W hat can be learned from decays ?

What can be learned from decays? Giulia Bampa

Contents

Historical introduction How did previous physicists learn so much about weak interaction by analyzing the decays?

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

The mostfamousβ-decay • Alpha decay is mono-energetic and was already well understood by the early 20th 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

Parityviolation • 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. Π

Parityviolation • 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 θand 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 !

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

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

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

Λ0 vs n Neutron decay Lambda decay ?

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

The suppression of K0μ+μ- … ΔS=1 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!

…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 ΔS=1

If this decay is STRONGLY suppressed, why we have a finite value for the BR (and not an upper limit)? Second order diagrams for K0 u-exchange graph c-exchange graph

Cabibbo-Kobayashi-Maskawa matrix A new family of quarks (nice analogy with leptons!): 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

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

D-mesons’ decays

Principalfeatures ofD-mesons C. Amsler et al. (Particle Data Group), Physics Letters B667, 1 (2008)

Leptonic and rare decays The possible decays are:

Flavor-changingneutralcurrents (FCNC) • Why are they interesting? • They are expected to be very rare in the standard model • some slides ago, we see that the K0 decay is strongly suppressed by the GIM! • Lepton family number violation is strictly forbidden We are looking for

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

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

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

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

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

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

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

Conclusions of the experiment • 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

Summary • 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 • We analyze a present experiment in the c-sector Questions?

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!

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

W hat can be learned from decays ?