Amnon harel aharel@fnal gov university of rochester
This presentation is the property of its rightful owner.
Sponsored Links
1 / 45

Amnon Harel [email protected] University of Rochester PowerPoint PPT Presentation


  • 79 Views
  • Uploaded on
  • Presentation posted in: General

International Workshop on Top Quark Physics La Biodola, Isola d’Elba, Italy 18-24 May, 2008. MC for top background at the Tevatron. Amnon Harel [email protected] University of Rochester.

Download Presentation

Amnon Harel [email protected] University of Rochester

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Amnon harel aharel@fnal gov university of rochester

International Workshop on Top Quark PhysicsLa Biodola, Isola d’Elba, Italy18-24 May, 2008

MC for top backgroundat the Tevatron

Amnon [email protected]

University of Rochester

Thanks to: Kirsten Tollefson, Lisa Shabalina, Christopher Neu, Tom Junk, Ann Heinson, Jason Nielsen, Michael Begel, Ulrich Husemann, Kevin Lanon, Gustavo Garzon, Gerald Grenier, and apologies to those whose names I accidentally left out


Outline

Outline

Backgrounds

Measurements

Matched MC

Reweighting the MC

The cost of matching

  • BTWs:

  • multijets

  • Detector simulation

Amnon Harel


The backgrounds top pair

The backgrounds – top pair

Hard to simulate misreconstructions  Data driven estimation

Dilepton channel

Small  A rough simulation suffices

A.k.a.: Z+jets

Lepton+jets channel

Haven’t discovered that one yet

Amnon Harel


V jets top pair

V+jets – top pair

Dilepton channel

  • Z(ee/μμ)+jets - main background in ee and μμ channels, Z()+jets in eμ

  • Very important at the early stages of event selection

  • After final selection Z+jets contribution is small

  • Most measurements do not use b tagging

    • Flavor composition usually not important

Lepton+jets channel

  • This channel provides the most precise measurements

  • W+jets - main background

  • Very important at all stages of selection

  • Most of measurements use b-tagging

  • Knowledge of flavor composition is a limiting factor for precision measurements

Amnon Harel


The backgrounds single top

The backgrounds – single top

H.F. fractions of W+jets crucial

W+light

Multijet

  • BTW: WH Higgs search is also sensitive to W+jets H.F. fractions

CDF

Amnon Harel


V jets cross sections

V+jets cross sections

Amnon Harel


W bb measurements

W+bb measurements

What have we measured? Though conclusions from W+HF measurements not yet applied to top analyses

CDF preliminary from Monday(1.9fb-1):

Harsh b-tagging and MET (>25GeV) selections

Jet definitions:

Jet selection:

Result:

Alpgen prediction: 0.78pb

, &

, &

DØ preliminary from 2005(382pb-1):

, , &

Amnon Harel


W c measurements

W+c measurements

CDF Data:

MCFM:

DØ Data:

Alpgen+Pythia:

Phys. Rev. Lett. 100, 091803 (2008)

arXiv: 0803.2259v1[hep-ex]

Recent MCFM calculations (John Campbell)

for DØ cuts yield R(LO)=3.7, R(NLO)=3.4

Amnon Harel


W jets measurement

W+jets measurement

Dedicated W+jets measurement.

SMPR = CKKW matched Madgraph + Pythia

MLM = MLM matched Alpgen + Herwig

MCFM = NLO predictions

will be a recurring theme

Amnon Harel


Matched mc

Matched MC

For top physics, we usually simulate W+jets (also Z+jets) using MLM matched Alpgen. Implementations differ:

  • light partons jets are MLM matched

    • with pT>8GeV

    • MLM stable to the chosen pT

  • Discard events with additional heavy quarks from the PS MC

    • done in post processing

  • Generate 14 samples:

  • W+bb+Nlp, with N=0,1,2, or ≥3

  • W+cc+Nlp, with N=0,1,2, or ≥3

  • W+Nlp, with N=0,1,2,3,4, or ≥5 (includes W+c+jets  massless c quarks)

