# Presentation: 922929

## Soft QCD Phenomena in High-ET Jet Events at Tevatron

_**Andrey Korytov**_

**for CDF Collaboration**

## Dissecting a High-ET Jet Event

** ****hard scattered partons**
** ****final state radiation**
** ****hadronization**
_**UNDERLYING EVENT (UE)**_
** ****initial state radiation (IR)**
** ****multi-parton interactions (MPI)**
** ****proton/antiproton remnants**
** ****hadronization**
****_**ALL ENTANGLED WITH COLOR CONNECTIONS (!)**_

**Dissecting a High-E****T**** Jet Event**

**Soft QCD Phenomena in High-E****T ****Jet Events at Tevatron**

_**JET FRAGMENTATION**_

## Jet Fragmentation

**Jet Fragmentation**
**Driven by soft QCD (k****T****<<1 GeV)    ******
**Why bother?**
**Good stuff:**
**Lots of phenomena**
**Theoretical challenge (pQCD at k****T****~********QCD****)**
**Hadronization stage remains a mystery**
**Many high-E****T**** physics analyses depend on good understanding of jet properties **
**Tools available **
**Re-summed pQCD approximations **
**analytic, but only for a few observables**
**Monte Carlo generators **
**generic, but many tuning knobs**

**k****T**

## Underlying Event

**Underlying event processes**
**IR, MPI, pp remnants, hadronization**
**all are very soft**
**Why bother?**
**Physics is poorly understood: **
**Implementations in MC’s are very different**
**Default predictions for LHC vary wildly        ******
**UE pollutes many analyses and is source of systematic errors**
**Tools available **
**any analytic predictions? **
**hardly any... (color flow away from jets)**
**Monte Carlo generators **
**generic, but many (too many?)  tuning knobs**

_**LHC:**_
****** ****25 events per bunch crossing**
****** ****Jet cone pollution (R=0.7):**
** ****15 GeV**
** ****10 charged particles**
**Different MC’s disagree by more than a factor of 2**

## Results presented in this talk

_**Jets Fragmentation    **__****__**    NLLA**_
**Momentum distribution of charged particles in jets**
**Multiplicities of charged particles in g- and q-jets ****	**

_**Underlying Event      **__****__**      MC**_
**Energy flow in transverse direction away from a jet****	**
**Charged particle multiplicity flow away from a jet****	**
**Momenta of away-from-jet charged particles****	****	**

## Jet fragmentation: doing it analytically

**Parton shower development: NLL**
**Modified Leading Log Approximation**
**	****with one k****T****-cutoff parameter: k****T**** > Q****cutoff****=********QCD ****(=Q****eff****)**
**Various next-to-NLL extensions**
**Hadronization: LPHD**
**Hypothesis of Local Parton Hadron Duality                                          with one parameter K****LPHD****=N****hadrons****/N****partons**
**MLLA+LPHD**
**cannot describe all details...**
**but all analytical...**
**and works surprisingly well...**

## Jets: particle momentum spectra

_**Momenta of charged particles in jets**_
**(MLLA+LPHD, next-to-NLL normalization corrections)**
_**Measurement**_
**central dijet events with M****jj**** ~ 80-600 GeV**
**di-jet center of mass frame: E****jet**** = 1⁄2M****jj**
**charged particles in cones with opening angle ******** from ~0.3 to ~0.5 rad**
**energy scale Q = 2E****jet****tan(********/2) ******** E****jet******
_**Results **__**	**__**	**__**	**__**          **__****_
**Phys. Rev. D 68, 012003 (2003)**
**Q****eff**** = 240 ******** 40 MeV**
**	****k****T****-cutoff can be set as low as ********QCD**
**K****LPHD(********)**** = 0.56 ******** 0.10**
**	****number of hadrons ******** number of partons**

## Qeff at CDF vs Other Experiments

## Jets: multiplicities in quark & gluon jets

**Jets: multiplicities in quark & gluon jets**

**C****A ****/C****F ****=9/4**

**Ratio r = N****ch****(gluon jet) / N****ch****(quark jet)**

## Jets: multiplicities in quark & gluon jets

**Theory:**
**Next-Leading-Log extensions**
**r = N****ch****(gluon jet) / N****ch****(quark jet) **
**       ****= 1.5-1.8 (Q=20-100 GeV)**
- e+e-** Data:**
**15+ papers (mostly last 10 yrs)**
**Results range from 1.0 to 1.6**
**Diversity of results:**
**biases due to using events of different topologies (qqg and qq)**
**energy scale confusion**
**“****catch-22” analyses**
**model-dependent analyses**
**recent unbiased/model-independent**** **

