Latest PHOBOS Results Bjorken is AWOL Mark D

Latest PHOBOS Results Bjorken is AWOL! Mark D. Baker (BNL) for the Collaboration Phobos data on d. N/dh New Phobos data on d. N/dh Particle production in pp&AA collisions – The source isn’t boost invariant – d. N/dh may teach us about QCD Mark D. Baker

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

What do I mean “Bjorken”? Boost-invariant Increasing E y y d. N/dy’ “Inside-out” & 1 dimensional y’=y-ybeam Mark D. Baker 0

Impact of “Bjorken” X X • d. N/dy distribution is flat over a large region except “near the target”. • v 2 is independent of y over a large region except “near the target”. (2 d-hydro. ) • p. T(y) described by 1 d or 2 d-hydro. • Usual HBT interpretation starts from a boost -invariant source. • T(t) described by 1 d-hydro. • Simple energy density formula Mark D. Baker

PHOBOS Apparatus 135, 000 Silicon Pad channels 12 meters of Beryllium beampipe Mark D. Baker

PHOBOS Silicon Detector Octagon Detector Vertex Detector Octagon Detector: 2. 7 x 8. 8 mm 2 Vertex Detector: 0. 4 x 12 mm 2 Mark D. Baker Ring Counter

Many ways to slice pz Rapidity: Generalized velocity Pseudorapidity: ~y: easier to measure Away from mid-rapidity: Mark D. Baker Feynman x: scaled pz

Single Event PHOBOS An event in in. PHOBOS Unrolled: tracklets f -5. 5 Mark D. Baker -3 0 h +3 +5. 5

PHOBOS results at 200 Ge. V Theory 1999 Theory 2000 PHOBOS 2001 PHOBOS 2000 PRL 85 (2000) 3100 PRL 88 (2002) 022302 Total Ncharged ~ 5000 particles Mark D. Baker

RHIC results so far PHOBOS, PRL 88 (2002) 022302 Brahms 200 Ge. V nucl-ex/0112001 Phenix 200 Ge. V preliminary Mark D. Baker

Centrality Dependence |h| < 1 PRC 65, 031901(2002) & submitted to PRC Saturation Model Kharzeev, Levin nucl-th/0108006 Peripheral Mark D. Baker Central

Implications: • Particle production on the low side. • Limited entropy production in late stages. • d. N/dh is a window to QCD 1 Colliding Nuclei Geometry/Saturation Mark D. Baker 2 3 4 Parton Cascade Hadron Gas & Freeze-out QGP? / Fragmentation Gentle Freeze-out Hard Collisions QCD

Analog and Digital Hit-Counting Energy Spectrum (DE) in Si pads f 1 hit Data MC 2 hits -5. 5 -3 Analog: • Sum energy • Divide by E/particle • Correct for material Mark D. Baker 0 +3 h Digital: • Count hits • Correct for multi-hits • Correct for material +5. 5

Latest PHOBOS results Peripheral h Mark D. Baker h h d. N/dh 200 Ge. V 130 Ge. V d. N/dh Typical Systematic Errors h Central h

Collider Kinematics Lab = cm frame S ybeam -ybeam Beam rest frame S’ y’=0 y’=-2 ybeam Fragmenting Target p y’>~0 Mark D. Baker

Results : Limiting Fragmentation Latest PHOBOS results (vs pp) UA 5, Z. Phys. C 33, 1 (1986) A+A PHOBOS 200 0 -6% PHOBOS 130 0 -6% EMU-13 17 0 -9. 4% p + p inelastic (different frame) Systematic errors not shown Au. Au and pp show the same behavior, the extent of the “fragmentation region” grows with collision energy Mark D. Baker

Results. Centrality : Limiting Fragmentation vs Centrality dependence. • Each centrality exhibits limiting fragmentation (E-independence) but the shape in this region depends on centrality ! 200 Ge. V 130 Ge. V Fragmentation Systematic errors not shown 200 Ge. V Data Mark D. Baker 130 Ge. V Data

Fragmentation: Energy dependence Low energy Confusion ANY High energy Fragmenting Target For E>Ethresh, d. N/dy’(y’~0) reaches a limiting value. Mark D. Baker

Let’s pick a different frame. . . Beam rest frame S’ y’=0 y’=-2 ybeam Arbitrary frame S’’ y’’=+3 y’’=3 -2 ybeam = Mark D. Baker

Makes sense for pp p etc. Limiting fragmentation could just be local string fragmentation Also works in e+e- But how about AA? ? Mark D. Baker

Can we see the “Limit Curves”. UA 5, Z. Phys. C 33, 1 (1986) p+p inel. A+A Systematic errors not included Line “p” to guide the eye 1. 45 x line “p” Systematic errors not shown Mark D. Baker

Boost-invariance vs. Outside-in evolution Mc. Lerran, Venugopolan Kharzeev, Nardi, Levin Kovchegov. . . Mark D. Baker

Can “we” calculate the limit curves? • Saturated initial state gives predictions about final state. Þ Nh = c x N g l~. 25 from HERA F 2 data Mark D. Baker Saturation Model Kharzeev, Levin nucl-th/0108006 Not really designed for frag. region. . .

Saturation model in h’ space. . . 900 546 Kharzeev, Levin nucl-th/0108006 (code from P. Steinberg) 200 130 More work needed. . . 53 Mark D. Baker h’

Results. It’s : the Energy Dependence Comparison to pp central AA& that agrees best w/ pp shape !? Despite: • Hard processes • “stopping” • Rescattering • Saturation Systematic errors not shown Mark D. Baker

Peripheral AA is NOT pp Mark D. Baker

Results : Limiting Fragmentation vs Centrality dependence again. • Each centrality exhibits limiting fragmentation (E-independence) but the shape in this region depends on centrality ! 200 Ge. V 130 Ge. V Fragmentation Systematic errors not shown 200 Ge. V Data Mark D. Baker 130 Ge. V Data

Elliptic Flow: A collective effect Beam’s eye view of a non-central collision: Asymmetric particle distribution: f Particles prefer to be “inplane” d. N/d(f -YR ) = N 0 (1 + 2 V 1 cos (f-YR) + 2 V 2 cos (2(f-YR)) +. . . ) Mark D. Baker Elliptic flow

Elliptic Flow PRL 86 (2001) 402 Particle asymmetry V 2 midrapidity : |h| < 1. 0 Hydrodynamic model Preliminary Normalized Multiplicity Mark D. Baker Peripheral Central

Pseudorapidity dependence of V 2 Averaged over centrality PHOBOS Preliminary Errors are statistical only (systematic errors ~ 0. 007) Mark D. Baker h

Summary • The source is not boost invariant – Limiting “fragmentation” works over a broad & growing region of h’. – Elliptic Flow is strongly dependent on h (y) – AA at 200 Ge. V is NOT very boost-invariant! • AA vs. pp – Central AA & pp shape very similar (? !) – Peripheral data shows high h excess. • Outlook – Compare AA & pp WELL at a few energies. . . – QCD understanding of d. N/dh’ in AA & pp? Mark D. Baker

For AA: How about boost invariance? = Stronger & different ansatz: global state between two ~irrelevant “black walls” Mark D. Baker