Discovery Potential of ATLAS for Extended Gauge Symmetries

1. Discovery Potential of ATLAS for Extended Gauge Symmetries Daisuke Naito (Okayama University, Japan) for the ATLAS Collaboration Nov. 1st, 2006 DPF/JPS-06@Hawaii

2. Outline • Extended gauge symmetries • Z’ production and decay at LHC • Discovery potential for new gauge bosons • Summary Discovery Potentail of ATLAS for Extended Gauge Symmetries

3. 1. Extended gauge symmetries • Extended Gauge Symmetries and the associated heavy neutral gauge bosons (Z’) are the feature of many extensions of the Standard Model (SM). • There are many models: • Z’y model, • Z’c model, • Z’h model, • The Left-Right symmetry model (LRM) • The Alternative LRM (ALRM), • The Kaluza-Klein model (KK) from Extra Dimension. • Little, Littlest Higgs model, • etc… from superstring-inspired E6 and/or SO(10) models Discovery Potentail of ATLAS for Extended Gauge Symmetries

4. u, d, s l g / Z / Z’ l u, d, s 2. Z’ production and decay at LHC • The dominant Z’ production process is . • The gauge bosons are produced via Drell-Yan process. • The Z’ decay into 2 leptons with large invariant mass. • The differential cross section for the process ppZ’ l+l-X depends on: • The effective Z’ mass s’, • The Z’ rapidity Y, • The angle q* between l- and q in the center of mass of the colliding partons. Ref: ATL-PHYS-PUB-2005-010 Sq and Aq are the model-dependent quantities. gSq and gAq involve the parton distribution function. Discovery Potentail of ATLAS for Extended Gauge Symmetries

5. Z’ resonance Ref: ATL-PHYS-PUB-2005-010 • Z’e+e-, m+m- • large invariant mass, • very clean, • sizable cross section. • The LHC design luminosity is 1033(1034)cm-2s-1 at low(high) luminosity. • 10fb-1/year (low luminosity), • 100fb-1/year (high luminosity). • In the channels one would be able to measure: • Mass MZ’, • Decay width GZ’, • Total cross section sZ’, • Spin of Z’. • The Tevatron experiment • a lower limit M(Z’) > 850 GeV for SSM (CDF) T. Rizzo, hep-ph/0610104v1 M(Z’)=1.5TeV To observe the resonance one has to detect 2 high energy leptons. Mll (GeV) Discovery Potentail of ATLAS for Extended Gauge Symmetries

6. ATLAS detector • High energy electrons are detected by LAr calorimeter. • Muons are detected by the Muon System. • Expected electron energy resolution is: • ~0.6% for E=500GeV, • ~0.5% for E=1000GeV. • Muon transverse momentum (pT) resolution is: • ~6% for pT=500GeV, • ~11% for pT=1000GeV. Electron energy resolution Muon System LAr Calorimeter End-caps Discovery Potentail of ATLAS for Extended Gauge Symmetries

7. High pT leptons from Z’ decay • The leptons pT distribution from Z’ decay has a Jacobian peak. • At high pT, the muon momentum resolution degrades. • For the muon pT resolution, calibration and alignment are critical. Muon spectrometer TDR (CERN/LHCC 97-22) Oliver Kortner (MPI), HCP2006 (Duke, May 22-26, 2006) Lepton pT distribution Muon pT resolution Discovery Potentail of ATLAS for Extended Gauge Symmetries

8. 3. Discovery potential for new gauge bosons • At LHC, the discovery limits at 5s confidence level are: • M(Z’)=3-4TeV for L ≈ 10fb-1 (low luminosity) • M(Z’)=4-5TeV for L ≈ 100fb-1 (high luminosity) • If Z’ exists, one would be able to measure: • Mass MZ’, • Decay width GZ’, by fitting the resonance, • Total cross section sZ’, • Spin of Z’. • One can discriminate between the underlying theories by measuring the forward-backward asymmetry. Discovery Potentail of ATLAS for Extended Gauge Symmetries

9. Fitting for the Z’ resonance: Z’e+e- • Electron channel: GZ’>~ DMe+e- • One can get the mass and the decay width by fitting to the Z’ resonance. • For the Drell-Yan background fitting, an exponential function was used. Convolution fitting of Breit-Wigner with Gaussian smearing • Z’e+e- • = 128fb L= 312fb-1 M(Z’) • model MZ’=1.5TeV Ref: ATL-PHYS-PUB-2005-010 Discovery Potentail of ATLAS for Extended Gauge Symmetries

10. Background (for electron channel) • The main background processes are: • Drell-Yan • W± g(may be easily reduced because of the high photon rejection factor.) • ttbar • bbbar (can be excluded by a pT cut.) • ZZ • ZW± • W+W- • Zg Ref: ATL-PHYS-PUB-2005-010 events • Z’e+e- • = 128fb L= 312fb-1 L = 100 fb-1 events • model MZ’=1.5TeV Discovery Potentail of ATLAS for Extended Gauge Symmetries

11. Fitting for the Z’ resonance: Z’m+m- • Muon channel: GZ’< DMm+m- • The fitting function is numerical convolution of a Gaussian with a Breit-Wigner. • This channel is almost background free. • Possible backgrounds: • DY process, • very small at high mass. • ttbarm+m-, • negligible. SSM model M(Z’)=1TeV. • Z’m+m- • = 501fb L= 7.81fb-1 Convolution fitting Discovery Potentail of ATLAS for Extended Gauge Symmetries

