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Direct photon production in Pb-Pb collisions at
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Direct photon production at mid-rapidity in Pb&ndash;Pb collisions at <span id="mmlsi1" class="mathmlsrc">source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si1.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=1c8185232f9746b818569f8c750d7a2c">ss="imgLazyJSB inlineImage" height="15" width="105" alt="View the MathML source" style="margin-top: -5px; vertical-align: middle" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0370269316000320-si1.gif">script>style="vertical-align:bottom" width="105" alt="View the MathML source" title="View the MathML source" src="http://origin-ars.els-cdn.com/content/image/1-s2.0-S0370269316000320-si1.gif">script><span class="mathContainer hidden"><span class="mathCode">si1.gif" overflow="scroll">sqrt>sub>sNNsub>sqrt>=2.76 TeVspan>span>span> was studied in the transverse momentum range <span id="mmlsi148" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si148.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=0013d3c9759dbd106f0bce3f81d2dae8" title="Click to view the MathML source">0.9<p<sub>Tsub><14 GeV/cspan><span class="mathContainer hidden"><span class="mathCode">si148.gif" overflow="scroll">0.9<sub>pTsub><14 GeVstretchy="false">/cspan>span>span>. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the <span id="mmlsi4" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si4.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=1166737fefdecce3d6290006e66f4e08" title="Click to view the MathML source">e<sup>+sup>e<sup>&minus;sup>span><span class="mathContainer hidden"><span class="mathCode">si4.gif" overflow="scroll">sup>e+sup>sup>e&minus;sup>span>span>span> pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0&ndash;20%, 20&ndash;40%, and 40&ndash;80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for <span id="mmlsi5" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si5.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=f876eeae8231e04158b538b59eb9bcfd" title="Click to view the MathML source">p<sub>Tsub>≳5 GeV/cspan><span class="mathContainer hidden"><span class="mathCode">si5.gif" overflow="scroll">sub>pTsub>≳5 GeVstretchy="false">/cspan>span>span>. Direct photon spectra down to <span id="mmlsi6" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si6.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=145471da8223133907c91fcc1011f6da" title="Click to view the MathML source">p<sub>Tsub>&asymp;1 GeV/cspan><span class="mathContainer hidden"><span class="mathCode">si6.gif" overflow="scroll">sub>pTsub>&asymp;1 GeVstretchy="false">/cspan>span>span> could be extracted for the 20&ndash;40% and 0&ndash;20% centrality classes. The significance of the direct photon signal for <span id="mmlsi7" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si7.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=4067fc100418e5d9dc8ef576fcc34c21" title="Click to view the MathML source">0.9<p<sub>Tsub><2.1 GeV/cspan><span class="mathContainer hidden"><span class="mathCode">si7.gif" overflow="scroll">0.9<sub>pTsub><2.1 GeVstretchy="false">/cspan>span>span> is 2.6σ   for the 0&ndash;20% class. The spectrum in this <span id="mmlsi8" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si8.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=aa58cb03581d230341ba1ff810fef8b0" title="Click to view the MathML source">p<sub>Tsub>span><span class="mathContainer hidden"><span class="mathCode">si8.gif" overflow="scroll">sub>pTsub>span>span>span> range and centrality class can be described by an exponential with an inverse slope parameter of <span id="mmlsi9" class="mathmlsrc"><span class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0370269316000320&_mathId=si9.gif&_user=111111111&_pii=S0370269316000320&_rdoc=1&_issn=03702693&md5=2e8066a435697eed598476449755381f" title="Click to view the MathML source">(297&plusmn;12<sup>statsup>&plusmn;41<sup>systsup>) MeVspan><span class="mathContainer hidden"><span class="mathCode">si9.gif" overflow="scroll">stretchy="false">(297&plusmn;sup>12statsup>&plusmn;sup>41systsup>stretchy="false">) MeVspan>span>span>. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.

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