REVIEW
Polarization of Prompt J/psi at the Tevatron
High Energy Physics - Phenomenology · Submitted 22 Nov 1999 · 19 references
Pith's verdict: The reference calculation for prompt J/psi polarization: the qualitative rise of alpha is solid, but the quoted 3-sigma significance leans on a quadrature band over a parameter that isn't Gaussian.
T0 · AI review
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Referee report
The paper
The paper claims
Prompt $J/psi$ mesons should become transversely polarized as their transverse momentum grows, and the size of that polarization can now be predicted including feeddown from heavier charmonium states.
The reading
What carries the argument
The machinery is the NRQCD factorization formula $d sigma_{H_lambda}(P) = d sigma_{c bar{c} n}(P) langle O_{H_lambda}(P)_n rangle$, in which the short-distance parton cross sections for each color and angular momentum state $n$ of the $c bar{c}$ pair are multiplied by nonperturbative matrix elements, and the spin dependence of the bound state enters only through the polarization tensor of $H_lambda$. The authors re-extract the color-octet parameters $langle O_8(^3S_1) rangle$, the combination $M_r = langle O_8(^1S_0) rangle + r langle O_8(^3P_0) rangle/m_c^2$, and $langle O_{chi_{c0}8}(^3S_1) rangle$ from CDF $p_T$ distributions for $J/psi$, $chi_c$, and $psi'$, add the gluon fragmentation contribution $g to c bar{c}_8(^3S_1)$ with its timelike evolution, and interpolate between the fusion and fragmentation regimes. New parton cross sections for the individual spin states of the $chi_{cJ}$ were required and are cited to a companion paper. The decay step then converts parent spin states into longitudinal $J/psi$ fractions, which is what turns a set of partonic polarization cross sections into a single measurable $alpha$ curve.
Extended reading notes
Core claim
The central claim is that within NRQCD factorization, the angular distribution parameter $alpha = (sigma_T - 2 sigma_L)/(sigma_T + 2 sigma_L)$ for leptons from prompt $J/psi$ at the Tevatron rises with transverse momentum, from near 0 at $p_T = 5$ GeV to roughly 0.5 at $p_T = 20$ GeV, once the $chi_{cJ}$ and $psi'$ feeddown contributions are computed spin state by spin state. Radiative $P$-wave decays reduce the transverse polarization relative to the direct-$J/psi$ case but do not eliminate it, because the decay probabilities into a longitudinal $J/psi$ are only $1/3$ for $chi_{c0}$, $1/2$ for the transverse $chi_{c1}$ and $chi_{c2}$ states, $2/3$ for $chi_{c2}(0)$, and zero for the rest. Comparing to the CDF measurement, the prediction agrees at intermediate $p_T$ but sits about three standard deviations high in the 15 to 20 GeV bin, where CDF finds $alpha = -0.29 plus or minus 0.23. The authors read that mismatch as a serious challenge to the formalism rather than a detail, since the qualitative rise is what the low-order spin dynamics of massless partons and heavy quarks requires.
Load-bearing premise
The longitudinal cross section for $psi(nS)$ depends on a combination of two color-octet matrix elements that the $p_T$-distribution fit does not determine, so the authors fix the mixing ratio $x = langle O_8(^1S_0) rangle / M_r$ at $1/2$ by hand and treat the whole range from 0 to 1 as an error bar; the polarization prediction therefore rests on a parameter no data constrains.
Editorial extensions
If this is right
- If the calculation is right, any future measurement of prompt $J/psi$ polarization at the Tevatron or the LHC should show $alpha$ increasing with $p_T$ rather than falling, so a continued downward trend would falsify the low-order spin-dynamics picture rather than merely refine it.
- The $psi'$ channel provides a cleaner test than the $J/psi$ channel, since no feeddown complicates it; a more precise direct-$psi'$ polarization measurement would discriminate between the present prediction and the earlier, higher curves.
- Improved $J/psi$ and $psi'$ cross-section data would shrink the dominant ambiguities, which the authors trace to the fitted color-octet matrix elements, the parton distribution choice, and the unmeasured mixing parameter $x$.
- Because the disagreement grows with $p_T$, a Run II sample extending the measurement beyond 20 GeV would decide whether the discrepancy is a statistical fluctuation in the top bin or a systematic failure of the factorization approach for charmonium.
Reading between the lines
- The paper's logic suggests a useful diagnostic separation: measure $alpha$ for $psi'$ and for $chi_{cJ}$-tagged events independently, since the two feeddown streams partly cancel at high $p_T$ in the prompt sample and could mask a real discrepancy in either one.
- The same spin-bookkeeping could be run in reverse, using measured polarization as an additional constraint that breaks the degeneracy between $langle O_8(^1S_0) rangle$ and $langle O_8(^3P_0) rangle$ that the $p_T$ distributions alone cannot resolve.
- If the anomaly survives, the most economical reading is not that QCD is wrong but that $m_c$ is too light for the velocity expansion to control the spin-dependent channels, which would predict the discrepancy to shrink for bottomonium, where the same calculation is more trustworthy.
- The three-sigma tension sits in a single bin with a large error bar, so a reanalysis combining bins or quoting a correlation-aware goodness of fit would be a cheap way to see how much of the tension is real.
Original abstract
The polarization of prompt J/psi at the Fermilab Tevatron is calculated within the nonrelativistic QCD factorization framework. The contribution from radiative decays of P-wave charmonium states decreases, but does not eliminate, the transverse polarization at large transverse momentum. The angular distribution parameter alpha for leptonic decays of the J/\psi is predicted to increase from near 0 at p_T = 5 GeV to about 0.5 at p_T = 20 GeV. The prediction is consistent with measurements by the CDF Collaboration at intermediate values of p_T, but disagrees by about 3 standard deviations at the largest values of p_T measured.
