International Journal of Applied Glass Science 4 [1] 1–4 (2013)
DOI:10.1111/ijag.12013
Communication
A New Glassy State of Matter: The Color
Glass Condensate
Steve Feller*,† and Ugur Akgun
Physics Department, Coe College, Cedar Rapids, Iowa 52402
This communication describes a newly reported state of matter: The color glass condensate. Analogies to glass science
are made for this nuclear physics discovery.
In response to recent announcements in the international scientific community, we are pleased to give a
brief overview of a new experimental claim that the
Color Glass Condensate, an additional sixth state of
matter, has been observed. The previously known states
of matter include solid, liquid, gas, plasma, and the
Bose–Einstein condensate. In the traditional use of the
words from our field of glass science, this new state of
matter is not a glass, has no color, and is not a condensate. These terms only describe the new state through
useful analogies to what we commonly think these
terms mean. This development is an exciting new result
from the Large Hadron Collider (LHC) that is nestled
on the Swiss-French border near Geneva.
The proposed new state of matter relies heavily on
some of the great ideas of modern physics including
length contraction from Einstein’s theory of relativity.
This theory has ample demonstrated experimental
agreement and predicts that objects contract in
*Member, The American Ceramic Society.
†
[email protected]
© 2013 The American Ceramic Society and Wiley Periodicals, Inc
the direction of their motion, as seen by stationary
observers.
First, we will briefly explain the connotation of
each term in the Color Glass Condensate. Afterward,
we describe briefly the new results from CERN.
Color
In particle physics, the term color refers to the type
of the field in strong interactions that is similar to
charge in electromagnetic interactions. Color is associated with the quarks and gluons that are the quanta of
the strong nuclear force (SNF) field as photons are for
the electromagnetic (EM) field. The term color is just an
unfortunate naming convention that has nothing to do
with differing wavelengths of visible light. Electric
charge comes in two qualities: positive and negative,
whereas in Quantum Chromodynamics (QCD, the theoretical framework of strong interactions), a quark’s color
is assigned one of the three values: red, green, and blue.
An antiquark has one of the three anticolors, called
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International Journal of Applied Glass Science—Feller and Akgun
antired, antigreen, and antiblue (represented as cyan,
magenta, and yellow, respectively). Gluons are mixtures
of color and anticolor components, such as red and
magenta. Hadrons are composite particles whose total
color is zero. Nature accomplishes this by using two or
three quarks to form the hadrons. The color is conserved to zero by using a quark- anti-quark pair or three
quarks of different colors yielding zero net color.1
Glass
In this new state of matter, glass refers to the disordered state of the gluons. The disorder arises, in part,
because of a so-called gluon wall that is built as a result
of length contraction due to the high relative velocity
seen by the colliding particles. The high velocity is with
respect to the target particle (the gluon wall) that other
particle collides with. In essence, the gluons pile on
each other in an amorphous manner as the interaction
dimension shrinks as viewed by the incoming particle.
Let us assume that the x-axis is in the direction of
motion. The shrinkage of the x dimension for the
observed particle is described using relativity by:
Dx ¼ Dxp =c
ð1Þ
where Dx is the contracted dimension as seen in the
opposing particle’s motion toward the observer due to
length contraction, Dxp is the proper length of a particle measured in the particle’s own frame of reference,
and c, the relativistic correction, is a number larger
than one that is given by
p
c ¼ 1= ð1 v 2 =c 2 Þ
ð2Þ
In Eq. (2), v is the relative velocity of the two particles, and c is the speed of light. As v approaches c,
the x shrinkage can be very large. For example, for
v = 0.9999c, c is about 70.7.
Condensate
In the context of the whole term Color Glass
Condensate, condensate refers to the large density of
the gluons as they pile up due to length contraction
and a resulting coherence in the wave function of the
Vol. 4, No. 1, 2013
gluons. It is not a chemical condensate in the manner
that atoms undergo.
The evidence for the new state of matter is based
on highly correlated directions of some of the released
particles from 2 million lead–proton collisions observed
in the Compact Muon Solenoid (CMS) experiment at
the LHC.2 The same behavior between generated
particle pairs was observed in CMS experiments during
proton–proton collisions in 20103 (see Fig. 1).
