Open Physics Journal




(Discontinued)

ISSN: 1874-8430 ― Volume 5, 2019

Physics with Tagged Forward Protons at RHIC§



Kin Yip*
Brookhaven National Lab., Collider-Accelerator Department, Upton, NY 11973, USA

Abstract

Here we describe diffractive measurements at RHIC in proton+proton collisions with a special optics run of β*~21 m at STAR, at the center-of-mass energy GeV during the the RHIC 2009 run. We present published results of single spin asymmetry as well as preliminary results on double spin asymmetries and central exclusive production.

PACS Number(s): 13.85.Dz and 13.88.+e.

Keywords:: Elastic scattering, spin asymmetry.


Article Information


Identifiers and Pagination:

Year: 2014
Volume: 1
Issue: Suppl 1: M7
First Page: 52
Last Page: 56
Publisher Id: PHY-1-52
DOI: 10.2174/1874843001401010052

Article History:

Received Date: 25/11/2013
Revision Received Date: 24/12/2013
Acceptance Date: 25/12/2013
Electronic publication date: 31/12/2014
Collection year: 2014

© Kin Yip; Licensee Bentham Open.

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.


* Address correspondence to this author at the Brookhaven National Lab., Collider-Accelerator Department, Upton, NY 11973, USA; Tel: 1-631-344-4116; Fax: 1-631-344-5954; E-mail: kinyip@bnl.gov§ Presented at the Low x workshop, May 30 - June 4 2013, Rehovot and Eilat, Israel.






1. INTRODUCTION AND THEORETICAL FORMALISM

Elastic scattering of polarized protons at small four momentum transfer squared  is described by interference of Coulomb and nuclear amplitudes. Coulomb amplitude is calculable by QED and such interference provides a unique opportunity to study the dynamics of the strong interaction in the nonperturbative region. The total cross section was measured to very high energy and turned out to be in a good agreement with the description by the Regge pole exchange. At ultra relativistic energies the main contribution comes from Pomeron or, in modern terms, multigluon exchange [1(a) Nussinov S. Colored-Quark Version of Some Hadronic Puzzles Phys Rev Lett 1975; 34: 1286-9.; (b) Nussinov S. Perturbative recipe for quark-gluon theories and some of its applications Phys Rev 1976; D14: 246-57.]. Most of the previous experiments were done with unpolarized beams and targets. The first measurement with polarized protons at high energies in the Coulomb nuclear interference (CNI) region GeV) was done in E704 experiment [2Akchurin N, Langland J, Onel Y , et al. Analyzing power measurement of pp elastic scattering in the Coulomb-nuclear interference region with the 200-GeV/c polarized-proton beam at Fermilab Phys Rev 1993; D48: 3026-6.] with moderate precision. RHIC with its polarized beams [3Bai M, Roser T, Ahrens L , et al. Polarized proton collisions at 205 GeV at RHIC Phys Rev Lett 2006; 96: 174801--4.] published a number of accurate measurements with  GeV [4(a) Okada H, Alekseev IG, Bravar A , et al. Measurement of the analyzing powerAN in pp elastic scattering in the CNI region with a polarized atomic hydrogen gas jet target Phys Lett 2005; B638: 450-.; (b) Alekseev IG, Bravar A, Bunce G , et al. Measurements of single and double spin asymmetry in pp elastic scattering in the CNI region with a polarized atomic hydrogen gas jet target Phys Rev 2009; D79: 094014-1-18., 5Bazilevsky A. Measurements of the energy dependence of the analyzing power in pp elastic scattering in the CNI region J Phys Conf Ser 2011; 295: 012096-1-5.] a few years ago. But only one measurement with a limited statistics exists so far in the collider energy range [6(a) Bültmann S, Chiang IH, Chrien RE , et al. First measurement of AN at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2006; B632: 167-72.; (b) Bültmann S, Chiang IH, Chrien RE , et al. Double spin asymmetries ANN and ASS at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2007; B647: 98-103.].

