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Status of EW symmetry breaking

Status of EW symmetry breaking. Introduction The SM Higgs boson Other scenarios Conclusions. 1)The Standard Model of particle physics : the ingredients. 12 elementary constituents. 6 leptons. 6 quarks. d. e -.  e. u.  -.  . c. s.  .  -. t. b. Fundamental interactions.

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Status of EW symmetry breaking

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  1. Status of EW symmetry breaking Introduction The SM Higgs boson Other scenarios Conclusions 17 April 2002

  2. 1)The Standard Model of particle physics : the ingredients 12 elementary constituents 6 leptons 6 quarks d e- e u -  c s  - t b Four seas conference

  3. Fundamental interactions • 3 interactions propagated by intermediatebosons of spin 1:  (massless) electromagnetic interaction W et Z (massive) weak interaction 8 gluons (massless) strong interaction one example : • The  and Z fields are linear combinations of two vector fields which do not know the difference between the em and weak interactions : before mass generation  that is, at high energy ElectroWeak symmetry Four seas conference

  4. EW symmetry breaking • is assumed to be spontaneous i.e. due to a non-zero v.e.v. • is therefore responsible for the generation of the particle masses : M = 0 MZ ~ 91 GeV MW ~ 80 GeV confirmed when discovering the W,Z at CERN in the 80’s • what is the exact mechanism of the breaking ? the SM minimal solution: Higgs mechanism with one doublet of scalar fields with a non-zero v.e.v. Pending question : Four seas conference

  5. EW symmetry breaking • the SM minimal solution: Higgs mechanism with one doublet of scalar fields acquiring a non-zero v.e.v.  one Higgs boson  all properties predicted its mass, which is poorly constrained by theory: 0  MH 1000 GeV  search ALL experimental clues to such a Higgs boson • Other more complicated scenarios exist, too … BUT Four seas conference

  6. 2) Status of the search for the SM Higgs boson Over the past decade, the search strategy was twofold: • Direct search for a Higgs boson actually produced in collisions: LEP sensitivity to low masses • Indirect constraints from precise EW measurements sensitive to the quantum corrections due to loops with the Higgs boson: LEP, SLC, TeVatron sensitivity to low and high masses see W.Adam’s talk Four seas conference

  7. Direct search at LEP: the environment • Initial S/B ratios: at LEP 1 : MH = 10 GeV: 10-3 MH = 60 GeV: 10-5 at LEP 2 : MH = 60 GeV: 10-2 MH = 115 GeV: 10-3 • LEP 1 result: • Before LEP: MH > 100 MeV (95%CL) MH > 60 GeV (95% CL) Four seas conference

  8. Direct search at LEP: experimental signature • LEP provided the ideal experimental environment to search for a light Higgs boson, i.e. with mass MH S – MZ : the dominant production process: main decay is H bb (80% at 100 GeV)  clean signature Z boson easy to tag: mass, decays: Z hadrons 70% Z charged leptons 10% Z neutrinos 20% Four seas conference

  9. Tools for the direct search at LEP: b-tagging After full alignement, hit precisions are : • ~10 m in R • ~15 m in Rz in the central part of the detector. Step 1: a silicon vertex detector Ex: DELPHI ~3 double-sided layers ~0.5 X0 Four seas conference

  10. SV PV IP Tools for the direct search at LEP: b-tagging Step 2: impact parameters • B hadron lifetimes <>= 1.6 ps  flight distances ~3 mm  impact parameters ~c  ~400 m • Experimental resolutions: IP/ (= significances) as basic inputs of b-tagging R:  = 20  60/p sin3/2m Rz:  = 39  71/p m ( ~90o) Four seas conference

  11. Tools for the direct search at LEP: b-tagging Step 2: secondary vertices • B hadron decays lead to tracks originating from secondary vertices  information fromreconstructed SV add more discrimination between b quarks and other flavours  more powerful b-tagging: e.g. SV masses c quarks b quarks Four seas conference

  12. Tools for the direct search at LEP: b-tagging Step 3: tuning and control of performance • Simulated IP distributions and resolutions tuned on data.  data/simulation agree within 5% before tuning after tuning R significances Four seas conference

  13. Tools for the direct search at LEP: b-tagging Step 3: control of performance • b-tagging performance checked on control samples: Z data data/simulation agree within 5% Z data b-tagging variable Four seas conference

  14. Tools: multidimensional analyses and kinematic fits L3 Hqq channel • To reach the best S/B discrimination: multidimensional analyses (NN, likelihood …) H channel likelihood • To improve on signal mass reconstructions: kinematic fits with E,p conservation and the Z mass constraint Four seas conference

  15. Tools… : statistical interpretation of the results • To make an unbiased and powerful statistical analysis of the search results: • stop selection of signal-like events at a loose level • test compatibility of data with B-only and S+B hypotheses  rates and 2d pdf’s (H mass vs a second variable such as b-tagging, NN …) e.g. Hqq channel, S = 206.5 GeV, DELPHI • Tools : likelihood ratio test-statistics (-2lnQ) and confidence levels (CLs,CLb) Four seas conference

