内容来源：https://www.quantamagazine.org/experiments-ring-the-death-knell-for-sterile-neutrinos-20260408/

内容总结：

中微子研究重大转折：“惰性中微子”假说遭遇实验“丧钟”

长久以来，中微子——这种几乎不与物质相互作用的“幽灵粒子”——以其微小质量和诸多未解之谜，深刻塑造了无数物理学家的职业生涯。过去十多年间，多项实验观测到的异常现象，似乎共同指向一种理论上存在但尚未被发现的“惰性中微子”。然而，近期一系列高精度实验的否定性结果，为这一备受期待的假说敲响了“丧钟”。

异常现象曾引发希望
20世纪末，物理学家意外发现中微子具有微小质量，并能在三种已知类型（电子中微子、μ子中微子、τ子中微子）之间相互转换（振荡），这直接挑战了粒子物理的“标准模型”。为解释其质量来源，理论物理学家提出了“惰性中微子”的假说。这种假想粒子几乎不与任何已知力发生作用，极难探测。

与此同时，多个独立实验接连报告了令人费解的异常：洛斯阿拉莫斯国家实验室的LSND实验、费米实验室的Miniboone实验均观测到超出预期的电子中微子；俄罗斯和意大利的“镓实验”则发现探测到的电子中微子比理论值少约20%；2011年，科学家发现基于核反应堆的中微子通量计算存在偏差（反应堆反中微子异常）。这些迹象似乎暗示，可能存在一种质量约为1-2电子伏特的惰性中微子，它能引发中微子在极短距离内快速振荡，从而解释上述所有异常。

精密实验给出否定答案
为寻找这一假想粒子，全球科学家开展了长达十余年的精密搜寻。德国卡尔斯鲁厄氚中微子实验（Katrin）利用巨型探测器，以前所未有的精度测量中微子质量，并搜寻惰性中微子可能带走额外能量的迹象。2025年12月公布的分析结果显示，未发现质量约1电子伏特的惰性中微子存在的证据。该实验共同负责人、物理学家蒂埃里·拉塞尔表示，这一结果“与用惰性中微子解释反应堆异常的想法不一致”。

在美国费米实验室，升级后的MicroBooNE实验利用能拍摄“亚原子烟火”的下一代技术，对Miniboone的异常进行复核。该实验同样未发现电子伏特质量范围的惰性中微子存在的任何痕迹。项目物理学家马克·罗斯-朗根坦言，这些结果“在我看来，是为惰性中微子敲响了丧钟”。

谜团未解，探索不止
尽管单一电子伏特惰性中微子的简洁理论已被实验数据否定，但诸多异常现象本身并未消失。反应堆异常可能源于理论预期的不确定性，但LSND、Miniboone和镓实验的异常信号依然显著且尚未得到解释。

物理学家们正在开辟新的探索方向。一种可能是，这些异常由更复杂的粒子家族（如多个质量不同的惰性中微子）导致；另一种可能是，它们源于尚未被理解的实验误差或巧合。未来十年，中国江门中微子实验（JUNO）、美国深层地下中微子实验（DUNE）以及麻省理工学院领导的Isodar实验等新一代研究，将获得海量新数据，有望更清晰地揭示中微子世界的奥秘。

尽管简洁的答案暂时落空，但中微子拥有质量这一事实本身，已确凿地指向了标准模型之外的新物理。追寻这些“幽灵粒子”秘密的旅程，依然激励着科学家在不确定性中持续求索。正如爱荷华大学物理学家马修斯·霍斯特特所言：“这正是这个领域非常激动人心的时刻，尤其对我们这些理论物理学家来说，得以对所有数据提出尖锐的问题。”

中文翻译：

实验敲响惰性中微子“丧钟”

中微子堪称粒子界最“与世无争”的存在。它们几乎没有质量，不带电荷，也没有“色荷”。因此中微子与自然界大多数作用力绝缘，能穿透整颗行星甚至恒星而不与任何原子碰撞。

但正是这种粒子，却屡屡改写科学家的人生轨迹。

上世纪90年代末，当物理学家意外发现中微子具有质量时，蒂埃里·拉塞尔毅然放弃宇宙学研究投身粒子领域。“这发现太激动人心，我根本无法抗拒。”现就职于德国海德堡马克斯·普朗克核物理研究所的拉塞尔回忆道。马克·罗斯-朗尼根原本立志成为气象学家，直到2010年与粒子物理的偶然邂逅改变了他的方向。与全球数千名研究者一样，他们将职业生涯奉献给了这个微小而近乎完全惰性的粒子。

