Dreams of the e+e- Linear Collider: Exploring the Constitution of Matter at the Smallest Scale

1. P. Grannis – Notre Dame 2/23/05 The next step for particle physics: The e+e- Linear Collider • I will describe the dreams of many particle physicists for taking the next steps toward understanding constitution of matter at the smallest scale. • The International Linear Collider (ILC) • Are the physics questions important enough to go there? • Why e+e-, why linear, and how would you do it? • How to organize to achieve it? • Is it worth the cost?

2. A. The case for the next step in understanding the fundamental construction of matter I will use an analogy (strained, as analogies often are) of the present state of high energy physics to an expedition to climb a mountain peak in a range not previously explored. HEP is at present at the lowest base camp for the assault, on a plateau with hidden peaks rising above us. Our base camp is mostly in fog and we have been here a long time, but we have occasional clearings with glimpses at the ridges above.

3. Our base camp: the Standard Model Over the past 30 years, the SM has been assembled and tested with 100’s of precision measurements. 3 doublets of quarks and leptons; 3 forces (Strong and unified EM and Weak). Gravity is left out of the SM. In my view, there are clear indications that the SM is inadequate, though we have tried unsuccessfully to find failures for decades. High energy lepton and hadron collisions, neutrino studies, rare decays will all give complementary views, enabling a real understanding of the SM, and what lies beyond it.

4. From our base camp, we see crevasses that suggest that our SM is not fully correct and stable. Some of these fissures: a) The SM is QCD x EW [SU(3)xSU(2)xU(1) ] but these pieces are just pasted together. We know that unification (coupling constants coming together) does not happen within the context of the SM. And gravity stands outside the SM. Wouldn’t it be elegant to have unifiedstrong and electroweak forces, and to bring gravity into the mix? New physics is needed to unify – supersymmetry, extra spatial dimensions … non-unification of coupling constants

5. q q H • The Weak and EM interactions are joined as the unified Electroweak (EW) force, but the symmetry is broken: mg ≠mW ≠ mZ in the SM. This symmetry breaking is transmitted to the matter particles (quarks and leptons) as well. In the SM, it is the Higgs field that does this: Higgs field interactions induce the symmetry breaking, give mass to W and Z, reduce the quark/lepton momenta at a given energy (give them masses) in proportion to the coupling gHqq . Larger Higgs couplings → larger mass. (So that’s an explanation for anything??) Why is the scale of the Higgs mass and EW symmetry breaking [ O(100 GeV) ] so different from the Planck or force unification scale (1016 – 1018 GeV)? The disparity is highly unstable; huge fine tuning is needed. • Is EW symmetry breaking really due to the SM Higgs boson ? Don’t we need something else ?

6. c) CP is violated: rate (K0→ p+ e-n) ≠ rate (K0 → p- e+ n). See it also in decays of b-quark mesons: rates of B0 and B0 into J/y KS are not equal. But the asymmetry observed in the K0 system is too small to explain the amount of matter in the universe –the question of why we arehere remains open! To get an asymmetry in baryon/antibaryons in the universe as observed, we need CP violation. Originally matter and antimatter are equally abundant. Red = mattergreen = antimatter equalin early universe It may be that CP violation in the neutrino sector could explain some of this, but it seems that new physics is required. Supersymmetry might provide the extra CP violation through new phases relating the partners of W, Z and g. us

7. d) Dark matter and dark energy are observed, but are not explained in the SM. Extensions to the SM could provide dark matter. Higgs vacuum energy might have been dark energy, but is 10120 times too large !

8. Occasionally the fog at our base camp clears a little and we can glimpse pathways upwards from our base camp. Several classes of theoretical models offer some guidance on the route we might take. But we very much need new experimental tools that will enable us to climb off the SM plateau and see what is up there.

