arXiv subjects

Marek Karliner

Publications and source records attributed to Marek Karliner.

At least 73 records · Page 4Linked to original sources

arXiv2007

New Quark Relations for Hadron Masses and Magnetic Moments - A Challenge for Explanation from QCD

Prompted by the recent surprising results in QCD spectroscopy, we extend the treatment of the constituent quark model showing that mass differences and ratios have the same values when obtained from mesons and baryons. We obtain several new successful relations involving hadrons containing two and three strange quarks and hadrons containing heavy quarks and give a new prediction regarding spin splitting between doubly charmed baryons. We provide numerical evidence for an effective supersymmetry between mesons and baryons related by replacing a light antiquark by a light diquark. We also obtain new relations between quark magnetic moments and hadron masses. Limits of validity of this approach and disagreements with experiment in properties of the Sigma and Xi baryons are discussed as possible clues to a derivation from QCD.

hep-ph↗

arXiv2006

A tetraquark model for the new X(1576) K+ K- resonance

We discuss the likely tetraquark interpretation of the X(1576) K+ K- isovector resonance recently reported by BES with J^{PC}=1^{--}. We point out that if this interpretation as a four-quark state is correct, the (s u sbar dbar) tetraquark decays might have striking signatures. We also provide predictions for possible analogous tetraquarks involving heavy quarks - (c q cbar qbar) and (b q bbar qbar).

hep-ph↗

arXiv2006

Diquarks and antiquarks in exotics: a menage a trois and a menage a quatre

A menage a trois is very different from an ordinary family. Similarly, exotic hadrons with both qq and qbar q pairs have important color-space correlations that are completely absent in ordinary mesons and baryons. The presence of both types of pairs requires attention to the basic QCD physics that the q qbar interaction is much stronger than the qq interaction. This new physics in multiquark systems produces color structures totally different from those of normal hadrons, for example the ud system is utterly unlike the ud diquark in the uds Lambda baryon. The color-space correlations produce unusual experimental properties in tetraquarks with heavy quark pairs which may be relevant for newly discovered mesons like the X(3872) resonance. Tetraquark masses can be below the two-meson threshold for sufficiently high quark masses. A simple model calculation shows the b q bbar ubar and b q cbar qbar tetraquarks below the B Bbar and B Dbar thresholds. Some of these states have exotic electric charge and their decays might have striking signatures involving monoenergetic photons and/or pions.

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arXiv2005

Direct Estimate of the Gluon Polarization in the Nucleon

We make a first crude direct estimate of the net gluon polarization in the proton, DeltaG, combining data on the asymmetries in high-p_T hadron production from the HERMES, SMC and COMPASS collaborations. Although these data sample a restricted range of x, they provide no hint that DeltaG is large. Fixing the normalizations of different theoretical parametrizations using the hadron asymmetry data, we find typical central values of DeltaG \sim 0.5, with uncertainties of similar magnitude. Values of DeltaG \ge 2 are disfavoured by Delta chi^2 \sim 9 to 20, depending on the parametrization used.

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arXiv2005

Isospin Analysis of pentaquark production forbids gamma p --> Theta^+ K_s and allows gamma n --> Theta^+ K^-

The discrepancy between Theta^+ photoproduction on proton vs. deuteron can be resolved if the photon couples much more strongly to K^+K^- than to K0 K0bar as indicated by the experimentally observed asymmetry between gamma p --> Lambda(1520) K^+ and gamma n --> Lambda(1520) K_s. Significant signal-background interference effects can occur in experiments like gamma N --> Kbar Theta^+ which search for the Theta^+ as a narrow I=0 resonance in a definite final state against a nonresonant background, with an experimental resolution coarser than the expected resonance width. We show that when the signal and background have roughly the same magnitude, destructive interference can easily combine with a limited experimental resolution to completely destroy the resonance signal. Whether or not this actually occurs depends critically on the yet unknown relative phase of the I=0 and I=1 amplitudes. We discuss the implications for some specific experiments.

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arXiv2005

On a possible tetraquark cousin of the Theta^+

If a narrow Theta^+ pentaquark exists, it is likely that a ud\bar s-ud triquark-diquark configuration is a significant component of its wave function. If so, the mechanism responsible for the binding of a triquark and a diquark is also likely to bind the triquark to an \bar s antiquark. We discuss the expected properties of such a ud\bar s-\bar s tetraquark meson. In particular, we point out that for a 0^+ isoscalar ud\bar s\bar s meson the lowest allowed decay mode is a four-body K K pi pi channel with a small phase space and a distinctive experimental signature.

