Study of DC3 Fully Study of DC3 Fully Simulated WSimulated W→→eeνν Samples Samples with an eye to Strange Sea with an eye to Strange Sea

Asymmetry AnalysisAsymmetry Analysis

Laura Gilbert, University of Oxford 20/09/06

Many thanks to: Jeff Tseng, Amanda Cooper-Sarkar, Tony Weidberg, Chris

Hayes

OUTLINEOUTLINE

1)1) Motivation: quark asymmetries in Motivation: quark asymmetries in the protonthe proton

2)2) Technique for strange sea analysisTechnique for strange sea analysis

3)3) Analysis sample: Analysis sample: WW→→eeννee W reconstructionW reconstruction ConclusionsConclusions

10-2

10-1

100

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

MRST2001

Q2 = 10 GeV2

ant

idow

n / a

ntiu

p

x

Motivation: Quark Motivation: Quark Asymmetries in the Asymmetries in the

ProtonProton u, d distributions in the proton predicted to be

almost flavour symmetric within pQCD. MNC measured the flavour nonsinglet structure

function [Fp2(x,Q2) − Fn

2(x,Q2)]. → large (~30%) violation of Gottfried sum rule:

0))()((1

0

dxxuxd

d/u Confirmed by the NA51, E866 and HERMES.

Various theoretical models proposed. Meson Cloud model Meson Cloud model (MCM) seems most successful in (MCM) seems most successful in explaining observations. explaining observations.

A symmetric s/s distribution is often assumed, but not A symmetric s/s distribution is often assumed, but not established theoretically or experimentally.established theoretically or experimentally.

MCM would seem to imply a strange momentum fraction MCM would seem to imply a strange momentum fraction asymmetry too.asymmetry too.

Signal & Cao showed that incorporating asymmetric Signal & Cao showed that incorporating asymmetric distribution into the MCM can reduce NuTeV anomaly distribution into the MCM can reduce NuTeV anomaly (measured sin(measured sin22θθWW of 3 of 3σσ above accepted value, reduced to above accepted value, reduced to 22σσ by MCM). by MCM).

Possible Strange Sea Possible Strange Sea Momentum Asymmetry?Momentum Asymmetry?

In the MCM the proton In the MCM the proton oscillates into virtual oscillates into virtual mesons/baryons mesons/baryons

sea q/qbar are in different sea q/qbar are in different environments carrying environments carrying different momenta.different momenta.

0))()((1

0

dxxsxsx

du

u qq

du

uoscillate

sq

du

u q

A note on measuring A note on measuring strange sea asymmetries at strange sea asymmetries at

ATLASATLAS Likely to be hard to see at ATLAS:Likely to be hard to see at ATLAS:

s(x)

s(x)

s(x) - s(x)

Physics Letters B 381 (1996) 317-324: Brodsky & Ma

Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)]

Ws at LHC sensitive to small x regime (<0.01). Difficult to

probe.

An isolated electron, η<2.4 precision region, pT>25GeV.

ETmiss>25GeV. Kπ(ππ)(π0) + bachelor pion s→W-D*+; s→W+D*-: Sign of πB will be

anticorrelated with sign of W.

Detecting a Strange Sea Detecting a Strange Sea AsymmetAsymmet ryry

Signal: W+D*

e-

ν

D*+

jet Kπ

c

s

g

d

d π+

D0

cWgscWsg

cWgscWsg

NN

NNA

W-

Analysis TechniqueAnalysis Technique Select W candidate Select W candidate ((isolated

electron, |η|<2.4, pT>25GeV, ETmiss>25GeV)

Reconstruct DReconstruct D00→K→K--ππ++ (also (also DD00→K→K--π π ++ππ00, D, D00→K→K--π π ++ππ--π π ++ππ0 0 etcetc))

DD00 flight length: c flight length: cττ=123=123μμm so m so vertex displaced.vertex displaced.

Add prompt (soft) pion.Add prompt (soft) pion. Signal has opposite sign Signal has opposite sign

combinations of W, combinations of W, ππBB..

