CMS-EXO-20-008

CMS-EXO-20-008 ; CERN-EP-2022-037
Search for narrow resonances in the b-tagged dijet mass spectrum in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
4 May 2022
Phys. Rev. D 108 (2023) 012009
Abstract: A search is performed for narrow resonances decaying to final states of two jets, with at least one jet originating from a b quark, in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The data set corresponds to an integrated luminosity of 138 fb$^{-1}$ collected with the CMS detector at the LHC. Jets originating from energetic b hadrons are identified through a b tagging algorithm that utilizes a deep neural network or the presence of a muon inside a jet. The invariant mass spectrum of jet pairs is well described by a smooth parametrization and no evidence for the production of new particles is observed. Upper limits on the production cross section are set for excited b quarks and other resonances decaying to dijet final states containing b quarks. These limits exclude at 95% confidence level models of Z' bosons with masses from 1.8 to 2.4 TeV and of excited b quarks with masses from 1.8 to 4.0 TeV. This is the most stringent exclusion of excited b quarks to date.
Links: e-print arXiv:2205.01835 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Simulated signal shapes of b* from the process $\mathrm{b} \mathrm{g} \to {\mathrm{b} ^{\ast}} \to \mathrm{b} \mathrm{g} $. Shown are the wide jets (see text) used to reconstruct the dijet mass spectra.
png pdf	Figure 2: The product of acceptance and efficiency of the event selection for a $\mathrm{Z'} \to \mathrm{b} {}\mathrm{\bar{b}} $ resonance as a function of the resonance mass.
png pdf	Figure 3: The observed differential cross sections as a function of the dijet mass, shown as fit with the background functions, for the four tagging categories (rows) and the three data-taking periods (columns). The number of parameters in the fit, and the goodness of fit "$\chi ^2/\text {ndf}$'', are listed where "ndf'' is the number of degrees of freedom. The lower panel within each row shows the pulls, $(\text {data}-\text {fit})/\text {uncertainty}$, in units of the statistical uncertainty in data. The upper three rows are used to search for Z' models, the bottom row is used to search for the b* model, and example shapes of these signal models are shown with the same arbitrary normalization for three choices of resonance mass.
png pdf	Figure 4: The observed 95% CL upper limits (solid curve) on the product of the cross section and branching fraction (left), and multiplied by signal acceptance (right), for a resonance decaying to $\mathrm{b} {}\mathrm{\bar{b}}$. The corresponding expected limits (dashed curve) and their variations at the 1- and 2-standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for Z' bosons from the sequential SM (SSM) and the heavy vector triplet (HVT) models A and B. The latter two models follow the parameter choices of $ {g_\text {V}} =$ 1 and $ {g_\text {V}} =$ 3 respectively.
png pdf	Figure 4-a: The observed 95% CL upper limits (solid curve) on the product of the cross section and branching fraction for a resonance decaying to $\mathrm{b} {}\mathrm{\bar{b}}$. The corresponding expected limits (dashed curve) and their variations at the 1- and 2-standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for Z' bosons from the sequential SM (SSM) and the heavy vector triplet (HVT) models A and B. The latter two models follow the parameter choices of $ {g_\text {V}} =$ 1 and $ {g_\text {V}} =$ 3 respectively.
png pdf	Figure 4-b: The observed 95% CL upper limits (solid curve) on the product of the cross section and branching fraction multiplied by signal acceptance, for a resonance decaying to $\mathrm{b} {}\mathrm{\bar{b}}$. The corresponding expected limits (dashed curve) and their variations at the 1- and 2-standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for Z' bosons from the sequential SM (SSM) and the heavy vector triplet (HVT) models A and B. The latter two models follow the parameter choices of $ {g_\text {V}} =$ 1 and $ {g_\text {V}} =$ 3 respectively.
png pdf	Figure 5: The coupling strengths to SM bosons (${g_\text {H}}$) and fermions (${g_\text {F}}$) of a Z' boson with mass 2.0 TeV (blue) and 2.5 TeV (red) that are excluded at 95% CL for the HVT model. The shading indicates the excluded side of the contour. The benchmark scenarios corresponding to HVT models A and B are represented by a purple cross and a red point, respectively. The gray shaded area corresponds to the region where the resonance natural width is predicted to be larger than the typical experimental resolution, and thus the narrow-width approximation is not fulfilled.
png pdf	Figure 6: The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for dijet resonances decaying to a b quark and a gluon (points). The corresponding expected limits (short dashed) and their variations at the 1- and 2-standard deviation levels (shaded bands) are also shown. Limits are compared to predictions for single b* production (blue, dot dashed), the resonant component of the b* production via contact interactions (magenta, long dashed), and the total b* signal from the sum of these two production modes (red, solid).

Summary

A search for heavy resonances decaying into b quarks has been presented and no excess has been found over the standard model (SM) expectations. The data were collected by the CMS experiment at $\sqrt{s} = $ 13 TeV during 2016-2018 and correspond to an integrated luminosity of 138 fb$^{-1}$. Model-independent upper limits are set on the product of the cross section of the resonance and its branching fraction to b quarks. Signals of Z' bosons decaying to pairs of b quarks are considered, for both the previously explored sequential standard model (SSM), and also for a new heavy vector triplet (HVT) model. The decays of Z' bosons in both the SSM and the HVT Model A are excluded at 95% confidence level for masses from 1.8 to 2.4 TeV, and limits are set on the coupling strengths of the HVT boson to SM bosons and fermions. Signals of an excited b quark are considered for a previously explored channel, $\mathrm{b}\mathrm{g}\to{\mathrm{b}^{\ast}} \to\mathrm{b}\mathrm{g}$, and a production mode via contact interactions. The excited b quark is excluded at 95% confidence level for masses from 1.8 to 4.0 TeV. This is the most stringent exclusion of excited b quarks to date.

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