REVIEW
Search for supersymmetry in hadronic final states with missing transverse energy using the variables AlphaT and b-quark multiplicity in pp collisions at 8 TeV
High Energy Physics - Experiment · Submitted 12 Mar 2013 · 62 references
Pith's verdict: A solid, honest 8 TeV alphaT search with real but bounded soft spots in the high-b-tag background extrapolation; worth refereeing.
T0 · AI review
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Referee report
The paper
The paper claims
In 11.7 fb$^{-1}$ of 8 TeV proton collisions, the CMS experiment finds no excess of jets-plus-missing-energy events over the standard model, and converts that null result into 95% confidence lower bounds on gluino and squark masses within a
The reading
What carries the argument
The load-bearing object is $alpha_T$, defined for a dijet as $E_T^{j_2}/M_T$ where $M_T$ is the transverse mass of the two-jet system, and generalised to any number of jets by clustering them into two pseudo-jets that minimise the transverse-energy imbalance $Delta H_T$: $$alpha_T = frac{1}{2} cdot frac{1 - $\Delta$ H_T / H_T}{sqrt{1 - ($H_T^{{miss}}$/H_T)^2}}.$$ Its virtue is that it never uses a direct missing-energy measurement, only jet energies and relative angles, so a jet whose energy is mismeasured pulls $alpha_T$ below 0.5 rather than above it. Two supporting pieces of machinery carry the rest of the argument. First, the background estimate: transfer factors, defined as the ratio of simulated signal-region to control-region yields and applied to observed $mu$+jets, $mu mu$+jets, and $gamma$+jets control samples, extrapolate into the signal region, with the b-tag multiplicity predicted from generator-level flavour counts combined with measured tagging and mistag efficiencies. Second, the multijet estimate: the ratio of yields above and below $alpha_T = 0.55$ is modelled as a falling exponential in $H_T$, $R_{alpha_T}(H_T) = A e^{-k H_T}$, with the decay constant $k$ fixed from a multijet-enriched sideband.
Extended reading notes
Core claim
The central claim is a null result with teeth. Using 11.7 $fb^{-1}$ of $sqrt{s} = 8$ TeV pp collisions, the CMS collaboration bins events with two or more energetic jets, $H_T > 275$ GeV, and $alpha_T > 0.55$ by jet multiplicity ($2$–$3$ versus $geq 4$), by $H_T$ (eight bins up to an open $H_T > 875$ GeV bin), and by the number of b-tagged jets ($n_b = 0, 1, 2, 3, geq 4$). The observed counts agree with the fitted standard-model expectation in all eight $n_{jet}$–$n_b$ categories, with p-values uniformly distributed and a minimum of 0.1, and no candidate event shows non-physical behaviour. Interpreting this in simplified models, the paper excludes at 95% CL gluino masses up to 950–1125 GeV for gluino-pair production followed by $tilde{g} to q bar{q} tilde{chi}^0_1$ with light, bottom, or top quarks in the final state, with the strongest gluino limits in the bottom-quark channel; excludes masses up to 750 GeV for eightfold-degenerate first- and second-generation squarks and 450 GeV for a single light $tilde{u}_L$; and excludes bottom squarks up to 600 GeV. For direct top-squark pair production no observed exclusion survives, but the expected reach extends to about 500 GeV in top squark mass and 100 GeV in LSP mass.
Load-bearing premise
The whole background estimate rests on the assumption that the simulation correctly predicts the ratio of signal-region to control-region yields, a ratio that is checked only by closure tests among the control samples themselves and never directly in the signal region.
Editorial extensions
If this is right
- For gluino-mediated squark production, gluino masses in the range 950–1125 GeV are excluded at 95% CL for the three decay flavours considered, so any model placing a gluino below roughly 1 TeV and decaying through an off-shell squark to a light LSP is ruled out in this topology.
- Directly pair-produced first- and second-generation squarks are excluded below 750 GeV under an eightfold mass degeneracy, and below 450 GeV if only a single light squark exists, tightening the reach of the earlier 7 TeV analysis by as much as 225 GeV in parent mass.
- Bottom squarks decaying to $b tilde{chi}^0_1$ are excluded below 600 GeV, with no exclusion possible once the LSP exceeds about 200 GeV.
- Direct top-squark pair production is not excluded by the observed data, but the expected sensitivity reaches roughly 500 GeV in top-squark mass and 100 GeV in LSP mass, so the naturalness-motivated region stays open.
- The limits on LSP mass, 300–650 GeV depending on channel, remain valid for compressed spectra with parent–LSP splittings as small as about 200 GeV.
