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arXiv · 2203.12354

Low-$p_T$ direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV

N. J. Abdulameer·U. Acharya·A. Adare·C. Aidala·N. N. Ajitanand·Y. Akiba·R. Akimoto·H. Al-Ta'ani·J. Alexander·M. Alfred·A. Angerami·K. Aoki·N. Apadula·Y. Aramaki·H. Asano·E. C. Aschenauer·E. T. Atomssa·T. C. Awes·B. Azmoun·V. Babintsev·M. Bai·B. Bannier·K. N. Barish·B. Bassalleck·S. Bathe·V. Baublis·S. Baumgart·A. Bazilevsky·R. Belmont·A. Berdnikov·Y. Berdnikov·L. Bichon·B. Blankenship·D. S. Blau·J. S. Bok·V. Borisov·K. Boyle·M. L. Brooks·H. Buesching·V. Bumazhnov·S. Butsyk·S. Campbell·V. Canoa Roman·P. Castera·C. -H. Chen·M. Chiu·C. Y. Chi·I. J. Choi·J. B. Choi·S. Choi·R. K. Choudhury·P. Christiansen·T. Chujo·O. Chvala·V. Cianciolo·Z. Citron·B. A. Cole·M. Connors·R. Corliss·Y. Corrales Morales·M. Csanád·T. Csörgő·L. D'Orazio·S. Dairaku·A. Datta·M. S. Daugherity·G. David·C. T. Dean·A. Denisov·A. Deshpande·E. J. Desmond·K. V. Dharmawardane·O. Dietzsch·L. Ding·A. Dion·M. Donadelli·V. Doomra·O. Drapier·A. Drees·K. A. Drees·J. M. Durham·A. Durum·S. Edwards·Y. V. Efremenko·T. Engelmore·A. Enokizono·R. Esha·K. O. Eyser·B. Fadem·W. Fan·D. E. Fields·M. Finger, Jr.·M. Finger·D. Firak·D. Fitzgerald·F. Fleuret·S. L. Fokin·J. E. Frantz·A. Franz·A. D. Frawley
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Abstract

The measurement of direct photons from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$ Gev/$c$ is presented by the PHENIX collaboration at the Relativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{_{NN}}}=62.4$ GeV is scaled with $(dN_{\rm ch}/d\eta)^{\alpha}$ for $\alpha=1.21{\pm}0.04$. This scaling also holds true for direct-photon spectra from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The spectra have a local inverse slope $T_{\rm eff}$ increasing with $p_T$ of $0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to $0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{_{NN}}}= 39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.

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N. J. Abdulameer, U. Acharya, A. Adare, C. Aidala, N. N. Ajitanand, Y. Akiba, R. Akimoto, H. Al-Ta'ani, J. Alexander, M. Alfred, A. Angerami, K. Aoki, N. Apadula, Y. Aramaki, H. Asano, E. C. Aschenauer, E. T. Atomssa, T. C. Awes, B. Azmoun, V. Babintsev, M. Bai, B. Bannier, K. N. Barish, B. Bassalleck, S. Bathe, V. Baublis, S. Baumgart, A. Bazilevsky, R. Belmont, A. Berdnikov, Y. Berdnikov, L. Bichon, B. Blankenship, D. S. Blau, J. S. Bok, V. Borisov, K. Boyle, M. L. Brooks, H. Buesching, V. Bumazhnov, S. Butsyk, S. Campbell, V. Canoa Roman, P. Castera, C. -H. Chen, M. Chiu, C. Y. Chi, I. J. Choi, J. B. Choi, S. Choi, R. K. Choudhury, P. Christiansen, T. Chujo, O. Chvala, V. Cianciolo, Z. Citron, B. A. Cole, M. Connors, R. Corliss, Y. Corrales Morales, M. Csanád, T. Csörgő, L. D'Orazio, S. Dairaku, A. Datta, M. S. Daugherity, G. David, C. T. Dean, A. Denisov, A. Deshpande, E. J. Desmond, K. V. Dharmawardane, O. Dietzsch, L. Ding, A. Dion, M. Donadelli, V. Doomra, O. Drapier, A. Drees, K. A. Drees, J. M. Durham, A. Durum, S. Edwards, Y. V. Efremenko, T. Engelmore, A. Enokizono, R. Esha, K. O. Eyser, B. Fadem, W. Fan, D. E. Fields, M. Finger, Jr., M. Finger, D. Firak, D. Fitzgerald, F. Fleuret, S. L. Fokin, J. E. Frantz, A. Franz, A. D. Frawley. 2022-03-23. Low-$p_T$ direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV. https://doi.org/10.1103/physrevc.107.024914

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