The Most Precise Measurement of Antimatter Yet Deepens the Mystery of Why We Exist

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The Most Precise Measurement of Antimatter Yet Deepens the Mystery of Why We Exist

By Aylin Woodward, Live Science Contributor | April 4, 2018 01:00pm ET

                               
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One of the biggest questions that keep physicists up at night is why there is more matter than antimatter in the universe.

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Scientists have made the most precise measurement of antimatter yet, and the results only deepen the mystery of why life, the universe, and everything in it exists.

The new measurements show that, to an incredibly high degree of precision, antimatter and matter behave identically.

Yet those new measurements can't answer one of the biggest questions in physics: Why, if equal parts matter and antimatter were formed during the Big Bang, is our universe today made up of matter?

Universe in balance

Our universe is predicated on the balance of opposites. For every type of "normal" particle, made of matter, there is a conjugate antiparticle of the same mass that has the opposite electric charge produced at the same time. Electrons have opposing antielectrons, or positrons; protons have antiprotons; and so on. [The 18 Biggest Unsolved Mysteries in Physics]

When matter and antimatter particles meet, however, they annihilate each other, leaving only leftover energy behind. Physicists posit that there should have been equal amounts of matter and antimatter created by the Big Bang, and each would have ensured the other's mutual destruction, leaving a baby universe bereft of life's building blocks (or anything, really). Yet here we are, in a universe made up almost wholly of matter.

But here's the kicker: We don't know of any primordial antimatter that made it out of the Big Bang. So why — if antimatter and matter behave the same way — did one type of matter survive the Big Bang and the other did not?

One of the best ways to answer that question is to measure the fundamental properties of matter and its antimatter conjugates as precisely as possible and compare those results, said Stefan Ulmer, a physicist at Riken in Wako, Japan, who was not involved in the new research. If there's a slight deviation between matter properties and correlated antimatter properties, that could be the first clue to solving physics' biggest whodunit. (In 2017, scientists found some slight differences in the way some matter antimatter partners behave, but the results weren't statistically strong enough to count as a discovery.)

But if scientists want to manipulate antimatter, they have to painstakingly make it. In recent years, some physicists have taken to studying antihydrogen, or hydrogen's antimatter counterpart, because hydrogen is "oneof the things we understand best in the universe," study co-author Jeffrey Hangst, a physicist at Aarhus University in Denmark, told Live Science. Making antihydrogen typically involves mixing 90,000 antiprotons with 3 million positrons to produce 50,000 antihydrogen atoms, only 20 of which are caught with magnets in an 11-inch-long (28 centimeters) cylindrical tube for further study.

Now, in a new study published today (April 4) in the journal Nature, Hangst's team has achieved an unprecedented standard: They've taken the most precise measurement of antihydrogen — or any type of antimatter at all — to date. In 15,000 atoms of antihydrogen (think doing that aforementioned mixing process some 750 times), they studied the frequency of light the atoms emit or absorb when they jump from a lower energy state to a higher one. [Beyond Higgs: 5 Elusive Particles That May Lurk in the Universe]

The researchers' measurements showed that antihydrogen atoms' energy levels, and the amount of light absorbed, agreed with their hydrogen counterparts, with a precision of 2 parts per trillion, dramatically improving upon the previous measurement precision on the order of parts per billion.

"It's very rare that experimentalists manage to increase precision by factor of 100," Ulmer told Live Science. He thinks that, if Hangst's team continues the work for an additional 10 to 20 years, they will be able to increase their level of hydrogen spectroscopy precision by a further factor of 1,000.

For Hangst — the spokesperson for the ALPHA collaboration at the European Organization for Nuclear Research (CERN), which produced these results — this achievement was decades in the making.

Trapping and holding antimatter was a major feat, Hangst said.

"Twenty years ago, people thought this would never happen," he said. "It's an experimental tour de force to be able to do this at all."

The new results are very impressive, Michael Doser, a physicist at CERN who was not involved in the work, told Live Science in an email.

"The number of trapped atoms for this measurement (15,000) is a huge improvement on [Hangst's group's] own records of only a few years ago," Doser said.

So what does the most precise measurement of antimatter even tell us? Well, unfortunately, not much more than we already knew. As expected, hydrogen and antihydrogen ­— matter and antimatter — behave identically. Now, we just know that they're identical at a measurement of parts per trillion. However, Ulmer said the 2-parts-per-trillion measurement does not rule out the possibility that something is deviating between the two types of matter at an even greater level of precision that has thus far defied measurement.

