大型強子對撞機揭示強子之間的強相互作用丨《自然》（20201210出版）一周論文導讀

編譯 | 未玖

Nature,10 December 2020,VOL 588,ISSUE 7837

《自然》2020年12月10日，第588卷，7837期

天文學Astronomy

Detection of large-scale X-ray bubbles in the Milky Way halo

探測銀河系光暈中的大尺度X射線氣泡

▲ 作者：P. Predehl, R. A. Sunyaev, W. Becker, H. Brunner, R. Burenin, A. Bykov, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2979-0

▲ 摘要

銀河系的光暈提供了一個實驗場所，用以研究由銀河系形成模型預測的激波熱氣的特性。有觀測證據表明，在過去銀河系核的活動中，能量注入光暈；然而，這種能量的來源（恆星形成或超大質量黑洞活動）尚不確定，而且核結構與大規模特徵之間的因果關係尚未明確建立。

研究組報導了軟X射線輻射的氣泡，這些氣泡在銀河系中心上下延伸約14千秒差距，且包括在南部天空中類似於北銀極支的結構。這些氣泡的尖銳邊界顯示無碰撞和無輻射衝擊，並證實了這樣一種觀點，即這些氣泡不是局部超新星的殘餘，而是與γ射線所見特徵密切相關的巨大星系級結構的一部分。

來自銀河系中心的大量能量注入是造成γ射線和X射線氣泡的最可能原因。後者的估計能量約為1056爾格，足以擾亂銀河系環星系介質的結構、能量含量和化學富集。

▲ Abstract

The halo of the Milky Way provides a laboratory to study the properties of the shocked hot gas that is predicted by models of galaxy formation. There is observational evidence of energy injection into the halo from past activity in the nucleus of the Milky Way; however, the origin of this energy (star formation or supermassive-black-hole activity) is uncertain, and the causal connection between nuclear structures and large-scale features has not been established unequivocally. Here we report soft-X-ray-emitting bubbles that extend approximately 14 kiloparsecs above and below the Galactic centre and include a structure in the southern sky analogous to the North Polar Spur. The sharp boundaries of these bubbles trace collisionless and non-radiative shocks, and corroborate the idea that the bubbles are not a remnant of a local supernova but part of a vast Galaxy-scale structure closely related to features seen in γ-rays. Large energy injections from the Galactic centre are the most likely cause of both the γ-ray and X-ray bubbles. The latter have an estimated energy of around 1056 erg, which is sufficient to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way..

物理學Physics

Unveiling the strong interaction among hadrons at the LHC

大型強子對撞機揭示強子之間的強相互作用

▲ 作者：ALICE Collaboration

▲ 連結：

https://www.nature.com/articles/s41586-020-3001-6

▲ 摘要

當今核物理面臨的主要挑戰之一，是要從第一性原理出發，理解具有不同夸克含量的強子之間的有效相互作用。利用求解離散時空晶格上夸克和膠子動力學的技術已獲得了初步成功。

實驗中通過強子彼此散射對強相互作用的動力學進行研究。對於不穩定的強子來說，這樣的散射實驗很困難，甚至不可能，因此高質量的測量僅存在於包含上下夸克的強子中。

研究組證明，在歐洲核子研究中心大型強子對撞機（LHC）的超相對論性質子-質子碰撞中，測量強子對之間的動量空間的相關性，提供了一種精確的方法，可獲得關於任何一對不穩定強子之間相互作用動力學的缺失信息。

具體來說，研究組討論了包含奇異夸克（超子）的重子相互作用的情況。研究組演示了如何使用質子-ω重子相關性的精確測量結果，用類似於晶格計算預測的精度對強子-強子對強相互作用的影響進行研究與比較。

大型強子對撞機的質子-質子碰撞中發現的大量超子，再加上對小（約一個飛米）粒子間距離的精確建模以及對相關性函數的精確預測，使人們能夠詳細確定核子-超子相互作用的短程部分。

