High Energy Physics for the 21 st Century

1. High Energy Physics forthe 21st Century Step one: into the unknown Christopher Lester

2. The Standard Model Where are we now?

3. Higgs not yet found Quark mixing not over-constrained yet Quark masses poorly measured Top-quark charge undetermined! No conflict with experiment (yet) Parts (QED) in extremely good agreement with experiment – even with atomic physics! (Lamb Shift, magnetic moments) Elementary particle content “reasonably” small … Standard Model Good Standard Model Bad

4. What is the charge of the top-quark? Based on 17 events. [Markus Klute] Preliminarily excludes exotic top-quark charge of -4/3 at 94% confidence. (365 pb-1) Spring 2006. Dark corners of the Standard Model

5. Higgs not yet found Quark mixing not over-constrained yet Top-quark charge undetermined! Quark masses poorly measured Fine-tuning / “hierarchy problem” (technical) – Why are particles light? Does not explain Dark Matter No gauge coupling unification No conflict with experiment (yet) Parts (QED) in extremely good agreement with experiment – even with atomic physics! (Lamb Shift, magnetic moments) Elementary particle content “reasonably” small … Standard Model Good Standard Model Bad New Physics, e.g. Supersymmetry, can help.

6. Four Questions: • What might the new physics be? (2) What sort of experiment will help us? (3) How will we go about extracting answers from the data? (4) Can we trust the answers? Will describe some later. Coming next! Very much the work of people in The Cavendish. … if time allows …

7. Simple experimental aim: Collide protons and see what happens.

8. Large Hadron Collider (LHC)

9. Inside the LHC

10. LHC protons 7 TeV protons 7 TeV The Semiconductor Tracker “ATLAS” Experiment

11. Note concerning units eV = electron-volt = 1.6 x 10-19 J GeV = 10 9 eV = 1.6 x 10-10 J TeV = 1012 eV = 1.6 x 10-7 J (= K.E. of 1.3mg mosquito at 0.5 m/s) Express most particle energies and masses in GeV … … but LHC proton beams are 7 TeV each (14 mosquitos in total)

12. Anatomy of the detector Layered like Onion Different layers for different types of particles Neutrino Muon

13. So main things we can do Average direction of things which were invisible • Distinguish the following from each other • Hadronic Jets, • B-jets (sometimes) • Electrons, Positrons, Muons, Anti-Muons • Tau leptons (sometimes) • Photons • Measure Directions and Momenta of the above. • Infer total transverse momentum of invisible particles. (eg neutrinos) electron Here Be Monsters Hadronic Jet photon

14. Muon Detector MAGNETIC FIELD MAGNETIC FIELD Muons bend away from us. Anti-muons bend toward us. Man for scale

15. Calorimetersand Central Solenoid

16. Right Honourable and Most Reverend Dr Rowan Douglas Williams, the 104th Lord Archbishop of Canterbury and Primate of All England Transition Radiation Tracker (TRT) – tracks charged particles and distinguishes electrons from pions

17. The SemiConductor Tracker (SCT) Records tracks of charged particles Most of the data-acquisition and calibration/monitoring software designed and written in Cambridge Many components designed and built in The Cavendish

18. 10cm SCT contains 4088 “Modules” 768 sensitive-strip diodes per side. (200 V) 3 infra-red communication channels. Collisions recorded @ 40MHz (every 25 ns) Neutron bombardment will degrade silicon over time. Individual strips will need recalibration. Optical properties need adjustment. May need to use redundant links.

19. SCT Data Acquisition Software • Present size: • 350,000 lines of code • ~6 developers • Much still to be done: • Have managed to control 500 modules at once • only 1/8th of final number • “multi-crate” development - parallelisation • Needs to become usable by non-experts • Needs to recover from anomalies automatically

20. Evidence that it will work: First cosmic rays seen in SCT and TRT! PRELIMINARY Data from morning of 18th May 2006

21. Back to the new physics • Fine-tuning / “hierarchy problem” (technical) – Why are particles light? • Does not explain Dark Matter • No gauge coupling unification Remember the aim was to fix some of these problems with the Standard Model Possibilities: • Supersymmetry • Minimal • Non-minimal • R-parity violating or conserving • Extra Dimensional Models • Large (SM trapped on brane) • Universal (SM everywhere) • With/without small black holes • “Littlest” Higgs ? • …. We will look at supersymmetry (SUSY)

22. Supersymmetry!CAUTION! • It may exist • It may not • First look for deviations from Standard Model! Experiment must lead theory. Gamble: IF DEVIATIONS ARE SEEN: • Old techniques won’t work • New physics not simple • Can new techniques in SUSY but can apply them elsewhere.

23. Electron Higgs Anti-Electron Higgs Selectron Higgsino Anti-selectron Higgsino What is Supersymmetry? Reverse the charges, retain the spins. Matter Antimatter Retain the charges,reverse the spins.(exchange boson with fermions). Supersymmetric Matter For technical reasons each sparticle can be heavier than its partner by no more than a TeV or so.

24. Neutralinos : The collective name of the supersymmetric partners of the photon, the Z-boson and the higgs boson. LSP : Lightest Supersymetric Particle. Often the lightest neutralino. Great! • Fix Hierarchy Problem • The Lightest Neutralino (LSP) is a prime candidate for neutral stable cold Dark Matter • Can have gauge coupling unification ΩCDMh2 = 0.103 ± 0.009(WMAP 3-year data)

25. Unfortunately • Doubling of particle content • Conservation of “R-parity” • LSPs generated in pairs • LSPs invisible to ATLAS • Large number of tuneable parameters • Assume just five of them exist for the moment – unification arguments

26. What might events look like? What we can see Here Be Monsters! (again) What we can see This is the high energy physics of the 21st Century!