Elastic scattering of two identical particles with spin  is described by 5 helicity amplitudes [7Buttimore NH, Gotsman E, Leader E. Spin-dependent phenomena induced by electromagnetic-hadronic interference at high energies Phys Rev 1978; D18: 694-716., 8Trueman TL. Hadronic spin dependence and the use of coulomb-nuclear interference as a polarimeter journal nam 1996; [arXiv hep-ph/9610316]]. Two amplitudes  and  produce no spin-flip, two other  and  produce double spin-flip and the last  produces single spin-flip. Each of the amplitudes can be written as a sum of hadronic and Coulomb amplitudes . Electromagnetic part is calculable from QED. It is believed that the main hadronic contribution to the cross section comes from non-flipping amplitudes so the optical theorem could be written as . Other hadron amplitudes are expected to be small and are parametrized in terms of :

               (1)

The differential cross section and asymmetries can be written in terms of the amplitudes:

    (2)

                               (3)

                            (4)

where  is the single spin asymmetry and  and  are the double spin asymmetries.

2. EXPERIMENT

The layout of the experiment is shown in Fig. (1).

Fig.(1)

Layout of the setup for small-t measurements with the STAR detector (in the center).



Protons scattered at very small angles at the interaction point (IP) travel inside the beam pipe until they reach the Roman Pot (RP) detectors located in the RHIC tunnel on both sides of the STAR detector. Each RP contains four silicon microstrip detectors and a trigger scintillation counter. During the 2009 run, we were able to insert RP detectors to be as close as about 12  (  being the beam size) or 10 mm from the center of the beam pipe. Two RP's with detectors inserted horizontally (at 55.5 m from IP) and another two RP's vertically (at 58.5 m) were used at each side of IP. More details of the detectors can be found in [9Bültmann S, Chen W, Chiang IH , et al. The PP2PP experiment at RHIC: silicon detectors installed in Roman Pots for forward proton detection close to the beam Nucl Instr Meth 2004; A535: 415-20.]. The coordinates measured by the detectors relate to the scattering angles at IP by the transport matrix:

                                                         (5)

where index  denotes a particular Roman Pot. The  at STAR during our special run was about 21 m and the RP's were positioned to give us a parallel-to-point focusing optics. As a result, the error introduced by unknown position of the interaction point was minimal. More details on the detector layout, alignment and performance can be found at [10Plyku D. Performance of silicon detectors in polarized proton-proton elastic scattering at RHIC J Phys Conf Ser 2011; 295: 012129-1-5.].

3. ANALYSIS

Elastically triggered events were selected for reconstructions and the cuts are briefly described below.

(A)           Clusters of consecutive strips with charge values above  from their pedestals were found. We ignore rare clusters larger then 5 strips, because there were a lot of noise among them.

(B)           A threshold depending on the cluster width was applied to the total charge of each cluster. This gave us better signal to noise ratio for clusters of 3 and 4 strips. After these cuts we had individual plane efficiencies above 99%.

Clusters in the planes of the same orientation (horizontal/x or vertical/y) within the same RP were merged and we required that their coordinates were within 200 µ (2 strips) from each other.

(C)           Clusters in the planes of the same orientation (horizontal/x or vertical/y) within the same RP were merged and we required that their coordinates were within 200  (2 strips) from each other.

(D)           Clusters in x and y orientations form a track and opposite pairs of tracks formed from each side of the IP were chosen.

(E)           Transport equation (5) was solved for each side.

(F)           The strongest criteria of elastic events selection is the collinearity cut which was realized by requiring , where  and  and  are typically  58µ rad, to be <9. The correlation between the angles can be seen in Fig. (2).

Fig.(2)

(color online) Distribution of δθ y vs δθ x for both detector pairs in horizontal RPs (a) and their projections in δθ y (b) and δθ x (c). The overlaid curves represent the fits with a Gaussian signal and a linear background. The σ values of distributions are ≈ 58 μ rad, consistent with beam angular divergence, and the background-to-signal ratio under the Gaussian distributions in ±3 σ is ≈ 0.4%.



