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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
2
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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W+D* SelectionW+D* Selection
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
Real D*s Full sample
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W+D* SelectionW+D* Selection
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
Real D*s Full sample
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W+D* SelectionW+D* Selection
Real D*s Full sample
Optimised Cuts:Optimised Cuts: m(D0reco)- m(D0true)< m(D0reco)- m(D0true)<
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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W+D* SelectionW+D* Selection
Real D*s Full sample
Optimised Cuts:Optimised Cuts: m(D0reco)- m(D0true)< m(D0reco)- m(D0true)<
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
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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W+D* SelectionW+D* Selection
Optimised Cuts:Optimised Cuts: m(D0reco)- m(D0true)< m(D0reco)- m(D0true)<
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
D0 impact parameter D0 impact parameter <0.2mm<0.2mm
D* pT>6GeV, |D* pT>6GeV, |ηη|<2.5|<2.5
Real D*s Full sample
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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.