9. B. Getting there – tools and paths to see more clearly As with mountain climbing, we want ‘belt and suspenders’ backup – complementary ways of gaining new understanding. Large Hadron Collider(LHC) 14 TeV (ECM) pp collisions (but lower and unknown constituent collision energy). Reactions occur by strong interaction; copious backgrounds to rare processes. Not a well specified initial state, and many spectator particles.

10. g, Z f e+ f e- with ECM=0.5 to 1 TeV. Lower total energy than LHC, but all energy in single colliding constituents. Initial state is specific quantum # (JP=1-). LC: e+e- linear collider Cross sections for interesting signal events and backgrounds are comparable. Beam polarization (e-) allows enhancement or suppression of reactions or backgrounds. Events are clean; little background. Lepton and hadron collisions give the complementary view that enables real understanding. LHC will start first (~2007-8)

11. Other routes: There are other routes to understanding the GUT scale besides studying EW symmetry breaking – other faces of the mountain we could attempt to scale. Neutrino masses, CP violation, proton decay, very rare meson decays, neutron electric dipole moment etc. all can offer some direction. But at present, these remain indirect views of the summit without any clear paths for actually going there. Increasing the CM energy and exploring EW symmetry breaking directly seems the most direct and accessible path to really understanding the new terrain.

12. Possible paths to the summit on the high energy routes: 1) Supersymmetry: boson-fermion symmetry with new fermionic space-time degrees of freedom. A host of new particles. The symmetry is clearly broken – how does that happen? Susy allows unification of Strong, EW, Weak couplings Susy can provide the dark matter candidate, and many other new particles Four (at least) Higgs states • Strong coupling composite models: new strong interactions with new heavy particles. In these, Higgs boson is a fermion condensate, like Cooper pairs for superconductivity • Bring the mountain to Mohammed – ‘large’ extra spatial dimensions in which some or all SM particles can move. Motivated by string theory. The real Planck scale is not so far above our current base camp at the TeV scale.

13. C. Main elements of LC physics case SM Higgs bounded by expt: 114 < mH < 230 GeV (1) Study the SM (or other) Higgs LHC will see it if it exists up to > 1 TeV. But for mH < 160 GeV, hard to observe the dominant decay H → bb. LHC will not measure Higgs quantum numbers (JP). Will measure branching ratios to only ~20%. LC gives precise profile of Higgs. Determines JP and total decay rate. Measures BRs to few %. These BRs distinguish between SM and other models – in pink shaded hypothetical LC measurements, indicates Susy with certain parameters. Allowed MA Possible BR measurements

14. Higgs: Higgs potential At the LC, it is possible to measure Higgs self coupling in e+e-→ ZHH, which determines the Higgs potential directly. Comparing with its mass, get self-consistency check. Can observe all Higgs decays, including invisible, from e+e-→ ZH, using only the Z decay products. The LC is the pathway to understanding the nature of the Higgs boson.

15. ~ ~ (2) Explore SUSY LHC will discover SUSY if it exists, seeing excess energy transverse to beam line due to SUSY particle decays. LHC mainly produces partners of strongly interacting quarks and gluons, but partners of leptons or (W,Z,g,H) are rare. LHC measures mainly mass differences (Q values in decays). LC makes the partners of leptons, neutrinos, gauge bosons copiously. Measures the lowest mass supersymmetric particle (partner of Z,g) – best candidate for dark matter. Simple production processes like e+e-→ m+m-. Can measure masses accurately and determine SUSY particle quantum #s. Can measure mixing matrices of the partners of the (W,Z,g,H) and CP violating phases.

16. SUSY symmetry breaking The LC can make accurate mass measurements from energy threshold scans (dm/m ~ 0.1%). Mapping out the supersymmetry spectrum would by itself be a great achievement. But this also opens the way to map the SUSY symmetry breaking terrain itself at very high energy and to get a clear glimpse into the grand unification scale. LC can measure the CP violating parameters for partners of top quark, or (Z, g). CP violation in Supersymmetry could offer the explanation for baryon-baryon asymmetry.

