data will be very valuable to constrain the magnitude of gluon PDFs at low x. These studies are being extended to pPb collisions leading to results on gluon distributions in heavy nuclei at low x, that is, deep into the saturation region. Unfortunately, Q² cannot be varied independently. Nevertheless, such data will be very valuable to test predictions from models based on gluon saturation.

Furthermore, open charm and open bottom production will also be studied in eA collisions at an EIC. Together with the results from the pPb program at the LHC this should allow fundamental tests of high-energy QCD predictions for energy loss of heavy quarks in a dense (gluonic) medium.

An emerging program at the LHC is the study of ultra-peripheral collisions in pPb and PbPb. These collisions (or, rather, near-misses) are experimentally defined such that the impact parameter is large compared to the sum of the radii of the colliding hadrons or nuclei. In such collisions, a photon from one of the colliding particles can penetrate into the other, providing an effective means to study photon-nucleus collisions at very high energy. Of particular interest are photonuclear collisions involving the exclusive production of light vector mesons (ρ, ω, Φ) and of heavy quarkonia (J/ψ and Υ particles). All of these can, and will, be studied with precision at the LHC, in particular in the ALICE and LHCb experiments. Since photonuclear cross sections scale, at leading order, as the square of the gluon distribution in the relevant nucleus, they provide an excellent tool to probe gluon distributions at low x, albeit at fixed (low) Q². First results have been presented,^{10,11,12,13,14,15} and many more are to come.

The future operation of the LHC and its upgrading to the High-Luminosity LHC (HL-LHC) is a central plank of the European Strategy for Particle Physics:¹⁶

The HL-LHC upgrades will be implemented in the 2020s and are expected to be exploited until around 2035. The upgrades include improvements to the injector complex and the LHC itself that will allow the heavy-ion program at the LHC to exceed its initial luminosity goal of 1 nb^–1 in two experiments (ALICE and CMS) to 10 nb⁻¹ in three experiments (ALICE, ATLAS, and CMS) plus LHCb. The present heavy-ion program is foreseen to continue until 2029.

The Nuclear Physics European Collaboration Committee (NuPECC), essentially the analogue of the Nuclear Science Advisory Committee (NSAC) in Europe,

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¹⁰ H. Paukkunen, 2017, Status of nuclear PDFs after the first LHC p-Pb run, Nucl. Phys. A967:241.

¹¹ S. Klein, 2017, Ultra-peripheral collisions and hadronic structure, Nucl. Phys. A967:249.

¹² E. Kryshen, 2017, Photoproduction of heavy vector mesons in ultra-peripheral Pb-Pb collisions, Nucl. Phys. A967:273.

¹³ M. Dyndal, 2017, Electromagnetic processes in ultraperipheral Pb-Pb collisions with ATLAS, Nucl. Phys. A967:281.

¹⁴ H. Mäntysaari et al., 2017, Proton structure fluctuations: Constraints from HERA and applications to pA collisions, Nucl Phys. A967:317.

¹⁵ D. d’Enterria et al., 2017, Physics with ions at the Future Circular Collider, Nucl. Phys. A967:888.

¹⁶ “The European Strategy for Particle Physics Update 2013,” CERN-Council-S/106, http://cds.cern.ch/record/1567258/files/esc-e-106.pdf?subformat=pdfa.

recently published its 2017 long-range plan.¹⁷ NuPeCC considers it “crucial that all aspects of the LHC heavy-ion program, including manpower support and completion of the detector upgrades, are strongly supported.”

OTHER FUTURE ELECTRON-HADRON COLLIDER PROPOSALS

LHeC

The Large Hadron-Electron Collider (LHeC) has been proposed¹⁸ as an extension of the present LHC that would provide ep or eA collisions simultaneously with the LHC’s pp or AA collisions. To achieve this, a large energy recovery linac (ERL) would have to be constructed in a new 9 km racetrack-shaped deep-underground tunnel. The electron-hadron collisions would occur at one of the present interaction points of the LHC, requiring the replacement of one of the present experiments with a new detector designed for these collisions. Parameters of the LHeC for ep collisions are given in the first two columns of Table 5.2.