十余年来，研究似乎即将迎来突破。实验反复记录到中微子诡异地出现与消失。这些异常现象与中微子的神秘质量，共同指向一种可能的解释：某种特定质量的“惰性”中微子或许正隐匿在已知物理图景之后。

研究人员耗费多年开展日益精密的实验，试图捕捉这个神秘粒子。然而随着零结果不断累积——尤其是2025年末发表的多项研究——如今多数物理学家认为这种惰性中微子并不存在。“在我看来，这敲响了惰性中微子的丧钟。”哥伦比亚大学物理学家罗斯-朗尼根表示，他正是近期关键研究的合著者之一。

这些进展反而加深了中微子的谜团。它们在部分实验中显现、又在其他实验中消失的能力依然无解。而中微子具有质量这一事实，本质上意味着它们必须与某种未知的物理现实产生联系。对物理学家而言，这种粒子的影响力仍在持续发酵。

“我们需要学会创造性地思考。”爱荷华大学物理学家马修斯·霍斯特特指出，“对这个领域而言，现在正是激动人心的时刻，尤其对我们这些需要从海量数据中提出尖锐问题的理论研究者。”

消失的魔术

物理学家对中微子的所有认知，都源自那些难以自洽的实验。“整个领域建立在异常现象的基石之上。”罗斯-朗尼根坦言。

1930年，沃尔夫冈·泡利通过研究放射性衰变首次推断出中微子的存在。在这类衰变中，元素原子转化为另一种元素，并以电子形式释放剩余能量。但在某些衰变中，电子携带的能量总是不足。泡利认为，必定存在某种不可见的粒子悄悄带走了剩余能量。他将这种粒子称为“微小的中性粒子”，推测其不带电荷且没有质量，仅通过弱力与物质世界相互作用——正是这种让亚原子粒子相互转化的力量，使得放射性衰变得以发生。

然而弱力作用极其微弱，中微子即便穿越数光年厚的铅墙也不会扰动任何原子。泡利曾以一箱香槟打赌人类永远无法探测到它。但约二十年后，天才的实验物理学家在南卡罗来纳州萨凡纳河核电站捕捉到了中微子的确凿踪迹。

此后不久，物理学家开始思考如何从这些弱力作用的“隐形信使”中获取新知，将目光从人工核反应堆转向天然核聚变源——太阳。

上世纪60年代末，小雷蒙德·戴维斯在近一英里深的地下矿井中监造了一个可容纳10万加仑干洗液的水箱，用以研究太阳中微子。实验联合负责人约翰·巴考尔计算出预期探测到的中微子数量，但实际探测值仅为预测值的三分之一。要么太阳活动未达预期，要么中微子在半途消失了。

这个谜题悬置了三十年。直到日本超级神冈探测器与加拿大萨德伯里中微子天文台揭晓答案，才引发物理学界震动。

中微子消失是因为它们会变身。中微子共有电子中微子、μ子中微子、τ子中微子三种类型。超级神冈与萨德伯里的实验表明，戴维斯实验能探测到的那类中微子，正在“振荡”转化为实验无法探测的其他类型。这一发现彻底颠覆了认知——根据描述所有已知粒子行为的“标准模型”，中微子振荡本不可能发生。振荡现象的前提是三种中微子质量各不相同，但当时理论认为所有中微子质量完全一致：都为零。

这是因为标准模型将粒子描述为穿越空间量子场的涟漪，而有质量粒子则是穿越两个场的重叠涟漪。其中“左旋”场产生倾向于某一方向自旋的粒子，“右旋”场产生反向自旋的粒子。以电子为例，其质量正源于左旋涟漪与右旋涟漪的耦合。但实验物理学家此前只观测到左旋中微子，故认定其无质量——直到超级神冈与萨德伯里的实验推翻了这个结论。

于是二十世纪的异常现象最终汇聚成至今未解的悬念：中微子为何具有质量？

多重谜题，一种解释

最简单的解释是存在第四种中微子，即右旋场的涟漪，这种粒子对现有实验几乎完全隐形。

弱力有个奇特性质：它只作用于左旋场，核衰变后仅出现左旋中微子。因此右旋中微子完全无法感知标准模型中的任何作用力，科学家称之为“惰性中微子”。

另一种可能是左旋中微子实际兼具微弱右旋特性，从而能自我赋予质量。但这个设想会从另一角度打破标准模型，最简单的修补方案就是引入另一种微幅双旋（但主要呈右旋）的惰性中微子。

