# Presentation: 512433

## Jet Fragmentation and Baryon Production

**Jet fragmentation**
**Why we should expect medium effects**
**Energy loss effects on the fragmentation function**
**Medium effects on baryon production **
**	****near a “bath” of quarks & antiquarks**

**Barbara Jacak**
**Stony Brook University**
**Nov. 8, 2004**

## Analog of hard x-ray probe of EM plasma

**Want to know**
**	*****pressure, viscosity, equation of state,***
**	*****thermalization time & extent***
***	******	*****determine from collective behavior**
**Other plasma properties**
**	*****radiation rate, collision frequency, conductivity, opacity, Debye screening length***
***	******	***** ****what is interaction ******** of q,g in the medum?**
**	****	**** ****need short wavelength strongly interacting probe**
**	**** *****transmission probability***
***	******	****** *****jet quenching via R****AA*****	******        ***
***high momentum q,g are the probes!***

## Hard quarks & gluons  jets

**Hard scattering happens early**
**	****affected by initial state nucleus**
**Scattered partons propagate**
**fast quarks, gluons traverse the interesting stuff**
**radiate gluons**
**interact with QGP partons**
**Fragmentation is last step **
**described by phenomenological fragmentation function**
**outside the medium (?)**

## Jet Fragmentation in vacuum

**String Breaking**
**Used in Lund Model  & PYTHIA**
** ****ARIADNE splits color dipoles**
**Shower gluons add kinks to strings**
**Hadrons formed when string breaks **
** ****into two (multiple times)**
** ****L-R symmetric splitting function**
**F(z) = (1-z)****a****/z exp(-bM****T****2****/z)**

## Formation time of fragmentation hadrons

**Uncertainty principle relates hadron formation time to hadron size, R****h**** and mass, m****h**
**In laboratory frame: ********f**** ~ R****h ****(E****h**** /m****h****)**
**consider 2.5 GeV p****T**** hadrons **
******f ****~ 9-18 fm/c for pions;  R****h****~0.5-1 fm**
******f ****~ 2.7 fm/c for baryons  (R****h****~1 fm)**
**Alternatively, consider color singlet dipoles from combination of q & q from gluon splitting**
**Using gluon formation time, can estimate**
******f**** ~ 2E****h**** (1-z)/(k****T****2****+m****h****2****)**
**	****for z = 0.6-0.8 and k****T**** ~ ********QCD****:****  ********f ****baryons ~ 1-2 fm/c**

***  ******R(Au nucleus) ~ 7 fm***
********* ******Baryon formation is INside the medium!***

## Energy loss effect: increased gluon radiation

**Initial state multiple scattering**
**Energy loss**

- _Induced Gluon Radiation_
- ~collinear gluons in cone
- “Softened” fragmentation

- Gyulassy *et al*., nucl-th/0302077

**1/N dN/dz**

## But, things are more complicated

**Radiated gluons are collinear **
**  ****(inside jet cone)**
**Can also expect a  **
**jet “wake” effect,**
**medium particles**
**“****kicked” alongside the jet by energy they absorb**

- Fries, Bass & Mueller
- nucl-th/0407102

***How is baryon number conservation ensured in these mechanisms?***

## And EVEN MORE complicated

**Edward’s conic flow: a pressure wave or “super wake”**
**	****i.e. medium response to the energy deposited by jets **

***Does jet fragmentation have a meaning in presence of medium?***
***Mechanisms mix up medium & radiated partons***
***New tool to see conductivity & correlations in medium at ~1 fm/c??***

## Data say: away side jet suppression/broadening

**And “interesting” shape changes**
** ****for correlations of sufficiently soft **
** ****associated particles**

## Yields on away side

**Integrated over 90 degrees**

## More partners on same side

- nucl-ex/0408007

**In 55****° ******** and small ******
**In large ******

## <Nch in jet>

**jet multiplicity unchanged with d+Au centrality vs. pp**
**STAR** ** ****1/N****trig**** dN/dp****T****assoc ****shown on previous slide**
** ****+ full jet reconstruction in dAu (shown at QM04)**

**PHENIX preliminary**

**Must measure own**
**reference!**
***Our jets are soft.***
***Trigger bias from***
***   ******high p******T****** hadron.***

***But, we need to figure out the QUESTION to ask in Au+Au!***

## What about baryons?

**Formed via diquarks in string fragmentation**
**Reduced phase space due to high mass in cluster decay**
**Suppressed relative to mesons by factor of ~10**

