1. YETI’11: The Standard Model at the Energy Frontier
Min-Bias and the Underlying Event at the LHC
Rick Field
University of Florida
1st Lecture
What is the “underlying event” and how is it related to “min-bias” collisions.
CMS
Lessons learned about “min-bias” and the “underlying event” at the TEVATRON.
Predicting the behavior of the “underlying event” at the LHC. What we expected to see.
ATLAS
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

2. 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!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

3. QCD Monte-Carlo Models: Lepton-Pair Production
“Hard Scattering” Component
“Underlying Event”
Start with the perturbative Drell-Yan muon pair production and add initial-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-state radiation.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

4. MPI, Pile-Up, and Overlap
MPI: Multiple Parton Interactions
MPI: Additional 2-to-2 parton-parton scatterings within a single hadron-hadron collision.
Pile-Up
Proton
Proton
Proton
Proton
Interaction Region Dz
Pile-Up: More than one hadron-hadron collision in the beam crossing.
Overlap
Overlap: An experimental timing issue where a hadron-hadron collision from the next beam crossing gets included in the hadron-hadron collision from the current beam crossing because the next crossing happened before the event could be read out.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

5. Proton-Proton Collisions
stot = sEL + sIN
stot = sEL + sSD + sDD + sHC ND
“Inelastic Non-Diffractive Component”
The “hard core” component contains both “hard” and “soft” collisions.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

6. 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)!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

7. The “Underlying Event”
Select inelastic non-diffractive events that contain a hard scattering
Hard parton-parton collisions is hard
(pT > ≈2 GeV/c)
1/(pT)4→ 1/(pT2+pT02)2
“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)!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

8. Model of sND + + + + … 1/(pT)4→ 1/(pT2+pT02)2
Allow leading hard scattering to go to zero pT with same cut-off as the MPI!
Model of the inelastic non-diffractive cross section!
1/(pT)4→ 1/(pT2+pT02)2
“Semi-hard” parton-parton collision
(pT < ≈2 GeV/c) + + +
+ …
Multiple-parton interactions (MPI)!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

9. UE Tunes + + + + … All of Rick’s tunes (except X2):
“Underlying Event”
Allow primary hard-scattering to go to pT = 0 with same cut-off!
All of Rick’s tunes (except X2):
A, AW, AWT,DW, DWT,
D6, D6T, CW, X1,
and Tune Z1,
are UE tunes!
Fit the “underlying event” in a hard scattering process.
1/(pT)4→ 1/(pT2+pT02)2
“Min-Bias” (add single & double diffraction)
“Min-Bias” (ND) + + +
Predict MB (ND)!
Predict MB (IN)!
+ …
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

10. MB Tunes + + + + … Most of Peter Skand’s tunes:
“Underlying Event”
Most of Peter Skand’s tunes:
S320 Perugia 0, S325 Perugia X,
S326 Perugia 6
are MB tunes!
Predict the “underlying event” in a hard scattering process!
“Min-Bias” (ND) + + +
Fit MB (ND).
+ …
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

11. MB+UE Tunes + + + + … “Underlying Event”
Most of Hendrik’s “Professor”
tunes: ProQ20, P329
are MB+UE!
Fit the “underlying event” in a hard scattering process!
“Min-Bias” (ND)
Simultaneous fit to both MB & UE + +
The ATLAS AMBT1 Tune is an MB+UE tune, but
because they include in the fit the ATLAS UE data
with PTmax > 10 GeV/c (big errors) the LHC UE data
does not have much pull (hence mostly an MB tune!). +
Fit MB (ND).
+ …
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

