REVIEW
Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in $\sqrt{s}=8$ TeV $pp$ collisions with the ATLAS detector
High Energy Physics - Experiment · Submitted 11 Mar 2015 · 93 references
Pith's verdict: A careful full-luminosity ATLAS search whose durable result is the off-Z null; the 3σ on-Z excess is real in the data but its size depends on which estimate of the electron-channel background you use, and the paper's own sideband fit points the weaker way.
T0 · AI review
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Referee report
The paper
The paper claims
ATLAS reports a three-standard-deviation excess of events with a Z-boson-mass lepton pair, jets and large missing momentum, while its companion search for a dilepton mass edge finds nothing beyond the Standard Model.
The reading
What carries the argument
Three pieces of machinery carry the argument. First, the flavour-symmetry method: because $tbar{t}$, $WW$, single-top and $Z to tau tau$ produce $ee$, $mu mu$ and $e mu$ pairs in a $1:1:2$ ratio, the same-flavour background is predicted as one half of the observed $e mu$ yield in a control region with identical cuts, corrected by lepton-efficiency factors $k_{ee}$, $k_{mu mu}$ and a trigger factor $alpha$ measured in data. For the off-$Z$ search this is extended with a mass-shape correction $S(i)$ fitted on simulated $tbar{t}$ so that the predicted background has a dilepton mass distribution, not just a normalisation. Second, the jet-smearing method for the peaking $Z$+jets background: well-measured $Z$+jets events from a low-missing-momentum seed region are re-smeared with a data-tuned jet response function to make pseudo-data, which is then normalised in a validation region and extrapolated into the signal region, giving $0.07 pm 0.05$ events there. Third, the statistical machinery: the $CL_s$ prescription implemented in HistFitter turns yields into 95% confidence limits on visible cross sections and on model parameters, and for the off-$Z$ search a sliding-window scan over 45 (21) dilepton mass windows picks, for each model point, the window with the best expected sensitivity — the right procedure when the endpoint position is a free parameter of the model.
Extended reading notes
Core claim
The paper's central claim is an excess, not a discovery. In the on-$Z$ signal region — two same-flavour opposite-sign leptons with $81 < m_{ll} < 101$ GeV, at least two jets, $E_T^{miss} > 225$ GeV, $H_T > 600$ GeV, and an azimuthal separation between the two leading jets and the missing-momentum direction above $0.4$ — $29$ events are observed against $10.6$ expected, giving a local significance of $3.0$ standard deviations. The excess is concentrated in the electron channel ($16$ observed, $4.2$ expected, $3.0$ standard deviations) and is weaker in the muon channel ($13$ observed, $6.4$ expected, $1.7$ standard deviations). The companion off-$Z$ search, which vetoes the $Z$ window and looks for a kinematic edge in the dilepton mass, sees data consistent with the Standard Model in all five signal regions, including the loose region that mirrors the CMS phase space where a $2.6$ standard deviation excess had been reported. Interpreting the on-$Z$ result in general gauge mediation, the observed limits are weaker than expected, excluding gluino masses up to about $850$ GeV for $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-window $Z$-boson decays at $m_{ll}$-w
Load-bearing premise
The whole excess rests on the assumption that the same-flavour background can be predicted as half the different-flavour yield, corrected by global lepton and trigger efficiency factors, and the paper's own sideband cross-check gives a somewhat higher background ($4.9$ and $5.3$ events against $2.8$ and $3.3$) that would shrink but not erase the excess.
Editorial extensions
If this is right
- A $3.0$ standard deviation local excess in the on-$Z$ region is not enough to claim discovery, but it is enough to justify re-examining the same final state at $13$ TeV, where the production cross sections for gluino pairs rise steeply.
- The observed general gauge mediation limits are weaker than expected, excluding gluino masses only up to roughly $850$ GeV for $tan beta = 1.5$ and $820$ GeV for $tan beta = 30$ in the parameter range where the $Z$-boson branching fraction is large.
- In the simplified squark and gluino models the off-$Z$ search excludes squarks up to about $780$ GeV and gluinos up to about $1170$ GeV, using the two-jet and four-jet b-vetoed signal regions respectively.
- The paper supplies model-independent upper limits on the visible cross section in each region, so that any future model predicting same-flavour dileptons plus jets plus missing momentum can be tested against these numbers without re-running the analysis.
- The null result in the region mirroring the earlier CMS phase space removes one candidate explanation for that experiment's $2.6$ standard deviation excess.
Reading between the lines
- The electron-channel excess and the muon-channel near-null result are combined assuming correlated flavour-symmetry uncertainties; if the electron channel's larger excess reflects an electron-specific efficiency or charge-misidentification effect rather than a signal, the combined significance is an overstatement rather than an understatement.
- Because the on-$Z$ and off-$Z$ control regions overlap each other's signal regions (up to $60\%$ of the on-$Z$ top control region comes from off-$Z$ signal events), a genuine signal in one search would leak into the other's background estimate; the paper assumes this away, and a joint fit would be the cleaner test.
