discovery potential of atlas for extended gauge symmetries daisuke naito (okayama university, japan)...

Discovery Potential of ATLAS for Extended Gauge Symmetries

Daisuke Naito (Okayama University, Japan)

for the ATLAS Collaboration

Nov. 1st, 2006

DPF/JPS-06@Hawaii

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

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Outline

1. Extended gauge symmetries

2. Z’ production and decay at LHC

3. Discovery potential for new gauge bosons

4. Summary


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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 E6 and/or SO(10) models


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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 ppZ’ 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.

Sq and Aq are the model-dependent quantities.gS

q and gAq involve the parton distribution function.

u, d, s

u, d, s

/ Z / Z’l

l

Ref: ATL-PHYS-PUB-2005-010

qq Z


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Z’ resonance• Z’e+e-, +-

– 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 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/0610104v1

M(Z’)=1.5TeV

To observe the resonance one has to detect 2 high energy leptons.


Mll (GeV)


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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.

LAr CalorimeterMuon System

End-caps

Electron energy resolution


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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.

Oliver Kortner (MPI), HCP2006(Duke, May 22-26, 2006)

Muon spectrometer TDR(CERN/LHCC 97-22)

Muon pT resolutionLepton pT distribution


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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 MZ’,– 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.


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Fitting for the Z’ resonance: Z’e+e-

• Electron channel: Z’ >~ Me+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-

= 128fbL= 312fb-1

modelMZ’=1.5TeV


Convolution fitting of Breit-Wigner with Gaussian smearing

M(Z’)


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Background (for electron channel)

• The main background processes are:– Drell-Yan – W± (may be easily reduced because of the high photon rejection factor.)

– ttbar

– bbbar (can be excluded by a pT cut.)

– ZZ– ZW±

– W+W-

– Z

even

ts even

ts


Z’e+e-

= 128fbL= 312fb-1

modelMZ’=1.5TeV

L = 100 fb-1


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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.

– ttbar+-,• negligible.

SSM modelM(Z’)=1TeV.

Z’+-

= 501fbL= 7.81fb-1

Convolution fitting


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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:

• 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).

* is angle between l- and quark in the CMS of the colliding partons.


N+: number of events with the lepton in the forwardN-: number of events with the lepton in the backward


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AFB(Mll) measurement(1)• Z’e+e- :

– high discriminating power of the asymmetry.


Plots for 1.4TeV < M(Z’) < 1.6TeVM(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.


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AFB(Mll) 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-161 23 May 1997

400 fb-1

~2fb-1 for SSM

~4fb-1 for E6


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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.

qq Z

Backup slides


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Forward and backward

cos* 0

cos* 0

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


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Observed AFB 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:



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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, 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


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Specific models• A popular model is:

– Effective SU(2) U(1)YU(1)’Y.• There are two additional neutral gauge bosons.

– The new gauge boson uniquely determined by:

– There are 3 special cases:• Z’ model: =0, E6SO(10) U(1)• Z’ model: =-/2, E6SO(10) U(1)SU(5) U(1)U(1)• Z’ model: =arctan(-sqrt(5/3))+/2,

E6SU(3)CSU(2)LU(1)YU(1)=SMU(1)(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…

2 independent U(1) bosons.

is a new mixing angle.



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TeV energy muon simulation• The high energy muons are simulated by Geant4.

Cu: =8.96g/cm3

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


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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 potential of atlas for extended gauge symmetries daisuke naito (okayama university, japan)...

Documents

z model

z decay

spin of z

z resonance z e e

decay width z

discovery potentail

process pp z

effective z mass s