Transcript strings_surf_talk - Caltech High Energy Physics

Search for Low Mass Strings at CMS
Andy Yen
Mentor: Harvey Newman
Co-Mentors: Marat Gataullin, Vladimir Litvine
SURF Seminar
October 17, 2009
The Standard Model of Particle Physics
 The Standard Model of
particle physics has been
extremely successful in
describing interactions
between elementary particles
 There are four known force
carriers and twelve known
quarks and leptons.
However, the model is far
from complete!
There is no theory for
quantum gravity.
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Superstring Theory

There are plenty of reasons to believe in physics beyond
the Standard Model.
 One possible alternate model is Superstring theory.
Superstring theory incorporates quantum gravity by
utilizing extra degrees of freedom.
Extension into large extra
dimensions also solves the
hierarchy problem.
 Recently demonstrated by
theorists that some superstring
scenarios can be detected at the
LHC.
String mass scale (Ms) < 2 TeV
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The Large Hadron Collider
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The Large Hadron Collider
Luminosity is proportional to the
rate at which collisions occur.
The LHC is scheduled to restart
operation in late 2009.
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The Compact Muon Solenoid
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A New Definition of “Compact”
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Detecting superstrings at LHC
Superstrings can lead to the following non-SM
process: gg→gγ
Extra contribution to the SM photon+jet.
Ms ~ 1 TeV, 100 events for 200pb-1
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Backgrounds
Standard Model photon+jet is the dominant
background.
There is also a significant background from QCD
Dijets
Sometimes, most of the jet energy is in an isolated
neutral meson.
These neutral hadrons decay into a photon pair
which can appear in the ECAL as a single energetic
object.
This can lead to jets being misidentified as
photons.
Misidentified jets can be rejected with high
efficiency, but the cross section is large.
Signal and backgrounds simulated with Monte
Carlo methods.
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Event Selection
Primary objective is to reject QCD
jet fakes.
Cut based selection on a series of
kinematic variables is used.
R9 = S9/S25
H/E = HCAL Energy
ECAL Energy
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Event Reconstruction
Photon reconstruction is close to 98% efficient with
0.5% energy resolution.
Jets are not as well reconstructed
 Poor HCAL granularity
 Intrinsic fluctuations in hadronic showers
Resolution of reconstructed invariant mass is
dominated by jet resolution.
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Reconstruction Energy Resolution
sisCone algorithm (“seedless infrared safe cone”)
Energy response = Reco Energy/“True” Energy
Peak location: 1.00
Sigma: 0.5%
Peak location: 0.94
Sigma: 8.7%
Photons
Jets
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Collinear Splitting
Even in the sisCone Jet, there is still a tail at low
values.
Approximately 10% of reconstructed Jets have 2nd
Jet within dR<1
Standard sisCone algorithm is optimized for low
energy jets.
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Collinear correction
Take the vector sum of the momentum of the two jets
Use Pt-weighted average of the direction of the two
jets.
Improvements in position and energy reconstruction!
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Invariant Mass Reconstruction

The invariant mass of the
photon+jet can be
calculated from:
2

 

M e*    E     p 

 

Luminosity = 100 pb-1
2
Selection efficiency of
68.10% was attained for the
string signal.
Reconstructed mass
resolution of approximately
6% is achieved.
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Estimating Signal Significance
Significance is evaluated based on the number of
observed events compared to the number of expected
background events in the region of the peak.
 Random fluctuations in the background can lead to
“accidental” peak-like structures in the spectrum.
Five sigma significance - the probability that a claimed
signal is caused by a local fluctuation of the background
is 2.9·10-7 (very, very improbable!)
 A log-likelihood approach is used.
 Finally, the significance is given by
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Results
100 pb-1 of integrated luminosity will lead to a discovery
with a significance of 9.84.
A 5 sigma discovery is possible with as little as 25.8 pb-1.
Signal+BG and BG only
distributions for 1000pb-1
of integrated luminosity
and center of mass
energy of 10 TeV.
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Conclusion
These results represent the string theory searches
done at the LHC.
The results show that CMS is capable of making a
discovery during the early running stages.
LHC is currently scheduled to restart in 2 months.
 100 pb-1 of integrated luminosity might be possible
within the first year.
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Acknowledgements
 Professor Harvey
Newman
 Dr. Marat Gataullin and
Vladimir Litvine
 Dr. Arunava Roy
 Professor Haim Goldberg
 Caltech SFP and Rose
Hills Foundation
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The End