An overview is presented of the planned physics program for JLab\u27s Hall A following the 12 GeV upgrade with emphasis on the equipment needed to achieve the desired experimental goals. Results of simulations of sample experiments with anticipated uncertainties are presented.Predstavlja se plan fizičkih istraživanja u Halli A JLaba nakon podizanja energije na 12 GeV, s naglaskom na potrebnu opremu za postizanje željenih eksperimentalnih ciljeva. Izlažu se ishodi simulacija nekih mjerenja s očekivanim pogreškama

LeRose, John J.

English

An overview is presented of the planned physics program for JLab's Hall A following the 12 GeV upgrade with emphasis on the equipment needed to achieve the desired experimental goals. Results of simulations of sample experiments with anticipated uncertainties are presented.Predstavlja se plan fizičkih istraživanja u Halli A JLaba nakon podizanja energije na 12 GeV, s naglaskom na potrebnu opremu za postizanje željenih eksperimentalnih ciljeva. Izlažu se ishodi simulacija nekih mjerenja s očekivanim pogreškama

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ISSN1330–0016CODEN FIZBE7JLAB’S HALL A AFTER THE 12 GEV UPGRADEJOHN J. LEROSEThomas Jefferson National Accelerator facility, Newport News, Virginia 23606Received 9 October 2003; Accepted 26 April 2004Online 14 November 2004An overview is presented of the planned physics program for JLab’s Hall A fol-lowing the 12 GeV upgrade with emphasis on the equipment needed to achievethe desired experimental goals. Results of simulations of sample experiments withanticipated uncertainties are presented.PACS numbers: 29.20.-c, 41.75.Lx, 13.60.-r UDC 539.1.076Keywords: inclusive electron scattering, near threshold charm production1. IntroductionThe 12 GeV upgrade planned for Jefferson Lab will make possible a very broadphysics research program. In Hall A, a versatile collection of instruments is plannedaddressing a wide variety of experiments. Reactions in the program include:• inclusive electron scattering,• semi-inclusive measurements,• exclusive process measurements,• near threshold charm production.JLab’s Hall A is already equipped with a wide array of proven precision instruments[1]. In addition, new instruments are planned in order to address the experimentalpossibilities presented by the upgraded beam. Planned new equipment includes:• The MAD (medium acceptance device): an 8 GeV/c magnetic spectrome-ter built from two large combined-function superconducting magnets. TheMAD would provide moderate momentum resolution (σp/p ≈ 1 × 10−3),large solid angle acceptance (∆Ω ≈ 30 msr), and large momentum accep-tance (∆P/P ≈ ±15%). The MAD will be equipped with a detector packagecommensurate with the particle ID requirements of the experimental programFIZIKA B 13 (2004) 2, 453–460 453lerose: jlab’s hall a after the 12 GeV upgradeand the performance parameters of the MAD itself. The choice of design pa-rameters for the MAD is driven by the array of experiments shown in Table 1.• A large acceptance (∆Ω ≈ 0.1 sr) high resolution photon calorimeter madeup of 676 PbF2 crystals (26 × 26 × 200 mm3). The layout of a real comptonscattering (RCS) experiment using the photon calorimeter at 12 GeV is shownin Fig. 1.TABLE 1. Array of envisioned experiments in Hall A at 12 GeV with experimentalrequirements.Experiments Pmax Angle ∆Ω∆PPPresAng.res.Lumi-nositye or h(GeVc)(msr) (%) (%) (×1037)d/u (3H/3He) 6 15–30◦ 15–30 30 0.3 1–3 10 eAn1 , g1n 6–7 15–30◦ 15–30 30 0.3 2–3 0.1 eg2n 6 15–30◦ 15–30 30 0.3 2–3 0.1 eAp1 , g1p 6–7 15–30◦ 15–30 30 0.3 2–3 0.01 eSpin Duality 6–7 12–25◦ 12–25 30 0.3 2–3 0.1 eg1 at high E 6–7 12◦ 12 30 0.3 1–3 0.1 eDIS-Parity 6–7 12–15◦ 12–15 30 0.3 1–3 100 eSemi-π+/π− 6 15–25◦ 15–25 30 0.3 2–3 5 edbar/ubar 6 15–25◦ 15–25 30 0.3 2–3 40 e∆u, d, s 6 15–25◦ 15–25 30 0.3 2–3 0.1 eTransversity 6 15–38◦ 15–30 30 0.3 2–3 0.1 eπ struc. Fun. 3 15–22◦ 15–25 30 0.3 2–3 0.1 eCharm 6–7 12–15◦ 12–15 30 0.3 1–3 40 eHadronization 6 12–30◦ 12–30 30 0.3 2–3 40 ex > 1 7 12–60◦ 12–30 30 0.3 1–3 40 eGen 6 15–20◦ 15–20 30 0.3 2–3 0.1 eGep/Gmp 7-8 15–35◦ 15–30 30 0.3 2–3 40 p, FPPCT (e,e′p) 7-8 15–35◦ 15–30 30 0.3 2–3 40 pCT with FPP 7 15–40◦ 15–30 30 0.3 2–3 10 p, FPPCT in π prod. 6 12–30◦ 12–30 30 0.3 2–3 20 pπ± photoprod. 6 12–30◦ 12–30 30 0.3 2–3 20 pπ0 photoprod. 7 12–90◦ 12–30 30 0.3 2–3 20 p, FPPKλ 6 12–90◦ 12–30 30 0.3 2–3 20 py-d 4 20–40◦ 20–40 30 0.3 2–3 20 p, FPP454 FIZIKA B 13 (2004) 2, 453–460lerose: jlab’s hall a after the 12 GeV upgradeFig. 