The design and performance of the ZEUS micro vertex detector

Abt²,

et al. 2014

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The production of beauty and charm quarks in ep interactions has been studied with the ZEUS detector at HERA for exchanged four-momentum squared 5 < Q 2 < 1000 GeV 2 using an integrated luminosity of 354 pb −1 . The beauty and charm content in events with at least one jet have been extracted using the invariant mass of charged tracks associated with secondary vertices and the decay-length significance of these vertices. Differential cross sections as a function of Q 2 , Bjorken x, jet transverse energy and pseudorapidity were measured and compared with next-to-leading-order QCD calculations. The beauty and charm contributions to the proton structure functions were extracted from the double-differential cross section as a function of x and Q 2 . The running beauty-quark mass, m b at the scale m b , was determined from a QCD fit at next-to-leading order to HERA data for the first time and found to be m b (m b ) = 4.07 ± 0.14 (fit) +0.01 −0.07 (mod.) +0.05 −0.00 (param.) +0.08 −0.05 (theo.) GeV.Keywords: Lepton-Nucleon Scattering, QCD, Jets, B physics The ZEUS collaboration 53 IntroductionThe measurement of beauty and charm production in ep collisions at HERA is an important testing ground for perturbative Quantum Chromodynamics (pQCD), since the heavy-quark masses provide a hard scale that allows perturbative calculations to be made. At leading order, the dominant process for heavy-quark production at HERA is boson-gluon fusion (BGF). In this process, a virtual photon emitted by the incoming electron interacts with a gluon from the proton forming a heavy quark-antiquark pair. When the negative squared four-momentum of the virtual photon, Q 2 , is large compared to the proton mass, the interaction is referred to as deep inelastic scattering (DIS). For heavy-quark transverse momenta comparable to the quark mass, next-to-leading-order (NLO) QCD calculations based on the dynamical generation of the massive quarks [1][2][3][4] are expected to provide reliable predictions. Beauty and charm production in DIS has been measured using several methods by the H1 [5][6][7][8][9][10][11][12][13][14][15][16] and ZEUS [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Inclusive jet cross sections in beauty and charm events are used in the analysis presented here to extract the heavy-quark contribution to the proton structure function F 2 with high precision, and to measure the b-quark mass. For this purpose, the long lifetimes of the weakly decaying b and c hadrons, which make the reconstruction of their decay vertices possible, as well as their large masses were exploited. Two discriminating variables, the significance of the reconstructed decay length and the invariant mass of the charged tracks associated with the decay vertex (secondary vertex), were used. This inclusive tagging method leads to a substantial increase in statistics with respect to previous ZEUS measurements.Differential cross sections as a function of Q 2 , the Bjorken scaling variable, x, jet transverse energy, E jet T , and pseudorapidit...

Section: Experimental Set-upmentioning

confidence: 99%

Measurement of beauty and charm production in deep inelastic scattering at HERA and measurement of the beauty-quark mass

Abt²,

et al. 2014

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“…In the kinematic range of the analysis, charged particles were tracked in the central tracking detector (CTD) [19][20][21] and the microvertex detector (MVD) [22]. These components operated in a magnetic field of 1.43 T provided by a thin superconducting solenoid.…”

Section: Jhep09(2013)058mentioning

confidence: 99%

Measurement of charm fragmentation fractions in photoproduction at HERA

Abt²,

et al. 2013

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The production of D 0 , D * + , D + , D + s and Λ + c charm hadrons and their antiparticles in ep scattering at HERA has been studied with the ZEUS detector, using a total integrated luminosity of 372 pb −1 . The fractions of charm quarks hadronising into a particular charm hadron were derived. In addition, the ratio of neutral to charged D-meson production rates, the fraction of charged D mesons produced in a vector state, and the stangenesssuppression factor have been determined. The measurements have been performed in the photoproduction regime. The charm hadrons were reconstructed in the range of transverse momentum p T > 3.8 GeV and pseudorapidity |η| < 1.6. The charm fragmentation fractions are compared to previous results from HERA and from e + e − experiments. The data support the hypothesis that fragmentation is independent of the production process. 6 Charm-hadron production cross sections 11 The ZEUS collaboration 24 IntroductionThe fragmentation fractions of charm quarks into specific charm hadrons cannot be predicted by Quantum Chromodynamics (QCD) and have to be measured. It is usually assumed that they are universal, i.e. the same for charm quarks produced in e + e − annihilation, in ep collisions and also in pp or other hadronic collisions, even though the charm production mechanisms are not the same: in e + e − collisions, cc pairs are produced dominantly by QED pair production, whereas in ep collisions, the main production mechanism is the QCD boson-gluon fusion process γg → cc. The fragmentation universality can be tested by measuring the fragmentation fractions at HERA and comparing the results with those obtained with e + e − collisions. Additionally, the values of the fragmentation fractions are crucial parameters used in comparisons of perturbative QCD (pQCD) calculations with measurements of charm production at HERA and elsewhere.-1 - JHEP09(2013)058In this paper, measurements of the photoproduction of charm hadrons in ep collisions at HERA are presented. The relative production rates of the most copiously produced charm ground states, the D In addition, the ratio of neutral to charged D-meson production rates, the fraction of charged D mesons produced in a vector state, and the strangeness-suppression factor were determined.The analysis presented here is based on an independent data set with an integrated luminosity over 4.5 times larger than the previous ZEUS measurement [1]. The new measurement benefits also from the ZEUS microvertex detector (MVD), which made it possible to identify the secondary decay vertices of the charm ground states and thereby to suppress background significantly. The new results are compared to the previous ZEUS measurement [1] in photoproduction, other HERA results from H1 [2] and ZEUS [3,4] in deep inelastic scattering, and to results from experiments at the e + e − storage rings CLEO [5,6], ARGUS [7][8][9] and the LEP experiments [10][11][12][13][14][15]. A summary is given in [16], with an update to 2010 branching ratios [17].

“…In the kinematic range of the analysis, charged particles were tracked in the central tracking detector (CTD) [16] and the microvertex detector (MVD) [19]. These components operated in a magnetic field of 1.43 T provided by a thin superconducting solenoid.…”

Section: Experimental Set-upmentioning

confidence: 99%

Measurement of D ± production in deep inelastic ep scattering with the ZEUS detector at HERA

Abt²,

et al. 2013

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Charm production in deep inelastic ep scattering was measured with the ZEUS detector using an integrated luminosity of 354 pb −1 . Charm quarks were identified by reconstructing D ± mesons in the D ± → K ∓ π ± π ± decay channel. Lifetime information was used to reduce combinatorial background substantially. Differential cross sections were measured in the kinematic region 5 < Q 2 < 1000 GeV 2 , 0.02 < y < 0.7, 1.5 < p T (D ± ) < 15 GeV and |η(D ± )| < 1.6, where Q 2 is the photon virtuality, y is the inelasticity, and p T (D ± ) and η(D ± ) are the transverse momentum and the pseudorapidity of the D ± meson, respectively. Next-to-leading-order QCD predictions are compared to the data. The charm contribution, F cc 2 , to the proton structure-function F 2 was extracted.