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Brown University Providence, RI

Alex Melnitchouk. Brown University University of Mississippi Oxford, MS March 25, 2004. Search for the Higgs Boson. Brown University Providence, RI . Alex Melnitchouk. Ph.D Thesis Defense September XX , 2003. OUTLINE.

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Brown University Providence, RI

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  1. Alex Melnitchouk Brown University University of Mississippi Oxford, MS March 25, 2004 Search for the Higgs Boson Brown University Providence, RI Alex Melnitchouk Ph.D Thesis Defense September XX , 2003

  2. OUTLINE • Brief Overview of some Particle Physics Basics • Luminosity and Cross Section • Units • Connection between theory and experiment • Why Look for Higgs • What is Mass ? Where does it come from ? • Standard Model of Elementary Particles • Electroweak Symmetry Breaking • What have we learned experimentally about Higgs so far ? • Tevatron proton-antiproton collider. • Higgs Production and Decay Modes • DØ Detector • hsearch at DØ. • Overview of current Higgs analyses • Beyond the Tevatron • Conclusions Fresh results !!!

  3. Z AN EXAMPLE: Collide bunches of protons and antiprotons at certain (high) energy to produce, e.g., Z-bosons At the end of the day the number of Z-bosons produced will depend on: • How many collisions happened • Intrinsic properties of Z-boson, proton, antiproton (that are independent of the number of collisions)

  4. Luminosity and Cross Section • Integrated LuminosityLdt(total number of collisions) Measured inInverse Picobarns (pb-1), e.g. DØ experiment at Fermi National Accelerator Laboratory (Fermilab) collected 100 pb-1of proton-antiproton collisions data during Run I (1992-1996) • Cross Section  (interaction probability) Measured inPicobarns (pb) e.g (pp  Z(ee)+X) 200 pb for collision energy of 1.8 TeV • Number of Interactions (that happened) = Cross Section  Integrated Luminosity e.g 20,000 of Zee events in Run I • Number of Interactions (observed) = Cross Section  Integrated Luminosity Geometrical Detector Coverage Fraction  Detector Efficiency 10,000 of observed Zee events in Run I

  5. Units • Useh = c = 1 convention • Use GeV (10 9 eV) unitsfor Energy, Momentum, and Mass

  6. Theory Experiment. One-Slide Review of Basics • Theoretical description needs to (be): • Quantum (small distances ~ 10–15 cm) • Relativistic (speeds close to c) • Accommodate transformations(production, decays) of particles  Realtivistic Quantum Field Theory • Definitions • A field = system with infinite number of degrees of freedom • An elementary particle = excitation of the field above its ground state(vacuum) • Lagrangian (total energy) expressed as a function of fields and their couplings • To relate Theory to Experiment: • Perturbative expansion of the Lagrangian (in terms of coupling constant) • Calculate expansion terms (Feynman diagrams) • Derive Experimentally Measurable Quantities: • Cross Sections, Lifetimes

  7. Matter andEnergy • Massive Structures (atoms, biological cells, living beings, planets) • Light (pure energy) QUESTIONS: • What is the difference between the two ? • What is mass anyway ?

  8. What Do We Know About Mass? • Measure of Inertia Galileo: speed of falling objects does not depend on mass Newton: a = F/m • Massive particles behave also as waves Double-slit QM experiment: electrons (particles of well defined and measured mass) form interference patterns • Mass is equivalent to energy: E = mc2 • Mass and Spin – two fundamental quantities V. Bargman and E.P.Wigner: all relativistic wave equations (i.e. particles) can be classified by mass and spin (e.g. massive fermions, massless bosons etc.) • Mass and Space-Time are connected distribution of mass in the Universe affects the geometry of space-time (General Relativity) • Where does mass come from ? Standard Model of elementary particles suggests that mass is not an intrinsic property of a particle but rather comes from the interaction with theHIGGS FIELD

