Emergence of Long-Range Angular Correlations in Low-Multiplicity Proton-Proton Collisions

This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msqrt><a:mrow><a:mi>s</a:mi></a:mrow></a:msqrt><a:mo>=</a:mo><a:mn>13</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>TeV</a:mi></a:mrow></a:math>. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mn>1.4</c:mn><c:mo><</c:mo><c:mo stretchy="false">|</c:mo><c:mi mathvariant="normal">Δ</c:mi><c:mi>η</c:mi><c:mo stretchy="false">|</c:mo><c:mo><</c:mo><c:mn>1.8</c:mn></c:mrow></c:math> and a transverse momentum of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mn>1</h:mn><h:mo><</h:mo><h:msub><h:mrow><h:mi>p</h:mi></h:mrow><h:mrow><h:mi mathvariant="normal">T</h:mi></h:mrow></h:msub><h:mo><</h:mo><h:mn>2</h:mn><h:mtext> </h:mtext><h:mtext> </h:mtext><h:mrow><h:mi>GeV</h:mi><h:mo>/</h:mo><h:mi>c</h:mi></h:mrow></h:mrow></h:math>, as a function of the charged-particle multiplicity measured at midrapidity. This Letter extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msup><k:mi mathvariant="normal">e</k:mi><k:mo>+</k:mo></k:msup><k:msup><k:mi mathvariant="normal">e</k:mi><k:mo>−</k:mo></k:msup></k:math> collisions at <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msqrt><o:mi>s</o:mi></o:msqrt><o:mo>=</o:mo><o:mn>91</o:mn><o:mtext> </o:mtext><o:mtext> </o:mtext><o:mi>GeV</o:mi></o:math> and <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mrow><q:msqrt><q:mrow><q:mi>s</q:mi></q:mrow></q:msqrt><q:mo>=</q:mo><q:mn>183</q:mn><q:mi>–</q:mi><q:mn>209</q:mn><q:mtext> </q:mtext><q:mtext> </q:mtext><q:mi>GeV</q:mi></q:mrow></q:math>, where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mn>8</s:mn><s:mo>≲</s:mo><s:mo stretchy="false">⟨</s:mo><s:msub><s:mi>N</s:mi><s:mi>ch</s:mi></s:msub><s:mo stretchy="false">⟩</s:mo><s:mo>≲</s:mo><s:mn>24</s:mn></s:math> where the <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:msup><w:mi mathvariant="normal">e</w:mi><w:mo>+</w:mo></w:msup><w:msup><w:mi mathvariant="normal">e</w:mi><w:mo>−</w:mo></w:msup></w:math> results have good precision, the measured ridge yields in <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mi>p</ab:mi><ab:mi>p</ab:mi></ab:math> collisions are substantially larger than the limits set in <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:msup><cb:mi mathvariant="normal">e</cb:mi><cb:mo>+</cb:mo></cb:msup><cb:msup><cb:mi mathvariant="normal">e</cb:mi><cb:mo>−</cb:mo></cb:msup></cb:math> annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"><gb:msup><gb:mi mathvariant="normal">e</gb:mi><gb:mo>+</gb:mo></gb:msup><gb:msup><gb:mi mathvariant="normal">e</gb:mi><gb:mo>−</gb:mo></gb:msup></gb:math> annihilations do not contribute significantly to the emergence of long-range correlations in <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"><kb:mi>p</kb:mi><kb:mi>p</kb:mi></kb:math> collisions.