17. (3) Large extra dimensions LHC can sense extra dimensions through production of jets that are unbalanced by other particles (due to graviton flying into the new dimensions) and measure the true Planck mass (<< 1019 GeV). LC, using unbalanced photons, can tell us how many extra dimensions there are (in models where graviton only moves in extra dimensions) • But if the extra dimension models are in general correct, there are many variations possible: • What is the metric in the extra dimensions? • Which fields live in the extra dimensions? • Size, number of the extra dimensons? • Information from both LHC and LC will be crucial

18. D. Organizing the expedition The physics case and its priority In 2001, advisory panels in Europe, US and Asia recommended that the next major project in High Energy Physics should be a 500-1000 GeV Linear Collider. In 2002, a subcommittee on science for the OECD concurred, and noted that the LC should overlap with the LHC. In 2003, the international community prepared a 15 page document that outlined the elements of the scientific case – for use by scientists. Nearly 3000 physicists worldwide signed it. http://blueox.uoregon.edu/~lc/wwstudy/ Understanding Matter, Energy, Space and Time: The Case for the Linear Collider

19. Guiding the international effort 2002: Formed a Steering Committee (ILCSC) to guide the LC project. 2003: ILCSC subcommittee report on the technical issues for all potential designs of LC. The main contenders were • room temperature copper accelerating structures= X-band • superconducting Nb rf cavities = SC. Alternate lower frequency, room temperature, (C-band) as backup if others won’t work. R&D on higher energy LC using an intense beam to produce rf power (CLIC)– not ready for consideration yet. This committee listed the critical R&D steps that remained before technology choice. By mid 2004, X-band and SC both completed most of this R&D.

20. Guiding the international effort • 2003: ILCSC drew up the scientific parameters and overall specification for the LC: • Baseline: operate in range 200 < ECM < 500 GeV with 0.1% control of energy and ability to scan energy with 80% e- polarization • Two experiments (push/pull) • Upgrade: to 1 TeV, acquire 1000 fb-1 in 4 years • Options: retain possibility for positron polarization, running at the Z boson mass (91 GeV), gg collisions (backscatter laser from primary e- beam) This baseline is necessary to do the Higgs boson physics, explore supersymmetry or other new physics, and refine the knowledge of the Z, W boson and top quark properties.

21. Guiding the international effort 2004: Panel formed to make a recommendation on which of the two primary technologies should be selected – recognizing that continuing on both paths was wasteful of manpower and money. 12 people: 4 Asians, 4 Europeans, 4 from US – like a trial by jury. Meetings in/near Oxford, Hamburg, Palo Alto, Tokyo, Los Angeles and Pohang, Korea from Jan. – Aug. 2004

22. we are here cost Circular Collider Linear Collider Energy E. How does one build a Linear Collider? Why a linear collider • Particle physics colliders to date have all been circular machines (with one exception – SLAC SLC). • Highest energy e+e- collider was LEP2: ECM=200 GeV • Synchrotron light sources are circular As energy increases at given radius DE ~ E4/r (synchrotron radiation)e.g. LEP DE=4 GeV/turn; P~20 MW Going higher in energy in a circular machine becomes prohibitively expensive – large power or huge tunnels. But LC needs long single pass linacs to reach desired energy.

23. The basic linear collider elements • Source of polarized electrons (create positrons in secndry target) • Damping rings to cool the beams: create small emittance • Compress bunch length to 100-300 mm (keeping low emittance!) • Long acceleration section to full energy (keeping low emittance!) • Final focus to squeeze beams transversely (sx /sy = 300/3 nm) and bring into collision (flat beams : max L, min. DE) ½ of Linear Collider

24. Two choices: rf frequency dictates the details L-band (SC) 1.3 GHz l= 3.7 cm TESLA X-band (Rm Temp) 11.4 GHz 0.42 cm NLC/GLC • Power lost in structure walls ~ E2/shunt impedence; SC wants low frequency for high shunt impedence, X-band wants high freq. for high shunt impedence. • SC has large Q; means energy dissipates slowly thus allowing long bunch trains • Accelerating structure size ~ accelerating rf wavelength. Disruptive wakefields are inversely related to structure size so low frequency better. • Bunch spacing, train length related to rf frequency (and Q). Damping ring design depends on bunch train spacing and length, hence frequency Frequency dictates many of the design issues

