# Presentation: 473636

## Low mass dilepton production 

**      **
**Low mass dilepton production **
**at high pT**
**as a probe of gluon density**

**Jianwei Qiu**
***Iowa State University***

**RIKEN/BNL Research Center Workshop on**
**High pT Physics at RHIC**
**December 2-6, 2003  **

- Jianwei Qiu, ISU

- December 3, 2003

## Outline of the Talk

**Outline of the Talk**

** ****Summary and Outlook**

-  **Approaching to parton saturation from **
**    ****pQCD factorization**

-  **How much low mass Drell-Yan dilepton**
**    ****production can help?**

** ****Phase diagram of parton densities**

- Find when and where pQCD factorization fails

- Jianwei Qiu, ISU

- December 3, 2003

## Parton distribution functions

-  Theorist’s definition:

-  DGLAP evolution for the μ2-dependence

-  They are non-perturbative but universal

- same O(αs)
- evolution kennel

- Jianwei Qiu, ISU

- December 3, 2003

## Phase diagram of parton densities

- CGC

- CGF

- pQCD
- factorization

-  How to probe the
-     boundary between
-     different regions?

-  Experiments measure
-     cross sections, not PDFs

-  PDFs are extracted
-     based on
-  factorization
-  truncation of perturbative
-     expansion

- Look for where pQCD
- factorization approach
- fails

- Jianwei Qiu, ISU

- December 3, 2003

## DIS structure functions at low xB

** ****Virtual photon of (x****B****,Q****2****) probes:  **

**Process-****independent**
**shadowing – ****leading twist**

**Transverse area**

**Longitudinal uncertainty**

** ****Interact coherently with more than one nucleon **

**if**

**Process-****dependent**
**Shadowing – ****high twist**

**Sea quark**

**Cannot be removed**
**by changing gauge**

**(*****x,k******T*****)**

**Power suppressed**

- Jianwei Qiu, ISU

- December 3, 2003

## Nuclear shadowing in structure functions

-  **Leading twist shadowing only:**

** ****Example:**

** ****DGLAP for the scaling violation in Q****2****-dependence **
**    ****-- neglect power corrections for Q>Q****0**
** ****Input parton distributions at Q****0****:**

**Universal**** leading twist nuclear shadowing in PDFs **

** ****Gribov-Regge model ****(Armesto etal, Frankfurt etal, etc.)**

** ****CGC approach: saturated pdfs + BFKL ****(MV, etc.)**

** ****QCD global fitting to fix input distributions at Q0 ****(EKS98, etc.)**
** ****Multiple scattering off Pomeron and Reggeon ****(Brodsky, Lu,1990)**

-  **Maximum high twist shadowing (no LT shadowing): **

**Maximum**** ****characteristic scale of power corrections:  ****ξ****2**

**Breakdown of collinear factorization if  ****ξ****2****~Q****2**

- Qs

- Jianwei Qiu, ISU

- December 3, 2003

## Resummed power corrections to SFs

-  resum all powers in (A1/3-1)ξ2/Q2
-  evaluate the ξ2 to leading power in αs

**Vitev’s talk on Thursday ****(Qiu and Vitev, hep-ph/0309094)**

- Jianwei Qiu, ISU

- December 3, 2003

## Scale for cold matter power corrections

**(Qiu and Vitev, hep-ph/0309094)**

- Jianwei Qiu, ISU

- December 3, 2003

## Estimate the saturation scale

-  Maximum characteristic scale of power corrections from
-     DIS shadowing:

-  Saturation scale for gluonic matter:

-  Separation between the process-dependent power
-     corrections and the universal nuclear effect
-     in the leading twist nPDFs

** ****Precise data on Q****2****-dependence of the structure functions**

** ****QCD sum rules – Gross-Llewellyn Smith sum rule **
** – ****nuclear effect in leading twist PDFs does not change valence qurak numbers**** **

- Jianwei Qiu, ISU

- December 3, 2003

## The Gross-Llewellyn Smith Sum Rules

**Qiu and Vitev, to appear on hep-ph**

**Prediction is compatible**** with the trend in the current data **

- Fully coherent final-state power
- corrections to the sum rule almost
- cancel due to the unitarity:

- Process-dependent power corrections are important!

- Jianwei Qiu, ISU

- December 3, 2003

## Nuclear effect in hadronic collisions

- Jianwei Qiu, ISU

- December 3, 2003

## Arguments for the hadronic factorization

-  Relies on our ability to factorize one parton distribution:

-  Scale of power corrections:

-  No hadronic factorization beyond 1/Q2:

- Jianwei Qiu, ISU

- December 3, 2003

## A-enhanced power corrections in pA

-  A-enhanced power corrections, A1/3/Q2, are factorizable:

-  But, power corrections are process-dependent, and
-     they are different from DIS

- Jianwei Qiu, ISU

- December 3, 2003

## Probes of nuclear effect in gluon distribution

-  need processes that are
-     sensitive to gluons

-  Processes are factorizable
-     and calculable

-  Poor knowledge on nuclear
-     gluon distribution

-  pQCD factorization seems
-     to work fine at RHIC energy

**Stratmann’s talk**

**Accardi et al hep-ph/0310274**

- Jianwei Qiu, ISU

- December 3, 2003

## Low mass Drell-Yan dilepton production

-  Clean probe without final-state strong interactions

**Large virtuality limits the interaction length in medium**
**for fragmenting partons**

**Lowest order**

**Berger,Qiu,Zhang, PRD65, 2002**

- Jianwei Qiu, ISU

- December 3, 2003

## Low mass Drell-Yan in forward region

-  Like direct photon production, gluon Compton dominates
-     the production rate for  pT > Q/2,
-             without complications of isolation cuts

-  Forward region to probe the small-x gluon distribution

- Jianwei Qiu, ISU

- December 3, 2003

## Event rate and nonperturbative effect

-  But, it suffers a lower production rate (Drell-Yan factor)

-  Power corrections
-     and nuclear effects:

- Guo, PRD62, (2000)

- Fries,PRD68, (2003)

- Qiu and Zhang, PL B522 (2002)

**Baier’s talk on Thursday**

- Jianwei Qiu, ISU

- December 3, 2003

## Large nonperturbative effect at low pT

-  resummation of large Sudakov logarithms

-  **predictive power relies on**
**    ****the convergence of large-b**
**    ****region**

** ****low mass Drell-Yan get a **
**    ****lot of nonperturbative **
**    ****contribution!**

- Jianwei Qiu, ISU

- December 3, 2003

## Also suffers low production rate at RHIC

- Include resummation of final-state fragmentation logs

**Berger, Qiu, Zhang, PRD65, 2002**

**Only perturbative contributions**

**Q=2GeV**

- Jianwei Qiu, ISU

- December 3, 2003

##  By comparing resummed power corrections with data on

-  By comparing resummed power corrections with data on
-     nuclear shadowing, we estimate the maximum value for
-     the characteristic scale of perturbative power corrections:

-  From the value of ξ2, we estimate the maximum value
-     of the saturation scale in cold matter:

-  Low mass Drell-Yan in forward region is a potentially
-     clean probe for small-x gluon, because the gluon initiated
-     subprocesses contribute more than 80% to cross section
-     But, it suffers the low production rate at RHIC

- Jianwei Qiu, ISU

- December 3, 2003