1 A Feasibility Study for a Strange Sea Asymmetry Analysis at ATLAS Laura Gilbert and Jeff Tseng 24/09/07

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A Feasibility Study for a A Feasibility Study for a Strange Sea Asymmetry Strange Sea Asymmetry

Analysis at ATLASAnalysis at ATLAS

Laura Gilbert and Jeff Tseng 24/09/07

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OUTLINE

1)1) Background and motivation: quark Background and motivation: quark asymmetries in the protonasymmetries in the proton

2)2) Detecting a strange sea asymmetryDetecting a strange sea asymmetry

3)3) Analysis technique: W+D* Analysis technique: W+D* Selection Selection

4)4) Electroweak Backgrounds: resultsElectroweak Backgrounds: results

5)5) Discussion of other backgroundsDiscussion of other backgrounds

6)6) Notes on missing pT Notes on missing pT

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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 Asymmetries in the

Proton 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 (MCM) seems physically intuitive as a way to explain observations.

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Motivation: Quark Asymmetries in the Proton

In the MCM the proton oscillates into virtual mesons/baryons

Sea q/q are in different environments thus carry different momenta.

Symmetric s/s distribution often assumed, but not established theoretically or experimentally.

MCM would imply a strange momentum fraction asymmetry too.

0))()((1

0

dxxsxsx

du

u qq

du

uoscillate

sq

du

u q

x(s(x) - s(x))

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

probe.

Phys.Lett. B590 (2004) 216-222: Ding & Ma

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

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Detecting a strange sea asymmetry in the proton

Feynman diagram sensitive to strange quark distribution needed. Use s+g→c+W, ie. NLO W production.

This mechanism is charge symmetric if the strange/anti-strange distributions are the same.

General W production at LHC already shows charge asymmetry in rapidity distributions of W.

Need to remove this bias and then look for limits on null hypothesis of signal channel.

s

c

W

g

s

g

W

c

cg

Ws

NLO Gluon production:10% of total

s

c

W

NLO W production

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D*D* + W Search: Technique

Select W candidate Reconstruct D0→K-π+ D0 vertex displaced. Add prompt (soft) pion. Consider 3 sign correlations: Consider 3 sign correlations:

(K(K-- with with ππ++, K, K-- with with ππBB++, , ππBB

+ + with ewith e--)) Plot reconstructed D*-D0 mass Plot reconstructed D*-D0 mass

difference = 145.4MeVdifference = 145.4MeV(small intrinsic (small intrinsic resolutions: D* width 96keV, D0 width resolutions: D* width 96keV, D0 width 1.6meV , small background)1.6meV , small background)

Consider backgrounds inc. Cabibbo suppressed wrong sign combinations

s

g

W

c

cg

Ws

Branching ratios: D*+→D0π+ 67.7%

D0 → K- π+ 3.8%c→D* 25.5%c→e 9.6%

cWgscWsg

cWgscWsg

NN

NNA

Asymmetry: Plot as a function of

rapidity. Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out confidence limits on null hypothesis

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W+D* SelectionW+D* Selection Sample of 3 million of each WSample of 3 million of each W++,W,W--→e→eνν

generated with MC@NLO, passed through generated with MC@NLO, passed through HERWIG and ATLFAST (software release HERWIG and ATLFAST (software release 12.0.6)12.0.6)

Preliminary Cuts:Preliminary Cuts: 1 electron with pT>25GeV, |1 electron with pT>25GeV, |ηη|<2.4|<2.4 MET>25GeVMET>25GeV Two oppositely signed tracks: assign one K, one Two oppositely signed tracks: assign one K, one ππ. . pT(K)>1.5GeV, pT(pT(K)>1.5GeV, pT(ππ)>1GeV)>1GeV Third track: assign bachelor Third track: assign bachelor ππBB, pT(, pT(ππBB)>0.5GeV)>0.5GeV ππB B charge opposite to e, opposite to Kcharge opposite to e, opposite to K

Further cuts indicated by sFurther cuts indicated by s22/(s+b) optimisation /(s+b) optimisation – compare efficiency of selecting “true” signal – compare efficiency of selecting “true” signal D*s with backgrounds of the same sign D*s with backgrounds of the same sign correlations.correlations.

