1 Analysis of Prompt Diphoton Production at the Large Hadron Collider. Andy Yen Mentor: Harvey Newman Co-Mentors: Marat Gataullin, Vladimir Litvine California Institute of Technology SCCUR 2008 November 22, 2008 November 22, 2008

2 Andy Yen 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) Andy Yen 3

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 searches Andy 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 LHC Andy Yen 5

The Large Hadron Collider Andy Yen 6

7 Luminosity is proportional to the rate at which collisions occur. The LHC is scheduled to restart operation in early The Large Hadron Collider Andy Yen

The Compact Muon Solenoid 8 Andy Yen

A New Definition of “Compact” Andy Yen 9

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. Andy Yen 10

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. Andy Yen 11

Selection cuts Andy Yen 12 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 (σ ηη ) varies according to whether the photon candidate was formed by a real photon or a neutral meson. Andy Yen 13

Template Method  We form two templates, (σ ηη ) 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! Photon Jets Andy Yen 14

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 Expected0.81 ± ± Fit0.84 ± ± 0.02 Andy Yen 15

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. Andy Yen 16

(σ ηη ) Backup Covetaeta is defined as where and and the sum is over all crystals in a 5x5 array centered at the supercluster seed.