Probing nuclear PDFs with dijets in ultra-peripheral Pb+Pb collisions

This is a sel -a chi ed e sion o an o iginal a icle. This e sion may di e om he o iginal in pagina ion and ypog aphic de ails. Au ho (s): Ti le: Yea : Ve sion: Copy igh : Righ s: Righ s u l: Please ci e he o iginal e sion: CC BY-NC-ND 4.0 h ps://c ea i ecommons.o g/licenses/by-nc-nd/4.0/ P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions © The Au ho s 2019 Published e sion Helenius, I. Helenius, I. (2019). P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions. In D. d'En e ia, A. Mo sch, & P. C oche (Eds.), Ha d P obes 2018 : In e na ional Con e ence on Ha d and Elec omagne ic P obes o High-Ene gy Nuclea Collisions (A icle 118). Sissa Medialab s l. POS P oceedings o Science, 345. h ps://doi.o g/10.22323/1.345.0118 2019 PoS(Ha dP obes2018)118 P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions Ilkka Helenius∗ Uni e si y o Jy askyla, Depa men o Physics, P.O. Box 35, FI-40014 Uni e si y o Jy askyla, Finland Ins i u e o Theo e ical Physics, Tübingen Uni e si y, Au de Mo gens elle 14, 72076 Tübingen, Ge many E-mail: [email p o ec ed] In his alk we apply he pho op oduc ion amewo k ecen ly implemen ed in o he PYTHIA 8 Mon e Ca lo e en gene a o o s udy he po en ial o pho o-nuclea dije s in ul a-pe iphe al Pb+Pb collisions a he LHC o u he cons ain he nuclea PDFs. These e en s can be desc ibed as γA collisions whe e he ele an pa o he lux o quasi- eal pho ons om hea y-ions is ob- ained by using he equi alen pho on app oxima ion and cu ing ou impac -pa ame e alues which would lead o had onic in e ac ions be ween he beam pa icles. In pa icula , we quan- i y he small-x each wi h di e en je kinema ics and show how well he alues o xde i ed om econs uc ed je momen a a e co ela ed wi h he ac ual alues o pa onic momen um ac- ions p obed in hese measu emen s. Also he con ibu ions om di ec and esol ed pho ons a e sepa a ely p esen ed. To demons a e he po en ial, we compa e he expec ed expe imen al un- ce ain ies o he cu en nuclea -PDF e o s and discuss o he heo e ical unce ain ies including he unce ain y a ising om poo ly-cons ained pho on PDFs. We ind ha such a measu emen would po en ially p o ide a conside able educ ion o he nuclea PDF unce ain ies in a egion 10−4.x.10−2. In e na ional Con e ence on Ha d and Elec omagne ic P obes o High-Ene gy Nuclea Collisions 30 Sep embe - 5 Oc obe 2018 Aix-Les-Bains, Sa oie, F ance ∗Speake . c Copy igh owned by he au ho (s) unde he e ms o he C ea i e Commons A ibu ion-NonComme cial-NoDe i a i es 4.0 In e na ional License (CC BY-NC-ND 4.0). h ps://pos.sissa.i / PoS(Ha dP obes2018)118 P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions Ilkka Helenius 1. In oduc ion The unce ain ies in he cu en nuclea PDF (nPDF) analyses [ 1 , 2 ] a e conside able especially a small alues o x due o lack o da a cons ain s. The mos p ecise way o pin down he nPDF unce ain ies would be o measu e he s uc u e unc ions in deep inelas ic sca e ing (DIS) p ocess in a high-ene gy elec on-ion collide whe e a i ual pho on p obes he pa onic s uc u e o a hea y nucleus [ 3 ]. Such collide s a e being conside ed (see e.g. Re . [ 4 ]) bu o some ex en simila measu emen s could be pe o med also in ul a-pe iphe al Pb+Pb collisions a he LHC. In ul a-pe iphe al collisions (UPCs) [ 5 ] he beam pa icles pass wi hou any s ong in e ac ions be ween he cons i u ing pa ons bu a quasi- eal pho on emi ed by one beam pa icle collide wi h a pa icle om he o he beam o wi h a pho on om he o he beam. Unlike in DIS p ocesses he i uali y o hese in e media e pho ons is small so he ha d scale jus i ying he use o pe u ba i e QCD mus be p o ided by a high- pT obse able. Fu he mo e, hese quasi- eal pho ons may luc ua e in o a had onic s a e wi h he same quan um numbe s. The pa onic s uc u e o hese esol ed pho ons can be desc ibed wi h PDFs and, simila ly as in he case o pu ely had onic collisions, gi es ise o mul iple pa onic in e ac ions (MPIs). Due o he ine i able con ibu ion o esol ed pho ons, he measu emen s in his low- Q2 (pho op oduc ion) egime a e no as clean as in high- Q2 (DIS) bu due o p esence o he di ec -pho on -pa on sca e ing, and di e en pa onic s uc u e, he unde lying e en is signi ican ly supp essed compa ed o had onic collisions. In his wo k we s udy he possibili y o u he cons ain he nPDFs using dije p oduc ion in UPCs a he LHC as p oposed in Re . [ 6 ]. The i s expe imen al esul s o he p ocess ha e been ecen ly published by he ATLAS collabo a ion [ 7 ]. Dije p oduc ion in p+Pb collisions a he LHC has al eady been conside ed as a p obe o nPDFs (see e.g. Re . [ 8 ]) bu he UPC measu emen would allow o s udy je p oduc ion in a cleane γA in e ac ion ha should enable je econs uc ion a lowe alues o pT . This inc eases he small- x each o he obse able and also lowe s he scale a which he nPDFs a e p obed whe e he unce ain ies a e la ge . In his s udy he dije c oss sec ions a e calcula ed applying he ecen pho op oduc ion amewo k o PYTHIA 8 [ 9 ] Mon e Ca lo e en gene a o ha has been alida ed agains a ious HERA da a [10]. 2. Ul a-pe iphe al Pb+Pb collisions a he LHC The pho on lux om hea y ions can be ob ained acco ding o he equi alen pho on app oxi- ma ion. Since he con igu a ions whe e he wo ions would in e ac had onically mus be ejec ed, i is con enien o de ine he pho on lux in impac -pa ame e ( b ) space. To a good p ecision, he ele an pa o he pho on lux can be calcula ed by in eg a ing he lux o e impac pa ame e s la ge han he sum o adii o he colliding ions (RA), gi ing xγ A γ(xγ) = 2αEMZ2 πξK1(ξ)K0(ξ)−ξ2 2K2 1(ξ)−K2 0(ξ),(2.1) whe e xγ is he momen um ac ion o he pho on w . he beam pa icle, ξ=2RAxγm/(¯ hc) , m (pe -nucleon) mass and Z he elec ic cha ge o he nucleus. Compa ed o he lux o pho ons om lep ons he lux om hea y ions is ampli ied by a ac o o Z2 bu , due o he ini e size o he nuclei aken in o accoun wi h he impac -pa ame e based ejec ion, he pho on spec um is so e as 1 PoS(Ha dP obes2018)118 P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions Ilkka Helenius shown in Fig. 1. The in a ian mass dis ibu ions o he γ -nucleon sys em, Wγp , o e en s ha ha e a pa onic je wi h pT>10 GeV/c is shown in Fig. 1 o se ups co esponding o he pho op oduc ion measu emen s a HERA and he UPCs a he LHC. As he pho on spec um om nuclei is a he so , la ge con ibu ion