Teasing strange matter from the ordinary

Date:
April 18, 2023
Source:
DOE/Thomas Jefferson National Accelerator Facility
Summary:
In a unique analysis of experimental data, nuclear physicists
have made observations of how lambda particles, so-called 'strange
matter,' are produced by a specific process called semi-inclusive
deep inelastic scattering (SIDIS). What's more, these data hint
that the building blocks of protons, quarks and gluons, are capable
of marching through the atomic nucleus in pairs called diquarks,
at least part of the time.


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FULL STORY ==========================================================================
In a unique analysis of experimental data, nuclear physicists have made
the first-ever observations of how lambda particles, so-called "strange matter," are produced by a specific process called semi-inclusive deep inelastic scattering (SIDIS). What's more, these data hint that the
building blocks of protons, quarks and gluons, are capable of marching
through the atomic nucleus in pairs called diquarks, at least part
of the time. These results come from an experiment conducted at the
U.S. Department of Energy's Thomas Jefferson National Accelerator
Facility.


==========================================================================
It's a result that has been decades in the making. The dataset was
originally collected in 2004. Lamiaa El Fassi, now an associate professor
of physics at Mississippi State University and principal investigator
of the work, first analyzed these data during her thesis project to earn
her graduate degree on a different topic.

Nearly a decade after completing her initial research with these data,
El Fassi revisited the dataset and led her group through a careful
analysis to yield these unprecedented measurements. The dataset comes
from experiments in Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF), a DOE user facility. In the experiment, nuclear
physicists tracked what happened when electrons from CEBAF scatter off
the target nucleus and probe the confined quarks inside protons and
neutrons. The results were recently published in Physical Review Letters.

"These studies help build a story, analogous to a motion picture,
of how the struck quark turns into hadrons. In a new paper, we report first-ever observations of such a study for the lambda baryon in the
forward and backward fragmentation regions," El Fassi said.

In like a lambda, out like a pion Like the more familiar protons and
neutrons, each lambda is made up of three quarks.

Unlike protons and neutrons, which only contain a mixture of up and
down quarks, lambdas contain one up quark, one down quark and one
strange quark.

Physicists have dubbed matter that contains strange quarks "strange
matter." In this work, El Fassi and her colleagues studied how these
particles of strange matter form from collisions of ordinary matter. To
do so, they shot CEBAF's electron beam at different targets, including
carbon, iron, and lead.

When a high-energy electron from CEBAF reaches one of these targets,
it breaks apart a proton or neutron inside one of the target's nuclei.

"Because the proton or neutron is totally broken apart, there is little
doubt that the electron interacts with the quark inside," El Fassi said.

After the electron interacts with a quark or quarks via an exchanged
virtual photon, the "struck" quark(s) begins moving as a free particle
in the medium, typically joining up with other quark(s) it encounters to
form a new composite particle as they propagate through the nucleus. And
some of the time, this composite particle will be a lambda.

But the lambda is short-lived -- after formation, it will swiftly decay
into two other particles: a pion and either a proton or neutron. To
measure different properties of these briefly created lambda particles, physicists must detect its two daughter particles, as well as the beam
electron that scattered off the target nucleus.

The experiment that collected this data, EG2, used the CEBAF Large
Acceptance Spectrometer (CLAS) detector in Jefferson Lab's Experimental
Hall B. These recently published results, "First Measurement of
? Electroproduction off Nuclei in the Current and Target Fragmentation Regions," are part of the CLAS collaboration, which involves almost 200 physicists worldwide.

SIDIS This work is the first to measure the lambda using this process,
which is known as semi-inclusive deep inelastic scattering, in the
forward and backward fragmentation regions. It's more difficult to use
this method to study lambda particles, because the particle decays so
quickly, it can't be measured directly.

"This class of measurement has only been performed on protons before,
and on lighter, more stable particles," said coauthor William Brooks,
professor of physics at Federico Santa Mari'a Technical University and co-spokesperson of the EG2 experiment.

The analysis was so challenging, it took several years for El Fassi
and her group to re-analyze the data and extract these results. It was
her thesis advisor, Kawtar Hafidi, who encouraged her to pursue the investigation of the lambda from these datasets.

"I would like to commend Lamiaa's hard work and perseverance in dedicating years of her career working on this," said Hafidi, associate laboratory director for physical sciences and engineering at Argonne National Lab and
co- spokesperson of the EG2 experiment. "Without her, this work would not
have seen fruition." "It hasn't been easy," El Fassi said. "It's a long
and time-consuming process, but it was worth the effort. When you spend
so many years working on something, it feels good to see it published."
El Fassi began this lambda analysis when she herself was a postdoc, a
couple of years prior to becoming an assistant professor at Mississippi
State University.

Along the way, several of her own postdocs at Mississippi State have
helped extract these results, including coauthor Taya Chetry.

"I'm very happy and motivated to see this work being published," said
Chetry, who is now a postdoctoral researcher at Florida International University.

Two for one A notable finding from this intensive analysis changes the way physicists understand how lambdas form in the wake of particle collisions.

In similar studies that have used semi-inclusive deep inelastic scattering
to study other particles, the particles of interest usually form after
a single quark was "struck" by the virtual photon exchanged between the electron beam and the target nucleus. But the signal left by lambda in
the CLAS detector suggests a more packaged deal.