12. Traditional Approach
CDF Run 1 Analysis
Charged Particle Df Correlations PT > PTmin |h| < hcut
Leading Calorimeter Jet or Leading Charged Particle Jet or
Leading Charged Particle or
Z-Boson
“Transverse” region very sensitive to the “underlying event”!
Look at charged particle correlations in the azimuthal angle Df relative to a leading object (i.e. CaloJet#1, ChgJet#1, PTmax, Z-boson). For CDF PTmin = 0.5 GeV/c hcut = 1.
Define |Df| < 60o as “Toward”, 60o < |Df| < 120o as “Transverse”, and |Df| > 120o as “Away”.
All three regions have the same area in h-f space, Dh×Df = 2hcut×120o = 2hcut×2p/3. Construct densities by dividing by the area in h-f space.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

13. ISAJET 7.32 (without MPI) “Transverse” Density
ISAJET uses a naïve leading-log parton shower-model which does not agree with the data!
ISAJET
“Hard”
Component
Beam-Beam
Remnants
Plot shows average “transverse” charge particle density (|h|<1, pT>0.5 GeV) versus PT(charged jet#1) compared to the QCD hard scattering predictions of ISAJET 7.32 (default parameters with PT(hard)>3 GeV/c) .
The predictions of ISAJET are divided into two categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants); and charged particles that arise from the outgoing jet plus initial and final-state radiation (hard scattering component).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

14. HERWIG 6.4 (without MPI) “Transverse” Density
HERWIG uses a modified leading-log parton shower-model which does agrees better with the data!
HERWIG
Beam-Beam
Remnants
“Hard”
Component
Plot shows average “transverse” charge particle density (|h|<1, pT>0.5 GeV) versus PT(charged jet#1) compared to the QCD hard scattering predictions of HERWIG 5.9 (default parameters with PT(hard)>3 GeV/c without MPI).
The predictions of HERWIG are divided into two categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants); and charged particles that arise from the outgoing jet plus initial and final-state radiation (hard scattering component).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

15. HERWIG 6.4 (without MPI) “Transverse” PT Distribution
HERWIG has the too steep of a pT dependence of the “beam-beam remnant” component of the “underlying event”!
Herwig PT(chgjet#1) > 30 GeV/c
“Transverse” <dNchg/dhdf> = 0.51
Herwig PT(chgjet#1) > 5 GeV/c
<dNchg/dhdf> = 0.40
Compares the average “transverse” charge particle density (|h|<1, pT>0.5 GeV) versus PT(charged jet#1) and the pT distribution of the “transverse” density, dNchg/dhdfdPT with the QCD hard scattering predictions of HERWIG 6.4 (default parameters with PT(hard)>3 GeV/c without MPI). Shows how the “transverse” charge particle density is distributed in pT.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

16. MPI: Multiple Parton Interactions
PYTHIA models the “soft” component of the underlying event with color string fragmentation, but in addition includes a contribution arising from multiple parton interactions (MPI) in which one interaction is hard and the other is “semi-hard”.
The probability that a hard scattering events also contains a semi-hard multiple parton interaction can be varied but adjusting the cut-off for the MPI.
One can also adjust whether the probability of a MPI depends on the PT of the hard scattering, PT(hard) (constant cross section or varying with impact parameter).
One can adjust the color connections and flavor of the MPI (singlet or nearest neighbor, q-qbar or glue-glue).
Also, one can adjust how the probability of a MPI depends on PT(hard) (single or double Gaussian matter distribution).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

17. Tuning PYTHIA 6.2: Multiple Parton Interaction Parameters
Default
Description
PARP(83)
0.5
Double-Gaussian: Fraction of total hadronic matter within PARP(84)
PARP(84)
0.2
Double-Gaussian: Fraction of the overall hadron radius containing the fraction PARP(83) of the total hadronic matter.
PARP(85)
0.33
Probability that the MPI produces two gluons with color connections to the “nearest neighbors.
PARP(86)
0.66
Probability that the MPI produces two gluons either as described by PARP(85) or as a closed gluon loop. The remaining fraction consists of quark-antiquark pairs.
PARP(89)
1 TeV
Determines the reference energy E0.
PARP(82)
1.9 GeV/c
The cut-off PT0 that regulates the 2-to-2 scattering divergence 1/PT4→1/(PT2+PT02)2
PARP(90)
0.16
Determines the energy dependence of the cut-off
PT0 as follows PT0(Ecm) = PT0(Ecm/E0)e with e = PARP(90)
PARP(67)
1.0
A scale factor that determines the maximum parton virtuality for space-like showers. The larger the value of PARP(67) the more initial-state radiation.
Hard Core
Determines the energy dependence of the MPI!
Determine by comparing
with 630 GeV data!
Affects the amount of
initial-state radiation!
Take E0 = 1.8 TeV
Reference point
at 1.8 TeV
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

