With both the US Nuclear Science Advisory Committee (NSAC) and the CERN Council Open Symposium on European Strategy for Particle Physics convening in early September — in Gaithersburg, Maryland, and Kraków, Poland, respectively — the 30 August Nature offers two news articles and a commentary on what may be coming for physics conducted with particle accelerators.
What's coming could be trouble, says Eugenie Samuel Reich in a news report, for at least one of three big US nuclear physics enterprises:
* The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory on Long Island, which has operated since 2000 to explore the properties of quark-gluon plasmas.
* The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab in Newport News, Virginia, which studies 'the arrangement of quarks and gluons in protons and neutrons' and has more than halfway completed an energy-doubling upgrade.
* The Facility for Rare Isotope Beams (FRIB), envisioned for Michigan State University, which would 'create new nuclear isotopes and study their properties.'
Reich observes that CERN's Large Hadron Collider (LHC) ended the need for Fermilab's Tevatron, and that RHIC is trying to avoid the same, given that on 13 August, researchers at the LHC's ALICE heavy-ion experiment 'announced that they had created the hottest-ever man-made plasma of quarks and gluons,' which 'eclipsed the record temperature achieved at RHIC two years earlier by 38%, and raised uncomfortable questions about RHIC's future.' NSAC, she says, will 'pit' RHIC against CEBAF and FRIB, with RHIC likely to be closed unless funding somehow rises unexpectedly.
Matthew Chalmers's news report 'After the Higgs: The new particle landscape' carries the subheadline 'Physicists are planning the powerful accelerators they will need to study the Higgs boson and its interactions in detail.' Chalmers says that a question defines the 'whole future' of particle physics: 'Is the particle a Higgs boson of maximum simplicity, as predicted by the 40-year-old standard model of particle physics? Or is it something more complex and interesting that will point towards a deeper, more complete theory?'
Researchers hope to get a lot more out of LHC, Chalmers writes, and they 'already have one piece of good news: the mass of the Higgs-like particle ... turns out to lie towards the light end of the range that theorists had estimated.' This finding 'has two important consequences: it means that a relatively modest new collider would be sufficient to produce the Higgs in bulk, and it gives the new particle a rich variety of decay modes that will make it easier for physicists to study its interactions with other standard-model particles.'
Chalmers explains the attractiveness of lepton colliders, which can 'sidestep the messiness' of 'strong quark-gluon interactions,' working 'more like scalpels than sledgehammers,' thereby yielding precise measurements. He describes the 'relatively cheap option' of adding an electron-positron collider to the LHC tunnel, which was designed to accommodate 'both types of collider running simultaneously.' But this 'LEP3' option, named for the predecessor collider that once occupied the LHC tunnel, would have limitations. So would the alternative of building a muon collider.
Another future development could be a linear electron-positron collider, its straightness inherently precluding the problem of curvature-induced synchrotron radiation losses. Chalmers summarizes the two big concepts:
The ILC [International Linear Collider], developed by a worldwide consortium of laboratories and universities, would be some 30 kilometres long, and would use proven superconducting accelerator technology to reach energies of 0.5 TeV, with the possibility of upgrading to 1 TeV. The ILC team is soon to publish a technical design report and the cost of the project is currently estimated at $6.7 billion. The Compact Linear Collider (CLIC), championed by CERN, would be almost 50 kilometres long, but would use novel acceleration techniques to reach energies of 3 TeV. CLIC's costs are less clear than the ILC's because only a conceptual design report is available, but its higher energies would open up new realms for discovery as well as for precision measurements.
Discussions are underway, Chalmers says, to agree on 'a proposal for a single linear collider by the end of 2015.' But the expense is high, and predictions are sketchy. For various reasons, neither CERN nor the US is likely to host this collider, the article says, but many observers think that Japan might. A 'dream scenario,' Chalmers observes, 'is the LHC exploring the high-energy frontier in Europe; multiple neutrino experiments exploring the intensity frontier in the United States; and a new lepton collider in Japan pinning down the details of all the exotic new particles that so far have not turned up in the LHC's collisions.'
In the commentary 'Beyond the Higgs,' the University College London physicist Jon Butterworth proposes that the 'discovery of the Higgs marks the dawn of a new era for researchers in extreme-energy physics,' for 'we need to understand why the masses of fundamental particles have the values they do, and why some of the patterns in these fundamental particles are as they are.' The Higgs 'vindication,' he declares, 'is strong encouragement to continue — to measure the Higgs boson and see whether it behaves as predicted, and whether it offers clues to other outstanding questions.' Butterworth announces, 'We have a wild new frontier of physics to explore.'
The LHC, he says, 'will get us partway into this unknown region, but to go beyond that we will need all the clues we can find.' One key question is why the Higgs is 'so light':
If one assembles the standard model without fine-tuning some parameters, quantum effects mean that the Higgs boson's mass should grow and end up near the Planck scale. This is clearly wrong, and it hints at gaps in the theory.
Supersymmetry and extra space-time dimensions have been proposed as solutions to this problem. Supersymmetry introduces a set of particles that cancel out the quantum effects that would otherwise make the Higgs heavier. Extra dimensions could bring the Planck scale closer to 100 GeV. There is no direct evidence for either theory, but new particles or deviations from the standard model's predictions could turn up at the LHC any time, especially after the machine doubles its energy in 2014.
Butterworth calls for accelerator technology research to investigate making higher-energy beams by 'using high-current, low-energy beams to drive higher-energy, lower-current ones,' by 'accelerating electrons in the electromagnetic wake of a proton beam,' and by colliding muons. He also mentions discoveries that can be made without higher energies, the potential of neutrino experiments, puzzles like dark matter that remain in the standard model, and the need for a 'better understanding of how the strong interaction binds quarks and gluons into hadrons.'
His conclusion merits quoting verbatim:
The standard model gives us a list of apparently fundamental particles, the only stuff in the Universe that is not made of other stuff. With the discovery of the Higgs, we now have a theory within which these particles can have mass, and this is a huge step forward.
Now we can relish the next steps, at the LHC and beyond. Perhaps researchers can build a linear collider to serve as a 'Higgs factory'. Perhaps another experiment will deliver a surprise that changes everything. Perhaps a breakthrough in the theory will explain the coincidences and patterns of the standard model.
The patterns seen in the periodic table before anyone knew about electrons and nuclei turned out to be a sign of the underlying structure of atoms. Maybe there is another layer of substructure that explains the patterns we see now in particle physics.
Steven T. Corneliussen, a media analyst for the American Institute of Physics, monitors three national newspapers, the weeklies Nature and Science, and occasionally other publications. He has published op-eds in the Washington Post and other newspapers, has written for NASA's history program, and is a science writer at a particle-accelerator laboratory.
