REVIEW
The Upsilon spectrum and m_b from full lattice QCD
High Energy Physics - Lattice · Submitted 13 Jul 2005 · 54 references
Pith's verdict: First bottomonium spectrum and bottom mass from 2+1-flavor lattice QCD; the scale-consistency argument is the real result, and the m_b error bar is the part to argue about.
T0 · AI review
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Referee report
The paper
The paper claims
With light and strange quark vacuum polarization included, two independent splittings in the Upsilon spectrum finally agree on a single lattice spacing, and the same bare bottom mass reproduces both the Upsilon and the B_s meson.
The reading
What carries the argument
The load-bearing device is a consistency test rather than a single number: two spin-independent splittings in the same meson family, one orbital and one radial, are each matched to experiment to give an inverse lattice spacing, and the ratio of the two answers is examined as a function of sea quark mass and lattice spacing. The bottom quarks are handled by lattice nonrelativistic QCD through next-to-leading order in $v^2$, with the relativistic, radiative and discretisation corrections estimated term by term and tabulated, so that the reader can see which uncertainty dominates at each lattice spacing. The same bare bottom mass then feeds the heavy-light sector, where the $B_s$ mass is extracted from a dispersion relation, making the Upsilon and B sectors a joint test rather than two separate calculations.
Extended reading notes
Core claim
The central claim is that a simulation with realistic sea quarks restores internal consistency to the bottomonium spectrum. On the coarse 2+1-flavor ensembles the ratio of the inverse lattice spacing from $2S-1S$ to that from $1P-1S$ is $0.984(20)(34)$; on the fine ensembles it is $1.021(23)(13)$; on quenched ensembles the same ratio is about $1.09$. Since the lattice spacing should be unique, the quenched disagreement was a symptom of missing quark vacuum polarization, and its disappearance is the paper's evidence that the 2+1-flavor configurations are close enough to the real world to fix QCD's parameters. The authors then show that the bare bottom mass tuned to reproduce the Upsilon mass also reproduces the $B_s$ mass to within the quoted errors, and convert the bare mass to the $MSbar$ scheme at one loop, obtaining $m_b^{MSbar}(m_b^{MSbar}) = 4.4(3)$ GeV. Along the way they quote predictions for states not yet measured: $M_ ext{Upsilon} - M_{eta_b} = 61(14)$ MeV, $M_{ ext{Upsilon}'} - M_{eta_b'} = 30(19)$ MeV, and a splitting between the $h_b$ and the spin average of the $chi_b$ states below 6 MeV.
Load-bearing premise
The step from the bare lattice bottom mass to the quoted $MSbar$ value rests on one-loop perturbative matching evaluated at a scale $mu = 2/a$ chosen by hand to make the first-order coefficient small, with the quoted error being an estimate of the uncomputed next-order terms rather than a calculated bound.
Editorial extensions
If this is right
- The lattice spacing can be fixed from the Upsilon $2S-1S$ splitting without the ambiguity that plagued quenched calculations, so dimensionful predictions from these ensembles become well defined.
- A bottom quark mass in the $MSbar$ scheme, $4.4(3)$ GeV, follows from one-loop matching of the bare lattice mass, removing the roughly 30 percent spread between bottom masses used in separate quenched Upsilon and B analyses.
- The $B_s$ mass comes out about 4 percent high, within one standard deviation, when the bottom mass is fixed from the Upsilon, which is the check that the same parameters describe both sectors.
- The heavy-quark potential scales come out as $r_0 = 0.469(7)$ fm and $r_1 = 0.321(5)$ fm, giving other lattice calculations a common ruler tied to an experimentally measured splitting.
- The predicted $eta_b$ mass of 9.399(14) GeV and the near-degeneracy of the $h_b$ with the $chi_b$ spin average are concrete targets for experiment.
Reading between the lines
- If the consistency of the two splittings really is driven by sea-quark content rather than by the improved gluon action or the improved operators, then other quantities that disagreed in the quenched approximation, such as the charmonium spectrum and heavy-light decay constants, should show the same repair on these ensembles.
- The prediction that the $h_b$ lies within 6 MeV of the $chi_b$ spin average is a sharp test that does not depend on the overall scale, so a future measurement of the $h_b$ mass would probe the spin-dependent part of the nonrelativistic action almost independently of the tuning.
- Because the quoted $4.4(3)$ GeV error is an estimate of uncomputed higher-order matching terms rather than a derived bound, the same ensembles combined with a two-loop self-energy calculation would convert this result from a plausible central value into a controlled one.
- The pattern here, where a long-standing discrepancy between two scale determinations dissolves once the sea-quark content is realistic, suggests that other reported tensions between lattice observables should first be checked against sea-quark content before being attributed to the discretisation or the effective theory.
