# Presentation: 174944

## QCD Results at CDF

**Andrey Korytov**
**(for CDF Collaboration)**

**CDF**

**D0**

**1.2 fb****-1**** on tape**

**p-pbar collisions**
**sqrt(s) = 1.96 TeV**
**peak L = 10****32**** cm****-2****s****-1**

## QCD Results at CDF in 12 min?

**	****High P****T**** QCD**
**observable: **_**inclusive jet production**_
**theory: exact NLO pQCD ME**
**	**
**	**
**	****Low P****T**** QCD**
**observable: **_**long-range momentum correlations of particles in jets**_
**theory: NLLA pQCD resummation**

## Inclusive Jet Production: Run I legacy

**	****Run I**
**Cone jet finding algorithm**
**Apparent excess at high PT, but within the overall systematic errors**
**	****Between Run I and Run II**
**Machinery for improved jet finding algorithms:**
**	****MidPoint Cone Algorithm**
- infrared safe
**	****kT Algorithm:**
- infrared and collinear safe
- no ad hoc splitting/merging of jets in experiment and theory
- might be more sensitive to UE
**PDFs are further tuned:**
**	****Excess is gone...**

## Run II Inclusive Jets: kT vs MidPoint

**   ****Jet finding algorithms**
**left: kT (D=0.7)**
**right: MidPoint (R=0.7)**
**both for central jets only: 0.1<|Y|<0.7**
**   ****Comparison to NLO:**
**both agree with NLO and have similar patterns in Data/Theory**
**  ****UE+Had Corrections:**
**UE+Hadronization are phenomenological models, not a theory!**
**matter only for P****T****<100**
**k****T**** algorithm is twice more sensitive**

**    ****kT jet-finding algorithm                                         MidPoint Cone algorithm**

## What do we really measure? (I)

**What do we really measure? (I)**

**   ****Pick two partons and their momenta**
**phenomenological parton density functions, PDF**
**   ****Hard Scattering: 2 ******** X**
**pQCD exact matrix element at LO, some at NLO,...**
**   ****Soft final state radiation**
**pQCD approximate resummation in all orders: LLA (leading log approximation), NLLA**
**   ****Underlying event**
**phenomenological models**
**   ****Hadronization**
**phenomenological models**
**   ****Calorimeter response**
**electromagnetic shower for photons**
**hadronic shower for “stable” hadrons**
**   ****Jet identification**
**jet finding algorithms**
**   ****Instrumental corrections**

## What do we really measure? (II)

**What do we really measure? (II)**

** ****PDF**** ****+**** ****pQCD ME**** ****+**** ****pQCD Approximation**** ****+**** ****UE + Had + Algo ****= Meas’ment**
**	****How much do we really learn about QCD?**
**	****Can we miss new physics by tuning PDFs, UE, Hadr. models, ...?**
**	****The only way to know is to study many more observables ****	****        to completely constrain phenomenological models                          (while keeping number of parameters constrained):**
**jets in different angular domains**
**multijet observables (e.g., dijet mass, dijet *********, etc.)**
**vector boson production (********, Z, W with n jets)**
**jet fragmentation**
**underlying event (average, explicit multiparton interactions)**
**...**

## Run II: forward jets (kT algorithm)

**All self-consistent!**

**0.7<|Y|<1.1**

**1.1<|Y|<1.6**

**1.6<|Y|<2.1**

## Jet fragmentation: Run I legacy

***=ln(E**jet**/p**particle**)*

*dN/**** per jet*

- 0       1       2      3       4       5       6      7      8

*Ratio = N**g-jet** / N**q-jet*

*Q = E**jet **** ****cone*

**	****Momentum distribution of charged hadrons in jets:**
**well described by MLLA**
** ****dijet mass range 80-600 GeV**
** ****cutoff Q****eff****=230 ******** 40 MeV**
** ****N********hadrons****/N****partons**** = 0.56 ******** 0.10**
**	****Ratio of charged hadron multiplicities in gluon and quark jets:**
**agrees with NNLLA**
** ****ratio = 1.6 ******** 0.2**
**	****Implications:**
**pQCD calculations carried out down  to scale Q~********QCD****~200 MeV**
**number of hadrons and their momenta well match those of partons**

## Run II: two-particle momentum correlations

**consider all particle pairs in cone ********=0.5 around jet axis**
******=ln(E****jet****/p****particle****)**
**theory:**

- =-0

**normalized to unity**

- c0 is always >1
- c1 is always positive
- c2 is always negative

## Run II: momentum correlations (cont’d)

**hadron correlations follow the pattern expected for partons**

## Run II: momentum correlations (cont’d)

**fit to c****1**** and c****2**** coefficients gives Q****eff**** ~ 140 ******** 80 MeV and       provides further support for local parton-hadron duality hypothesis**

## Summary

**	****QCD physics **
**very reach and still holds a challenge for experimentalists and theorists**
**better understanding of QCD is vital for discovering new physics:**
** ****new physics is likely to be born in a QCD processes**
** ****QCD-driven processes are often the dominant backgrounds**
**	****High P****T**** QCD: **
**no striking data.vs.theory discrepancies within errors**
**~10% precision remains a benchmark precision for both experimental and theoretical uncertainties**
**	****Low P****T**** QCD: **
**when resummation is possible, the domain of pQCD apparently can be pushed down as low as ~200 MeV**
**largely continues to be *****terra incognita***