Showing papers on "Proton spin crisis published in 2013"

Nanoscale Nuclear Magnetic Resonance with a Nitrogen-Vacancy Spin Sensor

Abstract: Extension of nuclear magnetic resonance (NMR) to nanoscale samples has been a longstanding challenge because of the insensitivity of conventional detection methods. We demonstrated the use of an individual, near-surface nitrogen-vacancy (NV) center in diamond as a sensor to detect proton NMR in an organic sample located external to the diamond. Using a combination of electron spin echoes and proton spin manipulation, we showed that the NV center senses the nanotesla field fluctuations from the protons, enabling both time-domain and spectroscopic NMR measurements on the nanometer scale.

The Spin Structure of the Nucleon

Abstract: This article reviews our present understanding of QCD spin physics: the proton spin puzzle and new developments aimed at understanding the transverse structure of the nucleon. Present experimental investigations of the nucleon's internal spin structure, the theoretical interpretation of the different measurements, and the open questions and challenges for future investigation are discussed.

Physics of the gluon-helicity contribution to proton spin.

Abstract: The total gluon helicity in a polarized proton, measurable in high-energy scattering, is shown to be the large momentum limit of a gauge-invariant but nonlocal, frame-dependent gluon spin $\stackrel{\ensuremath{\rightarrow}}{E}\ifmmode\times\else\texttimes\fi{}{\stackrel{\ensuremath{\rightarrow}}{A}}_{\ensuremath{\perp}}$ in QCD. This opens a door for a nonperturbative calculation of this quantity in lattice QCD and also justifies using free-field expressions in the light-cone gauge as physical observables.

Comment on "Proton spin structure from measurable parton distributions".

Abstract: We show that the recent claim that the expression 1/2 \\int x dx [ H_q (x,0,0) + E_q(x,0,0)], involving the generalized parton distributions H and E, measures the transverse angular momentum of quarks in a transversely polarized nucleon, is incorrect.

Comment on "Proton spin structure from measurable parton distributions".

Abstract: A Comment on the Letter by X. Ji, X. Xiong, and F. Yuan, Phys. Rev. Lett. 109, 152005 (2012). The authors of the Letter offer a Reply.

Resolving an individual one-proton spin flip to determine a proton spin state.

Abstract: Previous measurements with a single trapped proton (p) or antiproton (p) detected spin resonance from the increased scatter of frequency measurements caused by many spin flips. Here a measured correlation confirms that individual spin transitions and states are rapidly detected instead. The 96% fidelity and an efficiency expected to approach unity suggests that it may be possible to use quantum jump spectroscopy to measure the p and p magnetic moments much more precisely.

Octet spin fractions and the proton spin problem.

Abstract: The relatively small fraction of the spin of the proton carried by its quarks presents a major challenge to our understanding of the strong interaction. Traditional efforts to explore this problem have involved new and imaginative experiments and QCD based studies of the nucleon. We propose a new approach to the problem that exploits recent advances in lattice QCD. In particular, we extract values for the spin carried by the quarks in other members of the baryon octet in order to see whether the suppression observed for the proton is a general property or depends significantly on the baryon structure. We compare these results with the values for the spin fractions calculated within a model that includes the effects of confinement, relativity, gluon exchange currents, and the meson cloud required by chiral symmetry, finding a very satisfactory level of agreement given the precision currently attainable.

Polarized neutron Laue diffraction on a crystal containing dynamically polarized proton spins

Abstract: A polarized neutron Laue diffraction experiment on a single crystal of neodymium-doped lanthanum magnesium nitrate hydrate containing polarized proton spins is reported. By using dynamic nuclear polarization to polarize the proton spins, it is demonstrated that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.

Trigger electronics upgrade of PHENIX muon tracker

Abstract: The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) offers the unique capability to collide polarized protons at high energies. One of the highlights of the polarized proton program performed at s = 500 GeV is that it affords the direct measurement of sea quark contribution to the proton spin via W-boson production through the measurement of the parity violating single spin asymmetry. A new trigger electronics system for forward muons, which is especially capable of W-boson detection, was developed for the PHENIX experiment. The trigger was installed as an additional electronic circuit, and it was connected in parallel with the existing cathode readout electronics of the muon tracking chamber.

