Physics performance of the STAR zero degree calorimeter at relativistic heavy ion collider

Physics performance of the STAR zero degree calorimeter at relativistic heavy ion collider 1 2 first-author 1 corresp chenjinhui@sinap.ac.cn chenjinhui@sinap.ac.cn chenjinhui@sinap.ac.cn 1 3 corresp ygma@sinap.ac.cn ygma@sinap.ac.cn ygma@sinap.ac.cn 4 4 1 2 Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100049, China University of Chinese Academy of Sciences, Beijing 100049, China ShanghaiTech University, Shanghai 200031, China ShanghaiTech University, Shanghai 200031, China Brookhaven National Laboratory, Upton 11973, USA Brookhaven National Laboratory, Upton 11973, USA The zero degree calorimeter (ZDC) at RHIC-STAR was installed in the year 2000. After running for more than 10 years, the performance of the STAR-ZDC cannot maintain a proper status because of the radiation damage. The ZDC on RHIC-BRAHMS had been moved to STAR in 2011 after some tests. We present here the result of the tests as well as the physical performance of those ZDC modules between the 2011 to 2015 RHIC run. The excellent energy resolution of the ZDC in heavy ion collision provide a good candidate for future detecter development, such as the CSR experiment at CAS-Lanzhou facility. Zero degree calorimeter Calibration Energy resolution STAR 1 Introduction 1 The zero degree calorimeter (ZDC) at the Solenoidal Tracker at RHIC (STAR) are hadron calorimeters which are installed on both the east and west sides of STAR. The purpose of STAR-ZDC is to detect neutrons emitted from the interaction region and went along the beam direction with a divergence angle less than 4 mrad [ 1 1 ], and measure their total energy. From the measured total energy, one can calculate the multiplicity. The neutron multiplicity is known to be correlated with the event geometry and is used to measure the reaction centrality in mutual beam interactions [ 1 1 ]. The ZDC coincidence of the two beam directions is a minimal bias selection of heavy ion collisions, thus it is useful as an event trigger and a luminosity monitor [ 1 1 , 2 2 ]. The STAR ZDC have six identical modules in total, three of them are installed in the east side of the STAR interaction region and three are on the west. We also have shower maxi mum detector(ZDC-SMD) installed on both sides which provide the position information of the neutron beam. The ZDC-SMD combined detector was used extensively in data analysis for reaction plane determination, such as the charged particle direct and elliptic flow measurement [ 3 3 - 8 8 3-8 ]. As can be seen in Fig. 1 Fig. 1 [ 1 1 ], the ZDC modules consist of tungsten plates, fibers, and photon multiplier tubes. And the ZDCs are installed about 18 meters away from the intersection point along the beam line, behind the dipole magnets. The magnets will bend all charged particles and leave the neutrons and other neutral particles to hit the ZDC modules. More detailed technical information about ZDC could be found in Ref. [ 1 1 ]. 2 Test of ZDC gain versus high voltage 1 The current STAR ZDC modules are moved from the RHIC-BRAHMS experiment, which were idled since 2006. Before these modules were installed, we tested the relation of ZDC gains versus high voltage applied. Figure 2 Figure 2 shows the ZDC modules on RHIC-STAR experiment. 2.1 Electronics layout and General Procedure 2 In order to test the ZDC gain under different high voltages, we constructed an electronics layout as shown in Fig. 3 Fig. 3 . Muons from cosmic rays hit sequentially the five modules: the east SMD, the west SMD, the top trigger scintillator, the ZDC module, and the bottom trigger scintillator. Since there are background particles everywhere at anytime, it is essential to use the logic "and" result of the top and bottom scintillators to select events triggered by cosmic ray muons. When a muon hits the ZDC after traversing the top scintillator, it will produce Cherenkov light. The light is guided by the optical fibers and generates a signal in the ZDC. The muon will then hit the bottom scintillator and also generate a signal. The coincidence of the top and bottom scintillator signals is used as the system trigger and provides a gate to the ADC. 2.2 Result 2 The expected relation of PMT module gain with high voltage should be a power law: gain = a ×HV b , where a and b are parameters related to the PMT module and environment, and the gain is in unit of pC and voltage is in unit of kV. Ideally b is related only to the PMT, so as long as we are using PMT modules of the same standard, b should not vary much. The gain vs. HV plots of different ZDCs, and the fitting results, are shown in Fig. 4 Fig. 4 . The fit parameters are listed in Table 1 Table 1 . 