REVIEW
A Polarized View of the Top Asymmetry
High Energy Physics - Phenomenology · Submitted 18 May 2011 · 27 references
Pith's verdict: A solid, re-implementable sensitivity study whose qualitative chirality pattern is robust but whose 3σ numbers are statistical-only ceilings, with the PDF systematic on the forward rapidity cut left entirely unbounded.
T0 · AI review
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Referee report
The paper
The paper claims
The charged leptons from top decays carry enough spin information to tell rival explanations of the Tevatron's top forward-backward asymmetry apart, at about three sigma with existing or imminent data.
The reading
What carries the argument
The load-bearing object is the spin-analyzing power $kappa_i$ in the decay distribution $frac{1}{Gamma}frac{dGamma}{dcos heta_{i,n}} = frac{1}{2}(1 + P_n kappa_i cos heta_{i,n})$, where the charged lepton has $kappa = 1.0$, the neutrino $-0.3$ and the $b$ quark $-0.4$; this is what makes the lepton, the easiest top daughter to identify, also the most informative about polarization. The polarization itself is extracted as a simple counting asymmetry $P_n = [N(cos heta_{ell,n}>0) - N(cos heta_{ell,n}<0)]/[N(cos heta_{ell,n}>0) + N(cos heta_{ell,n}<0)]$ along three candidate axes (helicity, beam, and the off-diagonal axis that interpolates between them), and the helicity basis is found to separate models from the Standard Model best at the Tevatron because the new-physics polarization grows at high invariant mass where the top is relativistic. For the LHC the machinery is a redefinition: the t-tbar system's boost direction stands in for the unknown incoming quark direction, and the cuts $m_{tbar{t}} > 450$ GeV with $|y_t + y_{bar{t}}| > 2$ both enrich the quark-antiquark component against the symmetric gluon-fusion background and sharpen that correlation. Fully leptonic events are reconstructed by solving the eight neutrino constraints with the Dalitz-Goldstein weighting, which picks among the discrete solutions using the lepton energy spectrum in the top rest frame and the parton distribution functions.
Extended reading notes
Core claim
The paper's claim is that a compact set of charged-lepton observables — the leptonic charge asymmetry $A^ell_{FB}$ in semileptonic events, the dileptonic charge asymmetry $A^{Deltalell}_{FB}$, the net top polarization $P_n$ measured from $cos heta_{ell,n}$ in the top rest frame, and the spin correlation built from the product of the two lepton cosines — is sufficient both to establish that new physics is present in $tbar{t}$ production and to discriminate among competing explanations of it. The discriminating power comes from chirality: a model that couples dominantly to right-handed tops pushes the charged lepton forward and yields a lepton asymmetry comparable to or larger than the parent top asymmetry, while a left-handed coupling suppresses it, so the ratio of the two asymmetries reads off the chiral structure. The authors report that at the Tevatron the $W'$ reference model can be separated from the Standard Model and from the other benchmarks at $gtrsim 3sigma$ with the full dataset, that the left-handed axigluon gives a distinctive near-zero lepton asymmetry alongside a significant top asymmetry, and that at the 7 TeV LHC all benchmarks except the left-handed axigluon give a top asymmetry and a forward-central lepton asymmetry detectable at $gtrsim 3sigma$ in $5~mathrm{fb}^{-1}$.
Load-bearing premise
The claim that these observables discriminate generically rests on four hand-picked benchmark models — three 2.0 TeV axigluons sharing the same axial couplings and differing only in their vector coupling to the top, plus one 0.4 TeV $W'$ — and no scan over mass, width, or mixed chirality is shown to back the assertion that other models yielding a similar asymmetry would yield similar conclusions.
Editorial extensions
If this is right
- If the asymmetry is new physics, a published lepton charge asymmetry from CDF should sit away from zero in a way that correlates with the parent top asymmetry, and the correlation pattern should identify whether the coupling is right-handed, left-handed, or axial.
- At the 7 TeV LHC, the forward-central lepton asymmetry $A^ell_{FC}$ and the top asymmetry $A^{tbar{t}}$ should both rise above the Standard Model at roughly three sigma within the first $5~mathrm{fb}^{-1}$, with the $W'$-like benchmark giving the largest values (35% and 36% respectively after the rapidity cut).