About 21 millions events out of about 33 million elastic triggers written during the run were selected for asymmetry calculations.

Using the square root formula [6(a) Bültmann S, Chiang IH, Chrien RE , et al. First measurement of AN at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2006; B632: 167-72.; (b) Bültmann S, Chiang IH, Chrien RE , et al. Double spin asymmetries ANN and ASS at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2007; B647: 98-103., 11Ohlsen GG, Keaton PW Jr. Techniques for measurement of spin-½ and spin-½ polarization analyzing tensors Nucl Instr Meth 1973; 109: 41-59.], raw asymmetry as function of azimuthal angle  for only  and  bunch polarizations can be written as:

                        (6)

where ,  - number of events with bunch polarization pattern  at the azimuthal angle .  are polarizations of the blue and yellow beams, measured by HJET and pCarbon polarimeters [12http: //www4.rcf.bnl.gov/~cnipol/pubdocs/Run09Offline/ ]. The polarization values averaged for the time of our data taking were: ,  and  (errors shown here include global systematic uncertainties). From double spin asymmetries measured by [6(a) Bültmann S, Chiang IH, Chrien RE , et al. First measurement of AN at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2006; B632: 167-72.; (b) Bültmann S, Chiang IH, Chrien RE , et al. Double spin asymmetries ANN and ASS at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2007; B647: 98-103.], we know that  is less than 0.01. Using other different bunch polarization combinations, other raw asymmetries can be introduced similarly to (6); particularly, the so-called “wrong combination” is shown here:

                        (7)

The results of  for 5 -intervals and  for the whole -range are presented in Fig. (3).

Fig.(3)

(color online) The asymmetry ε (φ ) / (PB +PY ) for the five t -intervals (a) - (e). The asymmetry ε ′(φ ) for the whole measured t - range (f). The red curves represent the best fit to Eq. (6) (a) - (e) and Eq. (7) (f).



Using (6), we fitted the raw asymmetry to extract 's in 5 -bins.

Double spin raw asymmetries  and  can be extracted from the following equation (as there is no square root formula available):

                              (8)

Here  are relative luminosity monitors for the corresponding polarization pattern.

4. RESULTS ON ELASTIC SCATTERING MEASUREMENTS

The recently published results [13Adamczyk L, Agakishiev G, Aggarwal MM , et al. [STAR Collaboration] Single spin asymmetry ANin polarized proton-proton elastic scattering at √s=200 GeV Phys Lett 2013; B719: 62-9.] on the single spin asymmetry are shown in Fig. (4)

Fig.(4)

(color online) The measured single spin asymmetry AN for five −t intervals. Vertical error bars show statistical uncertainties. Statistical error bars in −t are smaller than the plot symbols. The dashed curve corresponds to theoretical calculations without hadronic spin-flip and the solid one represents the r5 fit.



Fig.(5)

(color online) Fitted value of r5 with contours corresponding to statistical error only (solid ellipse and cross) and statistical+systematic errors (dashed ellipse and cross) of 1σ.



in comparison with theoretical curve without hadron spin-flip and with the best fit allowing non-zero hadronic spin-flip (see [14Buttimore NH, Kopeliovich BZ, Leader E, Soffer J, Trueman TL. Spin dependence of high energy proton scattering Phys Rev 1999; D59: 114010--18.] for formula). Only statistical uncertainties have been included. The value of  resulting from the fit described above is shown in Fig. (5), together with both statistical and systematic uncertainties. The obtained values Re  = 0.0017 0.0063 and Im  = 0.007 0.057 are consistent with the hypothesis of no hadronic spin-flip contribution ( ) at the energy of this experiment. That is, only Pomeron exchange, which contributes only to spin non-flipping amplitudes  and , seems to survive at high energies.

The preliminary results on double spin raw asymmetries are shown in Fig. (6).

Fig.(6)

STAR Preliminary results on double spin raw asymmetry for the entire t-interval.



