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Sourav Sarkar

Publications and source records attributed to Sourav Sarkar.

At least 145 records · Page 8Linked to original sources

Coupling of \bar K*N to the Lambda(1520)

We study the coupling of the Lambda(1520)= Lambda* resonance to the \bar K* vector meson and nucleon. This coupling is not directly measured from the resonance decay, but is expected to be important in hyperon production reactions, in particular for the exotic Theta+ production. We compute the coupling in two different schemes, one in the chiral unitary model where the Lambda* is dominated by the quasibound state of mesons and baryons, and the other in the quark model where the resonance is a p-wave excitation in the three valence quarks. Although it is possible to construct both models such that they reproduce the \bar K N and pi Sigma decays, there is a significant difference between the Lambda* \bar K* N couplings in the two models. In the chiral unitary model $|g_{Λ^*\bar{K}^* N}| \sim 1.5$, while in the quark model $|g_{Λ^*\bar{K}^* N}| \sim 10$. The difference of the results stems from the different structure of the Lambda* in both models, and hence, an experimental determination of this coupling would shed light on the nature of the resonance.

hep-ph↗

Chiral dynamics of the $Λ(1520)$ in coupled channels tested in the $K^-p\toππΛ$ reaction

The $Λ(1520)$ resonance is generated dynamically in a unitary coupled channel framework with the $πΣ^*$ and $KΞ^*$ channels in s-wave and $πΣ$ and $\bar K N$ channels in d-wave. The dynamics of this resonance close to and above threshold is then tested through the reactions $K^-p\toππΛ$ and a good agreement with the experimentally measured cross section is observed.

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Dynamical generation of $J^P=3/2-$ resonances and the $Λ(1520)$ resonance

The lowest order chiral Lagrangian is used to study s-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons. The coupled channel Bethe Salpeter equation is used to obtain dynamically generated $\tfrac{3}{2}^-$ resonances in the partial wave amplitudes which provide a reasonable description to a number of 3 and 4-star resonances like $Δ(1700)$, $Λ(1520)$, $Σ(1670)$, $Σ(1940)$, $Ξ(1820)$, $Ω(2250)$ etc. The phenomenological introduction of d-wave channels in the coupled channel scheme along with the existing s-wave channels is shown to provide a much improved description of the $Λ(1520)$ resonance. The prediction of the absolute strength of the cross section in the reactions $K^-p\toΛπ^0π^0$ and $K^-p\toΛπ^+π^-$ provided by this scheme shows a good agreement with existing data.

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Chiral coupled channel dynamics of the Lambda(1520) and the K-P -> pi0 pi0 Lambda reaction

We study the $Λ(1520)D_{03}$ in a chiral coupled channel approach. This resonance appears as dynamically generated from the interaction of the decuplet of baryons and the octet of mesons in s-wave, and its treatment is improved here with the phenomenological inclusion of the $\bar{K} N$ and $πΣ$ channels in d-wave. Since the most important building block in the $Λ(1520)$ is the $π\Sgs(1385)P_{13}$ channel, we study the $K^- p \to π\Sgs(1385) (π^0 Λ)$ reaction in the region of the $Λ(1520)$ and above, and compare the results with recent experimental data. With the coupling of the $Λ(1520)$ to the $π\Sgs$ channel predicted by the theory we find a cross section in good agreement with the data and there is as well agreement for the invariant mass distributions which show a neat peak for the $\Sgs(1385)$ in the $(π^0 Λ)$ spectrum. Predictions are made of a strong $Λ(1520)$ resonant peak of the cross section, as a function of the $K^-$ momentum, in the region below the measured data which, if confirmed experimentally, would give a stronger support to the idea of the $Λ(1520)$ as a dynamically generated resonance.

hep-ph↗

Study of a possible S=+1 dynamically generated baryonic resonance

Starting from the lowest order chiral Lagrangian for the interaction of the baryon decuplet with the octet of pseudoscalar mesons we find an attractive interaction in the $ΔK$ channel with L=0 and I=1, while the interaction is repulsive for I=2. The attractive interaction leads to a pole in the second Riemann sheet of the complex plane and manifests itself in a large strength of the $ΔK$ scattering amplitude close to the $ΔK$ threshold, which is not the case for I=2. However, we also make a study of uncertainties in the model and conclude that the existence of this pole depends sensitively upon the input used and can disappear within reasonable variations of the input parameters. We take advantage to study the stability of the other poles obtained for the ${3/2}^-$ dynamically generated resonances of the model and conclude that they are stable and not contingent to reasonable changes in the input of the theory.

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Baryonic Resonances from Baryon Decuplet-Meson Octet Interactions and the Exotic Resonance S=1, I=1, J^P=3/2-

Using the lowest order chiral Lagrangian we study s-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons. We find two bound states in the flavour SU(3) limit corresponding to the octet and decuplet representations. These are found to split into eight different trajectories in the complex plane when the SU(3) symmetry is broken gradually. Finally, we are able to provide a reasonable description for a good number of 4-star 3/2- resonances listed by the Particle Data Group. In particular, the Xi(1820), the Lambda(1520) and the Sigma(1670) states are well reproduced. We predict a few other resonances and also evaluate the couplings of the observed resonances to the various channels from the residues at the poles of the scattering matrix from where partial decay widths into different channels can be evaluated.

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Resonances from Baryon decuplet-Meson octet interaction

The s-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons is studied in a unitarized coupled channel approach. We obtain a fair agreement for mass and width of several 3/2- resonances. In particular, the Xi(1820), the Lambda(1520) and the Sigma(1670) states are well reproduced. Other resonances are predicted and also the couplings of the observed resonances to the various channels are evaluated.

