Search for new phenomena in dijet events using $37\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of $pp$ collision data collected at $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$ with the ATLAS detector

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Search for new phenomena in dijet events using $37 {fb}^{- 1}$ of $p p$ collision data collected at $\sqrt{s} = 13 TeV$ with the ATLAS detector

M. Aaboud et al. (ATLAS Collaboration)

Phys. Rev. D 96, 052004 – Published 28 September 2017

Abstract

Dijet events are studied in the proton-proton collision data set recorded at $\sqrt{s} = 13 TeV$ with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to integrated luminosities of $3.5 {fb}^{- 1}$ and $33.5 {fb}^{- 1}$ respectively. Invariant mass and angular distributions are compared to background predictions and no significant deviation is observed. For resonance searches, a new method for fitting the background component of the invariant mass distribution is employed. The data set is then used to set upper limits at a 95% confidence level on a range of new physics scenarios. Excited quarks with masses below 6.0 TeV are excluded, and limits are set on quantum black holes, heavy $W^{'}$ bosons, $W^{*}$ bosons, and a range of masses and couplings in a $Z^{'}$ dark matter mediator model. Model-independent limits on signals with a Gaussian shape are also set, using a new approach allowing factorization of physics and detector effects. From the angular distributions, a scale of new physics in contact interaction models is excluded for scenarios with either constructive or destructive interference. These results represent a substantial improvement over those obtained previously with lower integrated luminosity.

Received 28 March 2017

DOI:https://doi.org/10.1103/PhysRevD.96.052004

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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Hadron-hadron interactions Hadronic decays Hypothetical particle physics models Quark & gluon jets

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Vol. 96, Iss. 5 — 1 September 2017

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Figure 1
The reconstructed dijet mass distribution $m_{j j}$ (filled points) is shown for events with $p_{T} > 440$ (60) GeV for the leading (subleading) jet. The spectrum with $| y^{*} | < 0.6$ is shown in (a) for events above $m_{j j} = 1.1 TeV$ while the selection with $| y^{*} | < 1.2$ is shown in (b) for events above $m_{j j} = 1.7 TeV$ . The solid line depicts the background prediction from the sliding-window fit. Predictions for benchmark signals are normalized to a cross section large enough to make the shapes distinguishable above the data. The vertical lines indicate the most discrepant interval identified by the bumphunter algorithm, for which the $p$ -value is stated in the figure. The middle panel shows the bin-by-bin significances of the data-fit differences, considering only statistical uncertainties. The lower panel shows the relative differences between the data and the prediction of pythia 8simulation of QCD processes, corrected for NLO and electroweak effects, and is shown purely for comparison. The shaded band denotes the experimental uncertainty in the jet energy scale calibration.
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Figure 2
Reconstructed distributions of the dijet angular variable $χ$ in different regions of the dijet invariant mass $m_{j j}$ for events with $| y^{*} | < 1.7$ , $| y_{B} | < 1.1$ , and $p_{T} > 440$ (60) GeV for the leading (subleading) jet. The data (points), pythia predictions with NLO and electroweak corrections applied (solid lines), and examples of the contact interaction (CI) signals discussed in the text (dashed lines) are shown. The theoretical uncertainties and the total theoretical and experimental uncertainties in the predictions are displayed as shaded bands around the SM prediction. The SM background prediction and corresponding systematic uncertainty bands are extracted from the best-fit to the data. Data and predictions are normalized to unity in each $m_{j j}$ bin.
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Figure 3
The 95% C.L. upper limits obtained from the dijet invariant mass ( $m_{j j}$ ) distribution on cross section times acceptance times branching ratio to two jets, $σ \times A \times BR$ , for the models described in the text. Clockwise from top left: $q^{*}$ , quantum black holes with $n = 6$ generated with blackmax, $W^{'}$ , and $W^{*}$ where the first three use the nominal selection and the last uses the widened $| y^{*} | < 1.2$ selection. The numerical values of the observed and expected limits are summarized in Table 2.
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Figure 4
The 95% C.L. exclusion limits for the $Z^{'}$ model described in the text, as a function of the coupling to quarks, $g_{q}$ , and the mass, $m_{Z^{'}}$ , obtained from the dijet invariant mass $m_{j j}$ distribution. For a given mass, the cross sections rise with $g_{q}$ , and thus the upper left unfilled area is excluded, as indicated by the direction of the hatched band. The exclusion applies up to $g_{q} = 0.5$ , in the sensitivity range of the method as explained in the text. Points were simulated with 0.5 TeV spacing in mass and spacing as fine as 0.05 in $g_{q}$ . A smooth curve is drawn between points by interpolating in $g_{q}^{2}$ followed by an interpolation in $m_{Z^{'}}$ .
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Figure 5
The 95% C.L. upper limits obtained from the dijet invariant mass $m_{j j}$ distribution on cross section times acceptance times branching ratio to two jets, $σ \times A \times BR$ , for a hypothetical signal with a cross section $σ_{G}$ that produces a Gaussian contribution to the particle-level $m_{j j}$ distribution, as a function of the mean of the Gaussian mass distribution $m_{G}$ . Observed limits are obtained for five different widths, from a narrow width to 15% of $m_{G}$ . The expected limit and the corresponding $\pm 1 σ$ and $\pm 2 σ$ bands are also indicated for a narrow-width resonance.
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Figure 6
Ratio $σ / σ_{th}$ of the observed and expected 95% C.L. upper limits on the cross section in the contact interaction model to the predicted cross section as a function of the compositeness scale $Λ$ , for constructive (top) and destructive (bottom) interference with QCD processes. The $Λ$ regions for which the observed and expected 95% C.L. lines are below the line at 1.0 represent the observed and expected exclusion regions, respectively. The numerical values of the observed and expected limits are summarized in Table 2.
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Physical Review D

covering particles, fields, gravitation, and cosmology

Search for new phenomena in dijet events using $37 {fb}^{- 1}$ of $p p$ collision data collected at $\sqrt{s} = 13 TeV$ with the ATLAS detector

M. Aaboud et al. (ATLAS Collaboration)

Phys. Rev. D 96, 052004 – Published 28 September 2017

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Physical Review D

covering particles, fields, gravitation, and cosmology

Search for new phenomena in dijet events using 37 fb−1 of pp collision data collected at s=13 TeV with the ATLAS detector

M. Aaboud et al. (ATLAS Collaboration)

Phys. Rev. D 96, 052004 – Published 28 September 2017

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Search for new phenomena in dijet events using $37 {fb}^{- 1}$ of $p p$ collision data collected at $\sqrt{s} = 13 TeV$ with the ATLAS detector