# Presentation: 401839

## Objectives for Spin Physics at  RHIC 

**Werner Vogelsang**
**RBRC & BNL Nuclear Theory**
**NSAC Review Meeting  06/03**

## Outline:

- What are the challenges in QCD spin physics             over the next  ~10 years?
**•**** ****What do we expect from RHIC, **
**    ****and what is the impact of other activities ?**

**Outline:**

## Exploring the nucleon:  at the forefront of QCD

**N**

**Hard probes,**
**asymptotic**
**freedom**

**low-energy**
**models: **
******SB, confinement, ...**

**lattice**

**Exploring the nucleon:  at the forefront of QCD**

## To understand the spin structure of 

**To understand the spin structure of **
** ****hadrons in terms of quarks and gluons**

- • The way to do it:

**Hard probe**

## Famous examples:

**quark spin**

**transversity**

## What are the probes ?

**Polarized DIS**

**SLAC, CERN, DESY, Jlab,  eRHIC**

## ?

- •  **the main lesson:**    The nucleon spin is quite subtle !

- Quark Model

- QCD

- gluons,  qq pairs,  orbital,  interactions,  renormalization

- 

##   Result of the hugely successful program of  polarized DIS 

## +  QCD scaling violations

**+  QCD scaling violations**

## Hirai, Kumano, Saito

## Challenges in QCD spin physics

-   • **What else carries the proton spin ?**** **
**        ******** ****How are gluons polarized ?**
**        ******** ****How large are parton orbital ang. mom. ?                           **

-   •** ****What are the detailed patterns of **
**     ****quark & antiquark  polarizations ?**
**       ******** ****Flavor asymmetries in sea ?**

-   • **Is there significant transversity in the nucleon ?**

-   • **What are the origins of large observed **
**     ****single-spin asymmetries ?**
**       ******** ****Correlations between transverse spin and **
**             ****parton k   ?**

**T**

** **

**Complementary information :****  **HERMES, COMPASS, Jlab, eRHIC

## Challenges in QCD spin physics

-   • What else carries the proton spin ?**  **

-   •** **What are the detailed patterns of
-      quark & antiquark  polarizations ?
**       ** Flavor symmetry breakings in sea ?

-   • Is there significant transversity in the nucleon ?

-   • What are the origins of large observed
-      single-spin asymmetries ?
**       ** Correlations between transverse spin and
-              parton k   ?

**T**

-   How large are parton orbital ang. mom. ?

-   How are gluons polarized ?

****** ****How are gluons polarized ? **

## Gluon polarization             

**• ****  ****a key contributor to the proton spin ? **

**• ****                                                                   ****in QCD**

**• ****  ****some model predictions  (Q ******** 0.5 GeV) :**

**...**

##  •  gluons subdominant in inclusive DIS

-   Q    evolution     need large lever arm

-  2

-   single out  “photon-gluon fusion”

** •**** **** ****gluons are “leaders” at hadron colliders ...**

- [very difficult theory !]

## Slide 15

## Slide 16

## Slide 17

## Near-term example:  spin asymmetry for jet  production

**Jager,Stratmann,WV**

## Mid / long-term at RHIC:

- •  gluon polarization from  **pp ******** ******** X**

**“****rare probes”**

**L=100/pb**

**Frixione,WV**

## •   main information is at                            

**      ****typically,   ****0.025 < x   < 0.3    @    ********s = 200 GeV****  **

** •**** **** ****to get integral, may need to expand this: **

**                  ****0.01 < x   <  0.1     @    ********s = 500 GeV****  **

**g**

**g**

-   forward pp scattering

-   scaling violations  at small-x at  **eRHIC**

## Challenges in QCD spin physics

-   • What else carries the proton spin ?**  **

-   •** **What are the detailed patterns of
-      quark & antiquark  polarizations ?

-   • Is there significant transversity in the nucleon ?

-   • What are the origins of large observed
-      single-spin asymmetries ?
**        ** Correlations between transverse spin and
-               parton k   ?

**T**

-   How large are parton orbital ang. mom. ?

-   How are gluons polarized ?

-  Flavor asymmetries in sea ?

****** ****Flavor asymmetries in sea ?**

## Detailed information on (anti)quarks

**Polarized case:**

**x**

**most models predict **

## •  remarkably,  there is a very powerful way at RHIC:     

**parity violating**

- unpol.

- u

## HERMES, COMPASS, JLab     eRHIC 

- HERMES, COMPASS, JLab     eRHIC

**• **** ****probe properties of nucleon on very different scales !   **

## Challenges in QCD spin physics

-   • What else carries the proton spin ?**  **

-   •** **What are the detailed patterns of
-      quark & antiquark  polarizations ?

-   • Is there significant transversity in the nucleon ?

-   • What are the origins of large observed
-      single-spin asymmetries ?
**        ** Correlations between transverse spin and
-               parton k   ?

**T**

-   How large are parton orbital ang. mom. ?

-   How are gluons polarized ?

-  Flavor symmetry breakings in sea ?

- •**  ****Is there significant transversity in the nucleon ?**

## Transversity

**•**** **** ****the unknown “leading-twist” distribution functions    **

## •  the physics involved:

-   relativistic / dynamical  effects

-   “odd chirality”    helicity-flip,  SB

-   tensor charge  (relevant for  EDM)

-   no mixing with gluons

## • Opportunities for measurement ?

-   not in inclusive DIS:

-   helicity-flip from final-state effect ?

## Challenges in QCD spin physics

-   • What else carries the proton spin ?**  **

-   •** **What are the detailed patterns of
-      quark & antiquark  polarizations ?

-   • Is there significant transversity in the nucleon ?

-   • What are the origins of large observed
-      single-spin asymmetries ?
**        ** Correlations between transverse spin and
-               parton k   ?

**T**

-   How large are parton orbital ang. mom. ?

-   How are gluons polarized ?

-  Flavor symmetry breakings in sea ?

**• ****What are the origins of large observed **
**   ****single-spin asymmetries ?**

## 

- L

- R

**STAR**

## •  not really understood so far 

-   transversity & Collins effect are probably involved

-   multi-parton correlations ?

-   exciting possibility:

-     knows about  **parton orbital angular momentum**
-     [connection to “generalized parton distributions” of  DVCS ?]

**Sivers**

-   simple collinear hard scattering:  **no asymmetry**

-   many exciting theory issues:  factorization, universality, ...

## jet

- lepton-nucleon:

- proton-proton:

**jet**

**jet**

**+ Drell-Yan, ...**

## Over  the  next ~10  years,  RHIC  will

**provide  new  and  groundbreaking **
**information  on  most  open  questions**** ****   **

****  **efforts to understand nucleon spin structure **
**    *****from***** QCD:    lattice,  models,  ... **

****  **complementary information from lepton scattering**

**• **** ****important interplay with:**

- fixed-target  HERMES, COMPASS, Jlab     eRHIC

**Conclusions**