Though some effects of the order of  could be seen, they are small. Here we have used relative luminosities obtained from counts of inelastic triggers produced by the vertex position detector and beam-beam counters (BBC). However, after more thorough studies, BBC coincidence counts were proved to be the least sensitive to double spin effects with negligible statistical uncertainty. The systematic error due to the normalization uncertainty of BBC coincidence counts on  is at the level of .

5. CENTRAL EXCLUSIVE PRODUCTION

We have also studied the invariant mass spectrum of the two oppositely charged pions produced in the Ce  ntral Exclusive Production process of . Our ability to tag protons in the Roman Pots on opposite sides of the IP helps reduce background in this measurement of double Pomeron exchange compared to standard hadronic production processes. Pions were obtained from tracks with dE/dx identification. It is also required that the protons be non-collinear with scattering angle separation more than 0.15 mrad to avoid cosmics. Moreover, the missing transverse momentum of the final states two pions and two protons) had to be less than 0.02 GeV.

In Fig. (7), the spectrum of the invariant mass of

Fig.(7)

(color online) STAR Preliminary results on invariant mass spectrum of π +π − pairs produced in the central exclusive process p + p→ p +π +π − + p at s = GeV.



 pairs produced with the above cuts is presented. The like-sign background is very small, which gives a measure of exclusiveness of the process. The spectrum is not corrected for acceptance, but preliminary acceptance study indicates that corrections would not change shape of the spectrum significantly. Presented spectrum is similar to the one published by the AFS Collaboration at ISR [15Akesson T, Albrow MG, Almehed S , et al. [Axial Field Spectrometer Collaboration] A search for glue balls and a study of double pomeron exchange at the CERN intersecting storage rings Nucl Phys 1986; B264: 154-84.] as it shows the same characteristic features :

•           it is dominated by low invariant mass pairs, , below 1 GeV;

•           it shows the same characteristic drop around 1 GeV which may be due to  rescattering or (980) interference with the S-wave background [16Dedonder J-P, Furman A, Kaminski R, Lesniak L, Loiseau B. S-P- and D-wave final state interactions and CP violation in B± → π± π ± π± decays Acta Phys Polon 2011; B42: 2013-43.].

6. SUMMARY AND FUTURE PROSPECTS

We had a very successful run with the physics program with tagged forward protons at RHIC in 2009, in which over 70 million events (including 33 million events with elastic triggers) were collected. We have published our results on single spin asymmetry (ANand r5) and we are finalizing our results on double spin asymmetries as well as the central exclusive production. In the mean time, we are also preparing for the Phase II of this physics program, in which we change our detector configurations so that we will not need a special optics of RHIC and thus this physics program can be carried out simultaneously with other physics programs of the STAR experiment. This will allow us to take much more data and explore other physics possibilities at RHIC.

CONFLICT OF INTEREST

The author confirms that this article content has no conflicts of interest.

ACKNOWLEDGEMENTS

We thank the RHIC Operations Group and RCF at BNL, the NERSC Center at LBNL, the KISTI Center in Korea and the Open Science Grid consortium for providing resources and support. This work was supported in part by the Offices of NP and HEP within the U.S. DOE Office of Science, the U.S. NSF, CNRS/IN2P3, FAPESP CNPq of Brazil, Ministry of Ed. and Sci. of the Russian Federation, NNSFC, CAS, MoST and MoE of China, the Korean Research Foundation, GA and MSMT of the Czech Republic, FIAS of Germany, DAE, DST, and CSIR of India, National Science Centre of Poland, National Research Foundation (NRF-2012004024), Ministry of Sci., Ed. and Sports of the Rep. of Croatia, and RosAtom of Russia.

