Plain English Summary of "Mesurement
of the Ratio of B+ and B0 Meson Lifetimes"
Life expectancy was supposed to be the same for neutral and charged B
mesons - it turned out not to be true as showed a recent precise measurement
performed at the DZero detector at Fermilab. The lifetime of these mesons
differs by eight percent, about by how much women live longer than men.
Neutral and charged B mesons have a lot of things in common. As all mesons
they are composed of a quark-antiquark pair. One quark is always a "bottom"
quark, the second heaviest of all existing quarks. The other partner is a
light quark, either of "up" or "down" type. The two quarks, heavy and
light, share their responsibilities for the properties of B mesons. Heavy
quark cannot easily disintegrate and this determines the long lifetime of
the mesons. For nuclear time scales this lifetime is amazingly long, 1.5
ps (one-trillionth of a second) and, in fact, B mesons hold a record among
other particles in the same weight category. Though long for high energy
physics scale, one and a half ps is about enough time for light to pass only
half a millimeter.
The light quark determines if the B meson is neutral or charged - also
an impotant property. A (b - anti-u) pair forms a charged B meson and a (b
- anti-d) pair forms a neutral B meson. This fact is simple to understand
because the charge of b quark is -1/3 and charge of anti-d is 1/3 so
there sum is 0. Similarly a combination of b and anti-u quarks has charge
(-1/3) + (-2/3) = -1. For a long time it was assumed that the light quark,
which is stable, plays only a "spectator" role in the disintegration of
the heavy partner. However, it turns out the life expectancy does
depend on who is your partner and exchanges between heavy and light quarks
matter. Initial theoretical calculations showed that the lifetimes should
be different by several percent. Recent progress in the theory allowed us
to determine all contributions to the lifetime calculations with more precision
than previously possible and predict the ratio of these lifetimes at 1.06
+- 0.02. Thus, the theory sais that charged B meson lives longer than neutral
B meson by approximately 6 percent. It is very important now to improve
the precision of experimental measurements in order to test the theoretical
assumptions used in the calculations. In Run II, the DZero experiment has
collected a large sample of B- and B0 decays and has made a precision measurement
of this difference using decays of B mesons to a muon, its neutrino and
a D meson as explained in more detail below.
Physicists at Fermilab have a long history of studying B mesons at the
Tevatron collider. These mesons are abundantly produced in the proton-antiproton
collisions and experiments use complicated trigger algorithms and electronics
to select those events out of billions of other collisions. At the analysis
phase B mesons are reconstructed combining a muon and two or three other
particles. Muons are very similar in properties to electrons but they are
about 200 times heavier. Experimentalists like muons because they do
not interact much with matter and reach the outmost part of the detector
penetrating through thick slabs of iron. This allows for easy identification
of the muons. The other particles in the same event have to form D mesons
and the exact type of D meson indicates the charge of the parent B meson.
D mesons are composed of a "charm" quark and a light anti-quark. These mesons
can be reconstructed as peaks in the invariant mass distributions as shown
in the figures below. If two particles, kaon and pion, tend to originate
from something that weighs like a D0 meson with mass about 1.86 GeV (left
plot) then with high probability their parent B meson was charged.
However is some cases there could be another pion which forms a D*+ meson
together with the two first particles. We can identify these cases looking
at the mass difference between the kaon-pion mass and kaon-pion-pion mass
(right plot). If this is true and these kaon and two pions form a D*+ peak
then with high probability their parent B meson was neutral.
We split these D0 and D* samples into intervals of time and measure the
ratio of lifetimes by taking the ratio of the number of D0 and D*+ events
in each interval. This ratio is shown at the plot below. The variable "Visible
Proper Decay Length" used for this plot is closely related to the lifetime
that we want to measure. If the lifetimes of charged and neutral B mesons
were the same, the data points in the below histogram would fall on a flat
line. We see that this is not the case - the number of neutral B mesons decreases
faster than the number of charged mesons with time and this proves
that the lifetimes are different. We use a refined mathematical procedure
that takes into account many other experimental and theoretical inputs and
their uncertainties to extract the exact value of the lifetime ratio from
this graph below.
Our result submitted for publication in Phys.Rev.Letters in October 2004,
R = 1.080 +- 0.016 (stat) +- 0.014 (sys), agrees with previous results,
and the precision of our measurement is better than the previously published
single best measurements from the Belle experiment at KEK and BaBar experiment
at SLAC. The new result of DZero is also in good agreement with the
latest theoretical prediction.
The preliminary result of this analysis made headlines in the "Fermilab
Today" daily newsletter in April 2004.
http://www.fnal.gov/pub/today/archive_2004/today04-04-29.html
The full text of the submitted publication can be found at the link below.
http://arxiv.org/abs/hep-ex/0410052
Please address your further inquires to the primary authors : Guennadi
Borissov, Sergey Burdin, Andrei Nomerotski.