Measuring Leptonic B Decays at BaBar
Michael Dahlstrom
Iowa State University REU Program
August 3, 2001
Abstract
Using data gathered from the BaBar experiment at the Stanford Linear
Accelerator Center, I searched for B mesons that decay into two charged leptons,
focusing on the dielectron, dimuon, and electron-muon decay channels. These decays are
interesting because their branching fractions have only upper limits. Also, the existence
of any electron-muon pair product, or eµ, would give strong support to several beyond
the standard model theories. So far, the data is in agreement with the results from the
CLEO and ARGUS collaborations. This analysis is still in progress, however, and this
paper is merely a report on the preliminary details.
Introduction
1.1 The BaBar Experiment
The BaBar detector at Pep-II was designed to study B mesons produced in
asymmetric electron-positron collisions. “Asymmetric" refers to the fact that the colliding
particles have different energies. This gives the resulting particles momentum in the
laboratory reference frame, allowing their lifetimes to be measured even if they carry
away most of the collision energy. In the current run at Pep-II, electrons are stored in one
ring at 9 GeV and positrons in the other at 3.1 GeV. This sets the collision energy right
at the upsilon (4S) resonance; a short- lived combination of a bottom quark and its antiquark. This resonance decays preferentially into a pair of mesons called B and B bar,
hence the name of the detector.
Mesons are short-lived systems made up of a quark and an anti-quark; B mesons are
mesons in which one quark is a bottom (or an anti-bottom) and the other is a light quark
(up, down, strange, or their corresponding anti-quarks). The BaBar detector is optimized
to measure the decay process of these B 's as precisely as possible.
B mesons decay into many different products. We now look into three certain types
of decays.
1.2 Leptonic B Decays
B mesons have hundreds of decay products and it is impossible to predict which
product will be produced. However, the probability of each product is measurable and
most are well known. This probability is called the branching fraction.
Three possibilities of decay products for the B meson are into two charged electrons,
two charged muons, and one e and one µ. Because this last channel violates lepton
number, it should have a zero branching fraction. The actual branching fractions of all
three channels are as of yet unknown, but there are upper limits from the CLEO
collaboration (Table 1).
Even though the actual branching fractions have yet to be measured, the Standard
Model predicts non-zero values for the dielecton and dimuon channels. On the other
hand, it strictly forbids any B meson to eµ decays because it violates lepton number. Any
non-zero eµ branching fraction, no matter how small, would have great significance, as it
would expose a fracture in the standard model.
Data
.
2.1 Event Selection
We began by using Monte Carlo data to reconstruct events where the B-meson
decayed into two electrons, two muons, or one electron and one muon. The background
was kept to a minimum by applying the following preliminary cuts (Table 2).
The invariant mass (Mll) must lie between 5.20 and 5.33 GeV/c^2. The total number
of charged tracks of each event must be greater than or equal to 4. Two oppositely
charged tracks must come from the common vertex. The angle between the lepton pair
direction and the thrust axis of the rest of the event ( | cos θ thrust | ) must be greater that
0.7. The ratio between the second and zeroth Fox-Wolfram moments, (R2) must be
greater that 0.5. Finally, the mass must be between 5.275 and 5.287 GeV and the ∆E must
be between -.0.038 and 0.05 GeV. These provide a 2-sigma variance.
I stress again that these cuts are preliminary and as the analysis continues, they most
likely will change.
2.2 Mass Fit
This is where the analysis stands now. The expected mass and ∆E signal for all three
lepton events are shown in Figures 1 through 3 (Fig. 1 , 2, 3).
Conclusions
This study is still far from finished, however progress is being made. The
simulated data is near completion and the real data will be compared with expectations
when it is finalized. In this experiment, any information is useful due to the fact that these
figures have never been measured before. And with BaBar’s greater number of events
than both CLEO and ARGUS, we hope for much tighter limits on the leptonic branching
fractions in the near future.
Acknowledgments
I thank Dr. Jim Cochran from Iowa State University who advised me in this
research. The data came from the BaBar group at the Stanford Linear Accelerator Center
in Palo Alto, California. The first two paragraphs of 1.1, marked with a (1), are a slightly
modified form of a similar section from Measuring L = 1 D mesons at BaBar, written by
Tom Plagge, July 31, 2000.
References
[1] Langenegger, Urs; “Looking for Leptonic B Decays”, (2001).
[2] Plagge, Tom; “Measuring L = 1 D mesons at BaBar”, (2000).
[3] Yang, Songhoon; BaBar note #394, “ A search for B0 → l + l – at BaBar”, (1997).
[4] Yang, Songhoon; BaBar note #447, “ Constraints on New Physics from B → l + l”,
(1998).
Table 1: Branching fractions for leptonic decays
Mode
Experimental Limit
SM Expectation
e+ e-
< 8.3 x 10 –7
1.9 x 10 –15
µ+ µ-
< 6.1 x 10 –7
8.0 x 10 –11
e+ µ-
< 15 x 10 –7
Table 2: Preliminary cuts
5.20 < Mll < 5.35 GeV/c2
Ntrack > 4
| cos θ thrust | < 0.7
R2 < 0.5
5.275 < M < 5.287 GeV/c2
0
-0.038 < ∆E < 0.050 GeV/c2
Figure 1: Mass and delta E of the Electrons
Figure 2: Mass and delta E of the Muons
Figure 3: Mass and delta E of the Emu