NNuclear Physics A 00 (2020) 1–7
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
Electromagnetic & Weak Probes: Experimental Overview
Frank Geurts
Department of Physics & Astronomy, Rice University, Houston TX 77005, USA
Abstract
Electromagnetic and electroweak probes are the most versatile probes in the study of heavy-ion collisions. Produced atevery stage in the evolution of QCD matter, its messengers are practically inert to the strongly interacting medium theytravel through. In this contribution, I will discuss a selection of new results from experiments at the LHC, RHIC, andSIS facilities, spanning almost four orders of magnitude in beam energy. I will conclude with a brief overview of theexperimental landscape in the near future.
Keywords: heavy-ion collisions, quark-gluon plasma, electromagnetic probes, dileptons, direct photons, electroweakbosons
1. Introduction
Colorless probes are the ideal carriers of information in a strongly interacting medium. While notcompletely immune to interactions with hadrons, or quarks for that matter, the typical mean free path ofleptons and photons is long enough to allow these particles to escape and make it to our detectors. Moreover,the reach into the TeV scale has made electroweak bosons the latest asset in the experimentalist’s toolbox,helping to provide constraints on nuclear parton distributions functions in p + A and A + A collisions. Thus,electromagnetic and weak probes can provide a complete picture with access to initial conditions and thesubsequent evolution of the system; ranging from hard process, such as the W ± , Z production and promptphotons, to soft process that include thermal photons and dileptons originating from leptonic decays andvirtual photons.The present experimental landscape of electroweak probes involves several facilities and many moreexperiments spanning almost four orders of magnitude in center-of-mass energies. The versatility of thesefacilities provided for electroweak and dilepton data from a wide range of collision systems, ranging fromheavy-ion collisions such as Pb + Pb and Au + Au, to smaller systems such as In + In and Cu + Cu, p + Pb, andpp. Detector improvements furthermore opened up access to a range of collision geometries from ultra-peripheral collisions (UPC), to nuclear overlaps in peripheral hadronic collisions, and high-multiplicity ppcollisions. It is a testament of a very active field to see at this conference so many results as well as plansfor the future presented. In what follows only a small selection of several exciting results can be discussed. a r X i v : . [ nu c l - e x ] A p r Author / Nuclear Physics A 00 (2020) 1–7 - - * y D a t a / T h e o r y pQCQ + NNPDF31 + nCTEQ15 <4.5) m h >20 GeV, 2< m T LHCb 8.16 TeV (p <4.5) m h >20 GeV, 2< m T LHCb 5.02 TeV (p <4) m h >20 GeV, 2.5< m T ALICE 5.02 TeV (p |<2.4) l h >20 GeV, | lT CMS 5.02 TeV (pATLAS 5.02 TeV (full lepton phase space)
LHCb Preliminary) - l + l fi (Z s pPb, y AA R = 5.02 TeV NN s Pb, −
90% Pb − ALICE Preliminary, 0 c > 20 GeV/ µ T, p − < µ η − , - µ + µ → Z NLO pQCD with CT14 as pp reference (Phys.Rev.D93 (2016) 033006)
CT14 (Eur.Phys.J. C (2017) 77:163)
CT14 + EPPS16
DataUncorr. syst. uncertainty
ALI−PREL−328632
Fig. 1. Electroweak boson production in p + Pb and Pb + Pb collisions. Left panel: Comparison of results on Z-boson production inp + Pb collisions at √ s NN = .
16 TeV measured by LHCb, compared with previous results from ATLAS, CMS, ALICE, and LHCb at √ s NN = .
02 TeV [2]. Right panel: Z-boson production in Pb + Pb at √ s NN = .
02 TeV as a function of dimuon y , compared withCT14 PDF and EPPS16 nPDF [3].
2. Electroweak Bosons
Electroweak bosons are created in high-momentum processes that take place in the initial stages of acollisions. This makes these bosons excellent probes of the parton distribution functions (PDFs) inside anucleon. Moreover, the electroweak boson’s fast decay into leptons allows the information of the partonicstructure of the initial state to be carried without being a ff ected by a strongly interacting medium in the caseof heavy ion collisions. The production of W bosons, primarily through q¯q annihilation, provides accessto the distribution of the light (anti-) quarks. Proton PDFs, based on global fit analyses, now include datafrom recent LHC results. In a bound nucleus, these distribution functions are observed to be modified. Thenuclear PDFs can be expressed in terms of f p / Ai ( x , Q ) = R Ai ( x , Q ) f pi ( x , Q ), where the nuclear modification R Ai ( x , Q ) shows at small Bjorken x the e ff ect of shadowing, at intermediate x that of anti-shadowing andthe EMC e ff ect, at high x followed by Fermi motion [1].At this conference, the CMS and ATLAS collaborations reported on its recently published results of W ± production from p + Pb collisions at √ s = .