18. PYTHIA default parameters
PYTHIA Defaults
MPI constant
probability
scattering
PYTHIA default parameters
Parameter
6.115
6.125
6.158
6.206
MSTP(81) 1
MSTP(82)
PARP(81)
1.4
1.9
PARP(82)
1.55
2.1
PARP(89)
1,000
PARP(90)
0.16
PARP(67)
4.0
1.0
Plot shows the “Transverse” charged particle density versus PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA (PT(hard) > 0) using the default parameters for multiple parton interactions and CTEQ3L, CTEQ4L, and CTEQ5L.
Default parameters give very poor description of the “underlying event”!
Note Change
PARP(67) = 4.0 (< 6.138)
PARP(67) = 1.0 (> 6.138)
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

19. Run 1 PYTHIA Tune A PYTHIA 6.206 CTEQ5L
CDF Default!
PYTHIA CTEQ5L
Parameter
Tune B
Tune A
MSTP(81) 1
MSTP(82) 4
PARP(82)
1.9 GeV
2.0 GeV
PARP(83)
0.5
PARP(84)
0.4
PARP(85)
1.0
0.9
PARP(86)
0.95
PARP(89)
1.8 TeV
PARP(90)
0.25
PARP(67)
4.0
Run 1 Analysis
Plot shows the “transverse” charged particle density versus PT(chgjet#1) compared to the QCD hard scattering predictions of two tuned versions of PYTHIA (CTEQ5L, Set B (PARP(67)=1) and Set A (PARP(67)=4)).
Old PYTHIA default
(more initial-state radiation)
Old PYTHIA default
(more initial-state radiation)
New PYTHIA default
(less initial-state radiation)
New PYTHIA default
(less initial-state radiation)
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

20. “Transverse” Cones vs “Transverse” Regions
“Cone Analysis”
(Tano, Kovacs, Huston, Bhatti)
Transverse Cone:
p(0.7)2=0.49p
Transverse Region:
2p/3=0.67p
Sum the PT of charged particles in two cones of radius 0.7 at the same h as the leading jet but with |DF| = 90o.
Plot the cone with the maximum and minimum PTsum versus the ET of the leading (calorimeter) jet.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

21. Energy Dependence of the “Underlying Event”
“Cone Analysis”
(Tano, Kovacs, Huston, Bhatti)
630 GeV
1,800 GeV
PYTHIA 6.115
PT0 = 1.4 GeV
PYTHIA 6.115
PT0 = 2.0 GeV
Sum the PT of charged particles (pT > 0.4 GeV/c) in two cones of radius 0.7 at the same h as the leading jet but with |DF| = 90o. Plot the cone with the maximum and minimum PTsum versus the ET of the leading (calorimeter) jet.
Note that PYTHIA is tuned at 630 GeV with PT0 = 1.4 GeV and at 1,800 GeV with PT0 = 2.0 GeV. This implies that e = PARP(90) should be around 0.30 instead of the 0.16 (default).
For the MIN cone 0.25 GeV/c in radius R = 0.7 implies a PTsum density of dPTsum/dhdf = 0.16 GeV/c and 1.4 GeV/c in the MAX cone implies dPTsum/dhdf = 0.91 GeV/c (average PTsum density of 0.54 GeV/c per unit h-f).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

