Linear Collider Workshop – ChargeLinear Collider Workshop – Charge

Johns Hopkins UniversityP. Grannis March 19, 2001

The coming months will be crucial for establishing a future for the Linear Collider …

In the US: “Linear Collider Physics Resource Book for Snowmass01” Snowmass workshop HEPAP Subpanel

In Europe: TESLA design report (Colloquium at DESY Mar. 23-24) (see talk of Ties Behnke)

In Japan: Work on cost, milestones is ongoing for 2003 design rept?

Globally: Expect technical review/comparison of proposed machines, costs, technical risks, upgradability; Start early 2002? ICFA umbrella

Snowmass 2001

Five Physics groups:

Electroweak symmetry breaking (SM Higgs, Susy, strong coupling, extra dimensions, whatever …

Flavor physics (both quark and lepton sectors)

Scales beyond 1 TeV (whatever is above the EW scale)

Astro/Cosmo/Particle Physics

QCD and Strong Interactions (perturbative, non-perturbative)

Seven ‘Experimental Approaches’ working groups:

Neutrino factories and Muon colliders

Electron positron colliders below the Z (phi, tau-charm, bb Giga-Z …)

Linear colliders (above the Z and Giga-Z for EW measurements)

Hadron and lepton-hadron colliders

Fixed target experiments

Astro/cosmo/particle experiments on the earth and in the sky

Particle physics and technology (detectors, accelerator technology, computing, advanced algorithms … )

Snowmass 2001

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The Matrix … connections between Physics and Experimental Approaches groups

Snowmass – Linear collider Working Group

The physics and experimental approaches groups will be run as a matrix – Physics in the AM and Experimental Approaches in the PM; Accelerator specific topics (e.g. Linear Collider machines) will run parallel to Physics groups. The NLC group will prepare its own 2001 baseline document for Snowmass.

Linear Collider working group (Marco Battaglia, John Jaros, James Wells, Ian

Hinchliffe – conveners) :

• Compile physics case for 500 GeV initial operation, with scenarios, run plan

• Special options; physics case for Z-pole, FEL, etc.

• Higgs factory case (300 GeV operation)

• Evaluate need for higher energy operation later (1, 2, … 5 TeV)

• R&D issues

• Beam/accelerator limits on performance

• Outline issues for technical review panel

• Upgrade paths from possible first stage LCs

• Realization of international collaboration

HEPAP Subpanel

Jon Bagger, Barry Barish: Paul Avery, Janet Conrad, Persis Drell, Glennys Farrar, Fred Gilman, Larry Gladney, Don Hartill, Norbert Holtkamp, George Kalmus, Rocky Kolb, Joe Lykken, Kevin McFarland, Bill Marciano, John Marriner, Jay Marx, Hitoshi Murayama, Yori Nagashima, Rene Ong, Tor Raubenheimer, Abe Seiden, Mel Shochet, Bill Willis

Charge:

1. Define the central questions (accelerator and non-accelerator based exp’ts) What present and future tools to explore them?

2. Role for the U.S. at the energy frontier. What should be the next facility?

3. R&D for a 20 year program

4. Impact of HEP on society

Also a briefing book for government officials; report by Oct. 1, 2001

HEPAP subpanel will use Snowmass deliberations as a primary input!

What should the North American Linear Collider group

be doing?

In past, the LC group activities have been devoted to developing the physics case for a linear collider, in conjunction with physicists in Europe and Asia. Enough attention to detector issues has been given to define the primary features of a LC detector and approximate cost. Recently, more attention has been focussed on the case for a first stage at up to 500 GeV.

This meeting comes at a transition time, as accelerator proposals become better defined and we enter the era of evaluating machines, working toward approval by governments, international cooperation arrangements.

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 future of the American Linear Collider working group

The American LC group has been in operation since 1995. Many physics studies have been performed and reported in numerous workshops. Detector studies and rather generic detector designs developed for the purpose of defining scope (with a conscious attempt to avoid usurping the role of eventual LC detector collaborations).

Since 1998, we’ve had an international coordinating group for European, Asian and American activities; this group serves also as the advisory body for the series of International LC Workshops.

In late 2000, ICFA agreed that an international technical review panel should be formed – evaluate LC designs from point of view of design parameters, technical risks, cost, R&D needs on a comparative basis.

Is it time to bring the regional physics and detector working groups together more closely, and operate them as a merged single group? Doing so would underscore the existence of an integrated international community that we foresee doing the experiments at whichever LC may be built. There may be an opportunity now, with the ECFA/DESY study completion of its Physics and Detector technical design report.

Book on Physics at the Linear Collider and Detector Issues

The book should be of real importance for the Snowmass Workshop, and for the discussions prior to Snowmass at various workshops, universities and Laboratories. The TESLA TDR – Detector and Physics sections – will be of importance for Snowmass.

We need it soon, and its not at all there yet! Plug in and get this document in order. (see M. Peskin talk) This should be a major theme at this workshop.

Addressing the concerns of our colleagues is, at this time, more important than most of the specific physics/detector studies that one could do. It is also imperative that each of us be able to present a cogent case for the physics of the linear collider, defend its first phase operation at about 500 GeV, and to place the LC in context with other initiatives.

