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,
CMS at the LHC
Summary & Conclusions.
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

2. Proton-AntiProton Collisions at 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 mb (4-7)mb + (47-44)mb
The “hard core” component contains both “hard” and “soft” collisions.
CDF “Min-Bias” trigger
1 charged particle in forward BBC
AND
1 charged particle in backward BBC
“Inelastic Non-Diffractive Component”
Beam-Beam Counters
3.2 < |h| < 5.9
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

3. Inelastic Non-Diffractive Cross-Section
My guess!
Lots of events!
Linear scale!
Log scale!
stot = sEL + sSD + sDD + sHC
The inelastic non-diffractive cross section versus center-of-mass energy from PYTHIA (×1.2).
sHC varies slowly. Only a 13% increase between 7 TeV (≈ 58 mb) and 14 teV (≈ 66 mb). Linear on a log scale!
Workshop November 6, 2009
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.
Workshop November 6, 2009
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.
Workshop November 6, 2009
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.
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

7. Min-Bias “Charged Particle Density
LHC14
LHC7
Tevatron
900 GeV
RHIC
0.2 TeV → 1.96 TeV
(dN/dh increase ~1.63 times)
1.96 TeV → 14 TeV
(dN/dh increase ~1.58 times)
LHC
RHIC
Tevatron
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 TeV predicted by PYTHIA Tune DW at the particle level (i.e. generator level).
Linear scale!
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

8. Min-Bias “Charged Particle Density
LHC14
LHC10
LHC7
Tevatron
900 GeV
RHIC
7 TeV → 14 TeV
(dN/dh ≈ 19% increase)
Log scale!
Linear on a log plot!
LHC7
LHC14
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 TeV predicted by PYTHIA Tune DW at the particle level (i.e. generator level).
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

9. Charged Particle Multiplicity
New
No MPI!
Tune A!
7 decades!
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).
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

10. Charged Particle Multiplicity
Tune A!
No MPI!
Tune S320!
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).
Workshop November 6, 2009
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)!
Workshop November 6, 2009
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)!
Workshop November 6, 2009
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)!
Workshop November 6, 2009
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).
Workshop November 6, 2009
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!
Workshop November 6, 2009
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!
Charged Particle Multiplicity
Tune A prediction at 900 GeV!
No MPI!
Tune A!
Tune A prediction at 2.2 TeV!
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).
Prediction from PYTHIA Tune A for proton-proton collisions at 900 GeV and 2.2 TeV.
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

17. Rick Field – Florida/CDF/CMS
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.
Workshop November 6, 2009
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!
LHC Predictions: 900 GeV
10 events
L = 0.24/mb!
100 events
L = 2.4/mb!
1,000 events
L = 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.
Workshop November 6, 2009
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 TeV as defined by PTmax, PT(chgjet#1), and PT(muon-pair) from PYTHIA Tune DW at the particle level (i.e. generator level). Charged particle jets are constructed using the Anti-KT algorithm with d = 0.5.
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

20. Min-Bias “Associated” Charged Particle Density
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
Tevatron
LHC
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 TeV predicted by PYTHIA Tune DW at the particle level (i.e. generator level).
Linear scale!
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

21. Min-Bias “Associated” Charged Particle Density
LHC14
LHC10
LHC7
Tevatron
900 GeV
RHIC
7 TeV → 14 TeV
(UE increase ~20%)
LHC7
LHC14
Linear on a log plot!
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 TeV predicted by PYTHIA Tune DW at the particle level (i.e. generator level).
Log scale!
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

22. Rick Field – Florida/CDF/CMS
sHC: PTmax > 5 GeV/c
Still lots of events!
Linear scale!
Log scale!
stot = sEL + sSD + sDD + sHC
In 1,000,000 HC collisions at 900 GeV you get 940 with PTmax > 5 GeV/c!
The inelastic non-diffractive PTmax > 5 GeV/c cross section versus center-of-mass energy from PYTHIA (×1.2).
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!
Workshop November 6, 2009
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 TeV as defined by PTmax and PT(chgjet#1) from PYTHIA Tune DW at 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 PYTHIA Tune DW at the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5.
Workshop November 6, 2009
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 TeV for |h| < 1 and |h| < 2 as defined PT(chgjet#1) from PYTHIA Tune DW at 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 TeV for |h| < 1 and |h| < 2 as defined PT(chgjet#1) from PYTHIA Tune DW at the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5.
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

25. QCD Monte-Carlo Models: High Transverse Momentum Jets
“Hard Scattering” Component
“Underlying Event”
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).
The “underlying event” consists of the “beam-beam remnants” and from particles arising from soft or semi-soft multiple parton interactions (MPI).
Of course the outgoing colored partons fragment into hadron “jet” and inevitably “underlying event” observables receive contributions from initial and final-state radiation.
The “underlying event” is an unavoidable background to most collider observables and having good understand of it leads to more precise collider measurements!
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

26. Drell-Yan Muon-Pair Cross-Section
Linear scale!
The Drell-Yan muon-pair cross section 70 < M(pair) < 110 GeV versus center-of-mass energy from PYTHIA (×1.3).
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!
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

27. Drell-Yan Muon-Pair Cross-Section
Linear scale!
4,700 events in 10/pb!
CMS acceptance!
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).
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!
Workshop November 6, 2009
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 TeV as defined by PTmax, PT(chgjet#1), and PT(muon-pair) for PYTHIA Tune DW at 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 PYTHIA Tune DW at the particle level (i.e. generator level). Charged particle jet are constructed using the Anti-KT algorithm with d = 0.5.
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

29. Z-Boson: “Towards” Region
RDF LHC Prediction!
Tevatron
LHC
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).
Extrapolations of PYTHIA Tune AW, Tune DW, Tune DWT, Tune S320, and Tune P329, and pyATLAS to the LHC.
Workshop November 6, 2009
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!
10 events
L = 21/nb!
1,000 events
L = 2.1/pb!
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).
Workshop November 6, 2009
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.
Workshop November 6, 2009
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),
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS

33. measure BOTH “min-bias” and the “underlying event”
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!
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!
Workshop November 6, 2009
Rick Field – Florida/CDF/CMS