aa r X i v : . [ h e p - e x ] J u l An Electron-Ion Collider at Jefferson lab
Anthony W. ThomasSuite 1, Jefferson Lab, 12000 Jefferson Ave., Newport News VA 23606 USAand College of William and Mary, Williamsburg VA 23187 USALong term plans for the investigation of the quark and gluon structure of matter havefor some time focussed on the possibility of an electron-ion collider, with the nuclearphysics communities associated with JLab and BNL being particularly active. Webriefly outline the current thinking on this subject at Jefferson lab.
As explained in the oral presentation [1], the original plans for an electron-ion collider(ELIC) at Jefferson Lab involved the construction of two figure-of-eight rings, intersectingat up to four collision points, with a proton energy of 30-225 GeV (30-100 GeV/A for ionsup to Pb) and electrons (and positrons) from 3 to 9 GeV [2]. Since the construction ofsuch a facility must await completion of the 12 GeV Upgrade at Jefferson Lab [3], as wellas the construction of FRIB, it is unlikely to begin much before the end of the next decade.The design for ELIC was appropriately ambitious for a world-leading machine that will nottake data until the third decade of this millenium, with a design luminosity approaching10 cm − sec − . The exploration of the physics case for such a machine, which shares atleast some common ground with the proposed eRHIC machine at BNL, involves an on-goingcollaboration under the heading of EIC [4], between the communities associated with bothnational laboratories.Since the cost of such very high energy colliders is likely to be rather high, at JeffersonLab considerable effort has recently gone into the design of possible staging options, whichpresent a strong, self-contained physics case, yet have a cost comparable to that of FRIB. Amultitude of novel suggestions for such a machine led to a very vigorous discussion betweenphysicists, engineers and machine designers and the full user community at Jefferson Labis just beginning to participate in these discussions. Nevertheless, there is already a veryexciting design which currently appears to optimize the opportunities for scientific discoverywhile satisfying reasonable cost constraints. Everything which I shall present is the resultof an impressive team effort by the group of people thanked in the acknowledgements. Thisnew machine is currently known as the MEIC, or the medium energy electron-ion collider. As illustrated in Fig. 1, MEIC is designed to serve as the natural first stage of full ELICconstruction. As far as possible the tunnel and components can be re-used in the ultimatemachine, should that be built. The circumference of the machine is 634m, with straightsections of 150m. The proton energies range from 12 to 60 GeV, in collision with electrons(or positrons) taken from the CEBAF 12 GeV Upgrade between 3 and 11 GeV. This allowsfor e-p collisions over an impressive range of cm energies, from s = 100 to 2640 GeV . Wealso note that if it were to be essential, even the MEIC can be staged with a warm ion ring,allowing proton momenta up to 12 GeV/c, being a somewhat less expensive first step. DIS 2009 igure 1: Layout of the ELectron Ion Collider (ELIC) and the MEIC at Jefferson Lab. Bothof these machines would use polarized electrons (or positrons) at energies up to 11 GeV fromthe upgrade of CEBAF which is currently underway.As outlined earlier, a key design criterion has been to ensure that the luminosity will beappropriate for a world-leading macine in the 2020’s and 30’s. The current design yieldsexcellent luminosity, around 10 cm − -sec −
1, for s ∈ (200 , . The luminosityremains above 10 over the rest of the range. In terms of physics reach, this means thatone can access the structure of polarized protons at luminosities of order 10 above x =0 . Q = 1 GeV and above x = 0 .
01 at 12 GeV . For the time being, the luminosityis bounded by detector and data acquisition limitations, with the repetition rate being keptat a conservative 500 MHz. There is an urgent need for R & D to explore whether one couldraise this rate and consequently the luminosity.The figure of 8 structure, illustrated in Fig. 1, is designed to ensure high polarizationfor both the light ion and electron beams. There are four possible intersection points. It isintended to also make a polarized positron capability with the same luminosity available.A particularly attractive feature of the MEIC design is that unlike ELIC or eRHIC, wheresome of the technical issues presently seem to be “very challenging” or worse, there is noissue associated with MEIC which ranks above “challenging” – with the electron cooling andtravelling focussing being the two major issues. Innovative features, such as crab crossingand crab cavities seem to be almost in hand. Other important issues which need work in thenear future, include beam-beam effects and the formation of the high intensity low energyion beam but these are not regarded as especially challenging. As the novel design of the MEIC is very recent, the detailed physics case matched to itsunique capabilities still needs considerable work and we certainly invite all interested mem-bers of the community to join this effort.