两条最简单的理论路径殊途同归。“理论直觉告诉我，这简直是完美风暴，惰性中微子必然存在于某处。”罗斯-朗尼根说道。

世纪之交，新一代实验发现了新一代异常现象。几乎所有这些异常都可解读为某种特定惰性中微子存在的迹象。

1993至1998年间，洛斯阿拉莫斯国家实验室的液体闪烁体中微子探测器在μ子中微子束流中观测到超量的电子中微子。后来费米实验室的迷你助推器中微子实验也发现同样现象。LSND/迷你助推器异常由此诞生。

同在1990年代，俄罗斯和意大利物理学家将强放射源紧贴盛有液态金属镓的大桶——这种材料对中微子异常敏感——以检验探测器性能。设备运转正常，但探测到的电子中微子数量却低了约20%，形成“镓异常”。2022年一项更精密的实验为此提供了新证据。

2011年，物理学家发现他们一直低估了核反应产生的电子中微子数量，误差达几个百分点。这意味着过去数十年间，每当在核反应堆外放置各类探测器并计数“正确”数量的中微子时，实际探测值其实始终不足。这个差异被称为“反应堆反中微子异常”。

这三类迹象都指向中微子振荡——中微子再次上演出现与消失的戏码。但这次振荡并非像日地间数百万英里距离上那般缓慢发生，中微子似乎能在穿越房间的瞬间完成快速变身。

中微子振荡速度取决于质量差异。三种已知中微子因质量近乎相同（都接近零），其振荡主要发生在数英里距离上。而米级距离的振荡现象，则暗示可能存在第四种质量更大的中微子——恰如理论家为解释中微子质量最初设想的右旋变体。具体而言，质量为一到两个电子伏特（质量能量单位）的惰性中微子似乎能完美串联所有线索。

“这些异常现象的证据类型迥异，却都能被同一种惰性中微子解释。”拉塞尔说道。

全球科学家展开了地毯式搜索：南极冰盖之下、核反应堆旁、矿井深处。2007年，德国物理学家将200吨飞艇形探测器经地中海绕道运至卡尔斯鲁厄氚中微子实验基地。费米实验室的物理学家则升级了迷你助推器探测器，启动了名为“微助推器”的新实验。

最新一批结果已然揭晓。搜寻行动无功而返，物理学家们陷入沉思：下一步该何去何从？

中微子之死

2000年，当振荡现象证实中微子具有质量时，拉塞尔刚完成宇宙学博士论文。深受吸引的他投身中微子研究，并于2011年协助发现反应堆异常。数年后他加入卡尔斯鲁厄实验团队，加入搜寻惰性中微子的行列。

该实验利用房屋大小的探测器捕捉氚原子衰变释放的电子。追随泡利的足迹，科学家仔细记录电子能量分布，精确测算生成伴生中微子所需的额外能量。

实验主要目标是推导产生中微子所需的最小能量——即其静止质量。2025年4月，在分析数亿个电子后，合作组宣布中微子质量不超过0.5电子伏特。（作为对比，普通电子质量约为50万电子伏特。）

“这也是搜寻惰性中微子的完美工具。”拉塞尔指出。若较重惰性中微子存在，有时会从电子处带走额外能量。但在2025年12月发表的分析中，卡尔斯鲁厄团队未发现质量约1电子伏特的惰性中微子迹象。拉塞尔称这是“否定该惰性中微子假说解释反应堆异常的重要一步”。他现在怀疑反应堆异常源于预期中微子数量的计算偏差，许多物理学家也持相同观点。

拉塞尔坦言，虽然发现惰性中微子会令人振奋，但能获得明确结论已值得庆幸。“我很欣慰，因为我们没有得出模棱两可的结果。谁都不愿带着未解之谜离开人世。”

这种释然却与罗斯-朗尼根无缘，他仍在为LSND和迷你助推器之谜苦思冥想。

罗斯-朗尼根分析着微助推器实验的数据。该实验采用能追踪中微子引发亚原子级反应的新一代技术，复核迷你助推器的发现。“我们能拍摄单个原子被击碎的影像，这些画面永远看不腻。”他说道。

