# Presentation: 826861

## Dijet Mass Distribution

- Robert M. Harris
- QCD Meeting
- Nov. 22, 2002

## Motivation & Strategy

- To search for new particles decaying to dijets, which appear as bumps in dijet mass distribution.
- Set limits on cross section & mass of new particles if no discovery.
- Example models are axigluons, q*, T, W’ ,Z’ , E6 Diquarks.
- Complete a physics result in time for Winter Conferences.
- Show that CDF is making progress understanding jets in run 2.
- Demonstrate run 2 is already more sensitive to new physics than run 1.
- Repeat run 1 analysis as closely as possible.
- We’ve done this before and can benefit from our run 1 experience
- References: PRD 55, R5263 (1997); PRL 74, 3538 (1995)
- Allows comparison of run 2 with run 1 data as a check.
- We calibrate the run 2 jet energy to the run 1 jet energy for expedience.

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass Analysis

- As in run1, use J20, J50, J70 & J100 triggers.
- 52 pb-1 of Frank Chlebana’s ntuples from Dec 2001 – Sep 2002.
- As in run 1, we apply the following cuts.
- | Z Vertex | < 60 cm  to insure cal towers project from vertex. Efficiency 94%.
- Missing ET/sqrt(ET) < 6.0 to eliminate cosmic rays.
- E < 2.2 TeV (2.0 TeV in run 1) to eliminate unphysical noise.
- Get the two leading jets (highest ET), with cone R=0.7, and correct the energy.
- Correction function from Bhatti & Flanagan applied to ntuple using code of G. Latino.
- Relative correction vs. detector  comes from dijet balancing in J20 sample.
- Absolute corrections for central responce, out-of-cone energy & underlying event from run 1.
- Apply E-scale corrections in same way as G. Latino.
- Increase CEM scale by 0.9% for average scintillator response over period (Nodulman).
- Increase CHA scale by 4% for run 2 / run 1 MIP peak.
- Calibrate jets using photon-jet balancing results in run 2 and run 1 from G. Latino.
- Increase jet energy by  4.41% +/- 0.50%. Completes “calibration” of run 2 jet energy to run 1.
- CDF6152 & http:/cdfsga.fnal.gov/internal/people/links/GiuseppeLatino/links/talk_11_13_02.ps.gz
- As in run 1, require each leading jet have || < 2,  |cos*| = | tanh([1-2]/2)| < 2/3.
- Reduces QCD background (t-channel) when searching for new particles (s-channel).
- As in run 1, define dijet mass M = sqrt(E2 –p2), where E=E1+E2, p=p1+p2 .

- Robert M. Harris, Fermilab CD/CDF

## Selection Cuts in J100 Sample

- z vertex cut is 94% efficient.
- Vertex strategy 1 algorithm fails on 2% of events and z=0.0 is assigned.
- Missing Et significance cut is crucial for elimination of cosmics rays
- Efficiency >> 99%
- Total energy cut for obvious junk.

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass Triggers

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass Trigger Efficiency

- Using same thresholds and mass bins as run 1, calculate cross section.
- Apply the run 2 luminosity, prescales, trigger efficiency and z vertex efficiency.

| Trigger | Mass Cut (GeV) | Efficiency at Threshold
Run 1A     Run 1B    Run 2 | Luminosity/Prescale (pb-1)
Run 1A    Run 1B    Run 2 |
| --- | --- | --- | --- |
| Jet 20 | 180 |     1                1            1 | 19.1/500   87.3/1000  51.7/240 |
| Jet 50 | 241 |     0.99           0.98       0.989 | 13.1/20     87.3/40      51.7/20 |
| Jet 70 | 292 |     0.95           0.96       0.997 | 19.1/6       87.3/8        51.7/8 |
| Jet 100 | 388 |     0.97           0.96       0.994 | 19.1/1       87.3/1        51.7/1 |

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass Distribution

- Robert M. Harris, Fermilab CD/CDF

## Angular Variables & Relative CorrectionsJ100 with M>388 GeV

- Robert M. Harris, Fermilab CD/CDF

## Miscellaneous VariablesJ100 with M>388 GeV

- Possible excess at
- 90 and 270 degrees
- is ~1% of total rate.

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass from Run 2 & Run 1

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass Ratio: Run 2 / Run 1

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass and Parametric Fit

- Robert M. Harris, Fermilab CD/CDF

## Dijet Mass Residuals: (Data – Fit) / Fit

- Robert M. Harris, Fermilab CD/CDF

## High Mass Dijet Event

- Run 151128 event 295868
- Dijet Mass = 1197 GeV (corr)
- cos * = 0.36
- z vertex = 54 cm

- J1 ET = 480 GeV (raw)
- = 561 GeV (corr)
- J1  = 0.42 (detector)
- = 0.20 (correct z)

- J2 ET = 471 GeV (raw)
- = 556 GeV (corr)
- J2  = -0.32 (detector)
- = -0.55 (correct z)

- Corrected ET and
- mass are preliminary

- Robert M. Harris, Fermilab CD/CDF

## Conclusions

- We have made a very preliminary measurement of the dijet mass distribution in run 2.
- The analysis was as close as possible to that in run 1.
- Same jet algorithm and jet correction procedure as run 1.
- Calibration of run 2 jet energy to run 1 using photon-jet balance.
**Any shift in energy scale from run 1 to run 2 should be corrected for.**
- Important check: The ratio of run 2 to run 1 dijet mass cross section agrees with LO QCD parton level prediction to a few percent.
- The run 2 to 1 calibration procedure works to within ~1% in energy.
- The increase of 4.41% to the run 2 jet energy from G. Latino was needed.
- We also used 0.9% CEM, 4% CHA, run 2 relative & run 1 absolute corrections.
- Run 2 dijet mass distribution is smooth.
- Well fit by the same parametric function used in run 1.
- Next: Fit for bumps and set preliminary limits on new physics.

- Robert M. Harris, Fermilab CD/CDF