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Event Shapes in NC DIS at ZEUS

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On Behalf of the ZEUS Collaboration. DIS 2006, Tsukuba, Japan ... Hessian method was used. Analysis Method. Event Shapes, Alexander Savin University of Wisconsin ... – PowerPoint PPT presentation

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Title: 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

2
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
3
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
4
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

5
Event-shape variables
Sums are over all momenta in the current
hemisphere of the Breit frame
6
Mean Event Shapes
  • Comparison with the ARIADNE
  • Parton level should be taken as indicative only

7
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
8
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
9
Shape Distributions
  • ARIADNE describes data well

Only high Q region is used
10
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
11
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.

12
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

13
Test of Q-independency
Consistent with being independent of the Q range
14
Event Shapes With Jets
  • y2 resolution cut parameter (21) to (11)
  • KOUT out-of-plane momentum

2 jets
15
y2 parameter
  • ARIADNE describes data well at high Q
  • NLO describes data without hadronization
    correction


16
Out-of-plane momentum

ARIADNE describes data well
indicating significant hadronization corrections
17
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
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