1. Layout of a real Compton scattering (RCS) experiment in Hall A at 12 GeV.Photons are detected in the calorimeter while coincident protons are detected inthe MAD.2. The MADThe MAD is actually a relatively simple device consisting of two combined-function superconducting magnets. Each magnet contains dipole and quadrupolemagnetic field components superimposed on each other. The superposition ofquadrupole and dipole are required to achieve the strong focussing needed to getreasonable solid angle (requires a quadrupole doublet with minimum space be-tween the magnets) and moderate momentum resolution with large momentumacceptance. Separating the quadrupole and dipole functions would result in eithera much larger spectrometer and detector array or greatly reduced acceptance. Thetotal vertical bend of 30◦ (10◦+20◦) is sufficient to eliminate any direct line ofsight from the detectors to the target and delivers the desired momentum resolu-tion. Preliminary studies (see Fig. 2) indicate that building the magnets is feasible,there being no overly large fields or forces on the conductors with currents lessthan the “critical currents” for existing superconducting wire. In the event thata particular experiment has needs not met by the “standard” tune of the MAD,the quadrupole and dipole field strengths in the magnets can be independently setallowing some latitude in the optical properties. Further versatility is achieved bychanging the initial distance between the target and the first magnet. Lengtheningthat initial drift makes it possible to get to smaller scattering angles with an associ-FIZIKA B 13 (2004) 2, 453–460 455lerose: jlab’s hall a after the 12 GeV upgradeFig. 2. Results of magnetostatic studies of MAD magnets. Upper left shows thecoil configuration for the dipole component. Lower left shows the coil configurationfor the quadrupole component. Upper right shows the dipole and quadrupole coilstogether with yoke iron surrounding them. Lower right shows the resulting modulusof the magnetic field in the bore of the magnet (note the bull’s-eye pattern indicativea dipole superimposed on a quadrupole).Fig. 3. Simulated mo-mentum (upper left),vertical angle (lowerleft), transverse targetlocation (upper right),and transverse (hori-zontal) angle resolu-tions for the MADat 6 GeV/c centralmomentum using theHall A polarized 3Hetarget and detectorperformance based onthe proposed detectorpackage.456 FIZIKA B 13 (2004) 2, 453–460lerose: jlab’s hall a after the 12 GeV upgradeated loss of solid angle, 30 msr at 35◦ goes to 15 msr at 15◦. Using magnetic fieldmaps generated while studying the feasibility of building the magnets, raytracingstudies have been performed to evaluate the resolution and acceptance capabilitiesof the MAD as well as providing a means of evaluating the potential backgroundproblems associated with doing experiments with the MAD. Figure 3 shows the an-ticipated resolution properties of the MAD taking into account multiple scatteringin the standard Hall A polarized 3He target, He atmosphere in the MAD itself, andthe detector system. The detector system planned for the MAD includes scintil-lators, drift chambers, gas and aerogel Čerenkovs, an electromagnetic calorimeter,and a focal plane polarimeter. Fig. 4 shows the anticipated layout and some of theparameters for the detectors.Fig. 4. MAD detector configuration for lepton detection on the left and hadrondetection on the right.3. Experimental programMany experiments have been studied in great detail and as mentioned abovecan be found in the Hall A PCDR [2]. Space limitations here prevent doing anykind of justice to that enormous body of work. Studies of the quark flavor andspin decomposition in the nucleon are presented here as an example of the kindof research that will be possible in Hall A after the 12 GeV upgrade. In the 80’s,technical advances for producing polarized beams and polarized targets triggereda new experimental effort which focused on the spin structure of the nucleon. Thiseffort culminated with the test of the Bjorken sum rule, a fundamental sum rule ofFIZIKA B 13 (2004) 2, 453–460 457lerose: jlab’s hall a after the 12 GeV upgradeQCD, and the determination of the quark contribution to the spin of the nucleon.Although a large experimental effort has gone into measuring the full kinematicregime, there has never been a facility where the valence quark region could bemeasured with precision. The statistical precision of the world data is quite poor,for xBj > 0.4. While the valence quark momentum distribution is peaked around0.3, the sea quarks and the gluons tarnish the experimental picture in this regime.A clean and unambiguous contribution from the “valence quarks” can only beexpected when x is