(Highly) Tentative Conclusions based on the Early LHC Data P. Skands (CERN TH) 1
The Power of Prediction • We are at a unique time in the LHC era • Predictions, without foreknowledge, can be tested with totally NEW data • This is right here and now, once and only • Attempt to learn as much as possible from these “blind” tests, which cannot be repeated 2
The Basic Four 900 GeV 900 GeV ALICE (INEL, NSD) ATLAS (N ≥ 1) CMS (NSD) ALICE (N ≥ 1,NSD) ATLAS (N ≥ 1) P(N) dN/d η 2.36 TeV 2.36 TeV CMS (NSD) ALICE (N ≥ 1,NSD) ALICE (INEL, NSD) 7 TeV 7 TeV ALICE (N ≥ 1) CMS (NSD) 900 GeV 900 GeV CMS (NSD) ATLAS (N ≥ 1) ATLAS (N ≥ 1) 2.36 TeV dN/dp ⊥ 〈 p ⊥ 〉 (N) 2.36 TeV - CMS (NSD) 7 TeV 7 TeV - CMS (NSD) (NSD): physical MB trigger + SD correction w/o physical SD definition C. Zampolli : different model for each of these! “Not satisfactory” Models should be “universal”. Inability to get universal tune → more physics? 3
dN/d η • Really d 〈 N 〉 /d η ⇒ most sensitive to first few bins of P(N) • 〈 N 〉 cannot be interpreted without • EITHER: a soft/zero trigger + good model of diffraction • OR: a hard trigger that suppresses diffraction • Cannot be interpreted at all without physical trigger (NSD!) • It is the least useful of the basic four • Mixes low-mult (diffractive/peripheral) and high-mult (non-diffractive/ hard-core) physics over its entire range • Danger: It is entirely possible to fit this variable while still mismodeling both diffraction and UE (the two wrongs → right effect) 4
dN/d η • Really d 〈 N 〉 /d η ⇒ most sensitive to first few bins of P(N) • 〈 N 〉 cannot be interpreted without • EITHER: a soft/zero trigger + good model of diffraction • OR: a hard trigger that suppresses diffraction • Cannot be interpreted at all without physical trigger (NSD!) • It is the least useful of the basic four • Mixes low-mult (diffractive/peripheral) and high-mult (non-diffractive/ hard-core) physics over its entire range • Danger: It is entirely possible to fit this variable while still mismodeling both diffraction and UE (the two wrongs → right effect) 4
dN/dp ⊥ • Each track = one entry ⇒ low-mult events relatively less important • Still mixes low-mult (diffractive/peripheral) and high-mult (non-diffractive/ hard-core) physics over its entire range • Compare p ⊥ spectrum under different trigger conditions • Mainly sensitive to (string) fragmentation processes. Some sensitivity to semi-hard (mini-)jet production • Soft models → too soft spectrum? • When tuning to 〈 p ⊥ 〉 (N), important to check tail of dN/dp ⊥ • To maximize fragmentation sensitivity: convert to x ⊥ spectrum? (~ UE-corrected p ⊥ /E ⊥ jet ) 5
dN/dp ⊥ • Each track = one entry ⇒ low-mult events relatively less important • Still mixes low-mult (diffractive/peripheral) and high-mult (non-diffractive/ hard-core) physics over its entire range • Compare p ⊥ spectrum under different trigger conditions • Mainly sensitive to (string) fragmentation processes. Some sensitivity to semi-hard (mini-)jet production • Soft models → too soft spectrum? • When tuning to 〈 p ⊥ 〉 (N), important to check tail of dN/dp ⊥ • To maximize fragmentation sensitivity: convert to x ⊥ spectrum? (~ UE-corrected p ⊥ /E ⊥ jet ) 5
dN/dp ⊥ • Fast turnaround. Data propagates quickly into HepDATA! • + set of standard MC curves in paper gives us a reproducible counter- check and benchmark for future comparisons. EXCELLENT! 10 10 ] ] 900 GeV p+p -2 -2 Inelastic, Non-Diffractive p > 500 MeV, | | < 2.5, n 1 ! # [ GeV [ GeV b) 1 1 ch 1/N ch dN ch /dp � T Charged Particle p � Spectrum (| � |<2.5, p � >0.5GeV) -1 -1 ATLAS 10 10 T T 1 1 ATLAS data p p s = 900 GeV -2 -2 d d 10 10 ! ! Perugia 0 /d /d -3 -3 -1 -1 10 10 ch ch 10 10 N N 2 2 -4 -4 10 10 ) d ) d T T -2 -2 -5 -5 p p 10 10 10 10 $ $ 1/(2 1/(2 -6 -6 10 10 Data 2009 -3 -3 ev ev -7 -7 10 10 10 10 N N PYTHIA ATLAS MC09 1/ 1/ PYTHIA ATLAS MC09c -8 -8 10 10 -4 -4 PYTHIA DW -9 -9 10 10 PYTHIA Perugia0 10 10 PHOJET -10 -10 10 10 -5 -5 10 10 Data Uncertainties 1.5 1.5 MC / Data -6 -6 Ratio Ratio 10 10 Pythia 6.423 1 1 Data from ATLAS Collaboration, Phys.Lett. B688(2010)21 -7 -7 10 10 0 0 5 5 10 10 15 15 20 20 0.5 0.5 p � ! GeV " 1 1 10 10 6 p p [GeV] [GeV] T T
