Unlike
electrons, quarks cannot wander freely in ordinary matter. They are confined by
the strong force within hadrons such as the protons and neutrons that make up
atomic nuclei. However, at very high energy densities, such as those that are
achieved in collisions between nuclei at the Large Hadron Collider (LHC), a
different phase of matter exists in which quarks and the mediators of the
strong force, gluons, are not confined within hadrons. This form of matter,
called a quark–gluon plasma, is thought to have filled the universe in the
first few millionths of a second after the Big Bang, before atomic nuclei
formed.
At the
Rencontres de Moriond conference today, the ALICE collaboration at the LHC
reported an analysis of head-on collisions between lead nuclei showing that
quark mass matters when quarks cross a quark–gluon plasma.
Hadrons
containing charm and beauty quarks, the heavier cousins of the up and down
quarks that make up protons and neutrons, offer an excellent way to study the
properties of the quark–gluon plasma, such as its density. A charm quark is
much heavier than a proton, and a beauty quark is as heavy as five protons.
These quarks are produced in the very first instants of the collisions between
nuclei, before the formation of the quark–gluon plasma that they then traverse.
Therefore, they interact with the plasma’s constituents throughout its entire
evolution.
Just
like electrically charged particles crossing an ordinary gas can tell us about
its density, through the energy they lose in the crossing, heavy quarks can be
used to determine the density of the quark–gluon plasma through the energy they
lose in strong interactions with the plasma’s constituents. However, before
using the energy loss in the plasma to measure the plasma’s density, physicists
need to validate the theoretical description of this loss.
A
fundamental prediction of the theory of the strong force is that quarks that
have a larger mass lose less energy than their lighter counterparts because of
a mechanism known as the dead-cone effect, which prevents the radiation of
gluons and thus of energy in a cone around the quark’s direction of flight.
In their
new study of head-on collisions between lead nuclei, the ALICE collaboration
tested this prediction using measurements of charm-quark-containing particles
called D mesons. They measured D mesons produced right after the collisions
from initial charm quarks, called ‘prompt’ D mesons, as well as ‘non-prompt’ D
mesons produced later in the decays of B mesons, which contain the heavier
beauty quarks. They presented the measurements in terms of the nuclear
modification factor, which is a scaled ratio of particle production in
lead–lead collisions to that in proton–proton collisions (figure below). They
found that the production of non-prompt D mesons (blue markers in the figure)
in lead–lead collisions is less suppressed than that of prompt D mesons (red
markers).
These results are described well by models in which beauty quarks lose less energy than charm quarks in the quark–gluon plasma, because of their larger mass. They thus confirm the theoretical expectations of the role of quark mass in the interactions of quarks with the quark–gluon plasma. In addition, the measurements are sensitive to B mesons that have low energies. This is crucial when it comes to using beauty quarks to determine the density and other properties of the plasma.
Further
measurements with the upgraded ALICE detector in the next run of the LHC, which
is scheduled to start this coming summer, will help to better understand the
theoretical description of the energy loss that quarks experience when they
cross the quark–gluon plasma.
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