CMS-B2G-16-006

CMS-B2G-16-006 ; CERN-EP-2016-319
Search for single production of vector-like quarks decaying into a b quark and a W boson in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
28 January 2017
Phys. Lett. B 772 (2017) 634
Abstract: A search is presented for a heavy vector-like quark, decaying into a b quark and a W boson, which is produced singly in association with a light flavor quark and a b quark. The analysis is performed using a data sample of proton-proton collisions at a center-of-mass energy of $ \sqrt{s} = $ 13 TeV collected at the LHC in 2015. The data set used in the analysis corresponds to an integrated luminosity of 2.3 fb$^{-1}$. The search is carried out using events containing one electron or muon, at least one b-tagged jet with large transverse momentum, at least one jet in the forward region of the detector, and missing transverse momentum. No excess over the standard model prediction is observed. Upper limits are placed on the production cross section of heavy exotic quarks: a T quark with a charge of $ 2/3 $, and a Y quark with a charge of $ -4/3 $. For Y quarks with coupling of 0.5 and ${\cal{B}}(\mathrm{Y} \to \mathrm{ b } \mathrm{ W }) = $ 100%, the observed (expected) lower mass limits are 1.40 (1.0) TeV. This is the most stringent limit to date on the single production of the Y vector-like quark.
Links: e-print arXiv:1701.08328 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading order Feynman diagram for singly produced Y or T quarks.
png pdf	Figure 2: Kinematic distributions in the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $-enriched control sample: $S_\mathrm {T}$ (left) and $M_{\mathrm {inv}}$ (right). The last bin includes overflow events. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot.
png pdf	Figure 2-a: $S_\mathrm {T}$ distribution in the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $-enriched control sample. The last bin includes overflow events. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot.
png pdf	Figure 2-b: $M_{\mathrm {inv}}$ distribution in the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $-enriched control sample. The last bin includes overflow events. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot.
png pdf	Figure 3: Kinematic distributions in the $\mathrm{ W } $+jets-enriched control sample: $S_\mathrm {T}$ (left) and $M_{\mathrm {inv}}$ (right). The last bin includes overflow events. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot.
png pdf	Figure 3-a: $S_\mathrm {T}$ distribution in the $\mathrm{ W } $+jets-enriched control sample. The last bin includes overflow events. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot.
png pdf	Figure 3-b: $M_{\mathrm {inv}}$ distribution in the $\mathrm{ W } $+jets-enriched control sample. The last bin includes overflow events. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot.
png pdf	Figure 4: The invariant mass $M_{\text {inv}}$ distribution of heavy quark candidates, reconstructed from their decay products: the lepton, the leading central jet, and the neutrino. The distribution is obtained after the fit, assuming the background-only hypothesis. The dashed histogram shows the event distribution expected for a Y quark with a mass of 1.0 TeV , coupling of 0.5 and ${\cal {B}}( {\mathrm {Y}} \to \mathrm{ b } \mathrm{ W } ) = $ 100%. The statistical and systematic uncertainties are represented by the hatched band on the ratio plot. In the last bin the data overflow event is an electron channel event with a mass of 2.22 TeV .
png pdf	Figure 5: Expected and observed limits on the single VLQ production ($ \mathrm{ pp \to Ybq } $ and $ \mathrm{ pp \to Tbq } $) cross section together with the one and two standard deviation uncertainty bands.

Tables
png pdf	Table 1: Summary of the systematic uncertainties used for the simulated backgrounds and the signal events in the signal region.
png pdf	Table 2: Data, background, and possible signal pre-fit event yields corresponding to 2.3 fb$^{-1}$ of integrated luminosity. The signal sample is the $M( {\mathrm {Y}} )=$ 1.0 TeV mass point using the NLO crosssection [30]. The percentage in the signal column indicates the signal efficiency. The background uncertainties include both the statistical and the systematic pre-fit components.
png pdf	Table 3: Expected and observed upper limits (UL) on the single VLQ production cross section, assuming ${\cal {B}} (\mathrm{VLQ} \to \mathrm{bW} )=$ 100%.

Summary

A search has been performed for single production of a vector-like quark decaying into a b quark and a W boson in the electron/muon + jets channels. The mass of the vector-like quark is reconstructed by forming the invariant mass of the leading b-tagged jet, electron or muon, and missing transverse momentum in the event, and a fit to the invariant mass spectrum is performed. No evidence of an excess due to new physics is observed. Upper limits at 95% CL are set on the cross sections for single production of vector-like Y and T quarks in the mass range from 0.70 to 1.80 TeV. In the framework of the model considered, Y quarks with a coupling of 0.5 and ${\cal{B}}(\mathrm{Y} \to \mathrm{ b } \mathrm{ W }) = 100%$ are excluded in the mass range from 0.85 to 1.40 TeV. This result may be compared with the expected region of excluded masses, which extends up to 1.0 TeV. These results represent the most stringent limits to date on the single production of a vector-like Y quark. In the case of T quarks with a coupling of 0.5, the theoretical cross section is the same as for Y quarks, but the expected decay branching fraction ${\cal{B}}(\mathrm{T} \to \mathrm{ b } \mathrm{ W })$ is 50%, only half that expected for ${\cal{B}}(\mathrm{Y} \to \mathrm{ b }\mathrm{ W })$. Thus mass exclusion limits similar to those achieved for the Y quark would only be obtained for ${\cal{B}} ( \mathrm{T} \to \mathrm{ b } \mathrm{ W }) = 100%$.

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