Reading between the lines
- Because $alpha_T$ depends only on jet energies and relative angles, the same discriminator transfers almost unchanged to higher centre-of-mass energies; the natural next step is to ask how far the 950–1125 GeV gluino boundary moves when the same selection is applied to 13 TeV data, where the production cross sections grow by roughly an order of magnitude.
- The paper declines to interpret the kinematic region with $m_{parent} - m_{LSP} < 175$ GeV or $m_{parent} < 300$ GeV, citing initial-state-radiation modelling; that leaves a soft-radiation-sensitive corner where monojet-style or soft-lepton observables, rather than $alpha_T$, would be the appropriate handle.
- The b-tag multiplicity is predicted from generator-level flavour counts combined with measured efficiencies, a template that could be reused to forecast the sensitivity of future high-luminosity runs without regenerating full detector simulations for every mass point.
- Since the background extrapolation is validated only by closure tests among control samples, an independent cross-check would be to measure the transfer factors in a signal-depleted $alpha_T$ sideband and confirm they match the simulated ratios there.
Original abstract
An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 inverse femtobarns collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, AlphaT, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950-1125 GeV are excluded depending on the assumed model. For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600\GeV for bottom squarks.
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
An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 inverse femtobarns collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, AlphaT, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950-1125 GeV are excluded depending on the assumed model. For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600\GeV for bottom squarks.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. | contribution 1.00 | 0.95 | 0.03 | asserted 0.98 |
| s1 | The data sample corresponds to an integrated luminosity of 11.7 inverse femtobarns collected by the CMS experiment at the LHC. | method 1.00 | 0.69 | 0.01 | asserted 1.00 |
| s2 | In this search, a dimensionless kinematic variable, AlphaT, is used to discriminate between events with genuine and misreconstructed missing transverse energy. | method 0.99 | 0.69 | 0.02 | asserted 1.00 |
| s3 | The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. | method 0.99 | 0.92 | 0.02 | asserted 1.00 |
| s4 | No significant excess of events over the standard model expectation is found. | result 1.00 | 0.96 | 0.84 | asserted 1.00 |
| s5 | Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. | result 0.91 | 0.94 | 0.13 | asserted 0.99 |
| s6 | For the case of gluino-mediated squark production, gluino masses up to 950-1125 GeV are excluded depending on the assumed model. | result 0.99 | 0.94 | 0.13 | asserted 1.00 |
| s7 | For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600\GeV for bottom squarks. | result 0.99 | 0.93 | 0.10 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:1303.2985 by the authors.
References
How later papers used it
124 later papers cite it
4 later papers by other authors are listed below: 1 used its method or tool and 3 used its data or values.
Uses its method or tool 1 of 9 papers with this label
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Processed its samples using all the available LHC searches, including the AlphaT-variable searches of this paper, to ascertain the viability of its benchmark points.
Signs of Tops from Highly Mixed Stops (2015) · 0.99 · paraphrased: the licence does not allow quoting
Uses its data or values 3 of 27 papers with this label
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Used the bounds from this paper to impose constraints on the mirror quark mass and branching ratios to light jets and b-jet.
The search for mirror quarks at the LHC (2015) · 0.99 · paraphrased: the licence does not allow quoting
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Utilized two CMS simplified model searches for light generation squarks, one of them based on 11.7 inverse femtobarns of this paper, for the dijet plus MET signal.
Effective WIMPs (2013) · 0.99 · paraphrased: the licence does not allow quoting
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Applied the results of this paper's CMS inclusive search for SUSY final states with large missing transverse energy and b-quark jets using the variable.
Dark matter and collider signatures of the MSSM (2013) · 0.89 · paraphrased: the licence does not allow quoting
How the literature uses it
124 arXiv papers cite this in 199 sentences
Papers citing it · 128 sentences without a confident label
Uses its method or tool 9 papers
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To bound $b̅ b + E_T$ signals, CMS [ref] retains events with 2-3 jets and 1 or 2 $b$-tags.
Natural Supersymmetry and Implications for Higgs physics · Direct sbottom searches · 0.91
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In order to solve this problem, we use [ref] , that uses Monte Carlo event generation to compare all production and decay channels for the neutralinos, charginos and sleptons with the current LHC analyses [ref] .
The Tiny (g-2) Muon Wobble from Small-$\mu$ Supersymmetry · Benchmark Points Explaining $Δa_μ$, Dark Matter and Avoiding LHC Constraints · 1.00
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We used Delphes 3.0.11 [ref] with Fastjet [ref] for detector simulation using the standard CMS settings included in this version of Delphes, but changed the $b$-tag misidentification rate to 0.01 [ref] .
