# Presentation: 736686

## Hadron pT Spectra by PHOBOS from 0.03 to 6 GeV/c

- Gábor I. Veres
***Massachusetts Institute of Technology**** *
- for the                   Collaboration
*International Workshop on Hot and Dense Matter in Relativistic Heavy Ion Collisions*
*March 24-27, 2004, Budapest*

- Hadron pT Spectra by PHOBOS from 0.03 to 6 GeV/c

##                     Collaboration (March 2004)

- Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley,
- Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski,
- Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell,
- Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman, Richard Hollis,
- Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich,
- Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Michael Ricci,
- Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals, Iouri Sedykh, Wojtek Skulski,
- Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov,
- Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Siarhei Vaurynovich, Robin Verdier, Gábor Veres, Edward Wenger, 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**

- Gábor I. Veres

## Outline

**Hadron p****T****-spectra: ****why**** and ****how**** to study them? **
**The extremes:**** low**** and ****high**** p****T**
**Small**** (d+Au) to ****large**** (Au+Au) colliding systems**
**Flavour dependence – ****charged ****and ****identified**** hadron spectra. Particle ratios.**
**Importance in the details: ****centrality**** and ****rapidity**** dependence**

- Gábor I. Veres

## Longitudinal and Transverse Dynamics

_**Low p**_**T**
** ****Probing long distances**
** ****Radial flow**
** “****soft” physics (N****part****?)**

**Longitudinal and Transverse Dynamics**

_**High p**_**T**
** ****Suppression, E loss, quenching...**
** ****Initial/final state effects    (different systems)**
** “****hard” physics (N****coll****?)**

- Gábor I. Veres

## The PHOBOS Detector (2001)

**The PHOBOS Detector (2001)**

**137000 Silicon Pad Channels**

**1m**

**12m Be Beampipe**

**Spectrometer**

**Octagon**

**Vertex**

**Ring Counters**

**Paddle Trigger Counter**

**Č****erenkov Counter**

**DX magnet**

**DX Magnet**

**ZDC**

**ZDC**

**NIM A 499, 603-623 (2003)**

- Au+Au

- Gábor I. Veres

## Silicon Detectors

**Octagon & Vertex**** **

**Spectrometer arm**

**Ring**

**Silicon ****Detectors**

- Gábor I. Veres

## Spectra and PID in

**dN/dp****T**

**PID**

**Mass +Charge**

**0.03   0.2                            1.0                    p****T****, GeV/c**

**up to ****6**** GeV/c**

**Stopping in Si    ****     ****  dE/dx in Si   ****      ****      ToF+Si     ****       ****        Si**

**Mass**

**Z**

**ToF**

**Spectra and PID in**

**Charge**

- Gábor I. Veres

## I.  Hadrons in the low pT range

**p****T**** = ****30 ****–**** 200 MeV/c   (depending on particle ****mass****)**** **

**I.  Hadrons in the low p****T**** range**

**Negligible  B field + multiple scattering:**
**                ****charge sign cannot be measured**** **

** ****Probing long distances**** (t****ruly non-perturbative QCD regime****)**** **
** ****Radial flow **
** ****DCC (enhanced pion yields??)**
** ****Dynamical fluctuations at phase transition??**

- Gábor I. Veres

**Notes:**
PHOBOS detector unique features: proximity to the IP, small amount of material between IP and detector.

## PHOBOS Capability of Low pT Measurements

- z

**-x**

**10 cm**

**y**

**70 cm**

**B=2T**

**Drawback:**

-  	**Advantages:**
-  Sensitive detector layers
-        close to the IP
-  Little material between the
-        IP and Si layers
-  High segmentation of the Si
-        detectors

-  small acceptance of the
-          spectrometer

- Gábor I. Veres

**Notes:**
PHOBOS detector unique features: proximity to the IP, small amount of material between IP and
 detector.

## Finding very low pT particles

**Mass measurements**
**(‘energy-range’ method)**

** ****Cuts on dE/dx**** ****per plane**
**    ****mass hypothesis****  **

-          0       10      20

- 0

- 10

- 20

**Z [cm]**

**Search for particles ****stopping**** in the 5****th**** spectrometer plane**** **

**  ****A ****    ****  B ****   ****   C ****    ****  D ****   ****  E**

**dE/dx**

******  ****E****k****= 8 MeV**

**P  E****k****=21 MeV**

**K  E****k****=19 MeV**

** ****Cuts on ****E****loss**** ****(E****k****=kinetic energy)**
**    ****momentum hypothesis****    **

**  ****E****i****kin****=****dE****i****+dE****i+1****+dE****i+2****...**
** ****M****i****p**** = dE/dx****i**** * ****E****i****kin**** **** ******** ****m**
**      ****  ****   ****(********1/********2****) ( ********m********2****)**

