A feasibility study on measuring a strange sea asymmetry in the proton in atlas
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A Feasibility Study on Measuring a Strange Sea Asymmetry in the Proton in ATLAS. Laura Gilbert – Graduate Symposium 15th March 2006. Quark Asymmetries in the Proton. u, d distributions in the proton predicted to be almost flavour symmetric within pQCD.

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A feasibility study on measuring a strange sea asymmetry in the proton in atlas l.jpg

A Feasibility Study on Measuring a Strange Sea Asymmetry in the Proton in ATLAS

Laura Gilbert – Graduate Symposium 15th March 2006


Quark asymmetries in the proton l.jpg
Quark Asymmetries in the Proton the Proton in ATLAS

  • u, d distributions in the proton predicted to be almost flavour symmetric within pQCD.

  • MNC measured the flavour nonsinglet structure function [Fp2(x,Q2) − Fn2(x,Q2)]. → large (~30%) violation of Gottfried sum rule:

  • Confirmed by the NA51, E866

    (NuSea), and HERMES.

  • Various theoretical models proposed.

d/u


Theoretical models to explain an asymmetric sea l.jpg
Theoretical models to explain an asymmetric sea the Proton in ATLAS

  • Meson Cloud model seems most successful in explaining observations: Proton oscillates into virtual mesons/baryons; valence anti-quarks in these contribute to sea in asymmetric way.

  • Bag model

  • Effective Chiral Quark Model

  • Pauli Exclusion considerations (“Pauli Blocking”)

  • Isospin breaking


Possible strange sea asymmetries l.jpg
Possible Strange Sea Asymmetries the Proton in ATLAS

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

  • Number of possible strange sea asymmetry measurements:

    • Mass: CCFR data might indicate due to asymmetric valence quark distribution in proton (Li, Zhang, Ma) ( is medium-induced mass)

    • Number asymmetry

    • Momentum fraction asymmetry


Strange sea momentum asymmetry l.jpg

s(x) the Proton in ATLAS

s(x)

s(x) - s(x)

Strange Sea Momentum Asymmetry

  • NuTeV anomaly: NuTeV experiment measured sin2θW of 3σ above accepted value.

  • Eg. Signal&Cao showed that incorporating a strange sea asymmetry into the meson cloud model can reduce this to 2σ.

  • Standard model explanation or new physics?

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

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


Detecting a strange sea asymmet ry l.jpg

e the Proton in ATLAS-

s

ν

c

g

d

π+

D*+

d

D0

Kπ(ππ)(π0)

jet

Detecting a Strange Sea Asymmet ry

Signal:

  • Tag with:

  • -electron

  • -missing Et

  • -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 asymmet ry7 l.jpg
Detecting a Strange Sea Asymmet ry the Proton in ATLAS

Signal: notes on W production

- At the LHC σ(W+ prod) > σ(W- prod)

-Cross section for pp→WX, W→(e/μ)ν is about 30nb

-Contribution from charm/strange initial states ~10%, mainly in central region

- Forward production is mostly due to up/down states

- The product x1x2 of parton momenta ~3x10-5at LO


Detecting a strange sea asymmet ry8 l.jpg

s(x) the Proton in ATLAS

s(x)

s(x) - s(x)

Detecting a Strange Sea Asymmet ry

Note: With Ws at LHC we are sensitive to small x regime (<0.01) – area in which we would be likely to see an excess of anti-strange if at all. Difficult to probe.


Analysis technique l.jpg
Analysis Technique the Proton in ATLAS

  • Reconstruct D0→K-π+(also D0→K-π +π0, D0→K-π +π-π +π0 etc)

  • Add soft (prompt) pion to reconstruct D*+.

  • Signal has opposite sign combinations of W, πB.

  • Same sign are backgrounds

  • Find signal, remove background, calculate systematics.

  • Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out CL on limits of null hypothesis


Mass difference plots l.jpg
Mass Difference Plots the Proton in ATLAS

Atlfast - unsmeared

Atlfast - smeared

Mass (KππB) - Mass(Kπ)

Mass (KππB) - Mass(Kπ)

  • sample of W→eνe at NLO.240k events before trigger cuts, ~210k after.

  • One electron track with Pt>25GeV, missing Et>25GeV, η<2.4 (W tagging)

  • Pt of Kaon candidate > 1.5GeV, pion pt > 1.0GeV (combined to D0)

  • Pt of batchelor pion > 0.9GeV

  • Remove background with track cuts (d0, z0, lxy)

  • Plotted mass difference: reconstructed D* - Kπ. Peak around 146MeV.


Qcd backgrounds l.jpg

Irreducible the Proton in ATLAS

background

if this is d or b

s

e-

c

ν

π+

g

D*+

D0

Kπ(ππ)(π0)

c

jet

QCD Backgrounds

1) cc sample:

signal + irreducible background + reducible

background:σ~7.8mb

Signal from cc and Irreducible Background:


Qcd backgrounds12 l.jpg

e the Proton in ATLAS-

ν

jet

c

s

π+

g

D*+

D0

Kπ(ππ)(π0)

c

jet

QCD Backgrounds

1) cc sample:

signal + irreducible background + reducible

background:σ~7.8mb

Reducible Background:

Other time ordering of signal above, now gives strange jet (difficult

to cut). The c will be virtual:

-if W virtual, eν pair is soft, removed by W selection cuts (high energy tail → systematic uncertainty)

-if the W is real c is far off shell so suppressed →systematic uncertainty


Qcd backgrounds13 l.jpg

e the Proton in ATLAS-

ν

D

K

c

jet

g

π+

D*+

D0

Kπ(ππ)(π0)

c

jet

QCD Backgrounds

1) cc sample:

signal + irreducible background + reducible

background σ~7.8mb

Reducible Background:

Remove with electron exclusion cuts? Cut electrons with close

tracks (from D) in the calorimeter. Hard jet vetos.


Qcd backgrounds14 l.jpg

l the Proton in ATLAS+

ν

b

jet

g

π+

b

D*+

D0

Kπ(ππ)(π0)

Background D*s

jet

Prompt Signal D*s

d0 of bachelor pion

QCD Backgrounds

2) bb sample:

reducible background: σ~0.5mb

c

c

e-

ν

Two charged leptons, one lost. Electron can

now be same or opposite sign as πB in equal

quantities. D* no longer prompt. Hard jet veto.


Qcd backgrounds15 l.jpg

l the Proton in ATLAS-

ν

t

g

t

QCD Backgrounds

2) tt sample:

reducible background: σ~0.8nb

...then as bb decay above. Two

extra leptons, four in total, three

must be lost. Equal numbers of

same and opposite sign

combinations again. Similar cuts

to B sample should remove this

background also.

b

b

l+

ν

(t→bW branching ratio ~100%)


Other sources of background l.jpg
Other sources of background the Proton in ATLAS

  • Electron charge identification (trigger studies)

  • Soft pion efficiency

  • Falsely-reconstructed D*s.

  • Pile-up

  • Z+1jet, missing lepton

  • W+extra jets: incl. W + cc (bb), one heavy quark lost: qq→Wg*→WQQ


Future work l.jpg
Future Work the Proton in ATLAS

  • Increase statistics (DC3)

  • Optimise cuts for maximum signal/background

  • Full simulation needed (look at alignment, multiple scattering for very low pt tracks to get soft pion efficiency, systematic uncertainties)

  • Study backgrounds and pile-up at low/high luminosity


Conclusion l.jpg
Conclusion the Proton in ATLAS

This will be a very challenging, but physically

extremely interesting and relevant study.

Thanks to Jeff Tseng and Mandy Cooper-Sarkar