**Proposal for Forward Hadron Calorimetry (FHC) Addition to STAR**

L.C.Bland^1^, E. Braidot^5^, H.J. Crawford^2^, A. Derevschikov^3^, J.
Engelage^2^, L. Eun^4^, A. Gordon^1^, S. Heppelmann^4^, E.G. Judd^2^, Y.
Matulenko^3^, A. Meschanin^3^, N. Minaev^3^, D.A. Morozov^3^, J.
Nelson^7,2^, M. Ng^2^, L. Nogach^3^, S. Nurushev^3^, A. Ogawa^1^, T.
Peitzmann^5^, C. Perkins^2^, M. Planinic^6^, N. Poljak^6^, A.
Vasiliev^3^, J. Wolf^2^...

^1^ *Brookhaven National Laboratory*, Upton, NY USA

^2^ *University of California, Bekeley/Space Sciences Institute*,
Berkeley CA USA

^3^ *IHEP*, Protvino Russia

^4^ *Pennsylvania State University*, State College, PA USA

^5^ *Utrecht University*, Utrecht, The Netherlands

^6^ *Zagreb University*, Zagreb, Croatia

^7^ *University of Birmingham*, Birmingham, England

13 November 2008

DRAFT version 4

I.  **Introduction and Goals**

**We propose to add for RHIC run 9 two 8×8 stacks of E864 hadronic
calorimeter modules immediately behind the forward meson spectrometer
(FMS). The physics goals motivating this proposal are**

1.  **to minimize the effects of spin-dependent fragmentation as a
    contribution to observed forward pion transverse single spin
    asymmetries by their measurement for complete jets;**

2.  **to extend STAR's large x~F~ capabilities from the meson to the
    baryon sector;**

3.  **to test next-to-leading-order (NLO) perturbative QCD (pQCD)
    expectations for polarization transfer coefficients through
    measurement of Λ polarization in the Λ→nπ^0^ channel.**

**The cost is \~\$25k plus \~16 man-weeks of technical effort, with the
additional labor of staging, operating, and analyzing borne by the
authors.**

Measurement of forward particle production at RHIC has already provided
many new results and stimulated the theory community. Measurements of
π^0^ cross sections were made by the STAR collaboration at large
rapidity for d+Au and p+p collisions at √s=200 GeV \[1\]. Systematic
measurements of π^±^, *K*^±^, *p* and$\overline{p}$cross sections at
large rapidity were measured by the BRAHMS collaboration for Au+Au, d+Au
and p+p collisions at  √*s*~NN~=200 GeV \[2\]. Principal findings from
these forward particle production studies are as follows.

-   Inclusive forward hadron production for p+p collisions at √*s*=200
    GeV can be described by next-to-leading order perturbative QCD, and
    provides important constraints in a global analysis of fragmentation
    functions \[3\] because of the large longitudinal momentum component
    of the produced particles, typically given by the Feynman-*x*
    scaling variable *x*~F~=2*p*~L~ / √*s*. As at midrapidity, forward
    proton and antiproton production cannot be explained by pQCD
    calculations.

-   Inclusive forward (relative to the deuteron beam) hadron production
    for d+Au collisions at √s~NN~=200 GeV is suppressed \[4,1\]. The
    possibility that saturation of the gluon density at low-*x* is
    responsible for this suppression was a primary motivation for RHIC
    run 8, scheduled after completion of a large-acceptance
    electromagnetic calorimeter \[5\], the Forward Meson Spectrometer
    (FMS), in the forward direction at STAR. With the FMS addition, STAR
    now has nearly hermetic electromagnetic detectors for the
    pseudorapidity interval -1 \< η \< 4.

-   Large transverse single spin (SSA) effects are found for forward π
    \[6-8\] and *K* \[8\] production. Similarly large transverse spin
    effects were observed in semi-inclusive deep inelastic scattering
    \[9\], and two dynamical origins of these effects have been
    identified. To date, the dynamical origins of these effects have not
    been identified in p~↑~+p collisions at RHIC, although model
    calculations support a common origin between the SIDIS results and
    the RHIC data.

