# Presentation: 904351

## Charged particle multiplicity distributions in Au+Au collisions up to 200 GeV from the PHOBOS detector at RHIC

- Pradeep Sarin
- For the PHOBOS CollaborationMay 25, 2002
*Meeting of the Division of Particles and Fields, *
*American Physical Society*

## The PHOBOS Collaboration

_**70 Collaborators from 3 countries**_

- 	_ARGONNE NATIONAL LABORATORY_	Birger Back, Alan Wuosmaa
- 	_BROOKHAVEN NATIONAL LABORATORY _	Mark Baker, Donald Barton, Alan Carroll, Nigel George,
- 		Stephen Gushue, George Heintzelman, Burt Holzman,
- 		Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov
- 	_INSTITUTE OF NUCLEAR PHYSICS, KRAKOW_	Andrzej Budzanowski, Roman Hołyński, Jerzy Michałowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski,
- 		Adam Trzupek, Barbara Wosiek, Krzysztof Woźniak
- _	__MASSACHUSETTS INSTITUTE OF TECHNOLOGY_	Maartin Ballintijn, Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane, Judith Katzy, Piotr Kulinich, Jang Woo Lee, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Carla Vale, Gerrit van Nieuwenhuizen, Gábor Veres, Robin Verdier, Bernard Wadsworth, Bolek Wysłouch
- 	_NATIONAL CENTRAL UNIVERSITY, TAIWAN_	Chia Ming Kuo, Willis Lin, Jaw-Luen Tang
- 	_UNIVERSITY OF ILLINOIS AT CHICAGO_	Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer
- 	_UNIVERSITY OF MARYLAND_	Abigail Bickley, Richard Bindel, Alice Mignerey, Marguerite Belt Jones
- 	_UNIVERSITY OF ROCHESTER_	Joshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly,
- 		Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs

## Probing the phase diagram of QCD..or..Why collide heavy ions?

- Probing the phase diagram of QCD..or.._Why collide heavy ions?_

- ICE

- WATER

- VAPOR

- ~1012 K

- NUCLEI

- QGP

- Melt IceBoil WaterVapor
- Do this at many pressures..
- ... *Thomson, Kelvin, Carnot (1840-1860)*Understand phase diagram of water... *Theories of Condensed Matter *

- Collide nucleiMake QGP
- Do this at many energies..
- ... *AGS, SPS, RHIC (1987-present)*Understand QCD phase diagram ... *Theories of Matter  *

*A question of scale!*

**Notes:**
I would like to start my talk by providing some motivation on why we collide heavy ions...

Lets look at an analogy with a very simple material – water: back in the mid 1800’s people like Thomson, Kelvin and Carnot were the first to perform experiments on water and determine the relations between different forms of this simple substance. By melting ice and boiling water into vapor at various pressures, they were able to map out this phase diagram (As you lower the pressure, the temperature at which ice melts decreases etc..)

Statistical and thermodynamical theories of condensed matter were then developed to understand and explain in detail the features of this phase diagram for water and other simple materials : orders of phase transitions were dervied, critical phenomena and critical points defined.

In QCD, the situation is slightly reversed. The theory has long predicted the existence of different phases of hadronic matter: nucleons, partons, quark gluon plasma, and recently color superconductors . But the complex (non-Abelian) nature of the theory prevents us so far from attacking the phase diagram in detail. 

This is where the experimentalists come in: what we do in the lab is very similar to ‘melting ice’: we collide nuclei to melt nuclear matter and probe the other predicted phases of QCD. These experiments have been done at the AGS, SPS accelerators, and the RHIC collider over the past 15 years.

Note the difference in scale in this analogy: water on the left side boils at 100C, nuclear matter is thought to undergo transition at 10^12 K!   