dN/dp ⊥ • Fast turnaround. Data propagates quickly into HepDATA! • + set of standard MC curves in paper gives us a reproducible counter- check and benchmark for future comparisons. EXCELLENT! 10 10 ] ] 900 GeV p+p -2 -2 Inelastic, Non-Diffractive p > 500 MeV, | | < 2.5, n 1 ! # [ GeV [ GeV b) 1 1 ch 1/N ch dN ch /dp � T Charged Particle p � Spectrum (| � |<2.5, p � >0.5GeV) -1 -1 ATLAS 10 10 T T 1 1 ATLAS data p p s = 900 GeV -2 -2 d d 10 10 ! ! Perugia 0 /d /d -3 -3 -1 -1 10 10 ch ch 10 10 N N 2 2 -4 -4 10 10 ) d ) d T T -2 -2 -5 -5 p p 10 10 10 10 $ $ 1/(2 1/(2 -6 -6 10 10 Data 2009 -3 -3 ev ev -7 -7 10 10 10 10 N N PYTHIA ATLAS MC09 1/ 1/ PYTHIA ATLAS MC09c -8 -8 10 10 -4 -4 PYTHIA DW -9 -9 10 10 PYTHIA Perugia0 10 10 PHOJET -10 -10 10 10 -5 -5 10 10 Data Uncertainties 1.5 1.5 MC / Data -6 -6 Ratio Ratio 10 10 Pythia 6.423 1 1 Data from ATLAS Collaboration, Phys.Lett. B688(2010)21 -7 -7 10 10 0 0 5 5 10 10 15 15 20 20 0.5 0.5 p � ! GeV " 1 1 10 10 6 p p [GeV] [GeV] T T
P(N) • One entry for each N: low-mult events clearly distinguishable from high-mult • Low peak sensitive to diffraction, dominated by peripheral (LEP-like) collisions (?), no collective effects? • Falloff of high-N tail sensitive to UE, dominated by hard, central collisions. Departures from LEP fragmentation? Collective effects? • Intermediate region (shape) sensitive to proton mass distribution 7
P(N) • Extrapolations from Tevatron have ~ too low tail already at � � � 900 GeV (cf UA5) - gets worse when we go → 2.36 → 7 TeV 900 GeV p+pbar Inelastic, Non-Diffractive 1 1 From C. Zampolli’s talk Probability(N ch ) Charged Particle Multiplicity (| � |<1.5, all p � , N ch � 5) UA5 data (NSD) <13.9> -1 -1 Perugia 0 <17.7> 10 10 Pro-pTO <17.8> Pro-Q2O <17.2> DW <16.1> -2 -2 10 10 -3 -3 10 10 -4 -4 10 10 Pythia 6.423 Data from UA5 Collaboration, Z Phys 43(1989)357 -5 -5 10 10 0 0 20 20 40 40 60 60 N ch (| � |<1.5, all p � , N ch � 5) 8 � � � �
P(N) • Extrapolations from Tevatron have ~ too low tail already at � � � 900 GeV (cf UA5) - gets worse when we go → 2.36 → 7 TeV 900 GeV p+pbar Inelastic, Non-Diffractive 1 1 From C. Zampolli’s talk Probability(N ch ) Charged Particle Multiplicity (| � |<1.5, all p � , N ch � 5) UA5 data (NSD) <13.9> -1 -1 Perugia 0 <17.7> 10 10 Pro-pTO <17.8> Pro-Q2O <17.2> DW <16.1> -2 -2 So! They already knew! Why didn’t they (we) lift the tail higher? 10 10 -3 -3 10 10 -4 -4 10 10 Pythia 6.423 Data from UA5 Collaboration, Z Phys 43(1989)357 -5 -5 10 10 0 0 20 20 40 40 60 60 N ch (| � |<1.5, all p � , N ch � 5) 8 � � � �
P(N) Tevatron tail tension. E.g., Perugia 0 already slightly high at both Tevatron energies - (more LHC data at ~ 2-3 TeV would be useful) 9
〈 p ⊥ 〉 (N) • One entry for each N: low-mult events clearly distinguishable from high-mult • Low N sensitive to diffraction, dominated by peripheral (LEP- like) collisions (?), no collective effects? • High N sensitive to UE, dominated by hard, central collisions. Departures from LEP fragmentation? Collective effects? • Intermediate region (shape) sensitive to proton mass distribution • Appears to be a sensitive probe of infrared dynamics. Higher moments also sensitive? 10
〈 p ⊥ 〉 (N) ��� • Non-trivial energy dependence. • A (partial) tradeoff with 〈 N 〉 appears possible. Sufficient? average transverse momentum <p T > 1.3 1.3 [ GeV ] [ GeV ] p > 500 MeV, | | < 2.5, n 1 ! # d) ch T 1.2 1.2 ATLAS % % T T p p s = 900 GeV & & 1.1 1.1 1 1 0.9 0.9 Data 2009 0.8 0.8 PYTHIA ATLAS MC09 PYTHIA ATLAS MC09c 0.7 0.7 PYTHIA DW PYTHIA Perugia0 PHOJET 0.6 0.6 10 10 20 20 30 30 40 40 50 50 60 60 1.1 1.1 Ratio Ratio 1 1 0.9 0.9 Data Uncertainties MC / Data 0.8 0.8 10 10 20 20 30 30 40 40 50 50 60 60 n n ch ch 11 At N >60 model shows a rise not
Conclusions • Question marks concerning energy scaling • Apparent tensions with Tevatron: not certain that “trivial” retunings sufficient to span all energies? • What is the cause? • Energy-dependent energy dependence? • Different scaling law? • Different scaling for diffraction vs non-diffractive? • Energy- vs x - dependence? • Other energy- or x -dependent phenomena? (e.g., mass distributions? collective effects? … ?) 12
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