Backgrounds inc. same sign combinations, QCD.Backgrounds inc. same sign combinations, QCD. Should find zero asymmetry in Monte-Carlo from accepted Should find zero asymmetry in Monte-Carlo from accepted

PDFs. Work out CL on limits of null hypothesisPDFs. Work out CL on limits of null hypothesis

Preliminary ATLFAST Preliminary ATLFAST search search

DC2 sample (A4) of WDC2 sample (A4) of W→→eeννee at NLO. at NLO. One electron track with Pt>25GeV, missing Et>25GeV, One electron track with Pt>25GeV, missing Et>25GeV, ηη<2.5<2.5

~211k events, cross section 8.4nb, luminosity 33pb~211k events, cross section 8.4nb, luminosity 33pb -1-1

D* selection cutsD* selection cuts Pt of Kaon candidate > 1.5GeV, pion pt > 1.0GeV (combined to DPt of Kaon candidate > 1.5GeV, pion pt > 1.0GeV (combined to D00)) Pt of batchelor pion > 0.9GeV Pt of batchelor pion > 0.9GeV

Atlfast - unsmeared Atlfast - smeared

Mass (K-π+πB+) - Mass(K-π+)

Mass difference Mass difference plots:plots:

Mass (K-π+πB+) - Mass(K-π+)

Current Sample: Current Sample: W→W→ee--νν @ NLO @ NLO W→W→ee--νν from MC@NLO, full simfrom MC@NLO, full sim 47800 events currently 47800 events currently

available available (reconstructed in 11.0.42, using CBNTs, DC3 (reconstructed in 11.0.42, using CBNTs, DC3 generation, 005250)generation, 005250)

Generated cross section is 8.4nb (cf. 30nb for Generated cross section is 8.4nb (cf. 30nb for WW→→llνν: TDR). : TDR). Luminosity (for 50k events) is 9.5pbLuminosity (for 50k events) is 9.5pb-1-1..

Cuts applied at any stage are listed on top right of slide.Cuts applied at any stage are listed on top right of slide. All truth plots are normalised to weighting of simulated data.All truth plots are normalised to weighting of simulated data.

Electron Selection CutsElectron Selection Cuts At least one electron with transverse energy > At least one electron with transverse energy >

25GeV.25GeV. Electron candidate has at least one matched track.Electron candidate has at least one matched track. ||ηη| < 2.4. | < 2.4. IsEM flag = 0 (electron isolation cut). IsEM flag = 0 (electron isolation cut).

- before cuts- after cuts

Initially tracks associated with egamma candidates include kaons, pions, muons.

After cuts only electrons and positrons remain.

ELECTRONSELECTRONSElectron ETElectron ET

- truth- full

sim

Cuts: ET(ele) > 25GeV At least one

track matched IsEM=0 η(ele) < 2.4

ELECTRONSELECTRONSIdentifying electrons with their partner in Identifying electrons with their partner in truth containers, matching (truth containers, matching (ηη,,φφ) space) space::

22 )()( TTR

If (ΔR<0.1) match is found

Cuts: ET(ele) > 25GeV At least one

track matched IsEM=0 η(ele) < 2.4

ELECTRONSELECTRONSElectron ET resolution: (sim-truth)/truth for Electron ET resolution: (sim-truth)/truth for matched sim:truth electron pairs.matched sim:truth electron pairs.

Cuts: ET(ele) > 25GeV At least one track

matched IsEM=0 η(ele) < 2.4 ΔR<0.1

(matching sim to truth)

Resolution: ~1.8%

ELECTRONSELECTRONSET distribution of "wrong sign" electron ET distribution of "wrong sign" electron candidates:candidates:

- +ve charge electrons (from track q/p)

- all electron candidates (for shape comparison)

Cuts: ET(ele) > 25GeV At least one

track matched IsEM=0 η(ele) < 2.4

Only 4 of these reconstructed e+ have "Trk_truthpdg = 11", meaning only 4 are charge

misidentified

ELECTRONSELECTRONS

What are the wrong sign electrons?What are the wrong sign electrons? Not generated (except in eg pion decays, low Not generated (except in eg pion decays, low

momenta).momenta). Tracks carry "truth id" of a positron, implies Tracks carry "truth id" of a positron, implies

mostly not charge misidentified. (Do I understand mostly not charge misidentified. (Do I understand Trk_ParticlePdg variable?)Trk_ParticlePdg variable?)

Probably hard brem or photon conversions in Probably hard brem or photon conversions in which one electron has significantly higher pT which one electron has significantly higher pT than the other.than the other.

Only ever one electron/positron passing cuts per Only ever one electron/positron passing cuts per eventevent

Check with HepVis…Check with HepVis…

BeampipBeampipee

v-atlas view of an event v-atlas view of an event with e+…with e+…

ElectronNeutrinoPhoton

Pixel hitsSCT hitsTRT hits

Photon Photon conversion conversion

with one soft with one soft e?e?

This electron This electron produced with produced with

high high ηη: probably : probably lostlost

Nothing particularly Nothing particularly fishy going on (?) – fishy going on (?) –

count these as count these as statistical errors in statistical errors in

the same sign the same sign combinations.combinations.

