SIDIS CLASNOTE 2013

Abstract

Semi-inclusive deep inelastic scattering (SIDIS) experiments may be used to identify the flavor of the quark that participates in the scattering process. Semi-inclusive scattering is defined as an electron scattering experiment in which the scattered electron and one hadron are detected in the final state. Ex- periments at Jefferson Lab have used longitudinally polarized electron beams to probe longitudinally polarized Hydrogen (15NH3) and Deuterium (15ND3) targets to investigate the quark’s contribution to the properties of a nucleon. This work reports a measurement of SIDIS pion asymmetries using the CEBAF Large Acceptance Spectrometer (CLAS) at Thomas Jefferson National Labo- ratory. The incident electron’s energy was 4.2 GeV and covered a kinematic region where the struck quark carries at least 30% of the nucleons total mo- mentum (xB ≥ 0.3). The electrons scatter mostly from valence quarks in this kinematic region allowing measurements which are less sensitive to the ocean of quark-antiquark pairs that are also inside a nucleon.

Data Analysis

Semi-inclusive deep inelastic scattering experiments using longitudinally polarized hydrogen (15NH3) and deuterium (15ND3) targets and a longitudinally polarized electron beam can be used to measure the ratio of the polarized va- lence quark distribution function to the unpolarized. Semi-inclusive scattering identifies an electron scattering experiment in which the scattered electron and one hadron are detected in the final state. This chapter describes the techniques used to analyze the data collected during the EG1b experiment and calculate semi-inclusive cross sections for the following reactions: ⃗e−N⃗ → e−π+X and ⃗e−N⃗ → e−π−X using NH3 and ND3 polarized targets respectively. The goal of this work is to measure charged pion asymmetries defined according to the incident electron helicity and the target polarization. This work focuses on a kinematic region where the struck quark carries at least 30% of the nucleons total momentum (xb > 0.3). The leptons scatter mostly from valence quarks in this kinematic region allowing contributions from sea quarks to be neglected. The above measurements are able to distinguish between the predictions made by the hyperfine perturbed quark constituent model (pQCM) and perturbative Quantum Chromodynamics (pQCD). As a result, the following study can be used to test the validity of the above models describing the structure of the nucleon.

The CLAS Data Selection

The data files from the EG1b experiment chosen for this analysis are listed in Table 1.1. During the experiment, 2.2 GeV, 4.2 GeV and 5.7 GeV longitudinally polarized electron beams were used to probe the polarized frozen ammonia NH3 and ND3 targets. This work will discuss the analysis of the 4.2 GeV electron beam data set as this energy provided the most statistics. The restrictions applied to the reconstructed events are described below. Run Set Target Type Torus Current(A) Target Polarization HWP 28100 - 28102 ND3 +2250 -0.18 +1 28106 - 28115 ND3 +2250 -0.18 -1 28145 - 28158 ND3 +2250 -0.20 +1 28166 - 28190 ND3 +2250 +0.30 +1 28205 - 28217 NH3 +2250 +0.75 +1 28222 - 28236 NH3 +2250 -0.68 +1 28242 - 28256 NH3 +2250 -0.70 -1 28260 - 28275 NH3 +2250 +0.69 -1 28287 - 28302 ND3 -2250 +0.28 +1 28306 - 28322 ND3 -2250 -0.12 +1 28375 - 28399 ND3 -2250 +0.25 -1 28407 - 28417 NH3 -2250 +0.73 -1 28456 - 28479 NH3 -2250 -0.69 +1 Table 1.1: EG1b runs analyzed for this work.

Particle Identification

Additional tests were performed on the electron and a pion candidates reconstructed by the standard CLAS software package. Electrons are identified by matching the charged particle hits in the Cherenkov counter, electromagnetic calorimeter, and the time of flight system. Geometrical and timing cuts are applied to improve electron identification [3]. In addition, cuts are applied on the energy deposited by the particle into the calorimeter and the number of photoelectrons produced in the Cherenkov counter. Charged pions are identi- fied by matching the hits in the drift chamber and ToF counter, along with a Cherenkov cut requiring that the number of photons for pions be less than two.