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
The polarization of prompt J/psi at the Fermilab Tevatron is calculated within the nonrelativistic QCD factorization framework. The contribution from radiative decays of P-wave charmonium states decreases, but does not eliminate, the transverse polarization at large transverse momentum. The angular distribution parameter alpha for leptonic decays of the J/\psi is predicted to increase from near 0 at p_T = 5 GeV to about 0.5 at p_T = 20 GeV. The prediction is consistent with measurements by the CDF Collaboration at intermediate values of p_T, but disagrees by about 3 standard deviations at the largest values of p_T measured.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The polarization of prompt J/psi at the Fermilab Tevatron is calculated within the nonrelativistic QCD factorization framework. | contribution 0.69 | 0.90 | 0.08 | asserted 1.00 |
| s1 | The contribution from radiative decays of P-wave charmonium states decreases, but does not eliminate, the transverse polarization at large transverse momentum. | result 0.99 | 0.78 | 0.30 | asserted 1.00 |
| s2 | The angular distribution parameter alpha for leptonic decays of the J/\psi is predicted to increase from near 0 at p_T = 5 GeV to about 0.5 at p_T = 20 GeV. | result 1.00 | 0.88 | 0.27 | asserted 0.98 |
| s3 | The prediction is consistent with measurements by the CDF Collaboration at intermediate values of p_T, but disagrees by about 3 standard deviations at the largest values of p_T measured. | result 0.64 | 0.59 | 0.29 | asserted 0.75 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:hep-ph/9911436 by the authors.
References
How the literature uses it
33 arXiv papers cite this in 48 sentences
Papers whose main result relates to this one's
States the same result 1 paper
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The prediction agrees with measurements from the CDF collaboration at intermediate values of p_T, but disagrees at the large values of p_T measured.
Papers citing it · 25 sentences without a confident label
Contests it 3 papers
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Unfortunately, the predictions of the polarizations of prompt $ψ(2S)$ [ref] and prompt $J/ψ$ [ref] were disfavored by the CDF measurement at Run I of the Fermilab Tevatron [ref] .
Recent Developments in Heavy-Quarkonium Phenomenology · Introduction · contest 0.87
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However, NRQCD predicts polarisation [ref] that is not observed in LHC data [ref] .
Non-Relativistic Quantum Chromodynamics in Parton Showers · Introduction · contest 0.77
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However, based on it, prediction of prompt $$ hadroproduction at leading order (LO) in $_s$ is significantly transversely polarized [ref] which contradicts with the measurement by the CDF Collaboration [ref] , where one found that the produced $$ is almost unpolarized and even slightly longitudinally polarized as $p_T$ increases.
Relativistic effect of $J/\psi$ hadroproduction in large $p_T$ region · Introduction · contest 0.79
Uses its method or tool 1 paper
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Employing the method given in Refs. [ref] , one can compute the polarized cross sections $σ_T$ and $σ_L$ in the hadron CM frame from Eq. ([ref]).
Polarization of prompt J/psi in proton-proton collisions at RHIC · Introduction · 1.00
Uses its data or values 8 papers
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The numerical values for the relevant NRQCD matrix elements are given in Table I of Ref. [ref] .[ The color-octet matrix elements $⟨O_8^H (^1S_0) ⟩$ and $⟨O_8^H (^3P_J) ⟩$ are determined not separately but as a linear combination $M_r^H=⟨O^H_8(^1S_0)⟩+ r ⟨O^H_8 (^3P_0) ⟩/m_c^2$ which depends on the variable $r$ given in Ref. [ref] .] For the PDF's, we choose MRST98LO [ref] as the default value and CTEQ5L [ref] for comparison.
Polarization of prompt J/psi in proton-proton collisions at RHIC · Introduction · 0.91
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In this work, we use $⟨O_8^J/ψ (^3S_1) ⟩= 3.9×10^-3$ $GeV^3$ [ref] , $⟨O_8^Υ (^3S_1) ⟩= 1.5×10^-1$ $GeV^3$ [ref] .
p p -> J/psi+Upsilon+X as a clean probe to the quarkonium production mechanism · Introduction · 1.00
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We use numerical values for the NRQCD matrix elements (ME) given in Ref. [ref] .
Polarization of prompt J/psi in pp -> J/psi+X at sqrt{s}=200 GeV · $p_T$ distributions · 1.00
Compares with it 1 paper
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However, the CO LDMEs used in the present paper are actually moderate compared to those in some other Refs. [ref] .
Double heavy quarkonia production with color-octet channels at Z factory and at the CEPC/FCC-ee · Uncertainties · 0.99
Cites it as background 7 papers
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The quarkonia directly produced at high transverse momentum, , are predicted to be transversely polarized [ref] with respect to the direction of their own momentum.
Measurement of the prompt J/psi and psi(2S) polarizations in pp collisions at sqrt(s) = 7 TeV · Introduction · 0.89
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For the production of $J/ψ$'s with large transverse momentum $p_T$, the most important contribution in NRQCD at leading order (LO) in $α_s$ comes from the $$ channel, which suggests that the produced $J/ψ$'s should be transversely polarized [ref] .
$J/\psi$ polarization in the CGC+NRQCD approach · Introduction · 0.83
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In spite of the great importance of Non-relativistic QCD(NRQCD) factorization in study of processes involving heavy quarkonia(see, for example, Refs. [ref] ), such factorization theorem for inclusive production of heavy quarkonia remains a non-trivial issue( [ref] ).
Cancellation of Infrared Divergence in Inclusive Production of Lepton Pair Near the Threshold of Heavy Quarkonia · Introduction · 0.98
Discussion
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