## Baryons already different in p+A

**Baryons already different in p+A**

## Nuclear medium modifies initial state

**Nuclear medium modifies initial state**

***Shouldn’t initial state scattering and fragmentation factorize?!***
***R. Hwa says medium already matters in d+Au***

**Cronin effect for baryons larger than for mesons**
**(as at lower energy)**

## In Au+Au baryons scale with Ncoll  !

**In Au+Au baryons scale with N****coll  ****!**

***Jet-like***

***J. Velkovska talk***

## do jet analysis with identified triggers

**CARTOON**

**flow**

- flow+jet

-  **dN**

**N****trig****  d******

**includes ALL triggers**
**(even those with no **
**associated particles in**
**the event)**

- jet

**1**

**combinatorial background large in Au+Au!**

## 2 particle correlations

**Select particles with p****T****= 2.5-4.0GeV/c**
**Identify them as mesons or baryons via**
**time-of-flight**
**Find second particle with p****T**** = 1.7-2.5GeV/c**
**Plot distribution of the pair opening angles;**
**integrate over 55****°**

## intermediate pT baryons ARE from jets

** ****Same side: slight decrease with centrality for baryons**
**Dilution from boosted thermal p, pbar?**
**Away side: partner rate as in p+p confirms jet source of baryons!**
***“******disappearance” of away-side jet into narrow angle for both baryons and mesons***

**intermediate p****T**** baryons ARE from jets**

## What’s going on?

**Thermal**
** ****quark**
** ****recombination**

**Radiated gluons**
**  ****inside jet cone**
**+ wake effects**
**Increases partner**
**  ****yield**

**Dilutes jet**
**   ****partner yield**

## Jet partner distribution on trigger side

**Corrected to jet yield**
** ****according to fragmentation**
** ****symmetric in ******
**Partner spectrum flatter, as expected for jet source**
**Partners soften in most central collisions**

**Jet partners**
**Inclusive**

## Compare to hard-soft recombination

-  trigger &  associated
- Hwa & Yang nucl-th/0407081

**Soft-hard recomb. also explains baryon Cronin effect! **
**No jet-correlated medium flow**

## Conclusions

**Baryon excess has a very significant jet component**
**Dilution becoming visible in most central collisions?**
**Jet fragmentation is modified by the medium!**
**Baryon production enhanced**
**Au+Au jets richer in soft hadrons than p+p or d+Au**
**Away side jet gets complicated**
**Moderate p****T**** associated particles have significant medium splash? Should we call them jet fragments??**
**A new probe!**
**Leading & association baryons ********  q,qbar correlations in the medium**
**Mapping the splash ******** how the medium conducts energy**

## Fun to come...

## Jets in PHENIX

**Large multiplicity of charged particles**
**--solution: find jets in a statistical manner **
**    ****using angular correlations of particles**
**	****mixed events give combinatorial background**
**2 x 90 degree acceptance in phi and |********|<0.35**
**	****  ****--solution: correct for azimuthal acceptance,**
**	****	****but not for ******** acceptance****	**
**Elliptic flow correlations **
**--solutions: **
**use published strength values**
**  ****and subtract **
**(could integrate over 90****° **
**to integrate all even **
**harmonics to zero)**

- PHENIX PRL 91 (2003) 182301

## So, do jet analysis on identified baryons

**Trigger:**
**hadron with p****T**** > 2.5 GeV/c**
**Identify as baryon or meson**
_***Biased, low energy, high z jets!***_
**Plot ******** of associated partners**
**Count associated lower p****T ****particles for each trigger **
** “****conditional yield”**
**Near side yield: number of jet associated particles from same jet in specified p****T**** bin**
**Away side yield: jet fragments from opposing jet**

## Compare p+p and d+Au to PYTHIA

## Are extras from the (soft) underlying event?

**Coalesce (recombine) **
**boosted quarks ******** hadrons**
**  ****enhances mid p****T ****hadrons**
**  ****baryons especially**

**Are extras from the (soft) underlying event?**

## Phase space filled with partons:coalesce into hadrons

**ReCo of hadrons: convolution of Wigner functions**
**Where does ReCo win?**

*W**ab**(1;2) = w**a**(1)w**b**(2)*

- fragmenting parton:
- ph = z p, z<1

- recombining partons:
- p1+p2=ph

*Use lowest Fock state, i.e. valence quarks*

- R. Fries

## Coalescence Model results

- Fries et al: Phys.Rev. C68 (2003) 044902

- Greco, Ko, Levai: PRC 68 (2003)034904

**particle ratios and spectra OK**
**intermediate p****T**** hadrons from coalescence of flowing partons NOT from jets, so no jet-like associated particles**

##  kT, jT at RHIC from p+p Data

** *****k******T******, j******T****** *****at RHIC from p+p Data**

- J. Rak, Wed.

- PHENIX preliminary

- J. Rak, DNP03

- di-hadron

- Statistical Errors Only

**near-side**

**away-side**

- 

## Pions in 3 detectors in PHENIX

**Charged pions from TOF**
**Neutral pions from EMCAL**
**Charged pions from RICH+EMCAL**

**Cronin effect gone at p****T ****~ 8 GeV/c**

## A puzzle at high pT

- Adler et al., nucl-ex/0206006

**A puzzle at high p****T**

**Still flowing at p****T**** = 8 GeV/c?**** Unlikely!!**

- Nu Xu