## Relativistic Heavy Ion Collider (RHIC)

-  2 rings, 3.83 km circumference
-  Can collide any two nuclear species
-  .. Up to a CMS energy 	        = 200 GeV for Au+Au                  = 500 GeV for p+p

-  **Luminosities:**    ~1026 cm-2 s-1 for Au+Au    ~1032 cm-2 s-1 for p+p
-  **4 Experiments** to observe   heavy ion collisions

**Notes:**
Possibly Remove

For those of you that may not familiar with the Relativistic Heavy Ion Collider (RHIC for short), here’s a bird’s eye view...

Well the first thing to note is the scale of things, a 10^12 increase in the scale of temperature from the phase diagram of water means also about a 10^12 jump in the cost! 

RHIC consists of two concentric rings in which two particle beams are simultaneously accelerated in clock-wise and counter-clock-wise directions.

The beams are bent and brought into collision at 6 points along the circumference: 4 of these currently have experiments in place to observe what comes out of these collisions. 
PHOBOS is located at 10 o’clock on the RHIC clock face

The collider is capable of accelerating many particle species, for Au+Au the maximum design energy in the CM system is 200 GeV,
For proton on proton collisions it’s 500 GeV. Polarized proton beams are also in the RHIC program.

## Melting the QCD Vacuum

- Colliding nuclei

- HardCollisions

- Parton Cascade

- Freezeout

- VNI Simulations: Geiger, Longacre, Srivastava, nucl-th/9806102 Animation: J. Mitchell (UW, Seattle)

**Notes:**
Maybe remove this slide?

This is an artist’s conception of what a ‘central’ heavy ion collision looks like in the center of mass system, 
which coincides with the lab system in the case RHIC.

The two nuclei traveling at speeds close 99.99% of the speed light are highly Lorenz contracted pancakes.
(The purple color represents what?)

When they collide at t=0, we may have lots of hard collisions between the constituent nucleons

But as the nuclei separate immediately afterward, they leave behind in their wake a region of ‘excited vacuum’: the soft gluons form a parton cascade

Which as time progresses, freezes out into hadrons that stream outward into the detectors.

(At least get correct description hints from Wit’s audio UCSB? The above sucks!)

## Why count charged particles?

- _Why count charged particles_?

- Charged-particle production in heavy-ion collisions:
_*Geometry of collision*_: impact parameter, participants/spectators
_*Entropy production*_: 		**Au+Au********1000’s of particles** 	Where does it come from? 	  Initial, partonic or hadronization stage? 	What are the mechanisms?	  Binary collisions, gluon saturation, fragmentation?
- Lots of existing data for pp, pA, AB, AA collisions at different energies
- How do RHIC data fit into this picture?

**Notes:**
From this picture of heavy ion collisions, it is clear that just about the simplest thing one can do with the output is to count how many particles come out of the collision
Now Charged particles are easy to detect, they deposit energy by ionization – so let’s just count the charged particles : what does this tell us?

The first thing to factor out is how is this number influenced by simple geometry: the nucleus is an extended object, so as the extent of overlap or ‘impact parameter’ varies, the number of participating nucleons also varies : does this cause the number of produced particles to vary in a sensible way?

The second important question is one of entropy production: we have two heavy nuclei with about 400 total constituents going in -> and thousands of particles come out..Where does the entropy come from?Is it mostly produced in the initial state or the partonic stage, or does it just come out of the final hadronization stageWhat are the mechanisms for producing this entropy?

One has to remember that entropy, once produced cannot be destroyed (even in a non-equilibrium system) : so the charged particle multiplcity which is proportional to the entropy of the final state, provides strong constraints on the entropy production at the various intermediate stages of a heavy ion collision 

There is also a lot of data (and phenomenology) for electron-electron, proton-proton, proton-nucleus and nucleus-nucleus collisions at different energies : we have to see how RHIC data fit in, and if we can form a unified picture of hadronic particle production mechanisms!

