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Observation of a Higgs-like particle at the LHC

Observation of a Higgs-like particle at the LHC. Stathes Paganis (The University of Sheffield) Oxford seminar, 29-Jan-2013. Introduction/Outline.

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Observation of a Higgs-like particle at the LHC

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  1. Observation of a Higgs-like particle at the LHC Stathes Paganis (The University of Sheffield) Oxford seminar, 29-Jan-2013

  2. Introduction/Outline On the 4th of July 2012, ATLAS and CMS experiments announced the observation of a new narrow resonance at a mass of ~125-126 GeV. Studies of the properties of this particle are now in full force with the aim to establish if the particle is the long sought Higgs boson of the Higgs mechanism responsible for the EW gauge symmetry breaking. Here I present the latest results from ATLAS/CMS (last update: December/2012) • Why Higgs? • The search, the discovery. • Interpretation, new physics?

  3. The Standard Model • Includes all elementary particles and their interactions. • Has passed all experimental tests and predicts observables to better than 1%. • Predicts the unification of EM & Weak interactions. • Predicts a new particle: the Higgs boson. • Is this the “final theory” ? • Can’t be: • there is no gravity • Cannot tell us why Higgs appears with this mass. • Cannot explain why the masses of the fermions are so different • Dark matter, Dark energy ? • ...

  4. The ElectroWeak part of the SM EM charges The Standard Model is a Chiral Theory

  5. Symmetries and the Std Model Rotations in space: cause rotations in the quantum spaces of quantum fields 1D quantum space  unaffected by rotations (i.e. Scalars) 2D quantum space  needs a 4pi rotation to return to itself (spin ½ ) 3D quantum space  needs a 2pi rotation (vector) ... Particles are labelled by quantum numbers (spin, charge, baryon number, ...) Fermions have chirality left and right: the way you rotate in the internal space. Clockwise or anti-Clockwise. Fermions have (conserved) spin 1/2: invariance after 4pi rotations in internal 2D space which has a twist.

  6. LocalSymmetries: towards the EM interaction Particles like the electron are complex fields: phase rotations are unobservable: • phase invariance  Charge conservation Can we change the phases arbitrarily at every point in space? No, it costs energy. However, we can “communicate” the phase difference from point to point (change it appropriately) so that the symmetry holds

  7. Interactions: due to phase rotation freedom U(1) rotations Observers cannot tell the difference between a (bare) electron and an electron together with a cloud of collinear massless vector fields. These vector fields are identified as the photons. They “couple” to the electrons. The strength of this coupling is the e-charge. This works ONLY for massless vector fields. “U(1) Gauge Symmetry”

  8. More gauge symmetries  More interactions But, we can have doublets, or triplets, etc, of fields in space. Is there more freedom? This is exactly like our familiar spin space (SU(2) rotations). Similar to the proton neutron isospin. Rotations in this 2D complex space that leave our system invariant are possible if we introduce 3 massless “photons”: two electrically charged and one neutral. Note that like the EM charge there is an isospin charge. Note2: they can mix “e” with “n” In fact one can combine these single phase (U(1)) and spin space rotations, SU(2) to one interaction, with 4 types of photons. It comes out that only left particles “feel” the weak force!

  9. Puzzle: W/Z bosons have mass How can we give mass to W/Z without breaking the weak interaction gauge invariance ? How about fermion masses? Dirac mass mixes left and right chiralities. But L-R fermions have different weak charges!

  10. How do we make a massive fermionbut conserve weak charge? left-handed mass flipschirality right-handed left-handed mass violates weak charge!!! right-handed left-handed Mass Violates Electroweak Gauge Symmetry!!!

  11. Introduce the Higgs boson field This must be a weak doublet. Fills the vacuum: It is a condensate, ie it has a non-zero density. It has weak charge (but not electric charge). It gives non-zero energy density to the vacuum (i.e. cosmological constant) Englert, Brout, Higgs (1964). Gauge bosons (W/Z) having weak charge acquire mass through interaction with the charged vacuum.  idea taken from Superconductivity BCS theory, and Landau-Ginzburgphenom.

  12. Couple to the Higgs time right-handed electron wc=-1 Higgs field wc=1/2 left-handed electron wc=-1/2 +z axis Weak charge is conserved !

  13. Fermion Masses in Electroweak Theory left-handed right-handed left-handed right-handed left-handed Fermion Mass requires Higgs to maintain Electroweak Gauge Symmetry!!!

