MinBias and the Underlying Event in Run 2
“Min-Bias” and the “Underlying Event” in Run 2 at CDF and the LHC Outline of Talk Æ Discuss briefly the components of the “underlying event” of a hard scattering as described by the QCD parton-shower Monte-Carlo Models. Æ Review the CDF Run 1 analysis which was used to tune the multiple parton interaction parameters in PYTHIA (i. e. Tune A and Tune B). Æ Study the “underlying event” in CDF Run 2 as defined by the leading calorimeter jet and compare with PYTHIA Tune A (with MPI) and HERWIG (without MPI). Æ Discuss the universality of PYTHIA Tune A. Direct Photon Production – Z-boson Production – etc. Æ Discuss extrapolations of HERWIG and PYTHIA Tune A to the LHC. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF Jet. Clu R = 0. 7 1
Beam-Beam Remnants Maybe not all “soft”! Æ The underlying event in a hard scattering process has a “hard” component (particles that arise from initial & final-state radiation and from the outgoing hard scattered partons) and a “soft? ” component (“beam-beam remnants”). Æ Clearly? the “underlying event” in a hard scattering process should not look like a “Min. Bias” event because of the “hard” component (i. e. initial & final-state radiation). Æ However, perhaps “Min-Bias” collisions are a good model for the “beam-beam remnant” component of the “underlying event”. Are these the same? Æ The “beam-beam remnant” component is, however, color connected to the “hard” component so this comparison is (at best) an approximation. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 2
MPI: Multiple Parton Interactions Æ 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”. Æ 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. Æ 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). Æ One can adjust the color connections and flavor of the MPI (singlet or nearest neighbor, q-qbar or glue-glue). Æ Also, one can adjust how the probability of a MPI depends on PT(hard) (single or double Gaussian matter distribution). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 3
The “Transverse” Regions as defined by the Leading Jet “Transverse” region is very sensitive to the “underlying event”! Charged Particle Df Correlations p. T > 0. 5 Ge. V/c |h| < 1 Look at the charged particle density in the “transverse” region! Æ Look at charged particle correlations in the azimuthal angle Df relative to the leading Æ calorimeter jet (Jet. Clu R = 0. 7, |h| < 2). Define |Df| < 60 o as “Toward”, 60 o < -Df < 120 o and 60 o < Df < 120 o as “Transverse 1” and “Transverse 2”, and |Df| > 120 o as “Away”. Each of the two “transverse” regions have area Dh. Df = 2 x 60 o = 4 p/6. The overall “transverse” region is the sum of the two transverse regions (Dh. Df = 2 x 120 o = 4 p/3). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 4
Particle Densities Dh. Df = 4 p = 12. 6 31 charged particles particle Charged Particles p. T > 0. 5 Ge. V/c |h| < 1 CDF Run 2 “Min-Bias” Observable Average Density per unit h-f Nchg Number of Charged Particles (p. T > 0. 5 Ge. V/c, |h| < 1) 3. 17 +/- 0. 31 0. 252 +/- 0. 025 PTsum (Ge. V/c) Scalar p. T sum of Charged Particles (p. T > 0. 5 Ge. V/c, |h| < 1) 2. 97 +/- 0. 236 +/- 0. 018 chg/dhdf = 1/4 p 3/4 p = 0. 08 0. 24 d. Nchg 13 Ge. V/c PTsum Divide by 4 p d. PTsum/dhdf = 1/4 p 3/4 p Ge. V/c = 0. 08 0. 24 Ge. V/c Æ Study the charged particles (p. T > 0. 5 Ge. V/c, |h| < 1) and form the charged particle density, d. Nchg/dhdf, and the charged scalar p. T sum density, d. PTsum/dhdf. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 5
