Showing papers on "Meson published in 2017"

Exotics: Heavy Pentaquarks and Tetraquarks

Abstract: For many decades after the invention of the quark model in 1964 there was no evidence that hadrons are formed from anything other than the simplest pairings of quarks and antiquarks, mesons being formed of a quark-antiquark pair and baryons from three quarks. In the last decade, however, in an explosion of data from both $e^+e^-$ and hadron colliders, there are many recently observed states that do not fit into this picture. These new particles are called generically "exotics". They can be either mesons or baryons. Remarkably, they all decay into at least one meson formed of either a $c\overline{c}$ or $b\overline{b}$ pair. In this review, after the introduction, we explore each of these new discoveries in detail first from an experimental point of view, then subsequently give a theoretical discussion. These exotics can be explained if the new mesons contain two-quarks and two-antiquarks (tetraquarks), while the baryons contain four-quarks plus an antiquark (pentquarks). The theoretical explanations for these states take three divergent tracks: tightly bound objects, just as in the case of normal hadrons, but with more constituents, or loosely bound "molecules" similar to the deuteron, but formed from two mesons, or a meson or baryon, or more wistfully, they are not multiquark states but appear due to kinematic effects caused by different rescatterings of virtual particles; most of these models have all been post-dictions. Both the tightly and loosely bound models predict the masses and related quantum numbers of new, as yet undiscovered states. Thus, future experimental discoveries are needed along with theoretical advances to elucidate the structure of these new exotic states.

Heavy-quark symmetry implies stable heavy tetraquark mesons $Q_iQ_j \bar q_k \bar q_l$

Abstract: A pair of theory papers use the recently discovered double-charmed baryon to predict double-beauty tetraquarks.

Setting the scale for the CLS 2+1 flavor ensembles

Abstract: We present measurements of a combination of the decay constants of the light pseudoscalar mesons and the gradient flow scale ${t}_{0}$, which allow us to set the scale of the lattices generated by CLS with $2+1$ flavors of nonperturbatively improved Wilson fermions. Mistunings of the quark masses are corrected for by measuring the derivatives of observables with respect to the bare quark masses.

Isoscalar ππ Scattering and the σ Meson Resonance from QCD.

Abstract: We present for the first time a determination of the energy dependence of the isoscalar ππ elastic scattering phase shift within a first-principles numerical lattice approach to QCD. Hadronic correlation functions are computed including all required quark propagation diagrams, and from these the discrete spectrum of states in the finite volume defined by the lattice boundary is extracted. From the volume dependence of the spectrum, we obtain the S-wave phase shift up to the KK[over ¯] threshold. Calculations are performed at two values of the u, d quark mass corresponding to m_{π}=236,391 MeV, and the resulting amplitudes are described in terms of a σ meson which evolves from a bound state below the ππ threshold at the heavier quark mass to a broad resonance at the lighter quark mass.

Status of the $B\to K^*\mu^+\mu^-$ anomaly after Moriond 2017

Abstract: Motivated by recent results by the ATLAS and CMS collaborations on the angular distribution of the $B \to K^* \mu^+\mu^-$ decay, we perform a state-of-the-art analysis of rare $B$ meson decays based on the $b \to s \mu \mu$ transition. Using standard estimates of hadronic uncertainties, we confirm the presence of a sizable discrepancy between data and SM predictions. We do not find evidence for a $q^2$ or helicity dependence of the discrepancy. The data can be consistently described by new physics in the form of a four-fermion contact interaction $(\bar s \gamma_\alpha P_L b)(\bar \mu \gamma^\alpha \mu)$. Assuming that the new physics affects decays with muons but not with electrons, we make predictions for a variety of theoretically clean observables sensitive to violation of lepton flavour universality.

