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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.

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Presentation on theme: "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."— Presentation transcript:

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 of Electroweak 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

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: Which is invariant under the local U(1) gauge transformation But this isn’t invariant under gauge transformation (*) so is not allowed Adding a gauge boson mass term could be attempted with a term like: where (*) Instead add a complex scalar field which couples to the gauge boson

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

7 Higgs field “eats Goldstone boson” Flat direction in potential usually represents zero-mass particle –“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 Gauge boson Example of a Feynman diagram showing a contribution to the gauge boson 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 ±, Z 0, mass terms leaving a single scalar for the physical Higgs boson. For full SU(2) treatment see e.g. Halzen & Martin section 14.9 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 ±, Z 0, mass terms leaving a single scalar for the physical Higgs boson. For full SU(2) treatment see e.g. Halzen & Martin section 14.9 φ'φ'

8 Constraints on the Higgs mass Higgs boson mass is the remaining unpredicted parameter in Standard Model: Higgs self-couplings not predicted So Higgs mass not predicted by Electroweak theory However there are: 1. Bounds from theory: Perterbative unitarity of boson-boson scattering 2. Indirect bounds Loop effects on gauge boson masses 3. Direct bounds Searches

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

10 Indirect Higgs bounds: LEP Electroweak data W (and Z) mass depends on m Higgs –Logarithmic loop corrections to masses –Also depends on top mass W (and Z) mass depends on m Higgs –Logarithmic loop corrections to masses –Also depends on top mass Measurements Prediction as a function of m H

11 Direct bounds: Higgs LEP No discovery Direct lower bound at GeV Phys.Lett. B565 (2003) Higgsstrahlung – dominant production ALEPH: Candidate vertex:

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: GeV < m SM Higgs < 199 GeV

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14 The Large Hadron Collider 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 Proton on Proton at √s = 14 TeV Design luminsoity ~ ~100 fb -1 / expt / year

15 General Purpose Detectors ATLAS Similarities: 1.Tracker 2.Calorimeter 3.Muon chambers Differences: Size : CMS “compact” Magnetic-field configuration ATLAS has muon toroids Electromagnetic-Calorimeter: CMS crystals. ATLAS Liquid Argon Outer tracker technology CMS all-silicon. ATLAS straw tubes

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

17 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

18 LHCb Asymmetric detector for B-meson physics For more information see Lazzeroni talk at:

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

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

21 Signals for QGP: –Jet quenching –Quarkonim (e.g. J/ψ) suppression (“melt bound states”) ALICE 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 QGP Jet No Jet J/ψ c c _

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

24 Producing a Higgs Higgs couplings  mass –u-ubar  H has very small cross-section –Dominant production via vertices coupling Higgs to heavy quarks or W/Z bosons Higgs couplings  mass –u-ubar  H has very small cross-section –Dominant production via vertices coupling Higgs to heavy quarks or W/Z bosons

25 Production cross-sections

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

27 Needle in a haystack… Higgs production QCD jet production at high energy Need to use signatures with small backgrounds: - Leptons - High-mass resonances - Heavy quarks to avoid being overwhelmed Need to use signatures with small backgrounds: - Leptons - High-mass resonances - Heavy quarks to avoid being overwhelmed

28 Example 1 : H  ZZ Only works when m Higgs >~ 2.M Z When the Z decays to leptons there are small backgrounds q q _ H Z Z e+ e- e+ e-

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

30 H  ZZ  e+e- e+e- Plot shows simulated distributions of [invariant mass of four electrons] for 3 different values of m Higgs (We wouldn’t see all of these together!) q q _ H Z Z e+ e- e+ e- 1.Find events consistent with above topology (four electrons) 2.Add together the four electron 4-vectors 3.Find the mass of the resultant 4-vector (  mass of the Higgs) m H =130 m H =170 m H =150 background

