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High p T physics at the LHC Lecture IV Searches

This lecture covers various searches conducted at the LHC, including Higgs searches, SUSY, extra dimensions, and inclusive searches. The focus is on the ATLAS and CMS experiments.

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High p T physics at the LHC Lecture IV Searches

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  1. High pT physics at the LHC Lecture IVSearches Miriam Watson, JurajBracinik (University of Birmingham) Warwick Week, April 2011 LHC machine High PT experiments – Atlas and CMS Standard Model physics Searches M. Watson, Warwick week

  2. Introduction • Topics I will cover today: • Higgs searches • SUSY • Extra Dimensions • Inclusive searches • I will not cover • All the details of every search! • I will concentrate on ATLAS and CMS M. Watson, Warwick week

  3. Why we think a Higgs field exists • The SM is really two separate theories - QCD and GSW electroweak • We know that the electroweak piece must be broken • Separate EM and weak forces • Unified electroweak theory involves massless gauge bosons only • Short range of the weak interaction  gauge bosons mediating the weak force must be quite massive • Something has to break the electroweak symmetry and something has to give the W,Z mass • All the fermions that are massless Something has to give them mass as well M. Watson, Warwick week

  4. Electroweak Symmetry Breaking • The gauge group for the GSW theory is SU(2)L⊗U(1) • This must be a broken symmetry, but do not want to destroy gauge invariance of theory (SM) • We want to add a new field to the SM that will initially have SU(2)L⊗U(1) symmetry. When this symmetry is broken, the massless bosons become the massive W,Z and a massless photon • The addition of a single SU(2) doublet of complex scalar fields satisfies these requirements: M. Watson, Warwick week

  5. Higgs Potential Vacuum expectation value (vev) = 246 GeV • Distance from the centre describes the strength of the Higgs field • Height denotes the energy of a particular field configuration. • The zero-field configuration (centre) is unstable to small perturbations • system will fall into the lower energy state in the moat • lowest energy state of space (the vacuum) is not empty, but is permeated by the Higgs field • in the ground state there is no symmetry in the radial direction • As the universe fell into the ground state electroweak symmetry was “spontaneously” broken M. Watson, Warwick week

  6. Theoretical constraints on the Higgs Mass (non-perturbative) Λ = cut-off scale at which new physics becomes important • In order to confirm the existence of a Higgs field and the Higgs mechanism, we need to find a quantum of this field (Higgs boson) • Theoretical bounds on the allowed Higgs mass  a chimney around 180 GeV extending to the Planck scale • Additional constraints from “fine tuning” limits  new physics O(TeV) M. Watson, Warwick week

  7. Indirect limits from electroweak precision data Karl Jakobs, 2010 W mass and top quark mass are fundamental parameters of the Standard Model There are well defined relationships between mW, mt and mH M. Watson, Warwick week

  8. W and top mass measurements DMW/MW ~ 3.10-4 Measurements up to July 2010 DMt/Mt ~ 6.10-3 These measurements favour a light Higgs boson: MH=89 +35 -26 GeV (68% CL) LEP2 direct search MH > 114.4 GeV (95% CL) M. Watson, Warwick week

  9. Tevatron constraints on the Higgs Mass • Recent CDF and D0 combination excludes158 < MH < 173 GeV at 95% CL M. Watson, Warwick week

  10. Higgs processes at the LHC • The Higgs will be produced through a variety of processes at the LHC • Some dominate (gg fusion) • Others are rare (ttH) • If a Higgs exists, it will be produced at the LHC • Finding it is another matter M. Watson, Warwick week

  11. SM Higgs production cross-sections • Cross-sections O(100 pb) significant no. of Higgs will be produced by the LHC in a very short time (weeks/months) • It will take longer than that to claim a discovery • We have seen the relative cross-sections of Higgs and QCD/EW processes M. Watson, Warwick week

  12. Standard Model Higgs decays • For mH < 1 TeV, divide into low, intermediate and high mass regions • Decay modes change as a function of mH since the Higgs couples to mass and will decay to the heaviest particle(s) • Low mass: dominant decay mode (bb) is essentially useless due to overwhelming QCD backgrounds  concentrate on Hgg M. Watson, Warwick week

