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STAR Forward Meson Spectrometer

This proposal aims to develop a Forward Meson Spectrometer for studying deep inelastic scattering (DIS) of electrons and muons. It focuses on determining the parton densities and gluon density in the proton using global analyses and QCD evolution equations. The project also aims to investigate new physics phenomena associated with high gluon density.

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STAR Forward Meson Spectrometer

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  1. STAR Forward Meson Spectrometer • NSF Major Research Initiative (MRI) Proposal • submitted January 2005 • [hep-ex/0502040] L.C. Bland Brookhaven National Laboratory RHIC-II Workshop, 27 April 2005

  2. Deep inelastic scattering Parton Densities in the Proton Deep inelastic scattering (DIS) of electrons and muons is the primary source of information about the quark and gluon structure of the proton. Kinematics defined for electron(muon) scattering from a fixed proton target. Global analyses use world data from DIS, neutrino scattering, Drell-Yan,… to determine parton distribution functions (PDF). L.C.Bland, RHIC-II Workshop

  3. Determining the gluon density The gluon density is determined by applying QCD evolution equations to account for the Q2 dependence (scaling violations) of structure functions measured in DIS. At low-x, the full QCD evolution equations can be simplified to approximate the gluon distribution by i.e., determine g(2x) by measuring the lnQ2 slope of F2(x,Q2) at fixed x. K. Prytz, Phys. Lett. B311 (1993) 286 L.C.Bland, RHIC-II Workshop

  4. Gluons in the Proton • DIS results from HERA ep collider provide accurate determination of xg(x) for the proton in the range 0.001<x<0.2 • the low-x gluon density is large and continues to increase as x0 over the measured range J. Pumplin, D.R. Stump, J. Huston, H.L. Lai, P. Nadolsky, W.K. Tung JHEP 0207 (2002) 012. L.C.Bland, RHIC-II Workshop

  5. Nuclear Gluon Density e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70 (2004) 044905 and data references therein World data on nuclear DIS constrains nuclear modifications to gluon density only for xgluon > 0.02 L.C.Bland, RHIC-II Workshop

  6. Figure 3 Diagram showing the boundary between possible “phase” regions in the t=ln(1/x) vs plane . New Physics at high gluon density • Shadowing. Gluons hidingbehind other gluons. Modificationof g(x) in nuclei. Modified distributionsneeded by codes that hope to calculateenergy density after heavy ion collision. • Saturation Physics. New phenomena associated with large gluon density. • Coherent gluon contributions. • Macroscopic gluon fields. • Higher twist effects. • “Color Glass Condensate” Edmond Iancu and Raju Venugopalan, review for Quark Gluon Plasma 3, R.C. Hwa and X.-N. Wang (eds.), World Scientific, 2003 [hep-ph/0303204]. L.C.Bland, RHIC-II Workshop

  7. Hard Scattering Hard scattering hadroproduction p Factorization theorems state that the inclusive cross section for p+p  p +X can be computed in perturbative QCD using universal PDF and fragmentation functions, and perturbatively calculated hard-scattering cross sections L.C.Bland, RHIC-II Workshop

  8. 0production at midrapidity p + p  p0 + X, s=200 GeV S.S. Adler et al. (PHENIX), PRL 91, 241803 (2003). NLO pQCD calculation, using CTEQ5M PDF and KKP fragmentation functions is found to be consistent with data down to surprisingly low pT. Universality tests at collider energies yield comparable results. L.C.Bland, RHIC-II Workshop

  9. Deep inelastic scattering Why Consider Forward Physics at a Collider?Kinematics Hard scattering hadroproduction How can Bjorken x values be selected in hard scattering? • Assume: • Initial partons are collinear • Partonic interaction is elastic pT,1  pT,2  Studying pseudorapidity, h=-ln(tanq/2), dependence of particle production probes parton distributions at different Bjorken x values and involves different admixtures of gg, qg and qq’ subprocesses. L.C.Bland, RHIC-II Workshop

  10. p+p  p0+X, s = 200 GeV, h=0 Simple Kinematic Limits • Mid-rapidity particle detection: • h10 and <h2>0 •  xq  xg  xT = 2 pT / s • Large-rapidity particle detection: • h1>>h2 • xq  xT eh1 xF(Feynman x), and xg xF e-(h1+h2) NLO pQCD (Vogelsang) 1.0 0.8 0.6 0.4 0.2 0.0 qq fraction qg gg 0 10 20 30 pT,p(GeV/c)  Large rapidity particle production and correlations involving large rapidity particle probes low-x parton distributions using valence quarks L.C.Bland, RHIC-II Workshop