31 Example (2): H  γγ No direct coupling of H to photon However allowed at loop level Branching ratio: ~ (at low m Higgs ) Important at low mass Actually a very clean way of looking for Higgs –Small backgrounds Production and decay of Higgs through ‘forbidden’ direct couplings

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

33 H  γγ Simulation by CMS for different Higgs masses for early LHC data (1 fb -1 ) Higgs signal scaled up by factor 10! Invariant mass of the pair of photons

34 H  γγ … backgrounds “Irreducible” 2 real photons “Reducible” e.g. fake photons γ gluon q q _ π0π0 γ γ Need v. good calorimeter segmentation to separate these “Born” “Box”

35 Recent find – vector boson fusion Smaller production cross-section –Weak couplings But low backgrounds Tagging Jet No QCD COLOUR exchange between q and qbar So no QCD irradiation into central part of detector

36 Significance H->ZZ Significance is a measure of the answer to the question “What is the probability that a background fluctuation would produce what I am seeing” 5-  means “probability that background fluctuation does this is less than 2.85·10 -7 ” 5-  is usually taken as benchmark for “discovery”

37 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!

38 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

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40 What is supersymmetry? 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

41 Extended higgs sector 2 cplx doublets  8-3 = 5 Higgs bosons! (S)Particles Standard Model Supersymmetric partners quarks (L&R) leptons (L&R) neutrinos (L&?) squarks (L&R) sleptons (L&R) sneutrinos (L&?)  Z 0 W ± gluon BW0BW0 h0H0A0H±h0H0A0H± H0H±H0H± 4 x neutralino 2 x chargino After Mixing gluino Spin-1/2 Spin-1 Spin-0 Spin-1/2 Spin-0 Bino Wino 0 Wino ± gluino ~ ~ (Higgsinos)

42 Why Supersymmetry? Higgs mass –Quantum corrections to m H –Would make “natural” mass near cut-off (Unification or Planck scale) –But we know m H <~ 1 TeV –m H = m H bare +  m H –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 –m H now natrually at electroweak scale top Δm 2 (h)  Λ 2 cutoff higgs λλ stop higgs λλ Quantum correction to m Higgs Cancelling correction to m Higgs

43 Further advantages 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” SM +SUSY Log 10 (μ / GeV) miss Hit! 1/α Big Bang relic abundance calculations are in good agreement with WMAP microwave background observations in regions of SUSY parameter space

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

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

46 Sparticle Interactions Interactions & couplings same as SM partners 2 SUSY legs for R P conservation Largely partner of W 0 boson Q: Does the gluino couple to: the quark? the slepton? the photino? Q: Does the gluino couple to: the quark? the slepton? the photino?

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

49 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 Invisible particles

50 Standard Model backgrounds: measure from LHC DATA Example: background to “4 jets + missing energy” –Measure background in control region –Extrapolate to signal region –Look for excess in signal region   Measure in Z -> μμ Use in Z -> νν R: Z  B: Estimated R: Z  B: Estimated μ With SUSY Missing P T / GeV

51 Constraining SUSY masses Mass constraints Invariant masses in pairs –Missing energy –Kinematic edges Observable:Depends on: Limits depend on angles between sparticle decays Frequently- studied decay chain

52 Mass determination Measure edges Variety of edges/variables Try various masses in equations C.G. Lester Narrow bands in ΔM Wider in mass scale Improve using cross- section information These measurements can tell us about SUSY breaking

53 Other things to do with SUSY Measure the sparticle spins –“prove” that it is really supersymmetric partners we are seeing Measuring the couplings & mixings –Use to “predict” Dark Matter relic density Find the extra Higgs bosons –Recall that SUSY predicts 5 Higgs bosons –Now we want to find H 0, h 0, A 0, H ± –Also measure their couplings, CP, …

54 Standard Model Physics The ATLAS and CMS experiments also potentially can measure: –Top mass –W mass –Rare B-meson decay rates –Jet physics To much higher precision that is currently achievable –Large number of e.g. top quarks produced –Small statistical errors –Systematic errors (such as jet energy scale determination) limiting Mass of hadronic top

55 Other things to look for… Leptoquarks –Motivated by Grand Unified Theories –Carry lepton and baryon number –E.g. LQ  bμ New heavy quarks –Predicted by some non-SM Higgs theories New heavy gauge bosons –Indications of new symmetry groups Extra dimensions –Large variety of models on the market!

