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Analysis of Prompt Diphoton Production at the Large Hadron Collider.

Andy YenAndy Yen Mentor: Harvey NewmanMentor: Harvey Newman

Co-Mentors: Marat Gataullin, Vladimir LitvineCo-Mentors: Marat Gataullin, Vladimir LitvineCalifornia Institute of TechnologyCalifornia Institute of Technology

SCCUR 2008SCCUR 2008 November 22, 2008November 22, 2008

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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 which have all been shown to exist.

One missing particle, the Higgs boson.

What is Prompt Diphoton Production?

Prompt Diphoton Production refers primarily to two processes

1. quark-antiquark collisions

(also known as the born process)

2. gluon-gluon collisions

(also known as the box process)

qq γγ

gg γγ

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Theoretical and Experimental Motivations

A good cross check for the accuracy of perturbative QCD predictions.

Many physically interesting processes involve a final state.

Examples include:• Heavy Graviton Decays• Extra Spatial Dimensions• Higgs searches

Discovery is only possible if a statistically significant signal is seen above the Standard Model background.

Understanding the prompt diphoton production rates allows us to estimate the level of background.

Particularly important for Higgs searchesAndy Yen 4

CMS Ecal TDR

What is the Higgs?

The Higgs is a hypothetical massive spin-0 boson.

Predicted to exist by the Standard Model.Experiments at the Tevatron and LEP

colliders have established the upper limit of the Higgs at 154 GeV with 95% confidence.

The Higgs would provide an explanation for the spontaneous breaking of electroweak gauge symmetry, a phenomena known as the Higgs mechanism.

This Higgs mechanism is the process through which elementary particles acquire mass in the Standard Model.

Higgs has already been spotted at LHCAndy Yen 5

The Large Hadron Collider

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Luminosity is proportional to the rate at which collisions occur.

The LHC is scheduled to restart operation in early 2009.

The Large Hadron Collider

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The Compact Muon Solenoid

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A New Definition of “Compact”

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Selecting Prompt Diphoton Events

Simulated data was produced using Monte Carlo methods.

Simulated data is used to develop a selection algorithm which can then be run over real data.

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Background from Jets

A jet is a narrow cone of hadrons and other particles, typically charged.

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.

A fake diphoton event can consist of either one or two jet(s) misidentified as a photon.

The rates of these fake events are much higher compared to the born/box processes.

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Selection cuts

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ECAL Isolation = amount of energy deposited in the ECAL in the vicinity of a photon candidate

H/E = Energy deposited in the HCAL divided by energy deposited in the ECAL

Separating Signal from Background

Invariant mass distribution is similar for prompt diphoton signal and jet fake background.

Need to develop a method of separating the diphoton signal from background.

A solution is ECAL electromagnetic shower profiles.

The energies of ECAL crystals in a 5x5 array around the photon candidate is weighted with the position of the crystal.

The resulting variable (σηη) varies according to whether the photon candidate was formed by a real photon or a neutral meson.

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Template Method

We form two templates, (σηη) for real photons and jet fakes.

There is now a prompt diphoton template and jet fake background template.

We can now fit the signal and background templates to the covariance eta eta distribution of the data using a linear log-likelihood fit.

Fit coefficients give us the signal to background ratio in the data!

PhotonJets

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Results

Template method is tested using a “data” template generated using Monte Carlo data where the signal and background fractions are known.

Template method is applied to determine the prompt diphoton fraction.

Results indicate template method using covariance eta eta works.

Viable method for isolating prompt diphoton signal.

Photon Fraction

Jet Fraction

Expected 0.81 ±

0.013

0.19 ±

0.004

Fit 0.84 ±

0.03

0.16 ±

0.02

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Conclusion

Successfully developed a selection algorithm which is capable of selecting out real diphoton events while rejecting the majority of jet fakes.

The results demonstrate that using a template based method can yield accurate results.

This is the first realistic study of prompt diphoton production at the CMS.

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(σηη) Backup

Covetaeta is defined as

where and

and the sum is over all crystals in a 5x5 array centered at the supercluster seed.

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