Determination Of Quark Masses Research Articles

Dispersive determination of fourth generation quark masses

We determine the masses of the sequential fourth generation quarks b′ and t′ in the extension of the Standard Model by solving the dispersion relations associated with the mixing between the neutral states Qq¯ and Q¯q, with Q (q) being a heavy (light) quark. The box diagrams responsible for the mixing, which provide the perturbative inputs to the dispersion relations, involve multiple intermediate channels, i.e., the ut and ct channels, u (c, t) being an up (charm, top) quark, in the b′ case, and the db′, sb′, and bb′ ones, d (s, b) being a down (strange, bottom) quark, in the t′ case. The common solutions for the above channels lead to the masses mb′=(2.7±0.1) and mt′≈200 TeV unambiguously. We show that these superheavy quarks, forming bound states in a Yukawa potential, barely contribute to Higgs boson production via gluon fusion and decay to photon pairs and bypass current experimental constraints. The mass of the b¯′b′ ground state is estimated to be about 3.2 TeV. It is thus worthwhile to continue the search for b′ quarks or b¯′b′ resonances at the (high-luminosity) Large Hadron Collider. Published by the American Physical Society 2024

Renormalization group improved determination of light quark masses from Borel-Laplace sum rules

Renormalization group improved determination of light quark masses from Borel-Laplace sum rules

Top quark pair production near threshold: single/double distributions and mass determination

We investigate top quark pair production near the threshold where the pair invariant mass {M}_{toverline{t}} approaches 2mt, which provides sensitive observables to extract the top quark mass mt. Using the effective field theory methods, we derive a factorization and resummation formula for kinematic distributions in the threshold limit up to the next-to- leading power, which resums higher order Coulomb corrections to all orders in the strong coupling constant. Our formula is similar to those in the literature but differs in several important aspects. We apply our formula to the {M}_{toverline{t}} distribution, as well as to the double differential cross section with respect to {M}_{toverline{t}} and the rapidity of the toverline{t} pair. We find that the resummation effects significantly increase the cross sections near the threshold, and lead to predictions better compatible with experimental data than the fixed-order ones. We demonstrate that incorporating resummation effects in the top quark mass determination can shift the extracted value of mt by as large as 1.4 GeV. The shift is much larger than the estimated uncertainties in previous experimental studies, and leads to a value of the top quark pole mass more consistent with the current world average.

Bilinear quark operators in the RI/SMOM scheme at three loops

We consider the renormalization of the matrix elements of the bilinear quark operators ψ¯ψ, ψ¯γμψ, and ψ¯σμνψ at next-to-next-to-next-to-leading order in QCD perturbation theory at the symmetric subtraction point. This allows us to obtain conversion factors between the MS‾ scheme and the regularization invariant symmetric momentum subtraction (RI/SMOM) scheme. The obtained results can be used to reduce the errors in determinations of quark masses from lattice QCD simulations. The results are given in Landau gauge.

Light quark masses in {N_mathrm{f}=2+1} lattice QCD with Wilson fermions

We present a lattice QCD determination of light quark masses with three sea-quark flavours (N_mathrm{f}=2+1). Bare quark masses are known from PCAC relations in the framework of CLS lattice computations with a non-perturbatively improved Wilson-Clover action and a tree-level Symanzik improved gauge action. They are fully non-perturbatively improved, including the recently computed Symanzik counter-term b_{mathrm{A}} - b_{mathrm{P}}. The mass renormalisation at hadronic scales and the renormalisation group running over a wide range of scales are known non-perturbatively in the Schrödinger functional scheme. In the present paper we perform detailed extrapolations to the physical point, obtaining (for the four-flavour theory) m_{mathrm{u}/mathrm{d}}(2~mathrm{GeV})= 3.54(12)(9)~mathrm{MeV} and m_{mathrm{s}}(2~mathrm{GeV}) = 95.7(2.5)(2.4)~mathrm{MeV} in the overline{mathrm{MS}} scheme. For the mass ratio we have m_{mathrm{s}}/m_{mathrm{u}/mathrm{d}}= 27.0(1.0)(0.4). The RGI values in the three-flavour theory are M_{mathrm{u}/mathrm{d}}= 4.70(15)(12)~mathrm{MeV} and M_{mathrm{s}}= 127.0(3.1)(3.2)~mathrm{MeV}.

