# Presentation: 677771

##  High pt Physics @ RHIC

- A.Stolpovsky
- Dissertation Advisor T.Cormier

-  High pt Physics @ RHIC

-  Physics motivation
-  STAR EMC
-  Physics results from Au+Au @      = 130 GeV
-   π0 reconstruction
-  Conclusions

## New with Heavy Ions at RHIC

-  New opportunity for heavy ion physics            _Hard parton scattering_

-  = 130 GeV at RHIC      vs            = 17 GeV at CERN SPS

- Jets and mini-jets from hard-scattering of partons
-  30-50% of particle production
-  high pt leading particles
-  azimuthal correlations

- As scattered partons propagate through colored medium they lose
- energy at ~GeV/fm
-  interaction of parton with partonic matter
-  suppression of high pt particles: "parton energy loss"
- or "jet quenching"

## High pt Particles and Jets

- High pt Particles and Jets

-  Probe for QGP
-  Model: pQCD
-  partonic energy loss increase with centrality
- compared to that of scaled NN data
-  affects final state pt

**Notes:**
High Pt partons leave the collision zone before the hadronization and retain information about initial state. Energy loss should increase with centrality.

## Final State

- Can we measure jets?

- Can’t tell jets from background @ AA collisions

- ~0.7

-  Finite radius used by jet finding
- codes: ~0.7 in η & φ

- jet

- Typical jet ~25GeV = 10% of

- (central)

## Jets in Heavy Ion Collisions at RHIC

**Notes:**
OPAL is a e+e- experiment at LEP (CERN)

## STAR EMC

- Full barrel EMC
- -1.0 < η < 1.0
- Full azimutal coverage
- 120 modules
- (Δη, Δφ)module~ (1.0, 0.1)
- 40 towers/module
- 21 X0
- (Δη, Δφ)tower~(0.05, 0.05)
- dE/E ~ 14%/√E
- Shower Max Detector
- Positioned at ~ 5 X0
- Larger spatial resolution
- (Δη, Δφ) ~(0.007, 0.007)
- Pre-shower detector
- 2 X0
- not avaliable this year

- one tower

- 2 towers, 15 strips in η

- 2 towers, 15 strips in φ

## EMC patch for HI and pp run

- 12 modules installed
- 480 towers
- ()** ~ (1.0, 1.2)**
**Running since late august**
- No SMD most of the time
**Running since middle november**
- pp run
- 22 modules installed
- 880 towers
- ()** ~ (1.0, 2.2)**
- SMD
- High-tower trigger

******=**** **HI run,  ****** ****+**** ******** ****=**** **pp run

## EMC - L0 Trigger

-  L3 trigger can only be considered as an
- event filter - it doesn't add anything new
-  CTB & ZDC - fast but no jet triggering capability
-  EMC - L0 trigger: selection of "interesting"
- events. Can trigger on

- Trigger for high pt physics

-  High towers
-  High energy deposition over a large
- area in the EMC

## Centrality Dependence Relative to UA1

- No NN reference at 130 GeV:
- use UA1 pbar-p, 200-900 GeV
-  normalize for NN
-  hard: binary collisions
-  soft: wounded nucleus
-  scale to √S = 130 GeV
-  flavor & isospin effects are
- negligible

- ~ # of binary collisions

- ~ # of participants

- hadron suppression in nuclear
- collisions @ high pt

**Notes:**
Two limits: hard and soft scaling. Binary collisions limit would be achieved if the outcome of a heavy-ion collision looked like a result of a bunch of independently colliding nucleons. In heavy ion collision there are many other effects present, like e.g. nucleon excitation, etc.
Taa – geometrical factor factor.

## UA1 Fit, Scaled to s=130 GeV

- UA1 Fit, Scaled to s=130 GeV

- Power fit agrees with pQCD calculations @ high pt

**Notes:**
Fitted 200GeV reference data was scaled to 130 GeV
Left plot: Invariant Cross-section
Right plot: Ratio of 130GeV data to 200GeV data, shows agreement with pQCD calculations

## Centrality Dependence Relative to UA1

- Multiple scattering in initial state(“Cronin effect”)

**Notes:**
Up to about 2GeV you see this increase from participant scaling up towards binary scaling limit, and than it turns over, and then as you go to less central it's not as striking of an effect until you get to the more peripheral, and these shaded regions are the errors predicted from the GLAUBER.
As we go to the most peripheral events, we see data points above 1. These points can be explained by Cronin Effect, which is multiple parton scattering in the initial state that gives a parton a certain mean value of Pt prior to hard scattering.

## High pt Azimuthal Correlations

-  Jets are not
- back-to-back in η
- due to parton energy
- distribution within the
- nucleons
-  Limited acceptance

- Jet quenching, 1GeV/fm

**Notes:**
High Pt azumuthal correlations in gold+gold collisions are different from those observed in p-pbar experiments in that in STAR we don't see the increase in number of tracks going in the opposite direction in phi. This could be because the jets are not back-to-back in eta due to parton energy distribution within the nucleons, and  because of the limited acceptance in eta we "miss" jets with rapidity>1. But we do see two peaks in STAR pp data, with the same acceptance, so the absence of a second peak in gold+gold data can be explained by jet quenching, as one of the jets may have to pass through larger amount of matter before it emerges the collision region, losing energy at a rate of 1GeV per fm.

## Leading particle: π0

-  Can be measured in EMC
-  HI run, towers only (no SMD)

- Leading particle: π0

**Notes:**
No other particles but pi0 can be measured because STAR doesn’t have any hadronic calorimetry.

## π0 with SMD

-  High Tower trigger
-  160k pp events
-  All pairs of EMC points

- Stephen Trentalange, UCLA

-  EMC cluster      finder works
-  SMD provides great spatial resolution

**Notes:**
EMC cluster finder works well

## π0 peak for different Pt(pair)

- Peak smears as Pt increases:

- Higher pair Pt

- Smaller opening angle

- Lower resolution in SMD

## π0 yield vs. Pt

- Very preliminary:
-  Uncorrected for efficiency and acceptance
-  Lousy PMT HV settings

**Notes:**
For example, the efficiency of each SMD plane at the lower pi zero energies is probably around 1/3, making a factor of 10 or so for the lower points.  Maybe a factor of 2 for the upper points.So the slope will get a lot steeper, inaccordance with what we observe for the charged particles.

## Conclusions

-   at the right place
-  SMD is highly anticipated for Au+Au
-  Spatial resolution
-  Shower width
-  More results after EMC is completed (including SMD)

## Mid-rapidity Multiplicity

-  Mid-rapidity multiplicity
-  Multiplicity is monotonic
- with centrality
-  Selection of high pt tracks
- biases for more central
- events

**Notes:**
Extra slide (I'm not going to show it)