▲ Abstract

One of the key challenges for nuclear physics today is to understand from first principles the effective interaction between hadrons with different quark content. First successes have been achieved using techniques that solve the dynamics of quarks and gluons on discrete space-time lattices. Experimentally, the dynamics of the strong interaction have been studied by scattering hadrons off each other. Such scattering experiments are difficult or impossible for unstable hadrons and so high-quality measurements exist only for hadrons containing up and down quarks. Here we demonstrate that measuring correlations in the momentum space between hadron pairs produced in ultrarelativistic proton-proton collisions at the CERN Large Hadron Collider (LHC) provides a precise method with which to obtain the missing information on the interaction dynamics between any pair of unstable hadrons. Specifically, we discuss the case of the interaction of baryons containing strange quarks (hyperons). We demonstrate how, using precision measurements of proton-omega baryon correlations, the effect of the strong interaction for this hadron-hadron pair can be studied with precision similar to, and compared with, predictions from lattice calculations. The large number of hyperons identified in proton-proton collisions at the LHC, together with accurate modelling of the small (approximately one femtometre) inter-particle distance and exact predictions for the correlation functions, enables a detailed determination of the short-range part of the nucleon-hyperon interaction.

Dipolar evaporation of reactive molecules to below the Fermi temperature

低於費米溫度的活性分子偶極蒸發

▲ 作者：Giacomo Valtolina, Kyle Matsuda, William G. Tobias, Jun-Ru Li, Luigi De Marco & Jun Ye

▲ 連結：

https://www.nature.com/articles/s41586-020-2980-7

▲ 摘要

分子的控制是研究量子相的關鍵，其中豐富的自由度可用來編碼信息，強相互作用可精確調諧。但分子碰撞中的非彈性損失極大地阻礙了低熵分子系統的工程設計。到目前為止，唯一的量子簡併分子氣體是通過結合兩種高度簡併原子氣體產生的。

研究組使用外部電場和光學晶格約束來創建自旋極化的鉀-銣（KRb）極性分子的二維費米氣體，其中彈性的、可調諧的偶極相互作用支配著所有非彈性過程。捕集阱中分子之間的直接熱化導致有效的偶極蒸發冷卻，造成相空間密度快速增加。

在量子簡併開始時，研究組觀察到費米統計量對分子氣體熱力學的影響。這些結果為偶極分子氣體的量子簡併提供了一種常規策略，其中強、長程和各向異性的偶極相互作用可以驅動奇異多體相的出現，例如層間配對和p波超流體。

▲ Abstract

The control of molecules is key to the investigation of quantum phases, in which rich degrees of freedom can be used to encode information and strong interactions can be precisely tuned. Inelastic losses in molecular collisions, however, have greatly hampered the engineering of low-entropy molecular systems. So far, the only quantum degenerate gas of molecules has been created via association of two highly degenerate atomic gases. Here we use an external electric field along with optical lattice confinement to create a two-dimensional Fermi gas of spin-polarized potassium–rubidium (KRb) polar molecules, in which elastic, tunable dipolar interactions dominate over all inelastic processes. Direct thermalization among the molecules in the trap leads to efficient dipolar evaporative cooling, yielding a rapid increase in phase-space density. At the onset of quantum degeneracy, we observe the effects of Fermi statistics on the thermodynamics of the molecular gas. These results demonstrate a general strategy for achieving quantum degeneracy in dipolar molecular gases in which strong, long-range and anisotropic dipolar interactions can drive the emergence of exotic many-body phases, such as interlayer pairing and p-wave superfluidity.

