Discovery of the Higgs Boson

Transcription

1 Discovery of the Higgs Boson Seminar: Key Experiments in Particle Physics Martin Vogrin Munich, 22. July 2016

2 Outline Theoretical part Experiments Results Open problems

3 Motivation The SM is really two separate theories - QCD and electroweak (EW) 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

4 Electroweak Symmetry Breaking The gauge group for the EW theory is SSSS(2) LL UU(1) This must be a broken symmetry, but we do not want to destroy gauge invariance of the theory We want to add a new field to the SM that will initially have SSSS(2) LL UU(1) symmetry. When this symmetry is broken, the massless bosons become the massive WW, ZZ and a massless photon The addition of a single SSSS(2) doublet of complex scalar fields satisfies these requirements

5 Higgs Potential SSSS(2) doublet: The potential: φφ 0 = 1 2 φφ 1 + iiφφ 2 φφ 3 + iiφφ 4 VV φφ = μμφφ φφ + λλ φφ φφ 2 A fluctuation around the minimum v spontaneously breaks the rotational symmetry of the Higghs field. Choose φφ 0 = 1 2 Fluctuation around the minimum: 0 vv 1 2 φφ 1 + iiφφ 2 φφ 3 + iiφφ vv = 246 GGGGGG 0 vv + h(xx) Physical Higgs boson

6 Masses Expanding the Higgs potential to 2 nd order in h: VV = VV 0 + λλvv 2 h 2 EW bosons coupling: gg 2 ττ WW + gg 2 BB φφ 0 LL HH = 1 8 MM h = 2λλvv ggww 3 + gg BB gg WW 1 iiww 2 gg WW 1 + iiww 2 ggww 3 + gg BB 0 vv 2 Fermion coupling: MM WW = vvvv 2 MM ZZ = vv gg2 + gg 2 2 LL FF = gg ff 2 ff LL ff RR + ff RR ff LL vv mm ff = vvgg ff 2 Note that the coupling to AA 00 = ggww 33 + gg BB to φφ 00 is zero!

7 Interactions Important for M W Important for m top Higgs couples to all fermions in proportion to their mass

8 Interactions H W H W W H W Higgs couples to W and Z WWH vertex ZZH vertex Important for M W Higgs quartic coupling to W and Z WWHH vertex ZZHH vertex

9 Higgs production The Higgs will be produced through the following processes: Mediated by virtual particles proton-antiproton proton-proton

10 Higgs decay Decay modes change as a function of m H since the Higgs couples to mass and will decay to the heaviest particle(s) Low mass: dominant decay mode (bb) is not useful due to overwhelming QCD backgrounds concentrate on H γγ

11 What Is Detected? Detect Electrons Muons Taus(not so easy) Photons Jets Original quark type(b,c,s) sometimes Neutrinos or other noninteracting particles How? Electromagnetic calorimetry/tracking Absorber/tracking Tracking/calorimeter Electromagnetic calorimetry/tracking Calorimeter/tracking Secondary vertices/tracking Calorimeter Detection Onion

12 Detecting the Higgs Discover the Higgs boson directly at a high energy collider Measure the couplings Reconstruct the Higgs potential PARTS 1 AND 2:

13 Key experiments Early measurements: LEP I ( ) LEP II ( ) Tevatron LEP I (missing mass experiment) Focus on the ee + ee ZZ 0 ll + ll HH decay Result: mm h > 46 GGGGGG Higher energy needed!

14 Key experiments LEP II (missing mass experiment) Focus on the ee + ee ZZ 0 ll + ll HH decay Result: mm h > 114 GGGGGG Higher energy needed!

15 Tevatron Three different channels: Key experiments Gluon-gluon fusion: gggg HH WW + WW ll + ll νννν W-H production: qq qq WWHH llllll bb Z-H production: qq qq ZZHH llllll bb Result: mm h < 140 GGGGGG

16 Result Look in the range: 114 GGGGGG < mm h < 140 GGGGGG

17 LHC Look in the range: 114 GGGGGG < mm h < 140 GGGGGG Two independent measurements

18 Cost: $4.1 billion LHC 14 years = time taken to build LHC ~10000 people working on the project 22 member states M(protons)= grams One ten-thousandth of a second = time taken by proton to circle the ring 8.3 tesla = top field strength of each of the LHC's 1232 superconducting dipole magnets 6 million = number of DVDs needed to hold all of the data generated by the LHC

19 ATLAS and CMS

20 Results: γγγγ

21 Results: 4ll

22 Results 6σσ

23 Nobel prize in Physics 2013 Robert Brout ( )

24 Is it really Higgs? FERMI: weak interactions, e.g. μμ ee νν ee νν μμ LL = GG FF 2 νν μμγγ λλ 1 γγ 5 μμ eeγγ λλ 1 γγ 5 νν ee GG FF GGGGVV 2 At high energies: MM νν μμ ee μμ νν ee ~ GG FF 2 2ππ ss VIOLATES UNITARITY Solution: interaction mediated by a heavy boson WW ±

25 Is it really Higgs? WWWW WWWW MM WW LL WW LL WW LL WW LL ~ss VIOLATES UNITARITY Solution: new scalar particle HH

26 TO DO: Is it really Higgs? Build a e+e- collider and check for the missing mass (no possible quantum corrections to the channel independence of the production mechanism) Show that unitarity is not violated

27 Need for a theory beyond the Standard Model 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 Dark matter and dark energy SM appears to be a low-energy approximation of a fundamental theory