Quark Model Predictions Research Articles

Wounded quarks inA+A,p+A, andp+pcollisions

We explore predictions of the wounded quark model for particle production and properties of the initial state formed in ultrarelativistic heavy-ion collisions. The approach is applied uniformly to A+A collisions in a wide collision energy range, as well as for p+A and p+p collisions at the CERN Large Hadron Collider (LHC). We find that generically the predictions from wounded quarks for such features as eccentricities or initial sizes are close (within 15\%) to predictions of the wounded nucleon model with an amended binary component. A larger difference is found for the size in p+Pb system, where the wounded quark model yields a smaller (more compact) initial fireball than the standard wounded nucleon model. The inclusion of subnucleonic degrees of freedom allows us to analyze p+p collisions in an analogous way, with predictions that can be used in further collective evolution. The approximate linear dependence of particle production in A+A collisions on the number of wounded quarks, as found in previous studies, makes the approach based on wounded quarks natural. Importantly, at the LHC energies we find approximate uniformity in particle production from wounded quarks, where at a given collision energy per nucleon pair similar production of initial entropy per source is needed to explain the particle production from p+p collisions up to A+A collisions. We also discuss the sensitivity of the wounded quark model predictions to distribution of quarks in nucleons, distribution of nucleons in nuclei, and to the quark-quark inelasticity profile in the impact parameter. In our procedure, the quark-quark inelasticity profile is chosen in such a way that the experiment-based parametrization of the proton-proton inelasticity profile is properly reproduced. The parameters of the overlaid multiplicity distribution is fixed from p+p and p+Pb data.

Towards an assessment of the ATLAS data on the branching ratioΓ(Λb0→ψ(2S)Λ0)/Γ(Λb0→J/ψΛ0)

Recently the ATLAS Collaboration at CERN reported on the measurement of the branching ratio R(Lambda_b) = Gamma(Lambda_b(0) -> psi(2S) Lambda(0))/Gamma(Lambda_b(0) -> J/psi Lambda(0)). The measured branching ratio R(Lambda_b) = 0.501 pm 0.033$ (stat) pm 0.019 (syst) was found to be lower than the covariant quark model prediction of R(Lambda_b) = 0.8 pm 0.1 calculated by us recently. We present a detailed analysis of the branching ratio R(Lambda_b) using a model-independent framework for the heavy-to-light form factors based on results from our previos papers.

Measurements ofep→e′π+nat1.6<W<2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

Differential cross sections of the exclusive process $e p \to e^\prime \pi^+ n$ were measured with good precision in the range of the photon virtuality $Q^2 = 1.8 - 4.5$ GeV$^2$, and the invariant mass range of the $\pi^+ n$ final state W = 1.6 - 2.0 GeV using the CEBAF Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the $n\pi^+$ center-of-mass system. More than 37,000 cross section points were measured. The contributions of the isospin $I = {1\over 2}$ resonances $N(1675){5\over 2}^-$, $N(1680){5\over 2}^+$ and $N(1710){1\over 2}^+$ were extracted at different values of $Q^2$ using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-$t$ dispersion relations, were employed in the analysis. We observe significant strength of the $N(1675){5\over 2}^-$ in the $A_{1/2}$ amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the $N(1680){5\over 2}^+$ we observe a slow changeover from the dominance of the $A_{3/2}$ amplitude at the real photon point ($Q^2=0$) to a $Q^2$ where $A_{1/2}$ begins to dominate. The scalar amplitude $S_{1/2}$ drops rapidly with $Q^2$ consistent with quark model prediction. For the $N(1710){1\over 2}^+$ resonance our analysis shows significant strength for the $A_{1/2}$ amplitude at $Q^2 < 2.5$ GeV$^2$.