Precision Determination of the Light Quark Masses from QCD Finite Energy Sum Rules

The light quark masses are determined from a QCD finite energy sum rule, using the pseudoscalar correlator to six-loop order in perturbative QCD, with the leading vacuum condensates and higher order quark mass corrections included. Both the fixed order perturbation theory (FOPT) method and contour improved perturbation theory (CIPT) method are explored. The results in the latter framework exhibit good convergence and stability in the window s0 = 3.0 5.0 GeV2 for the strange quark and s0 = 1.5 4.0 GeV2 for the up and down quarks; where s0 is the radius of the integration contour in the complex s-plane. The results are: , and the sum is . These proceedings critically explore how the current results - computed precisely in a modern computer language, Mathematica compare to past determinations in literature.

Confronting SUSY SO(10) with updated Lattice and Neutrino data

We present an updated fit of supersymmetric SO(10) models to quark and lepton masses and mixing parameters. Including latest results from lattice QCD determinations of quark masses and neutrino oscillation data, we show that fits neglecting supersymmetric threshold corrections are strongly disfavoured in our setup. Only when we include these corrections we find good fit points. We present χ2-profiles for the threshold parameters, which show that in our setup the thresholds related to the third generation of fermions exhibit two rather narrow minima.

Colour Reconnection in WW Events and the Models with It

The colour reconnection effects were intensively studied with LEP2 data and are a dominant sources of systematic uncertainty in the W boson mass in e+e− annihilation at LEP2 and one of a dominant sources of systematic uncertainty in the top quark mass determination at hadronic colliders. With the discovery of Higgs boson, a new arena for the effects studies opened up. The effects are discussed within the existent different models and what future tests may come with a future FCC-ee.

NLO and off-shell effects in top quark mass determinations

We study the impact of different theoretical descriptions of top quark pair production on top quark mass measurements in the di-lepton channel. To this aim, the full NLO corrections to ppto {W}^{+}{W}^{-}boverline{b}to left({e}^{+}{nu}_eright)left({mu}^{-}{overline{nu}}_{mu}right)boverline{b} production are compared to calculations in the narrow width approximation, where the production of a top quark pair is calculated at NLO and combined with three different descriptions of the top quark decay: leading order, next-to-leading order and via a parton shower. The different theory predictions then enter the calibration of template fit functions, which are used for a fit to pseudo-data. The offsets in the top quark mass resulting from the fits based on the various theoretical descriptions are determined.

Quarks in a polar model

Current-quark masses are compared to the rest masses allowed by the Helmholtz equation in a polar model. Within the uncertainty of the current u quark mass determination, the current quark mass coincides with the rest mass allowed by the Helmholtz equation in the polar model in accordance with the second root of the zero Neumann function. Current d quark mass coincides with the rest mass calculated in accordance with the third root of the Bessel zero function. On the basis of a comparison of these results with the results obtained earlier for ordinary real particles u and d quarks stability is discussed.

Non-perturbative quark mass renormalisation and running in N_{scriptstyle mathrm{f}}=3 QCD

We determine from first principles the quark mass anomalous dimension in N_{scriptstyle mathrm{f}}=3 QCD between the electroweak and hadronic scales. This allows for a fully non-perturbative connection of the perturbative and non-perturbative regimes of the Standard Model in the hadronic sector. The computation is carried out to high accuracy, employing massless text{ O }(a)-improved Wilson quarks and finite-size scaling techniques. We also provide the matching factors required in the renormalisation of light quark masses from lattice computations with text{ O }(a)-improved Wilson fermions and a tree-level Symanzik improved gauge action. The total uncertainty due to renormalisation and running in the determination of light quark masses in the SM is thus reduced to about 1%.