  • MLM matched (pT>15GeV) within each class of events

  • cc / bb pairs within the same parton jet are taken only from the PS MC; those in different jets taken only from the ME MC

  • Generate 15 samples:

  • W+bb+Nlp, with N=0,1, or ≥2

  • W+cc+Nlp, with N=0,1, or ≥2

  • W+c+Nlp, with N=0,1,2, or ≥3

  • W+Nlp, with N=0,1,2,3, or ≥4

Amnon Harel


Z jets simulation

Alpgen qfac=ktfac=0.5,could be a JES issue

Z+jets Simulation

  • Z+jets appears at a lower rate (~×10), but has much less background  a good process for tuning the simulations.

  • NormalizationUsually it suffices to normalize according to cross sections predicted from MCFM or NNLO calculations.

  • Dependency on kinematic cuts!

  • Some analyses require normalization from data, e.g., CDF’s top FCNC search in Z+jets

  • KinematicsCan be tuned using data

  • Example: ResBos described DØ’s Z pT data well. DØ is starting to use it as a surrogate to the data and to re-weight ALPGEN+PYTHIA to match the same spectrum. The same re-weighting is carried over to W+jets.

Alpgen qfac=ktfac=1

arXiv:/0712.0803

Amnon Harel


Z jets shape pythia sherpa i

Z+jets Shape: Pythia & Sherpa - I

Sherpa uses CKKW matching

T

Normalized to data without parametrizing in Njet

Pythia is a parton shower generator  not enough radiation

JES uncertainties dominate systematics (and are a bit conservative)

Amnon Harel


Z jets shape pythia sherpa ii

Z+jets Shape: Pythia & Sherpa - II

Again Pythia spectrum is too soft

Amnon Harel


Z jets shape pythia sherpa iii

Z+jets Shape: Pythia & Sherpa - III

Again Pythia spectrum is too soft

Amnon Harel


Z jets shape pythia sherpa iv

Z+jets Shape: Pythia & Sherpa - IV

We’ll return to the simulation of dijet angles…

Amnon Harel


Z jets vs nlo

Z+jets vs. NLO

Dedicated Z+jets measurement.

Phys. Rev. Lett. 100, 102001 (2008)

MCFM hiding behind data points

Amnon Harel


Normalizing w jets

Normalizing W+jets

W+jets and W+HF are normalized to data, after other backgrounds (multijets, dibosons, etc.) are subtracted.

  • multijets estimated from data samples with leptons that pass only a looser selection (mostly looser isolation)

    • “loose-tight”

  • several approaches, typically:

    • W+jets normalized to data before b tagging

      • or fitted in signal samples

    • W+HF fractions normalized to data after b tagging

  • multijets estimated from data samples with leptons that pass only a looser selection

    • “anti-electrons”, ”non-isolated”

  • Studies with dijet & multijet events

  • “Method 2” inherited from Run I:

    • W+jets normalized to data before b tagging

    • W+HF fraction fitted to data after b tagging

Assumptions: Kbb=Kcc, and Kc=1 (consistent with studies)

Amnon Harel


W jets normalization @ cdf

W+jets Normalization @ CDF

  • Three parameter fit to Bottom-Charm-Light templates of jet-flavor separating distributions (NN output, SecVtx mass) in W+2 jet data

    • yields KHF=1.4±0.4

      • relative to Alpgen H.F. fraction

  • Light flavor yield with prediction from

    • per jet fake b tag rates (estimated from inclusive multijet data)

    • either W LF MC or data, pre b tag yields

Amnon Harel


Amnon harel aharel fnal university of rochester

W+jets Normalization @ DØ

PRL 98, 181802 (2007)

  • For W+jets:

  • Normalize to pretag data

  • Channel dependence?

    • single-top vs. top-pair analysis

    • same in e+jets & μ+jets within uncertainties

  • Njet dependence?

