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Predicting Min-Bias and the Underlying Event at the LHC: Early Physics Measurements

This talk outlines the measurements and predictions of the min-bias and underlying event at different energies at the LHC. It includes comparisons with Monte-Carlo tunes and explores the relationship between the hard scattering process and min-bias collisions.

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Predicting Min-Bias and the Underlying Event at the LHC: Early Physics Measurements

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  1. Predicting “Min-Bias” and the “Underlying Event” at the LHC Early Physics Measurements Rick Field University of Florida Outline of Talk • “Min-Bias” at 900 GeV, 2.2 TeV, 7 TeV, 10 TeV, and 14 TeV. • New CDF charged multiplicity distribution and comparisons with the QCD Monte-Carlo tunes. CERN November 6, 2009 • Relationship between the “underlying event” in a hard scattering process and “min-bias” collisions. CDF Run 2 • The “underlying event” at 900 GeV, 7 TeV, 10 TeV, and 14 teV. • The “underlying event” in Drell-Yan production at 7 TeV, UE&MB@CMS CMS at the LHC • Summary & Conclusions. Rick Field – Florida/CDF/CMS

  2. Proton-AntiProton Collisionsat the Tevatron The CDF “Min-Bias” trigger picks up most of the “hard core” cross-section plus a small amount of single & double diffraction. stot = sEL + sIN stot = sEL + sSD+sDD+sHC 1.8 TeV: 78mb = 18mb + 9mb + (4-7)mb + (47-44)mb CDF “Min-Bias” trigger 1 charged particle in forward BBC AND 1 charged particle in backward BBC The “hard core” component contains both “hard” and “soft” collisions. “Inelastic Non-Diffractive Component” Beam-Beam Counters 3.2 < |h| < 5.9 Rick Field – Florida/CDF/CMS

  3. Inelastic Non-Diffractive Cross-Section • The inelastic non-diffractive cross section versus center-of-mass energy from PYTHIA (×1.2). My guess! Lots of events! Linear scale! Log scale! stot = sEL + sSD+sDD+sHC • sHC varies slowly. Only a 13% increase between 7 TeV (≈ 58 mb) and 14 teV (≈ 66 mb). Linear on a log scale! Rick Field – Florida/CDF/CMS

  4. Charged Particle Density: dN/dh • Charged particle (all pT) pseudo-rapidity distribution, dNchg/dhdf, at 1.96 TeV for inelastic non-diffractive collisions from PYTHIA Tune A,Tune DW, Tune S320, and Tune P324. • Charged particle (pT>0.5 GeV/c) pseudo-rapidity distribution, dNchg/dhdf, at 1.96 TeV for inelastic non-diffractive collisions from PYTHIA Tune A,Tune DW, Tune S320, and Tune P324. Rick Field – Florida/CDF/CMS

  5. Charged Particle Density: dN/dh RDF LHC Prediction! Tevatron LHC • Charged particle (all pT) pseudo-rapidity distribution, dNchg/dhdf, at 1.96 TeV for inelastic non-diffractive collisions from PYTHIA Tune A,Tune DW, Tune S320, and Tune P324. • Extrapolations (all pT) of PYTHIA Tune A,Tune DW, Tune S320,Tune P324. and ATLAS to the LHC. Rick Field – Florida/CDF/CMS

  6. Charged Particle Density: dN/dh RDF LHC Prediction! Tevatron LHC • Charged particle (pT > 0.5 GeV/c) pseudo-rapidity distribution, dNchg/dhdf, at 1.96 TeV for inelastic non-diffractive collisions from PYTHIA Tune A,Tune DW, Tune S320, and Tune P324. • Extrapolations (pT > 0.5 GeV/c) of PYTHIA Tune A,Tune DW, Tune S320,Tune P324. and ATLAS to the LHC. Rick Field – Florida/CDF/CMS

  7. Min-Bias “Charged Particle Density • Shows the “min-bias” charged particle density, dN/dh, for charged particles (pT > 0.5 GeV/c) for at 0.2 TeV, 0.9 TeV, 1.96 TeV and 14 TeVpredicted by PYTHIA Tune DW at the particle level (i.e. generator level). LHC14 LHC7 Tevatron 900 GeV RHIC 1.96 TeV → 14 TeV (dN/dh increase ~1.58 times) 0.2 TeV → 1.96 TeV (dN/dh increase ~1.63 times) LHC Tevatron RHIC Linear scale! Rick Field – Florida/CDF/CMS

