# Presentation: 132812

## Recent Results from PHOBOS

- 19th Winter Workshop on Nuclear Dynamics
- February 2003

- Edmundo García - UIC
- for the PHOBOS Collaboration

##  Collaboration (Jan 2002)

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

**Notes:**
This is the phobos collaboration, 70 collaborators from 8 different institutions

Talking about number of collaborators, I found the other day in one of my kids book this riddle that I thought sharing with you

## Phobos apparatus (2001)

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

**Notes:**
Phobos detector for the 2001 run, it consists of
Upper figure
Trigger detecotrs: Paddle counters (array of scintillators), cerenkov counters (rings of lucite plastic), and ZDC
Multiplicity detector: Be beam pipe, Octagon, and Ring detectors
Spectrometer: 16 layers of sensors, providing tracking inside and outside of a 2 Telsa fieal of the Phobos Magnet
Two layer silicon vertex detector.
Lower figure
Scale of phobos detector

## Trigger

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- CN

- CP

- Event Selection (average number of participants <Npart>)

**Notes:**
Use as Trigger the time coincidence between several detectors
 Paddle counters time difference define a collision, 
Cerenkov counters  (hardware) trigger on events with in a given (choosen +- 15 cm) range of vertices
ZDC timing is used to reject background
- Sensitive to 97% of inelastic cross section for Au+Au at sNN = 200 GeV
Ones we have the events on tape they are classified by the centrality
which is characterized by the average  number of participants <Npart>
The number of participants is calculated from the truncated mean of the signals from the paddle counters and the response of these detectors from Monte Carlo simulations
(Glauber Model). 
In order to extract Npart for a given fraction of the cross-section we relay only the fact that the multiplicity in the paddles increases monotonically with centrality, verified by correlations between paddles and zdcs)

## Multiplicity Measurement

- Acceptance
- (Single Event)

****

**Notes:**
Multiplicity measurement
Use Octagon, vertex the rings for the multiplicity measurements
 Upper left corner is an example of the energy spectrum
in the Si pads compared to Monte Carlo simulation
Lower figures the acceptance of the detectors is shown, for a single
Event. The different colors in the one dimensional histogram show the
eta coverage of the individual elements up to 5.4 , and in the right one
Shows in addition the two pi phi acceptance 

## dN/dEta

**dN****ch****/d****h**

****

**19.6 GeV***

**130 GeV**

**200 GeV**

**PHOBOS:  nucl-ex/0210015 Oct  2002 - PRL (submitted) **

- centrality

- Centrality:
-  0 - 6%
-  6 - 15 %
- 15 - 25%
- 25 – 35%
- 35 – 45%
- 45 – 55%

- *For 19.6 GeV - instead of  Paddle Counters –
-   Eoct = SDE(path length corrected energy deposited in octagon)
-   for | **** |< 3.2 was used to determine centrality (Npart).

- Typical systematic errors

**Notes:**
Charged particle pseudorapidity distribution measured for Au+Au  at three energies, for 6 different centrality bins
Statistical errors are negligible and the systematic errors are shown as bands for selected bins
As mentioned before, the centrality is calculated with respect to the number of participants, for example for 200 GeV AuAu the most central bib 0- 6% centrality corresponds to <Npart>= 344 and for the most peripheral centrality bin 45-55%  <Npart> = 65 
For 19.6 GeV the much lower beam rapidity precludes the use of the paddle counters for centrality determination so
instead of paddle counters to calculate centrality (Npart) we constructed a different quantity, which is also proportional
to the multiplicity 
 Eoct = SDE(path length corrected energy deposited in octagon) for | h |< 3.2

## Limiting Fragmentation

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

**dN/d******

- selected systematic errors

- 0 – 6 % central

- 35 – 40 % central

- PHOBOS Au + Au

**Notes:**
Taking advantage of the large pseudo rapidity coverage of phobos  we can study the properties of the multiplicity distributions in the fragmentation region
This is done looking at the data at different energies in rest frame of one of the colliding nuclei
 For reference in p+ pbar collisions (showed in the left frame), at high enough collision energy the multiplicity reaches a limiting value becoming independent of energy  at ~ y’ = 0, this behavior refered to as  “limiting fragmentation” and has been fount to be valid for pA and pp collisions 
The limiting value of d2N/dy’dp implies limiting value of dN/dh’ where (h’ = h – ybeam)
For Au Au after dividing the dN/dh’ distributions by <Npart/2> to scale the different number of participant nucleons at different centralities we observe that the limiting fragmentation holds for all the centrality bins as shown in the lower right figure 
Limiting fragmentation applies over broad range of h’, the distributions reach ~90% of maximum before deviating from limiting curve 
Result in contrast with the boos-invariance scenario which predicts fixed fragmentation region and a central rapidity plateau that grows with increasing energy
Limiting fragmentation also present for non-central collisions as shown in lower frame where the extent of the limiting fragmentation is nearly independent of centrality

