1. 1 Searching for Z’ and model discrimination in ATLAS ● Motivations ● Current limits and discovery potential ● Discriminating variables in channel Z’  e + e - - Natural width and cross section - Forward – backward asymmetry B. Trocmé (LSPC Grenoble)

2. 2 Motivations for a Z’ ● Additional gauge bosons emerge in many extended gauge models: - E6 models : E6  SU(3) C x SU(2) L x U(1) Y x U(1)  x U(1)  Lightest Z’ : Z’ = cos(   ) Z’  – sin(   )Z’  - LR model : SO(10)  SU(3) C x SU(2) L x SU(2) R x U(1) Relative coupling strengths given by a parameter  = g R /g L ● But also in extra dimensions models : - Kaluza Klein excitations. ● A sequential standard model is also often considered : - SM + 1 additional massive boson with the same couplings constants.

3. 3 hep-ph/9504216 Current limits and discovery potential ● Better limits now come from the direct searches at Tevatron : - E6 : ~ 600-700 GeV - depends on    (LEP indirect : 400-600GeV). - LR : ~ 600 GeV Moriond EW 2005

4. 4 Discovery potential in Atlas ● Possibility to discover a Z’ up to 4-5 TeV in Atlas in several channels: ee, , jets (  ). 100 fb -1 ● Assuming that a Z’ is discovered, how could one determine its nature? ● In all the following, focus on the golden channel: Z’  e + e - LArg calorimeter :  (M ee ) ~ 8GeV (M ee = 1.5TeV) (M rec -M gen )/M gen

5. 5 Disentangling Z’ models ● Relevant observables sensitive to Z’ couplings : - Natural width & cross section - Forward backward asymmetry ● Considered models : ● Atlas full simulation (Geant 3) + official reconstruction one small extra dimension compactified on S1/Z2. EPJ C39 (2005) 1-11  = 1 E6 models

6. 6 The width & leptonic cross sections ● Partial decay widths (light fermions at tree level): ● Width / branching ratios variations in E6 models, assuming no exotic decays. ● Resonance shape for several models (arbitrary normalization) ● Caveat : total width altered if Z’ decays in exotic particles, e.g gauginos  Consider instead the product :  ee x 

7. 7 The width extraction in Atlas ● Analytical fit of di-electron mass (convoluted with resolution function):  model M Z’ = 1.5TeV  th. = 9.5 GeV ● Summary of measurements and discrimination power

8. 8 Forward backward asymmetry :the potential ● Typical spin 1 particle behaviour : Wide mass bins (off peak analysis) ● Asymmetry at generation level for several models with M Z’ = 1.5 TeV and 100fb -1 Narrow mass bins (on peak analysis)

9. 9 Angles definition ● Main challenge at LHC (pp collider): - Determination of the quark direction. ● Z’ mainly originates from the annihilation of a valence quark with a sea antiquark.  cos  * approximated by cos  , the angle between the outgoing electron and the reconstructed Z’. - If P quark > P antiquark : unbiased estimator only degraded by E/position resolution, ISR. - Otherwise : maximally biased estimator (cos   = - cos  *).

10. 10 Angles definition (2) ● Study at Monte Carlo level to extract the probability to be in the “maximal bias” configuration :  (Y). - Differences between models explained by the different u/d couplings in the initial state (source of systematic error). ● The impact of the imperfect knowledge of the quark direction: - : roughly computed with cos   - - Artificial reduction of the observed asymmetry. Known as the dilution effect. ● A new corrected asymmetry is defined

11. 11 The A FB extraction in Atlas ● A FB is deduced with a differential method : - Compute A FB (cos  ) by the basic counting method (N + - N - /N + +N - ) in several bins of cos  (  * : generation /   : observed-corrected). - Extract A FB by fitting ● Example :  model at M Z' = 1.5TeV (1.48TeV<M ll <1.52TeV)

12. 12 ● The results for all models in the central mass bins ● Possible to precisely measure the forward backward asymmetry in Atlas: - dilution correction works well, even in the case of an off peak analysis (wider mass bins and reduced statistics – not shown here). - Systematic error associated to the  parameterization estimated around 10%. - very promising discriminating potential (especially when including analysis of all mass bins – cf slide 8). The A FB extraction in Atlas (2)

13. 13 Conclusion ● An additional neutral gauge boson is predicted in many models. Discovery potential of Atlas very promising up to 4-5TeV. ● A method to extract the natural width and the forward backward asymmetry was presented in Z’  e + e - channel. - With realistic statistics, the underlying model can be largely constrained. ● The discrimination potential will be improved when including : - more channels decay : ,  - more variables : rapidity is especially sensitive to Z’ couplings to initial quarks.

14. 14 Back Up Slides

15. 15 Moriond 2005

16. 16 The on peak analysis ● The summary of the whole analysis of 5 models in the 5 mass bins (therefore 25 independent analyses). Double peak structure (cf |A FB gen | > |A FB obs |) Dilution effect amplitude : 1.4  on average The correction is an efficient and unbiased way to correct from the dilution

17. 17 The off peak analysis ● Preliminary feasibility study : - Only at M Z' = 1.5TeV. A single mass bin :800Gev-1400GeV. ● Correction procedure still efficient but less powerful than for the on peak analysis: - Due to the incorrect  estimate (large mass bin and too few statistic) - Will be improved by an increased number of MC events to extract .