Index
We corrected numerical values of gravitational radii in Phys. Rev. D 97, 014020 (2018) [arXiv:1711.08088]. Because a separate arXiv number different from arXiv:1711.08088 is not possible, we explain the erratum in the appendix section of the revised (version 3) arXiv:1711.0808.
Generalized parton distributions (GPDs) are 3-dimensional (3D) structure functions for hadrons, and they are important for solving the proton spin puzzle including partonic orbital-angular-momentum contributions. The s-t crossed quantities of the GPDs are generalized distribution amplitudes (GDAs). Here, s and t are Mandelstam variables. The GDAs can be studied in two-photon processes (gamma^* gamma-> h hbar) at KEKB. A GDA describes the amplitude from quark and antiquark to the hadron pair h-hbar. In 2016, the Belle collaboration reported measurements for pion-pair production in electron-positron collision, and the pion GDAs were determined in this work by analyzing the Belle data. In our analysis, the pion GDAs are expressed by a few parameters, which are determined by analyzing the Belle data. From the obtained GDAs, form factors of energy-momentum tenor, so called gravitational form factors, are calculated for pion in the timelike region. The spacelike gravitational form factors are calculated from the timelike ones by using the dispersion relation. Then, the mass radius is calculated as 0.32-0.39 fm and the mechanical radius, defined by the slope of the form factor Theta_1, is calculated as 0.82-0.88 fm for the pion by using the spacelike form factors. This is the first study on gravitational form factors and radii of hadrons from actual experimental measurements. In 2019, the Belle II collaboration will start collecting data by the higher luminosity Super KEKB, so that the GDAs of other hadrons should also be investigated in the near future. Our studies are valuable in understanding 3D structure and gravitational properties of hadrons.
We report recent theoretical progress on a polarization asymmetry in the proton-deuteron Drell-Yan process with a polarized-deuteron target and the tensor-polarized structure function b_1. Experimental measurements are possible at JLab for b_1 and at Fermilab for the Drell-Yan process. First, we show a theoretical estimate for the proton-deuteron Drell-Yan asymmetry in the Fermilab-E1039 experiment. We evolved tensor-polarized parton distribution functions, which explain existing HERMES b_1 data, at Q^2=2.5 GeV^2 to the $Q^2$ range of the Fermilab Drell-Yan measurements. Then, we predicted that the asymmetry is of the order of a few percent. The Drell-Yan experimenthas an advantage to probe the tensor-polarized antiquark distributions, which were suggested by the HERMES experiment as a finite sum for b_1 (int dx b_1 (x) ne 0). Second, we predicted b_1 for the JLab experiment by the standard convolution model of the deuteron. Our theoretical b_1 structure function seems to be much different from the HERMES data. Furthermore, a significant distribution exists at very large x (>1) beyond the kinematical limit x_{max}=1 for the proton. Because the standard deuteron-model estimate is much different from the HERMES data, there could be an interesting development as a new hadron-physics field if future JLab data will be much different from our conventional prediction.
This report is the summary of the Eighth International Conference on Quarks and Nuclear Physics (QNP2018). Hadron and nuclear physics is the field to investigate ultimate high-density quantum many-body systems bound by strong interactions. It is intended to clarify matter generation of universe and properties of quark-hadron many-body systems. The QNP is an international conference which covers a wide range of hadron and nuclear physics, including quark and gluon structure of hadrons, hadron spectroscopy, hadron interactions and nuclear structure, hot and cold dense matter, and experimental facilities. First, I introduce the current status of the hadron and nuclear physics field related to this conference. Next, the organization of the conference is explained, and a brief overview of major recent developments is discussed by selecting topics from discussions at the plenary sessions. They include rapidly-developing field of gravitational waves and nuclear physics, hadron interactions and nuclear structure with strangeness, lattice QCD, hadron spectroscopy, nucleon structure, heavy-ion physics, hadrons in nuclear medium, and experimental facilities of EIC, GSI-FAIR, JLab, J-PARC, Super-KEKB, and others. Nuclear physics is at a fortunate time to push various projects at these facilities. However, we should note that the projects need to be developed together with related studies in other fields such as gravitational physics, astrophysics, condensed-matter physics, particle physics, and fundamental quantum physics.