The MSR mass and the mathcal{O}left({Lambda}_{mathrm{QCD}}right) renormalon sum rule

We provide a detailed description and analysis of a low-scale short-distance mass scheme, called the MSR mass, that is useful for high-precision top quark mass determinations, but can be applied for any heavy quark Q. In contrast to earlier low-scale short-distance mass schemes, the MSR scheme has a direct connection to the well known overline{mathrm{MS}} mass commonly used for high-energy applications, and is determined by heavy quark on-shell self-energy Feynman diagrams. Indeed, the MSR mass scheme can be viewed as the simplest extension of the overline{mathrm{MS}} mass concept to renormalization scales ≪ mQ. The MSR mass depends on a scale R that can be chosen freely, and its renormalization group evolution has a linear dependence on R, which is known as R-evolution. Using R-evolution for the MSR mass we provide details of the derivation of an analytic expression for the normalization of the mathcal{O}left({varLambda}_{mathrm{QCD}}right) renormalon asymptotic behavior of the pole mass in perturbation theory. This is referred to as the mathcal{O}left({varLambda}_{mathrm{QCD}}right) renormalon sum rule, and can be applied to any perturbative series. The relations of the MSR mass scheme to other low-scale short-distance masses are analyzed as well.

Masses and decay constants of pions and kaons in mixed-action staggered chiral perturbation theory

Lattice QCD calculations with different staggered valence and sea quarks can be used to improve determinations of quark masses, Gasser-Leutwyler couplings, and other parameters relevant to phenomenology. We calculate the masses and decay constants of flavored pions and kaons through next-to-leading order in staggered-valence, staggered-sea mixed-action chiral perturbation theory. We present the results in the valence-valence and valence-sea sectors, for all tastes. As in unmixed theories, the taste-pseudoscalar, valence-valence mesons are exact Goldstone bosons in the chiral limit, at non-zero lattice spacing. The results reduce correctly when the valence and sea quark actions are identical, connect smoothly to the continuum limit, and provide a way to control light quark and gluon discretization errors in lattice calculations performed with different staggered actions for the valence and sea quarks.

Controlling quark mass determinations non-perturbatively in three-flavour QCD

The determination of quark masses from lattice QCD simulations requires a non-perturbative renormalization procedure and subsequent scale evolution to high energies, where a conversion to the commonly used MS-bar scheme can be safely established. We present our results for the non-perturbative running of renormalized quark masses in Nf=3 QCD between the electroweak and a hadronic energy scale, where lattice simulations are at our disposal. Recent theoretical advances in combination with well-established techniques allows to follow the scale evolution to very high statistical accuracy, and full control of systematic effects.

Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD

We analyze the energy spectra of $single$ b-jets and B-hadrons resulting from the production and decay of top quarks within the SM at the LHC at the NLO QCD. For both hadrons and jets, we calculate the correlation of the peak of the spectrum with the top quark mass, considering the "energy-peak" as an observable to determine the top quark mass. Such a method is motivated by our previous work where we argued that this approach can have reduced sensitivity to the details of the production mechanism of the top quark, whether it is higher-order QCD effects or new physics contributions. As part of the NLO improvement over the original proposal, we assess the residual sensitivity of the extracted top quark mass to perturbative effects both in top quark production and decay. For a 1% jet energy scale uncertainty (and assuming negligible statistical error), the top quark mass can then be extracted using the energy-peak of b-jets with an error +- (1.2 (exp) + 0.6(th)) GeV. We note that recently the CMS collaboration reported a top quark mass measurement based on the original proposal (with b-jets) so that our result contributes to a precise evaluation of the associated theory uncertainty. In view of the dominant jet energy scale uncertainty in the measurement using b-jets, we also investigate the extraction of the top quark mass from the energy-peak of the corresponding B-hadrons which, in principle, can be measured without this uncertainty. The calculation of the B-hadron energy spectrum is carried out using fragmentation functions at NLO. The dependence on the fragmentation scale turns out to be the largest theoretical uncertainty in this extraction of top quark mass. Future improvement of the treatment of bottom quark hadronization can reduce this uncertainty, rendering methods based on the B-hadron energy-peak competitive for the top quark mass measurement.