  14. Higgs in SM: a single c weak doublet With SU(2) x U(1) invariant scalar potential • for m<0 the minimum is at With a single real excitation particle H surviving:

  15. Higgs Field vs Higgs Boson Particle Field Use the ATLAS detector the find the particle Manifestation of Higgs field: Interaction carriers acquire mass: W/Z bosons ~100 x the proton mass

  16. The ATLAS Detector

  17. muons travelling through ATLAS Muons are reconstructed using the tracker and the muon chambers. Here we are interested in Zmm These are “isolated” without much hadronic debris. Background (from top and b decays) is removed by using isolation cuts. But we also have FSR (photon lost?):

  18. Trigger – pileup - DAQ

  19. Zee and Zmm reconstruction

  20. Effects of these photons on the Z mass Add photon Background 0.3% (!) These photons check the EM Calo energy scale to 0.3% ! Electrons are checked down to 0.1% using the idea we introduced in 2004: E1/E2 vs PS. NIM A614 (2010) 400-432

  21. Higgs production at the LHC Golden channel: Higgs4 leptons

  22. Higgs branching ratios Important: for a 125-135GeV Higgs all decay channels are open! Can nature be so kind?

  23. Higgs decays to gauge bosons Higgs couples directly to the ZZ Higgs goes through a loop to gg, Zg WW, ZZ, gg, and Zg all available at 125GeV ! WW has no peak, Zg has huge backgrounds

  24. Measured Higgs decays (CMS) Same channels have been analyzed by ATLAS (Hgg with 5+13 fb-1 luminosity)

  25. Higgs gg

  26. gg event selection summary (ATLAS) Photons: converted, unconverted. Photon energy: from LAr EM cluster energy Photon position: h from calorimeter and the primary vertex, f from calorimeter Dijet category improves sensitivity to VBF. Search performed in the 110-150 GeV gg mass range.

  27. gg signal mass resolution Higgs Mass Resolution: for different methods of longitudinal vertex position reconstruction. Calorimetric pointing and likelihood lead to improved resolution. Not affected by increased levels of pileup. Varies from category to category.

  28. gg invariant mass and local Significance Combination of all categories leads to a 6s significance.

  29. gg invariant mass from CMS

  30. 4-lepton event selection summary (ATLAS)

  31. Lepton Reconstruction Electron reconstruction/identification improved in 2012: New pattern finding/track fitting, Improved track-cluster matching, to recover electrons undergoing hard bremsstrahlung , GSF. Muon reconstruction: use ID tracks matched with partial or complete track segments in the muon spectrometer, and ID tracks+energy deposits in the calo (|h|<0.1 pt>15GeV). Standalone muons (2.5<|h|<2.7).

  32. 4-lepton invariant mass Observed excess at 123-124 GeV

  33. CMS (new) result

  34. HWWenmn gluon-gluon Fusion Weak-boson fusion Frank Wilczek

  35. HWWenmn: ATLAS and CMS

  36. Higgstt Mauro Donega Zurich Workshop 13 Binned likelihood fit in 5 categories

  37. Htt: ATLAS and CMS At 125GeV Observed (exp) upper limit: 1.6 (1.0) xSM At 125GeV Observed (exp) upper limit: 1.9 (1.2) xSM Observed (exp) local significance: 1.1 (1.7)s

  38. V+HV+bb

  39. Hbb: CMS non VV subtracted CMS report an excess in bb

  40. Hbb: ATLAS ATLAS reports NO excess in bb

  41. Combination • Is this the SM Higgs ? • Hints for new physics BSM ? Must measure its properties

  42. ATLAS: Combined Significance Combination of all channels (including 2011 tt, bb, etc) leads to a 7s significance.

  43. CMS combination CMS observe a 6.9s significance

  44. Mass and Signal strength mwrt Standard Model ATLAS mass:125.2 ± 0.3(stat) ± 0.6(syst) GeV (using only 4-lepton and gg channels) CMS mass:125.8 ± 0.5(stat) ± 0.4(syst) GeV ATLAS Combined Signal Strength:1.35 ± 0.24

  45. CMS Signal strength mwrt Standard Model Tension again between gg and the rest. Very interesting to look at the HZg channel!

  46. Higgs Zg : our gateway to beyond the Std Model? Z In SM, the W loop is dominant. New BSM physics heavy particles may run in the loop increasing the Hgg yield with respect to the SM. The Zg yield is related to the gg yield. Note that the yield of Zg relative to gg depends on the spin and other properties of the new particles. So, measuring both Zg and gg yields is significant for understanding the properties of particles running in the loop.

  47. Higgs Zg : CMS with ~10 fb-1 Last December CMS reported a 15xSM limit with low luminosity. Results from both experiments expected for Moriond2013 !

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