The LHeC is designed to have 10 to 20 times higher center-of-mass energy, and nearly 1,000 times higher luminosity, than HERA. Therefore, the LHeC extends the kinematic range accessed with HERA on the proton from a maximum momentum transfer squared, Q², of about 0.03 (TeV/c)² to above 1 and from a maximum x of about 0.6 to 0.9. Furthermore, the low x range extends down to 10⁻⁶. In addition, the LHeC would have the ability to study nuclei in electron-ion collisions, which was not possible at HERA.

Selected science highlights of the LHeC Study Group¹⁹ are described in the following sections.

High-Precision Studies of QCD and Electroweak Physics

Thanks to the wide kinematic range, high luminosity, and possibility of beam variations, the LHeC would provide the necessary constraints on all parton (quark and gluon) distributions to determine PDFs completely, free of conventional QCD fit assumptions, which has hitherto not been possible.

The strong coupling constant α_S(M_Z²) can be measured to parts-in-a-thousand precision, as compared to the percent level today. Such a measurement would put attempts to study whether the strong, electromagnetic, and weak forces become

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¹⁷ A. Bracco et al., eds., 2016, “NuPECC Long Range Plan 2017 Perspectives in Nuclear Physics,” European Science Foundation, http://www.nupecc.org/lrp2016/Documents/lrp2017.pdf.

¹⁸ The LHeC Study Group, 2012, A Large Hadron Electron Collider at CERN, J. Phys. G: Nuclear and Particle Physics 39:075001.

¹⁹ Ibid.

TABLE 5.2 Baseline Parameters for e-p Collisions at Three Potential Future Lepton-Hadron Colliders that might be built as Extensions of the CERN Complex

Parameter	LHeC CDR	ep at HL-LHC	ep at HE-LHC	FCC-he
Ep [TeV]	7	7	12.5	50
Ee [GeV]	60	60	60	60
	1.3	1.3	1.7	3.5
Bunch spacing [ns]	25	25	25	25
Protons per bunch [1011]	1.7	2.2	2.5	1
γ εp [µm]	3.7	2	2.5	2.2
Electrons per bunch [109]	1	2.3	3.0	3.0
Electron current [mA]	6.4	15	20	20
IP beta function	10	7	10	15
Hourglass factor Hgeom	0.9	0.9	0.9	0.9
Pinch factor Hb−b	1.3	1.3	1.3	1.3
Proton filling Hcoll	0.8	0.8	0.8	0.8
Luminosity [1033 cm−2s−1]	1	8	12	15

NOTE: This reference also provides parameters for electron-ion collisions. With the proton and heavy-ion beams foreseen for the future High-Luminosity Large Hadron Collider (HL-LHC) (third column), it would be possible to provide higher luminosity than foreseen at the time of the Large Hadron-Electron Collider (LHeC) Critical Design Review (CDR) (second column). Realizations of an electron-hadron collider by constructing a large energy recovery linac (ERL) next to the High-Energy LHC (HE-LHC) or Future Circular Collider (FCC) hadron colliders could yield the performance indicated in the fourth and fifth columns.

SOURCE: O. Bruning, J.M. Jowett, M. Klein, D. Pellegrini, D. Schulte, and F. Zimmermann, 2017, “Future Circular Collider Study FCC-he Baseline Parameters,” CERN-ACC-2017-0019, April, http://cds.cern.ch/record/2260408, accessed August 13, 2018.

comparable in strength at the grand unification scale, some 10¹⁶ GeV, on a firm footing.

