Finding The Higgs Boson

1. FindingThe Higgs Boson A (hopefully) slightly better explained version of the events around July 4, 2012 Dr. B. Todd Huffman, Oxford UniversityDr. A. Weidberg, Oxford University

2. Explanation in two parts • Particle Physicists  coffee • Finding the Higgs • Standard Model Higgs properties • How to Find the Boson • Bump Hunting • Special Relativity • July 4th: the data • Detector performance • CMS and ATLAS results • Stat. Confidence of Discovery

3. Standard Model Higgs(part 1) • Start with Higgs boson as a given. • Standard Model is a quantitative theory. • Predicts Probability of a Higgs boson at the LHC • Prediction is the cross-section (sh) in “barns” • What does this mean for us? Standard Model does not predictthe Higgs Mass though. B. Todd Huffman, Oxford University

4. Cross Section is an area. 10 pb = 10-35 cm2. Brightness = Lum. # Particles per cm2 per second = 1034 n/(cm2s)

5. Reason for Radiation Hard Electronics Mh = 125 GeV/c2SM Higgs Production Rate = 10-35 cm2 x 1034 cm-2s-1 = 0.1 Hz or one every 10 seconds. But Hang on! spp ~ 100 mbthat’s “millibarns” With L = 1034 cm-2s-1random interactions a billion times a second. (not Higgs)Beam bunches cross once every 50 ns. 50 interactions/crossing B. Todd Huffman, Oxford University

6. The LHC Once the Energy is fixed (ring size) Then the only thing we can tweak is Luminosity. This is a hard problem.

7. More Predictions:Higgs Decays g g

8. Irreducible photon processes time Photon (g) Standard Model shape:Number of photon pairs vs. energy quark B. Todd Huffman, Oxford University

9. Irreducible ZZ* processes l+ l- l- l+ Zo Zo quark Standard Model shape:Number of 4-lepton pairs vs. energy Anti-quark B. Todd Huffman, Oxford University

10. Why did we find it in the decays that are so rare? Higgs to gg → 100 per year Higgs to ZZ* → 1000 per year (but Z to ee, mm means ~5 per year)

11. One Step back: Special Relativity But what if we were moving really fast to the left? Two things happen! Decay B Explosion A At time t0 and location x0 At time t1 and location x1 B. Todd Huffman, Oxford University

12. V The order and distance depends on the speed you travel! Two things happen! Decay B Explosion A At time t1’ and location x1’ At time t0’ and location x0’ B. Todd Huffman, Oxford University

13. Special Relativity But this quantity is the same in ALL frames of reference. Time t0 ; location x0 Invariant Scalar t1 ; x1

14. Special Relativity • Momentum and Energy do this too! E2 - p2c2 = m2c4 • No momentum, P = 0, then you get E = MC2 • Throw in this fact of nature: • Energy and momentum are conserved. ALWAYS g1 g0 Mhiggs

15. Invariant Mass E2-p2c2=m2c4 (E0 + E1)2 – (P0 + P1)2c2 = M2higgsc4 e- Works for any number of particles.Works no matter how fast or slow the Higgs is moving in the lab. Does not work if they did not come from a Higgs m- m+ e+ Mhiggs B. Todd Huffman, Oxford University

16. Irreducible ZZ* processes time l+ l- l- l+ Zo Zo quark Standard Model shape:Number of 4-lepton pairs vs. energy Anti-quark B. Todd Huffman, Oxford University

17. Higgs Bump Hunting 6 months Many events have 4 lepton or two photon candidates. So just plug E and p of eachone into the formula to find their scalar invariant mass. Mostly not Higgs. The scalarformula then puts a pip randomly on this histogram If there really is a parent  ALL combinations land at Mhiggs; every time.

18. 2 years B. Todd Huffman, Oxford University

19. 15 years Glad I did not book a flight to Stockholm. Last Paper for TheoristPrior to Managing a Hedge Fund B. Todd Huffman, Oxford University

20. ATLAS • Features: • Standalone muon spectrometer (air-core toroid). • Conventional EM calorimeter (Pb/LAr). B. Todd Huffman, Oxford University

21. CMS • Some Powerful detectors (e.g. tracker). • Less demanding on muon chamber technology. B. Todd Huffman, Oxford University

22. Why The Pain is Worth It • Backgrounds • H  g g • Protons have quarks with electric charge. Two photons can result when q-qbar’s annihilate • Neutral pions decay to photons po  g g • Bad News; Quark jet could fake a photon • CMS and ATLAS detectors built to ID pions this way. • H  Z Z* then Z  e+ e- or m+ m- • Proton-Proton  Z Z* happens too • No Higgs involved • “Irreducible Background” • We must deal with Backgrounds • Careful Detector design. B. Todd Huffman, Oxford University

23. Geometric exploitspo g g Fine strip segmentation Very Useful! B. Todd Huffman, Oxford University

25. B. Todd Huffman, Oxford University

26. The Data – gg CMS ATLAS B. Todd Huffman, Oxford University

27. The Data - ZZ* Next: Detector Resolution ATLAS CMS B. Todd Huffman, Oxford University

28. Accurate Measurement Much Pain: to obtain track resolutions less than ten microns. To measure mand eenergy as accuratelyas possible B. Todd Huffman, Oxford University

29. Meaning: What if the momentum we measure is further away from the true momentum? What would happen if tracking resolution was worse? LHC 2 years B. Todd Huffman, Oxford University

30. B. Todd Huffman, Oxford University

31. Would have published earlier. B. Todd Huffman, Oxford University

32. How do we know this is real? “The Data were inconclusive, so we applied Statistics” (A quote taken from Louis Lyons’ book) B. Todd Huffman, Oxford University

33. 15 years Basic Question: What is the probability, if it IS just random, that this “signal” is just a fake? Random events can, occasionally,fake a signal. B. Todd Huffman, Oxford University

34. Discovery!And Limits ATLAS CMS

35. Nobel Choices P. Higgs F. Englert G. Guralnik T. Kibble T. Hagen Who will win the prize? Any other questions? R. Brout(deceased) B. Todd Huffman, Oxford University

36. References • David Griffiths, "Introduction to Elementary Particles, 2nd ed.", Wiley-VCH, Weinheim, Germany, 2008. • F. Halzen and A. D. Martin, “Quarks & Leptons: An Introductory Course in Modern Particle Physics” John Wiley & Sons. • I. J. R. Aitchison and A. J. G. Hey, “Guage Theories in Particle Physics, 2nd Ed.”, Adam Hilger, Bristol. B. Todd Huffman, Oxford University