16 TeV [4, 5]. These results not only demonstrate that nPDFsare clearly favored over the CT14 PDF but with experimental uncertainties smaller than those of the modelsit is possible to di ff erentiate between EPPS16 and CTEQ15. New results from Z boson measurements inp-Pb at √ s = .
16 TeV and Pb-Pb at √ s = .
02 TeV by the LHCb and ALICE collaborations, respectively,are shown in Fig. 1 and were shown to be compatible with CTEQ15 and EPPS16 predictions.
3. Direct Photons
Photons are produced throughout the entire space-time evolution of a strongly interacting system. Itssources include prompt hard parton scattering, thermal radiation, and jets - collectively referred to as directphotons [6]. In addition, photons can originate from the electromagnetic decay of final state hadrons. Themajor experimental challenge is to disentangle these contributions. Electromagnetic decays from final statehadrons by far provide the largest contribution and as such form a substantial backgrounds to the measure-ment of direct photons. The PHENIX collaboration reported on new measurements of R γ ( p T ) in Au + Auat √ s NN =
200 GeV. These new measurements, shown with red symbols in the two left panels of Fig. 2 fordi ff erent centralities use the external conversion method and are based on the large RHIC Run-14 data sam-ple. The results show a clear enhancement in the direct photon yields for p T ≤ / c , which continues topersist in the semi-peripheral data (middle panel). At high momenta, the results show consistency with N coll -scaled p + p results. In another important consistency check, PHENIX’s new results show good agreementwith previously published results that based on di ff erent data sets [8], and / or di ff erent methods such as thevirtual- γ [9] and the calorimeter methods [10]. In the right panel of Fig. 2, the invariant yield of photons is uthor / Nuclear Physics A 00 (2020) 1–7 + Au collisions at √ s NN =
200 GeV [7]. Left panel: for 0-20% central collisions. Middle panel: for20-40% semi-peripheral collisions. Right panel: universal scaling of low- p T direct photon yields dN γ / dy with respect to the numberof charged particles at midrapidity dN ch / d η | η = . plotted as a function of the charged hadron multiplicity dN ch / d η at midrapidity. The new data from PHENIXare in line with the recently observed scaling [11]. In the same figure, data from the STAR experiment isadded. While the scaling appears to be similar, the rates are systematically lower. It is expected that datafrom the ongoing STAR Beam Energy Scan (BES) Phase-2 program, see Sect. 5, will be able to add severalnew points at the lower charged hadron multiplicities using a similar conversion technique [12].
4. Dileptons
At this conference, a wealth of new experimental dilepton data has been being presented comprisingmore than three orders of magnitude in √ s NN . Dilepton invariant-mass spectra bring a plethora of physicschannels from di ff erent stages of the evolution of the medium that can be ”tuned in” by selecting the rele-vant mass window and thus not only include the leptonic decay channels of various light, strange, and charmmesons but also allow for the measurement of virtual direct photons from similar sources as mentioned pre-viously in Sect. 3. The HADES collaboration recently published [13] its results from measurements ofdielectron production in Au + Au collisions at √ s NN = .
42 GeV . Its findings confirm at this energy thestrong in-medium modification of the ρ meson, first reported at SPS energies by the NA60 collaboration[14]. After careful removal of the hadronic contributions to the invariant mass spectrum the HADES col- Fig. 3. Thermal dielectron measurements from Au + Au collisions at √ s NN = .
42 GeV by the HADES collaboration [15]. Leftpanel: invariant-mass spectra for low- p T dielectrons. Middle and right panels: transverse momentum distributions for 0 . < M ee < . / c and 0 . < M ee < .