Low x Physics

The most pressing issue in low x physics is the need for a mechanism to tame the growth of the parton density, which, from very general considerations, is expected to be modified in the region of LHeC sensitivity. There is a wide, though nonuniversal, consensus that nonlinear contributions to parton evolution (e.g., via gluon recombination gg → g) eventually become relevant and the parton densities saturate. The LHeC offers the unique possibility of observing these nonperturbative dynamics at sufficiently large Q² for weak-coupling theoretical methods to be applied, suggesting the exciting possibility of an understanding at the parton-level of the collective properties of QCD.

Nuclear Structure at High Energy

Structure function measurements and their flavor decompositions in eA will allow nuclear parton densities at small x to be measured, testing current methods of extraction, particularly for the gluon density for x < 10⁻², and the unknown charm and beauty densities in nuclei, quantities which are presently almost unconstrained by experimental data.

Exclusive vector meson production in eA collisions will offer a handle complementary to ep, on the possible evidence of nonlinear dynamics and saturation of partonic densities, as these effects increase with A.

The dynamics of hadronization and QCD radiation will be clarified in e-A collisions through semi-inclusive measurements of both particles and jets, of which large yields will be produced up to high transverse momenta. The effects of the nuclear environment will be explored through the modification of yields, the variety of hadron species produced, jet substructure, and so on, compared to equivalent ep measurements.

The science motivating the LHeC has significant overlap with that motivating the EIC. However, there is also substantial complementarity between the projects both in terms of scientific motivation and timescale for realization. While the LHeC would push to lower x, an EIC would have high luminosity polarized electron and nucleon beams. In electron-ion collisions, an EIC will probe the approach to saturation while the LHeC would be more likely to reach the saturation regime. Reasonably, an EIC would be realized first and a later LHeC would still have a first-rate science case.

Status of the LHeC Proposal

In Europe, there is currently no plan for an EIC-like facility. After the 2013 European Strategy for Particle Physics process, the plans for the LHeC project at CERN were not pursued actively. However, a significant amount of work is ongoing to prepare discussions on such an accelerator for the upcoming 2019-2020 European Strategy for Particle Physics deliberations. In any case, such a facility could only be realized for the final phase of LHC operations in the 2030s.

HE-LHC-he

The High-Energy LHC (HE-LHC)²⁰ is another potential future hadron collider based on replacing the present LHC superconducting magnets with higher-field

___________________

²⁰ O. Bruning, J.M. Jowett, M. Klein, D. Pellegrini, D. Schulte, and F. Zimmermann, 2017, Future Circular Collider Study FCC-he Baseline Parameters, CERN-ACC-2017-0019, April, http://cds.cern.ch/record/2260408, accessed August 13, 2018.

magnets, employing Nb-Sn technology, in the existing LHC tunnel. It could potentially provide ep and eA collisions simultaneously with pp or AA collisions. It would use the same ERL to provide electron beams similar to those envisaged for the LHeC but would collide them with hadron beams of up to twice the energy. This might succeed the LHC to become operational in the late 2030s.

FCC-he

A further potential long-term step at CERN is the Future Circular Collider (FCC-hh) a hadron collider built with Nb-Sn magnets in a new 100 km tunnel in the Geneva area. It would use the existing CERN complex as its injectors. This collider might succeed the LHC to become operational in the 2040s. Again, with the additional construction of a large ERL, it could provide ep and eA collisions simultaneously with pp or AA collisions.

ELECTRON-POSITRON COLLIDERS

The majority of particle colliders built for elementary particle physics research since the early 1960s have been electron-positron colliders. In terms of energy reach, these culminated in the Large Electron Positron Collider (LEP) at CERN which attained a center-of-mass energy of 200 GeV thanks to massive deployment of superconducting RF cavities. Its 27 km tunnel is now occupied by the LHC. More recent e⁺e⁻ colliders, such as DAΦNE in Frascati (Italy), and the B-factories at SLAC and KEK (Japan), have been built to explore the intensity frontier by revisiting lower energies with much higher luminosity than their predecessors. Indeed, these machines have pioneered much of the technology needed for the present EIC proposals: high-intensity, multibunch electron rings, SRF technology, crab cavities, electron-cloud mitigation, and advanced interaction region designs. The electron rings of the EIC design concepts have much in common with them.

