1. J. Nielsen1 What is the Higgs Boson? Jason Nielsen SCIPP / UC Santa Cruz VERTEX 2004 June 25, 2007 And how do we search for it?

2. J. Nielsen2 Challenge of Particle Physics Particle collisions at the energy frontier enable us to pursue these and other questions about nature Unification of the basic forces and the origin of mass for the fundamental particles Unexpected new physics or extra dimensions not included in Standard Model Unknown new physics (forces or particles) hinted at by cosmology

3. J. Nielsen3 Fundamental Particles & Forces

4. J. Nielsen4 Force Carrier Quanta Gauge symmetry is fundamental to electrodynamics when extended to electroweak theory, requires massless W,Z how to accomodate their large masses? Photon (electromagnetic) verified 1922 mass of photon = 0 W,Z bosons (weak force) verified 1983 m W, m Z : 80 GeV/c 2, 91 GeV/c 2

5. J. Nielsen5 Higgs Mechanism in Field Theory Additional fields with constructed potential just like gravitational field, electric field Electroweak “Standard Model” relies on broken symmetry Introduction of a pervasive Higgs field Rotationally symmetric potential But the stable minimum breaks the symmetry!

6. J. Nielsen6 Spontaneous Symmetry Breaking Came to particle physics from condensed matter physics Theory has rotational invariance; ground state is not invariant  Symmetry has been broken by external factor Pencil on pointHeisenberg ferromagnet above T c below T c

7. J. Nielsen7 Higgs Mechanism in Field Theory Goldstone bosons give mass to W ±,Z One physical scalar boson: Higgs boson whose mass is unknown Discovery of the Higgs boson would help verify this approach Otherwise, much head-scratching and new theories! Spontaneous symmetry breaking Lost degree of freedom -> Goldstone bosons

8. J. Nielsen8 Why is the Top Quark So Massive? Note: Higgs couplings explain fundamental fermion mass but not everyday mass! m t =175 GeV/c 2 Schwinger (1957): a coupling produces effective mass terms through the action of the vacuum fluctuations (Higgs boson) Top quark most affected by this “Higgs field molasses” Interaction with Higgs quantum defines mass of fermions

9. J. Nielsen9 So What IS the Higgs boson? Higgs boson is a physical condensate of the pervasive postulated Higgs field Similar to photon, except Higgs boson is not a force carrier What kinds of particles do it couple to? Its couplings are proportional to the fermion masses So it couples most strongly to the most massive particles This makes it clear how to search for it, if it exists…

10. J. Nielsen10 Wringing Out the Higgs Condensate H Z Z* e+e+ e-e- But Higgs boson is fleeting: decays immediately to characteristic “final state” Physical Higgs bosons can be produced, given enough energy (Here E CM > m H + m Z ) That’s where the collider comes in That’s where the detector comes in H Z b b q q

11. J. Nielsen11 Recent Physics Results m H < 182 GeV/c 2 at 95% CL (including previous searches) Updated winter 2007 with new Tevatron m W =80.4±0.04 GeV Effects of the Higgs boson are felt via loop interactions Precision measurements are sensitive to the Higgs mass

12. J. Nielsen12 How does the Higgs Boson Decay? Notice coupling to massive particles (bb, , WW, ZZ) For low mass Higgs, expect decay to b quark pairs; For very high mass Higgs expect decay to ZZ

13. J. Nielsen13 Rare Higgs Decays (?) (Claus Grupen)

14. J. Nielsen14 Identifying b Quarks from Higgs proton-antiproton Interaction point B hadron B hadrons have lifetimes of 1.5 ps: find the decay vertex! Fit tracks together to form secondary vertex measure flight distance of B hadron typical flight distance is 0.5 cm from interaction point close, precise measurement provided by silicon is crucial

15. J. Nielsen15 One Provocative Candidate Event 3 NN b-tagged jets E CM =206.7 GeV HZ  bbbb selection Reconstructed m H = 110 ± 3 GeV/c 2

16. J. Nielsen16 Bumps in the Mass Spectrum Decay products of the Higgs boson form a mass resonance - similar to resonances from past discoveries of new particles Strategy for identifying Higgs boson production: 1.Excess of events in Wbb signature (or other signature) 2.Higgs decay products form a invariant mass peak

17. J. Nielsen17 Tevatron Cross Section Hierarchy In proton-antiproton collisions at  s = 1.96 TeV: b-jet pairs from QCD high-energy leptons 0.05 1 Particle production rates vary widely: the Higgs is the “needle in the haystack!”

18. J. Nielsen18 What kind of unit is a “barn?” Photo: Reidar Hahn, Fermilab the term “barn” wasn't officially declassified until 1948 Apparently there was also a unit called the “shed”: 10 - 48 cm 2 This summer CDF will have collected 3 giga-sheds of data! Manhattan Project physicists gave the name to the typical nuclear cross-section defined as 10 -24 cm 2 Practically “as big as a barn” where (sub)-nuclear processes are concerned

19. J. Nielsen19 bb Dijet Invariant Mass Distribution

20. J. Nielsen20 Large Hadron Collider at CERN Italy pp 14 TeV Next generation collider: startup scheduled for 2008 ATLAS CMS Luminosity target: 10 34 cm -2 s -1 Increased production of heavy particles like Higgs, top quark More particles at higher energy requires new detector design and technology

21. J. Nielsen21 Higgs Decay to Photons Rare decay in SM LHC detectors have been optimized to find this peak! H t t  

22. J. Nielsen22 Higgs Decay to ZZ Requires precise measurement of muon curvature

23. J. Nielsen23 ATLAS Experiment at LHC

24. J. Nielsen24 ATLAS Experiment at LHC ATLAS collaborator

25. J. Nielsen25 ATLAS DETECTOR Nov. 2005

26. J. Nielsen26 Installation of inner detector end-cap

27. J. Nielsen27 Prospects for SM Higgs at LHC Should discover SM Higgs regardless of mass value Low-mass Higgs channels: H  ! (  m =1.5 GeV/c 2 ) W,Z boson fusion to Higgs: then H  WW or H  ttH: top quark again! High-mass Higgs channels: golden mode 4e/  opens >2m Z

28. J. Nielsen28 Identifying Particle Signatures ATLAS trigger system can identify specific signatures online

29. J. Nielsen29 “Hunt for Higgs” WWW Site http://www.sciencemuseum.org.uk/antenna/bigbang/huntforhiggs/index.asp Let’s have a look together at the “Hunt for Higgs” One of the best I’ve seen at describing what really happens

30. J. Nielsen30 Future of the Higgs Search Tevatron experiments still searching LHC turns on in 2008 –Commissioning and calibrating detectors Understand non-Higgs backgrounds Find the Higgs boson peak above the bkgd! My guess is that it will take a few years to collect enough events to convince ourselves

31. J. Nielsen31