REFERENCES

[1] (a) Nussinov S. Colored-Quark Version of Some Hadronic Puzzles Phys Rev Lett 1975; 34: 1286-9.; (b) Nussinov S. Perturbative recipe for quark-gluon theories and some of its applications Phys Rev 1976; D14: 246-57.
[2] Akchurin N, Langland J, Onel Y , et al. Analyzing power measurement of pp elastic scattering in the Coulomb-nuclear interference region with the 200-GeV/c polarized-proton beam at Fermilab Phys Rev 1993; D48: 3026-6.
[3] Bai M, Roser T, Ahrens L , et al. Polarized proton collisions at 205 GeV at RHIC Phys Rev Lett 2006; 96: 174801--4.
[4] (a) Okada H, Alekseev IG, Bravar A , et al. Measurement of the analyzing powerAN in pp elastic scattering in the CNI region with a polarized atomic hydrogen gas jet target Phys Lett 2005; B638: 450-.; (b) Alekseev IG, Bravar A, Bunce G , et al. Measurements of single and double spin asymmetry in pp elastic scattering in the CNI region with a polarized atomic hydrogen gas jet target Phys Rev 2009; D79: 094014-1-18.
[5] Bazilevsky A. Measurements of the energy dependence of the analyzing power in pp elastic scattering in the CNI region J Phys Conf Ser 2011; 295: 012096-1-5.
[6] (a) Bültmann S, Chiang IH, Chrien RE , et al. First measurement of AN at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2006; B632: 167-72.; (b) Bültmann S, Chiang IH, Chrien RE , et al. Double spin asymmetries ANN and ASS at √s=200 GeV in polarized proton–proton elastic scattering at RHIC Phys Lett 2007; B647: 98-103.
[7] Buttimore NH, Gotsman E, Leader E. Spin-dependent phenomena induced by electromagnetic-hadronic interference at high energies Phys Rev 1978; D18: 694-716.
[8] Trueman TL. Hadronic spin dependence and the use of coulomb-nuclear interference as a polarimeter journal nam 1996; [arXiv hep-ph/9610316]
[9] Bültmann S, Chen W, Chiang IH , et al. The PP2PP experiment at RHIC: silicon detectors installed in Roman Pots for forward proton detection close to the beam Nucl Instr Meth 2004; A535: 415-20.
[10] Plyku D. Performance of silicon detectors in polarized proton-proton elastic scattering at RHIC J Phys Conf Ser 2011; 295: 012129-1-5.
[11] Ohlsen GG, Keaton PW Jr. Techniques for measurement of spin-½ and spin-½ polarization analyzing tensors Nucl Instr Meth 1973; 109: 41-59.
[12] http: //www4.rcf.bnl.gov/~cnipol/pubdocs/Run09Offline/
[13] Adamczyk L, Agakishiev G, Aggarwal MM , et al. [STAR Collaboration] Single spin asymmetry ANin polarized proton-proton elastic scattering at √s=200 GeV Phys Lett 2013; B719: 62-9.
[14] Buttimore NH, Kopeliovich BZ, Leader E, Soffer J, Trueman TL. Spin dependence of high energy proton scattering Phys Rev 1999; D59: 114010--18.
[15] Akesson T, Albrow MG, Almehed S , et al. [Axial Field Spectrometer Collaboration] A search for glue balls and a study of double pomeron exchange at the CERN intersecting storage rings Nucl Phys 1986; B264: 154-84.
[16] Dedonder J-P, Furman A, Kaminski R, Lesniak L, Loiseau B. S-P- and D-wave final state interactions and CP violation in B± → π± π ± π± decays Acta Phys Polon 2011; B42: 2013-43.

Endorsements



"Open access will revolutionize 21st century knowledge work and accelerate the diffusion of ideas and evidence that support just in time learning and the evolution of thinking in a number of disciplines."


Daniel Pesut
(Indiana University School of Nursing, USA)

"It is important that students and researchers from all over the world can have easy access to relevant, high-standard and timely scientific information. This is exactly what Open Access Journals provide and this is the reason why I support this endeavor."


Jacques Descotes
(Centre Antipoison-Centre de Pharmacovigilance, France)

"Publishing research articles is the key for future scientific progress. Open Access publishing is therefore of utmost importance for wider dissemination of information, and will help serving the best interest of the scientific community."


Patrice Talaga
(UCB S.A., Belgium)

"Open access journals are a novel concept in the medical literature. They offer accessible information to a wide variety of individuals, including physicians, medical students, clinical investigators, and the general public. They are an outstanding source of medical and scientific information."