微助推器合作组首先统计了出现电子（即电子中微子）的事件，未发现异常。去年他们分析了两束不同来源的中微子，仍未找到电子伏特级惰性中微子的踪迹。

卡尔斯鲁厄实验与微助推器实验的结果，结合其他实验发现及宇宙学调查的强烈暗示，共同传递出清晰信息：物理学家无法用某个巧妙理论解释一切。单粒子电子伏特级惰性中微子假说是错误的。

于是单一谜题裂变为多重谜团。反应堆异常似乎日益与中微子无关，但LSND、迷你助推器和镓实验的异常仍悬而未决。“这些信号的显著性都很强。”麻省理工学院中微子物理学家珍妮特·康拉德指出，“肯定不是电子伏特级惰性中微子。问题在于：那到底是什么？”

新的可能

一种可能是这些异常纯属错误与巧合的不幸叠加。物理学异常现象时有发生，通常可追溯至微妙的系统效应。“我们对异常现象往往持合理怀疑态度。”西北大学理论物理学家安德烈·德古维阿表示，他专攻中微子研究。

但至今无人能构建足以解释迷你助推器异常的错误组合。“人们竭尽全力试图推翻它。”康拉德说道。镓异常同样难以简单解释。

另一种可能是这些异常（单独或共同）确实指向中微子的异常行为，但并非由单一电子伏特级惰性中微子引起。物理学家目前缺乏足够的数据和计算能力来判断：包含两个、三个或更多电子伏特级惰性中微子——抑或质量更大的惰性中微子——的扩展中微子家族，能否解释这些异常现象。

微助推器实验仍有探索空间。未来十年，物理学家将获得海量新数据：中国已投入运行的江门中微子实验、美国费米实验室主导的深地下中微子实验预计2030年代开始采集数据。康拉德正领导名为“伊索达尔”的实验，专门搜寻由任意数量轻惰性中微子引发的快速振荡，预计2028年投入运行。

借助新数据，物理学家有望拼凑出更清晰的中微子世界图景。“以往我们要么获得少量优质数据，要么得到大量低质数据。”德古维阿说，“因此海量优质数据将为我们开启新世界。”

无论异常现象最终如何解释，中微子具有质量这一事实意味着它们直通未知领域。而惰性中微子——如果以某种可探测形式存在——可能仅是冰山一角。物理学家深知标准模型并不完整，例如它无法解释宇宙大部分质量。困难在于如何在已知粒子与作用力的喧嚣中捕捉细微的新现象。爱荷华大学的霍斯特特比喻道：“这好比要在曼哈顿车流轰鸣中分辨空调的微弱嗡鸣。”

但几乎不参与相互作用的中微子，以及更“羞涩”的惰性中微子，“提供了安静得多的倾听环境”。霍斯特特说。尽管他希望当前及未来的实验能捕捉到那微弱讯号，但也清楚成功并无保证。

面对不确定性，有些物理学家选择了坦然接受。“终其一生可能难有重大突破，这确实令人沮丧。”德古维阿坦言。但他认为思考异常现象的可能影响具有启发意义，“说到底，我们投身于此的隐秘动力，不正是为了探索新知吗？”

康拉德则从挑战中汲取能量。她在这个领域起步时，正值四分之一世纪前那场预示中微子质量发现的异常现象迷雾期——又一位被无形粒子改写人生轨迹的物理学家。她感到这个领域此刻与当年同样充满可能。“最艰难的时期往往最有趣。既然选择这个领域，不正是为了迎接挑战吗？”

