1 HEP 2008, Olympia, Greece Ariadni Antonaki Dimitris Fassouliotis Christine Kourkoumelis Konstantinos Nikolopoulos University of Athens Studies for the detection of new heavy bosons with the ATLAS detector

2 Search motivation Several theoretical models beyond SM predict the existence of new, heavy gauge bosons  GUTs  E6 models  Left-Right Symmetric models  Little Higgs models  Kaluza-Klein models Z’ (neutral) / W’ (charged)

3 Mass limits Not well defined by the theoretical models  experiment searches in a broad mass region Current Limits (Tevatron):  Z’ mass > 95% C.L.  W’ mass > 1 95% C.L. LHC will search up to ~4TeV. The reference model For our analysis we use the Sequential Standard Model (SSM)  Z’,W’ identical to Z,W (same couplings to fermions) but with much larger mass We have worked with two different mass samples for each case : 1 TeV and 2 TeV

4 What’s new in this analysis  All samples with Full Simulation  Initial Detector Layout  MS Trigger Study (Isol. μ, P T >20GeV)  90% eff. W’, 95% eff. Z’ (first time)  NLO  Evaluation of systematic effects

5 W’, Z’ cross sections (LO) σ x BR (pb) 1 TeV σ x BR (pb) 2TeV SM-like W’  μν SM-like Z’  μμ

6 Transverse mass distribution (M T ) W’  μν (MSc thesis of Zacharias Roupas) 1TeV2TeV Background to W’ decay: W-boson tail, ttbar production, di-jets…

7 W’->μν : clear signature, consisting of high-energy isolated muon + large E Tmiss σ (pb) (NLO) CutW’ 1W’ 2W>200GeVttbarDi-jet Νο Cuts x10 10 Preselection (1μ) x10 7 P T >50GeV x10 3 E Tmiss >50GeV Isolation(ΔR<0.3,ΣP T <5% P Tmuon ) Lepton Fraction Event Selection W’ boson

8 Transverse Mass (after all Cuts) --- W tail : dominant

9 Significance Integrated Luminosity for a 5σ W’ discovery ~3pb -1 ~70pb -1

10 Z’  μμ The most distinctive signature comes from dilepton decay. The dominant background is the (irreducible) Drell-Yan process to muons. Other contributions (ttbar prod., Z  ττ, dijets e.t.c.) negligible in high-mass region & after appropriate selection criteria

11 Cut P T >30GeV |η|<2.5 Isolation Event Selection Cut Efficiency (NO isol. Cut) Sample(%) Drell-Yan73 Zprime 1TeV68 Zprime 2TeV70 Z’ boson At least 2 opposite charged muons with:

12 M(GeV) Mass Plots (after Cuts) * Drell-Yan * Zprime 1TeV * Zprime 2TeV

13 The main goal is to test two hypotheses:  the data is compatible with non-signal SM background (“null hypothesis” - H0 )  the data is compatible with signal+background (“alternative/test hypothesis” – H1 ) Significance Studies

14 Significance Studies (methods) 1). Mass Spectrum Input: Mass distributions for signal and bkg, normalized at the luminosities under study. Output: FFT signal significance for several luminosities under study. Systematics (Ptmuon resolution, ±5% efficiency) can be included. 1). Mass Integral Input: Total number of signal and bkg events in the same mass region (normalized at luminosities under study). Output: signal significance for several luminosities under study. Systematics (P T muon resolution, ±5% efficiency) can be included. 2). Mass Spectrum Input: Mass distributions for signal and bkg, normalized at the luminosities under study. Constructs the p.d.f.’s (FFT) for the signal and the bkg for some discriminant variable (e.g. Inv.Mass) and counts the Likelihood Ratio Output: signal significance for the several luminosities under study. Systematics (P T muon resolution, ±5% efficiency) can be included.

15 (s+b) median (20pb -1 ) fixed mass Likelihood Ratio Significance Studies (methods) 3). Toy MC  Input: The signal and bkg distributions, normalized at 1pb -1. Perform an expo fit for the bkg and assume gaussian for the signal.  Perform pseudo-experiments and count the integral of signal Gaussian and bkg. The Gaussian can be performed in a fixed region (“fixed mass”) or everywhere (“floating mass”). For each hypothesis, we produce: Count how many b above s+b median  Output: Significance for “fixed mass” and “floating mass” case. * Expo fit(DY) * Gaussian(1TeV) * Gaussian(2TeV) H0 H1

16 Significance for 1TeV (SM-like) Zprime For 5σ : Luminosity 12-13pb -1

17 For 5σ : Luminosity ~300pb -1 Significance for 2TeV (SM-like) Zprime

18 Significance for several Z’ models SSM: most optimistic case Difference < factor of 10  stable in all mass range Z’  e + e -

19 Conclusions & Future Work SM-like Z’ and W’ bosons can be detected in ATLAS, even in early data taking (~10pb -1 after a month) The expected luminosities for SM-like Z’ and W’ at 5σ discovery have been estimated:  A SM-like Z’  μμ 1TeV mass at ~13pb -1, 2TeV mass ~300pb -1  A SM-like W’  μν 1TeV mass at ~3pb -1, 2TeV mass ~70pb -1 Future:  W’ : Mass measurements  Z’ : More masses, different Z’ models (when produced)

20 Back up Slides

21 W’ boson data samples Process Generator σ ΧBR [fb] Comments Events 1TeVW → l ν PYTHIA 9430 min(√s) = 300GeV 30K 2TeVW → l ν PYTHIA 437 min(√s) = 300GeV 30K 3TeVW → l ν PYTHIA 54 min(√s) = 300GeV 10K SMW → ν PYTHIA GeV< m(W) <500GeV 20K SMW → ν PYTHIA m(W) >500GeV20K t ¯t K Dijet J0 PYTHIA 1.76Χ1013 pˆT = 8−17GeV 380K Dijet J1 PYTHIA 1.38Χ1012 pˆT = 17−35GeV 380K Dijet J2 PYTHIA 9.33Χ1010 pˆT = 35−70GeV 390K Dijet J3 PYTHIA 5.88Χ109 pˆT = 70−140GeV 380K Dijet J4 PYTHIA 3.08Χ108 pˆT = 140−280GeV 390K Dijet J5 PYTHIA 1.25Χ107 pˆT = 280−560GeV 370K Dijet J6 PYTHIA 3.60Χ105 pˆT = 560−1120GeV 380K Dijet J7 PYTHIA 5.71Χ103 pˆT = 1120−2240GeV 430K W’  μν

22 W’ boson P T -1 Resolution E Tmiss Resolution 1TeV2TeV W’  μν

23 Systematics (theor.) Systematics (exp.) SourceRelative Uncertainty Muon Recon. Efficiency ±5% Muon Momentum Resolution Uncertainties in Jets (E Tmiss ) (W’) ---- : Coulomb scattering, low energies ---- : Alignment, high energies (P T Resolution Uncertainty: 10->20%) SourceRelative Uncertainty PDF’s6-7% K-factor (~30%)~8%

24 Cut Flow