Multiquark States

Abstract: Why do we see certain types of strongly interacting elementary particles and not others? This question was posed over 50 years ago in the context of the quark model M Gell-Mann and G Zweig proposed that the known mesons were $q \bar q$ and baryons $qqq$, with quarks known at the time $u$ ("up"), $d$ ("down"), and $s$ ("strange") having charges (2/3,-1/3,-1/3) Mesons and baryons would then have integral charges Mesons such as $qq \bar q \bar q$ and baryons such as $qqqq \bar q$ would also have integral charges Why weren't they seen? They have now been seen, but only with additional heavy quarks and under conditions which tell us a lot about the strong interactions and how they manifest themselves The present article describes recent progress in our understanding of such "exotic" mesons and baryons

Light-by-light scattering sum rules in light of new data

Abstract: We evaluate the light-quark meson contributions to three exact light-by-light scattering sum rules in light of new data by the Belle Collaboration, which recently has extracted the transition form factors of the tensor meson $f_2(1270)$ as well as of the scalar meson $f_0(980)$. We confirm a previous finding that the $\eta, \eta^\prime$ and helicity-2 $f_2(1270)$ contributions saturate one of these sum rules up to photon virtualities around 1 GeV$^2$. At larger virtualities, our sum rule analysis shows an important contribution of the $f_2(1565)$ meson and provides a first empirical extraction of its helicity-2 transition form factor. Two further sum rules allow us to predict the helicity-0 and helicity-1 transition form factors of the $f_2(1270)$ meson. Furthermore, our analysis also provides an update for the scalar and tensor meson hadronic light-by-light contributions to the muon's anomalous magnetic moment.

K*(892)(0) and phi(1020)meson production at high transverse momentum in pp and Pb-Pb collisions at root sNN=2.76 TeV

Abstract: The production of K∗(892)0 and φ(1020) mesons in proton-proton (pp) and lead-lead (Pb-Pb) collisions at sNN=2.76TeV has been analyzed using a high luminosity data sample accumulated in 2011 with the ALICE detector at the Large Hadron Collider (LHC). Transverse momentum (pT) spectra have been measured for K∗(892)0 and φ(1020) mesons via their hadronic decay channels for pT up to 20GeV/c. The measurements in pp collisions have been compared to model calculations and used to determine the nuclear modification factor and particle ratios. The K∗(892)0/K ratio exhibits significant reduction from pp to central Pb-Pb collisions, consistent with the suppression of the K∗(892)0 yield at low pT due to rescattering of its decay products in the hadronic phase. In central Pb-Pb collisions the pT dependent φ(1020)/π and K∗(892)0/π ratios show an enhancement over pp collisions for pT≈3GeV/c, consistent with previous observations of strong radial flow. At high pT, particle ratios in Pb-Pb collisions are similar to those measured in pp collisions. In central Pb-Pb collisions, the production of K∗(892)0 and φ(1020) mesons is suppressed for pT>8GeV/c. This suppression is similar to that of charged pions, kaons, and protons, indicating that the suppression does not depend on particle mass or flavor in the light quark sector.

Constraining the hadronic spectrum through QCD thermodynamics on the lattice

Abstract: Fluctuations of conserved charges allow us to study the chemical composition of hadronic matter. A comparison between lattice simulations and the hadron resonance gas (HRG) model suggested the existence of missing strange resonances. To clarify this issue we calculate the partial pressures of mesons and baryons with different strangeness quantum numbers using lattice simulations in the confined phase of QCD. In order to make this calculation feasible, we perform simulations at imaginary strangeness chemical potentials. We systematically study the effect of different hadronic spectra on thermodynamic observables in the HRG model and compare to lattice QCD results. We show that, for each hadronic sector, the well-established states are not enough in order to have agreement with the lattice results. Additional states, either listed in the Particle Data Group booklet (PDG) but not well established, or predicted by the quark model (QM), are necessary in order to reproduce the lattice data. For mesons, it appears that the PDG and the quark model do not list enough strange mesons, or that, in this sector, interactions beyond those included in the HRG model are needed to reproduce the lattice QCD results.