  • For W+HF:

  • Normalize to data after b-tagging

    • in 0 b tag bin  negligible signal

  • Defined relative to Alpgen H.F. fraction

First data driven normalization: single-top evidence analysis

Amnon Harel


W jets normalization @ d ii

Sensitive to extra backgrounds included

Sensitive to selection cuts on jets

single top ||<3.4; ttbar ||<2.5

Consistent between e/μ channels

Studied several cuts on b tag output

Measured HF scale factor (uncertainty from all deviations observed in the studies):

KHF = 1.170.18

Used for summer results

W+jets Normalization @ DØ - II

Procedure refined for later ttl+jets measurements

  • tighter selection cuts

  • normalize MC to match the fraction of 0 tag events in 1 and 2 jet bins

Studies of systematic uncertainties

  • Separated Wc for W+lp and varied by 20% (not enough?)

  • Dependence on the multijet background contribution

  • Tried to extract corrections to Wcc and Wbb separately

Results

  • Switched from Alpgen 2.05 to 2.12

    • Several bug fixes including one in Wcc / Wbb generation

    • Compared shapes: the only noticeable difference is R(j,j)

    • Cross sections differ by a factor of 2!

  • New KHF factor ~1.9

Amnon Harel


Amnon harel aharel fnal university of rochester

W+jets Normalization for BYSM

Normalizing W+jets to data with the same jet multiplicity as signal is not trivial for BYSM searches, or even for cross section measurements.

Examples

  • searches for resonant top pair production

    • Iterative procedure in CDF’s

    • Analytical work around in D0’s

  • Charged Higgs search

Amnon Harel


W jets other reweightings

W+jets: Other Reweightings

Some discrepancies between Alpgen and data are showing up with the increasing statistics.

Corrected for / treated as systematic uncertainties in recent measurements.

E.g. possible missimulation of the angular distributions of the jet with the 2nd highest pT

Plot from latest single top analysis

Amnon Harel


D experience with matching

DØ experience with matching

At the time, Alpgen was the only matched MC that CDF & DØ could: run, integrated with their software, mass produce.

It allows us to produce physics results!

Large scale use + cutting edge technology = pain

Outlining the DØ experience to identify lessons.

  • Must add together the parton-jet bins with the correct weights. Unfortunately, weights are sample dependent

    • Must freeze samples

    • Post processing in DØ due to overlaid zero bias data & HF removal

    • Book keeping nightmare (ttbar, Z+jets in HF & MZ* bins, systematic variations)

    • Error prone

  • Large relative weights

    • necessary for multiple (additional) jets

    • but complicates statistics (some physics required)

Amnon Harel


D experience with matching1

DØ experience with matching

  • Using matched Alpgen extensively for the last couple of years:

  • We’ve made several mistakes, e.g.,

    • Random seeds outside legal range

    • Imperfect HF removal

  • Study Interplay with MC tunes

  • Some bugs found along the way, had to find workarounds

    • Best case: need to look at the right plot in the right channel

    • Recent case: “why does that matching weight look odd?”

  • Slow turn around times

    • Alpgen release  DØ release  Production (large 0lp samples, slow 5lp samples)  Postproduction  Analyses

    • 6-12 months

  • Limits our ability to generate sufficient samples to study systematics

Amnon Harel


Btw1 mc in multijet modeling

Work in progress – Example of failed model

BTW1: MC in multijet modeling

  • Though multijet modeling is data driven, it is (mostly) based on samples with 3 jets which are reconstructed as lepton + 2 jets.

  • Can check the methodology using simulated multijets

  • Lepton ID cuts

  • Lepton ID efficiencies and their parametrization

  • MET (“triangle”) cuts

  • Sample composition

Example of bad agreement from recent DØ studies:

Amnon Harel


Btw2 detector simulation

BTW2: Detector simulation

  • An ubiquitous problem, often taken for granted.Data based modeling for:

  • misreconstructed leptons, i.e., multijet background

  • b tagging rates

  • relative jet energy scale

  • This talk focused on the generator parts of simulation rather than on detector simulation.

  • Modeling of multijet background is covered in analysis presentations

  • the last two items will be covered in two talks about “tools for top” later today (CDF @ 15:30, D0 @ 16:00).