DIS 2009 n many ways the proposed MEIC is the perfect complement to the 12 GeV Upgradecurrently underway at Jefferson Lab [5]. While the latter aims to define the spin and flavordependence of nucleon and nuclear parton distribution functions in the valence region, theformer is ideally suited to serve the same function for the sea. Indeed, with its capabilityto clinically examine the debris left from the target, this collider should enable a far deeperunderstanding of the origin and structure of the non-perturbative sea than we have everbeen able to imagine before.The generalized parton distributions, which will be thoroughly explored at 12 GeV andthen exploited as source of information about the distribution of angular momentum onthe valence quarks [6, 7], will serve as a vital source of information on the orbital angularmomentum carried by sea quarks. It remains to be seen whether they can also be used toinvestigate the gluon angular momentum but if a method were to be found it would be veryvaluable indeed. The understanding of the potential for transversity to yield information onthe distribution of orbital angular momentum within the proton is at an earlier stage butthere are clear indications of its potential which needs to be developed further.As a tool to investigate the quark and gluon structure of atomic nuclei, a fundamentalissue for modern nuclear physics, the MEIC offers some remarkable new possibilities. Thestudy of the iso-vector EMC effect [8] could be dramatically advanced if one could make acomparison of the ( e − , ¯ ν e ) and ( e + , ν e ) reactions on a variety of heavy nuclei. The abilityto identify fragments of the final nucleus offers potentially novel ways to test explanationsof the EMC effect. We expect the beam quality to be such that this machine should alsobe suitable for investigations involving parity violation, which also offers a novel look insidehadron and nucleon structure.A number of groups have just begun to explore the potential of this machine for studyingcharmed systems – from the production on free nucleons, to in-medium modification to theirrole as a possible tool to determine the gluon angular momentum. This is an extremelypromising area that merits detailed examination.Finally, one cannot imagine constructing a machine such as MEIC, with its high levelof polarization and luminosity, as well as its expanded range of invariant mass, withoutexploring its potential for precision tests of the Standard Model. For the present there isnothing to report in this area, but I would certainly encourage some of the younger membersof the community who may actually live long enough to finish such an experiment, to beginto contemplate the possibilities. It is clear that one or more electron-ion colliders may well have an important role to playin the development of nuclear physics over the next few decades [9]. The MEIC, which isunder intense study at Jefferson Lab, represents a cost effective first stage towards the veryhigh energy ELIC. Nevertheless, MEIC appears to have a very impressive physics programassociated with it. We certainly welcome all members of the DIS community who are excitedby its potential and would like to contribute.
It is a pleasure to acknowledge the tremendous amount of effort that has so far been devotedto this important project by a number of staff and users at Jefferson Lab. I would particularly
DIS 2009 ike to thank S. Bogacz, P. Chevtsov, Ya. Derbenev, R. Ent, G. Krafft, T. Horn, A. Hutton,C. Hyde-Wright, R. Li, B. Yunn, Y. Zhang, F. Klein and P. Nadel-Turonski and C. Weiss,without whom this presentation would not have been possible. This work was supported bythe the U.S. Department of Energy under Contract No. DE-AC05-06OR23177, under whichJefferson Science Associates, LLC operates Jefferson Laboratory. [1] Slides: http://indico.cern.ch/getFile.py/access?contribId=28&sessionId=22&resId=3&materialId=slides&confId=53294 [2] S. Bogacz et al. , In the Proceedings of Particle Accelerator Conference (PAC 07), Albuquerque, NewMexico, 25-29 Jun 2007, pp 1935 .[3] [4] http://web.mit.edu/eicc/ [5] A. W. Thomas, Eur. Phys. J. ST (2007) 117.[6] X. D. Ji, Phys. Rev. Lett. , 610 (1997) [arXiv:hep-ph/9603249].[7] A. W. Thomas, Phys. Rev. Lett. , 102003 (2008) [arXiv:0803.2775 [hep-ph]].[8] I. C. Cloet, W. Bentz and A. W. Thomas, arXiv:0901.3559 [nucl-th].[9] “The Frontiers of Nuclear Science”, the NSAC Long Range Plan for Nuclear Physics (2007) ;Report of the OECD Global Science Working group on Nuclear Physics, May 2008 ;IUPAP Report No. 41, “Research Facilities in Nuclear Physics” http://trshare.triumf.ca/~ramsay/IUPAP-WG9/IUPAP-Report-41.pdfhttp://trshare.triumf.ca/~ramsay/IUPAP-WG9/IUPAP-Report-41.pdf