Forward photoproduction of <math><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math> pairs with invariant mass of the order of <math><msub><mi>m</mi><mi>ρ</mi></msub><mo>∼</mo><mn>770</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math> is traditionally attributed to Pomeron exchange. Based on a detailed analysis of the CEBAF Large Acceptance Spectrometer photoproduction data collected at photon energies below 4 GeV, it is shown from a study of the angular moments that the dynamics of two-pion photoproduction for <math><mo>|</mo><mi>t</mi><mo>|</mo><mo>≳</mo><mn>0.5</mn><mtext> </mtext><mtext> </mtext><msup><mi>GeV</mi><mn>2</mn></msup></math> cannot be explained by Pomeron exchange alone. This motivates the development of a new theoretical model of two-pion photoproduction which incorporates both two-pion and pion-nucleon resonant contributions. After fitting free parameters, the model provides an excellent description of the low moments of the angular distribution measured at CLAS and enables an assessment of the relative contributions of particular production mechanisms and an interpretation of the various features of the data in terms of these mechanisms.

Poland

Łukasz Bibrzycki

Spain

Nadine Hammoud

Vincent Mathieu

Robert J. Perry

United States

Alex Akridge

César Fernández-Ramírez

Gloria Montaña

Italy

Alessandro Pilloni

Arkaitz Rodas

Vanamali Shastry

Wyatt A. Smith

Germany

Daniel Winney

Adam P. Szczepaniak

Studying <math><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math> photoproduction beyond Pomeron exchange

The Reggeized pion is expected to provide the main contribution to the forward cross section in light meson photoproduction reactions with charge exchange at high energies. We discuss the Reggeization of pion exchange in charged pion photoproduction with an emphasis on consistency with current conservation. We show that the gauge-invariant amplitude for the exchange of a particle with a generic even spin <math><mi>J</mi><mo>≥</mo><mn>2</mn></math> in the <math><mi>t</mi></math> channel is analytic at <math><mi>J</mi><mo>=</mo><mn>0</mn></math> and that it can be interpreted in terms of the nucleon electric current. This enables us to reconcile the dynamics in the <math><mi>s</mi></math> and <math><mi>u</mi></math> channel, which involves also nucleon exchanges, with the amplitude expressed in terms of <math><mi>t</mi></math>-channel partial waves, as required by Regge theory.

Glòria Montaña

Giorgio Foti

Revisiting gauge invariance and Reggeization of pion exchange

We apply a model-independent reconstruction method to experimental data in order to identify complex poles of overlapping resonances. The algorithm is based on the Schlessinger Point Method where data points are interpolated using a continued-fraction expression. Statistical uncertainties of the experimental data are propagated with resampling. In order to demonstrate the feasibility of this method, we apply it to the S-wave <math><mi>J</mi><mo>/</mo><mi>ψ</mi><mo>→</mo><mi>γ</mi><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup></math> decay. We benchmark the method on known analytic models, which allows us to estimate the deviation from the true value. We then perform the pole extraction from BESIII data, and identify the <math><msub><mrow><mi>f</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>(</mo><mn>1500</mn><mo>)</mo></math>, <math><msub><mrow><mi>f</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>(</mo><mn>1710</mn><mo>)</mo></math>, and <math><msub><mrow><mi>f</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>(</mo><mn>2020</mn><mo>)</mo></math> scalar states. Our results are in reasonable agreement with recent results, which suggests the proposed method as a promising model-independent alternative for the determination of resonance poles that is solely based on available experimental data.

