1,805 research outputs found

### The Rare Decays $K\to\pi\nu\bar\nu$, $B\to X\nu\bar\nu$ and $B\to l^+l^-$ -- An Update

We update the Standard Model predictions for the rare decays
$K^+\to\pi^+\nu\bar\nu$ and $K_L\to\pi^0\nu\bar\nu$. In view of improved limits
on $B_s$--$\bar B_s$ mixing we derive a stringent and theoretically clean
Standard Model upper limit on $B(K^+\to\pi^+\nu\bar\nu)$, which is based on the
ratio of $B_d$--$\bar B_d$ to $B_s$--$\bar B_s$ mixing, $\Delta M_d/\Delta
M_s$, alone. This method avoids the large hadronic uncertainties present in the
usual analysis of the CKM matrix. We find $B(K^+\to\pi^+\nu\bar\nu)< 1.67\cdot
10^{-10}$, which can be further improved in the future. In addition we consider
the extraction of $|V_{td}|$ from a future measurement of
$B(K^+\to\pi^+\nu\bar\nu)$, discussing the various sources of uncertainties
involved. We also investigate theoretically clean constraints on
$B(K_L\to\pi^0\nu\bar\nu)$. We take the opportunity to review the
next-to-leading order (NLO) QCD corrections to $K\to\pi\nu\bar\nu$,
$K_L\to\mu^+\mu^-$, $B\to X\nu\bar\nu$ and $B\to l^+l^-$, including a small
additional term that had been missed in the original publications. The
phenomenological impact of this change is negligible, the corresponding
numerical shift being essentially within the small perturbative uncertainties
at the NLO level.Comment: 19 pages, no figure

### Effects from the charm scale in K+ -> pi+ nu nubar

We consider contributions to the rare decay K+ -> pi+ nu nubar which become
nonlocal at the charm scale. Compared to the leading term, such amplitudes are
suppressed by two powers of mK/mc and could potentially give corrections at the
level of 15%. We compute the leading coefficients of the subleading dimension
eight operators in the effective theory below the charm mass. The matrix
elements of these operators cannot all be calculated from first principles and
some must be modeled. We find that these contributions are likely to be small,
but the estimate is sufficiently uncertain that the result may be as large as
the existing theoretical uncertainty from other sources.Comment: 9 pages, 3 figures; Eq. (15) fixed, all results and conclusions
unaltere

### Theoretical Update on Rare K Decays

We review the status of rare kaon decays, concentrating on modes with
sensitivity to short-distance flavour physics.Comment: Invited talk presented at Daphne 99, Frascati, Nov. 1999, 13 pages, 3
figure

### Heavy Quark Theory

These lectures describe the most important theoretical methods in b-physics.
We discuss the formalism of effective weak Hamiltonians, heavy quark effective
theory, the heavy quark expansion for inclusive decays of b-hadrons and,
finally, the more recent ideas of QCD factorization for exclusive nonleptonic B
decays. While the main emphasis is put on introducing the basic theoretical
concepts, some key applications in phenomenology are also presented for
illustration.Comment: 50 pages, Lectures given at the 55th Scottish Universities Summer
School in Physics, 7-23 August 2001, St. Andrews, Scotlan

### QCD and Heavy Hadron Decays

We review recent developments in QCD pertaining to its application to weak
decays of heavy hadrons. We concentrate on exclusive rare and nonleptonic
B-meson decays, discussing both the theoretical framework and phenomenological
issues of current interest.Comment: 13 pages, 8 figures, talk at Lepton Photon 03, 11-16 August 2003,
Fermila

### CP Violation in K Decays and Rare Decays

The present status of CP violation in decays of neutral kaons is reviewed. In
addition selected rare decays of both K and B mesons are discussed. The
emphasis is in particular on observables that can be reliably calculated and
thus offer the possibility of clean tests of standard model flavor physics.Comment: LaTeX, 21 pages, 4 figure

### CP Violation and Rare Kaon Decays

We summarize both the study of CP violation with $K$ and $B$ mesons, as well
as rare decays of kaons, emphasizing recent developments. The topics discussed
include the unitarity triangle, $\epsilon'/\epsilon$, $K\to\pi\nu\bar\nu$ and
other rare $K$ decays, T-odd asymmetries in kaon physics, theoretical aspects
of CP violation in B decays, $B\to J/\Psi K_S$, $B\to\pi\pi$, $B\to\pi K$ and
inclusive CP asymmetries.Comment: Plenary talk at EPS-HEP99, Tampere, Finland, 21 pages, 6 figure

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