Decomposing p+p Events at √s = 200 GeV with STAR

1. Decomposing p+p Events at √s = 200 GeV with STAR Helen Caines – Yale University, for the STAR Collaboration Abstract When studying p-p collisions we are interested in understanding the fundamental constituents of matter and how they form into colorless objects. By measuring the inclusive jet cross-sections and fragmentation properties as a function of collision energy we test the accuracy of our understanding of nature via pQCD. Jet measurements to date have confirmed that QCD is a good description. However, as our analysis of jets has improved it has become clear that there is significant contribution to these measurements from something other than the hard scattering - the so-called underlying event. This underlying event consists of several contributions: the beam-beam remnants, and initial and final state radiation. The jets are the hard scattered parton's fragmentation plus initial and final state radiation. These two sets of processes are strikingly different in both their particle compositions and pT distributions. Only by understanding both components can we fully describe a p-p collision. Interestingly the underlying event at mid-rapidity displays no dependence on the energy of the hard scattering and very little on the incoming beam energy. Preliminary results comparing the jet and underlying event distributions using both unidentified and identified particles at √s = 200 GeV will be shown. Comparison of the results to PYTHIA predictions and earlier results from the Tevatron at √s = 1.96 TeV will also be made. Jet Reconstruction Algorithms The 2006 p-p Dataset Sampled luminosity: ~8.7 pb-1 8.3 M Jet-Patch triggered events Jet-Patch: EMCal - h x f = 1x1 region with ET>8 GeV Charged particles (TPC tracks pT>0.1GeV/c) and neutral energy (EMC towers ET>0.1 GeV) are combined into jets using three seedless algorithms from the Fastjet package [Cacciari, Soyez, arXiv:0704.0292]. Jet pointing vectors are required to be within |h|<1-Rjetcone Jet energies are corrected for MIP and e- double counting in the EMCal only. Seedless Cone - SISCone Recombination kT – starts from low pT particles and merges those close in phase space, weighted by 1/pT i.e. high pT is dis-favored, not bound to circular structure. Anti-kT– starts from high pT particles and merges those close in phase space, weighted by pT, i.e. low pT is dis-favored, circular structure for high pT jets. R=√(Δφ2+Δη2) tracks or towers Rcone • Circular in structure unless splitting and/or merging algorithms applied. seed [CDF Note 7703] CDF UE studies at 1.96 TeV Event Samples and Their Sensitivities The two plots below show preliminary results from CDF. Randomly sampling a Poisson distribution with a mean of 0.4 results in TransMax~0.6 and TransMin~0.2, this is what is observed for the Back-to-Back data. The Leading jet data is strikingly different. This is attributed to the TransMax region including significant initial/final state The data are divided into a Leading jet collection and a Back-to-Back collection. Leading jet : At least one jet in the acceptance Back-to-Back : Sub-set of Leading jet collection. |Df| > 150 and pTAway/pTLead > 0.7. Requiring that the two jets have similar energies in the Back-to-Back case suppresses the probability that the hard scattering produces any large angle, high energy loss, initial or final state radiation. TransMax : enhanced probability of containing hard initial and final state radiation component. TransMin: very sensitive to beam-beam remnants and soft multiple parton interactions. Defining the Underlying Event (UE) “Toward” |Df| < 60o “Away” |Df| > 120o “Transverse” 60o<|Df|<120o TransMin – Trans. region with smaller SpT or Sntrack in event TransMax – Trans. region with higher SpT or Sntrack in event radiation from the hard scattering. Such a contribution also explains the increase in the charge density with lead jet pT. The <pT> in the transverse regions is shown below Figure from Rick Field TransMin vs TransMax at RHIC There is evidence that the Leading data contain not only additional particles but have a higher <pT>. All 3 plots show that both PYTHIA and Herwig, do a SISCone R=0.7Jet, |h|<0.3, Particle pT>0.2GeV/c, data uncorrected The UE - data in the Transverseregions The UE consists of beam remnants, soft or semi-hard multiple parton interactions, and initial and final state radiation Properties of the UE and Jets at RHIC - Poisson mean = 0.36 Preliminary comparisons of the TransMin and TransMax regions and the jet are shown below. Charged tracks, excluding identified e-, in with pT>0.2GeV/c and |h|<1 are considered. Only statistical errors are shown. In all cases no significant difference is seen between the different jet finders. reasonable job at describing the trends of the data Identified particle pT spectra in jets, UE, and Min-Bias events at RHIC No strong differences are seen in either the particle density or <pT> of the UE for Leading and Back-to-Back events for the jet energies studied. The particle density is approximated by sampling a Poisson distribution with a mean of 0.36. While there is a small drop in the UE particle density as a function jet energy the <pT> stays constant. The Leading and Back-to-Back data, within errors, are the same, suggesting that at √s = 200GeV, and for these jet pT, the contribution from large angle radiation is small. The UE is largely decoupled from the jet hard scattering. Back-to-Back Leading 15<pTjet<20 GeV/c Charged Particles 15<pTjet<20 GeV/c K0s Leading Back-to-Back 15<pTjet<20 GeV/c L 15<pTjet<20 GeV/c L Summary These preliminary studies show that the UE contributes ~900 MeV per unit area to a jet. While this does not have a large effect on the jet energy scale it should not be neglected. The UE has only minor contributions from initial/final state radiation from hard scattered partons and in fact seems largely decoupled from the jet. Finally the UE has properties similar to that measured at 1.96 TeV and close to that of our Min-bias events. As expected the particle density and <pT> within the jet increase significantly with jet pT. The UE is smaller and largely independent of jet pT. PYTHIA gives a reasonable representation of the UE but slightly over predicts the jets. Interestingly the Back-to-Back UE data at √s=200 GeV and 1.96 TeV are very similar. Shown above are raw uncorrected pT spectra. Yield comparisons cannot be made between the species but the shapes can be compared between Jet, UE, and Min-Bias for a given specie. While the jet spectra are clearly the hardest, the UE pT spectra “surround” the Min-Bias, especially for the inclusives. Yale University The STAR Collaboration: http://drupal.star.bnl.gov/STAR/presentations