Transverse double spin asymmetries in proton-proton elastic scattering
at sqrt(s)=200 GeV and small momentum transfer
Target Journal: Physics Letters B
PAs: Igor Alekseev, Andrzej Sandacz, Dmitry Svirida
Abstract:
Precise measurements of transverse double spin asymmetries A_NN and A_SS in proton-proton elastic scattering at very small values of four-momentum transfer squared, t, have been performed using the Relativistic Heavy Ion Collider (RHIC) polarized proton beams. The measurements were made at the center-of-mass energy sqrt(s) = 200 GeV and in the region 0.003 < |t| < 0.035 (GeV/c)^2 , which was accessed using Roman Pot devices incorporated into the STAR experimental setup. The measured asymmetries are sensitive to the poorly known hadronic double spin-flip amplitudes. While one of these amplitudes, \phi_4 , is suppressed as t \to 0 due to angular momentum conservation, the second double spin-flip amplitude, \phi_2 , was found to be negative and small, but significantly different from zero. Combined with our earlier result on the single spin asymmetry A_N, the present results provide significant constraints for the theoretical descriptions of the reaction mechanism of proton-proton elastic scattering at very high energies.
Figure 1:
Figure 1: Difference in R2 double spin normalization ratio for BBC and ZDC as a function of the RHIC fill number during the experiment data taking.
Figure 2:
Figure 2: (a) STAR BBC small tiles: white - inner, hatched - outer; the central circle and the dot show the beam pipe and the beam; (b) hit multiplicity distribution in STAR BBC.
Figure 3:
Figure 3: Difference in R2 ratio for various BBC parts as a function of the RHIC fill number during the experiment data taking: (a) east and west arms compared for high multiplicity events; (b),(c) and (d) - correspondingly: 'inner' tiles, 'outer' tiles and high multiplicity events compared to the BBC as a whole.
Figure 4:
Figure 4: Angular distributions for the asymmetry \epsilon2(\phi)/(P_B P_Y) and their fit with A_2+ + A_2- cos 2\phi: top left - full t-range of the experiment, other panels - individual t-intervals.
Figure 5:
Figure 5: Magnified background part of the \chi2 distribution for one of the t intervals and its fit with a sum of exponent and linear function.
Figure 6:
Figure 6: Dependence of the extracted asymmetries on the \chi^2_cut value for one of the t-intervals and their fits with quadratic polynomials.
Figure 7:
Figure 7: Results on the double spin asymmetries A2+ (left) and A2- (right) and their fits to extract relative amplitudes r2 and r4 .
Figure 8:
Figure 8: 1\sigma confidence level ellipses for the relative amplitudes r2 (left) and r4 (right).
Paper Conclusions:
In conclusion, we present precise measurements of transverse double spin asymmetries in elastic proton-proton scattering at the CNI region and sqrt(s) = 200 GeV. The experimental uncertainty of the result is about a factor of 10 smaller than that of the previous measurements at the same energy.
Extensive studies were performed to evaluate and reduce systematic uncertainties originating from the relative luminosity and a background asymmetry. A detailed analysis of the data from various detectors and processes was carried out in a search for an optimal monitor of the relative bunch luminosity, which should be insensitive to double spin asymmetries. It led to the choice of the BBC detectors. The background asymmetry is not related to any physics process, but is dominated by accidental coincidences of scattered protons with beam halo particles. This background effect was studied and subtracted using two approaches, each with its own advantages and disadvantages. The conservative estimate of the corresponding uncertainty was obtained by comparing the results from both approaches.
The measured asymmetry (A_NN - A_SS )/2 is compatible with zero. On the contrary the values of (A_NN + A_SS )/2 are significantly below zero. Its t-dependence is flat and the absolute values are of the order of 0.005. Our results are at variance, both for the sign and t-dependence, with the latest predictions [4] of the model based on the Regge theory. Using the extracted values of the relative double spin-flip amplitudes r2 and r4 , we conclude that the hadronic double spin-flip amplitudes \phi_2^had and \phi_4^had are different at our kinematic range. This indicates that the exchange mechanism is more complex than an exchange of Regge poles only. This conclusion is further supported by comparing \phi_2^had with the STAR result on the single spin-flip amplitude \phi_5^had [3].
The STAR measurements of the double and single spin asymmetries, with small uncertainties and at high energy, provide important constraints for theoretical models aiming to describe the spin-dependence of elastic scattering.
Recent Presentations:
Early stage discussion
PWGC preview
DUBNA-SPIN2013 talk on asymmetries
DUBNA-SPIN2013 talk on asymmetry uncertainties
SPIN2014 talk on asymmetries
SPWGC paper draft discussion
Comments exchange / paper discussion
Supporting Documents:
Relative luminosity and normalization uncertainties
Double spin asymmetries analysis note
Paper Drafts:
Collection of Paper Draft revisions
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