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Searches for the SM Higgs Boson

Searches for the SM Higgs Boson. g g s. Matthew Herndon, Dec 2008 University of Wisconsin University of Massachusetts Amherst Physics Colloquium. Forces of Nature Unification of the Forces and the Higgs Particle Searching for the Higgs/Higgs Searches Results. BEACH 04. J. Piedra. 1.

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Searches for the SM Higgs Boson

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  1. Searches for the SM Higgs Boson g g s Matthew Herndon, Dec 2008 University of Wisconsin University of Massachusetts Amherst Physics Colloquium • Forces of Nature • Unification of the Forces and the Higgs Particle • Searching for the Higgs/Higgs Searches Results BEACH 04 J. Piedra 1

  2. The Atom • In the early twentieth century atomic physics was well understood • The atom had a nucleus with protons and neutrons. • An equal number of electrons to the protons orbited the nucleus • The keys to understanding this were the electromagnetic(EM) force and the new ideas of quantum mechanics • The EM force held the electrons in their orbits • Quantum mechanics told us that only certain quantized orbits were allowed • Allowed detailed understanding of the properties of matter M. Herndon 2

  3. The Periodic Table Elements in same column have similar chemical properties Different types of quantum orbits M. Herndon 3

  4. We Observed New Physics Needed a new theory Already there were some clear problems • One type of atom could convert itself into another type of atom • Nuclear beta decay • Charge of atom changed and electron emitted • How could the nucleus exist? • Positive protons all bound together in the the atomic nucleus M. Herndon 4

  5. The Forces How do we understand the Forces? Fundamental differences in strengths! • Best way to think about the problem was from the viewpoints of the forces • Needed two new forces and at first glance they were not very similar to the familiar electromagnetic and gravitational forces! M. Herndon 5

  6. How Do the Forces Work electron photon electron • Relativistic quantum field theory (QFT): Quantum electrodynamics(QED) • Unification of relativity(the theory space time and gravity) and quantum mechanics(the theory of atoms as described by the EM Force) • Description of the particles and the forces at one time • Allowed for a possible unification of the forces - description by one theory • Electromagnetic force comes about from exchange of photons. Electromagnetic repulsion via emission of a photon Exchange of many photons allows for a smooth force(EM field) • For a very quick interaction we can see individual photon exchanges

  7. Particle Annihilation or Creation ??? electron ??? photon electron photon electron electron Also learned from studying EM force that the proton and neutron were made of smaller particles. up and down quarks. p=uud, n=udd • The new QED EM Theory has one very interesting additional feature • Can rotate diagrams in any direction Antiparticles! Anti-electron or positron. This is going to be a useful way to make new particles. Time goes from left to right. What is an electron going backward in time?

  8. Unification! Still working to fully understand EW=EM+Weak Unification • The Standard Model of Particle Physics • QFTs for EM, Weak and Strong • Unified EM and Weak forces - obey a unified set of rules with strengths quantified by single set of constants • All three forces appear to have approximately the same strength at very high energies • So far just a theory - though a successful one • Maxwell had unified electricity and magnetism • Both governed by the same equations with the strengths of the forces quantified using a set of constants related by the speed of light 1eV = 1.6x10-19 J M. Herndon 8

  9. Electroweak Symmetry Breaking • Consider the Electromagnetic and the Weak Forces • SM says that they are two aspects of one force and governed by the same rules • They should be the same strength, but EM always active, weak decays can take thousands of years! • Coupling probabilities at low energy: EM: ~2, Weak: ~2/(MW,Z)4 • Fundamental difference in the coupling strengths at low energy, but apparently governed by the same constant • Difference due to the massive nature and short lifetime of the W and Z bosons. • At high energy the strengths become the same. We say the forces are symmetric • SM postulates a mechanism of electroweak symmetry breaking via the Higgs mechanism • Predicts a field, the Higgs field, and an associated particle, the Higgs boson. • Introduces terms where particles interact with themselves: self energy or mass • Directly testable by searching for the Higgs boson A primary goal of the Tevatron and LHC

