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LHC Physics

LHC Physics. Alan Barr UCL. This morning’s stuff…. Higgs – why we expect it, how to look for it, …. Supersymmetry – similar questions!. Smorgasbord of other LHC physics. Physics at TeV-scale. Dominated by the physics of Electroweak Symmetry Breaking Answering the question:

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LHC Physics

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  1. LHC Physics Alan Barr UCL

  2. This morning’s stuff… Higgs – why we expect it, how to look for it, … Supersymmetry – similar questions! Smorgasbord of other LHC physics

  3. Physics at TeV-scale • Dominated by the physics ofElectroweak Symmetry Breaking • Answering the question: • “Why do the W and Z bosons have mass?” • Standard Model suggests: Higgs mechanism • However Higgs boson predicted by SM not yet observed

  4. Higgs mechanism - history • 1964 Demonstration that a scalar field with appropriate interactions can give mass to gauge bosons • Peter Higgs (Edinburgh, previously UCL) • Independently discovered by Francois Englert and Robert Brout (Brussels) • Not until 1979 that Salam, Weinberg and Glashow use this in a theory of electroweak symmetry breaking • For a biographic article on P. Higgs see http://physicsweb.org/articles/world/17/7/6

  5. Higgs mechanism: why needed? • Example of P. Higgs – give mass to a U(1) boson (heavy “photon” in a QED-like theory) Start with QED Lagrangian: where Which is invariant under the local U(1) gauge transformation (*) Adding a gauge boson mass term could be attempted with a term like: But this isn’t invariant under gauge transformation (*) so is not allowed Instead add a complex scalar field which couples to the gauge boson

  6. Pictorial representation Excitations in this direction produce physical Higgs boson Quartic term self-couplingpositive Excitations in this direction = gauge transformation- Globaltransformationsunobserved- Local transformations give mass to gauge bosons Quadratic coupling termnegative Degenerate minimumVacuum (field strength≠0) If you don’t understand this, study Phys.Lett.12:132-133,1964 Scalar field strength = 0

  7. Higgs field “eats Goldstone boson” • Flat direction in potential usually represents zero-massparticle • “Goldstone boson” • But in Higgs theory this direction is coupled to the gauge boson • No massless Goldstone boson • Instead mass term generated for gauge boson φ φ φ' Gauge boson Example of a Feynmandiagram showing a contribution to the gaugeboson mass term N.B. Our example here was for a single complex scalar and for a U(1) field. In the Standard Model the Higgs is an electroweak SU(2) doublet field, with 4 degrees of freedom. 3 of these are ‘eaten’ by W±, Z0, mass terms leaving a single scalar for the physical Higgs boson. For full SU(2) treatment see e.g. Halzen & Martin section 14.9

  8. Constraints on the Higgs mass • Higgs boson mass is the remaining unpredicted parameter in Standard Model: • Higgs self-couplings not predicted • So Higgs massnot predicted by Electroweak theory • However there are: • Bounds from theory: • Perterbative unitarity of boson-boson scattering • Indirect bounds • Loop effects on gauge boson masses • Direct bounds • Searches

  9. Perturbative limit Vector Boson scattering Without other new physics the Higgs boson must exist & have mass < 1 TeV Phys.Rev.D16:1519,1977 Halzen & Martin section 15.6

  10. W (and Z) mass depends on mHiggs Logarithmic loop corrections to masses Also depends on top mass Indirect Higgs bounds: LEP Electroweak data Measurements Prediction as a function of mH http://lepewwg.web.cern.ch/LEPEWWG/

  11. Direct bounds:Higgs searches @ LEP • No discovery • Direct lower bound at 114.4 GeV Higgsstrahlung – dominant production ALEPH:Candidate vertex: Phys.Lett. B565 (2003) 61-75

  12. Higgs-Hunter Situation Report • Something very much like the Higgs must exist with:~100 GeV < m < ~1 TeV • No discovery as yet • If it is a Standard Model Higgs the constraints are tighter:114.4 GeV < mSM Higgs <199 GeV

  13. Large 27 km circumference Built in LEP tunnel Hadron Mostly protons Can also collide ions Collider ~ 7 x higher collision energy ~ 100 x increase in luminosity Compared to Tevatron The Large Hadron Collider Proton on Protonat √s = 14 TeVDesign luminsoity ~ ~100 fb-1 / expt / year

  14. General Purpose Detectors • Similarities: • Tracker • Calorimeter • Muon chambers ATLAS Differences:Size : CMS “compact” Magnetic-field configurationATLAS has muon toroidsElectromagnetic-Calorimeter: CMS crystals. ATLAS Liquid ArgonOuter tracker technology CMS all-silicon. ATLAS straw tubes

