# Presentation: 572789

## Forward Spin Physics at STARRHIC, BNL

- Spin-dependent forward particle correlations in p+p collisions at      = 200GeV

*Nikola Poljak**University** of Zagreb**(For the STAR Collaboration)*

***Forward Spin Physics at ******STAR******RHIC, BNL***

## Single Spin Asymmetry

- Definition:
- dσ↑(↓) – differential cross section of  when incoming proton has spin up(down)

_**positive A**_**N****: **
**more ********0 ****going **
**left ****to ****polarized beam**

## Published measurements at STAR

**STAR:  arXiv:hep-ex/0801.2990**
**accepted for publication in PRL**
**RUN 6**

- Large transverse single-spin asymmetries at large xF
- xF dependence matches Sivers effect expectations qualitatively
- pT dependence at fixed xF not consistent with 1/pT expectation of pQCD-based calculations

## Developments in theory and experiment

- Expectations that the Collins effect is suppressed in p↑+p →  +X (PRD 71 014002) were found incorrect due to a sign error (_[arXiv:0804.3047](http://arxiv.org/abs/0804.3047)_)

- The need remains to separate Collins and Sivers effects in  p↑+p →  +X

- new phenomenological analyses within a generalized parton model can explain both Sivers moments in semi-inclusive deep inelastic scattering and many features of p↑+p →  +X. (PRD 77 051502(R))

- ~20% of the COMPASS transversely polarized proton data has been analyzed and reported.  COMPASS finds non-zero Collins moments and Sivers moments compatible with zero, although expected Sivers moments are small in the *x*,*Q*2 range of their experiment. (_[Levorato](http://www.fe.infn.it/transversity2008/body/pdf/SSA_on_proton_at_COMPASS.pdf)__[, ](http://www.fe.infn.it/transversity2008/body/pdf/SSA_on_proton_at_COMPASS.pdf)__[for](http://www.fe.infn.it/transversity2008/body/pdf/SSA_on_proton_at_COMPASS.pdf)__[ COMPASS; ](http://www.fe.infn.it/transversity2008/body/pdf/SSA_on_proton_at_COMPASS.pdf)__[Ferrara](http://www.fe.infn.it/transversity2008/body/pdf/SSA_on_proton_at_COMPASS.pdf)__[, 2008](http://www.fe.infn.it/transversity2008/body/pdf/SSA_on_proton_at_COMPASS.pdf)_).

## Separating Sivers and Collins effects

**Collins mechanism:** asymmetry in the forward jet fragmentation

**Sivers mechanism:** asymmetry in the forward jet or γ production

**S****P**

**k****T,q**

**p**

- p

**S****P**

**p**

- p

**S****q**

**k****T,****π**

- To discriminate between the two effects we need to go beyond π0 detection to **jet****-like events**

- Sensitive to **proton spin** – parton **transverse motion** correlations

- Sensitive to **transversity**

## Detectors – from FPD to FMS

**Run****s ****3-****6:**** FPD****,FPD++**** **
** ****Inclusive ********0**** cross sections **
** ****A****N**** for inclusive ********0 ****production**

**Detectors – from ****FPD to FMS**

**Run8 and beyond:  FMS **
**FMS will provide full azimuthal coverage for range 2.5 ******** ******** ******** 4.0**
**broad acceptance in x****F****-p****T**** plane for inclusive ********,************,********,K********,... production in p+p and d(p)+Au**
**broad acceptance for ************ and   ******************** from forward jet pairs**

**20x**** more a****cceptance**
**than previous detectors**** **

**uses 3 different settings**
**of modular detectors**** **

**uses a single **
**monolithic detector**

## Forward Meson Spectrometer (FMS)

**New FMS Calorimeter**
**Lead Glass From FNAL E831**
**804 cells of 5.8cm********5.8cm********60cm**
**Schott F2 lead glass**

**Forward Meson Spectrometer (FMS)**

## Calibration

- Event selection done with:

- Offline calibration done** **cell-by-cell
- included energy corrections

**the calibration methodologies employed for the FPD have been successfully adapted to the FMS**

- minbias condition
- Hightower ADC threshold
- (400/200 cts. for small/large cells)

- <0.7 (small) ; <1.0 (large cells)
- fiducial volume cut (0.5 cell)

## Details of data analysis - calibration

- Minimal run-by-run dependence in mass peak observed

- LED system : critical calibration tool

- MIT (LED optics)
- UC Berkeley/SSL (flasher boards)
- Texas / Protovino / BNL (assembly)
- SULI program (Stony Brook students) / BNL
-                   (control electronics)

- Details of data analysis - calibration

## Association analysis – energy corrections

-  comparison of generated quantities to reconstructed GEANT simulations
-  We consider

**Eliminating energy dependence in ************ mass peak gives the correct average neutral pion energy**

**UNCORRECTED**

**CORRECTED**

## Distributions comparison

-  Full PYTHIA/GEANT simulations have adequate statistics to reach moderate xF at large pT
-  Cell mass resolution in data is reasonable, given run-6 FPD performance
-  Simulations have somewhat better resolution than data

**DATA**

**SIMULATION**

**Present understanding sufficient; further investigations to be done**

## Summary & goals

-  verify spin information from STAR local polarimeter
-  extract transverse single spin asymmetries from FMS from
-  data for p↑+ p -> 0 + X as a point of contact with previous work
-  extract transverse single spin asymmetries from FMS from
-  data for p↑+ p -> “jet-like” + X final state.

