arXiv subjects

Kenji Morita

Publications and source records attributed to Kenji Morita.

At least 37 records · Page 2Linked to original sources

arXiv2016

Hadron-Hadron Correlation and Interaction from Heavy-Ion Collisions

We investigate the $ΛΛ$ and $K^-p$ intensity correlations in high-energy heavy-ion collisions. First, we examine the dependence of the $ΛΛ$ correlation on the $ΛΛ$ interaction and the $ΛΛ$ pair purity probability $λ$. For small $λ$, the correlation function needs to be suppressed by the $ΛΛ$ interaction in order to explain the recently measured $ΛΛ$ correlation data. By comparison, when we adopt the $λ$ value evaluated from the experimentally measured $Σ^0/Λ$ ratio, the correlation function needs to be enhanced by the interaction. We demonstrate that these two cases correspond to the two analyses which gave opposite signs of the $ΛΛ$ scattering length. Next, we discuss the $K^-p$ correlation function. By using the local $\bar{K}N$ potential which reproduces the kaonic hydrogen data by SIDDHARTA, we obtain the $K^-p$ correlation function. We find that the $K^-p$ correlation can provide a complementary information with the $K^{-}p$ elastic scattering amplitude.

nucl-th↗

arXiv2016

Effects of kinematic cuts on net-electric charge fluctuations

The effects of kinematic cuts on electric charge fluctuations in a gas of charged particles are discussed. We consider a very transparent example of an ideal pion gas with quantum statistics, which can be viewed as a multi-component gas of Boltzmann particles with different charges and masses. Cumulants of net-electric charge fluctuations $χ_n^Q$ are calculated in a static and expanding medium with flow parameters adjusted to the experimental data. We show that the transverse momentum cut, $p_{t_\text{min}}\leq p_t\leq p_{t_\text{max}}$, weakens the effects of Bose statistics, i.e. contributions of effectively multi-charged states to higher order moments. Consequently, cuts in $p_t$ modify the experimentally measured cumulants and their ratios. We discuss the influence of kinematic cuts on the ratio of mean and variance of electric charge fluctuations in a hadron resonance gas, in the light of recent data of the STAR and PHENIX Collaborations. We find that the different momentum cuts of $p_{t_\text{min}}=0.2$~GeV (STAR) and $p_{t_\text{min}}=0.3$~GeV (PHENIX) are responsible for more than 30\% of the difference between these two data sets. We argue that the $p_t$ cuts imposed on charged particles will influence the normalized kurtosis {$κ_Qσ_Q^2 = χ_4^Q/χ_2^Q$} of the electric charge fluctuations. In particular, the reduction of $κ_Qσ_Q^2$ with increasing $p_{t_\text{min}}$ will lead to differences between PHENIX and STAR data of ${\cal O}(6\%)$ which currently are buried under large statistical and systematic errors. We furthermore introduce the relation between momentum cut-off and finite volume effects, which is of relevance for the comparison between experimental data and lattice QCD calculations.

hep-ph↗

arXiv2015

Lambda-Lambda interaction from two-particle intensity correlation in relativistic heavy-ion collisions

We investigate $ΛΛ$ interaction dependence of the $ΛΛ$ intensity correlation in high-energy heavy-ion collisions. By analyzing the correlation data recently obtained by the STAR collaboration based on theoretically proposed $ΛΛ$ interactions, we give a constraint on the $ΛΛ$ scattering length, $-1.25~\text{fm} < a_0 < 0$, suggesting that $ΛΛ$ interaction is weakly attractive and there is no loosely bound state. In addition to the fermionic quantum statistics and the $ΛΛ$ interaction, effects of collective flow, feed-down from $Σ^0$, and the residual source are also found to be important to understand the data. We demonstrate that the correlation data favor negative $ΛΛ$ scattering length with the pair purity parameter $λ=(0.67)^2$ evaluated by using experimental data on the $Σ^0/Λ$ ratio, while the positive scattering length could be favored when we regard $λ$ as a free fitting parameter.

nucl-th↗

arXiv2015

Temperature dependence of dimension 6 gluon operators and their effects on Charmonium

