CMS-TOP-22-003

CMS-TOP-22-003 ; CERN-EP-2024-027
Search for baryon number violation in top quark production and decay using proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
29 January 2024
Phys. Rev. Lett. 132 (2024) 241802
Abstract: A search is presented for baryon number violating interactions in top quark production and decay. The analysis uses data from proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC with an integrated luminosity of 138 fb$ ^{-1} $. Candidate events are selected by requiring two oppositely-charged leptons (electrons or muons) and exactly one jet identified as originating from a bottom quark. Multivariate discriminants are used to separate the signal from the background. No significant deviation from the standard model prediction is observed. Upper limits are placed on the strength of baryon number violating couplings. For the first time the production of single top quarks via baryon number violating interactions is studied. This allows the search to set the most stringent constraints to date on the branching fraction of the top quark decay to a lepton, an up-type quark (u or c), and a down-type quark (d, s, or b). The results improve the previous bounds by three to six orders of magnitude based on the fermion flavor combination of the baryon number violating interactions.
Links: e-print arXiv:2402.18461 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagrams for single top quark production (left) and top quark decays (right) via BNV interactions. The red circles mark the BNV vertices.
png pdf	Figure 2: The BDT output distributions for data (points) and backgrounds (histograms) for the $ \mathrm{e}^+\mathrm{e}^- $ (left), $ \mu^{+}\mu^{-} $ (right), and $ \mathrm{e}^\pm\mu^\mp $ (lower) channels, including the ratio of data to the predicted total background yield. The hatched bands indicate the total uncertainty (statistical and systematic added in quadrature) for the SM background predictions. The predicted yields of the backgrounds and the uncertainty bands are shown after the simultaneous fits for the signal-plus-background hypothesis. Examples of the predicted signal contribution for the BNV interactions via $ \mathrm{t}\mathrm{e}\mathrm{u}\mathrm{d} $ (solid gray line) and $ \mathrm{t}\mu\mathrm{u}\mathrm{d} $ (dashed black line) vertices are shown.
png pdf	Figure 2-a: The BDT output distributions for data (points) and backgrounds (histograms) for the $ \mathrm{e}^+\mathrm{e}^- $ (left), $ \mu^{+}\mu^{-} $ (right), and $ \mathrm{e}^\pm\mu^\mp $ (lower) channels, including the ratio of data to the predicted total background yield. The hatched bands indicate the total uncertainty (statistical and systematic added in quadrature) for the SM background predictions. The predicted yields of the backgrounds and the uncertainty bands are shown after the simultaneous fits for the signal-plus-background hypothesis. Examples of the predicted signal contribution for the BNV interactions via $ \mathrm{t}\mathrm{e}\mathrm{u}\mathrm{d} $ (solid gray line) and $ \mathrm{t}\mu\mathrm{u}\mathrm{d} $ (dashed black line) vertices are shown.
png pdf	Figure 2-b: The BDT output distributions for data (points) and backgrounds (histograms) for the $ \mathrm{e}^+\mathrm{e}^- $ (left), $ \mu^{+}\mu^{-} $ (right), and $ \mathrm{e}^\pm\mu^\mp $ (lower) channels, including the ratio of data to the predicted total background yield. The hatched bands indicate the total uncertainty (statistical and systematic added in quadrature) for the SM background predictions. The predicted yields of the backgrounds and the uncertainty bands are shown after the simultaneous fits for the signal-plus-background hypothesis. Examples of the predicted signal contribution for the BNV interactions via $ \mathrm{t}\mathrm{e}\mathrm{u}\mathrm{d} $ (solid gray line) and $ \mathrm{t}\mu\mathrm{u}\mathrm{d} $ (dashed black line) vertices are shown.
png pdf	Figure 2-c: The BDT output distributions for data (points) and backgrounds (histograms) for the $ \mathrm{e}^+\mathrm{e}^- $ (left), $ \mu^{+}\mu^{-} $ (right), and $ \mathrm{e}^\pm\mu^\mp $ (lower) channels, including the ratio of data to the predicted total background yield. The hatched bands indicate the total uncertainty (statistical and systematic added in quadrature) for the SM background predictions. The predicted yields of the backgrounds and the uncertainty bands are shown after the simultaneous fits for the signal-plus-background hypothesis. Examples of the predicted signal contribution for the BNV interactions via $ \mathrm{t}\mathrm{e}\mathrm{u}\mathrm{d} $ (solid gray line) and $ \mathrm{t}\mu\mathrm{u}\mathrm{d} $ (dashed black line) vertices are shown.
png pdf	Figure 3: The observed upper limits on the branching fractions of the top quark BNV decays are shown with circle and triangle shapes for electron and muon couplings, respectively. The observed limits corresponding to the $ C_t $ and $ C_s $ coefficients are shown with filled and open markers, respectively. The yellow light (green dark) bands indicate the range within plus or minus one (two) standard deviations bands around the expected limits.

Tables
png pdf	Table 1: Theoretical inclusive cross sections, in units of pico barn (pb), for single top quark production (ST) and top quark-antiquark pair production with the decay (TT) via BNV interactions, assuming a top quark mass of 172.5 GeV, the top quark decay width 1.33 GeV, $ \Lambda= $ 1 TeV, and $ C_t = $ 1 or $ C_s= $ 1. The uncertainties arising from the choice of the renormalization and factorization scales and PDFs are given as ($ \sigma{\pm\text{Scale}}{\pm\text{PDF}} $). Here, the sum of the two cross sections is given where $\ell =$ e or $ \mu $.
png pdf	Table 2: Expected and observed 95% CL upper limits on the BNV effective couplings and top quark BNV branching fractions.

Summary

In summary, a search for baryon number violation (BNV) in events with top quarks is performed using the LHC proton-proton collision data at a center-of-mass energy of 13 TeV. The analysis explores baryon number violating effects in single top quark production for the first time. Data were collected by the CMS experiment in 2016--2018 and correspond to an integrated luminosity of 138 fb$ ^{-1} $. Events with a lepton pair and exactly one b-tagged jet are selected. A boosted decision tree (BDT) is used to separate signal events from background events. A binned maximum likelihood fit to the BDT output distribution is performed to search for the BNV processes. Considering BNV vertices in the production of top quarks dramatically increases the sensitivity of this search. No significant excess of events over the background prediction is observed. Upper limits are placed on the strengths of the BNV couplings, which are multiple orders of magnitude more stringent than the previous limits [11].

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