- A top polarization measurement in the helicity basis, which needs the top rest frame, should show a right-handed excess growing with $m_{tbar{t}}$ for right-handed models while staying near zero or negative for left-handed ones.
- The spin correlation asymmetry is the weakest of the suite and will need more luminosity than the charge asymmetries before it constrains anything, so it should be measured but not relied on for an early verdict.
- Because the lepton is color-neutral, the lepton asymmetry can be compared to next-to-leading-order QCD predictions more cleanly than the top asymmetry itself, making it a better bridge between measurement and theory.
Reading between the lines
- The paper's logic suggests an inverted use of the same observables: if the LHC measures a large top asymmetry with a lepton asymmetry near zero, that combination is close to the left-handed axigluon signature, which is exactly the case the authors say their cuts handle worst — so the null-lepton-asymmetry corner of parameter space is where the method is least tested and most needed.
- The same spin-analyzing logic applies to any resonance decaying to top pairs, so the polarization and correlation tables here could be reused as a template for distinguishing a heavy color-octet resonance from a t-channel exchange in a resonance search, not just in the nonresonant region the authors deliberately restrict to.
- The deliberate exclusion of the axigluon pole region ($m_{tbar{t}} < 1.5$ TeV) for conservatism means the most dramatic LHC signatures of these models — the resonance itself — are left on the table; a companion analysis using the pole would likely give far stronger discrimination than the three-sigma figures quoted here.
- Since acceptances differ so sharply between forward-produced t-channel tops and central s-channel tops, the lepton asymmetry is partly an acceptance measurement, and a detector-level unfolding will be needed before the numbers in these tables can be compared to data.
Original abstract
Recent experimental results from the CDF collaboration which study the top forward-backward asymmetry have strengthened the case that new physics is playing a role in t-tbar production. Here we propose a set of measurements, built from the charged lepton kinematics in semileptonic and fully leptonic t-tbar events, designed to further probe the underlying causes of this asymmetry both at the Tevatron and at the LHC. Using a set of conservative reference models we find that measurements of the charged lepton asymmetry, top polarization, and t-tbar spin correlation can establish the existence of new physics and distinguish between competing models both at the Tevatron and the LHC. At the Tevatron, discrimination between models is possible at the three sigma level. At the LHC, we demonstrate that a top forward-backward asymmetry can be established at roughly three sigma in the first five inverse-femtobarns of data, and show how competing explanations can be further disentangled.
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
Recent experimental results from the CDF collaboration which study the top forward-backward asymmetry have strengthened the case that new physics is playing a role in t-tbar production. Here we propose a set of measurements, built from the charged lepton kinematics in semileptonic and fully leptonic t-tbar events, designed to further probe the underlying causes of this asymmetry both at the Tevatron and at the LHC. Using a set of conservative reference models we find that measurements of the charged lepton asymmetry, top polarization, and t-tbar spin correlation can establish the existence of new physics and distinguish between competing models both at the Tevatron and the LHC. At the Tevatron, discrimination between models is possible at the three sigma level. At the LHC, we demonstrate that a top forward-backward asymmetry can be established at roughly three sigma in the first five inverse-femtobarns of data, and show how competing explanations can be further disentangled.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Recent experimental results from the CDF collaboration which study the top forward-backward asymmetry have strengthened the case that new physics is playing a role in t-tbar production. | background 1.00 | 0.04 | 0.08 | hedged possibility 0.52 |
| s1 | Here we propose a set of measurements, built from the charged lepton kinematics in semileptonic and fully leptonic t-tbar events, designed to further probe the underlying causes of this asymmetry both at the Tevatron and at the LHC. | contribution 0.95 | 0.84 | 0.07 | asserted 0.86 |
| s2 | Using a set of conservative reference models we find that measurements of the charged lepton asymmetry, top polarization, and t-tbar spin correlation can establish the existence of new physics and distinguish between competing models both at the Tevatron and the LHC. | result 0.80 | 0.91 | 0.74 | asserted 0.89 |
| s3 | At the Tevatron, discrimination between models is possible at the three sigma level. | result 0.99 | 0.85 | 0.11 | hedged possibility 0.82 |
| s4 | At the LHC, we demonstrate that a top forward-backward asymmetry can be established at roughly three sigma in the first five inverse-femtobarns of data, and show how competing explanations can be further disentangled. | result 0.99 | 0.92 | 0.24 | asserted 1.00 |
What was examined, and what was not
- arXiv listing abstract v2
- 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:1105.3743 by the authors.