larger than 0.5. Unfortunately, in this kinematic region theprobability of finding any of the valence quarks becomes rather small leading toa poor statistical determination of the key observables. Taking advantage of theenergy upgrade and the unprecedented polarized luminosity in Hall A, this situationwill be improved dramatically. A detailed mapping of the spin structure functionof the proton and neutron as a function of the scale probed is expected to have aprofound impact on our understanding of the structure of the nucleon.For example, in most dynamical models of the nucleon, its polarization asym-metry An,p1 , which reflects the quark spin wave-function, is expected to be large andpositive in the valence quark region. At large momentum transfers, the asymmetry,An,p1 , is expected to approach 1 when xBj → 1. This reflects the fact that in thevalence region the struck quark, which carries most of the nucleon momentum, alsocarries all of its spin. A detailed examination of the present neutron data shows nosign of the expected behavior (see Fig. 5). However, a dramatic improvement can beachieved in the measurement of the neutron asymmetry using the 11 GeV polarizedbeam and a polarized 3He target combined with the proposed MAD spectrometerin Hall A, as shown in Fig. 5.Fig. 5. A measure-ment of the neutronpolarization asymme-try An1 , determinedby the spin struc-ture of the valencequarks made possibleby the combination ofan 11 GeV beam andthe MAD spectrome-ter in Hall A. Theshaded area representsa range of valencequark models; thesolid line is a predic-tion of a pQCD light-cone quark model.458 FIZIKA B 13 (2004) 2, 453–460lerose: jlab’s hall a after the 12 GeV upgradeValues of xBj greater than 0.8 cannot be reached at 11 GeV due to kinemat-ics limitations, nor can they be reached at high-energy facilities due to luminositylimitations. However, if the validity of duality between the spin distribution mea-sured in DIS and that measured in the resonance region has been verified, valuesof An,p1 for xBj up to 0.9 can be achieved. These measurements are important tounderstand how to reconcile a constituent quark picture of the nucleon with one in-volving current quarks and gluons. The knowledge of the polarization asymmetriesAn,p1 makes possible a flavor decomposition of the valence quark spin distributionsassuming a value for the ratio d/u. In the constituent quark model, it is expectedthat ∆u/u → 1 while ∆d/d → 1/4 when xBj → 1. A different result is obtained inthe quark-parton model when pQCD quark helicity conservation is used, namely∆u/u → 1 and ∆d/d → 1. These predictions can be tested and spin distributionsextracted in a comprehensive analysis if one complements the inclusive data withsemi-inclusive asymmetry measurements of charged pions on the proton, deuteriumand 3He. Fig. 6 shows the level of precision achievable in determining the valencequark distributions using the 11 GeV beam and the MAD spectrometer in HallA. Factorization of the quark distributions and the fragmentation functions hasbeen assumed. The validity of this assumption will be tested in a separate series ofstudies.Fig. 6. A semi-inclusive measurement of π+ and π− production on the protonand 3He makes possible a spin and flavor decomposition of the nucleon spin quarkdistributions. Factorization is assumed but will be tested and quantified by severaladditional measurements.FIZIKA B 13 (2004) 2, 453–460 459lerose: jlab’s hall a after the 12 GeV upgrade4. SummaryHall A at 12 GeV (the nominal 12 GeV in Hall D will actually give 11 GeVin Hall A) will have a versatile complement of experimental equipment capable ofaddressing a broad spectrum of interesting physics.AcknowledgementsThe development of the plans for Hall A after the 12 GeV Upgrade involved alot of work by many dedicated people. The entire Hall A staff and many members ofthe Hall A Collaboration contributed to the work presented here. The SoutheasternUniversities research Association (SURA) operates the Thomas Jefferson NationalAccelerator facility under DOE Contract No. DE-AC05-84-ER4015.References[1] J. Alcorn et al., Nucl. Instrum. Meth. A 522 (2003) 171.[2] Hall A 12 GeV Upgrade PCDR, http://hallaweb.jlab.org/12GeV/.HALLA A JLABA NAKON POSTIZANJA 12 GeVPredstavlja se plan fizičkih istraživanja u Halli A JLaba nakon podizanja energije na12 GeV, s naglaskom na potrebnu opremu za postizanje željenih eksperimentalnihciljeva. Izlažu se ishodi simulacija nekih mjerenja s očekivanim pogreškama.460 FIZIKA B 13 (2004) 2, 453–460

Halla A JLaba nakon postizanja 12 GeV

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