Abstract

1. ANGULAR CORRELATION OF NEUTRONS EMITTED FROM DECAY OF GIANT DIPOLE RESONANCE IN ULTRA-PERIPHERAL COLLISIONS AT RHIC Kimberly Kirchner (for the STAR Collaboration) Creighton University Abstract ZDC SMD In an ultra peripheral collision the two nuclei do not physically overlap. However two virtual photons are exchanged, which can excite each nucleus into a giant dipole resonance state, which decays via neutron emission. The neutrons are detected in STAR’s Zero Degree Calorimeters (ZDC). Each ZDC has a shower maximum detector (SMD). The two SMDs each consist of vertical and horizontal slats that give spatial information about the location of the neutron in the ZDC. The predicted angular correlation for the emitted neutrons is C(ΔΦ) = 1 + ½ cos(2ΔΦ). Here I present a preliminary measurement of the correlation function obtained from the Au-Au collisions at √sNN = 200 GeV in RHIC Run IV. This work is supported in part by U.S. DoE-EPSCoR Grant # DE-FG02-05ER46186. In 2004, a Shower Max Detector (SMD) was installed in each ZDC at about one third of the depth into the ZDC. The SMD works by detecting the energy deposited by neutrons. As the neutron travels through the ZDC it excites the tungsten plates of the ZDC causing a shower which is spread over the surface of the SMD. To detect spatial information the SMD is divided into seven vertical and eight horizontal slats. By looking at where the vertical and horizontal slats with the most energy overlap it is possible to pinpoint the location where the neutron hit in the ZDC. Figures 3 and 4 show the arrangement of the slats in the SMD and show how the angle is measured in both the East and West ZDC SMDs from a known beam center location. Φ1 Φ2 Ultra-Peripheral Collisions φ2 φ1 In an Ultra-Peripheral Collision the two accelerated gold ions do not physically overlap. In this type of collision there are no nuclear interactions, since the nuclei are too far apart. The nuclei interact via the long-range electromagnetic force, by exchanging two virtual photons. Each nucleus is left in an excited state. This state is thought to be a giant dipole resonance (GDR) state, which decays via neutron emission [1]. Figure 4 West SMD Figure 3 East SMD Figure 5 Image of the SMD Triggering and Data Selection Figure 1 This is a representation of an UPC, where b is the impact parameter, which is greater then twice the radii of the gold nuclei. The trigger that is used to study these ultra-peripheral collisions is activated when at least one neutron is detected in each zero degree calorimeter, but has no requirement on the lower limit on the number of tracks that are seen in the detector. This guarantees that a collision has occurred, though not necessarily an ultra-peripheral collision. For a giant dipole resonance interaction there would only be two emitted neutrons and nothing else. So I want to select events with no charged tracks in the detector. The figures below show an event after pedestal and gain correction. The two dimensional graphs were created by plotting the product of the horizontal and vertical slats at intersecting points. Looking at these graphs it is easier to visualize the location of the neutron. If we know the location of the beam center it is then possible to measure the value of delta phi. The yellow stars show the location of the beam center for this event. To do this more precisely I use a Gaussian fit to plots of the separate vertical and horizontal slats for both the East and West SMD. Where the mean positions for the horizontal and vertical directions intersect gives the location of the neutron in the SMD. Using that location and a the known position of the beam center, ΔΦ can be calculated. The beam center position was determined for each fill separately. This was done by finding the mean neutron position for each event, and then averaging over all the events [3]. Angular Correlation between the emitted neutrons Figure 2 The yellow dots are the gold nuclei. The black dot denotes that that nucleus is coming out of the page while the one with the x is going in to the page. b is the impact parameter; it lies along the dashed line. The red dots are the emitted neutrons. The angles F1 and F2are the angles at which the neutrons are emitted with respect to the impact parameter. The angles φ1 and φ2 are measured to STAR’s x axis and are what is experimentally obtained. The two sets of angles differ by a rotation from the impact parameter. ΔΦ is the same in either coordinate system. STAR Φ1 φ1 b Φ2 φ2 STAR Preliminary STAR Preliminary Nuclei Figure 7 Figure 6 ΔΦ Plot The distribution for the emitted neutron a(b) from one nucleus goes like, a(b)~sinΘcosΦ where b is the impact parameter, Θ is the polar angle and Φ is the azimuthal angle [2]. These angles are measured with respect to the impact parameter b. Figure 2 shows the coordinates of this system. The polar angle points from the z axis in towards the page. For two nuclei the distribution of the emitted neutrons is the product of the individual distributions, a12(b) = a1(b)a2(b) = sinΘ1cosΦ1sinΘ2cosΦ2 where the subscripts 1 and 2 represent the two interacting nuclei. Since both nuclei have the same impact parameter there should be an angular correlation between the two ejected neutrons [2]. However since the impact parameter cannot be measured and will be different for each collision the difference of the two angles, ΔΦ = φ1 – φ2 becomes an important quantity since it can be measured. In this case the polar angle is approximately 90° so the sinΘ terms go to 1. The predicted angular correlation is: C(ΔΦ) = 1 + ½ cos(2ΔΦ) The most probable angular correlations are expected when ΔΦ is equal to 0 or ±π. • This is a very preliminary plot of ΔΦ. The data were selected to have • zero charged tracks • total reading in the SMD greater then zero • least a peak in the horizontal and vertical directions • only one neutron in each ZDC • The plot was made with less than 1% of the triggered data. It may be improved with better statistics and refining the methods of selecting and analyzing the events. STAR Preliminary Figure 8 References [1] Bertulani, Carlos A. and Baur, Gerhard. “Electromagnetic Processes in Relativistic Heavy Ion Collisions.” Physics Reports (Review Section of Physics Letters) 163 (1988): 299-408. [2] Baur, Gerhard., et al. “Multiphoton Exchange Processes in Ultraperipheral Relativistic Heavy Ion Collisions.” Nuclear Physics A729 (2003): 787-808. [3] Wang, Gang. “Correlations Relative to the Reaction Plane at the Relativistic Heavy Ion Collider Based on Transverse Deflection of Spectator Neutrons.” Doctoral Dissertation. Kent State University, April 2006.