Original abstract
We show results for the Upsilon spectrum calculated in lattice QCD including for the first time vacuum polarization effects for light u and d quarks as well as s quarks. We use gluon field configurations generated by the MILC collaboration. The calculations compare the results for a variety of u and d quark masses, as well as making a comparison to quenched results (in which quark vacuum polarisation is ignored) and results with only u and d quarks. The b quarks in the Upsilon are treated in lattice Nonrelativistic QCD through NLO in an expansion in the velocity of the b quark. We concentrate on accurate results for orbital and radial splittings where we see clear agreement with experiment once u, d and s quark vacuum polarisation effects are included. This now allows a consistent determination of the parameters of QCD. We demonstrate this consistency through the agreement of the Upsilon and B spectrum using the same lattice bare b quark mass. A one-loop matching to continuum QCD gives a value for the b quark mass in full lattice QCD for the first time. We obtain m_b^{\bar{MS}}(m_b^{\bar{MS}}) = 4.4(3) GeV. We are able to give physical results for the heavy quark potential parameters, r_0 = 0.469(7) fm and r_1 = 0.321(5) fm. Results for the fine structure in the spectrum and the Upsilon leptonic width are also presented. We predict the Upsilon - eta_b splitting to be 61(14) MeV, the Upsilon^{\prime} - eta_b^{\prime} splitting as 30(19) MeV and the splitting between the h_b and the spin-average of the chi_b states to be less than 6 MeV. Improvements to these calculations that will be made in the near future are discussed.
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
We show results for the Upsilon spectrum calculated in lattice QCD including for the first time vacuum polarization effects for light u and d quarks as well as s quarks. We use gluon field configurations generated by the MILC collaboration. The calculations compare the results for a variety of u and d quark masses, as well as making a comparison to quenched results (in which quark vacuum polarisation is ignored) and results with only u and d quarks. The b quarks in the Upsilon are treated in lattice Nonrelativistic QCD through NLO in an expansion in the velocity of the b quark. We concentrate on accurate results for orbital and radial splittings where we see clear agreement with experiment once u, d and s quark vacuum polarisation effects are included. This now allows a consistent determination of the parameters of QCD. We demonstrate this consistency through the agreement of the Upsilon and B spectrum using the same lattice bare b quark mass. A one-loop matching to continuum QCD gives a value for the b quark mass in full lattice QCD for the first time. We obtain m_b^{\bar{MS}}(m_b^{\bar{MS}}) = 4.4(3) GeV. We are able to give physical results for the heavy quark potential parameters, r_0 = 0.469(7) fm and r_1 = 0.321(5) fm. Results for the fine structure in the spectrum and the Upsilon leptonic width are also presented. We predict the Upsilon - eta_b splitting to be 61(14) MeV, the Upsilon^{\prime} - eta_b^{\prime} splitting as 30(19) MeV and the splitting between the h_b and the spin-average of the chi_b states to be less than 6 MeV. Improvements to these calculations that will be made in the near future are discussed.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We show results for the Upsilon spectrum calculated in lattice QCD including for the first time vacuum polarization effects for light u and d quarks as well as s quarks. | contribution 0.99 | 0.96 | 0.17 | asserted 1.00 |
| s1 | We use gluon field configurations generated by the MILC collaboration. | method 1.00 | 0.34 | 0.01 | asserted 1.00 |
| s2 | The calculations compare the results for a variety of u and d quark masses, as well as making a comparison to quenched results (in which quark vacuum polarisation is ignored) and results with only u and d quarks. | method 0.86 | 0.89 | 0.05 | asserted 0.96 |
| s3 | The b quarks in the Upsilon are treated in lattice Nonrelativistic QCD through NLO in an expansion in the velocity of the b quark. | method 1.00 | 0.79 | 0.04 | asserted 1.00 |
| s4 | We concentrate on accurate results for orbital and radial splittings where we see clear agreement with experiment once u, d and s quark vacuum polarisation effects are included. | result 0.90 | 0.96 | 0.41 | asserted 1.00 |
| s5 | This now allows a consistent determination of the parameters of QCD. | interpretation 0.98 | 0.52 | 0.26 | asserted 0.92 |
| s6 | We demonstrate this consistency through the agreement of the Upsilon and B spectrum using the same lattice bare b quark mass. | result 0.64 | 0.94 | 0.15 | asserted 1.00 |
| s7 | A one-loop matching to continuum QCD gives a value for the b quark mass in full lattice QCD for the first time. | contribution 0.62 | 0.94 | 0.19 | asserted 1.00 |
| s8 | We obtain m_b^{\bar{MS}}(m_b^{\bar{MS}}) = 4.4(3) GeV. | result 1.00 | 0.97 | 0.30 | asserted 1.00 |
| s9 | We are able to give physical results for the heavy quark potential parameters, r_0 = 0.469(7) fm and r_1 = 0.321(5) fm. | result 1.00 | 0.96 | 0.08 | asserted 1.00 |
| s10 | Results for the fine structure in the spectrum and the Upsilon leptonic width are also presented. | result 0.76 | 0.91 | 0.03 | asserted 0.99 |
| s11 | We predict the Upsilon - eta_b splitting to be 61(14) MeV, the Upsilon^{\prime} - eta_b^{\prime} splitting as 30(19) MeV and the splitting between the h_b and the spin-average of the chi_b states to be less than 6 MeV. | result 1.00 | 0.94 | 0.08 | asserted 0.93 |
| s12 | Improvements to these calculations that will be made in the near future are discussed. | future work 1.00 | 0.24 | 0.02 | asserted 0.55 |
What was examined, and what was not
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Figures
Showing 15 of the paper's 19 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-lat/0507013 by the authors.