MISSING ENERGYMISSING ENERGY

- truth: calculated from non-interacting particles

- full sim: corrected, inc. muons

Missing ET (cut on 25GeV to select Ws)Missing ET (cut on 25GeV to select Ws)

Probably not properly

calibrated?

Cuts: Ptele > 25GeV At least one

track matched IsEM=0 Electron η < 2.4 MET>25GeV

Cut on MET>25GeV

MISSING ENERGYMISSING ENERGYCuts:

Ptele > 25GeV At least one

track matched IsEM=0 Electron η < 2.4 Jet η < 2.4 ΔR (e-jet) > 0.7 MET>25GeV

Missing ET Resolution:Missing ET Resolution:

Asymmetry reflecting right-

shift of data w.r.t. truth

Resolution: ~17%

MISSING ENERGYMISSING ENERGYCuts:

Ptele > 25GeV At least one

track matched IsEM=0 Electron η <

2.4 MET>25GeV

Missing ET Resolution:Missing ET Resolution: Expect this to vary with the ET of the Expect this to vary with the ET of the

rest of the event, barring the electron.rest of the event, barring the electron.

Total ET - electron jet ET

(recoil)

Missing ET Resolution as a function of Missing ET Resolution as a function of ΣΣEET T (recoil):(recoil):ΣΣEET T == total Etotal ETT-electron E-electron ETT

Error in MET resolution ≈ A(event recoil)B

Expect MET resolution to increase with event Expect MET resolution to increase with event recoil: power law. recoil: power law.

σMET(ΣET) ≈ 0.10 (ΣET)0.27

20GeV< ΣET <30GeV

90GeV< ΣET <100GeV

0

0.05

0.1

0.15

0.2

0.25

5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155

recoil ET (GeV)

ME

T r

eso

luti

on

(G

eV)

MISSING ENERGYMISSING ENERGY

- truth- full sim

Missing ET parallel to leptonMissing ET parallel to lepton

Cuts: Ptele > 25GeV At least one

track matched IsEM=0 Electron η < 2.4 Jet η < 2.4 ΔR (e-jet) > 0.7 MET>25GeV

Missing ET perpendicular to leptonMissing ET perpendicular to leptonMISSING ENERGYMISSING ENERGY

Cuts: Ptele > 25GeV At least one

track matched IsEM=0 Electron η < 2.4 Jet η < 2.4 ΔR (e-jet) > 0.7 MET>25GeV

W RECONSTRUCTIONW RECONSTRUCTION

- truth- full

sim

Cuts: Ptele > 25GeV At least one

track matched IsEM=0 Electron η < 2.4 Jet η < 2.4 ΔR (e-jet) > 0.7 MET>25GeV

W transverse mass reconstructed from W transverse mass reconstructed from Missing ET and highest ET electron:Missing ET and highest ET electron:

)cos1(2 Tmiss

Te

TW ppm TDR:

ATLFAST

Rapidity Distributions of Ws Rapidity Distributions of Ws

There should be a charge asymmetry There should be a charge asymmetry between Wbetween W++, W, W- - rapidity distributions.rapidity distributions.

Ws produced with heavy quarks should be Ws produced with heavy quarks should be produced preferentially at central rapidities.produced preferentially at central rapidities.

Leptons from electron decays continue to Leptons from electron decays continue to display the charge asymmetry.display the charge asymmetry.

The WThe W++ sample half as large (errors): only sample half as large (errors): only 25100 events (same type of sample, run # 25100 events (same type of sample, run # 005254)005254)

Rapidity Distributions of Ws: Rapidity Distributions of Ws: generator levelgenerator level

- all- heavy quarks - light quarks

(~2.5 times more light than heavy)

Hmm!

Pseudorapidity Distributions of Pseudorapidity Distributions of electrons: reconstructedelectrons: reconstructed

- all- heavy quarks - light quarks

Conclusions:Conclusions: There is enough CSC data to start looking in There is enough CSC data to start looking in

detail at W detail at W →→eeνν events.events. Missing energy doesn’t seem best corrected to Missing energy doesn’t seem best corrected to

match truth, but behaves as expected.match truth, but behaves as expected. High-energy positrons probably consistent with High-energy positrons probably consistent with

hard brem or conversions.hard brem or conversions. Ws produced from heavy quark slightly more Ws produced from heavy quark slightly more

central than light as expected. Not clear whether central than light as expected. Not clear whether this effect can be seen in the decay products.this effect can be seen in the decay products.

WW- - distribution requires further investigation.distribution requires further investigation. Much more data will be needed for strange Much more data will be needed for strange

asymmetry study in full simulation.asymmetry study in full simulation.