1 search for excited leptons with the cms detector at the large hadron collider. andy yen, yong...
Post on 21-Dec-2015
213 views
TRANSCRIPT
1
Search for Excited Leptons with the CMS Detector at the Large Hadron Collider.
Andy Yen, Yong Yang, Marat Gataullin, Andy Yen, Yong Yang, Marat Gataullin, Vladimir LitvineVladimir Litvine
California Institute of TechnologyCalifornia Institute of Technology
JTerm III, Photon+XJTerm III, Photon+XJanuary 14, 2009January 14, 2009
LHC Goals
LHC has many capabilities beyond Higgs.Both ATLAS and CMS are designed to be general
purpose detectors with good sensitivity towards new phenomena, e.g. SUSY, MSSM, Compositeness, etc
CMS in particular with its precision ECAL is an effective tool for discovering new physics involving photon or electron final states.
2Andy Yen
Compositeness theories
In compositeness models, the known quarks and leptons have structure and share common constituents
These common constituents are point-like particles called preons (“pre-fermions”)
This is similar to protons and neutrons being the bound state of three quarks.
According to this approach, a quark or lepton might be a bound state of three preons.
The SM quarks and leptons would be the ground states of a rich spectrum of fermions
Andy Yen 3
Motivation
Reasons for why preons might exist:
1) There is no explanation for why there are three generations of matter.
2) Most of the fundamental particles are unstable.
This raises a logical conundrum, how can Nature’s most fundamental objects decay into equally fundamental objects?
Andy Yen 4
• If compositeness models are correct, some excited states, such as excited electrons and muons, can be produced copiously at the high center of mass energy of the LHC and thus detected.
Excited Quarks and Leptons
Excited leptons are much more massive compared to their Standard Model counterparts. ZEUS: Me* > 200 GeV OPAL: Me* > 306 GeV
There should exist new interactions between quarks and leptons at the scale of constituent binding energy.
At energies below the compositeness scale Λ, interactions are effectively contact interactions
At energies above Λ, we expect new particles and interactions. ALEPH: Λ > 6.2 TeV at 95% CL D0: Λ > 4.2 TeV at 95% CL
Λ may be within reach of the LHC
Andy Yen 5
Excited Electron Production at the LHC
Excited electrons can be produced in two ways at the LHC – through contact interactions or through gauge mediated interactions.
Contact interactions are described with the word “contact” because the force carriers involved have not yet been observed so the mechanism of the interaction is not well understood.
Andy Yen 6
Excited Electron Decays
Contact interactions are believed to make up over 99% of the excited electron production so this study will focus exclusively one excited electrons produced through contact interactions.
Excited electrons cannot be directly observed as they are highly unstable.
They must be detected through their decays.
Andy Yen 7
Why the e*eγ decay channel?
For theoretically probable Me*< Λ scenario, gauge decays will dominate.
In the e* vW channel, the neutrino from the decay cannot be detected by the CMS.
The e* eZ channel cannot be used because it does not allow for direct e* reconstruction.
The excited electron can be reconstructed in the ECAL only if the Z decays into an electron-positron pair.
Andy Yen 8
Z decaying into hadrons
Z decaying into e+ e-
Detecting e*eγ decays at the CMS
The process can be summarized by the following:
Cross section varies inversely with mass (Λ=Me* shown)200 GeV: 188400 pb
500 GeV: 2355 pb1 TeV: 63.43 pb
Due to the high masses of excited electrons, the final products will show up in the ECAL as three particles with high Pt.
*qq ee e e
9Andy Yen
Standard Model Background
The dominant Standard Model background process is:
The Z can decay into an electron-
positron pair leading to the exact same final state as the
qqee*e-e+γ process. Other backgrounds:
Z+jetttbar (846 pb)
Z+gamma reduced
with Z mass cut.
γqq Z
Andy Yen 10
Selecting qqee*e-e+γ events
Reconstructed electron, positron , and photon transverse momentum cut.
Simulated qqee*e-e+γ event
Andy Yen 11
Overall selection efficiency ranges from 50-70% depending on Me*
Reconstructing e* invariant mass
Identify e+/e- using a track requirement.
Both e+gamma possibilities are considered.
Correct combination contributes to peak, incorrect combo contributes to flat tails.
CMS gives very good Me* resolutions of 1-2% for all e* masses.
Better resolution for high Me* consistent with expected ECAL behavior.
Andy Yen 12
e* invariant mass distributions
Background relatively flat, the Me* peaks are the main features.
Lends itself to a bump hunting analysis approach.
For lower masses, good resolution of the CMS can significantly effect discovery potential.
At high mass (above 1 TeV), background becomes negligible.
Andy Yen 13
Luminosity = 1 fb-1
Statistical Analysis
If a peak in the reconstructed mass spectrum is observed, its significance will be evaluated based on the number of observed events compared to the number of expected background events in the region around the peak.
Window width of +- 4 σM Random fluctuations in the background can lead to
“accidental” peak-like structures in the spectrum. The standard peak significance of five sigma is required.
(probability of 2.9·10-7) The signal significance is given by
Also, we know
2 ln 1s
S s b sb
s ss L
Andy Yen 14
Results
Andy Yen 15
Solid line: theoretical σ for different values of Λ
Dotted line: σ required for a 5 sigma discovery for different
amounts of luminosity
CMS e* Discovery Potential
16Andy Yen
The CDF experiment at Tevatron has a 1.8 TeV center-of-mass energy compared to the 14 (10) TeV LHC.
17
Production of * at LHC (Yong Yang)
Contact interaction dominates the production of single * at LHC.
cteq5L
18
Decay of *
Decay to has high branching ratio for small value, my current analysis
Full decay channels of excited states implemented in CompHEP
Automatic computations from Lagrangians to events, Nucl. Instrum. Meth. A534 (2004) 250
12 decay channels
3 decay channels
ElectroWeak
Contact Interaction
19
* Search potential with CMS
(left) Measured cross section x branching ratio limit, compared to the contact interaction prediction for different choices of
(right) Expected discovery region in the plane of and M by present analysis
D0 380pb-1
5
Conclusion
These results represent the first excited lepton studies done at the CMS.
The results show that CMS is capable of extending the excited lepton search to regions of the theoretical parameter space far beyond those excluded at any previous experiment.
LHC physics runs are currently scheduled to commence in mid-2009 making 1 fb-1 of integrated luminosity possible by the end of 2009.
20Andy Yen