Searches for scalar leptoquarks in pp collisions at [Formula: see text] = 8 TeV with the ATLAS detector

Abstract

Searches for pair-produced scalar leptoquarks are performed using 20 fb[Formula: see text] of proton-proton collision data provided by the LHC and recorded by the ATLAS detector at [Formula: see text] TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to [Formula: see text] and [Formula: see text] final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to [Formula: see text] 1050 GeV for first-generation leptoquarks, [Formula: see text] 1000 GeV for second-generation leptoquarks, [Formula: see text] 625 GeV for third-generation leptoquarks in the [Formula: see text] channel, and 200 [Formula: see text] 640 GeV in the [Formula: see text] channel.

Fig. 1

Distributions of the dilepton invariant mass ($m_{\ell \ell }$) in the eejj  (left) and $\mathit{\mu }\mathit{\mu }$ jj  (right) channels after applying the pre-selection cuts. The signal model assumes $\mathit{\beta }=1.0$. The last bin includes overflows. The ratio of the number of data events to the number of background events (and its statistical uncertainty) is also shown. The hashed bands represent all sources of statistical and systematic uncertainty on the background prediction

Fig. 2

Distributions of the total scalar energy ($S_{\mathrm{T}}$) in the eejj  (left) and $\mathit{\mu }\mathit{\mu }$ jj  (right) channels after applying the pre-selection cuts. The signal model assumes $\mathit{\beta }=1.0$. The last bin includes overflows. The ratio of the number of data events to the number of background events (and its statistical uncertainty) is also shown. The hashed bands represent all sources of statistical and systematic uncertainty on the background prediction

Fig. 3

Distributions of the lowest reconstructed LQ mass ($m_{\mathrm{LQ}}^{\min }$) in the eejj  (left) and $\mathit{\mu }\mathit{\mu }$ jj  (right) channels after applying the pre-selection cuts. The signal model assumes $\mathit{\beta }=1.0$. The last bin includes overflows. The ratio of the number of data events to the number of background events (and its statistical uncertainty) is also shown. The hashed bands represent all sources of statistical and systematic uncertainty on the background prediction

Fig. 4

The cross-section limits on scalar LQ pair-production times the square of the branching ratio as a function of mass (left) and the excluded branching ratio as a function of the LQ mass (right) to eq for the eejj channel (top) and to $\mathit{\mu }q$ for the $\mathit{\mu }\mathit{\mu }$ jj channel (bottom). The $\pm 1(2)\mathit{\sigma }$ uncertainty bands on the expected limit represent all sources of systematic and statistical uncertainty. The expected NLO production cross-section ($\mathit{\beta }=1.0$) for scalar LQ pair-production and its corresponding theoretical uncertainty due to the choice of PDF set and renormalisation/factorisation scale are also included. The exclusion limits on $\mathrm{LQ}1$ [12] and $\mathrm{LQ}2$ [13] set by ATLAS in the eejj $+$ e $\mathit{\nu }$ jj and $\mathit{\mu }\mathit{\mu }$ jj $+$ $\mathit{\mu }\mathit{\nu }$ jj search channels using 1.03 fb${}^{-1}$ of data collected at $\sqrt{s}=7$ TeV are also shown

Fig. 5

The expected (dashed) and observed (solid) 95 % CL upper limits on third-generation scalar LQ pair-production cross-section times the square of the branching ratio to $b{\mathit{\nu }}_{\mathit{\tau }}$ as a function of LQ mass, for the $b{\mathit{\nu }}_{\mathit{\tau }}\overline{b}\overline{{\mathit{\nu }}_{\mathit{\tau }}}$ channel. The $\pm 1(2)\mathit{\sigma }$ uncertainty bands on the expected limit represent all sources of systematic and statistical uncertainty. The expected NLO production cross-section ($\mathit{\beta }=0.0$) for scalar LQ pair-production and its corresponding theoretical uncertainty due to the choice of PDF set and renormalisation/factorisation scale are also included

Fig. 6

The expected (dashed) and observed (solid) 95 % CL upper limits on the third-generation scalar LQ pair-production cross-section times the square of the branching ratio to $t{\mathit{\nu }}_{\mathit{\tau }}$ as a function of LQ mass, for the $t{\mathit{\nu }}_{\mathit{\tau }}\overline{t}\overline{{\mathit{\nu }}_{\mathit{\tau }}}$ channel. The $\pm 1(2)\mathit{\sigma }$ uncertainty bands on the expected limit represent all sources of systematic and statistical uncertainty. The expected NLO production cross-section ($\mathit{\beta }=0.0$) for third-generation scalar LQ pair-production and its corresponding theoretical uncertainty due to the choice of PDF set and renormalisation/factorisation scale are also included