Deciphering the z g distribution in heavy ion collisions P. Caucal, - PowerPoint PPT Presentation

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Deciphering the z g distribution in heavy ion collisions P. Caucal, E. Iancu and G. Soyez Institut de Physique Th´ eorique Quark Matter 2019 - November 6 - Wuhan Talk based on JHEP 10(2019)273 0 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion A new pQCD picture for jet evolution in the QGP Caucal, Iancu, Mueller, Soyez, 2018 - QM 2018 ωθ Phase space for vacuum-like emissions (VLEs) =log dictated by pQCD principles. ω = E t θ log k c ω = ω Vetoed region for VLEs: only k ⊥ > ˆ qt f or t f > L allowed. c inside Angular ordering of VLEs inside and outside medium the medium. VETOED outside medium Factorization between VLEs and medium-induced emissions (MIEs). non perturbative ⇒ Independent energy loss for VLEs inside with � log 1/ θ θ > θ c = 2 / ˆ qL 3 . Monte-Carlo implementation In this talk, first Monte-Carlo results based on this factorized picture (including coherence effects ) for the z g distribution and R AA . 1 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion z g distribution in a nutshell Larkoski, Marzani, Soyez, Thaler, 2014 primary Lund plane For a given jet of radius R , 6 recluster the constituents of the jet using C/A (ordered in angles). 4 log( k /GeV) = Then iteratively decluster the jet p T 3 = 2 z cut until the SD condition is met: cut p T 1 = 2 0 z 12 ≡ min ( p T 1 , p T 2 ) ≥ z cut p T 1 + p T 2 0 2 4 6 8 Additional constraint: log( R / ) � ∆ y 2 12 + ∆ φ 2 ∆ R 12 ≡ 12 ≥ θ cut 1 2 p T, 2 Add the value of z g to the 3 corresponding bin in your histogram ∆ R 12 = θ g p T, 1 and normalize it either to 1 or to p T z g = min ( p T, 1 ,p T, 2 ) θ 1 ≫ θ 2 ≫ θ 3 = θ g p T, 1 + p T, 2 the total number of jets . 2 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Why focusing on the z g distribution ? Not IRC safe but Sudakov safe ⇒ can be calculated in pQCD. Larkoski, Marzani, Thaler, 2015 Example: in the vacuum, with fixed coupling α s : measure splitting function ! � �� � 1 dN i ∼ 1 ¯ ∝ P i ( z g ) N jets dz g z g 3 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Why is the z g distribution interesting in heavy ion collisons ? In the presence of the medium, controlled by two basic phenomena : • SoftDrop condition triggered either by a vacuum-like emission or by a medium induced emission . Mehtar-Tani, Tywoniuk 2017 • both subjets lose energy via medium induced emissions, independently as long as � qL 3 . Mehtar-Tani, Salgado, Tywoniuk, 2010-11 ; Casalderrey-Solana, Iancu, 2011 θ g ≥ θ c = 2 / ˆ 4 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Two regimes Red region: Phase space for vacuum-like emissions inside the medium . qL 2 maximal energy and Q s = ω c θ c = √ ˆ ω c = 1 2 ˆ qL maximal k ⊥ for MIEs. primary Lund plane - high p T regime primary Lund plane - low p T regime 6 6 4 4 cut = z cut p T log( k /GeV) log( k /GeV) = = p T cut = 2 2 = z cut p T p T = k = c c ( c , c ) ( c , c ) 0 0 0 2 4 6 8 0 2 4 6 8 log( R / ) log( R / ) High- p T regime: p T z cut ≫ ω c Low- p T regime: p T z cut � ω c SD can only select VLEs. SD can select both VLEs and MIEs. 5 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion High p T regime z cut p T ≫ ω c : energy loss shift effect p T, 1 = zp T − E g zp T − E g z g = p T − E g − E i ⇒ z = z g + E g − z g ( E g + E i ) p T, 2 = (1 − z ) p T − E i ≥ z g p T p T Incoherent energy loss Relate the physical z fraction before energy loss to the measured z g balance. 6 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion High p T regime z cut p T ≫ ω c : energy loss shift effect p T, 1 = zp T − E g zp T − E g z g = p T − E g − E i p T, 2 = (1 − z ) p T − E i p T ⇒ z = z g + E g − z g ( E g + E i ) ≥ z g p T N jets -norm z g distribution - high-p T jets 1.05 anti-k t (R=0.4), SD( β =0,z cut =0.1) [mMDT], θ g >0.1 =1.5 GeV 2 /fm, L=4 fm ^ q Assume E g and E i constant , 1 p T0,gluon =1 T eV 0.95 R(z g ) ¯ P i ( z ) z ≃ 1 − ∆ z P i ( z g ) ∼ z g 0.9 R AA ( z g ) ∝ ¯ z g 0.85 ε =16.5 GeV Ratio increases with z g . 