  16. Direct search for the SM Higgs boson at LEP: results • The HZ production cross-section rises fast once the kinematic threshold is crossed  a few pb-1 are enough to test a given MH hypothesis as soon as SMH+MZ e.g. july 2000: hypothesis MH=110 GeV is excluded  LEP 2 result : MH > 114.1 GeV (95% CL) Four seas conference

  17. Direct search for the SM Higgs boson at LEP: results expected behaviour from background only(mean, ±1 and ±2 bands) • A possible signal ? data: consistent with a signal of mass: MH = 115.6 ± 0.8 GeV expected behaviour from a a 115 GeV signal + bkg likelihood ratio test-statistics mass hypothesis compatibility with the hypothesis of a background fluctuation: 3.4% compatibility with the hypothesis of a 115.6 GeV signal: 44.% a handful of events makes most of the effect Four seas conference

  18. One event consistent with the SM Higgs boson production 4 jets of particles 2 b-jets Final state: e+e- HZ qq bb Comparing signal and background probabilities: ln(1+s/b) = 1.73 Reconstructed mass: MH = 114.3  3 GeV Four seas conference

  19. Summary about the SM Higgs boson: EW precise measurements: (LEP,SLC,TeVatron..) (95% CL) Direct searches (LEP): (95% CL) MH  196 GeV MH 114.1 GeV MH = 115.6 GeV ? Four seas conference

  20. 3) Other more complicated scenarios One Higgs boson with non-standard properties: • Same decays but different cross-section: • Non-b hadronic decays and different cross-section: 114.1 GeV MH105 GeV: BR /SM 20% 112.9 GeV MH105 GeV: BR /SM 30% Four seas conference

  21. One Higgs boson with non-standard properties: 114.4 GeV • Invisible decays and different cross-section: • Photonic decays and different cross-section: MH105 GeV: BR /SM 25% 115. GeV MH105 GeV: BR /SM 5% Four seas conference

  22. More Higgs bosons: h, A, H, H+, H- from SUperSYmmetry • two representative scenarios: results in the (mh, tan) plane e+e- h A ratio of the two Higgs doublet v.e.v.’s e+e- h Z Mh 91.5 GeV, MA 92.2 GeV (95% CL) 0.7  tan  10.5 excluded (95% CL) Mh 91.0 GeV, MA 91.9 GeV (95% CL) 0.5  tan  2.4 excluded (95% CL) Four seas conference

  23. More Higgs bosons: h, A, H, H+, H- from supersymmetry • the same two scenarios: results in the (mA, tan) plane • the mass limits in the representative scenarios have been checked to be valid in most scenarios • Large mu scenario (h, A decoupled from b’s): completely excluded • reinterpretation of the analyses in models with explicit CP violation : under progress Four seas conference

  24. More Higgs bosons: h, A, H, H+, H- in general 2HD models assuming : Br(H±) + Br(H± cs) = 1 MH 78.6 GeV (95% CL) • charged Higgs bosons: • New decays open at high mass (H+ W A) : dedicated analyses under progress Four seas conference

  25. More Higgs bosons: h, A, H, H+, H- in type II 2HD models • neutral Higgs bosons: • masses and couplings no longer constrained as in SUSY Models,  more final states to be expected and hence analysed, e.g. is forbidden in SUSY models (Mh ~ MA when cos(-) is large) but allowed in 2HD models is negligible in the SM and experimentally excluded in SUSY models but possible in 2HD models • More general analyses of LEP data to cover less constrained topologies than in SM or SUSY-driven analyses Four seas conference

  26. Neutral Higgs bosons in type II 2HD models: examples of results enhancement factor of the bb h/A couplings reduction factor 0.1 Z bb h/A bb reduction factor 1 • Other results: analyses of other final states (non-b hadrons, 4’s), reinterpretation of the existing analyses in models with two doublets and a singlet … Four seas conference

  27. No elementary Higgs bosons: technicolor models • EW symmetry breaking due to condensation in the vacuum of strongly-interacting fermions (technifermions): • most models are disfavoured by EW precision constraints • some models fulfill them  direct searches TC models SM MT 79.8 GeV (95% CL) MT 206.7GeV (95%CL) data J.Ellis et al., Phys. Lett. B343 (1995) 282. Four seas conference

  28. Conclusions • The past decade did open the era of the search for the exact EW symmetry breaking mechanism with both the precise EW measurements and the direct searches (LEP, SLC, TeVatron) • SM Higgs boson: • Many other scenarios have also been investigated • The main question : is the EW symmetry breaking due to doublet(s) of scalar fields or not ? TeVatron run II, LHC, LC MH < 196 GeV MH > 114 GeV MH = 115.6 GeV ? Four seas conference

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