By CMS Collaboration
Quark Matter 2019-Han Show Theater in Wuhan, China

The CMS collaboration presented over ten brand-new results in the latest installment of the Quark Matter conference, one of the largest conferences in the field of heavy-ion physics.

In heavy-ion physics, the properties of the quark-gluon plasma, a hot and dense QCD medium created during the heavy-ion collision, are studied.  Even though we are less than a year since the latest data taking period for lead-lead (PbPb) collisions in 2018, which increased the recorded integrated luminosity by a factor of four, many new results already take advantage of this huge new data set.

A heavy-ion collision where a candidate top quark and a candidate antitop quark were produced. From the top decays there are two b-jet candidates (shown by the cones), a muon (the isolated red line), an electron (the green line and green tower).

Preliminary results show for the first time strong evidence for top quark production in nucleus-nucleus collisions.  Top quarks are very short-lived and are the only quarks to decay before forming hadrons, making them unique probes of the initial stages of the collision and of the quark-gluon plasma.  This new result opens up numerous potential future measurements. link


Also presented for the first time in heavy-ion collisions is the evidence of the elusive X(3872) resonance.  Since its discovery in the last decade, the exact nature of this resonance is yet unknown: it could be a tetraquark state, or a loosely-bound molecule consisting of two D⁰ mesons, or something potentially more exotic.  The quark-gluon plasma will modify the production rate of this heavy resonance, depending on its physical size. If the resonance is large, it is easier to get destroyed by the surrounding activity, and CMS physicists plan to use this interaction with the medium to provide valuable input into understanding the composition of the X(3872) resonance. link

The family of Upsilon (Y) mesons (containing a b quark and antiquark) continues to be a source of fascination.  CMS measures the amount of collective behavior for the Y(1S) meson in PbPb collisions with unprecedented precision.  For the first time, the amount of collective behavior of the Y(2S) meson is also measured, which is consistent with zero within uncertainties.  link - link

A probe for the strangeness enhancement, which is a classical signature of the quark-gluon plasma, is presented in terms of production rates of the Ds meson and comparisons with the D⁰ meson.  The heavy charm quark is expected to be produced early in the evolution of the quark-gluon plasma and serves as an excellent tag for the strangeness content. link

The collective behavior of the D⁰ meson is studied in detail with the huge new dataset.  By comparing the collective behavior of the D⁰ meson with its anti-particle, one can probe potential effects from the large electric field from the 'bystanders', the field created by other quarks in a heavy-ion collision during the early stages of the collision.  So far no indication is seen for this initial electric field. CMS also examined, in proton-lead data, the difference between prompt and non-prompt D⁰ meson, the latter being decay products of B hadrons. One can infer the potential flavor-dependent hierarchy of collective behavior between b and c quarks. link - link

Yeonju Go presents the measurement of photon suppression as a flash talk on Saturday.

Image: CMS physicist Yeonju Go presents the measurement of photon suppression as a flash talk on Saturday (credit: Quark Matter 2019)

Objects that only subject to electro-weak interactions, such as photons are not expected to be modified in the Quark-Gluon Plasma.  At the Quark Matter 2019 conference, CMS presented an update of the measurement of the amount of suppression (called the nuclear modification factor) of photons produced in PbPb collisions.  It is the most precise measurement of this quantity, with an improvement in precision by more than a factor of five. The result has been selected as one of the six flash talks featured in the Saturday plenary session, out of more than 300 potential topics. link

Jets (sprays of particles coming from high-energy partons) are excellent tools to probe the quark-gluon plasma.  For the first time in heavy-ion collisions, the charge of jets is measured. It can potentially be used as input to understand the composition of the samples, such as up quark, down quark and gluon, that we observe in the heavy-ion collisions.  This measurement also provides independent information on how jets are quenched, ie., how a high-energy quark or gluon interacts with the quark-gluon plasma. link

The amount of suppression of large-radius jets in heavy-ion collisions (also measured as a nuclear modification factor) was also presented for the first time.  Even though it is a simple observable, it is sensitive to many different physics effects affecting how jets are modified in the quark-gluon plasma, from coherent/incoherent energy loss to wide-angle medium response, and many more. link

These new exciting pieces of information help to deepen our understanding of the physics of heavy-ion collisions. With the large dataset in hand, the CMS Collaboration is planning to release many more results in the coming months!


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