Electron Identification

The CLAS trigger system required a particle to deposit energy in the electromagnetic calorimeter and illuminate the Cherenkov counter within a 150 ns time window (Figure 1.1). Unfortunately, this trigger suffers from a back- ground of high energy negative pions that may be misidentified as electrons. The pion contamination of the electron sample is reduced using cuts on the energy deposited in the electromagnetic calorimeter and the momentum mea- sured by reconstructing the particles track in the known magnetic field. The energy deposition mechanism for the pions and electrons in the electromagnetic calorimeter is different. The total energy deposited by the electrons in the EC is proportional to their kinetic energy, whereas pions are minimum ionizing parti- cles and the energy deposition is independent of their momentum (Figure 1.2). The pion background is further suppressed using geometrical and time match- ing cuts between the Cherenkov counter hit and the measured track in the drift chamber.

Fig. 1.1: Example of electron passing through the drift chambers and creating the signal in the Cherenkov counter and electromagnetic calorimeter. Electron track is highlighted by the blue line (Run number 27095, Torus Current +2250 (inbending)). EC CUTS The CLAS electromagnetic calorimeter was used to reduce the misidenti- fication of electron and negative pion candidates. The electromagnetic calorime- ter contains thirteen layers of lead-scintillator sandwiches composed of ∼ 2 mm thick lead and 10 mm thick scintillator. Each set of thirteen layers are subdi- vided into five inner and eight outer layers that are named the inner and outer calorimeter respectively. Electrons interact with the calorimeter producing electromagnetic show- ers that release energy into the calorimeter. The deposited energy is propor- tional to the momentum of the electrons. Figure 1.3 shows the correlation of

Fig. 1.2: Momentum versus ECtotal. the inner and outer calorimeter electron candidate’s energy measured by the calorimeter and divided by the particles momentum reconstructed by the drift chamber. As shown in the Figure 1.3, there is an island near E/p = 0.2, which contains most of the electron candidates as well as some regions below 0.2 which will be argued to be negative pions misidentified as electrons. Pions entering the calorimeter are typically minimum ionizing parti- cles, loosing little of their incident energy in the calorimeter at a rate of 2 MeV g−1cm2. Electrons, on the other hand, deposit a larger fraction of their momentum into the calorimeter. As a result, the energy deposited into the elec- tromagnetic calorimeter is different for electrons and pions. Pions loose about 0.08 GeV of energy traversing the calorimeter independent their momentum thereby producing the constant signal in the calorimeter around 0.08 GeV. In order to reduce misidentified pions from the electron sample, the following cut

ECinner > 0.08 × p, (1.1) where p represents a particle’s momentum and ECinner the energy deposited into the inner part of the calorimeter. Since the energy loss of pions is related to the calorimeter thickness, a correlation can be established between the energy deposited into the inner and outer layers of the calorimeter: ECtot = 13, (1.2) ECinner 5 and results in the following cut for the energy deposition into the outer layer of the calorimeter: ECtot > 0.2 × p. (1.3) Cherenkov Counter Cut The Cherenkov counter has been used to further reduce the negatively charged pion background in the reconstructed electron sample. When the veloc- ity of a charged particle is greater than the local phase velocity of light or when it enters a medium with different optical properties, the charged particle will emit photons. Cherenkov light is emitted at the critical angle θc representing the angle of Cherenkov radiation relative to the particle’s direction. It can be

has been applied:  (a) Before cuts. (b) After cuts. Fig. 1.3: ECinner/p versus ECtot/p before and after EC cuts (ECtot > 0.2p and ECinner > 0.08p). After applying EC cuts about 46% of the events have been removed from the electron sample. shown that the cosine of the Cherenkov radiation angle is inversely proportional to the velocity of the charged particle cosθc = 1 , (1.4) nβ