45 GeV / c , respectively. Author / Nuclear Physics A 00 (2020) 1–7 laboration extracted, based on a black-body spectral function fit, the average temperature of the radiatingfireball of 71 . ± . ff erential measure-ments of the dielectron invariant-mass and p T spectra [15]. In Fig. 3, a comparison of the invariant-massyield (left panel) and momentum spectra in two mass windows (middle and right panels) are compared withseveral model descriptions and shows to have su ffi cient sensitivity to the details of the model descriptions.Dilepton-based measurements of the azimuthal anisotropy v as a function of p T in di ff erent invariantmass regions have been long been proposed as an alternative way to study medium at the di ff erent stages[16]. However, measuring the dielectron v is a statistics-hungry challenge, see e.g. [17]. Here, HADESreported on its first findings based on 2.6 billion events, and found a consistent comparison of its preliminaryresults in the π Dalitz mass range the v of charged pions [15]. Fig. 4. Dielectron invariant mass spectra in Au + Au and Pb + Pb collisions from STAR and ALICE, respectively. Left panel: high-statistics measurements at √ s NN =
27 and 54.4 GeV (red symbols) by STAR [18]. Middle and right panels: ALICE results for p + pand R pPb in collisions at √ s NN = .
02 TeV, respectively [19].
The STAR collaboration reported its new dielectron results from three high-statistics data sets [18]. Inthe left panel of Fig. 4 the dielectron invariant mass for the √ s NN =
27 GeV and 54.4 GeV energies isoverlaid with STAR’s results from other collision energies, including the first RHIC BES [20]. A ten-foldincrease in event statistics compared to the BES data is expected to better constrain the cocktail by directmeasurements of the ω and φ mesons, and allow for virtual direct photon measurements. The uncertaintiesin these new results are a considered good indicators of the expected precision for the BES Phase-2 energiesbetween √ s NN = . + p collisions at √ s = .
02 TeV. The vacuum baseline in the p + p data is found to be welldescribed by the expectations from the hadronic cocktail. The distinct shape of the charm and beautycontributions in the intermediate mass range (1 . ≤ M ee ≤ . / c ) is used to extract the charm andbeauty cross sections which is found to be consistent with independent heavy-flavor measurements [19].In the right panel of Fig. 4, the ALICE collaboration used the p + Pb invariant mass spectra to verify initialstate nuclear modification, R pPb = (cid:104) N coll (cid:105) dN / dM ee | pPb dN / dM ee | pp at √ s NN = .
02 TeV. In the intermediate mass range, theresults do not show significant modifications in agreement with previous D-meson measurements from theALICE collaboration. However, in the low mass range ( M ee ≤ / c a deviation from unity is observed.This deviation is expected as light-flavor production at low p T does not scale with N coll and is also observedwhen comparing to cocktail ratios that include scaling of light flavor in p + Pb.Coherent γ -N and γ − γ interactions are conventionally studied in UPC interactions. Recently, theSTAR and ATLAS experiments have published observations of low- p T dilepton excess in hadronic heavy-ion collisions [22, 23]. In the left panel of Fig. 5 new measurements in Pb + Pb collisions at 5.02 TeV fromthe ALICE collaboration show a 3 σ low- p T excess in the intermediate mass range for the 70-90% centralityclass. Meanwhile, both STAR and ATLAS have further expanded their measurements by including low- uthor / Nuclear Physics A 00 (2020) 1–7 p T dilepton measurements in peripheral Pb + Pb collisions at √ s NN = .
02 TeV. Left panel: ALICE low- p T dielectronmeasurements [19]. Right panel: centrality dependence of k ⊥ from dimuon measurement from ATLAS [21]. p T dimuons, and combining the 5.02 TeV data sets, respectively. Based on the combined 2014 and 2018data sets, the ATLAS collaboration showed a distinct centrality dependence of (cid:104) k ⊥ (cid:105) in Pb + Pb collisions at √ s NN = .
