Flavor Tagging at Tevatron incl. calibration and control Page: 1 of 6
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FERMI LAB-CONF-06-495-E
1
Flavor Tagging at Tevatron incl. calibration and control
T. Moulik (DO Collaboration) a* .
aDepartment of Physics and Astronomy, University of Kansas, Lawrence, KS 66045
This report summarizes the flavor tagging techniques developed at the CDF and DO experiments. Flavor
tagging involves identification of the B meson flavor at production, whether its constituent is a quark or an anti-
quark. It is crucial for measuring the oscillation frequency of neutral B mesons, both in the B0 and B system. The
two experiments have developed their unique approaches to flavor tagging, using neural networks, and likelihood
methods to disentangle tracks from b decays from other tracks. This report discusses these techniques and the
measurement of B0 mixing, as a means to calibrate the taggers.1. INTRODUCTION
Flavor tagging is important in studies of neu-
tral B meson mixing and CP violation in the B
system. At hadron colliders, because of minimum
bias events and multiple interactions per cross-
ing, the background level is higher as compared to
the more cleaner environment at e+e- colliders.
Also, at hadron colliders the b-jets may not al-
ways be well separated (as in the case of g -+ bb),
while at e+e- colliders, one gets well balanced
and separated jets. Therefore, in order to iden-
tify b-jets or decays one needs sophisticated tech-
niques to achieve a good efficiency and purity of
the tags. Broadly speaking, flavor tagging meth-
ods can be categorized into Opposite Side Tag-
ging (OST) and Same Side Tagging (SST) [1].
In the case of OST, one uses the decay products
of the opposite B in the event, and since the B
mesons are produced in pairs in a bb event, the
B flavor of the tag side is ideally opposite to the
B flavor of the decay side (The B meson which
is fully or partially reconstructed is referred to
as the "decay" or "reconstruction" side B me-
son). The tag side B meson however could oscil-
late if it was neutral, independent of the recon-
struction side B meson and therefore in such a
case the tag would incorrect. OST flavor taggers
are sub-categorized into (i) Lepton taggers, where
one exploits the charge correlation of the lepton
and the quark flavor in semileptonic b decays, (ii)
Jet charge taggers, where one use the fact that the
*MS 352, Fermilab Box 500, Batavia, IL-60510kinematically weighted charge of the b-jet is corre-
lated with the charge of the b, and (iii) Opposite
Side Kaon Tagger (OSKT), where one exploits
the correlation of the charge of the kaon with the
B flavor in the decay chain b -+ c -+ XK-. In
the case of SST, one utilizes the charge correla-
tion of the fragmentation tracks produced on the
same side as the "reconstructed" B. For SST
the wrong sign contribution due to the "tag" B
meson oscillating is absent, and it has a high effi-
ciency, but it has a somewhat worse purity than
the OST tagger, as it is difficult to identify the
correct fragmentation track from a large number
of decay product tracks.
Flavor taggers are optimized to obtain high effi-
ciencies (e) and purities, where e is defined as the
fraction of reconstructed events (Ntot) that are
tagged (Ntag): e = Ntag/Ntot. The term dilution
(D) is more commonly used, which is related to
the purity, P, as D = 2P -1 and is defined as the
normalized difference of correctly and wrongly
tagged events: D = Ncor-Nwr _ NcorNwr where,
Ncor+Nwr Ntag
P = Ncor/Nag. The terms "correctly" and
"wrongly" refer to the determination of the de-
cay B meson flavor. The effective tagging power
of a tagging algorithm is given by eD2 and the
goal is to maximize this quantity.
2. CDF AND DO DETECTOR
Leptons from B decays have low momenta as
compared to leptons from top or electroweak de-
cays, and require special treatment to identify
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Moulik, T. Flavor Tagging at Tevatron incl. calibration and control, article, January 1, 2007; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc878718/m1/1/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.