1 1 Module 1 1 a 1 1 b 1 1 2-1 1 1 0.0191 ± 0.0014 1 1 4.6616 ± 0.0676 1 1 2-2 1 1 0.0095 ± 0.0006 1 1 4.4533 ± 0.0480 1 1 2-3 1 1 0.0243 ± 0.0094 1 1 4.2267 ± 0.3787 1 1 2-4 1 1 0.0198 ± 0.0075 1 1 4.1288 ± 0.3778 1 1 2-5 1 1 0.0111 ± 0.0039 1 1 4.5443 ± 0.3384 1 1 2-6 1 1 0.0153 ± 0.0086 1 1 4.3839 ± 0.5660 2.3 Conclusion 2 As we can see from the result shown in Fig. 4 Fig. 4 , different fit lines are nearly parallel to each other, which is reasonable since the slope b is a PMT-related parameter and it should have nearly the same value. From the relation between the gain and high voltage, we can get separate values of high voltages necessary for different ZDCs to have the same gain. 3 Experimental performance 1 After installed into STAR, the ZDC modules encountered different beam types in different energies. Here we collect five different runs, as shown in Table 2 Table 2 , to show the ZDC tower gain ratios and the single neutron peak distributions. We then will discuss both the high voltage calibration of ZDC by investigating the gain ratios and the energy resolution by plotting single neutron peaks. The basic information of those runs are listed in Table 2 Table 2 . 1 1 Run number 1 1 Year 1 1 Energy ( <math id="M1"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> ) 1 1 Beam type 1 1 12130083 1 1 2011 1 1 200 GeV 1 1 Au + Au 1 1 15067001 1 1 2014 1 1 15 GeV 1 1 Au + Au 1 1 15186001 1 1 2014 1 1 200 GeV 1 1 3 He + Au 1 1 15122044 1 1 2014 1 1 200 GeV 1 1 Au + Au 1 1 16134022 1 1 2015 1 1 200 GeV 1 1 p + Au 3.1 Tower gains and high voltage applied 2 When a neutron flies through three ZDC modules, the gain in each tower decreases from the first tower to the last one due to the energy loss in ZDC material. Ideally, the ratio of ZDC tower gains should roughly be 6:3:1. However, it is difficult to tune the high voltage perfectly in practice as the gains will be affected by many factors like beam conditions, shower leakage, etc. So in this article, we consider the HV setting to be good if the ratios are not far from the ideal value. With the runs listed in Table 2 Table 2 , we will show the ratios with different beam types in Figs. 5 Figs. 5 , 6 6 and 7 7 . For Au+Au runs at <math id="M2"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in the year 2011, as shown in Fig. 5 Fig. 5 , the gain ratios between tower 1 & 2 and 2 & 3 for both sides are shown in four upper panels. The mean gain ratios vs. ADC are then shown in the four lower panels. The slope of a linear fit will indicate the ratio between towers. The uncertainties of the slopes are always on the order of 10 -3 , so they are not shown. The fitting range varies from plot to plot, because, in the low-x range, the small signal may introduce large errors, and in the high- x range, the shower leakage and other effects will break the linear relation. Figure 5 Figure 5 shows that, on east side, the slope for tower2/tower1 is 0.84 and for tower3/tower2 is 0.36. For west side, the slopes are 0.84 and 0.39 for tower2/tower1 and tower3/tower2, respectively. For Au+Au runs at <math id="M3"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =15 GeV in the year 2014, the neutrons emitted from the peripheral collisions will not carry much energy at this low energy. So it’s not possible to obtain the slopes as we did for the run above due to the small gains on towers. We counted the raw gains for each tower and calculated the ratio directly between towers: the ratio is 6.38:1.61:1 on the east, and is 7.35:2.37:1 on the west, where we take the gain of third tower as unit of 1. For Au+Au runs at <math id="M4"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in the year 2014, we still plot the gains and the slopes between towers as shown in Fig. 6 Fig. 6 . In the figure, the slopes for tower2/tower1 and tower3/tower2 are 0.61 and 0.33 for east side, and for west side they are 0.61 and 0.28. We also have 3 He+Au runs at <math id="M5"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in the year 2014. In this run, we have asymmetric beam types, which is quite different from runs above. After collisions, the neutral residual of the gold ions will hit the east ZDC modules, while the residual of <math id="M6"><mrow><msup><mrow/><mtext>3</mtext></msup><mtext>He</mtext></mrow></math> will hit the west modules. The ZDC gains are quite different from east to west. The gold ions contain more neutrons, so the east ZDC gains are larger than west. In Fig. 7 Fig. 7 we show the ZDC gains and the slopes, the west side is not shown due to the low ZDC gains. From the figure we can see that the slope of tower2/tower1 is 0.70, while that of tower3/tower2 is 0.30. For p+Au runs at <math id="M7"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in the year 2015, the beam types are also asymmetric. From the raw gains, the ratio between towers are 6.88:3.10:1 on the east side and 9.80:2.75:1 on the west side. As mentioned above, it is quite difficult to tune the high voltages to get ideal gain ratios. In this article, by fitting, or by direct use of raw gains, we evaluate the HV settings by calculating the gain ratios of towers. Considering that the tower gains are affected by many factors, we think those HV settings are acceptable. 