arXiv subjects

T. J. Burns

Publications and source records attributed to T. J. Burns.

At least 19 recordsLinked to original sources

arXiv2022

Production of $P_c$ states in $Λ_b$ decays

We develop a model for the production of the Pc states observed at LHCb in Lambdab - Jpsi p K- decays. With fewer parameters than other approaches, we obtain excellent fits to the Jpsi-p invariant mass spectrum, capturing both the prominent peaks, and broader features over the full range of invariant mass. A distinguishing feature of our model is that whereas Pc(4312), Pc(4380), and Pc(4440) are resonances with Sigmac*-D* constituents, the nature of Pc(4457) is quite different, and can be understood either as a Sigmac-D* threshold cusp, a Lambdac(2595)-D enhancement due to the triangle singularity, or a Lambdac(2595)-D resonance. We propose experimental measurements that can discriminate among these possibilities. Unlike in other models, our production mechanism respects isospin symmetry and the empirical dominance of colour-enhanced processes in weak decays, and additionally gives a natural explanation for the overall shape of the data. Our model is consistent with experimental constraints from photoproduction and Lambdab - Lambdac-D(*)0 K- decays and it does not imply the existence of partner states whose apparent absence in experiments is unexplained in other models.

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arXiv2022

The LHCb state $P_{ψs}^Λ(4338)$ as a triangle singularity

We present a model for the $J/ψΛ$ spectrum in $B^-\to J/ψΛ\bar{p}$ decays, including the $P_{ψs}^Λ(4338)$ baryon recently observed by the LHCb collaboration. We assume production via triangle diagrams which couple to the final state via non-perturbative interactions which are constrained by heavy-quark and $SU_3$-flavor symmetry. The bulk of the distribution is described by a triangle diagram with a color-favored electroweak vertex, while the sharp $P_{ψs}^Λ(4338)$ enhancement is due to the triangle singularity in another diagram featuring a $1/2^-$ baryon consistent with $Σ_c(2800)$. We predict a comparable $P_{ψs}^Λ(4338)$ signal in $η_c Λ$, and anticipate possible large isospin mixing effects through decays to $J/ψΣ^0$ and $η_c Σ^0$

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arXiv2022

Experimental constraints on the properties of $P_c$ states

Using new experimental data on photoproduction and $Λ_b^0$ decays, we derive constraints on the properties of the LHCb $P_c$ states. We conclude that $P_c(4312)$, $P_c(4380)$ and $P_c(4440)$ can be described as $Σ_c\bar D$, $Σ_c^*\bar D$ and $Σ_c\bar D^*$ molecules, but that $P_c(4457)$ does not fit into the same picture. Based on the apparent absence of additional partner states, and the striking disparity between $Λ_c^+\bar D^0$ and $Λ_c^+\bar D^{*0}$ decays, we conclude that $P_c(4440)$ has $3/2^-$ quantum numbers. Using heavy-quark symmetry we predict large experimental signals for $P_c$ states in $η_c p K^-$, $Λ_c^+\bar D^{*0}K^-$, and $Σ_c^{(*)}\bar D K^-$. We also argue that current experimental data on photoproduction is almost at the level of sensitivity required to observe $P_c$ states.

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arXiv2021

Discriminating among interpretations for the $X(2900)$ states

We make predictions for the production and decays of $X(2900)$ states, and their possible charged partners, in $B^+$ and $B^0$ decays, considering a number of competing models for the states, including triangle diagrams mediated by quark exchange or pion exchange, and resonance scenarios including molecules and tetraquarks. Assuming only isospin symmetry and the dominance of colour-favoured weak decays, we find characteristic differences in the predictions of the different models. Future experimental studies can therefore discriminate among the competing interpretations for the states.

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arXiv2021

Kinematical Cusp and Resonance Interpretations of the $X(2900)$

We examine whether the LHCb vector $ud\bar{c}\bar{s}$ state $X(2900)$ can be interpreted as a kinematical cusp effect arising from $\bar D^*K^*$ and $\bar D_1 K^*$ interactions. The production amplitude is modelled as a triangle diagram with hadronic final state interactions. A satisfactory fit to the Dalitz plot projection is obtained that leverages the singularities of the production diagram without the need for $\bar{D}K$ resonances. A somewhat better fit is obtained if the final state interactions are strong enough to generate resonances, although the evidence in favour of this scenario is not conclusive.