Tuning PYTHIA: Multiple Parton Interaction Parameters Parameter Default PARP(83) 0. 5 Description Double-Gaussian: Fraction of total hadronic matter within 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 PARP(89) 1 Te. V PARP(90) 0. 16 PARP(67) 1. 0 Te. V 4 LHC Fermilab September 16, 2004 d Har Core Determine by comparing with 630 Ge. V data! Probability that the MPI produces two gluons either as described by PARP(85) or as a closed gluon loop. remaining fraction consists of Affects the The amount of quark-antiquark pairs. initial-state radiation! Determines the reference energy E 0. Determines the energy dependence of the cut-off PT 0 as follows PT 0(Ecm) = PT 0(Ecm/E 0)e with e = PARP(90) Take E 0 = 1. 8 Te. V A scale factor that determines the maximum parton virtuality for space-like showers. The larger the value of PARP(67) the more initialstate radiation. Rick Field - Florida/CDF Reference point at 1. 8 Te. V 7
Tuned PYTHIA 6. 206 Double Gaussian PYTHIA 6. 206 CTEQ 5 L Parameter Tune B Tune A MSTP(81) 1 1 MSTP(82) 4 4 PARP(82) 1. 9 Ge. V 2. 0 Ge. V PARP(83) 0. 5 PARP(84) 0. 4 PARP(85) 1. 0 0. 9 PARP(86) 1. 0 0. 95 PARP(89) 1. 8 Te. V PARP(90) 0. 25 PARP(67) 1. 0 4. 0 New PYTHIA default (less initial-state radiation) Te. V 4 LHC Fermilab September 16, 2004 Run 1 Analysis Æ Plot shows the “Transverse” charged particle density versus PT(chgjet#1) compared to the QCD hard scattering predictions of two tuned versions of PYTHIA 6. 206 (CTEQ 5 L, Set B (PARP(67)=1) and Set A (PARP(67)=4)). Old PYTHIA default (more initial-state radiation) Rick Field - Florida/CDF 9
Tuned PYTHIA 6. 206 “Transverse” PT Distribution PT(charged jet#1) > 30 Ge. V/c PARP(67)=4. 0 (old default) is favored over PARP(67)=1. 0 (new default)! Can we distinguish between PARP(67)=1 and PARP(67)=4? No way! Right! Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1) and the PT distribution of the “transverse” density, d. Nchg/dhdfd. PT with the QCD Monte-Carlo predictions of two tuned versions of PYTHIA 6. 206 (PT(hard) > 0, CTEQ 5 L, Set B (PARP(67)=1) and Set A (PARP(67)=4)). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 10
PYTHIA 6. 206 Tune A (CDF Default) Describes the rise from “Min-Bias” to “underlying event”! Set A PT(charged jet#1) > 30 Ge. V/c “Transverse” <d. Nchg/dhdf> = 0. 60 “Min-Bias” Set A Min-Bias <d. Nchg/dhdf> = 0. 24 Æ Compares the average “transverse” charge particle density (|h|<1, PT>0. 5 Ge. V) versus PT(charged jet#1) and the PT distribution of the “transverse” and “Min-Bias” densities with the QCD Monte-Carlo predictions of a tuned version of PYTHIA 6. 206 (PT(hard) > 0, CTEQ 5 L, Set A). Describes “Min-Bias” collisions! Describes the “underlying event”! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 11
PYTHIA Min-Bias PYTHIA Tune A “Soft” + ”Hard” CDF Run 2 Default Tuned to fit the “underlying event”! 12% of “Min-Bias” events have PT(hard) > 5 Ge. V/c! 1% of “Min-Bias” events have PT(hard) > 10 Ge. V/c! Æ PYTHIA regulates the perturbative 2 -to-2 parton-parton cross sections with cut-off parameters which allows one to run with Lots of “hard” scattering PT(hard)in>“Min-Bias”! 0. One can simulate both “hard” and “soft” collisions in one program. Æ The relative amount of “hard” versus “soft” depends on the cut-off and can be tuned. Æ This PYTHIA fit predicts that 12% of all “Min-Bias” events are a result of a hard 2 -to-2 parton-parton scattering with PT(hard) > 5 Ge. V/c (1% with PT(hard) > 10 Ge. V/c)! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 12
Charged Particle Density Df Dependence Run 2 Log Scale! Leading Jet Min-Bias 0. 25 per unit h-f Æ Shows the Df dependence of the charged particle density, d. Nchg/dhdf, for charged particles in the range p. T > 0. 5 Ge. V/c and |h| < 1 relative to jet#1 (rotated to 270 o) for “leading jet” events 30 < ET(jet#1) < 70 Ge. V. Æ Also shows charged particle density, d. Nchg/dhdf, for charged particles in the range p. T > 0. 5 Ge. V/c and |h| < 1 for “min-bias” collisions. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 13
