Event Shapes in NC DIS at ZEUS

1
Event Shapes in NC DIS at ZEUS

• Alexander A. Savin
• University of Wisconsin
• On Behalf of the ZEUS Collaboration
• DIS 2006, Tsukuba, Japan

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HERA Kinematic Variables

• Breit Frame Definition
• q 2xBP 0

Q2, xBj, y
27.5 GeV
920 GeV
ZEUS 98-00 (82.2 pb-1) 80 lt Q2 lt 20 840 GeV2
0.0024 lt x lt 0.6 DESY-06-042, hep-ex/0604032
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Power Corrections

• Use power corrections to correct for
  non-perturbative effects in infrared and
  collinear safe event shape variable.

Used to determine the hadronization corrections
Universal non-perturbative constant
Similar for the means and differential
distributions
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Analysis Method

• Test validity of the power correction method by
  extracting
• from a fit to the data.
• For means NLO PC as a function of Q
• (DISASTER/DISENT)
• For differentials NLO NLL PC in bins of
  (x,Q2)
• (DISASTER/DISPATCH/DISRESUM)
• Hessian method was used

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Event-shape variables
Sums are over all momenta in the current
hemisphere of the Breit frame
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Mean Event Shapes

• Comparison with the ARIADNE
• Parton level should be taken as indicative only

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Mean Event Shapes

• Results of the NLOPC fit
• Simultaneous fit for ?s and ?0
• Each shape fit separately
• NLO calculation using DISASTER

Negative power correction
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Mean Event Shapes

• Extracted parameters
• Consistent with the previous measurement
• Extracted ?0 are consistent (except Tg ) with
  each other, slightly lower than H1
• Extracted ?s values are consistent with each
  other, except BT and Tg
• High sensitivity of T? to PDF
• Theoretical errors dominate.

World Average
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Shape Distributions

• ARIADNE describes data well

Only high Q region is used
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Shape Distributions

• The differential distributions are fitted over a
  limited range.
• Bins for which theoretical calculations are not
  expected to be valid are omitted from fit.

NLONLLPC
NLONLL
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Shape Distributions

• The differential distributions are fitted over a
  limited range.
• Bins for which theoretical calculations are not
  expected to be valid are omitted from fit.

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Extracted Parameters

• Extracted parameters
• Extracted ?s values are consistent with each
  other and H1

• Major theoretical uncertainties
• Fit range
• Theoretical parameters renormalization scale,
  logarithmic rescale factor, power factor in
  modification term etc

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Test of Q-independency
Consistent with being independent of the Q range
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Event Shapes With Jets

• y2 resolution cut parameter (21) to (11)
• KOUT out-of-plane momentum

2 jets
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y2 parameter

• ARIADNE describes data well at high Q
• NLO describes data without hadronization
  correction

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Out-of-plane momentum

ARIADNE describes data well
indicating significant hadronization corrections
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Summary

• Mean values and differential distributions of
  event-shape variables have been measured at ZEUS
  in the kinematic region
• 80 lt Q2 lt 20 840 GeV2 and 0.0024 lt x lt 0.6
• The data are well described by the ARIADNE MC
• Power-correction method provides a reasonable
  description of the data for all event-shape
  variables with ?0 0.4-0.5 for most of the
  variables.
• The lack of consistency in the (aS,a0) extraction
  suggests the importance of high-order processes
  that are not yet included in the model
• Need some theoretical input to proceed with the
  jets event shapes