  15. y Definitions φ x Particle η = 0 η = -1 η = +1 η = -2 η = +2 η = -3 η = +3 θ Beam pipe proton proton z Endcap“Forward” Barrel“Central” Endcap“Forward” Differences in rapidity are conservedunder Lorentz boosts in the z-direction * Rapidity: Good approximation to rapidity if E>>m Pseudorapidity: * pT = (px, py) *prove these! “Transverse” |pT| = √(px2, py2)

  16. Making particles in hadron colliders • Hadron-Hadron collisions complicated • See lectures by Mark Lancaster(“Hadron Collider Physics”) • QCD  Lots of background events with jets • QCD  Lots of hadronic “rubbish” in signal events • Hard scatters are largely from q-qbar or glue-glue • Proton structure is important – See lectures by Robert Thorne • But they provide the highest energies available • Often these are the discovery machines proton proton

  17. LHCb • Asymmetric detector for B-meson physics For more information see Lazzeroni talk at: http://indico.cern.ch/conferenceDisplay.py?confId=5426

  18. LHCb Physics Quark flavour e-states are not the same as mass e-states: mixing: • VCKM must be unitary: V.V† = V †.V = 1 • Multiply out rows & columns: Do this!

  19. LHCb Physics • Measurements of decay rates and kinematics tell us about squark mixings • Over-constraining triangles gives sensitivity to new physics through loop effects

  20. Signals for QGP: Jet quenching Quarkonim (e.g. J/ψ) suppression(“melt bound states”) Designed to examine collisions of heavy ions(e.g. lead-lead or gold-gold) Theorised to produce a new state of matter – a quark-gluon plasma Quarks no longer confined inside colourless baryons ALICE QGP Jet No Jet c _ J/ψ c

  21. Couplings of the SM Higgs • Couplings proportional to mass • What does this mean for the Higgs-hunter?

  22. Producing a Higgs • Higgs couplings  mass • u-ubar  Hhas very small cross-section • Dominant production via vertices coupling Higgs to heavy quarks or W/Z bosons

  23. Production cross-sections

  24. Decay of the SM Higgs • Width becomes large as WW mode opens • Branching ratios change rapidly as new channels become kinematically accessible

  25. Needle in a haystack… QCD jet productionat high energy Higgs production • Need to use signatures with small backgrounds: • Leptons • High-mass resonances • Heavy quarksto avoid being overwhelmed

  26. e+ e- Z q _ H q e+ Z e- Example 1 : H  ZZ • Only works when mHiggs >~ 2.MZ • When the Z decays to leptons there are small backgrounds

  27. H  ZZ CMS H  ZZ  e+e- e+e- Electrons have track (green ) & energy deposit (pink)

  28. e+ e- Z q _ H q e+ Z e- H  ZZ  e+e- e+e- mH=150 background mH=130 mH=170 • Find events consistent with above topology(four electrons) • Add together the fourelectron 4-vectors • Find the mass of the resultant4-vector ( mass of the Higgs) Plot shows simulated distributions of [invariant mass of four electrons] for 3 different values of mHiggs(We wouldn’t see all of these together!)

  29. Example (2): H  γγ • No direct coupling of H to photon • However allowed at loop level • Branching ratio: ~ 10 -3(at low mHiggs) • Important at low mass • Actually a very clean way of looking for Higgs • Small backgrounds Production and decay of Higgsthrough ‘forbidden’ direct couplings

  30. γ γ H γγ CMS simulation. Physics TDR, 2006

  31. H  γγ • Simulation by CMS for different Higgs massesfor early LHC data (1 fb-1) Higgs signalscaled up by factor 10! Invariant mass of the pair of photons

  32. _ q H  γγ … backgrounds “Irreducible”2 real photons “Born” “Box” “Reducible” e.g. fake photons γ q Need v. good calorimetersegmentationto separate these γ γ π0 gluon

  33. Significance Significance is a measureof the answer to the question“What is the probabilitythat a backgroundfluctuation would producewhat I am seeing” H->ZZ 5- means “probabilitythat backgroundfluctuation does this is less than 2.85·10-7 ” 5- is usually takenas benchmarkfor “discovery”

  34. After discovery of Higgs? • Measure Higgs mass • The remaining unconstrained parameter of the Standard Model • Measure Higgs couplings to fermions and vector bosons • All predicted by Standard Model • Check Higgs mechanism • Couplings very important since there may be more than one Higgs boson • Theories beyond the Standard Model (such as Supersymmetry) predict multiple Higgs bosons. • In such models the couplings would be modified • Do direct searches for further Higgs bosons!

