Parity Partners Research Articles

Vector mesons in matter

One consequence of the chiral restoration is the mixing of parity partners. We look for a possible signature of the mixing of vector and axial vector mesons in heavyion collisions. We suggest an experimental method for its observation. The dynamical evolution of the heavy-ion collision is described by a transport equation of QMD-type evolving nucleons,N* and Δ resonances, Λ’s and gS baryons, and furthermore,π’s,η’sρ’sσ’sΩ’s and kaons with their isospin degrees of freedom. The input cross-sections and resonance parameters of the model are fitted to the available nucleon-nucleon and pion-nucleon cross-sections

Light hadrons with improved staggered quarks: Approaching the continuum limit

We have extended our program of QCD simulations with an improved Kogut-Susskind quark action to a smaller lattice spacing, approximately 0.09 fm. Also, the simulations with $a\ensuremath{\approx}0.12$ fm have been extended to smaller quark masses. In this paper we describe the new simulations and computations of the static quark potential and light hadron spectrum. These results give information about the remaining dependences on the lattice spacing. We examine the dependence of computed quantities on the spatial size of the lattice, on the numerical precision in the computations, and on the step size used in the numerical integrations. We examine the effects of autocorrelations in ``simulation time'' on the potential and spectrum. We see possible effects of decays, or coupling to two-meson states in the ${0}^{++}$ and ${1}^{+}$ meson propagators. A state consistent with $\ensuremath{\pi}+K$ is seen as a ``parity partner'' in the Goldstone kaon propagator, and we make a preliminary mass computation for a radially excited ${0}^{\ensuremath{-}}$ meson.

Chiral doublers of heavy-light baryons

We discuss the consequences of the chiral doubling scenario for baryons built of heavy and light quarks. In particular, we use the soliton description for baryons, demonstrating why each heavy-light baryon should be accompanied by the opposite parity partner. Our argumentation holds both for ordinary baryons and for exotic heavy pentaquarks which are required by the symmetries of QCD to appear in parity doublets, seperated by the mass shift of the chiral origin. Interpreting the recently observed by BaBaR, CLEO and Belle charmed mesons with assignment $(0^+,1^+)$ as the chiral partners of known $D$ and $D^*$ mesons, allows us to estimate the parameters of the mesonic effective lagrangian, and in consequence, estimate the masses of ground states and excited states of both parities. In particular, we interpret the state recently reported by the H1 experiment at HERA as a chiral partner $\tilde{\Theta}_c^0(3099)$ of yet undiscovered ground state pentaquark $\Theta_c^0(2700)$.

Chiral multiplets of heavy-light mesons

The recent discovery of a narrow resonance in ${D}_{s}{\ensuremath{\pi}}^{0}$ by the BaBar Collaboration is consistent with the interpretation of a heavy ${J}^{P}{(0}^{+}{,1}^{+})$ spin multiplet. This system is the parity partner of the ground state ${(0}^{\ensuremath{-}}{,1}^{\ensuremath{-}})$ multiplet, which we argue is required in the implementation of ${\mathrm{SU}(3)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{SU}(3)}_{R}$ chiral symmetry in heavy-light meson systems. The ${(0}^{+}{,1}^{+})\ensuremath{\rightarrow}{(0}^{\ensuremath{-}}{,1}^{\ensuremath{-}})+\ensuremath{\pi}$ transition couplings satisfy a Goldberger-Treiman relation, ${g}_{\ensuremath{\pi}}=\ensuremath{\Delta}{M/f}_{\ensuremath{\pi}},$ where $\ensuremath{\Delta}M$ is the mass gap. The BaBar resonance fits the ${0}^{+}$ state, with a kinematically blocked principal decay mode to $D+K.$ The allowed ${D}_{s}+\ensuremath{\pi},$ ${D}_{s}+2\ensuremath{\pi},$ and electromagnetic transitions are computed from the full chiral theory and found to be suppressed, consistent with the narrowness of the state. This state establishes the chiral mass difference for all such heavy-quark chiral multiplets, and precise predictions exist for the analogous ${B}_{s}$ and strange doubly heavy baryon states.

Negative-parity baryons in quenched anisotropic lattice QCD

We study negative-parity baryon spectra in quenched anisotropic lattice QCD. The negative-parity baryons are measured as the parity partner of the ground-state baryons. In addition to the flavor octet and decuplet baryons, we pay much attention to the flavor-singlet negative-parity baryon as a three-quark state and compare it with the Λ (1405) baryon. For the flavor octet and decuplet negative-parity baryons, the calculated masses are close to experimental values of corresponding lowest-lying negative-parity baryons. On the other hand, the flavor-singlet baryon is found to be much heavier than Λ(1405), which indicates the multi-quark state of Λ(1405) such as the N K molecule rather than the flavor-singlet three-quark state.

