12-17 February 2007 Winter Workshop on Nuclear Dynamics STAR identified particle measurements at large transverse momenta in Cu+Cu collisions at RHIC Richard

12-17 February 2007

Winter Workshop on Nuclear Dynamics

STARSTAR identified particle measurements at large transverse momenta in Cu+Cu collisions at RHIC

Richard Hollis

for the STAR collaboration

212-17 February 2007

Richard HollisUniversity of Illinois at Chicago

Winter Workshop onNuclear Dynamics

STARSTAR

outline

previous results and motivation

data analysis

data, comparison and interpretation

high-pT and low-pT phenomena across energy and system-size

summary




STARSTAR

previous results and motivation




STARSTAR

two effects: relative to p+p collisions spectral suppression B/M enhancement in Au+Au

effect are turned on and off dependent on centrality

how can one understand these effects? system size dependence energy* dependence

* discussed in the previous talk by Bedanga

Phys.Rev.Lett. 91 (2003) 072304

Phys.Rev.Lett. 97 (2006) 152301

Phys.Lett. B637 (2006) 161previous results:

inclusive spectra & B/M ratio




STARSTAR motivation:

1. why high-pT?

why do we look at high-pT? cartoon showing the

relative effects of Npart and Ncoll scaling

calculable by pQCD





2. why identified particles? why do we look at identified

particles? at high-pT affords insight into

gluon and quark jet production

gluon jets produce more baryons than quark jets

a probe relative energy loss for quark and gluon jets expected, but not seen

Phys.Lett. B606 (2005) 303





3. why Cu+Cu?

smaller system-size is Central Cu+Cu ~

Central Au+Au? or Central Cu+Cu ~

Peripheral Au+Au?

observe path length effects of energy loss

with Cu+Cu can explore the turn on of the suppression

Phys.Lett. B639 (2006) 38

pT>6GeV/c




STARSTAR

data analysis




STARSTAR data: analysis

analysis summary

low-pT (0.2<pT<1.2) energy loss in TPC clear particle separation

low- to intermediate-pT (0.2<pT<3.0) TOF clear particle separation

high-pT (pT>2.5) relativistic rise of energy loss in TPC statistical separation

1012-17 February 2007



STARSTAR data: analysis @ low-pT

analysis summary direct identification from

separated bands at low-pT

4-gaus fits determine the yield

• -, e-, K-,pbar

data are then corrected for feed-down track efficiency

Negative Particles

1112-17 February 2007



STARSTAR data: analysis @ high-pT

analysis summary use the relativistic rise of

energy loss at high-pT

estimate K contribution from bin-counting

3-gaus fits (with K fixed)

data are then corrected for feed-down track efficiency

variety of methods for the kaon contribution are used: bin counting, K0 data and TOF data

1212-17 February 2007



STARSTAR

preliminary results

1312-17 February 2007



STARSTAR data: preliminary results

spectra

1<pT<2 GeV/c gap will be filled by TOF - analysis underway in STAR

spectra proton spectra

1412-17 February 2007




anti-particle to particle ratios

and pbar/p ratios: no centrality

dependence similar pT dependence

as found in Au+Au collisions

pbar/p consistent with Vitev model

Phys.Rev.C 71 (2005) 041901

1512-17 February 2007




nuclear modification factor

systematic differences in RAA between and p data data are flatter with pT (at high-pT) p data have a weak centrality dependence

1612-17 February 2007




proton/pion ratios

baryon/meson enhancement

intermediate-pT enhancement in baryon production is observed maximal enhancement at

pT~2GeV/c

centrality independent for pT>5GeV/c

Phys.Lett. B37 (2006) 161

1712-17 February 2007



STARSTAR

data interpretation

how do the new data compare to existing data?