-  FMS - a new device, with many more channels (1264 detectors compared to 98 for north/south FPD modules).
-  FMS has 20x more acceptance than the previous modular detectors
-  the FMS involves the large cells, not used in the FPD
-  methodologies used in FPD successfully adapted to FMS

## BBC polarization time dependence samples

- Star preliminary

- Flat line fits shown

- Correlation of multiplicity topology in beam-beam counter (BBC) with polarization direction turns out to be good polarimeter for s = 200 GeV
- see J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1
- For run-8 data, analysis of BBC asymmetries, using effective analyzing powers from run-6, is effective quality assurance for the FMS analysis
- Every run for which there is FMS data, also has BBC data.

**Polarization**

- Relative polarimetry consistent with CNI

## First look at analysis results 

- First look at analysis results

- Octant subdivision of FMS for inclusive  spin sorting.

- AN comparable to prior measurements
- Azimuthal variation appears to be as expected
- Systematic errors being evaluated
- First estimate tot.        1.2 stat.

- STAR preliminary

- stat.errors only

- 75% of run-8 data

## First look at “jet-like” events

- Event selection done with:

- >15 cells with energy > 0.4GeV in the event (no single pions in the event sample)
- cone radius = 0.5 (eta-phi space)
- “Jet-like” pT > 1 GeV/c ; xF > 0.2
- 2 perimeter fiducial volume cut (small/large cells)

- The agreement between data and simulation looks convincing

## Conclusions

-  FMS is complete and in place. Commissioned and operated in run-8. It has 20x the acceptance of FPD
-  Reconstruction and calibration procedures successfully ported from FPD to FMS
-   Calibration is mostly complete and data shows good agreement with the simulated sample of events
-  Inclusive 0  AN(xF) from FMS is comparable to FPD precision measurements
-  analysis of jet-like events is under way

## Outlook

-  Complete analysis of “jet-like” events
-  Determine AN(pT) for p↑+ p -> 0 + X
-  Determine AN for final state that contains 0 pairs
-  Determine AN for final states with heavier mesons
-  Run-9 - Go beyond  detection to direct photons + jet final state AN

- THANK YOU

## BACKUP

## Possible mechanisms

- Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]:
-      Flavor dependent correlation between the proton spin (Sp), proton momentum (Pp) and transverse momentum (kT) of the unpolarized partons inside.  The unpolarized parton distribution function fq(x,kT) is modified to:
- Collins effect [Nucl. Phys. B396, 161 (1993)]:
-      Correlation between the quark spin (sq), quark momentum (pq) and transverse momentum (kT) of the pion.  The fragmentation function of transversely polarized quark q takes the form:

## How can the  cross section depend on the proton transversity?

- Proton  quark scattering is insensitive to transverse spin. However, the quark retains its initial spin after a hard scattering, and *the* *quark **** **π**0** **fragmentation** can have azimuthal dependence on the** **transverse spin of the** **quark. *This process is referred to as the* *Collins Effect. [Nucl. Phys. B396, 161 (1993)]
- A quark inside a proton may have orbital angular momentum that is correlated to the spin of the proton. If two quarks with opposite transverse momentum contribute different scattering amplitudes to the same final state, a case can be made where *the **proton **** quark** **scattering** is sensitive to the** **transverse spin of the proton.**** ***This process is referred to as the* *Sivers Effect. [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]

## Collins Effect

**S****P**

**p**

**π****0**

**π****0**

**p****q**

**s****q**

**k****T****π**

**P (any polarization)**

- sq = Spin of the struck quark
- pq = Momentum of the struck quark
- kTπ = Transverse momentum of the neutral pion

- p + p   + X

- The spin of the scattered quark is correlated with the spin of the proton

- The fragmentation of the quark to 0 has sq dependence

- Spin of the proton affects the scattering angle through the spin of the large x quark

- x

- y

- z

## Sivers Effect

**S****P**

**P****p**

**π****0**

**π****0**

**p****q**

**k****T****q**

**P (any polarization)**

- Sp = Spin of the proton
- Pp = Momentum of the proton
- kTq = Transverse momentum of the quark inside the proton

- Quark Parton Distribution Function has kTq dependence

- p + p   + X

- Spin of the proton affects the scattering angle through the quark transverse momentum

- Quark transverse momentum is correlated with the spin of the proton

- x

- y

- z

## Background fitting

- d*f*/d*x* = S / [σ(2π)1⁄2] exp[-(*x*-μ)2/2σ2] + B[β2 / (*t*1 -* t*2)](*x *- *x*0)exp[-β(*x* - *x*0)];

- S: Gaussian peak integral,μ: Gaussian peak centroid,σ: Gaussian width,B:  integral of background function for |*x*i - μ| < 3σ,*x*P:  background peak position,β:  background exponential falloff parameter.

- S and B are spin dependent

- Tuned 2-γ fit, especially for Large cells.  Reduced from 6 to 5 parameters by fixing Eγγ

## Energy-dependent corrections

- 0 peak position depends on the energy
-  Linear correction extracted from 0 peak position and being applied to photon energies
-  works for both 0s and ηs, and significantly decreases shift from zero in E = Esimu - Ereco.

## Resolution smearing

-  A data-driven model is applied to introduce irresolution to the simulation
-  This smearing is taken from the individual detector performance, as measured from high-tower associated invariant mass
-  Applying this to the full PYTHIA/GSTAR simulations of the small cells results in a better match between simulation and data

## Simulation and search algorithm for 00 in FMS and its engineering protoype

-  Without Z vertex information in the calculation above, it is possible to find events where the 0 pair originated at a significant distance from the origin
-  One source of such events are decays KS→00 (31% branching fraction)
-  Plot shows the mass distribution for displaced vertices above 100 cm from the BBC vertex. A pronounced KS mass bump is visible