# Presentation: 250604

## CMS Sensitivity to Dijet Resonances

Robert M. Harris 	(Fermilab)
Kazim Gumus and Nural Akchurin (Texas Tech)
Selda Esen	 (Brown)

DPF Meeting 
 November 1, 2006

- CMS Sensitivity to Dijet Resonances
- Robert M. Harris 	(Fermilab)
- Kazim Gumus and Nural Akchurin (Texas Tech)
- Selda Esen	 (Brown)
- DPF Meeting
-  November 1, 2006

## Introduction

- This study performed at the LHC Physics Center at Fermilab (LPC)
- A center of CMS software and analysis expertise in the U.S.
- Where students and postdocs learn CMS analysis from Tevatron experts.
- Publicly available as CMS Note 2006 / 070
- http://cms.cern.ch/iCMS/jsp/openfile.jsp?type=NOTE&year=2006&files=NOTE2006_070.pdf
- Ph.D thesis of Kazim Gumus at Texas Tech University.
- Uses a jet trigger proposed in CMS Note 2006 / 069
- http://cms.cern.ch/iCMS/jsp/openfile.jsp?type=NOTE&year=2006&files=NOTE2006_069.pdf
- Part of the Ph.D thesis of Selda Esen now at Brown.
- She talked earlier today on “CMS sensitivity to contact interactions using dijets”.
- Analysis
- Estimate 5 discovery and 95% CL exclusion sensitivity for dijet resonances.
- √s=14 TeV and ∫Luminosity of 100 pb-1 (2008?), 1 fb-1 (2009?), 10 fb-1 (2011?).
- Uses a full simulation of signal and background in the CMS detector.

- Robert Harris, Fermilab

## Motivation

- Theoretical Motivation
- Dijet resonances are in **many models** that address important questions:
- Why Flavor ?   Technicolor or Topcolor  Octet Technirho or Coloron
- Why Generations ?  Compositeness  Excited Quarks
- Why So Many Forces ?  Grand Unified Theory  W’ & Z’
- Can we include Gravity ?   Superstrings   E6 Diquarks
- Why is Gravity Weak ?  Extra Dimensions  Randall-Sundrum Gravitions
- Experimental Motivation
- LHC is a parton-parton resonance factory in a previously unexplored region.
- We search for generic narrow dijet resonances, not specific models.
- Nature may surprise us with unexpected new particles. It wouldn’t be the first time ...

- Dijet Resonances
- S-channel parton-parton resonances.
- Observe two high pT jets: Dijets.
- Dijet mass forms narrow resonance peak.

- Robert Harris, Fermilab

## Dijet Analysis 

- Jets are reconstructed using a cone algorithm
- Energy inside a circle of radius R centered on jet axis is summed:

****

****

- Dijet is two highest pT jets.
- Require each jet to have |  | < 1
- Reduces t-channel QCD background.
- Dijet mass
- Jet E and p are corrected for
- Calorimeter non-linear response
- Pile-up of extra soft proton-proton collisions on top of our dijet event
- Correction varies from 33% at pT= 75 GeV to 7% at  pT =2.8 TeV.

****

****

****

**Jet **
**2**

**Jet **
**1**

****

**BARREL**

**ENDCAP**

**ENDCAP**

- Robert Harris, Fermilab

## Narrow Resonance Shape in CMS

**Model narrow resonance line shape at CMS with Z’ Simulation. **
**All resonances with a width less than our resolution look like this in CMS.**
**Corrected dijet mass peaks around generated  value **
**Gaussian core with resolution  **
**Long tail to low mass comes mainly from QCD radiation.**
**Data here is in bins equal to the measured mass resolution above.**

- Robert Harris, Fermilab

## Signal and Background

- QCD cross section falls smoothly as a function of dijet mass.
- Resonances produce mass bumps we can see if xsec is big enough.

- Robert Harris, Fermilab

## Signal / QCD 

- Many resonances give obvious signals above the QCD error bars
- Resonances produced via color force
- Excited Quark (shown)
- Axigluon
- Coloron
- Color Octet T
- Resonances produced from valence quarks of each proton
- E6 Diquark (shown)
- Others may be at the edge of our sensitivity.

- Robert Harris, Fermilab

## Statistical Sensitivity to Dijet Resonances

- Sensitivity estimates
- Statistical likelihoods done for both discovery and exclusion
- 5 Discovery
- We see a resonance with 5 significance
- Likelihood found from a sample that has a large signal.
- 95% CL Exclusion
- We don’t see anything but QCD at 95% CL.
- Likelihood found from a sample with QCD only.
- Plots show resonances at 5 and 95% CL
- Compared to statistical error bars from QCD.

**5 TeV**

**2 TeV**

**0.7 TeV**

**2 TeV**

**0.7 TeV**

- Robert Harris, Fermilab

## Systematic Uncertainties

- We include all these systematic uncertainties in our sensitivity estimates.

- Uncertainty on QCD Background
- Dominated by jet energy uncertainty (±5%) which produces rate uncertainty.
** ****Background will be measured.**
- Trigger prescale edge effect
- Jet energy uncertainty has large effect at mass values just above where trigger prescale changes.
- Resolution Effect on Resonance Shape
- Bounded by difference between particle level jets and calorimeter level jets.
- Radiation effect on Resonance Shape
- Long tail to low mass which comes mainly from final state radiation.
- Luminosity

- Robert Harris, Fermilab

## Sensitivity to Resonance Cross Section

- Cross Section for Discovery or Exclusion
- Shown here for 1 fb-1
- Also for 100 pb-1, 10 fb-1
- Compared to cross section for 8 models
- CMS expects to have sufficient sensitivity to
-  Discover with 5 significance any model above solid black curve
- Exclude with 95% CL any model above the dashed black curve.
- Can discover resonances produced via color force, or from valence quarks.

- Robert Harris, Fermilab

## Sensitivity To Resonance Models

**CMS can discover the strongly produced models up to many TeV.**
**Discoveries with only 100 pb****-1 ****!**

- Robert Harris, Fermilab

## Conclusions

- CMS will perform a generic search for narrow dijet resonances
- We’ve presented sensitivity estimates for 100 pb-1, 1 fb-1 and 10 fb-1
- CMS capability to discover (5) or exclude (95% CL) resonances.
- We can discover some models up to many TeV.
- Axigluon, Coloron, Excited Quark, Color Octet Technirho or E6 Diquark
- Produced via the color force or from the valence quarks of each proton.
- Sensitivity to any new narrow resonance can be determined.
- Compare the model’s cross section for ||<1 to results in our CMS Note.
- Theorists can compare their latest ideas with CMS sensitivity estimates.
- We look forward to the discovery of new physics at the LHC.

- Robert Harris, Fermilab