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Physics at Future Colliders

Physics at Future Colliders. Lecture 1 An historical perspective (1964-2014): The need for precision and energy A strategy for the future: Towards the precision and energy frontier The short-term perspectives (2020-2030) Lecture 2 The quest for precision (2030-2045): Linear or Circular ?

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Physics at Future Colliders

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  1. Physics at Future Colliders • Lecture 1 • An historical perspective (1964-2014): The need for precision and energy • A strategy for the future: Towards the precision and energy frontier • The short-term perspectives (2020-2030) • Lecture 2 • The quest for precision (2030-2045): Linear or Circular ? • The energy frontier (2045-2060): Leptons or Hadrons ? Physics at Future Colliders

  2. Lecture 2 Mid-term perspectives (2030-2045) The quest for precision: Linear or Circular ? Physics at Future Colliders

  3. Precision with e+e- colliders (1) • Historically, e+e- colliders have been used for precision measurements • The accuracy of e+e- colliders led to predictions at higher scales (mtop, mH, limits on NP) • And to unexpected discoveries (e.g., c quark, gluon, tau lepton, …) Circular ? FCC-ee, CEPC Linear ? ILC, CLIC Physics at Future Colliders

  4. Precision with e+e- colliders (2) • The European Strategy update in 2013 does not say otherwise • Electrons are not protons, i.e., do not interact strongly: no pile-up collisions • Corollary #1: Final state is clean and cosy, triggering is easy (100% efficient) • Corollary #2: No huge QCD cross section: All events are signal. • Electrons are leptons, i.e., elementary particles: no underlying event • Corollary: Final state has known energy and momentum: (√s, 0, 0, 0) • E.g., can determine jet energies from jet directions and conservation laws - - A e+e- → HZ → qqbb candidate at LEP with ALEPH Remember: A m+m-candidate event at LHC with CMS Physics at Future Colliders

  5. Precision with e+e- colliders (3) • For 20 years, there was only one such project on the market • A 500 GeVe+e- linear collider, now called “ILC”, proposed in the early 1990’s • Why not a 500 GeV circular collider ? Total length: 31 km Physics at Future Colliders

  6. Linear or Circular ? (1) • Why not a 500 GeV circular collider ? • Synchrotron radiation in circular machines • Energy lost per turn grows like , e.g., 3.5 GeV/turn at LEP2 • Must compensate with R and accelerating cavities Cost grows like E4 too. • A 500+ GeVe+e- collider can only be linear. Cost of a circular collider is prohibitive. • “Up to a centre of mass energy of 350 GeV at least, a circular collider with superconducting accelerating cavities is the cheapest option” Author: HerwigSchopper (Former CERN DG) Foreword: Rolf Heuer (Current CERN DG) ~500 GeV H. Schopper, private communication, 2014 Physics at Future Colliders

  7. Linear or Circular ? (2) • Interest for circular collider projects grew up again after first LHC results • The Higgs boson is light – LEP2 almost made it: only moderate √s increase needed • Need to go up to the top-pair threshold (~350 GeV) anyway to study the top quark • There seems to be no heavy new physics below 500 GeV • The interest of √s = 500 GeV (and even 1 TeV) is now very much debated • Way out: study with unprecedented precision the Z, W, H and top quark • Highest luminosities at 91, 160, 240 and 350 GeV are needed LEP2 Physics at Future Colliders

  8. Linear or Circular ? (3) • The ILC is designed for √s = 500 GeV • It is supported by 20 years of R&D and innovation • With a complete technical design report delivered in 2013 • In principle, ready for construction as soon as decision is taken • This machine has many technological challenges • A 24 km-long, high-gradient (31 MV/m), RF system • A very low b* opticsdelivering small beam spot sizes at high intensity • Never demonstrated to be achievable- ATF2 still struggling • A positron source with no precedent • Its performance cannot be verified before the construction is complete • A green-field project • It can deliver data to only one detector at a time • It is in principle upgradeable to √s = 1 TeV • And possibly more : CLIC or Plasma acceleration in the same tunnel (?) D. Schulte Physics at Future Colliders