  9. Standard Model of Elementary Particles • Standard Model is a relativistic quantum field theory based on SU(3)  SU(2)  U(1) gauge group • SM contains: • Spin-1/2 fermions, spin-1 bosons, spin-0 boson Higgs Boson Bound states  structures in the Universe

  10. e  e Fermions Interact via Gauge Boson Exchange • electron-electron (Möller) scattering • Attraction between the nucleus and atomic electron that leads to a bound state (atom) 

  11. Gauge Symmetries and Interactions • Existence and properties of force carriers follow from the requirement of the local gauge invariance on the fermion field (Dirac) Lagrangian. • Gauge groups  Interactions: U(1): Electromagnetic SU(2): Weak SU(3): Strong • e.g. U(1)  Photon (Electromagnetic interaction) • Dirac Lagrangian is not invariant under • To preserve the invariance need to introduce additional vector field Am( photon field) • Photon field is massless • How do we explain massive W and Z gauge bosons ? Mass terms break the local gauge invariance and make the theory non-renormalizable

  12. Electroweak Theory. Higgs Mechanism • Electromagnetic and weak interactions are unified under SU(2) U(1) gauge group • Introduce complex scalar (Higgs) field doublet • Its Lagrangian is invariant under SU(2)  U(1) • But a choice of particular ground state e.g. • 1=0, 2=0, 4=0, 32=-m2/=v2breaks the symmetry in such a way that massive gauge bosons appear W1 W2W3 B Massless weak and electromagnetic mediators

  13. mass = 80.4 GeV W photon mass = 0 Higgs Mechanism. EW Symmetry Breaking • Symmetry breaking reveals three extra degrees of freedom(in the unbroken theory they correspond to zero-energy excitations along the ground state surface) Singlet illustration of spontaneous symmetry breaking V() which get absorbed as additional (longitudinal) polarizations of W,Z 1 2 - Weak gauge bosons acquire mass vev - Photon remains massless

  14. Higgs Boson • Unstable weakly interacting massive spin 0 particle Higgs boson (Higgs field excitation) is also predicted – need to find it to verify Higgs hypothesis (1960’s) P.W. Higgs, Phys. Rev. Lett. 12 508 (1964); F. Englert and R. Brout, Phys. Rev. Lett. 13 321 (1964); G.S. Guralnik, C.R. Hagen, and T.W.B. Kibble, Phys. Rev. Lett. 13 585 (1964).

  15. Higgs Field Parameters • There are three parameters that describe the Higgs field : , , and v (vacuum expectation value) • v can be expressed in terms of Fermi coupling constant GF(which has been determined from muon lifetime measurement) v = (2 GF ) –1/2 = 246 GeV and related to the other parameters via v 2 = -  2 /  • There remains a single independent parameter, which can not be determined without experimental information about the Higgs boson • This parameter can be rewritten as the Higgs boson mass mH = (-2  2) 1/2

  16. What have we found out about mHfrom the experiments so far • Electro-weak precision measurements : mH < 211 GeV • LEP* direct searches : mH > 114 GeV Well defined target ! • Summer and Autumn 2000: Hints of a Higgs? • the LEP data may be giving some indication of a Higgs with mass 115 GeV (right at the limit of sensitivity) • despite these hints, CERN management decided to shut off LEP operations in order to expedite construction of the LHC† Before LHC turns on (end of this decade) the place to look for Higgs is Tevatron** !!! LEP* = Large Electron-Positron Collider at CERN LHC† = Large (proton-proton) Hadron Collider at CERN Tevatron** = Proton-antiproton collider at Fermilab

  17. CDF Chicago DØ DØ DØ Batavia, Illinois Run I 1992-95 Run II 2001-09(?) 100  larger dataset at increased energys =1.96 TeV ; t = 396 ns CDF `p p Booster Tevatron `p source Main Injector & Recycler Tevatron Collider and Detectors

