Measurement of isolated prompt photon production in pp and p–Pb collisions at the LHC

Abstract

Abstract This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p–Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p–Pb collisions at centre-of-mass energies per nucleon pair of $$\sqrt{s_{\text {NN}}}={5.02}\,\textrm{TeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mtext>NN</mml:mtext> </mml:msub> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mrow> <mml:mn>5.02</mml:mn> </mml:mrow> <mml:mspace/> <mml:mtext>TeV</mml:mtext> </mml:mrow> </mml:math> and 8.16 TeV, as well as in pp collisions at $$\sqrt{s}={5.02}\,\textrm{TeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mrow> <mml:mn>5.02</mml:mn> </mml:mrow> <mml:mspace/> <mml:mtext>TeV</mml:mtext> </mml:mrow> </mml:math> and 8 TeV. The cross section is obtained at midrapidity $$(|y|&lt;0.7)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>|</mml:mo> <mml:mi>y</mml:mi> <mml:mo>|</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>0.7</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> using a charged-track based isolation momentum $$p_\textrm{T}^{\text {iso,~ch}}&lt;{1.5}\,\textrm{GeV}/c$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>p</mml:mi> <mml:mtext>T</mml:mtext> <mml:mrow> <mml:mtext>iso, ch</mml:mtext> </mml:mrow> </mml:msubsup> <mml:mo>&lt;</mml:mo> <mml:mrow> <mml:mn>1.5</mml:mn> </mml:mrow> <mml:mspace/> <mml:mtext>GeV</mml:mtext> <mml:mo>/</mml:mo> <mml:mi>c</mml:mi> </mml:mrow> </mml:math> in a cone with radius $$R=0.4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>R</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:math> . The data for both collision systems are well reproduced by perturbative QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor $$R_{\text {pA}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>R</mml:mi> <mml:mtext>pA</mml:mtext> </mml:msub> </mml:math> for both collision energies is consistent with unity for $$p_{\textrm{T}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>p</mml:mi> <mml:mtext>T</mml:mtext> </mml:msub> </mml:math> $$&gt;{20}\,\textrm{GeV}/c$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>&gt;</mml:mo> <mml:mn>20</mml:mn> <mml:mspace/> <mml:mtext>GeV</mml:mtext> <mml:mo>/</mml:mo> <mml:mi>c</mml:mi> </mml:mrow> </mml:math> . However, deviations from unity ( $$R_\textrm{pA}&lt;1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mtext>pA</mml:mtext> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> ) of up to 20% are observed for $$p_{\textrm{T}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>p</mml:mi> <mml:mtext>T</mml:mtext> </mml:msub> </mml:math> $$&lt;{20}\,\textrm{GeV}/c$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>&lt;</mml:mo> <mml:mn>20</mml:mn> <mml:mspace/> <mml:mtext>GeV</mml:mtext> <mml:mo>/</mml:mo> <mml:mi>c</mml:mi> </mml:mrow> </mml:math> with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing $$p_{\textrm{T}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>p</mml:mi> <mml:mtext>T</mml:mtext> </mml:msub> </mml:math> with a s

Affiliated Institutions

• Indian Institute of Technology Bombay IN
• Yale University US
• IMT Atlantique FR
• Laboratoire de Physique Subatomique et de Cosmologie FR
• Hospital Universitário da Universidade de São Paulo BR
• Istituto Nazionale di Fisica Nucleare, Sezione di Torino IT
• Istituto Nazionale di Fisica Nucleare, Sezione di Bologna IT
• Universidad Nacional Autónoma de México MX
• University of Kansas US
• Korea Institute of Science & Technology Information KR
• University of Derby GB
• University of Oslo
• Aligarh Muslim University IN
• University of Copenhagen DK
• Technical University of Munich DE
• CEA Paris-Saclay FR
• Centre National de la Recherche Scientifique FR
• Nantes Université FR
• University of Science and Technology of China CN
• Pontifical Catholic University of Peru PE
• Laboratoire de Physique Subatomique et des Technologies Associées FR
• Horia Hulubei National Institute for R and D in Physics and Nuclear Engineering RO
• European Organization for Nuclear Research CH
• Istituto Nazionale di Fisica Nucleare, Sezione di Bari IT
• Istituto Nazionale di Fisica Nucleare, Sezione di Padova IT
• Université de Strasbourg FR
• Sungkyunkwan University KR
• Commissariat à l'Énergie Atomique et aux Énergies Alternatives FR
• Goethe University Frankfurt DE
• Lawrence Berkeley National Laboratory US
• Jagiellonian University PL
• Benemérita Universidad Autónoma de Puebla MX
• HUN-REN Wigner Research Centre for Physics HU
• Universidad Autónoma de Sinaloa MX
• Central China Normal University CN
• Max Planck Institute for Nuclear Physics DE
• Institut Pluridisciplinaire Hubert Curien FR
• Homi Bhabha National Institute IN
• Institute of Particle Physics CA
• University of Pavol Jozef Šafárik SK
• Czech Academy of Sciences, Institute of Physics CZ
• Université Grenoble Alpes FR
• Indian Institute of Technology Indore IN
• University of Jammu IN
• Istituto Nazionale di Fisica Nucleare, Sezione di Catania IT
• University of Houston US
• GSI Helmholtz Centre for Heavy Ion Research DE
• AGH University of Krakow PL
• Institut de Recherche sur les Lois Fondamentales de l'Univers FR
• University of Bergen
• The University of Tokyo JP
• Gangneung–Wonju National University KR
• Heidelberg University DE
• National and Kapodistrian University of Athens GR

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