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Baryonic Resonances from Baryon Decuplet-Meson Octet Interaction

We study $S$-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons using the lowest order chiral Lagrangian. In the S=1 sector, we find an attractive interaction in the $ΔK$ channel with I=1 while it is repulsive for I=2. The attractive interaction leads to a pole in the second Riemann sheet of the complex plane and is manifested as a large strength in the scattering amplitude close to the $ΔK$ threshold, which is not the case for I=2. We use the unitarized coupled channel approach to also investigate all the other possible values of strangeness and isospin. We find two bound states in the SU(3) limit corresponding to the octet and decuplet representations. These are found to split into eight different trajectories in the complex plane when the SU(3) symmetry is broken gradually. Finally, we are able to provide a reasonable description for a good number of 4-star ${3/2}^-$ resonances listed by the Particle Data Group. In particular, the $Ξ(1820)$, the $Λ(1520)$ and the $Σ(1670)$ states are well reproduced. We predict a few other resonances and also evaluate the couplings of the observed resonances to the various channels from the residues at the poles of the scattering matrix from where partial decay widths into different channels can be evaluated.

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Lepton interferometry in relativistic heavy ion collisions - a case study

We propose intensity interferometry with identical lepton pairs as an efficient tool for the estimation of the source size of the expanding hot zone produced in relativistic heavy ion collisions. This can act as a complementary method to two photon interferometry. The correlation function of two electrons with the same helicity has been evaluated for RHIC energies. The thermal shift of the rho meson mass has negligible effects on the HBT radii.

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On the nucleon self-energy in nuclear matter

We consider the nucleon self-energy in nuclear matter in the absence of Pauli blocking. It is evaluated using the partial-wave analysis of $NN$ scattering data. Our results are compared with that of a realistic calculation to estimate the effect of this blocking. It is also possible to use our results as a check on the realistic calculations.

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Thermal photons from $πρ\ra πγ$ revisited

We evaluate the photon spectra from the reaction $πρ\ra πγ$ for the exchange of $π$, $ρ$, $ω$, $ϕ$ and $a_1$ as intermediary mesons. It is found that the contributions from the intermediary $a_1$ is more than any other meson exchange processes up to photon energies 2.5 GeV.

hep-ph↗

Pion parameters in nuclear medium from chiral perturbation theory and virial expansion

We consider two methods to find the effective parameters of the pion traversing a nuclear medium. One is the first order chiral perturbation theoretic evaluation of the pion pole contribution to the two-point function of the axial-vector current. The other is the exact, first order virial expansion of the pion self-energy. We find that, although the results of chiral perturbation theory are not valid at normal nuclear density, those from the virial expansion may be reliable at such density. The latter predicts both the mass-shift and the in-medium decay width of the pion to be small, of about a few MeV.

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Indication of a Co-Existing Phase of Quarks and Hadrons in Nucleus - Nucleus Collisions

The variation of average transverse mass of identified hadrons with charge multiplicity have been studied for AGS, SPS and RHIC energies. The observation of a plateau in the average transverse mass for multiplicities corresponding to SPS energies is attributed to the formation of a co-existence phase of quark gluon plasma and hadrons. A subsequent rise for RHIC energies may indicate a deconfined phase in the initial state. Several possibilities which can affect the average transverse mass are discussed. Constraints on the initial temperature and thermalization time have been put from the various experimental data available at SPS energies.

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Thermal Radiation from Nucleons and Mesons

Thermal photon emission rates due to meson-nucleon interactions have been evaluated. An exhaustive set of reactions involving p(\bar p), n(\bar n), rho, omega, a_1, pi and eta is seen to provide a sizeable contribution to the emission rate from hot hadronic matter. Contributions from baryonic resonances are found to be negligibly small.

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Photon interferometry and size of the hot zone in relativistic heavy ion collisions

The parameters obtained from the theoretical analysis of the single photon spectra observed by the WA98 collaboration at SPS energies have been used to evaluate the two photon correlation functions. The single photon spectra and the two photon correlations at RHIC energies have also been evaluated, taking into account the effects of the possible spectral change of hadrons in a thermal bath. We find that the ratio $R_{side}/R_{out} \sim 1$ for SPS and $R_{side}/R_{out} <1$ for RHIC energy.

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Spectral Red-Shift Versus Broadening from Photon and Dilepton Spectra

We estimate the photon and dilepton emission rates from hot hadronic matter with in-medium spectral shift and broadening of vector mesons. It is observed that both the WA98 photon data and CERES/NA45 dilepton data can be well reproduced with similar initial conditions. The freeze-out condition has been constrained by the transverse mass spectra of pions and protons measured by the NA49 collaboration. We argue that simultaneous measurement of the $p_T$ spectra of single photons as well as invariant mass distribution of dileptons is crucial to understand the in-medium spectral function of the vector mesons.

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Shadowing in photo-production : role of in-medium hadrons

We study the effects of in-medium hadronic properties on shadowing in photon-nucleus interactions in Glauber model as well as in the multiple scattering approach. A reasonable agreement with the experimental data is obtained in a scenario of downward spectral shift of the hadrons. Shadowing is found to be insensitive to the broadening of the spectral functions. An impact parameter dependent analysis of shadowing might shed more light on the role of in-medium properties of hadrons.

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Spectral Representation at Finite Temperature

This is a short review on the thermal, spectral representation in the real-time version of the finite temperature quantum field theory. After presenting a clear derivation of the spectral representation, we discuss the properties of its spectral function. Two applications of this representation are then considered. One is the solution of the Dyson equation for the thermal propagator. The other is the formulation of the QCD sum rules at finite temperature.

hep-th↗