  9. Linear or Circular ? (4) • The FCC-ee is designed to be a Z, W, H, and top factory (√s = 90-400 GeV) • It is a project in its infancy • Less than two years old • This machine has at least as many technological challenges • A high-power (200 MW), high-gradient (15-20 MV/m), 1 km-long, RF system • Loads of synchrotron radiation (100 MW) to deal with • A booster (for top up injection), and probably a double ring for e+ and e- • An optics with very low b*, and large momentum acceptance • Polarization (transverse and longitudinal ?) • Up to four experiments to serve • … and much more • It is supported by 50 years of experience and progress with e+e- circular machines • Most of the above challenges will be addressed at SuperKEKB as of 2015 • FCC-ee will have to build on this experience • It is the first step towards a 100 TeV proton-proton collider D. Schulte Physics at Future Colliders

  10. Linear or Circular ? (5) • Performance target for e+e- colliders • Complementarity • Ultimate precision measurements with circular colliders (FCC-ee) • Ultimate e+e- energies with linear colliders (CLIC) D. Schulte Possible upgrades with dashed lines CEPC (2 Ips) Physics at Future Colliders

  11. Precision Higgs physics at FCC-ee and ILC (1) • Dominant production processes for √s ≤ 500 GeV • Effect of beam polarization (exercise) • Higgs-strahlung cross section multiplied by 1 - P-P+- Ae× (P-- P+) • Boson fusion cross section multiplied by (1-P-)× (1+P+) Higgs-strahlung Boson fusion 300 200 100 0 FCC-ee ILC Cross section (fb) 200 250 300 350 400 450 500 √s (GeV) Physics at Future Colliders

  12. Precision Higgs physics at FCC-ee and ILC (2) • Number of Higgs events expected / year • The plan is to run ~5 years at each centre-of-mass energy, in order to • Determine all Higgs couplings in a model-independent way • Infer the Higgs total decay width • Evaluate (or set limits on) the Higgs invisible or exotic decays • Through the measurements of with Y = b, c, g, W, Z, g, t, m , invisible Physics at Future Colliders

  13. Precision Higgs physics at FCC-ee and ILC (3) • Reminder: Predicted Higgs branching fractions • mH = 126 GeV is a very good place to be for precision measurements ! • All decay channels open and measurable – can test new physics from many angles Physics at Future Colliders

  14. Precision Higgs physics at FCC-ee and ILC (4) • Physics backgrounds are “small” • For example, at √s = 240 GeV • “Green” cross sections decrease like 1/s • “Purple” cross sections increase slowly with s • To be compared to gg → qq, l+l- m > 30 GeV - - - e+e- → qq, l+l- e+e- → W+W- e+e- → Ze+e- e+e- → Wen e+e- → ZZ e+e- → Znn 30 pb 32 fb 60 pb 16 pb 3.8 pb 1.4 pb 1.3 pb - Add e+e-→ tt for √s > 340 GeV 0.6 pb • Only one to two orders of magnitude smaller • vs. 11 orders of magnitude in pp collisions • Trigger is 100% efficient (no need for trigger with ILC – all crossings are recorded) • All Higgs events are useful and exploitable • Signal purity is large 200 fb Physics at Future Colliders

  15. Precision Higgs physics at FCC-ee and ILC (5) • Example: Model-independent measurement of sHZand kZ • The Higgs boson in HZ events is tagged by the presence of the Z → e+e-, m+m- • Select events with a lepton pair (e+e-, m+m-) with mass compatible with mZ • No requirement on the Higgs decays: measure sHZ× BR(Z→ e+e-, m+m-) • Apply total energy-momentum conservation to determine the “recoil mass” • mH2 = s + mZ2- 2√s (p+ + p-) • Plot the recoil mass distribution – resolution proportional to momentum resolution • Provides an absolute measurement of kZ and set required detector performance Exercise ! mH=125 GeV √s=240 GeV ZH → l+l-X Events / 0.2 GeV ILD simulation Physics at Future Colliders

  16. Precision Higgs physics at FCC-ee and ILC (6) • Repeat the search in all possible final states • For all exclusive decays of the Higgs boson: measure sHZ× BR(H → YY) • Including invisible decays, just tagged by the presence of the lepton pair & mmiss • For all decays of the Z (hadrons, taus, neutrinos) to increase statistics • For the WW fusion mode (Hnn final state): measure sWW→H× BR(H → YY) - - - ZH → qq bb, 0.25 ab-1 ZH → l+l- + nothing, 0.5 ab-1 BR(H → invis) = 100% mH=125 GeV √s=240 GeV ILD simulation CMS simulation Mmiss(GeV) Mbb(GeV) Physics at Future Colliders

