1. 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. 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.

3. 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

4. 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

5. 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

6. The Large Hadron Collider Andy Yen 6

7. 7 Luminosity is proportional to the rate at which collisions occur. The LHC is scheduled to restart operation in early 2009. The Large Hadron Collider Andy Yen

8. The Compact Muon Solenoid 8 Andy Yen

9. A New Definition of “Compact” Andy Yen 9

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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 ± 0.013 0.19 ± 0.004 Fit0.84 ± 0.03 0.16 ± 0.02 Andy Yen 15

16. 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

17. (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.