Soft-Dielectron Excess in Proton-Proton Collisions at \ns=13\u2009\u2009TeV

Abstract

A measurement of dielectron production in proton–proton (pp) collisions at √ 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 invariant mass mee and pair transverse momentum pT,ee that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of mee, pT,ee, and event multiplicity dNch/dη . The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured η/π0 ratio in pp and proton-nucleus (p–A) 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<mee < 0.6 GeV/c2 and for pT,ee < 0.4 GeV/c indicates an enhancement of soft dielectrons, reminiscent of the ’anomalous’ soft-photon and -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. Acceptancecorrected excess spectra in mee and pT,ee are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach. *See Appendix A for the list of collaboration members ar X iv :2 00 5. 14 52 2v 2 [ nu cl -e x] 1 0 M ay 2 02 1 Soft-dielectron excess in proton–proton collisions at √ s = 13 TeV ALICE Collaboration The study of lepton pair production is an important tool to investigate the properties of hadronic and nuclear collisions as they can leave the strongly interacting system at any stage of its evolution. In order to single out possible medium contributions to the dilepton yield in nucleus–nucleus collisions on top of those from hadron decays, studies in hadronic collision systems are instrumental to obtain a mediumfree reference. Recent measurements of dielectron (e+e−) production at midrapidity in proton–proton (pp) collisions at the Large Hadron Collider (LHC) at CERN [1–3] and at the Relativistic Heavy-Ion Collider (RHIC) at BNL [4–6] are compatible with the expectations from hadron decays, i.e., with the hadronic cocktail, and show no indication of medium effects within the experimental uncertainties. In contrast to this, recent measurements of hadronic observables in small collision systems at the LHC [7– 10] and at RHIC [11–13] reveal signs of collectivity and equilibration of the final-state particles at high multiplicities. This suggests that considerable interaction in an intermediate state may indeed be at work even in pp collisions, which should also give rise to the emission of electromagnetic radiation. The production of soft photons in hadronic collision systems was extensively studied in fixed-target experiments at beam momenta ranging from 10.5 to 450 GeV/c. Except for the lowest collision energies [14], most experiments reported an excess of soft photons compared with the expectation from hadron decays that could not be explained by initialand final-state bremsstrahlung [15–17]. The emergence of a photon excess in a transverse momentum (pT) range far below 0.2 GeV/c was dubbed the soft-photon puzzle because bremsstrahlung from initialand final-state particles should dominate over the radiation from any intermediate state in the soft limit, as stated by the Low theorem [18]. This raised speculations about the existence of a radiating intermediate state with characteristic time and length scales well above 1 fm [19]; a scenario that can be largely ruled out by more recent measurements of the source size in pp collisions from particle interferometry [20–22]. Several possible mechanisms were proposed to explain the observations, including the annihilation of soft partons [23–28], the production of a cold non-equilibrium state of quarks and gluons [29, 30], and the emission of synchrotron radiation off quarks that are accelerated in the chromomagnetic fields of the colliding hadrons [31, 32]. A final conclusion on the interpretation of the soft-photon excess has not been reached though [33, 34]. In the dilepton sector, an enhancement over the hadronic cocktail was observed for both electron and muon pairs at small invariant masses in pp collisions at the Intersecting Storage Rings (ISR) [35], and in fixed-target experiments with π and p beams from 10 to 400 GeV/c [36–46]. Similarly to the case of real photons, the excess yield could not be reconciled with the expectation from hadronic bremsstrahlung. These observations are supported by findings of an enhanced e+/π ratio at the ISR [47]. However, the observations in the dilepton sector remained controversial because other experiments reported results that were compatible with bremsstrahlung and hadron decays only [48–50]. The question of anomalous soft-dilepton production in hadronic collisions awaits further experimental input since three decades. In a dedicated campaign during pp operation at √ s = 13 TeV, the ALICE central-barrel detectors [51] were operated inside a lower magnetic solenoid field, which increased the sensitivity for electrons at low pT (the term ’electron’ is used here for electrons and positrons). This makes a reassessment of soft dielectron production possible that could not be performed in a previous analysis at nominal field [2]. A detailed description of the ALICE apparatus and its performance can be found in [52]. The tracking of charged particles is performed by the Inner Tracking System (ITS) [53] and by the Time Projection Chamber (TPC) [54], which are located in the central barrel and are surrounded by a solenoid, providing a homogeneous magnetic field along the beam direction. The TPC is used for particle identification (PID) via the measurement of the specific ionization energy loss (dE/dx). Additional PID information is provided by the Time-Of-Flight (TOF) [55] system. Collision events are selected using the V0 detectors located on either side of the interaction point. Furthermore, the events are classified on the basis of the V0 signal amplitude. The event classes are reported in terms of dNch/dη at midrapidity [56]. The data samples analyzed for this Letter were recorded in 2016–2018 in pp collisions at √ s = 13 TeV