# Presentation: 908209

## Energy Flow Within Jets

**Dan Green**
**Fermilab**
**July, 2002**

## MLLA - Theory

**QCD in LLA predicts the x distribution of partons. LPD then gives hadron distribution in x in terms of basically 1 parameter. The value of Q is taken from CDF fits to data. The theory appears in J. Mod Phys. A 1875 (1992) and Z Phys C, 55, 107 (1992). The CDF fits appear in Fermilab – Pub – 02/096-E (June 2002). **

## MLLA - Numerical

**Numerical results were calculated. <n> increases rapidly with cone angle up to ~ 0.8. We expect ~ 30 hadrons in the jet at M=700 GeV., ~ 12 at M = 120 GeV. We can use D(z,M) as a guide to pileup in extracting jets. Basically angular ordering means that soft jet fragments occur at wide angles w.r.t. the parton axis.**

## Z’(120) Clustering at Low L

**20 seeds > 1 GeV. 8 cones, R = 0.9, 8 jets, Rsep = 0.9. 27, 19 clusters (Et > 0.5 GeV in 2 highest Et jets). Find all seeds, then attach all clusters to cones, Rc, centered on seeds. Finally merge cones within Rsep. Etj1 = 49 GeV, Etj2 = 37.9 GeV, Mjj = 85 GeV**

## Z’(120) at Design L

**Clearly, towers are still sparsely populated. Equally clearly, we need to work harder to preserve the dijet mass resolution.**

## Seeds, Clusters, Cones

**<Ns> = 30.8/event**
**<Nc> = 9.6/event**
**<Nj> = 9.3/event**
**Ets> 1.0 GeV, Etc > 0.25 GeV, Rc = 0.9, Rsep = 0.9.**
**There are many jets if seed threshold is made low.**

## Leading Jet

**<Ns>j1 = 21 = # clusters in lead jet**
**<Et>sj1 = 25.0 GeV = Et of seed cluster in lead jet**
**<Et>j1 = 55 GeV ( 45.0 GeV for j2)**

## Pileup in Z’(120) at Low L

**Use log(x) as the appropriate variable (MLLA). The jet shows the small angle, high momentum component. A cone at 90 degrees to the jet shows soft and wide angle underlying event and pileup behavior. Cuts are picked for high L.**
** ****The sigma/mean is 17.8% at low L.**

## Contours in Z’(120)

**Estimate the true contour in (logx,dR) for a jet by subtracting the cones contour at 90 degrees to the jet for Z’(120) at low L. The pileup and the jet populate different regions of the energy-angle flow phase space.**

## Pileup Subtraction

**Must go to cone radius ~ 0.6 to capture all the jet fragments on average. However, the number of clusters is ~ 29 then (10.3 real + 18.3 pileup) and the mean dijet mass is 258 GeV**

## Z’(120) at Design L

**After cuts on dR and logx the fitted mean is 119 GeV with a fitted sigma = 20.26 GeV or sigma/mean = 17.0 %. Thus the dijet mass resolution is maintained at design L. There is some residual high mass tail, however.**

## Z’(700) at Design L

## Z’(700) at Design L

**Seed Et > 10 GeV, <Ns> = 10.4 , <ETs1> = 198 GeV (leading seed cluster).**
**Cone Rc = 0.8, <Nc> = 3.33**
**Jet – Merge Cones, <Nj> = 2.72**
**Clusters in Jet - <Ncl> = 55.5**
**Clusters at 90 degrees to jet, <Nclo> = 36.4. Difference = 19.1 (expect more in LLA, but there may be cluster merging at high density within the jet).**
**<Etj1> = 352.2 GeV**

## Clustering

**For Z’(120) there are 24 clusters, Rc = 0.9,  within the jet and 10 at 90 degrees – leading to 14 after subtraction. This is close to MLLA estimates.  For Z’(700) at design L, the number of clusters within a cone is much larger due to pileup, 55.8.  For 1/5 luminosity it is 21 clusters. For design luminosity it is ~ 56 – 36 clusters or ~ 19.**

## dR and log(x) for Z’(700)

**Clearly the pileup has dNdR~R (area), while the jet shows a peak at R ~ 0.2. The pileup is al low x, while there is a peak at high x.**

## Pileup Subtracted (logx,dR) Jet contours

**For both Z’(120) at low L and for Z’(700) at design L there is a core of the jet at small dR and large x.**

## Fake Jets at Design L

**Look at fake jets by taking cone at 90 degrees to jet at design L and normalizing x now to total Et within that cone. There is no jet “core” of high momentum at small angles. **

## Dijet Mass in Z’(700)

**Mean ~ 614 GeV, sigma ~ 74 GeV. No major improvements.**
**Rc = 0.4 or log(x) > -4 or a combined cut all about the same.**

## Conclusions

**The pileup at design luminosity is not too severe for Z’(700).**
**The appearance of a low Pt inclusive pileup ~ uniform in cone area is confirmed.**
**Pileup for Z’(120) is severe but it can be alleviated with well designed cuts on energy – angle flow within the jet.More incisive cuts than just a small cone or a threshold work well.**
**Direct subtraction event by event does not work well.**