# Presentation: 422151

## CMS Sensitivity to Quark Contact Interactions with Dijets

- Selda Esen (Brown)
- Robert M. Harris (Fermilab)
- DPF Meeting
- Nov 1, 2006

## Quark Contact Interactions

- New physics at a scale  above the observed dijet mass is effectively modeled as a contact interaction.
- Quark compositeness.
- New interactions from massive particles exchanged among partons.
- Contact interactions sometimes look different than QCD.
- QCD is predominantly t-channel

**Quark Contact Interaction**

****

**M ~******

**Quark Compositeness**

**New Interactions**

**M ~ ******

**Dijet Mass << ******

**q**

**q**

**q**

**q**

**q**

**q**

**q**

**q**

## Contact Interactions in Mass Distribution 

- Contact interaction produces rise in rate relative to QCD at high mass.
- Observation in mass distribution alone  requires precise understanding of QCD cross section.
- Hard to do
- Jet energy uncertainties give large cross section uncertainties
- Parton distribution uncertainties are significant at high mass = high x and Q2.

## Contact Interactions in Angular Distribution

- Contact interaction is often more isotropic than QCD.
- For example, the standard contact interaction among left-handed quarks introduced by Eichten, Lane and Peskin (Phys. Rev. Lett. 50, 811-814 (1983)).
- Angular distribution has much smaller systematic uncertainties than cross  section vs. dijet mass.
- We use a simple single measure (one number) for the angular distribution as a function of dijet mass.
- See the effect emerge at high mass.

## The CMS Detector and Jets

**Hadronic**

**Electromagnetic**

**CMS**
**Calorimeters**

- Measure jets in the CMS Calorimeter
- Energy inside a cone of radius R centered on jet axis

- Jet energy calibrated to equal the energy of particles in jet cone
- Dijet is the two leading jets in the event
- Two jets with highest transverse momentum, PT.
- Dijet mass m is

## Sensitive Variable for Contact Interactions

- Dijet Ratio =
-      N(||<0.5) / N(0.5<||<1)
- Number of events in which each leading jet has ||<0.5, divided by the number in which each leading jet has 0.5<||<1.0
- Dijet Ratio is the variable we use
- Simple measure of the most sensitive part of the angular distribution.
- We measure it as a function of mass.
- It was first introduced by D0 (hep-ex/980714).
- We will show systematics on the dijet ratio are small.

**Our lowest order calculation of both signal**
**and background.**

## Dijet Ratio and Statistical Uncertainty(Smoothed CMS Simulation)

- CMS simulation of  QCD is flat at 0.6
- Error bars are the expected statistical uncertainty for 100 pb-1, 1 fb-1 and 10 fb-1
- This uses expected jet triggers and prescales (CMS PTDR vol. II, 2006)
- We compare to the contact interaction signal for =5, 10 & 15 TeV
- Calculate 2 for significance estimates.

## Dijet Ratio and Systematic Uncertainty

- Systematics are small
- Because they cancel out in the ratio.
- Upper plot shows systematics & statistics.
- Lower plot shows zoomed vertical scale.
- Absolute Jet Energy Scale
- No effect on QCD dijet ratio: flat vs. dijet mass.
- Causes 5% uncertainty in . (included)
- Relative Energy Scale
- Energy scale in ||<0.5 vs. 0.5 < || < 1.
- Estimate **+/- 0.5 %** is achievable in Barrel.
- Changes ratio between **+/-.01** and **+/-.03.**
- Resolution
- No change to ratio when changing resolution
- Systematics bounded by MC statistics: **0.02.**
- Parton Distributions
- We’ve used CTEQ6.1 uncertainties.
- Systematics on ratio less than 0.02.

## Significance of Contact Interaction Signal

- Significance found from 2
- 5 Discovery
- 95% CL Exclusion
- Effect of dijet ratio systematics on the significance is small.

## Sensitivity to Contact Interactions

| Left-Handed
Quark Contact
Interaction |  | + for
100 pb-1
(TeV) | + for 
1 fb-1
(TeV) | + for
 10 fb-1
(TeV) |
| --- | --- | --- | --- | --- |
| 
95% CL
Exclusion | Stat Only | 6.4 | 10.6 | 15.1 |
|  | All Sys | 6.2 | 10.4 | 14.8 |
| 5σ 
Discovery | Stat Only | 4.7 | 8.0 | 12.2 |
|  | All Sys | 4.7 | 7.8 | 12.0 |

- Numbers in **red** are our  sensitivity estimates.
- Published Limit from D0:+ > 2.7 TeV at 95% CL (hep-ex/980714).

## Conclusions

- CMS plans to use the dijet ratio to search for quark contact interactions
- We’ve estimated the signal and QCD background from 0.3 to 6.5 TeV
- From jet triggers for 100 pb-1, 1 fb-1 and 10 fb-1
- Using the full CMS detector simulation.
- We have presented CMS 95% CL exclusion and 5discovery sensitivity to a quark contact interaction.
- Including both statistical and systematic uncertainties.
- CMS can discover a quark contact interaction  = 12 TeV  with 10 fb-1.
- Corresponds to a quark radius of order 10-18 cm if quarks are composite.