Using Drell-Yan to Probe the

Using Drell-Yan to Probe the Underlying Event in Run 2 at CDF Deepak Kar University of Florida (On behalf of the CDF Collaboration) Experimental Particle Physics Seminar 30 th October 2008

Prologue: We need a good Understanding of ordinary QCD Need to be able to simulate ordinary QCD and Standard Model events at the collider. Finding new physics requires a good understanding of the old Physics (Not only to have a good model of the hard scattering part of the process but also of underlying event ). 2

Outline Collider Events Underlying Event. Drell-Yan Process Advantages. Tuning PYTHIA. Leading Jet and Drell-Yan Results. Looking ahead to the LHC. 3

TeVatron CDF 4

TeVatron Luminosity 5

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Collider Detector at Fermilab (CDF) 7

Collider Detector at Fermilab (CDF) 8

Collider Coordinates Positive z-axis along the incident proton beam direction. Phi - Azimuthal angle P T - component of momentum in the transverse plane 9

Pseudo-Rapidity Instead of the polar angle, pseudo-rapidity, is used: η = log(tan θ/2). 10 η θ cm 0 90 o 1 40 o 2 15 o 3 6 o 4 2 o The sphere has lines drawn at pseudo-rapidity intervals. The cylinder is after the transformation to pseudorapidity space, θ η.

Proton-AntiProton Collisions at the Tevatron Elastic Scattering Single Diffraction Double Diffraction M M1 M2 σ tot = σ EL + σ SD +σ DD +σ HC tot EL SD DD HC The hard core component contains both hard and soft collisions. Hard Core Hard Hard Core (hard scattering) Outgoing Parton PT(hard) Soft Hard Core (no hard scattering) Proton AntiProton Proton AntiProton Underlying Event Underlying Event Outgoing Parton Final-State Radiation Initial-State Radiation 11

Minimum Bias Events Events collected with a trigger that is not very restrictive ideally with totally inclusive trigger. Typically contain some single and double diffraction as well as most of the hard core component of the inelastic cross section. In principle contains all types of interactions proportionally to their natural production rate. Which is not the same as 12

Hard The Underlying Event Scattering Component Hard Scattering Outgoing Parton Initial-State Radiation PT(hard) Outgoing Parton Final-State Radiation Proton AntiProton Underlying Event Underlying Event Underlying Event Everything except the two outgoing hard scattered components. (Beam-beam remnants, multiple parton interactions ) 13

The Underlying Event in a Hard-Scattering Process is not the same as Min-Bias Events The underlying event produces tracks in the detector and energy in the calorimeter, thus affecting the measurement of the hard scattering component. Presence of initial and final state radiation. Color interactions between the hard scattering and the underlying event might occur. 14

At the Tevatron about 1% of min-bias events contain a jet with 10 GeV transverse energy. At the LHC we expect this fraction increase by more than a factor of 10. The CDF UE analysis showed that the density of particles in the UE in jet events is about a factor of two larger than the density of particles in a typical MB collision. At the LHC the difference might be even greater. 15

So what is the problem with the Underlying Event? The environment at Hadron Colliders are dominated by hard scattering events. These hard scattering events are contaminated by the underlying event. The underlying event is an unavoidable background to most collider observables. Increasing luminosity implies more hadronic collisions which also complicates things. (pile-up) The underlying event is not well understood since non perturbative physics is involved. 16

From an experimental point of view, on an event by event basis, it is impossible to separate these two components. However, modeling these softer physics is very important. By looking at Min-Bias and Underlying Event data. By looking at the correlation between <p T > and charged multiplicity to discriminate different scenarios. 17

Precision measurements of hard interactions where soft effects need to be subtracted. Inclusive jet cross-section, b-jet cross section, missing energy, isolation cuts QCD Monte-Carlo tuning. Higher the precision, higher the accuracy of physics measurements. 18

Monte-Carlo Simulation of Hadron-Hadron Collisions Proton AntiProton Proton AntiProton Beam Remnants color string color string Beam Remnants Beam Remnants color string color string Beam Remnants 19

Monte-Carlo Simulation of Hadron-Hadron Collisions Jet quark-antiquark pairs Beam Remnants Beam Remnants Beam Remnants Beam Remnants quark-antiquark pairs Jet The resulting event consists of hadrons and leptons in the form of two large transverse momentum outgoing jets plus the beam-beam remnants. 20

Dividing up the Central Region Charged Jet #1 or Z-Boson Direction φ We define Toward Transverse Transverse Away φ < 60 o as Toward 60 o < φ < 120 o as Transverse φ > 120 o as Away PT > 0.5 GeV η < 1 Azimuthal angle φ relative to the leading calorimeter jet (or the Z-boson) 21