----------------------------------------------------------
dN/dh is proportional to entropy density at freezeout and provides a bound on the initial state entropy(entropy doesn’t require equilibrium)
It constrains the entropy production in the initial state and its subsequent evolution 
Interplay between hard and soft processes
Effects of shadowing, jet quenching
Reaction dynamics, re-scattering

## PHOBOS Experimental Setup

-  137,000 Silicon Readout Channels
-  1,300 Scintillator Readout Channels

**Nominal Interaction Point**

## Triggering on Collisions & Classifying them

- - Coincidence between Paddle counters near |tp-tn| = 0 defines a collision
- - Paddle + ZDC timing reject background
- - Sensitive to 97% of inelastic cross section for Au+Au at sNN = 200 GeV

-  Impact parameter *b *not accessible
-  Classify events by mean number   of participants <Npart>
-  Compare Paddle detector signal   response to Monte Carlo simulations   to estimate <Npart> 	        ..... ***Geometry!******                         (Glauber model)***

**Notes:**
Number of colliding nucleons relates to physics...

## 4 Silicon Multiplicity Detector

- 4 Silicon Multiplicity Detector

- Ring detector module8 x 8 pads each

- Octagon detector module4 x 32 pads each

- 12m of Be beam-pipe

## Choosing phase space variables..

- Fragmentation
- Region

- Fragmentation
- Region

- 0

- 1

- -1

- xF~0.1

**All CMS Energies**

- 0

- ybeam

- -ybeam

- ~(ybeam-2)

- 0

- ybeam

- -ybeam

**A+B********C+X**

**Experimentally, easier to measure!**

- A

- B

- C

- X

- 

- ~4.8 (sNN = 130GeV)

- ~5.4 (sNN = 200GeV)

**Notes:**
Y=tanh-1(beta_z)

##  Coverage of Detectors

** Coverage of Detectors

**

- For vertex at Z=0.

- 1m

- 2m

- 5m

- 3.2m

- Octagon

- Rings

- Paddle Counters

## Measuring dNch/dOverview

_***Single Event Display***_* **Energy deposition*
*in multiplicity*
*detectors for 1 event.*

- Measuring dNch/dOverview

*N*(***,b*)* *×*O*(**,*b*) × *f**B*(**,*b*)

*A*(**,*Z**VTX*)

- 

-   z or ******

- Correct for detector Acceptance ***A*****(***********,Z******VTX*****)*** *

- Correct for the occupancy per hit pad ***O*****(***********,b*****)**

- Fold in a background correction factor ***f******B***(******,*b*)

- Count hit pads ***N*****(***********,b*****)** binned in **, centrality (*b*)

## Occupancy Correction O(,b) 

- Occupancy Correction ***O*****(***********,b*****)**

***N *****= number of tracks/pad**
********* *****= mean number of tracks/pad**

- z

-   z or ******

- 

**Notes:**
keep description very short

## Background Correction

- MC, reconstructed Output

- MC Input(primaries)

- 

- Compare PHOBOS Monte Carlo input to reconstructed output  the ratio gives corrections for background hits fromsecondaries generated in thebeampipe and decay particles.

***f*****B****=MC****Truth****/MC****OccCorr**

- dNch/d**

- 200

- 400

- 600

**Notes:**
Condense
compare monte carlo input to output – only
Correction for non-primaries, -  secondaries from beampipe (most), decays (little)

## Putting it all together...

- Au+Au at 200 GeV : Preliminary

- AuAu at 130 GeV  :  PHOBOS PRL 87, 102303 (2001)

- _Putting it all together_...

- Typical systematic errors

**130 GeV: <N****ch****>=4100 ± 210              200 GeV: <N****ch****>= 4960 ± 250**

**Notes:**
Spend more time here – first results!

## Results & Conclusions - I

- _Results & Conclusions - I_

- Fragmentation

- Systematic errors not shown

- UA5, Z.Phys.C33, 1 (1986)

- 53 GeV

- 200 GeV

- 545 GeV

- 900 GeV

- pp

- Central AA

- Fragmentation

- _Limiting Fragmentation in AA collisions!_*“At high energies the number of particles **produced by the fragmenting target is independent of the **target, projectile and beam energy”-*Benecke et al, PRC 188 (1969) 2159
- Chou-Yang geometrical model of hadronic collisions * *

- SPS data: PRC 62, 014903 (2000)

- The extent of the limiting curve **grows** with increasing energy

**Notes:**
Define and explain division by N_part/2, shifting of eta by beam rapidity.

Point about fragmentation region..(dima): when you go to the rest frame of the target nucleus : you have the following : you have the wavefunction of the oncoming nucleus at very small x, and this one is at x very close to one. So you have a very dilute system of valence quarks being hit by a very dense system of gluons : beyond a point, it doesn’t matter how dense!