A Proposal for the Muon Piston Calorimeter Extension (MPC-EX) to the PHENIX Experiment at RHIC

Abstract: The PHENIX MPC-EX detector is a Si-W preshower extension to the existing PHENIX Muon Piston Calorimeters (MPC). The MPC-EX will consist of eight layers of alternating W absorber and Si mini-pad sensors and will be installed in time for RHIC Run-15. Covering a large pseudorapidity range, 3.1 80 GeV, a factor of four improvement over current capabilities. Not only will the MPC-EX strengthen PHENIX's existing forward neutral pion and jet measurements, it also provides the necessary neutral pion rejection to make a prompt photon measurement feasible in both p+A and p+p collisions. With this neutral pion rejection, prompt (direct + fragmentation) photon yields at high p_T, p_T > 3 GeV, can be statistically extracted using a double ratio method. In p+A collisions direct photons at forward rapidities are optimally sensitive to the gluon distribution because, unlike pions, direct photons are only produced by processes that are directly sensitive to the gluon distribution at leading order. A measurement of the forward prompt photon R_pA will cleanly access and greatly expand our understanding of the gluon nuclear parton distribution functions and provide important information about the initial state in heavy ion collisions. In transversely polarized p+p collisions the MPC-EX will make possible a measurement of the prompt photon single spin asymmetry A_N, and will help elucidate the correlation of valence partons in the proton with the proton spin.

Measuring proton spin polarizabilities with polarized compton scattering

Abstract: MEASURING PROTON SPIN POLARIZABILITIES WITH POLARIZED COMPTON SCATTERING FEBRUARY 2013 PHILIPPE PAUL MARTEL B.Sc., UNIVERSITY OF MASSACHUSETTS, AMHERST M.Sc., UNIVERSITY OF MASSACHUSETTS, AMHERST Ph.D., UNIVERSITY OF MASSACHUSETTS, AMHERST Directed by: Professor Rory Miskimen Polarized nuclear Compton scattering on a proton target provides a test of low energy QCD. The beam-target asymmetries of a circularly polarized Bremsstrahlung photon beam on a transversely polarized butanol target (Σ2x) and on a longitudinally polarized butanol target (Σ2z), and the beam asymmetry of a linearly polarized Bremsstrahlung beam on an unpolarized hydrogen target (Σ3) are sensitive to the proton spin polarizabilities, third order terms in the energy expansion of the Compton scattering amplitude. This experiment consisted of the Σ2x measurement, both just below and above two-pion threshold.

The Higgs-like Bosons and Quark Compositeness

Abstract: Considering that each quark is composed of two prequarks, called primons, it is shown that the recently found neutral Higgs-like boson belongs to a triplet constituted of a neutral boson and two charged bosons. The quantum numbers of these bosons are calculated and shown to be associated to a new kind of hypercharge which is directly related to the weak decays of hadrons and to the CKM matrix elements. Solutions to the proton spin puzzle and to other problems of particle physics are presented.

Magnetic resonance force microscopy : harnessing nuclear spin fluctuations

Abstract: Over the past few years, a wide variety of nuclear spin preparation techniques using hyperfine interaction-mediated dynamics have been developed in systems including gate-defined double quantum dots, self-assembled single quantum dots and nitrogen-vacancy centers in diamond. Here, we present a novel approach to nuclear spin state preparation by harnessing the naturally occuring stochastic fluctuations in nanoscale ensembles of nuclear spins in a semiconductor nanowire. Taking advantage of the excellent sensitivity of magnetic resonance force microscopy (MRFM) to monitor the statistical polarization fluctuations in samples containing very few nuclear spins, we develop real-time spin manipulation protocols that allow us to measure and control the spin fluctuations in the rotating frame. We focus on phosphorus and hydrogen nuclear spins associated with an InP and a GaP nanowire and their hydrogen-containing adsorbate layers. The weak magnetic moments of these spins can be detected with high spatial resolution using the outstanding sensitivty of MRFM. Recently, MRFM has been used to image the proton spin density in a tobacco mosaic virus with a sensitivity reaching up to 100 net polarized spins. We describe how MRFM together with real-time radio frequency (RF) control techniques can also be used for the hyperpolarization, narrowing and storage of nuclear spin fluctuations and discuss how such nuclear spin states could potentially be harnessed for applications in magnetic resonance and quantum information processing. In addition to presenting the experimental results on nuclear spin order, the theory of nuclear spin resonance and nanomechanical resonators is briefy discussed. The physical concepts explained provide the necessary background for the understanding of our MRFM experiments. The MRFM experimental apparatus, both sample-on- cantilever and magnet-on-cantilever, is also presented in considerable detail.

