Show threadChuck DarwinDec 28

The gluon cloud is exactly what QCD predicts.

“The HERA data are direct experimental proof that QCD describes nature,” Milner said.

But the young theory’s victory came with a bitter pill:

While QCD beautifully described the dance of short-lived quarks and gluons revealed by HERA’s extreme collisions,

the theory is useless for understanding the three long-lasting quarks seen in SLAC’s gentle bombardment.

QCD’s predictions are easy to understand only when the strong force is relatively weak.

And the strong force weakens only when quarks are extremely close together,
as they are in short-lived quark-antiquark pairs.

#Frank #Wilczek, #David #Gross and #David #Politzer identified this defining feature of QCD in 1973,
winning the Nobel Prize for it 31 years later.

But for gentler collisions like SLAC’s, where the proton acts like three quarks that mutually keep their distance,
these quarks pull on each other strongly enough that QCD calculations become impossible.

Thus, the task of further demystifying the three-quark view of the proton has fallen largely to experimentalists.
(Researchers who run “digital experiments,” in which QCD predictions are simulated on supercomputers,
have also made key contributions.)

And it’s in this low-resolution picture that physicists keep finding surprises.

Recently, a team led by #Juan #Rojo of the National Institute for Subatomic Physics in the Netherlands and VU University Amsterdam
analyzed more than 5,000 proton snapshots taken over the last 50 years,
using machine learning
to infer the motions of quarks and gluons inside the proton
in a way that sidesteps theoretical guesswork.

The new scrutiny picked up a background blur in the images that had escaped past researchers.

In relatively soft collisions just barely breaking the proton open,
most of the momentum was locked up in the usual three quarks:
two ups and a down.

But a small amount of momentum appeared to come from a “#charm” #quark and charm #antiquark
— colossal elementary particles that each outweigh the entire proton by more than
one-third❗️

Short-lived charms frequently show up in the “quark sea” view of the proton
(gluons can split into any of six different quark types if they have enough energy).

But the results from Rojo and colleagues suggest that the charms have a more permanent presence,
making them detectable in gentler collisions.

In these collisions, the proton appears as a quantum mixture,
or superposition,
of multiple states:

An electron usually encounters the three lightweight quarks.

But it will occasionally encounter a rarer “molecule” of five quarks,
such as an up, down and charm quark grouped on one side and an up quark and charm antiquark on the other.

Such subtle details about the proton’s makeup could prove consequential.

At the Large Hadron Collider, physicists search for new elementary particles by bashing high-speed protons together and seeing what pops out;

to understand the results, researchers need to know what’s in a proton to begin with.

The occasional apparition of giant charm quarks would throw off the odds of making more exotic particles.

And when protons called #cosmic #rays hurtle here from outer space and slam into protons in Earth’s atmosphere,
charm quarks popping up at the right moments would shower Earth with extra-energetic #neutrinos, researchers calculated in 2021.

These could confound observers searching for high-energy neutrinos coming from across the cosmos.

Rojo’s collaboration plans to continue exploring the proton by searching for an imbalance between charm quarks and antiquarks.

And heavier constituents,
such as the #top quark, could make even rarer and harder-to-detect appearances.

Next-generation experiments will seek still more unknown features.

Physicists at Brookhaven National Laboratory hope to fire up the
"Electron-Ion Collider"
in the 2030s
and pick up where HERA left off,
taking higher-resolution snapshots that will enable the first 3D reconstructions of the proton.

The #EIC will also use spinning electrons to create detailed maps of the spins of the internal quarks and gluons,
just as SLAC and HERA mapped out their momentums.

This should help researchers to finally pin down the origin of the proton’s spin,
and to address other fundamental questions about the baffling particle that makes up most of our everyday world.

https://www.quantamagazine.org/inside-the-proton-the-most-complicated-thing-imaginable-20221019/

Inside the Proton, the ‘Most Complicated Thing’ Imaginable

The positively charged particle at the heart of the atom is an object of unspeakable complexity, one that changes its appearance depending on how it is probed. We’ve attempted to connect the proton’s many faces to form the most complete picture yet.

Quanta Magazine
Reymond AguinaldoNov 24, 2022

#ICYMI: On July 5, 2022, #CERN announced the discovery of three new exotic particles, one of which is a pentaquark while the other two are tetraquarks. With this discovery, physicists hope to better understand how quarks bind with each other to form various particles underlying the bedrock of particle physics.

