# Presentation: 412218

## Elena Bruna, for the STAR Collaboration
Yale University

- Elena Bruna, for the STAR Collaboration
- Yale University

*Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009*

## Going from pp...

- Topological Jets in pp at RHIC energies
- pT spectrum up to 50 GeV
- What we know:
- Good agreement of jet spectrum with NLO/PYTHIA at RHIC energies
- Jet spectra and fragmentation functions measured at Tevatron  good agreement with theory
- Perspectives in STAR:
- Measure jet Fragmentation Functions at RHIC energies
- Compare conceptually different jet algorithms (kT, anti-kT, cone)
- Energy resolution using PYTHIA simulation
- Understand the Trigger Bias
- Goal:
-  compare Fragmentation functions in pp and Au+Au

- Elena Bruna

## ...to Au+Au

- Topological jets measured in Au+Au in STAR at RHIC energies!
- What we want to measure:
- Jet pT spectrum
- Jet Fragmentation Functions
- What we expect with an unbiased jet population:
- pT spectrum Nbin scaling (jet production is a hard process)
- Fragmentation functions  modification

- Elena Bruna

## Jets in high-energy collisions

- High-pT partons produced in hard scatterings

**a, x****a**

**b, x****b**

**σ****ab**

**c, x****c**

**d, x****d**

- Full jet (spray of collimated hadrons) after parton fragmentation gives access to:
- partonic kinematics
- jet cross sections
- TOOL: Jet-Finding algorithms

- Elena Bruna

## Jet Reconstruction

- Seedless, not bound to a circular structure
-  kT: starts from merging low pT particles close in the phase-space
-  Anti-kT: starts from merging high pT particles close in the phase-space

**R**cone

*seed*

*tracks or towers*

- R=√(Δφ2+Δη2)

**Seed Cone**:
- ‘seed’ (E>Ethreshold)
-  iterative approach
**Seedless Cone** (SIS cone):
-  all the particles used as seeds
-  Splitting/Merging applied

**Cone  Algorithms**

**Recombination  Algorithms**

- [Cacciari, Salam, Soyez, arXiv:0802.1189]

- [Cacciari, Soyez, arXiv:0704.0292]

*outgoing parton*

*fragmentation*

*seed*

- Elena Bruna

## EMC

-  charged particle pT (**TPC**)
-  neutral tower Et 0.05x0.05 (ηxφ) (**EMC**)
-  corrected for hadronic energy.
-  Electron correction for double counting
-  EMC provides fast trigger.
-  Two trigger setups with the EMC:
-  Jet Patch Trigger (JP):
-  1x1(ηxφ), Et>8 GeV
-  High Tower Trigger (HT):
- tower 0.05x0.05 (ηxφ) Et> 5.4 GeV (pp)
-  cluster 0.1x0.1 (ηxφ) Et> 7.5 GeV (AuAu)

**** 40 towers

******: **120 towers in =0-2

- Elena Bruna

- =-1

- =0

- =+1

**EMC**

## Analyzed STAR data-sets:

*Analyzed STAR data-sets:*
-  p+p (2006) **High-Tower (HT) trigger** (single tower Et>5.4 GeV)
-  p+p (2006) **Jet-Patch (JP) trigger** (ηxφ=1x1 with sum Et>8 GeV)

- Elena Bruna

## Fragmentation functions for charged hadrons

- Definition:  =ln (Ejet/phadr)
- We use: ******=ln (p****T,jet****/p****T,hadr****) :** no assumptions on the particle mass

| ξ | pT,hadr  (GeV/c)(pTjet=30 GeV) |
| --- | --- |
| 1 | 11 |
| 2 | 4 |
| 3 | 1.5  |
| 4 | 0.5  |
| 5 | 0.2  |

**30< p****T,jet****<40 GeV**

- p+p √s=200 GeV
- JP trigger

**Fragmentation function in qualitative agreement for all Jet-Finders**
***Statistical errors only***
***Systematic studies ongoing***