56 Extra dimensions models Motivated by need for ED in string theory and m-theory –Logical a possibility for a LHC discovery Different models… –Which particles are localised where (bulk/brane) –Form of space-time metric (flat/warped) –Geometry and size of extra dimensions …make different predictions –Kalazua-Klein resonances of SM particles –Graviton states –Stringy resonances –Effects of strong gravity (micro Black Holes) –Energy loss into extra dimensions More information:

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58 General sources Higgs at the LHC: talk by Zeppenfeld Physics at the LHC: Higgs talk by Harlander: ATLAS physics Technical Design Report (TDR) ccess.html (1999) ccess.html CMS physics Technical Design Report (TDR) (2006) Supersymmetry:

59 Constraints on m Higgs Scale at which new physics enters Unstable vacuum No perturbative unitarity

60 Producing a LHC Higgs couplings  mass –Direct e.g. u-ubar  H very small cross-section Dominant production via vertices coupling Higgs to heavy quarks or W/Z bosons Higgs couplings  mass –Direct e.g. u-ubar  H very small cross-section Dominant production via vertices coupling Higgs to heavy quarks or W/Z bosons top H g g W/Z H q q _ top H g g W/Z H q q _

61 Higgs’ mechanism Add a complex scalar field –In fact he adds 2 real scalar fields, (fermion part of L now ignored) This is gauge invariant when the scalars have covariant derivatives: Now if the potential, V, has a degenerate minimum at φ≠0 we get interesting consequences… N.B. scalar field must couple to gauge field like this for the Higgs mechanism to work

62 mSUGRA – “super gravity” A.K.A. cMSSM Gravity mediated SUSY breaking –Flavour-blind (no FCNCs) Strong expt. limits –Unification at high scales Reduce SUSY parameter space –Common scalar mass M 0 squarks, sleptons –Common fermionic mass M ½ Gauginos –Common trilinear couplings A 0 Susy equivalent of Yukawas Programs include e.g. ISASUSY, SOFTSUSY GeV EW scale Iterate using Renormalisation Group Equations Unification of couplings Correct M Z, M W, …

63 Other suggestions for SUSY breaking Gauge mediation –Gauge (SM) fields in extra dimensions mediate SUSY breaking Automatic diagonal couplings  no EWSB –No direct gravitino mass until M pl Lightest SUSY particle is gravitino Next-to-lightest can be long-lived (e.g. stau or neutralino) Anomaly mediation –Sequestered sector (via extra dimension) Loop diagram in scalar part of graviton mediates SUSY breaking Dominates in absence of direct couplings –Leads to SUSY breaking  RGE β-functions Neutral Wino LSP Charged Wino near-degenerate with LSP  lifetime Interesting track signatures Not exhaustive!

64 Producing exotics? Time standard exotic Time standard exotic Time standard exotics Time standard exotics If exotics can be produced singly they can decay –No good for Dark Matter candidate If they can only be pair- produced they are stable –Only disappear on collision (rare) Require an even number of exotic legs to/from blobs (Conserved multiplicative quantum number) If we want a good dark matter candidate Require an even number of exotic legs to/from blobs (Conserved multiplicative quantum number) If we want a good dark matter candidate No R P With R P

65 How do they then behave? Events build from blobs with 2 “exotic legs” A pair of cascade decays results Complicated end result Events build from blobs with 2 “exotic legs” A pair of cascade decays results Complicated end result Time standard 2 exotics Production part Time standard heavy exotic lighter exotic Decay part Time Complete “event” = exotic = standard


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