0.8 0 0.1 0.2 0.3 0.4 0.5 z g 6 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion How much energy is lost ? In previous slide, constant energy loss for a jet without VLEs: E g ∝ ω br ≡ α 2 s ω c Not realistic ! p T ր ⇒ phase space for in-medium VLEs ր ⇒ more sources for energy loss This simple fact explains R AA for the jet cross-section pattern... Caucal, Iancu, Soyez 2019 average energy loss - p T0 dependence R AA : varying uncontrolled parameters 70 70 MIEs only 1 =1.5 GeV 2 /fm =1.5 GeV 2 /fm ^ ^ MIEs+VLEs q q ATLAS 60 60 0.9 21.4+10.6 log(p T0 / ω c )+1.46 log 2 (p T0 / ω c ) L=4 fm L=4 fm ours average energy loss [GeV] α s =0.24 α s =0.24 gluon gluon 0.8 50 50 =1.5 GeV/fm 2 =1.5 GeV/fm 2 ^ ^ q q 0.7 40 40 L=4 fm L=4 fm R AA α s =0.24 α s =0.24 0.6 30 30 0.5 20 20 0.4 anti-k t (R=0.4), |y|<2.8 anti-k t (R=0.4), |y|<2.8 0.3 √ s=5.02 T √ s=5.02 T eV, 0-10% centrality eV, 0-10% centrality 10 10 anti-k t (R=0.4) anti-k t (R=0.4) θ max =1(0.75,1.5), k ⊥ ,min =0.25(0.15,0.5) GeV θ max =1(0.75,1.5), k ⊥ ,min =0.25(0.15,0.5) GeV θ max =R, k ⊥ ,min =0.25 GeV θ max =R, k ⊥ ,min =0.25 GeV 0.2 100 200 500 1000 0 0 2 2 5 5 10 10 20 20 50 50 100 100 200 200 500 500 1000 1000 p T,jet [GeV] p T0 [GeV] 7 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion How much energy is lost ? In previous slide, constant energy loss for a jet without VLEs: E g ∝ ω br ≡ α 2 s ω c Not realistic ! p T ր ⇒ phase space for in-medium VLEs ր ⇒ more sources for energy loss ... and R AA for the N jets normalized z g distribution ! [ E → E ( zp T )] average energy loss - p T0 dependence 70 70 N jets -norm z g distribution - high-p T jets MIEs only 1.05 MIEs+VLEs 60 60 anti-k t (R=0.4), SD( β =0,z cut =0.1) [mMDT], θ g >0.1 21.4+10.6 log(p T0 / ω c )+1.46 log 2 (p T0 / ω c ) =1.5 GeV 2 /fm, L=4 fm ^ q average energy loss [GeV] 1 gluon gluon 50 50 =1.5 GeV/fm 2 =1.5 GeV/fm 2 p T0,gluon =1 T eV ^ ^ q q 0.95 40 40 L=4 fm L=4 fm R (z g ) α s =0.24 α s =0.24 0.9 30 30 MC 20 20 0.85 ε = ε fi t (p T ) ε =16.5 GeV 10 10 anti-k t (R=0.4) anti-k t (R=0.4) 0.8 θ max =R, k ⊥ ,min =0.25 GeV θ max =R, k ⊥ ,min =0.25 GeV 0 0.1 0.2 0.3 0.4 0.5 0 0 z g 2 2 5 5 10 10 20 20 50 50 100 100 200 200 500 500 1000 1000 p T0 [GeV] 7 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Low p T regime z cut p T � ω c : intrajet medium-induced emissions At low p T , the energy loss shift effect remains . Contradiction with the data ? No, because there is another effect: SD can select relatively hard medium-induced emissions remaining inside the jet cone. z g distribution - normalized by 1/N jets anti-k t (R=0.4), p T ∈ [80,120] GeV, | η |<0.5 1 SD( β =0,z cut =0.1) [mMDT], θ g >0.1 ALICE Collaboration, 2018 ^ = 1.5 GeV 2 /fm, L = 4fm, α s =0.24 q 0.8 Ratio PbPb/pp 0.6 0.4 MC - √ s=5.02 T eV 0.2 MC - incoherent energy loss only 0 0.1 0.2 0.3 0.4 0.5 z g 8 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Analytical insight For MIEs, emission probability is: Baier, Dokshitzer, Mueller, Peign´ e, and Schiff; Zakharov 1996–9 � d 2 P MIE = α s C R 2 ω c dz z 3 / 2 × P broad ( z , θ ) d θ π p T � �� � Gaussian-like broadening distribution ⇒ more singular at small z than the vacuum splitting function. 9 / 13

Introduction High p T regime: energy loss only Low p T regime: energy loss and MIEs Conclusion Analytical insight The z g distribution without energy loss shift is enhanced by a medium-induced term: � R (1 + ν ) d 2 P MIE 1 dN 1 dN ∆ VLE ( R , θ g )∆ MIE = + d θ g ( R , θ g ) i i N jets dz g N jets dz g | vac dz g d θ g θ cut � �� � no emission with z > z cut between R and θ g Multiplicity factor 1 + ν because each VLE inside the medium can further radiate a MIE. With the energy loss shift, one has to relate the physical z to the measured z g as before. zp T − E g z g = p T − E g − E i 9 / 13