- Elena Bruna

- STAR Preliminary

## Jet-Patch vs High Tower triggers

**10< p****T,jet****<15 GeV**

**20< p****T,jet****<30 GeV**

**30< p****T,jet****<40 GeV**

**p****T,jet****>40 GeV**

**Low p****Tjet****: **Multiplicity of charged particles influenced by Neutral Energy Fraction and high z fragmenting jets  stronger trigger BIAS in the HT sample
**High p****Tjet****: **JP and HT do not show difference due to a smaller bias of different trigger selections  JP and HT fragmentation functions similar for higher jet energy

- STAR preliminary

- STAR preliminary

- STAR preliminary

- STAR preliminary

|  | 10GeV jet pThadr (GeV/c)
 | 20GeV jet 
pThadr (GeV/c)  |
| --- | --- | --- |
| 1 | 3.7 | 7.4 |
| 3 | 0.5 | 1 |
| 5 | 0.07 | 0.1 |

|  | 30GeV jet 
pThadr (GeV/c) | 40GeV jet 
pThadr (GeV/c) 
 |
| --- | --- | --- |
| 1 | 11 | 14.7 |
| 3 | 1.5 | 2 |
| 5 | 0.2 | 0.26 |

- STAR Preliminary

- STAR Preliminary

- STAR Preliminary

- STAR Preliminary

- p+p √s=200 GeV

- p+p √s=200 GeV

- p+p √s=200 GeV

- p+p √s=200 GeV

##  for different jet energies (1)

-  for different jet energies (1)

- Elena Bruna

**10< p****T,jet****<15 GeV**

**20< p****T,jet****<30 GeV**

**30< p****T,jet****<40 GeV**

**p****T,jet****>40 GeV**

-  Uncorrected spectra
-  Different Jet Finders show similar performance for a given R

- p+p √s=200 GeV
- JP trigger

- p+p √s=200 GeV
- JP trigger

- p+p √s=200 GeV
- JP trigger

- p+p √s=200 GeV
- JP trigger

##  for different jet energies (2)

*p**T**>=1 GeV/c*

-  for different jet energies (2)

**10< p****T,jet****<15 GeV**

**20< p****T,jet****<30 GeV**

**30< p****T,jet****<40 GeV**

**p****T,jet****>40 GeV**

- p+p √s=200 GeV
- JP trigger

- p+p √s=200 GeV
- JP trigger

- p+p √s=200 GeV
- JP trigger

- p+p √s=200 GeV
- JP trigger

*p**T**>=1 GeV/c*

*p**T**>=1 GeV/c*

*p**T**>=1 GeV/c*

-  Conceptually different Jet Finders  Similar performance for different cone radii and for different jet pT  suggests no significant NLO effects at RHIC energies

## p+p Fragmentation function vs. PYTHIA

- Increasing
- Cone R

- Increasing
- Jet Energy

- 20<Ereco<30 GeV

- 30<Ereco<40 GeV

- 40<Ereco<50

- R<0.4

- R<0.7

**Good agreement with PYTHIA especially at low R**

**JP trigger**
- R=0.7
- |ηjet|<0.3

- Elena Bruna

## pp: reference for Au+Au

- Jets in pp are a safe baseline for Au+Au
- Good agreement with NLO/PYTHIA
- Free choice of  jet algorithms in pp
- Systematic studies ongoing (e- identification, hadronic shower, trigger bias)

- Elena Bruna

*Analyzed STAR data-sets:*
-  Au+Au (2007) **High-Tower (HT) trigger **(cluster ET>7.5 GeV)
-  Au+Au (2007) **Minimum-Bias (MB) trigger **

## Jet Finding in Heavy-Ion collisions

- GOAL: Fully reconstruct the jet in high-multiplicity environment
- How to suppress background:
- Reduce the jet area
- Apply a pTcut on tracks and towers

- Elena Bruna

-  Background estimation:
- Mean energy in out-of-cone areas

## Background in Au+Au 0-20%

- Background fluctuations [Gev]