  17. Precision Higgs physics at FCC-ee and ILC (7) • Indirect determination of the total Higgs decay width • From a counting of HZ events with H → ZZ at √s = 240 GeV : sHZ× BR(H → ZZ) • sHZis proportional to kZ2 • BR(H → ZZ) = G(H → ZZ) / GH is proportional to kZ2/GH • sHZ× BR(H → ZZ) is proportional to kZ4 / GH • In practice, also use the WW fusion process: WW → H • Measure cross section at √s ≥ 350 GeV • Use BR(H → bb, WW) measured at 240 GeV • Better precision for GH Z e- Z* Final state with three Z’s Almost background free H Z e+ • Measured with the Hl+l- final state • (see slide 16) Z* Physics at Future Colliders

  18. Precision Higgs physics at FCC-ee and ILC (8) • Grand summary • Results from a model-independent determination of the couplings • Mostly from measurements of sHZ, sHZ×BR(H → YY), and GH FCC-ee, 10 years 0.1-1% precision The 10B$ ILC, 15 years 1-10% precision Physics at Future Colliders

  19. Precision Higgs physics at FCC-ee and ILC (9) • Comparison with LHC Model-independent results • Sensitive to new physics at tree level • Expected effects < 5% / L2NP • 1% precision needed for LNP~1TeV • Sub-percent needed for LNP>1TeV Sensitive to new physics in loops Sensitive to light dark matter (sterile n, c, …) and to other exotic decays Need higher energy to improve on LHC (*) indirect Physics at Future Colliders

  20. Precision Higgs physics at FCC-ee and ILC (10) • Sensitivity to new physics (example) • Compare the difference between the predictions of a few simple SUSY models and the SM for a few Higgs branching fractions • With LHC, HL-LHC, ILC and FCC-ee expected precision • With the current SM prediction uncertainties • Basic messages • The statistics proposed by the FCC-ee are needed to distinguish these SUSY models from the Standard Model • The SM theoretical uncertainties (dominated by QCD) must be reduced to match the experimental potential • Feasible by FCC-ee / ILC timescales ? FCC-ee Physics at Future Colliders

  21. Precision electroweak physics at FCC-ee (1) • The FCC-ee goals in numbers (after commissioning) • FCC-ee is the ultimate Z, W, Higgs and top factory • 10 to 10,000 times the ILC targeted statistics at the same energies • 105 more Z’s and 104 more W’s than LEP1 and LEP2 • Potential statistical accuracies are mind-boggling ! • It is the tool of choice for precision electroweak physics (*) Estimate Physics at Future Colliders

  22. Precision electroweak physics at FCC-ee(2) • The Z line-shape measurement with 1012 Z • Repeat the LEP1 programme every 15 minutes ! • Remember, after 5 years at LEP: Nn = 2.984 ±0.008 GZ = 2495.2 ± 2.3MeV mZ = 91187.5 ± 2.1 MeV Rl= 20.767± 0.025 • aS = 0.1190 ± 0.0025 • Predicting accuracies with 300 times smaller statistical precision than at LEP is difficult • Conservatively used LEP experience for systematics. This is just the start. • Example: The uncertainty on EBEAM (1 MeV) was the dominant uncertainty on mZ, GZ • Can we do significantly better at FCC-ee ? √s ~ 91 GeV Physics at Future Colliders

  23. Precision electroweak physics at FCC-ee(3) • Measurement of the beam energy at LEP • Ultra-precise measurement unique to circular colliders (crucial for mZ, GZ) The electrons get transversally polarized (i.e., their spin tends to align with B) Slow process (~ 1 hour to get 10% polarization) NB. Polarization can be kept in collision (was tried only once at LEP). Electron with momentum pin a uniform vertical magnetic field B: In real life, B non uniform, LEP ring not circular Bdipole  p e- R LEP L = 2pR = 27km E  p = e B R = (e/2p) B L To be measured Physics at Future Colliders

  24. Precision electroweak physics at FCC-ee(4) • Measurement of the beam energy at LEP (cont’d) • The spin precesses around B with a frequency proportional to B (Larmor precession) • Hence, the number of revolutions nS for each LEP turn is proportional to BL (or Bdl) • Resonant intrinsic precision of the method DEbeam<100 keV ! • However, mZ and GZ measured at LEP with a precision of 2.2 MeV … • Extrapolation uncertainty from measurements performed w/o collisions Resonant depolarization: Bdipole 0 1 2  p 3 -Bx e- Bx Bx: oscillating field with frequency n in one point of the ring Vary n until Pol = 0 (n=nS) Physics at Future Colliders

