A science fiction story I read in my younger years opened by saying that both High Energy Physics (HEP) and Optical Astronomy were no longer funded because their facilities had gotten too expensive. Watching the effects of the US and European economies and of the cost of accelerators on the so-called energy frontier has brought that story back to haunt me.
The question of cost is relevant now. It's also relevant to even broader questions about coordinated world programs in many areas of science and technology. The ITER fusion experiment, for example, will cost about $15 billion, and it will take 15 years until the reactor is ready to burn deuterium and tritium. No one country was willing to take it on, but a worldwide collective could and did. The real questions are whether we should have such a world collaboration in HEP, and if we should, should CERN be the central organization.
Before I get to those questions, there are a few things about CERN that should be kept in mind. Its budget this year in US dollars is about $1.25 billion. According to CERN’s website, its facilities now have about 10 000 scientist-users; 16% are from the US, and 22% are from other nonmember states, so member states with 62% of the scientist users pay most of the bills. Member states contribute to the CERN budget in proportion to their GNP, with a cap of about 23%.
In 1997 the US negotiated a special agreement with CERN on the Large Hadron Collider (LHC) project. Of the $531 million the US agreed to contribute, $200 million went to the machine and $331 million went to the detectors. In 2004, when the project was well under way, the US had 538 scientists involved in the experiment, about 9% of the total. Based on our fraction of the users then, we under-contributed to the construction of the machine and over-contributed to the detectors. The balance was to be reasonable. If the US were a member state now, our contribution would be nearly $300 million this year.
Physics issues: Proton accelerators
The US long-range plan for proton colliders lost its focus by the cancellation of the Superconducting Super Collider (SSC) in 1993. That 40-TeV machine (the LHC will be 14 TeV when it reaches its design energy in 2015) fell to a rare fit of budget cutting by Congress.
The energy frontier has belonged to Fermilab, with its 1.9-TeV Tevatron, for nearly 30 years, much longer than thought when the SSC plan was being developed. Completed in 1983 under the leadership of Leon Lederman, Fermilab’s second director, the Tevatron was supposed to be superseded by the SSC in about the year 2000. Instead, the Tevatron remained the world’s frontier facility until the startup of the LHC in 2010.
To increase the LHC’s discovery potential, CERN plans to upgrade its luminosity by about a factor of five starting in 2020. CERN also has a small R&D program on 15-tesla, high-temperature superconducting magnets that might allow an energy upgrade to 30 TeV beginning around 2030. If both upgrades are successful, CERN will have the energy lead until 2040, and the LHC will have had a lifetime as long as the Tevatron’s.
The upgrades will be costly, not only because of the work on the machine but also because the detectors will need major overhauls to be useable under the new operating conditions. The LHC now yields about 25 events, per beam crossing, dominated by relatively uninteresting low-energy interactions of the protons. More than a billion uninteresting events have to be filtered out for every interesting one that occurs.
Doubling the machine’s energy—which has begun with the current shutdown and be completed at the start of 2015—will also increase the luminosity by about a factor of two, thereby doubling the events per beam crossing. The luminosity upgrade planned for around the year 2020 will cause the event rate to go up by an additional factor of five, overwhelming the current detector tracking systems. New systems will be required.
The energy upgrade requires much higher field magnets than are now used in accelerators. Superconducting magnets can already deliver fields of 15 T, but so far only in small systems that do not require high-quality fields. In my view, developing 15-T magnets for the LHC will require an R&D program on the scale of the one that Bob Wilson ran at Fermilab. That program resulted in the technology developments that made today’s superconducting magnets possible. Incidentally, the SSC, whose main ring would have been three times longer than that of the LHC, could have reached 120 TeV with 15-T magnets.
But who would undertake such a program? I don’t know what one would cost. I also don’t know if the requisite detectors can be built. Worst of all, I don’t know if the upgraded LHC would yield interesting results.
Physics issues: Electron accelerators
If the US joins CERN, the center will become a world laboratory. The future of electron–positron accelerators must therefore be considered alongside that of proton accelerators, in which CERN already leads the world. From the physics perspective, there are only two electron–positron machines that are under discussion. One is a Higgs factory at an energy of roughly 250 GeV. The other is a few-TeV machine to look beyond what can be seen with the proton machines.
Some physicists love the Higgs factory for its potential to uncover which Higgs-like particle was seen at the LHC earlier this year. The standard-model Higgs has a specific relation between branching fractions—that is, the rates at which the Higgs decays in its various modes. Most of the theories “beyond the standard model” have branching fractions that are different from those in the plain vanilla model. But the fractions differ only by a few percent. Given their horrible backgrounds, there is no way that proton colliders can get the branching fractions nailed down with the precision necessary for scientists to decide which version of the standard model is the correct one. Of course, the direct discovery of supersymmetry or one of the other standard-model variants would render the question moot.
