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New particle discovery at LHC and first properties measurements

New particle discovery at LHC and first properties measurements. Estelle Scifo Laboratoire de l’Accélérateur Linéaire. OUTLINE. Theory Higgs Status before LHC LHC and its detectors Data taking and results. 1. THEORY. The atom. 3 elementary particles : e, u, d. Neutrinos.

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New particle discovery at LHC and first properties measurements

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  1. New particlediscoveryat LHCand first propertiesmeasurements Estelle ScifoLaboratoire de l’Accélérateur Linéaire

  2. OUTLINE • Theory • HiggsStatusbefore LHC • LHC and its detectors • Data taking and results

  3. 1. THEORY

  4. The atom 3 elementaryparticles: e, u, d

  5. Neutrinos • An exemple of complementaritybetweentheory and experiment; • Postulated in 1930 by Wolfang Pauli (and called neutrino in 1933 by Enrico Fermi) to explain an observedmissingenergy in beta decay; • Weaklyinteractingparticle; • Discovered in 1956 by Frederick Reines and Clyde Cowan (Nobel prize 1995)

  6. Interactions • Interaction = exchange of mediatorparticles • Electromagnetic: betweenelectricallychargedparticles (photons γ) • Strong: needed to explain nucleus coherence (gluons g) • Weak: reponsible for some radioactive decays (W± & Z0) • Gravity: (graviton ?)

  7. Standard Model of particlephysics Fermions: Bosons:

  8. Mass hierarchy • Photons (m<10-18 eV) and gluons massless • Neutrinos: massless in minimal SM but somehints (such as neutrinos oscillations) indicatetheyshould have a verysmall mass Order of magnitude me = 0.5 GeV ≈ 10-30 kg

  9. Higgs mechanism • Problem: • In the theory so far (described by quantum field theory), W and Z bosons have no mass, • but experiments found masses ~ 80 GeV • Solution: • Nice suggestion by (Brout-Englert) Higgs in the 60’s : • Vacuum is filled with a ‘’Higgs’’ field that interact with particles and give mass to elementary particles. • The mediator associated to this interaction: the Higgs boson

  10. SpontaneousSymmetryBreaking • Higgspotential: • Mexicanhat • Stable solutions symmetric • The system has to chose one of them spontaneoussymmetrybreaking(= the stable solution do not have the samesymmetrythan the physics) • Other SSB in physics: • Phase transition • Columnbuckling

  11. Analogywithsuperconductivity • For T<Tc, magneticfieldlockedoutside the material → photons have a mass ≠ 0 • Critical temperature for SSB related to the BEH mechanism corresponds to ~100 GeV and is therefore equal  to ~ 1015 K. • It happened at a time of  ~  10-10 s after the  big bang

  12. ‘’QCD mass’’ D U U • WithoutHiggs, youwouldalso have a mass! • Ex: a proton is made of 3 quarks • Mproton = 938 MeV • ∑mquarks = 9.4 MeV • Explanation: more realisticview of protons D D D D g g g U U U U g g g g U U U • A proton at time t: • The proton mass ismainly due to energy: m=E/c² U

  13. Towardsexperiments • Need to observe this ‘’(BE)Higgs boson’’ to validate the theory; • All propertiespredicted by the theoryexceptits mass; • In particular, we know itslifetime (≈10-24s !!) and itsdecay modes; • The (BE)Higgs boson will therefore be observed through its decay modes and not directly. →severalchannelscanbeanalyzed

  14. 2. HIGGS STATUS BEFORE LHC

  15. Laboratories • Several laboratories have contributed to the construction and evolution of particle physics • Accelerators : LHC, TeVatron • Detectors (experiments): Atlas, CMS, D0, CDF

  16. CERN • European organisation for NuclearResearch • Created in 1954 • Member states: (at the beginning) (joined) • Contributions fromoutside Europe Country (Japan, USA…) • Budget in 2012: 887 million CHF 16 ≈ 707 million €

  17. First Higgssearches: LEP • LEP • Electron-positron collider • 27km circumference • Center of mass energy up to 200 GeV • 4 detectors • Results about Higgs: • No discovery • But allow to excludesome mass range…

  18. Limit on the Higgs mass • LEP • TeVatron (1 TeV proton-antiproton collider in Chicago)

  19. 3. LHC AND ITS DETECTORS

  20. Large Hadron Collider (LHC) • Proton-protoncollider • In the same tunnel than LEP (27 km) • Each proton energy: 7 TeV(nominal value) • Previous CERN facilitiesstillused to prepare the beambefore injection in the LHC • Collisions each 25ns (nominal value), 50 ns (today) →eachexperimentneeds a very effective trigger system to select interesting signature (lowenergyeventseg are skipped)

