Can we reach the Zeptouniverse with rare K and B\_s,d decays?
(04.08.2014)Zusammenfassung
The Large Hadron Collider will directly probe distance scales as short as
10^-19m, corresponding to energy scales at the level of a few TeV. In order
to reach even higher resolutions before the advent of future high-energy
colliders, it is necessary to consider indirect probes of New Physics (NP), a
prime example being Delta F=2 neutral meson mixing processes, which are
sensitive to much shorter distance scales. However Delta F=2 processes alone
cannot tell us much about the structure of NP beyond the LHC scales. To
identify for instance the presence of new quark flavour-changing dynamics of a
left-handed (LH) or right-handed (RH) nature, complementary results from Delta
F=1 rare decay processes are vital. We therefore address the important question
of whether NP could be seen up to energy scales as high as 200 TeV,
corresponding to distances as small as 10^-21m -- the Zeptouniverse -- in
rare K and B\_s,d decays, subject to present Delta F=2 constraints and
perturbativity. We focus in particular on a heavy Z' gauge boson. If restricted
to purely LH or RH Z' couplings to quarks, we find that rare K decays, in
particular KL-> pi^0 nu nubar and K^+->pi^+ nu nubar, allow us to probe the
Zeptouniverse. On the other hand rare B\_s and B\_d decays, which receive
stronger Delta F=2 constraints, allow us to reach about 15 TeV. Allowing for
both LH and RH couplings a loosening of the Delta F=2 constraints is possible,
and we find that the maximal values of M\_Z' at which NP effects that are
consistent with perturbative couplings could be found are approximately 2000
TeV for K decays and 160 TeV for rare B\_s,d decays. Finally we present a
simple idea for an indirect determination of M\_Z' that could be realised at the
next linear e^+e^- or mu^+ mu^- collider and with future precise flavour data.