Dissecting the Higgs Discovery: The Anatomy of a 21st Century Scientific Achievement

Transcription

1 Dissecting the Higgs Discovery: The Anatomy of a 21st Century Scientific Achievement Lauren Tompkins Arthur H. Compton Lectures October 19th, 2013 Lecture 6 Adding particles with a little help from Einstein

2 Schedule of Lectures [10/5] Welcome to the Theater: Introduction to the Standard Model and Higgs Boson [10/12] Accelerators: Creating particles out of (very) thin air [10/19] Seeing the Higgs with light [10/26] Guest Lecture: Martin Bauer On Theory [11/2] Seeing the Higgs with heavy particles [11/9] Adding particles with a little help from Einstein [11/16] Digesting the Data: Triggering, Data Processing [11/23] Finding the Needle in the Haystack: Data Analysis and Statistics [11/30] No Lecture for Thanksgiving [12/7] No Lecture for Physics with a Bang [12/14] What's next for the LHC? 2

3 Outline Brief Review of last week A (very) brief introduction to special relativity Constructing an invariant mass Detecting quarks and gluons The bulk of Higgs decays 3

4 Last week 4

5 Last week Higgs decay to a wide variety of particles b tau W Z photon other 0.2% 11% 3% 22% 6% 58% 4

6 Last week Higgs decay to a wide variety of particles b tau W Z photon other 0.2% 11% 3% 22% 6% 58% Higgs decay through a pair of Z bosons, one real and one virtual 4

7 Last week Higgs decay to a wide variety of particles b tau W Z photon other 0.2% 11% 3% 22% 6% 58% Higgs decay through a pair of Z bosons, one real and one virtual Tracking detectors are critical to electron and muon identification 4

8 H ZZ Event Displays 5

9 Outline Brief Review of last week A (very) brief introduction to special relativity Constructing an invariant mass Detecting quarks and gluons The bulk of Higgs decays 6

10 Special Relativity in Nutshell Einstein s insight: The laws of physics are the same in all non-accelerating reference frames. 7

11 Frames of Reference 8

12 Problems with reference frames in E&M Magnet and Conductor at rest 9

13 Problems with reference frames in E&M Magnet and Conductor at rest Magnet and Conductor moving 9

14 Problems with reference frames in E&M Magnet and Conductor at rest Magnet and Conductor moving 9

15 Another Thought Experiment Conductor moving Magnet moving 10

16 Special Relativity in Nutshell Consequence: the speed of light is the same in all reference frames

17 Special Relativity in Nutshell Consequence: space and time should be treated equally: spacetime Time is not absolute! History and implications: index.html 12

18 Outline Brief Review of last week Constructing an invariant mass Detecting quarks and gluons The bulk of Higgs decays A (very) brief introduction to special relativity 13

19 Invariants Certain quantities are invariant under spacetime transformations: I.e. they are the same in all reference frames! Examples: Distance between two points in spacetime (interval) Combination of E&B field magnitudes (B 2 -E 2 /c 2 ) Combination of Energy and momentum 14

20 Energy Conservation 15

21 Energy Conservation Energy is always conserved in every reference frame Total energy always stays the same in a reference frame 15

22 Energy Conservation Energy is always conserved in every reference frame Total energy always stays the same in a reference frame Two different observers will not agree on total energy (i.e. Energy is not invariant) 15

23 Momentum Conservation 16

24 Momentum Conservation Momentum is always conserved in every reference frame 16

25 Momentum Conservation Momentum is always conserved in every reference frame Two different observers will not agree on the momentum of objects (i.e. Momentum is not invariant) 16

26 Invariant Mass E = mc 2 17

27 Invariant Mass E 2 = m 2 c 4 + p 2 c 2 18

28 Invariant Mass E 2 = m 2 c 4 + p 2 c 2 E 2 - p 2 c 2 = m 2 c 4 19

29 Invariant Mass E 2 = m 2 c 4 + p 2 c 2 E 2 - p 2 c 2 = m 2 c 4 20

30 Invariant Mass in Particle Collisions Higgs reference frame. E1,0, p1,0 γ Higgs Higgs mh, EH,0 γ E2,0, p2,0 EH,0 2 -ph,0 2 c 2 = mh 2 c 4 21

31 Invariant Mass in Particle Collisions Higgs reference frame. E1,0, p1,0 γ Higgs Higgs mh, EH,0 γ E2,0, p2,0 EH,0 2 = mh 2 c 4 22

32 Invariant Mass in Particle Collisions Higgs reference frame. E1,0, p1,0 γ Higgs Higgs mh, EH,0 γ E2,0, p2,0 EH,0 2 = mh,0 2 c 4 = (E1,0+E2,0) 2 -(p1,0+p2,0) 2 c 2 23

33 Invariant Mass in Particle Collisions Higgs reference frame. E1,0, p1,0 γ Higgs Higgs mh, EH,0 γ E2,0, p2,0 EH 2 -ph 2 c 2 = mh 2 c 4 = (E1+E2) 2 -(p1+p2) 2 c 2 Lab frame. γ E1, p1 Higgs Higgs γ E2, p2 mh, EH, ph 24

34 Outline Brief Review of last week Constructing an invariant mass Detecting quarks and gluons The bulk of Higgs decays A (very) brief introduction to special relativity 25

35 Quarks and Gluons Reminder: Quarks and gluons cannot exist by themselves Produces interesting phenomena... 1: Showering 26

36 Quarks and Gluons Reminder: Quarks and gluons cannot exist by themselves Produces interesting phenomena... 2: Fragmentation 27

37 Quarks and Gluons Reminder: Quarks and gluons cannot exist by themselves Produces interesting phenomena... 3: Hadronization 28

38 Quarks and Gluons Reminder: Quarks and gluons cannot exist by themselves Produces interesting phenomena

39 on Level In the detector: Jets 30

40 ATLAS Jet Events 31

41 CMS Jet Events 32

42 Outline Brief Review of last week A (very) brief introduction to special relativity Constructing an invariant mass Detecting quarks and gluons The bulk of Higgs decays 33

43 Finding Higgs Decays to Quarks and Gluons Higgs decays mostly to quarks! Should be easy, right? Wrong! Jets are the most common thing produced at the LHC 10000x more common than Higgses! σ (nb) proton - (anti)proton cross sections M H =125 GeV σ tot σ b σ jet (E T jet > s/20) σ W σ Z σ jet (E jet T > 100 GeV) σ WW σ σ { t ZZ σ ggh σ WH σ VBF Tevatron LHC events / sec for L = cm -2 s WJS s (TeV)

44 Any hope for the Higgs? Yes! b-quarks form special jets B-hadrons live long enough to travel O(1mm) before decaying 35

45 b-jets in the detector 36

46 Still not enough... Even just looking at b-jets, there are too many background events Look for Higgs + something else instead q H b b q W/Z 37

47 ZH Candidate Event 38

48 Fermilab s Role On July 2nd, 2012, Fermilab announced it has statistical evidence for Higgs decaying to 2 b- quarks in events with Z&W bosons 39

49 Fermilab s Role On July 2nd, 2012, Fermilab announced it has statistical evidence for Higgs decaying to 2 b- quarks in events with Z&W bosons 39

50 Conclusions Special Relativity: laws of physics are the same in all reference frames Speed of light is constant Rest mass is an invariant quantity Quarks and Gluons make jets in the detector Higgs are very hard to find in jet events, but possible with b-quarks! 40