Heavy Flavor Production tagged in m -e coincidences

1. Number of events that generated a trigger request Total number of HFP events Number of min bias events Min bias events that generated trigger response Open Heavy Flavor Physics with a Central Arm Trigger at PHENIX Kenneth N. Barish for the PHENIX Collaboration Proton Structure from polarized p-p collisions RHIC is a large accelerator facility situated at Brookhaven National Lab (BNL) on Long Island. Originally it was designed for the study of quark-gluon plasma, an exotic state of matter that existed in the very first moments after Big-Bang and it is assumed to exist in the interior of neutron stars. Its creation requires extremely high temperatures (~1013K) which can be achieved by Proton Structure Polarized p-p collisions are a powerful tool for the study of proton spin structure. In each collision a constituent (quark or gluon) of the one proton will interact with a constituent of the other in an process that is usually spin dependent. Experiments at SLAC (Stanford Linear Accelerator) in the late 60s provided solid evidence for the existence of point like constituents inside the proton. These particles, called quarks, are bound together with strong interaction whose exchange particle, the gluon, has been observed indirectly in 3-jet events. The interaction of quarks and gluons is described by Quantum Chromo-dynamics (QCD) which is a powerful tool for understanding proton structure. If the quark or gluon spins have, on average, a certain orientation in comparison to proton spin orientation, this will be reflected in a difference in cross section of the corresponding subprocess for different proton bean polarization and thus in a non zero value of the cross section asymmetry: colliding highly energetic beams of heavy nuclei. RHIC will also be the first machine to collide highly energetic polarized proton beams, and thus be a powerful tool in the study of proton spin structure. For that reason RHIC collides polarize4dproton beams (70% polarization) with 500GeV center of mass energy. RHIC consists of two rings (blue and yellow) where the two beams travel at opposite directions. The rings intersect at 6 points and it is there where the collisions take place. In four of them large detectors have been installed whereas the others are aimed to be used in the future. One of the most interesting interaction processes is the so-called Heavy Flavor Production (HFP) in which two gluons, one from each proton, couple to each other and in the final state a quark-antiquark pair emerges. According to perturbative QCD proton is far more complicated than three quarks revolving around their center of mass. Apart from these quarks (uud), called “valence quarks” there are also gluons that are exchanged between them and guarantee the proton stability. A gluon may couple to another gluon or it can split to a quark-antiquark pair. The virtual quarks of the proton are usually called “sea quarks” and may be of any flavor. It is obvious that all these particles contribute to proton properties such as energy, momentum and spin. The fact that not only the quarks but also the field (gluons) contributes to proton properties is something we are familiarizes with already from Maxwell's theory of electromagnetism where fields posses energy momentum and angular momentum. Proton spin is one of its most interesting features because unlike elementary particles it is not intrinsic but it can be derived by adding up the the angular momentum contribution of its constituents. More specifically we can write: The quarks can either be independent (open HFP) or they may emerge in a bound state (quarkonium). The spin dependence can be qualitatively understood in the following simple argument: if the initial gluons have parallel spins their total angular momentum is 2 and as a result they cannot couple to each other because there is no-two quark state with spin 2. Only gluons with antiparticle can couple to each other. Open heavy flavor production (OHFP) allows access to the measurement of quark gluon contribution ΔG. It can be identified experimentally in the following ways: The Electromagnetic Calorimeter The Electro Magnetic Calorimeter (EMCal) is one of the main subsystems that can be used for electron triggering purposes. It consists of eight sectors, six of which are based on PbSc technology whereas the other two on PbGl technology. Each PbSc sector consists of 18 (6z  3φ) subsections called “supermodules”. Each supermodule consists of 36 (6z  6φ) PbSc blocks each being read by 4 photo multipliers (PMTs). As a result there are 144 PMTs in each PbSc sector in square arrangement. Quark spin contribution Angular momentum • Single electrons • Electron-muon coincidences • Electron-D0 coincidence (D0Kπ, 4%). Gluon spin contribution A PbGl block (4.0  4.0  40.0cm) The PbGl sector consists of 32 (8z  4φ) subsections called super-dupermodules, each consisting of 6 (2z  3φ) structures called supermodules. Supermodules have 24 (6z  4φ) blocks of PbGl (each of which read by one PMT) and thus one PbGl super-dupermodule has 144 PMTs in square arrangement. PbSc supermodules and PbGl super-dupermodules are equivalent, they have the same number of channels and are read by the same electronics, so they will be referred both as “supermodules”. There are in total 172 EMCal supermodules with 144 PMTs each. PHENIX PHENIX is one of the large experiments at RHIC. It is a general purpose detector with excellent energy momentum and position resolution. It consists of two muon arms (South-North) and two central arms (East-West). The high resolution of the detector make it strong in studying the gluon spin contribution in proton spin in several physics channels such as heavy flavor production, prompt photon production, jet production etc. • The region where the sum is referred is called trigger tile. There are obviously three kinds of trigger tiles in EMCal: • A2x2: 2x2 non overlapping • Α4x4: 4x4 overlapping • Α12x2: 12x2 non overlapping • The main advantage of overlapping design is that there is at least one trigger tile that contains all the particle energy. In the non overlapping trigger tiles, if a particle hits the boundaries, the energy reported by each trigger tile will only be a fraction of the total energy. In the worst case, the particle hits the corner and that fraction is 25%. PbSc block (10.5  10.5  37cm) Ring Imaging Cherenkov Counter (RICH) Electron Trigger for PHENIX • The PHENIX trigger was designed for Heavy Ions: • For Heavy-Ion, data reduction done in Level-2 • But in proton-proton interaction rate will be high • Expected Level-1 DAQ Bandwidth is 12 kHz • Need to be shared among 10 different physics