  10. Weak and EM Force: Strength q momentum of the W or Z bosons Coupling strength: Same as EM force 30 years of searching and no luck yet! P  2/(q2+M2)2 • For EM force • For weak force P  2/(q2+MW2)2 • Mass of the photon is 0, mass of the W and Z bosons is large • When the mass of the W boson is large compared to the momentum transfer, q, the probability of a weak interaction is low compared to the EM interaction! • At high energy when q was much larger than the mass of the weak bosons the the weak and EM interaction have the same strength However it’s only a theory. Have to find the Higgs boson! M. Herndon 10

  11. The Forces Revisited M. Herndon 11

  12. The Standard Model Only observing the Higgs Boson is left to complete the experimental program associated with the SM • What is the Standard Model? • Explains the hundreds of common particles: atoms - protons, neutrons and electrons • Explains the interactions between them • Basic building blocks • 6 quarks: up, down… • 6 leptons: electrons… • Bosons: force carrier particles • All common matter particles are composites of the quarks and leptons and interact by exchange of the bosons 12

  13. Searching for the Higgs positron Higgs Boson electron • How do we search for the Higgs Boson • Use the idea of particle anti-particle annihilation • Annihilate high energy electrons and positrons or high energy quarks and anti-quarks inside of protons and anti-protons • Problem: The probability or strength of Higgs interactions is proportional to the mass of the particle. Electrons and u and d quarks are very light! M. Herndon 13

  14. Searching for the Higgs: Production t _ t • The Higgs will couple best to the most massive particles and the W and Z • W and Z bosons: 80 and 91 GeV • The top quark: 172.6 GeV: Gold atom • We need to produce Higgs using interactions with those particles! 10 orders of magnitude smaller cross section than total inelastic cs M. Herndon 14

  15. Searching for the Higgs: Decay • We need decays of the Higgs involving massive particles • Higgs particle is probably not massive enough to decay to top quarks • So we look for the interactions involving the W and Z and the next most massive particle, the b quark, 4.5GeV M. Herndon 15

  16. Higgs Search at LEP Final Result mH > 114.4 GeV • Searched for the Higgs using an electron positron collider • Achieved an energy of 209GeV which allowed it to search for Higgs particle up to a mass of ~115GeV M. Herndon 16

  17. Indirect Higgs Search Current Result mH < 160 GeV • Measuring the mass of the most massive quarks and boson should allow you to calculate the Higgs mass. M. Herndon 17

  18. Tevatron Higgs Search • The search for Higgs continues of the Tevatron Accelerator • 1.96TeV proton anti-proton accelerator • Enough energy to produce the Higgs. • However, the rate is expected to be very small - 3fb-1 of data per experiment • Two experiments designed to find the Higgs: CDF and DØ • Wisconsin participates in the Higgs search at the CDF experiment • The stage is set. • We can produce the Higgs • We know where to look • The Higgs boson mass is between 114.4 and ~160GeV M. Herndon 18

  19. The CDF Detector Higgs analysis uses most of the capabilities of the CDF detector • CDF Tracker • Silicon detector: 1 million channel solid state device! • 96 layer drift chamber • Dedicated systems for finding different types of particles • Electrons and muons • Measurement of the energy of quarks(jets) • And if any energy is missing Detector designed to measure all the SM particles M. Herndon 19

  20. The Real CDF Detector Wisconsin Colloquium M. Herndon 20

  21. Searching for the Higgs: Low Mass • At Higgs masses well below 160GeV we search for Higgs decays to b quarks. • b hadrons are long lived. • Low efficiency to tag long lifetime. • Many different searches. • Associated production with a vector boson, VH: Leptonic decays W and Z are distinctive M. Herndon 21

  22. Higgs Search: WHlbb • Example: CDF WHlbb - signature: high pT lepton, MET and b jets • Key issues: Maximizing lepton acceptance and b tagging efficiency • Backgrounds: W+bb, W+qq(mistagged), single top, Non W(QCD) • Single top: yesterdays new physics signal is today’s background • Innovations: acceptance from isolated/forward tracks. Multiple or NN b tagging methods. Multivariate discriminants: example - Matrix Element Method (probability of any decay configuration based on the SM calculation compared between signal and background) • Factor of 1.5 improvement in the expected limits in the last year from innovations Results at mH = 115GeV: 95%CL Limits/SM Worlds most sensitive low mass Higgs search - Still a long way to go!

