Physics and technological highlights of the current and near future activities of the Italian JLab12 collaboration working at the Jefferson National Accelerator Facility are shortly summarized

Cisbani, E.

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DOI 10.1393/ncc/i2013-11441-6Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Experimental physics at JLab: Where confinement meetsasymptotic freedomE. CisbaniINFN Rome, Sanita` group and Italian National Institute of Health - Rome, Italyricevuto il 31 Agosto 2012Summary. — Physics and technological highlights of the current and near fu-ture activities of the Italian JLab12 collaboration working at the Jefferson NationalAccelerator Facility are shortly summarized.PACS 12.38.-t – Quantum chromodynamics.PACS 29.40.-n – Radiation detectors.1. – IntroductionThe Thomas Jefferson National Accelerator Facility (JLAB) is one of the most impor-tant nuclear physics laboratory in the world, where QCD is extensively investigated bymeans of a high-intensity, longitudinally polarized, 6GeV electron beam of the Continu-ous Electron Beam Accelerator Facility (CEBAF); the beam is delivered simultaneouslyin 3 experimental halls. Within the next two years, the beam energy will be doubled,a new hall will be available for real photon physics and the equipments of the existinghalls will be upgraded or renewed.With this update JLAB will enter the 12GeV era and will offer substantial opportuni-ties to improve our understanding of the nature of the strong interaction in the hadrons,nucleus and sub-nucleon (quark and gluon) scales, at low energies as well as small dis-tances, where QCD shows its remarkable peculiarities of confinement and asymptoticfreedom, respectively. Moreover, the high luminosity achievable in the JLAB experi-ments and the excellent control of the beam parameters give access to the measurementof the parity-violating processes of the electroweak interaction, and therefore they permitthe test of the Standard Model, at low energies, with high accuracy.In this scientific context operates the Italian JLab12 Collaboration; it involves 50Italian researchers, funded primarily by the third INFN Scientific Commission. Thecollaboration activities cover large part of the JLAB scientific program; they are mainlydevoted to the experimental studies on the structure and dynamics of the nucleon, theorigin of the confinement and the role of the gluon in the hadronic spectrum, the innerc© Societa` Italiana di Fisica 210EXPERIMENTAL PHYSICS AT JLAB: WHERE CONFINEMENT MEETS ETC. 211behavior of (hyper)nuclei and the measurement of the nuclear/nucleon properties by theelectron scattering, both electromagnetic and the parity violating weak interaction.2. – Physics2.1. Nucleon structure. – The current picture of the nucleon structure consists of threequantities [1]: the nucleon form factors, the parton distribution functions (PDF), andthe generalized parton distribution functions (GPD). All of them can be derived fromthe phase-space parton distribution Wigner functions reviewed in [2]; according to thepresent understanding the Wigner functions cannot be measured directly.Form factors, PDFs and GPDs express fundamental properties of the nucleon andits internal dynamics, such as the origin of the spin and the orbital angular momentum(OAM) of its constituents, and therefore of the complex aspects of Quantum Chromo-dynamics (QCD).New data from elastic electron-proton scattering experiments performed at JLABaround 2000, clearly demonstrated, for the first time, a firm negative slope of the protonelectric to magnetic form factor ratio versus the momentum transfer Q2 [3], in contrastto the existing results obtained by the traditional Rosenbluth separation method; apossible explanation of this discrepancy is the inadequacy of the single-photon exchangemechanism in describing the elastic scattering. New experiments at JLAB have beenapproved to measure, by different techniques, the neutron and proton electromagneticform factors at high Q2, up to ∼ 15GeV2 where quark OAM (unknown and expected toprovide a significant contribution to the total spin of the nucleon), may play a noticeablerole, perturbative QCD can provide reliable predictions [4] and different models havequite different behaviors.PDFs are mainly accessed by the measurement of the cross section asymmetries on po-larized beam and/or target, in DIS (inclusive and semi inclusive, SIDIS) and, in Drell-Yanprocesses (assuming some sort of PDF universality is valid). Of the 8 leading twist PDFs,five are transverse-momentum–dependent (TMD) distribution functions accounting forthe probability of the quark with transverse momentum (kT ) and a given polarizationin a nucleon with defined polarization relative to the direction of the nucleon. Most ofthem are related, rather directly, to the quark OAM. The remaining 3 PDFs are thewell-known unpolarized distribution f1(x;Q), the rather known helicity g1(x;Q) and therecently measured transversity h1(x;Q) being x the momentum fraction of the quarkrelative to the momentum of the nucleon and Q the energy scale (momentum transfer inDIS). Investigation