Uraltsev's and other sum rules, theory and phenomenology of D**'s

We first discuss Uraltsev's and other sum rules constraining the B → D**(L = 1) weak transitions in the infinite mass limit, and compare them with dynamical approaches in the same limit. After recalling these well established facts, we discuss how to apply infinite mass limit to the physical situation. We provide predictions concerning semileptonic decays and non-leptonic ones, based on quark models. We then present in more detail the dynamical approaches: the relativistic quark model à la Bakamjian–Thomas and lattice QCD. We summarise lattice QCD results in the infinite mass limit and compare them to the quark model predictions. We then present preliminary lattice QCD results with finite b and c quark masses. A systematic comparison between theory and experiment is performed. We show that some large discrepancies exist between different experiments. Altogether the predictions at infinite mass are in fair agreement with experiment for non-leptonic decays contrary to what happens for semileptonic decays. We conclude by considering the prospects to clarify both the experimental situation, the theoretical one and the comparison between both.

Spectroscopy of higher bottomonia

In this contribution, we discuss our recent unquenched quark model results for the spectrum of $b \bar b$ mesons with self energy corrections, due to the coupling to the meson-meson continuum. Our unquenched quark model predictions for the masses of the recently discovered $\chi_b(3P)$ states are compared to those of a re-fit of Godfrey and Isgur's relativized quark model to the most recent experimental data. The possible importance of continuum effects in the $\chi_b(3P)$ states is discussed. Finally, we show our quark model results for the radiative decays of the $\chi_b(3P)$ system and the open-bottom decays of $b \bar b$ mesons.

Self-Consistent Covariant Description of Twist-3 Distribution Amplitude of a Pseudoscalar Meson in the Light-Front Quark Model

We discuss the light-front zero-mode issue in the light-front quark model prediction of the twist-3 distribution amplitude of a pseudoscalar meson from the perspective of the vacuum fluctuation consistent with the chiral symmetry of QCD.

B̄s → K semileptonic decay from an Omnès improved nonrelativistic quark model

We study the f+ form factor for the B̄s → K+ℓ−v̄ℓ semileptonic decay in a nonrelativistic quark model. The valence quark contribution is supplemented with a B̄*-pole term that dominates the high q2 region. To extend the quark model predictions from its region of applicability near q2max = (MBs – MK)2 we use a multiply-subtracted Omnès dispersion relation. We fit the subtraction constants to a combined input from previous light cone sum rule results in the low q2 region and the quark model results (valence plus B̄*-pole) in the high q2 region. From this analysis, we obtain Γ(B̄s → K+ℓ−v̄ℓ) = (5.47+5.54−0.46)|Vub|2 × 10−9 MeV, which is about 10% and 20% higher than predictions based on Lattice QCD and QCD light cone sum rules respectively.

Formula omitted] semileptonic decay from an Omnès improved constituent quark model

We study the f+ form factor for the semileptonic B¯s→K+ℓ−ν¯ℓ decay in a constituent quark model. The valence quark estimate is supplemented with the contribution from the B¯⁎ pole that dominates the high q2 region. We use a multiply-subtracted Omnès dispersion relation to extend the quark model predictions from its region of applicability near qmax2=(MBs−MK)2∼23.75 GeV2 to all q2 values accessible in the physical decay. To better constrain the dependence of f+ on q2, we fit the subtraction constants to a combined input from previous light cone sum rule by Duplancic and Melic (2008) [11] and the present quark model results. From this analysis, we obtain Γ(B¯s→K+ℓ−ν¯ℓ)=(5.47−0.46+0.54)|Vub|2×10−9 MeV, which is about 10% and 20% higher than the predictions based on Lattice QCD and QCD light cone sum rules respectively. The former predictions, for both the form factor f+(q2) and the differential decay width, lie within the 1σ band of our estimated uncertainties for all q2 values accessible in the physical decay, except for a quite small region very close to qmax2. Differences with the light cone sum results for the form factor f+ are larger than 20% in the region above q2=15 GeV2.

