1. B TAGGING
A b quark has a chance of decaying into a low energy (soft) lep-
ton and an up or charm quark. B tagging, the process of identifying a
jet as a b jet, is done via algorithms that pair quarks with soft muons
that are close enough to the jet, appear to originate from a point in
space near the jet, and have energy and momentum such that it is
likely that said soft muon came from a b quark in that jet.
EXPERIMENTAL METHOD AND RESULTS
We hypothesize that γ+jets events (similar to W+jets, but pro-
ducing a photon that does not decay instead of a decaying W boson)
produce the same mixture of jet flavors as W+jets events. Since the
γ+jets is large and relatively pure (there is no relatively pure W+jets
data), we measured the probability, referred to as the tag rate, in
γ+jets data that a given track that is close to jet will be a soft muon
that causes said jet to be tagged. We create the tag rate as a function
of pT, the muon’s momentum in the direction orthogonal to the
beamline, and |ηdet|, a value representing the muon’s position in the
detectorb
.
Predicting W+jets background to top-antitop events in the Large Hadron Collider
Ken Bloom1
, Aaron Dominguez1
, Maxwell Gregoire1
, Jason Keller1
, Tuula Maki2
, Helena Malbouisson1
, Jeff Temple3
,
Don Teo4
, and Peter Wittich4
1 University of Nebraska-Lincoln, United States; 2 CERN; 3 University of Maryland, United States; 4 Cornell University, United States
INTRODUCTION
The Large Hadron Collider (LHC) at CERN, Switzerland, is cur-
rently colliding beams of particles with record-high center-of-mass
energy of 7 TeV, much higher than the energy of the collisions at the
Tevatron in Fermilab, IL, which peak at 1.96 TeV[1][2]
. At higher ener-
gies, we are able to observe more particles of higher massa
. tt̅ events,
events in which a top quark and anti-top quark are produced, are es-
pecially important because they are plentiful, have an accurately-
predicted production rate, and involve the production of many differ-
ent particles. We choose to analyze tt̅ events in order to better under-
stand the detector. One of the first steps to analyzing tt̅ events is un-
derstanding the background, or non tt̅ events that appear to be tt̅
events.
THE COMPACT MUON SOLENOID
The detector we work with is the Compact Muon Solenoid
(CMS), a general purpose detector optimized for muon detection.
CMS uses a silicon tracker, muon detectors, and electromagnetic and
hadronic calorimeters to detect and identify particles. Muons are de-
tected by the tracker and muon detectors and leave few traces in the
calorimeters. Quarks and gluons appear as jets, collimated streams of
many types of particles (mostly hadrons), in the calorimeters and the
trackers.
TT̅ EVENTS AND W+JETS EVENTS
Figure 2 diagrams a typical W+jets event and tt̅ event and
shows how those events appear to the detector. In each event the de-
tector detects a muon, a neutrino, and four jets; the only difference is
that we expect two b jets in tt̅ events, while the jets in W+jets events
are much less likely to include two b jets. Since non-b jets are often
mistaken for b jets, and since the jets in W+jets events sometimes are
b jets, we expect to have W+jets events sometimes look like as tt̅
events in the detector. W+jets events comprise the largest portion of
the tt̅ background.
REFERENCES
[1] “LHC Machine Outreach” http://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/
(retrieved March 1, 2011)
[2] “LHC Beam” http://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/beam.htm
(retrieved March 1, 2011)
[3] “Overview of CMS Physics Goals and Detector” https://twiki.cern.ch/twiki/bin/view/
CMSPublic/WorkBookCMSExperiment (retrieved March 1, 2011)
Fig. 1[3]
: Description of how different particles are detected in the CMS detector.
The tag rate is roughly-constant for all pT except high pT, in which case
the tag rate decreases drastically. The tag rate increases subtly as
|ηdet| with fluctuations at 1.4 and 2.2.
To apply the tag rate to a data sample, we scale a parameter of
our choice in each event by the factor 1 - ∏i
all tracks
[ 1 - Pi(pT , ηdet) ],
which represents the probability that the event will contain at least
one b tagd
. For our analysis, we choose jet multiplicity per event as
our parameter because we expect a dependence of tt̅ and tt̅ back-
ground levels on jet multiplicity.
