Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 1 Correlations...

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-pT physics at the LHC, Tokaj ’08

1

Correlations with a high-pT triggerover a broad η range

Gábor Veres

Eötvös Loránd University, Budapest

for the collaboration

-based on a PHOBOS talk at QM’2008 by E. Wenger -

Támogatók: OTKA F 49823, NKTH-OTKA H07-C 74248 Magyary Zoltán Felsőoktatási Közalapítvány


2

The Collaboration

Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton,

Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Vasundhara Chetluru,

Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson,

David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova,

Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey,

Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak,

Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Peter Steinberg,

George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek,

Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger,

Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Bolek Wysłouch

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER


3

Motivation

PHOBOS, PRL 96, 212301 (2006)

One of the most fundamental discoveries at RHIC is that partons strongly interact as they traverse the produced medium

• single-particle spectra, RAA

STAR, PRL 91, 072304 (2003)

• high-pT azimuthal correlations


4

Beep! Beep!

Associated high-pT particles

Where does the energy go?

Look at associated particles at lower momentum!


5

Associated low-pT particlesExisting triggered correlation measurements show novel features in heavy-ion collisions

STAR, arXiv:nucl-ex/0701074v2

• Enhanced correlation (“ridge”) at =0 and large

Au+Au

d+Au

• There is no “ridge”without medium:

PHENIX, arXiv:0705.3060v2

• broadening in of away-side compared to p+p


6

Interpretations of the “ridge”Different proposed mechanisms qualitatively describe “ridge” at |Δη|<2:

• Coupling of induced radiation to longitudinal flow

• Recombination of shower + thermal partons

• Anisotropic plasma

• Turbulent color fields

• Bremsstrahlung + transverse flow + jet-quenching

• Splashback from away-side shock

• Momentum kick imparted on medium partons

Armesto et al., Phys. Rev. Letters 93, 242301

Hwa, Nucl. Phys. A783, 57 (2007)

Shuryak, Phys. Rev. C76, 047901 (2007)

Romatschke, Phys. Rev. C 75, 014901

Majumder, Muller, Bass, Phys. Rev. Lett. 99, 042301 (2007)

Pantuev, arXiv:0710.1882v1 [hep-ph]

Wong, Phys. Rev. C 76, 054908 (2007)


7

The advantage of

• Can use the uniquely broad acceptance of the PHOBOS multiplicity detectors to measure the ridge at large Δ…

• …to constrain the possible explanations for correlated particle production far in from a high-pT trigger

?

?


8

Experimental layoutHigh pT trigger trackpT > 2.5 GeV/c

0 < ηtrig < 1.5

Small acceptance in

Octagon holes are filled using hits from the first layers of the

Spectrometer and

Vertex detectors

Associated hitsFull coverage

Broad η coverage (-3 < η < 3)

Single layer of silicon

No pT information

pT > 35 MeV/c (at η=0)

TRIGGER

PARTICLE


9

Constructing the correlation

= { - a [ ] }

V = <v2trig><v2

assoc>

“Background”:Acceptance-corrected mixed-event pairs (per trigger particle)

Signal/Background.Detector acceptance cancels in the ratio

= - · a [ ]

Modulation from elliptic flow

How can these three terms be measured?


10

1. Normalized background

is the mixed-event pair distribution corrected for the pair acceptance, per trigger particle

In other words, it is the single-particle distribution (dN/dη) convoluted with the ηtrigger distribution

PHOBOS: PRL 91, 052303 (2003)

Δηηtriggerη

Au+Au200 GeV


11

2. Signal and mixed events15-20% central

3mm < vz < 4mm

averaged over the vertex position:

-15cm < vz < 10cm

Au+Au


12

3. Estimating the flow contributionParametrize the published PHOBOS v2 results in a factorized form:

v2(Npart,pT,η) = A(Npart) B(pT) C(η)

PHOBOS: PRC 72, 051901 PHOBOS: PRC 72, 051901

• Correct v2(Npart,<pTtrig>,ηtrig) for occupancy and

v2(Npart,<pTassoc>,ηassoc) for secondaries

V = <v2trig><v2

assoc>

A(Npart) B(pT) C(η)

• V2 is available at any Npart, pT, η


13

Subtracting elliptic flow

The scale factor, a, is calculated such that the yield after subtraction is zero at its minimum (ZYAM)

- a [ ]

Ajitanand et al. PRC 72, 011902(R) (2005)

a

PHOBOS preliminary10-30% central-0.5 < Δη < 0.0


14

Systematic uncertaintiesPHOBOS preliminary

10-30% central-0.5 < Δη < 0.0

PHOBOS preliminary

0-6% central-0.5 < Δη < 0.0

Dominant systematic error: the uncertainty of v2

trig v2assoc

• ~14% error on v2trig v2

assoc (η=0)

• ~20% error on v2trig v2

assoc (η=3)

• In the most central collisions - where flow is small compared to the correlation - the relative error on v2

trig v2assoc can exceed 50%.


15

PYTHIA: p+p reference• p+p data from PHOBOS is limited by statistics • We will compare our Au+Au results to PYTHIA, which

reproduces STAR p+p data reasonably well:

STAR, PRL 95, 152301 (2005)

|η| < 14 < pT

trig < 6 GeV/c0.15 < pT

assoc < 4 GeV/c


16

Results

PHOBOS preliminary

p+p PYTHIA v6.325 Au+Au 0-30% central

pTtrig > 2.5 GeV/c

pTassoc 20 MeV/c


17

A closer look…Correlated yield, short-range (|Δη| < 1):

Significant broadening on the away side

Au+Au


18

The ridge extends to high Correlated yield, long-range (-4 < Δη < -2):

Ridge

Broadening, away side

Au+Au


19

Short-range; centrality dependence |Δη| < 1

The broadeningis growing withcentrality

CENTRAL

PERIPHERAL

Au+Au


20

Long-range; centrality dependence

-4 < Δη < -2

Both featuresare growing withcentrality

CENTRAL

PERIPHERAL

Au+Au


21

Integrated yield of the “ridge”

-4 < < -2

Integrate ridge over || < 1

|| < 1 rad


22

The extent of the ridge in Correlated yield on near-side (|| < 1):

Au+Au

Ridge persiststo high ||!


23

Comparison to predictions

C.Y. Wong, PRC 76, 054908 (2007)

Momentum kick

Trigger

σy = rapidity width of partons

q ||<1

C.Y. Wong, private communication


24

ConclusionsCorrelations with a high-pT trigger were measuredin an extended pseudorapidity region.

Looking forward to LHC, where similar studies willbe feasible with a comparable region.

• Broadening of the away-side correlation in (relative to p+p) persists over the complete range covered

• The near-side correlation at 0 and large (“ridge”) extends to 4

• The yield of the “ridge” at large gradually disappears going from central to peripheral Au+Au collisions

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 1 Correlations...

Documents

Transcript of Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 1 Correlations...