# Presentation: 400355

## High pT physics in the forward direction 

**Michael Murray **
**University of Kansas**

- CIPANP2006  High Energy Hadron 1-June-2006

## How far forward is forward?

**At 200GeV the longitudinal scaling region is about 3.1 units wide. Therefore one might say y>2.3 is forward at RHIC.   Extrapolating to  LHC this implies y>3.6 is forward.**

******-y****beam**

- CIPANP2006  High Energy Hadron 1-June-2006

## What pT is high pT? 

**Spectra have an exponential shape below p****T ****<1GeV then a power law for p****T ****>2 GeV **

**X****T ****= 2p****T****/√s**** **** spectra deviate from a power law for p****T  ****below 1-2GeV**

**Phenix found that the index of the power law depends on centrality**

- CIPANP2006  High Energy Hadron 1-June-2006

## Are definitions of high pT and forward rapidity independent?

**For the sake of this talk I will assume that they are and take p****T ****> 1.5GeV and y> 2.3**
**These could be tested at different rapidities or x values by comparing spectra from NA49 and BRHAMS at 20, 63 and 200GeV. **

- CIPANP2006  High Energy Hadron 1-June-2006

## Particle ID at high momentum

**STAR Lead-glass 3.4<********<4.0**

**BRAHMS **
**RICH**

**Phenix muons **
**1.2<y<2.4**

- CIPANP2006  High Energy Hadron 1-June-2006

## Can we understand forward pp physics?

**We would like to be able to explain spectra in terms of parton distribution functions plus a fragmenation scheme.**

- CIPANP2006  High Energy Hadron 1-June-2006

## pp 0  at y~3-4 consistent with NLO

**STAR **
**nucl-ex/0602011**

**KKP frag. func. has higher g->******** than Kretzer**

**pp******** ********0 **** at y~3-4 consistent with NLO**

- CIPANP2006  High Energy Hadron 1-June-2006

**Notes:**
Spectra at 3.3 and 3.8 obtained with a smaller FPD

## pp, k, p @y=3  √S=200GeV

**Geometrical acceptance corrections applied as well as absorption and decay in flight. Trigger bias (~20%) is also corrected.**** ****Normalization to total inelastic cross-section 41 mb****  **

- BRAHMS Preliminary

**pp********, k, p @y=3 **** ****√S=200GeV**

**Brahms Preliminary**

****

**K**

**p**

- CIPANP2006  High Energy Hadron 1-June-2006

## NLO roughly consistent with -/+, k-/k+ @ y=3

**The pions are not corrected for weak decays (small).**

**At high p****T**** the    ********-****/********+**** ratio is consistent with dominance of valence quarks**

**NLO roughly consistent with ********-****/********+****, k****-****/k****+**** @ y=3**

**pp at 200GeV**

**p****T ****(GeV/c)**

- CIPANP2006  High Energy Hadron 1-June-2006

## Forward p/+  is large

**BRAHMS Preliminary**

**p/p contradicts strong g -> p or p fragmentation **

**e****+****e****-**** p/******

**Forward p/********+ **** is large**

**y=3.0**

**y=3.3**

**p/********+**

**p/********-**

- CIPANP2006  High Energy Hadron 1-June-2006

**Notes:**
E+e- from Aleph, mention balanceAKK may explain ratios

The difference between p and pbar at high pT rules  out fragmentation. 

## NLO pQCD can explain  pp spectra at y=3

- BRAHMS Preliminary

**NLO pQCD can explain  pp spectra at y=3**

**p****T ****(GeV)**

******-**

**k****-**

**pbar**

**p**

***Calc by Vogelsang***

- CIPANP2006  High Energy Hadron 1-June-2006

**Notes:**
Calculations done by W. Vogelsang. Only one scale =pT and the same fragmentation functions as used for the PHENIX comparison.
KKP has only 0 frag. Modifications were needed to calculate charged pions
KKP FF does a better job suggesting gg and gq dominance at y=3 for pion production

## Summary of forward pp results

**Using KKP fragmentation functions NLO pQCD can describe ************-****, k****-****. This implies a dominance of g q and g g processes at y=3 as was found for ************0**** at y=0. **
**This dosen’t work for protons at ****y=3. The large p/********+****,**** at high p****T  ****is interesting.**

**Summary of forward pp results**

- CIPANP2006  High Energy Hadron 1-June-2006

## For AA life is more complicated

**What is  the initial state of the nucleus?**
**How important is energy loss at forward angles?**
**What about radial, elliptic and directed flow?**
**Does recombination of quarks dominate high p****T**

- CIPANP2006  High Energy Hadron 1-June-2006

## Probing the initial state of the nucleus

**Increasing p****T**

**Increasing y or √s**

- CIPANP2006  High Energy Hadron 1-June-2006

**Notes:**
Catani-Ciafaloni-Fiorani-Marchesini equations(*) resum DGLAP (~alpha_s log(Q^2)) and BFKL (~alpha_s log(1/x)) evolutions. it's an improved version of DGLAP to take into account BFKL effects at small-x. but still they are linear equations (no gluon saturation, fusion considered). it works at low-x *and* high Q^2

## Suppression in dAu at Forward Rapidities

- CIPANP2006  High Energy Hadron 1-June-2006

## Energy Loss in hot medium

**The number of interactions (each emitting a gluon) depends on the density of the medium.**

**Gluon density of the formed medium (rapidity dependent)**

**X. N. Wang et al.**

**Energy Loss in hot medium**

- CIPANP2006  High Energy Hadron 1-June-2006

**At √s**** ****= 200GeV, **
**R****AA**** is independent of ********  **

- CIPANP2006  High Energy Hadron 1-June-2006

## But dN/dy falls by factor of 2 

**In the parton-hadron duality approach, each parton produces one hadron. **

**But dN/dy falls by factor of 2 **

- CIPANP2006  High Energy Hadron 1-June-2006

## AuAu vs CuCu

|  | Centrality | Npart |
| --- | --- | --- |
| AuAu | 33-43% | 96.3 |
| CuCu | 0-10% | 96.8 |
| AuAu | 59-79% | 15.7 |
| CuCu | 40-60% | 17.2 |

- = 3.2

- AuAu

- CuCu

- Now when we explore the forward CuCu system it behaves similarly to the AuAu.

