A STUDY OF TRIGGER ALGORITHMS FOR ULTRA PERIPHERAL

COLLISIONS 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.

AcknowledgementsTravel supported by Creighton’s Dean of Arts and Sciences Work supported by Department of Energy through grant DE-FG02-05ER46186

1Acceptance plots from Yury Gorbunov’s talk at the 6th Small X and Diffraction Workshop 2007

LHC

STAR

ALICE

What Are Ultra Peripheral Collisions?

Ultra peripheral collisions (UPCs) in heavy ion reactions occur when two nuclei miss physically with an impact parameter, b~20 fm, more than twice the radius, R :

Interact electromagnetically

Different types include: photon-photon, photon-parton, parton-parton, …

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: ρ, φ, ω, 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, λ, and transverse momentum, Pt

is lower;

Example of a photon-parton UPC with a ρ-meson decaying into π+π- :

h

Pt

RHIC 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 √sNN = 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.

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/Ψ:

Low multiplicity events with e+e- pair separated between opposite sides of the detector, and neutrons from the decay of excited gold nuclei

STAR Trigger Results

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 world’s largest Time Projection Chamber.

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/Ψ and Υ

Possibility to study inelastic production of jets and heavy quarks like cċ

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

Potential Triggers At ALICEVector mesons from elastic production (ex: Υ→e+e-)

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: γ + g→cċ)

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

Examples Of Vector Mesons J/Ψ And Υ

Conclusion

Utilizing the LHC’s energies, our objective is to measure J/Ψ and Υ 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/Ψ→ee,18.4% for J/Ψ→μμ, 23.6% for Υ→ee, and 24.1% for Υ→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.

Particle Accelerators To Study UPCs

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%

Total ~16000 reconstructed events

A) B)

Pb

Pb

high-pT jet 1

high-pT jet 2

Studying Trigger Possibilities With Monte Carlo

Simulated events J/Ψ→e+e-, J/Ψ→μ+μ-, Υ→e+e-, and Υ→μ+μ-

To determine geometrical acceptance and reconstruction efficiency of the events and the resolution of the detector

Determination of background from directly produced e+e- and μ+μ- pairs in order to optimize trigger selection for background suppression

A B C D

Process Inv. Mass (GeV) Acceptance Resolution (GeV)

A J/Ψ→ee 2.946 16.4 % 0.15

B J/Ψ→μμ 3.035 18.4 % 0.06

C Υ→ee 9.161 23.6 % 0.30

D Υ→μμ 9.331 24.1 % 0.13

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

Min bias Topology

STAR preliminarySTAR preliminary