Precision Measurement of Leptonic Forward Backward Asymmetries Nafisa Tasneem, Michael Roney

1. Precision Measurement of Leptonic Forward Backward Asymmetries Nafisa Tasneem, Michael Roney University of Victoria Canada May 15, 2012 BaBar Collaboration Meeting, Ferrara, Italy.

2. Overview of the Talk • Historical Overview • e+e- + -and Forward-Backward Asymmetry (AFB) • AFB below Z Resonance • Theoretical Calculation using Kk2f MC Generator • Contribution from All processes • Selection Procedure • Analysis Procedure • Result from Run 1 & 6 • Futures Steps

3. Historical Overview • First observation of weak neutral current in 1973. • First AFB measurement reported in at . • Next AFB measurement reported at in 1982. • Another contemporaneous AFB measurement reported at . Mark II detector in the PEP storage ring at s= 29 GeV Magnetic detector in the SPEAR storage ring at s= 5.8 -7.4 GeV • Then a number of papers reported AFB measurement at Z resonance (1989-1995) • after the first direct observation of W and Z particles in 1983. • Our aim is to measure the forward backward asymmetry in muon pair • using BaBar data at .

4. Previous Experiments Measured AFB

5. e+e- Annihilation Z exchange dominant at Z resonance Photon Exchange dominant well below Z BaBar High Energies: WW production

6. Forward Backward Asymmetry (AFB) Where is the forward events w.r.to some chosen direction. and is the backward events. Also is the cross-section for the forward events. and is the cross-section for the backward events. A non-zero asymmetry because the couplings of the Z boson to LH and RH particles are different. AFB  value inconsistent with the Standard Model would be a signature of new physics.

7. Forward Backward Asymmetry (AFB) Differential Cross Section: At Z Resonance: Forward and Backward Cross Sections in terms of angle: Which gives, At Z, this can be written as, with

8. AFB below Z resonance Dominant contributions below Z resonance  exchange -Z interference Where, After appropriate calculation and integration with limits Analytical expression for Forward Backward Asymmetry at GeV = -0.00379 (for -) without any radiative correction.

9. Radiative Corrections • In practice, ISR/FSR radiation distort the measurement. • ISR reduces the centre of mass energy of e+e- collision. • Corrected cross-section become : • Vertex correction: Where, and and • Propagator correction: Where, : -0.0054 • The corrected calculation for AFB at GeV Ref: Precision electroweak measurements on the Z resonance; Physics Reports, Volume 427, Issue 5-6, p. 257-454.

10. Theoretical Calculation of AFB using Kk2f Generator • Real data deals with higher order terms & need radiation corrections. • Keeping track of the large number of details involved in predicting the effects of radiative corrections is not easy. • Kk2f is a MC event generator running in standalone mode. • It includes up to third order QED processes. • Kk2f shows better agreement with data than KORALB. • We’ll used Kk2f to estimate the theoretical prediction for • All (QED+interference+Weak) Processes • Only QED (with ISR*FSR on + Higher Orders) Process • Weak Process • AFB calculation has done by event counting. • Result is produced from 10M Kk2f MC sample.

11. Polar Angle Distribution from the contribution of all processes using Kk2F Generator Forward Events Counting Backward Events Counting

12. Data and MC Samples Integrated Luminosity (fb-1) • BaBar Data Sample R(1-6): • L=424.78 for On Resonance. • L=45 for Off Resonance. • Monte Carlo Sample R(1-6): for On Resonance , , , & • Generic : 441.64 • Generic : 16.49 • Generic :1761.00 • Generic :1642.98 • Generic : 918.32 • R24 skimmed datasets were used under analysis 52. • Current study with Run 1 & 6 • Blind analysis later with Run 2, 3, 4 and 5

13. Selection Criteria ( ) • Event Topology • We require 1 charged track in one hemisphere. • We require the other charged track in opposite hemisphere. • Require events to pass Tau11 skim to keep -pairs classified as 1-1 prong events. • While Tau11 skim classified the -pair events as 1-N(3) prong events. • GoodTracksVeryLoose (GTVL) was used as a muon selector. • Require Level3 trigger slice at DCH and EMC for getting the events of -pairs. • (L3OutDch||L3OutEMC) • Require BGFMuMu background filter for -pairs • We require 2 tracks with and . • || • Either one of the track has . • Number of neutrals in each hemisphere (with energy >50 MeV) is required to be <6. • Total event mass < 3 GeV/c2 (combined mass of all particles including the neutral • and charged particles). • cos of the two most energetic tracks are required to lie within the ECAL • acceptance (-0.76, +0.96) • No PID selection was used.

14. Comparison between Data-MC Polar Angle Distribution, Run1 (28%) Polar Angle Distribution, Run 6 (53%)

15. Analysis Procedure Fitting Function Forward and Backward Events from the Fitting Polynomials AFB from the Fitting Polynomials

16. Analysis Procedure Run1 2nd Order Polynomial Fit over the Polar Angle Distribution, (Purple: Data, Green: MC) Values of the Fitting Polynomials

17. Analysis Procedure Run6 2nd Order Polynomial Fit over the Polar Angle Distribution, (Purple: Data, Green: MC) Values of the Fitting Polynomials

18. Preliminary Result of AFB at BaBar

19. Background and Systematic Uncertainties Backgrounds Systematic Uncertainties • Discrepancy in the calculated values of AFB for + - • Boost Correction • cos Measurement • Backgrounds • Radiation Corrections • Signal Efficiency etc • Bhabas • pairs • Fermion & anti-fermion pairs as • , etc • Two Photons • Cosmic muon etc

20. Future Plan • Data and MC production for the rest of the Run (2, 3, 4 & 5) and background events. • Revision in selection criteria & estimate the cosmic muon background contamination. • Background events separation • Radiative Corrections • Precise determination of systematic uncertainties • Using Kk2f we will also estimate • The contribution from only QED process • The contribution from only WEAK process • The contribution from the interferences between QED & WEAK processes • The contribution of ISR and FSR radiation in AFB. • Calculate the combined AFB for an integrated luminosity of 424.78 fb-1 • after background events separation and minimize the systematic errors. Thanks

21. Backup slide: 1 (Analytical Expression for AFB)