References
How the literature uses it
44 arXiv papers cite this in 61 sentences
Papers citing it · 41 sentences without a confident label
Cites it as support 3 papers
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The $W^'$ model and other models [ref] with more right- than left-handed top quarks tend to be preferred by the data provided that the constraint of the overall rate is satisfied.
The Top Quark Production Asymmetries $A_{FB}^t$ and $A_{FB}^{\ell}$ · Introduction · 0.99
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This implies that models which generate large $$ also predict that observables related to top quark polarization will be affected [ref] .
Spinning the Top · Introduction · 0.96
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Dedicated collider studies, e.g. Ref. [ref] , indeed show that non-zero (and different) top polarizations will be measured from various models including the SM and the axigluon that theoretically produce zero polarizations.
Renormalization group-induced phenomena of top pairs from four-quark effective operators · Independent observables of top pair · 0.97
Cites it as background 12 papers
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In some of these works they also use polarization, spin correlation and relative rapidity of the tops [ref] and the $tt̅$ transverse momentum, $p_T(tt̅)$ [ref] .
Improving top quark induced charge asymmetries at the LHC using ttbar transverse momentum · Introduction · 0.93
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Comparative studies of these models have also been carried out [ref] , and their implications for top observables at the Tevatron observed [ref] .
Tevatron Top $A_{FB}$ Versus LHC Top Physics · Introduction · 0.87
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(Previous studies [ref] have focused on specific points in the parameter space of octet couplings.) The method used for the fit and the observables used as input are explained in section [ref].
Portrait of a colour octet · Introduction · 0.93
What it cites · 29 sentences without a confident label
Builds on 1 paper
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In sec:lhc we will build upon prior works [ref] and present a detailed set of cuts and observables which allow for useful probes of relevant BSM physics within the first $1-5 fb^-1$ of $7 TeV$ LHC data.
On measurement of top polarization as a probe of $t \bar t$ production mechanisms at the LHC · Introduction · 0.92
Uses the method or tool of 3 papers
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Therefore, we further shower the parton-level events in Pythia [ref] , cluster the visible particles into $0.1×0.1$ calorimeter cells between $-5<η<5$, and form $R=0.7$ anti-$k_T$ [ref] jets using Fastjet [ref] [For comparison, parton level results are tabulated in Appendix [ref].].
The anti-k_t jet clustering algorithm · LHC Studies · 1.00
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Therefore, we further shower the parton-level events in Pythia [ref] , cluster the visible particles into $0.1×0.1$ calorimeter cells between $-5<η<5$, and form $R=0.7$ anti-$k_T$ [ref] jets using Fastjet [ref] [For comparison, parton level results are tabulated in Appendix [ref].].
PYTHIA 6.4 Physics and Manual · LHC Studies · 1.00
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All results are simulated using the full $2→6$ matrix elements of $$ production as computed in Madgraph [ref] .
MadGraph/MadEvent v4: The New Web Generation · Tevatron Analysis · 1.00
Uses data or values from 1 paper
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To gauge the statistical significance of the asymmetries, note that in 5.3 fb$^-1$ CDF observes $976.7 ±91.2$ signal events passing their semileptonic cuts [ref] .
Evidence for a Mass Dependent Forward-Backward Asymmetry in Top Quark Pair Production · Tabulated Results for Leptonic Observables · 1.00
Cites as background 2 papers
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One especially interesting variable is the leptonic charge asymmetry [ref]
Standard Model Top Quark Asymmetry at the Fermilab Tevatron · Tevatron Analysis · 0.83
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During this time the theory community has dedicated much effort to model building aimed at explaining these excesses [ref] .
The forward-backward asymmetry of top quark production at the Tevatron in warped extra dimensional models · Introduction · 0.96
Discussion
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