As for Hangst, he's less concerned with answering the question of why our universe of matter exists as it does without antimatter — what he calls "the elephant in the room." Instead, he and his group want to focus on making even more precise measurements, and exploring how antimatter reacts with gravity — does it fall down like normal matter, or could it fall up?

And Hangst thinks that mystery could be solved before the end of 2018, when CERN will shut down for two years for upgrades. "We have other tricks up our sleeve," he said. "Stay tuned."

Original article on Live Science.

对反物质最精确测量，却发现宇宙的存在更加神秘
Original
2018-04-06
App：博科园

科学家们已经对反物质进行了最精确的测量，结果只会加深关于生命、宇宙以及它存在的一切的神秘。新测量结果表明，在相当高的精确度上，反物质和物质的行为是相同的。然而这些新的测量方法不能回答物理学中一个最大的问题：为什么如果在宇宙大爆炸时产生了等量的物质和反物质，那么我们今天的宇宙是由物质组成的吗？宇宙是建立在对称平衡之上的，对于每一种由物质构成的“正常”粒子，都有一个共轭的反粒子，它与同时产生相反的电荷，电子有相反的反电子或正电子，质子反质子等等。

                               
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让物理学家夜不能寐最大问题之一就是为什么宇宙中有比反物质更多的（正）物质。图片版权：Shutterstock

当物质和反物质粒子相遇时会相互湮灭只留下能量。物理学家认为宇宙大爆炸产生的物质和反物质应该是等量的，而且每一种物质都能保证对方的相互毁灭，让一个婴儿宇宙失去生命的积木。然而在一个几乎完全由物质组成的宇宙中是这样。但问题是：不知道有什么原始反物质在宇宙大爆炸中产生。那么如果反物质和物质的行为是一样的，为什么一种物质能在大爆炸中幸存，而另一种物质却没有呢？

日本Wako的Riken的物理学家Stefan Ulmer说：一个最好的回答这个问题的方法是测量物质的基本性质和它的反物质共轭，尽可能精确地比较这些结果。如果物质属性和反物质属性之间存在细微的偏差，这可能是解决物理学最大“whodunit”的第一个线索（2017年科学家们发现一些物质的反物质行为方式存在一些细微的差异，但统计结果并不足以算作一项发现）。但如果科学家想要操纵反物质就得煞费苦心地制造反物质。

该研究的共同作者丹麦奥尔赫斯大学的物理学家Jeffrey Hangs说：最近几年一些物理学家已经开始研究反氢或氢的反物质，因为氢是“我们在宇宙中最了解的东西之一”。制造反氢原子通常需要将9万个反质子与300万个正电子混合，从而产生5万个反氢原子，其中只有20个能被捕获，在一个11英寸长的圆柱形管中进行进一步研究。现在发表在《自然》杂志上的一项新研究中，Hangst的团队达到了一个前所未有的标准：已经对反氢原子或任何类型的反物质进行了最精确的测量。在15000个反氢原子的原子中(想想前面提到的混合过程大约750次)，他们研究了原子从低能级跃迁到高能级时所发射或吸收的光的频率。

研究人员测量结果显示，反氢原子能量水平，以及吸收光量与它们的氢原子一致，其精度为每万亿分之2，大大提高了之前测量精度的每十亿分之一。很罕见的是，实验者设法提高100倍的精确度，如果Hangst的团队继续进行10到20年的工作，他们将能够将氢谱精度提高1000倍。对于Hangst——欧洲核子研究组织(CERN)的ALPHA合作的发言人来说，这一成果是几十年来的成果。Hangst说：捕获和持有反物质是一个重大壮举。20年前人们认为这永远不会发生，这是一个实验性的杰作，现在能够做到这一点。

欧洲核子研究中心的物理学家迈克尔·多瑟(Michael Doser)说：新的研究结果令人印象深刻。这种测量(15000)被捕获的原子数量是(Hangst的)几年前自己记录的一个巨大改进。那么，对反物质最精确的测量究竟是什么呢？不幸的是知道的不多。氢和反氢原子——物质和反物质——行为是一样的。现在只知道它们在每万亿分之一测量中是相同的。

然而Ulmer表示，每万亿次的测量并不排除某些东西在更大程度上偏离测量精度的可能性。团队想要集中精力做更精确的测量，探索反物质与重力的反应——它像正常物质一样下降，还是会怎样？Hangst认为这个谜案可以在2018年底之前解决，届时CERN将关闭两年进行升级，拭目以待吧。

博科园-科学科普｜文 / Aylin Woodward / Live Science