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arXiv2019

Molecular Interpretation of the $P_c(4440)$ and $P_c(4457)$ States

A molecular model of the $P_c(4457)$ and $P_c(4440)$ LHCb states is proposed. The model relies on channels coupled by long range pion-exchange dynamics with features that depend crucially on the novel addition of the $Λ_c(2595)\bar D$ channel. A striking prediction of the model is the unusual combination of quantum numbers $J^P(4457) = 1/2^+$ and $J^P(4440) = 3/2^-$. Unlike in other models, a simultaneous description of both states is achieved without introducing additional short-range interactions. The model also gives a natural explanation for the relative widths of the states. We show that the usual molecular scenarios cannot explain the production rate of $P_c$ states in $Λ_b$ decays, and that this can be resolved by including $Λ_c(2595)\bar D$ and related channels. Experimental tests and other states are discussed in the conclusions.

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arXiv2016

Interpreting the $X(5568)$

A variety of options for interpreting the DØ state, $X(5568)$, are examined. We find that threshold, cusp, molecular, and tetraquark models are all unfavoured. Several experimental tests for unravelling the nature of the signal are suggested.

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arXiv2015

Phenomenology of $P_c(4380)^+$, $P_c(4450)^+$ and related states

The $P_c(4380)^+$ and $P_c(4450)^+$ states recently discovered at LHCb have masses close to several relevant thresholds, which suggests they can be described in terms of meson-baryon degrees of freedom. This article explores the phenomenology of these states, and their possible partners, from this point of view. Competing models can be distinguished by the masses of the neutral partners which have yet to be observed, and the existence or otherwise of further partners with different isospin, spin, and parity. Future experimental studies in different decay channels can also discriminate among models, using selection rules and algebraic relations among decays. Among the several possible meson-baryon pairs which could be important, one implies that the states are mixtures of isospins 1/2 and 3/2, with characteristic signatures in production and decay. A previous experimental study of a Cabibbo-suppressed decay showed no evidence for the states, and further analysis is required to establish the significance of this non-observation. Several intriguing similarities suggest that $P_c(4450)^+$ is related to the $X(3872)$ meson.

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arXiv2014

Meson mass splittings in unquenched quark models (EEF70)

General results are obtained for meson mass splittings and mixings in unquenched (coupled-channel) quark models. Theorems derived previously in perturbation theory are generalised to the full coupled-channel system. A new formula is obtained for the mass splittings of physical states in terms of the splittings of the valence states. The S-wave hyperfine splitting decreases due to unquenching, but its relation to the vector $e^+e^-$ width is unchanged; this yields a prediction for the missing $η_b(3S)$. The ordinary (quenched) quark model result that the P-wave hyperfine splitting vanishes also survives unquenching. A ratio of mass splittings used to discriminate quarkonium potential models is scarcely affected by unquenching.

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arXiv2014

Angular momentum coefficients for meson strong decay and unquenched quark models

In most meson strong decay and unquenched (coupled-channel) quark models, the pair-creation operator is a scalar product of vectors in the spin and spatial degrees of freedom. While differing in the spatial part, most models have the same spin part, which creates a qq* pair coupled to spin triplet, with the spins of the initial quarks as spectators. This is a basic assumption of the 3P0 model, and is well-known to arise also in the flux tube model, starting from the strong coupling expansion of lattice QCD. In this article the same structure is shown to emerge in the Cornell model, in the dominant contributions of a more general microscopic decay model, and in the pseudoscalar-meson emission model. A solution is obtained for arbitrary matrix elements in these ``non-flip, triplet'' models, expressed as a weighted sum over spatial matrix elements. The coefficients in the expansion, which involve the spin degrees of freedom and the associated angular momentum algebra, are model-independent. Tables of the angular momentum coefficients are presented which can be used in future calculations, avoiding tedious Clebsch-Gordan sums. The symmetry and orthogonality properties of the coefficients are discussed, as well as their application to transitions involving hybrid mesons and states of mixed spin. New selection rules are derived, and existing ones generalised. The coefficients lead to model-independent relations among decay amplitudes and widths which can be tested in experiment and lattice QCD. They can also be used to explain how mass shifts in the unquenched quark model do not spoil successful predictions of the ordinary (quenched) quark model.

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arXiv2013

Angular momentum coefficients for meson strong decay and the unquenched quark model (CHARM 2013)

Most models for the strong decay of mesons, as well as unquenched quark models which incorporate the effect of coupling to meson-meson channels, assume that the coupling is driven by the creation of a quark-antiquark pair in spin triplet. Their matrix elements can be factorised into a sum over (model-dependent) spatial matrix elements multiplied by (model-independent) coefficients. A general expression for these coefficients is obtained, and their properties are studied. Numerical tables of the coefficients can be used as a starting point for future calculations. The coefficients lead to model-independent predictions for decay amplitudes and widths, and can explain how mass shifts in the unquenched quark model do not spoil successful predictions of the ordinary (quenched) quark model. This article is based on work which will be presented in a forthcoming paper.

