CP
asymmetry is the only non-trivial difference between matter and antimatter
found so far. Its discovery in neutral kaon decays in 1964 came as a big
surprise to the physics community, but today it is an essential component of
the Standard Model of particle physics. Without CP asymmetry the Big Bang would
have created equal amounts of matter and antimatter, which would all have then
annihilated, leaving behind an empty Universe filled with radiation. To produce
a matter-dominated Universe like the one we live in, an excess of matter must
have formed and survived this annihilation. But to produce such an excess, some
difference between matter and antimatter must be present: enter CP asymmetry.
Unfortunately, the amount of CP asymmetry present in the Standard Model of
particle physics is not enough to explain the observed composition of the
Universe, driving extensive studies of this phenomenon and searches for other
sources of CP asymmetry.
This
week, at the Rencontres de Moriond Electroweak conference and during a seminar
held at CERN, the LHCb collaboration presented new results from studies of CP
asymmetry in charmless three-body decays of charged B mesons. These decays
involve a charged B meson, consisting of a beauty quark and an up quark,
transforming into a combination of π and K mesons. The name “charmless” refers
to the absence of charm quarks in the final state: π± mesons (pions) contain
only up and down quarks, and K± mesons (kaons) contain a strange and an up
quark. Charmless decays involve the transformation of a beauty quark into an up
quark, which is an unlikely process, as the beauty quark predominantly decays
into a charm quark. In this rare process the effects of CP violation are
expected to be enhanced.
The new
LHCb results focus on “direct” CP violation: a phenomenon where the same decay
process has a different probability for a particle than for an antiparticle.
The strongest global asymmetry was observed for the decay into two kaons and
one pion, where the probability of a B+→π+K+K- decay is about 20% higher than
for the B-→π-K+K-decay (corresponding to a measured CP asymmetry ACP of
-0.114). A global CP asymmetry has also been observed with a significance of
more than five standard deviations for the first time in decays into three
pions and decays into three kaons. For the final state with two pions and one
kaon, CP violation is still not confirmed.
The
three-particle final state can, however, be studied further in order to extract
more information. The process of a B meson transforming into three particles
can occur in several steps, with intermediate short-lived particles
(“resonances”) forming and subsequently decaying into the pions and kaons seen
in the final state. These processes can make different contributions to the CP
asymmetry and can be disentangled by taking into account the momenta of the
final state particles in what’s known as “phase space analysis”. One
spectacular result of such an analysis is the indication of a χhc0 meson
(containing a charm-anticharm quark pair) being formed during the B→πππ decay.
The χhc0 was not expected to contribute to CP violation but the results show
the presence of a significant asymmetry. In fact, the subset of data containing
the χhc0 events features the highest CP asymmetry ever observed: the B- meson
makes an almost 7 times greater contribution to this process than its
B+counterpart, as can be seen in the plot below.
The results presented provide important clues about the mechanism of CP asymmetry generation in the Standard Model, which is not yet fully understood. Even more detailed studies will be performed in the upcoming LHC Run 3 with the newly-upgraded LHCb detector.
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