This proposal is to add modular hadron calorimeter stacks behind the
FMS. The Pb-scintillator hadron calorimeter modules would be stacks of
detectors originally built for the E864 experiment \[10\] at the AGS.
E864 detectors that had been used by the PHOBOS collaboration, are now
the property of one of the proponents. The primary motivation for this
addition is to minimize the bias in triggering on and reconstructing
jets that are produced at large rapidity in p~↑~+p~↑~ collisions. In a
pQCD description, jets are products of the hard scatterings of quarks
and gluons. Symmetric integration about the thrust axis of hadronic
fragments of the produced jet will eliminate spin-dependent
fragmentation (Collins mechanism) \[11\] as a contribution to the
forward pion (kaon) transverse single-spin asymmetries. Consequently, a
non-zero SSA for inclusive jet production isolates contributions from
the Sivers mechanism \[12\] that explains transverse SSA forward pion
(kaon) production by a spin-correlated transverse momentum of the
partons within the proton (orbital motion).

![](media/image1.png){width="2.8895833333333334in"
height="2.9006944444444445in"}
![](media/image2.png){width="2.901388888888889in" height="2.90625in"}

**Figure 1** Simulations of forward jet reconstruction with the existing
FMS and a forward hadron calorimeter (FHC) addition. Jets are
reconstructed, using a standard jet finder used for previous studies,
for electromagnetic energy and observable hadrons within the acceptance
of an FMS half and a FHC module. The reconstructed jets are then
subjected to acceptance cuts in η−φ space, to assure good acceptance for
their hadronic fragments. The resulting narrow acceptance, centered at
\<η\>\~2.7, induces a strong x~F~-p~T~ correlation for the jets.

The forward direction at RHIC remains a frontier for QCD studies. Run-8
results from the FMS for p+p and d+Au collisions at √*s*~NN~=200 GeV
will substantially extend our knowledge of forward meson production to
heavier particles produced primarily at moderate *p*~T~ and large
*x*~F~, and for light mesons over a broad range of *x*~F~ and *p*~T~.
Addition of hadronic calorimetry behind the FMS can extend large *x*~F~
capabilities from the meson sector to the baryon sector.

Combined information from the FMS and the FHC will allow the study of
inclusive lambda production at large *x*~F~ and moderate *p*~T~, by
reconstruction of Λ through its *n*+π^0^ decay. Measurement of the
angular distribution of Λ→*n*+π^0^ decay products determines the
longitudinal or transverse polarization of the produced Λ,  because of
the parity-violating weak decay of the Λ. In a pQCD description,
spin-dependent fragmentation functions relate Λ polarization to the
scattered quark polarization. The scattered quark polarization is
related to the incident quark polarization in a robust way via
polarization transfer coefficients, computed at next-to-leading order by
pQCD for quark and gluon hard scatterings. Consequently, measurements of
the longitudinal polarization transfer coefficient, *D*~LL~, for
$\overrightarrow{p} + \overrightarrow{p} \rightarrow \overrightarrow{\Lambda} + X$at
large *x*~F~ can provide an important test that quark polarization
observed in deep inelastic scattering is universally applicable to all
hard scattering processes. Measurements of the transverse polarization
transfer coefficient, *D*~NN~, for
$p + p_{\uparrow} \rightarrow \Lambda_{\uparrow} + X$can provide a
unique window onto the transversity structure function, particularly if
the transverse spin-dependent fragmentation function for Λ production
can be extracted from *e*^+^*e*^--^ collision data. The optimal
collision energy for these studies is √*s* = 500 GeV. Feasibility for
future *D*~LL~ and *D*~NN~ measurements for large-*x*~F~ Λ production
can be established during the engineering test of first polarized proton
collisions at √*s* = 500 GeV, planned for RHIC run 9.

