Looking down the road ahead
November 19, 2012


As humans, we are often short-sighted. Sometimes, this may be out of necessity. When it is a struggle to provide food on the table each day, it’s hard to take a long view. However, in conversations and exchanges with many people in the lab, there is a desire to know what is going to come next? What does our future hold?

A starting point for such a discussion might be to ask what does the present hold. Or what does the near-term future hold? Well, of course, we have been planning that for a long time. In the early part of the previous decade, discussions started about an upgrade to CEBAF that would provide beams and physics at 12 GeV, a substantial increase from the initial designs at 4 GeV and twice what eventually turned out to be realistic aims to operate at 6 GeV. By now, we are four years into the construction of the 12 GeV Upgrade Project. We are in the middle of a long shutdown of accelerator operations. When that shutdown ends, we will turn to commissioning the new accelerator. I say new not because every bolt and screw will be new, but because there will have been many changes; lots of the cryogenics plant is augmented; there are 10 new cryomodules; and most of the magnets will have been refurbished and re-installed.

But what comes next? We had many discussions during the year, and while our strategic plan report is not yet out, we are talking about its primary conclusions. We see that following the 12 GeV Upgrade and the stellar physics during more than a decade of experimentation that will result from it, an electron-ion collider would be a natural successor to our current fixed-target program with CEBAF.

An electron-ion collider would involve building a new accelerator of protons and other nuclei so that we could collide them using a system of storage rings, with the electrons coming from CEBAF. Such a plan is ambitious, but it has emerged from discussions within the nuclear physics community that started before the 2007 Nuclear Science Advisory Committee’s December 2007 Long Range Plan. In that plan, an electron-ion collider (EIC) was, indeed. flagged as a promising future path for U.S. nuclear physics. Between now and then groups of experimentalists, accelerator designers and theoreticians have been developing the idea, and over the past 18 months a rather coherent set of requirements has emerged. These are requirements that would continue the exploration work already under way with CEBAF, but would move it to an entirely new kinematic regime. We would be exploring the strong interaction and the structure of the nucleons and nuclei in a region, that, until now, has been lightly touched.

The physics is articulated in terms of partons, quarks and gluons, inside nucleons and nuclei. The 12 GeV machine will probe in fine detail the region where it is mainly quarks that carry large fractions of the momentum of the target nucleon. The electron-ion collider would concentrate on probing the partons that carry a small fraction, typically less than one percent, of the momentum of the target nucleon. There, gluons dominate and it is the study of their dynamics that pushes us to a higher energy machine. The machine HERA, at DESY in Hamburg, Germany delved into those regions, but without the benefit of spin and high luminosity with which we would hope to imbue the EIC.

As you might imagine, we are not the only ones to have this idea. Both the Jefferson Lab and Brookhaven Relativistic Heavy Ion Collider communities have worked together on the specifications. The development of a united view of the physics goals is thought to be a prerequisite for broader support within nuclear physics. There have been many studies over the past years, and most recently a White Paper making the physics case is emerging; a draft is “on the street” for community comment. Meanwhile, there are separate design efforts at the two labs and our Jefferson Lab team, with collaborators from several other national labs, released a design report in August. The particular version of the machine described in that document is a Medium Energy electron-Ion Collider (MEIC).

The plans need careful review and also careful costing to understand the practicalities of our ambitions, and this work will go on for some time. Some of the ideas also require significant advances in accelerator and detector technology, which will only be achieved with a substantial research and development program. As suggested by the name “Medium Energy. .. “ the design could accommodate a higher energy version; we are looking into the best way to layout the new machines on the current site and adjoining land. We will also look at other staging options in order to fully understand all the possibilities for the machine.

We are concentrating hard on bringing the 12 GeV Upgrade Project to a successful conclusion. Nevertheless, we anticipate that over the course of the next year, there will be a definite up-tick in the public discussions in seminars and conferences around the country looking down the road at the plans for an electron-ion collider at Jefferson Lab.

Bibliography:
Gluons and the quark sea at high energies: distributions, polarization, tomography. The EIC Science case: a report on the joint BNL/INT/JLab program. Ed: D. Boer, M. Diehl, R. Milner, R. Venugopalan, W. Vogelsang, September- November, 2010.

Nuclear Physics in an electron-ion collider, A. Accardi, V. Guzey, A. Prokudin and C. Weiss, Eur. Phys. J. A 48, 92 (2012).

Science Requirements and Conceptual Design for a Polarized Medium Energy Electron-Ion Collider at Jefferson Lab. Ed: Y. Zhang and J. Bisognano.

The Electron Ion Collider: A Next QCD Frontier: Understanding the Glue which binds us. Ed: A. Deshpande, Z.-E. Meziani. J. Qiu.

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