## ’

******’**

******’**

**dN/d********’/<N****part****>/2>**

- PHOBOS Au + Au

- systematic errors not shown

- 200 GeV

- 19.6 GeV

- systematic errors on error bars

**Notes:**
More on limiting fragmentation
Top figure shows the centrality dependence of the dN/dh’(Npart/2) distributions at two different the two extreme energies
Data suggests that the limiting curve in the fragmentation region changes significantly with the centrality
 This is due mainly to excess of particles at high h’ for peripheral events  and narrowing of overall distribution
Lower figure is the ratio of the dn/h’ per participant pair between non – central (35 –40%) and ventral (0-6%) data, shows that 
The eta distribution is broader for  peripheral collisions than in central collisions   

## Similarity of AA and e+e-  at mid rapidity

- Similarity of AA and e+e-  at mid rapidity

**PHOBOS:  nucl-ex/0301017 Jan  2003 - PRL (submitted) **

**Notes:**
Another empirical observation that we derived from the multiplicity measurements is the comparison
Of the multiplicity distributions in High energy Heavy ion collisions with e+e- and pp/pp- data as shown in this figure.
Here the total pseudo rapidity dristributions at mid rapidity for pp, ppbar, e+e-, and central Au+Au events is shown as function of
the particle-particle center of mass energy. The central AuAu data are scaled by Npart/2 
The AuAu data seem to follow the same trend from AGS to RHIC energies. The ppbar or pp data
On the other hand lays about 30% below the AuAu data at 200 GeV and the difference decreases at lower energies 
At high energies the multiplicity measured for Au+Au scales in similar way to e+e- data.
This may suggest that the yield mechanism for these  systems is  controlled mainly by the amount of energy available 
for particle production

(total charged multiplicity per participant pair).

## Comparison <NCh> vs. Energy 

**<N****ch****>/ <N****part****>/2>**

**Notes:**
Total charged multiplicity per participant pair for pp, pbar e+e- and central Au+Au events as a function of sqrt(s)
The squares for pp and ppbar data are rescaled by data rescaled to effective energy = sqrt(s) – E(leading particle)
Pp e+e- data scale after 20 GeV or so, below tha gold data lays lower in the curve

## Total charged multiplicity scaling with Npart

- Shaded band is uncertainty on extrapolation procedure
- Errors include contributions from overall  Nch and Npart scaling

- Open symbols are UA5 data at 200 GeV and results from an interpolation at lower energies
- The dotted lines show the values
- From the e+e- fit

**Notes:**
This transparency shows the total charge multiplicity per participant pair as a function of the number of participants for AuAu at three different
energies,. Also shown in the figure are the equivalent results for pp- and e+e-, at the same valuse of sqt(s)
Total charged multiplicity in high energy AA approximately sales with Npart (wounded nucleons).
Total particle production in AA per participant pair is the same as total particle production in e+e- at the
Same energy for a broad range of centralities
Paricle production in pp and ppbar is reduced compared to AA and e+e-

## Particle Spectra

**Notes:**
So far I summarized the global results from phobos, I will complement them now with more specific features, turning to the particle spectra
Unique features of phobos for the momentum-measures particles are the close proximity of the detector to the interaction region (10 –80cm), the precize vertex determination (0.1-3mm), good segmentation (0.4-1mm), and the small amount of material between ir and detectors (Be beam pipe) these allows measurement of particles with good momentum resulution with a range of 0.03 – 5 GeV 
Particle spectra measurement are performed with a two arm phobos spectrometer shown (1/2) on the lower right panel.
Upper left panel is the distribution of average energy loss as function of particle momentum, the bands correspond to pions, kaons and protons.
Upper right pannel shows the magnetic field of the PHOBOS magnet and its relation with the spectrometer.
Lower left panel is the contours of the typical acceptance as function of the rapidity. Left for particles bending towards the beam pipe and right opposite bending

## Particle Ratios

- ratio

**<p->/<p+>**

**<K->/<K+>**

**< ****->/< ****+>**

- 12% most central Au+Au

- systematic errors in bars

- PHOBOS:  nucl-ex/0301017,  PRL 87(102301)2001,

- b = 45  5 at 130 GeV to b =  27  2 at 200 GeV    (Tch  160 – 170 MeV)

- 200 GeV:
- 0.2 <  y  < 0.8 K,p
- 0.35 < y < 1.5 

**Notes:**
One of the important measurements that we can do with the spectrometer is the multiplicity ratios of antiparticles to particles
The results are shown in the left graph, where the different ratios are shown as function of the energy.
Note steeper increase for protons, then kaons and finally for pions
For 200 GeV the rapidity coverage for ratio measurements extends from about 0.2 to 0.8 for kaons and protons and 0.35 to 1.3 for pions 
 For anti-protons/protons the ratio increases about 25% from 130 GeV to 200 GeV, this indicates a rapidly decreasing net-baryon density with the energy near mid-rapidity
 Using measured <K>/<K+>,<p>/<p> and a statistical model with Tch  160 – 170 MeV the estimated baryon chemical 
potential goes from 45  5 at 130 GeV to 27  2 MeV at 200 GeV
In right hand figure, we show the results in perspective to different measurements