22. “Transverse” Charged Densities Energy Dependence
Increasing e produces less energy dependence for the UE resulting in less UE activity at the LHC!
Lowering PT0 at 630 GeV (i.e. increasing e) increases UE activity resulting in less energy dependence.
Shows the “transverse” charged PTsum density (|h|<1, PT>0.4 GeV) versus PT(charged jet#1) at 630 GeV predicted by HERWIG 6.4 (PT(hard) > 3 GeV/c, CTEQ5L) and a tuned version of PYTHIA (PT(hard) > 0, CTEQ5L, Set A, e = 0, e = 0.16 (default) and e = 0.25 (preferred)).
Also shown are the PTsum densities (0.16 GeV/c and 0.54 GeV/c) determined from the Tano, Kovacs, Huston, and Bhatti “transverse” cone analysis at 630 GeV.
Rick Field Fermilab MC Workshop
October 4, 2002!
Reference point
E0 = 1.8 TeV
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

23. Run 1 vs Run 2: “Transverse” Charged Particle Density
“Transverse” region as defined by the leading “charged particle jet”
Excellent agreement between Run 1 and 2!
Shows the data on the average “transverse” charge particle density (|h|<1, pT>0.5 GeV) as a function of the transverse momentum of the leading charged particle jet from Run 1.
Compares the Run 2 data (Min-Bias, JET20, JET50, JET70, JET100) with Run 1. The errors on the (uncorrected) Run 2 data include both statistical and correlated systematic uncertainties.
PYTHIA Tune A was tuned to fit the “underlying event” in Run I!
Shows the prediction of PYTHIA Tune A at 1.96 TeV after detector simulation (i.e. after CDFSIM).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

24. Run 1 vs Run 2: “Transverse” Charged PTsum Density
“Transverse” region as defined by the leading “charged particle jet”
Excellent agreement between Run 1 and 2!
Shows the data on the average “transverse” charged PTsum density (|h|<1, pT>0.5 GeV) as a function of the transverse momentum of the leading charged particle jet from Run 1.
Compares the Run 2 data (Min-Bias, JET20, JET50, JET70, JET100) with Run 1. The errors on the (uncorrected) Run 2 data include both statistical and correlated systematic uncertainties.
PYTHIA Tune A was tuned to fit the “underlying event” in Run I!
Shows the prediction of PYTHIA Tune A at 1.96 TeV after detector simulation (i.e. after CDFSIM).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

25. “Underlying Event” as defined by “Calorimeter Jets”
Charged Particle Df Correlations pT > 0.5 GeV/c |h| < 1
Look at the charged particle density in the “transverse” region!
“Transverse” region is very sensitive to the “underlying event”!
Perpendicular to the plane of the 2-to-2 hard scattering
Away-side “jet”
(sometimes)
Look at charged particle correlations in the azimuthal angle Df relative to the leading JetClu jet.
Define |Df| < 60o as “Toward”, 60o < |Df| < 120o as “Transverse”, and |Df| > 120o as “Away”.
All three regions have the same size in h-f space, DhxDf = 2x120o = 4p/3.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

26. “Transverse” Charged Particle Density
“Transverse” region as defined by the leading “calorimeter jet”
Shows the data on the average “transverse” charge particle density (|h|<1, PT>0.5 GeV) as a function of the transverse energy of the leading JetClu jet (R = 0.7, |h(jet)| < 2) from Run 2.
, compared with PYTHIA Tune A after CDFSIM.
Compares the “transverse” region of the leading “charged particle jet”, chgjet#1, with the “transverse” region of the leading “calorimeter jet” (JetClu R = 0.7), jet#1.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

27. “Transverse” Charged PTsum Density
“Transverse” region as defined by the leading “calorimeter jet”
Shows the data on the average “transverse” charged PTsum density (|h|<1, PT>0.5 GeV) as a function of the transverse energy of the leading JetClu jet (R = 0.7, |h(jet)| < 2) from Run 2.
, compared with PYTHIA Tune A after CDFSIM.
Compares the “transverse” region of the leading “charged particle jet”, chgjet#1, with the “transverse” region of the leading “calorimeter jet” (JetClu R = 0.7), jet#1.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