  18. DØ detector.The work of many people… The DØ detector was built and is operated by an international collaboration of ~ 670 physicists from 80 universities and laboratories in 19 nations> 50% non-USA~ 120 graduate students

  19. Pseudorapidity  = - log (tan /2) y UnderlyingEvent x  g q z u u q d u d u Hard Scatter Coordinate System • Center-of-mass energy is not fixed • Energy balance can not be used •  use pT = psin r p p

  20. r-z View of the DØ Detector Muon System 5 0 5 Tracking System Calorimeter -10 -5 0 5 10 (m) protons anti-protons

  21. = 2 TeV Leading SM Higgs Production Processes at Tevatron gluon fusion : cross-section ~ m2  the top-quark loop is dominant Cross-Section, pb 10.0 W/Z associated 1.0 (Z) (Z*) 0.1 0.01 80 100 120 140 160 W/Z fusion Higgs Mass, GeV • quark-antiquark fusion cross-section is small : • Higgs-fermion coupling ~ mf • Masses of u,d quarks are small

  22. Higgs Decay Modes • why gg ? • very clean experimental signature • gg decays can be enhanced

  23. Examples of Enhancement of hgg decays hggBranching Fraction no couplings to fermions (Fermiophobic Higgs) no couplings to top,bottom quarks no couplings to down-type fermions Standard Model Higgs Mass, GeV S.Mrenna, J.Wells, Phys. Rev. D63, 015006 (2001) in general we should be prepared for any hgg branching fraction ( up to 1.0 ) due to new physics

  24. hggSearch Strategy Focus on 2 Scenarios • Fermiophobic Higgs (does not couple to fermions) • Production: W/Z associated + W/Z fusion • Main signature with diphotons : gg + 2jets • Topcolor Higgs (of allfermions couplesonly to top) • Production: all three leading processes • Main signature with diphotons : gg • Remaining models would give similar signal to one of the two scenarios: e.g. no couplings to down-type fermions  topcolor no t, b couplings  fermiophobic; Goal : setting limits on Cross-Section  B(gg) for both scenarios assuming SM couplings to W/Z and top-quark (in case of Topcolor) NEXT QUESTION : How do we identify photons in the D0 detector?

  25. p p s =1.96 TeV Energy, keV 2.0 16.5 31.0 45.0 60.0 The Scale of Photon Energies Atomic Spectra: ~ eV X -rays: ~ keV (103 eV)  -rays: ~ MeV (106 eV) h : ~ 100 GeV (1011 eV) Mh=120 GeV  Higgs 

  26. Interaction point Muon detector Calorimeter Induces shower in dense material Tracking system Magnetized volume Innermost tracking layers use silicon EM layers fine sampling Absorber material Hadronic layers Jet A Slice of the DØ Detector Electron EM showers developing via e+e- pair production and bremsstrahlung Photon Experimental signature of a Photon :EM-like shower in the calorimeter + NO associated track

  27. DØ Calorimeter • Uranium/Liquid Argon Sampling Calorimeter • Three modules: -- Central Calorimeter (CC) -- Two End Calorimeters (EC) Unit cell

  28. DØ Calorimeter (Cont’d) Several unit cells = readout cell   Hadronic EM (0,0,0) Hadronic EM Using Cell information – reconstruct clusters of deposited energy to identify photons

  29. Hadronic point Identification of a Photon Shower. Isolation Photon-induced shower is smaller than quark/gluon shower both transversely and longitudinally

  30. g QCD jet misidentified as g Photon ID Tools (Monte Carlo Distributions) EM fraction ratio of EM cluster energy deposited in EM calorimeter and total energy Isolation (previous slide) measure of cluster narrowness multi-variable shower shape tool - layer energy fractions -width at shower maximum

  31. DØ Tracking System • Central Fiber Tracker • Silicon Microstrip Tracker (0,0,0) Silicon Tracker • Focus on Silicon Tracker

