arXiv subjects

K. P. Khemchandani

Publications and source records attributed to K. P. Khemchandani.

At least 73 records · Page 4Linked to original sources

arXiv2016

A QCD sum rule calculation of the $X^\pm(5568) \to B_{s}^0π^\pm$ decay width

To understand the nature of the $X(5568)$, recently observed in the mass spectrum of the $B_{s}^0π^\pm$ system by the D0 Collaboration, we have investigated, in a previous work, a scalar tetraquark (diquak-antidiquark) structure for it, within the two-point QCD sum rules method. The result found for its mass agrees well with the experimental value. Encouraged by this finding we now extend our calculations to obtain the decay width of $X(5568)$ to $B_{s}^0π^\pm$ using the three-point QCD sum rule. We obtain a value of $(20.4\pm8.7)\MeV$, which, on comparing with the experimental value of $21.9\pm6.4 (\mbox{sta})^{+5.0}_{-2.5}(\mbox{syst}) \MeV/c^2$, reinforces the scalar four quark nature of $X(5568)$.

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arXiv2016

$X(3872)$ production and absorption in a hot hadron gas

We calculate the time evolution of the $X(3872)$ abundance in the hot hadron gas produced in the late stage of heavy ion collisions. We use effective field Lagrangians to obtain the production and dissociation cross sections of $X(3872)$. In this evaluation we include diagrams involving the anomalous couplings $πD^*\bar{D}^*$ and $X \bar{D}^{\ast} D^{\ast}$ and also the couplings of the $X(3872)$ with charged $D$ and $D^*$ mesons. With these new terms the $X(3872)$ interaction cross sections are much larger than those found in previous works. Using these cross sections as input in rate equations, we conclude that during the expansion and cooling of the hadronic gas, the number of $X(3872)$, originally produced at the end of the mixed QGP/hadron gas phase, is reduced by a factor of 4.

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arXiv2016

A QCD sum rule study for a charged bottom-strange scalar meson

Using the QCD sum rule approach we investigate the possible four-quark structure for the new observed $B_{s}^0π^\pm$ narrow structure (D0). We use a diquak-antidiquark scalar current and work to the order of $m_s$ in full QCD, without relying on $1/m_Q$ expansion. Our study indicates that although it is possible to obtain a stable mass in agreement with the state found by the D0 collaboration, a more constraint analysis (simultaneous requirement of the OPE convergence and the dominance of the pole on the phenomenological side) leads to a higher mass. We also predict the masses of the bottom scalar tetraquark resonances with zero and two strange quarks.

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arXiv2015

A study of the $KN$-$K^*N$ coupled systems

We study the strangeness $+1$ meson-baryon systems to obtain improved $KN$ and $K^*N$ amplitudes and to look for a possible resonance formation by the $KN$-$K^*N$ coupled interaction. We obtain amplitudes for light vector meson-baryon systems by implementing the $s$-, $t$-, $u$- channel diagrams and a contact interaction. The pseudoscalar meson-baryon interactions are obtained by relying on the Weinberg-Tomozawa theorem. The transition amplitudes between the systems consisting of pseudoscalars and vector mesons are calculated by extending the Kroll-Ruderman term for pion photoproduction replacing the photon by a vector meson. We fix the subtraction constants required to calculate the loops by fitting our $KN$ amplitudes to the data available for the isospin 0 and 1 $s$-wave phase shifts. We provide the scattering lengths and the total cross sections for the $KN$ and $K^* N$ systems obtained in our model, which can be useful in future in-medium calculations. Our amplitudes do not correspond to formation of any resonance in none of the isospin and spin configurations.

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arXiv2015

Problems with meson spectroscopy involving perturbative loop corrections

In this talk we review the limitations of including meson loops as perturbative corrections in a solvable quark model. We first discuss meson-meson scattering within a formalism which treats confined quark pairs and mesons on an equal footing. The interaction between the mesons proceeds through $s$-channel meson-exchange diagrams. Next, we develop a perturbative expansion of the model, and show that the resonance poles found in such a treatment, even by accounting for contributions up to fourth order, do not coincide with those obtained with the full model. We conclude that the resonance predictions based on perturbative approximations in quark models are not reliable, especially in those cases where the coupling to the scattering channels is large.