28. Rick Field University of Chicago July 11, 2006
“Leading Jet”
“Back-to-Back”
Shows the data on the tower ETsum density, dETsum/dhdf, in the “transMAX” and “transMIN” region (ET > 100 MeV, |h| < 1) versus PT(jet#1) for “Leading Jet” and “Back-to-Back” events.
Compares the (corrected) data with PYTHIA Tune A (with MPI) and HERWIG (without MPI) at the particle level (all particles, |h| < 1).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

29. Rick Field University of Chicago July 11, 2006
“Leading Jet”
“Back-to-Back”
Neither PY Tune A or HERWIG fits the ETsum density in the “transferse” region!
HERWIG does slightly better than Tune A!
Shows the data on the tower ETsum density, dETsum/dhdf, in the “transMAX” and “transMIN” region (ET > 100 MeV, |h| < 1) versus PT(jet#1) for “Leading Jet” and “Back-to-Back” events.
Compares the (corrected) data with PYTHIA Tune A (with MPI) and HERWIG (without MPI) at the particle level (all particles, |h| < 1).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

30. Rick Field University of Chicago July 11, 2006
“Leading Jet”
“Back-to-Back”
“transDIF” is more sensitive to the “hard scattering” component of the “underlying event”!
Use the leading jet to define the MAX and MIN “transverse” regions on an event-by-event basis with MAX (MIN) having the largest (smallest) charged PTsum density.
Shows the “transDIF” = MAX-MIN ETsum density, dETsum/dhdf, for all particles (|h| < 1) versus PT(jet#1) for “Leading Jet” and “Back-to-Back” events.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

31. Rick Field University of Chicago July 11, 2006
Possible Scenario??
PYTHIA Tune A fits the charged particle PTsum density for pT > 0.5 GeV/c, but it does not produce enough ETsum for towers with ET > 0.1 GeV.
It is possible that there is a sharp rise in the number of particles in the “underlying event” at low pT (i.e. pT < 0.5 GeV/c).
Perhaps there are two components, a vary “soft” beam-beam remnant component (Gaussian or exponential) and a “hard” multiple interaction component.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

32. Charged Particle Multiplicity
No MPI!
Tune A!
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 (non-diffractive cross-section).
The data are compared with PYTHIA Tune A and Tune A without multiple parton interactions (pyAnoMPI).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

33. PYTHIA Tune A Min-Bias “Soft” + ”Hard”
New
Ten decades!
12% of “Min-Bias” events have PT(hard) > 5 GeV/c!
1% of “Min-Bias” events have PT(hard) > 10 GeV/c!
Lots of “hard” scattering in “Min-Bias” at the Tevatron!
Comparison of PYTHIA Tune A with the pT distribution of charged particles for “min-bias” collisions at CDF Run 1 (non-diffractive cross-section).
pT = 50 GeV/c!
PYTHIA Tune A predicts that 12% of all “Min-Bias” events are a result of a hard 2-to-2 parton-parton scattering with PT(hard) > 5 GeV/c (1% with PT(hard) > 10 GeV/c)!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

34. CDF: Charged pT Distribution
Erratum November 18, 2010
No excess at large pT!
Excess events at large pT!
50 GeV/c!
Published CDF data on the pT distribution of charged particles in Min-Bias collisions (ND) at 1.96 TeV compared with PYTHIA Tune A.
CDF consistent with CMS and UA1!
CDF inconsistent with CMS and UA1!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

35. Min-Bias Correlations
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).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

36. 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!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