25. Important parameters Many of design parameters follow from the frequency choice: Parameter X-bandSuperconducting Bunch spacing 1.4 ns337 ns Bunch train length 267 ns 950 ms Train rep rate 120 Hz5 Hz Accel. Gradient 65 MV/m35 MV/m (52 MV/m bm loaded) 2 linac length (1 TeV) 27.6 km37.5 km Wall plug effic. 2.5% 23% Site power (500 GeV) 200 MW140 MW

26. I. Getting the energy – rf acceleration

27. Accelerating structures Travelling wave structure; need phase velocity = velectron = c Circular waveguide mode TM01 has vp> c ; no good for acceleration! Need to slow wave down (phase velocity = c) using irises. Bunch sees constant field Ez=E0cosf Group velocity < c, controls the filling time in cavity. Ez c z SC cavity Room temp Cu structure

28. rf power system • Modulator converts wall ac power to HV dc pulse for klystron • Klystron tube is rf pulse power amplifier --accelerate electrons through bunching cavities to generate high power microwave pulse • For X-band, must shorten & intensify the pulse train; use dual delay lines and rf switching to chop and fold the rf pulse on itself • rf pulse injected into set of rf structures to accelerate particles rf pulse compression needed for high power, short bunch X-band, not for SC

29. klystrons X-band PPM klystrons – small but need ~4500 with 75 MW pulse power; 1.6 ms pulse length; permanent magnet focussing to reduce power SC (TESLA) multibeam klystron Need 572; 10 MW pulse power; 1.4ms pulse length Klystron is larger, but simpler than X-band

30. Accelerating structures X-band Iris size = 4.5 mm 106 Cu structures SC pure Nb 9-cell cavity; Iris size = 3.5 cm ~20,000 Cavities

31. Issues for X-band accelerating structures There have been problems with breakdown of Cu structures at high power. Improved design, going to lower group velocity has helped. Recent structures have met the specified breakdown rates. X-band specification on gradient and breakdown rate (controls linac up time).

32. Issues for SC accelerating structures Learning how to prepare smooth pure Nb surfaces to get the design gradient was a decade-long effort, now achieved. Recent advance uses electropolishing instead of chemical polishing. One still worries about field emission from imperfections on the surface that lead to current draw, and unacceptable loads on cryogenic systems. SC specification on gradient and Q value. Now exceeding spec.

33. II. The problem of herding the electrons: Must transmit low emittance through the accelerator to get small beams at collision for high luminosity

34. Damping rings Prepare the very small emittances needed for small beams at collision in the damping rings. Send few GeV e- or e+ beams through wiggler magnets to radiate energy (lose energy in x, y and z directions) and then restore pz with rf acceleration. Must keep very careful control of optics, vacuum, instabilities to avoid emittance dilution. Damping Ring for X-band has been built in KEK and achieved necessary emittance.

35. Damping rings Long train in SC (~ 300 km) means that one must fold the train on itself, and then kick out a selected bunch without disturbing the neighboring bunches. Need special optics to transform flat round beams. Need very fast kicker magnet with no residual B field at succeeding bunch.