- The mass-edge scan is a template for how a search with an unknown endpoint should be reported: quoting limits per mass window, rather than per region, makes the result reusable by anyone whose model predicts a different endpoint.
- A natural next check is whether the on-$Z$ excess persists when the $Z$+jets background is estimated without the $Delta phi$ requirement, since that cut is the main handle on mismeasured-jet fake missing momentum and the paper explicitly repeats the estimate both ways.
Original abstract
Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy $\sqrt{s}=8$ TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Two leptonic production mechanisms are considered: decays of squarks and gluinos with $Z$ bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the $Z$-boson mass; and decays of neutralinos (e.g. $\tilde{\chi}^{0}_{2} \rightarrow \ell^{+}\ell^{-}\tilde{\chi}^{0}_{1}$), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of 3 standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy $\sqrt{s}=8$ TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Two leptonic production mechanisms are considered: decays of squarks and gluinos with $Z$ bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the $Z$-boson mass; and decays of neutralinos (e.g. $\tilde{\chi}^{0}_{2} \rightarrow \ell^{+}\ell^{-}\tilde{\chi}^{0}_{1}$), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of 3 standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. | contribution 1.00 | 0.91 | 0.05 | asserted 0.95 |
| s1 | The proton-proton collision data used in these searches were collected at a centre-of-mass energy $\sqrt{s}=8$ TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. | method 1.00 | 0.67 | 0.02 | asserted 1.00 |
| s2 | Two leptonic production mechanisms are considered: decays of squarks and gluinos with $Z$ bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the $Z$-boson mass; and decays of neutralinos (e.g. $\tilde{\chi}^{0}_{2} \rightarrow \ell^{+}\ell^{-}\tilde{\chi}^{0}_{1}$), resulting in a kinematic endpoint in the dilepton invariant mass distribution. | method 0.86 | 0.23 | 0.03 | asserted 0.92 |
| s3 | For the former, an excess of events above the expected Standard Model background is observed, with a significance of 3 standard deviations. | result 1.00 | 0.95 | 0.21 | asserted 1.00 |
| s4 | In the latter case, the data are well-described by the expected Standard Model background. | result 1.00 | 0.90 | 0.13 | asserted 0.98 |
| s5 | The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos. | interpretation 0.98 | 0.86 | 0.09 | asserted 0.95 |
What was examined, and what was not
- arXiv listing abstract v3
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:1503.03290 by the authors.
References
How the literature uses it
53 arXiv papers cite this in 123 sentences
Papers citing it · 75 sentences without a confident label
Uses its method or tool 7 papers
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We then applied the cuts for the on-$Z$ region of the ATLAS search for final states containing a pair of opposite-sign dileptons, jets and MET [ref] to these simulated SUSY samples.
The ATLAS Z + MET Excess in the MSSM · Analysis · 0.91
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In this section we apply the main requirements of the ATLAS selection in [ref] to Monte Carlo generated samples of the process $pp→G^* →B_Z B̅_Z →ZZ bb̅$ in fig.
Z-peaked excess from heavy gluon decays to vector-like quarks · Applying the ATLAS selection · 0.94
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In order to set bounds on the parameter space of the model, we use the latest CheckMATE validated analyses [ref] : * ATLAS search for squarks and gluinos with jets and missing momentum [ref] , * ATLAS search for third generation squarks via charm quarks or compressed supersymmetric scenarios [ref] , * ATLAS search for new phenomena with high energetic jets and large missing transverse momentum [ref] for the (a) topologies, and * ATLAS search for direct top-squark pair production in final states with two leptons [ref] , * ATLAS search for top squark pair production with one isolated lepton and missing transverse momentum [ref] , * ATLAS search for supersymmetry in events containing a same-sign dilepton pair, jets and large missing transverse momentum [ref] , * ATLAS search for direct slepton and chargino production in final states with two opposite-sign leptons, missing energy and no jets
Implications of a Electroweak Triplet Scalar Leptoquark on the Ultra-High Energy Neutrino Events at IceCube · LHC Constraints · 0.95
Uses its data or values 15 papers
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For $20.3 fb^-1$ of integrated luminosity at a center-of-mass energy of $8 TeV$, the number of the observed leptonic-$Z$ events is 29 (summing over electron and muon pairs) in comparison with $10.6 ±3.2$ events expected in the SM [ref] .
Explanation of the ATLAS Z-peaked excess in the NMSSM · Introduction · 1.00
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ResultsIn Fig. [ref] we show the parameter regions where the ATLAS excess in the leptonic-$Z +jets +E_T$ channel [ref] is reproduced within 1$σ$ (dark-red shaded region) and 2$σ$ (light-red shaded region), corresponding to the signal event number of $12.1 ≤N_sig ≤24.7$ and $5.8 ≤N_sig ≤31$ [ref] , respectively.