of the spin related PDF and TMDs will greatly improve with the newJLAB beam in terms of larger statistics, flavor decomposition and extended phase space,especially toward the high-x valence region (unique feature offered by JLAB), where seapartons effects are marginal and lattice QCD may be predictable. In this direction apretty large program on nucleon PDF, both on neutron and proton, will interest theJLAB activities [5].The most recent advance in the theoretical description of the nucleon structure, thatgathered considerable experimental attention, are the GPDs, which somehow connectform factors and PDFs and are measurable in hard exclusive processes. Experimen-tal knowledge of GPD’s is still modest; the Deep Virtual Compton Scattering (DVCS)process is the main tools of investigation and will be largely used also at JLAB [6].2.2. Hadron spectroscopy . – The gluon role in the determination of the hadron spec-trum is of fundamental importance in the understanding of the QCD. The interaction ofquarks in terms of gluon flux tube (string) implies the possibility of existence of mesons212 E. CISBANIwith exotic quantum numbers due to the excitation of the force mediators; in these casesthe gluonic degree of freedom seems to contribute explicitly to the color singlet. LatticeQCD predicts the lowest exotics states (hybrids) in the range of 1.4–3.0GeV [7], whichoptimally matches the energy range that will be available at JLab in 2014. Search ofexotics meson states is going on in different laboratories especially by hadron probes withspin 0 like pions and kaons; the use of photon beam is expected to favor the creation ofexotic states due to the higher possible number of final states related to the spin 1 of theprobe. A comprehensive study of the meson spectrum will be carried on in Hall B [8]using the CLAS12 detector extended by a Forward Tagger (mentioned later) that willpermit to run experiments with quasi-real photons.The hadron spectroscopy in general is the primary source of information in questionslike: the origin of the quark and gluon confinement and therefore the origin of thehadron formation, the origin of the hadron masses, the effective degrees of freedom whenasymptotic freedom is not applicable.2.3. Parity violation. – The longitudinally polarized, very stable, and intense CEBAFelectron beam permits accurate and precise measurements of the parity violating electronscattering (PVES) asymmetry of the weak interaction, directly related to the Weinbergmixing angle θW , as well as the weak currents in the nucleons and nuclei; while thelatter gives access to properties of the hadrons largely suppressed in electromagneticinteraction, the measurement of θW probes the validity of the Standard Model.The most precise determination of the quark s contribution to the nucleon form factorshas been obtained by a series of PVES experiments at JLAB [9]; in 2012 the first directdetermination of the neutron skin (difference of the radii of the neutron and protondistributions) in Lead has been published by the PREX experiment [10], probably oneof the most challenging experiment at JLAB. New, more precise, PVES measurementsare scheduled to run after the 12GeV upgrade.2.4. Nuclear structure and dynamics by electron scattering . – The above studies coversa large part of the JLAB physics program; however investigation on traditional nuclearand hypernuclear physics aspects will continue after the 12GeV upgrade: relativisticeffects and short range correlation in nuclei, electrodisintegration of the deuteron, colortransparency, as well as the reaction mechanism in electron scattering off nucleons andnuclei [11].3. – Technological developmentThis extended physics program requires new detectors and equipment able to permitthe full exploitation of the high energy beam (up to 11GeV), its intense current (up to100μA) and large longitudinal polarization (up to ∼ 90%). The JLab12 collaboration,is therefore promoting quite a few new devices, covering basically all main component ofa scattering experiment: beam, target and detectors; they are shorty introduced in thissection.3.1. Forward photon tagger . – The new Hall D with its GlueX [12] detector will bededicated to real photon physics and therefore the JLAB laboratory of election for hadronspectroscopy. However limited resolution and particle identification will be complementedby a new photon forward tagger that is under development to extend the CLAS12 detectorcapability in HallB. The device, shown in the left of fig. 1 will consist of a calorimeter,a scintillator hodoscope and a MicroMegas tracker that will detects position (angle)EXPERIMENTAL PHYSICS AT JLAB: WHERE CONFINEMENT MEETS ETC. 213ChargedparticlePhotonDetectorPhotonAerogel MirrorMirrorVertex400 cm800 cmProximityFocusingFig. 