  25. Precision electroweak physics at FCC-ee (5) • Measurement of the beam energy at the FCC-ee • LEP was colliding 4 bunches of e+ and e-; FCC-ee will have 10,000’s of bunches. • Use ~100 “single” bunches to measure EBEAM with resonant depolarization • Each measurement gives 100 keV precision, with no extrapolation uncertainty • Examples of targeted accuracy • Many opportunities for direct searches for new physics through rare decays • 1012 (1013) Z, 1011 b, c or t Fantastic potential that remains to be explored. mZ to < 100 keV! + many asymmetries for sin2qW to 10-5 - 10-6 Physics at Future Colliders

  26. Precision electroweak physics at FCC-ee(6) • E.g, search for heavy sterile neutrino in Z decays • Number of events depend on mixing between N and n, and on mN Z → Nni, with N → W*l or Z*nj 4×106 Z decays BAU maybeachievablewith 1010 - 1013 Z decays? See-saw Physics at Future Colliders

  27. Precision electroweak physics at FCC-ee(7) • Longitudinally polarized beams at √s = mZ: ALR measurement ? • SLC demonstrated the feasibility of longitudinally polarized beams at linear colliders • ILC design values: P- = 80% and P+ = 30% • FCC-ee would need a specific setup for this measurement • Reduced luminosity (~1% of nominal) to avoid top-up injection (depolarizing) • Wigglers to get transverse polarization faster (natural time: 15h for 5% polarization) • Spin rotators on both sides of the detector(s) to get longitudinal fragmentation • With 1010 Z (1% design, still a factor 10 more than ILC) • Measurement of sin2qW to 2×10-6 • Added value to be weighed with the experimental challenge Physics at Future Colliders

  28. Precision electroweak physics at FCC-ee(8) • Oku-W: 108 WW events at production threshold Measurement of the W mass Measurement of the number of neutrinos with cross section at threshold (with single photon events) • Will bring final answer to the current Nn deficit (sterile neutrinos ?) - Nn ~ s(e+e-→ nng) / 2s(e+e- → m+m-g) n - n mW to < 500 keV! Physics at Future Colliders

  29. Precision electroweak physics at FCC-ee(9) - • Mega-top: a million tt events at threshold • Measure top quark mass and width • From cross section at threshold 10 x the ILC statistics No beamstrahlung systematics • Measure the top Yukawa coupling FCC-ee mtop to 10 MeV! Physics at Future Colliders

  30. Precision electroweak physics at FCC-ee(10) • Comparison with potential precision measurements at linear colliders • Beam energy measurement (compton back-scattering, spectrometer) to ~10-4 • mZ and mW not measured to better than 5-10 MeV at ILC • In absence of new physics, the (mtop, mW) plot would look like this • Constraints on new physics ? SM with mH = 126 GeV Without mZ@FCC-ee, the SM line would have a 2.2 MeV width Physics at Future Colliders

  31. Precision electroweak physics at FCC-ee(11) • Higher-dimensional operators as relic of new physics ? • Possible corrections to the standard model LEP constraints: LNP > 10 TeV After FCC-ee: LNP > 100 TeV ? Sensitivity to Weakly-coupled NP Physics at Future Colliders

  32. Precision electroweak physics at FCC-ee(12) • Theoretical uncertainties • Today’s measurements of mZ, mtop, mH, aQED(mZ) and aS(mZ) and lead to the predictions • First uncertainty is parametric, and arises from experimental uncertainties • Will need to reduce uncertainties on mZ, mtop, aQEDand aSby a factor 10 mZ, mtop, and aSwill be addressed by FCC-ee measurements Need to find ways to improve aQED (experimentally and theoretically) • Second uncertainty comes from the lack of higher-order electroweak corrections • Will need order of magnitude improvement here too • Requires a new generation of electroweak calculations to higher orders Beyond the state of the art today … But within reach on the time scale required by FCC-ee • Until recently, theory efforts were motivated by the precision needed by ILC The FCC-ee brings an entirely renewed ambition towards better results Physics at Future Colliders

