Particle Physics From an experiment-driven to a theory-driven field

1. Particle PhysicsFrom an experiment-driven to a theory-driven field Manfred Jeitler Institute of High Energy Physics of the Austrian Academy of Sciences

2. Particle Physicsin a nutshell • elementary particles first postulated in antiquity • Democritus called them ἄτομος (undividable = elementary), atoms • what we call atoms today is not at all undividable! • philosophical reasoning, no experimental evidence • maximally underdetermined • indirect evidence for atoms • elements react in ratios of small whole numbers • John Dalton 1803: “law of multiple proportions” • erratic movement of small objects • Brownian motion 1827 (Robert Brown)

3. e- direct evidence for first elementary particle: the electron the electron Thomson 1897

4. e- another constituent of matter:the proton p the proton Rutherford 1914 1897 1900-1924

5. e- g p the particle of light:the photon the photon Einstein Planck Compton 1900-1924 1897

6. e- g p there is still something else in the nucleus:the neutron Chadwick n the neutron 1932 1914 1897 1900-1924

7. a particle theory of the world - all aspects nicely determined • a few particles nicely described our world • very simple: for a scientist, beautiful ! • Occam’s razor • each of them had been accurately determined by experiment • almost everything explained • except some details: for instance, what keeps the particles in a nucleus together? • but experimentalists continued their observations!

8. e- g p n e+ Who ordered this ? more particles appearing • Hess • Anderson, • Neddermeyer µ the muon 1937 1914 1897 1900-1924 1932

9. L e- g K n p µ n p e+ S too many particles? In his Nobel prize speech in 1955, Willis Lamb expressed nicely the general attitude at the time: „I have heard it said that the finder of a new elementary particle used to be rewarded by a Nobel Prize, but that now such a discovery ought to be punished by a $10,000 fine.“ Lamb 1947-... 1947 1914 1932 1897 1937 1900-1924

10. the theorists’ turn • get some order into this “particle zoo” • after some more work (details later) everything was again classified into a model: The Standard Model (~1970)

11. two extra generations t ne nm nt interactions u c strong g strong g electromagnetic u d u u d d d s b m t e weak W, Z ? gravitation weak force carriers = bosons (spin 1) the Standard Model fermions (spin ½) leptons quarks charge 0 +2/3 -1 -1/3 +1 0 proton neutron baryons

12. u c u u u d d d The Standard Model explaining all the particles p+ K-  D+ d u d b ... mesons s b ... u u baryons s u u d proton neutron D++ L0 atom nucleus He nucleus (a-particle) matter

13. where did the “extra generations” come from? • “leptons” (like electron) and “quarks” (like in proton and neutron) have heavier, unstable partners • predicted by theory ? • found as a surprise by experiment ?

14. the second generation • muon (μ) • 2nd generation of “leptons” (the electron’s partner) • observed by chance, not expected • “strange” quark (s-quark) • 2nd generation of “hadrons” (the up and down quarks’ partner) • observed by chance, not expected • puzzle observed: no “flavor-changing neutral currents” • particle decays described by strange quark (inside an unstable particle) decaying into up quark, but never into down quark • why ?!

15. the GIM mechanism Glashow - Iliopoulos -Maiani

16. discovery of the “charm” quark • predicted by theory (1970) • but experimentalists payed no attention ! • discovered accidentally (1974) • in the “J/ψ” particle • Sam Ting and Burt Richter

17. decay of a “charmed” baryon (Σc++)

18. bubble chamber

19. drift chamber

20. another question:are particles completely symmetrical ? • imagine as little billiards balls • no room for any asymmetry (left - right)? • few people doubted symmetry • but this conviction was underdetermined • and turned out wrong K p e

21. parity violation • parity (= mirror-image) symmetry had not been proved for all interactions • C.N.Yang and T.D.Lee conjectured that parity symmetry might be broken in Weak interactions • based on experimental evidence • C.S.Wuproved this experimentally • in the same year (1956) •  the world’s mirror image differs from the world itself

22. parity violation Chien-Shiung Wu Chen Ning Yang and Tsung-Dao Lee (Nobel prize 1957)

23. save the symmetry! • parity violation came as a shock ! • physicists hoped to find the lost symmetry again on a higher level

24. left-handed neutrino right-handed neutrino X Parity CP Charge charge conjugation: replace particles by anti-particles right-handed anti-neutrino Charge-Parity symmetry In “Weak Interactions”, P and C “maximally violated” but the combined CP symmetry is mostly conserved

25. p K0L K0L p p CP = -1 CP = -1 K0S CP = -1 p p p CP = +1 CP = +1 p CP = +1 the next shock:CP symmetry is also broken! • but (rarely) K0L also decays into two π’s

26. 1964 experiment the first signal: K0L p+p-

27. CP-violation • people were unhappy and proposed other explanations for the experimental findings (1964) • but soon had to accept CP-violation as a fact • theories were developed to explain it • one theory predicted a further “generation” of quarks (1973)

28.  Makoto Kobayashi Toshihide Maskawa  Nobel prize 2008 (together with Yoichiro Nambu)

29. the Cabibbo-Kobayashi-Maskawa matrixand the”unitarity triangle“ • a 3 x 3 “quark mixing matrix” can explain CP-violation • so, there should be two more quarks (“b” and “t”) • “beauty”, or “bottom” • “truth”, or “top” • “c” (“charm”) had not yet been found in 1973 !