  13. Low mass Higgs: Hgg Simulation With good segmentation • Low branching ratio, but take advantage of the excellent photon resolution to see a narrow peak above continuum background • Need at least 10 fb-1 M. Watson, Warwick week

  14. Low mass Higgs: vector boson fusion Simulation • Tag two forward jets • Select Higgs bosons in the channel H→tt (tl or thad) • Decay products in central region, i.e. high pT • Make a collinear approximation (assume neutrinos in tau decays are in same direction as visible decay products) • Reconstruct Higgs mass  excess if sufficient luminosity M. Watson, Warwick week

  15. High mass Higgs: H 4 leptons CMS simulation • Finding a high mass Higgs is much easier • Both H→WW→lnln, and H→ZZ→4l are viable search modes (l = e, m) • Multi-lepton signatures are relatively easy to discern above background • Both are easier if bosons are on-shell (WW: mH > 160 GeV, ZZ: mH > 180 GeV) • H→ZZ→4l is considered to be the “golden mode” for Higgs searches • Low backgrounds (ZZ,Zbb,tt) M. Watson, Warwick week

  16. What has the LHC found so far? Close to SM sensitivity in HWWlnln (1.2 x SM) with 35 pb-1 HWWlnln HWWlnln 2010 Note different mH ranges on plots HZZllqq/llnn Hgg M. Watson, Warwick week

  17. Prospects for SM Higgs in 2011-12 Indicates contributions from different channels Could exclude down to LEP limit with <4fb-1 ! (possibly) M. Watson, Warwick week

  18. Higgs boson properties MH measurement dominated by ZZ4l and Hgg modes Eventual precision ~0.1% over large mass range • If the Higgs boson is discovered, want to measure its properties: • mass, width • spin, CP (SM predicts 0++) • coupling to other bosons and to fermions • self-coupling • … and check whether it is a SM Higgs, or if it is compatible with theories beyond the SM (e.g. SUSY) • in principle there could be more than one Higgs boson • perform direct searches for extra Higgs bosons M. Watson, Warwick week

  19. Need for a theory beyond the Standard Model SM appears to be a low-energy approximation of a fundamental theory De Santo, 2007 • Gravity is not included in the Standard Model • Hierarchy problem: • In order to avoid the significant fine-tuning required to cancel quadratic divergences of the Higgs mass, some new physics is required (below ~10 TeV) • Unification of gauge coupling constants M. Watson, Warwick week

  20. Supersymmetry Spin differs by ½ Identical gauge numbers Identical couplings • One favoured idea to solve the hierarchy problem is supersymmetry (SUSY) • Space-time symmetry between fermions and bosons • To make the SM lagrangiansupersymmetric requires each bosonic particle to have a fermionicsuperpartner and vice-versa • These contribute with opposite sign to the loop corrections to the Higgs mass providing cancellation of the divergent terms! M. Watson, Warwick week

  21. Supersymmetric particles Now have unification of gauge couplings: • Superpartners have not been observed! • Minimal Supersymmetric SM (MSSM): • Gauginos and higgsinos mix  2 charginos, 4 neutralinos • Two Higgs doublets  5 Higgs bosons (h,H; A, H±) M. Watson, Warwick week

  22. R-parity • SUSY allows for proton decay to occur via p → e+p0 • But proton decay experiments have established that tp > 1.6 x 1033 yrs • This can be prevented by introducing a new symmetry in the theory, called R-parity: • All SM particles have even R-parity (R = 1) • All SUSY particles have odd R-parity (R= -1) • R-parity conservation  proton cannot decay • Two consequences: • Lightest SUSY particle (LSP) is stable • Sparticles can only be pair-produced M. Watson, Warwick week

  23. The LSP and Dark Matter • The LSP would make a very good dark matter candidate: • Stable • Electrically neutral • Non-strongly interacting (weak and gravitational interactions only) • This is why many models are popular in which the LSP is the lightest neutralino, • Whenever SUSY particles are produced they always cascade down to the massive but stable LSP  Missing energy is the canonical SUSY signature M. Watson, Warwick week

  24. SUSY Phenomenology • Some possible constraints: • Impose boundary conditions at higher energy scale and evolve down to the weak scale via Renormalisation Group Equations (mSUGRA) • Constraints related to the way SUSY is broken (e.g. GMSB) • – we know it must be broken, because there are no sparticles with same mass as particles • There are a very large (>100) number of free parameters in the MSSM! • e.g. none of the masses are predicted • Impossible to make any phenomenological predictions without making further assumptions M. Watson, Warwick week

