We examine the potential of the COMPASS experiment at CERN to study color
transparency via exclusive coherent vector meson production in hard
muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity
to test this important prediction of perturbative QCD.Comment: Feasibility study for COMPASS collaboration, 3 pages, no figures, 1
  table. This contribution is based on talk presented at the Workshop on Spin
  Physics, Trento, Italy, July 2001. The conference www site is
  http://ECTstar.ect.it/contents.html The proceedings of the workshop will be
  published as a special issue of Nuclear Physics B (proc suppl), eds. S Bass,
  A De Roeck and A Deshpande. A more complete 26 page feasibility study with 7
  figures and 3 tables is available as A. Sandacz et al., hep-ex/0106076
  Revision of 26 Sept. for Ref. 11 modificatio

A. Sandacz

Ackerstaff

Adams

Aitala

Amaudruz

Arneodo

Bradamante

Brodsky

Carroll

E. Piasetzky

Frankfurt

M. Moinester

Makins

O'Neill

O.A. Grajek

Renk

Sokoloff

English

arXiv

We examine the potential of the COMPASS experiment at CERN to study color transparency via exclusive coherent vector meson production in hard muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity to test this important prediction of perturbative QCD

Moinester, M.

Grajek, O.A.

Piasetzky, E.

Sandacz, A.

