Coherentlowtransversemomentume+e− pairproductioninhadronicAu+Aucollisionsat 𝑠""� = 200 GeVandU+Ucollisionsat 𝑠""� = 193 GeVatSTAR

Shuai Yang, for the STAR CollaborationBrookhaven National Laboratory

The STAR Collaborationdrupal.star.bnl.gov/STAR/presentations

AbstractDileptons (l+l−) are produced in all the stages of the heavy-ion collisions, and escape with minimum interaction with the strongly interactingmedium. Thus, l+l− pair measurements play an essential role in the study of hot and dense nuclear matter, created in heavy-ion collisions.Recently, a significant excess of J/ψ yield at very low transverse momenta (pT < 0.3 GeV/c) was reported by the ALICE [1] and STARcollaborations in peripheral A+A collisions. These observations may point to evidence of coherent photoproduction of J/ψ in hadronicinteractions which conflicts with traditional knowledge of the coherent photoproduction mechanism. It is interesting to investigate the e+e−pair production in a wider invariant mass region (Mee < 4 GeV/c2) at very low pT in heavy-ion collisions for different centrality bins in order tostudy the production mechanism.In this poster, we will present e+e− spectra with various invariant mass and pT differentials in Au+Au collisions at 𝑠""� = 200 GeV and U+Ucollisions at 𝑠""� = 193 GeV. The structure of the t (t = 𝑝+,) distributions of these mass regions will be shown and compared with the samedistributions in ultra-peripheral collisions.

Photoninteractioninultra-peripheralA+Acollisions[2]

ØPhoton-photoninteractionØPhoton-nucleus interactionü Coherent photoproduction• Restrictsthefinalstatestolowtransversemomenta• SignificantdestructiveinterferenceforpT <<1/<b>

ü Incoherentphotoproduction

)2 (GeV/ceeM0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

)-1 )2

((G

eV/c

eedN

/dM

0

0.2

0.4

0.6

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13−10×

AuAu@200 GeVUU@193 GeVSum

ρIn_Med QGP

Data - CocktailCentrality: 60-80%

<0.15 GeV/cT

0< p

STAR Preliminary|<1ee|<1, |yeη>0.2 GeV/c, |e

Tp

Physics results

SummaryØ SignificantenhancementatverylowpT inthemostperipheralcollisionsis

observedintheentiremassregionforthefirsttimeØ TheenhancementatverylowpT hasasimilarstructuretothatinUPC

References[1]J.Adametal.(ALICECollaboration),Phys.Rev.Lett.116(2016)222301[2]C.A.Bertulani, S.R.Klein, andJ.Nystrand,Annu.Rev.Nucl.Part.Sci.55(2005) 271[3]R.Rapp,Adv.HighEnergyPhys.2013(2013)148253

)-2

N/(d

tdy)

((G

eV/c

)2 d

)2 ((GeV/c)2T

t = p

4−10

3−10

2−10

1−10 2 <0.76 GeV/ceeAuAu@200 GeV, 0.4< M/ndf: 3.07/42χ

-2 31 (GeV/c)±Slope: 371

Centrality: 60-80%

)2 ((GeV/c)2T

t = p

2 <2.6 GeV/ceeAuAu@200 GeV, 1.2< M/ndf: 0.55/32χ

-2 37 (GeV/c)±Slope: 287

STAR preliminary

)2 ((GeV/c)2T

t = p0 0.01 0.02 0.03

4−10

3−10

2−10

1−10 2 <0.76 GeV/ceeUU@193 GeV, 0.4< M/ndf: 5.08/42χ

-2 53 (GeV/c)±Slope: 506

)2 ((GeV/c)2T

t = p0 0.01 0.02 0.03

2 <2.6 GeV/ceeUU@193 GeV, 1.2< M/ndf: 1.26/42χ

-2 81 (GeV/c)±Slope: 411

WeobservedSimilarstructuretothatinultra-peripheralcollisions (UPC)• Indicationofinterferencestructure• Similarslopeparameter

ØEnhancementcannotbedescribedbybroaden𝞀modelcalculation[3]

ØNeedadditionalsource(s)

STARexperimentTimeProjectionChamber

TimeOfFlight

Ø TPC:tracking,momenta,andenergylossmeasurementØ TOF:rejectsslowhadronsandenablescleanelectron

identificationupto3GeV/c

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)-1 )2

((G

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/dM

11−10

9−10

7−10

5−10

3−10

1−1060-80%

-2 10×40-60% -4 10×10-40%

Cocktail

Au + Au @ 200 GeV

|<1ee|<1, |yeη>0.2 GeV/c, |eT

p

)2 (GeV/ceeM0.5 1 1.5 2 2.5 3 3.5

U + U @ 193 GeV <0.15 GeV/cT

0< p

STAR preliminary

(GeV/c)T

p0 0.2 0.4 0.6 0.8

)-1

((G

eV/c

)T

dN/d

p

8−10

6−10

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2−10

1−1020.4-0.76 GeV/c

0.02× 21.2-2.6 GeV/cCocktail

Au + Au @ 200 GeV

|<1ee|<1, |yeη>0.2 GeV/c, |eT

p

(GeV/c)T

p0 0.2 0.4 0.6 0.8

U + U @ 193 GeV Centrality: 60-80%

STAR preliminary

UPC 𝞀0 :Phys.Rev.C 77 (2008)34910Slope:388± 24

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9𝑩𝒄𝒐𝒉𝒕 + 𝑨𝒊𝒏𝒄𝒆9𝑩𝒊𝒏𝒄𝒕