Forward Physics Initiative at RHIC

1
Forward Physics Initiative at RHIC
F.Videbaek BNL
2
Overview

• Background
• Physics and experimental issues.
• Further explore low-x physics (Gluon Saturation)
• Polarized spin asymmetries at large xF
• Fixed Target opportunities
• Implications for RHIC planning

3
Background

• Forward physics is at present being explored at
  RHIC by BRAHMS (charged identified hadrons) STAR
  (?0 ), PHENIX (muons), PHOBOS (charged particles)
  and via ZDC, PCAL measurements.
• Physics opportunities exist for additional
  measurements at RHIC that are not covered in the
  present program, and should lead to new
  measurements with existing or modified detectors.
• As of now a subset people from Brahms and Phobos
  have been discussing ideas a broader group would
  be helpful to carry this to a proposal and de
  facto experiment stage.

4

• Such ideas were discussed at a one-day workshop
  in October, and in the BNL Physics Departments
  Planning Groups White-Paper.
• The ideas brought forward cover a broad range of
  physics interests. It is not clear the execution
  of these can be covered within a single
  experi-mental setup.
• Some of these ideas have connections to the
  future program with eRHIC In part via
  exploration at large y, small x and through
  possible common detector elements.

5

• Heavy Ions
• Low-x physics
• Gluon saturation is best explored at forward
  rapidities or high energies corresponding to
  small x. The CGC approach is an intriguing
  description of initial state strong interactions
  and offers a connection between RHIC, HERA and
  eRHIC.
• Extended measurements at forward rapidities will
  offer an additional experimental test of this.
  Particularly important will be p(d)-A, and A-d(p)
  measurements

6
d-Au Nuclear Modification factor at ? 3.2
RdAu compares the yield of negative particles
produced in dAu to the scaled number of
particles with same sign in p-p For d-Au min.bias
data Ncoll7.2 Error shown is systematic for
?3.2.
PRL 91 072305 (2003)
BRAHMS preliminary (DNP, Tucson)
7
What is needed to extend these measurements
significantly

• Extend pt reach to 6 GeV/c for inclusive
  spectra.
• Kinematic limit restrict such measurements to h
  2-3.5
• Momenta and PID determination in range of 20-60
  GeV/c

8
Yields and rates needed.

• A much larger solid angle than presently in
  Brahms is required.
• Typical cross section at 5-6 GeV/c is d2N/dptdh
  10-6
• With a factor 10 in d? and having IP2 with b2
  and with luminosities achieved in d-Au run one
  can extend reach to 6 GeV/c with a small but
  significant statistics (100) events/GeV/c) in a
  nominal RHIC run (10-14 wks) of running.

Ref spectrum Hijing 1.383
9
Straw man Detector
Magnet
Calorimeter
Rich
8
4
1.5
Tracking
Such a design can probably give a factor 10 in
solid angle. For larger rates, and forward jets a
radically different design is needed.
10
Benefits

• Such a new/modified detector will bring the d-A
  measurements into a pt region where one might
  expect partonic hard scattering, and can quantify
  observed differences.
• Observing these high-pt particles with the
  opposite side particle production might
  illuminate the question of di-jet vs. gluon
  fusion processes.
• Making such quantitative measurements of the d-A
  and pp with detailed comparisons to theoretical
  descriptions is relevant to the future e-A
  program at eRHIC.

11

• Forward Spin Physics
• Needed measurements in Transverse polarization
  (AN)
• analyzing power for ? and ?- .
• pT dependence for fixed xF
• Analyzing power for jet production
• ?0 AN have been measured by STAR
• Additional measurements are possible with Brahms
  but the range in xF is limited due to solid
  angles.

12
Polarized pp AN measurements
STAR, hep-ex/0310058 (PRL)
Low energy data (AGS) show clear differences
between pi-0. At higher energies the models
used to describe the data differ.Large spin
effects reported by STAR for ?s 200 GeV pp
collisions This makes it an appealing
measurement within the RHIC spin program, and has
been discussed within the RHIC spin group.
13
Fixed Target _at_ RHIC

• Using internal targets, a whole new energy regime
  can be opened up. Potential exists to expand
  RHICs coverage of strongly- interacting matter
  under extreme conditions.
• Good reason to believe the QCD phase diagram has
  interesting features in the region probed by
  10-100 GeV beams on fixed targets
• Good reason to think that high(est??) baryon
  densities are created in collisions of beams
  around 20-30 GeV on fixed targets
• RHIC will carry out some energy scans in coming
  years likely restricted to a few energies
  between 20 and 200 GeV.

14
Motivation

• Recent intriguing SPS data from NA49 in Kaon
  yields and inverse slopes.

RHIC BEAM Colliding
RHIC BEAM Fixed
15
Some Details

• Target options gas jet (only few available
  targets) or possibly atomic beam
• The Physics of first interest have high cross
  sections.
• Europeans are building a low energy machine
  (30GeV) partly justified by interest in this
  physics But it wont be completed for a decade
  or so.
• Potential exists to expand the RHIC program into
  a whole new regime via dedicated runs.
• Work is needed to develop target machine
  operations

16
Other connections

• Other opportunities are present in diffractive
  physics and Ultra peripheral collisions. These
  are options that could be implemented within an
  area with an open geometry like IP2.
• Relation to Cosmic ray physics
• Large uncertainties in knowledge on atmospheric
  interactions, in particular inelasticity
  parameter. Regions of particular interest are
  large xF (0.4-0.9) and lighter ion beams (Fe, N,
  p)
• Part of this community may be attracted to a
  forward physics program.

17
Incorporating this into RHIC decadal planning

• The proposed d-A program will require other such
  run(s) within the frame of the decadal plan (and
  a new/modified detector, size to be determined).
• The spin program would fall into a time-scale
  where polarization and intensities have been
  developed so a (relative) short program can make
  the AN measurements for p- to larger XF
• A fixed target program is envisioned using an
  existing detector system on a shorter time-scale.

18
Conclusions

• This initiative is clearly a work in progress,
  and has several options with details still have
  to be worked out. There are though several
  important physics issues that ought to be
  addressed by the RHIC physics program.
• The region of forward physics do have a clear
  connection, experimental, instrumental as well as
  physics interest to the envisioned program at
  eRHIC.