Excited baryons from the FLIC fermion action

Masses of positive and negative parity excited nucleons and hyperons are calculated in quenched lattice QCD using anO( a 2) improved gluon action and a fat-link clover fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier N * simulations with improved actions, and exhibit a clear mass splitting between the nucleon and its parity partner, as well as a small mass splitting between the two low-lying J P =½ − N * states. Study of different A interpolating fields suggests a similar splitting between the lowest two½ −Λ * states, although the empirical mass suppression of theΛ * (1405) is not seen.

Excitations of the nucleon with dynamical fermions

We measure the spectrum of low-lying nucleon resonances using Bayesian fitting methods. We compare the masses obtained in the quenched approximation to those obtained with two flavours of dynamical ferinions at a matched lattice spacing. At the pion masses employed in our simulations, we find that the mass of the first positive-parity nucleon excitation is always greater than that of the parity partner of the nucleon.

Negative-parity baryon masses using an (a)-improved fermion action

We present a calculation of the mass of the lowest-lying negative-parity J=1/2- state in quenched QCD. Results are obtained using a non-perturbatively O(a)-improved clover fermion action, and a splitting is found between the mass of the nucleon and its parity partner. The calculation is performed on two lattice volumes and at three lattice spacings, enabling a study of both finite-volume and finite lattice-spacing uncertainties. A comparison is made with results obtained using the unimproved Wilson fermion action.

The nuclear structure of 229Th

The γ-rays following the β − decay of 229Ac have been investigated by means of γ-ray singles and γγ-coincidence measurements using Ge detectors. Multipolarities of 40 transitions in 229Th have been established by measuring conversion electrons with a mini-orange electron spectrometer. The half-lives of the 146.35, 164.53 and 261.96 keV levels have been measured using the advanced time delayed βγγ(t) method. The low-lying states in 229Th and observed transition rates have been interpreted within the quasiparticle–phonon model with inclusion of Coriolis coupling. Two octupole correlated parity partner bands, with K π =5/2 ± and K π =3/2 ±, were identified in 229Th.

N* spectrum using an O( a) improved fermion action

The construction of operators and calculational methods for the determination of the N* spectrum are discussed. The masses of the parity partners of the nucleon and delta are computed from the O ( a)-improved data of the UKQCD Collaboration, and a clear splitting observed between the mass of the nucleon and its parity partner.

Discovering mirror particles at the Large Hadron Collider and the implied cold universe

The Mirror Matter or Exact Parity Model sees every standard particle, including the physical neutral Higgs boson, paired with a parity partner. The unbroken parity symmetry forces the mass eigenstate Higgs bosons to be maximal mixtures of the ordinary and mirror Higgs bosons. Each of these mass eigenstates will therefore decay 50% of the time into invisible mirror particles, providing a clear and interesting signature for the Large Hadron Collider (LHC) which could thus establish the existence of the mirror world. However, for this effect to be observable the mass difference between the two eigenstates must be sufficiently large. In this paper, we study cosmological constraints from Big Bang Nucleosynthesis on the mass difference parameter. We find that the temperature of the radiation dominated (RD) phase of the universe should never have exceeded a few 10's of GeV if the mass difference is to be observable at the LHC. Chaotic inflation with very inefficient reheating provides an example of how such a cosmology could arise. We conclude that the LHC could thus discover the mirror world and simultaneously establish an upper bound on the temperature of the RD phase of the universe.

The parity partner of the nucleon in quenched QCD with domain wall fermions

We present preliminary results for the mass spectrum of the nucleon and its low-lying excited states from quenched lattice QCD using the domain wall fermion method which preserves the chiral symmetry at finite lattice cutoff. Definite mass splitting is observed between the nucleon and its parity partner. This splitting grows with decreasing valence quark mass. We also present preliminary data regarding the first positive-parity excited state.

The parity partner of the nucleon in quenched QCD with domain wall fermions

The parity partner of the nucleon in quenched QCD with domain wall fermions

Enhanced global symmetries and the chiral phase transition

We examine the possibility that the physical spectrum of a vectorlike gauge field theory exhibits an enhanced global symmetry near a chiral phase transition. A transition from the Goldstone phase to the symmetric phase is expected as the number of fermions ${N}_{f}$ is increased to some critical value. Various investigations have suggested that a parity-doubled spectrum develops as the critical value is approached. Using an effective Lagrangian as a guide, we note that parity doubling is associated with the appearance of an enhanced global symmetry in the spectrum of the theory. The enhanced symmetry would develop as the spectrum splits into two sectors, with the first exhibiting the usual pattern of a spontaneously broken chiral symmetry, and the second exhibiting an additional, unbroken symmetry and parity doubling. The first sector includes the Goldstone bosons and other states such as massive scalar partners. The second includes a parity-degenerate vector and axial vector along with other possible parity partners. We note that if such a near-critical theory describes symmetry breaking in the electroweak theory, the additional symmetry suppresses the contribution of the parity-doubled sector to the S parameter.