英文来源：

Experiments Ring the ‘Death Knell’ for Sterile Neutrinos
Introduction
Neutrinos have about as little influence as a particle can have. They have essentially no heft, no electric charge, and no “color” charge. As a result, the neutrino has no connection with most of nature’s forces; it can slip through whole planets and stars without striking a single atom.
But neutrinos have proven more than capable of bending the life path of a scientist.
In the late 1990s, when physicists unexpectedly discovered that neutrinos have mass, Thierry Lasserre abandoned cosmology to go all in on the particles. “It was so exciting I just couldn’t resist,” said Lasserre, now a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. And Mark Ross-Lonergan was planning to be a meteorologist until a chance encounter with particle physics in 2010 inspired him to switch fields. Lassere and Ross-Lonergan, along with thousands of others, have devoted their careers to investigating this tiny and almost perfectly inert speck.
For more than a decade, their investigations seemed to be closing in on a breakthrough. Experiments reported strange acts of neutrinos appearing and disappearing. These results, along with neutrinos’ mysterious mass, all pointed to a single potential explanation: A particular “sterile” type of neutrino, of a particular mass, seemed to lurk undiscovered behind the scenes.
Researchers spent years running increasingly sophisticated experiments to pin down the interloper. However, in the face of an increasing number of null results, most notably in studies published in late 2025, most physicists now agree that this sterile neutrino doesn’t exist. “This is, in my opinion, the death knell for sterile neutrinos,” said Ross-Lonergan, a physicist at Columbia University and co-author of one of the latest studies.
These developments have only deepened the mysteries of neutrinos. Their apparent ability to appear in some experiments and vanish from others remains unexplained. And the fact that they have mass essentially requires them to be in contact with some undiscovered aspect of reality. For physicists, the particle’s influence continues unabated.
“It’s on us to learn how to get creative,” said Matheus Hostert, a physicist at the University of Iowa. “This is a very exciting time for the field, especially for theorists like myself who get to ask hard questions about all this data.”
Disappearing Act
Everything physicists know about neutrinos, they’ve learned through experiments that didn’t quite add up. “The whole field is built on a backbone of anomalies,” Ross-Lonergan said.
Wolfgang Pauli first inferred the presence of the neutrino in 1930 from a study of radioactive decays. In these decays, an atom of one element transforms into another while releasing its remaining energy in the form of an electron. But in certain decays, the electron doesn’t have enough zip. Pauli argued that some additional, invisible particle must be smuggling the leftover energy into the world. This particle, which he called “little neutral one,” would have no electric charge and no mass. It would interact with the atoms of our world only through the weak force, which makes radioactive decay possible by turning certain subatomic particles into others.
The weak force is so weak, however, that a neutrino could travel through light-years of lead without altering a single atom. Pauli bet a case of champagne that no one would ever detect one. But some 20 years later, ingenious experimentalists caught unmistakable signs of neutrinos at the Savannah River Site nuclear power plant in South Carolina.
Soon after, physicists started brainstorming about what they could learn from these nigh-invisible heralds of weak force transformations. They turned their focus from artificial nuclear reactors to a natural one — the sun.
In the late 1960s, Raymond Davis Jr. oversaw the installation of a 100,000-gallon tank of dry-cleaning fluid in a mine nearly a mile underground, where he planned to study solar neutrinos. John Bahcall, the co-leader of the experiment, calculated the number of neutrinos the experiment should see. But the tank picked up just one-third of the number of neutrinos that Bahcall had predicted it should. Either the sun was underperforming expectations, or neutrinos were going missing.
The anomaly took 30 years to resolve. But when the resolution came, via the Super-Kamiokande experiment in Japan and the Sudbury Neutrino Observatory (SNO) in Canada, it delivered a bombshell.
Neutrinos were disappearing because they were changing form. Neutrinos come in three varieties, dubbed electron, muon, and tau. And Super-Kamiokande and SNO showed that neutrinos of one type, visible in the Davis experiment, were “oscillating” into neutrinos of another, which the Davis experiment could not see. This finding was a major twist, because according to the Standard Model — the playbook that accounts for all the known behavior of all the known particles — neutrino oscillation was not allowed. Oscillation could take place only if the masses of the three types were different from one another. But all neutrinos were supposed to have exactly the same mass: none.
That’s because the Standard Model describes a particle as a ripple traveling through a quantum field in space, and a massive particle as two overlapping ripples traveling through two fields. There’s a “left-handed” field generating particles that tend to corkscrew one way, and a “right-handed” field producing particles that tend to corkscrew the other way. The electron, for instance, has mass because it is a left-handed ripple linked with a right-handed ripple. But experimentalists had seen only left-handed neutrinos, so the particle was thought to be massless — until Super-Kamiokande and SNO proved otherwise.