where βc is the particle’s velocity and n the index of refraction of the medium. The charged particle in time t travels a distance βct, while the electromagnetic waves travel c t. For a medium with given index of refraction n, there is a n threshold velocity βthr = 1 , below which no radiation is emitted. This process n may be used to distinguish between the highly relativistic electrons and the less relativistic pions based on the number of photons produced in the Cherenkov detector. The number of photons produced per unit path length of a particle with charge Ze and per unit energy interval of the photons is proportional to the sine of the Cherenkov angle [2] d2NPE αz2 2 αz2􏰝 1 􏰞 dEdx = 􏱯c sin θc = 􏱯c 1− β2n2(E) d2NPE 2παz2 1 dλdx = λ2 [1 − β2n2(λ)] (1.5) (1.6) β=v=􏰦 pc . (1.7) c (pc)2 + (mc2)2 Taylor expanding Eq. 1.6 and keeping only the first two terms we get following d2NPEαz22 αz222 dEdx = 􏱯c sin θc = 􏱯c [β n (E)−1]. (1.8) The gas used in the CLAS Cerenkov counter is perfluorobutane C4F10 with index of refraction equal to 1.00153. Approximately thirteen photoelec- trons are produced by electrons traversing the Cherenkov detector. On the

other hand, calculations show that the number of photons produced by the negatively charged pions in the Cherenkov detector is approximately two. The theoretical results of the number of photons produced by the electrons and pions when passing through the Cherenkov counter are shown on Figure 1.4. (a) For electrons. (b) For pions. Fig. 1.4: Theoretical Calculation of the Number of Photoelectrons for electrons and pions. The distribution of the number of photoelectrons measured in the Cherenkov detector and the energy deposition dependence on number of photoelectrons are shown on Figure 1.5 and Figure 1.6. Pions, misidentified as electrons appear on Figure 1.5 at nphe<2.5.

Fig. 1.5: The number of photoelectrons without cuts. Fig. 1.6: The total energy deposited into the Calorimeter versus the Number of Photoelectrons. Geometric and Timing cuts Negative pions may be produced when the lepton scatters at a polar angle close to zero and is not observed by the detector. In order to reduce the electron sample contamination by those pions, geometrical cuts on the location of the particle at the entrance to the Cherenkov detector and time matching cuts have been developed by Osipenko [3]. For each CLAS Cherenkov detector segment the following cut has been applied |θp − θpcenter − θpoffset| < 3σp, (1.9) where θp represents the measured polar angle with respect to a projectile plane for each electron event and σp the width of the polar angle θp. The Cherenkov counter’s projective plane is an imaginary plane behind the Cherenkov detector where Cherenkov radiation would have arrived if it had moved the same distance from emission point to the PMT, without reflections in the mirror system. θpcenter is the polar angle from the CLAS detector center to the image of Cherenkov counter segment center and θpoffset is the shift in the segment center position. In addition to geometrical cuts, timing cuts have been applied to match the time between a Cherenkov counter hit and the time of flight system. The pion contamination in an electron sample was estimated by fitting the photoelectron distribution using two Gaussian distributions convoluted with a Landau distribution [4]: −0.5“ x−p1 ”2 1 −0.5“ x−p7 ”2 Npe =p0e p2 +p4 􏰙x−p5􏰚+p6e p8 . (1.10) 1− p6 The fits in Figure 1.7.(a) suggest that the pion contamination in the electron sample is 9.63% ± 0.01% before applying the OSI cuts and after the OSI cuts the contamination is about 4.029% ± 0.003% (Figure 1.7.(b)).

(a) Before Cuts. (b) After OSI Cuts. Fig. 1.7: The number of photoelectrons before and after geometrical and time matching cuts.

[SIDIS_PionAsym_EG2000]