Soft-Dielectron Excess in Proton-Proton Collisions at sqrt[s]=13 TeV.
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Vermunt, R Vernet, R Vértesi, L Vickovic, Z Vilakazi, O Villalobos Baillie, G Vino, A Vinogradov, T Virgili, V Vislavicius, A Vodopyanov, B Volkel, M A Völkl, K Voloshin, S A Voloshin, G Volpe, B von Haller, I Vorobyev, D Voscek, J Vrláková, B Wagner, M Weber, S G Weber, A Wegrzynek, S C Wenzel, J P Wessels, J Wiechula, J Wikne, G Wilk, J Wilkinson, G A Willems, E Willsher, B Windelband, M Winn, W E Witt, J R Wright, Y Wu, R Xu, S Yalcin, Y Yamaguchi, K Yamakawa, S Yang, S Yano, Z Yin, H Yokoyama, I-K Yoo, J H Yoon, S Yuan, A Yuncu, V Yurchenko, V Zaccolo, A Zaman, C Zampolli, H J C Zanoli, N Zardoshti, A Zarochentsev, P Závada, N Zaviyalov, H Zbroszczyk, M Zhalov, S Zhang, X Zhang, Z Zhang, V Zherebchevskii, Y Zhi, D Zhou, Y Zhou, Z Zhou, J Zhu, Y Zhu, A Zichichi, G Zinovjev, N ZurloA measurement of dielectron production in proton-proton (pp) collisions at sqrt[s]=13 TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron (ee) invariant mass m_{ee} and pair transverse momentum p_{T,ee} that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of m_{ee}, p_{T,ee}, and event multiplicity dN_{ch}/dη. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured η/π^{0} ratio in pp and proton-nucleus collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at 0.15<m_{ee}<0.6 GeV/c^{2} and for p_{T,ee}<0.4 GeV/c indicates an enhancement of soft dielectrons, reminiscent of the "anomalous" soft-photon and soft-dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to 1.61±0.13(stat)±0.17(syst,data)±0.34(syst,cocktail) in the ALICE acceptance. Acceptance-corrected excess spectra in m_{ee} and p_{T,ee} are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.
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