Discovery Potential of ATLAS for Extended Gauge Symmetries Daisuke Naito (Okayama University, Japan) for the ATLAS Collaboration Nov. 1st, 2006

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 2 Outline 1.Extended gauge symmetries 2.Z ’ production and decay at LHC 3.Discovery potential for new gauge bosons 4.Summary

Nov. 1st, 2006Discovery 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 ’  model, –Z ’  model, –Z ’  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 E 6 and/or SO(10) models

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 4 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  * between l - and q in the center of mass of the colliding partons. S q and A q are the model-dependent quantities. g S q and g A q involve the parton distribution function. u, d, s  / Z / Z’ l l Ref: ATL-PHYS-PUB

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 5 Z ’ resonance Z ’  e + e -,  +  - –large invariant mass, –very clean, –sizable cross section. The LHC design luminosity is (10 34 )cm -2 s -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 M Z ’, –Decay width  Z’, –Total cross section  Z’, –Spin of Z ’. The Tevatron experiment –a lower limit M(Z ’ ) > 850 GeV for SSM (CDF) T. Rizzo, hep-ph/ v1 M(Z’)=1.5TeV To observe the resonance one has to detect 2 high energy leptons. Ref: ATL-PHYS-PUB Mll (GeV)

Nov. 1st, 2006Discovery 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 (p T ) resolution is: –~6% for p T =500GeV, –~11% for p T =1000GeV. LAr Calorimeter Muon System End-caps Electron energy resolution

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 7 High p T leptons from Z ’ decay The leptons p T distribution from Z ’ decay has a Jacobian peak. At high p T, the muon momentum resolution degrades. For the muon p T resolution, calibration and alignment are critical. Oliver Kortner (MPI), HCP2006 (Duke, May 22-26, 2006) Muon spectrometer TDR (CERN/LHCC 97-22) Muon p T resolutionLepton p T distribution

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 8 3. Discovery potential for new gauge bosons At LHC, the discovery limits at 5  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 M Z ’, –Decay width  Z ’, by fitting the resonance, –Total cross section  Z’, –Spin of Z ’. One can discriminate between the underlying theories by measuring the forward-backward asymmetry.

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 9 Fitting for the Z ’ resonance: Z ’  e + e - Electron channel:  Z ’ >~  M e+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. Z’  e + e -  = 128fb L= 312fb -1  model M Z’ =1.5TeV Ref: ATL-PHYS-PUB Convolution fitting of Breit-Wigner with Gaussian smearing M(Z’)

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 10 Background (for electron channel) The main background processes are: –Drell-Yan –W ±  (may be easily reduced because of the high photon rejection factor.) –tt bar –bb bar (can be excluded by a p T cut.) –ZZ –ZW ± –W + W - –Z  events Ref: ATL-PHYS-PUB Z’  e + e -  = 128fb L= 312fb -1  model M Z’ =1.5TeV L = 100 fb-1

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 11 Fitting for the Z ’ resonance: Z ’  +  - Muon channel:  Z ’ <  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. –tt bar  +  -, negligible. SSM model M(Z’)=1TeV. Z’  +  -  = 501fb L= 7.81fb -1 Convolution fitting

Nov. 1st, 2006Discovery 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 cos  *. And if Z ’ has spin 1, the differential cross section is given by: A FB (M ll ) quantity can be deduced by a counting method: This quantity A FB (M ll ) is model-dependent. One can discriminate between the underlying models by measuring A FB (M ll ).  * is angle between l - and quark in the CMS of the colliding partons. Ref: ATL-PHYS-PUB N + : number of events with the lepton in the forward N - : number of events with the lepton in the backward

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 13 A FB (M ll ) measurement(1) Z ’  e + e - : –high discriminating power of the asymmetry. Ref: ATL-PHYS-PUB Plots for 1.4TeV < M(Z’) < 1.6TeV M(Z’) = 1.5TeV L = 100fb -1, |eta|<2.5 Asymmetry at generation level Correction: Taking into account mis-estimation of quark direction. Fractions of the mis-estimation of quark direction is parameterized by simulation.

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 14 A FB (M ll ) measurement(2) Z ’  +  - –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. Ref: ATLAS Internal Note Muon-NO May fb -1 ~2fb -1 for SSM ~4fb -1 for E 6

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 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.

Backup slides

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 17 Forward and backward Forward Backward quark direction negative charged lepton direction ** quark direction negative charged lepton direction ** 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

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 18 Observed A FB correction We can obtain a quantity, 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

Nov. 1st, 2006Discovery 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: –E 6, –SO(10), –SU(5). W, Z,  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

Nov. 1st, 2006Discovery 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 ’  model:  =0, E 6  SO(10)  U(1)  Z ’  model:  =-  /2, E 6  SO(10)  U(1)   SU(5)  U(1)   U(1)  Z ’  model:  =arctan(-sqrt(5/3))+  /2, E 6  SU(3) C  SU(2) L  U(1) Y  U(1)  =SM  U(1)  (E 6 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 … 2 independent U(1) bosons.  is a new mixing angle. Ref: ATL-PHYS-PUB

Nov. 1st, 2006Discovery Potentail of ATLAS for Extended Gauge Symmetries 21 TeV energy muon simulation The high energy muons are simulated by Geant4. Cu:  =8.96g/cm 3 1TeV mu+ Length 3m 1TeV muon runs through 3m copper. Some times, muon radiates, and electromagnetic showers are developed. The deposited energy of simulated muon has Landau distribution. Event Display

Nov. 1st, 2006Discovery 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, (2001) Red points are the simulated stopping power.