Protons have more quantum innards than we previously knew, transforming our view of these particles that sit at the heart of every atom.
Researchers have long known that protons are made up of smaller particles called quarks and gluons, but the details of their behaviour and distribution have been challenging to pin down. Yet these details really matter 鈥� how well we understand the proton鈥檚 insides affects how well we can interpret experiments in which they are smashed, which aim to search for new particles or uncover the secrets of fundamental forces.
Physicists鈥� current best mathematical tool for understanding the proton鈥檚 insides is the parton model. This allows them to calculate the density of particles inside the proton probabilistically, pinpointing what its make-up is most likely to be like in accelerator experiments, for instance.
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鈥淭he parton model is extremely powerful: it鈥檚 the main source of all our knowledge about the internal structure of the proton. But if you look at the modern state of it, you realise the picture behind it is pretty classical,鈥� says at the Complutense University of Madrid. 鈥淭his is, of course, false because we know quantum mechanics exists.鈥�
Researchers have theorised about and looked for quantum effects within protons since the 1980s, says Vladimirov, but over the years, different experiments led to unclear and, at times, contradictory conclusions.
To get a clearer picture, he and his colleagues zeroed in on one process that quarks and gluons could be undergoing that is a hallmark of quantumness: interference. Quantum particles have a wave-like nature and interference is equivalent to two quantum matter waves overlapping and amplifying each other in some places, but cancelling each other out in others. Mathematically, where classically two probabilities would simply add, when there is interference, combining them produces more mathematical terms than just a sum.
The researchers devised a way to extract mathematical signatures of quantum interference from a combination of more than a dozen very varied datasets. For example, some data came from the CERN particle physics laboratory near Geneva, Switzerland, while some was collected at German national laboratory聽DESY across rather different kinds of experiments, none of which was designed to look for interference.
鈥淎ll previous investigations failed because people tried to look at just one source [of data],鈥� says Vladimirov. While it was a big technical challenge to put all the data together, it paid off in new clarity. The researchers could identify specific instances of quantum interference within a proton, such as a gluon interfering with a combination of a quark and an antiquark, which is its antimatter companion.
Vladimirov says this underlines how physicists ought to be more keenly aware of the proton鈥檚 quantumness. 鈥淚t鈥檚 commonly accepted that the proton is almost like a classical bag of particles, and the fact that we demonstrate that it is not so means that it鈥檚 more quantum than is usually thought,鈥� he says.
鈥淭he value of this study is that it collects a lot of information from different sources and says: 鈥楲ook, now we see definitely that these quantum mechanical effects, these small effects due to quantum interference, are present in physical processes鈥�,鈥� says at the University of Regensburg in Germany.
This is the first time that an analysis of this scale has been completed and it ought to motivate further experiments, says Braun. For example, future experiments at facilities such as the Thomas Jefferson National Accelerator Facility in Virginia could find further signatures of quantum interference and connect them to other effects that happen in fast particle collisions.
Although he never doubted the proton鈥檚 quantumness, Braun was still surprised by the details of the interference effects seen most clearly in the new work. 鈥淚 was working on this kind of physics for 40 years, and I expected that the big effects will be in different place,鈥� he says.
Going forward, the team wants to expand its dataset and analysis, and further increase the certainty of its findings. There is also the big question of how to interpret the findings and tell a more complete story of just what the proton鈥檚 insides are actually like. For example, the interference effect is related to quarks and gluons鈥� quantum mechanical spin, which has so far not been understood or measured very precisely.
Physicists have also yet to resolve how interference connects to forces that act on quarks and gluons, says Vladimirov. Notably, the strong nuclear force that binds quarks together is one of the four fundamental forces of nature, so understanding this would give us a better grasp of the most basic laws of our universe.
Physical Review Letters