W selection

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W+D* SelectionW+D* Selection

Optimised Cuts:Optimised Cuts: m(D0reco)- m(D0true)< 40MeVm(D0reco)- m(D0true)< 40MeV

Real D*s Full sample

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Optimised Cuts:Optimised Cuts: m(D0reco)- m(D0true)< 40MeVm(D0reco)- m(D0true)< 40MeV Signed Lxy > 0.35mmSigned Lxy > 0.35mm

D0

D0 cτ=123μm K

πLxy

(Lxy –ve is tracks point towards vertex)

Reconstruct vertex: straight line approx


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Optimised Cuts:Optimised Cuts: m(D0reco)- m(D0true)< m(D0reco)- m(D0true)<

40MeV40MeV Signed Lxy > 0.35mmSigned Lxy > 0.35mm D0 impact parameter D0 impact parameter

significance d0/significance d0/σσ(d0)<3(d0)<3D* lifetime < 10-20s

Therefore batchelor π should be prompt: sanity cut at 3 σ


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40MeV40MeV Signed Lxy > 0.35mmSigned Lxy > 0.35mm ππBB impact parameter impact parameter

significance d0/significance d0/σσ(d0)<3(d0)<3 d0(K)*d0(d0(K)*d0(ππ)<0mm)<0mm22

Impact parameter is signed according to which side of the vertex it passes.

Therefore K, π have oppositely signed impact parameters.

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D0 impact parameter D0 impact parameter <0.2mm<0.2mm

D* lifetime < 10-20s, therefore D0 impact parameter should be small

Cut is not very effective, probably redundant with previous cut.

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D0 impact parameter D0 impact parameter <0.2mm<0.2mm

D* pT>6GeV, |D* pT>6GeV, |ηη|<2.5|<2.5


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Signal sample: ResultsSignal sample: Results

(NB. 90% of real passing D*s have pT > 8GeV. Relevant later…)

No. signal events =86±22No “real” D*s in window = 76No. W- events = 45 ±14No “real” D*s = 40

No. W+ events = 41 ±13No “real” D*s = 36

Reconstructed Unsmeared Real D*s

NB. Just two of the passing events come from gluon splitting:s

c

W

g cc

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W→eW→eνν estimation using Comphep: estimation using Comphep:q

g

W-

cq

νe

e-

Comphep: cross sections without cuts qg→W-c ≈ 10900pb, qg→W+c ≈

10250pb Which implies:

σ (qg→e-νe Kππ) ≈ 0.823pb

σ (qg→e+νe Kππ) ≈ 0.773pb

Comphep: Applying cuts pT(e)>25GeV |η(e)|<2.5 pT(c)>8GeV |y(c)|<2.5 pT(νe) >25GeV

Bσ(W-,cuts)=0.136pb Bσ(W+,cuts)=0.132pb (ie. 17% of signal events pass these cuts)

q No. W- signal events / fb-1

No. W+

signal events / fb-1

sum 136 132

d 13 9

s 123 123

b 0.1 0.1

Inherent 1.5% asymmetry

NB: around 30% of these numbers pass real selection

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QED BackgroundsQED Backgrounds W→W→τντν: Additional signal: Additional signal ZZ→ee→ee ZZ→→ττττ WWWW WZ WZ ZZZZ

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Signal: Signal: W→W→τντνs

g

W-

cs

W-

ντ

τ-

ντ

νe

e-

Comphep: cross sections without cuts qg→W-c ≈ 10900pb qg→τ-ντ c ≈ 1140pb

B(W→τ-ντ)=10.74%

Implies qg→ e-νeντ ντ c ≈ 200pb

B(τ- → e- νe ντ)=17.84% Mc@NLO with ATLFAST: 3 million of each W-,

W+. 0.9 W+ events and 2.0 W- events pass cuts, ie. ~3

total, <~8 at 95%CL.

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Background: Background: Z→eeZ→ee

MC@NLO with ATLFAST: (2 million events: Lepton Filter applied so one electron required pT(e)>10GeV, |η(e)|<2.7 ) Without MpT>25GeV cut 18 events pass per fb-1 (allow more

than one electron) With MpT>25GeV cut 0 events pass per fb-1. Would we lose more electrons in full simulation?

