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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 quark.

Preliminary BABAR 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)

However, enterprising 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 of matter.

Stanford graduate 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!”

BABAR Data Analysis—Form Factors Function Set

The BABAR 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 Weinstein.

“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 Dingfelder.

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.

The 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.