**R****c**

-  Event-by-event basis:
-  <pT,Bkg>=mean pT in out-of-cone area
-  Bkg for Fragmentation Functions = mean FF in out-of-cone area for a given pTjet
- Background energy in R=0.4 ~ 45 GeV (no pT cut)
-  Substantial region-to-region background fluctuations ⇒ significantly reduced by applying a **p****T**** cut**

- Elena Bruna

- STAR Preliminary

- STAR Preliminary

## Simulation

- 1) Jet Finder on PYTHIA events
- 2) a. PYTHIA event embedded in Au+Au real event
-         b. Jet Finder on PYTHIA + AuAu
- 3) Compare  (1) and (2)
- detector effects neglected

- Elena Bruna

## Simulation

_**Simulation**_

## Simulation: ξ distribution for 30 GeV (pt,cut>2 GeV)

- _Simulation_: ξ distribution for 30 GeV (pt,cut>2 GeV)

- Charged particle FF:
- Rc(FF)=0.7 and pt>0 GeV

-  ξ background subtraction method and jet energy resolution in Au+Au 0-20% causes deviations < 10-20% for ξ<2-2.5 _**using Pythia fragmentation**_**  **
-  Systematic deviations in the ξ shape ratio at low ξ are caused by jet-energy resolution
-  Only statistical errors

## Data: ξ distribution for jet energies > 30 GeV in Au+Au

- Cone and kt algorithm give similar fragmentation-function measurements for ptcut > 2 GeV and reconstructed jet pt above 30 GeV in Au+Au 0-10%

- Elena Bruna

## Fragmentation Function in Au+Au 0-20% and p+p for 30 GeV jets

**1)  **pt,jetrec.(pp) > 30 GeV
**2)  **pt,jetrec.(Au+Au)>31 GeV for ptcut>2 GeV
**3)  **pt,jetrec.(Au+Au)>35 GeV for ptcut>1 GeV

- No apparent modification in the fragmentation function with respect to p+p

- Elena Bruna

-  AuAu jet energies should correspond to 30 GeV pp jets

*Statistical errors only*

## Au+Au results:

- _Simulation _ jet Finding works in high multiplicity environment!
-                           background reasonably under control
- _Data_:
**SPECTRA** *(MinBias data)*: **Binary scaling **
-     observed with no pT cut (S. Salur talk)
*PYTHIA fragmentation assumed*
**FRAGMENTATION FUNCTIONS**
**    ** *(HT trigger data)*:
- 	-- no apparent modification w.r.t. pp
*Corrections for energy resolution (bkg fluctuations)*
*Corrections based on PYTHIA fragmentation*
- -- we would expect a modification (high-pT hadron suppression)
- Measurements consistent: why?

## What’s happening!

_**Effect A:**_** **Biased sample of jets due to the High-Tower Trigger:
- the HT trigger favors “surface” jets that are not modified by the medium
- Ejet (AuAu) = Ejet (pp)  FF unmodified
- If this is true  HT jets should not binary scale
-      even without pTcut!
_**Effect B**_: Biased sample of jets due to energy loss and pTcut
- The jet softens in the medium
- Its energy is not recovered with pTcut _AND_
-       assuming PYTHIA fragmentation
- Its energy is UNDERESTIMATED  ξ=ln(ptjet/pt)
-     should be larger
- If this is true  Quenching models could address this issue

- dN/dξ

- ξ

- Au+Au
- p+p

- dN/dξ

- ξ

- Au+Au
- p+p

## Conclusions

- Full jet reconstruction feasible at RHIC
- Jet Fragmentation Functions:
- Measured and under control in pp
- Powerful tool to study medium effects in Au+Au

- Elena Bruna

- We do not see modification of the Fragmentation Functions
- _Surface effect _on HT events or _Softening_ of jets in the medium or_ both_?

## What’s next

- How to address this issue:
- High-Tower jet-pT spectrum
- di-jets in Min Bias and HT events
- Quenching models will help
- Systematic studies (detector effect,...) to have more control on our understanding of jet finder in Au+Au
- Jets are a precious tool to explore the medium: exciting physics is coming!

- Elena Bruna

*Thank you !*