Starting from an earlier representation of the independent dimension 6 gluon operators in terms of color electric and magnetic fields, we estimate their changes near the critical temperature $T_c$ using the temperature dependence of the dimension 4 electric and magnetic condensates extracted from pure gauge theory on the lattice. We then improve the previous QCD sum rules for the $J/ψ$ mass near $T_c$ based on dimension 4 operators, by including the contribution of the dimension 6 operators to the OPE. We find an enhanced stability in the sum rule and confirm that the $J/ψ$ will undergo an abrupt change in the property across $T_c$.

hep-ph↗

arXiv2015

Stable Yang-Lee zeros in truncated fugacity series from net-baryon number multiplicity distribution

We investigate Yang-Lee zeros of grand partition functions as truncated fugacity polynomials of which coefficients are given by the canonical partition functions $Z(T,V,N)$ up to $N \leq N_{\text{max}}$. Such a partition function can be inevitably obtained from the net-baryon number multiplicity distribution in relativistic heavy ion collisions, where the number of the event beyond $N_{\text{max}}$ has insufficient statistics, as well as canonical approaches in lattice QCD. We use a chiral random matrix model as a solvable model for chiral phase transition in QCD and show that the closest edge of the distribution to real chemical potential axis is stable against cutting the tail of the multiplicity distribution. The similar behavior is also found in lattice QCD at finite temperature for Roberge-Weiss transition. In contrast, such a stability is found to be absent in the Skellam distribution which does not have phase transition. We compare the number of $N_{\text{max}}$ to obtain the stable Yang-Lee zeros with those of critical higher order cumulants.

hep-ph↗

arXiv2015

Net baryon number fluctuations across the chiral phase transition at finite density in the strong coupling lattice QCD

We investigate the net-baryon number fluctuations across the chiral phase transition at finite density in the strong coupling and chiral limit. Mesonic field fluctuations are taken into account by using the auxiliary field Monte-Carlo method. We find that the higher-order cumulant ratios, $Sσ$ and $κσ^2$, show oscillatory behavior around the phase boundary at $μ/T\gtrsim 0.2$, and there exists the region where the higher-order cumulant ratios are negative. The negative region of $κσ^2$ is found to shrink with increasing lattice size. This behavior agrees with the expectations from the scaling analysis.

hep-lat↗

arXiv2015

Momentum scale dependence of the net quark number fluctuations near chiral crossover

We investigate properties of the net baryon number fluctuations near chiral crossover in a hot and dense medium of strongly interacting quarks. The chirally invariant quark-antiquark interactions are modeled by an effective quark-meson Lagrangian. To preserve remnants of criticality in the O(4) universality class, we apply the functional renormalization group method to describe thermodynamics near chiral crossover. Our studies are focused on the influence of the momentum cuts on the critical behavior of different cumulants of the net quark number fluctuations. We use the momentum scale dependence of the flow equation to examine how the suppression of the momentum modes in the infrared and ultraviolet regime modifies generic properties of fluctuations expected in the O(4) universality class. We show, that the pion mass $m_π$ is a natural soft momentum scale at which cumulants are saturated at their critical values, whereas for scales larger than $2m_π$ the characteristic O(4) structure of the higher order cumulants get lost. These results indicate, that when measuring fluctuations of the net baryon number in heavy ion collisions to search for a partial restoration of chiral symmetry or critical point, a special care have to be made when introducing kinematical cuts on the fluctuation measurements.

hep-ph↗

arXiv2015

Charmonium Spectroscopy in Strong Magnetic Fields by QCD Sum Rules: S-Wave Ground States

We investigate quarkonium mass spectra in external constant magnetic fields by using QCD sum rules. We first discuss a general framework of QCD sum rules necessary for properly extracting meson spectra from current correlators computed in the presence of strong magnetic fields, that is, a consistent treatment of mixing effects caused in the mesonic degrees of freedom. We then implement operator product expansions for pseudoscalar and vector heavy-quark current correlators by taking into account external constant magnetic fields as operators, and obtain mass shifts of the lowest-lying bound states $η_c$ and $J/ψ$ in the static limit with their vanishing spatial momenta. Comparing results from QCD sum rules with those from hadronic effective theories, we find that the dominant origin of mass shifts comes from a mixing between $η_c$ and $J/ψ$ with a longitudinal spin polarization, accompanied by other subdominant effects such as mixing with higher excited states and continua.

