Linear Collider Workshop 2002: Addressing the Case for the Next Major International High Energy Physics Project

1. Linear Collider P. Grannis: Workshop charge Chicago, Jan. 7 – 9, 2002 Since the Johns Hopkins workshop Mar. 19 – 21, 2001, a good deal has happened in the Linear Collider world. From the charge at JHU: “ In the US, the HEP community has not yet articulated its support for the LC; our chief focus now should be to make the case for the LC within the physics community as fairly and as completely as possible. Only with a recommendation from the community that the LC is the right next choice can we address the case with other branches of science, the government, and international partners. It will be hard enough even if we are unanimous! ” The pressing problems for the LC community have changed considerably since then. This workshop is intended to give an overview of the LC physics and detector issues, and to provide a forum for discussing the way forward.

2. I. Milestones: • March 23,24: TESLA Colloquium. The TESLA proposal is now under consideration by the German Science Council. TESLA TDR: http://tesla.desy.de/new_pages/TDR_CD/start.html • July: SNOWMASS 2001 – N.A. LC group prepared a 400 page “Linear Collider Sourcebook”: http://www.slac.stanford.edu/grp/th/LCBook/ 1.Introduction 2. The Case for a 500 GeV Linear Collider (from 2000) 3. Sourcebook for Linear Collider Physics 4. Pathways beyond the Standard Model 5. Experimental Program Issues 6. Detectors for the Linear Collider 7. Suggested Study Questions (a big effort by many. Thanks particularly to M. Peskin) • July: Snowmass Physics Groups recommendation: “There are fundamental questions concerning electroweak symmetry breaking and physics beyond the Standard Model that cannot be answered without a physics program at a Linear Collider overlapping that of the Large Hadron Collider. We therefore strongly recommend the expeditious construction of a Linear Collider as the next major international High Energy Physics project. ”

3. Milestones, cont’d: • August: ECFA Working Group on Future of Accelerator-based Particle Physics in Europe recommended: “the realization, in as timely a fashion possible, of a world-wide collaboration to construct a high-luminosity e+e- linear collider with an energy range up to at least 400 GeV ” • September: ACFA statement on the Linear Collider: “The e+e- LC must start operation when the high luminosity run of LHC starts around 2009-2010. The center of mass energy of the LC should be 250-500 GeV where urgent and critical physics is expected. Including its energy upgrade to higher than 1 TeV, the project as a whole is foreseen to evolve for a quarter of a century. ACFA strongly endorses the plan to construct such a collider in the Asian-Pacific region with Japan as the host, and urges KEK to take initiative to investigate possible and practical form of globalization for the construction, commissioning and operation of the collider.” ACFA/LC Working Group report : htts://acfahep.kek.jp/and hep-ph/0109166

4. Milestones, cont’d: • October: HEPAP subpanel draft statement: “We recommend that the highest priority of the U.S. program be a high-energy, high-luminosity, electron-positron linear collider, wherever it is built in the world…” “We recommend that the United States prepare to bid to host the linear collider, in a facility that is international from the inception, with a broad mandate in fundamental physics research and accelerator development…” “We recommend the formation of a steering committee to oversee all linear collider activities in the U.S. … ” Workshop Plans for near future: Europe : 12 – 15 April, St. Malo, France U.S. : May/June, site not yet fixed Asia : July , Tokyo World LC Workshop: 26 – 30 August, Jeju Island, Korea

5. So, the playing field has changed – we have a strong statement of priority for the LC in all regions. This does not mean that all physicists in the US/elsewhere share the sense of LC as their avenue to future experimentation. Many other initiatives of merit exist: Although those in the LC camp argue that the physics issues of EWSB are the most ripe for fundamental new understanding in the near term, we should recognize that these other projects are also of fundamental importance. • Superbeam/long baseline n studies; mSR • Underground/sea/ice labs for p decay, n astronomy • Studies of rare decays of K, B … and insights into flavor • Innovative astroparticle experiments – SNAP, GLAST, LISA, Pierre Auger, CMB … • Very high energy hadron collider

6. II. The Physics Case for LC • The broad case for the LC – “500 GeV, upgradable” is now quite clearly delineated (talks by M. Peskin, S. Heinemeyer, L. Gibbons, J. Lykken): • Something playing the role of the SM Higgs boson will exist and be accessible at the LC. The LHC will not tell us its full character, so we will need the LC. • Some new physics beyond the SM Higgs is needed, and its signs should be seen at a 500 – 1000 GeV e+e- (gg, eg, e-e-)LC • LHC will give only fragmentary understanding of Susy; LC will delineate and give understanding of Susy-breaking • LHC and LC both will see evidence for Strong Coupling or Extra Dimensions. LHC and LC are complementary, with LC offering many unique observables. • There are other physics topics of considerable importance – QCD studies in new regimes, precision EW measurements, new flavor studies in the Susy sector, but: • The main rationale for the LC is understanding EWSB.

7. Physics, cont’d: • There remain many physics studies that are critical for filling in areas of the physics program (see S. Dawson talk), but the main outlines are now there. • Some of the areas for more physics study: • For representative Susy benchmark points – what sparticle states are clearly observable ( masses, BR’s) with realistic luminosity? How to measure the cross sections, and with what accuracy? How accurately can the chargino & neutralino matrices be experimentally reconstructed? (mixings, CP phases etc. in an unconstrained MSSM) • For the tree of Extra dimension models, how incisive will LC experiments be? (different assumptions of no. and size of extra dimensions, the particles that go into the bulk, metric of bulk … ) • What is the optimum way to delineate the heavy Higgs sector in MSSM models? What energy, beam particles, polarizations etc. What indirect measurements are crucial?