Refer to this as a “Leading Jet” event Charged Particle Density Df Dependence Run 2 Subset Refer to this as a “Back-to-Back” event Æ Look at the “transverse” region as defined by the leading jet (Jet. Clu R = 0. 7, |h| < 2) or Æ by the leading two jets (Jet. Clu R = 0. 7, |h| < 2). “Back-to-Back” events are selected to have at least two jets with Jet#1 and Jet#2 nearly “back-to-back” (Df 12 > 150 o) with almost equal transverse energies (ET(jet#2)/ET(jet#1) > 0. 8). Shows the Df dependence of the charged particle density, d. Nchg/dhdf, for charged particles in the range p. T > 0. 5 Ge. V/c and |h| < 1 relative to jet#1 (rotated to 270 o) for 30 < ET(jet#1) < 70 Ge. V for “Leading Jet” and “Back-to-Back” events. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 14
“Transverse” PTsum Density versus ET(jet#1) Run 2 “Leading Jet” “Back-to-Back” Min-Bias 0. 24 Ge. V/c per unit h-f Æ Shows the average charged PTsum density, d. PTsum/dhdf, in the “transverse” region (p. T > 0. 5 Ge. V/c, |h| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events. Æ Compares the (uncorrected) data with PYTHIA Tune A and HERWIG after CDFSIM. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 15
“Trans. MIN” PTsum Density versus ET(jet#1) “Leading Jet” “Back-to-Back” “trans. MIN” is very sensitive to the “beam-beam remnant” component of the “underlying event”! Æ Use the leading jet to define the MAX and MIN “transverse” regions on an event-byevent basis with MAX (MIN) having the largest (smallest) charged particle density. Æ Shows the “trans. MIN” charge particle density, d. Nchg/dhdf, for p. T > 0. 5 Ge. V/c, |h| < 1 versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 16
“Transverse” PTsum Density PYTHIA Tune A vs HERWIG “Leading Jet” “Back-to-Back” Now look in detail at “back-to-back” events in the region 30 < ET(jet#1) < 70 Ge. V! Æ Shows the average charged PTsum density, d. PTsum/dhdf, in the “transverse” region (p. T Æ > 0. 5 Ge. V/c, |h| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events. Compares the (uncorrected) data with PYTHIA Tune A and HERWIG after CDFSIM. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 17
Charged PTsum Density PYTHIA Tune A vs HERWIG (without multiple parton interactions) does not produces enough PTsum in the “transverse” region for 30 < ET(jet#1) < 70 Ge. V! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 18
“Transverse” PTmax versus ET(jet#1) “Leading Jet” Highest p. T particle in the “transverse” region! “Back-to-Back” Min-Bias Æ Use the leading jet to define the “transverse” region and look at the maximum p. T charged particle in the “transverse” region, PTmax. T. Æ Shows the average PTmax. T, in the “transverse” region (p. T > 0. 5 Ge. V/c, |h| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events compared with the average maximum p. T particle, PTmax, in “min-bias” collisions (p. T > 0. 5 Ge. V/c, |h| < 1). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 20
Min-Bias “Associated” Highest p. T charged particle! Charged Particle Density “Associated” densities do not include PTmax! Æ Use the maximum p. T charged particle in the event, PTmax, to define a direction and Æ is “associated” more probable to findd. N a chg particle look at the It the density, /dhdf, in “min-bias” collisions (p. T > 0. 5 PTmax than it is to Ge. V/c, |h| <accompanying 1). find a particle in the central region! Shows the data on the Df dependence of the “associated” charged particle density, d. Nchg/dhdf, for charged particles (p. T > 0. 5 Ge. V/c, |h| < 1, not including PTmax) relative to PTmax (rotated to 180 o) for “min-bias” events. Also shown is the average charged particle density, d. Nchg/dhdf, for “min-bias” events. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 21
Min-Bias “Associated” Rapid rise in the particle density in the “transverse” region as PTmax increases! Charged Particle Density PTmax > 2. 0 Ge. V/c Transverse Region Ave Min-Bias 0. 25 per unit h-f Transverse Region PTmax > 0. 5 Ge. V/c Æ Shows the data on the Df dependence of the “associated” charged particle density, d. Nchg/dhdf, for charged particles (p. T > 0. 5 Ge. V/c, |h| < 1, not including PTmax) relative to PTmax (rotated to 180 o) for “min-bias” events with PTmax > 0. 5, 1. 0, and 2. 0 Ge. V/c. Æ Shows “jet structure” in “min-bias” collisions (i. e. the “birth” of the leading two jets!). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 22