o he dije c oss sec ion in UPCs is coming om Wγp alues p obed a HERA bu due o highe √sNN a he LHC, he Wγp dis ibu ion ex ends s ill up o a ac o o wo highe ene gies han wha was eached a HERA. Fo he UPC e en s he pa on showe s and MPIs a e gene a ed o he γ -nucleon sys em so u he sca e ings o he pho on emnan wi h he o he nucleons a e no conside ed. 10 410 310 210 1100 x 10 2 10 1 100 101 102 103 104 105 x i ( x ) * / EM lep ons Pb nucleus a bi . uni s Wγp[GeV] HERA: ep@318 GeV LHC: PbPb@5000 GeV E en s wi h pje T>10 GeV Figu e 1: Le : The pho on lux om lep ons (o ange) and om Pb-nucleus (blue) as a unc ion o pho on momen um ac ion xγ . Righ : In a ian -mass dis ibu ions o γ -nucleon sys em in e+p collisions a HERA (o ange) and ul a-pe iphe al Pb+Pb collisions a he LHC (blue) o e en s ha ha e a pa onic je wi h pT>10 GeV/c. 3. Dije sensi i i y o nPDFs We s udy he pho o-nuclea dije p oduc ion in wo di e en je kinema ics. Fi s is based on he p elimina y ATLAS s udy [ 7 ] whe e he pT o he leading je was abo e 20 GeV/c and he sub-leading je s had pT>15 GeV/c . In he second case he co esponding limi s o he pT o he leading and sub-leading je s a e 8 GeV/c and 6 GeV/c which esembles he je kinema ics analyzed a HERA [ 11 ]. In bo h cases he je s a e econs uc ed wi h he an i- kT algo i hm wi h R=0.4 and a e equi ed o be wi hin |η|<4.4 and e en s wi h a leas wo accep ed je s a e conside ed. As in he ATLAS s udy we cons uc he ollowing e en -le el a iables om he ou momen a o he econs uc ed je s mje s =q(ΣiEi)2−|Σi~pi|2HT=ΣipTi yje s =1 2logΣiEi+Σipzi ΣiEi−Σipzi xA=mje s √se−yje s , whe e he sums un o e accep ed je s. Fo he nPDF s udies pa icula ly in e es ing obse able is he xA a iable, which, in leading o de pa on-le el kinema ics would co espond o x2 , he momen um ac ion o he pa on in he nucleus. Howe e , his connec ion is smea ed due o pa on-showe emissions, had oniza ion e ec s and unde lying e en om he MPIs. To s udy he co ela ion be ween xA and x2 wi hin he applied MC amewo k we plo he wo-dimensional c oss sec ion in 2 PoS(Ha dP obes2018)118 P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions Ilkka Helenius e ms o hese a iables in Fig. 2. Fo a mo e quan i a i e esul we also plo he c oss sec ion as a unc ion o x2in one xAbin in linea scale in Fig. 2. The esul s show a s ong co ela ion be ween xAand x2. dσ/dx2[mb] x2 Pb+Pb √sNN = 5.0 TeV an i-kT, R = 0.4 plead T>20 GeV/c mje s >35 GeV 10−2< xA<1.5·10−2 Figu e 2: Le : Co ela ion be ween xA and he ac ual x2 a which he nPDFs a e p obed wi h dije s in he ATLAS kinema ics. Righ : C oss sec ion o e en s wi hin a gi en xAbin as a unc ion o x2. To calcula e he dije c oss sec ion we apply NNPDF2.3 LO [ 12 ] p o on PDFs wi h EPPS16 NLO [ 1 ] nuclea modi ica ions. The esul ing c oss sec ions co esponding o he wo di e en cu s o je pT ’s a e p esen ed in Fig. 3as a unc ion o xA . The esol ed-pho on con ibu ion domina es he c oss sec ion a la ge- xA egion bu he di ec con ibu ion akes o e a a ound xA<0.01(0.002) wi h he ha de (so e ) je cu s. The impac o nPDFs is quan i ied by showing a a io o he calcula ion wi h he p o on PDFs only. The nPDF-o igina ing unce ain ies om EPPS16 