38 Scientific American, June 2019
BROOKHAVEN NATIONAL LABORATORYof quantum mechanics, which governs particle inter-
actions. By carefully measuring the en ergy and angle
of the electron as it recoils, we gain information about
what it hit.
The virtual photon’s wavelength in DIS ex periments
is on the order of femtometers (10–15 meter)—the dis-
tance scale of the proton diameter. The higher the ener-
gy of the collision, the smaller the virtual photon’s wave-
length, and the smaller the wavelength, the more pre-
cise and localized the probe. If it is small enough, the
electron in essence bounces off one of the quarks inside
the proton (rather than the whole proton itself ), provid-
ing a peek at the particle’s inner structure.
The first DIS experiment was the SLAC-M.I.T. proj-
ect at the facility then called the Stanford Linear Accel-
erator Center (SLAC). In 1968 it provided the first evi-
dence of quarks—a discovery that won the experi-
ment’s leaders the 1990 Nobel Prize in Physics. Similar
experiments discovered that quarks inside free pro-
tons and neutrons and those inside nuclei behave very
differently. Furthermore, they found that proton and
neutron spin does not come from the spins of the con-
stituent quarks, as scientists had expected. This find-
ing was first made in protons and initially called the
“proton spin crisis.” The first DIS collider, in which
both electrons and protons were accelerated before
crashing, was the Hadron-Electron Ring Accelerator
(HERA) at the German Electron Synchrotron (DESY)
research center in Hamburg, Germany, which ran from
1992 to 2007. The HERA experiments showed that
what we thought was a simple configuration of three
quarks inside each proton and neutron could in fact
become a particle soup in which many quarks and glu-
ons instantly appear and disappear. HERA significant-
ly advanced our understanding of the structure of
nucleons but could not address the Spin Crisis and
lacked the beams of nuclei necessary to study quark
and gluon behavior in the nuclei.A major factor complicating all observations at
this scale is the weirdness of quantum mechanics.
These rules describe subatomic particles as hazes of
probability: they do not exist in specific states at spe-
cific places and times. Instead we must think of
quarks as existing in an infinite number of quantum
configurations simultaneously. Furthermore, we must
consider the quantum-mechanical phenomenon of
entanglement, in which two particles can become
connected so that their fates are intertwined even
after they separate. Entanglement could pose a funda-
mental problem for observing at the nuclear scale
because the quarks and gluons we would like to
observe are at risk of becoming entangled with what-
ever probe we use to look at them—in the case of DIS,
the virtual photon. It seems impossible to define what
we mean by nucleon structure when what we find
depends on how we probe it.
Luckily, by the 1970s QCD had advanced enough
for scientists to figure out that the probe and the tar-
get in DIS experiments can be separated—a condition
called factorization. At high-enough energies, scien-
tists can essentially ignore the effects of quantum
entanglement under certain circumstances—enough
to describe the structure of the proton in one dimen-
sion. This meant that they could extract from DIS
experiments a measurement of the probability that
any given quark inside a proton is contributing a par-
ticular share of its forward momentum.
Recently theoretical advancements have enabled
us to push further and describe the inner structure of
nucleons in more than one dimension—not just how
much quarks and gluons contribute to its forward
momentum but how much they move side to side
inside the nucleon as well.
But the real step forward will come with the EIC.ELECTRON-ION COLLIDER
the eic will make a three-dimensional map of the in-
terior of a nucleon. We expect the collider to deliver
measurements of the positions and momenta of
quarks and gluons and the amount each contributes
to the nucleon’s overall mass and spin.
The key advance of the EIC compared with previ-
ous DIS experiments is its brightness: it will produce
between 100 and 1,000 more collisions per minute
than HERA, for instance. In addition, the high ener-
gies of the colliding beams at the EIC will resolve dis-
tances of several hundredths the diameter of a proton,
enabling us to investigate the regions where a large
number of quarks and gluons each carry roughly 0.01
percent of the proton’s forward momentum. The EIC
will also let us control the alignment of the spin of the
particles in its beams so that we can study how the
spin of the proton arises from the QCD interactions of
quarks and gluons. When incorporated into our mod-
ern theoretical framework, the EIC’s measurements
will allow us to create a truly 3-D image of the proton
in terms of quarks and gluons.HEAVY IONS
and polarized
protons accel
erate in side
Brook haven
National Labo
ratory’s Relativ
istic Heavy Ion
Collider (RHIC).