Back-to-Back “Associated” Charged Particle Densities Maximum p. T particle in the “transverse” region! “Associated” densities do not include PTmax. T! Æ Use the leading jet in “back-to-back” events to define the “transverse” region and look at the maximum p. T charged particle in the “transverse” region, PTmax. T. Æ Look at the Df dependence of the “associated” charged particle and PTsum densities, d. Nchg/dhdf and d. PTsum/dhdf for charged particles (p. T > 0. 5 Ge. V/c, |h| < 1, not including PTmax. T) relative to PTmax. T. Æ Rotate so that PTmax. T is at the center of the plot (i. e. 180 o). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 25
Back-to-Back “Associated” Charged Particle Density “Associated” densities do not include PTmax. T! Jet#2 Region ? ? Log Scale! Æ Look at the Df dependence of the “associated” charged particle density, d. Nchg/dhdf for charged particles (p. T > 0. 5 Ge. V/c, |h| < 1, not including PTmax. T) relative to PTmax. T (rotated to 180 o) for PTmax. T > 0. 5 Ge. V/c, PTmax. T > 1. 0 Ge. V/c and PTmax. T > 2. 0 Ge. V/c, for “back-to-back” events with 30 < ET(jet#1) < 70 Ge. V. Æ Shows “jet structure” in the “transverse” region (i. e. the “birth” of the 3 rd & 4 th jet). Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 26
Jet Topologies QCDThree Four Jet QCD Jet Topology 0. 5 1. 0 1. 5 2. 0 Polar Plot Æ Shows the Df dependence of the “associated” charged particle density, d. Nchg/dhdf, p. T > 0. 5 Ge. V/c, |h| < 1, PTmax. T > 2. 0 Ge. V/c (not including PTmax. T) relative to PTmax. T (rotated to 180 o) and the charged particle density, d. Nchg/dhdf, p. T > 0. 5 Ge. V/c, |h| < 1, relative to jet#1 (rotated to 270 o) for “back-to-back events” with 30 < ET(jet#1) < 70 Ge. V. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 30
“Associated” Charge Density PYTHIA Tune A vs HERWIG (without multiple parton interactions) too few “associated” particles in the direction of PTmax. T! And HERWIG (without multiple parton interactions) too few particles in the direction opposite of PTmax. T! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 31
“Associated” Charge Density PYTHIA Tune A vs HERWIG For PTmax. T > 2. 0 Ge. V both PYTHIA and HERWIG produce slightly too many “associated” particles in the direction of PTmax. T! Next Step Look at the jet topologies (2 jet vs 3 jet vs 4 jet etc). See if there is an excess of But HERWIG (without multipledue to multiple 4 jet events parton interactions) produces too few particles in the interactions! parton direction opposite of PTmax. T! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 33
The Universality of PYTHIA Tune A Æ We would like to have a “universal” tune of PYTHIA! § QCD Hard Scattering § Direct Photon Production § Z-Boson Production § Heavy Flavor Production Æ I working on a “universal” PYTHIA Run 2 tune! § § Must specify the PDF! Must specify MPI parameters! Must specify Q 2 scale! Must specify intrinsic k. T! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 35
Refer to this as a “Leading Jet” event New CDF Run 2 Analysis Photon and Z-boson Refer to this as a “Leading Photon” event Refer to this as a “Zboson” event Æ Study the Df distribution of the charged particle density, d. Nchg/dhdf, and the charged scalar p. T sum density, d. PTsum/dhdf, for charged particles in the region p. T > 0. 5 Ge. V/c, |h| < 1) in “leading jet” events. and “leading photon” events! and “Z-boson” events! Æ Study the average charged particle and PTsum density in the “toward”, “transverse”, and “away” regions versus ET(jet#1) in “leading jet” events. and “leading photon” events! and “Z-boson” events! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 36
