Multi-boson production Page: 2 of 8
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boson decays are utilized to extract limits on the TGC
parameters. These distributions are compared with MC
predictions for different set of coupling constants in order
to evaluate 95% C.L. limits.
The CDF and DO collaborations are producing many
competitive measurements in the diboson sector. In the
following the most updated results are presented. Only
peculiar features will be discussed.
The measurement of the cross section for the pro-
duction of Zy events, with the Z boson decaying in
two neutrinos and the photon with ET > 90 GeV and
Iyr < 1.1, has been performed by the DO collaboration
using a sample of data corresponding to an integrated
luminosity of 3.6 fb-1 . The analysed events have
been collected with a trigger from a set of high-ET sin-
gle EM-cluster triggers, which are (99 1)% efficient
for photons of ET > 90 GeV. The events selection re-
quires also the presence of a single photon candidate
and Yr > 70 GeV. The background to the -y+ Yr sig-
nal is mainly due to misidentified W or Z events and
to events in which muons from the beam halo or cos-
mic rays undergo bremsstrahlung and produce energetic
photons. This non-collision background is reduced using
a pointing algorithm which utilizes the transverse and
longitudinal energy distributions in the electromagnetic
(EM) calorimeter and central preshower detector, to esti-
mate the position of production vertex (zEM) along the
beam direction (z axis). The algorithm assumes that
given EM showers are initiated by photons and utilizes
the distance of closest approach of the direction of the
EM shower to the z axis. Selected events are required to
have IzEM - zVI < 10 cm, where zV is the z position of
the reconstructed primary vertex of the event.
After applying all selection criteria 51 candidate
events are observed, with a predicted background of
17 0.6(stat.) 2.3(syst.) events. The measured cross
section for the production of Zy events with Z -> v
and the photon with ET > 90 GeV is:
o- 32 9(stat. + syst.) 2(lumi.) fb
This result is in good agreement with the next-to-leading
order (NLO) prediction of 39 4 fb. The statistical signif-
icance of this measurement has been estimated perform-
ing 108 background-only pseudo-experiments: the p-
value is evaluated as the fraction of pseudo-experiments
with an estimated cross section above the measured one.
This probability is found to be 3.1 10--7, which corre-
sponds to a statistical significance of 5.1 standard devia-
tions, making this the first observation of the Zy -> uVVy
process at the Tevatron.
The limits on the anomalous triple gauge couplings
(aTGC) are evaluated comparing the photon ET spec-
trum in data with that from the sum of the background
and the predicted signal for each pair of couplings
for a grid in which h30 runs from -0.12 to 0.12 with
a step of 0.01 and h40 runs from -0.08 to 0.08 with
step of 0.001. The MC samples are generated with a
leading order (LO) Zy generator (corrected for the NLO
with an ET-dependent K-factor) for the form-factor
A 1.5 TeV. The resulting one-dimension limits in the
neutrino channel alone are h20 < 0.036, Ih 0 < 0.0019,
Ih30 < 0.035, Ih30 < 0.0019. To further improve the
sensitivity these measurements are combined with the
limit evaluated for the Zy -> lly (l e, ) channels on
100 150 200 250 300
Fig. 2. Photon ET distribution in data, compared with the sum
of backgrounds, the sum of MC signal and background for the
SM prediction (solid line) and for the aTGC prediction with
h30 = 0.09 and h40 = 0.005 (dashed line). The shaded band
corresponds to the +1 standard deviation total uncertainty
on the predicted sum of SM signal and background.
a 1 fb- data sample, previously analysed  by the DO
collaboration. The combination of all three channels
yields the most stringent limits on the aTGC set at
a hadron collider to date, reported in Table 2. The
limits on the h30, h30 and hjo couplings improve on the
constraints from LEP II, and are the most restrictive to
Table 2. One-dimension 95% C.L. limits obtained by the DO
collaboration in the analysis of the ZZy and Zyy couplings.
Parameter 95% C.L.
h30 (-0.033, +0.033)
h40 (-0.0017, +0.0017)
h30 (-0.033, +0.033)
h o (-0.0017,+0.0017)
0.1 -0.05 0 0.05 0.1
Fig. 3. Two-dimensional bounds (ellipses) at 95% C.L. on CP-
conserving Zyy (left) and ZZ; (right) couplings. The crosses
represent the one-dimension bounds at the 95% C.L. setting
all other couplings to zero. The dashed lines indicate the
unitarity limits for A = 1.5 TeV.
- t DO, 3.6 Wu
-... Sum of backgrounds
- SM signal MC + backgrounds
. ATGC signal MC + backgrounds
(a) Zyy DO, 3.6 fh'
(b) ZZy DO, 3.6 M'3
A = 1.5 TeV...._......_....
01 -05 0 0-05 0.1
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Mastrandrea, Paolo. Multi-boson production, article, September 1, 2010; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc1012956/m1/2/: accessed April 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.