Quark structure of theX(3872)andχb(3P)resonances

We discuss the nature of the $\chi_b(3P)$ and $X(3872)$ mesons: Are the $\chi_b(3P)$'s standard $b \bar b$ mesons or $b \bar b$ states with a significative continuum component? Is the $X(3872)$ a $c \bar c$ state with continuum coupling effects or a meson-meson molecule? To do that, we compare quark model and unquenched quark model results for the mass barycenter and splittings of the $\chi_b(3P)$ multiplet. Future and more precise experimental results will discriminate between the two interpretations. In the case of the $X(3872)$, we interpret it a $c \bar c$ core plus higher Fock components due to the coupling to the meson-meson continuum, thus we think that it is compatible with the meson $\chi_{c1}(2P)$, with $J^{PC} = 1^{++}$. The $J^{PC} = 1^{++}$ quantum numbers are in agreement with the experimental results found by the LHCb collaboration. In our view, the $X(3872)$'s mass is lower than the quark model's predictions because of self energy shifts. We also provide an estimation of the open charm/bottom strong decay modes of the $X(3872)$ and $\chi_b(3P)$ mesons, such as $X(3872) \rightarrow D \bar D^*$ and $\chi_{b2}(3P) \rightarrow B \bar B$, and radiative transitions.

Self-consistent covariant description of vector meson decay constants and chirality-even quark-antiquark distribution amplitudes up to twist 3 in the light-front quark model

Although the meson decay amplitude described by a two-point function may be regarded as one of the simplest possible physical observable, it is interesting that this apparently simple amplitude bears abundant fundamental informations on QCD vacuum dynamics and chiral symmetry. The light-front zero-mode issue of the vector meson decay constant $f_V$ is in this respect highly non-trivial and deserves careful analyses. We discuss the zero-mode issue in the light-front quark model (LFQM) prediction of $f_V$ from the perspective of the vacuum fluctuation consistent with the chiral symmetry of QCD. We extend the exactly solvable manifestly covariant Bethe-Salpeter model calculation to the more phenomenologically accessible realistic LFQM and present a self-consistent covariant description of $f_V$ analyzing the twist-2 and twist-3 quark-antiquark distribution amplitudes with even chirality.

Systematic study of longitudinal and transverse helicity amplitudes in the hypercentral Constituent Quark Model

The predictions of the hypercentral Consituent Quark Model for the nucleon helicity amplitudes are briefly reported. Some future perspectives are also discussed.

Light-Front Zero-Mode Issue on the Vector Meson Decay Constant

We discuss the light-front zero-mode issue in the light-front quark model (LFQM) prediction of a vector meson decay constant from the perspective of the vacuum fluctuation consistent with the chiral symmetry of QCD. We extend the exactly solvable manifestly covariant Bethe-Salpeter model calculation to the more phenomenologically accessible realistic LFQM and present a self-consistent covariant description of the vector meson decay constant analyzing the twist-2 and twist-3 quark-antiquark distribution amplitudes with even chirality.

POLARIZATION OBSERVABLES FOR K+Λ AND K+Σ0 PHOTOPRODUCTION FROM POLARIZED PROTONS

The search for undiscovered excited states of the nucleon continues to be a focus of experiments at Jefferson Lab. Recent LQCD calculations have confirmed long-standing quark-model predictions of many more states than have so far been identified.1 A large effort for the N* program has been launched using the CLAS detector to provide the database that will allow nearly model-independent partial wave analyses to be carried out in the search for such states. Polarization observables play a crucial role in this effort, as they are essential in disentangling overlapping resonant and non-resonant amplitudes. In 2010, double-polarization data were taken at JLab using circularly polarized photons incident on a transversely polarized frozen-spin butanol target.2 Our current analysis yields preliminary data of the T and F asymmetries of the K+Λ and K+Σ0 final states, which are compared to predictions of recent multipole analyses. This work is the first of its kind and will significantly broaden the world database for these reactions.

Measurements of the γ*p → Δ reaction at low Q2

We report new p$(\vec{e},e^\prime p)\pi^\circ$ measurements in the $\Delta^{+}(1232)$ resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.

Multichannel parametrization ofK¯Nscattering amplitudes and extraction of resonance parameters

We present results of a new multichannel partial-wave analysis for $\bar K N$ scattering in the c.m.\ energy range 1480 to 2100 MeV. Resonance parameters were extracted by fitting partial-wave amplitudes from all considered channels using a multichannel parametrization that is consistent with $S$-matrix unitarity. The resonance parameters are generally in good agreement with predictions of the Koniuk-Isgur quark model.

