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