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Scientific Report 2007-2009<br />
Particle physics<br />
P19. Kaon physics with the KLOE experiment<br />
The KLOE experiment at the DAΦNE e + e − collider,<br />
the Frascati ϕ-factory, completed the first data-taking<br />
campaign in 2006, with a total integrated luminosity of ∼<br />
2.5 fb −1 , corresponding to ∼ 8×10 9 ϕ-mesons produced.<br />
At a ϕ-factory the ϕ → K 0 ¯K0 decay - with a branching<br />
fraction of ∼ 34% - produces the neutral kaon pair in a<br />
coherent quantum state with quantum numbers J P C =<br />
1 −− :<br />
|K 0 ¯K0 ⟩ = 1 √<br />
2<br />
{|K 0 ⟩| ¯K 0 ⟩ − | ¯K 0 ⟩|K 0 ⟩}<br />
= N √<br />
2<br />
{|K S ⟩|K L ⟩ − |K L ⟩|K S ⟩} (1)<br />
where N ≃ 1 is a normalization factor. Detection of a<br />
K L thus signals the presence of a K S , and vice-versa.<br />
This is a unique feature at a ϕ-factory, not possible at<br />
fixed target experiments, that can be exploited to select<br />
pure K S and K L beams of precisely known momenta<br />
(event by event) and flux.<br />
The ϕ meson also decays 50% of the times into K + K −<br />
pairs. As for neutral kaons, the identification of a K ∓<br />
decay tags a K ± beam. Therefore the clean selected<br />
samples of K S , K L , K + , and K − can be used to measure<br />
most, if not all, of the properties of the kaon system<br />
with high accuracy, e.g. lifetimes, masses, and absolute<br />
branching ratios (BR).<br />
From the study of semileptonic decays the |V us | matrix<br />
element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix<br />
has been measured with the best accuracy in a single<br />
experiment, and the lepton universality has been tested<br />
measuring the parameter r µe = 1.000 ± 0.008 [1], being<br />
the Standard Model (SM) prediction r µe = 1. From<br />
the leptonic and semileptonic decays, combined with results<br />
from nuclear β decay and pion decays, it is possible<br />
to test with high precision a fundamental property<br />
of charged-current weak interactions in the SM, i.e. the<br />
unitarity of the CKM matrix, which results to be verified<br />
to O(0.1%) (see Fig.1) [1].<br />
At KLOE all the BRs for kaon dominant decays have<br />
been measured, as well as several rare kaon decays.<br />
For instance the BR for the K S → γγ decay, measured<br />
to a few percent precision, BR(K S → γγ) =<br />
(2.26 ± 0.12 ± 0.06) × 10 −6 [2], constituted an important<br />
test for Chiral perturbation theory predictions.<br />
Another important example is the decay K ± → e ± ν,<br />
strongly suppressed in SM (O(10 −5 )), which offers<br />
the possibility of detecting minute contributions from<br />
physics beyond the SM. In particular the ratio R K =<br />
Γ(K → eν)/Γ(K → µν) could deviate from SM prediction<br />
up to a few percent in minimal supersymmetric<br />
models. The measurement R K = (2.493 ± 0.025 ±<br />
0.019)×10 −5 [3] has been found in good agreement with<br />
SM expectations.<br />
The state in eq.(1) has a quite straightforward formal<br />
analogy with the singlet state of two spin 1/2 particles.<br />
Figure 1: KLOE results for |V us| 2 , |V us/V ud | 2 , and |V ud | 2<br />
from β decay measurements, shown as 2σ bands. The ellipse<br />
is 1σ contour from a fit. The unitarity constraint is illustrated<br />
by the dashed line [1].<br />
It can be exploited to perform several tests of Quantum<br />
Mechanics (QM) and CP T symmetry [4]. For instance<br />
the decoherence parameter ζ in the K 0 − K ¯0<br />
basis, signalling<br />
a loss of coherence of the bipartite state and a<br />
departure from the standard QM time evolution, has<br />
been bounded at the level of O(10 −6 ), being ζ = 0 the<br />
QM prediction.<br />
A second data-taking campaign (KLOE-2) is going<br />
to start at DAΦNE upgraded in luminosity. The KLOE<br />
detector has been upgraded with small angle electron<br />
taggers for γγ physics, while the installation near the<br />
interaction point (IP) of an inner tracker is planned<br />
for the next year. The KLOE-2 scientific program<br />
aims, among the several items, to further improve the<br />
experimental studies on kaon physics, and it will greatly<br />
benefit of the improved reconstruction with the inner<br />
tracker of charged tracks and decay vertices near the IP.<br />
References<br />
1. F. Ambrosino, et al., JHEP 04, 059 (2008).<br />
2. F. Ambrosino, et al., JHEP 05, 051 (2008).<br />
3. F. Ambrosino, et al., Eur. Phys. J. C 64, 627 (2009).<br />
4. A. Di Domenico (ed.), Handbook on neutral kaon<br />
interferometry at a ϕ-factory, Frascati Phys. Series 43<br />
(2007).<br />
Authors<br />
C. Bini, V. Bocci 1 , A. De Santis, G. De Zorzi, A. Di<br />
Domenico, S. Fiore, P. Franzini, P. Gauzzi, F. Lacava, E.<br />
Pasqualucci 1 , M. Testa, P. Valente 1<br />
http://www.roma1.infn.it/exp/kloe/<br />
<strong>Sapienza</strong> Università di Roma 126 Dipartimento di Fisica