Recent Results for the Unquenched Quark Model

Abstract: We discuss a new generation of unquenched quark models for baryons is presented in which the effects of quark-antiquark pairs are taken into account in an explicit form by means of a 3 P0 quark-antiquark creation mechanism. This provides the possibility to address many open problems in baryons structure and spectroscopy. The applications to magnetic moments and to the spin and flavor content of baryons will be reviewed here. 1 Open Problems in Baryons Physics There is compelling evidence for the existence of exotic degrees of freedom, i.e. other than valence quarks, in baryons, in particular for the inclusion of the effects of quark-antiquark pairs (virtual intermediate meson- baryon states). The importance of these quark-antiquark configurations (or higher Fock components in baryon wave functions) is evident from measurements of the ¯ d/ ¯ u asymmetry in the nucleon sea (1), parity-violating electron scattering experiments (2,3), the proton spin crisis (4-6), as well as from analysis of helicity ampli- tudes (7,8) and strong couplings of baryon resonances (9-11). The coupling with the continuum and higher Fock configurations can also be the key solution of the missing resonances problem, which are resonances predicted by all quark models based on three constituent quarks, such as the (non)relativistic quark model, bag models and chiral soliton models, but which have never been observed (12). This problem is common to all quark models even below 2 GeV, as well as for most quark-diquark models (see (13) and references therein for a review on quark-diquark models), even though in this case the number of missing states is much smaller, since they are based on a reduced number of effective degrees of freedom. The aim of this article is to present a short overview of constituent quark models (CQM) and the need for unquenching of the quark model. The recently introduced unquenched quark model for baryons (14,15) will be reviewed and briefly and some recent results for the magnetic moments, and the spin and flavor contents of octet baryons will be discussed.

Recent results of high-energy spin phenomena of gluons and sea-quarks in polarized proton-proton collisions at RHIC at BNL

Abstract: The STAR experiment at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory is carrying out a spin physics program in high-energy polarized proton collisions at $\sqrt{s}=200\,$GeV and $\sqrt{s}=500\,$GeV to gain a deeper insight into the spin structure and dynamics of the proton. One of the main objectives of the spin physics program at RHIC is the precise determination of the polarized gluon distribution function. The STAR detector is well suited for the reconstruction of various final states involving jets, $\pi^{0}$, $\pi^{\pm}$, e$^{\pm}$ and $\gamma$, which allows to measure several different processes. Recent results suggest a gluon spin contribution to the proton spin at the same level as the quark spin contribution itself. The production of $W$ bosons in polarized p+p collisions at $\sqrt{s}=500\,$GeV opens a new era in the study of the spin-flavor structure of the proton. $W^{-(+)}$ bosons are produced in $\bar{u}+d\;(\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^{-}+\bar{ u}_{e}\;(e^{+}+ u_{e})$, where only the respective charged lepton is measured. Results of $W^{-(+)}$ production suggest a large asymmetry between the polarization of anti-$u$ and anti-$d$ quarks.

Fragmentation Functions measurement at COMPASS

Abstract: Fragmentation functions represent a key ingredient to address the proton spin structure in semi-inclusive deep-inelastic scattering and proton-proton collisions. They can not be determined from perturbative Quantum Chromodynamics and have to be extracted from experimental data in different processes. The COMPASS experiment at CERN provides a large data sample and covers a wide kinematic range for precise measurement of hadron multiplicities, directly connected to fragmentation functions. Recent, full-differential results on pion and kaon multiplicities are presented and discussed

From form factors to generalized parton distributions

Abstract: I present an extraction of generalized parton distributions from selected data on the electromagnetic nucleon form factors. The extracted distributions can in particular be used to quantify the contribution to the proton spin from the total angular momentum carried by valence quarks, as well as their transverse spatial distribution inside the proton.