Hypernuclei and massive neutron stars

Abstract: Background: The recent accurate measurement of the mass of two pulsars close to or above $2\phantom{\rule{0.16em}{0ex}}{\mathrm{M}}_{\ensuremath{\bigodot}}$ has raised the question of whether such large pulsar masses allow for the existence of exotic degrees of freedom, such as hyperons, inside neutron stars.Purpose: In the present work, we will investigate, within a phenomenological relativistic mean field approach, how the existing hypernuclei properties may constrain the neutron star equation of state and confront the neutron star maximum masses obtained with equations of state calibrated to hypernuclei properties with the astrophysical $2\phantom{\rule{0.16em}{0ex}}{\mathrm{M}}_{\ensuremath{\bigodot}}$ constraint.Method: The study is performed using a relativistic mean field approach to describe both the hypernuclei and the neutron star equations of state. Unified equations of state are obtained. A set of five models that describe $2\phantom{\rule{0.16em}{0ex}}{\mathrm{M}}_{\ensuremath{\bigodot}}$ when only nucleonic degrees of freedom are employed. Some of these models also satisfy other well-established laboratory or theoretical constraints.Results: The $\mathrm{\ensuremath{\Lambda}}$-meson couplings are determined for all the models considered, and the $\mathrm{\ensuremath{\Lambda}}$ potential in symmetric nuclear matter and $\mathrm{\ensuremath{\Lambda}}$ matter at saturation are calculated. Maximum neutron star masses are determined for two values of the $\mathrm{\ensuremath{\Lambda}}\text{\ensuremath{-}}\ensuremath{\omega}$ meson coupling, ${g}_{\ensuremath{\omega}\mathrm{\ensuremath{\Lambda}}}=2{g}_{\ensuremath{\omega}N}/3$ and ${g}_{\ensuremath{\omega}\mathrm{\ensuremath{\Lambda}}}={g}_{\ensuremath{\omega}N}$, and a wide range of values for ${g}_{\ensuremath{\phi}\mathrm{\ensuremath{\Lambda}}}$. Hyperonic stars with the complete baryonic octet are studied, restricting the coupling of the $\mathrm{\ensuremath{\Sigma}}$ and $\mathrm{\ensuremath{\Xi}}$ hyperons to the $\ensuremath{\omega},\ensuremath{\rho},$ and $\ensuremath{\sigma}$ mesons due to the lack of experimental data, and maximum star masses calculated.Conclusions: We conclude that, within a phenomenological relativistic mean field approach, the currently available hypernuclei experimental data and the lack of constraints on the asymmetric equation of state of nuclear matter at high densities set only a limited number of constraints on the neutron star matter equation of state using the recent $2\phantom{\rule{0.16em}{0ex}}{\mathrm{M}}_{\ensuremath{\bigodot}}$ observations. It is shown that the $\mathrm{\ensuremath{\Lambda}}$ potential in symmetric nuclear matter takes a value of $\ensuremath{\sim}30\ensuremath{-}32\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ at saturation for the ${g}_{\ensuremath{\omega}\mathrm{\ensuremath{\Lambda}}}$ coupling given by the SU(6) symmetry, being of the order of the values generally used in the literature. On the other hand, the $\mathrm{\ensuremath{\Lambda}}$ potential in $\mathrm{\ensuremath{\Lambda}}$ matter varies between $\ensuremath{-}14$ and $\ensuremath{-}8$ MeV, taking for vector mesons couplings the SU(6) values, at variance with generally employed values between $\ensuremath{-}1$ and $\ensuremath{-}5$ MeV. If the SU(6) constraint is relaxed and the vector meson couplings to hyperons are kept to values not larger than those of nucleons, then values between $\ensuremath{-}13$ and $+9$ MeV are obtained.

Measurement of the prompt J/ψ pair production cross-section in pp collisions at √s = 8 TeV with the ATLAS detector

Abstract: The production of two prompt $J/\psi$ mesons, each with transverse momenta $p_{\mathrm{T}}>8.5$ GeV and rapidity $|y| < 2.1$, is studied using a sample of proton-proton collisions at $\sqrt{s} = 8$ TeV, corresponding to an integrated luminosity of 11.4 fb$^{-1}$ collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised $J/\psi$ production, is measured as a function of the transverse momentum of the lower-$p_{\mathrm{T}}$ $J/\psi$ meson, di-$J/\psi$ $p_{\mathrm{T}}$ and mass, the difference in rapidity between the two $J/\psi$ mesons, and the azimuthal angle between the two $J/\psi$ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two $J/\psi$ mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be $\sigma_{\mathrm{eff}} = 6.3 \pm 1.6 \mathrm{(stat)} \pm 1.0 \mathrm{(syst)} \pm 0.1 \mathrm{(BF)} \pm 0.1 \mathrm{(lumi)}$ mb.

