# Presentation: 720403

## Jet Measurements at  using a kT Algorithm

- Ursula Bassler
- LPNHE,Paris
- on behalf of the DØ collaboration

**Notes:**
I am going to present results on W and Z production at the Tevatron
I will start telling you why it is important to study Ws and Zs, followed by a description of how we did it at D0 during Run 1.
 I will concentrate in 4 new D0 results, two published in early 2000, and two just submitted for publication (papers 99 and 100).

## Outline

**          **Run IB (94-95)            Run 1C (95)                                            ~90 pb-1                              ~600 nb-1

**Outline**

- Introduction: k**T** jet algorithm @ D0
- ABS 350: **Inclusive Jet Cross Section**                             *Phys. Lett. B525  211, 2002*  *(hep-ex/0109041**)*
- ABS 407**: Sub-jet Multiplicities**
*     **Phys. Rev. D65 052008, 2002  (hep-ex/0108054)*
- ABS 421:** Thrust Cross Sections**                                                   *Preliminary*

## - Calorimeter

**- Calorimeter**

-  **Liquid Ar/Ur calorimeter:**
**high granularity, good hermeticity, quasi-compensated**

******ln [ tan (********/ 2) ]******
******< 4.2       ********int********total)**

-  **Transverse segmentation ****(towers****)**

- Electrons:**    ********E**** / E = 15% /**************
- Pions:**           ********E**** / E = 45% /**************

**Z**

**y**

**x**

****

- 

## Run I kT Algorithm 

-  Fixed cone: most DØ Run I results, all CDF results
-  kT -algorithm: New DØ Run I results:
** ****fewer split-merge ambiguities, **
** ****infrared safe to all orders in perturbation theory.**

*Ellis, Soper Phys. Rev. D48  3160, 1993   *
*Catani, Dokshitzer, Seymour, Webber Nucl. Phys B406  187, 1993*** **

**For each object and pair of objects:**

**Run I k****T**** Algorithm **

- If **d****min****=d****ij**
-  merge particles

**order all ****d****ii**** and ****d****ij****:**

- If  **d****min****=d****ii**
-  jet

**Inclusive Jet Cross Section**

** ****pQCD predictions, proton structure, quark compositeness**

**Stat Errors only**

**k****T**** algorithm (D=1)**

**Tot. Err = 14 (27)% at 60 (450) GeV**

**dominant**

## Comparison with Cone Result 

**k****T**** vs. cone agreement is reasonable; marginal at low P****T **
**NLO predictions: ********(k****T****, D=1) = ********(cone, R=0.7) within 1%         **
**the data is corrected back to particle level**** **
**error correlations are large point-to-point in p****T****, but largely uncorrelated between the two measurements.**

- Each result is compared to its own NLO prediction

## Hadronization effects

** ****k****T**** collects more energy**
** ****cone losses energy**

**Hadronization effects**

**k****T**** jets are 7 (3)% more energetic at 60 (200) GeV than cone jets:**
**consistent with HERWIG at high p****T****, at 2******** at low p****T****  **

**applying correction to cone-jets improves agreement between the 2 algorithms **

## Comparisons with pdf’s

| all data points |  |  |
| --- | --- | --- |
| pdf | 2(ndf=24) | prob(%) |
| CTEQ3M | 37.6 | 3.8 |
| CTEQ4M | 31.2 | 15 |
| CTEQ4HJ | 27.2 | 29 |

| pT> 100 GeV only |  |  |
| --- | --- | --- |
| pdf | 2(ndf=20) | prob(%) |
| CTEQ3M | 17.4 | 62.7 |
| CTEQ4M | 15.8 | 72.7 |
| CTEQ4HJ | 15.1 | 77.3 |

-  MRST: nearly constant offset
-  CTEQ4M: improved description at high pT
-  CTEQ4HJ: better 2, especially at high pT

## Multiplicity in Quark & Gluon Jets

-  **Test of QCD: difference between quark & gluon jets**
** ****ratio of color factors of gluons radiated from gluons/quarks = 9/4 **
** ****multiplicity of objects in gluon/quark jets at asymptotic limit**
** ****particles in a gluon jet are softer than in a quark jet**
** ****Separate quark from gluon jets: top, Higgs, W+Jets events**

- at fixed Jet ET quark/gluon jet contribution to the total x-section vary with s

-  measure the sub-jet multiplicity in quark and gluon jets

**Notes:**
D0 measures the subjet multiplicity in quark and gluon jets.
The goal of the analysis to separate jets produced from a quark parton from jets produced by a gluon parton. If we were able to separate  quark jets from gluon jets, this knowledge could then be used in various analyses (like top, higgs and W+Jet production). In itself the measurement can be used to test the dynamics of SU(3), for instance QCD color factors.
The first thing that needs to be done, is to select a quark enriched and a gluon enriched jet sample. We can then compare jet properties in the two samples. The real challenge is not to bias the jet properties by the sample selection.
This plot shows the contributions of the different initial states to the cross section as a function of jet Et for two different cme: 1800 and 630GeV. D0 compares jets with the same Et and eta produced at different cmes and assumes that the relative quark/gluon content is known.