Status and plans for the polarized hadron collider RHIC

Abstract: As the world’s only high energy polarized proton collider, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) has been providing collisions of polarized proton beams at beam energy from 100 GeV to 255 GeV for the past decade to explore the proton spin structure as well as other spin dependent measurements. With the help of two Siberian Snakes per accelerator plus outstanding beam control, beam polarization is preserved up to 100 GeV. About 10% polarization loss has been observed during the acceleration between 100 GeV and 255 GeV due to several strong depolarizing resonances. Moderate polarization loss was also observed during a typical 8 hour physics store. This presentation will give an overview the achieved performance of RHIC, both polarization as well as luminosity. The plan for providing high energy polarized He-3 collisions at RHIC will also be covered.

Measuring the contribution of low Bjorken-x gluons to the proton spin with polarized proton-proton collisions

Abstract: The PHENIX experiment is one of two detectors located at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, NY. Understanding the spin structure of the proton is a central goal at RHIC, the only polarized proton-on-proton collider in existence. The PHENIX spin program has two primary objectives. The rst is to improve the constraints on the polarized parton distributions of the anti-u and anti-d quarks within the proton. The second objective is to improve the constraint on the gluon spin contribution to the proton spin, $\Delta G$ . The focus of this thesis is the second objective. The motivation to study G originates with polarized Deep Inelastic Scattering (DIS) experiments, in which a polarized lepton is scattered o of a polarized proton. Polarized DIS scattering experiments have found that the quark polarization is signicantly less than expected and too small to account for the proton spin of 1 2~. This result was published rst in 1989 by the European Muon Collaboration. After it was further conrmed through DIS experiments at SLAC, CERN and DESY, it was natural to try measuring G, as a non-zero value would imply that the missing quark spin was due to gluon spin. Polarized DIS experiments are able to indirectly access gluons, however, the kinematic reach has so far been insucient to put a strong constraint on G. Even extreme scenarios for the gluon polarization could not be excluded by these experiments. At a polarized proton-on-proton collider, one gains direct access to gluons and more powerful constraints to the gluon polarization become possible with relatively small data samples. The PHENIX experiment has been successful at providing the rst meaningful constraints on G, along with STAR, the other detector located at RHIC. These constraints have, in fact, eliminated the extreme scenarios for gluon polarization through measurements of the double spin asymmetry, ALL, between the cross section of like and unlike sign helicity pp interactions. ALL measurements can be performed with a variety of nal states at PHENIX. Until 2009, these nal states were only measured for pseudo-rapidities of jj < 0:35. This range of is referred to as mid-rapidity. These mid-rapidity measurements, like the polarized DIS measurements, suer from a limited kinematic reach. Final states containing a measured particle with pT & 1 GeV=c are considered to have occurred in the hard scattering domain where the pp interaction is well approximated as an interaction of a quark or gluon in one proton and a quark or gluon in the second proton. Each of these interacting particles has a momentum fraction, x, of its parent proton's momentum. The gluon polarization is dependent on the momentum fraction and the net gluon polarization can be written as the integral of the momentum fraction dependent polarization: G = R 1 0 g(x)dx. The momentum fractions of the two interacting particles give information about the nal state jets. Likewise, one can work backwards. By measuring the kinematics of nal state hadrons or jets, information about quark and gluon momentum fractions can be learned. It turns out that mid-rapidity measurements of ALL are primarily sensitive to pp collisions in which the gluon momentum fraction was in the range 0:05 < x < 0:2. Therefore, mid-rapidity measurements are capable of constraining g(x) only within this range and the polarization of gluons having a momentum fraction outside this range do not play a signicant role in the observed ALL. This leaves a large gap in understanding as the gluon number density at low-x, x < 0:05, grows rapidly. It is, therefore, precisely the region not constrained by mid-rapidity ALL measurements that is the most interesting place to look for a potentially large gluon polarization. This provides the motivation to build a new calorimeter for PHENIX that is able to measure nal states of pp interactions in which a low-x gluon was a participant. Like a fast moving car crashing into a slow moving car and the debris ending up mostly along the line of motion of the fast moving car, the debris of a high-x quark interacting with a low-x gluon will result in debris at forward rapidity at small angles to the initial quark momentum. The Muon Piston Calorimeter (MPC) was installed in 2006 and 2007 at forward rapidity, 3:1 < jj < 3:9, with the intention of giving PHENIX the ability to constrain g(x) for x < 0:05. In this thesis, the rst two measurements of ALL using the MPC to measure a single hadron in the nal state will be presented. Following this, an electronics upgrade to the MPC will be described which enables the selection of events with two hadrons detected in the MPC. This requirement favors gluons at even lower x than the single hadron event selection. The di-hadron measurement that this upgrade makes possible will allow PHENIX to produce an ALL measurement that constrains g(x) in the range of 5 10􀀀4 < x < 0:01. Finally, we discuss the most important systematic uncertainty common to all ALL measurements which arises from the determination of the relative luminosity. A precision ALL measurement requires measuring the nal state yield from the portions of the proton beams that collide like and unlike sign helicity protons separately. It also requires understanding the ratio of the collision rates of these two portions of the beam exquisitely well. This is a long standing problem and, until recently, had threatened to severely restrict the ability of PHENIX to utilize the large data sets that have been acquired in the last two years to improve the constraints on G. We will conclude this thesis with a comprehensive overview of the relative luminosity systematic uncertainty and present a new framework within which this uncertainty can be determined. It will be demonstrated that not only were very large eects previously overlooked, but that by accounting for these eects the systematic uncertainty is reduced by an order of magnitude, from O(10􀀀3) to O(10􀀀4). This improvement has consequences for all high statistics measurements at PHENIX which were previously limited by their systematic uncertainty. The measurement of the gluon contribution to the proton spin at the PHENIX experiment is a multifaceted problem which requires a multi-faceted solution. This thesis describes several aspects of the solution as the single- and di-hadron measurements from MPC data are likely to provide the best constraints to G at low-x for the next decade. Eventually, an Electron-Ion Collider (EIC) will be designed and commissioned that will further extend the kinematic reach of the polarized DIS experiments that motivated the spin program at RHIC. In the meantime, the goal of PHENIX in general, and the MPC in particular, is to glean as much information about the gluon polarization as possible before the EIC era arrives.