Interpretation of theX(3872)as a charmonium state plus an extra component due to the coupling to the meson-meson continuum

We present a quark model calculation of the charmonium spectrum with self energy corrections due to the coupling to the meson-meson continuum. The bare masses used in the calculation are computed within the relativized quark model by Godfrey and Isgur. The strong decay widths of 3S, 2P, 1D and 2D $c \bar c$ states are also calculated, in order to set the values of the $^3P_0$ pair-creation model's parameters we use to compute the vertex functions of the loop integrals. Finally, the nature of the X(3872) resonance is analyzed and the main possibilities ($c \bar c$ state or $D \bar D^*$ molecule) are discussed. According to our results, the X(3872) is compatible with the meson $\chi_{c1}(2P)$, with $J^{PC} = 1^{++}$, and is thus interpreted as a $c \bar c$ core plus higher Fock components due to the coupling to the meson-meson continuum. These $J^{PC} = 1^{++}$ quantum numbers are in agreement with the experimental results found by the LHCb collaboration. In our view, the X(3872)'s mass is lower than the quark model's predictions because of self energy shifts.

Analysis ofγ*Λ→Σ0transition in QCD

The ${\ensuremath{\gamma}}^{*}\ensuremath{\Lambda}\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{0}$ transition form factors are investigated within the light-cone QCD sum rules method. Using the most general form of the interpolating current of ${\ensuremath{\Sigma}}^{0}$ baryon and the distribution amplitudes of $\ensuremath{\Lambda}$ baryon we calculate the ${Q}^{2}$ dependence of the electromagnetic form factors. Our results are compared with the predictions of the covariant spectator quark model.

Unravelling the Excitation Spectrum of the Nucleon

The low-energy structure of QCD lies encoded in the excited states of the nucleon, a complicated overlap of many resonances. Recent Lattice calculations have confirmed the longstanding quark model predictions of many more excited states than have been identified. Reactions that probe the spectrum are clouded by effects that dress the interactions and complicate the identification of excited levels and the interpretation of their structure. Recent theoretical work has exposed dramatic effects from such dressings. On the experimental side, new complete measurements of pseudoscalar meson photo-production are being pursued at several laboratories, where here the designation of complete refers to measurements of most if not all of the 16 possible reaction observables. This has been the focus of a series of experiments at Jefferson Lab culminating in the recently completed g9/FROST and g14/HDice runs which are now under analysis. With realistic errors, the number of observables needed to constrain the production amplitude is many more than required of a mathematical solution.

Lattice study onηc2andX(3872)

Properties of ${2}^{\ensuremath{-}+}$ charmonium ${\ensuremath{\eta}}_{c2}$ are investigated in quenched lattice QCD. The mass of ${\ensuremath{\eta}}_{c2}$ is determined to be 3.80(3) GeV, which is close to the mass of $D$-wave charmonium $\ensuremath{\psi}(3770)$ and in agreement with quark model predictions. The transition width of ${\ensuremath{\eta}}_{c2}\ensuremath{\rightarrow}\ensuremath{\gamma}J/\ensuremath{\psi}$ is also obtained with a value of $\ensuremath{\Gamma}=3.8(9)\text{ }\text{ }\mathrm{keV}$. Since the possible ${2}^{\ensuremath{-}+}$ assignment to $X(3872)$ has not been ruled out by experiments, our results help to clarify the nature of $X(3872)$.

Systematic study of longitudinal and transverse helicity amplitudes in the hypercentral constituent quark model

We report in a systematic way the predictions of the non-relativistic hypercentral Constituent Quark model for the electromagnetic excitations of baryon resonances. The longitudinal and transverse helicity amplitudes are calculated with no free parameters for fourteen resonances, both for proton and neutron. The calculations lead to an overall fair description of data, specially in the medium $Q^2$ range, where quark degrees of freedom are expected to dominate.