  11. <z> <xq> <xg> Forwardp0production in hadron collider Ep p0 p d EN qq qp p Au xgp xqp qg EN (collinear approx.) • Large rapidity p production (hp~4) probes asymmetric partonic collisions • Mostly high-x valence quark + low-x gluon • 0.3 < xq< 0.7 • 0.001< xg < 0.1 • <z> nearly constant and high 0.7 ~ 0.8 • Large-x quark polarization is known to be large from DIS • Directly couple to gluons = A probe of low x gluons NLO pQCD Jaeger,Stratmann,Vogelsang,Kretzer L.C.Bland, RHIC-II Workshop

  12. STAR detector layout • TPC: -1.0 <  < 1.0 • FTPC: 2.8 <  < 3.8 • BBC : 2.2 <  < 5.0 • EEMC:1 <  < 2 • BEMC:0 <  < 1 • FPD: || ~ 4.0 & ~3.7 L.C.Bland, RHIC-II Workshop

  13. Di-photon Mass Reconstruction and calibrationPb-glass reconstruction (no SMD) Time/luminosity dependent gain shift corrections PMT Gain Matching p0 reconstructionefficiency Cluster categorization MC & Data comparison FTPC-FPD matchingPhoton conversion in beam pipe Track in FTPC • Clustering and moment analysis • Fitting with parametrized shower shape • Number of photons found >= 2 • Fiducial volume > 1/2 cell width from edge • Energy sharing zgg=|E1-E2| / (E1+E2) < 0.7 • Absolute gain determined from  peak position for each tower • Energy dependent gain correction • Run/luminosity dependent gain correction • Checking with MC (PYTHIA+GEANT) 2 photon cluster example p + p (+ X)   (+ )  e+ e- Hit in FPD Luminosity vs PMT gain Beam pipe Limit with zgg<0.5 cut Try both Dh 1g Cluster Gain stability (before correction) from reconstruction of MC(PYTHIA+GEANT) Energy Geometrical limit Df Dh 2g Cluster • FPD position known relative to STAR Df Gain stability (after correction) 2nd moment of cluster (long axis) High tower sorted mass distributions Mass resolution ~ 20MeV We understand gain ~2% level Efficiencies is almost purely geometrically determined L.C.Bland, RHIC-II Workshop

  14. STAR xF and pT range of FPD data L.C.Bland, RHIC-II Workshop

  15. ppp0X cross sections at 200 GeV • The error bars are point-to-point systematic and statistical errors added in quadrature • The inclusive differential cross section for p0 production is consistent with NLO pQCD calculations at 3.3 < η < 4.0 • As η increases, systematics regarding the comparison with NLO pQCD calculations begin to emerge. The data at low pT are more consistent with the Kretzer set of fragmentation functions. Similar to what was observed by PHENIX. D. Morozov (IHEP), XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 L.C.Bland, RHIC-II Workshop

  16. STAR -FPD Preliminary Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189. L.C.Bland, RHIC-II Workshop

  17.  q g  q g g PYTHIA: a guide to the physics Subprocesses involved: Forward Inclusive Cross-Section: q+g g+g and q+g  q+g+g STAR FPD Soft processes • PYTHIA predictionagrees well with the inclusive 0 cross section at 3-4 • Dominant sources of large xF production from: • q + g  q + g (22) + X • q + g  q + g + g (23)+ X L.C.Bland, RHIC-II Workshop

  18. FPD Detector and º reconstruction • robust di-photon reconstructions with FPD in d+Au collisions on deuteron beam side. • average number of photons reconstructed increases by 0.5 compared to p+p data. L.C.Bland, RHIC-II Workshop

  19. y=0 As y grows Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003)  Dependence of RdAu G. Rakness (Penn State/BNL), XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 See also J. Jalilian-Marian, Nucl. Phys. A739, 319 (2004) • From isospin considerations, p + p  h is expected to be suppressed relative to d + nucleon  h at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)] • Observe significant rapidity dependence similar to expectations from a “toy model” of RpA within the Color Glass Condensate framework. L.C.Bland, RHIC-II Workshop

  20. For 22 processes Log10(xGluon) TPC Barrel EMC FTPC FTPC FPD FPD Gluon Constraining the x-values probed in hadronic scattering Guzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173 (2004). Log10(xGluon) Collinear partons: • x+ = pT/s (e+h1 + e+h2) • x = pT/s (eh1 + eh2) • FPD: ||  4.0 • TPC and Barrel EMC: || < 1.0 • Endcap EMC: 1.0 <  < 2.0 • FTPC: 2.8 <  < 3.8 CONCLUSION: Measure two particles in the final state to constrain the x-values probed L.C.Bland, RHIC-II Workshop