ATLAS on-Z Excess Through Vector-Like Quarks · Introduction · 0.90
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At the 8 TeV LHC with an integrated luminosity of $20.3 fb^-1$, the observed number of leptonic-$Z$ events for combined electron and muon channels is 29 while $10.6 ±3.2$ events are expected in the SM [ref] , which corresponds to $3σ$ deviation.
The ATLAS Leptonic-$Z$ Excess from Light Squark Productions in the NMSSM Extension with a Heavy Dirac Gluino · Introduction · 0.99
Compares with it 8 papers
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To further check the consistency with the data we compare our $m _ ℓℓ ,, H _T ,$ and $n _jets$ distributions with those measured by the ATLAS on-$Z$ search (these correspond to Fig.6 and 7 in [ref] ).
Mixed Stops and the ATLAS on-Z Excess · Kinematic distributions · 0.99
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In this appendix, we show the validations of our simulations on the ATLAS searches for the leptonic-$Z$ $+ jets + E_T^miss$ signal [ref] and the $2∼6$ jets + $E_T^miss$ signal [ref] .
Explanation of the ATLAS Z-peaked excess in the NMSSM · Validation of Monte Carlo simulations · 0.91
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Finally, we show in fig.s [ref],[ref],[ref] the kinematic distributions of $G^* →B_Z B̅_Z$ events, to be compared with the distributions of the observed events shown by ATLAS in [ref] – which we report for the reader convenience on the right side of fig.s [ref],[ref],[ref].
Z-peaked excess from heavy gluon decays to vector-like quarks · Results · 0.96
Cites it as support 1 paper
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These complementary SFOS processes, in particular, are the main motivation of this analysis [ref] .
Searches for Beyond the Standard Model Physics at the LHC: Run1 Summary and Run2 Prospects · Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transv · 0.98
Cites it as background 6 papers
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The on-$Z$ signal was investigated originally to search for the supersymmetry (SUSY) [ref] , and most of the theoretical works have been performed within the framework of SUSY.
ATLAS on-Z Excess Through Vector-Like Quarks · Introduction · 0.88
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Recently, results of searches for SUSY particles in events with dileptons, jets and $E^miss_T$ have been published by the CMS and ATLAS collaborations [ref] .
Possible explanation of excess events in the search for jets, missing transverse momentum and a Z boson in pp collisions · Introduction · 0.98
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Searches in this final state have been performed by the ATLAS [ref] and CMS [ref] experiments using data collected at $√(s)=8$ and 13.
Search for supersymmetry in final states with two oppositely charged same-flavor leptons and missing transverse momentum in proton-proton collisions at $\sqrt{s} =$ 13 TeV · Introduction · 0.95
What it cites · 27 sentences without a confident label
Uses the method or tool of 36 papers
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The asymptotic $CL_S$ prescription [ref] , implemented in the HistFitter program [ref] , is used to determine upper limits at 95 % confidence level (CL).
HistFitter software framework for statistical data analysis · Interpretation of results · 1.00
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The results are obtained using asymptotic formulae [ref] in the case of the off-$Z$ numbers.
Asymptotic formulae for likelihood-based tests of new physics · Interpretation of results · 1.00
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These include leptons from $b$-hadron decays, misidentified hadrons or converted photons, and are estimated from data using a matrix method, which is described in detail in Ref. [ref] .
Measurement of the top quark-pair production cross section with ATLAS in pp collisions at $\sqrt{s}=7\TeV$ · Fake-lepton contribution · 0.99
Uses data or values from 1 paper
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Total cross-section uncertainties of 22 % [ref] and 50 % are applied to $+W$/$Z$ and $+WW$ sub-processes, respectively.
t \bar{t} W production and decay at NLO · Theoretical uncertainties on background processes · 0.97
Compares with 1 paper
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No evidence for an excess is observed in the region similar to that in which CMS reported a 2.6$σ$ excess [ref] .
Search for physics beyond the standard model in events with two leptons, jets, and missing transverse momentum in pp collisions at sqrt(s) = 8 TeV · Summary · 0.98
Cites as background 3 papers
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Searches for a dilepton mass edge have also been performed by the CMS Collaboration [ref] .
Search for new physics in events with opposite-sign leptons, jets, and missing transverse energy in pp collisions at sqrt(s) = 7 TeV · Introduction · 0.92
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Searches for a dilepton mass edge have also been performed by the CMS Collaboration [ref] .
Search for physics beyond the standard model in events with two leptons, jets, and missing transverse momentum in pp collisions at sqrt(s) = 8 TeV · Introduction · 0.92
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Previous searches for physics beyond the Standard Model (BSM) in the $Z+jets+$ final state have been performed by the CMS Collaboration [ref] .
Search for physics beyond the standard model in events with a Z boson, jets, and missing transverse energy in pp collisions at sqrt(s) = 7 TeV · Introduction · 0.94
Discussion
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