Abstract
Recent unexpected evidence for collectivity in high multiplicity pp and pPb collisions at LHC energies has challenged the notion that such small systems do not exhibit any of the properties that have been used to study the quark gluon plasma in heavy ion collisions. An overview of recent results concerning particle production and collectivity in such collisions using the CMS detector at the LHC is presented.
Highlights
IntroductionFor roughly the first decade of heavy ion physics at RHIC and the first few years of similar studies at the LHC, heavy ion physicists assumed that no interesting physics (in the quark gluon plasma, QGP, sense) happened in pp collisions and almost no interesting physics happened in pA (or dA) collisions
For roughly the first decade of heavy ion physics at RHIC and the first few years of similar studies at the LHC, heavy ion physicists assumed that no interesting physics happened in pp collisions and almost no interesting physics happened in pA collisions
Some pA data were found to be useful to constrain nuclear PDFs, but this usage served more as a fine tuning of assumptions about the initial state, as opposed to being directly related to QGP physics. These attitudes towards these small systems have been challenged by recent observations of evidence for collectivity, first in very high multiplicity pp collisions [1] and later in high multiplicity pA collisions [2]
Summary
For roughly the first decade of heavy ion physics at RHIC and the first few years of similar studies at the LHC, heavy ion physicists assumed that no interesting physics (in the quark gluon plasma, QGP, sense) happened in pp collisions and almost no interesting physics happened in pA (or dA) collisions These systems were viewed as too small for any possibility of forming a system of quarks and gluons that could be considered a medium. Some pA data were found to be useful to constrain nuclear PDFs, but this usage served more as a fine tuning of assumptions about the initial state, as opposed to being directly related to QGP physics These attitudes towards these small systems have been challenged by recent observations of evidence for collectivity, first in very high multiplicity pp collisions [1] and later in high multiplicity pA collisions [2]. Details of all heavy ion results from CMS can be found on the heavy ion physics website [3]
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