STAR physicists track sequential 'melting' of upsilons
Findings provide evidence for 'deconfinement' and insight into seething temperature of the hottest matter on Earth

Date:
March 14, 2023
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists using the Relativistic Heavy Ion Collider (RHIC) to
study some of the hottest matter ever created in a laboratory have
published their first data showing how three distinct variations
of particles called upsilons sequentially 'melt,' or dissociate,
in the hot goo.


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FULL STORY ========================================================================== Scientists using the Relativistic Heavy Ion Collider (RHIC) to study
some of the hottest matter ever created in a laboratory have published
their first data showing how three distinct variations of particles called upsilons sequentially "melt," or dissociate, in the hot goo. The results,
just published in Physical Review Letters, come from RHIC's STAR detector,
one of two large particle tracking experiments at this U.S. Department of Energy (DOE) Office of Science user facility for nuclear physics research.


==========================================================================
The data on upsilons add further evidence that the quarks and gluons
that make up the hot matter -- which is known as a quark-gluon plasma
(QGP) -- are "deconfined," or free from their ordinary existence locked
inside other particles such as protons and neutrons. The findings will
help scientists learn about the properties of the QGP, including its temperature.

"By measuring the level of upsilon suppression or dissociation we
can infer the properties of the QGP," said Rongrong Ma, a physicist
at DOE's Brookhaven National Laboratory, where RHIC is located, and
Physics Analysis Coordinator for the STAR collaboration. "We can't
tell exactly what the average temperature of the QGP is based solely
on this measurement, but this measurement is an important piece of a
bigger picture. We will put this and other measurements together to get
a clearer understanding of this unique form of matter." Setting quarks
and gluons free Scientists use RHIC, a 2.4-mile-circumference "atom
smasher," to create and study QGP by accelerating and colliding two beams
of gold ions -- atomic nuclei stripped of their electrons -- at very high energies. These energetic smashups can melt the boundaries of the atoms' protons and neutrons liberating the quarks and gluons inside.

One way to confirm that collisions have created QGP is to look for
evidence that the free quarks and gluons are interacting with other
particles. Upsilons, short-lived particles made of a heavy quark-antiquark
pair (bottom-antibottom) bound together, turn out to be ideal particles
for this task.

"The upsilon is a very strongly bounded state; it's hard to dissociate,"
said Zebo Tang, a STAR collaborator from the University of Science
and Technology of China. "But when you put it in a QGP, you have
so many quarks and gluons surrounding both the quark and antiquark,
that all those surrounding interactions compete with the upsilon's own quark-antiquark interaction." These "screening" interactions can break
the upsilon apart -- effectively melting it and suppressing the number
of upsilons the scientists count.

"If the quarks and gluons were still confined within individual protons
and neutrons, they wouldn't be able to participate in the competing interactions that break up the quark-antiquark pairs," Tang said.

Upsilon advantages Scientists have observed such suppression of other quark-antiquark particles in QGP -- namely J/psi particles (made of a charm-anticharm pair). But upsilons stand apart from J/psi particles,
the STAR scientists say, for two main reasons: their inability to reform
in the QGP and the fact that they come in three types.

Before we get to reforming, let's talk about how these particles
form. Charm and bottom quarks and antiquarks are created very early
in the collisions - - even before the QGP. At the instant of impact,
when the kinetic energy of the colliding gold ions is deposited in a
tiny space, it triggers the creation of many particles of matter and
antimatter as energy transforms into mass through Einstein's famous
equation, E=mc2. The quarks and antiquarks partner up to form upsilons
and J/psi particles, which can then interact with the newly formed QGP.

But because it takes more energy to make heavier particles, there are
many more lighter charm and anticharm quarks than heavier bottom and
antibottom quarks in the particle soup. That means that even after
some J/psi particles dissociate, or "melt," in the QGP, others can
continue to form as charm and anticharm quarks find one another in the
plasma. This reformation happens only very rarely with upsilons because
of the relative scarcity of heavy bottom and antibottom quarks. So,
once an upsilon dissociates, it's gone.

"There just aren't enough bottom-antibottom quarks in the QGP to
partner up," said Shuai Yang, a STAR collaborator from South China
Normal University. "This makes upsilon counts very clean because their suppression isn't muddied by reformation the way J/psi counts can be."
The other advantage of upsilons is that, unlike J/psi particles, they
come in three varieties: a tightly bound ground state and two different
excited states where the quark-antiquark pairs are more loosely bound. The
most tightly bound version should be hardest to pull apart and melt at
a higher temperature.

"If we observe the suppression levels for the three varieties are
different, maybe we can establish a range for the QGP temperature,"
Yang said.

First time measurement These results mark the first time RHIC scientists
have been able to measure the suppression for each of the three upsilon varieties.

They found the expected pattern: The least suppression/melting for
the most tightly bound ground state; higher suppression for the
intermediately bound state; and essentially no upsilons of the most
loosely bound state -- meaning all the upsilons in this last group may
have been melted. (The scientists note that the level of uncertainty in
the measurement of that most excited, loosely bound state was large.)
"We don't measure the upsilon directly; it decays almost instantly,"
Yang explained. "Instead, we measure the decay 'daughters.'" The team
looked at two decay "channels." One decay path leads to electron-
positron pairs, picked up by STAR's electromagnetic calorimeter. The
other decay path, to positive and negative muons, was tracked by STAR's
muon telescope detector.

In both cases, reconstructing the momentum and mass of the decay daughters establishes if the pair came from an upsilon. And since the different
types of upsilons have different masses, the scientists could tell the
three types apart.

"This is the most anticipated result coming out of the muon
telescope detector," said Brookhaven Lab physicist Lijuan Ruan, a STAR co-spokesperson and manager of the muon telescope detector project. That component was specifically proposed and built for the purpose of tracking upsilons, with planning back as far as 2005, construction beginning
in 2010, and full installation in time for the RHIC run of 2014 --
the source of data, along with 2016, for this analysis.

"It was a very challenging measurement," Ma said. "This paper is
essentially declaring the success of the STAR muon telescope detector
program. We will continue to use this detector component for the next
few years to collect more data to reduce our uncertainties about these results." Collecting more data over the next few years of running STAR,
along with RHIC's brand new detector, sPHENIX, should provide a clearer
picture of the QGP.

sPHENIX was built to track upsilons and other particles made of heavy
quarks as one of its major goals.

"We're looking forward to how new data to be collected in the next few
years will fill out our picture of the QGP," said Ma.

Additional scientists from the following institutions made significant contributions to this paper: National Cheng Kung University, Rice
University, Shandong University, Tsinghua University, University of
Illinois at Chicago.

The research was funded by the DOE Office of Science (NP), the
U.S. National Science Foundation, and a range of international
organizations and agencies listed in the scientific paper. The STAR team
used computing resources at the Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) at DOE's Lawrence Berkeley National Laboratory, and the Open
Science Grid consortium.

* RELATED_TOPICS
o Matter_&_Energy
# Quantum_Physics # Physics # Detectors # Nature_of_Water
# Nuclear_Energy # Nanotechnology # Chemistry #
Materials_Science
* RELATED_TERMS
o Particle_physics o Subatomic_particle o Mass o Quark o
Chelation o Chemistry o Earth_science o Heat

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. B. E. Aboona et al. (STAR Collaboration). Measurement of
Sequential U Suppression in Au+Au Collisions at
SQRTsNN=200  GeV with the STAR Experiment. Phys. Rev.

Lett., 2023 DOI: 10.1103/PhysRevLett.130.112301 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230314205334.htm

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