channels • A rejection factor of 10,000 is needed in Level-1 to fully utilize beam When a particle travels in a media with velocity larger than the velocity of the light in that media, blue light is emitted in a cone around the particle trajectory. This radiation is called Cherenkov radiation and can be used for the detection of particles such as in the RICH. The RICH consists of two gas vessels containing CO2 in which there are two identical mirrors. The mirrors are segments of a sphere and are in a way constructed to focus the Cherenkov radiation of a particle on two PMT arrays. The image of the Cherenkov radiation is a circular ring. In each array there are in total 16z  80φ. Trigger Performance Simulations Efficiency of the electron trigger for 1 GeV single electron events as a function of EMCal threshold (RICH threshold is fixed = 3 npe). Electron Trigger parameters: Efficiency The fraction of the events that generate a response in the trigger is called trigger efficiency and it can be defined by the formula: Geometrical Coincidence between EMCal and RICH An effective electron can be formed by requiring a geometric coincidence between the EMCal and RICH. For triggering purposes the analog sum in a 4z  5φ region, this is what we call trigger tile, is performed which is compared to a programmable threshold Rejection power for different trigger granularities. Rejection Power This is the factor by which the data rate is reduced : Spin Physics with an Electron Trigger Heavy flavor production, cc and bb, is dominated by gluon-gluon interactions and gives rise to a double spin asymmetry from which DG can be extracted. A measurement using heavy flavor production extends the accessible xg-range down to 0.02 or 0.01, depending on the center of mass energy. Even more importantly, however, it provides an alternative way to access the gluon polarization with different systematic and theoretical uncertainties. This will permit a cross check of the results obtained from direct photon production. Experimentally, heavy flavor production can be tagged in PHENIX using (a) single electrons, (b) m-e coincidences, and (c) e-D0 coincidences. In all four channels much of the statistics falls below a pT-cut of 2 GeV, i.e. below the threshold accessible with an EMCal-only based high pT: The following is based on a analysis of heavy flavor production in PHENIX by Wei Xie and Hiroki Sato. Simulations are based on the event generator Pythia and PHENIX acceptances. We have simulated the full response and reconstruction of the MVD. Single Electron Measurements Single electron samples provide large numbers of charmed events with significant backgrounds from p0-Dalitz decays and g conversions. The charm production cross section is not well known at RHIC center of mass energies and has been estimated to be between 200<sbb<350mb at s=200 GeV the branching ratio for leptonic charm decays is about 10. Single electron yields from different sources including charm decays, bottom decays, p0- Dalitz decays, and conversion electrons are shown: PHENIX's multiplicity vertex detector (MVD) can be used to help identify electrons which have come from conversions in the beam pipe or Dalitz decay electrons. We find that a pulse height in association with a separation cut between charged particle tracks (10 degrees) rejects 68% of the Dalitz decay electrons and 75% of the beam pipe conversion electrons, while keeping 78% of the signal electrons: We have estimated our sensitivity to ALL for heavy flavor production taking into account the diluting effect of the conversion and Dalitz electrons which are not rejected by the MVD. Statistical and background subtraction errors based on a 32 pb-1 and 320 pb-1. Events have been tagged online by an electron with pT>1GeV in the central arm, and an offline MVD cut which rejects Dalitz and conversion electrons has been applied. The MVD cuts can be inverted to produce a sample of events which contain mostly electrons from conversions and Dalitz decays. These come from QCD jet events with p0's. Again, we can estimate the asymmetry from this sample: Events have been tagged online by an electron with pT>1GeV in the central arm, and an offline MVD cut enhancing conversion and Dalitz electrons which have come from p0 QCD jet events. The asymmetry at low transverse momentum has flipped sign, giving us a handle on false asymmetries caused by acceptance effects. Further, the asymmetry can be used in conjunction with the direct p0 measurement in a global analysis that will give us a handle on our systematic errors. Simulations performed by Dimitris Galanakis, Ben Lillie, and Wei Xie Heavy Flavor Production tagged in m-e coincidences In addition to the electron in the central detector it is possible to require a muon detected in one of the forward muon arms in coincidence. This requirement removes all background from conversions and Dalitz decays and it enhances the bottom yield in the event sample. The xg-distributions are shown: In the m-e channel the kinematic range reaches down to xg~ 0.02. Above are the expected experimental asymmetries for 10 weeks of data taking at design luminosity based on Gehrmann Stirling A and B. The pure charm and bottom asymmetries are also shown in the plots. At high transverse momentum, bottom begins to dominate In 320pb-1 of e-m coincidences we expect approximately 230K charm events and 142K bottom events if we require the electron to have pT>1GeV and the muon to have a momentum >2 GeV into the muon arm acceptance. The statistics allows us to differentiate between Gehrmann Sterling A and B. Further, the e-m channel will allow us to distinguish between charm and bottom using the asymmetry at high pT and comparisons between like and unlike sign electron muon pairs. Heavy Flavor Production tagged in m-e coincidences Finally it is possible to identify open charm production by requiring in addition to the electron (trigger) that the second charm quark fragments into a D0-meson and then decays: D0 Kp (4% branching ratio); where the D0 is identified offline by reconstructing it's invariant mass. For 320 pb-1 only 31k events will be reconstructed. However, background rejection is more efficient and independent from the single electron or electron-muon channels. This provides a valuable cross check for the charm/background yields in single electron and muon-electron channels. Pythia 2 GeV Fraction of Events above Threshold 50% 10% Electron Momentum [GeV] pT spectrum of electrons from open charm and open bottom production. Relative rates of single electrons from bb and cc QCD jet events and conversion and Dalitz processes from minimum bias Pythia events versus transverse momentum in GeV at s=200 GeV