The Transverse Regions For Jet Production we use the Transverse Regions to study the underlying event. φ 2π Away Region Transverse Region 1 Leading Jet The transverse region is almost Perpendicular to the plane of 2-2 hard scattering and therefore very sensitive to the underlying event! Charged Jet #1 Direction Toward Toward-Side Jet φ Transverse Transverse Toward Region Transverse Region 2 Away Away Region 0-1 η +1 Away-Side Jet 22

Drell Yan Process Quarks and Anti Quarks annihilate to produce a virtual Photon or Z, which then decays into a lepton pair (µ + µ - or e + e - ). Z 23

A Bit of History Initial studies on muon pair production in hadron-hadron collisions showed rapid fall in cross section with increasing dilepon mass. Predictions of Drell and Yan (1970,1971) explained most features of this process. Christenson et al, 1970 (and subsequent results) 24

Drell-Yan Event COT view CAL view 25

The Underlying Event Drell-Yan Production Lepton Lepton-Pair Production Hard Scattering Component Anti-Lepton Initial-State Radiation Proton AntiProton Underlying Event Underlying Event Initial-State Radiation Lepton Anti-Lepton Proton AntiProton Underlying Event Underlying Event Underlying Event Everything except the two outgoing hard scattered components. (Beam-beam remnants, multiple parton interactions ) 26

Low p T Drell-Yan Production Drell-Yan Production Lepton Proton AntiProton Underlying Event Underlying Event Initial-State Radiation Anti-Lepton 27 High p T Drell-Yan Production Drell-Yan Production Lepton-Pair PT(pair) Proton AntiProton Underlying Event Underlying Event Outgoing Parton Final-State Radiation Initial-State Radiation

Z-Boson Production at Tevatron Single Z Bosons are produced with large p T via the ordinary QCD sub processes: Z-Boson Direction φ They generate additional gluons via bremsstrahlung resulting in multi-parton final states fragmenting into hadrons and forming away-side jets. Toward Transverse Transverse Away Away-Side Jet CDF (pb) NNLO (pb) σ(z l + l - ) 254.9±3.3(stat)±4.6(sys)±15.2(lum) 252.3±5.0 CDF: Phys. Rev. Lett. 94, 091803 (2005) NNLO Theory: Stirling, Van Neerven 28

Comparison with Jet Production Drell-Yan Production Lepton Proton AntiProton Underlying Event Underlying Event Initial-State Radiation For Hard scattered jets we have studied the transverse regions, which inevitably receive contributions from initial and final state radiation (i.e. multijet production). The central region is very sensitive to the underlying event! For DY production It is easy to identify and remove leptons, so it is a cleaner way to study the Anti-Lepton 0 underlying event. -1 +1 φ 2π Central Region η 29

Why Study Drell-Yan? Very nice probe of the underlying event. Can study the underlying event as a function of the lepton-pair invariant mass. Can study the underlying event as a function of the lepton-pair transverse momentum. Can compare with high p T jet production. Also 30

Importance of z -> l + l - Apart from tuning QCD MC Large W/Z background in new physics searches. Clean experimental signature for different measurements. Leptons, originating from Z bosons with large transverse momentum, may fake the signature of predicted new particles, which decay into high energetic leptons. SUSY Searches: 31

Observables Observable Particle Level Detector Level Lepton Pair P T P T of the Lepton Pair. P T of the Lepton Pair, formed by at least one Tight Lepton. No of Charged Number of charged particles (P T > 0.5 GeV/c, η < 1) Number of good charged tracks (P T > 0.5 GeV/c, η < 1) P T Sum Scalar P T sum of charged Scalar P T sum of good particles charged tracks (P T > 0.5 GeV/c, η < 1) (P T > 0.5 GeV/c, η < 1) < P T > Average P T of charged particles (P T > 0.5 GeV/c, η < 1) Require at least 1 charged particle Average P T of good charged tracks (P T > 0.5 GeV/c, η < 1) Require at least 1 charged track P T Max Maximum P T of charged particle (P T > 0.5 GeV/c, η < 1) Require at least 1 charged particle Maximum P T of good charged tracks (P T > 0.5 GeV/c, η < 1) Require at least 1 good charged track 32

PYTHIA Apollo's priestess, Pythia, performing the duty of the oracle: 33 For underlying event studies, the only tool we have is to compare the data and the predictions from various Monte Carlo event generators, i.e. PYTHIA.