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arXiv2005

New tests for experiments producing pentaquarks

The distribution of the squared momentum difference |P_A|^2 - |P_B|^2 between the momenta in the laboratory system of two experimentally observed particles A and B provides a test for whether an observed mass peak indicates a real resonance rather than nonresonant background or kinematic reflection. The angular distribution of the relative momenta in the center-of-mass system exhibits a forward-backward symmetry in the production and decay of any resonance with a definite parity. This symmetry is not expected in other nonresonant processes and can be expressed without needing angular distributions in terms of the easily measured momenta in the laboratory frame that are already measured and used to calculate the invariant mass of the system. Our test is especially useful for low statistics experiments where the full angular distribution cannot be determined. It can be applied to both fixed-target and collider searches for the Theta^+ and Theta_c pentaquarks.

hep-ph↗

arXiv2005

Coalescence model for Theta_c pentaquark formation

We present a model for the formation of the charmed pentaquark Theta_c in hard scattering processes such as deep inelastic scattering, e^+e^- annihilation, and high-energy p pbar collisions. The model assumes that the cross section for Theta_c formation is proportional to the rate of production of p D^{*-} (or pbar D^{*+}) pairs in close proximity both in momentum space and in coordinate space. The constant of proportionality is determined from the Theta_c cross section in deep inelastic scattering as reported by the H1 experiment. The HERWIG Monte Carlo is used to generate simulated DIS events and also to model the space-time structure of the final state. Requiring the proton and the D^* be within a 100 MeV mass window and separated by a spacelike distance of no more than 2 fm, we find that a large "coalescence enhancement factor" F_co \sim 10 is required to account for the H1 signal. The same approach is then applied in order to estimate the number and characteristics of Theta_c events produced at LEP and the Tevatron.

hep-ph↗

arXiv2004

Why the Theta+ is seen in some experiments and not in others - a possible explanation

The contradiction between evidence for and against the existence of the Theta+ pentaquark might be resolved if it only appears as a result of a particular production mechanism which is present in some experiments and absent in others. We examine the implications of Theta+ production via decay of a cryptoexotic N* resonance with a mass of about 2.4 GeV corresponding to a peak in the experimental data for the invariant mass of the (Theta+,K-) system. Further experimental checks are suggested.

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arXiv2004

Chiral-Soliton Predictions for Exotic Baryons

We re-analyze the predictions of chiral-soliton models for the masses and decay widths of baryons in the exotic antidecuplet of flavour SU(3). The calculated ranges of the chiral-soliton moment of inertia and the π-nucleon scattering Σterm are used together with the observed baryon octet and decuplet mass splittings to estimate 1430 MeV < m_{Θ^+} < 1660 MeV} and 1790 MeV < m_{Ξ^{--}} < 1970 MeV. These are consistent with the masses reported recently, but more precise predictions rely on ambiguous identifications of non-exotic baryon resonances. The overall decay rates of antidecuplet states are sensitive to the singlet axial-current matrix element in the nucleon. Taking this from polarized deep-inelastic scattering experiments, we find a suppression of the total Θ^+ and Ξ^{--} decay widths that may not be sufficient by itself to reproduce the narrow widths required by experiments. We calculate SU(3) breaking effects due to representation mixing and find that they tend to suppress the Θ^+ decay width, while enhancing that of the Ξ^{--}. We predict light masses for some exotic 27 baryons, including the I=1, J^P={3/2}^+ Θ^+ and I=3/2, J^P={3/2}^+ Ξmultiplets, and calculate their decay widths.

hep-ph↗

arXiv2004

A Mass Inequality for the $Ξ^*$ and $Θ^+$ Pentaquarks

We derive an upper bound on the mass difference between the $Ξ^*$ and $Θ^+$ pentaquarks which are the manifestly exotic members of the $SU(3)_f$ antidecuplet. The derivation is based on simple assumptions about $SU(3)_f$ symmetry breaking and uses the standard quantum mechanical variational method. The resulting rather robust bound is more than 20 MeV below the experimentally reported $Ξ^*-Θ^+$ mass difference, emphasizing the need for confirmation of the experimental mass values and placing strong constraints on quark models of the pentaquark structure.

hep-ph↗

arXiv2004

A Diquark-Triquark Model for the KN Pentaquark

We propose a model for the recently discovered Theta^+ exotic KN resonance as a novel kind of a pentaquark with an unusual color structure: a color triplet (ud) diquark, coupled to a color anti-triplet (ud s-bar) triquark in a relative P-wave. The state has J^P=1/2^+, I=0 and is an antidecuplet of SU(3)_f. A rough mass estimate of this pentaquark is close to experiment.