II. **Requirements**

Observation of inclusive jet production in p~↑~+p collisions at RHIC
requires a detector with acceptance for both hadronic and
electromagnetic jet fragments emitted in a cone of radius at least 0.5
radians in η−φ space, centered on the thrust axis. In its closed
position, the FMS provides electromagnetic calorimetry over 2π in
azimuth for 2.5 \< η \< 4, and so satisfies the acceptance requirement.
The FMS is built from lead-glass detectors having \~18 radiation lengths
but only \~1 hadronic interaction length. Consequently, the FMS alone is
sensitive primarily to the electromagnetic decay fragments from jets. A
modular 8×8 stack of the 10×10×117 cm^3^ detectors from E864 mounted
immediately behind the FMS can measure the hadronic jet fragments in a
cone radius of 0.5 radians.

To minimize systematic errors in the measurement of the analyzing power
requires at least two detector modules, so that symmetries allow use of
the so-called cross ratio method where spin-up and spin-down yields are
separately measured by left and right detectors.

Jet triggers not biased by specific types of fragmentation (for example,
triggers based only on electromagnetic energy deposition in the FMS)
require the addition of calibrated electromagnetic energy deposited in
the FMS and the calibrated energy deposition in hadronic calorimeter
modules. This requirement can be met by the STAR trigger system if the
hadron calorimeter detectors are encoded by existing QT boards, but
forces online calibrations for both the electromagnetic and hadronic
calorimeters. The FMS has been calibrated from reconstruction of neutral
pions into pairs of photons. That calibration, determined in offline
reconstructions, needs to be used to set individual photomultiplier
gains, in a procedure successfully employed for the Forward Pion
Detector prior to RHIC run 6. The hadronic calorimeter detectors would
initially be calibrated by their response to cosmic-ray muons.

Technical requirements are imposed by the physics goals. Left/right
symmetry by staging of two 8×8 stacks of E864 detectors, will require
modification of the present vapor barrier, especially to the south of
the DX magnet, west of the STAR interaction point. Furthermore,
permanent mounting of the 8×8 stacks behind the FMS will likely require
revision of the procedures to prepare the ring vacuum system, following
a STAR rollout. A revision where vacuum preparation is done on the east
side of STAR should be workable.

Furthermore, calibration requirements of the hadron calorimeter
detectors, impose a testing requirement for individual detectors prior
to making the stack in the wide angle hall.

![](media/image3.png){width="5.997222222222222in"
height="3.936111111111111in"}

**Figure** **2** Plan view of the FHC in relation to only the STAR
magnet near midrapidity, showing multiple issues to resolve: (1) note
the interference with the existing vapor barrier and an FHC stack on the
south side of the west DX magnet; (2) this z location of the FHC stacks
would provide only a \~0.45 radian cone radius. Moving the stacks closer
to the IP would provide the required jet acceptance for 8×8 stacks, and
allow integration compatibility of the existing shielding blocks on the
north side with the north FHC stack. To overlap FMS and FHC acceptances,
the two halves of the FMS would need to be opened.

III. **Resources**

The following resources are required to stage FHC:

-   128 co-axial cables to transmit signals from individual FHC
    detectors to 4 QT boards mounted in open slots in the existing VME
    crates used for FMS readout. Estimated fabrication cost: \$15,000

-   computer-controlled low-voltage to power Nanometrics bases, that are
    an existing part of the E864 detector system. The best solution
    would be production of eight more of the 16-channel "Yale controller
    boards", originally designed and fabricated by Penn State for the
    FMS. Estimated cost: \$6,000

-   we estimate expenses for tests and mounting of the FHC to be
    \$4,000. Items included here are a mini-crane for detector tests,
    materials for adaptation of the enclosure, etc.

-   mechanical design support to adapt existing enclosures used for
    hadron calorimeters at PHOBOS, for use in FHC. Mechanical design
    assistance is also required to integrate the FHC enclosures behind
    with existing systems (FMS, west DX magnet, tunnel shielding, vapor
    barriers, etc.). Estimated time for mechanical design work: 4
    man-weeks.

