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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)

, ,u d s

, , 'Z Z

, ,u d s

e

e

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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 = gR/gL

● 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.

3hep-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

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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 :

)eGeV750%(8.0

%7.0E%5.9

E)E(

(Mee) ~ 8GeV(Mee = 1.5TeV)

(Mrec-Mgen)/Mgen

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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

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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

Mgg481

cosgNff'Z 2

A2V2

2

C

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The width extraction in Atlas

● Analytical fit of di-electron mass (convoluted with resolution function): model

MZ’ = 1.5TeVth. = 9.5 GeV

llDYllint McDY

Mc22222

ll

22BW

ll eaeMMM

MaMf

● Summary of measurements and discrimination power

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Forward backward asymmetry :the potential

● Typical spin 1 particle behaviour :

cosAcos18

3cosdd

FB2

Wide mass bins (off peak

analysis)

● Asymmetry at generation level for several models with MZ’ = 1.5 TeV and 100fb-1

Narrow mass bins (on peak

analysis)

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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 Pquark > Pantiquark : unbiased estimator

only degraded by E/position resolution, ISR.

- Otherwise : maximally biased estimator (cos = - cos *).

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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

observedFB

correctedFB A21

1A

generationFB

observedFB A21A

observedFBA

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The AFB extraction in Atlas

● AFB is deduced with a differential method :- Compute AFB (cos ) by the basic counting method (N+- N-/N++N-)

in several bins of cos (* : generation / : observed-corrected).

- Extract AFB by fitting

● Example : model at MZ' = 1.5TeV (1.48TeV<M ll<1.52TeV)

2FBFB cos1

cosA38cosA

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● 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 AFB extraction in Atlas (2)

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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.

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Back Up Slides

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Moriond 2005

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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 |AFB

gen| > |AFBobs|)

Dilution effect amplitude : 1.4 on average

The correction is an efficient and unbiased way to correct from the dilution

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The off peak analysis● Preliminary feasibility study :

- Only at MZ' = 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 .