A high-energy electron–positron collider can identify new particles much more easily than a proton machine can. That’s because of what I call a kind of new physics democracy. In an electron–positron collider, all new particles are produced with comparable cross sections and without the need to filter out a billion-times-larger background. Whereas at a proton collider the backgrounds are so high that you can find only what you are looking for, at an electron–positron collider you can find whatever is there.
The ongoing International Linear Collider (ILC) program is aimed at building and running a 500-GeV machine by 2020. A new ILC design study is scheduled for release in a few months, but by 2020 the LHC should have delivered enough cumulative output to make anything the ILC can produce irrelevant beyond what its lower-energy Higgs-factory option can do.
The US–CERN relationship
Why the US needs some sort of formal relationship with CERN is simple. I doubt that CERN would continue to welcome the 16% of LHC users who are based in the US without some sort of commitment for shared support. Not so long ago, there was a kind of balance of trade in which European users of US facilities numbered as many as US users of European facilities. With the shutdown of the B factory at SLAC and the Tevatron at Fermilab, that balance no longer exists.
Two modes of collaboration are possible: The US becomes either a member state or an appropriate-scale contributor to ongoing and future projects through some sort of agreement less than full membership. CERN is a formal international organization that was set up by a treaty ratified by the governments of its member states. In Switzerland, where CERN is headquartered, the organization even has diplomatic status, like that of the United Nations. CERN’s constitution and status make unilateral budgetary action by one of its members extremely difficult, which is one of the main reasons for the stability of CERN’s budget.
For example, some years ago Germany sought to reduce its payment by introducing an amendment to lower the maximum contribution of member states. That amendment required the agreement of all the other members. After negotiation, the maximum contribution was indeed reduced—from 25% to about 23%, giving Germany its reduction while preserving the treaty.
The CERN Council, the organization’s governing body, is set up as a one-member, one-vote system. Although it is not impossible that the US Senate would ratify a treaty to join CERN, it seems extremely unlikely. We have joined one such system, the United Nations, but there, thanks to our permanent membership on the Security Council and our veto power, we enjoy more power than do most of the UN’s 193 member countries. Perhaps something could be worked out with CERN, but it would not be easy.
Also, it is no surprise that CERN is Eurocentric, since it was set up as a European organization. Although CERN is expanding beyond Europe—Israel has applied to become a member—it retains its European focus. While the CERN Council has changed the rules to allow money to be spent on projects outside of Europe, funding for any such project has yet to be approved.
CERN is not a world lab now and is unlikely to evolve into one without a major change in its organization to make it more like ITER, in which the European Union is one of the members and has one vote like the others. By contrast, on the CERN Council, each European member has one vote. Together, the Europeans would form the dominant party. I do not see CERN becoming more like ITER any time soon, and so I would recommend that the US not become a member state.
I do recommend, however, that the US negotiate a new association agreement that incorporates an appropriate level of support for LHC operations and a role in the development of the technology required to enhance the program. For example, US laboratories could help create the 15-T magnets required for the LHC’s energy upgrade. If the US continues to participate in the program, our scientists will be involved in the detector upgrades like they were during the development of the first round of LHC detectors.
There are other projects on the horizon, like the 250-GeV electron–positron collider to understand the Higgs particle. It is already an international project—scientists from around the world are helping to design it—and it could be built in the ITER mode. The site could be constructed in numerous places. At the moment, Japan is emerging as the most likely location. CERN could possibly serve as the representative of the European Union. Clearly, Asia is becoming more of an economic and scientific powerhouse, and any discussion about a world lab needs to address how the continent’s countries can be included in a major way.
A final word on physics
The standard model has survived all attempts to find “new physics” beyond it. The model stands like a wall with what might be cracks, but they also might only be marks on the paint. I say the wall is covered with Post-it notes that cover things that can be accommodated but are not explained by it.
Examples include three generations of quarks and leptons, three colors, and CP violation. Until this year, the standard model had a problem: A particular process became too big at high energy unless a low-mass Higgs boson existed. Now that the LHC has found the particle, the standard model can be extended to energies beyond anything that is conceivable from Earth-bound accelerators.
But there remain mysteries. The standard model accommodates the strong, electromagnetic, and weak forces but has no room for gravity. It has nothing to identify with the dark matter that we know permeates our universe and which holds galaxies together. It cannot explain the accelerating expansion of the universe, whose discoverers received the 2011 Nobel Prize in Physics.
There are experiments on the ground, under the ground, in space, and at accelerators that attempt to find out more about those open questions. But if the next generation of accelerators contributes nothing new to answering them, I doubt that we will ever see a 100-TeV collider or a giant electron–positron machine. Then the premise of the science fiction book that still haunts me will come true. If our only theory of everything comes down to the landscape model, where we are only one of a zillion universes with the parameters we see as only a statistical accident necessary for life, the game is over. I hope not.
Burton Richter currently holds two positions at Stanford University: Senior Fellow at the Freeman Spogli Institute for International Studies and the Paul Pigott Professor in the Physical Sciences Emeritus. He was the director of SLAC from 1984 to 1999.