Thus the 20th-century anomalies culminated in a cliffhanger that remains unsolved: Why do neutrinos have mass?
Many Mysteries, One Explanation
One simple explanation is that there is a fourth type of neutrino, a ripple in a right-handed field, one almost perfectly invisible to experiments to date.
A bizarre quirk of the weak force is that it affects only left-handed fields; only left-handed neutrinos show up after nuclear decays. So right-handed neutrinos would be completely barren of anything that would let them feel the forces of the Standard Model. Scientists call them sterile.
Alternatively, the left-handed neutrinos could turn out to be slightly ambidextrous and therefore capable of giving themselves mass. But this idea cracks the Standard Model in a different way, and the simplest patch is to add another slightly ambidextrous, but mostly right-handed, sterile neutrino.
So the two simplest logical paths for explaining neutrino mass led to the same place. “The theorist in me says it’s a perfect storm and clearly sterile neutrinos exist somewhere,” Ross-Lonergan said.
And around of the turn of the century, a new generation of experiments uncovered a new generation of anomalies. Almost all of them could be interpreted as hints that there should be one particular type of sterile neutrino.
From 1993 to 1998, an experiment at Los Alamos National Laboratory called the Liquid Scintillator Neutrino Detector, or LSND, saw what looked like too many electron neutrinos in a beam of mostly muon neutrinos. Later, the Miniboone experiment at Fermilab saw the same thing — way too many electron neutrinos. The LSND/Miniboone anomalies were born.
Also in the 1990s, physicists in Russia and Italy had put highly radioactive sources right next to huge vats of gallium, a metallic liquid that’s especially sensitive to neutrinos, to test whether the vats were working as neutrino detectors. They were, but their counts of electron neutrinos were about 20% too low. This became known as the gallium anomaly. A more refined experiment found further evidence for the gallium anomaly in 2022.
And in 2011, physicists found that they had been underestimating the number of electron neutrinos that should be produced in a nuclear reaction by a few percent. This meant that every time that physicists had plunked down any type of detector outside a nuclear reactor and counted the “right” number of neutrinos in previous decades, in reality there had not been enough. This discrepancy came to be known as the reactor antineutrino anomaly.
All three of these signs pointed toward neutrino oscillation — neutrinos were, again, appearing and disappearing. But the oscillation wasn’t happening slowly over millions of miles between the sun and the Earth. This time, the neutrinos seemed to be changing fast enough to oscillate while crossing a room.
How quickly neutrinos oscillate depends on the difference between the neutrino masses. Oscillations between the three types show up mainly over miles of travel because their masses are all almost the same — nearly zero. But oscillations occurring over meters could be explained by the existence of a fourth, beefier neutrino — one quite like the right-handed variety theorists needed to account for neutrino mass in the first place. Specifically, a sterile neutrino weighing one or two electron volts, a unit of mass and energy, seemed to tie everything together.
“These [anomalies] were very different types of evidence, but they would all be explained by the same kind of sterile neutrino,” Lasserre said.
Scientists searched the world for this neutrino. They hunted it under the ice in Antarctica, next to nuclear reactors, and down in mines. In 2007, German physicists shipped a 200-ton dirigible-shaped detector across the Mediterranean sea on a roundabout odyssey to a lab on the other side of the country — the Karlsruhe Tritium Neutrino Experiment, or Katrin. And Fermilab physicists upgraded Miniboone’s detector, launching a new experiment called Microboone.
The latest batch of results have come in. The hunt has come up short, leaving physicists puzzling over what to do next.
Death of a Neutrino
Back in 2000, when oscillations had proved that neutrinos have mass, Lassere had just completed his Ph.D. thesis in cosmology. Intrigued, he dove into the neutrino world, and in 2011, he helped discover the reactor anomaly. A few years later, he joined Katrin to hunt for sterile neutrinos.
Katrin uses its house-size detector to look for electrons released during radioactive decays of tritium atoms. Following in Pauli’s footsteps, scientists involved in the effort carefully tabulate the electrons’ energies to make stringent measurements of any excess energy that went into making the counterpart neutrinos.
The experiment’s main goal is to deduce the smallest amount of energy required to produce a neutrino — its resting mass. In April 2025, after scrutinizing hundreds of millions of electrons, the collaboration found that the neutrino mass can’t exceed half an electron volt. (An ordinary electron, by contrast, has a mass of around half a million electron volts.)
