Search for Magnetic Monopoles at RHIC
by Vasily Dzhordzhadze
The separate existence of two opposite electric charges is a
well known phenomenon in Physics. Examples are electron -
proton, positron - antiproton etc. At the same time Nature
shows, that magnetic charges or magnetic poles exist only in
pairs, North and South. No single magnetic poles or Magnetic
Monopoles (MM) have been seen so far. Any attempt to break
magnets [1], gives again objects with two magnetic charges.
A complete description of all known classical electromagnetic phenomena is represented in Maxwell’s laws of electrodynamics [2], which doesn’t contain magnetic charges. It is however easy to modify these equations by including a magnetic charge density and the resulting equations thus become symmetric. Symmetry has always played an important role in guiding physicists towards the development of successful theories. One could thus argue that the existence of MM is favored for aesthetic reasons.
Dirac [3] made a fundamental breakthrough establishing the role MMs would play in explaining the quantization of electric charge. He showed that the quantization of all electric charges is explained and required if even single magnetic pole exits. He established the basic relation between the elementary electric charge e and the basic magnetic charge g: eg = nħc/2, where n is an integer, ħ the plank constant and c the velocity of light. From this equation one can get the magnetic charge: g = nħc/2e = ne/2α = 68.5en = gDn, gD= 68.5e, |n|=1, 2, 3, and so on. The smallest possible magnetic charge is 68.5 times of the electrical charge. Dirac predicted only electromagnetic properties of the MMs, but he didn’t predict its mass.
The next strong support for the existence of MMs is related to the Grand Unified Theories (GUT) [4]. In these theories MMs arise as a consequence of the Higgs mechanism and symmetry breaking. In these theories the MM mass is related to the GUT mass scale and it is of order of 1016 - 1017 GeV. This mass value is too high to search for at the terrestrial accelerators.
The searches for the MMs can be divided in two broad classes: GUT type MMs are searched for in cosmic radiation, while Dirac type MMs are searched for at accelerators.
The detectors used in MM search experiments are based either on induction or on ionization or excitation. The induction method is based on the long-range electromagnetic interaction between the magnetic charge and the macroscopic quantum state of a superconducting loop. A Magnetic monopole, moving through the loop, induces an electromotive force and a current (Δi ). If the coil has N turns and its inductivity is L, the current Δi = 4πNgD /L = 2 Δo, where Δo is the current change corresponding to a change of one unit of the flux quantum of superconductivity. A superconducting induction detector consists of the detection coil, which is coupled to a SQUID (Superconducting Quantum Interference Device).
Ionization detectors used for MM detection use the excitation loss technique and is based on high ionization losses of the MMs (~5000 MIP) in matter. Induction methods are widely used in cosmic search experiments, while ionization detectors are used in the accelerator searches. The induction technique doesn’t require any assumption about MMs, while the ionization technique assumes some facts about the MMs behavior in matter, which are difficult to prove.
There is no field theoretical model for the production of monopoles. The most widely used model is the Drell-Yan mechanism, where the dilepton is replaced by a pair of oppositely charged monopoles. This approach is also used in the R20 Experiment [5] at RHIC, which plans to search for MMs in Gold-Gold interactions. If the Drell-Yan mechanism is correct for MM production, then the expected detectable mass region is up to 40 GeV. The MM production is significantly enhanced in λλ interactions of the Gold Ions, but the MM mass coverage is less, about 19 GeV in this case.
The experiment R20 at BNL is planned to search for MMs using the induction technique. This method has never been used before in any accelerator based experiments and represents a new challenge. In Run 7 a test setup with induction devices coupled to SQUIDs showed that this technique can work in the harsh RHIC environment created by the 200 GeV/nucleon Gold Ions. Currently a design of a real detector is underway and Figure 1 shows a proposed set up for the MM search at the RHIC BNL.
Figure 1. Cross Section of the proposed Magnetic Monopole detector cryostat at RHIC. The Vacuum vessel measures 36” (914 mm) in diameter and 74" (1900 mm) in both length and height.
The MM search will be accomplished by placing several superconducting inductive detectors in coincidence inside the chamber where the collisions take place with a clear line of sight between the collision point and the detectors. This experimental arrangement has never been used before in accelerator based monopole searches. In addition, a silicon detector is placed behind the inductive detectors and is used to monitor and measure the energy loss of particles produced by collisions.
It is possible that the RHIC beam energy per nucleon may not be sufficient to produce monopoles and, hence, one may see no monopole-generated events in our SQUID based detector. However, the functioning of this new type of detector will be demonstrated in the high energy accelerator environment and this will enable us to propose continuing the search at the Large Hadron Collider (LHC), where, in the 14 TeV energy regions, MM production is predicted by some Electroweak and Superstring theories.
[1] Pierre de Maricourt, On the Magnet, letter to Siger de
Foucaucourt, (1269). In The Letter of Petrus Peregrinus on the
Magnet, New York, 1904. McGraw-Hill. Translated by Brother
Arnold.
[2] J. C. Maxwell "A Treatise on Electricity and Magnetism",
Oxford Classic Text in Physical Sciences (1873)
[3] P.L.M. Dirac, Proc. Roy. Soc. A133, 60 (1934).
[4] G. 't Hooft, Nucl. Phys. B 79, 276 (1974). M. Polyakov, JETP
Letters 20, 430 (1974).
[5] P. Chaudhari, V. Dzhordzhadze, V. Radeka, M. Rehak, P. Rehak,
S. Rescia, Y. Semertzidis, J. Sondericker, and P. Thieberger.
Search for Magnetic Monopoles at the Reletavistic Heavy Ion
Collider (RHIC). (pdf)
P. Chaudhari et al., R20 Response to PAC. (pdf)
V. Dzhordzhadze et al., Accelerator Based Magnetic Monopole Search Experiments (pdf)
The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
funded by the U.S. Department of Energy Office of Science. Hundreds
of physicists from around the world use RHIC to study what the
universe may have looked like in the first few moments after its
creation. What physicists learn from these collisions may help us
understand more about why the physical world works the way it does,
from the smallest subatomic particles, to the largest stars.