36. tail head Wakefields Wakefields: Off axis beam particles induce currents in cavity walls; these cause deflections of the tail of the same bunch, and on subsequent bunches. Betatron oscillation in head of bunch creates a wakefield that resonantly drives the oscillation of the tail of same bunch. Can be cured by reducing tail energy; quads oversteer and compensate for beam size growth. Wakefield effects on subsequent bunches in X-band are cured by detuning the structures to cancel the fields on later bunches. SC bunch spacing is large enough that it’s not a problem. tail head Beam growth due to single bunch wakefield

37. Beam Alignment Beam alignment: cavities to a few mm, focusing quadrupolesto ~100 nm, and final lens to ~ few nm! Requires careful initial survey to 100m level; measure the kick of the beam with precision position monitors to 10m as it passes down the linac and correct the magnet positions. Correction needed at intervals of minutes. Ground motion takes the beam out of alignment; must re-establish the ‘gold orbit’ ~ daily: non-invasive tuning. Getting and keeping alignment is big deal – harder for X-band linac than SC owing to larger wakefield effects. But the biggest problem is final focus – similar for both.

38. F. Making the technology decision Evaluation Panel analyzed technology choice with matrix having six general categories with specific items for each technology: • Meeting the scope and parameters • Technical issues and risks • Costs • Schedule • Operation for physics • General considerations reflecting the impact on other sciences, technology and society Evaluated each of these categories with the help of answers to questions to the proponents.

39. Technology decision • Both technologies meet desired scope; cost and schedule issues do not discriminate the two technologies at current precision. • Technical issues: SC cavities now being built with major industrial involvement. The same cavities are the basis for the DESY free electron laser project, but need more effort on dark currents. DR is complex. • X-band damping rings have been demonstrated, but few klystrons and only a small section of linac shown to operate at full power with acceptable breakdown. Still worry about industrialization of klystrons, rf bunch compressor. • Larger bunch Dt, smaller wakefields make SC linacs less risky • Positron production scheme for X-band is more straightforward. Main risk for X-band is in high cost linac. Main risks for SC is in lower cost subsystems.

40. Technology decision • Experimental conditions are slightly better in SC due to long time between bunches (little event pileup) and slightly smaller energy spread. Not a major discriminant. • Lower power costs for SC. • Impact on other sciences, society: X-band linacs will benefit medical industry (use of smaller systems), communications (airborne radar) SC linacs now coming into use in a wide range of scientific applications: XFELs, ERLs, Rare Isotope Accelerator, proton linacs for n spallation sources, n production …

41. Technology decision Recommend superconducting technology This is a choice of technology, not of the specific TESLA design. Recommendation accepted by ICFA and supported by major funding agencies as an important demonstration that hard choices can be made. Retire the old names – NLC, TESLA, GLC: call the new entity International Linear Collider = ILC Recommend that the world HEP/Accelerator community optimize a design of the superconducting rf linear collider that capitalizes on the best features of all R&D to date. Ask for capability to reach the highest energy feasible (at least 1 TeV) through upgrades to the baseline 500 GeV machine.

42. G. Towards a fully international project How do we organize the ILC so that all regions of the world feel that they are full partners and gain from participation? Herding cats: must give value added for all participants An important 1st step was the first ILC Workshop in Japan last November; integrated working groups from all regions started vigorous examination of a new optimized design.

43. Next steps 1. Organize the “Global Design Initiative” Central Team: Director to oversee design, parameter lists, cost methodology, regional R&D and industrialization Regional teams in Asia, Europe and North America to carry out the subsystem R&D, design, cost and industrialization. Keep accelerator science strong in all three regions. • Now choosing Central Team Director and site. Director candidate list announced Feb. 14; now finalizing site. • Phase I: Conceptual design (and preliminary costs) in ~1 year. • Phase II: Engineering design, ready for site and full agreement on international participation by ~2008 • Final approval by governments based on early LHC results; • Start construction ~2010 (if all goes well!!) • ILC Accelerator Workshop in Snowmass in August 2005 to establish baseline parameters and start the CDR.

44. Next steps • Detector R&D and experiment design • Several different detector concepts (call for 2 exp’ts) • large TPC or jet chamber gas-based tracking with ‘conventional’ calorimeters • Smaller silicon microstrip-based tracking with SiW compact calorimetry • Choose experiments by time of project start Workshop on ILC physics and detectors concurrent with accelerator workshop in Snowmass this summer. • Engage funding agencies in setting up project Meetings for past year with high level funding agency representatives from Canada, Europe/CERN, Germany, India, Japan, Korea, UK, US-DOE & US-NSF to make plans for organizing, establishing procedure for site, etc. Funding agencies are supportive of decisions taken so far.