Amnon Harel


Conclusions

Conclusions

  • Simulation of W+jets and Z+jets

  • is not trivial

  • can be analysis dependent:

    • One size can almost fit all

    • Don’t always need the most sophisticated treatment

    • Several approaches to estimating the heavy flavor contribution

    • Can fit and/or normalize W+jets

    • New physics can complicate matters

  • Using matched Alpgen extensively for the last couple of years

  • Able to meet all our physics needs!

    • Higgs / Top pairs / single top / W+HF / Z+HF

    • Lepton+jets+MET / di-lepton final states (Z+jets important)

  • possible inaccuracies (e.g. Z pT, 2nd jet ΔR)

  • difficulties adapting to this technology

    • Technical lesson: avoid any post processing that can break the matching

  • Other generators seem promising

  • but have received much less scrutiny

Amnon Harel


Amnon harel aharel fnal university of rochester

Back up slides

Amnon Harel


Amnon harel aharel fnal university of rochester

29

Other weights

To estimate PDF uncertainties we reweight our MC after the fact, rather than regenerate it.

Is this compatible with MLM matching?

Yes [private communications with Mangano]

We simulate additional collisions by overlaying “zero-bias” data (i.e. free of trigger biases) onto simulated collisions. Since instantaneous luminosities are still rising, we “update” the simulation by giving more weight to simulated events whose overlaid data event had a high inst. lumi.

Amnon Harel


Amnon harel aharel fnal university of rochester

30

Matching @ CDF

Amnon Harel


Calibrating the simulation

Calibrating the Simulation

  • We verify each aspect the simulation, mostly on appropriate data samples, and correct the simulation as needed.

  • Jets

    • Relative energy scale

    • Energy resolution (ET smearing)

    • Reco. & ID Efficiencies

  • b-tagging rates

    • Corrections for gluon splitting

  • Instantaneous Luminosity

  • Primary Vertex Z-coordinate

  • Electrons

    • Resolutions (ET smearing)

    • Reco. & ID efficiencies

  • Muons

    • Resolutions (pT smearing)

    • Reco., ID and isolation efficiencies

Amnon Harel


Amnon harel aharel fnal university of rochester

Normalizing to Data @ DØ

In several double-top l+jets “property” analyses

  • W+HF normalized to data as in previous slides

  • W+jets fit to data

    • topological likelihood separates top pair from W+jets

Example from PRL 100, 062004 (2008)

Backgrounds (mostly W+jets)

Top Pair

Amnon Harel


Amnon harel aharel fnal university of rochester

Normalizing W+jets @ DØ HZ

Another example: Higgs search in HZbbX channel

  • Heavy Flavor in (W/Z)+jet Alpgen+Pythia predictions were multiplied by K-factors (NLO/LO) calculated with MCFM

  • (W/Z)+jets normalized to data before b-tagging

After b-tagging

Before b-tagging

Amnon Harel


Terminology

Terminology

  • DØ is trying to converge on terminology for normalization factors:

  • K-factor: normalizes LO to match (N)NLO

  • K’-factor: normalizes MC to match (N)NLO

  • S-factor: normalizes MC to match “pretag” data

  • SHF: normalized heavy-flavor MC to match b-tagged data

  • At DØ:

  • For Z+lp and Z+HF:

  • Using K’-factors, normalize to theory

  • Some analyses (e.g. ) override with S-factors

  • For W+lp: Using S factors

  • For W+HF: Using S & SHF factors

Amnon Harel


Know how i

Know-how I

  • Generate 14 samples:

    • W+bb+Nlp, with N=0,1,2 or ≥3

    • W+cc+Nlp, with N=0,1,2 or ≥3

    • W+Nlp, with N=0,1,2,3,4 or ≥5 (includes W+c+jets)

  • Individual samples can not be used any more!

Add all parton-jet bins together with weights Fi

Amnon Harel


Know how ii

Know-how II

  • Sample has to be frozen

  • Large relative weights

  • Complicates statistics

  • Post processing:

    • Data quality selection due to zero bias overlay

    • Discard events with additional heavy quarks created by Pythia

  • Generate 14 samples:

    • W+bb+Nlp, with N=0,1,2 or ≥3

    • W+cc+Nlp, with N=0,1,2 or ≥3

    • W+Nlp, with N=0,1,2,3,4 or ≥5

No skimming

Book keeping is a nightmare!