Some questions that need addressing

We should frame the questions that Snowmass groups can work on. People appreciate the validity of the LC better through doing studies than reading reports!

These questions should emerge naturally from the ‘Orange Book’, and we should develop the list of them this week.

More generally, our colleagues have many concerns that influence their view of the Linear Collider as the appropriate next step for the US to take. We need to listen and take these seriously -- and provide thoughtful input to these concerns.

In what follows, I give some of the questions that I hear from the sceptics (some are interrelated). We need to have sensible answers to these.

(See J. Jaros’ talk)

Some examples for further studies

1. Study on t from the F-B asymmetry in tt

2. Study of gttH Yukawa coupling from tt cross section

3. How can one probe CP violation from top production at threshold?

4 Can one improve mtop resolution for tt production above threshold (use meff ?)

5. Study the anomalous top couplings at some fixed positron polarization (including zero); what is the utility of positron polarization here?

These are specific topics for LC working group study taken from reading of the top quark chapter:

There will be similar lists from the other chapters

Physics questions …

What is the real probability that the Higgs is inaccessible at a 500 GeV LC? What is the phase I program if it is? (Make the July White Paper case accessible and understandable.) If there is no Higgs in Phase I, is the program ‘worth $5B’ ? Are precision measurements, anomalous couplings enough?

The precision EW data tell us a lot and constrain potential models, but there is no guarantee of observable manifestations of EWSB at a 500 GeV linear collider. Suppose the strong coupling or large extra dimension models do in fact describe nature, and the action is mostly at a few TeV or above. Don’t we want to go as fast as possible to high energy, with either hadrons or leptons?

Physics questions …

Suppose there is a Higgs at 170 – 350 GeV (no Susy, Higgs decay dominantly to WW or ZZ): Is there really a good enough case for a ‘$5B’ linear collider then, when one cannot access the Higgs fermion couplings and probably cannot access the particles of the new physics?

People probably accept that if there is a light Higgs and supersymmetry at a observable scale, the case is compelling. But even here, they worry that the number of energies, polarizations, beam particles is so large that the time to acquire the needed luminosity is very long. What’s the program?

What is the likelihood that we will want significant operation at the Z? Can one really include a new Z collider as a part of the rationale for a new machine when we just turned two Z factories off?

Physics questions …

Will the linear collider address the flavor puzzle in a substantive way? (for example, if there is Susy, will the study of CP violation and flavor physics for sparticles be the next flavor frontier from which we might learn something fundamental?

If there is Susy, will sparticle flavor studies gain us more insight into the origin of flavor than study of the quark or neutrino flavor sector? (Flesh this out)

“The (g-2) ‘signal’ + b s + Higgs ‘signal’ + dark matter ‘requirements’ need LC at 1.2 TeV ”. Please comment !!

Can LC Susy measurements really point the way to the Susy breaking mechanism in a model independent way? How definitive can such a case be?

OK, the goal of High Energy physics should be to experiment at the highest partonic energy possible by any technique! We can imagine 150 TeV hadron collisions. Should that not be our next goal? Take a stepwise path if need be.

Political & sociological questions …

How can you advocate to governments to spend ~$5B ( 7X the current yearly HEP budget in the US) on a project that is not guaranteed to have some major discoveries? Remember TRISTAN ! Can you set $5B in terms of previous machines? LEP? Tevatron? Note the difficulty with getting SNS – with a well-defined, sure program of research and cost of $1.5B ! Isn’t proposing a machine of this magnitude so impossible in the political climate that asking for it damages our credibility for any new initiative?

Isn’t it clear that TESLA is the better machine? -- more forgiving with large aperture, widely spaced bunches. And TESLA is further along. So should we focus US effort more on TESLA, possibly in the US? Or should we not agree to having a Linear Collider in Europe and focus efforts here on the Muon Storage Ring or on VLHC?

The SR/ Collider or the VLHC offer more flexible paths for future options (intense hadron beams, circular Z/Higgs factory, p colliders, etc. The LC has fewer such options. Should this be a factor for planning the future?

Is there really scientific and political value added from the FEL? Does the FEL add-on really broaden the support base? How should this be pursued?

Machine and detector questions …

How much should we weight the energy upgradability of a linear collider? What are the sustainable accelerating gradients and what is the impact on future upgrades? Why is the NLC (CLIC !!) not dead in the water if the copper damage issues are not resolved?

Should there be two IRs? Should they both be high energy? Or is there real utility to having an IR that can go to 5 TeV? If one IR is low energy ( < 1 TeV ), is there enough physics to make it an attractive opportunity? Can 1 detector do all the physics? Is 1 detector politically realistic?

What are the issues that drive very good jet energy resolution? Are they worth the cost of silicon tungsten calorimetry? Are there suitable compromises?

What is the case for silicon (outer) tracking ?

What is the real physics payoff of the options (e-e-, , e+ polarization)? How much is it worth ($$) to build in their possibility? Further quantify the e+ polarization benefit.

This Meeting

The main jobs are:

Prepare as honest and compelling a case for the community to evaluate Linear Colliders as we can make. Get that book out!

Start to think more globally about where we need to go.

Review the recent studies, and formulate the questions for the next round of physics and detector studies.