The first discussion on how to interpret the LHC
result was focused on the possible formation of a
quark-gluon plasma (QGP). A QGP, a hot soup of
quarks and gluons, is believed to be the state of the
universe shortly after the Big Bang. The wave of QGP
created during the heavy ion collisions sweeps some of
the resulting particles in the same direction, which
explains the two and three particle that correlations
have been observed in heavy ion (lead, gold, copper)
collisions at the LHC (see Fig. 1), and at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National
Laboratory.4–7 However, based on calculations that protons in the LHC should not create such plasma, the
QGP interpretation was dismissed for this mysterious
correlation in proton collisions.
The theoretical model proposed by R. Venugopalan to explain this phenomenon suggests that proton–
proton collisions produce a liquid-like wave of gluons,
known as the Color Glass Condensate, shortly before
the particle direction correlation was seen. Although
protons consist of three quarks with gluons between
them as they gain energy, up to 7 TeV in the LHC,
many complementary gluons are added, see Fig. 2. The
increase in the number of gluons means a consequent
decrease in individual gluon energy, hence creating a
very dense medium filled with low energy gluons. This
causes the color field to be perceived as a classical field,
like the Coulomb field, by the other fast-moving particles. In the rest frame of the target proton, a fast-moving particle sees these classical fields as contracted to sit
atop one another and act coherently. This saturated
gluon medium is the Color Glass Condensate; the color
is due to the color of the gluons, it is a glass because of
the random packing of slow gluons in the medium
during collisions, and it is a condensate due to phase
space density coherence.8–10
The correlated wave functions of the gluon field
are the source of the entanglement between two particles generated during the collision. This quantum
entanglement manifests itself in the correlated direction
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A New Glassy State of Matter
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Fig. 1. Two-particle correlation functions for 7 TeV pp (A), 2.76 TeV PbPb (B), and 5.02 TeV pPb (C) collisions. The peak shows
the long-range correlations at small Dφ. Dφ is a measure of the angle between two tracks in the transverse plane. (Image is courtesy of
the CMS Collaboration at CERN).
In summary, glass shows up where you least expect
it—even in the new LHC CMS detector!
Acknowledgments
The authors would like to thank Edwin Norbeck
of the physics department of the University of Iowa for
valuable insights on the subject. The NSF is thanked
for support under grant NSF-DMR 0904615.
References
Fig. 2. Increasing gluon density within a proton with increasing energy. Quarks are not displayed
of the particles generated in the collision, which is the
experimental evidence for this new state of matter. (the
peaks seen in Fig. 1)
1. http://en.wikipedia.org/wiki/Color_charge.
2. CMS Collaboration, “Observation of Long-Range Near-Side Angular
Correlations in Proton-Lead Collisions at the LHC,” arXiv:1210.5482
[nucl-ex], Accepted by Physics Letters B (2012).
3. CMS Collaboration, “Observation of Long-Range, Near-Side Angular
Correlations in Proton-Proton Collisions at the LHC,” J. High Energy
Phys., 09 091 (2010).
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International Journal of Applied Glass Science—Feller and Akgun
4. CMS Collaboration, “Long-Range and Short-Range Dihadron
Angular Correlations in Central PbPb Collisions at a Nucleon-Nucleon
Center of Mass Energy of 2.76 TeV,” J. High Energy Phys., 07 076
(2011).
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6. PHENIX Collaboration, “Dense-Medium Modifications to Jet-Induced
Hadron Pair Distributions in Au + Au Collisions at √sNN = 200 GeV,”
Phys. Rev. Lett., 97 052301 (2006).
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High Transverse Momentum Particles in pp and Au + Au collisions at
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8. A. H. Mueller and J.-W. Qiu, “Gluon Recombination and Shadowing at
Small Values of x,” Nucl. Phys., 268 427 (1986).
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(1994).
10. L. V. Gribov, E. M. Levin, and M. G. Ryskin, “Semihard Processes in
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