X-DIM group of SUSY GDR1

F. Ledroit, B. Trocmé, J. Morel(LPSC – Grenoble)

Search for Z’ →e+e-

with ATLAS detector at LHC

, ,u d s

, , 'Z Z

, ,u d s

e

e

X-DIM group of SUSY GDR2

Introduction

Z’ = generic notation for additionnal neutral gauge bosons

new bosons in GUTs (e.g. E6)

excited states of existing bosons (e.g. KK states)

…

Aims of our study: • determine discovery potential of new models with Z’• assuming a discovery, can we infer the underlying theory ?

X-DIM group of SUSY GDR3

Outline

already studied models

discovery potential

observables allowing to infer the underlying theory

what we have done so far:

generators used

decay width reconstruction

AFB measurement

Z’ rapidity fit

outlook

X-DIM group of SUSY GDR4

•LHC nominal CMS energy = 14 TeV•Colliding beams in ATLAS = pp•LHC nominal low luminosity = 1033 cm-2 s-1

high 1034 cm-2 s-1

•Integrated luminosity ∫Ldt1 year of running at low luminosity = 10 fb-1

high = 100 fb-1

Nb of events = x ∫Ldt•ATLAS can detect efficiently photons, electrons, muons, jets.Taus are decaying in the beam pipe and thus only detected indirectly via their products.Very good energy resolution can be obtained quickly for photons and electrons. Jet energy scale will take more time.

Reminder for our theoretician friends:

X-DIM group of SUSY GDR5

Already studied models

One X-dimensions model: T.G. Rizzo, Phys.Rev. D 61 (2000) 055005

ADD model.

Only fermions confined to 3-brane (all on same orbifold point D=0)

Gauge fields propagate in 1 small extra dimension with compactification radius ~1 TeV-1; one single parameter Mc.

Masses of the KK modes Mn2= M0

2 + (nMc)2

Couplings = √2x SM couplings

Invariant mass Invariant mass

M1

Azuelos&Polesello

X-DIM group of SUSY GDR6

Already studied models (cont’d)

E6:E6 … SU(3)C x SU(2)L x U(1)Y x U(1) x U(1) Lightest Z’ : Z’ = cos(E6) Z’ – sin(E6)Z’models

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 = 1Rem: W’

X-DIM group of SUSY GDR7

Discovery potential (ATLAS parameterized sim.)

ATLAS detector and physics performance TDR, CERN/LHCC-99-14

G. Azuelos and G. Polesello, Eur.Phys.J.C39S2:1-11,2005

Discover a resonance (5)(any model)

Nb of DY events at low energy (Rizzo model)

Ultimate limit (300fb-1) :Mc < 13.5 TeV

M1

X-DIM group of SUSY GDR8

Strategies proposed long time ago(e.g. M. Cvetič and S. Godfrey, hep-ph/9504216)Latest update = M. Dittmar, A.-S. Nicollerat and A. Djouadi, Phys.Lett.B583:111-120,2004

Observables allowing to infer couplings =

Z’ → l+l- decays (l=e or ):• total decay width , • forward-backward asymmetryAFB = (F-B)/(F+B), F/B = ∫0/-1 1/0 dcos ∂/∂cosangle between q and l- in Z’ rest frame• Z’ rapidity distributions

How to infer the underlying theory ?

X-DIM group of SUSY GDR9

Z’ → ffbar decays (f= or q):• polarization, • jet-jet cross-section

4 fermion final states:• rare decays Z’ → Wl• “associated production” pp → Z’V, V=Z,W

we concentrate on the ‘golden channel’ Z’→e+e-

How to infer the underlying theory ? (cont’d)

X-DIM group of SUSY GDR10

Generators used

E6, LR:

ffbar→/Z/Z’ subprocess implemented in PYTHIA;

possibility to set non universal couplings.

X-dimensions model:

• either Pythia, stop at (2)/Z(2)

• or (private) generator from T. Rizzo interfaced with PYTHIA matrix element calculated with full interference for , (1), (2), Z, Z(1), Z(2) + resummation of higher lying states

X-DIM group of SUSY GDR11

Total decay width reconstruction xith ATLAS Resolution on the invariant mass of the 2 electrons: ~ 30 GeV (at 4 TeV)

Fitting function = BW convoluted with gaussian resolution+ exponential background

= 173±8 GeV = 168±14 GeV

∫Ldt = 500 fb-1

Mc = 4 TeV

Generated level Simulated level

X-DIM group of SUSY GDR12

Forward backward asymmetry

Typical spin 1 particle behaviour :

cosAcos183

cosdd

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)

SSM

X-DIM group of SUSY GDR13

Forward backward asymmetry (cont’d)

Must take care of the fact that q(bar) side unknownin the case of pp collisions

Examples of ‘on peak’ analyses for 300fb-1:

SSMmodel

X-DIM group of SUSY GDR14

• Br(Z’ qqbar) and thus Prop(Z’ qqbar) depend on Z’ couplings on model• Possibility to separate uu and dd contributions to Z’

signal thanks to the ≠ PDFs producing ≠ rapidity distributions

• ZSSM example:

'SSMZY

'dd Z

'SSMZY

'ss Z

'SSMZY

'uu Z

'SSMZY

3 model independent shapes 1 model dependent combination

qqZ’

Z’ rapidity

X-DIM group of SUSY GDR15

Outlook

Our main expectation in the GDR context:find additionnal X-Dim models to be studied,both from the discovery potential and from the discrimination point of view.

Could consider including other decay channels( straightforward, e easy, e and jet-jet much more difficult).