1. What can be learned from decays? Giulia Bampa

2. Contents β-decay Parity violation Existence of charm quark Electro-weak phenomenology Fermi theory Cabibbo theory GIM mechanism Kobayashi-Maskawa mixing Electro-weak theory General overview Flavor-changing neutral currents Theoretical predictions Experimental activities Decays of D-mesons 2

3. 3 How did previous physicists learn so much about weak interaction by analyzing the decays?

4. 4 Why are we studying decays? The analysis of decays is very important since it gives “real-world” tests of the theory Does a particular decay - allowable in theory – occur in practice? ‘If it is permitted, it must happen’ – or our theory is incomplete Do the decays we observe have the characteristics we expect? If they don’t, we need to figure out why

5. 5 The most famous β-decay Alpha decay is mono-energetic and was already well understood by the early 20 th century: Simple conservation of the four-momenta was enough to predict that alpha decay is mono-energetic, and experiments confirmed this Against this background, one might have expected beta decay to be similar: In fact, experiments showed them to be characterized by an energy distribution – suggesting the existence of new particles which “share” the energy with the electrons: neutrinos (1930, W. Pauli) exp

6. Parity violation 6 τ-θ puzzle The τ-θ puzzle:  the τ and the θ must be different particles since their spin-parity are different;  the τ and the θ are not different particles, since they have the same masses and lifetimes.

7. Parity violation 7 τ-θ puzzle The τ-θ puzzle:  the τ and the θ must be different particles since their spin-parity are different;  the τ and the θ are not different particles, since they have the same masses and lifetimes. In 1956 Lee and Yang suggested that θ and τ were different decay modes of the same particle (K-meson), and that parity was not conserved in the weak interaction While the strong and electromagnetic interactions conserve parity, weak interactions do not !

8. 8 Fermi theory Analogy with the electromagnetic interaction: e.m. charge propagator Dirac spinors interaction First approximation: POINT-LIKE interaction

9. 9 scalar (S) vector (V) tensor (T) axial vector (A) pseudoscalar (P) There is no a priori reason why the weak current should be a vector current Every covariant current is in principle a possible candidate NOTE that Fermi hypothesis cannot account for the parity violation… Perhaps not surprisingly, given that it had not yet been discovered!

10. 10 M.me Wu’s experiment  Parity violation  V-A interaction low energy recoil high energy recoil high energy recoil  right-handed electron  left-handed electron  right-handed electron  left-handed electron S V T A

11. Λ 0 vs n Neutron decayLambda decay 11

12. 12 Cabibbo theory B. Povh, K. Ritz, C. Scholtz, F. Zetsche, Teilchen und Kerne, Springer-Verlag (1995) are the eigenstates of the weak interaction are the eigenstates of the strong interaction where

13. According to the Cabibbo theory, …but although theory predicts the amplitude for the decay should be proportional to, experiment suggests a rate many orders of magnitude weaker! The suppression of K 0  μ + μ - … 13 ΔS=1

14. 14 …and the existence of the charm quark In 1970 Glashow, Iliopoulos and Maiani solved this problem by proposing the existence of a new quark which belongs to a “second generation” doublet According to the GIM mechanism, …meaning the neutral current makes no contribution to strangeness- changing decays ! where

15. Second order diagrams for K 0 15 u-exchange graph c-exchange graph If this decay is STRONGLY suppressed, why we have a finite value for the BR (and not an upper limit)?

16. 16 Cabibbo-Kobayashi-Maskawa matrix A less “theoretical” and more “experimental” CKM matrix: E. Golowich (talk at II° Int. Conf. on B-physics and CP-Violation), arXiv:hep-ph/9706548v1 A new family of quarks (nice analogy with leptons!):

17. 17 C. Amsler et al. (Particle Data Group), Physics Letters B667, 1 (2008) Standard parameterization Standard parameterization: where The factor δ is the so-called “Kobayashi-Maskawa phase”. and are the family labels. Features Features: 1.in the limit of where ! 2.the phase-term is responsible for all CP-violating phenomena in flavor changing processes in the standard model Numbers Numbers:

18. 18

19. 19 Principal features of D-mesons PAnti-P quark.- const. Rest mass [MeV] SCB Lifetime [s] D+D+ D-D- cdcd1869.40+1010.6x10 -13 D0D0 D0D0 cucu1864.60+104.2x10 -13 D+sD+s D-sD-s cscs1969+1 0 4.7x10 -13 C. Amsler et al. (Particle Data Group), Physics Letters B667, 1 (2008)

20. 20 Leptonic and rare decays The possible decays are:

21. 21 Why are they interesting? They are expected to be very rare in the standard model  some slides ago, we see that the K 0 decay is strongly suppressed by the GIM!K 0 decay Lepton family number violation is strictly forbidden Flavor-changing neutral currents (FCNC) We are looking for

22. There are two different diagrams which contribute to the : Theoretical calculations (QCD…) provide this order of magnitude for the branching ratio: 22 A. Freyberger et al., Phys. Rev. Lett. 76, 17 (1996)

23. 23 D0 detector at FERMILAB Projection End view of the collision, with charged particle tracks in the silicon detector, the energy deposited in the calorimeters, and possibly hits in the muon detectors.  The inner part, with the concentric circles, shows the locations, to scale, of the tracking detectors.  The outer concentric ring is a histogram of deposited energies

24. 24 Analysis of the data V. Abazov et al., Phys. Rev. Lett. 100, 101801 (2008)

25. Check of the detector 25 In order to check this detector, we can focus on a “known” reaction: V. Abazov et al., Phys. Rev. Lett. 100, 101801 (2008)

26. 26 V. Abazov et al., Phys. Rev. Lett. 100, 101801 (2008) From the last graph, it is possible to extract the branching ratio: The yield ratio is related to the branching ratio by: which is consistent with expected value given by the product of

27. 27 From the last graph, it is possible to extract the branching ratio: The yield ratio is related to the branching ratio by: which is consistent with expected value given by the product of …what would have happened if it wasn’t consistent?

28. 28 Now, we search for the continuum decay of D + mediated by FCNC interactions, eliminating the condition on the dimuon invariant mass. V. Abazov et al., Phys. Rev. Lett. 100, 101801 (2008) This is ≈ 500 times above the SM expected rate.

29. Conclusions of the experiment 29  This is the most stringent limit to date in a decay c  u μ + μ -  It’s 500 times above the Standard Model expected rate SM pass the test!  Other models can be ruled out little Higgs model, SUSY, etc

30. Summary 30 fundamental role played by the studies on decays  We saw the “step-by-step” historical evolution of the theory for the weak interaction and the fundamental role played by the studies on decays present experiment  We analyze a present experiment in the c-sector

31. References S. Bianco, F. Fabbri, D. Benson, I. Bigi, A Cicerone for the Physics of Charm, hep-ex/0309021 (2008) C. Amsler et al. (Particle Data Group), Phys. Lett. B667, 1 (2008) V. Abazov et al., Phys. Rev. Lett. 100, 101801 (2008) W. E. Burcham, M. Jobes, Nuclear and Particle Physics, Prentice Hall (1979) B. Povh, K. Rith, C. Scholz, F. Zetsche, Particles and Nuclei, Springer (1996) H. Frauenfelder, Subatomic Physics, Prentice-Hall (1974) … and of course, Wikipedia! 31

32. How to handle with the J P 32 1.Let’s take a very common reaction: 2.If the parity is conserved, I would aspect: 3.Consider also the conservation of J: 4.Consider the total asymmetry 5....and then, give a number for So called “intrinsic parity” Parity related to the relative motion τ-θτ-θ