** ****Corrections**
**    ****acceptance, efficiency**** **
**    ****absorption, ****background****    **

**silicon plane**

**Finding very low p****T**** particles**

- MC

- Gábor I. Veres

## Measuring particle mass

**Test of the method:**

**Reconstruction of low**
**momentum MC particles**

**MC**

**Measuring particle mass**

**p+****p**

**K****+****+****K****–**** **

** ********+****+************ **

**DATA**

- Gábor I. Veres

## Invariant yields (Au+Au)

**-0.1< y <0.4**

**Invariant yields (Au+Au)**

- nucl-ex/0401006, submitted to PRL

**Momentum and energy:**
**   ****From carefully calibrated MC**
** ****Yields: binned in p****T**** and**
**        ****corrected**

- Gábor I. Veres

**Notes:**
Insert plot dN/dpT, may be some numbers

## Comparison to the Spectra Measured at Intermediate pT Range

**Comparison to the Spectra Measured at Intermediate p****T**** Range**

**PHENIX - open symbols**
**PHOBOS –closed symbols**

*Log scale!*

**Fit PHENIX ****spectra ****(nucl-ex/0307022)**
**for m****T****<1**** ****GeV/c****2****:**

- +1 for baryons
- -1 for mesons

- Fits: solid curves
- Extrapolations: dashed curves

**Extrapolation of the fits to low p****T ****agrees with our low-p****T**** yields.**
**T****fit**** increases with mass ******** consistent with the collective transverse expansion**

- 1/[exp(mT/Tfit)±1]

- Gábor I. Veres

## Model Comparisons

**Event generators are n****ot a****ble to consistently describe low p****T**** yields.**
**HIJING overpredicts all yields**

- Gábor I. Veres

## I. Low pT: Summary

-   No enhancement of low pT yields  is observed (compared to
-       extrapolations of intermediate pT spectra).

-   Spectra flatten at low pT          transverse expansion

-   Constraints for models and integrated yields.

-  Centrality dependence of the low pT yields
-  Negatively charged particle yields
-  Attempt to measure BE correlations at very low mT

- Future (high statistics Au+Au run):

- Gábor I. Veres

## II.  High-pT spectra. Tracking

- x

**10 cm**

- 1

- 2

**B****y**

- z

- Beam

**1) find straight tracks in the field-free region**
**2) curved tracks found in B field by clustering in (1/p, ********) space**
**3) Pieces matched**
**4) Momentum fit using the full track, and detailed field map**
**5) Quality cuts, DCA cuts**

**Very clean track sample with high efficiency**

- Gábor I. Veres

## High-pT spectra: acceptance

**Acceptance**

**Momentum resolution**

**2001 Au+Au run (200 GeV): Data Sample:**
**        ******** ****7.8 M minimum bias Au+Au events (2004: over 200 M)**
**        ******** ****32 M reconstructed particles**

- Gábor I. Veres

## Triggering on Collisions & Centrality 

- Triggering on Collisions & Centrality** **

**     ****HIJING + GEANT**
**    ****Model of paddle trigger**

- Data

**Centrality Determination**

**%**** ********(****dN****ch**** s****ignal)                    %**** ********(N****part****, N****coll****, b)**

**3% uncertainty in  ********TOT ****(trigger efficiency) **
****** ****less than 10% uncertainty in N****part ****for N****part****>100**

- Paddle Signal (a.u.)

- Gábor I. Veres

**Notes:**
Total cross-section
Fraction of what? (models have a correct TOT) 
RHIC: experiments do not measure TOT
Instead of TOT, estimate of the trigger efficiency based on model (e.g. HIJING)
Systematic error of this estimate is important
Large for small Npart!

## Why Centrality Matters?

**“****Participant” Scaling**

**N****coll****= # of NN collisions: ****~A****4/3**

**N****part****/2 ~ ****A**

**“****Collision” Scaling**

**Why Centrality Matters?**

**N****coll**

**N****part**

**b [fm]**

- Gábor I. Veres

## PHOBOS-Spectra @ 200GeV Au+Au

**PHOBOS-Spectra @ 200GeV Au+Au**

**(GeV/c)****-2**

**Phys.Lett. B 578 (2004) 297**

- Gábor I. Veres

## Scaled Au+Au Spectra / p+p-Fit 

**Centrality range:**
** ****<b> from 10 to 3 fm**
** ****<********> from 3 to 6**

**Phys.Lett. B 578 (2004) 297**

- Gábor I. Veres

## Evolution with Centrality (Au+Au)

- gradual change of shape
- peak develops at 1.5 GeV/c

**Phys.Lett. B 578 (2004) 297**

**Spectra normalized to a fit to the p****T**** spectrum at N****part**** = 65                     (most peripheral bin)**

**Low and high p****T****:  approximate scaling with N****part**

- Gábor I. Veres

## Is Suppression an Initial or Final State Effect?

**Strong suppression of hadron yields at high p****T****!**
- high density strongly interacting matter (final state)? OR
- multi-partonic effects in the nuclear wave-function
-        (initial state)?