analysis a e o he o de 10% a xA>0.01 bu g ow up o 30% a smalle alues o xA wi h he so e je cu s. By de aul PYTHIA 8 akes he PDFs o he esol ed pho ons om CJKL analysis [ 13 ] bu we ha e conside ed also wo o he widely-used se s, GRV [ 14 ] and SaSgam (1D V2) [ 15 ] o s udy he unce ain ies ela ed o p ecision o he pho on PDFs. The esul ing unce ain y is up o 10% a he highes alues o xA whe e he esol ed con ibu ion domina es bu is negligible a smalle alues o xA whe e he nuclea PDF unce ain ies a e la ge. In Fig. 3we also es ima e he expec ed s a is ical p ecision o he so e pT je s assumming in eg a ed luminosi y o L=1.0 nb−1 co esponding o he hea y-ion p og am a he LHC and L=13 nb−1 co esponding o he high-luminosi y LHC (HL- LHC) p oposal. The expec ed s a is ical unce ain ies become la ge han he nPDF unce ain ies a xA.2·10−4. 4. Conclusions We ha e s udied he pho o-nuclea dije p oduc ion in ul a-pe iphe al Pb+Pb collisions a he LHC and i s po en ial o u he cons ain he nPDFs. We ind ha xA de i ed om he econs uc ed je s co ela e well wi h he ac ual alues o he x p obed in he nucleus and ha a small- xA egion he dije p oduc ion is domina ed by he clean di ec -pho on con ibu ion. Compa ing he nPDF unce ain ies wi h he expec ed s a is ical p ecision o he measu emen a he LHC show ha his obse able could po en ially p o ide p ecise nPDF cons ain s down o x∼10−4. 3 PoS(Ha dP obes2018)118 P obing nuclea PDFs wi h dije s in ul a-pe iphe al Pb+Pb collisions Ilkka Helenius dσ/dxA[nb] PbPb,√sNN = 5.02 TeV an i-kT, R = 0.4 plead T>20 GeV/c mje s >35 GeV Py hia NNPDF2.3 EPPS16 esol ed di ec Ra io o NNPDF2.3 xA SaSgam GRV dσ/dxA[nb] PbPb,√sNN = 5.5 TeV an i-kT, R = 0.4 plead T>8 GeV/c mje s >14 GeV Py hia NNPDF2.3 EPPS16 esol ed di ec Ra io o NNPDF2.3 xA L= 1 nb−15.0 TeV L= 13 nb−15.5 TeV Figu e 3: Di e en ial c oss sec ion o pho o-nuclea dije p oduc ion in ul a-pe iphe al Pb+Pb collisions a he LHC as a unc ion o xA . Resul s wi h (blue) and wi hou ( ed) EPPS16 nPDFs a e shown sepa a ely as also he con ibu ions om di ec (o ange) and esol ed p ocesses (g een). The lowe panels show a ios o he esul wi hou he EPPS16 nPDFs. Wi h he highe pT cu s (le ) also he a ios o di e en pho on PDFs a e shown and he expec ed s a is ical unce ain ies co esponding o he in eg a ed luminosi ies o he LHC and HL-LHC a e es ima ed o lowe pTcu s ( igh ). Acknowledgmen s The wo k has been suppo ed by he Ca l Zeiss Founda ion and he Academy o Finland, P ojec 308301. Re e ences [1] K. J. Eskola, P. Paakkinen, H. Paukkunen and C. A. Salgado, Eu . Phys. J. C 77 (2017), 163. [2] K. Ko a ik e al., Phys. Re . D 93 (2016), 085037. [3] E. C. Aschenaue , S. Fazio, M. A. C. Lamon , H. Paukkunen and P. Zu i a, Phys. Re . D 96 (2017), 114005. [4] A. Acca di e al., Eu . Phys. J. A 52 (2016), 268. [5] A. J. Bal z e al., Phys. Rep . 458 (2008) 1. [6] M. S ikman, R. Vog and S. N. Whi e, Phys. Re . Le . 96 (2006) 082001. [7] The ATLAS collabo a ion [ATLAS Collabo a ion], ATLAS-CONF-2017-011. [8] K. J. Eskola, H. Paukkunen and C. A. Salgado, JHEP 1310 (2013) 213 [9] T. Sjös and e al., Compu . Phys. Commun. 191 (2015) 159. [10] I. 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