## Gluon vs Quark jets at Tevatron

**Dijet and ********-jet events with M****jj**** and M********j ****~80-100 GeV**
**tracks are not used in jet reconstruction**
**di-jet events (~60% gluon jets) and ********-jet events (~80% quark jets)**
**very stable for the energies used**
**di-jet or ********-jet center of mass frame: E****jet**** = 1⁄2M****jj**** or 1⁄2M********j**
**N****ch**** multiplicity in cones with opening angle ******** from ~0.3 to ~0.5 rad**
**Energy scale Q = 2E****jet****tan(********/2) ******** E****jet********                 **

**Leading diagrams for dijet events**

**Leading diagrams for ********-jet events**

## Nch multiplicities in gluon and quark jets

**Tevatron and ****e****+****e****-**** data (except for CLEO) agree **
**Tevatron and ****e****+****e****-**** data (except for CLEO) follow 3NLL trends**

## Nch multiplicities in gluon and quark jets

**Tevatron: Ratio r=1.6********0.2 for typical Q********20 GeV**
**Tevatron and recent OPAL “unbiased” data (r=1.51********0.04 at Q=80 GeV) agree **
**and follow trends obtained in the recent NLL extensions**
**CLEO “unbiased” results at small Q~7 GeV fall out...**

## Underlying Event (UE)

**Event sample: **
**min-bias**
**central jet events**
**In direction transverse to the leading jet, we measure:**
**n = dN / dA****	****    -- average density of charged particles: ****	**
**dn / dp****T****          -- p****T ****spectrum of charged particles:         ****	**
**dP****T    ****/ d********        -- average density of energy flow (summed over charged particles)**
**                       ****-- compare sides with maximum and minimum activities**
**Confront Monte Carlo with Data:**
**identify relevant importance of various Monte Carlo knobs/parameters**

**“****transverse” charged particles**
**as a probe of the underlying event**
**dA=d********d******

**sum**

## UE: Default Pythia 6.206 fails to reproduce data

**Number of charged tracks in transverse direction is underestimated**
**P****T**** distribution of charged tracks in transverse direction is OK**
**Average energy flow in transverse direction is no good**

## UE: Default Herwig 6.4 fails to reproduce data

**Number of charged tracks in transverse direction is OK**
**No (or too soft) tail in the spectrum for particles in transverse direction   in minbias events and events with small E****T**** jets **
**	******** ****missing multi-parton interaction being added...**
**Average energy flow in transverse direction is more or less OK**

## UE: Tune Pythia to match CDF data

**      **_**CDF Tune A    vs.    Pythia 6.206 default parameters**_
**Enhance initial state radiation**
**from PARP(67) =1    to    4**
**Smooth out probability of Multi-Parton Interactions vs. impact parameter**
**from solid opaque disk to 50-50 double-Gaussian distribution (r=0.4R)**
**Make more di-gluons than q-qbar pairs in Multi-Parton Interactions **
**from 66% to 95%**
**Enhance color connection probability for MPI gluons and p-pbar remnants**
**from 33% to 90%**
**Reduce Multi-Parton Interaction dependence on CM Energy**
**to better connect Tevatron and lower energy data**
**this approximately halves the LHC underlying event activity...**

## UE: Pythia Tune A describes Data

**   ****Number of Charged Particles n=dN/dA      **

**sum**

## UE: Pythia Tune A describes Data

**          ****Charged Energy Flow dP****T     ****/ dA             **

**sum**

## UE: Pythia Tune A describes Data

**    ****Number of Charged Particles n=dN/dA      **

**          ****Charged Energy Flow dP****T     ****/ dA           **

**sum**

**       ****P****T**** spectrum of charged particles dn/dP****T      **

##  Summary (Soft QCD in High-ET Jet Events...)

**Jet fragmentation**
**Momenta of charged particles in jets are well described by NLLA pQCD:**
**MLLA k****T****-cutoff Q****eff****=240 ******** 40 MeV**
**	**** *****k******T******-cutoff can be set as low as ~************QCD***
**LPHD N****hadrons****/N****partons**** = K****KLPHD(********)**** = 0.56 ******** 0.10**
**	*****number of hadrons ~ number of partons***** **
**Ratio of charged particle multiplicities in gluon/quark jets *****r *****= 1.6 ******** 0.2****                                                   **
**multiplicities and their ratio agree with next-to-NLLA and recent LEP data**
**Underlying event**
**MC generators with default parameters do not quite work,                      but can be tuned to match data... **
**Pythia tuned for Run I continues to work in Run II for broader range...**
**Did we gain any insights into UE physics?**
**    ****-- Need to compare Pythia Tune A and Herwig Tune “X” (work in progress)**