A STUDY OF TRIGGER ALGORITHMS FOR ULTRA PERIPHERAL COLLISIONS WITH THE ALICE DETECTOR

1
A STUDY OF TRIGGER ALGORITHMS FOR ULTRA
PERIPHERALCOLLISIONS WITH THE ALICE DETECTOR

• Joey Butterworth, Dr. Yury Gorbunov, Dr. Janet
  Seger
• Department of Physics, Creighton University for
  STAR and ALICE Experiments

The ALICE (A Large Ion Collider Experiment)
detector located at the Large Hadron Collider
(LHC) at CERN (European Organization for Nuclear
Research) will study collisions of lead nuclei
traveling more than 99.9999 the speed of light.
These collisions will have a center-of-mass
energy of 2.76 TeV/nucleon. With these
record-high energies, we extend our studies of
ultra peripheral collisions that started at
Brookhaven National Laboratory (BNL). Ultra
peripheral collisions occur when the nuclei pass
one another without overlapping. The intense
electric fields present can be treated as a flux
of photons these photons can interact with the
other nucleus, producing a range of particles,
including vector mesons (Upsilon, J/Psi, rho,
phi, omega) and pairs of oppositely charged
pions. Studies from BNL focused on collisions of
gold nuclei at energies of 130 and 200
GeV/nucleon. By using trigger methods and
theoretical models developed from these studies,
we will extend our understanding of ultra
peripheral collisions and forecast Upsilon and
J/Psi production for ALICE. We have produced
Monte Carlo simulations of J/Psi and Upsilon
production at the LHC. The simulations are being
used to study the effectiveness of the available
trigger detectors in ALICE. The information
learned from these experiments will help
distinguish between current vector meson
photoproduction models. This, in turn, forms a
small piece of the larger puzzle that we call the
physical world. This work supported by DoE grant
DE-FG02-05ER46186.
Particle Accelerators To Study UPCs

• Potential Triggers At ALICE
• Vector mesons from elastic production (ex
  ??ee-)
• Low multiplicity events
• Low track transverse momentum, production in the
  central part of the detector
• What to trigger on
• 2 charged tracks throughout the detector
• No tracks on the sides of the detector due to
  central production
• e e- in opposite sides of the central part of
  the detector
• Detection of emitted neutrons via nuclei
  excitation
• Inelastic production of jets and heavy quarks
  (ex ? g?cc)
• Photon emitting nucleus has a gap between the
  nucleus and produced final states
• What to trigger on
• Low multiplicity events tracks on only one side
  of the detector to account for the gap

• Photonuclear Interactions
• Photon emitted by nucleus fluctuates to virtual
  qq (bar) pair
• qq (bar) pair elastically scatters from nucleus
  (absorb part of photon wave function) and real
  vector meson emerges
• Examples ?, f, ?, and mesons
• Meson decays while conserving momentum, spin, and
  charge
• Interactions can be coherent, which occur when
  the nucleus acts as a whole to emit photons
• Coherent photons have a longer wavelength, ?, and
  transverse momentum, Pt is lower
• Example of a photon-parton UPC with a ?-meson
  decaying into pp-

RHIC
STAR
STAR

• RHIC is located on Long Island. The collider
  accelerates protons and ions to speeds close to
  the speed of light. The particles are split
  into 2 beams with vsNN 200 GeV, which
  counter-rotate and collide at 6 locations around
  the ring. STAR is one of the locations for beam
  collision.
• STAR is comprised of several subsystems and has
  particle tracking and particle identification
  capabilities.

LHC
ALICE

• Current Triggers At STAR Used As A Basis For
  ALICE Triggers
• Used to select events most likely to produce
  vector mesons in UPCs
• Provide an example of experimentally tested UPC
  triggers and a basis for approaching UPCs at
  ALICE
• Minimum Bias
• At least one neutron detected from the decay of
  excited gold nuclei
• Topology
• Events with tracks in the top and bottom are
  vetoed
• Suppresses background from cosmic rays
• Rho candidates with tracks in the north and south
• Access to candidates with and without excited
  gold nuclei decay
• Four Prong
• Low multiplicity but higher than minimum bias
  events
• Neutrons detected from the decay of excited gold
  nuclei
• J/?
• Low multiplicity events with ee- pair separated
  between opposite sides of the detector, and
  neutrons from the decay of excited gold nuclei

• Studying Trigger Possibilities With Monte Carlo
• Simulated events J/??ee-, J/??µµ-, ??ee-, and
  ??µµ-
• To determine geometrical acceptance and
  reconstruction efficiency of the events and the
  resolution of the detector
• Determination of background from directly
  produced ee- and µµ- pairs in order to optimize
  trigger selection for background suppression

• LHC spans the border of France and Switzerland.
  It is designed to accelerate particles near the
  speed of light and for Pb to reach
  2.76TeV/nucleon.
• ALICE is comprised of 13 subsystems to track the
  collisions taking place, and has the worlds
  largest Time Projection Chamber.

Examples Of Vector Mesons J/? And ?

• What Are Ultra Peripheral Collisions?
• Ultra peripheral collisions (UPCs) in heavy ion
  reactions occur when two nuclei miss physically
  with an impact parameter, b20 fm, more than
  twice the radius, R
• Interact electromagnetically
• Different types include photon-photon,
  photon-parton, parton-parton,

A
B
C
D
Process Inv. Mass (GeV) Acceptance Resolution (GeV)
A J/??ee 2.946 16.4 0.15
B J/??µµ 3.035 18.4 0.06
C ??ee 9.161 23.6 0.30
D ??µµ 9.331 24.1 0.13

• Physics We Aim To Study In UPCs With ALICE
• LHC gives the opportunity to study photonuclear
  and photo-nucleon interactions at energies higher
  than any existing accelerator
• Interactions enable the study of subatomic
  structure of hadrons and photons
• Possibility to study elastic production of vector
  mesons like J/? and ?
• Possibility to study inelastic production of jets
  and heavy quarks like cc
• Unique chance to study photon-parton events from
  hadronic interactions.
• Goal is to trigger on (select) events that focus
  on UPCs as they occur in order to conserve
  computing resources and to concentrate efforts on
  desired candidates
• Need to know UPC characteristics and what has
  been successful in the past to make an educated
  first attempt at a trigger for UPCs in ALICE

• Background events from ee and µµ are excluded
  through kinematical limitations at a rate of 99

Conclusion Utilizing the LHCs energies, our
objective is to measure J/? and ? production in
UPC to explore subatomic structures. With
trigger algorithms similar to ones used at STAR,
we expect to have an acceptance of 16.4 for
J/??ee,18.4 for J/??µµ, 23.6 for ??ee, and
24.1 for ??µµ events, and detector resolutions
ranging from 0.06 to 0.30 GeV. These physics
studied will ultimately provide important input
on available theoretical models.
STAR Trigger Results

• A1) Invariant mass taken with the minimum bias
  trigger, which had a trigger efficiency of 40
• B1) Invariant mass taken with the topology
  trigger, which had a trigger efficiency of 12

Min bias
Topology

• Acknowledgements
• Travel supported by Creightons Dean of Arts and
  Sciences
• Work supported by Department of Energy through
  grant DE-FG02-05ER46186
• 1Acceptance plots from Yury Gorbunovs talk at
  the 6th Small X and Diffraction Workshop 2007

STAR preliminary
STAR preliminary
B)
A)
Total 16000 reconstructed events