3.2 Energy resolution 2 The single neutron peak from peripheral collisions is used to determine the energy resolution of the ZDC. The energy resolution of ZDC modules has been simulated and tested in a test beam before. In those tests, the resolution is around E / E =20% at E n =100 GeV [ 1 1 ]. In this section, we will show energy resolution of ZDC in different beam types and energies since 2011. We require a very low Time of Flight detector multiplicity (less than 2) in order to select the peripheral collisions. We also require the TDC value to be within [500, 2500] in order to reduce the background. For Au+Au runs at <math id="M8"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in 2011, the single neutron peak is shown in Fig. 8 Fig. 8 . In this figure, the panel (a) is the single neutron peak for the east side ZDC and panel (b) is for the west side ZDC. A Gaussian function was used to fit the peak and the fitting parameters are shown also in the plot. For both east and west side ZDCs, the E / E =26%. For Au+Au runs at <math id="M9"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =15 GeV in 2014, the single neutron peak can hardly be extracted due to the low beam energy. As shown in Fig. 9 Fig. 9 , we can see that on east side, E / E =29%, and on west side, E / E =27%. For Au+Au runs at <math id="M10"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in 2014, this is consistent with results from the year 2011 run and the early beam test. In 3 He+Au runs at <math id="M11"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in the year 2014, <math id="M12"><mrow><msup><mrow/><mtext>3</mtext></msup><mtext>He</mtext></mrow></math> is formed by one neutron and two protons, so it’s rare to have multiple neutrons hit the west ZDC modules in one collision. This is why we can see a relatively clean single neutron peak on the west side ( Fig. 10b Fig. 10b ). On the east, the peak is not as well as the east one due to high background ( Fig. 10a Fig. 10a ). From the Gaussian fit, on the west, E / E =40%. This E / E is much larger than the same year and the year 2011 Au+Au collisions results. This may be related to the larger beam-beam crossing angle in 3 He+Au runs than in Au+Au runs. In p+Au runs at <math id="M13"><mrow><msqrt><mrow><msub><mi>s</mi><mrow><msub><mrow/><mrow><mtext>NN</mtext></mrow></msub></mrow></msub></mrow></msqrt></mrow></math> =200 GeV in the year 2015, the proton contains no neutron, so it’s rare to have neutrons hit on the west side of ZDC modules. And the gold side, like the 3 He+Au runs, has large background. The single neutron peak cannot be seen on either side. 3.3 Conclusion 2 We report the beam test and physics performance of the new installed ZDC in STAR experiment since 2011. With the detail runs listed in Table 2 Table 2 , the ZDC gains between towers were obtained and the ratios between towers were calculated. The ratios in these runs are close to expectation, which indicate that the high voltage applied on these modules are acceptable. 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Tech . 2015 , 26 : 030502 http://www.j.sinap.ac.cn/nst/EN/10.13538/j.1001-8042/nst.26.03050210.13538/j.1001-8042/nst.26.030502 http://www.j.sinap.ac.cn/nst/EN/10.13538/j.1001-8042/nst.26.03050210.13538/j.1001-8042/nst.26.030502 Neutron time-of-flight spectrometer based on HIRFL for studies of spallation reactions related to ADS project We thank the STAR collaboration, the RHIC Operations Group, the Collider-Accelerator Department, and the RHIC Computing Facility at BNL. 10.1007/s41365-016-0129-z This work was supported by the Major State Basic Research Development Program in China (2014CB845400), the National Natural Science Foundation of China (Nos. 11421505, 11520101004, 11322547 and 11275250) 2016-05-19 2016-08-05 2016-08-12 2016-09-17 2016-12-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0129-z http://dx.doi.org/10.1007/s41365-016-0129-z . 2016 Nuclear Science and Techniques 06 27