The experiment at RHIC

Peter Steinberg

Brookhaven National Laboratory

for the collaboration

DNP2000, Williamsburg, VA

October 4, 2000

Physics of RHIC

Study of hadronic matter at the highest energies

Possible creation of a Quark-Gluon Plasma (QGP)

Data-taking in RHIC Year-1

First Measurements

dN/dh at h=0 vs. N_part

dN/dh over full rapidity range (vs. N_part)

Baryon asymmetry at midrapidity (p/p)

Elliptical & Directed Flow

Charged Multiplicity at h=0

Important physics measurement

Initial entropy: S ~ dN/dy

Energy density

Data (pp,pp) exists at all energies

Models vary by factor of 2

Strong constraint on physics scenarios

Relation: dy = b dh

Centrality: Participants vs. Spectators

Measuring Centrality in PHOBOS

In simulations, signal in paddles correlates monotonically with N_part.

Estimate of N_part

Combining all sources of statistical and systematic error, we measure:

At lower energy, larger relative fluctuations in paddles depletes most central bin

Uncertainty on N_part

Measurement sensitive to trigger bias

“Minimum-bias” still has bias

Affects most peripheral events

Tracklets to measure dN/dh

Tracklets are 3-point tracks

Vertex + 2 planes in SPEC, VTX

Quality controlled by cuts in

Pseudorapidity residual: dh

Azimuthal angle residual: df

Good efficiency

Reduced sensitivity

Non-vertex backgrounds

Strangeness / gamma conversion

Measuring Vertex

Spectrometer tracks extrapolated to vertex (x,y,z)

Combinations of Vertex hits resonate at vertex (y,z)

Octagon detector hits are strongly peaked in Z around main vertex (z)

Time difference between the paddle counters and ZDCs

Corrections to dN/dh

a(Z) comes from simulations

90% of the correction comes from geometrical acceptance

Other considerations

Background subtraction

Uncertainty on a due to model differences

feed-down from strange decays

stopping particles

Total uncertainty on dN/dh is ±8%

Results

Results compared with pp/pp

Progress on new results

dN/dh at h=0 vs. N_part

dN/dh over full rapidity range (vs. N_part)

Baryon asymmetry at midrapidity (p/p)

Elliptical & Directed Flow

Predictions for dN/dh vs N_part

HIJING

Non-linear scaling

Rises by 30% over full range

EKRT (saturation)

Constant over full range

Full geometrical calculation rises less steeply @ low N_part!

Tracklets in the VTX detector

PR00 setup has 4x acceptance

Hundreds of tracks per central event

dN/dEta in the Multiplicity Array

Octagon and Ring detectors cover

Tracking and Particle ID

If central region of RHIC collisions has zero net baryon number

Particle ID

TOF wall

dE/dx in silicon

Two B-field settings

Many systematic effects cancel in the ratio

Elliptic Flow

Expectation:

Asymmetry in initial-state collision geometry

Þ ellipsoidal distribution in final state momentum distribution

Estimate reaction plane

Clear signal observed in |h|<2

Currently extending analysis to use full coverage |h| < 5

Look for directed flow at large h

Conclusions

PHOBOS has performed the first multiplicity measurement at RHIC

Highest particle density measured in man-made collisions

70% higher than pp collisions at similar energies

Strong constraint on model pre/post-dictions

Upcoming results will answer questions about:

Centrality dependence (variation with N_part)

Particle production vs h

Baryon asymmetry at midrapidity

Directed and Elliptic flow over full rapidity range


	Peter Steinberg
	Brookhaven National Laboratory
	for the collaboration

	DNP2000, Williamsburg, VA
	October 4, 2000


	Study of hadronic matter at the highest energies






	Possible creation of a Quark-Gluon Plasma (QGP)


		dN/dh at h=0 vs. N_part
		dN/dh over full rapidity range (vs. N_part)
		Baryon asymmetry at midrapidity (p/p)
		Elliptical & Directed Flow


	Important physics measurement
		Initial entropy: S ~ dN/dy
		Energy density



	Data (pp,pp) exists at all energies
	Models vary by factor of 2
	Strong constraint on physics scenarios

	Relation: dy = b dh


	In simulations, signal in paddles correlates monotonically with N_part.


	Combining all sources of statistical and systematic error, we measure:




	At lower energy, larger relative fluctuations in paddles depletes most central bin


	Measurement sensitive to trigger bias
		“Minimum-bias” still has bias
		Affects most peripheral events


	Tracklets are 3-point tracks
		Vertex + 2 planes in SPEC, VTX
	Quality controlled by cuts in
		Pseudorapidity residual: dh
		Azimuthal angle residual: df
	Good efficiency
	Reduced sensitivity
		Non-vertex backgrounds
		Strangeness / gamma conversion







	Spectrometer tracks extrapolated to vertex (x,y,z)





	Combinations of Vertex hits resonate at vertex (y,z)





	Octagon detector hits are strongly peaked in Z around main vertex (z)






	Time difference between the paddle counters and ZDCs



	a(Z) comes from simulations



		90% of the correction comes from geometrical acceptance
	Other considerations
		Background subtraction
		Uncertainty on a due to model differences
		feed-down from strange decays
		stopping particles
	Total uncertainty on dN/dh is ±8%


	HIJING
		Non-linear scaling
		Rises by 30% over full range
	EKRT (saturation)
		Constant over full range
		Full geometrical calculation rises less steeply @ low N_part!


	PR00 setup has 4x acceptance
	Hundreds of tracks per central event


	If central region of RHIC collisions has zero net baryon number



	Particle ID
		TOF wall
		dE/dx in silicon
	Two B-field settings
		Many systematic effects cancel in the ratio


	Expectation:
		Asymmetry in initial-state collision geometry
		Þ ellipsoidal distribution in final state momentum distribution




	Estimate reaction plane
	Clear signal observed in \|h\|<2
	Currently extending analysis to use full coverage \|h\| < 5
		Look for directed flow at large h


	PHOBOS has performed the first multiplicity measurement at RHIC
		Highest particle density measured in man-made collisions
		70% higher than pp collisions at similar energies
		Strong constraint on model pre/post-dictions
	Upcoming results will answer questions about:
		Centrality dependence (variation with N_part)
		Particle production vs h
		Baryon asymmetry at midrapidity
		Directed and Elliptic flow over full rapidity range