  21. LHC tunnel

  22. LHC Control Room

  23. Luminosity • Caracteristic of the accelerator • Related to the number of collisions delivered • The higherluminosity, the higher the number of recordedevent, the better for search for rare process • Dimension: L-2T-1; Current unit: inverse barn per second • Integratedluminosity Number of collisions per second Luminosity Probability of interaction btw 2 protons

  24. Higgs production at LHC • Modes: • Important becauseinvolvedifferentHiggscouplings (to W, Z or quarks)

  25. Sensible Higgsdecay modes • H→bb: • Highestbranching ratio • But: veryhigh background • Study possible in veryspecific cases  lowernumber of events and smaller mass resolution • H→ZZ: the ‘’golden channel’’; • H→γγ: clear signature, another important channelat LHC; • Other (less important because poor mass resolution): • H →WW , • H→τ τ

  26. What to do withdecayproducts ? Signal: Part. 1 A Mass=mA Ψ Part. 2 A Background: Part. 1 Ψ Part. 2

  27. What to do withdecayproducts ? Total:

  28. H→ZZ→4l: the golden channel • 3 possibilities: ZZ→eeee; ZZ→µµµµ; ZZ→eeµµ • Signal over Background ratio veryhigh; • But very few expectedevents(due to the BR(Z→ll)=3%); • Expected m4l distribution: • mass resolution ≈1.5 GeV l l H Z Z l l

  29. H→gg: the diphotonchannel • Vertex Hgg impossible in the SM becausephotons are massless (= no coupling to Higgs boson) • Processallowedthroughloopsinvolving massive particles: • Clear signature • Expectedmggdistribution • mass resolution ≈1.5 GeV γ H γ

  30. Interpretation:Statisticaltools • Important variable: p-value (p0): estimate‘’the probability for the background to fluctuateat least as much as the observed data’’ • If p-value ishigh: observed data are consistent with the hypothesis of background only • If p-value issmall(<10-7): there are more events in data than the expected background, eventakingintoaccount fluctuations →theremightbe a signal !

  31. Experiments on the LHC • 4 detectors: • ATLAS: generalpurpose • CMS: generalpurpose • LHCb: asymmetrymatter/antimatter (CP-violation) • ALICE: heavy ions studies (Quarks and gluons plasma) • ATLAS and CMS designedmainly for Higgs (and SUSY) searches

  32. A general detector description • Need to measure: • Trajectories • Energies: calorimeters (need to destroy incident particle) • Ex: CMS • And need to identifyparticles

  33. The ATLAS detector

  34. ATLAS duringassembling

  35. CMS Silicon Tracker The Silicon tracker (200m2) has 10M channels Limoges 14-1-2013

  36. CMS EM calorimeter more than 75000 cristals of PbW04 (E)/E = 3%/EGeV  0.7 % Limoges 14-1-2013

  37. Importance of granularity • Ex. of ATLAS calorimeter: 3 longitudinal layers • 1st samplingvery fine transverse granularity • Allow to distinguishbetween photons and their main background: two close photons coming from the decay of energetic π°'s • Photon π° γ π° γ

  38. 4. DATA TAKING AND RESULTS

  39. LHC schedule 10th september 2008 : first beamsaround 19th september 2008 : incident 2008 14 months of major repairs and consolidation New Protection system 20th november 2009 : first beamsaround (again) december 2009 : collisions at 2.36 TeV 2009 2010 30th march 2010 : first collisions at 7 TeV august 2010 : luminosity of 1031 cm-2 s-1 2011 november2011 : integrated luminosity ~ 5 fb-1 13thdecember 2011 : first ‘signal’ around 126 GeV march 2012 : start again at 8 TeV 4th July 2012 : evidence for a new boson ( total 8 TeV integrated luminosity  for discovery : 6 fb-1)  luminosity = 8 1033  cm-2s-1 now at 8 TeV : 13 fb-1 published  20 fb-1 taken 2012

  40. Data taking conditions • Btw 2010 and 2012, LHC increasesits performance: more and more eventscollected per unit of time • Good point for search for rare phenomenon • Drawback:more and more pile-up: lot of particles to identify in a single event 2010 2012 2011

  41. First ATLAS results(2010 data) • All knownparticleswererediscoveredat the right mass →give confidence in the new results

  42. Higgsdiscoverysteps • 13th December 2011: observation of an excess • 4th July 2012: discovery

  43. Results per decay modeInvariant mass distribution H→ZZ→4l H→gg NB: plots shown are updated plots fromDecember 2012

  44. p0 distributions H→ZZ→4l H→gg

  45. Combinedresults CMS • ATLAS p0 < 10-7 = discovery For both ATLAS and CMS  There is a new particle

  46. Evolution of the excess

  47. Update December 2012 • Update with more data 6→13 fb-1 • Blue line ≈ July value

  48. Event Display: H→γγ

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