We have proposed to use an effective theory to describe interactions of an $Nbar N$-system. The effective theory can be constructed in analogy to the existing effective theory for an $NN$-system. In this work we study the next-to-leading order correc
tion to $Nbar N$ scattering near the threshold in the effective theory. We find that the experimental data can be well described with the effective theory.
The observed enhancement of $pbar p$-production near the threshold in radiative decays of $J/psi$ and $e^+e^-$-annihilations can be explained with final state interactions among the produced $Nbar N$ system, where the enhancement is essentially deter
mined by $Nbar N$ elastic scattering amplitudes. We propose to use an effective theory for interactions in a $Nbar N$ system near its threshold. The effective theory is similar to the well-known one for interactions in a $NN$ system but with distinctions. It is interesting to note that in the effective theory some corrections to scattering amplitudes at tree-level can systematically be summed into a simple form. These corrections are from rescattering processes. With these corrected amplitudes we are able to describe the enhancement near the threshold in radiative decays of $J/psi$ and $e^+e^-$-annihilations, and the $pbar p$ elastic scattering near the threshold.
Glauber gluons in Drell-Yan processes are soft gluons with the transverse momenta much larger than their momentum components along the directions of initial hadrons. Their existence has been a serious challenge in proving the factorization of Drell-Y
an processes. The recently proposed soft collinear effect theory of QCD can provide a transparent way to show factorizations for a class of processes, but it does not address the effect of glauber gluons. In this letter we first confirm the existence of glauber gluons through an example. We then add glauber gluons into the effective theory and study their interaction with other particles. In the framework of the effective theory with glauber gluons we are able to show that the effects of glauber gluons in Drell-Yan processes are canceled and the factorization holds in the existence of glauber gluons. Our work completes the proof or argument of factorization of Drell-Yan process in the framework of the soft collinear effective theory.
A new method is proposed to calculate wave functions in $k_T$-factorization in cite{LiMi} as a comment about our paper cite{FMW}. We point out that the results obtained with the method are in conflict with the translation invariance and depend on the
chosen contours for loop-integrals. Therefore, the method is in principle unacceptable and the results with the method cannot be correct.
Using partonic states we verify the transverse momentum dependent factorization for single transverse spin asymmetries in semi inclusive DIS at leading but nontrivial order of alpha_s. The factorization has been previously derived in a formal way by
using diagram expansion at hadron level. We find with our partonic results that two relevant structure functions satisfy the factorization. Our results also satisfy the collinear factorization but with the perturbative coefficient different than that derived formally.
In the $k_T$-factorization for exclusive processes, the nontrivial $k_T$-dependence of perturbative coefficients, or hard parts, is obtained by taking off-shell partons. This brings up the question of whether the $k_T$-factorization is gauge invarian
t. We study the $k_T$-factorization for the case $pi gamma^* to gamma$ at one-loop in a general covariant gauge. Our results show that the hard part contains a light-cone singularity that is absent in the Feynman gauge, which indicates that the $k_T$-factorization is {it not} gauge invariant. These divergent contributions come from the $k_T$-dependent wave function of $pi$ and are not related to a special process. Because of this fact the $k_T$-factorization for any process is not gauge invariant and is violated. Our study also indicates that the $k_T$-factorization used widely for exclusive B-decays is not gauge invariant and is violated.
Single transverse-spin asymmetries have been studied intensively both in experiment and theory. Theoretically, two factorization approaches have been proposed. One is by using transverse-momentum-dependent factorization and the asymmetry comes from t
he so called Sivers function. Another is by using collinear factorization where the nonperturbative effect is parameterized by a twist-3 hadronic matrix element. However, the factorized formulas for the asymmetries in the two approaches are derived at hadron level formally by diagram expansion, where one works with various parton density matrices of hadrons. If the two factorizations hold, they should also hold at parton level. We examine this for Drell-Yan processes by replacing hadrons with partons. By calculating the asymmetry, Sivers function and the twist-3 matrix element at nontrivial leading order of $alpha_s$, we find that we can reproduce the result of the transverse-momentum-dependent factorization. But we can only verify the result of the collinear factorization partly. Two formally derived relations between Sivers function and the twist-3 matrix element are also examined with negative results.
We study the observed enhancement of a $pbar p$ system near the threshold in the process $J/psi to gamma pbar p$ and $e^+ e^- to pbar p$. From early studies the enhancement can be explained by final state interactions, which are in general taken into
account with some potential models. In this work we offer a simple approach within quantum field theory to explain the observed enhancement. We point out that among different final state interactions the rescattering in a $Nbar N$ system though exchange of $pi$ is the most important. The effects of the rescattering is completely fixed by the well-known coupling $g_{pi NN}$. Our results show that the enhancement in $J/psi to gamma pbar p$ and $e^+ e^- to pbar p$ can be well described with the rescattering effects.
The recently discovered $X$(3872) has many possible interpretations. We study the production of $X$(3872) with PANDA at GSI for the antiproton-proton collision with two possible interpretations of X(3872). One is as a loosely-bound molecule of $D$-me
sons, while another is a 2P charmonium state $chi_{c1}$ (2P). Using effective couplings we are able to give numerical predictions for the production near the threshold and the production associated with $pi^0$. The produced $X$(3872) can be identified with its decay $J/psi pi^+pi^-$. We also study the possible background near the threshold production for $X(3872) to J/psi pi^+pi^-$. With the designed luminosity $1.5{rm fb}^{-1}$ per year of PANDA we find that the event number of $pbar p to J/psi pi^+pi^-$ near the threshold is at the order of $10^6 sim 10^8$, where the large uncertainty comes from the total decay width of X(3872). Our study shows that at the threshold more than about 60% events come from the decay of X(3872) and two interpretations are distinguishable from the line-shape of the production. With our results we except that the PANDA experiments will shed light on the property of X(3872).