  23. Low Mass Higgs Searches • We gain our full sensitivity by searching for the Higgs in every viable production and decay mode • With all analysis combined we have a sensitivity of about ~2.4xSM at low mass. • A new round of DØ analysis, 2x data and 1.5x improvements will bring us to SM sensitivity.

  24. Searching for the Higgs: High Mass - • At Higgs masses around 160GeV we search for Higgs decays to W bosons. • Leptonic W decay • Uses the excellent charged lepton fining ability of our detectors • Also a primary channel for the LHC M. Herndon 24

  25. Higgs Search: HWW W+ H μ+ W+ W- ν e- W- • HWWll - signature: Two high pT leptons and MET • Key issue: Maximizing lepton acceptance • Primary backgrounds: WW and top in di-lepton decay channel • Innovations: CDF/DØ : Inclusion of acceptance from VH and VBF CDF : Combination of ME and NN approaches Spin correlation: Charged leptons go in the same direction ν

  26. SM Higgs Search: HWW • Most sensitive Higgs search channel at the Tevatron Results at mH = 165GeV : 95%CL Limits/SM Both experiments Approaching SM sensitivity! Let’s Combine the Results.

  27. SM Higgs Combination High mass only Exp. 1.2 @ 165, 1.4 @ 170 GeV Obs. 1.0 @ 170 GeV

  28. SM Higgs Excluded: mH = 170 GeV • We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV SM Higgs Combination • Result verified using two independent methods(Bayesian/CLs) 95%CL Limits/SM

  29. Projections • Goals for increased sensitivity achieved • Goals set after 2007 Lepton Photon conference • First stage target was sensitivity for possible exclusion • Second stage goals still in progress • Expect large exclusion, or evidence, with full Tevatron dataset and further improvements. Run II Preliminary

  30. Discovery • Discovery projections: chance of 3 or 5 discovery • Two factors of 1.5 improvements examined relative to summer Lepton Photon 2007 analyses. • First 1.5 factor achieved for summer ICHEP 2008 analysis • Resulted in exclusion at mH = 170 GeV.

  31. Conclusions • Finding the Higgs Boson would add fundamental information to our understanding of the forces of nature • Without the Higgs boson we don’t understand the nature of the weak force: Why it is so much weaker than the electromagnetic force? • The Higgs boson search is in its most exciting era ever • The Tevatron experiments have achieved sensitivity to the SM Higgs boson production cross section at high mass • We exclude at 95%C.L. the production of a SM Higgs boson of 170 GeV • Expect large exclusion, or evidence, with full Tevatron data set and improvements SM Higgs Excluded: mH = 170 GeV M. Herndon 31

  32. Backup

  33. SM Higgs Combined Limits • Limits calculating and combination • Using Bayesian and CLs methodologies. • Incorporate systematic uncertainties using pseudo-experiments (shape and rate included) (correlations taken into account between experiments) • Backgrounds can be constrained in the fit • Winter conferences combination April: hep-ex/0804.3423

  34. HWW Systematic Uncertainties • Shape systematic evaluated for • Scale variations, ISR, gluon pdf, Pythia vs. NL0 kinematics, jet energy scale: for signal and backgrounds. Included in limit setting if significant. • Systematic treatment developed in collaboratively between CDF and DØ

  35. LHC Prospects: SM Higgs • LHC experiments have the potential to observe a SM Higgs at 5 over a large region of mass • Observation: ggH, VBF H, HWWll, and HZZ4l • Possibility of measurement in multiple channels • Measurement of Higgs properties • Yukawa coupling to top in ttH • Quantum numbers in diffractive production All key channels explored Exclusion at 95% CL CMS ATLAS preliminary

  36. Example HEP Detector 36

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