Perspectives for Polarized Antiprotons

Abstract: One of the most challenging aspects of hadron physics is represented by the proton spin and its interpretation in terms of its internal constituents. The issue rose dramatically in the late 1980s when the European Muon Collaboration (EMC) conducted experiments suggesting that, contrary to the naive expectation, the spin carried by quarks is only a small fraction of the total spin of the proton. The problem of where the missing spin lies is referred to as the “proton spin crisis.” Although more than twenty years have passed since these first pioneering experiments, during which our theoretical and experimental knowledge have enormously deepened, the proton-spin has not yet revealed all of its mystery.

Physics of polarized protons in accelerators

Abstract: Polarized proton beams have been one of the essential elements in fundamental research such as unveiling the deep secret of proton spin structure. Polarized proton beams can also be the tool for direct measurement of the proton’s electric dipole moment (EDM). However, due to the interaction between spin motion and electric and magnetic fields, it is very challenging to overcome various depolarizing mechanisms through acceleration, and necessary spin manipulations at a store energy to meet the physics program requirements. Several decades of efforts have been devoted to develop techniques to preserve polarization and spin manipulation, as well as further our understanding of spin dynamics. These efforts directly led to the successful spin program at the Brookhaven Relativistic Heavy Ion Collider (RHIC), the world’s first high energy polarized proton collider. This tutorial presentation introduces basic physics which governs the spin dynamics in accelerators. A brief history of polarized protons development, as well as what have been achieved at RHIC are also reported.