陽子と中性子は質量とスピンをどこから得ているのか？
意外なことに誰にもわかっていない。
これらの粒子の内部を探る新装置で、その答えが見つかりそうだ。
核子と原子核の内部構造を探るための電気的および磁気的形状因子は、 電子散乱で求められる。これと同様に、ハドロンや原子核内部の質量や 圧力分布を示す重力形状因子を求めることが、最近の ハドロン・トモグラフィー研究の進展により可能 になった。重力相互作用は、他の基本相互作用と比較して 極端に弱い相互作用であるため、電磁相互作用を直接用いる 電子散乱とは異なり、直接的に重力相互作用を使用して 重力形状因子を測定することはできない。 ところが、ハドロンや原子核の重力相互作用の源である、 クォークとグルーオンのエネルギー運動量テンソルの行列要素は、 ハドロン・トモグラフィーに用いられる３次元構造関数に含まれており、 実験的に研究可能である。３次元構造関数の実験研究は、 欧州のCERN-COMPASS, 米国のJefferson研究所、 Brookhaven研究所、Fermi研究所などで研究が行われている。 日本国内においては、この課題に関して、 すでにKEKのB中間子研究施設で実験結果があり、 J-PARCとILCにおいても可能なプロジェクトである。 この研究において、ハドロンの電荷半径と質量半径がかなり異なることが 示唆されており、その物理的原因の解明が必要である。 ３次元構造関数と重力形状因子の研究は、 核子スピン起源の研究のみならず、 クォーク・グルーオンの自由度によるハドロン質量起源の解明に繋がる。 また、重力形状因子から求まるハドロン内の圧力は、 中性子星の圧力をはるかに上回る、 自然界最高圧力である$10^{35}$パスカルの現象であるとともに、 圧力分布を通じたハドロンの安定性に関する課題であるため、 今後の発展が期待される。 ニュートン以来、重力の研究はマクロな物体を対象に行われて きたが、クォーク・グルーオンの自由度によるミクロの世界での 解明の時代になった。
Nucleon spin structure functions have been investigated mainly
by longitudinally-polarized ones for finding the origin of
the nucleon spin. Other types of spin structure functions are
transversely-polarized ones. In particular, quark transversity
distributions in the nucleons have very different properties
from the longitudinally-polarized quark distribution functions,
especially in scaling violation, because they are decoupled from
the gluon transversity, due to the fact that they are
helicity-flip (chiral-odd) distributions.
Such studies are valuable for finding not only the origin
of the nucleon spin but also a signature on physics
beyond the standard model, because the electric dipole moment
of the neutron is proportional to the transversity distributions.
Now, there is experimental progress on the quark transversity
distributions; however, there is no experimental information
on gluon transversity. In fact, the gluon transversity does not
exist for the spin-1/2 nucleon due to the helicity-conservation constraint.
One needs a hadron with spin more than or equal to one,
so that the helicity flip of two units is allowed.
A stable spin-1 target is, for example, the deuteron for studying
the gluon transversity. In this work, we propose a possibility
for finding the gluon transversity at hadron-accelerator facilities,
especially in the proton-deuteron Drell-Yan process
with the linearly-polarized deuteron, by showing theoretical formalism
and numerical results. In the experiment, the information
on the angular distribution of the dimuon is necessary
in the final state; however, the proton beam does not have to be polarized.
We show the dependencies of the Drell-Yan cross section
on the dimuon-mass squared $M_{\mu\mu}^{\,2}$,
the dimuon transverse-momentum $q_T$,
the dimuon rapidity $y$ in the center-of-momentum frame,
and the magnitude of the gluon transversity $\Delta_T g$.
We also show typical spin asymmetries in the Drell-Yan process.
Since the internal spin-1/2 nucleons within the deuteron
cannot contribute directly to the gluon transversity,
it could be a good observable to find a new non-nucleonic
component beyond the simple bound system of nucleons in nuclei.
!==================================================================
Preface: Eighth International Conference on Quarks and Nuclear Physics (QNP2018)
The eighth International Conference on Quarks and Nuclear Physics was held in Tsukuba, Japan on November 13-17, 2018. This conference follows the series of meetings previously held in Adelaide, Jülich, Bloomington, Madrid, Beijing, Palaiseau, and Valparaiso. Experimentalists and theorists discussed recent developments in the field of hadron and nuclear physics, and the following topics were covered:
There were 216 participants from 25 countries.We are happy that 89 young students and postdocs
came to the conference. There were a number of top scientists in hadron and nuclear physics at this
conference. It is particularly nice that young people had opportunities to listen to their talks and to
discuss their own studies with them.We would like to thank the members of the international advisory
committee for their precious suggestions on many aspects of the conference. We thank the local organizers,
secretaries, and students for their dedicated works on organization matters. The conference
was financially supported by APCTP, J-PARC/KEK, JSPS Grant on Innovative Area on clustering
as a window on the hierarchical structure of quantum systems, RCNP at Osaka University, Tsukuba
Tourism and Convention Association/Tsukuba City. It was also supported by JAEA, RIKEN, and
SOKENDAI. Finally, we thank all the participants to make this conference successful by excellent
presentations and active discussions.
Shunzo Kumano and Shinya Sawada
Institute of Particle and Nuclear Studies, KEK
May 2019, Tsukuba