References
How the literature uses it
49 arXiv papers cite this in 90 sentences
Papers citing it · 36 sentences without a confident label
Builds on it 1 paper
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Here we improve on the calculations in [ref] which used results from MILC super-coarse, coarse and fine ensembles and compared ensembles with and without sea quarks.
Precise determination of the lattice spacing in full lattice QCD · $m_Υ^'-m_Υ$ · 0.86
Uses its method or tool 9 papers
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We therefore initialise $N_smear$ NRQCD $b$ propagators by setting [equation] and evolve with [equation] We use an improved lattice NRQCD Hamiltonian [ref] : [equation] [equation]
Precision $B_c$ and $B_s$ mass calculations · Heavy-light 2-point functions · 1.00
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Here, improved non-relativistic QCD (NRQCD) [ref] is employed instead, following closely the methods used in the calculation of the bottomonium spectrum on the MILC gauge configurations in [ref] (these configurations use an improved staggered fermion action for the sea quarks and a one-loop Symanzik-improved gluon action).
The bottomonium spectrum from lattice QCD with 2+1 flavors of domain wall fermions · Introduction · 0.96
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For the heavy quark fields, $ψ(𝐱,t)$, we use the NRQCD action of [ref] , which is improved through $O(1/M_0^2)$ and $O(a^2)$ and includes the leading relativistic $O(1/M_0^3)$ correction.
Matching lattice and continuum axial-vector and vector currents with NRQCD and HISQ quarks · Heavy Quark Action · 0.99
Uses its data or values 13 papers
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In our analysis we use simulation values for $r_1/a$, from the static-quark potential, to determine ratios of scales from different configuration sets, and simulation values for the $Υ^'$–$Υ$ mass difference to set the overall scale [ref] .
Update: Accurate Determinations of alpha_s from Realistic Lattice QCD · Conclusions · 0.95
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The reader is referred to [ref] and [ref] for the details.
The bottomonium spectrum from lattice QCD with 2+1 flavors of domain wall fermions · Radial/orbital splittings and the lattice spacing · 0.92
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The absolute scale from the $Υ$ $2S$-$1S$ splitting was determined on most of our ensembles by the HPQCD Collaboration [ref] .
Quarkonium mass splittings in three-flavor lattice QCD · Configuration details · 0.96
Compares with it 11 papers
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Our result is $1.5σ$ from our previous analysis [ref] using only the $Υ$ $2S-1S$ splitting on fewer ensembles and combined with less accurate $r_1/a$ values.
Precise determination of the lattice spacing in full lattice QCD · Conclusions · 0.90
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[ref] (pre-dating the discovery of the $1$) and Meinel [ref] are consistent with all of the measured values and the PDG averages; Dowdall .
Hyperfine splitting and the experimental candidates for \eta_b(2S) · Lattice QCD · 0.88
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Our calculation improves on a previous determination of $m_b(m_b) = 4.4(3)$ GeV from unquenched lattice QCD simulations using NRQCD valence b quarks [ref] .
The b quark mass from lattice nonrelativistic QCD · Introduction · 0.94
Cites it as background 10 papers
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Currently most high-precision lattice work on $b$ physics relies upon nonrelativistic effective field theories, like NRQCD [ref] .
High-Precision c and b Masses, and QCD Coupling from Current-Current Correlators in Lattice and Continuum QCD · Introduction · 0.89
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The bottomonium spectrum was calculated using improved lattice NRQCD of order $v^4$ on the MILC gauge field ensembles in [ref] .
Bottomonium spectrum at order v^6 from domain-wall lattice QCD: precise results for hyperfine splittings · Introduction · 0.81
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Many phenomenological researches on $η_b$ have been done by some groups [ref] .
Study on the radiative decays of $\Upsilon(nS)\to \eta_b+\gamma$ · introduction · 1.00
Discussion
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