KnE Energy | The 3rd International Conference on Particle Physics and Astrophysics (ICPPA) | pages: 326–332

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1. Introduction

Spin correlation parameter A NN was measured earlier in diffractive region at the momentum less than 12 GeV/c [1,2,3]. We propose to measure the beam and target analyzing power and the initial state spin-spin correlation parameter in proton-proton elastic scattering at -t from 0.075 to 0.6 (GeV/c) 2 using a 45 GeV polarized proton beam at the new IHEP beam line 24A [4].

The results of this experiment can be described by a set of spin parameters, which are related to the pure-initial-spin state differential elastic cross sections by the formula:

dσdΩij=dσdΩ01+PBijAB+PTijAT+PB(ij)PTijAnn

where P B and P T are the beam and target polarizations, A B and A T are the beam and target analyzing powers, A NN is the initial state spin-spin correlation parameter. The A NN, A B , and A T can be expressed by equations (2-4), respectively:

ANN=1PBPTNNN+NN+N+N+N
AB=1PBN+NNNN+N+N+N
AT=1PTNN+NNN+N+N+N,

where N(ij) is the number of events normalized to the incident proton flux with the beam's (i) and target's (j) spin direction up ( ) or down ( ). Our main goal is to determine the momentum dependence of the spin-correlation parameters in the range of 12-45 GeV/c in the diffractive kinematic region.

2. Existing experimental data of the A N and A NN in proton-proton elastic scattering

A N is fairly well measured at various energies (see Figure 1 which was presented originally in Ref. [5]). The solid line is the fitting function with poles, suggested by the Regge model, namely, A N =a1+a2/ p beam + a3/p beam [6].

Measured value of A N at 45 GeV/c is equals to (2.3 ± 0.2) % [7], and this value is used here for estimating the required beam time. The experimental data on the spin correlation parameter A NN in the diffractive region are very scarce. This parameter has been measured only at the momenta of 3.3; 6; 11.75 GeV/c [1,2,3] in laboratory system (see Figure 2). The solid line presents the fit to experimental data with the function A NN = a(p/p 0 ) bbeam , where a=0.57 ± 0.23 and b=-0.86 ± 0.33, and p0=1 GeV/c. The expected value for A NN at 45 GeV/c is (2.2 ± 1.1)%.

Figure 1

A N for pp elastic scattering at –t=0.15 (GeV/c) 2 as a function of beam momentum.

fig-1.jpg
Figure 2

A NN for pp elastic scattering at –t=0.6 (GeV/c) 2 as a function of beam momentum.

fig-2.jpg

3. Beam characteristics and beam polarization measurement

The polarized proton and antiproton beams are described in detail in Ref. [8]. We plan to do these measurements using the 45 Gev/c polarized proton beam with the narrow momentum width Δ p/p=1,2% to fit into the polarized target of a diameter 18 mm [9].The beam properties are as follows: an average beam polarization P B =40%, transverse dimensions σ(x) × σ(y) = 11 × 8.7 mm 2 , angular divergences σ(x') × σ(y)=1.4 × 1.7 mrad 2 , intensity I=5 × 10 6 protons per cycle hitting the target.

The transvers polarization dependence on the beam-ray position along the vertical axis in the intermediate focus is presented in Figure 3. It can be described by the function P N =ay+by 3 , where a=(0.025 ± 0.027) mm -1 , b= -(0. 68 ± 0.19) × 10 -5 mm -3 . The beam-tagging system will be used for measuring the polarization of each particle at the accuracy of about 1% [10]. The beam-tagging system will also be used for the polarized beam tune up at commissioning time.

4. Experimental setup

The design of the experimental setup is presented in Figure 4. The main detectors to be used are the trigger scintillator counters (S1-S8) and hodoscopes (H1-H8).

The parameters for all hodoscopes are presented in table 1. The hodoscopes are constructed in such a manner that each scintillator strip overlaps its two adjacent neighbors by one-third of the width. This kind of overlapping arrangement provides twice as many of the spatial segments for the same number of channels of encoding electronics as for the edge-butted array. It also eliminated the loss of tracking efficiency by eliminating the inter-segment gaps.

Figure 3

Correlation between the average vertical transverse polarization of protons and the vertical ray coordinate y in the plane of intermediate focus.

fig-3.jpg
Figure 4

Design of the experimental setup.

fig-4.jpg

In order to suppress inelastic events, a special fast trigger will be organized on the base of coincidences between scattered and recoil protons in forward (H3-H4) and backward ((H5-H8) detectors. It worth mentioning, that elastic scattering measurements can be carried out simultaneously with the inclusive double-spin studies, using the main detectors of SPASCHARM experiment.

Table 1

Design of the hodoscopes.


Hodoscope Distance from target (mm) Total dimensions (mm) Dimension of scintillator strips: Width × Thickness × Length (mm 3 ) Numbers of channels
Z X × Y X      Y X      Y
Beam Hodoscopes
H1 -9000 40 × 40 6 × 3 × 40      6 × 3 × 40 9       9
H2 -2800 40 × 40 6 × 3 × 40       6 × 3 × 40 9      9
Forward Hodoscopes
H3 850 62 × 45 6 × 3 × 62       6 × 3 × 45 15       11
H4 2850 182 × 86 6 × 3 × 182      6 × 3 × 86 45       21
Recoil Hodoscopes
X Z × Y Z       Y Z       Y
H5 - H7 300 322 × 228 9 × 5 × 322       9 × 5 × 228 51      37
H6 - H8 600 432 × 430 9 × 5 × 432      9 × 5 × 420 71      69

5. Beam time estimation

Numerical calculations for the number of elastic scattering events has been done for the worst case scenario with the maximum value of |t|= 0.6 (GeV/c) 2 and Δ t= ± 0,1 (GeV/c) 2 , where the cross-section is the smallest. For measuring the t-dependence of A NN , the following input parameters have been assumed in simulations: the main beam characteristics as they are described in Sec. 3, the beam duty factor ν=0.1, the polarized target proton density ρ = 0.12 g/sm 3 ,the intensity I=5 × 10 6 protons per cycle, the vertical component of magnetic field in the frozen polarized target zone – 0.45 T, the target polarization P T = (75 ± 5)%. The corresponding luminosity is L=10 28 sm -2 s -1 [8].

An event rate is defined as n=L Δ t dσ/dt Δφ/φ , where dσ/dt =0.23 mb/(GeV/c) 2 [11] and Δφ/φ =0.1. Finally, at the assumptions above, the counting rate n=0.04644 s -1 . In order to measure A N with the statistical error Δ A NN =A NN = 2.2%, and taking into account both the beam and target polarizations the required beam time would be about 10 days. The parameters A B and A T will be measured at the same time with the precision of 50% and 30%, respectively, for the same |t|= 0.6 (GeV/c) 2 . The much better precisions for these observables are expected at the smaller |t|-values.

6. Summary

We proposed the experiment which allows to measure simultaneously three spin observables in elastic pp scattering at 45GeV/c in the |t| interval of 0.07 ÷ 0.60 (GeV/c) 2 .

10 days are required for measuring the t-dependence of A NN in order to reach the precision Δ A NN =A NN at maximum -value. The time required for measuring the A B and A T with 10% relative errors is significantly shorter at the smaller |t|.

The momentum dependence of these parameters can be studied in the wider momentum range from 15- to 45 GeV/c.

The work on estimations of the possibility to carry out the similar measurements with the polarized-antiproton beam is in progress.

This work has been supported in part by the RFBR Grant No. 16-02-00667 and by the Competitiveness Programme of National Research Nuclear University MEPhI.

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