Charged Particle Density Df Dependence rdfsoft! PY Tune A Æ Shows the Df dependence of the density, d. Nchg/dhdf, for charged particles in the range Æ Æ p. T > 0. 5 Ge. V/c and |h| < 1 relative to jet#1 (rotated to 270 o) for ET(jet#1) > 30 Ge. V for “Leading Jet” events from PYTHIA Tune A. Shows the Df dependence of the density, d. Nchg/dhdf, for charged particles in the range p. T > 0. 5 Ge. V/c and |h| < 1 relative to pho#1 (rotated to 270 o) for PT(pho#1) > 30 Ge. V for “Leading Photon” events from PYTHIA Tune A. Shows the Df dependence of the density, d. Nchg/dhdf, for charged particles in the range p. T > 0. 5 Ge. V/c and |h| < 1 relative to the Z (rotated to 270 o) for PT(Z) > 30 Ge. V for “Zboson” events from PYTHIA Tune A. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 37
Not “Blessed” Yet! Preliminary Results Æ PYTHIA Tune A agrees well with the density, d. Nchg/dhdf, for charged Æ Æ particles and the scalar PTsum density, d. PTsum/dhdf, for the region p. T > 0. 5 Ge. V/c and |h| < 1 in the “toward” and “transverse” regions for both direct photon and Z-boson events at 1. 96 Te. V. However, I probably should increase the intrensic k. T. PYTHIA Tune A uses the default value. In addition to specifying the PDF and the MPI parameters, one must specify the Q 2 scale for each process. For Tune A Q 2 = 4 p. T 2 for QCD jets and direct photons and Q 2 = Mz 2 for Z-boson production. The data looks like PYTHIA Tune A! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 38
PYTHIA Tune A LHC Predictions Factor of 2! Æ Shows the average “transverse” charge particle and PTsum density (|h|<1, PT>0) versus PT(charged jet#1) predicted by HERWIG 6. 4 (PT(hard) > 3 Ge. V/c, CTEQ 5 L). and PYTHIA Tune A (PT(hard) > 0, CTEQ 5 L) at 1. 8 Te. V and 14 Te. V. Æ At 14 Te. V tuned PYTHIA Tune A predicts roughly 2. 3 charged particles per unit h-f (PT > 0) Æ in the “transverse” region (14 charged particles per unit h) which is larger than the HERWIG prediction. At 14 Te. V tuned PYTHIA Tune A predicts roughly 2 Ge. V/c charged PTsum per unit h-f (PT > 0) in the “transverse” region at PT(chgjet#1) = 40 Ge. V/c which is a factor of 2 larger than at 1. 8 Te. V and much larger than the HERWIG prediction. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 39
PYTHIA Tune A LHC Predictions 12% of “Min-Bias” events have PT(hard) > 10 Ge. V/c! LHC? Æ Shows the center-of-mass energy dependence 1% of “Min-Bias” events have PT(hard) > 10 Ge. V/c! of the charged particle density, d. Nchg/dhdfd. PT, for “Min-Bias” collisions compared with PYTHIA Tune A with PT(hard) > 0. Æ PYTHIA Tune A predicts that 1% of all “Min-Bias” events at 1. 8 Te. V are a result of a hard 2 -to-2 parton-parton scattering with PT(hard) > 10 Ge. V/c which increases to 12% at 14 Te. V! Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 43
LHC Predictions Summary & Conclusions Tevatron LHC Warning! 12 times more likely These predictions cannot to find be a 10 Ge. V Æ PYTHIA Tune A predict a 40 -45% rise in d. Nchg/dhdf at h = 0 in going from the “jet” in “Min-Bias” trusted! We need to improve the at the LHC! Tevatron (1. 8 Te. V) to the LHC (14 Te. V). 4 charged particles per unit h at the Tevatron modeling of the “beam-beam” becomes 6 per unit h at the LHC. remnants “multiple-parton Æ PYTHIA Tune A predicts that 1% ofand all “Min-Bias” events at the Tevatron. Twice (1. 8 as. Te. V) muchare activity in the result of a hard 2 -to-2 parton-parton scattering with PT(hard) > 10 Ge. V/c whichthe interactions”. “underlying event” What we are learning at the Tevatron will result in improved models! Æ Æ increases to 12% at LHC (14 Te. V)! at the LHC! For the “underlying event” in hard scattering processes the predictions of HERWIG and PYTHIA Tune A differ greatly (factor of 2!). HERWIG predicts a smaller increase in the activity of the “underlying event” in going from the Tevatron to the LHC. PYTHIA Tune A predicts about a factor of two increase in the charged PTsum density of the “underlying event” in going from the Tevatron to the LHC. Te. V 4 LHC Fermilab September 16, 2004 Rick Field - Florida/CDF 44
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