The e⁺e⁻ colliders, LEP in particular, have among numerous other achievements made a profound impact on the understanding of how color-carrying quarks transform into color-neutral hadrons. This knowledge, encoded in what are called “fragmentation functions,” is essential in relating observations at other facilities to the underlying physics. The knowledge of fragmentation functions has made it possible, for example, to relate neutral pion production in polarized proton collisions to the gluon spin distribution in the polarized proton, thereby complementing the insights gained from jet measurements at RHIC. Recent fragmentation measurements with Belle at the KEK B-factory have revealed a rich interplay between spin and transverse momenta, which enable determinations of quark transversity, a quark spin distribution related to the nucleon tensor charge and electric dipole moment from the azimuthal distributions of hadrons produced

in polarized deep-inelastic lepton-nucleon collisions, such as those at JLab. These and other fragmentation measurements will continue to have important roles at a future EIC, where they form also the vacuum baseline for studies of hadronization in nuclei (see Figure 2.3.1 in Box 2.3).

RELATED NUCLEAR PHYSICS FACILITIES

The preceding sections covered facilities whose scientific programs have quite direct relations to that of the proposed EICs. Among facilities, existing, planned, or proposed worldwide, there are a few others that have some less direct connections. For the sake of completeness, they are covered in this section.

FAIR at GSI

The Facility for Antiproton and Ion Research (FAIR) is currently under construction at the GSI Helmholtz Center for Heavy Ion Research at Darmstadt, Germany. It will come online around 2024, while a phase-0 program of experiments using the detectors and accelerators already available will start in 2018. The FAIR accelerators will provide intense beams of heavy ions and antiprotons in a wide energy range up to 10 GeV/nucleon. Its main physics focus will be in four research areas: atomic physics, plasma physics, and applications; nuclear matter physics with the High Acceptance Dielectron Spectrometer and Compressed Baryonic Matter detectors; nuclear structure, astrophysics, and reactions with the Nuclear Structure, Astrophysics, and Reactions detectors; and physics with high-energy antiprotons with the PANDA detector.

While much of the FAIR physics program is concentrated on areas outside the focus of an EIC, the PANDA experiment plans to measure processes like proton + antiproton → 2 photons and proton + antiproton → dileptons. The resulting data should be interesting for and complementary to EIC physics in that they open new avenues to measure deeply virtual Compton scattering and to probe distributions in the nucleon that change sign under time reversal.

HIAF and an EIC in China

In the past decade or so, the Chinese central and local governments started to make major investments in large-scale accelerator-based facilities. One such example is the Shanghai Synchrotron Radiation Facility, which was built and supported jointly by the Chinese Academy of Sciences and the Shanghai government. The High Intensity Heavy-Ion Accelerator Facility (HIAF), officially approved by the Chinese government at the end of 2015, is one of the 16 large-scale research facilities proposed to boost China’s capabilities in basic science research during the

country’s twelfth 5-year plan. HIAF will address a number of important questions in nuclear physics and nuclear astrophysics such as the following: What are the limits to nuclear existence? What are new forms of nuclear matter far from stability? How were elements from carbon to uranium created? How is energy generated in stars and stellar explosions? The construction of HIAF is currently under way in Huizhou, a city in the southeast part of China.

In the past several years, Chinese physicists, together with collaborators in the United States, proposed a concept of a polarized EIC at HIAF. EIC@HIAF would be an extension to the originally proposed HIAF. The China EIC would include 3 to 5 GeV polarized electrons on 12 to 23 GeV polarized protons (and ions about 12 GeV/nucleon), with luminosities of 1 to 2 × 10³³cm⁻² s⁻¹ for stage 1 design. Such a facility would allow for the studies of the spin and the exploration of three-dimensional nucleon structure in both the valence and sea quark regions, the studies of QCD dynamics, and advance understanding of the strong force. While this plan for an EIC in China received strong support from the Chinese high-energy and nuclear physics communities, the project has not been funded, and the timing for its construction is uncertain.