Tuning PYTHIA Since some physics aspects, like hadronization and multiple interactions cannot be derived from first principles, this program contains many parameters that represent a true uncertainty in our understanding of nature. Need to produce tunes, not of one parameter at a time, but simultaneously for a group of them. Given the many PYTHIA parameters to be tuned, it is convenient to divide the task into subtasks. 1. If we assume jet universality, hadronization and final-state parton showers should be tuned to e^+e^- annihilation data, notably from LEP1, since this offers the cleanest environment. 2. With such parameters fixed, hadron collider data should be studied to pin down multiple interactions and other further aspects, such as initial-state radiation. 34

PYTHIA UE Parameter Definition MSTP(81) MPI on/off MSTP(82) 3 / 4: resp. single or double gaussian hadronic matter distribution in the p / pbar PARP(67) ISR Max Scale Factor PARP(82) MPI pt cut-off PARP(83) Warm-Core: parp(83)% of matter in radius parp(84) PARP(84) Warm-Core: PARP(85) prob. that an additional interaction in the MPI formalism gives two gluons, with colour connections to NN in momentum space PARP(86) prob. that an additional interaction in the MPI formalism gives two gluons, either as described in PARP(85) or as a closed gluon loop. Remaining fraction is supposed to consist of qqbar pairs. PARP(89) ref. energy scale PARP(90) energy rescaling term for PARP(81-82)~E CM^PARP(90) 35

PYTHIA UE Parameter Definition MSTP(81) MPI on/off MSTP(82) 3 / 4: resp. single or double gaussian hadronic matter distribution in the p / pbar PARP(67) ISR Max Scale Factor PARP(82) MPI pt cut-off PYTHIA uses MPI to enhance the UE. Multiple parton interaction more likely in a hard (central) collision. ISR Max Scale Factor affects the amount of initialstate radiation. Increasing the cut-off decreases the multiple parton interaction. 36

CDF Run 1 Tune (PYTHIA 6.2 CTEQ5L) UE Parameters Parameter Tune A Tune AW MSTP(81) 1 1 0.12 0.08 0.04 Z-Boson Transverse Momentum CDF Run 1 Data PYTHIA Tune A PYTHIA Tune AW CDF Run 1 published 1.8 TeV Normalized to 1 Distribution 1/N dn/dpt MSTP(82) 4 4 PARP(82) 2.0 GeV 2.0 GeV PARP(83) 0.5 0.5 PARP(84) 0.4 0.4 PARP(85) 0.9 0.9 PT PARP(86) 0.95 0.95 ISR Parameters PARP(89) 1.8 TeV 1.8 TeV PARP(90) 0.25 0.25 0.00 0 2 4 6 8 10 12 14 16 18 20 Z-Boson PT (GeV/c) PARP(62) 1.0 1.25 Intrensic KT PARP(64) 1.0 0.2 PARP(67) 4.0 4.0 MSTP(91) 1 1 PARP(91) 1.0 2.1 PARP(93) 5.0 15.0 Both tunes reveal a remarkably good agreement of the data and PYTHIA. 37

Bringing PYTHIA in Good Agreement with the Data The initial state radiation had to be adjusted. The dependence of the probability of multi-parton (secondary) interactions on the impact parameter had to be smoothed out. Probability of di-gluon production in multi-parton secondary interactions had to be substantially enhanced over di-quark production. The probability of color connections of products of secondary interactions with p p-remnants had to be increased. Soft QCD phenomena in events with high-et jets at Tevatron - Andrey Korytov, Eur Phys J C 33, s01, s425 s426 (2004) 38

CDF Run 2 Tune (PYTHIA 6.206 CTEQ5L) Parameter Tune A Tune DW Tune DWT MSTP(81) 1 1 1 "Transverse" Charged Average PT UE Parameters MSTP(82) 4 4 4 1.7 1.5 1.3 1.1 0.9 "Transverse" <PT> (GeV/c) PARP(82) 2.0 GeV 1.9 GeV 1.9409 GeV PARP(83) 0.5 0.5 0.5 PARP(84) 0.4 0.4 0.4 PARP(85) 0.9 1.0 1.0 PARP(86) 0.95 1.0 1.0 PARP(89) 1.8 TeV 1.8 TeV 1.96 TeV 0.7 CDF Run 2 Preliminary data corrected to particle level PY Tune A, DW 1.96 TeV HERWIG PY-ATLAS "Leading Jet" MidPoint R = 0.7 η(jet#1) < 2 Charged Particles ( η <1.0, PT>0.5 GeV/c) 0 50 100 150 200 250 300 350 400 450 PT(jet#1) (GeV/c) ISR Parameters Intrensic KT PARP(90) 0.25 0.25 0.16 PARP(62) 1.0 1.25 1.25 PARP(64) 1.0 0.2 0.2 PARP(67) 4.0 2.5 2.5 MSTP(91) 1 1 1 PARP(91) 1.0 2.1 2.1 PARP(93) 5.0 15.0 15.0 PYTHIA Tune DW is very similar to Tune A except that it fits the CDF P T (Z) distribution and it uses the DØ prefered value of PARP(67) = 2.5. 39