First physics results at the physical pion mass from N f =2 Wilson twisted mass fermions at maximal twist

Abstract: We present physics results from simulations of QCD using Nf 1⁄4 2 dynamical Wilson twisted mass fermions at the physical value of the pion mass These simulations are enabled by the addition of the clover term to the twisted mass quark action We show evidence that compared to previous simulations without this term, the pion mass splitting due to isospin breaking is almost completely eliminated Using this new action, we compute the masses and decay constants of pseudoscalar mesons involving the dynamical up and down as well as valence strange and charm quarks at one value of the lattice spacing, a ≈ 009 fm Further, we determine renormalized quark masses as well as their scale-independent ratios, in excellent agreement with other lattice determinations in the continuum limit In the baryon sector, we show that the nucleon mass is compatible with its physical value and that the masses of the Δ baryons do not show any sign of isospin breaking Finally, we compute the electron, muon and tau lepton anomalous magnetic moments and show the results to be consistent with extrapolations of older ETMC data to the continuum and physical pion mass limits We mostly find remarkably good agreement with phenomenology, even though we cannot take the continuum and thermodynamic limits

Towards an automated tool to evaluate the impact of the nuclear modification of the gluon density on quarkonium, D and B meson production in proton–nucleus collisions

Abstract: We propose a simple and model-independent procedure to account for the impact of the nuclear modification of the gluon density as encoded in nuclear collinear PDF sets on two-to-two partonic hard processes in proton–nucleus collisions. This applies to a good approximation to quarkonium, D and B meson production, generically referred to $$\mathcal {H}$$ . Our procedure consists in parametrising the square of the parton scattering amplitude, $${\mathcal {A}}_{gg \rightarrow {\mathcal {H}} X}$$ and constraining it from the proton–proton data. Doing so, we have been able to compute the corresponding nuclear modification factors for $$J/\psi $$ , $$\Upsilon $$ and $$D^0$$ as a function of y and $$P_T$$ at $$\sqrt{s_\mathrm{NN}}=5$$ and 8 TeV in the kinematics of the various LHC experiments in a model independent way. It is of course justified since the most important ingredient in such evaluations is the probability of each kinematical configuration. Our computations for D mesons can also be extended to B meson production. To further illustrate the potentiality of the tool, we provide – for the first time – predictions for the nuclear modification factor for $$\eta _c$$ production in $$p$$ Pb collisions at the LHC.

Charmed Hadrons from Coalescence plus Fragmentation in relativistic nucleus-nucleus collisions at RHIC and LHC

Abstract: In a coalescence plus fragmentation approach we calculate the heavy baryon/meson ratio and the $p_T$ spectra of charmed hadrons $D^{0}$, $D_{s}$ and $\Lambda_{c}^{+}$ in a wide range of transverse momentum from low $p_T$ up to about 10 GeV and discuss their ratios from RHIC to LHC energies without any change of the coalescence parameters. We have included the contribution from decays of heavy hadron resonances and also the one due to fragmentation of heavy quarks which do not undergo the coalescence process. The coalescence process is tuned to have all charm quarks hadronizing in the $p_T\rightarrow 0$ limit and at finite $p_T$ charm quarks not undergoing coalescence are hadronized by independent fragmentation. The $p_T$ dependence of the baryon/meson ratios are found to be sensitive to the masses of coalescing quarks, in particular the $\Lambda_{c}/D^{0}$ can reach values of about $\rm 1÷1.5 $ at $p_T \approx \, 3$ \mbox{GeV}, or larger, similarly to the light baryon/meson ratio like $p/\pi$ and $\Lambda/K$, however a marked difference is a quite weak $p_T$ dependence with respect to the light case, such that a larger value at intermediate $p_T$ implies a relatively large value also for the integrated yields. A comparison with other coalescence model and with the prediction of thermal model is discussed.

