Брукхэйвений үндэсний лабораторийн эрдэмтэд алтан цөмийг гэрлийн хурдаар мөргөлдүүлснээр орчлон ертөнцийн эхэн үед оршин байсан кварк-глюоны плазмын төлөв байдлын өвөрмөц хэв шинжийг олж илрүүллээ.
RHIC хурдасгуурын STAR туршилтын хүрээнд эрдэмтэд нэг тэрбум орчим алтан ионы мөргөлдөөнийг судалж, цөмийн материйн төлөв байдлын тэгшитгэлийг тодорхойлохыг зорьжээ. Энэхүү судалгаа нь материйн температур, даралт, нягтралын харилцан хамаарлыг ойлгох “дүрэм”-ийг гаргахад чиглэдэг. Туршилтын явцад эрдэмтэд 3-аас 7.7 GeV хүртэлх энергийн түвшинд бөөмс хэрхэн харилцан үйлчлэлцэж байгааг хэмжсэн байна.
Судалгааны явцад мөргөлдөөний энергийн түвшин өөрчлөгдөхөд бөөмсүүдийн харилцан хамаарал жигд бус өөрчлөгдөж, тодорхой хэсэгт огцом уналт үүсэж байгааг ажиглажээ. Энэхүү үзэгдэл нь статистикийн хувьд таван сигма буюу өндөр ач холбогдолтой үр дүн гэж үзэж байгаа ч одоогийн ашиглаж буй компьютерийн загварчлалуудаар бүрэн тайлбарлагдахгүй байна.
Охайо мужийн их сургуулийн судлаач Рутик Маникандхан болон бусад эрдэмтэд энэхүү үр дүнг цөмийн материйн “критик цэг”-ийн анхны шинж тэмдэг байж болзошгүй гэж таамаглаж байгаа юм. Гэсэн хэдий ч энэ нь батлагдсан баримт биш бөгөөд өөр бусад хүчин зүйлс нөлөөлсөн байх магадлалтай тул цаашид протоны тооны хэлбэлзэл зэрэг бусад хэмжилтүүдтэй харьцуулан судлах шаардлагатай байна.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
A particle collider experiment at Brookhaven National Laboratory has uncovered an unusual pattern in matter created under conditions similar to those that existed just after the Big Bang. The signal, spotted by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC).
The experiment involves smashing gold nuclei together at almost the speed of light and studying the particles that fly out afterward. In doing so, physicists can briefly recreate a state of matter known as quark-gluon plasma, which is thought to have filled the universe during its first few microseconds.
What scientists want to understand is how this hot, dense matter changed as the universe cooled. One way to do that is by mapping the equation of state of nuclear matter, which links properties such as temperature, pressure and density. According to study co-author Rutik Manikandhan of The Ohio State University, it is basically a “rulebook” for how matter behaves under some of the most extreme conditions in nature.
Scientists Probe for a Critical Point
Protons and neutrons are built from quarks, which are held together by gluons through the strong force. Under extreme heat or pressure, those particles can break free from their usual arrangement and form quark-gluon plasma.
Physicists have long suspected that nuclear matter may have a critical point, a special set of conditions where the way matter changes from one state to another shifts.
The idea is a bit similar to water. At certain conditions, the boundary between liquid water and steam disappears. For nuclear matter, researchers think there may be a comparable point separating a gradual transition from a much sharper one. Manikandhan stressed that this remains uncertain.
“All of this is still conjectured and there is nothing concrete yet, either from the experimentalists or theorists,” he said.
Some recent calculations suggest that this critical point could sit within the range explored by RHIC’s lower-energy collisions, making these measurements especially interesting.
One Billion Collisions Reveal An Unexpected Dip
The STAR team analyzed around 1 billion gold-ion collisions at energies between 3 and 7.7 GeV per pair of colliding protons or neutrons. RHIC can reach much higher energies, up to 200 GeV.
For the lowest-energy runs, researchers used a fixed-target setup. Instead of firing two beams at each other, a beam of gold nuclei hit a thin gold foil placed inside the detector. According to Manikandhan, this setup creates the densest matter RHIC can currently produce.
The study, available on Physical Review Letters, then measured transverse momentum, essentially how strongly charged particles were kicked sideways after each collision. They also looked at how particles from the same collision behaved together. If many of them receive a stronger or weaker push at the same time, that can reveal changes in the fireball’s temperature and expansion.
“Those correlations reflect how much the temperature and the flow of the fireball fluctuate,” Manikandhan said.
What caught the researchers’ attention was that the pattern did not change smoothly with collision energy. Instead, in the most head-on collisions, the correlations dropped and then rose again, forming a clear dip.
The Signal Is Strong, But The Mystery Is Not Solved
The dip reached a statistical significance of five sigma. If the underlying trend were actually smooth, random fluctuations would produce a deviation this large only about once in 3.5 million attempts.
As reported by the researchers, a commonly used computer simulation managed to reproduce the overall behavior of the collisions but not the dip itself. That model does not contain a critical point. The same feature appeared only weakly in off-center collisions, where the signal was not strong enough to count as evidence on its own. That is why the team is staying cautious.
“The result is suggestive, not proof of a critical point,” he said. Other effects could still influence the fluctuations seen in the data.
The next step is to use these correlations to estimate the specific heat of the hot matter and compare it with supercomputer calculations. Researchers also plan to study the result alongside other measurements, including fluctuations in the number of protons produced. As Manikandhan put it:
“Only when different measurements agree can we say confidently whether a critical point exists.”
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