02 TeV, as can be seen in the right panel of Fig. 5 [21].At the 2018 Quark Matter conference, the ALICE collaboration reported on its potential for studyingin p + p collisions at √ s =
13 TeV a soft dielectron enhancement [24] that was first reported at the ISRby the Axial Field Spectrometer collaboration for p + p at √ s =
63 GeV [25]. At the time of the previousconference, large uncertainties on the contribution of the η meson to the hadronic cocktail and limitedstatistics did not allow for a quantitative conclusion. At this conference, the collaboration reported onits findings from a special run in which the field in its solenoid magnet was lowered to B = . p T reach for electrons to drop to 75 MeV / c . Additionally, ALICE presented a reevaluationof the η contribution as is shown in the left panel of Fig. 6. Combined with the new low B-field run, theseimprovements now show a significant enhancement over the cocktail for p T , ee < . / c in the η massrange as can be seen in the middle panel. Interestingly, and shown in the right panel of Fig. 6, a comparisonof the data for π s in a similar momentum range and η mesons at higher p T is consistent with the hadroniccocktail calculations. The physical mechanism for this enhancement is not yet understood. Fig. 6. Soft dielectron production in p + p collisions at √ s =
13 TeV measured by ALICE [19]. Left panel: new parametrization of the η/π ratio. Middle panel: low dielectron invariant-mass spectrum in p + p collisions at √ s =
13 TeV. Right panel: data-over-cocktailratio for low- p T η mesons compared to π mesons in the same low- p T range, η mesons in a higher p T range.
5. The Near-Future Experimental Landscape
Measurements of electromagnetic and weak probes have posed particular challenges to experimentsin the past and present in terms of e.g. material budgets and event statistics. At this conference several
Author / Nuclear Physics A 00 (2020) 1–7 experiments, located at various facilities, presented updates on the current state of their general upgradeplans and future designs. Many of the planned upgrades and designs specifically relate to improvements inthe measurements that are relevant to the topic of this paper. At the same time, several collaborations showedvery encouraging glimpses into what to expect from very recently collected data sets. Starting at the lowercenter-of-mass energies, the HADES collaboration showed its first raw dielectron spectra, collected in 2019,of Ag + Ag collisions at 2.42 GeV and 2.55 GeV with 1.3 and 14 billion events, respectively [15]. The STARcollaboration embarked on the second phase of its BES program where it successfully collected large datasets of Au + Au collisions at √ s NN = .
61, 14.6, and 11.5 GeV. These, and the proposed 9.2 and 7.7 GeVdata sets will allow the collaboration to complete its beam energy scan of the low M ee excess yields in arange where the total baryon density is expected to increase as beam energies are lowered [20]. The BESPhase-2 data sets will provide su ffi cient statistics in the intermediate mass range to allow a simultaneousextraction of the medium temperature. Fig. 7. Left panel: excess yields scaled by dN / dy | π ± from SIS to RHIC energies [18]. Right and middle panels: ALICE ITS2 and ITS3upgrade simulations of dielectron measurements in Pb + Pb [26].
In the left panel of Fig. 7, normalized excess yields of HADES, NA60, and STAR [13, 18, 20, 27, 28] arecombined with the energies and projected precisions of four proposed, future detectors. The MPD detector atNICA will install its ECAL detector starting 2020 which will complement its proposed program to measureelectromagnetic probes in addition to the conversion-based methods [29]. The proposed E16 pilot run at J-PARC [30] aims to use its dielectron spectrometer to study vector-meson modifications in p + A collisions atvery high rates. The CBM experiment at FAIR is configurable for either dimuon of dielectron measurementsand is expected slated for operations in 2025 [31]. At the SPS, the NA60 + collaboration proposes a detectorthat will allow for high-precision, high statistics measurements of the full dimuon spectrum, which willinclude the opportunity to measure the e ff ects of chiral ρ − a mixing.At higher energies, the LHC experiments presented the status of their upgrades for the upcoming Run-3and plans for Run-4. Proposed LHCb upgrades will enable Drell-Yan measurements from low-mass dimuonat forward rapidities which can probe the gluon nPDF at small x [32]. Of particular interest to the dileptonmeasurements, the ALICE collaboration presented its expectations for Run 3. The new ITS will significantlyimprove the tracking resolution, while the TPC readout upgrade would increase the data rate by two ordersof magnitude. Simulations, shown in the middle and right panels of Fig. 7, demonstrate the significantimprovements that these upgrades will bring to the Run-3 and Run-4 dielectron measurements, respectively.In conclusion, electromagnetic and weak probes are the most versatile probes available in heavy-ioncollisions with precision data collected over almost four orders of magnitude in collision energies. Manyupgrades and new detector proposals make the future of weak and electromagnetic probes truly look verybright. at the time of this writing uthor / Nuclear Physics A 00 (2020) 1–7 Acknowledgements
The author would like to thank the organizers of the Quark Matter 2019 conference for the invitationto present this overview. This work is in part supported by the U.S. Department of Energy under grantNo. DE-FG02-10ER41666.
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