  8. Min-Bias “Charged Particle Density • Shows the “min-bias” charged particle density, dN/dh, for charged particles (pT > 0.5 GeV/c) for at 0.2 TeV, 0.9 TeV, 1.96 TeV and 14 TeVpredicted by PYTHIA Tune DW at the particle level (i.e. generator level). LHC14 LHC10 LHC7 Tevatron 900 GeV RHIC 7 TeV → 14 TeV (dN/dh ≈ 19% increase) Log scale! Linear on a log plot! LHC7 LHC14 Rick Field – Florida/CDF/CMS

  9. Charged Particle Multiplicity New • Data at 1.96 TeV on the charged particle multiplicity (pT > 0.4 GeV/c, |h| < 1) for “min-bias” collisions at CDF Run 2. Tune A! No MPI! 7 decades! • The data are compared with PYTHIA Tune A and Tune A without multiple parton interactions (pyAnoMPI). Rick Field – Florida/CDF/CMS

  10. Charged Particle Multiplicity • Data at 1.96 TeV on the charged particle multiplicity (pT > 0.4 GeV/c, |h| < 1) for “min-bias” collisions at CDF Run 2. Tune A! No MPI! Tune S320! • The data are compared with PYTHIA Tune A and Tune A without multiple parton interactions (pyAnoMPI). Rick Field – Florida/CDF/CMS

  11. The “Underlying Event” Select inelastic non-diffractive events that contain a hard scattering Hard parton-parton collisions is hard (pT > ≈2 GeV/c) “Semi-hard” parton-parton collision (pT < ≈2 GeV/c) The “underlying-event” (UE)! + + + … Given that you have one hard scattering it is more probable to have MPI! Hence, the UE has more activity than “min-bias”. Multiple-parton interactions (MPI)! Rick Field – Florida/CDF/CMS

  12. The Inelastic Non-Diffractive Cross-Section Occasionally one of the parton-parton collisions is hard (pT > ≈2 GeV/c) Majority of “min-bias” events! “Semi-hard” parton-parton collision (pT < ≈2 GeV/c) + + + + … Multiple-parton interactions (MPI)! Rick Field – Florida/CDF/CMS

  13. The “Underlying Event” Select inelastic non-diffractive events that contain a hard scattering Hard parton-parton collisions is hard (pT > ≈2 GeV/c) “Semi-hard” parton-parton collision (pT < ≈2 GeV/c) The “underlying-event” (UE)! + + + … Given that you have one hard scattering it is more probable to have MPI! Hence, the UE has more activity than “min-bias”. Multiple-parton interactions (MPI)! Rick Field – Florida/CDF/CMS

  14. Min-Bias Correlations New • Data at 1.96 TeV on the average pT of charged particles versus the number of charged particles (pT > 0.4 GeV/c, |h| < 1) for “min-bias” collisions at CDF Run 2. The data are corrected to the particle level and are compared with PYTHIA Tune A at the particle level (i.e. generator level). Rick Field – Florida/CDF/CMS

  15. Min-Bias: Average PT versus Nchg • Beam-beam remnants (i.e. soft hard core) produces low multiplicity and small <pT> with <pT> independent of the multiplicity. • Hard scattering (with no MPI) produces large multiplicity and large <pT>. • Hard scattering (with MPI) produces large multiplicity and medium <pT>. This observable is sensitive to the MPI tuning! = + + The CDF “min-bias” trigger picks up most of the “hard core” component! Rick Field – Florida/CDF/CMS

  16. Charged Particle Multiplicity The charged multiplicity distribution does not change between 1.96 and 2.2 TeV and proton-proton is the same as proton-antiproton! • Data at 1.96 TeV on the charged particle multiplicity (pT > 0.4 GeV/c, |h| < 1) for “min-bias” collisions at CDF Run 2. The data are compared with PYTHIA Tune A and Tune A without multiple parton interactions (pyAnoMPI). Tune A prediction at 900 GeV! Tune A prediction at 2.2 TeV! Tune A! No MPI! • Prediction from PYTHIA Tune A for proton-proton collisions at 900 GeV and 2.2 TeV. Rick Field – Florida/CDF/CMS

  17. LHC Predictions: 900 GeV • Charged multiplicity distributions for proton-proton collisions at 900 GeV (|h| < 2) from PYTHIA Tune A, Tune DW, Tune S320, and Tune P329. Rick Field – Florida/CDF/CMS

  18. LHC Predictions: 900 GeV If we get 10,000 HC collisions we could measure <Nchg> BUT we also want to measure the activity in the “underlying event” which would take ~2,000,000 HC collisions! 1,000 events L = 24/mb! 100 events L = 2.4/mb! 10 events L = 0.24/mb! • Charged multiplicity distributions for proton-proton collisions at 900 GeV (pT > 0.5 GeV/c, |h| < 2) from PYTHIA Tune A, Tune DW, Tune DWT, Tune S320, and Tune P329. Rick Field – Florida/CDF/CMS