37. CDF Run 1 PT(Z) PYTHIA 6.2 CTEQ5L
Tune used by the
CDF-EWK group!
PYTHIA 6.2 CTEQ5L
Parameter
Tune A
Tune AW
MSTP(81) 1
MSTP(82) 4
PARP(82)
2.0 GeV
PARP(83)
0.5
PARP(84)
0.4
PARP(85)
0.9
PARP(86)
0.95
PARP(89)
1.8 TeV
PARP(90)
0.25
PARP(62)
1.0
1.25
PARP(64)
0.2
PARP(67)
4.0
MSTP(91)
PARP(91)
2.1
PARP(93)
5.0
15.0
UE Parameters
ISR Parameters
Shows the Run 1 Z-boson pT distribution (<pT(Z)> ≈ 11.5 GeV/c) compared with PYTHIA Tune A (<pT(Z)> = 9.7 GeV/c), and PYTHIA Tune AW (<pT(Z)> = 11.7 GeV/c).
Effective Q cut-off, below which space-like showers are not evolved.
Intrensic KT
The Q2 = kT2 in as for space-like showers is scaled by PARP(64)!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

38. Jet-Jet Correlations (DØ)
Df Jet#1-Jet#2
Jet#1-Jet#2 Df Distribution
MidPoint Cone Algorithm (R = 0.7, fmerge = 0.5)
L = 150 pb-1 (Phys. Rev. Lett (2005))
Data/NLO agreement good. Data/HERWIG agreement good.
Data/PYTHIA agreement good provided PARP(67) = 1.0→4.0 (i.e. like Tune A, best fit 2.5).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

39. CDF Run 1 PT(Z) PYTHIA 6.2 CTEQ5L
Parameter
Tune DW
Tune AW
MSTP(81) 1
MSTP(82) 4
PARP(82)
1.9 GeV
2.0 GeV
PARP(83)
0.5
PARP(84)
0.4
PARP(85)
1.0
0.9
PARP(86)
0.95
PARP(89)
1.8 TeV
PARP(90)
0.25
PARP(62)
1.25
PARP(64)
0.2
PARP(67)
2.5
4.0
MSTP(91)
PARP(91)
2.1
PARP(93)
15.0
UE Parameters
ISR Parameters
Shows the Run 1 Z-boson pT distribution (<pT(Z)> ≈ 11.5 GeV/c) compared with PYTHIA Tune DW, and HERWIG.
Tune DW uses D0’s perfered value of PARP(67)!
Intrensic KT
Tune DW has a lower value of PARP(67) and slightly more MPI!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

40. Tune A energy dependence!
PYTHIA 6.2 Tunes
All use LO as
with L = 192 MeV!
Parameter
Tune AW
Tune DW
Tune D6
PDF
CTEQ5L
CTEQ6L
MSTP(81) 1
MSTP(82) 4
PARP(82)
2.0 GeV
1.9 GeV
1.8 GeV
PARP(83)
0.5
PARP(84)
0.4
PARP(85)
0.9
1.0
PARP(86)
0.95
PARP(89)
1.8 TeV
PARP(90)
0.25
PARP(62)
1.25
PARP(64)
0.2
PARP(67)
4.0
2.5
MSTP(91)
PARP(91)
2.1
PARP(93)
15.0
UE Parameters
Uses CTEQ6L
Tune A energy dependence!
(not the default)
ISR Parameter
Intrinsic KT
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

41. PYTHIA 6.2 Tunes These are all old PYTHIA 6.2 Tunes!
There are now many PYTHIA 6.4 tunes
(S320 Perugia 0, Tune Z1)
and some PYTHIA 8 tunes
(Tune 1, Hendrik tunes).
All use LO as
with L = 192 MeV!
Parameter
Tune DWT
Tune D6T
ATLAS
PDF
CTEQ5L
CTEQ6L
MSTP(81) 1
MSTP(82) 4
PARP(82)
GeV
GeV
1.8 GeV
PARP(83)
0.5
PARP(84)
0.4
PARP(85)
1.0
0.33
PARP(86)
0.66
PARP(89)
1.96 TeV
1.0 TeV
PARP(90)
0.16
PARP(62)
1.25
PARP(64)
0.2
PARP(67)
2.5
MSTP(91)
PARP(91)
2.1
PARP(93)
15.0
5.0
UE Parameters
Tune B
Tune AW
Tune A
Old ATLAS energy dependence!
(PYTHIA default)
Tune BW
ISR Parameter
Tune DW
Tune D6
Tune D
Tune D6T
Intrinsic KT
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