  35. If no Higgs found? • Arguably more exciting than finding Higgs • Look at WW scattering process • Look for whatever is “fixing” the cross-section • E.g. exotic resonances

  36. Nature permits only particular types of symmetry: Space & time Lorentz transforms Rotations and translations Gauge symmetry Such as Standard Model force symmetries SU(3)c x SU(2)L x U(1) Supersymmetry Anti-commuting (Fermionic) generators Changes Fermions into Bosons and vice-versa Consequences? Supersymmetric theory has a Boson for every Fermion and vice-versa Doubles the particle content Partners to Standard Model particles not yet observed Examples of Supersymmetric partner-states What is supersymmetry?

  37. (S)Particles StandardModel Supersymmetricpartners quarks (L&R)leptons (L&R) neutrinos (L&?) squarks (L&R)sleptons (L&R)sneutrinos (L&?) Spin-1/2 Spin-0 AfterMixing Z0W± gluon BinoWino0Wino± gluino BW0 Spin-1 4 x neutralino Spin-1/2 gluino ~ h0 H0 A0 H± H0H± ~ 2 x chargino Spin-0 (Higgsinos) Extended higgs sector 2 cplx doublets  8-3 = 5 Higgs bosons!

  38. Higgs mass Quantum corrections to mH Would make “natural” mass near cut-off (Unification or Planck scale) But we know mH <~ 1 TeV mH = mH bare + DmH Severe fine tuning required between two very big numbers Enter Supersymmetry (SUSY) Scalar partner of quarks also provide quantum corrections Factor of -1 from Feynman rules Same coupling, λ Quadratic corrections cancel mH now natrually at electroweak scale stop top λ λ λ λ higgs higgs higgs higgs Δm2(h) Λ2cutoff Why Supersymmetry? Quantum correction to mHiggs Cancelling correction to mHiggs

  39. Further advantages Big Bang relic abundance calculations are in good agreement with WMAP microwave background observations in regions of SUSY parameter space • Lightest SUSY particle is: • Light • Weakly interacting • Stable • Massive • Good dark matter candidate • Predicts gauge unification • Extra particles modify running of couplings • Step towards “higher things” 1/α 1/α +SUSY miss Hit! SM Log10 (μ / GeV) Log10 (μ / GeV)

  40. R-parity • Multiplicative discrete quantum number • RP= (-1)2s+3B+L • S=spin, B=baryon number, L=lepton number • Standard Model particles have RP = +1 • SUSY Model particles have RP = -1 • If RP is conserved then SUSY particles must be pair-produced • If RP is conserved then the Lightest Supersymmetric Particle (LSP) is stable Example of a Feynmandiagram for proton decaywhich is allowed if the RP-violating couplings (λ) are not zero

  41. How is SUSY broken? Weakcoupling (mediation) • Direct breaking in visible sector not possible • Would require squarks/sleptons with mass < mSM • Not observed! • Must be strongly broken “elsewhere” and then mediated • Soft breaking terms enter in visible sector • (>100 parameters) Strongly broken sector Soft SUSY-breaking termsenter lagrangianin visible sector Various models offer different mediation e.g.Gauge  “GMSB”Gravity  “mSUGRA” (supergravity) Anomaly  “AMSB”

  42. Sparticle Interactions • Interactions & couplings same as SM partners • 2 SUSY legs for RP conservation Largely partnerof W0 boson Largely partnerof W0 boson Q: Does the gluino couple to: the quark? the slepton?the photino?

  43. General features Mass/GeV Production dominatedby squarks and gluinos • Complicated cascade decays • Many intermediates • Typical signal • Jets • Squarks and Gluinos • Leptons • Sleptons and weak gauginos • Missing energy • Undetected Lightest Susy Particle “typical” susy spectrum(mSUGRA)

  44. Invisibleparticles The “real thing”(a simulation of…) • Two high-energy jets of particles • Visible decay products • “Missing” momentum • From two invisible particles • these are the invisible Dark Matter guys Proton beams perpendicular to screen

  45. Example: backgroundto “4 jets + missing energy” Measure background in control region Extrapolate to signal region Look for excess in signal region m m n n Standard Model backgrounds: measure from LHC DATA μμ With SUSY • Measure in Z -> μμ • Use in Z -> νν R: Z -> nn B: Estimated Missing PT / GeV

  46. Constraining SUSY masses Frequently- studieddecay chain • Mass constraints • Invariant masses in pairs • Missing energy • Kinematic edges Observable: Depends on: Limits depend on angles betweensparticle decays

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