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arXiv2013

Hyperfine splitting and the experimental candidates for η_b(2S)

Predictions for the hyperfine splitting of 2S bottomonia are compared with the two recent experimental candidates for the η_b(2S). The smaller splitting of the Belle state is consistent with unquenched lattice QCD computations, many potential models, and a model-independent relation which works well for charmonia. The larger splitting for the state extracted from CLEO data is inconsistent with most predictions.

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arXiv2011

P-wave spin-spin splitting and meson loops

In quark potential models the hyperfine splitting of P-wave mesons is zero in the nonrelativistic limit, a prediction strikingly confirmed by experiment in both charmonia and bottomonia. The result, however, ignores the coupling of bare quarkonia to meson-meson pairs. This coupling causes mass shifts among the states and so could potentially spoil the quark model prediction. This turns out not to be the case: in a variety of models the hyperfine splitting remains small despite large mass shifts. This is shown to be a generic feature of models in which the coupling involves the creation of a light quark pair with spin-one and the quark spin wavefunctions are conserved. This talk reports on the results of Phys. Rev. D84, 034021 (2011).

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arXiv2011

How the small hyperfine splitting of P-wave mesons evades large loop corrections

The recent discoveries of the bottomonia states h_b(1P) and h_b(2P) confirm the quark model prediction, already verified in the charmonia sector, that the hyperfine splitting of P-wave mesons is very small. The striking agreement is somewhat surprising because the non-relativistic result, for which the splitting is zero, may be modified due to large mass shifts from coupling to open flavour meson pairs. This paper is based on the observation that in most models hyperfine splitting remains small despite what are in many cases large mass shifts. This effect is shown to be a generic feature of models in which the coupling is driven by the creation of a spin-one pair.

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arXiv2011

Rethinking the X(3872)

The BaBar analysis which favours $2^{-+}$ quantum numbers for the X(3872) implies that it may be none other than the 1D2 charmonia state. In that case the isospin breaking in closed flavour modes may be the result of re-scattering from open flavour. However, the observed production cross section in proton-antiproton collisions is much larger than expectations, while the mass of the state, compared to the predictions of a string model, is too high. The 1D2 assignment would imply a 3D2 partner nearby in mass, which may be the X(3875). In the tetraquark interpretation the $2^{-+}$ assignment implies a rich spectrum of partner states, although the X(3872) may be among the few which are narrow enough to be observable. This talk is based in part on "The 2-+ assignment for the X(3872)" by T. J. Burns, F. Piccinini, A. D. Polosa & C. Sabelli (Phys. Rev. D 82, 074003 (2010); arXiv:1008.0018 [hep-ph]).

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arXiv2010

The 2^-+ assignment for the X(3872)

Very recently the BaBar collaboration has put forward a claim that the X(3872) is not a 1^++ resonance, as most of the phenomenological work on the subject was relying on, but rather a 2^-+ one. We examine the consequences of this quantum number assignment for the solution of the X(3872) puzzle. The molecular interpretation appears less likely, and the conventional charmonium interpretation should be reconsidered. There are several well-known difficulties with this interpretation, to which we add a new one: the production cross section at CDF is predicted to be much smaller than that observed. We also confirm, using a relativistic string model, the conclusion from potential models that the mass of the state is not consistent with expectations. In the tetraquark interpretation the 2^-+ assignment implies a rich spectrum of partner states, although the X(3872) may be among the few which are narrow enough to be observable.

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arXiv2008

Dynamics of hadron strong production and decay

We generalize results of lattice QCD to determine the spin-dependent symmetries and factorization properties of meson production in OZI allowed processes. This explains some conjectures previously made in the literature about axial meson decays and gives predictions for exclusive decays of vector charmonia, including ways of establishing the structure of Y(4260) and Y(4325) from their S-wave decays. Factorization gives a selection rule which forbids $e^+e^- \to D^* D_2$ near threshold with the tensor meson in helicity 2. The relations among amplitudes for double charmonia production $\e^+e^-\to ψχ_{0,1,2}$ are expected to differ from the analagous relations among light flavour production such as $\e^+e^-\to ωf_{0,1,2}$.

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arXiv2007

Hadron production in $ψ$, $η_c$ and $χ$ decays

We derive relations among branching fractions in the exclusive decay of charmonia to light flavour meson pairs assuming factorization between the quark spin and spatial degrees of freedom. With the further assumption that these amplitudes can be described by flux-tube models, we assess prospects for production of hybrid mesons in charmonium decays.

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