The experiment is also “a perfect tool” for searching for sterile neutrinos, Lasserre said. If those heavier neutrinos were to exist, they would sometimes pull additional energy away from the electrons. But in an analysis published in December 2025, Katrin scientists saw no sign of a sterile neutrino with a mass of around an electron volt. Lasserre called it “a major step that is inconsistent with this sterile neutrino idea” as an explanation for the reactor anomaly. He now suspects that the reactor anomaly arises from not knowing exactly how many neutrinos to expect, an opinion many physicists share.
While discovering a sterile neutrino would have been thrilling, Lasserre said he feels grateful to at least have a sense of closure. “I am very happy, because we don’t have some ambiguous results,” he said. “I would not want to die and have it be completely open.”
That satisfaction eludes Ross-Lonergan, who continues to puzzle over the LSND and Miniboone mysteries.
Ross-Lonergan analyzes data from Microboone, which checks Miniboone’s work by using next-generation technology capable of tracking the subatomic fireworks neutrinos can produce. “We get to take photos of individual atoms being broken apart,” Ross-Lonergan said. “I never get tired of looking at them.”
First, the Microboone collaboration counted the events where electrons (and therefore electron neutrinos) appeared, but they saw nothing out of the ordinary. Last year, they analyzed neutrinos coming from two different beams but still saw no trace of electron-volt sterile neutrinos.
The Katrin and Microboone results, along with findings from other experiments and strong hints from cosmological surveys, converge to deliver a clear message: Physicists can’t explain everything with one slick idea. The theory of the single electron-volt sterile neutrino is wrong.
So one mystery has fractured into multiple mysteries. The reactor anomaly increasingly seems unrelated to neutrinos. But the other experiments — LSND, Miniboone, and gallium — remain unexplained. “The significance of the signals, they’re all very large,” said Janet Conrad, a neutrino physicist at the Massachusetts Institute of Technology. “It’s not [the electron-volt sterile neutrino] for sure. And so the question is: What else is it?”
Bulking Up
One possibility is that LSND, Miniboone, and gallium are just an unlucky alignment of mistakes and coincidences. Anomalies in physics appear regularly, and physicists can usually trace them back to subtle systemic effects. “We tend to be very skeptical about anomalies, which I think is the healthy thing to do,” said André de Gouvêa, a theoretical physicist at Northwestern University who focuses on neutrinos.
But so far, no one has managed to cook up even a constellation of mistakes that could account for the Miniboone anomaly. “People work really, really hard to try to kill it,” Conrad said. The gallium anomaly remains similarly tough to explain away.
Another possibility is that the anomalies — either individually or collectively — do point to neutrino mischief, but not mischief of the simplest variety, caused by a single electron-volt sterile neutrino. Physicists don’t yet have the data or the computational power to say whether a more extended neutrino family containing two, three, or more electron-volt sterile neutrinos — or heavier sterile neutrinos weighing many electron volts — could help explain LSND, Miniboone, or the gallium anomaly.
Microboone has ground left to cover. And over the next decade, physicists will gain a deluge of data from new research, including a reactor experiment in China called JUNO, which is already operational, and a Fermilab-managed experiment in the United States called DUNE, which should begin taking data in the 2030s. For her part, Conrad is leading an experiment called Isodar, which will look specifically for fast neutrino oscillation caused by any number of light sterile neutrinos. She hopes to have it up and running in 2028.
With this new information, physicists expect to assemble a much clearer picture of the neutrino realm. “We usually get a little bit of good data or a lot of crappy data,” de Gouvêa said. “So lots of good data is a new world for us.”
Whatever happens with the anomalies, the fact that neutrinos have mass means that the particles have a direct line to the unknown. And sterile neutrinos, if they’re out there in some detectable form, could be just the beginning. Physicists know the Standard Model to be incomplete — it’s missing most of the universe’s mass, for instance. It’s just that detecting subtle new stuff among the blaring effects of known particles and forces is tough. Hostert, of the University of Iowa, likens it to picking out the faint hum of an air conditioner over the din of Manhattan traffic.
But the barely interacting neutrino, and the even more bashful sterile neutrino, “offer a much quieter place” to listen, Hostert says. Of course, while he hopes that current and upcoming experiments will pick up on that quiet crackle, he knows they have no guarantee of success.
In the face of this uncertainty, some physicists adopt a stance of resigned acceptance. “It can be frustrating that in your lifetime you may not make a lot of progress,” de Gouvêa said. But thinking about the possible implications of anomalies can be instructive, he said, “and somehow we’re all secretly in it just to learn new stuff.”
Conrad, meanwhile, feels energized by the challenge. She entered the field during the era of confusing anomalies that foreshadowed the discovery of neutrino mass a quarter century ago — another physicist with a life path shaped by the incorporeal particle. And she thinks the field feels just as full of possibility now as it did then. “I think the most interesting times are the hard times,” she said. “I mean, why are you in this field, if you don’t love hard?”