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arXiv2014

$X(3872)$ production from reactions involving $D$ and $D^*$ mesons

In this proceeding we show the results found for the cross sections of the processes $\bar D D\toπX(3872)$, $\bar D^* D\to πX(3872)$ and $\bar D^* D^*\toπX(3872)$, information needed for calculations of the $X(3872)$ abundance in heavy ion collisions. Our formalism is based on the generation of $X(3872)$ from the interaction of the hadrons $\bar D^0 D^{*0} - \textrm{c.c}$, $D^- D^{*+} - \textrm{c.c}$ and $D^-_s D^{*+}_s - \textrm{c.c}$. The evaluation of the cross section associated with processes having $D^*$ meson(s) involves an anomalous vertex, $X\bar D^* D^*$, which we have determined by considering triangular loops motivated by the molecular nature of $X(3872)$. We find that the contribution of this vertex is important. Encouraged by this finding we estimate the $X\bar D^* D^*$ coupling, which turns out to be $1.95\pm 0.22$. We then use it to obtain the cross section for the reaction $\bar D^* D^*\toπX$ and find that the $X\bar D^* D^*$ vertex is also relevant in this case. We also discuss the role of the charged components of $X$ in the determination of the production cross sections.

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arXiv2014

$X(3872)$ production in high energy heavy ion collisions

We show the results obtained for the cross sections of the processes $\bar D D$, $\bar D^* D$, $\bar D^* D^*\toπX(3872)$, information which is necessary to determine the $X(3872)$ abundance in heavy ion collisions. Our formalism is based on the generation of $X(3872)$ from the interaction of the hadrons $\bar D^0 D^{*0} - \textrm{c.c}$, $D^- D^{*+} - \textrm{c.c}$ and $D^-_s D^{*+}_s - \textrm{c.c}$ an the calculation of the $X\bar D^* D^*$ anomalous vertex considering $X(3872)$ as a molecule of the above hadrons channels.

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arXiv2014

Prediction of an $I=1$ $D \bar D^*$ state and relationship to the claimed $Z_c(3900)$, $Z_c(3885)$

We study here the interaction of $D \bar D^*$ in the isospin $I=1$ channel in the light of recent theoretical advances that allow to combine elements of the local hidden gauge approach with heavy quark spin symmetry. We find that the exchange of light $q \bar q$ is OZI suppressed and, thus, we concentrate on the exchange of heavy vectors and of two pion exchange. The latter is found to be small compared to the exchange of heavy vectors, which then determines the strength of the interaction. A barely $D\bar{D}^*$ bound state decaying into $η_cρ$ and $πJ/ψ$ is found. At the same time we reanalyze the data of the BESIII experiment on $e^+ e^- \to π^{\pm} (D \bar D^*)^\mp$, from where a $Z_c(3885)$ state was claimed, associated to a peak in the $(D \bar D^*)^\mp$ invariant mass distribution close to threshold, and we find the data compatible with a resonance with mass around $3875$ MeV and width around $30$ MeV. We discuss the possibility that this and the $Z_c(3900)$ state found at BESIII, reconfirmed at 3896 MeV at Belle, or 3885 MeV at CLEO, could all be the same state and correspond to the one that we find theoretically.

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arXiv2014

On $X(3872)$ production in high energy heavy ion collisions

We have determined the production cross sections of the $X(3872)$ state in the reactions $\bar D D\toπX$, $\bar D^* D\to πX$ and $\bar D^* D^*\toπX$, information which is useful for studies of the $X(3872)$ meson abundance in heavy ion collisions. We construct a formalism considering $X$ as a molecular bound state of $\bar D^0 D^{*0} - \textrm{c.c}$, $D^- D^{*+} - \textrm{c.c}$ and $D^-_s D^{*+}_s - \textrm{c.c}$. To obtain the amplitudes related to these processes we have made use of effective field Lagrangians. The evaluation of the cross section of the processes involving $D^*$ meson(s) requires the calculation of an anomalous vertex, $X\bar D^* D^*$, which has been obtained by considering triangular loops motivated by the molecular nature of $X(3872)$. Proceeding in this way, we have evaluated the cross section for the reaction $\bar D^* D\to πX$, and find that the diagrams involving the $X\bar D^* D^*$ vertex give a large contribution. Encouraged by this finding we estimate the $X\bar D^* D^*$ coupling, which turns out to be $1.95\pm 0.22$. We then use it to obtain the cross section for the reaction $\bar D^* D^*\toπX$ and find that, in this case too, the $X\bar D^* D^*$ vertex is relevant. We also discuss the role of the charged components of $X$ in the determination of the production cross sections.

