,
on the
centre-of-mass energy dependence of both calibration sample statistics and new
physics reach.
These plots were made for presentation at the LHC machine workshop.
They are intended to illustrate the interest of operating LHC at different centre-of-mass
energies and for different integrated luminosity, not to give detailed assessments of ATLAS
physics capabilities.
For those, see the recently released summary volume:
.
.
These slides contain a little background information and a summary.
.
Yield of Z decays to electron pairs usable for calibration and physics studies, for an
integrated luminosity of 50 pb-1.
Yield is evaluated with full simulation at 14 TeV, as described in
CERN-OPEN-2008-020
and scaled to lower centre-of-mass energies using a fast simulation with
simple fiducial and kinematic acceptance cuts.
Errors shown on the yield are quite conservative and include a
~15% luminosity uncertainty.
To set the scale, it should be possible to inter-calibrate, in 0.2x0.4 eta-phi bins,
the EM calorimeter to 0.7% statistical error with 50k well-measured
Z to ee decays, with the
error scaling approximately as 1/sqrt(N).
Systematic errors, arising in particular from the knowledge of the upstream
material, will have to be well understood to achieve this goal.
For muons, 20k Z to muon-pair decays should be statistically sufficient to
test the average muon system momentum scale to better than 1% (systematic
uncertainties are expected from alignment, knowledge of the magnetic field
and energy loss).
A more detailed understanding, and region dependent corrections (B-field,
material, alignment), will require much larger statistics.
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Yield of J/psi decays to muon pairs usable for calibration and physics studies, for an
integrated luminosity of 50 pb-1.
The plot shows the yield within a fiducial acceptance: one muon with
pT>6 GeV,
the second with pT>4 GeV, and both muons within pseudo-rapidity
range +-2.5.
An approximate detection efficiency factor of 70% is included.
The experimental error is estimated to be 10-20% on the usable yield.
J/psi and Upsilon decays to muons and electrons will also be important for early
detector understanding, such as particle identification and reconstruction,
inter-calibration, material effect and detailed alignment studies.
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Yield of top quarks in selected samples, for an
integrated luminosity of 50 pb-1.
The expected Tevatron (one experiment) statistics with 8 fb-1
are shown as a comparison benchmark.
For the di-lepton ('llbb') analysis, no
b-tagging is used, neither for Tevatron, nor for ATLAS. For the
semi-leptonic ('l+jets') analysis, a mature Tevatron analysis requiring 1
b-tagged jet is compared to an ATLAS analysis tuned for early data, without
b-tagging, but providing a useful S/B ratio.
The study was performed by
scaling 14 TeV full simulation results to lower Ecm values according to the
top-pair production cross-section.
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Expected sensitivity to production of a sequential Standard Model-like Z' for Z' masses
of 1 and 1.5 TeV, as a function of centre-of-mass energy.
The current Tevatron 95% CL lower limit is around 1 TeV.
The discovery potential shown corresponds to a signal of at least ten events, which is
expected to be somewhat more significant than 5 sigma.
Study carried out with a fast simulation (Atlfast), using in addition the
reconstruction efficiency (~90%) evaluated with a fuller simulation (Atlfast-II) at
10 TeV.
Systematic uncertainties, coming mainly from the alignment of the muon system,
are not included.
Around 100 pb-1 are needed to obtain discovery potential just beyond
the Tevatron limits at 10 TeV centre-of-mass energy.
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Expected sensitivity to production of a sequential Standard Model-like W' for W' masses
of 1 and 1.5 TeV, as a function of centre-of-mass energy.
The current Tevatron 95% CL lower limit is around 1 TeV.
The discovery potential shown corresponds to a signal of at least ten events, which is
expected to be rather more than 5 sigma.
Study carried out with fast simulation at 10 and 14 TeV and a simple rescaling
at lower centre-of-mass energies.
Around 20 pb-1 are needed to obtain discovery potential just beyond
the Tevatron limits at 10 TeV centre-of-mass energy.
This analysis will require a reasonable understanding of backgrounds and detector
performance, which will require time to obtain.
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Discovery sensitivity possible for SUSY, in an mSUGRA model with equal mass squarks and
gluinos.
The lepton plus jets plus missing-ET channel is employed, as this
should be understood
more rapidly than the statistically more powerful inclusive jets+missing-Et channel.
The current published Tevatron limits in this model are around 400 GeV
- discovery sensitivity beyond this should be accessible with a few tens of
pb-1 at 10 TeV.
The sensitivity drops away rather quickly below 8 TeV.
Study carried out with fast simulation.
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Signal significance possible for the decay of a Standard Model Higgs to W pairs, summing both
gluon-fusion and vector-boson fusion channels.
Only leptonic W decays are used.
Significance is shown as a function of centre-of-mass energy for 1 fb-1 of well-understood data.
Other channels (H to ZZ and H to two photons) will also contribute, but are not
included.
Mixed fast and full (GEANT4) simulation study for the mixed electron/muon W decays
using a cut-based approach, with a simple scale factor sqrt(2) applied on the signal significance
to account for the double-electron and double-muon decay channels.
Five sigma discovery sensitivity opens up with 1 fb-1 and a centre-of-mass energy of close to
10 TeV, for the most favourable Higgs masses (160-170 GeV).
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Integrated luminosity needed, as a function of centre-of-mass energy, to approach the reach
of a single Tevatron experiment with 8 fb-1 of data, for a Standard Model Higgs
mass of 160 GeV.
This Tevatron experiment sensitivity was estimated in December 2008, and improvements
in Tevatron
analyses will tend to make it pessimistic.
For the ATLAS estimate only the gluon-fusion Higgs production channel, and leptonic
W decays, are considered.
The overall sensitivity is approximately 1.9 sigma.
Full (GEANT4) simulation study for the mixed electron/muon W decays, using a cut-based
approach, with a simple scale factor 2 applied on the signal and background yields
to account for the double-electron and double-muon decay channels.
Systematic errors have not been re-evaluated from the studies done in the CSC book.
The overall luminosity scale uncertainty is significant, perhaps +-50%.
With 200 pb-1 of 10 TeV data, it should be possible to
approach the Tevatron sensitivity for this Higgs mass, assuming the
systematic uncertainty on the knowledge of the backgrounds would
remain under control.
Note that the Tevatron analyses have been updated since this plot was made.
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