  19. “Transverse” Charged Density • Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5 GeV/c, |h| < 1) at 7 TeVas defined by PTmax, PT(chgjet#1), and PT(muon-pair) from PYTHIATune DWat the particle level (i.e. generator level). Charged particle jets are constructed using the Anti-KT algorithm with d = 0.5. Rick Field – Florida/CDF/CMS

  20. Min-Bias “Associated”Charged Particle Density • Shows the “associated” charged particle density in the “transverse” region as a function of PTmax for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events at 0.2 TeV, 0.9 TeV, 1.96 TeV, 7 TeV, 10 TeV, 14 TeVpredicted by PYTHIA Tune DW at the particle level (i.e. generator level). LHC14 LHC10 LHC7 Tevatron 900 GeV RHIC 0.2 TeV → 1.96 TeV (UE increase ~2.7 times) 1.96 TeV → 14 TeV (UE increase ~1.9 times) RHIC LHC Tevatron Linear scale! Rick Field – Florida/CDF/CMS

  21. Min-Bias “Associated”Charged Particle Density • Shows the “associated” charged particle density in the “transverse” region as a function of PTmax for charged particles (pT > 0.5 GeV/c, |h| < 1, not including PTmax) for “min-bias” events at 0.2 TeV, 0.9 TeV, 1.96 TeV, 7 TeV, 10 TeV, 14 TeVpredicted by PYTHIA Tune DW at the particle level (i.e. generator level). LHC14 LHC10 LHC7 Tevatron 900 GeV RHIC 7 TeV → 14 TeV (UE increase ~20%) LHC7 LHC14 Linear on a log plot! Log scale! Rick Field – Florida/CDF/CMS

  22. sHC: PTmax > 5 GeV/c • The inelastic non-diffractive PTmax > 5 GeV/c cross section versus center-of-mass energy from PYTHIA (×1.2). Still lots of events! Log scale! Linear scale! stot = sEL + sSD+sDD+sHC In 1,000,000 HC collisions at 900 GeV you get 940 with PTmax > 5 GeV/c! • sHC(PTmax > 5 GeV/c) varies more rapidly. Factor of 2.3 increase between 7 TeV (≈ 0.56 mb) and 14 teV (≈ 1.3 mb). Linear on a linear scale! Rick Field – Florida/CDF/CMS

  23. “Transverse” Charged Density With 1/nb of “min-bias” data at 7 TeV we could study the UE out to PTmax = 25 GeV/c or PT(chgjet#1) = 45 GeV/c ! • Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5 GeV/c, |h| < 1) at 7 TeVas defined by PTmax and PT(chgjet#1) from PYTHIATune DWat the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5. • Shows the leading charged particle jet, chgjet#1, and the leading charged particle, PTmax, differential cross section, ds/dPT (pT > 0.5 GeV/c, |h| < 1) from PYTHIATune DWat the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5. Rick Field – Florida/CDF/CMS

  24. “Transverse” Charged Density • Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5 GeV/c) at 7 TeVfor |h| < 1 and |h| < 2 as defined PT(chgjet#1) from PYTHIATune DWat the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5. • Shows the charged particle density in the “transMAX” and “transMIN” region for charged particles (pT > 0.5 GeV/c) at 7 TeVfor |h| < 1 and |h| < 2 as defined PT(chgjet#1) from PYTHIATune DWat the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5. Rick Field – Florida/CDF/CMS

  25. “Hard Scattering” Component QCD Monte-Carlo Models:High Transverse Momentum Jets • Start with the perturbative 2-to-2 (or sometimes 2-to-3) parton-parton scattering and add initial and final-state gluon radiation (in the leading log approximation or modified leading log approximation). “Underlying Event” • The “underlying event” consists of the “beam-beam remnants” and from particles arising from soft or semi-soft multiple parton interactions (MPI). The “underlying event” is an unavoidable background to most collider observables and having good understand of it leads to more precise collider measurements! • Of course the outgoing colored partons fragment into hadron “jet” and inevitably “underlying event” observables receive contributions from initial and final-state radiation. Rick Field – Florida/CDF/CMS

  26. Drell-Yan Muon-Pair Cross-Section • The Drell-Yan muon-pair cross section 70 < M(pair) < 110 GeV versus center-of-mass energy from PYTHIA (×1.3). Linear scale! • The Drell-Yan cross-section varies rapidly. Factor of 2.2 increase between 7 TeV (≈ 0.9 nb) and 14 teV (≈ 2 nb). Linear on a linear scale! Note nb not mb! Rick Field – Florida/CDF/CMS