**Turn off final state ****to discriminate between ****the two scenarios   ********  ****d+Au collisions**

- Gábor I. Veres

## Predictions for  d+Au

- Vitev, Phys.Lett.B 562 (2003) 36
- Vitev and M.Gyulassy, Phys.Rev.Lett. 89 (2002)

- Kharzeev, Levin, McLerran,
- Phys.Lett.B 561 (2003) 93

***“******~30%suppression of high p******T****** particles”***
**(central vs peripheral)**

**16% increase central vs peripheral**

**Parton Saturation**
**(initial state)**

**pQCD **
**(final state)**

- Gábor I. Veres

## PHOBOS Results from d+Au

**Centrality  ****  <****N****part****>****  **** **** **** ****    <****N****coll****>**
- 70-100%      3.30.7      2.20.6
-  40-70%       6.70.9       5.40.8
-  20-40%      10.90.9      9.70.8
-    0-20%      15.51.0     14.60.9

- Gábor I. Veres

## Cronin Effect in d+Au vs. Centrality

**Cronin Effect in d+Au vs. Centrality**

**  ****6% most**
**central Au+Au**

**Phys.Rev.Lett. 91, 072302 (2003)**

## PRL 91, 072302 (2003)

**PRL 91, 072302 (2003)**

## No high-pT suppression at y~0 in d+Au

**Initial state effects may show up at  HIGHER rapidities?!**

**(small-x region of the Au nucleus is probed)**

- Gábor I. Veres

## Cronin Effect as a Function of (d+Au)

- - all centrality bins together -

## Cronin Effect as a Function of (d+Au)

- - all centrality bins together -

##  Evolution of RdAu with   

## II. Inclusive pT spectra: Summary

- Charged hadron spectra measured in d+Au and Au+Au
-       collisions vs. pT and centrality
-  High-pT suppression in Au+Au observed
-        (compared to Ncoll scaling)
-  Control experiment: d+Au spectra
-               Suppression is not an initial state effect
-                  (strongly interacting quark-gluon liquid?)
-    Latest findings show suppression at high rapidities in d+Au!

- Gábor I. Veres

## III.  Identified Hadrons

-  Identified particle spectra

**III.  Identified Hadrons**

-  Flavour dependence of the effects shown
-  Baryon transport in small and large systems
-  Properties of the system at chemical freezeout
-  Scaling features of different species (mT)

**   ****Important to compare more elementary (****d+Au****)**
**                ****and heavy ion (****Au+Au****) collisions**

**Motivation:**

- Gábor I. Veres

**Notes:**
PHOBOS detector unique features: proximity to the IP, small amount of material between IP and detector.

## PHOBOS PID Capabilities

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

**0.05**

**0.5**

**5.0**

**Stopping**
**particles**

**dE/dx**

**TOF**

**PHOBOS PID Capabilities**

**1/v (ps/cm)**

- Gábor I. Veres

## Antiparticle to particle ratios in Au+Au

**Reversible 2T magnetic field**
**Two symmetric spectrometer arms **

**Independent measurements **
**Acceptance & efficiency **
**           ****corrections cancel**

- B=2T

**A****ntiparticle to**** ****particle ratio****s**** in Au+Au**

- Careful corrections for feed-down,
- absorption in the material, secondaries

- Gábor I. Veres

## Result: ratios at sNN = 200 GeV Au+Au

**<K****–****>/<K****+****> **** ****= 0.95  **** ****±**** 0.03(stat.)   ****±**** 0.03(syst.)**
** ****<p>/<p>  ****  ****= 0.73 **** **** ****±**** 0.02(stat.)  **** **** ****±**** 0.03(syst.)**

**Au+Au ********s****NN**** = 200 GeV****, ****12% most central**** **

**High precision measurements**

- Corrections to the measured ratios :
**                 **+3.7% absorption
-                            +0.7% secondary                                        negligible
-                             -1.2% feed-down

