# Presentation: 248193

## George S.F. Stephans
Massachusetts Institute of Technology
for the                 i
collaboration

- Massachusetts Institute of Technology
- for the                 i
- collaboration

- Two-particle angular correlations in p+p and d+Au collisions

## Two-particle angular correlations in p+p and d+Au collisions

- A hint of

- Plus results for fluctuations of rapidity distributions in Au+Au

##                   Collaboration (August 2005)

- Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel,
-  Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,Piotr Kulinich, Chia Ming Kuo,
- Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen,
- Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland,
- Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg,
- George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek,
- Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm,
- Frank Wolfs,  Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

## Sample 2-particle correlation

- Raw data

-              d+Au @ 200 GeV  MinBias

- 

- 

- Unique to Phobos

## Dominant physics in raw correlation

- Detector effects (-e****, etc.)

- 

- 

-              d+Au @ 200 GeV  MinBias

## Dominant physics in raw correlation

- Momentum conservation

- 

- 

-              d+Au @ 200 GeV  MinBias

## Status of Phobos Results

- Why do this in Phobos?
- Large  available
- Can look at  or  at large ||
- Work ongoing to remove uninteresting effects.

## Change of Topic

- In addition to looking at correlations in , Phobos can study essentially all of dN/d either averaged or on event-by-event basis.
- Many physics possibilities:
- Dependence on energy, system, and centrality
- Event-by-event correlations and fluctuations

## Extended longitudinal scaling - I

- When effectively viewed in the rest frame of one of the colliding nuclei, dN/d appears to be independent of energy over a very large range of ',
- denoted “extended longitudinal scaling” (previously “limiting fragmentation”).

- Au+Au

- 19.6

- 62.4 (Prelim)

- 130

- 200

- Similar scaling observed for flow:
- Implies effect is set at an early stage.

## dN/d for Cu+Cu_x0010_

- dN/d for Cu+Cu_x0010_

- Extended longitudinal scaling is also observed to hold for Cu data.

- 62.4

- 200

- Cu+Cu
- Phobos
- Prelim

## Extended longitudinal scaling - II

- The shape is a function of centrality but the scaling with energy is repeated for each bin.

## Extended longitudinal scaling - III

- The factorization of the centrality and energy dependence is quite remarkable.

- Take the peripheral dN/d,
- normalize by Npart,
- divide by the central dN/d,
- also normalized by Npart

## Factorization of Energy and Centrality

**dN/d******** for 35-40% over 0-6%, each normalized by N****part**

- Take
- Periph
- over
- Central

## Observations on dN/d

- Complete factorization of centrality and energy observed in all data studied to date.
- It seems inappropriate to separate longitudinal phase space into distinct “fragmentation” and “central” regions governed by different physics.
- Differences in particle density will produce variations in final-state effects but the overall shape is set by the initial energy and centrality.

- “As we discovered on the train, tomorrow never happens, it’s all the same %#&*% day”  J. Joplin

## Some Related Studies

- Do regions of  correlate event-by-event?
- P.Steinberg talk last Saturday
- Are there events with very large multiplicity?
- Does the shape of dN/d vary event-by-event?
- These constitute the remainder of this talk...

## What we did - I

- Used 3% most central event sample in high statistics 200 GeV Au+Au data set.
- About 1.96M events pass all quality cuts
- Looked at the distribution of the total number of hits in the multiplicity detectors.
- Note that these analyses required multiple passes through the entire data sample (not just the 3%), each pass took about 2-4 hours using PROOF and distributed disk storage.
- See poster by M. Ballintijn

## Results - Ia

- There is a tail on the high-total-hit side

**Cut**

- 570 evts

- More than
- one event??

## Could it be pileup? 

- “Pileup” means a single event in which the data is affected by more than one actual “collision”, including beam-gas and halo from upstream.
- Scintillators and Si have different integration times.  Result also depends strongly on vertex location and is different for beam gas, beam-beam, halo, etc.
- Collisions from different or same bunches.
- Rate of each depends on how the beam is distributed into bunches, bunch crossing time, etc.
- Pileup within the Si detector integration time, or in the same bunch crossing, estimated at 6000 and 800 events, respectively, out of 2M.

## Results - Ib

- We found that these events are strongly correlated with the beam rate ➩ Pileup?
- Rate of events extrapolated to low luminosity is approximately consistent with zero

## What we did - II

- Divided dN/d distribution into individual bins and calculated average and variation.
- Compared each event to the average and looked for highly unusual events...

## Some details - IIa

- Normalized each event total to remove remaining fluctuations in total yield.
- Binned events in Z&Y vertex location.
- X vertex didn’t vary significantly.
- Used number of hit pads in bins in 

## Some Details - IIb

- Compared raw dN/d

## Some details - IIc

- Important to use the measured variance in each  bin, distributions are not Poisson

## Results - IIa

- 2 distribution shows a distinct tail

- 200 evts

## Results - IIb

- However, these events are also strongly correlated with the beam rate ➩ Pileup again?
- Rate of events extrapolated to low luminosity is again approximately consistent with zero

## Summary

- Many                   results are in progress for correlations and fluctuations.
- Extended longitudinal scaling indicates that mid- and far-from-mid-rapidity cannot be treated as totally distinct.
- First event-by-event investigation of total number of hits and overall shape of dN/d in the most central Au+Au collisions @ 200 GeV indicates that both are very stable (at the rate of ~104 or possibly lower).