Solving the J/ψ puzzle – NRQCD factorization at next-to-next-to-leading order

解决 J/Ï 难题 â NRQCD 因式分解

• 批准号: 409060694
• 负责人: Professor Dr. Bernd A. Kniehl
• 金额: --
• 依托单位: II. Institut für Theoretische Physik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/409060694?language=en
• 关键词: Solving; puzzle; NRQCD; factorization; next

Despite its great corroboration by the recent discovery of the Higgs boson at the CERN Large Hadron Collider (LHC), the Standard Model (SM) of elementary particle physics, which unifies the electromagnetic, weak, and strong forces of nature by a relativistic, renormalizable quantum field theory, is faced by severe challenges in the quantitative description of experimental data. The most striking discrepancies today appear in the production of heavy quarkonia, i.e. heavy quark-antiquark pairs QQ̅ bound by gluons. In fact, the world data of heavy-quarkonium yield and polarization taken in particle collision experiments at high energies, including those at the LHC, elude a coherent interpretation by the best available predictions of quantum chromodynamics (QCD), which governs the strong interactions within the SM. Discrepancies by more than 20 standard deviations are found for the otherwise very familiar J/ψ meson, discovered already in 1974! Such discrepancies between theory and experiment are absolutely unacceptable. We propose to tackle this longstanding fundamental problem by pushing the Bodwin-Braaten-Lepage factorization approach to nonrelativistic QCD (NRQCD), the effective field theory derived from QCD to describe QQ̅ bound-state dynamics, to the next level of precision. This amounts to including quantum and relativistic corrections at the next-to-next-to-leading order, i.e. at relative orders αs2, αsv2, v4 in the strong-coupling constant αs and the velocity v of Q in the QQ̅ rest frame. The quantum corrections involve two-loop Feynman diagrams with two incoming and three outgoing particles, one-loop 2 → 4 diagrams, and tree-level 2 → 5 diagrams. There are at least three different mass scales: two Mandelstam variables s and t, measuring the center-of-mass energy and the scattering angle, respectively, and the mass of Q. The relativistic corrections involve one more mass scale, q2, where q is the momentum of Q. While two-loop calculations are now state of the art in many other areas of particle phenomenology, heavy-quarkonium production is an exception. This is due to the spin and color projections onto the various QQ̅ Fock states, which immensely blow up the analytic expressions. An especially aggravating feature is the appearance of derivatives w.r.t. q via spin projectors with orbital angular momentum L ≥ 1and relativistic corrections. These generate novel classes of infrared singularities unknown outside NRQCD, which require extended patterns of cancellations between virtual and real corrections. The proposed research is likely to solve the J/ψ puzzle because the state-of-the-art predictions are plagued by very sizeable next-to-leading-order corrections seriously impugning their reliability. The envisaged calculations will also probe the underlying factorization hypothesis, universally demarcating the unavoidable non-perturbative phenomena of QCD, in an unexplored region and may necessitate modifications to our understanding of nature.

尽管最近在CERN大型强子对撞机（LHC）上发现的希格斯玻色子（Higgs boson）为基本粒子物理学的标准模型（SM）提供了有力的佐证，但它在实验数据的定量描述方面面临着严峻的挑战。今天，最显著的差异出现在重夸克偶素的产生上，即由胶子束缚的重夸克-反夸克对。事实上，在高能粒子碰撞实验（包括大型强子对撞机的实验）中获得的重夸克偶素产额和极化的世界数据，无法用量子色动力学（QCD）的最佳预测进行连贯的解释，而量子色动力学（QCD）控制着SM中的强相互作用。1974年发现的J/π介子的差异超过了20个标准差！理论和实验之间的这种差异是绝对不能接受的。我们建议通过将Bodwin-Braaten-Lepage因式分解方法推向非相对论QCD（NRQCD）来解决这个长期存在的基本问题，NRQCD是从QCD导出的描述QQ束缚态动力学的有效场论，精确到下一个水平。这相当于包括了次-次-领先阶的量子和相对论修正，即在QQ坐标系中强耦合常数αs和Q的速度v中的相对阶αs2、α sv 2、v4。量子修正涉及两个输入粒子和三个输出粒子的双圈费曼图，单圈2 → 4图和树级2 → 5图。至少有三种不同的质量尺度：两个曼德尔维变量s和t，分别测量质心能量和散射角，以及Q的质量。相对论修正涉及到另一个质量标度q2，其中q是Q的动量。虽然双圈计算现在在粒子唯象学的许多其他领域都是最先进的，但重夸克偶素的产生是一个例外。这是由于自旋和颜色投影到各种QQ量子态上，极大地放大了解析表达式。一个特别令人恼火的特点是衍生品w.r.t.的出现。q的轨道角动量L ≥ 1和相对论修正。这些产生了NRQCD之外未知的红外奇异点的新类别，这需要虚拟和真实的校正之间的消除的扩展模式。拟议的研究可能会解决J/J的困惑，因为国家的最先进的预测是困扰非常大的下一个领先的顺序校正严重质疑其可靠性。设想的计算也将探测潜在的因子分解假设，普遍划分QCD不可避免的非微扰现象，在一个未探索的区域，并可能需要修改我们对自然的理解。