# Presentation: 855150

## Nuclear modification of two-hadron correlations from HERMES 

- Nicola Bianchi
- Bianchi@lnf.infn.it

- Fragmentation function and Semi-Inclusive DIS
- Single hadron suppression in DIS  on nuclei
- Single hadron suppression in HIC at RHIC
- Two hadron fragmentation function
- Two hadron suppression in DIS on nuclei

- Hard Probes 2006.  Monterrey, June 10 – 16, 2006

## Deep Inelastic Scattering

- DIS and SIDIS are powerful tools to study parton distribution and fragmentation functions in the vacuum
- Underlying effects in the nuclear medium are better tested due to the static property and known density and dimension of the system
- Input for HIC in modification of partonic distribution functions (EMC valence quark at large x, shadowing effects, gluon saturation at low x ..)
- Input for HIC in modification of partonic fragmentation functions (parton energy loss, pre-hadronic formation and interaction, hadron formation time ..)
- Virtuality (Q2) is exactly measured in DIS/SIDIS
- All nuclear effects in SIDIS are Final State Interaction

## Fragmentation Function in DIS

- HERMES: EPJ C21(2001) 599

- Agreement with KKP (BKK) FFs evolved from LEP data

## Single hadron multiplicity ratio

- Leptonic variables :  (or x) and Q2
- Hadronic  variables : z and  Pt2
- Different nuclei : size and density
- Different hadrons : flavors and mixing of FFs

- Experimental observable: hadron multiplicity ratio in nuclei and deuterium

## The energy range ( 3-25 GeV) is well suited to study medium effects.  

- The energy range ( 3-25 GeV) is well suited to study medium effects.

- Measurements over the full z range

- Possibility to use several different gas targets

## HERMES @ HERA

**E=27.5  GeV e****+**** (e****-****)**
**I ~ 30 mA**
- p beam of 920 GeV, not used by HERMES

## The Spectrometer

- momentum resolution of 1%
- e+ identification: 99% efficiency and < 1% of contamination
- PID: RICH, TRD, Preshower, e.m. Calorimeter
- For N target: by Cerenkov   ID   4<p<14 GeV
- For He, Ne, Kr, Xe targets: by RICH   , K, p  ID 2.5<p<15 GeV
-  ID by e.m. Calorimeter (5% E (GeV))

- (NIM A417 (1998) 230)

## Particle Identification

- _Double radiator RICH_: Aerogel + C4F10. Cerenkov photons detected by ~4000 PMTs._Detection efficiency_: 99% (), 90% (K), 85-95% (p)

## Single hadrons vs transfer energy 

- HERMES, PLB 577 (2003) 37

- Single hadrons vs transfer energy 

## Different hadrons vs z 

- 

- Different hadrons vs z

## Leading pions vs A 

- 

- Leading pions vs A

## Leading pions vs Q2 

- 

- Leading pions vs Q2

## Leading hadrons vs pt 

- 

- Leading hadrons vs pt

- -

-  Cronin effect as FSI in DIS
-  Dependence of the Cronin effect on the hadron type
-  Cronin effect for proton larger than for pion
-  Similar effect at large pt as in HIC (SPS, RHIC non central)

## Interpretation of DIS data 

- B. Kopeliovich et al. (NPA 740, 211 (2004))
- T. Falter et al. (PLB 594, 61 (2004))
- Accardi et al. (NPA 720, 131 (2003))

- _Models based on partonic energy loss_

- X.N. Wang et al. (PRL 89, 162301 (2002))
- F. Arleo et al. (EPJ C 30, 213 (2003))

- -Color neutralization inside the medium-Pre-hadron formation and interaction-Time? Cross section? Absorption  Rescattering-Hadron formation mainly outside the nucleus-Induced radiation << absorption or rescatteringSee A.Accardi talk on wednesday

- -Energy loss mechanism for the hadron suppression -Parton rescattering  enhancenment at large pT

- Interpretation of DIS data

## Pre-hadronic FSI and formation times

- RM is very sensitive to the pre-h ; (pre-h=0.33 h)

- 84Kr

- 20Ne

- Falter et al.