CERN Document Server

arXiv:hep-ex/0109010v2  26 Sep 20011Color Transparency via Coherent Exclusive ρ0 ProductionM. Moinestera, O. A. Grajekb, E. Piasetzkya, A. Sandaczb ∗aSchool of Physics and Astronomy, R. and B. Sackler Faculty of Exact Sciences, Tel Aviv University,69978 Ramat Aviv, IsraelbSo ltan Institute for Nuclear Studies, ul. Hoz˙a 69, PL 00-681 Warsaw, PolandWe examine the potential of the COMPASS experiment at CERN to study color transparency via exclusivecoherent vector meson production in hard muon-nucleus scattering. It is demonstrated that COMPASS has highsensitivity to test this important prediction of perturbative QCD.1. INTRODUCTIONOne prediction of pQCD is that highQ2 longitudinally polarized virtual pho-tons γ∗L ﬂuctuate into hadronic components,e.g. qq¯ pairs, whose transverse size b =| r¯⊥q − r¯⊥q¯ | decreases with Q2, b2 ∝ (1/Q2). Atlarge Q2 the values of b are signiﬁcantly smallerthan the size of the nucleon. Such a Small SizeConﬁguration (SSC) interacts with hadrons withsmall cross sections, a phenomenon known asColor Transparency (CT) [1–3].Cross section for the interaction of SSC with ahadron target has been calculated in QCD usinga factorization theorem [2,3]. Observing CT inparticular kinematics would prove experimentallythe applicability of the QCD factorization theo-rem for those kinematics. Such a proof providesimportant complementary support to a class ofspin physics experiments, for example measure-ments of generalized parton distributions [4].The prerequisite for observing CT is to select asample containing SSC mini-mesons via large Q2,high pt, or large produced mass. For hard exclu-∗The authors gratefully acknowledge useful discussionswith L. Frankfurt and M. Strikman. The work was sup-ported in part by the Israel Science Foundation (ISF)founded by the Israel Academy of Sciences and Humani-ties, Jerusalem, Israel and by the Polish State Committeefor Scientific Research (KBN SPUB Nr 621/E-78/SPUB-M/CERN/P-03/DZ 298/2000 and KBN grant Nr 2 P03B113 19). One of us (A.S.) acknowledges support from theRaymond and Beverly Sackler Visiting Chair in Exact Sci-ences during his stay at Tel Aviv University.sive ρ0 leptoproduction, in addition to large Q2,selection of the longitudinally polarized mesons isrequired. In order to clearly observe CT, it is alsonecessary that the SSC lives long enough whilepropagating through the nucleus. This require-ment is characterized in terms of the coherencelength lc [5,6].Strong recent evidence for CT comes from Fer-milab E791 experiment on the A-dependence ofcoherent diﬀractive dissociation of pions into twohigh-pt jets [7]. Also the E691 [8] and NMC [9]results on A-dependence of coherent J/ψ photonand muon production are consistent with CT.The HERMES [6], NMC [10], and E665 [11] lep-toproduction experiments have made extensivestudies of CT via incoherent vector meson pro-duction (VMP), most of the data for ρ0 produc-tion. The BNL (p,2p) [12] and SLAC (e,e’p) [13]high four momentum transfer quasi-elastic datawere also compared to CT predictions.We proposed [14] to study CT at COMPASS[15,16] via exclusive coherent VMP via reactionssuch as µA→ µAρ on C and Pb nuclei. The se-lections of coherent or incoherent production willbe mainly using the t-distribution; at the lowest|t | values coherent events predominate.The basic observable for each process stud-ied is the ratio of the nuclear transparen-cies for lead and carbon, RT = TPb/TC =(σPb/APb)/(σC/AC). T is the ratio of the crosssection per nucleon on a nucleus A to the corre-sponding cross section on a free nucleon.22. SIMULATION OF EXCLUSIVE ρ0EVENTSSimulations were carried out [14] with a ded-icated fast Monte Carlo program which gener-ates deep inelastic exclusive coherent ρ0 eventswith subsequent decay ρ0 → pi+pi−. Experimen-tal eﬀects such as acceptance, kinematic smear-ing, etc., were included. The diﬀerential cross sec-tions for the proton targetdσNdt is related to thatfor coherent production on the nucleus A by:(dσcohAdt)i= A2eff, coh · e<R2A> t/3 ·(dσNdt)i. (1)Here <R2A> is the radius squared, Aeff, coh takesaccount of nuclear screening for the coherent pro-cess, and i = L or T designates the polarization.An experimentally based parameterization of thecross section was used for the production on thefree nucleon, µN → µNρ0. Simulations weredone as well [14] for incoherent ρ0 production.Coherent cross sections were generated for twomodels. For the complete color transparencymodel (CT model) Aeff, coh = A was used. Inanother model, a substantial nuclear absorption(NA model) was assumed, with Aeff, coh = A0.75.The experiment measures the t-integrated coher-ent cross section, for which the CT model viaintegration of Eq. 1 predicts an approximateA4/3 dependence. For the NA model, the ex-pected A-dependence is weaker; A5/6 similarly asfor the pion-nucleus cross section. For productionof mesons not having the normal qq¯ structure, theA-dependence of the cross section may be diﬀer-ent. In that case, the A-dependence might evenbecome a tool for investigating 4-quark or otherexotic meson structure.3. RESULTS ON EXCLUSIVE ρ0PRODUCTIONWe considered 190 GeV muon beam. Simula-tions were done for carbon (A = 12) and leadtargets (A = 207). For each target we assumedthe CT and the NA models. The kinematic rangeconsidered for Q2 and ν, the energy of the virtualphoton in the laboratory system, was:2 < Q2 < 20 GeV2, 35 < ν < 170 GeV. (2)We observe clear coherent peaks at small t ≈ p2t(< 0.05 GeV2). The numbers of accepted eventsfor the carbon and