First, we apply the tag rate to the γ+jets sample from which it
was created to verify the accuracy of our method. We compare the
scaled jet multiplicity to the jet multiplicity for only events containing
at least one b tag.
The results of this closure test show that our method can accurately
predict the γ+jets contribution to a given data set.
Next, we applied the tag rate to a sample of jet data.
The high level of agreement of this application verifies our hypothe-
sis; Our tag rate produced from γ+jets data is capable of predicting
other jet backgrounds, particularly W+jets background.
Finally, we use the tag rate to predict the tt̅ and W+jets contri-
butions to the l+jets data (events containing at least one high-energy
lepton and any number of jets). We use the following iterative
method:
Fig. 4: The tag rate as a function of pT (top) and ηdet (bottom)c
.
FOOTNOTES
a. The equation E=mc2
illustrates how higher energies are likely to produce higher numbers of
more-massive particles.
b.ηdet = -ln( tan( θ / 2 ) ), where θ is the angle between the beamline and the vector from the cen-
ter of the detector to the point at which the particle entered the detector.
c. The tag rate is actually a multivariable function of pT and ηdet, as opposed to the two separate
single-variable functions of each parameter shown in figure 3. However, the displayed plots
provide a better visual representation of the tag rate.
d.The scaling factor 1 - ∏i
all tracks
[ 1 - Pi(pT , ηdet) ] is different for each event since the number of
tracks and their respective pT and ηdet are different for each event.
Since events with low jet multiplicity are very unlikely to be tt̅ events,
our results show that our method correctly estimates the non-tt̅ con-
tributions to the data. This agreement implies that we can also esti-
mate the non-tt̅ contributions to events with higher jet multiplicity
which we do expect to have significant tt̅ contribution.
CONCLUSIONS
We can correct for the geometric limitations of the detector,
the limited efficiency of kinematic requirements, and the fact that
some tt̅ events contain b jets that do not have tagging muons to ob-
tain an estimate of the number of tt̅ events. We can divide this num-
ber by the integrated luminosity, 36 pb-1
, to obtain the tt̅ cross sec-
tion.
σtt̅ = 150 ± 9 (stat.) ± 17 (syst.) ± 6 (lumi.) pb
This matches the theoretical prediction of 164-13
+10
pb within uncer-
tainty. The greatest part of our uncertainty is derived from how accu-
rately we are able to describe the tt̅ background using the tag rate.
Fig. 2: A diagram of a tt̅ event and W+jets event in the CMS detector. Objects be-
low the “detector” line represent particles that reach the detector. Quarks mani-
fest themselves in the detector as jets.
Fig. 6: Application of the tag rate to the jet data.
——— jet multiplicity scaled by 1 - ∏i
all tracks
[ 1 - Pi(pT , ηdet) ]
● jet multiplicity for events with at least one b tag
Fig. 5: Application of the tag rate to the γ+jets data from which it was created.
——— jet multiplicity scaled by 1 - ∏i
all tracks
[ 1 - Pi(pT , ηdet) ]
● jet multiplicity for events with at least one b tag
Fig. 7: Flowchart depicting iterative method that predicts the tt̅ and W+jets con-
tributions to the l+jets data.
l+jets data
1. apply tag rate
prediction of
amount of tagged
W+jets data
2. subtract from
actual amount of
tagged W+jets data
amount of tt̅ in
tagged W+jets data
4. use as
correction
factor on
3. impliesamount of tt̅
in l+jets data
START
First iteration: assume all
l+jets data is W+jets data.
Repeat until
converges
Fig. 8: The composition of l+jets data.
● total l+jets data
primary muon,
pT: 45.1 GeV/c
b-tagged jet,
pT: 38.4 GeV/c
soft muon,
pT: 5.5 GeV/c
Fig. 3: Diagram of an event containing a high-energy muon, a b-tagged jet, and a
tagging soft muon.
cones = jets; lines = muons; bars = energy depositions in the calorimeters
ACKNOWLEDGEMENTS
 UCARE and the National Science Foundation, for funding.
 The LHC accelerator physicists, for the excellent performance of the machine.