Update on E12-07-109: Large Acceptance Proton Form Factor ... - Infn

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0.76 0.74 0.72 0.7 0.68 0.66 0.64 0.62 0.6 0.58 !G E /G M 2.5 GeV 2 BLW hadronic GPD COZ LO pQCD Born approx. GEP-I 2.47 GeV 2 GEP-2G PRELIMINARY 0.56 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ! Figure 3: Preliminary result for the proton form factor ratio measured in GEp-2γ experiment [7] as a function of the virtual photon polarization ε. The plot shows also the GEp(1) [26] result and hadronic [9], GPD [34], and pQCD [35] two-photon exchange calculations. Vertical lines indicate the lower limit of the validity of the corresponding calculations. for the GPD calculations, the two-photon exchange effects increase very slowly with Q 2 . Interestingly, the pQCD calculations show the opposite: a slight decrease of the two-photon effects with Q 2 . 6 <strong>Update</strong>d kinematics of the measurements Q 2 Ebeam beam time pp θSBS Ee θBigCal ∆[ µGE/GM] GeV 2 GeV days GeV deg GeV deg optics 6.6 4 28.0 5.0 6.6 1 3.48 28.0 3.94 26.3 0.023 10.0 8.8 10 6.20 16.7 3.47 35.3 0.065 14.5 11. 45 8.61 12.0 3.27 39.0 0.135 Table 1: The kinematics of the proposed data points. Because of the importance of the precise measurements at high Q 2 , we decided to have one precise measurement at 14.5 GeV 2 (instead of two neighboring points at 13 and 15 GeV 2 as in the original proposal) and two points at about 5 and 10 GeV 2 . This will be achieved by reducing the horizontal sizes of the detectors and thus, effectively reducing the Q 2 acceptance, but increasing the vertical aceptance and the event rates per unit Q 2 interval, while keeping the detector area and the number of channels the same. The optimized acceptance and use of beam time provides more power to discriminate among different theoretical predictions, while keeping the same total beam time as in the original proposal. The kinematical quantities and anticipated errors for these points are summarized in Table 1. Including four days for optical and spin transport studies, we have in total 60 days of beam on target, as it was requested in the approved experiment E12-07-109. 8
References [1] M.K. Jones et al., Phys. Rev. Lett. 84, 1398 (2000). [2] Ch. Perdrisat, V. Punjabi, M. Jones, E. Brash et al., Jefferson Lab experiment E04-108 (GEp(3)). [3] E. Brash, E. Cisbani, M. Jones, M. Khandaker, L. Pentchev, Ch. Perdrisat, V. Punjabi, and B. Wojtsekhowski et al., Jefferson Lab experiment E12-07-109 (GEp(5)). [4] E. Brash, M. Jones, Ch. Perdrisat, V. Punjabi et al., Jefferson Lab experiment E12-09-001 (GEp(4)). [5] G. Cates, N. Liyanage, and B. Wojtsekhowski et al., Jefferson Lab experiment E02-013. [6] I.A. Qattan et al., Phys. Rev. Lett. 94, 142301 (2005). [7] R. Gilman, L. Pentchev, C.F. Perdrisat, and R. Suleiman et al., Jefferson Lab experiment 04-019 (2004). [8] P.A.M. Guichon and M. Vanderhaeghen, Phys. Rev. Lett. 91, 142303 (2003). [9] P.G. Blunden et al., Phys. Rev. Lett. 91 (2003) 142304. [10] J. Arrington, W. Melnitchouk, J.A. Tjon, Phys. Rev. C 76, 035205 (2007). [11] M. Burkardt, Int. J. Mod. Phys. A 18, 173 (2003). [12] G.A. Miller, Phys. Rev. Lett. 99, 112001 (2007). [13] C.E. Carlson and M. Vanderhaeghen, Phys. Rev. Lett. 100, 032004 (2008). [14] C.D. Roberts et al., Eur. Phys. J. ST 140, 53-116 (2007); I.C. Cloët et al., Few-Body Systems 46, 1-36 (2009). [15] B. Aubert et al., Phys. Rev. D 73, 012005 (2006). [16] C.B. Crawford et al., Phys. Rev. Lett. 98, 052301 (2007); E. Geis et al., Phys. Rev. Lett. 101, 042501 (2008). [17] G. Ron et al., Phys. Rev. Lett. 99, 202002 (2007). [18] B. Aubert et al., Phys. Rev. D 80, 052002 (2009). [19] The Super-Bigbite Spectrometer (SBS) Conceptual Design Report and SBS Technical review report can be found in http://hallaweb.jlab.org/12GeV/SuperBigBite/SBS-CDR/ [20] K. de Jager, The Super Bigbite Project: a Study of Nucleon Form Factors, arXiv:0911.3268v1 [nuclex] 2009; [21] X. Zheng et al.[Jefferson Lab Hall A Collaboration], Phys. Rev. C 70, 065207 (2004). [22] G. A. Miller and M. R. Frank, Phys. Rev. C 65, 065205 (2002); G. A. Miller, Phys. Rev. C 66, 032201 R (2002). [23] A. V. Belitsky, X. Ji and F. Yuan, Phys. Rev. Lett. 91, 092003 (2003). [24] E. L. Lomon, Phys. Rev. C 64, 035204 (2001). 9
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