-   Technical support is required for relocation of the E864 detectors
    from their present stackups in IP10 to the STAR Assembly Building
    for tests, and hopefully can be arranged with the
    Collider-Accelerator department. Technical support is required to
    modify and mount enclosures for the FHC detectors. Technical support
    is required to modify the existing vapor barrier. Technical support
    would be useful to help route co-axial cables from the FHC
    enclosures to the readout electronics. Following detector tests,
    technical support is required to help relocate E864 detectors from a
    test stand in the Assembly Building to FHC enclosures mounted behind
    the FMS. It is likely that mini-cranes are required in the handling
    of the E864 detectors for tests and then for stacking in the FHC.
    Finally, survey is required following installation. Estimated total
    technical support: 12 man-weeks.

-   It is assumed that four existing QT boards (value=\$20K) and an
    existing DSM, made available by QT implementation for the forward
    pion detector, will be available for readout/triggering. The QT
    boards provide the primary data encoding and perform module sums, as
    part of the triggering, and would be mounted in FMS VME crates. Data
    transfer from the four QT boards to the DSM board, which serves as a
    layer-1 board, provide the path to trigger. Output from the DSM
    would then go to the existing FP201 board, to enable summing of
    hadronic energy from the FHC with electromagnetic energy from the
    FMS, thereby providing a jet trigger.

It is estimated that \>32 man-weeks of effort will be required and
provided by the proponents for the testing and staging of FHC, in
addition to the mechanical design and technical support listed above.

**IV. Timeline for FHC**

  --------------------------- ---------- ------- -------- ----------------
  Item                        Effort MW  Start   End      People

  Set up test lab in Ass'y    1          11/10   11/24    NM,LE
  bldg                                                    

  Bring cells to Ass'y Bldg   2          11/17   11/21    LB,CAD

  Design Enclosure on         1          11/17   11/21    JS
  platform                                                

  Fabricate platform          3          11/24   12/15    BS
  enclosure                                               

  Design changes to vapor     0.4        11/20   11/26    CAD
  barrier                                                 

  Change vapor barrier        2          12/1    12/15    CAD

  Spec and order cables       0.2        11/17   11/21    LB

  Fab HV modules              8          11/24   01/15    SH,LE

  Test 128 HCAL modules       8          11/24   01/09    NM,LE,Auth.

  Stage modules to platform   8          12/15   01/16    BS,CAD

  Install electronics (QTs,   0.4        12/15   01/09    HC,JE,CP
  DSM)                                                    

  VHDL                        2          12/15   01/23    EJ,CP,MN

  Install and test HV         2          12/15   01/23    PSU

  Analysis code               4          12/01   01/30    LB,AO,HC,JN

  Connect and test modules    4          12/15   01/23    AO,AG, Auth

                                                          

                                                          

                                                          

                                                          

                                                          
  --------------------------- ---------- ------- -------- ----------------

Auth = Authors on proposal

CAD = Collider Accelerator Personnel

LB = Les Bland

HC = Hank Crawford

JE = Jack Engelage

LE=Len Eun

AG = Andrew Gordon

SH = Steve Hepplemann

EJ = Eleanor Judd

NM= Nikolai Minaev

JN = John Nelson

MN = Michael Ng

AO = Akio Ogawa

CP = Chris Perkins

JS = John Scheblein

BS = Bob Soja

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252001.

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\[5\] L.C. Bland *et al.*, Eur. Phys. J **C43** (2005) 427.

\[6\] J. Adams *et al.* (STAR), Phys. Rev. Lett. **92** (2004) 171801.

\[7\] B.I. Abelev *et al.* (STAR), accepted for publication in PRL
\[arXiv:0801.2990\]

\[8\] I. Arsene *et al.* (BRAHMS), Phys. Rev. Lett. **101** (2008)
042001.

\[9\] A. Airapetian *et al.* (HERMES), Phys. Rev. Lett. **94** (2005)
012002.

\[10\] T.A. Armstrong *et al.*, Nucl. Instr. and Meth. A **406** (1998)
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\[11\] J. Collins, Nucl. Phys. B **396** (1993) 161.

\[12\] D. Sivers, Phys. Rev. D **41** (1990) 83; **43** (1991) 261.