  25. mSUGRA LHC experiments have agreed to examine 13 points in mSUGRA space • 9 at low mass (LM1->LM9) • 4 at high mass (HM1->HM4) • Only five parameters: • m0 — universal scalar mass • m1/2 — universal gaugino mass • A0 — soft breaking parameter • tanβ — ratio of Higgs vevs • sgn(μ) — sign of SUSY mH term • Highly predictive – masses determined mainly by m0 and m1/2 • Useful framework to provide benchmark scenarios M. Watson, Warwick week

  26. Searches for SUSY • Signatures for SUSY: • Several high-pT jets; • High missing ET (R-conservation); • Possibly leptons and/or b-jets • LEP and the Tevatron have set the most stringent limits to date on sparticle masses. Roughly speaking these are: • m_sleptons/charginos > ~ 95 GeV • m_LSP(neutralino) > ~ 45 GeV • m_gluino > ~290 GeV • m_squark > ~375 GeV M. Watson, Warwick week

  27. Searching for SUSY at the LHC Expected limits with 100 pb-1 – 1 fb-1 • If any of the more common variants of SUSY do exist, the LHC will find it • Should be found relatively quickly in one or more modes • Plot is for multi-jets + missing ET M. Watson, Warwick week

  28. Example LHC Search Mode - Squark/Gluino Production De Santo • These particles are strongly produced and thus have cross-sections comparable to QCD processes (at the same mass scale) • Will produce an experimental signature of multi-jets + leptons + missing ET • A useful variable is the effective mass • Typicalselection: • njets ≥ 4, ET > 100,50,50,50 GeV • 2 leptons ET > 20 GeV, • MET >100 GeV M. Watson, Warwick week

  29. Examples of results Jets + MET+ b tagging 3 leptons + jets Some LHC SUSY limits are already similar to or better than TEVATRON M. Watson, Warwick week

  30. Measuring SUSY masses m(ll) / GeV M. Watson, Warwick week • If SUSY is found, how can the underlying model be disentangled? • Aim to map out the SUSY mass spectrum • One strategy is to measure the endpoint of cascade decays • Make as many such measurements as possible • Other combinations within this chain: m(lq), m(llq) • Different decay chains

  31. MSSM Higgs searches Mtt • There are five Higgs bosons in the MSSM: h0, H0, H±, A0 • In nearly all models, the lightest neutral SUSY Higgs needs to be light (mh < ~130 GeV) • The phenomenology is sensitive to SUSY parameters, e.g. tanβ • If tanβ is large, couplings to down-type fermions are enhanced and the role of b jets and t leptons become increasingly important • Production cross-sections are enhanced by (tanβ)2 • Event rates can be large M. Watson, Warwick week

  32. An alternative to SUSY – Extra Dimensions • The hierarchy problem: the weak force is much stronger than gravity (1/MPlanck:1/MEW ~ 10-17) • Supersymmetry gives one solution to this problem • Can also be addressed as a geometrical space-time phenomenon: • Our 3D space could be a 3D “membrane” embedded in a much larger extra dimensional space • Two examples of models: • ADD (Arkani-Hamed, Dimopoulos, Dvali) • RS (Randall-Sundrum) M. Watson, Warwick week

  33. “Large” Extra-Dimensions (ADD) MPl is a smaller number in ADD Hierarchy problem is solved Electroweak interactions have been probed down to 1/MEW ~ O(10-15 m) Gravitational interactions had only been studied to ~1 mm Gravity may diverge from Newton’s Law at small distances For r << R, gravity behaves as if it were 4+n dimensonal (field lines spread out uniformly throughout the bulk) and is stronger For r ≥ R gravitational field lines are deformed since they are confined to the 4 dimensions (represented by a 3-D cylinder in the picture) M. Watson, Warwick week

  34. Detecting ADD extra dimensions De Santo Missing transverse energy plus single jet Dedicated experiments have also measured consistency with Newtonian gravity to scales < 10-100 μm • Gravitons can escape into the extra dimensions and appear as missing energy at the LHC  Search for an overall excess of ETmiss Or an excess of monojet + ETmiss events M. Watson, Warwick week