  32. North South 1/2-cylinder 6 Barrels 12 F-Disks and 4 H-Disks Silicon Tracker. Longitudinal View In z-coordinate large region has to be covered -- protons and antiprotons collide in bunches: interaction point is Gaussian-distributed about z=0 with  = 30 cm  Barrel/Disk Design:  50 cm

  33. Silicon Tracker. x-y View Barrel x-y view beam line a hit SMT Outer support structure a track a ladder

  34. Ladders Installed in Barrels barrel with ladders cabling cooling system outlets

  35. Selection of gg Candidate Events • Trigger: di-EM* high pT trigger • Offline:(on both objects) • Kinematic cuts: pT > 25GeV • Acceptance cuts: Central or End Cap Calorimeter up to ||=2.4 • Photon ID: -shower shape consistent with EM* shape (EMfraction, Isolation, H-matrix 2) - track veto • *EM = Electromagnetic Object (Photon or Electron)

  36. Event Displays of gg Candidate • 14 Mass = 125.8 GeV Topcolor hgg event is generally expected to look like this one

  37. Z/ge+e-with e+e- misidentified as photons (lost tracks) e.g. • QCD processesthat in the final state contain : 1. two photons 2. a photonanda hadronic jet misidentified as photon Major Backgrounds : Drell-Yan and QCD e.g. 3. two hadronic jets misidentified as photons e.g.

  38. Observed gg Events and Predicted Backgrounds Spring-Summer 2003(Ldt=52pb-1)

  39. (Ldt=52pb-1) Results. No B(hgg) limits yet  Fermiophobic Topcolor

  40. (Ldt  190pb-1) Results(end of last week !) Diphoton PT cut

  41. (Ldt 190pb-1) B(hgg) Limits (end of last week !)

  42. SM Higgs Search Strategy • Light Mass Region (M<~140 GeV) • Use qqW/Z+H(bb) • For ggH(bb) QCD background is very large ! • High Mass Region (M>~140 GeV) • Use inclusive production • Look for HWW

  43. Low Mass Region: (DØ) Study SM backgroundsto WH(We, Hbb) We+ two or more quark/gluon jets (no b-quark jet requirement) We+ two b-quark jets: Expect: 5.5 1.6 events Observe: 3 events Consistent with SM background

  44. Low Mass Region: WH(We(), Hbb) search at CDF • We()+ at least one b-tagged jet • use 162 pb-1 • Improved limits on the Cross Section  Branching Fraction over Run I but sensitivity of current search is still limited by statistics

  45. High Mass Region:Look for Excess in WW(ee,e,) (DØ) Missing Et in dimuon events (ee)

  46. Dielectron Mass in WW(ee) events (DØ) Dielectron Invariant Mass

  47. DØB(HWW) Limits (end of last week !)

  48. SUSY Higgs • Supersymmetry (SUSY) is a symmetry between spin degrees of freedom  any ordinary particle has a (much heavier) supersymmetric partner particle (to be discovered yet) • SUSY Higgs sector consists of more than one Higgs particle • e.g. Minimal Supersymmetric Model (MSSM) : • two complex scalar Higgs doublets • two VEV’s v1 and v2 (tan=v1/v2) • 5 Higgs particles : h0, H0, A0, H+, H-

  49. DØSearch for Neutral SUSY Higgs Bosons (h,A,H) • Production cross section ~ (tan)2 • High tan (>~30) models are motivated by Grand Unification Neutral Higgs Production can be enhanced • look for a signal in the invariant mass spectrum of the two jets with the highest transverse energy in triple b-tagged multi-jet events

  50. DØ Search for Neutral SUSY Higgs Bosons (Cont’d) Invariant mass spectrum for > = 4 jets (two b-tagged) . Backgrounds Higgs signal at the exclusion limit Invariant mass spectrum for >=3 jets (three b-tagged)

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