Operation of an optical atomic clock with a Brillouin laser subsystem

布裡淵雷射子系統操作光學原子鐘

▲ 作者：William Loh, Jules Stuart, David Reens, Colin D. Bruzewicz, Danielle Braje, John Chiaverini, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2981-6

▲ 摘要

傳統上，微波原子鐘是精確測量時間和頻率的「金標準」。然而，在過去的十年中，光學原子鐘的精確度已比微波原子鐘高兩個數量級及以上。現有的光學時鐘佔用體積超過1立方米，使這些時鐘能夠在現場環境中運行是一項重大挑戰，這需要原子基準和時鐘雷射器及其支持雷射器和電子設備的堅固性和小型化。

使用受激布裡淵散射雷射子系統，它有著縮小的腔體體積，且在非真空的情況下運行，研究組展示了一個頗有前途的可攜式光學原子鐘結構的組件。研究組用受激布裡淵散射雷射器探測88Sr+離子，獲得了一個在1秒鐘內具有3.9×10-14短期穩定性的時鐘，這比最先進的微波時鐘提高了一個數量級。

這種潛在可攜式系統中的性能提升為大幅改進現有技術提供了一個新途徑，例如全球定位系統，地球大地測量，對暗物質的搜尋，以及研究基本物理常數的可能長期變化等。

▲ Abstract

Microwave atomic clocks have traditionally served as the 『gold standard』 for precision measurements of time and frequency. However, over the past decade, optical atomic clocks have surpassed the precision of their microwave counterparts by two orders of magnitude or more. Extant optical clocks occupy volumes of more than one cubic metre, and it is a substantial challenge to enable these clocks to operate in field environments, which requires the ruggedization and miniaturization of the atomic reference and clock laser along with their supporting lasers and electronics. Here, using a stimulated Brillouin scattering laser subsystem that has a reduced cavity volume and operates without vacuum, we demonstrate a promising component of a portable optical atomic clock architecture. We interrogate a 88Sr+ ion with our stimulated Brillouin scattering laser and achieve a clock exhibiting short-term stability of 3.9 × 10−14 over one second—an improvement of an order of magnitude over state-of-the-art microwave clocks. This performance increase within a potentially portable system presents a compelling avenue for substantially improving existing technology, such as the global positioning system, and also for enabling the exploration of topics such as geodetic measurements of the Earth, searches for dark matter and investigations into possible long-term variations of fundamental physics constants.

材料科學Materials Science

Capillary condensation under atomic-scale confinement

原子尺度約束下的毛細凝聚

▲ 作者：Qian Yang, P. Z. Sun, L. Fumagalli, Y. V. Stebunov, S. J. Haigh, Z. W. Zhou, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2978-1

▲ 摘要

水的毛細凝聚在自然界和工藝中無處不在。它通常發生在顆粒狀和多孔性介質中，可以極大地改變諸如粘合、潤滑、摩擦和腐蝕等性質，在微電子、製藥、食品和其他行業的許多過程中都很重要。

已有百年歷史的開爾文方程常用於描述凝聚現象，且被證明可適用於直徑小至幾個納米的液體彎月面。對於在環境溼度下凝聚的更小直徑以及具有特殊實際意義的毛細通道，由於其直徑與水分子大小相當，開爾文方程可能會失效。

研究組使用二維晶體的範德華組裝來創建原子尺度毛細管，並研究其中的凝聚。他們最小的毛細管直徑不到4埃，只能容納單層水。令人驚訝的是，即使在這種尺度下，研究組發現使用體相水特性的宏觀開爾文方程竟在強親水性（雲母）毛細管中準確描述了凝聚轉變，且對弱親水性（石墨）毛細管的定性仍然有效。

研究組證明這種一致性是偶然的，可歸因於毛細管壁的彈性變形，它抑制了原子尺度毛細管和水分子之間的巨大振蕩行為。該工作為在極限小尺度上更好地理解毛細效應奠定了基礎，這在許多實際情況中都很重要。

