Discovery of the Higgs boson at the Large Hadron Collider

Transcription

1 Discovery of the Higgs boson at the Large Hadron Collider HEP101-9 March 25, 2013 Al Goshaw 1

2 The Higgs What? 2

3 Outline 1. Some history: From electromagnetism to the weak interaction and the Higgs boson 2. Past searches for the Higgs boson: From the LEP collider at CERN to the Tevatron at Fermilab 3. Using a bigger hammer: The Large Hadron Collider and the experiments at CERN. 4. The July 4 th discovery: Observation of a new Higgs-like particle. 5. Recent results from the ATLAS experiment 3

4 A short history: From electromagnetism to the weak interaction and the Higgs boson 4

5 The electromagnetic force Two hundred years after Newton s formulation of classical gravitation (1666) Maxwell pulled together the complete EM field equations. Prediction of EM waves, the speed of light, etc. and the manual for our modern electronic and optical based society For the E&M force the messenger particle is the mass-less spin 1 boson required by quantum electrodynamics => the photon 5

6 Discovery of the weak interaction At the end of the 19 th century and the beginning of the 20 th century the modern physics revolution including the discovery of forces beyond gravitation and electromagnetism. The discovery of natural radioactivity was the first observation of the weak interaction force from nuclear beta decay It took many years to understand the character of the weak interaction and to discover the messenger particles which are massive spin 1 bosons => Z 0, W +, W

7 Pulling it all together => electroweak theory In the early 1960 s progress was made in unifying electromagnetism and the weak interaction. Sheldon Glashow, Adbus Salam and Steve Weinberg constructed a gauge invariant theory with four messenger particles (the photon, Z, W + and W - ). The basis of the theory was the assumption of an exact U Y (1)xSU L (2) symmetry that required all particles (bosons and fermions) to be mass-less. This was the BIG problem the theory was beautiful for a world of zero mass particles, but this is not our world. The force carriers of the weak interaction (W and Z particles) are very massive GeV, as of course are the quarks and charged leptons. 7

8 Enter the Higgs field In 1964 several theorists working independently (1-4) postulated a new ubiquitous field carried by a spin zero boson with an unusual interaction potential. One of the theorists was Peter Higgs from the University of Edinburgh. His first publication attempt was rejected as not warranting rapid communication to the science community. But in the end his name stuck for this new field and the associated 0 spin boson => the Higgs boson. (1) P.W. Higgs, Phys. Lett. 12 (1964) 132. (2) F, Englert and R. Brout, Phys. Rev. Lett. 13 (1964) 321. (3) P.W. Higgs, Phys. Rev. Lett. 13 (1964) 508. (4) G.S. Guralnik, C.R. Hagen and T.W.B. Kibble, Phys. Rev. Lett. 13 (1964) 585 8

9 Completing the SM electroweak theory Introducing the Higgs field into the initially zero-mass electroweak Lagrangian, with an interaction with both the gauge bosons and fermions had miraculous effects: Mass terms for the W and Z bosons could be generated while preserving the zero-mass photon. electroweak symmetry breaking Mass terms for the fermions could be generated with a value proportional to the strength of the coupling of the Higgs boson to the fermion s mass. All the couplings of the Higgs bosons to the W/Z bosons and the fermions were defined => all production and decay rates. This was accomplished with the addition of one parameter to the electroweak theory the mass of the Higgs boson: m H = (2µ 2 ) 1/2 where µ is a parameter entering the Higgs potential. 9

10 Searches for the Higgs Boson 10

11 What to look for There are 2 classes of affects that can be used for the experimental verification of the existence of a Higgs boson (as for many other new particle searches) 1. Direct production: Just as photons are produced via bremstrahlung from an accelerated charged particle, Higgs bosons will be produced from the acceleration of any massive particle => Higgs strahlung. 2. Virtual loop affects: Quantum loops containing a Higgs boson will modify precision electroweak predictions. 11

12 The decay modes are known Given the Higgs boson s mass M H, the decay width and branching ratios are predicted. You know exactly what to look for. The Higgs boson favors decays to the most massive particles allowed by energy conservation since the coupling is proportional to the particle s mass. For M H ~ 125 GeV, Γ H ~ 5 MeV and H -> b b has the highest branching ratio. Rare decay modes such as Z Z* and γ γ have low decay rates but are very clean to detect. 12

13 Searches at the CERN Large Electron-Positron Collider The LEP Collider provided a perfect production mechanism for Higgs bosons limited by the e+e- collision cm energy. The LEP center of mass energy was pushed to a limit of 209 GeV in a dedicated search for the Higgs boson. This allowed a search for masses up to M H ~ 209 GeV M Z 2σ The 95% confidence limit limit obtained combining data from 4 LEP experiments is M H > GeV. ( If the LEP cm energy could have been increased by ~ 5% the Higgs boson would have been discovered.) LEP stopped November 2,