lead targets, assuming the twomodels for the nuclear absorption, are given inTable 1, for a projected 150 day run.Table 1Number of accepted events for the two models.model NC NPbCT 70 000 200 000NA 28 000 20 000As Q2 increases, the approach to the CT limitis expected to be diﬀerent if the ρ0 is producedby longitudinally or transversely polarized virtualphotons. We plan to study the A-dependenceof the cross sections for samples with diﬀerentρ0 polarizations, which will be selected by cutson the cos θ distributions for pions from ρ0 de-cays. These studies with different polarizationsof the virtual photons are important for the cleardemonstration of CT.Another important aspect for CT studies is thecovered range of the coherence length lc. It isimportant to disentangle eﬀects due to CT fromthose caused by the modiﬁed absorption at smalllc values. We may use the combined data at lc val-ues exceeding the sizes of the target nuclei, withthe selection lc > lminc ≃ 2· < r2Pb>1/2= 11 fm.About 50% of events survive that cut on lc. Theycover the range of Q2 < 6 GeV2, which is ex-pected to be suﬃcient to observe CT.We estimated values and statistical precisionof RT, the ratio of the nuclear transparenciesfor lead and carbon, for diﬀerent Q2 bins forp2t < 0.02 GeV2. One expects large diﬀerencesin RT for the two considered models. For coher-ent samples RT ≈ 5 for CT model and ≈ 1 for NAmodel. At Q2 ≃ 5 GeV2 the precision of the mea-surement of RT for coherent events will be betterthan 17%, thus allowing excellent discriminationbetween the models.34. COMPARISON WITH PREVIOUSEXPERIMENTSThe NMC [9], and and E691 [8] cross sectionsfor coherent J/Ψ production have A-dependencesclose to A4/3. The NMC and E665 data [10,11]on incoherent ρ0 production cover a large rangeof Q2 and lc values. Due to the moderate statis-tics of the data, the Q2 dependence of nuclearabsorption could not be obtained at suﬃcientlylarge or at ﬁxed lc, which may obscure the ex-pected CT eﬀects in incoherent production [5,6].The NMC and E665 experiments published onlylimited data [10,11] for coherent ρ0 production.The COMPASS ρ0 production data for C andPb will cover the kinematic range similar to thatof the NMC ρ0 production data, and with thehigh statistics given in Table 1. Due to largestatistics, splitting of COMPASS data in severalQ2 and lc bins as well as the selection of coherentand incoherent events, and with longitudinal ortransverse ρ0 polarization, will be possible.5. CONCLUSIONSWe simulated exclusive coherent ρ0 muopro-duction (µA→ µ ρ0A) events for the COMPASSexperiment, using thin nuclear targets of carbonand lead. Good resolutions in Q2, lc, t (p2t ) andcos θ are feasible. An eﬃcient selection of coher-ent events is possible by applying cuts on p2t . Inorder to obtain the samples of events initiated byeither γ∗L or γ∗T , the cuts on the ρ0 decay angulardistribution of cos θ will be used. The search forCT could be facilitated by using the events withlc values exceeding the sizes of the target nuclei.The fraction of such events is substantial and thecovered Q2 range seems suﬃcient to observe CT.We showed high sensitivity of the measuredratio RT for diﬀerent models of nuclear absorp-tion. Good statistical accuracy may be achievedalready during a 150 day run. These measure-ments, taken at diﬀerent Q2 intervals, may allowto discriminate between diﬀerent mechanisms ofthe interaction of the hadronic components of thevirtual photon with the nucleus, and should un-ambiguously demonstrate CT.REFERENCES1. S.J. Brodsky, L.L. Frankfurt, J.F. Gunion,A.H. Mueller, M.I. Strikman,Phys. Rev. D50 (1994) 3134.2. L.L. Frankfurt, G.A. Miller and M.I. Strik-man, Phys. Lett. B304 (1993) 1; L.L. Frank-furt, G.A. Miller and M.I. Strikman, Ann.Rev. Nucl. Part. Sci. 45 (1994) 501.3. L.L. Frankfurt, A.V. Radyushkin and M.I.Strikman, Phys. Rev. D55 (1997) 98.4. J. Pochodzalla, L. Mankiewicz, M. Moinester,G. Piller, A. Sandacz, M. Vanderhaeghen,Exclusive Meson Production at COMPASS,hep-ph/9909534.5. T. Renk, G.Piller and W. Weise,Nucl. Phys. A689 (2001) 869.6. HERMES Collab., K. Ackerstaﬀ et al,Phys. Rev. Lett. 82 (1999) 3025.7. E791 Collab., E.M. Aitala et al, Phys. Rev.Lett. 86 (2001) 4773; E791 Collab., E.M.Aitala et al, Phys. Rev. Lett. 86 (2001) 4768.8. FNAL E691 Collab., M.D. Sokoloﬀ et al,Phys. Rev. Lett. 57 (1986) 3003.9. CERN NMC Collab., P. Amaudruz et al,Nucl. Phys. B371 (1992) 553.10. NMC, M. Arneodo et al, Phys. Lett. B332(1994) 195; Nucl. Phys. B429 (1994) 503.11. FNAL E665 Collab., M.R. Adams et al,Phys. Rev. Lett. 74 (1995) 1525; G. Y. Fanget al, FERMILAB-Conf 93/305 (1993), Proc.PANIC, Perugia, Italy, 1993, ed. A. Pascolini(World Scientiﬁc, Singapore, 1994) p.332.12. BNL E850 Collab., A.S. Carroll et al, Phys.Rev. Lett. 61 (1988) 1698; A. Leksanov et al,Energy Dependence of Nuclear Transparencyin C(p,2p) Scattering, hep-ex/0104039.13. SLAC NE18 Collab., N. C. R. Makins etal, Phys. Rev. Lett. 72 (1994) 1986; T. G.O’Neill et al, Phys. Lett. B351 (1995) 87.14. A. Sandacz, O. A. Grajek, M. Moinester, E.Piasetzky, Color Transparency at COMPASS,hep-ex/0106076.15. G. Baum et al, CERN/SPSLC 96-30 (1996),CERN/SPSLC/96-14,SPSC/P297 (1996),wwwcompass.cern.ch/compass/proposal/16. F. Bradamante,Prog. Part. Nucl. Phys. 44 (2000) 339.

Color Transparency via Coherent Exclusive rho Production

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Color transparency via coherent exclusive ρ0 production

Color Transparency via Coherent Exclusive rho Production

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