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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.

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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

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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?

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• 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

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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.

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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.

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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.

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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

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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

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Selecting qqee*e-e+γ events

Reconstructed electron, positron , and photon transverse momentum cut.

Simulated qqee*e-e+γ event

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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.

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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.

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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

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Results

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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

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The CDF experiment at Tevatron has a 1.8 TeV center-of-mass energy compared to the 14 (10) TeV LHC.

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Production of * at LHC (Yong Yang)

Contact interaction dominates the production of single * at LHC.

cteq5L

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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

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* 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

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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.

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