Scalar correlator, Higgs decay into quarks, and scheme variations of the QCD coupling

In this work, the perturbative QCD series of the scalar correlation function $\Psi(s)$ is investigated. Besides ${\rm Im}\Psi(s)$, which is relevant for Higgs decay into quarks, two other physical correlators, $\Psi^{"}(s)$ and $D^L(s)$, have been employed in QCD applications like quark mass determinations or hadronic $\tau$ decays. $D^L(s)$ suffers from large higher-order corrections and, by resorting to the large-$\beta_0$ approximation, it is shown that this is related to a spurious renormalon ambiguity at $u=1$. Hence, this correlator should be avoided in phenomenological analyses. Moreover, it turns out advantageous to express the quark mass factor, introduced to make the scalar current renormalisation group invariant, in terms of the renormalisation invariant quark mass $\hat m_q$. To further study the behaviour of the perturbative expansion, we introduce a QCD coupling $\hat\alpha_s$, whose running is explicitly renormalisation scheme independent. The scheme dependence of $\hat\alpha_s$ is parametrised by a single parameter $C$, being related to transformations of the QCD scale parameter $\Lambda$. It is demonstrated that appropriate choices of $C$ lead to a substantial improvement in the behaviour of the perturbative series for $\Psi^{"}(s)$ and ${\rm Im}\Psi(s)$.

Precision QCD measurements in DIS at HERA

New and recent results on QCD measurements from the H1 and ZEUS experiments at the HERA ep collider are reviewed. The final results on the combined deep-inelastic neutral and charged current cross-sections are presented and their role in the extractions of parton distribution functions (PDFs) is studied. The PDF fits give insight into the compatibility of QCD evolution and heavy flavor schemes with the data as a function of kinematic variables such as the scale Q2. Measurements of jet production cross-sections in ep collisions provide direct proves of QCD and extractions of the strong coupling constants are performed. Charm and beauty cross-section measurements are used for the determination of the heavy quark masses. Their role in PDF fits is investigated. In the regime of diffractive DIS and photoproduction, dijet and prompt photon production cross-sections provide insights into the process of factorization and the nature of the diffractive exchange.

Heavy quark masses from lattice QCD

Progress in quark mass determinations from lattice QCD is reviewed, focusing on results for charm and bottom mass. These are of particular interest for precision Higgs studies. Recent determinations have achieved percent-level uncertainties with controlled systematics. Future prospects for these calculations are also discussed.

New Charm and Bottom Quark Mass Determinations from QCD Sum Rules

In this talk we discuss results of a new extraction of the MS‾ charm quark mass using relativistic QCD sum rules at O(αs3) based on moments of the vector and the pseudoscalar current correlators, and using the available experimental measurements from e+e− collisions and lattice results, respectively. The analysis of the perturbative uncertainties is based on different implementations of the perturbative series and on independent variations of the renormalization scales for the mass and the strong coupling, following a work we carried out earlier. Accounting for the perturbative series that result from this double scale variation is crucial, since some of the series can exhibit extraordinarily small scale dependence, if the two scales are set equal. The new aspect of the work reported here addresses the problem that double scale variation might also lead to an overestimate of the perturbative uncertainties. We supplement the analysis by a convergence test that allows to quantify the overall convergence of QCD perturbation theory for each moment, and to discard series that are artificially spoiled by specific choices of the renormalization scales. We also apply the new method to an extraction of the MS‾ bottom quark mass using experimental moments that account for a modeling uncertainty associated to the continuum region where no experimental data is available. We obtain mc(m‾c)=1.288±0.020 GeV and m‾b(m‾b)=4.176±0.023 GeV.

Analytical determination of the QCD quark masses

The current status of determinations of the QCD running quark masses is reviewed. Emphasis is on recent progress on analytical precision determinations based on finite energy QCD sum rules. A critical discussion of the merits of this approach over other alternative QCD sum rules is provided. Systematic uncertainties from both the hadronic and the QCD sector have been recently identified and dealt with successfully, thus leading to values of the quark masses with unprecedented accuracy. Results currently rival in precision with lattice QCD determinations.