  33. Precision with e+e-colliders: Summary • The small mass of the Higgs boson allows two options to be contemplated • A 250 – 500 GeV linear collider: ILC (possibly CLIC at low energy) • A 90-400 GeV circular collider: FCC-ee(also CEPC at √s = 240 GeV) • Precision measurements at the EW scale are sensitive to new physics • To potentially very high scales (up to ~100 TeV with FCC-ee) • To potentially very small couplings (sterile neutrinos, dark matter, …) • Through a study of the Z, W, H, and top properties with unprecedented statistics • Both options will be technologically / politically / financially challenging • But can potentially be ready for collisions in the 2030’s • Go through the slides again to form a personal opinion – at this level. • Access to the high-energy frontier is essential to evaluate these options • The likelihood of new physics below 1 TeV has reduced considerably with LHC Run1 • An new evaluation will have to be made after LHC Run2 Physics at Future Colliders

  34. Lecture 2, cont’d Long-term perspectives (2045-2060) The energy frontier: Leptons or Hadrons ? HE-LHC (27 km) pp collisions at 33 TeV) ILC (50km) e+e- at 1TeV CLIC (50km) e+e- at 0.5 - 3 TeV FCC (100 km) [Future Circular Colliders] Ultimate goal: FCC-hh (100 TeV) [Access to highest energies] Physics at Future Colliders Physics at Future Colliders

  35. Higgs physics at high energy (1) • Production in e+e-collisions • Improve the precision on all couplings through vector-boson fusion • See slide 18 – FCC-ee precision still standing out • Access to direct ttH coupling and Higgs self coupling measurements • Only possible at high energy e+ Z Z e- H H Physics at Future Colliders

  36. Higgs physics at high energy (2) • Production in pp collisions Large cross section increase with √s FCC-hh lt 4% 14% 4% 2% 4% < 4% 3% < 1% Physics at Future Colliders

  37. WW scattering at high energy • Why WW scattering (and Higgs pair production) ? • In the SM, Z and H exchange diagrams diverge, but exactly cancel each other • Anomalous couplings, as relics of new physics, would have dramatic effects • Total WW scattering / Higgs pair cross section diverge with m4WW,HH Precision on a and b ~30% at HL-LHC ~30% with CLIC 3 TeV ~1% with FCC-hh 100 TeV NB. “a” can be measured with 0.1% (1%) precision with FCC-e (ILC) Physics at Future Colliders

  38. Supersymmetryat high energy (1) • Production in e+e- collisions • If the spectrum is light enough • CLIC can produce a whole bunch of new particles with masses below 1.5 TeV • And measure the masses with sub-per-cent precision • Unique opportunity to probe the supersymmetry breaking mechanism Example: Heavy Higgses Physics at Future Colliders

  39. Supersymmetry at high energy (2) • Production in pp collisions • If the spectrum is heavier (as hinted at by the so-far negative LHC searches) • Higher energy will be needed • Example: gluino discovery reach ~ 11 TeV at FCC-hh (5 TeV at HE-LHC) • Discovery reach of stop searches Up to 3 TeV with HE-LHC Up to 6 TeV with FCC-hh Physics at Future Colliders

  40. Other exotica at high energy (1) • Example: Searches for new bosons (Z’) • FCC-hh directly sensitive up to mZ’ ~ 30-35 TeV (vs. 1.5 TeV for CLIC) • By looking for di-lepton resonances • CLIC 3-TeV indirectly sensitive up to mZ’ ~ 15-20 TeV • By looking for deviations in the dilepton mass distribution at high mass Physics at Future Colliders

  41. Other exotica at high energy (2) • 14 TeV (300 fb-1) → 100 TeV (3 ab-1) with pp collisions • Rule of thumb: a factor 5 in mass reach from LHC to FCC-hh Physics at Future Colliders

  42. Rare decays and precision measurements • E.g., at FCC-hh, 10 ab-1 at 100 TeV imply • 1010 Higgs bosons = 104 today’s statistics, 104 FCC-ee statistics • More precision measurements • Rare decays, FCNC probes, e.g., H→em… • 1012 top quarks = 5×104 today’s statistics, 106FCC-ee statistics • Rare decays, FCNC probes, e.g., t→cZ, cH • CP violation • 1012 W and 1012 b from top decays • 1011t from t → W → t • Rare decays, e.g., t → 3m, mg, CP violation … • BSM decays : any interesting channels to consider ? • Example: Majorana neutrino search in top decays Physics at Future Colliders

