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Search for excited quarks in the $\gamma+\mathrm{jet}$ final state in proton proton collisions at $\sqrt{s}=$ 13 TeV
Abstract: The study presents a search for excited quarks $({\rm q^{*}})$ decaying into a $\gamma+\mathrm{jet}$ final state at $\sqrt{s}=$ 13 TeV with the CMS experiment, using the dataset corresponding to an integrated luminosity of 2.7 fb$^{-1}$ collected during 2015 data taking at the LHC. High transverse momentum photons and jets are selected to search for a resonance peak in the continuous invariant mass distribution of $\gamma+\mathrm{jet}$. The 95% confidence level upper limits on cross section times branching ratio are evaluated as a function of excited quark mass ($M_{\mathrm q^{*}}$). We exclude at 95% CL excited quarks with mass 1.0 $< M_{\mathrm q^{*}} <$ 4.37 TeV and coupling strength $f=$ 1.0, and present exclusions of excited quark mass as a function of coupling strength.
Figures & Tables Summary References CMS Publications

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Figure 1:
Signal for production of an excited quark in the $s$-channel and subsequent decay into a quark and a photon.

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Figure 2:
The $ {\gamma + \text {jet}}$ invariant mass distribution in data (points) and MC (histogram) prediction after full selection. The result of the fit to the data using the parameterization of Eq.2 is shown with the red dotted curve. The bin-by-bin pull, (data-fit)/$\sigma _{data}$, is shown at the bottom. Simulations of excited quark signals ($ {\mathrm {q^* }} $) for coupling multiplier $f=$ 1.0 are shown at masses of 1, 2 and 3 TeV (dashed curves).

Figure 3:
Event display of the highest invariant mass $ {\gamma + \text {jet}}$ candidate with mass of 3.29 TeV observed in data in the 3-dimensional plane. The ${p_{\mathrm {T}}}$, $\eta $, and $\phi $ values of the photon and jet are indicated.

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Figure 4:
The reconstructed resonance mass spectrum generated using PYTHIA-8 simulation for $ {\gamma + \text {jet}}$ resonances modeled by $ {\mathrm {q}} {\mathrm {g}}\to {\mathrm {q}} {\gamma }$ for coupling multipliers $f=$ 1.0 (filled histogram) and $f=$ 0.1 (dotted histogram) having resonance masses ranging from 1.0 to 4.0 TeV .

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Figure 5:
The expected and observed 95% CL upper limits on $\sigma \times \mathcal {B}$ for excited quark search in $ {\gamma + \text {jet}}$ final state corresponding to coupling $f=$ 1.0. The limits are also compared with the theoretical predictions for excited quark production, shown for coupling values $f=$ 1.0, 0.5, and 0.1. The uncertainty at 1$\sigma $ and 2$\sigma $ levels are shown as green and yellow shaded bands around expected limit.

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Figure 6:
The observed (yellow filled) and expected (blue dashed line) excluded regions at 95% CL as a function of excited quark mass and the coupling strength for $\Lambda = {\mathrm {M_{q^* }}} $ (left axis) or the mass divided by compositeness scale for coupling strength of $f=1$ (right axis). The excluded region from Run-I is also shown (red shaded).

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Table 1:
The cumulative efficiency for signal event selection for ${\mathrm {q^* }}$ resonance mass of 2 TeV and 4 TeV and coupling multiplier $f=$ 1.0.
We have presented a search for excited quarks in the $\gamma+\mathrm{jet}$ final state. The data set collected at $\sqrt{s}= $ 13 TeV corresponds to an integrated luminosity of 2.7 fb$^{-1}$. The data are found to be consistent with the predictions of the standard model and no evidence is found for an excited quark resonance. The largest excess of events in data is seen at a $\gamma+\mathrm{jet}$ invariant mass of 2.0 TeV. A 95% CL upper limit is placed on $\sigma\times\mathcal{B}$ for $({\rm q^{*}})$ production in the $\gamma+\mathrm{jet}$ final state. Comparing these upper limits with the theoretical predictions, an excited quark state with mass 1.0 $ < {\rm q^{*}} <$ 4.37 TeV is excluded at 95% CL for $f =$ 1.0. Also, excited quarks with masses 1.0 $ < {\rm q^{*}} < $ 3.64 (1.36) TeV are excluded for $f=$ 0.5 (0.1). We also present the excluded mass as a function of coupling strength in Fig. 6.
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