27. (What they really look like) b soft gluon radiation? An example of an event where a higgs boson decayed to a pair of b-quarks/ b

28. So main EASY signatures are: • Lots of missing energy • Lots of leptons • Lots of jets • ATLAS Trigger: ETmiss > 70 GeV, 1 jet>80 GeV. (or 4 lower energy jets). Gives 20Hz at low luminosity. Just Count Events! Indicates deviation from The Standard Model.

29. events Signal S.M. Background Squark/gluon mass scale What you measure: Peak of Meff distribution correlates well with SUSY scale “as defined above” for mSUGRA and GMSB models. (Tovey)

30. The real test comes when you want to measure individual masses etc.

31. Technique 1: Kinematic Edges Plot distributions of the invariant masses of what you can see

32. ll llq ll llq S5 lq high lq low lq high lq low O1 llq Xq llq Xq Technique 1: Kinematic Edges

33. Technique 1: Kinematic Edges Account for all ambiguities: Both look the same to the detector

34. Determine how edge positions depend on sparticle masses

35. Technique 1: Kinematic Edges Use custom Markov-Chain algorithms to sample efficiently from the high dimensional parameter spaces of the model according to the Bayesian posterior probability. Shape of typical set is often something quite horrible.

36. Technique 1: Kinematic Endpoints Finally, project onto space of interest: Correlation between slepton mass measurement and neutralino mass measurement. Slepton mass

37. Other Techniques: • Look at the shapes of the distributions • Systematic errors harder to control • Create new variables • “Cambridge MT2 Variable”now international used methodfor sparticle mass measurementin pair production • Incorporate cross sections and branching ratio measurements • again, Cambridge “leading the way” as home to the most developed samplers for H.E.P.

38. Can even bring these techniques to bear on the data we have today • Don’t know • Know • m0 • M1/2 • A0 • Tan beta • Sgn mu • mb • mt • αs(Mz) SUSY params SM params

39. 2D Slices of 5D SUSY parameter space tell you very little … Roszkowski et.al.

40. Even worse news:Standard Model errors are very important!

41. Top Quark Mass Standard Model uncertainty: Experiment: mtop = 178 ± 4.3 GeVin 2006 (was 174.3 ± 3.2 GeV in 2004) mtop = 170 GeV mtop = 180 GeV

42. Bottom Quark Mass Standard Model uncertainty: Experiment: mbot = 4.1 to 4.4 GeVin MS scheme mbot = 4.0 GeV mbot = 4.5 GeV

43. h0 pole region Pseudoscalar higgs A0 s-channel annihilation region Slepton-neutralino co-annihilation region The parts of Supersymmetric Parameter Space are consistent with Today’s data: First analysis able to fold everything together was from Cambridge: “Multi-Dimensional mSUGRA Likelihood Maps”, B.C. Allanach & C.G. Lester (Phys.Rev. D73 (2006) 015013)

44. What if the Dark Matter isn’t all SUSY? Dark matter is just made of SUSY neutralinos: Other sources of Dark Matter allowed in addition to SUSY: Favoured regions of SUSY model don’t change an awful lot! Prediction fairly robust.

45. R.I.P.S.C.T.2017 Future plans • The whole programme is about the future. • If we knew what the experiment will tell us we wouldn’t need to build it. Experiment must lead. • In short term, must continue to integrate further with CERN physics analysis teams. • Analysis will be in collaboration • In 10 years the SCT will have been radiation damaged beyond repair, and the LHC may be upgraded. • Need to start work on “SCT version 2” long before 10 year lifetime of “version 1” is reached • LHC luminosity upgrade will place more demands on tracking systems • Cavendish HEP group in ideal position to play leading role in that endeavour. • Must strive to draw maximum inference from LHC data!

46. Conclusions • Expect new particles, new physics and other discoveries at the LHC • May include a Dark Matter candidate ? • Many competing physical theories: • Supersymmetry is one possibility • There are many others: • (UED, Large Extra Dimensions, Littlest Higgs …) • An example experimental technique was presented in the context of Supersymmetry • Kinematic endpoints and other measurements + care + efficient sampling from Posterior Distribution on parameter space • Supersymmetry may not be what nature has chosen! • Techniques will be applicable to any theory with large particle content and Dark Matter candidate – and to others too • Many more things I would like to have shown you: • How to measure particle spins and distinguish SUSY from UED etc ….

47. The End, and the ATLAS Collaboration Cambridge Office Christopher Lester 2006

48. Spare slides

49. Posterior maps

50. Progress in the last Century • 19th Century • 1897: Electron (Thomson) • 20th Century: • 1911: Nucleus (Rutherford) • 1930: Neutrino postulated (Pauli, beta decay) • 1936: Muon (Anderson, cosmic rays) • 1956: Neutrino observed (Cowan, Reines, et al) • 1960s and 1970s: Growing support for light quarks • 1960s: Higgs boson postulated • 1970s: Tau discovery • 1996: Top quark discovered (Tevatron) • 21st Century • Something’s coming, something good, (West Side Story) 25 year wait for neutrino 20-30 year wait for top quark 45 year wait for Higgs ??