Neutrino mixing and R K anomaly in U(1) X models: a bottom-up approach

Abstract: We identify a class of U(1) X models which can explain the R K anomaly and the neutrino mixing pattern, by using a bottom-up approach. The different X-charges of lepton generations account for the lepton universality violation required to explain R K . In addition to the three right-handed neutrinos needed for the Type-I seesaw mechanism, these minimal models only introduce an additional doublet Higgs and a singlet scalar. While the former helps in reproducing the quark mixing structure, the latter gives masses to neutrinos and the new gauge boson Z ′. Our bottom-up approach determines the X-charges of all particles using theoretical consistency and experimental constraints. We find the parameter space allowed by the constraints from neutral meson mixing, rare b → s decays and direct collider searches for Z ′. Such a Z ′ may be observable at the ongoing run of the Large Hadron Collider with a few hundred fb−1 of integrated luminosity.

Measurement of the B± Meson Nuclear Modification Factor in Pb-Pb Collisions at √sNN=5.02 TeV

Abstract: The differential production cross sections of B-+/- mesons are measured via the exclusive decay channels B-+/- -> J/psi K-+/- -> mu(+)mu K--(+/-) as a function of transverse momentum in pp and Pb-Pb collisions at a center-of-mass energy root s(NN) = 5.02 TeV per nucleon pair with the CMS detector at the LHC. The pp(Pb - Pb) data set used for this analysis corresponds to an integrated luminosity of 28.0 pb(-1) (351 mu b(-1)). The measurement is performed in the B-+/- meson transverse momentum range of 7 to 50 GeV/c, in the rapidity interval vertical bar y vertical bar < 2.4. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from pp reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Observation of Y(1S) Pair Production in Proton-Proton Collisions at √s = 8 TeV

Abstract: Pair production of Upsilon(1S) mesons is observed at the LHC in proton-proton collisions at sqrt(s) = 8 TeV by the CMS experiment in a data sample corresponding to an integrated luminosity of 207 inverse-femtobarns Both Upsilon(1S) candidates are fully reconstructed via their decays to mu+ mu- The fiducial acceptance region is defined by an absolute Upsilon(1S) rapidity smaller than 20 The fiducial cross section for the production of Upsilon(1S) pairs, assuming that both mesons decay isotropically, is measured to be 688 +/- 127 (stat) +/- 74 (syst) +/- 28 (B) pb, where the third uncertainty comes from the uncertainty in the branching fraction of Upsilon(1S) decays to mu+ mu- Assuming instead that the Upsilon(1S) mesons are produced with different polarizations leads to variations in the measured cross section in the range from -38% to +36%

Finite-volume QED corrections to decay amplitudes in lattice QCD

Abstract: We demonstrate that the leading and next-to-leading finite-volume effects in the evaluation of leptonic decay widths of pseudoscalar mesons at $O(\ensuremath{\alpha})$ are universal; i.e. they are independent of the structure of the meson. This is analogous to a similar result for the spectrum but with some fundamental differences, most notably the presence of infrared divergences in decay amplitudes. The leading nonuniversal, structure-dependent terms are of $O(1/{L}^{2})$ [compared to the $O(1/{L}^{3})$ leading nonuniversal corrections in the spectrum]. We calculate the universal finite-volume effects, which requires an extension of previously developed techniques to include a dependence on an external three-momentum (in our case, the momentum of the final-state lepton). The result can be included in the strategy proposed in Ref. [N. Carrasco et al.,Phys. Rev. D 91, 074506 (2015).] for using lattice simulations to compute the decay widths at $O(\ensuremath{\alpha})$, with the remaining finite-volume effects starting at order $O(1/{L}^{2})$. The methods developed in this paper can be generalized to other decay processes, most notably to semileptonic decays, and hence open the possibility of a new era in precision flavor physics.