Download - Recent Spin and FMS Results at STAR Andrew Gordon Brookhaven National Laboratory Moriond-QCD March 14-21, 2009 STAR.

Transcript

Recent Spin and FMS Results at STAR

Andrew GordonBrookhaven National LaboratoryMoriond-QCDMarch 14-21, 2009

STARSTAR

2

s=200 GeV

1) Longitudinal spin program at STAR

Strong constraint on the size of Δg from RHIC data for 0.05<x<0.2.

Run 9 currently running polarized protons at s=500 GeV for the first time. This will allow start of measurements of W bosons.

STAR data contributes strongly to global fits as in D. deFlorian et al., PRL 101 072001, 2008.

Colliding-Beam Spin Physics: Only at RHIC

Inclusive Jets

3

2) Transverse Spin program at STAR

Run 8 (2008) and earlier runs show large asymmetries in the forward region.

New detector will allow higher kinematic reach and extensions beyond inclusive 0 data.

Colliding-Beam Spin Physics: Only at RHIC

4

Located at far West side of Hall, at the opening to RHIC tunnel. Faces blue beam.

7.5 meters from interaction point.

New for Run 8: Forward Meson Spectrometer (FMS)

Stack of 1264 lead glass cells, roughly 18 X0 in z.

FPD (runs 3+)

STAR Detector

5

FMS newly commissioned for Run 8

Run 5 FPD

FMS provides nearly 20x the coverage of previous forward detectors at STAR

Nearly contiguous coverage for 2.5<<4.0.

North-half, view from the hall

Run 8 FMS

6

East BBC(yellow)

West BBC(blue)

Statistical uncertainties only

~3.5 (statistical) measurement of polarization per bunch per hour

Bunch-by-bunch polarization from colliding beams at STAR

See also J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1, 28 Jan 2005

Expected spin up

Expected spin down

STAR Preliminary

Pol

ariz

atio

n x

Asy

mm

etry

(un

cali

bra

ted

) Run 8 Data

Bunch number at STAR

7

<z>

<xq>

<xg>

NLO pQCD Jaeger, Stratmann, Vogelsang, Kretzer

High rapidity ’s (~4) from asymmetric partonic collisions

Mostly high-x valence quark on low-x gluon

p+p =3.8, √s=200GeV

Fragmentation z nearly constant and high 0.7 ~ 0.8

Why high XF at a Collider?

pp

If Polarization here: XF>0 Valence quark spin effects

Det

ecto

r

If Polarization here: XF<0 Low x gluons and other partons

Which beam is polarized (not averaged over) probes different regions

8

(STAR), PRL 92 (2004) 171801

Data from Jan 2002

Suggests that asymmetry data can be described within the context of pQCD.

Cross sections well described within pQCD framework at √s=200 GeV

Distributions steeply falling with PT and XF.

“Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at sqrt(s)=200 GeV,” PRL 101 (2008) 222001

Runs 2, 3, 5, and 6 forward 0 asymmetry from FPD

FPD set at different distances from beam for the different < ranges.

PT vs XF coverage of data

9

Data: B.I. Abelev et al. (STAR), PRL 101 (2008) 222001

Theory (red): M. Boglione, U. D’Alesio, F. Murgia [arXiv:hep-ph/0712.4240]

Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013

Run 3, 5, and 6 asymmetry data: Theory can predict XF dependence based on Sivers function fits to asymmetries…

dσ↑(↓)=differential cross section when proton has spin up (down).

AN =d - d

d + d

Run 6 Data

Yellow beam polarization = 562.6 %

L=6.8 pb-1

10…but rising PT dependence is not predicted by the same fits

Data broken out in XF bins

XF>0.4

B.I. Abelev et al. (STAR) PRL 101 (2008) 222001

11

AN versus <cos φ> for positive (blue beam)

and negative (yellow beam) XF

AN as a function of xF integrated over

the FMS acceptance.

Plots from Nikola Poljak, for STAR collaboration, “Spin-dependent Forward Particle Correlations in p+p Collisions at s = 200 GeV,” hep-ex/0901.2828, to be published as Spin 2008 conference proceedings.

FMS Acceptance allows azimuthal dependence to be measured

Run 8 dataRun 8 data

Important confirmation of previous data

L~6.2 pb-1 in plot

P(blue)=45.5±3.3%

12“Jet-like” events in the FMS can potentially allow integration over fragmentation products to measure Sivers effect.

