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Discovery Potential of ATLAS
for Extended Gauge Symmetries
Daisuke Naito (Okayama University, Japan)
for the ATLAS Collaboration
Nov. 1st, 2006
DPF/JPS-06@Hawaii
Outline
1.
2.
3.
4.
Nov. 1st, 2006
Extended gauge symmetries
Z’ production and decay at LHC
Discovery potential for new gauge bosons
Summary
Discovery Potentail of ATLAS for
Extended Gauge Symmetries
2
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,
from superstring-inspired E6 and/or SO(10) models
– 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…
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Extended Gauge Symmetries
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2. Z’ production and decay at LHC
•
•
•
•
–
–
–
The dominant Z’ production process is qq  Z .
The gauge bosons are produced via Drell-Yan process.
The Z’ decay into 2 leptons with large invariant mass.
u, d, s
The differential cross section 
for the process ppZ’ l+l-X depends on:
g / Z / Z’
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.
u, d, s
l
l
Ref: ATL-PHYS-PUB-2005-010
Sq and Aq are the model-dependent quantities.
gSq and gAq involve the parton distribution function.
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Discovery Potentail of ATLAS for
Extended Gauge Symmetries
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•
Z’e+e-,
m+m-
Z’ resonance
Ref: ATL-PHYS-PUB-2005-010
– 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’.
T. Rizzo, hep-ph/0610104v1
M(Z’)=1.5TeV
• The Tevatron experiment
– a lower limit M(Z’) > 850 GeV for SSM (CDF)
To observe the resonance one has to detect 2
high energy leptons.
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Discovery Potentail of ATLAS for
Extended Gauge Symmetries
Mll (GeV)
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ATLAS detector
• High energy electrons are detected by LAr calorimeter.
• Muons are detected by the Muon System.
Electron energy resolution
• 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.
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Discovery Potentail of ATLAS for
Extended Gauge Symmetries
Muon System
LAr Calorimeter
End-caps
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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)
Lepton pT distribution
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Muon spectrometer TDR
(CERN/LHCC 97-22)
Muon pT resolution
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Extended Gauge Symmetries
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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.
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Extended Gauge Symmetries
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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+es= 128fb
L= 312fb-1
M(Z’)
hmodel
MZ’=1.5TeV
Nov. 1st, 2006
Ref: ATL-PHYS-PUB-2005-010
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Extended Gauge Symmetries
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Background (for electron channel)
• The main background processes are:
events
Drell-Yan
W± g (may be easily reduced because of the high photon rejection factor.)
Ref: ATL-PHYS-PUB-2005-010
ttbar
bbbar (can be excluded by a pT cut.)
Z’e+eZZ
s= 128fb
L = 100 fb-1
L= 312fb-1
ZW±
W+WZg
events
–
–
–
–
–
–
–
–
hmodel
MZ’=1.5TeV
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Extended Gauge Symmetries
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Fitting for the Z’ resonance: Z’m+mSSM model
M(Z’)=1TeV.
Z’m+ms= 501fb
L= 7.81fb-1
• 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.
Convolution fitting
– ttbarm+m-,
•
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Discovery Potentail of ATLAS for
Extended Gauge Symmetries
negligible.
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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 cosq*.
• And if Z’ has spin 1, the differential cross section is given by:
q* is angle between l- and quark
in the CMS of the colliding partons.
• AFB(Mll) quantity can be deduced by a counting method:
N+: number of events with the lepton in the forward
N-: number of events with the lepton in the backward
• This quantity AFB(Mll) is model-dependent.
• One can discriminate between the underlying models by
measuring AFB(Mll).
Ref: ATL-PHYS-PUB-2005-010
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Extended Gauge Symmetries
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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
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Extended Gauge Symmetries
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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
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Extended Gauge Symmetries
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4. Summary
• There are many models that predict new gauge bosons.
• The dominant Z’ production process is qq  Z .
• 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.
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Extended Gauge Symmetries
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Backup slides
Forward and backward
negative charged lepton direction
Forward
cos q *  0
Backward
cos q *  0
q*
quark direction
negative charged lepton direction
q*

quark 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 <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
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Extended Gauge Symmetries
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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, 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
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Extended Gauge Symmetries
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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:
q is a new mixing angle.
– There are 3 special cases:
2 independent U(1) bosons.
• 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…
Ref: ATL-PHYS-PUB-2005-010
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Extended Gauge Symmetries
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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.
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Extended Gauge Symmetries
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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.
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Extended Gauge Symmetries
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