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Surprise from the quark sea reveals deeper complexity in the proton spin puzzle



The Proton Spin Puzzle: Scientists want to know how different constituents of the proton contribute to its rotation, a fundamental property that plays a role in how these building blocks lead to almost all visible matter in the universe. Pieces of the puzzle make up the orbital angular impulse of quarks and glones (top left), the Gluon spin (top right) and the Quark and antiquark spin (bottom). The latest RHIC data reveal that the contribution of antiquaries is more complex than previously thought. Credit: National Laboratory Brookhaven

New data from the STAR experiment in the relativistic heavy ion collider (RHIC) adds detail and complexity – to an intriguing puzzle that scientists are trying to solve: how building blocks that make up a proton contribute to its spinning. The results, just published as a quick communication in the magazine Physical examination D., for the first time reveal that the various "flavors" of antiquaries contribute differently to the full proton spin – and in a way that is contrary to the relative abundance of these flavors.


"This measurement shows that the quark piece of the proton spin trap is made of several pieces," said James Drachenberg, STAR's deputy spokesman at Abilene Christian University. "It is not a boring puzzle, it is not evenly divided. It has a more complicated image and this result gives us a glimpse of what the image looks like."

It's not the first time that scientists have changed the proton spin. In the 1980s, when an experiment at the European Center for Nuclear Research (CERN) found that the sum of quarks and antiquark spins within a proton could respond, at best, to a quarter of the overall spin. RHIC, an American nuclear physics office at the Nuclear Physics Science Office at the Brookhaven National Laboratory, was built partly, so scientists were able to measure the contributions of other components, including antiquaries and gluons (which "glued" collectively or bind quarks and antiquarks to form particles such as protons and neutrons).

Antiquaries have only transient existence. They are formed as a pair of quark-antiquark when the deafons divide.

"We call these pairs the Quark Sea," Dracenberg said. "At any moment, you have quarks, glones and the sea of ​​quark-antiquark pairs that contribute to some way of describing the proton. We understand the role that these marine quarks play in some aspects, but not in terms of spin."

Exploring the taste in the sea

One key point is whether different "flavors" of marine quarks contribute to a different turn.

This STAR detector model displays the main components of the detector used in this result. The electrons of W-boson decays (or positons of W + decay) are tracked inside the magnetic field using the Time Projection Chamber (TPC). The magnetic field causes negative and positive particles to curve in opposite ways, enabling scientists to identify who is who. The electromagnetic barrel calorimeter (BEMC) measures the energy of the particles resulting from collisions normally from collision beams, while the electromagnetic calendar of the end (EEMC) does the same for particles appearing in the advanced direction. This image shows a simulated electronic track (red) indicating a large localized accumulation of energy in BEMC (also red). Credit: T. Sakuma

The beans come in six flavors – up and down sorts that make up protons and neutrons from ordinary visible matter, and four other more exotic species. Separation gluons can produce up to quark / antiquark pairs, down quark / antiquark pairs – and sometimes even more exotic quark / antiquark pairs.

"There is no reason why the gloucine would like to split on one or the other of these flavors," said Ernst Sichterman, an associate of STAR from the Lawrence Berkeley National Laboratory (DON) Lawrence Berkeley (LBNL), who played a leading role in marine research quarks. "We were expecting an equal number [of up and down pairs], but that's not what we see. "Measurements at Laboratory Labs for CERN and DOE labs have repeatedly discovered more antecedents than antimicrobials.

"Because this is a surprise – asymmetry in the abundance of these two tastes – we thought it could also be a surprise in their turning role," said Dracenberg. Indeed, previous RHIC results indicate that there may be a difference in how the two flavors contribute to turning, prompting the STAR team to do more experiments.

Achieving goals on a spin

This result is an accumulation of data from the 20-year RHIC sleep program. It is the final result of one of the two starting pillars that motivates the spin program at the dawn of RHIC.

For all these experiments, STAR analyzed the results of polarized proton collisions in collisions with RHIC, where the entire direction of rotation of the two RHIC protists was aligned in certain ways. Looking for differences in the number of specific particles produced when the direction of centrifugation of a polarized proton beam is turned over can be used to monitor the spin alignment of various constituents – and thus their contribution to the full proton spin.

For the measurements of marine quarks, STAR physicists have calculated the electron and positron-antimatter versions of electrons that are the same in every way, except that they carry a positive, not a negative electric charge. The electrons and positrons come from the decomposition of particles called W bosons, which also come in negative and positive varieties, depending on whether they contain an up or down antiquark. The difference in the number of electrons produced when the rotation of the proton spin is turned over indicates a difference in W-production and serves as an attitude for measuring the centrifugation of the touch antikvari. Similarly, the difference in positrons arises from the difference in production W + and serves as an input in the role of measuring the spin contribution of a particular antiquark.

The collisions of the polarized protons (input beam to the left) and non-polarized protons (right) result in the production of W bosons (in this case, W-). The RHIC detectors identify the particles emitted as a decay of the W bosons (in this case, electrons, e-) and the angles in which they appear. Colored arrows represent different possible directions that examine how different quark varieties – for example, "to" antiquark (u) and "down" quark (d) – contribute to the proton spin. Credit: National Laboratory Brookhaven

A new detector, added precision

The latest data includes signals captured by STAR's endcap calorimeter, which takes the particles traveling close to the ship back and forth from each collision. With these new data added to particulate data that emerge normally in the collision zone, scientists have reduced the uncertainty in their results. The data show definitely, for the first time, that the spinning of antiquarks gives greater contribution to the entire proton spin of the spins of the lower antiquark.

"This" asymmetry of taste ", as scientists call it, is surprising in itself, but even more in view of the fact that there are more antiques compared to antiquaries," said Kinghua Xu of Shandong University, another scientist who oversaw one of the graduates the analysis was necessary for the newspaper.

As Sichtermann remarked, "If you go back to the original proton spin-puzzle, where we found out that the sum of quarks and antiquark spins only for a fraction of the proton spin, the following questions are what is the contribution of the glion, the contribution of the orbital motion of the quarks and the gluons? But also why is the contribution of the quark so small? Is it because of the quark between the contributions of quark and antiquark? Or is it because of the differences between the various quark's tastes?

"Previous RHIC results have shown that the gluten plays a significant role in the proton spin. This new analysis provides a clear indication that the sea also plays a significant role. It's far more complicated than just the glutenous ones that are shared by any taste you want – and very good reason to look deeper into the sea. "

Bernd Suruwe, a physicist at the University Temple, who helped develop the W boson method and oversaw two graduates whose analysis led to a new publication, agrees. "After years of experimental work by RHIC, this exciting new result provides a much deeper understanding of the quantum fluctuations of quarks and glans in the proton. These are the types of fundamental issues that attract young minds – students who will continue to spread the boundaries of our knowledge."

Additional STAR measurements can offer insight into the spin contribution of the exotic pieces of quark / antiquark. In addition, US scientists hope to persist deeper in the mystery of the proposed future electron-ion collider. This particle accelerator would use electrons to directly test the spin structure of the internal components of the proton, and ultimately solve the proton spin puzzle.


Explore further:
Particle physicists measure the spin-contribution of an antique proton

More information:
J. Adam et al., Measurement of longitudinal spin asymmetries for weak production of bosons in collisions of proton-proton at s = 510 GeV, Physical examination D. (2019). DOI: 10.1103 / PhysRevD.99.051102

Reference in the newspaper:
Physical examination D.

Provided by:
National Laboratory Brookhaven


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