  27. Drell-Yan Muon-Pair Cross-Section • The Drell-Yan muon-pair cross section 70 < M(pair) < 110 GeV (|h(m)| < 2.4, pT(m) > 5 GeV/c) versus center-of-mass energy from PYTHIA (×1.3). Linear scale! 4,700 events in 10/pb! CMS acceptance! • The CMS Drell-Yan cross-section varies rapidly. Factor of 1.9 increase between 7 TeV (≈ 0.5 nb) and 14 TeV (≈ 0.9 nb). Linear on a linear scale! Rick Field – Florida/CDF/CMS

  28. “Transverse” Charged Density Note at CMS “min-bias” is pre-scaled by a factor of 5,000 so this really corresponds to 5/pb delivered ! With 10/pb of data at 7 TeV we could study the UE in Drell-Yan production out to PT(pair) = 15 GeV/c ! • Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5 GeV/c, |h| < 1) at 7 TeVas defined by PTmax, PT(chgjet#1), and PT(muon-pair) for PYTHIATune DWat the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5. • Shows the leading charged particle jet, chgjet#1, and the leading charged particle, PTmax, differential cross section, ds/dPT (pT > 0.5 GeV/c, |h| < 1), and the Drell-Yan differential cross-section (70 < M(pair) < 110 GeV) from PYTHIATune DWat the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5. Rick Field – Florida/CDF/CMS

  29. Z-Boson: “Towards” Region • Data at 1.96 TeV on the density of charged particles, dN/dhdf, with pT > 0.5 GeV/c and |h| < 1 for “Z-Boson” events as a function of PT(Z) for the “toward” region from PYTHIA Tune AW, Tune DW, Tune S320, and Tune P329 at the particle level (i.e. generator level). RDF LHC Prediction! Tevatron LHC • Extrapolations of PYTHIA Tune AW, Tune DW, Tune DWT, Tune S320, and Tune P329, and pyATLAS to the LHC. Rick Field – Florida/CDF/CMS

  30. Drell-Yan Charged Multiplicity It would be nice to have 2/pb at 7 TeV (acquired) which might mean 3/pb (delivered)! 100 events L = 210/nb! 1,000 events L = 2.1/pb! 10 events L = 21/nb! • Prediction from PYTHIA Tune DW, Tune S320, and Tune P329 for Drell-Yan muon-pair production (70 < M(pair) < 110 GeV) for proton-proton collisions at 7 TeV for the number of charged particles with pT > 0.5 GeV and |h| < 2 (excluding the lepton-pair). Rick Field – Florida/CDF/CMS

  31. Drell-Yan Charged PT & PTsum • Prediction from PYTHIA Tune DW, Tune S320 for Drell-Yan muon-pair production (70 < M(pair) < 110 GeV) for proton-proton collisions at 7 TeV for the charged pT distribution |h| < 2 (excluding the lepton-pair). The plot shows the <Nchg> per unit pT versus pT. • Prediction from PYTHIA Tune DW, Tune S320 for Drell-Yan muon-pair production (70 < M(pair) < 110 GeV) for proton-proton collisions at 7 TeV for the PTsum distributions with pT > 0.5 GeV and |h| < 2 (excluding the lepton-pair). The plot shows the probability of having a given PTsum in the event. Rick Field – Florida/CDF/CMS

  32. Early LHC Measurements • The amount of activity in “min-bias” collisions (multiplicity distribution, pT distribution, PTsum distribution, dNchg/dh). • The amount of activity in the “underlying event” in hard scattering events (“transverse” Nchg distribution, “transverse” pT distribution, “transverse” PTsum distribution for events with PTmax > 5 GeV/c). • The amount of activity in the “underlying event” in Drell-Yan events (Nchg distribution, pT distribution, PTsum distribution, excluding the lepton-pair), Rick Field – Florida/CDF/CMS

  33. Summary & Conclusions It is very important to measure BOTH “min-bias” and the “underlying event” at 900 GeV! I hope we get enough events. • We are making good progress in understanding and modeling the “underlying event”. RHIC data at 200 GeV are very important! • The new Pythia pT ordered tunes (py64 S320 and py64 P329) are very similar to Tune A, Tune AW, and Tune DW. At present the new tunes do not fit the data better than Tune AW and Tune DW. However, the new tune are theoretically preferred! Py64 S320 = LHC “Reference Tune”! • It is clear now that the default value PARP(90) = 0.16 is not correct and the value should be closer to the Tune A value of 0.25. • The new and old PYTHIA tunes are beginning to converge and I believe we are finally in a position to make some legitimate predictions at the LHC! • All tunes with the default value PARP(90) = 0.16 are wrong and are overestimating the activity of min-bias and the underlying event at the LHC! This includes all my “T” tunes and the old ATLAS tunes! UE&MB@CMS • Need to measure “Min-Bias” and the “underlying event” at the LHC as soon as possible to see if there is new QCD physics to be learned! Rick Field – Florida/CDF/CMS

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