▲ Abstract

Capillary condensation of water is ubiquitous in nature and technology. It routinely occurs in granular and porous media, can strongly alter such properties as adhesion, lubrication, friction and corrosion, and is important in many processes used by microelectronics, pharmaceutical, food and other industries. The century-old Kelvin equation is frequently used to describe condensation phenomena and has been shown to hold well for liquid menisci with diameters as small as several nanometres. For even smaller capillaries that are involved in condensation under ambient humidity and so of particular practical interest, the Kelvin equation is expected to break down because the required confinement becomes comparable to the size of water molecules. Here we use van der Waals assembly of two-dimensional crystals to create atomic-scale capillaries and study condensation within them. Our smallest capillaries are less than four ångströms in height and can accommodate just a monolayer of water. Surprisingly, even at this scale, we find that the macroscopic Kelvin equation using the characteristics of bulk water describes the condensation transition accurately in strongly hydrophilic (mica) capillaries and remains qualitatively valid for weakly hydrophilic (graphite) ones. We show that this agreement is fortuitous and can be attributed to elastic deformation of capillary walls, which suppresses the giant oscillatory behaviour expected from the commensurability between the atomic-scale capillaries and water molecules. Our work provides a basis for an improved understanding of capillary effects at the smallest scale possible, which is important in many realistic situations.

氣候學Climatology

Quantification of an efficiency–sovereignty trade-offin climate policy

量化氣候政策中能效-主權的權衡

▲ 作者：Nico Bauer, Christoph Bertram, Anselm Schultes, David Klein, Gunnar Luderer, Elmar Kriegler, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2982-5

▲ 摘要

《巴黎協定》呼籲採取合作對策，目標是將全球變暖限制在比工業化前水平高兩個攝氏度以下，同時重申平等、共同但有區別的責任和能力的原則。儘管目標很明確，但實現目標所需的方法並不明確。

研究組表明，以適度偏離統一碳定價為指導的國際金融轉移戰略可以實現該目標，且不會給各國的經濟或主權帶來壓力。研究組使用綜合評估模型REMIND-MAgPIE來分析替代政策：統一碳定價系統中的金融轉移，不存在金融轉移情況下的差別化碳定價，或將金融轉移和差別化碳定價相結合的混合方案。

在統一碳價下，未來80年到2100年期間，需要4.4萬億美元的國際金融轉移現值來平衡努力。相比之下，在不進行金融轉移的情況下實現同等努力，需要發達國家的碳價格比發展中國家高100倍，從而導致2.6萬億美元的能效損失。混合解決方案顯示出成本效率與主權之間的強烈非線性權衡：適度偏離統一碳價會在能效損失相對較小的情況下大大減少金融轉移，而適度金融轉移則會通過縮小碳價差來顯著降低低能效。

研究組還確定了由於市場扭曲而導致的碳價格差異的風險和不利後果，這可能破壞環境可持續性目標。總之，量化碳價格差異的優勢和風險，有助於深入了解氣候和特定部門的政策組合。

▲ Abstract

The Paris Agreement calls for a cooperative response with the aim of limiting global warming to well below two degrees Celsius above pre-industrial levels while reaffirming the principles of equity and common, but differentiated responsibilities and capabilities. Although the goal is clear, the approach required to achieve it is not. Here we show that a strategy of international financial transfers guided by moderate deviations from uniform carbon pricing could achieve the goal without straining either the economies or sovereignty of nations. We use the integrated assessment model REMIND-MAgPIE to analyse alternative policies: financial transfers in uniform carbon pricing systems, differentiated carbon pricing in the absence of financial transfers, or a hybrid combining financial transfers and differentiated carbon prices. Under uniform carbon prices, a present value of international financial transfers of 4.4 trillion US dollars over the next 80 years to 2100 would be required to equalize effort. By contrast, achieving equal effort without financial transfers requires carbon prices in advanced countries to exceed those in developing countries by a factor of more than 100, leading to efficiency losses of 2.6 trillion US dollars. Hybrid solutions reveal a strongly nonlinear trade-off between cost efficiency and sovereignty: moderate deviations from uniform carbon prices strongly reduce financial transfers at relatively small efficiency losses and moderate financial transfers substantially reduce inefficiencies by narrowing the carbon price spread. We also identify risks and adverse consequences of carbon price differentiation due to market distortions that can undermine environmental sustainability targets. Quantifying the advantages and risks of carbon price differentiation provides insight into climate and sector-specific policy mixes.