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arXiv2004

The narrow width of the Θ^+ - a possible explanation

The narrow width of the exotic narrow baryon resonance $Θ^+$ might be explained by mixing between the two nearly degenerate states that arise in models with two diquarks and an antiquark. The only open $Θ^+$ decay channel is $KN$. When two states both coupled to a single dominant decay mode are mixed by the loop diagram via this decay mode, diagonalization of the loop diagram decouples one mass eigenstate from this decay mode as in some treatments of the $ρ-π$ decay from the mixed singlet-octet $ω-ϕ$ system, the $K^* -π$ decay of the strange axial vector mesons and the $NK$ couplings of some baryons. This mechanism can explain the narrow width and weak coupling of $Θ^+ \to KN$ while allowing a relatively large production cross section from $K^*$ exchange. Interesting tests are suggested in $K^-p$ reactions where backward kaon production must go by exotic baryon exchange.

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arXiv2003

The anticharmed exotic baryon Theta_c and its relatives

Motivated by the recent discovery of the exotic S=+1 narrow baryon resonance Theta^+ at 1540 MeV, with quark content {uudd sbar}, we conjecture the existence of its anti-charmed analogue Theta_c, with quark content {uudd cbar}, and compute its likely properties. We rely on the recently constructed model of a novel kind of a pentaquark with an unusual color structure which provides a good approximation to the Theta^+ mass. We expect that Theta_c is an isosinglet with J^P=1/2^+ and estimate its mass at 2985 \pm 50 MeV. We also discuss another possible exotic baryon resonance containing heavy quarks, the Theta_b^+, a {uudd bbar} state, and estimate m_{Theta_b^+}=6398 \pm 50 MeV. These states should appear as unexpectedly narrow peaks in D^-p, Dbar^0 n, B^0 p and B^+ n mass distributions.

hep-ph↗

arXiv2003

The Constituent Quark Model Revisited - Quark Masses, New Predictions for Hadron Masses and KN Pentaquark

Prompted by the recent surprising results in QCD spectroscopy, we extend to heavy flavors the hadron mass relations showing that the constituent quark mass differences and ratios have the same values when obtained from mesons and baryons. We obtain several new successful relations involving heavy quarks and provide some related predictions. We discuss in detail the apparent sharp decrease in m_s and m_c, when a light partner quark in a meson is replaced by a heavy one and construct a potential model which qualitatively reproduces this pattern through wave function effects. We apply these ideas to the recently discovered theta^+ exotic KN resonance and propose its interpretation as a novel kind of a pentaquark with an unusual color structure, J^P=1/2^+, I=0 and an antidecuplet of SU(3)_f. A rough mass estimate of this pentaquark is close to experiment.

hep-ph↗

arXiv2003

Meson-Baryon Scattering in QCD_2 for any Coupling

Extending earlier work on strong-coupling meson-baryon scattering in QCD_2, we derive the effective meson-baryon action for any value of the coupling constant, in the large-N_c limit. Colour degrees of freedom play an important role, and we show that meson-baryon scattering can be formulated as a relativistic potential problem. We distinguish two cases that are non-trivial for unequal quark masses, and present the resulting equations for meson-baryon scattering amplitudes.

hep-ph↗

arXiv2002

Scattering and Resonances in QCD_2

Extending previous works on the spectrum of QCD_2, we now investigate the 2D analogue of meson-baryon scattering. We use semi-classical methods, perturbing around classical soliton solutions. We start with the abelian case, corresponding to one flavor,and find that in this case the effective potential is reflectionless. We obtain an explicit expression for the forward phase shift. In the non-abelian case of several flavors, the method yields a potential which depends on the momentum of the incoming particle. In this case there is both transmission and reflection. In both cases no resonances appear. As a byproduct, we derive the general conditions for a 2D quantum field theoretical action to yield a reflectionless effective potential when one expands in small fluctuations about the classical solution.

hep-ph↗

arXiv2002

QCD and e+e- --> Baryon + anti-Baryon

We discuss the QCD constraints on e+e- --> baryon-anti-baryon close to threshold, in light of the puzzling experimental data which indicate that close to threshold sigma (e+e- --> n n-bar) > sigma (e+e- --> p p-bar). We focus on the process e+e- --> Delta Delta-bar, which is particularly simple from the theoretical point of view. In this case it is possible to make exact QCD predictions for the relative yields of the four members of the Delta multiplet, modulo one crucial dynamical assumption.

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