  21. Back-to-back Azimuthal Correlationswith large  Fit LCP normalized distributions and with Gaussian+constant Beam View Top View Trigger by forward   ] • E > 25 GeV •  4 ] Coicidence Probability [1/radian] Midrapidity h tracksin TPC • -0.75 < < +0.75 Leading Charged Particle(LCP) • pT > 0.5 GeV/c LCP S = Probability of “correlated” event under Gaussian B = Probability of “un-correlated” event under constant s = Width of Gaussian L.C.Bland, RHIC-II Workshop

  22. STAR STAR Preliminary STAR Preliminary PYTHIA (with detector effects) predicts • “S” grows with<xF> and <pT,> • “s” decrease with <xF> and <pT,> PYTHIA prediction agrees with p+p data Larger intrinsic kT required to fit data 25<E<35GeV 45<E<55GeV Statistical errors only L.C.Bland, RHIC-II Workshop

  23. Expectation from HIJING (PYTHIA+nuclear effects) X.N.Wang and M Gyulassy, PR D44(1991) 3501 with detector effects • HIJING predicts clear correlation in d+Au • Small difference in “S” and “s” between p+p and d+Au • “B” is bigger in d+Au due to increased particle multiplicity at midrapidity 25<E<35GeV 35<E<45GeV L.C.Bland, RHIC-II Workshop

  24.  “Mono-jet” PT is balanced by many gluons Dilute parton system (deuteron) Dense gluon field (Au) 25<E<35GeV Beam View Top View   • E > 25 GeV •  4 STAR Preliminary 35<E<45GeV Statistical errors only dAu Correlations: probing low x L.C.Bland, RHIC-II Workshop

  25. dAu Correlations: probing low x Large 0+h± correlations • Suppressed at small <xF> , <pT,> Consistent with CGC picture • Consistent in d+Au and p+p at larger <xF> and <pT,> More data are needed… 25<E<35GeV Fixed as E & pT grows STAR Preliminary 35<E<45GeV Statistical errors only L.C.Bland, RHIC-II Workshop

  26. Three Highlighted Objectives High In FMS Proposal(not exclusive) • A d(p)+Aup0p0+X measurement of the parton model gluon density distributions xg(x) in gold nucleifor0.001< x <0.1. For 0.01<x<0.1, this measurement tests the universality of the gluon distribution. • Characterization of correlated pion cross sections as a function of Q2 (pT2) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au) • Measurements withtransversely polarized protonsthat are expected toresolve the origin of the large transverse spin asymmetriesin reactions for forward  production. (polarized pp) L.C.Bland, RHIC-II Workshop

  27. FMS Design • FMS increases areal coverage of forward EMC from 0.2 m2 to 4 m2 • FMS to be mounted at roughly the same distance from the center of the STAR interaction region as the FPD, and would face the Blue beam • Addition of FMS to STAR provides nearly continuous EMC from -1<h<+4 FPD Calorimeters L.C.Bland, RHIC-II Workshop

  28. FMS: 2.5<< 4.0 STAR detector layout with FMS TPC: -1.0 <  < 1.0 FTPC: 2.8 <  < 3.8 BBC : 2.2 <  < 5.0 EEMC:1 <  < 2 BEMC:-1 <  < 1 FPD: || ~ 4.0 & ~3.7 L.C.Bland, RHIC-II Workshop

  29. FMS MRI Proposal Details • Full azimuthal EM coverage 2.5<<4.0 • Extending STAR coverage to -1<<4.0 • 684 3.8 cm  3.8 cm  45 cm lead glass inner cells (IHEP, Protvino). • 756 5.8 cm  5.8 cm  60 cm Schott F2 lead glass outer cells (FNAL-E831). • New Photinis XP2202 (outer cells) • Cockroft Walton Bases. • Readout Electronics L.C.Bland, RHIC-II Workshop

  30. Frankfurt, Guzey and Strikman, J. Phys. G27 (2001) R23 [hep-ph/0010248]. • constrain x value of gluon probed by high-x quark by detection of second hadron serving as jet surrogate. • span broad pseudorapidity range (-1<h<+4) for second hadron  span broad range of xgluon • provide sensitivity to higher pT for forward p0 reduce 23 (inelastic) parton process contributions thereby reducing uncorrelated background in Df correlation. L.C.Bland, RHIC-II Workshop Pythia Simulation