J-PARC

The Hadron Experimental Facility of the Japan Proton Accelerator Research Complex (J-PARC) provides the world’s highest-power beams for particle and nuclear experiments. The primary proton beam of 30 GeV at J-PARC is slowly extracted from the Main Ring accelerator and transported to the production target in the experimental hall. Various secondary particles, such as K and π mesons produced in the target, are transported through the secondary beam lines to the experimental area and are used for particle and nuclear physics experiments. The construction of the facility was started in 2004, and the first beam was extracted to the hall on January 27, 2009. The formal beam operation for users started in January 2010.

The J-PARC particle physics experiments on rare decay and searches for lepton flavor violation will shed light on unanswered fundamental questions, such as the mechanism to realize the dominance of matter over antimatter in the universe and the nature of dark matter. Nuclear physics experiments investigate the nature of hadrons and nuclear matter in various environments—such as the high temperature in the early universe and the high density in the core of neutron stars—to clarify the origins of matter in stars (as well as in humans) in the universe.

Plans in the original J-PARC conceptual designs to extend the Hadron Experimental Facility and construct new beam lines for future upgrade are currently being updated and revised. They will include the extension of the experimental hall, additional targets for producing secondary beams, new beam lines and the

increase of secondary beam intensities to maximize the physics impact from both nuclear and particle experiments.

The NICA Project at JINR, Dubna, Russia

The Nuclotron-Based Ion Collider Facility (NICA) at the Joint Institute for Nuclear Research (JINR) is a new superconducting accelerator complex under construction at Dubna, Russia, and is expected to be in operation by about 2020. Beams will be injected into NICA from the existing nuclotron machine. NICA will deliver intense beams of ions from protons to gold as well as polarized protons and deuterons with maximum energy GeV (for Au⁷⁹⁺) and 27 GeV (for protons). The expected luminosity is 10²⁷ cm⁻²s⁻¹ for gold and 10³² cm⁻²s⁻¹ for protons.

The scientific motivation is to study hot, dense baryonic matter and to investigate polarization phenomena, including nucleon spin structure. NICA will explore the QCD phase diagram in the terra incognita of highest net baryon density and will be complementary to RHIC, LHC, and FAIR. It will have the potential to discover a critical end-point, the restoration of chiral symmetry, and a hypothetical “quarkyonic phase” in the phase diagram of dense matter. Together with FAIR, it can be regarded as part of a third generation of heavy ion experiments.

A general multi-purpose detector (MPD) concept has been developed. The MPD Collaboration consists of about 200 physicists from 19 institutions in 9 countries. Furthermore, a consortium has been established between the experimental collaborations from Compressed Baryonic Matter/FAIR and MPD/NICA. The MPD detector includes a 0.5 T superconducting solenoidal magnet, charged particle tracking, particle identification, and calorimetry. Design constraints include hermeticity, homogeneous solenoidal field, good tracking performance, high event rate capability, and careful event characterization. It is planned to measure hadrons (π, K, anti-p, anti-hyperons, light anti-nuclei), and dilepton spectra as a function of energy, system size, centrality, transverse momentum p_T, rapidity and azimuthal angle. By about 2020, it is the aim of the MPD Collaboration to localize the QCD critical end point (if it exists) and to investigate it in detail. Measurements of low mass dileptons will also be a priority with the aim to probe for evidence of chiral symmetry restoration.

A second experiment with the aim of studying nucleon spin structure is also under development. It is planned to study the spin-dependent Drell-Yan process using both longitudinally and transversely polarized protons and deuterons to extract new parton distribution functions in a much lower kinematic range than at an EIC.