New particle discovery at LHC and first properties measurements

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

24. 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

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

26. 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

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

28. What to do withdecayproducts ? Total:

29. 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

30. 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

31. 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 !

32. 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

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

34. The ATLAS detector

35. ATLAS duringassembling

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

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

38. 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

39. 4. DATA TAKING AND RESULTS

40. 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

41. 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

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

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

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

46. p0 distributions H→ZZ→4l H→gg

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

48. Evolution of the excess

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

50. Event Display: H→γγ