Probes B Quark Coupling
By Heather Rock Woods
In the world of elementary particles,
beauty couples preferentially to charm. BABAR
studies these common ’charm’ decays, where B mesons containing a b quark
(b for beauty, or bottom) decay to charm mesons containing a charm
measurement of the decay rate as function of q2,
the invariant mass squared of the lepton-neutrino system, for
B-meson decays to a lepton, a neutrino and a pi meson. The
measured distribution (data points) is compared to the
predictions of various form-factor calculations (histograms).
(Image courtesy of Jochen C. Dingfelder)
experimentalists have been intrigued by the less favored couplings
between b quarks and non-charm quarks because they are less understood
and not fully predictable. Physicists expect that exploring charmless
decays will provide new insight into the world of quarks bound in states
student Amanda Weinstein (now at UCLA), Jochen Dingfelder, Mike Kelsey
and Vera Lüth (all EC) have sifted through 83 million BABAR
events looking for those where B mesons decay to a pi meson (an up (u)
quark plus an anti-down quark) and to a pair of leptons—an electron or
muon plus an undetectable neutrino. Such decays are called
‘semi-leptonic’ because the products include leptons and a meson.
“Experimentally, the challenge is two-fold,” said Lüth.
“First, the neutrino cannot be detected, thus its presence has to be
inferred from the fact that a sizable fraction of the B meson energy and
momentum appears to be lost. This is a direct application of Einstein’s
famous expression, E=mc2.
Second, charmless decays make up only 0.2 percent of all the semi-leptonic
B decays, and thus one has to look very hard to find them!”
Data Analysis—Form Factors Function Set
group found more than 500 of the rare decays among the very common
decays to a charm meson and a lepton pair. Because the leptons are
simple in nature and well understood, they can serve as a probe of the
properties of the very heavy B meson and its transition to the very
light pi meson (about 37 times lighter). “We use the part we
understand—the weak decay to the electron and neutrino—to examine the
part we don’t understand,” Dingfelder said.
One thing that makes this study complicated is the fact
that the quarks are not free and detectable because the strong force
binds quarks inside the mesons. Thus scientists have to infer the quark
properties from the observed meson, and this requires theoretical models
or calculations that are notoriously difficult.
“We can summarize the aspect of the problem which is the
least understood—the part of the decay involving the strong force and
bound quarks—in terms of a set of functions known as form factors,” said
“There are various
predictions for form factors, ranging from models to sophisticated
calculations, and their predictions vary a lot,” said Lüth. “We want to
disentangle this puzzle.”
PEP-II Luminosity Helps
Nail Down Vub
“Thanks to PEP-II’s
generous luminosity, for the first time we have enough data to measure
the form factor for this particular decay, which describes how the
quarks in the initial B meson turn into a u quark in the pi meson,” said
It turns out that
the model experimenters have relied on for many years now appears to be
the least likely, while the data agree well with two calculations
released last year that required huge computer farms to solve highly
sophisticated mathematical approximations.
A precise understanding of the form factor contributes to
a way of nailing down ‘Vub,’ the coupling strength between b and u
quarks. Vub determines the probability that a B meson will decay into a
pi meson. Its precise measurement will also test physicists’
understanding of the observed asymmetry between matter and anti-matter
in B meson decays.
researchers presented their results, the most precise to date, at the
Moriond Conference in Italy and the CKM Workshop in San Diego, both held
in March. Several BABAR
groups are now busy updating and improving the measurement of Vub and
the form factors using the three-fold increase in data gathered through
2004. Significant progress depends on close collaboration with
theorists. The competition, the Belle experiment in Japan, is likewise
aiming to do better.