Don Lincoln Fermilab

1. When a Proton Meets an Antiproton Don Lincoln Don Lincoln Fermilab

2. What’s the Point? High Energy Particle Physics is a study of the smallest pieces of matter. It investigates (among other things) the nature of the universe immediately after the Big Bang. It also explores physics at temperatures not common for the past 15 billion years (or so). It’s a lot of fun.

3. Helium Neon u d u u d d Periodic Table All atoms are made of protons, neutrons and electrons Electron Neutron Proton Gluons hold quarks together Photons hold atoms together

4. Anti-Matter • All particles have ‘anti-particles’, which have similar properties, but opposite electrical charge • Particles • u,c,t +2/3 • d,s,b -1/3 • e,, -1 • Anti-particles • u,c,t -2/3 • d,s,b +1/3 • e,, +1

6. Fermilab 4x10-12 seconds Stars form (1 billion years) Now (15 billion years) Atoms form (300,000 years) Nuclei form (180 seconds) Nucleons form (10-10 seconds) Quarks differentiate (10-34 seconds?) ??? (Before that)

9. The Main Injector upgrade was completed in 1999. • The new accelerator increases the number of • possible collisions per second by 10-20. • DØ and CDF have undertaken massive • upgrades to utilize the increased • collision rate. • Run II began March 2001 Expected Number of Events Huge statistics for precision physics at low mass scales 1000 Formerly rare processes become high statistics processes 100 Increased reach for discovery physics at highest masses 10 Run II 1 Run I Increasing ‘Violence’ of Collision

10. How Do You Detect Collisions? • Use one of two large multi-purpose particle detectors at Fermilab (DØ and CDF). • They’re designed to record collisions of protons colliding with antiprotons at nearly the speed of light. • They’re basically cameras. • They let us look back in time.

11. Typical Collider Detector: Run II • Weighs 5000 tons • Can inspect 3,000,000 collisions/second • Will record 50 collisions/second • Records approximately 10,000,000 bytes/second • Will record 1015(1,000,000,000,000,000) bytes in the next run (1 PetaByte). 30’ 30’ 50’

12. Remarkable Photos In this collision, a top and anti-top quark were created, helping establish their existence This collision is the most violent ever recorded. It required that particles hit within 10-19 m or 1/10,000 the size of a proton

13. Coolest Detector at Fermilab Collider Detector at Fermilab

14. AØ: The High Rise FØ: The RF BØ: CDF EØ: This Space For Rent CØ: Future BTeV DØ: Fermilab’s Best Detector

15. Why Colliders? Momentum Conservation Energy Conservation

16. Special Relativity Low momentum means large mass Clever (lazy?) Theorists c = 1 Note:

17. Why Colliders? 900 GeV 0 GeV 900 GeV 900 GeV Mmax = 42 GeV Mmax = 1800 GeV Note: GeV is an energy unit

18. u u u u d d u u d u u u d d u Proton AntiProton Proton-AntiProton Scattering d u u u d u

19. Collision Facts • Very messy collisions • Hundreds of objects after collision • Need to simplify the measurement Experimentalist's View

20. Feynman Diagrams Scatter particles Make particles Note: In subsequent slides, only representative diagrams are shown, not all. Concentrate on scattered particles, ignore spectators.

21. Quantum ChromoDynamics quark gluon quark quark quark gluon quark quark antiquark gluon antiquark gluon quark gluon • Stuff to Study • Violence of Collision • Frequency of Collision • Exotic Particles? • Number of Output Objects quark gluon quark

22. q q Quarks can’t exist, except when they are confined We’re in luck! Miracle As quarks leave a collision, they change into a ‘shotgun blast’ of particles called a ‘jet’ 

23. Most Likely:Least Likely 107:1! Expected Number of Events 1000 100 10 Data 1 Hiding things? Increasing ‘Violence’ of Collision

24. Theory agrees within ~20% over 7 orders of magnitude! CDF reports a slight excess of events at high p as compared to DØ Difference is not understood, but within global errors

25. ElectroWeak electron quark quark quark Z boson W boson antielectron antiquark antiquark antiquark electron quark • Stuff to Study • Frequency of W & Z Boson Creation • Mass of W & Z Boson • Frequency of Decay Modes W boson neutrino antiquark