1812-17 February 2007



STARSTAR Cu+Cu data:

questions:

how do Cu+Cu and Au+Au compare? fractional cross-section number of collisions/participant number of particles produced (Nch at mid-rapidity)

i.e. Cu+Cu (0-20%) =? Au+Au (0-20%) or Cu+Cu (Npart) =? Au+Au (Npart)* or Cu+Cu (Nch) =? Au+Au (Nch)

* Note that from a Glauber

Model calculation, the

functional form of Ncoll is the

same for Au+Au and Cu+Cu

collisions

1912-17 February 2007



STARSTAR data: comparison

nuclear modification factor

Cross-section Npart

similar dependence for same Npart

Phys.Rev.Lett. 97 (2006) 152301

Phys.Lett. B37 (2006) 161

2012-17 February 2007




baryon/meson enhancement

Cross-section Npart

similar dependence for same Npart

Phys.Lett. B37 (2006) 161

Phys.Rev.Lett. 97 (2006) 152301

2112-17 February 2007




path length effects

centrality dependence probes different jet propagation lengths

system-size dependence adds additional new dimension

Phys.Lett. B37 (2006) 161

Phys.Rev.Lett. 97 (2006) 152301

2212-17 February 2007




path length effects enhancement of

baryons gluon jets are more easily

propagated? more gluon jets initially produced or induced for more central data?

need further differential analysis to investigate this versus reaction plane?

• current statistics too limited for this analysis

Phys.Lett. B37 (2006) 161

Phys.Rev.Lett. 97 (2006) 152301

2312-17 February 2007




observed scaling properties at low-pT

Mean-pT Kinetic Freeze-out temperature

for same energy: scaling observed for same Npart

energy dependence: scaling observed with Nch

A.Iordanova, QM2006 poster

2412-17 February 2007




high-pT

similar scaling trends? scaling with Nch (at mid-rapidity) for high-pT data

data from B.Mohanty, QM2006

STAR

Preliminary

STAR

Preliminary

Au+Au 200GeV

Au+Au 62.4GeVAu+Au 200GeV

Au+Au 62.4GeV

++- ++-

2512-17 February 2007



STARSTAR

summary we have a systematic data sample of

identified and p(p) measured at RHIC

systematic effects are observed versus the system size of the collision suppression at large transverse

momenta (RAA) for larger system sizes(i.e. more central data)

enhanced p/ ratio at intermediate pT

Nch scaling for low- and high-pT particles

further differential measurements are needed to help determine the precise nature of the collision dynamics

2612-17 February 2007



STARSTAR

auxiliary slides

2712-17 February 2007



STARSTAR

why more baryons in gluon jets?

e++e- collisionsgluon jets result in:

• softer fragmentation function• higher multiplicities

softer → energy must go into particle productionmore particlesheavier particles

2812-17 February 2007



STARSTAR

why more mesons in quark jets?

in fragmentation: much more likely to produce a than qq qqqq

uddu uu

uddddu uu

2912-17 February 2007



STARSTAR

recombination and fragmentation

competing processes: recombination fragmentation

high-pT

dominated by fragmentation?

intermediate-pT

dominated by recombination?

Figs from B. Muller: nucl-th/0404015, R. Fries: QM 2004, PRL 90 202303 (2003)

DATA and FITS

Thermal Exponential

pQCD power-law

3012-17 February 2007



STARSTAR

recombination and fragmentation

Figs from B. Muller: nucl-th/0404015, R. Fries: QM 2004, PRL 90 202303 (2003)

fragmenting parton:ph = z p, z<1

recombining partons:p1+p2=ph

fragmentation: lessens B/M

recombination: enhances B/M

3112-17 February 2007



STARSTAR

comparison of data

comparison of PHENIX low- to intermediate-pT to STAR high-pT data

note: PHENIX: 0-10%

Central STAR: 0-20%

Central

Documents

12-17 February 2007 Winter Workshop on Nuclear Dynamics STAR identified particle measurements at large transverse momenta in Cu+Cu collisions at RHIC Richard