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Register a new userLeading order determination of the gluon polarisation from DIS events with high-p_T hadron pairs
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We present a determination of the gluon polarisation Delta g/g in the nucleon, based on the longitudinal double-spin asymmetry of DIS events with a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon beam scattering off a polarised ^6LiD target. The gluon polarisation is evaluated by a Neural Network approach for three intervals of the gluon momentum fraction x_g covering the range 0.04 < x_g < 0.27. The values obtained at leading order in QCD do not show any significant dependence on x_g. Their average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (syst.) at x_g=0.09 and a scale of mu^2 = 3 (GeV/c)^2.
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Using a novel analysis technique, the gluon polarisation in the nucleon is re-evaluated using the longitudinal double-spin asymmetry measured in the cross section of semi-inclusive single-hadron muoproduction with photon virtuality $Q^2>1~({rm GeV}/c)^2$. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/$c$ polarised muon beam impinging on a polarised $^6$LiD target. By analysing the full range in hadron transverse momentum $p_{rm T}$, the different $p_{rm T}$-dependences of the underlying processes are separated using a neural-network approach. In the absence of pQCD calculations at next-to-leading order in the selected kinematic domain, the gluon polarisation $Delta g/g$ is evaluated at leading order in pQCD at a hard scale of $mu^2= langle Q^2 rangle = 3 ({rm GeV}/c)^2$. It is determined in three intervals of the nucleon momentum fraction carried by gluons, $x_{rm g}$, covering the range $0.04 !<! x_{ rm g}! <! 0.28$~ and does not exhibit a significant dependence on $x_{rm g}$. The average over the three intervals, $langle Delta g/g rangle = 0.113 pm 0.038_{rm (stat.)}pm 0.036_{rm (syst.)}$ at $langle x_{rm g} rangle approx 0.10$, suggests that the gluon polarisation is positive in the measured $x_{rm g}$ range.
We present the QCD analysis of the out-of-event-plane momentum distribution in DIS events with high $p_t$ jets. The achieved accuracy allows the measurement of the running coupling and the study of non-perturbative effects, in particular the test of universality of power corrections in a new experimental regime.
The gluon polarisation in the nucleon was measured using open charm production by scattering 160 GeV/c polarised muons off longitudinally polarised protons or deuterons. The data were taken by the COMPASS collaboration between 2002 and 2007. A detailed account is given of the analysis method that includes the application of neural networks. Several decay channels of D^0 mesons are investigated. Longitudinal spin asymmetries of the D meson production cross-sections are extracted in bins of D^0 transverse momentum and energy. At leading order QCD accuracy the average gluon polarisation is determined as (Delta g/G)^LO=-0.06 +/- 0.21 (stat.) +/- 0.08 (syst.) at the scale <mu^2> ~13 (GeV/c)^2 and an average gluon momentum fraction <x>~ 0.11. For the first time, the average gluon polarisation is also obtained at next-to-leading order QCD accuracy as (Delta g/G)^NLO = -0.13 +/- 0.15 (stat.) +/- 0.15 (syst.) at the scale <mu^2> ~ 13 (GeV/c)^2 and <x> ~ 0.20.
Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, tbar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such particles. The identification of very energetic top quarks is crucial in such an analysis. We consider in detail the tbar ttoell^pm u bbar bqbar q (ell=e, mu) final state for high p_T top quarks. In this phase space region, two or more of the final state quarks can merge into a single jet due to the large Lorentz boost of the parent top quark. As a result, a large fraction of tbar ttoell^pm u bbar bqbar q events with an invariant mass in the TeV region contains less than four observable jets. Requiring one or two tagged b-quarks, we calculate the W+jets, Wb+jets, Wbbar b+jets, Wbt, and single top plus jets backgrounds for these final states, and identify cuts which help to suppress them. In particular, we discuss whether a cut on the jet invariant mass may be useful in reducing the background in the ell u+2 jets channel. We also investigate how next-to-leading order QCD corrections affect high p_T top quark production at the LHC. We find that the ell u+2 jets and ell u+3 jets final states with one or two $b$-tags will significantly improve the chances for discovering new heavy particles in the tbar t channel at the LHC.
We measured the longitudinal double spin asymmetries $A_{LL}$ for single hadron muo-production off protons and deuterons at photon virtuality $Q^2$ < 1(GeV/$it c$)$^2$ for transverse hadron momenta $p_T$ in the range 0.7 GeV/$it c$ to 4 GeV/$it c$ . They were determined using COMPASS data taken with a polarised muon beam of 160 GeV/$it c$ or 200 GeV/$it c$ impinging on polarised $mathrm{{}^6LiD}$ or $mathrm{NH_3}$ targets. The experimental asymmetries are compared to next-to-leading order pQCD calculations, and are sensitive to the gluon polarisation $Delta G$ inside the nucleon in the range of the nucleon momentum fraction carried by gluons $0.05 < x_g < 0.2$.
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