Jeffrey M. Weinberg
(St. Luke's-Roosevelt Hospital Center, USA)

"Open access journals are extremely useful for graduate students, investigators and all other interested persons to read important scientific articles and subscribe scientific journals. Indeed, the research articles span a wide range of area and of high quality. This is specially a must for researchers belonging to institutions with limited library facility and funding to subscribe scientific journals."


Debomoy K. Lahiri
(Indiana University School of Medicine, USA)

"Open access journals represent a major break-through in publishing. They provide easy access to the latest research on a wide variety of issues. Relevant and timely articles are made available in a fraction of the time taken by more conventional publishers. Articles are of uniformly high quality and written by the world's leading authorities."


Robert Looney
(Naval Postgraduate School, USA)

"Open access journals have transformed the way scientific data is published and disseminated: particularly, whilst ensuring a high quality standard and transparency in the editorial process, they have increased the access to the scientific literature by those researchers that have limited library support or that are working on small budgets."


Richard Reithinger
(Westat, USA)

"Not only do open access journals greatly improve the access to high quality information for scientists in the developing world, it also provides extra exposure for our papers."


J. Ferwerda
(University of Oxford, UK)

"Open Access 'Chemistry' Journals allow the dissemination of knowledge at your finger tips without paying for the scientific content."


Sean L. Kitson
(Almac Sciences, Northern Ireland)

"In principle, all scientific journals should have open access, as should be science itself. Open access journals are very helpful for students, researchers and the general public including people from institutions which do not have library or cannot afford to subscribe scientific journals. The articles are high standard and cover a wide area."


Hubert Wolterbeek
(Delft University of Technology, The Netherlands)

"The widest possible diffusion of information is critical for the advancement of science. In this perspective, open access journals are instrumental in fostering researches and achievements."


Alessandro Laviano
(Sapienza - University of Rome, Italy)

"Open access journals are very useful for all scientists as they can have quick information in the different fields of science."


Philippe Hernigou
(Paris University, France)

"There are many scientists who can not afford the rather expensive subscriptions to scientific journals. Open access journals offer a good alternative for free access to good quality scientific information."


Fidel Toldrá
(Instituto de Agroquimica y Tecnologia de Alimentos, Spain)

"Open access journals have become a fundamental tool for students, researchers, patients and the general public. Many people from institutions which do not have library or cannot afford to subscribe scientific journals benefit of them on a daily basis. The articles are among the best and cover most scientific areas."


M. Bendandi
(University Clinic of Navarre, Spain)

"These journals provide researchers with a platform for rapid, open access scientific communication. The articles are of high quality and broad scope."


Peter Chiba
(University of Vienna, Austria)

"Open access journals are probably one of the most important contributions to promote and diffuse science worldwide."


Jaime Sampaio
(University of Trás-os-Montes e Alto Douro, Portugal)

"Open access journals make up a new and rather revolutionary way to scientific publication. This option opens several quite interesting possibilities to disseminate openly and freely new knowledge and even to facilitate interpersonal communication among scientists."


Eduardo A. Castro
(INIFTA, Argentina)

"Open access journals are freely available online throughout the world, for you to read, download, copy, distribute, and use. The articles published in the open access journals are high quality and cover a wide range of fields."


Kenji Hashimoto
(Chiba University, Japan)

"Open Access journals offer an innovative and efficient way of publication for academics and professionals in a wide range of disciplines. The papers published are of high quality after rigorous peer review and they are Indexed in: major international databases. I read Open Access journals to keep abreast of the recent development in my field of study."


Daniel Shek
(Chinese University of Hong Kong, Hong Kong)

"It is a modern trend for publishers to establish open access journals. Researchers, faculty members, and students will be greatly benefited by the new journals of Bentham Science Publishers Ltd. in this category."


Jih Ru Hwu
(National Central University, Taiwan)


Browse Contents




Webmaster Contact: info@benthamopen.net
Copyright © 2019 Bentham Open