******=3.2**

- CIPANP2006  High Energy Hadron 1-June-2006

## RAA for , p independent of y

**R****AA ****for ********, p independent of y**

****

**p**

- CIPANP2006  High Energy Hadron 1-June-2006

## RAA at y=3 increases with mass

**R****AA ****at y=3 increases with mass**

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## RAA vs. “L dL”, for 200GeV AuAu y=0

**?**

**PHENIX changes L by varying centrality and reaction plane.**

**I=**

**I**

**I**

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## What controls RAA?

**What controls R****AA****?**

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## Brahms see a rough scaling with energy density

**E****T*****(dN/dy)/********R****2**

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## At 63GeV AuAu Rcp drops near kinementic limit

**At 63GeV AuAu R****cp ****drops near kinementic limit**

**Brahms Preliminary**

- CIPANP2006  High Energy Hadron 1-June-2006

## AuAu 200 GeV v2 vs 

- CHARGED HADRONS

**Underlying spectra**
** ****seem to be affecting **
**the v****2**** dependence on ******.

- CIPANP2006  High Energy Hadron 1-June-2006

## Recombination versus Fragmentation

**p**

**hadron momentum**

- suppressed by power-law

**Recombination ****versus Fragmentation**

_**dN**_
**dq**

**log**

- CIPANP2006  High Energy Hadron 1-June-2006

## Pbar/- falls with rapidity

**The shapes of the p****T**** dependence have been reproduced by calculations involving recombination****.**

**Pbar/********- ****falls with rapidity**

- CIPANP2006  High Energy Hadron 1-June-2006

## p/ - increases with centrality

**p/ ********-**** ****increases ****with centrality**

**In recombination model probability for N quarks to combine increases as density****N****.**

- CIPANP2006  High Energy Hadron 1-June-2006

## Recombination may explain this

**P**
****

**p****T**

**Hwa et al**

- CIPANP2006  High Energy Hadron 1-June-2006

## P/ versus system at 200GeV

**P/******** versus system at 200GeV**

- CIPANP2006  High Energy Hadron 1-June-2006

**Notes:**
Central AuAu and CuCu have similar p/ - ratios. But volumesdiffernt

## Pbar/ for depends only on Npart

**Pbar/******** for depends only on N****part**

- CIPANP2006  High Energy Hadron 1-June-2006

## p/ -  versus Npart 

**p/ ********-****  ****versus N****part **

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## Recombination

- “Forward production of protons is favored in recombination because of scarcity of anti-quarks” R. Hwa et al. nucl_th/0605037

- Dramatic difference between y=0 and 3. There are ~2.5 more protons than pions at ~ 2.5 GeV/c

**Recombination**

- CIPANP2006  High Energy Hadron 1-June-2006

## Pushing the kinematic limit

**If these particles have p****T ****> 350 MeV they are beyond the kinematic limit. This would be strong evidence for recombination**.

- CIPANP2006  High Energy Hadron 1-June-2006

## Beyond the kinematic limit at 62GeV

- CIPANP2006  High Energy Hadron 1-June-2006

## CMS + TOTEM forward detectors

**CMS IP**

- 5.32 ≤ |η| ≤ 6.86

**ZDC**

**RPs@220m**

** ****HF,CASTOR,ZDC + TOTEM: ****Quasi-full acceptance**** at LHC**

- Acceptance (neutral):
- θ<~(5.cm) /140.m <~400 μrad
**|**η**| **_**>**_** 8.5 , p****T ****<**** O****(2. GeV/c)**

**RPs@150m**

**T1/T2, Castor  **

**HF**

- CIPANP2006  High Energy Hadron 1-June-2006

## 

Good capabilities for heavy-Q, 
   QQbar  at low-x:

**Good capabilities for ****heavy-Q, **
**   ****QQbar****  at low-x:**

- 2.5< η< 4

- 2 < η < 5

**Alice, LHCb**

- CIPANP2006  High Energy Hadron 1-June-2006

## Summary	

**Forward measurements suggest an interplay of energy loss and initial state effects**
**It is possible that recombination will be able to explain data**
**One needs to carefully understand flow**
**LHC offers many possibilities in 2008 **

- CIPANP2006  High Energy Hadron 1-June-2006

## Backup slides

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## Particle Nuclear Modification Factors

**200 GeV**

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## Comparing two systems at low energy near mid-rapidity

**Comparing two systems at low energy near mid-rapidity**

- These results are consistent with weak effects of a dense medium and a return of strong SPS Cronin type enhancements

- CIPANP2006  High Energy Hadron 1-June-2006

## Slide 43

- CIPANP2006  High Energy Hadron 1-June-2006

##    p/+ ratio integrated over 1.5<pT<2.5 GeV/c 

-    p/+ ratio integrated over 1.5<pT<2.5 GeV/c
-    at forward rapidity increase with centrality;
-    smaller dependence seen at midrapidity

- CIPANP2006  High Energy Hadron 1-June-2006

## pion

- kaon

- CIPANP2006  High Energy Hadron 1-June-2006

## Slide 46

- CIPANP2006  High Energy Hadron 1-June-2006