The authors' analysis showed that when forming a lambda, the virtual
photonhas been absorbed part of the time by a pair of quarks, known as
a diquark, instead of just one. After being "struck," this diquark went
on to find a strange quark and forms a lambda.

"This quark pairing suggests a different mechanism of production and interaction than the case of the single quark interaction," Hafidi said.

A better understanding of how different particles form helps physicists
in their effort to decipher the strong interaction, the fundamental
force that holds these quark-containing particles together. The dynamics
of this interaction are very complicated, and so is the theory used to
describe it: quantum chromodynamics (QCD).

Comparing measurements to models of QCD's predictions allows physicists
to test this theory. Because the diquark finding differs from the model's current predictions, it suggests something about the model is off.

"There is an unknown ingredient that we don't understand. This is
extremely surprising, since the existing theory can describe essentially
all other observations, but not this one," Brooks said. "That means there
is something new to learn, and at the moment, we have no clue what it
could be." To find out, they'll need even more measurements.

Data for EG2 were collected with 5.014 GeV (billion electron-volt)
electron beams in the CEBAF's 6 GeV era. Future experiments will use
electron beams from the updated CEBAF, which now extend up to 11 GeV
for Experimental Hall B, as well as an updated CLAS detector known as
CLAS12, to continue studying the formation of a variety of particles,
including lambdas, with higher-energy electrons.

The upcoming Electron-Ion Collider (EIC) at DOE's Brookhaven National Laboratory will also provide a new opportunity to continue studying
this strange matter and quark pairing structure of the nucleon with
greater precision.

"These results lay the groundwork for upcoming studies at the upcoming
CLAS12 and the planned EIC experiments, where one can investigate the
diquark scattering in greater detail," Chetry said.

El Fassi is also a co-spokesperson for CLAS12 measurements of quark
propagation and hadron formation. When data from the new experiments is
finally ready, physicists will compare it to QCD predictions to further
refine this theory.

"Any new measurement that will give novel information toward understanding
the dynamics of strong interactions is very important," she said.

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========================================================================== Story Source: Materials provided by DOE/Thomas_Jefferson_National_Accelerator_Facility.

Original written by Chris Patrick. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. T. Chetry, L. El Fassi, W. K. Brooks, R. Dupre', A. El
Alaoui, K.

Hafidi, P. Achenbach, K. P. Adhikari, Z. Akbar, W. R.

Armstrong, M. Arratia, H. Atac, H. Avakian, L. Baashen, N. A.

Baltzell, L. Barion, M. Bashkanov, M. Battaglieri, I. Bedlinskiy, B.

Benkel, F. Benmokhtar, A. Bianconi, A. S. Biselli, M. Bondi,
W. A. Booth, F. Bossu`, S. Boiarinov, K.-Th. Brinkmann,
W. J. Briscoe, D. Bulumulla, V. D. Burkert,
D. S.

Carman, J. C. Carvajal, A. Celentano, P. Chatagnon,
V. Chesnokov, G. Ciullo, P. L. Cole, M. Contalbrigo,
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A. Deur, S. Diehl, C. Djalali, H.

Egiyan, L. Elouadrhiri, P. Eugenio, S. Fegan, A. Filippi,
G. Gavalian, Y.

Ghandilyan, G. P. Gilfoyle, D. I. Glazier,
A. A.

Golubenko, G. Gosta, R. W. Gothe, K. A. Griffioen, M.

Guidal, L. Guo, H. Hakobyan, M. Hattawy, T. B. Hayward,
D. Heddle, A. Hobart, M. Holtrop, Y. Ilieva, D. G. Ireland,
E. L.

Isupov, D. Jenkins, H. S. Jo, M. L. Kabir,
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X. Li, K.

Livingston, I. J. D. MacGregor, D. Marchand,
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S. Migliorati, T.

Mineeva, M. Mirazita, V. Mokeev, C. Munoz Camacho,
P. Nadel-Turonski, K.

Neupane, S. Niccolai, M. Nicol, G. Niculescu, M. Osipenko,
A. I.

Ostrovidov, P. Pandey, M. Paolone, L. L. Pappalardo, R.

Paremuzyan, E. Pasyuk, S. J. Paul, W. Phelps, N. Pilleux, M.

Pokhrel, J. Poudel, J. W. Price, Y. Prok, B. A. Raue,
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Reed, J. Richards, M. Ripani, J. Ritman, G. Rosner, F. Sabatie', C.

Salgado, S. Schadmand, A. Schmidt, R. A. Schumacher,
Y. G.

Sharabian, E. V. Shirokov, U. Shrestha, P. Simmerling,
D. Sokhan, N. Sparveris, S. Stepanyan, I. I. Strakovsky,
S. Strauch, J. A. Tan, N. Trotta, R. Tyson, M. Ungaro,
S. Vallarino, L.

Venturelli, H. Voskanyan, E. Voutier, X. Wei,
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Williams, R. Wishart, M. H. Wood, M. Yurov,
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of L Electroproduction off Nuclei in the Current and Target
Fragmentation Regions. Physical Review Letters, 2023; 130 (14)
DOI: 10.1103/PhysRevLett.130.142301 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/04/230418101434.htm

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