Daniele Binosi

Ralf-Arno Tripolt

Study for a model-independent pole determination of overlapping resonances

Tetraquarks <math><mi>Q</mi><mi>Q</mi><mover><mrow><mi>q</mi></mrow><mrow><mo>¯</mo></mrow></mover><mover><mrow><mi>q</mi></mrow><mrow><mo>¯</mo></mrow></mover></math> are found to be described remarkably well with the Quantum Chromodynamics version of the Hydrogen bond, as treated with the Born-Oppenheimer approximation. We show the robustness of the method by computing the mass of the observed <math><msub><mrow><mi>T</mi></mrow><mrow><mi>c</mi><mi>c</mi></mrow></msub></math> tetraquark following two different paths. Relying on this, we provide a prediction for the mass of the expected <math><msub><mrow><mi>T</mi></mrow><mrow><mi>b</mi><mi>b</mi></mrow></msub></math> particle. The average sizes of tetraquarks are estimated to be approximately 3–<math><mn>5</mn><msup><mrow><mspace width="0.20em"></mspace><mtext>GeV</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math>. As a consequence hyperfine separations are not expected to be sizeable. We discussed possible reasons why LHCb has observed only one state in the <math><mi>D</mi><msup><mrow><mi>D</mi></mrow><mrow><mo>⁎</mo></mrow></msup></math> spectrum.

CERN

Luciano Maiani

Antonio D. Polosa

Veronica Riquer

Doubly heavy tetraquarks in the Born-Oppenheimer approximation

We present multiple results on the production of loosely bound molecules in bottomonium annihilations and <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> collisions at <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>10.58</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>. We perform the first comprehensive test of several models for deuteron production against all the existing data in this energy region. We fit the free parameters of the models to reproduce the observed cross sections, and we predict the deuteron spectrum and production and the cross section for the <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo>→</mo><mi>d</mi><mover><mi>d</mi><mo>¯</mo></mover><mo>+</mo><mi>X</mi></math> process both at the <math><mi>ϒ</mi><mo>(</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn><mi>S</mi><mo>)</mo></math> resonances and at <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>10.58</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>. The predicted spectra show differences but are all compatible with the uncertainties of the existing data. These differences could be addressed if larger datasets are collected by the Belle II experiment. Fixing the source size parameter to reproduce the deuteron data, we then predict the production rates for <math><mi>H</mi></math> dibaryon and hypertriton in this energy region using a simple coalescence model. Our prediction on the <math><mi>H</mi></math> dibaryon production rate is below the limits set by the direct search at the Belle experiment, but in the range accessible to the Belle II experiment. The systematic effect due to the MC modeling of quarks and gluon fragmentation into baryons is reduced, deriving a new tuning of the pythia 8 Monte Carlo generator using the available measurement of single- and double-particle spectra in <math><mi>ϒ</mi></math> decays.

Davide Marietti

Umberto Tamponi

Production of loosely bound hadron molecules from bottomonium decays

The scalar glueball G is the lightest particle of the Yang–Mills sector of QCD, with a lattice predicted mass of about  $$m_{G}\simeq 1.7\,{\mathrm{GeV}}$$  <math> <mrow> <msub> <mi>m</mi> <mi>G</mi> </msub> <mo>≃</mo> <mn>1.7</mn> <mspace width="0.166667em"></mspace> <mi>GeV</mi> </mrow> </math>  . It is natural to investigate glueball-glueball scattering and the possible emergence of a bound state, that we call glueballonium. We perform this study in the context of a widely used dilaton potential, that depends on a single dimensionful parameter  $$\Lambda _G$$  <math> <msub> <mi>Λ</mi> <mi>G</mi> </msub> </math>  . We consider a unitarization prescription that allows us to predict the lowest partial waves in the elastic window. These quantities can be in principle calculated on the lattice, thus offering possibility for testing the validity of the dilaton potential and an independent determination of its parameter. Moreover, we also show that a stable glueballonium exists if  $$\Lambda _{G}$$  <math> <msub> <mi>Λ</mi> <mi>G</mi> </msub> </math>   is small enough. In particular, for  $$\Lambda _{G}$$  <math> <msub> <mi>Λ</mi> <mi>G</mi> </msub> </math>   compatible with the expectations from the gluon condensate, the glueballonium has a mass of about  $$3.4\,{\mathrm{GeV}}$$  <math> <mrow> <mn>3.4</mn> <mspace width="0.166667em"></mspace> <mi>GeV</mi> </mrow> </math>  .