26. For a particle (A), energy, momentum and mass are related: Let this particle (A) decay into two particles (1) & (2) High School Physics  Energy and momentum are conserved. Measuring Mass: Quicky Review of Special Relativity Lorentz Invariant 1 { { { A Not Lorentz Invariant 2 Lorentz Invariant

27. Transverse momentum is conserved them? do you detect Neutrinos can penetrate light-years of lead Need a ‘fully surrounding’ (4) detector How

28. Sometimes Z bosons are created in collisions with other objects quark quark e gluon Z e antiquark antiquark

29. The Needle in the Haystack: Run I • There are 2,000,000,000,000,000 possible collisions per second. • There are 300,000 actual collisions per second, each of them scanned. • We write 4 per second to tape. • For each top quark making collision, there are 10,000,000,000 other types of collisions. • Even though we are very picky about the collisions we record, we have 65,000,000 on tape. • Only 500 are top quark events. • We’ve identified 50 top quark events and expect 50 more which look like top, but aren’t. Run II ×10

30. Top Facts • Discovery announced March 1995 • Produced in pairs • Decays very rapidly ~10-24 seconds • You can’t see top quarks!!! • Six objects after collision Theorist’s View

31. Top Facts • In each event, a top and anti-top quark is created. • ~100% of the time, a top quark decays into a bottom quark and a W boson. • A W boson can decay into two quarks or into a charged lepton and a neutrino. • So, an event in which top quarks are produced should have: • 6 quarks • 4 quarks, a charged lepton and a neutrino • 2 quarks, 2 charged leptons and 2 neutrinos

32. Combining Viewpoints

33. 4 quarks, 1 lepton, 1 neutrino The Challenge End on viewtop+antitop Jets in “God Mode” Jets in “Don Mode”    Algorithm + Reality Algorithm    Guess!!!! Don’t know who goes with what Know (1) W   +  MW2 = (Em + En)2 - (pm +pn)2 (2) Mt = Manti-t (3) t  W + b anti-t  W + b Note: combinations jet m n jet jet jet

34. Top Quark Run I: The Summary • The top quark was discovered in 1995 • Mass known to 3% (the most accurately known quark mass) • The mass of one top quark is 175 times as heavy as a proton (which contains three quarks) Why???

35. In 1964, Peter Higgs postulated a physics mechanism which gives all particles their mass. This mechanism is a field which permeates the universe. If this postulate is correct, then one of the signatures is a particle (called the Higgs Particle). Fermilab’s Leon Lederman co-authored a book on the subject called The God Particle. bottom top Undiscovered!

36. “LEP observes significant Higgs candidates for a mass of 115 GeV with a statistical significance of 2.7 and compatible with the expected rate and distribution of search channels.” Chris Tully, Fermilab Colloquium 13-Dec-2000 Non-Expert Translation: Maybe we see something, maybe we don’t. What we see is consistent with being a Higgs Particle. But it could end up being nothing. It’s Fermilab’s turn.

37. Higgs Hunting at the Tevatron • If you know the Higgs mass, then the production cross section and decays are all calculable within the Standard Model • inclusive Higgs cross section is quite high: • ~ 1pb = 1000 events/year • but the dominant decay H  bb is swamped by background • thus the best bet appears to be associated production of H plus a W or Z • leptonic decays of W/Z help give • the needed background rejection • ~ 0.2 pb = 200 events/year

38. Is a Fermilab Higgs Search Credible? mH probability density, J. Ellis (hep-ph/0011086) • LEP incorrect • Rule out with 95% certainty by ~2003 • LEP correct • Similar quality evidence ~2004-2005 • “Discovery” quality evidence ~2007 • Higgs exists but is heavier than LEP suggests • Depends on how heavy • DØ and CDF have a good shot on seeing ‘maybe’ and possibly ‘absolutely’ quality evidence

39. Good News • ×10 more data than Run I • Bad News • ×1/10 less likely to be created than top quark Is a Fermilab Higgs Search Credible?: Good News/Bad News • So it’s a wash...similar problem to Run I top search • Except... • Events which look like Higgs but aren’t are much more numerous. • An irony...top quarks are a big piece of the ‘noise’ obscuring Higgs searches.

40. Run II: What are we going to find? I don’t know! Improve top quark mass and measure decay modes. Do Run I more accurately Supersymmetry, Higgs, Technicolor, particles smaller than quarks, something unexpected?