  35. “Warped” Extra Dimensions (RS Model) ONE small, highly curved (“warped”) extra dimension connects the SM brane at O(TeV) to the Planck scale brane Gravity is weak on the “weak brane” where SM fields are confined but increases in strength exponentially in the extra dimension (since space-time is accordingly “warped”) Signature: a series of narrow, high-mass resonances M. Watson, Warwick week

  36. Extra Dimensions in the gg channel R = compactification radius, k = curvature, coupling defined by k/MPL M. Watson, Warwick week

  37. Micro Black Holes MPl is the energy scale at which gravitational interactions become important We normally assume this scale is 1019 GeV and we completely ignore the gravitational interaction of the colliding particles But if, due to extra-dimensions, MPl ~ MEW then gravitational interactions will be important In fact, at length scales below 1/MPl, gravity will dominate, and a micro-black hole will form M. Watson, Warwick week

  38. Micro Black Hole signature ST is the scalar sum of the ET of the N individual objects (jets, electrons, photons, and muons) Excludes the production of black holes with minimum mass of 3.5 -4.5 TeV These micro black holes will rapidly evaporate via Hawking radiation and will radiate like a “black body” Democratic decays to all sorts of particle at the same time M. Watson, Warwick week

  39. Inclusive searches: di-jets Very early search for numerous non-SM resonances: string resonance, excited quarks, axi-gluons, colorons, E6 diquarks, W’ & Z’, RS gravitons.... M. Watson, Warwick week

  40. Di-jet centrality and angular distributions Excludes quark compositeness for Λ<4.0TeV (95%CL) Lower limit on scale of contact interaction Λ=5.6 TeV (95% CL) Di-jet centrality ratio: evts with two leading jets in |η|<0.7 compared to events with both leading jets in 0.7<|η|<1.3 Sensitive to deviations from the SM due to quark sub-structure, i.e. Compositeness Angular distribution sensitive to contact interactions M. Watson, Warwick week

  41. Inclusive searches: dileptons • Study invariant mass spectrum to look for dilepton resonances (Z') • Also • String-theory-inspired E6 models • ADD extra dimensions M. Watson, Warwick week

  42. Inclusive searches: leptons+MET W’ MW’>1.56TeV Example: W’ search W’ has W-like fermionic couplings W’ does not couple to other gauge bosons Tevatron limits: mW’ > 1.1TeV M. Watson, Warwick week

  43. Leptoquarks LQ LQ LQ LQ Leptoquarks possess both lepton and quark quantum numbers Pair produced: search for qqll or qqlν daughters Look at sum of transverse energy: M. Watson, Warwick week

  44. Other models • There are many other exotic possibilities... • Stopped gluinos • Split SUSY models • Hidden sectors • ..... • It would be impossible to cover all of these in one lecture (and too confusing!) → Please go and find out more! → Or, better still, find a particle... M. Watson, Warwick week

  45. Summary • With ~40 pb-1the LHC experiments have begun detailed measurements of Standard Model physics • The SM processes give a solid basis for understanding the detectors and the “background” to searches at higher mass and high ET • Numerous analyses are in place for searches • With 1-5 fb-1 in 2011-12 we could have • A firm discovery of the Higgs • Indications of SUSY • New resonances • Other new physics • And we could find something completely unexpected! M. Watson, Warwick week

  46. Additional material (and acknowledgements) • Last year’s lectures: • http://www2.warwick.ac.uk/fac/sci/physics/staff/academic/gershon/gradteaching/warwickweek/material/lhcphysics • CERN Academic Training lectures (Sphicas and Jakobs): • http://indico.cern.ch/conferenceDisplay.py?confId=124047 • http://indico.cern.ch/conferenceDisplay.py?confId=77835 • London lectures (de Santo et al.): • http://www.hep.ucl.ac.uk/~mw/Post_Grads/2007-8/Welcome.html • ATLAS and CMS public results: • https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome • Moriond Electroweak and QCD: • http://indico.in2p3.fr/conferenceOtherViews.py?view=standard&confId=4403 • http://moriond.in2p3.fr/QCD/2011/MorQCD11Prog.html M. Watson, Warwick week

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