Inclusive 0 asymmetries can not distinguish these.

“Collins effect”: asymmetry in fragmentation

“Sivers effect”: asymmetry from initial state KT-spin correlations

“Jet-shape” distribution of energy within jet-like objects in the FMS as a function of distance from the jet axis.

Caveat for Run 8: FMS data was acquired with a “high tower” trigger, which creates a bias towards “jets” that derive from a small number of high-z initial fragments

Plot from hep-ex/0901.2828.

STAR Preliminary

Simulation and Data agree well for “jet-like” events

13

p p M X M 200s GeV

STAR 2006 PRELIMINARY

Heavier mesons also accessible at high XF

Di-photons in FPD with E(pair)>40 GeV

No “center cut” (requirement that two-photon system point at middle of an FPD module)

With center cut and Z<0.85

Average Yellow Beam Polarization=56%

S. Heppelmann, PANIC 2008

14

Inclusive 0 asymmetries include feed-down from higher mass decays.

One source of 0s is the spin 1 (782 MeV)

decays are accessible in the FMS through their 0 (BR=9%) decay channel.

These events are closer to the original fragmentation product.

Spin-1 production is potentially interesting, and explictly occurs in some models of string fragmentation that provide an intuitive picture of asymmetry in fragmentation.

Measurement of vector mesons at large xF

15

Look at all triplets of clusters with E>6 GeV. Apply fiducial cuts of 1/2 cell from all module boundaries

Associate two of clusters with π0 from the ω decay and one with Choose π0 to be pair with mass closest to 0.135 GeV, and photon from ω decay to be the third cluster. A fast, photons-only simulation indicates that this produces the correct identification for nearly 100% of ω→π0γ decays

M(π0 clusters) (GeV/c2)

Run 8 data

STAR Preliminary

All cuts applied (see below)

Require that M(π0) be within 0.1 GeV of 0.135 GeV.

Selection

Pair mass for all triplesKinematic cuts to reduce QCD

background (real π0 decays with a third EM-rich hadronic cluster in the FMS):

•PT(triplet)>2.5 GeV •E(triplet)>30 GeV •PT(photon cluster)>1.5 GeV

•PT(π0)>1 GeV.

16

Run 8 dataPythia+GEANT (background only) (simulation generation ongoing)

STAR Preliminary

Mass distribution of all triples

For this plot simulation is background only

Fit is gaussian + cubic polynomial μ=0.784±0.008 GeV σ=0.087±0.009 GeV Scale=1339±135 Events

Significant (10) 0 signal seen in the data.

Comparison to Run 8 dAu data may be interesting.

17

ConclusionsLarge-acceptance FMS is online

“jet-like” events hold the promise of a direct measurement of the Siver’s effect.

Data should allow asymmetry measurements beyond inclusive 0s

Outlook

Run 9 (running now) should allow STAR to extend constraints on G and also to begin W measurements.

Possibility of future direct- AN measurements with FMS.

Possibility of future Drell-Yan AN with FMS.

Run 8 measurements of AN vs PT for inclusive s at high XF.

Run 8 pp and dAu comparisons.

18

End

19Kinematic cuts to reduce QCD background (real π0 decays with a third EM-rich hadronic cluster in the FMS):

•PT(triplet)>2.5 GeV •E(triplet)>30 GeV •PT(photon cluster)>1.5 GeV

•PT(π0)>1 GeV.

STAR Preliminary STAR Preliminary

Comparison of data (black) to simulation (red)

Run 8 dataPythia+GEANT

Run 8 dataPythia+GEANT

20

Can use large asymmetries to measure (relative) bunch-by-bunch polarization

STAR BBC

Inner tiles cover ~3.5<<5

Sizeable asymmetries can be used to measure beam polarization

See also J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1, 28 Jan 2005

21

} {East-West Coincidence

Bunch Crossing (7-bits){

Scaler Boards

}

24-bit word is histogrammed every clock cycle

Inner tiles of BBC accumulated every clock cycle

Discriminated phototube outputs

22

Inclusive Jets

Phys. Rev. Lett. 97 (2006) 252001

p

p Jet X

Smeared x-range for jets at a few pTs

Jets at each pT are a different mix ofsubprocesses