42. Min-Bias “Associated” Charged Particle Density
35% more at RHIC means 26% less at the LHC!
~1.35
~1.35
0.2 TeV → 14 TeV
(~factor of 70 increase)
RHIC
LHC
Shows the “associated” charged particle density in the “transverse” regions 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 and 14 TeV from PYTHIA Tune DW and Tune DWT at the particle level (i.e. generator level). The STAR data from RHIC favors Tune DW!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

43. Min-Bias “Associated” Charged Particle Density
~1.9
~2.7
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, 1.96 TeV and 14 TeV predicted by PYTHIA Tune DW at the particle level (i.e. generator level).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

44. The “Underlying Event” at STAR
At STAR they have measured the “underlying event at W = 200 GeV (|h| < 1, pT > 0.2 GeV) and compared their uncorrected data with PYTHIA Tune A + STAR-SIM.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

45. Min-Bias “Associated” Charged Particle Density
If the LHC data are not in
the range shown here then
we learn new (QCD) physics!
Rick Field October 13, 2009
RDF LHC Prediction!
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 1.96 TeV from PYTHIA Tune A, Tune S320, Tune N324, and Tune P329 at the particle level (i.e. generator level).
Extrapolations of PYTHIA Tune A, Tune DW, Tune DWT, Tune S320, Tune P329, and pyATLAS to the LHC.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

46. “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.
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

47. 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!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

48. 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!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

49. Rick Field – Florida/CDF/CMS
Conclusions November 2009
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!
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!
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

50. “Transverse” Charged Particle Density
Leading Charged Particle Jet, chgjet#1.
Prediction!
Leading Charged Particle, PTmax.
Fake data (from MC) at 900 GeV on the “transverse” charged particle density, dN/dhdf, as defined by the leading charged particle (PTmax) and the leading charged particle jet (chgjet#1) for charged particles with pT > 0.5 GeV/c and |h| < 2. The fake data (from PYTHIA Tune DW) are generated at the particle level (i.e. generator level) assuming 0.5 M min-bias events at 900 GeV (361,595 events in the plot).
Rick Field
Workshop
CERN, November 6, 2009
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

51. “Transverse” Charge Density
Rick Field
Workshop
CERN, November 6, 2009
factor of 2!
Prediction!
900 GeV → 7 TeV
(UE increase ~ factor of 2)
LHC
900 GeV
LHC
7 TeV
~0.4 → ~0.8
Shows the charged particle density in the “transverse” region for charged particles (pT > 0.5 GeV/c, |h| < 2) at 900 GeV and 7 TeV as defined by PTmax from PYTHIA Tune DW and at the particle level (i.e. generator level).
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS

52. YETI’11: The Standard Model at the Energy Frontier
Min-Bias and the Underlying Event at the LHC
Rick Field
University of Florida
2nd Lecture
How well did we do at predicting the behavior of the “underlying event” at the LHC (900 GeV and 7 TeV)?
CMS
In lecture 2 we will examine how well we did at predicting the “underlying event” and “min-bias” at the LHC and look at some new tunes that came after seeing the LHC data.
How well did we do at predicting the behavior of “min-bias” collisions at the LHC (900 GeV and 7 TeV)?
ATLAS
PYTHIA 6.4 Tune Z1: New CMS 6.4 tune (pT-ordered parton showers and new MPI).
New Physics in Min-Bias?? Observation of long-range same-side correlations at 7 TeV.
Strange particle production: A problem for the models?
YETI'11 Durham IPPP - Part January 10-12, 2011
Rick Field – Florida/CDF/CMS