  31. d+Au  p0+p0+X, pseudorapidity correlations with forward p0 HIJIING 1.381 Simulations • increased pT for forward p0 over run-3 results is expected to reduce the background in Df correlation • detection of p0 in interval -1<h<+1 correlated with forward p0 (3<h<4) is expected to probe 0.01<xgluon<0.1  provides a universality test of nuclear gluon distribution determined from DIS • detection of p0 in interval 1<h<4 correlated with forward p0 (3<h<4) is expected to probe 0.001<xgluon<0.01  smallest x range until eRHIC • at d+Au interaction rates achieved at the end of run-3 (Rint~30 kHz), expect 9,700200 (5,600140) p0-p0 coincident events that probe 0.001<xgluon<0.01 for “no shadowing” (“shadowing”) scenarios. L.C.Bland, RHIC-II Workshop

  32. TimelineCompletionByFall 2006 L.C.Bland, RHIC-II Workshop

  33. Other Possible Applications of FMS • p0-p0 correlations with transversely polarized p+p collisions to discriminate Collins/Heppelmann and Sivers mechanisms for large spin effects. • forward p0/g reconstruction in heavy-ion collisions • direct photon detection at large rapidity • reconstruction of other mesons decaying to g or e produced in p+p or d(p)+Au (and heavy-ion?) collisions • h  gg • w  p0g • Kshort p0p0  4g • h gg ? • J/ e+e- ? Reconstruction of HIJING/GSTAR simulations Limited sample of events obtained in Cu+Cu run with good view of interaction region L.C.Bland, RHIC-II Workshop

  34. New FMS Calorimeter Lead Glass From FNAL E831 Loaded On a Rental Truck for Trip To BNL L.C.Bland, RHIC-II Workshop

  35. Backup slides L.C.Bland, RHIC-II Workshop

  36. Possible Problems at Forward Angles • Is it possible to access large enough pT where NLO pQCD is applicable? • Large xF means high energy particles. Detection is best accomplished using electromagnetic + hadronic calorimetry + charge-sign determination from tracking through a magnetic field. • For increasing pT at large xF, faced with increasingly steep falloff of dN/dh distributions. Although aS does not vary much over accessible scales at RHIC, large h will primarily probe small pT need to understand scale dependence of fixed order pQCD calculations. L.C.Bland, RHIC-II Workshop

  37. larger spin effects at more forward angles. Expect at even more forward angles that the sensitivity (convolution ) will increase. Since large h probes small xgluon, gluon polarization may decrease because of sharp increase of unpolarized gluon density as xgluon 0. • expect the (p0+h0)/g ratio to be more favorable at forward angles than at midrapidity. • expect sensitivity to gluon polarization for forward jet (as well as g) production. h dependence of ALL for inclusive g production LCB, hep-ex/9907058 L.C.Bland, RHIC-II Workshop

  38. p0 q g p0 q g g Partonic Correlations from PYTHIA • Large energy deposited at h=3.8 • one parton in hard scattering with peak in forward direction +broad h range • other parton spread over broad h range + L.C.Bland, RHIC-II Workshop

  39. But, do we understand forward p0 production in p + p?At s << 200 GeV, not really…. √s=23.3GeV √s=52.8GeV Data-pQCD difference at pT=1.5GeV 2 NLO calculation with different scale: pT and pT/2 Ed3s/dp3[mb/GeV3] Ed3s/dp3[mb/GeV3] q=6o q=10o q=15o q=53o q=22o xF xF Bourrely and Soffer (hep-ph/0311110, Data references therein): NLO pQCD calculations underpredict the data at low s from ISR sdata/spQCD appears to be function of q, √s in addition to pT

  40. STAR Large Analyzing Powers at RHIC Similar to FNAL E704 result at s = 20 GeV In agreement with several models including different dynamics: STAR collaboration, hep-ex/0310058, Phys. Rev. Lett. 92 (2004) 171801 First measurement of AN for forward π0 production at s=200GeV • Sivers: spin and k correlation in initial state (related to orbital angular momentum?) • Collins: Transversity distribution function & spin-dependent fragmentation function • Qiu and Sterman (initial-state) / Koike (final-state) twist-3 pQCD calculations L.C.Bland, RHIC-II Workshop

  41. Integral Matter (Rad. Length) 8 6 4 2 0 3 4 5 h Forward Physics at STAR <1 radiation length between interaction region and large rapidity region (2.2<h<4.5) STAR has significant space available in the forward direction.  well suited to forward particle detection. L.C.Bland, RHIC-II Workshop

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