Antimatter really matters?

1. Antimatter really matters? Thanks to A. Höcker for material from his Summer Studen Lectures 2005

2. Paul Dirac (1933) “If we accept the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kind of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods.” e+ Carl Anderson in 1932 Also in the 30s: • Universe is expanding with finite size and age….  Mind boggling “existential” questions arise

3. Particle-antiparticle tracks in a bubble chamber Particle/antiparticle dynamics • A particle can annihilate with its antiparticle to form gamma rays • An example whereby matter is converted into pure energy by Einstein’s formula E = mc2 • Conversely, gamma rays with sufficiently high energy can turn into a particle-antiparticle pair What happens if particles and antiparticles are brought together?

4. p + p  g + g 5, 4, 3, 2, 1, Big Bang! • Initial state of the Universe • Soup of energy, and particle and antiparticles • “Quasithermal” equilibrium • Two starting possibilities for the baryon number (Bp = 1, Bp = -1) • <B(t=0)> /= 0 • Very unnatural…. • Even if it had been the case it would have been driven to zero during the initial stages • <B(t=0)> = 0 - Two possibilities • All annihilates leaving behind a Universe void of matter but filled with photons at 2.7K today • Anthropic principle rules this out…. • Annihilation rate falls below expansion rate before all is annihilated • Regions of antimatter in the Universe •  Antimatter search! • …unless…..

5. Naturally existing antimatter? No evidence of large antimatter regions at less that 20 Mpc (600 Mly) • Indirect search • No peaks in gamma-ray diffuse background from nucleon-antinucleon annihilation • No distorsions compatible with matter and antimatter annihilations in Cosmic Microwave Background • Direct search for antimatter • Single anti-helium, anti-carbon nucleon  anti-stars! • BESS experiment, Balloon-borne Experiment with Superconducting Spectrometer • Antihelium / helium < 10-6 • AMS – Alpha Magnetic Spectrometer

6. …unless… Baryogenesis! ? B(t=0)=0  B(t > 10-11s) /= 0 • Spontaneous symmetry breaking of matter/antimatter symmetry • Baryon asymmetry parameter h = as required by nucleosynthesis to generate abundance of light elements in the Universe (if B(t=0) = 0 would have been preserved, after freeze-out h = O (10-18)) 1 baryon out of 1010 pairs did not annihilate and survived • Where did the photons go? Naïve estimate of h by comparing the estimated atom density in the universe (~1.6/m3) with the photon gas density at 2.73 K cosmic background radiation temperature (~4.2108/m3)

7. Universe in the making The Universe begun as an extremely hot and dense soup of energy and particles The rapidly expanding Universe is already endowed with an excess of matter over antimatter by one in a billion and small density variations The soup of quarks condenses into protons, neutrons,.. The first light nuclei are formed Atoms are formed This marks the limit between the use of telescopes and accelerators! Recent research shows that we only know what 4% of the Universe is and that its expansion is accelerating CERN

8. What’s behind baryogenesis? • Andrei Sakharov’s criteria for baryogenesis (1967): • Baryon number violation h(t=0) = 0  h(today) /= 0 • C and CP violation otherwise the process generating excess of baryons will be exactly equal to process generating excess of antibaryons • Departure from thermal equilibrium

9. Discrete symmetries • Invariance under the discrete parity, charge conjugation, time reversal transformations requires: • Fundamental consequences of CPT theorem • Relation between spin and statistics: fields with integer spin (“bosons”) commute and fields with half-numbered spin (“fermions”) anticommute  Pauli exclusion principle • Particles and antiparticles have equal mass and lifetime, equal magnetic moments with opposite sign, and opposite quantum numbers • Best test • Electromagnetic and strong interactions are C, P and T invariant

10. handedness of the electron: suppressed suppressed handedness of the positron: B. Cahn, LBL C and P in weak interaction • Weak interaction violates both C and P symmetries • Consider the decay of a polarized muon: P The preferred emission direction of an electron is opposite to the muon polarization. P transformation (i.e. reversing all three directions in space) yields constellation that is suppressed in nature. C Similar situation for C transformation (i.e. replace all particles with their antiparticles). Applying CP, the resulting reaction—in which an antimuon preferentially emits a positron in the same direction as the polarization—is observed.