30. mechanism of CP-violaiton • one theory (“Standard Model”) predicted: • 3 generations of quarks • ε’ not equal 0 • another theory (“superweak model”) predicted: • nothing concerning a 3rd generation of quarks • nothing concerning ε’ • question to philosophers: • what would the “realist” conclude ? • what would the “antirealist” conclude ?

31. three generations - why so complex? • CP-violation, three generations ... just a whim of nature? • no! • CP-violation is of fundamental importance for our universe • but nobody had thought of this before

32. particle physics and cosmology: the big bang

33. the Big Bang, antimatter, and us • according to present understanding, the Universe was created in the “Big Bang” • matter and antimatter were created in equal quantities • there is almost no antimatter in the Universe • both would have disappeared if no matter excess had developed • CP-violation is necessary condition! • Sakharov, 1965

35. but why? • we wouldn’t be around without CP-violation • and lots of other facts • “fine tuning” of constants of nature • ... and so we wouldn’t be able to ask these questions! • anthropic principle • don’t invoke it - it’s not politically correct! • physicists are supposed to understand everything - not just to show it can’t be otherwise • although we sometimes use it and don’t care • ever wondered why we live on Earth, and not on Venus or Jupiter? • people don’t like it - but it might still be the right answer!

36. welcome to the Multiverse ! • anthropic principle made socially acceptable • maybe our “Universe” is just a bubble • among uncountably infinitely many other bubbles in the Multiverse • in each bubble universe, one set of laws of physics and natural constants is realized • just “one point in the parameter landscape” • unless we manage to communicate with other universes: • how? • or find some indirect proof … • is this science or theology?

37. completing the Standard Model:the W± and Z0 bosons (1983)

38. completing the Standard Model:the top quark (1995)

39. Standard Model:Complete and proved? No alternatives? • very good description of nature • many predictions but: • does not describe everything: • e.g., neutrino masses not predicted • one important ingredient still not found in experiment: the HIGGS BOSON • predicted by the mechanism proposed to give mass to quarks

40. to Higgs or not to Higgs ... • very good evidence for Standard Model in all other respects • but the “allowed mass window” (masses not yet excluded by experiments or theory) is getting smaller • also, it’s taking so long ... • merely psychological factor? •  “theoreticians are getting cold feet” • John Ellis (one of the chief theoreticians at CERN) •  lots of new theories (no Higgs after all, invisible Higgs, little Higgs, ... you name it) • much time and little data - theoreticians leave no stone unturned (?)

41. Supersymmetry SUSY bosons fermions for each known elementary particle there should exist a supersymmetric partner SUSY particles. green: known particles of the Standard Model red: hypothetical new particles

42. dark matter:MACHOS vs WIMPS • massive astrophysical cosmic halo objects? • weakly interacting massive particles? • questions of cosmology to particle physics: • Why is there more matter than anti-matter in the universe? • What is the universe made of? What is dark matter? • What is dark energy? • answers to these questions concerning the largest scales might come from the physics of the smallest scales - elementary particle physics

43. ... and much more • superstrings • extra dimensions

44. data dearth • so many theories • so many explored alternatives • probably some unexplored alternatives (?) • give us data !!

45. CMS just starting !

46. how to observe particles just starting ! Tracks of particles in a typical collider experiment (CMS, CERN)

47. just starting !

48. measurement of gravitational waves LIGO (Laser Interferometer Gravitational Wave Observatory, USA) still waiting for signal!

49. important questions of today’s particle physics (ongoing experiments) • • Where do particles get their mass from? • (by interaction with the Higgs particle?) • Why are these masses so different? • • Is there an overall (hidden) symmetry such as supersymmetry (SUSY)  “mirror world” of all known particles?. • What is the nature of “dark matter” and “dark energy” in the universe? • • Why is there more matter than anti-matter? • • Why have neutrinos such small mass? • • Is there a Grand Unification which combines all interactions, including gravitation? • • Are there extra dimensions, D > 4 ? ( string theory, …)

50. where are we now ? • many theories in particle physics are strongly underdetermined at present • numerous conceived alternatives • in violation of Latin grammar, and often also common sense • maybe nature has some unconceived alternatives in store for us? • finally, the Large Hadron Collider is online, and we may hope for some answers