Amnon Harel


From generated mc to data

Generate multi-parton MEs with Alpgen

Add showering and hadronization from Pythia

b-fragmentation model

underlying event model

Run full D0 detector simulation and reconstruction

Add zero bias events to match luminosity profile in data

Apply to simulated events:

JES

Jet removal

Smear jets, electrons and muon

Propagate to missing ET

Correction factors:

Trigger efficiency

Electron and muon ID efficiency

For b-tagging:

Taggability RF

Tag rate function

Reweightings: lumi, z vertex, etc

From generated MC to data

Amnon Harel


Z jets @ cdf i

Alpgen qfac=ktfac=1

Z+jets @ CDF - I

FCNC analysis used this for a systematic uncertainty.

negligible – doesn’t appear in the tables

Alpgen qfac=ktfac=2.0

Alpgen qfac=ktfac=0.5

Amnon Harel


Mcfm z jet @ d

MCFM Z+jet @ DØ

An example:

Results for 60-75 GEV

Amnon Harel


Mcfm w jets @ d

MCFM W+jets @ DØ

Method

  • All calculations come from the MCFM author (John Campbell)

  • K-factor = sigma(NLO)/sigma(LO)

  • Parameters:

    • PDF: CTEQ6L1(LO), CTEQ6M (NLO)

    • Factorization scale = Renormalization scale = MW

      Conclusions

  • MCFM calculations show that K-factor for W+light is stable

  • K-factors for Wc, Wcc, Wbb decrease as jet pT increases

  • MCFM does not support any additional HF scale factor in addition to W+light k-factor

  • The last conclusion contradicts our observation from data

Amnon Harel


D z jets normalizations

DØ Z+jets Normalizations

  • When estimating the W+jets background in Lepton+jets+MET channels, we predict Z+jets from the simulation + studies on di-lepton channels.

  • For Z+lp and Z+HF:

  • Typically weigh Alpgen+Pythia to MCFM / NNLO

    • NNLO from Hamberg et.al., Nucl.Phys.B359 + Martin et.al. hep-ph/0308087

    • Agree to within 10%

  • Recently calculate the K-factors with MCFM (is shaping needed?)

    • K-factors in the theoretical sense: NLO/LO

    • Cuts crucial in W+2HF (e.g. )

    • Evaluate effect of quark masses at LO: 10-40%

Amnon Harel


2nd jet angles @ 1 fb 1

2nd jet angles @ 1 fb-1

Clearly there’s nothing to be excited about in this plot.

Very preliminary analysis of later data shows similar trends.

Amnon Harel


D z jets shape corrections

DØ Z+jets Shape Corrections

arXiv:/0712.0803

  • Starting to use ResBos as a surrogate to our data

  • Reweighting Alpgen+Pythia MC to fit the ResBos spectrum

  • Applying the lesson from Z+jets to W+jets…

  • Other approaches

  • Scale directly to data as a function of Njet

    • Consider other event source

  • Measure & reweight as a function of:

    • ZpT (reconstruction vs. particle level)

    • jet observables

Amnon Harel


W c measurements1

1

4

+

(

)

(

)

(

)

l

b

i

9

8

2

8

0

6

t

t

t

§

:

§

1

4

+

s

a

s

y

s

u

m

p

(

(

)

(

%

)

)

%

b

P

D

F

1

4

7

1

1

4

0

0

1

3

7

§

§

1

6

¡

.

:

:

:

:

:

:

p

:

3

0

¡

:

:

:

:

:

:

W+c measurements

CDF Data:

MCFM:

DØ Data:

Alpgen+Pythia:

Phys. Rev. Lett. 100, 091803 (2008)

arXiv: 0803.2259v1[hep-ex]

Recent MCFM calculations (John Campbell)

for DØ cuts yield R(LO)=3.7, R(NLO)=3.4

Amnon Harel


Title

Title

  • Text

Amnon Harel


  • Login