1 a feasibility study for a strange sea asymmetry analysis at atlas: update laura gilbert and jeff...

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

Analysis at ATLAS: updateAnalysis at ATLAS: update

Laura Gilbert and Jeff Tseng 10/10/07

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OUTLINE

1)1) Reminder: Detecting a strange Reminder: Detecting a strange sea asymmetrysea asymmetry

2)2) Reminder: Analysis technique: Reminder: Analysis technique: W+D* Selection W+D* Selection

3)3) Electroweak Backgrounds: Electroweak Backgrounds: resultsresults

4)4) Discussion of QCD backgroundsDiscussion of QCD backgrounds

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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

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 (<~3 at 95%

CL)

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

W + cc (bb), Z + cc (bb): in current samples (gluon splitting), mainly removed by ET cuts. <8 events/fb<8 events/fb--

11 (95% CL). Need a larger NLO sample to (95% CL). Need a larger NLO sample to study further: cut on angle between D* and study further: cut on angle between D* and W in transverse plane?W in transverse plane?

qqbar: bb: MC@NLO ~3mb tt: MC@NLO ~0.8nb? cc: Not available at NLO. Pythia ~5mb.

Should consider pileup and missing jets Should consider pileup and missing jets

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Bbbar backgroundBbbar background MC@NLO for reasonably correct topology. Generated 1x107 events, none pass selection cuts

(very few pass epT, MET, isolation cuts: <1/10000). This implies <~3 events events pass at 95% CL.

However the cross section still large → in order to drop the bbbar background below 10 events/ fb-1 I need 1011 events, not feasible.

Currently regenerating sample with appropriately higher pT cut on outgoing quarks.

Probably most significant background. ccbar has higher cross section, but c semileptonic decays tend to produce significantly less energetic electrons and neutrinos, so they are more likely to fail W selection cuts, and the electron is more likely to be within the c jet.

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Reduce qqbar backgrounds with tighter electron isolation cuts? (using ATLFAST defaults)

In signal: large angle between reconstructed D* and W in transverse plane. Remove gluon splitting?

Further Background Rejection?Further Background Rejection?

c

c D*-

s jet

W+

b

b

D*+

cW-

b jet

t

t

bW+

bW-qq Background

D*sPrompt Signal

D*s

d0 of bachelor pion

Sign correlations: ccbar: D*, e from different quarks bbbar: D*, e from same quarks ttbar: several options for

combinations but unlikely to pass MET cuts.

Some backgrounds will remain:

cc

bb

tt

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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

Back-of-envelope: to exclude null hypothesis to Back-of-envelope: to exclude null hypothesis to 95% CL at 1fb95% 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.

1 a feasibility study for a strange sea asymmetry analysis at atlas: update laura gilbert and jeff...

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mev real d

s gc w

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