Cancer in Rwanda

Data are presented on the frequency of malignant tumours registered at the population-based cancer registry in the southern prefecture of Butare, Rwanda, from May 1991 until 2 months before the outbreak of civil war in April 1994. Beginning in 1992, subjects were also interviewed about socio-demographic and life-style factors that have been associated with cancer risk in the West. The distribution of cancer in Rwanda is similar to that in other countries in sub-Saharan Africa. The most frequent cancers are those with possible infectious aetiologies: liver cancer (12%), cervical cancer (12%) and stomach cancer (9%). In addition, cancers known to be associated with HIV infection are relatively frequent (Kaposi's sarcoma [6%] and non-Hodgkin's lymphoma [3%]). Chronic infection, including infection with HIV, high parity and multiple sexual partners are important determinants of cancer incidence in this population. Tobacco consumption is low in Rwanda and there are few tobacco-related tumours, such as lung and laryngeal cancer. Other tumours believed to be associated with aspects of Western life-style, such as colorectal and breast cancer, are also relatively infrequent.

The Singlet Axial Vector Coupling in the Effective Meson Baryon Lagrangian11The project supported in part by the ICSC-World Laboratory

The Goldberger–Treiman (GT) relation in the singlet channel, being consistent with the EMC experimental results, can be manifested in an extended model in which the chiral symmetry in the singlet channel is realized in the parity doublet model by assuming that the excited nucleon state is the parity partner of the nucleon state. The model gives a natural explanation of the smallness of the singlet axial vector coupling without the necessity to assume a large strange sea contribution, and is unique in explaining the decay rate of as well.

Full QCD hadron spectroscopy with two flavors of dynamical Kogut-Susskind quarks on the lattice.

A full lattice QCD simulation is carried out with two flavors of Kogut-Susskind staggered dynamical quarks using lattices of a size ranging from ${4}^{4}$ to ${20}^{4}$ at the gauge coupling constant $\ensuremath{\beta}=\frac{6}{{g}^{2}}=5.7$ with the quark mass of ${m}_{q}=0.01 \mathrm{and} 0.02$ in lattice units. Primary emphasis is given to the study of finite-lattice-size effects in the hadron mass spectrum. It is found that hadron masses suffer from substantial finite-size effects even for a lattice size of the order of 2 fm, showing the importance of a quantitative control of the effect for a comparison with the experimental spectrum at the accuracy of a few percent level. The finite-size correction is found to be well described by a power law in the lattice size, rather than an exponential form predicted by analytic formulas derived for point particles. It is suggested that the effect arises from the size of hadrons squeezed on a finite lattice. Finite-size effects on the realization of chiral symmetry are also studied. The behavior of the pion mass, the chiral condensate, and the mass splitting between parity partners all support a spontaneous breakdown of chiral symmetry for a large lattice size. Prediction from chiral Lagrangians on the size dependence of the chiral condensate does not describe the simulation results well, however, at least for the quark mass employed for the present study. Calculation of the pion decay constant with various relations derived from current algebra and partial conservation of axial-vector current gives ${f}_{\ensuremath{\pi}}=94(8)\ensuremath{-}105(9)$ MeV, with a method-dependent uncertainty contained within 10%. An examination is also made of the question of the dependence of hadron masses on hadron operators. Meson masses are basically operator independent, while baryon masses exhibit some operator dependence, necessitating further studies to resolve systematic uncertainties of this origin in the determination of the hadron mass spectrum.

Effective chiral lagrangians with baryons—the mass splitting of the spin 1/2 parity partners

There are two inequivalent spin 1/2 baryon field operators that can be constructed directly out of three quark field operators. These are associated with a pair of parity partners. Their chiral transformation properties yield some unexpected results. In particular.U(1) axial transformations can mix the two operators. With these chiral transformation properties the spin 1/2 baryon parity partners are incorporated in an effective chiral lagrangian together with the spin 0 meson fields. The mass splitting of these parity partners is then found to be related to their coupling to the flavour singlet pseudoscalar, $$M_ - - M_ + \simeq F_\pi g_{\mathop {flavour}\limits_{singlet} N_ - N_ + } $$ The further selection rule, in the exact chiral limit, $$g_{\pi N_ + N_ - } = 0$$ is also found. Using these results, and their extension to the non-chiral limit, the enhancement of the decay ofN− intoN+ η, as opposed toN+ π, is elucidated even though it is suppressed by phase space. Two flavours is given principle consideration although three flavours is also briefly discussed.

Linearly rising soliton trajectories

Linearly rising soliton trajectories without parity partners are obtained from the nonlinear field equations of a Dirac field in four dimensions with universal Fermi self-interaction. The model is Lorentz and global-gauge invariant.

Models with quark confinement and linear trajectories without parity doubling

We consider simple dynamic models where the quarks satisfy the Dirac equation with a confining potential. We show that if the confining potential is a Lorentz scalar, the solutions to the Dirac equation satisfy the MacDowell symmetry. We discuss the slope of Regge trajectories in these models and show that in order for the trajectory to be linear (in $s={E}^{2}$) the potential must have a specific $j$ dependence, which produces a cut in $j$ of the type first proposed by Carlitz and Kislinger. In other words, the same mechanism which produces linear trajectories in the first place also produces the cut which removes the parity partners. The specific $j$ dependence is expected in "bag" models, as well as relativistic solition solutions.