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arXiv2013

Role of vector and pseudoscalar mesons in understanding $1/2^-$ $N^*$ and $Δ$ resonances

A study of nonstrange meson-baryon systems has been made with the idea of understanding the properties of the low-lying $1/2^-$ $N^*$ and $Δ$ resonances. The coupled channels are built by considering the pseudoscalar and vector mesons together with the octet baryons. The formalism is based on obtaining the interactions from the lowest order chiral Lagrangian when dealing with pseudoscalar mesons and relying on the hidden local symmetry in case of the vector mesons. The transition between the two systems is obtained by replacing the photon by a vector meson in the Kroll-Ruderman theorem for the photoproduction of pseudoscalar mesons. The subtraction constants, required to calculate the loop-function in the scattering equations, are constrained by fitting the available experimental data on some of the reactions with pseudoscalar meson-baryon final states. As a consequence, we find resonances which can be related to $N^*(1535)$, $N^*(1650)$ (with a double pole structure), $N^*(1895)$ and $Δ(1620)$. We conclude that these resonances can be, at least partly, interpreted as dynamically generated resonances and that the vector mesons play an important role in determining the dynamical origin of the low-lying $N^*$ and $Δ$ states.

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arXiv2013

Study of the $D^*ρ$ system using QCD sum rules

In this talk I present a study of the $D^* ρ$ system made by using the method of QCD sum rules. Considering isospin and spin projectors, we investigate the different configurations and obtain three $D^*$ mesons with isospin $I=1/2$, spin $S=0$, $1$, $2$ and with masses $2500\pm 67$ MeV, $2523\pm60$ MeV, and $2439\pm119$ MeV, respectively. The last state can be related to $D^*_2(2460)$ (spin 2) listed by the Particle Data Group, while one of the first two might be associated with $D^*(2640)$, whose spin-parity is unknown. In the case of $I=3/2$ we also find evidences of three states with spin 0, 1 and 2, respectively, with masses $2467\pm82$ MeV, $2420\pm128$ MeV, and $2550\pm56$ MeV.

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arXiv2013

Interpreting the peak structures around 1800 MeV in the BES data on $J/Ψ\to ϕπ^+ π^-$, $J/Ψ\to γωϕ$

In this talk we present an interpretation for the experimental data available on two different processes, namely, $J/Ψ\to ϕπ^+ π^-$, $J/Ψ\to γωϕ$, which seem to indicate existence of two new resonances with the same quantum numbers ($J^{πc}=0^{++}, I = 0$) and very similar mass (~1800 MeV) but with very different decay properties. However, our studies show that the peak structure found in the $ωϕ$ invariant mass, in $J/Ψ\to γωϕ$, is a manifestation of the well known $f_0(1710)$ while the cross section enhancement found in $J/Ψ\to ϕπ^+ π^-$ is indeed a new $f_0$ resonance with mass near 1800 MeV. We present an explanation for the different decay properties of these two scalar resonances.

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arXiv2013

Relating $D^* \bar{D}^*$ currents with $J^π= 0^+,1^+$ and $2^+$ to $Z_c$ states

In this work we study the $D^* \bar{D}^*$ current with QCD sum rules. We write the correlation function using the general current corresponding to the $D^* \bar{D}^*$ system and then use spin projectors to obtain the correlation function in the $0^+, 1^+$ and $2^+$ spin-parity configurations. The purpose of the present work is to investigate the possibility of explaining the recently reported $Z_c(4025)$ as a $D^* \bar{D}^*$ moleculelike state. As a result we find a state for each spin case with a very similar mass: $M^{S=0} = \left(3943 \pm 104\right)$ MeV, $M^{S=1} = \left(3950 \pm 105\right)$ MeV, $M^{S=2} = \left(3946 \pm 104 \right)$ MeV. We discuss that our mass results, within error bars, for $1^+$ or $2^+$ are both compatible with $Z_c(4025)$. However, our results are also compatible with a possible $D^* \bar{D}^*$ bound state, in agreement with predictions of some previous works. We have also calculated the current-state coupling which turns out to be larger in the $2^+$ case.

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arXiv2013

Reanalysis of the $e^+ e^- \to (D^* \bar D^*)^{\pm} π^{\mp}$ reaction and the claim for the $Z_c (4025)$ resonance

In this paper we study the reaction $e^+ e^- \to (D^* \bar D^*)^{\pm} π^{\mp}$ in which the BESIII collaboration has claimed the existence of a $1^+$ resonance, named $Z_c(4025)$, in the $D^*\bar D^*$ invariant mass spectrum with a mass around 4026 MeV and width close to 26 MeV. We determine the $D^*\bar D^*$ invariant mass distribution and find that although the explanation considered by the BESIII collaboration is plausible, there are others which are equally possible, like a $2^+$ resonance or a bound state. Even more, we find that the data can be explained without the existence of a resonance/bound state. In view of the different possible interpretations found for the BESIII data, we try to devise a strategy which could help in identifying the origin of the signal reported by the BESIII collaboration. For this, we study the dependence of the $D^*\bar D^*$ spectrum considering the different options as a function of the total center of mass energy. We arrive to the conclusion that increasing the center of mass energy from 4.26 GeV to 4.6 GeV can be useful to distinguish between a resonance, a bound state or just a pure background as being responsible for the signal found. This information should be useful for future experiments.