Goal of the Analysis The goal of the analysis is to produce data on the underlying event that is corrected to the particle level so that it can be used to tune the QCD Monte-Carlo models without requiring CDF detector simulation (i.e. CDFSIM). Also by looking at the measurements sensitive to the underlying event, we would be able to better constrain our underlying event models. 40

Steps Are: 1. Calculate the observables by Monte Carlo event generator in particle level and in (by running through CDFSIM) detector level. 2. Correct the observables back to particle level in real data by calculating the correction factor from Monte Carlo. 3. Compare with different Monte Carlo event generators (PYTHIA, HERWIG ). 41

Compare Results with Compare the Tevatron Drell-Yan underlying event data with the Tevatron Jet underlying event data. Produce Drell-Yan data on the underlying event at the Tevatron to compare with a similar analysis done at the LHC by the Florida CMS group. 42

Our Analysis High p T Electron and Muon Data, yielding ~65,000 electron and muon (oppositely charged) pairs each, passing all selection cuts. Charged Particles with p T > 0.5 GeV/c and lηl <1 Using events with the lepton pair invariant mass in the Z region: 70 GeV/c 2 < M(ll) < 110 GeV/c 2 Errors coming both from statistical and systematic (due to lepton selection and charged particle selection) sources. 43

Negligible Background at Z 44

Transverse Region 45 Z-boson Direction φ Toward Transverse Transverse Away

Transverse Region Z-boson Direction φ Toward Transverse Transverse Away 46

Transverse Region Z-boson Direction φ Toward Transverse Transverse Away 47

Toward Region 48 Z-Boson Direction φ Toward Transverse Transverse Away

Toward Region Z-Boson Direction φ Toward Transverse Transverse Away 49

Toward Region Z-Boson Direction φ Toward Transverse Transverse Away 50

Away Region 51 Z-Boson Direction φ Toward Transverse Transverse Away

Away Region Z-Boson Direction φ Toward Transverse Transverse Away 52

Away Region Z-Boson Direction φ Toward Transverse Transverse Away 53

Overlaying All Three Regions 54

Overlaying All Three Regions 55

Combining All Three Regions Z-Boson Direction φ Overall 56

Comparing with Jet Transverse Region 57

Comparing with Jet Transverse Region 58

Comparing with Jet Away Region Differences due to the fact we do not require the awayside jet in dijet events to be in the away region. Charged Jet #1 Direction Toward φ Transverse Transverse Away Toward-Side Jet Z-Boson Direction Toward φ Transverse Transverse Away Away-Side Jet Away-Side Jet 59

Mean p T vs Charged Multiplicity Fermilab Result of the Week <p T > versus Nchg is a measure of the amount of hard versus soft processes contributing and it is sensitive to the modeling of the multiple-parton interactions. 60

Mean p T vs Charged Multiplicity 80 60 40 20 0 CDF Run 2 Preliminary data corrected generator level theory "Drell-Yan Production" 70 < M(pair) < 110 GeV PT(Z-Boson) versus Nchg HW pyaw 0 5 10 15 20 25 30 35 40 Number of Charged Particles JIM Fermilab Result of the Week Average PT(Z) (GeV/c) ATLAS Charged Particles ( η <1.0, PT>0.5 GeV/c) excluding the lepton-pair <p T > versus Nchg is a measure of the amount of hard versus soft processes contributing and it is sensitive to the modeling of the multiple-parton interactions. 61

Moving Forward to LHC The UE measurement plan at the LHC benefits from the solid experience of studies at Tevatron. Predictions on the amount of activity in transverse region at the LHC are based on extrapolations from lower energy data All the UE models have to be tested and adjusted at the LHC, in particular we know very little about the energy dependence of MPI in going from the Tevatron to the LHC. 62

Step by Step 1. The first pb 1 of collected data: Mainly to calibrate the different analysis tools, look at minbias data. 2. With 10 pb 1 and a partially calibrated detector: Look at underlying event observables, take control of systematics. 3. Extending the statistics to 100 pb 1 : Distinguish between differences of the investigated underlying models models. CMS plan is to measure MB and UE at low luminosity and tune QCD MC models for all other physics analysis. Retune as the measurements become more reliable. 63

The Underlying Event in Drell-Yan Production in LHC Underlying Event much more active at LHC 64

Summary Observed excellent agreement with PYTHIA Tune AW predictions. Close Match with leading jet underlying event results perhaps indicating an universality of underlying event models independent of hard scattering event. 65

Epilogue One will learn a lot about the energy dependence of the underlying event (i.e. multiple parton interactions) by comparing the Tevatron results with the LHC. Early analyses in LHC would be geared towards tuning the QCD Monte Carlo models. An important first step in LHC would be to rediscover the standard model Drell Yan process is one of the cleanest signature for that. 66

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