Link to article: https://home.cern/news/news/physics/lhcb-discovers-three-new-exotic-particles

#Science #Physics #ParticlePhysics #ExoticParticles #Quarks #Antiquark

LHCb discovers three new exotic particles

The international LHCb collaboration at the Large Hadron Collider (LHC) has observed three never-before-seen particles: a new kind of “pentaquark” and the first-ever pair of “tetraquarks”, which includes a new type of tetraquark. The findings, presented today at a CERN seminar, add three new exotic members to the growing list of new hadrons found at the LHC. They will help physicists better understand how quarks bind together into these composite particles. Quarks are elementary particles and come in six flavours: up, down, charm, strange, top and bottom. They usually combine together in groups of twos and threes to form hadrons such as the protons and neutrons that make up atomic nuclei. More rarely, however, they can also combine into four-quark and five-quark particles, or “tetraquarks” and “pentaquarks”. These exotic hadrons were predicted by theorists at the same time as conventional hadrons, about six decades ago, but only relatively recently, in the past 20 years, have they been observed by LHCb and other experiments. Most of the exotic hadrons discovered in the past two decades are tetraquarks or pentaquarks containing a charm quark and a charm antiquark, with the remaining two or three quarks being an up, down or strange quark or their antiquarks. But in the past two years, LHCb has discovered different kinds of exotic hadrons. Two years ago, the collaboration discovered a tetraquark made up of two charm quarks and two charm antiquarks, and two “open-charm” tetraquarks consisting of a charm antiquark, an up quark, a down quark and a strange antiquark. And last year it found the first-ever instance of a “double open-charm” tetraquark with two charm quarks and an up and a down antiquark. Open charm means that the particle contains a charm quark without an equivalent antiquark. The discoveries announced today by the LHCb collaboration include new kinds of exotic hadrons. The first kind, observed in an analysis of “decays” of negatively charged B mesons, is a pentaquark made up of a charm quark and a charm antiquark and an up, a down and a strange quark. It is the first pentaquark found to contain a strange quark. The finding has a whopping statistical significance of 15 standard deviations, far beyond the 5 standard deviations that are required to claim the observation of a particle in particle physics. The two new tetraquarks, illustrated here as single units of tightly bound quarks. One of the particles is composed of a charm quark, a strange antiquark and an up quark and a down antiquark (left), and the other is made up of a charm quark, a strange antiquark and an up antiquark and down quark (right) (Image: CERN) The second kind is a doubly electrically charged tetraquark. It is an open-charm tetraquark composed of a charm quark, a strange antiquark, and an up quark and a down antiquark, and it was spotted together with its neutral counterpart in a joint analysis of decays of positively charged and neutral B mesons. The new tetraquarks, observed with a statistical significance of 6.5 (doubly charged particle) and 8 (neutral particle) standard deviations, represent the first time a pair of tetraquarks has been observed. “The more analyses we perform, the more kinds of exotic hadrons we find,” says LHCb physics coordinator Niels Tuning. “We’re witnessing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons started being discovered and ultimately led to the quark model of conventional hadrons in the 1960s. We’re creating ‘particle zoo 2.0’.” “Finding new kinds of tetraquarks and pentaquarks and measuring their properties will help theorists develop a unified model of exotic hadrons, the exact nature of which is largely unknown,” says LHCb spokesperson Chris Parkes. “It will also help to better understand conventional hadrons.” While some theoretical models describe exotic hadrons as single units of tightly bound quarks, other models envisage them as pairs of standard hadrons loosely bound in a molecule-like structure. Only time and more studies of exotic hadrons will tell if these particles are one, the other or both. Further information:  Read more on the LHCb website. Illustrations: https://cds.cern.ch/record/2814136

CERN
Reymond AguinaldoNov 24, 2022

• Tetraquarks - two quarks and two antiquarks.

• Pentaquarks - four quarks and an antiquark.

Physicists know very little about the internal structure of these exotic particles & how they could resemble molecules—a proton bound to a meson, for example—or simply be a tightly bound collection of quarks. Studying tetraquarks and pentaquarks could improve our knowledge of quantum chromodynamics—the theory of how quarks interact with each other.

#Science #Physics #ParticlePhysics #Quarks #Antiquark