## Fragmentation function modification

- Importance to measure the full kinematical/dynamical dependence :
- transverse broadening  : high energy
- mixing of hadron species : good PID
- longitudinal effect (hadron suppression at large z/ enhancement at low z) : full momentum acceptance

- Rescattering with another *q *: mix of *quark* and *gluon* FF.

*Gluon*-rescattering including *gluon*-radiation: dominant contribution in QCD evolution of FF.

- FF and their QCD evolution are described in the framework of multiple parton scattering and induced radiation  : see X.Guo talk on wednesday

## Leading hadrons in SIDIS

-  1 free parameter Cquark-gluon correlation strength in nuclei
-  From 14N data C=0.0060 GeV2:
-  HERMES : cold but static nuclei Esta  0RA2 ;     0 gluon  density and RA6 fm
-  RHIC : hot but expanding  exp  Esta (20/RA);   0 initial medium formation time
-  Gluon density at RHIC ~ 30 times higher than in cold matter

- Parton energy loss :
- Landau-Migdal-Pomeranchuk interference pattern
- H-T term in the QCD evolution equation of FFs

- X-N Wang et al.

## Partonic energy loss + transport

- Soft gluons radiated in the dense QCD medium (gluon transport coefficient from DY)
- Energy loss of 0.6 GeV/fm in agreement with X-N Wang
- Nice agreement with both HERMES and old EMC data

- With formation time effect

- Without formation time effect

## DIS vs HIC 

- HERMES kinematics is relevant to Ion-Ion mid-rapidity

- E q =  = Ee-Ee'   15 GeV

- E h = z   ****** 3 - 15 GeV**

- E h = pT  ****** 2 - 20 GeV**

- E q = pT / z

**q**

**h**

- DIS vs HIC

## Hadron correlation in HIC and DIS

- DIS in cold nuclear matter                        double- hadron in the same jet

- HIC with hot matter : two jets and double- hadron correlation in the same jet
- Quenching on hadrons (low pt) from away side jet
- Small effect on hadrons from the same side jet

**Notes:**
PURE ENERGY LOSS: EACH HADRON SUFFERS THE SAME ENERGY LOSS ON AVERAGE

## Two hadrons in DIS 

- 

- Two hadrons in DIS

## Multiple hadrons in fragmentation 

- 

- Multiple hadrons in fragmentation

- All h rank 1,2,3 No +- and -+  no rank 2,  only 1 (leading), 3 (early formed)No +- reduces the Vector meson contribution to fragmentation

- Space-time evolution within Lund string model of fragmentation:

- Small nuclear effect in R2h compared to single hadron multiplicity
- No big difference between rank 2 and 3 hadrons
- Small A-dependence

-  No +- and -+  no rank 2,  only 1,3

- Pre-hadronic FSI described by a transport codePurely absorptive treatment of hadronic FSI

- Two hadrons vs transport model

*HERMES PRL 96 (2006) 162301*

## Dihadron fragmentation function 

- Dihadron fragmentation function

- Factorization into hard scattering and soft dihadron FF
- Dihadron FFs in terms of parton matrix elements and DGLAP at leading log

- Majumder and X-N Wang
- PR D 72 (2005) 034007

## Two hadron FFs in the medium 

- All h rank 1,2,3

- Pure partonic energy loss

- Two hadron FFs in the medium

- Reproduce the size and the kinematic dependence for Nitrogen
- Overestimate the A-dependence

- See A. Majumder talk on sunday

- Modified 2h FF due to quark scattering and gluon radiation

## Summary

- HERMES is providing new results on single and double hadron production in e-nucleus interaction:

-  Nuclear modification of the quark Fragmentation Functions
-  Pre-hadronization and final hadronization times
-  Partonic energy loss and scattering

## Gluon Bremsstrahlung 

- Kopeliovich et al

- FF modification: Nuclear Suppression + Induced Radiation

- _Nuclear suppression: _interaction of the qq in the medium. E_nergy loss: induced gluon radiation by multiple parton scattering in the medium_

- _