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Partikel tuhan higgsboson1


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Partikel tuhan higgsboson1

  1. 1. What is the Higgs Boson? And how do we search for it? Jason Nielsen SCIPP / UC Santa Cruz June 25, 2007 VERTEX 2004J. Nielsen 1
  2. 2. Challenge of Particle Physics • 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 Particle collisions at the energy frontier enable us to pursue these and other questions about natureJ. Nielsen 2
  3. 3. Fundamental Particles & ForcesJ. Nielsen 3
  4. 4. Force Carrier Quanta Photon (electromagnetic) W,Z bosons (weak force) • verified 1922 • verified 1983 • mass of photon = 0 • mW, mZ: 80 GeV/c2, 91 GeV/c2 Gauge symmetry is fundamental to electrodynamics • when extended to electroweak theory, requires massless W,Z • how to accomodate their large masses?J. Nielsen 4
  5. 5. Higgs Mechanism in Field Theory Electroweak “Standard Model” relies on broken symmetry Additional fields with constructed potential • just like gravitational field, electric field QuickTime™ an d a TIFF (Uncompressed) decompressor are need ed to see this p icture . Introduction of a pervasive Higgs field • Rotationally symmetric potential • But the stable minimum breaks the symmetry!J. Nielsen 5
  6. 6. Spontaneous Symmetry Breaking Came to particle physics from condensed matter physics above Tc below Tc Pencil on point Heisenberg ferromagnet Theory has rotational invariance; ground state is not invariant Symmetry has been broken by external factorJ. Nielsen 6
  7. 7. Higgs Mechanism in Field Theory Spontaneous symmetry breaking • Lost degree of freedom -> Goldstone bosons 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!J. Nielsen 7
  8. 8. Why is the Top Quark So Massive? 180 160 140 mt=175 GeV/c2 Interaction with Higgs quantummass (GeV/c2) 120 100 80 defines mass of fermions 60 40 20 0 u d s c b t 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” Note: Higgs couplings explain fundamental fermion mass but not everyday mass!J. Nielsen 8
  9. 9. 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…J. Nielsen 9
  10. 10. Wringing Out the Higgs Condensate e+ H Physical Higgs bosons can be produced, given enough energy Z* QuickTime™ and a TIFF (LZW) decompressor are needed to see this pic ture. (Here ECM > mH + mZ) e- Z That’s where the collider comes in But Higgs boson is fleeting: b b decays immediately to H characteristic “final state” Z q q That’s where the detector comes inJ. Nielsen 10
  11. 11. Recent Physics Results Effects of the Higgs boson are felt via loop interactionsPrecision measurementsare sensitive to the Higgs massUpdated winter 2007 with newTevatron mW=80.4±0.04 GeV mH < 182 GeV/c2 at 95% CL (including previous searches)J. Nielsen 11
  12. 12. 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 ZZJ. Nielsen 12
  13. 13. Rare Higgs Decays (?) (Claus Grupen)J. Nielsen 13
  14. 14. Identifying b Quarks from Higgs B hadrons have lifetimes of 1.5 ps: find the decay vertex! proton-antiproton Interaction point B hadron 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 crucialJ. Nielsen 14
  15. 15. One Provocative Candidate Event HZ bbbb selection ECM=206.7 GeV 3 NN b-tagged jets Reconstructed mH = 110 ± 3 GeV/c2J. Nielsen 15
  16. 16. Bumps in the Mass Spectrum Decay products of the Higgs boson form a mass resonance - similar to resonances from past discoveries of new particles QuickTime™ and a TIFF (LZW) decomp resso r are neede d to see this picture. Strategy for identifying Higgs boson production: 1. Excess of events in Wbb signature (or other signature) 2. Higgs decay products form a invariant mass peakJ. Nielsen 16
  17. 17. Tevatron Cross Section Hierarchy In proton-antiproton collisions at s = 1.96 TeV: b-jet pairs from QCD high-energy leptons 1 Particle production rates vary widely: 0.05 the Higgs is the “needle in the haystack!”J. Nielsen 17
  18. 18. What kind of unit is a “barn?” Manhattan Project physicists gave the name to the typical nuclear cross-section defined as 10-24 cm2 Practically “as big as a barn” where (sub)-nuclear processes Photo: Reidar Hahn, Fermilab are concerned the term “barn” wasnt officially declassified until 1948 Apparently there was also a unit called the “shed”: 10- 48 cm2 This summer CDF will have collected 3 giga-sheds of data!J. Nielsen 18
  19. 19. bb Dijet Invariant Mass DistributionJ. Nielsen 19
  20. 20. Large Hadron Collider at CERN Next generation collider: startup scheduled for 2008 Italy Luminosity target: 1034cm-2 s-1 p 14 TeV p Increased production of heavy particles like Higgs, top quark ATLAS CMS More particles at higher energy requires new detector design and technologyJ. Nielsen 20
  21. 21. Higgs Decay to Photons Rare decay in SM t H t LHC detectors have been optimized to find this peak!J. Nielsen 21
  22. 22. Higgs Decay to ZZ Requires precise measurement of muon curvatureJ. Nielsen 22
  23. 23. ATLAS Experiment at LHCJ. Nielsen 23
  24. 24. ATLAS Experiment at LHC ATLAS collaboratorJ. Nielsen 24
  25. 25. ATLAS DETECTOR Nov. 2005J. Nielsen 25
  26. 26. Installation of inner detector end-capJ. Nielsen 26
  27. 27. Prospects for SM Higgs at LHC Should discover SM Higgs regardless of mass value Low-mass Higgs channels: •H !( m =1.5 GeV/c2) • W,Z boson fusion to Higgs: then H WW or H • ttH: top quark again! High-mass Higgs channels: • golden mode 4e/ opens >2mZJ. Nielsen 27
  28. 28. Identifying Particle Signatures ATLAS trigger system can identify specific signatures onlineJ. Nielsen 28
  29. 29. “Hunt for Higgs” WWW Site One of the best I’ve seen at describing what really happens Let’s have a look together at the “Hunt for Higgs”J. Nielsen 29
  30. 30. 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 ourselvesJ. Nielsen 30
  31. 31. J. Nielsen 31