14 Searches at the Fermilab Tevatron The Tevatron Collider provided proton antiproton collisions up to a center of mass energy of 1.96 TeV. However the elementary partonic collisions (quarks and gluons) are typically at much lower center of mass energy. This opens a rich range of Higgs boson production mechanisms in addition to Higgs strahlung. The most sensitive decay channels for detection by the CDF and D0 experiments at the Tevatron are: H > b b and H > W + W - where the b quarks are detected using hadronic jets with a beauty hadron and the W boson from µ/e + neutrino decays. 14

15 Searches at the Fermilab Tevatron The combined data from the CDF and D0 experiments exclude broader range of Higgs boson masses. Using the Tevatron data the allowed mass range was limited to: GeV < M H < 147 GeV or M H > 179 GeV (95% confidence limits) excess In addition the Tevatron collaborations observed an excess of events at the level of 2.7 σ in the search channel: p + p -> W/Z + H with H -> b b for M H ~ GeV. Tevatron stopped Sept Hint of a low mass Higgs? 15

16 Higgs quantum loop corections There has been evidence for a light Higgs boson for many years from fits to measurements of electroweak phenomena. The EWK fits favored a low mass Higgs boson. M H = GeV 16

17 Using a bigger hammer The Large Hadron Collider And the ATLAS and CMS experiments 17

18 Discovery of the Higgs Boson 18

19 Seminar at CERN on July 4, 2012 The ATLAS and CMS experiments presented results of searches for the Higgs boson based upon data collected in 2011 and the first few months of The vibe was that this was important. Peter Higgs and other theory colleagues were invited. The most sensitive channels for the LHC search are the rare decay H -> γ + γ and H -> Z* Z with the Z bosons decaying to µ + µ - or e + e -. Students before seminar These channels allow a precise measurement of the Higgs mass from calculation of the invariant mass of the fully-measured γ/e/µ. 19

20 The optical Higgs boson decay mode H -> γ γ Candidate event from CMS detector Collect events with 2 isolated photons with energies > ~ GeV Calculate the invariant mass of the 2 photons: M γγ = 2E 1 E 2 ( 1 cos θ 12 ) For the case of a Higgs boson production M γγ = M H 20

21 The optical Higgs boson decay mode H -> γ γ CMS experiment ATLAS experiment Both experiments see a signal of a boson with mass ~ 125 GeV 21

22 Search in another decay mode: H -> Z* Z -> µ+µ- µ+ µ- (µ+µ- e+ e-) µ µ e e M4l ~ 125 GeV 22

23 Combine data from Searches in H-> γγ, ZZ and WW Calculate the probability that the background would fluctuate to the observed data versus M H A signal is observed with a statistical significance of 5.9σ corresponding to a background fluctuation of 2 x 10-9 The best mass fit is: M H = GeV (ATLAS experiment) arxiv: v2 [hep-ex] The best mass fit is: M H = GeV (CMS experiment) arxiv: v1 [hep-ex] 23

24 24

25 Recent results from LHC experiments 25

26 Higgs production at the LHC There is now direct experimental evidence for a Higgs field from the observation of a Higgs boson. Here are some results from a conference early in March: 26

27 Discovery channel H -> γ γ H -> γ + γ 27

28 Discovery channel H -> Z Z H -> Z +Z 28

29 Search channel H -> Z γ H -> Z + γ 29

30 Search channel H -> Z γ 30

31 Recent LHC results on Higgs boson observation The conclusions from the ATLAS measurements are: The mass is GeV using the combined H -> γ γ and H -> ZZ channels. The production cross section as measured in several channels is consistent with the SM prediction. The spin-parity of the observed boson is favored to be 0 +. So far, everything checks with the properties of the simple SM Higgs boson postulated in

32 Summary and what s next A new boson of mass ~ 125 GeV has been discovered. The meson s production and decay properties are so far consistent with those predicted for a Standard Model Higgs boson: the production cross sections decays into expected final states So it is quite tempting to declare success, and identify this discovery with the Higgs boson predicted on the backboard by Peter Higgs and others in BUT 32

33 Summary and what s next So far the precision of the branching ratio measurements and the number of channels detected is limited. Many cross checks need to be done. A critical measurement is the spin and parity of the SM Higgs boson. It must have the quantum numbers of the vacuum : J PC = Recent measurements favor this. In addition there are attractive theories beyond the SM that have multiple Higgs-like bosons (so-called super-symmetry theories). One of the 5 Higgs bosons of the simplest version of these SUSY theories is very similar to the SM Higgs boson. Are we seeing one of these? 33

34 Summary and what s next Data accumulated from the LHC experiments in 2012 is being analyzed to obtain as much information as possible. But the current data has limited statistics in very interesting Higgs decay channels. The LHC is now shutdown in for repairs that will allow the proton-proton cm energy to increase from 8 to ~ 14 TeV. This will result in Higgs bosons produced with a higher cross section and allow collection of over 10x as much data. There will also be improvements in the ATLAS and CMS detectors. There is every expectation with this data the question of THE HIGGS WHAT? will be definitely answered. 34

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