12. Forward-backward asymmetry • As a probe of the underlying model, one can measure the forward-backward asymmetry. • The differential cross section of Z’ depends on cosq*. • And if Z’ has spin 1, the differential cross section is given by: • AFB(Mll) quantity can be deduced by a counting method: • This quantity AFB(Mll) is model-dependent. • One can discriminate between the underlying models by measuring AFB(Mll). q* is angle between l- and quark in the CMS of the colliding partons. N+: number of events with the lepton in the forward N-: number of events with the lepton in the backward Ref: ATL-PHYS-PUB-2005-010 Discovery Potentail of ATLAS for Extended Gauge Symmetries

13. AFB(Mll) measurement(1) • Z’e+e- : • high discriminating power of the asymmetry. • Correction: • Taking into account mis-estimation of quark direction. • Fractions of the mis-estimation of quark direction is parameterized by simulation. L = 100fb-1, |eta|<2.5 Plots for 1.4TeV < M(Z’) < 1.6TeV M(Z’) = 1.5TeV Asymmetry at generation level Ref: ATL-PHYS-PUB-2005-010 Discovery Potentail of ATLAS for Extended Gauge Symmetries

14. AFB(Mll) measurement(2) • Z’m+m- • With 200fb-1, the ATLAS can distinguishes the underlying theories with accuracy better than 3% using the asymmetry for M(Z’) less than 2TeV. • At higher masses, we need much more luminosity. 400 fb-1 ~2fb-1 for SSM ~4fb-1 for E6 Ref: ATLAS Internal Note Muon-NO-161 23 May 1997 Discovery Potentail of ATLAS for Extended Gauge Symmetries

15. 4. Summary • There are many models that predict new gauge bosons. • The dominant Z’ production process is . • The Z’ is produced via Drell-Yan process and decays into 2 leptons with high invariant mass. • At LHC, the discovery limits are: • M(Z’)=3-4TeV for L ≈ 10fb-1 • M(Z’)=4-5TeV for L ≈ 100fb-1 • The measurement of forward-backward asymmetry shows the high discriminating power for underlying theories. Discovery Potentail of ATLAS for Extended Gauge Symmetries

16. Backup slides

17. negative charged lepton direction q* quark direction negative charged lepton direction q* quark direction Forward and backward Forward Backward When cos theta* is positive, we call forward, and when cos theta* is negative we call backward. The quark direction is not directly accessible in the data. Therefore the Z’ momentum defines the quark direction, because of the quark generally being at a higher momentum than the antiquark Discovery Potentail of ATLAS for Extended Gauge Symmetries

18. Observed AFB correction We can obtain a quantity <e>, defined as the probability to be wrong, when taking the Z’ direction as the quark direction. Hence we can say that the observed N+(N-) equals to the generation level N+(N-) times fraction of correct estimation, plus the generation level N-(N+) times fraction of incorrect estimation. Then we can obtain the observed AFB given by: Therefore we define the corrected value: Ref: ATL-PHYS-PUB-2005-010 Discovery Potentail of ATLAS for Extended Gauge Symmetries

19. Extended gauge symmetries • Extended Gauge Symmetries and the associated heavy neutral gauge bosons (Z’) are feature of many extensions of the Standard Model (SM). • Grand Unified Theories (GUTs) postulate: • the SU(3), SU(2) and U(1) symmetry groups of the SM have a common origin as subgroups of some larger symmetry group. • At sufficiently large scale, • this large symmetry is supposed to be unbroken, • all interactions are described by the corresponding local gauge theory, • all running couplings coincide. • Some candidate of GUT symmetries: • E6, • SO(10), • SU(5). • W, Z, g and g are not enough to secure local gauge invariance within a larger group. • So GUT models predict additional gauge bosons. Ref: ATL-PHYS-PUB-2005-010 Discovery Potentail of ATLAS for Extended Gauge Symmetries

20. Specific models • A popular model is: • Effective SU(2) U(1)YU(1)’Y. • There are two additional neutral gauge bosons. • The new gauge boson uniquely determined by: • There are 3 special cases: • Z’y model: q=0, E6SO(10) U(1)y • Z’c model: q=-p/2, E6SO(10) U(1)ySU(5) U(1)cU(1)y • Z’h model: q=arctan(-sqrt(5/3))+p/2, E6SU(3)CSU(2)LU(1)YU(1)h=SMU(1)h(E6 breaks directly down to a rank 5 model.) • Other popular models: • The Left-Right model from the breaking of the SO(10) group, • The Kaluza-Klein model (Extra Dimension). • etc… q is a new mixing angle. 2 independent U(1) bosons. Ref: ATL-PHYS-PUB-2005-010 Discovery Potentail of ATLAS for Extended Gauge Symmetries

21. TeV energy muon simulation • The high energy muons are simulated by Geant4. Event Display The deposited energy of simulated muon has Landau distribution. Cu: r =8.96g/cm3 1TeV mu+ Length 3m 1TeV muon runs through 3m copper. Some times, muon radiates, and electromagnetic showers are developed. Discovery Potentail of ATLAS for Extended Gauge Symmetries

22. Muon stopping power • The simulated muon stopping power corresponds to PDG plot. PDG D. E. Groom et al., Atomic Data and Nuclear Data Table 78,183-356(2001) Red points are the simulated stopping power. Discovery Potentail of ATLAS for Extended Gauge Symmetries