1. – Left: 3D view of the forward photon tagger. Right: conceptual side view of theCLAS12 aerogel RICH with two overlapping focusing and proximity optics that minimize thephoton detector size; only one Cherenkov photon is shown.and energy of the forward scattered electrons thus providing energy and polarization ofthe quasi-real radiated photon. High-intensity quasi-real photon beam will be thereforeavailable for the comprehensive physics program on meson spectroscopy mentioned above.3.2. Hydrogen-deuteride polarized target . – One of the most interesting, and challeng-ing, polarized target proposed for the next generation experiments at JLAB is the solid,frozen spin, hydrogen-deuteride neutron and proton target previously used at LEGS onphoton beam [13], which may provide longitudinal and transverse polarization up to 75%on H and 40% on D, with extremely long relaxation time. Signal to background ratio ofthe HD target is largely superior to conventional polarized target, however stability ofthe target under relatively intense electron beam has still to be demonstrated.3.3. RICH for CLAS12 . – The baseline CLAS12 detector lacks for an effective π/K/pidentification in the ∼ 3–8GeV/c; to overcome this limitation and permit a extensiveprogram of TMDs measurements toward a flavor decomposition, a challenging large areaRICH detector is under development [14]. The new RICH is based on high-qualityaerogel radiator and an hybrid optics (proximity and focusing, see the right drawing offig. 1) with double-crossing of the aerogel by part of the Cherenkov photons; this complexgeometrical configuration will minimize the expensive photon detector area which willlikely consist of a regular array of multianode photomultipliers or silicon photomultipliers.Intense R&D study is in progress to prove the feasibility of the detector at acceptablecost.3.4. GEM/SiD trackers. – Relevant part of the physics program on form factors athigh Q2 and neutron PDF’s measurement in Hall A needs a new spectrometer able tooperate in high luminosity environment and with moderately high acceptance at forwardangles. Such a spectrometer is under development and will include a large area trackerbased on gas chambers in GEM technology sitting behind a 2 T·m dipole magnet. Twosmall silicon detector planes upstream to the dipole, will significantly extend the effectivelength of the measured trajectory, thus improving remarkably the track reconstructionaccuracy.The GEM tracker [15] is in advanced development and will provide a hit spatialresolution at the level of 70μm, capability to support large background flux (up to250MHz/cm2 photons and 160 kHz/cm2 charged particles), transverse active area of∼ 40× 150 cm2. The tracker will consist of up to 6 GEM chambers, each chamber madeby three adjacent triple GEM 40× 50 cm2 modules, with minimal dead area.214 E. CISBANI4. – ConclusionThe above-presented physics and technological programs are a partial excerpt of theactivities carried on at JLAB by the JLab12 collaboration with the purpose of improvingour understanding of the strong interaction and its currently accepted theory: the QCD.The approaches are diversified and reflect the great complexity of the QCD interactionand its consequences on hadrons. JLAB, with the 12GeV upgrade, its high-intensity,longitudinally polarized and stable beam, combined to new complementary equipmentsand dedicated detectors, is going to be one of the most advanced facilities to carry onextensively and comprehensively the QCD investigation and other relevant studies ontesting the standard model and the existence of dark matter.∗ ∗ ∗Thanks to all JLab12 collaborators for the excellent research they are carrying on andthe contributions provided for this work, probably not enhanced enough as deserved.REFERENCES[1] Burkardt M., Miller A. and Novak W.-D., Rep. Prog. Phys., 73 (2010) 016201.[2] Xiangdong Ji, Annu. Rev. Nucl. Part. Sci., 54 (2004) 413-450.[3] Jones M. K. et al., Phys. Rev. Lett., 84 (2000) 1398; Gayou O. et al., Phys. Rev. Lett.,88 (2002) 092301.[4] JLAB approved experiments on nucleon form factors (refer to [11]): E12-07-108, E-12-09-019, E12-09-016, E12-07-109; E12-07-104; E12-11-009.[5] Some JLAB approved experiments on nucleon PDFs (refer to [11]): E12-10-006, E12-09-018, C12-11-108; E12-06-112, E12-09-008, E12-07-107, E12-09-009; E12-09-017, C12-12-102.[6] Some JLAB approved experiments on GPDs (refer to [11]): E12-06-114; E12-06-119, E12-11-003, PR12-11-104, E12-12-007, PR12-12-010.[7] Dudek J. et al., Phys. Rev. D, 82 (2010) 034508.[8] JLab proposal E12-11-005 for CLAS12 (refer to [11]).[9] Ahmed Z., Phys. Rev. Lett., 108 (2012) 102001.[10] Abrahamyan S. et al., Phys. Rev. Lett., 108 (2012) 112502; Horowitz C. J. et al., Phys.Rev. C, 85 (2012) 032501(R).[11] JLAB12GeVapproved experimentsweb pages by hall: Hall A: http://hallaweb.jlab.org/12GeV/, Hall B/CLAS12: http://www.jlab.org/exp prog/generated/12GeV/hallb.html,Hall C: http://www.jlab.org/exp prog/generated/12GeV/apphallc.html.[12] Hall D and GlueX home page: http://www.jlab.org/Hall-D/.[13] Hoblit S. et al., arXiv hep-ex 0808.2183v1 (2008).[14] Contalbrigo M. et al., Nucl. Instrum. Methods A, 639 (2011) 302.[15] Bellini V. et al., JINST, 7 (2012) C05013.

Experimental physics at JLab: Where confinement meets

asymptotic freedom

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Experimental physics at JLab: Where confinement meets asymptotic freedom

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