## Transverse Momentum and Centrality dependence of particle ratios

**Preliminary**

**Preliminary**

- PHOBOS 200 Au+Au

- Errors bars are systematic uncertainties

- 10% most central

**Notes:**
Particle ratios as function of transverse momentum and average number of participants
Ratios, within the errors, seem independent of pT and Npart

## Particle Spectra – low pT

** ****(Ullrich QM2002)**

**Centralities:**
-    Brahms 10 %
-    Phenix 5 %
-    Star 5%
-    Phobos 15%

**nucl-ex/0210037 July 2003**

**Notes:**
One of the unique capabilities of the phobos spectrometer is the identification of particles with low transverse momentum
Identified particle invariant yields as a function of pt for central (0-15%) central Au+Au
Collisions, the boxes show systematic uncertainties.
(These particles are mass identified but not charge identified.)
Right hand transparency is to put this measurements in perspective, taken from talk 9in QM02,
From these transparency we can see that
There is no enhancement in low pT yields
 pbar extrapolate with Phenix at higher pT,  this flattening of spectra may be due to collective radial expansion

## Particle Spectra – high pT

- Particle Spectra – high pT

- PHOBOS:  nucl-ex/0210006 Oct  2002

- Particle rapidities between 0.2 and 1.4  assuming pion mass to calculate y

**Notes:**
Invariant cross-section of charged hadrons, averaging the yields of positive and negative hadrons as function of transverse momentum. The range of of rapidity is between 0.2 to 1.4 assuming the pion mass for calculating y.
 Consecutive bins are scaled by a factor of 10
 6 centrality bins shown ranging from <Npart> = 65 to <Npart>= 344, with associated uncertainty of 4 and 11 respectively
 All spectra shows smooth descent with a slightly concave shape, and agree wityh data from the other experiments. 
Numbers Number of gosth tracks 4% at pt ~ 0.5 GeV/and les than 2% at 2.5 GeV/c

## Particle spectra notes

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

- Multiplicity yield per participant pair divided by a fit to the invariant  cross-section for proton proton collisions (200 GeV UA1)

**Notes:**
In order to study further the evolution of the spectra with centralities the following figures are presented 
This plot is the yield per participant pair divided by a fit to the invariant  cross-section for proton proton collisions (200 GeV UA1)
The gray lines are the systematic uncertainties at 90% confidence level
We can see that even for most peripheral bins the yield ratio increases considerably, relative to p+pbar collisions. 
There is no dramatic increase in particle production per participant beyond  pt = 2 GeV 

## Multiplicity yield per participant pair divided by a fit to the most peripheral bin

**Yield/<N****part****/2>/peripheral fit**

**Notes:**
Multiplicity yield per participant pair for six centrality bins divided by a fit to the most peripheral bin
Notice that going from peripheral to central events there is only a moderate change in the spectral shape.

## Particle Spectra Npart

- Evolution of the charged hadron yield in Au + Au scaled by <Npart>/2, normalized to the most peripheral bin as function of Npart, for selected pT bins

**Notes:**
Finally. We have the evolution of the charged hadron yield in Au + Au scaled by <Npart>/2, normalized to the most peripheral bin as function of Npart.
 Ratio was determined by interpolating data to power law fits
The yields scale approximately with the number of participant nucleons

## Energy independence of dN/d’ over a large range of ’ - Limiting Fragmentation. dN/d’ is not independent of centrality at high ’

- Energy independence of **dN/d********’**** **over a large range of ******’ **- Limiting Fragmentation. **dN/d********’ **is not independent of centrality at high ******’**
-  The fragmentation region becomes a dominant feature of the pseudo-rapidity distributions at high energy
- At high energies the Au+Au multiplicity per participant pair scales in similarly to e+e- data.
- Total charged multiplicity in high-energy Au+Au scales approximately with number of wounded nucleons (Npart).Total particle production in Au+Au per participant pair is the same as total particle production in e+e- at the same sqrt(s).
- Particle Ratios
-  For anti-protons/protons the data indicates a rapidly decrease of the net-baryon density near mid-rapidity
-  Ratios, within the errors, are independent of pT and Npart.

**Notes:**
We have performed a comprehensive examination of pseudorapidity distributions of charged particles produced in Au+Au at RHIC energies from sqrt(sNN) = 19.6 to 200 GeV, including an estimate of the full charged particle multiplicity at three energies. Among the interesting features we have:

## 	pp Data with PHOBOS at 200 GeV	

**  **No enhancement of low pT yields
-   pbar + p extrapolate to PHENIX data at higher pT. Flattening of spectra due to collective radial expansion.
- Particle Spectra (high pT)
-  Found significant changes in the transverse momentum spectra between ppbar and peripheral AuAu
-  Comparing peripheral to central AuAu collisions, the yields scale with the number of participant nucleons
- To Come:
- 	pp Data with PHOBOS at 200 GeV
- 	pp and dA data with PHOBOS at 200 GeV with enhanced trigger 	capabilities and forward proton calorimeter