8. Physics, cont’d: Such studies should continue, in part because they educate us all on the power of the LC program. But they do not constitute the dominant need now as we enter the era of achieving an approved LC. One question that I still do not feel has had as clear answer as is needed: Why should the 500 GeV LC be started before the LHC has given physics results? The bureaucrat with the most meager scientific understanding can – and will – ask this of us. Without a compelling answer, our ship may go dead in the tortured channels of the Congressional/OMB deliberation ! I propose a competition – for the best short essay in lay language to answer the questions: “Why do we need the LC” and “Why start now”. (and bear in mind the recent news articles that declare that the Higgs does not exist, and CERN wasted $9B on its search ! )

9. III. Detector Issues • We need more effort on detector R&D and simulations. • Europe and Japan have given more attention to this than the US. Although it is too early to produced detailed experiment designs (or form collaborations), now is the time to develop the new ideas that will be needed. (See R. Heuer talk.) • Some may say that a LC detector is not especially challenging on the scale of LHC detectors, so can rest on proven techniques. But: • We now understand that there is great premium on having the best vertex detector one can acquire to separate b,c,t, (uds), g cleanly. • Excellent dE/E for jets is paramount for some physics (e.g. Higgs potential). LC detectors are free of some of the LHC constraints (radiation damage, event pileup). How well can one do? • Detector integration is different in e+e-than pp. How to optimize signal handling techniques for the lower rates of the LC? (see plenary talks by K. Riles, R. Frey, G. Fisk, N. Graf for more detail)

10. Detector Issues, cont’d: If we are going to capitalize on new detector ideas that expand the LC reach, now is the time to do it – while the project is going through its political approval process, and before detector proposals are prepared. • The US needs an expanded program of detector R&D. • The three regions of the world should collaborate closely on detector R&D projects. The issues are the same world-wide, and for any type of LC. The financial resources of all regions are stretched. Available test beams are scattered across the globe. • Such collaboration is an important precursor to the full LC collaboration on both detector and accelerator fronts.

11. IV. Some remaining LC issues, needing work by the combined experimental and accelerator communities • Beam energy calibration – how do we achieve dE/E of ~50 ppm? • Polarization – how to get the needed 0.1% precision on effective polarization? What is the best scheme for positron polarization? How well can it be calibrated? • Is the Low-E / High-E IR strategy really optimal? • What is the physics (and political) rationale for 2 IRs vs. 1 IR? • And, how can we enhance the outreach to other sciences by enabling other uses of the LC components (linac tunnels, e+, e- sources, intense g beams, preaccelerators, damping rings, spent beams) ? • X-ray FELs and conventional light sources • Medical diagnostic/treatment facilities • Nanoscale instrumentation center • Laser interferometry projects • Material /biological science with g beams

12. Achieving the Linear Collider: • We should never underestimate the difficulty in getting the Linear Collider approved. • The LC project has many extremely challenging technical aspects (talks by C. Adolphsen, R. Brinkmann, T. Markiewicz) and much remaining R&D (talks by K. Kubo, S. Holmes) . The experimental community needs to participate in solving these. • Cost is well beyond the single project cost for basic science in the past. We say it must be fundamentally international to afford it, but do not yet have a blueprint for this. • Preoccupation of governments on other issues Terrorism and security needs Economic recession threats worldwide Costs of unification of Germany … • Competition with other science and technology projects (bureaucrats don’t distinguish laser fusion, space experiments, and accelerators). Our colleagues in other areas of science must be convinced that HEP in general, and the LC in particular, makes scientific sense in broad terms, and is worth large expenditures.

13. Achieving the Linear Collider, cont’d: The big steps: 1. Get realistic costs – for R&D, LC project, infrastructure (the political process, environmental protection, community outreach), manpower, contingency, escalation. The SSC, and now LHC, have been plagued by unexpected cost overruns. Our political capital is thereby severely damaged, and our costing must be correct! 2. Choose the accelerator technology: TESLA vs. NLC/JLC. The Loew panel will help define the tradeoffs (G. Loew talk) .We need to start now on a process to make this decision. We will find it hard to sell the project to governments until it is made. If there is clear preference from the ICFA evaluation process, make the decision! If not, it does not matter so much! Some of the communities will have to swallow their pride, but better sooner than later so we can mount a collective effort on a common project earlier.

14. Achieving the Linear Collider, cont’d: • Establish the process for reaching political international decisions on: • Shall we convene a working group of physicist-statespersons to give these issues a draft framework? • (see paper by G. Trilling on Workshop web page; talk by M. Tigner, and the panel discussion) • 4. Articulate the rationale for the LC to governments – it will not be the last project we request! It has to be sold on the basis of understanding fundamental makeup of universe (“structure of space, time, energy and matter”) and not on spin-offs. • funding arrangements – the shares for host and participant nations. • the way to reach the site decision. • organizational structure (is it just for LC, or envisioned as a structure for future projects?) • how to retain the health of accelerator/particle physics in all regions with a LC in only one.

15. Conclusion: We have done the easier part by coming to consensus that a Linear Collider is what HEP should aim for. Now comes the much harder part in making it occur !