hep-ph↗

arXiv2015

Lambda-Lambda interaction from relativistic heavy-ion collisions

We investigate the two-particle intensity correlation function of $Λ$ in relativistic heavy-ion collisions. We find that the behavior of the $ΛΛ$ correlation function at small relative momenta is fairly sensitive to the interaction potential and collective flows. By comparing the results of different source functions and potentials, we explore the effect of intrinsic collective motions on the correlation function. We find that the recent STAR data gives a strong constraint on the scattering length and effective range of $ΛΛ$ interaction as, $-1.8 \mathrm{fm}^{-1} < 1/a_0 < -0.8 \mathrm{fm}^{-1}$ and $3.5 \mathrm{fm} < r_\mathrm{eff} < 7 \mathrm{fm}$, respectively,if $Λ$ samples do not include feed-down contribution from long-lived particles. We find that feed-down correction for $Σ^0$ decay reduces the sensitivity of the correlation function to the detail of the $ΛΛ$ interaction. As a result, we obtain a weaker constraint $1/a_0 <-0.8$ fm$^{-1}$. Implication for the signal of existence of $H$-dibaryon is discussed. Comparison with the scattering parameters obtained from the double $Λ$ hypernucleus may reveal in-medium effects in the $ΛΛ$ interaction.

nucl-th↗

arXiv2014

Criticality of the net-baryon number probability distribution at finite density

We compute the probability distribution $P(N)$ of the net-baryon number at finite temperature and quark-chemical potential, $μ$, at a physical value of the pion mass in the quark-meson model within the functional renormalization group scheme. For $μ/T<1$, the model exhibits the chiral crossover transition which belongs to the universality class of the $O(4)$ spin system in three dimensions. We explore the influence of the chiral crossover transition on the properties of the net baryon number probability distribution, $P(N)$. By considering ratios of $P(N)$ to the Skellam function, with the same mean and variance, we unravel the characteristic features of the distribution that are related to $O(4)$ criticality at the chiral crossover transition. We explore the corresponding ratios for data obtained at RHIC by the STAR Collaboration and discuss their implications. We also examine $O(4)$ criticality in the context of binomial and negative-binomial distributions for the net proton number.

hep-ph↗

arXiv2014

QCD sum rules for magnetically induced mixing between $η_c$ and $J/ψ$

We investigate the properties of charmonia in strong magnetic fields by using QCD sum rules. We show how to implement the mixing effects between $η_c$ and $J/ψ$ on the basis of field-theoretical approaches, and then show that the sum rules are saturated by the mixing effects with phenomenologically determined parameters. Consequently, we find that the mixing effects are the dominant contribution to the mass shifts of the charmonia in strong magnetic fields.

hep-ph↗

arXiv2013

Net baryon number probability distribution near the chiral phase transition

We discuss properties of the net baryon number probability distribution near the chiral phase transition to explore the effect of critical fluctuations. Our studies are performed within Landau theory, where the coefficients of the polynomial potential are parametrized, so as to reproduce the mean-field (MF), the $Z(2)$ and $O(4)$ scaling behaviors of the cumulants of the net baryon number. We show, that in the critical region, the structure of the probability distribution changes, dependently on values of the critical exponents. In the MF approach, as well as in the $Z(2)$ universality class, the contribution of the singular part of the thermodynamic potential tends to broaden the distribution. By contrast, in the model with $O(4)$ scaling, the contribution of the singular part results in a narrower net baryon number probability distribution with a wide tail.

hep-ph↗

arXiv2013

Net quark number probability distribution near the chiral crossover transition

We investigate properties of the probability distribution of the net quark number near the chiral crossover transition in the quark-meson model. The calculations are performed within the functional renormalization group approach, as well as in the mean-field approximation. We find, that there is a substantial influence of the underlying chiral phase transition on the properties of the probability distribution. In particular, for a physical pion mass, the distribution which includes the effect of mesonic fluctuations, differs considerably from both, the mean-field and Skellam distributions. The latter is considered as a reference for a non-critical behavior. A characteristic feature of the net quark number probability distribution is that, in the vicinity of the chiral crossover transition in the O(4) universality class, it is narrower than the corresponding mean-field and Skellam function. We study the volume dependence of the probability distribution, as well as the resulting cumulants, and discuss their approximate scaling properties.