The Proton Spin-Dependent Structure Function, g 2 , at Low Q 2

Abstract: The Jefferson Laboratory accelerator has been used to great effect in the study of the polarized structure of nucleons. Measurements of the spin-dependent structure functions have proven to be powerful tools in testing the validity of a number of effective theories of Quantum Chromodynamics. While the neutron spin structure functions, g 1,2, and longitudinal proton spin structure function, g p 1 , have been measured over a wide kinematic range, the second proton spin structure function, g 2 , has not. In this talk I will present the E08-027 (g2p) experiment, which was an inclusive measurement of the proton’s spin structure function, g 2 , in the resonance region at Jefferson Lab’s Hall A. This is the first measurement of g 2 covering 0.02 GeV 2 < Q < 0.2 GeV. The experiment will allow us to test the Burkhardt-Cottingham Sum Rule at low Q as well as extract the longitudinal-transverse generalized spin polarizability and compare it to predictions made by Chiral Perturbation Theory. In addition, the data will reduce the systematic uncertainty of calculations of the hyperfine splitting of hydrogen and extractions of the proton charge radius. An update on the status of the analysis, along with preliminary results, will be presented.

Prompt photon measurements with PHENIX's MPC-EX detector

Abstract: The MPC-EX detector is a Si-W preshower extension to the existing Muon Piston Calorimeter (MPC). The MPC-EX consists of eight layers of alternating W absorber and Si mini-pad sensors. Located at forward rapidity, 3.1 80 GeV, a factor of four improvement over current capabilities. Not only will the MPC-EX strengthen PHENIX's existing forward ?0 and jet measurements, it will provide sufficient prompt photon and ?0 separation to make a prompt photon measurement possible. Prompt photon yields at high pT, pT > 3 GeV/c, can be statistically extracted using the double ratio method. In transversely polarized p+p collisions, the measurement of the prompt photon single spin asymmetry, AN, will resolve the sign discrepancy between the Sivers and twist-3 extractions of AN. In p+Au collisions, the prompt photon RpAu will quantify the level of gluon saturation in the Au nucleus at low-x, x ~ 10?3, with a projected systematic error band a factor of four smaller than EPS09's current allowable range. The MPC-EX detector will expand our understanding of the gluon nuclear parton distribution functions, providing important information about the initial state of heavy ion collisions, and clarify how the valence parton's transverse momentum and spin correlates to the proton spin.

Study of the proton structure by measurements of polarization transfers in Real Compton scattering at the Th. Jefferson Natl. Lab

Abstract: The research activity proposed in present PhD thesis project is devoted to the study of the structure of the proton. The first direct evidence that the proton has an internal structure came from a measurement of its magnetic moment in 1933 by O. Stern. In the 1950s, a series of experiments led by R. Hofstadter at SLAC (Stanford) using elastic electron scattering to explore nuclear structure, measured for the first time the electric and magnetic form factors of the proton, firmly establishing that the proton has an extended charge and magnetic distributions. In the late 1960s the first inclusive Deep Inelastic Scattering (DIS) experiments were performed at SLAC with an electron beam. They showed the scaling behavior of the structure functions for high momentum transfer Q, interpreted by Bjorken and Feynman as evidence of charged constituents inside the proton. The DIS experiments allowed also the extraction of the unpolarized parton distribution functions (PDF). Existence of the gluon (the gauge bosons in Quantum Chromodynamics) in the nucleon has been observed at DESY (Hamburg). Another remarkable discovery made at CERN by the EMC Collaboration [1] showed that the total spin of the quarks contributes at most by 25% to the spin of the nucleon (“spin crisis”); the nucleon spin SN can be decomposed into three different components: SN = 1 2∆Σ + ∆G + Lz = 1 2 , where ∆Σ is the total spin of the quarks, ∆G the total spin of gluons, and Lz the quarks and gluons orbital angular momentum. Recent measurements of ∆Σ seem to indicate a contribution of order 30% (HERMES and COMPASS [2]). While a vanishing value of ∆G has been measured, even if with low accuracy, there is almost absolute ignorance on Lz. In order to give more deep insights in the hadron structure, a new theoretical framework, based on the so-called Generalized Parton Distributions (GPDs),