HERA Scientists Fight to Extend Strong Interaction Studies

When should a productive machine be turned off?

A particle physics drought looms in Germany, from the time the plug is pulled on the Hadron Electron Ring Accelerator Facility (HERA) at the end of 2006 until whenever activity on the world's next linear collider revs up. "We face the danger of shutting down a unique accelerator irrevocably, and then sitting here for a very long time without particle physics," says Christian Kiesling of the Max Planck Institute for Physics in Munich and a member of the team that is pushing to extend HERA's lifetime. For the extension to happen, proponents will have to scrape together money and manpower and win over the management of the accelerator's parent lab, the German Electron Synchrotron (DESY) in Hamburg.

DESY

Nicknamed the "super electron microscope" because, by colliding high-energy electrons (or positrons) with protons, it provides a glimpse of the proton interior, the 11-year-old HERA has undergone an upgrade to higher luminosity and is on the verge of starting its second phase. The main aims of HERA-II are to flesh out details of proton structure at scales approaching a thousandth of the proton radius and to study the dependence of electron- quark scattering on the orientation of the electron's spin. After that, some 200 HERA scientists from 50 institutions around the world want to optimize HERA--through modifications to the machine and a new or upgraded detector--for a closer look at the dense quark-gluon clouds inside protons. HERA-III would also probe neutrons through electron-deuteron collisions and be used for other deep inelastic scattering experiments and investigations of strong interactions.

"The picture of the proton is getting more and more complicated," says Allen Caldwell of Columbia University and the Max Planck Institute for Physics. "We found that there are a huge number of quarks, antiquarks, and gluons in a proton and that each carries a small fraction (x) of the total momentum. How do we reconcile the picture of protons made up of three valence quarks with this new picture of the proton?" (See the story on page 19.) The most exciting results to come out of HERA were unexpected, he adds. "It often happens that way. If we want to get the most out of HERA, we should focus on small-x physics--that is, the radiative processes which bring about the large density of quarks and gluons."

But other DESY plans stand in the way of extending HERA to a third phase. About two years ago, DESY management decided to convert HERA's injector, PETRA, into a third-generation synchrotron light source. This year, the German government gave its blessing and said it would pay the estimated €120 million (roughly $136 million) for the conversion, which will entail updating the vacuum system, electronics, and other components and remodeling about one-eighth of the 2.3-kilometer ring. The PETRA makeover is supposed to begin at the start of 2007, but may be delayed to accommodate HERA-II, which is about a year behind schedule. Without the injector, HERA can't run.

At the time of the decision to convert PETRA, hopes were high that the German government would take a decisive step toward supporting DESY as the host site for the next linear collider, a multibillion-dollar global project. "Everyone agrees that such a machine should be built," says Max Klein, spokesman for HERA's H1 detector. "If we were in a situation where it was clear that the linear collider could be built here in a few years' time, I don't think we would argue." But in February, the government said it was not ready to commit to the linear collider (see Physics Today, April 2003, page 35). With that project's timing now uncertain, HERA-III proponents say the decision to stop their experiments should be revisited.

No one disputes the value of HERA science. In a letter to DESY management in May, several dozen theorists wrote that "HERA has the potential to provide data crucial to our developing understanding of QCD [quantum chromodynamics]. These data cannot be obtained at other facilities. . . . We therefore strongly support further experimentation with the HERA collider beyond the currently planned high luminosity phase."

DESY's international advisory bodies have praised the proposed HERA-III program, although they ranked it below the linear collider.

But focusing on a project that is not approved and has no date in sight is not good for the rest of particle physics, says Caldwell. "How can you compare experiments that are orders of magnitude different in cost? Huge projects--especially while they are undecided--tend to block smaller projects." Up until Germany announced it wouldn't yet commit to the linear collider, adds Alfred Mueller, a theorist at Columbia University and one of the letter's signatories, "I understood [DESY Director Albrecht] Wagner's logic about pushing the linear collider and not talking about other physics. I don't understand his position now--when it's clear to everybody that the machine will never be built at DESY, and he is willing to let high-energy physics disappear from DESY. It's a disaster for high-energy physics."

For his part, Wagner is more optimistic about the linear collider, and he stresses that DESY wants to play a large role no matter where the collider is built. "It's still doable by 2015," he says, "if the international community can reach a technology choice in 2004 and if political decisions are taken around 2007." And, between R&D on the collider, the new PETRA light source, and a planned x-ray free electron laser, manpower and money will be stretched thin at DESY. Says Wagner, "We have approved projects. We have to make progress in all of these projects, find European partners--we have work up to our nose. So there are no free resources to tackle new problems. Within the present financial framework, I don't see the possibility [of extending HERA]."

DESY isn't alone in facing a dry period in particle physics, Wagner says. "Experiments at KEK, SLAC, Fermilab--all of the major accelerator labs except CERN--will come to an end in 2007 to 2010. We have strategy groups. The common goal is the linear collider. We have to think how we'll keep active if we don't host it." German particle physicists, he adds, are heavily involved in accelerator physics programs worldwide, and it's not true that "the German particle physics program will collapse if HERA is stopped."

Still, HERA-III proponents are not giving up. "Most of us are ready to fight. It would be a real shame to stop," says Kiesling. The accelerator modifications would be relatively minor and participating institutions would foot the bill for the detector. So the main challenges--besides getting the lab's backing--are coming up with funds for a replacement injector plus 150 or so people to run the machine. At the moment, HERA-III proponents are waiting to see if DESY management mentions extending HERA in a five-year plan this fall. Matters are complicated by DESY's having a new oversight agency, the Helmholtz Society, which will review the lab for the first time next year. If HERA-III is not in the plan, says Kiesling, then it would be tough for DESY to go back later to argue for it.

"It seems that the future is sufficiently unclear, with hopes and desperation concerning the HERA-III project intimately mixed," says Kiesling.

2003 American Institute of Physics