hep-ph↗

arXiv2013

Free energy versus internal energy potential for heavy quark systems at finite temperature

Using the QCD sum rule with its operator product expansion reliably estimated from lattice calculations for the pressure and energy density of the quark-gluon plasma, we calculate the strength of the wave function of J/psi at the origin and find that it decreases with temperature above the transition temperature. This result is shown to follow exactly that obtained from the solution of the Schrödingier equation for a charm and anticharm quark pair using the free energy from lattice calculations as the potential and is in sharp contrast to that using the deeper potential associated with the internal energy, which shows an enhanced strength of the J/psi wave function at the origin. Our result thus has resolved the long-standing question of whether the free energy potential or the internal energy potential should be used in analyzing the spectrum of heavy quark systems at finite temperature.

nucl-th↗

arXiv2012

Modification of hadronic spectral functions under extreme conditions: An approach based on QCD sum rules and the maximum entropy method

Studies of quarkonium spectral functions at finite temperature, based on an approach combining QCD sum rules and the maximum entropy method are briefly reviewed. QCD sum rules for heavy quarkonia incorporate finite temperature effects in form of changing values of gluonic condensates that appear in the operator product expansion. These changes depend on the energy density and pressure at finite temperature, which we extract from quenched lattice QCD calculations. The maximum entropy method then allows us to obtain the most probable spectral function from the sum rules, without having to introduce any specific assumption about its functional form. Our findings suggest that the charmonium ground states of both S-wave and P-wave channels dissolve into the continuum already at temperatures around or slightly above the critical temperature T_c, while the bottomonium states are less influenced by temperature effects, surviving up to about 2.5 T_c or higher for S-wave and up to about 2.0 T_c for P-wave states.

hep-ph↗

arXiv2012

Thermal modification of bottomonium spectra from QCD sum rules with the maximum entropy method

The bottomonium spectral functions at finite temperature are analyzed by employing QCD sum rules with the maximum entropy method. This approach enables us to extract the spectral functions without any phenomenological parametrization, and thus to visualize deformation of the spectral functions due to temperature effects estimated from quenched lattice QCD data. As a result, it is found that Υand η_b survive in hot matter of temperature up to at least 2.3T_c and 2.1T_c, respectively, while χ_{b0} and χ_{b1} will disappear at T<2.5T_c. Furthermore, a detailed analysis of the vector channel shows that the spectral function in the region of the lowest peak at T=0 contains contributions from the excited states, Υ(2S) and Υ(3S), as well as the ground states Υ(1S). Our results at finite T are consistent with the picture that the excited states of bottomonia dissociate at lower temperature than that of the ground state. Assuming this picture, we find that Υ(2S) and Υ(3S) disappear at T=1.5-2.0T_c.

hep-ph↗

arXiv2012

A renewed look at eta' in medium

We revisit the question of whether the U_A(1) symmetry is effectively restored in hot and dense medium. In particular, by generalizing the Witten-Veneziano formula to finite temperature, we investigate whether the mass of eta'-meson will change in medium due to the restoration of chiral symmetry.

nucl-th↗

arXiv2012

Charmonium mass in hot and dense hadronic matter

We investigate mass shifts of charmonia driven by change of the gluon condensate below but near transition temperatures at finite baryonic chemical potential. Extending previous prescription on the relation between gluon condensates and thermodynamic quantities, we model the gluon condensates of hadronic matter at finite temperature and baryonic chemical potential such that the scalar gluon condensate fits with the latest lattice QCD data. By making use of the QCD sum rule and the second order Stark effect, we find that the smoother transition in the full QCD can lead to moderate mass shifts of charmonia even below the transition temperature. We also find larger mass shift at fixed temperature as chemical potential increases. Existing data on charmonium-charmonium ratio is found to be consistent with the statistical hadronization scenario including the obtained mass shift.

hep-ph↗