11. CP violation in neutral K • Strange particles: • Experimentally one does not observe the neutral “flavor eigenstates” K0 and K0 but rather long- and short-lived neutral states: KLand KS • The observed pionic decays were thought to be cleanly • KS(CP=+1) (pp)0 (CP=+1) • KL(CP=-1)  (ppp)0 (CP = -1) • Cronin, Fitch et al. discovered in 1964 CP-violating decay • KL(CP=-1)  p0p0 (CP = +1) KL +– Today’s most precise measurement for amplitude ratio:

12. Three types of CP violation • CP Violation in mixing (indirect CP violation) • Oscillating systems • CP Violation in interference between decays with and without mixing (indirect CP violation) • CP Violation in the decay (direct CP violation) • Due to the smallness of the effect, it took several experiments and over 30 years of effort to observe this phenomenon Oscillations!

13. CP violation in B • A much richer repertoire of CP phenomena available in b-hadrons • Start of “B-factories” at the end of 90s (BaBar (SLAC), BELLE (KEK)) • Three ingredients B0 J/y K0 (‘Gold-plated channel’) BaBar 2005 = K0 mixing B0 mixing B0 decay

14. d sb d’ s’b’ d sb = V Lcc = 2-1/2g (u, c, t)LgmVW+m+ h.c. VP violation in SM • How can we incorporate this phenomena in the SM? • Weak decay through transitions between families of quarks mediated by W • Flavor quarks (‘weak eigenstates’) are not the same as the physical quarks (‘mass eigenstates’) • Only if quarks are massive! (SM CP violation after appearance of Higgs field) • Quark mixing matrix - Cabibbo-Kobayashi-Maskawa (CKM) matrix, 1973 (KM) •  KM introduced third family which gave a complexe phase to generate CP violation

15. u c t d s b t b c s u Quark mixing • The |Vij |2 are transition probabilities and hence the matrix has to be unitary • For example, a t quark can decay into a d, s, or b quark and nothing else; thus, the sum of the decay probabilities into these quarks must be one: • Unitarity constraints!

16. Im relative size of CP-violating effect Unitarity triangles • Wolfstein parametrization of CKM matrix

17. ‘Flavour’ of what we do…. • Measuring all parameters in as many processes as possible and confront them with SM and with different measurements of the same parameters

18. What have we achieved? • Experiments up to now has established the KM mechanism as the dominant source of CP violation at the electroweak scale • Remarkable agreement • SM CP violation is always a rare phenomenon: • either the CP asymmetry is small • or/and the decay rate is suppressed • In terms of cosmology, this CP violation is OFF by O(1010) !! • The Universe would have contained ONE galaxy….. • Proof of New Physics! • But where and what? • Tip of the iceberg  Rock the iceberg to see what is below!

19. LHCb Experiment at LHC • Cross section for bb production at 14 TeV: sbb ~ 500 mb • Enormous production rate at LHCb: ~ 1012 bb pairs per year! much higher statistics than the current B factories • Expect ~ 200,000 reconstructed B0 J/y KS events/yearcf current B-factory samples of ~ 4000 events precision on sin 2b ~ 0.02 in one year for LHCb (similar to current world average precision) • But in addition, all b-hadron species are produced: B0, B+, Bs, Bc , Lb …In particular can study the Bs (bs) system, inaccessible at the B factories • ATLAS and CMS are also planning to do B physics but will only have a lepton trigger, and poor hadron identification

20. bb events b and b quarks are produced in pairs (mostly in the forward direction) • Need to measure proper time of B decay: t=mB L / pchence decay length L (~ 1 cm in LHCb)and momentum p from decay products (which have ~ 1–100 GeV) • Also need to tag production state of B: whether it was B or BUse charge of lepton or kaon from decay of the other b hadron in the event

21. LHCb in its cavern Offset interaction point (to make best use of existing cavern) Shielding wall(against radiation) Electronics + CPU farm Detectors can be moved away from beam-line for access

22. LHCb detector

23. Low energy antimatter studies Antihydrogen • 1996: • First production of 9 antihydrogen atoms at LEAR (CERN), moving with 90% of the speed of light • 2002: • Production of 1,000,000 antihydrogen atoms at the AD (CERN), moving with 1/1 000 000 of the speed of light

24. Antiproton Deccelerator • 188 m long • Antiproton Production, • Storage and Deceleration • 3 - 0.005 GeV energy

25. ? = Antimatter traps • Trap for antiparticle storage • E.g. spectroscopy

26. Summary It is ‘(not un-)likely’ that precision measurements on CP violation will discover New Physics indirectly before the direct observations come!