## Sub-jets with the kT algorithm

-  re-run kT algorithm on all particles already assigned to a jet

- Cone jet

- kT jet

- ycut=10-3
- ycut=1: no splitting
- ycut=0: no merging

**Sub-jets with the k****T ****algorithm**

-  determine gluon jet fraction** ƒ****g**** **from MC:** **ƒ1800=0.59  ƒ630= 0.33 *(55<E**T**<100 GeV)*
-  sub-jet multiplicity defined as:** *****M****** = ******f******g******M******g****** ******+ ******(1-******f******g ******)M******q****** ***
-  assuming sub-jet multiplicity independent of s
-  extract gluon (***M******g***)/quark (***M******q***) sub-jet multiplicities from data at 1800 and 630 GeV

- D= 0.5

## Sub-jets in Quark & Gluon Jets

**Sub-jets in Quark & Gluon Jets**

**HERWIG:1.91**
**ALEPH (e****+****e****-****): 1.7 ****±****0.1**

**gluon jets have a higher sub-jet multiplicity as expected**
**good description by HERWIG **
**dominant uncertainties: **
**quark/gluon jet fraction, dependent on pdf**
** ****jet energy scale**
**result qualitatively in agreement with resummation calculation from Forshaw & Seymore**

**Notes:**
The method is as follows:
D0 will measure the subjet multiplicity that can be written as a linear combination of the gluon and quark multiplicity multiplied by the fraction of gluon and quark jets. The gluon jet fractions are obtained from theoretical predictions and are 33% at 630GeV and 59% for 1800GeV for a 50GeV Jet. Assuming that the subjet multiplicity is independent of cme (which is verified with MC), one can work out expressions for the quark and gluon observable that can be measured.
D0 uses the kT algorithm to do this analysis. Jets are unraveled until all subjets are separated by a relative kT greater than .1% of the total jet energy.
The result for the data before any corrections for detector effects is shown in this plot. The subjets multiplicity for gluon jets is shown in red, the one for quark jets is shown in blue. The average subjet multiplicity for quark jets is 2.28, and for gluon jets it is 3.08. 
After correcting for detector effects the ratio of multiplicities becomes 1.91+-0.04+0.23-0.19. The systematic uncertainty is dominated by the knowledge of the gluon jet fraction and the choice of minimum jet Et. 
The MC prediction for this ratio is 1.86+-0.08, in good agreement with our measurement.
This result shows that we are able to distinguish quarks from gluon jets on a statistical bases and we may be able to do it on a jet by jet basis in the future. 

## Event Shape Variable: Thrust

**Thrust characterizes sphericity of an event**

**Event shape variables allow to:**
** ****study spatial distribution of hadronic final states**
** ****test perturbative QCD, verify resummation calculations**
** ****extract ********s **

## Thrust at hadron colliders

**Thrust at hadron colliders**

**difficulties: ****underlying event, pile-up, multiple interactions **
** ****define thrust from 2 leading jets  in thrust T****2**
**thrust is not Lorentz invariant**
** ****introduce Transverse Thrust T****2****T**** computed from p****t**

## Dijet Transverse Thrust x-section

* ***Disagreement with JETRAD calculation in 2 regions:***** ***
*** *********2/2 ******** T****2****T ******** ********3/2:** LO calculation is *O*(s4), *O*(s3) not over whole range
** ****limit (1-T) « 1**  emission of soft and collinear gluons  logarithmic terms in ln(1-T)  resummation needed

## Summary

**comparison of the k****T**** x-section, cone x-section and NLO calculations at low p****t**** opened a discussion  on matters such us hadronization, underlying event and algorithm definition  **
**quark & gluon jets have a different structure, consistent with HERWIG predictions**
** ****thrust distributions offer an excellent opportunity to **** test the recently developed NLO 3-jet generators**
**Run 2 offers a unique opportunity to improve the understanding of jets in hadron colliders!**

**Summary**

## Systematic Uncertainties

- (2nd Bin)