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arXiv2013

$N^*$'s and $Δ$'s generated in vector, pseudoscalar meson-baryon systems

We have recently studied coupled channel interactions of vector and pseudoscalar mesons with octet baryons having total strangeness null and spin-parity $1/2^-$. We, thus, considered $πN$, $ηN$, $K Λ$, $K Σ$, $ρN$, $ωN$, $ϕN$, $K^* Λ$, and $K^*Σ$ with total isospin 1/2 and 3/2. The interactions between pseudoscalar mesons and baryons are obtained by using the Weinberg-Tomozawa theorem. For the case of vector mesons, we calculate $s$-, $t$-, $u$-exchange diagrams and a contact term. The subtraction constants, required to calculate the loop-function in the scattering equations, are constrained by fitting the available experimental data on some of the reactions with pseudoscalar meson-baryon final states. We end up finding resonances which can be related to $N^*(1535)$, $N^*(1650)$ (with a double pole structure), $N^*(1895)$ and $Δ(1620)$. We conclude that these resonances can be, at least partly, interpreted as dynamically generated resonances and that the vector mesons play an important role in determining the dynamical origin of the low-lying $N^*$ and $Δ$ states.

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arXiv2013

Three-body hadron systems with strangeness

Recently, many efforts are being put in studying three-hadron systems made of mesons and baryons and interesting results are being found. In this talk, I summarize the main features of the formalism used to study such three hadron systems with strangeness $S=-1,0$ within a framework built on the basis of unitary chiral theories and solution of the Faddeev equations. In particular, I present the results obtained for the $π\bar{K}N$, $K\bar{K}N$ and $KK\bar{K}$ systems and their respective coupled channels. In the first case, we find four $Σ$'s and two $Λ$'s with spin-parity $J^P=1/2^+$, in the 1500-1800 MeV region, as two meson-one baryon s-wave resonances. In the second case, a $1/2^+$ $N^*$ around 1900 MeV is found. For the last one a kaon close to 1420 MeV is formed, which can be identified with K(1460).

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arXiv2013

Interaction of eta mesons with nuclei

Back in the mid eighties, a new branch of investigation which was related to the interaction of eta mesons with nuclei came into existence. It started with the theoretical prediction of possible exotic states of eta mesons and nuclei bound by the strong interaction and later developed into an extensive experimental program to search for such unstable states as well as understand the underlying interaction via eta meson producing reactions. The vast literature of experimental as well as theoretical works which studied various aspects of eta producing reactions such as the $π^+$ $n$ $\to ηp$, $p d \to ^3$He $η$, $p \,^6$Li $\to ^7$Be $η$ and $γ^3$He $\to η$ X, to name a few, had but one objective in mind: to understand the eta - nucleon ($ηN$) and hence the $η$-nucleus interaction which could explain the production data and confirm the existence of some $η$-mesic nuclei. In spite of these efforts, there remain uncertainties in the knowledge of the $ηN$ and hence the $η$-nucleus interaction. The present review is hence an attempt to bind together the findings in these works and draw some global and specific conclusions which can be useful for future explorations.

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arXiv2013

The role of $f_0(1710)$ in the $ϕω$ threshold peak of $J/Ψ\to γϕω$

We study the process $J/Ψ\to γϕω$, measured by the BES experiment, where a neat peak close to the $ϕω$ threshold is observed and is associated to a scalar meson resonance around 1800 MeV. We make the observation that a scalar resonance coupling to $ϕω$ unavoidably couples strongly to $K \bar K$, but no trace of a peak is seen in the $K \bar K$ spectrum of the $J/Ψ\to γK \bar K$ at this energy. This serves us to rule out the interpretation of the observed peak as a signal of a new resonance. After this is done, a thorough study is performed on the production of a pair of vector mesons and how its interaction leads necessarily to a peak in the $J/Ψ\to γϕω$ reaction close to the $ϕω$ threshold, due to the dynamical generation of the $f_0(1710)$ resonance by the vector-vector interaction. We then show that both the shape obtained for the $ϕω$ mass distribution, as well as the strength are naturally reproduced by this mechanism. The work also explains why the $ϕω$ peak is observed in the BES experiment and not in other reactions, like $B^\pm\to K^\pm ϕω$ of Belle.

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