45. Can we afford the ILC? The ILC cost is not exactly understood. Lets take the estimate for the 500 GeV TESLA project which was $3.1B€ (~$4B) (not including salaries of professionals). Increment to $6B to account for energy increase to 1 TeV, operations (~$250M/year), additional needs, remaining R&D etc. Divide by 3000 physicists (those signing the consensus document) and by 20 years for building + initial operation project duration: Cost per physicist/year = $100,000 Cost to be shared across all three regions of the world.

46. Can we afford the ILC? -- my guesses at other project costs ILC is a large total cost, but yearly cost per physicist is not far out of line with those for facilities in other areas: Rare Isotope Accelerator study of nuclei far from stability cost (NSAC estimate) is ~ $750M. 10 year operations cost $650M. ~750 people in Users Group. For 20 yr construct/operate: $95K/physicist/yr IceCube: detecting high energy astronomical neutrinos: $272M for 300 physicists. 10 year construct/operate: $90K/physicist/yr Linear Coherent Light Source: femtosecond sub-nm level XFEL for material studies, structural biology, fast chemical processes, plasmas. $300M for ~10 yr construction/operation. Support ~300 scientists: $100K/scientist/year

47. Can we afford the ILC? -- my guesses at other project costs GLAST: Satellite g detection in 10 MeV– 100 GeV range to look for point sources. Large Angel Telescope is $137M (not including rocket); duration ~2002 – 2012; 140 scientists; $100K/scientist/yr SNAP: The supernova SNIa search in space to explore dark energy (part of JDEM). Estimate $500M - $1B; 15 years build and operate; 200 physicists. $250K/yr/physicists. (Not to mention the Congressional budget office estimates of $130B for manned and robotic moon and Mars missions through 2020). Many proposed large scientific projects are in the $100K/yr/person range. One must of course weigh the prospective scientific payoff of any of these .

48. ILC in wider context • Physics expands along lines with differing approaches: • Areas with increasingly clever experiments on very complex systems (brains, BEC, quantum computers, etc.), often with clear applications to society. • Studies of the structure of matter and energy, space and time, at the simplest levels. Often with little practical benefit other than technology spinoffs, but with large impact on the way we view the world. And these often provide the magnet that attracts young people to physics. Both approaches are critical to our field. Each benefits from the health of the other. It is important that we understand the potential for advances in all areas, for we are truly interconnected. Both types of physics need to be sustained.

49. ILC in wider context – interconnectivity of physics • Nuclear, Astrophysics, Particle Physics connections: • NP/HEP → Astro: cross sections for n’s, nuclear reactions as input to stellar evolution, supernovae nucleosynthesis etc. • Astro→ HEP/NP: number of neutrinos from primordial cosmology; the wonderful puzzles of dark matter and dark energy. HEP may find what dark matter is. • HEP→ Astro:Inflation as a source of the cosmological isotropy and homegeneity • HEP→ NP:Quantum chromodynamics • NP  HEP: chiral symmetry breaking, lattice gauge theory, instantons etc. • NP → HEP:the field of particle physics itself !

50. ILC in wider context – interconnectivity of physics • Interconnections between high energy and condensed matter physics • CM → HEP: Cooper pairs for superconductivity, exploited in HEP and elsewhere as fermion condensates. (Maybe the Higgs boson is a fermion condensate?) • HEP → CM: Field theories; diagrammatic techniques, use of the renormalization group • CM  HEP: Spontaneous symmetry breaking is seen in both disciplines; insights in one field stimulate advances in the other. • And HEP has developed accelerator technology, now pervasive for studies of nuclear and heavy ion physics, materials, biological systems, environment etc. The totality of physics is greater than the sum of its parts.