  43. Detector challenges at FCC-hh (1) • Detectors will be a formidable challenge, well beyond HL-LHC • 171 in-time pile-up events with 25 ns bunch spacing, bunch length 5 cm • High-granularity calorimetry, tracking and vertexing required • Reduced to 35 in-time pile-up events with 5 ns bunch spacing • Ultra fast detectors required (out-of time pile-up) • Large longitudinal event boost • Enhanced coverage at large rapidity required (with tracking and calorimetry) • Also need for forward-jet tagging in boson fusion production • Zs, Ws, Higgses, tops, will also be boosted • Again, high-granularity detectors needed • Very energetic charged particles • Precise momentum measurement up to 10 TeV require large coil and tracker • Very energetic jets • Energy containment require thicker calorimeter Bigger, thicker, faster, clever detectors Physics at Future Colliders

  44. Detector challenges at FCC-hh(2) • Bigger, thicker, faster, clever … • Example : long solenoid (6T) + dipoles (2T) + return yoke (a la CMS) • + shield coil (3T) to avoid 120 kt yoke ! Exercise: How do the fields add up ? Experimental cavern and maintenance scenario Physics at Future Colliders

  45. Conclusions of the second lecture (1) • The discovery of H(125) brought new light on the next large machine • There are now many ideas around for the what is the best path to follow • With thorough and precise measurements of the known particles • With a thorough exploration of the TeV scale • To uniquely address the key questions of our field • A whole set of new discoveries may be waiting for us at LHC@14 TeV • … or it may be decades before the next big discovery • Meanwhile, the LHC will set the scene for precision physics and rare decay searches • Future options will need a long time to materialize • None of the options is cheap (from ~5 B$ to 20 B$) • Clear and global planning and funding are probably needed • Criteria for choice include Scientific potential Cost, funing availability, sociology Technological maturity Physics at Future Colliders

  46. Conclusions of the second lecture (2) • Two very ambitious visions for the future • PLAN A CLIC 3 TeV drive beam NC 12 GHz 100 MV/m 50 km ILC 1 TeV SC 1.3 GHz Klystrons 45 MV/m? 50 km ILC 500 GeV SC 1.3 GHz klystrons 31.5 MV/m 31 km Plasma acceleration? > 1 GV/m 4 km Precision studies of Higgs and top 10 B$, 2030 ? Energy Frontier 20 B$, 2060 ? ttH (HHH) couplings +10B$, 2045 ? ≥ 50 years of e+e-(e-e-, gg) collisions up to √s ~ 3 TeV Physics at Future Colliders

  47. Conclusions of the second lecture (3) • Two very ambitious visions for the future (cont’d) • PLAN B FCC-ee(80-100 km, e+e-, √s from 90 to ~400 GeV) LEP (26.7 km) LHC HL-LHC PSB PS (0.6 km) Precision studies of Z, W, H, top 3-5 B$, 2035 ? SPS (6.9 km) FCC-hh (pp, up to 100 TeVc.m.) ttH, HHH couplings Energy Frontier 20 B$, 2055 ? & e±(50-175 GeV) – p(50 TeV) collisions (FCC-he) ≥ 50 years of e+e-, pp and epcollisions at highest energies Physics at Future Colliders

  48. Conclusions of the second lecture (4) • Two very ambitious visions for the future (cont’d) • Both visions have different, but solid, technological maturity • The linear collider vision is the most advanced from the design point-of-view • Many test facilities have proven the acceleration technology to work But the associated risk is still quite high (final focus, positron source, …) • The circular collider vision is much younger • But is strongly backed up by 50 years of experience, by historical successes And by projects that will prove its feasibility (SuperKEKB, HL-LHC) • The scientific case is obvious • Precision studies for new physics with small couplings • Energy frontier for new physics at high mass and large couplings • My own impression • Beyond the LHC Run2, the combination of FCC-ee and FCC-hh offers, for a great cost/infrastructure effectiveness, the best precision and the best search reach • This impression will have to be reviewed after the LHC Run2 and 5 years of FCC study Physics at Future Colliders

  49. Conclusions of the second lecture (5) • Beyond these two visions, the R&D for the far future must not stop • Muon colliders, and even more so • Plasma wakefield acceleration • Might be the only way to collide leptons at the highest energies (> 5 TeV) • For acceptable construction and operation costs (?) • Other, even more innovative, ideas are obviously needed • You have the opportunity to become key players for the future of our field • The journey towards the future of HEP will probably be long and tortuous • You can make it enjoyable for yourself: • Always keep your passion for science • Apply healthy practical common sense • And follow your dreams ! Physics at Future Colliders

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