- p/p

**K****-****/K****+**
******-****/********+**

**Result: ratios at ********s****NN**** = 200 GeV Au+Au**

**Phys.Rev.C 67, 021901R, 2003**

- Gábor I. Veres

## Particle Ratios Using dE/dx PID

**Particle Ratios Using dE/dx PID**

**Submitted to Phys.Rev.C, nucl-ex/0309013**

**PHOBOS 200 GeV**

**Mean number of collisions per projectile nucleon  <********>**

- Gábor I. Veres

## ...p/p Compared to Models (p+p, d+Au)

**200 GeV**

**PHOBOS Preliminary**

**Mean number of collisions per projectile nucleon  <********>**

## Particle Ratios Using dE/dx PID

**Particle Ratios Using dE/dx PID**

**PHOBOS 200 GeV**

**Subm. to Phys.Rev.C nucl-ex/0309013**

**Mean number of collisions per projectile nucleon  <********>**

- Gábor I. Veres

## Identified spectra in d+Au

- Only ToF wall can identify above 1 GeV momentum in PHOBOS
- Many experimental challenges to solve
- 1.) high pT-reach desired
- 2.) high collision rate (10-100 kHz)
-  and low multiplicity in d+Au, p+p

- x150 – x500

- improving TOF RESOLUTION:
-  new start time detector
-  increased distance from                                       interaction point
- improving STATISTICS:
-  new high-pT trigger system  (x15 – x50)
-  DAQ upgrade (x10)

- Gábor I. Veres

## Upgrades in PHOBOS for the d+Au run (2003)

**SPECTRIG**

**T0**

**T0**

**mini-pCal**

**pCal**

**Moved TOF walls far (5 m) from IP **
**New, on-line high p****T  ****Spectrometer Trigger**
**New start-time (T0) ****Č****erenkov detectors  **
**On-line vertexing and ToF start time**
**Forward proton calorimeters on Au and d sides**
**DAQ upgrade (x10 higher rate!)**

**Response to Importance of High P****T**** Studies**

**Upgrades in PHOBOS for the d+Au run (2003)**

**Au+Au**

**d+Au, p+p**

**TOF**

**TOF**

- Gábor I. Veres

## Trigger detectors (d+Au)

**Segmented scintillator detectors at 45 and 90 degrees from beam line**

**Combined with the ToF walls: **

** ****selects events with particle**
**     ****hitting ToF and SpecTrig walls**
** ****enhances high-p****T**** (straight) **
**     ****tracks: “online” tracking**
** ****decision-making in 50 ns**

**SpecTrig**

**ToF**

**rejected**

**accepted**

- Gábor I. Veres

## High statistics d+Au track sample

- p

- K

****

**positives, **
**1.6<p<1.8 GeV/c**

- p

- K

****

- Gábor I. Veres

## Particle/Antiparticle Ratios using the TOF

**d+Au**

**per projectile nucleon <********>**

## Identified pT -spectra in d+Au

**Identified p****T**** -spectra in d+Au**

- Not feed-down corrected

- Scale uncertainty: 15%

## Particle Composition in d+Au

**Particle Composition in d+Au**

- Not feed-down corrected

## Comparison: Low Energy d+Au

**Comparison: Low Energy d+Au**

**Cronin, PRD 11, 3105 (1975)**

**y**

**0.2<   <1.2**

**   ********lab****=3.26**

- Not feed-down corrected

## Identified mT-spectra in d+Au

**Identified m****T****-spectra in d+Au**

- Scale uncertainty: 15%

- Not feed-down corrected

## Identified mT-spectra at 200 GeV

**Identified m****T****-spectra at 200 GeV**

**Subm. to Phys.Rev.Lett.**
**nucl-ex/0401006**

**d+Au**

- Scale uncertainty: 15%

- Not feed-down corrected

**Au+Au**

**  ****Spectra **
**normalized**
** ****at 2 GeV/c**

**200 GeV**

- 

## Identified mT-spectra at 200 GeV

**Identified m****T****-spectra at 200 GeV**

**d+Au**

- Scale uncertainty: 15%

- Not feed-down corrected

**Au+Au**

**  ****Spectra **
**normalized**
** ****at 2 GeV/c**

**200 GeV**

- 

**Subm. to Phys.Rev.Lett.**
**nucl-ex/0401006**

## III. Identified Hadrons: Summary

**PHOBOS has PID coverage from 0.03 to 3.5 GeV/c in p****T**
- First PID ratios and spectra shown from the PHOBOS TOF
- Surprisingly small centrality dependence of p/p ratios
- Approximate mT scaling in d+Au as opposed to Au+Au
- Particle composition in d+Au: similar pT-dependence to lower energy data (but different overall proton fraction)

- Gábor I. Veres

## Outlook

**Future of the PHOBOS hadron spectra program:**
** ****Further R****AA ****measurements**
**	****Energy scan (63 GeV, ...?)**
**	****Species scan (Cu+Cu ?   Si+Si ?)**
** ****Identified spectra in p+p and Au+Au at 200GeV**
** ****Charged spectra at very low p****T**** with charge ****	****separation**

**...****and many other observables besides spectra!**

- Gábor I. Veres

## Slide 53

- Gábor I. Veres