The interaction occurs with probability 1 and indeed you expect such limiting curves.

What is surprising is the extent over which this limiting behaviour is seen. And this extent grows with increasing c-o-m energy

## Results & Conclusions - II

- _Results & Conclusions - II_

-  The extent of the limiting   curve is **independent** of   centrality of collision
-  The shape in this region   depends on centrality: in   the far-forward  region,    the yield is larger for    peripheral collisions!

- Expect Feynman scalingand limiting fragmentation to work here..                    .. *It does!*

**Notes:**
Entire shape evolves, not just piling up particle production at mid-rapidity

## Results & Conclusions - III

- _Results & Conclusions - III_

**Charged particle multiplicities at mid-rapidity, **_**Centrality Dependence**_

- PHOBOS PRC, In Press (nucl-ex/0201005)

- Model curves: Kharzeev and Levin,  Phys. Lett. B523, 79 (2001)

- X~0.09-0.11npp~2.2-2.4

-  PHOBOS measurements   at 2 energies
-  _Pictures of initial state_:   Initial state gluon   saturation          	      or   2-component model:   “hard” binary collisions of   nucleons+“soft” color   exchange among   wounded nucleons      *Hard to discriminate...** *
* *In any case: very little    left for *final state* entropy   production!

**Notes:**
Small rise at most central – from dima – accountable by the fact npart is bounded from above. So when we map dndeta (unbounded) by npart (bounded), there’s bound to be a rise at the edges.

## Results & Conclusions – IV

**pp**

- _Results & Conclusions – IV_

**Charged particle multiplicities at mid-rapidity, **_**Energy Dependence**_

- PHOBOS PRL **88**, 22302 (2002)

-  Smooth ln(sNN ) evolution of mid-rapidity charged particle multiplicities
-  Yield at mid-rapidity is 55% higher than pp collisions at similar   center-of-mass energies

**Notes:**
Slip 2nd figure into new slide?

## Summary

- _Summary_

** ****dN****ch****/d******** in 4******
-  200 GeV : <Nch> = 4960 +/- 250  (||< 5.4) for 6% central
-  130 GeV : <Nch> = 4100 +/- 210  (||< 5.4) for 6% central
-  Evidence for limiting fragmentation in AA collisions 
** ****dN****ch****/d******** at mid-rapidity**
-  Au+Au 55% higher yield  than pp at maximum RHIC energy
-  Au+Au data at 200 & 130 GeV are in good agreement with initial state parton saturation model predictions and the two-component fits to the data (hard + soft)
-  Smooth evolution of yields as a function of ln(sNN)

## ...Some extra slides...

## 1 – Counting Hit Pads N(,b) 

- 1 – Counting Hit Pads ***N*****(***********,b*****)**

-  Merge energies shared in adjacent pads

- E1

- E2

- E3

- +

- +

-  Reject pads hit by   background
-    particles

- E (“MIP”)

- Si

- From vertex

- Not fromvertex

**

**Notes:**
Possibly remove 1, 2 ,3, 4

## 2 – Calculating Acceptance A(ZVTX) 

- 2 – Calculating Acceptance ***A*****(*****Z******VTX*****)*** *

-  Simple ray-tracing: 3-vectors drawn from a   given ***Z******VTX *** to random values of    ||<6,0<<2.

-  For a given [***Z******VTX ***],  the ratio of numbers   vectors that hit an  active pad to total   thrown gives the  acceptance ***A*****(*****Z******VTX*****)*** *