Francesco Giacosa

Enrico Trotti

Glueball–glueball scattering and the glueballonium

From a study of the line shape of the <math><mi>X</mi><mo>(</mo><mn>3872</mn><mo>)</mo></math>, the LHCb collaboration measures a sizeable negative effective range. This cannot be reconciled with a shallow <math><mi>D</mi><msup><mover><mi>D</mi><mo>¯</mo></mover><mo>*</mo></msup></math> bound state hypothesis. Based on Weinberg’s compositeness criterion, together with a theorem by Smorodinsky, it follows that the <math><mi>X</mi></math> has to have a compact hidden charm structure interacting with unbound <math><mi>D</mi><msup><mover><mi>D</mi><mo>¯</mo></mover><mo>*</mo></msup></math> pairs via short-distance color forces. This conclusion is strengthened by the general pattern recently emerging from exotic mesons.

Switzerland

Angelo Esposito

Verónica Riquer

From the line shape of the <math><mi>X</mi><mo>(</mo><mn>3872</mn><mo>)</mo></math> to its structure

Ultraperipheral heavy ion collisions constitute an ideal setup to look for exotic hadrons because of their low event multiplicity and the possibility of an efficient background rejection. We propose to look for four-quark states produced by photon-photon fusion in these collisions at the center-of-mass energy per nucleon pair <math><msqrt><msub><mi>s</mi><mi>NN</mi></msub></msqrt><mo>=</mo><mn>5.5</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math>. In particular, we focus on those states that would represent a definite smoking gun for the compact tetraquark model. We show that the <math><mi>X</mi><mo>(</mo><mn>6900</mn><mo>)</mo></math>, a likely <math><mi>c</mi><mi>c</mi><mover><mi>c</mi><mo>¯</mo></mover><mover><mi>c</mi><mo>¯</mo></mover></math> compact state, is a perfect candidate for this search, and estimate a production cross section ranging from around 250 nb to 1150 nb, depending on its quantum numbers. Furthermore, we discuss the importance of ultraperipheral collisions to the search for the scalar and tensor partners of the <math><mi>X</mi><mo>(</mo><mn>3872</mn><mo>)</mo></math> predicted by the diquarkonium model, and not yet observed. The completion of such a flavor-spin multiplet would speak strongly in favor of the compact tetraquark model.

Claudio Andrea Manzari

Antonio Davide Polosa

Hunting for tetraquarks in ultraperipheral heavy ion collisions

The structure of exotic resonances that do not trivially fit the usual quark model expectations has been a matter of intense scientific debate during the last two decades. A possible way of estimating the size of these states is to study their behavior when immersed in QCD matter. Recently, LHCb has measured the relative abundance of the exotic  $$X(3872)$$  <math> <mrow> <mi>X</mi> <mo>(</mo> <mn>3872</mn> <mo>)</mo> </mrow> </math>   over the ordinary  $$\psi (2S)$$  <math> <mrow> <mi>ψ</mi> <mo>(</mo> <mn>2</mn> <mi>S</mi> <mo>)</mo> </mrow> </math>  . We use the comover interaction model to study the yield of a compact  $$X(3872)$$  <math> <mrow> <mi>X</mi> <mo>(</mo> <mn>3872</mn> <mo>)</mo> </mrow> </math>  . To confirm the reliability of the model in high-multiplicity pp collisions, we describe the suppression of excited over ground  $$\Upsilon $$  <math> <mi>Υ</mi> </math>   states. With this at hand, we show that the size of the compact  $$X(3872)$$  <math> <mrow> <mi>X</mi> <mo>(</mo> <mn>3872</mn> <mo>)</mo> </mrow> </math>   would be slightly larger than that of the  $$\psi (2S)$$  <math> <mrow> <mi>ψ</mi> <mo>(</mo> <mn>2</mn> <mi>S</mi> <mo>)</mo> </mrow> </math>  . If the  $$X(3872)$$  <math> <mrow> <mi>X</mi> <mo>(</mo> <mn>3872</mn> <mo>)</mo> </mrow> </math>   is instead assumed to be a meson molecule of large size, we argue that its evolution in QCD matter should be described via a coalescence model, as suggested by data on deuteron production. We show that the predictions of this model for the  $$X(3872)$$  <math> <mrow> <mi>X</mi> <mo>(</mo> <mn>3872</mn> <mo>)</mo> </mrow> </math>   are in contrast with data.

Elena Ferreiro

Antonio Polosa

Carlos Salgado

The nature of X(3872) from high-multiplicity pp collisions