# Presentation: 682725

## Search for the Effects of the QCD Color Factor in High-Energy Collisions at RHIC 

- Bedanga Mohanty
- LBNL

-   Motivation
-   Color Factors
-   Search for Color Factor Effects at RHIC
-   Summary and Outlook

- Outline

## Theory of Strong Interactions : QCD

- Very successful
- Examples -

- STAR : PRL 97 (2006) 252001

- CP-PACS : PRL 84 (2000) 238

## Color Factors

- QCD : For SU(3) :
-  Nc = 3
-  CA = 3, CF = 4/3

- If  Nc is the dimension of the group (Lie) - CA = Nc, CF = (Nc2-1)/2Nc and TF = 1/2 A and F represent adjoint and fundamental representations.

- Color factors reflect basic properties of QCD. They are therefore measured to prove SU(3) is the gauge group of QCD

## Measurement of Color Factors

- Three basic vertices in four jet production in e+e-
-  Spin-1 or spin-1/2 particles in different
-     configurations. Leads to different angular
-     distributions in the final states.
- Observed jet angular distributions are fitted
-    to  theoretical predictions with  CA, CF, TF
-    as free parameters.

- ALEPH : ZPC 76 (1997) 1
- OPAL :EJPC 20 (2001) 601

- What are the expectations of effect of color factor on observables in HI collisions ?

## Color Factor Effect in QCD Matter at RHIC ?

-  Relate fundamental aspect of QCD to some observables
-  A tool to study the properties of the hot/dense medium at  RHIC
-  Application of pQCD basics to high- energy  nuclear collisions
- Look for effects of difference in color factor of quarks and gluons.

- Interpretation : Energy loss (interaction) of
-  partons (quarks and gluons) in a dense medium

- STAR : PLB 637 (2006) 161
- STAR : PRL 97 (2006) 152301

## Energy Loss and QCD

- Suppression in high pT particle production is due to energy loss of
-  partons in medium formed in nucleus-nucleus  collisions

- An opportunity to relate experimental observable (of Eloss) to
-  basic ingredient of QCD - Gauge Group  through Color Factors

- R. Baier et al., NPB 483 (1997) 291

- M. Gyulassy et al., PRL 85 (2000) 5535

- S. Wicks et al, nucl-th/0512076

## Dominat Source of high pT hadrons : quarks or gluons 

- High pT particle production well explained by NLO pQCD calculations

- At high pT range measured :
- Large gluon contribution (~ 90%) to produced baryons
- Substantial quark contribution (~ 40%) to produced mesons

- STAR : PLB 637 (2006) 161

- S. Albino et al, NPB 725 (2005) 181

- Ng(i)/ (Ng(i) + Nq(i)); i = , K, p...

- B. Mohanty (for STAR) nucl-ex/0705.9053
- L. Ruan (for STAR) nucl-ex/0701070  and WWND

## Expectations

-  Recall : Nucleus-Nucleus collisions produces a dense medium where
-               gluons loose more energy than quarks.
-               No such dense medium expected in p+p and d+Au collisions

- Then naïve expectation at high pT  :
- pbar/p  (pp or dAu)  > pbar/p (central Au+Au)
- pbar/ (pp or dAu) > pbar/ (central Au+Au)
- Rcp()  > Rcp (pbar+p)

- Do we see the color factor effect in experimental observables ?

## Observations In High pT Particle Production

- Anti-particle to particle ratio

- Observations different from expectation -
- Why particle ratios at high pT in Au+Au
-  similar to d+Au and p+p ?
- Why  have similar RCP as p+pbar  ?
- Where is the color factor effect ?

- Anti-Baryon to meson ratio

- Baryon & meson NMF

- STAR : PLB 637 (2006) 161

- STAR : PRL 97 (2006) 152301

- STAR : nucl-ex/0703040

## Model Comparison To Data

- Model calculation with partonic energy loss in heavy ion collisions
- + Color factor effect not consistent with measurements

- Absence of color factor effect in data ?
- What could be the possible reasons ?
- What are the new probes to explore in future ?

- STAR : nucl-ex/0703040

- STAR : PLB 637 (2006) 161
- STAR : PRL 97 (2006) 152301
- Wang et al, PRC 70 (2004) 031901

## Possible Reasons

-  Gluon dominated initial conditions in heavy ion collisions at RHIC ?
-  Possibility of conversions between quark and gluon jets in the
-     medium ?
- Eg/Eq ~ 9/4  only apparent for the limit E/Ejet tending to zero ?
-  High S and a low Q2 regime at RHIC ?
-  Sensitive to different energy loss scenarios ?

- B. Mohanty (for STAR) nucl-ex/0705.9053
- L. Ruan (for STAR), nucl-ex/0701070 and WWND

- W. Liu et al., nucl-th/0607047
- I.Vitev PLB 639 (2006) 38

- T. Renk and K.J. Eskola hep-ph/0702096

## Physics Possibilities : Quark and Gluon Jet Conversions

- Conversions between q- and g- jets via both inelastic (qqbar -- gg) and elastic (gq(qbar) -- q(qbar)g) scatterings with thermal partons in the QGP

- NMF for partons

- W. Liu et al., nucl-th/0607047

## Summary of Search of Color Factor Effect

- Anti-particle to particle, anti-baryon-to-meson ratios
-    are similar in central, peripheral Au+Au, d+Au and p+p
- Rcp of  is similar to Rcp of p+pbar
- Observation different from that expected due to color factor
-    difference between quarks and gluons

- Observations at high pT

- Possibilities

- May be jet conversions in medium is the reason
- May be we need to go higher pT or jet energy to see the effect
- May be giving us more information on energy loss mechanism

- Outlook .....

## Outlook - Heavy Flavor Sector 

- High pT  ratio of heavy to light NMF ratio is sensitive to color factor effect

- Similar idea can also applied for R (/h)

- N. Armesto et al., PRD 71 (2005) 05427

- Both the observable are very promising to see the color factor effects

## Outlook - PID di-Hadron Correlations 

- Suppression pattern in the away side of identified di-hadron correlations.

- Expectation is something like  ..
-           (This is not data)

- Choosing different particles may reflect varying contribution of quark and gluon at high pT.
- Will be interesting to see heavy-flavor correlations - probing quark energy loss.

- Radians

## Outlook - Energy Dependence 

- Similar Eloss for quarks and gluons

- QCD type (color charge effect) Eloss

- Probing quark dominated jet production at lower energy  to gluon dominated jet production at higher energy

- Wang et al., PRC 58 (1998) 2321
- Wang et al., PRC 71 (2005) 014903

## Thanks Thanks to OrganizersThanks to STAR Collaboration 

## Back up slides

## Physics Possibilities : Eloss Scenarios

- Energy loss : absorption of fraction of partons (Dense core + pp like corona)
-                                          or
-                       energy loss of every parton
-                                          or
-                       some combination of the two

- What is the relative contribution of different energy loss scenario ?

- T. Renk and K.J. Eskola
-  hep-ph/0702096

## Eloss Formalism : BDMPS & GLV

## Definition Jets