Constraining supersymmetry at the LHC with simplified models for squark production · Event generation and parameter scan · 0.94
Uses its data or values 27 papers
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The lower bound on the gluino mass from CMS [ref] and ATLAS [ref] searches are $M_g̃ ≳1 - 1.2$ TeV, where the actual bound depends on the search strategy.
Gluino bounds: Simplified Models vs a Particular SO(10) Model (A Snowmass white paper) · Introduction · 0.81
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CMS also did a R-hadron search [ref] which excluding long-lived gluino (stop) LSP masses of 1322 GeV (820 GeV).
Long-Lived Particle Searches in R-Parity Violating MSSM · Relevant LHC Searches · 0.80
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In our analysis, we have used the bounds from Ref. [ref] to impose constraints on the mirror quark mass and branching ratios to light jets and b-jet.
The search for mirror quarks at the LHC · Phenomenology: new physics at the LHC · 0.99
Compares with it 6 papers
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However, the excellent agreement between the gluino bounds from Refs. [ref] and our validation (cf. [ref] and [ref]) encourages us to believe that this is indeed a physical effect and not a relic of our analysis procedure.
LHC Phenomenology of SO(10) Models with Yukawa Unification · Conclusions · 0.95
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In fig:at_8tev_smsfig:a we present the bounds obtained from our likelihood in the gluino-LSP plane for model T1 of pair-produced gluinos decaying into a neutralino and quarks of the first two generations [ref] .
Dark matter and collider signatures of the MSSM · Likelihood for LHC direct SUSY searches · 0.96
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For comparison we show moreover the following official exclusion curves: for gluino ($g̃g̃$) topologies, we show the CMS exclusion curves for $g̃ →qq+χ̃_1^0$ (Fig. 6b of [ref] ) and $g̃ →t̅t+χ̃_1^0$ (Fig. 13 of [ref] ) as well as the ATLAS exclusion for $g̃ →b̅b+χ̃_1^0$ (Fig. 12a of [ref] ); regarding squark production, we show the exclusion curves for $q̃ q̃ →qq + χ̃_1^0 χ̃_1^0$ from both ATLAS and CMS, Figs. 6a of [ref] and 19c of [ref] ; for the 3rd generation, we show the exclusion curves for $t̃ t̅̃̅$ with $t̃ →t + χ̃_1^0$ from CMS (Fig. 10a of [ref] ) and $t̃ →b + χ̃_1^+ →b + W^+ + χ̃_1^0$ from ATLAS (Fig. 6 of [ref] ) and the curves for $b̃ b̅̃̅ →bb + χ̃_1^0 χ̃_1^0$ from both ATLAS and CMS (Fig. 4 of [ref] and Fig. 10c of [ref] ).
SModelS: a tool for interpreting simplified-model results from the LHC and its application to supersymmetry · Results from Scan-II (gluino and squark focus) · 0.97
Cites it as support 1 paper
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This choice is motivated by the absence of any signal from squark or gluino production in dedicated SUSY searches performed by the ATLAS [ref] and CMS [ref] collaborations. - All slepton mass parameters have been set to the same scale as the first- and second-generation squarks.
ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider · Interpretations in pMSSM models · 0.99
Cites it as background 11 papers
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This spectrum is well-known [ref] , and the LHC experiments have already provided outstanding constraints on simplified models involving light stops [ref] , light sbottoms [ref] , and gluinos that decay into these sparticles [ref] .
Natural Supersymmetry and Implications for Higgs physics · Introduction · 0.85
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Regarding the breaking of supersymmetry, whilst gauge mediation is favoured (and some recent work on gauge mediated supersymmetry breaking in a five dimensional context may be found in [ref] ), ultimately the universality of squark massses in GMSB mean that even though the gaugino mediated limit [ref] might allow for light squarks (and 5D RGE evolution allows for a large $A_t$ and the observed Higgs mass), the collider bounds on first and second generation squarks [ref] , in the supra-TeV range would apply also to the $3^rd$ generation squarks, i.e. the stops, which as discussed before, is both phenomenologically less interesting and unnatural.
Large $A_{t}$ Without the Desert · The setup · 0.86
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At the LHC, both the CMS and ATLAS collaborations have performed SUSY searches in all-hadronic final states [ref] .
Search for new physics in the multijet and missing transverse momentum final state in proton-proton collisions at sqrt(s) = 8 TeV · Introduction · 0.86
What it cites
Uses the method or tool of 1 paper
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Inclusive, process-dependent, NLO calculations of SUSY production cross sections, with next-to-leading-logarithmic (NLL) corrections, are obtained with the program [ref] .
Supersymmetry production cross sections in pp collisions at sqrt{s} = 7 TeV · Interpretation of the results · 0.99
Discussion
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