Comphep: Cuts: σ(cg→e-e+c) = 31.9pb

pT(e-)>25GeV, pT(e+)>25GeV |η(e-)|<2.5 AND/OR |η(e+)|<2.5 |y(c)|<2.5 pT(c)>8GeV

< 22 events/fb-1 (inc BRs)

c

gZ

c

c

e-

e+

Lost→MET

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Comphep: cross sections without cuts σ(cg→Zc) ≈ 2000pb σ(cg→τ-τ+ c) ≈ 60pb

B(Z→ τ-τ+ )=3.37%

Therefore σ(cg→ e+νeντ τ- c )≈ 11pb

B(τ- → e- νe ντ)=17.84%

Background: Background: Z→Z→ττττ

ZZ→→ττττ certainly negligible when certainly negligible when compared with compared with ZZ→ee results.→ee results.

c

g Z

c

c

τ+

τ-

W+

ντ

νe

e+

Lost→MET

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Backgrounds: Backgrounds: WW, WZ, ZZWW, WZ, ZZTotal

HERWIG xsect σ (pb)

Branching Ratio B

fractional cross

section σxB(pb)

No. events

/fb-1

WW 70 2(W→eν,W→cXc→Kππ)

=5.04x10-5

3.5x10-3 3.5

WZ 27 (W→eν, Z→cc) +(W→cX, Z→ee)

c→Kππ=1.68x10-5

4.5x10-4 0.45

ZZ 11 2(Z→ee, Z→cc, c→Kππ)

=5.56x10-6

6.1x10-5 0.061

W→eν=10.72%W→cX=33.6%Z→ee=3.36%Z→cc=11.81%c→Kππ=0.07%

These sum to <4 event /fb-1 (~5% of signal) with *no cuts* applied

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Signal and Electroweak Signal and Electroweak Backgrounds: SummaryBackgrounds: Summary

W→eW→eνν: Signal: 84: Signal: 84±22±22 events/fb events/fb-1-1

W→W→τντν: Signal: <8 events/fb: Signal: <8 events/fb-1-1 (95% CL) (95% CL) ZZ→ee: < 3 events/fb→ee: < 3 events/fb-1-1 pass cuts 95% CL pass cuts 95% CL ZZ→→ττττ: << 1 : << 1 event /fb-1 likely WW: WW: <1 event /fb-1 WZ: WZ: <<1 event /fb-1 ZZ: ZZ: <<1 event /fb-1

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QCD and other backgroundsQCD and other backgrounds QCD backgrounds:

D* + fake W: Sample 5802 dijet + fake electron (W, Z, t, γ). σ=191μb

bb: MC@NLO tt: MC@NLO cc: Pythia? Not available at NLO W + cc (bb), Z + cc (bb): in current samples, mainly

removed by ET cuts. <8 events/fb<8 events/fb-1-1 (95% CL) (95% CL) Should consider pileup and missing jets Should consider pileup and missing jets

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Notes on Missing pTNotes on Missing pT At LO the W is produced with momentum along the direction

of the beampipe Electron and neutrino from W decay produced back-to-back in

transverse plane Resolve MpT along the direction of travel of the electron:

perpendicular to line of flight of electron we expect MpT perp = 0 at generator level.

Including detector smearing this results in a sharp Gaussian. At NLO W is produced at any angle so electron and neutrino

tend to be approximately back to back, but angle is no longer 180 degrees at generator level

The NLO distribution will be much wider so this could be useful to select NLO diagrams.

Probable LO contribution

Probable NLO contribution

Plot from DC3 sample 005250 (MC@NLO), v 11.0.42

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Notes on Missing pTNotes on Missing pT

Can consider MET parallel as well as perpendicular to lepton line of flight.

Missing pT parallel to electron line of flight + electron pT = 0 at LO (gen level).

Parallel case is less well resolved in full simulation than perpendicular, also mean displaced from 0 since the electron calorimeter corrections are not perfectly tuned.

In signal we expect W with relatively low pT (e, missing energy ~back to back) which may not be true in QCD backgrounds so revisit later.

Probable LO contribution

Probable NLO contribution

Plots from DC3 sample 005250 (MC@NLO), v 11.0.42

Reconstructed GEANT truth

This cut is not useful for event selection in the signal sample

No improvement if calculated as the first cut, or if the MET >25GeV cut is entirely removed

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Final Thoughts Signal selection looking promising compared

to EW backgrounds QCD backgrounds likely to be more

significant but we have further rejection possibilities to work with (MET, stronger electron isolation criteria – currently using ATLFAST default)

Back-of-envelope: to exclude null hypothesis Back-of-envelope: to exclude null hypothesis to 95% CL at 1fbto 95% CL at 1fb-1 -1 (approx. 100 signal events (approx. 100 signal events passing) we need around 60% asymmetry passing) we need around 60% asymmetry (80:20).(80:20).

1fb-1 insufficient for convincing asymmetry calculations – probably need at least 100 fb-1.

Documents

1 A Feasibility Study for a Strange Sea Asymmetry Analysis at ATLAS Laura Gilbert and Jeff Tseng 24/09/07