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

Abstract

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...

Charm production and QCD analysis at HERA and LHC

“…The scale dependence of the charm and beauty running masses has been measured at LEP and HERA 4 [29][30][31] and is shown in Fig. 2.…”

“…ZEUS measured the production of charm and beauty with at least one jet using the invariant mass of the charged tracks associated with secondary vertices and the decaylength significance of these vertices [30]. The measurement was based on the full HERA-II data of 354 pb −1 .…”

“…29 29 In fact small correlations exist between inclusive measurements and those where full final states were reconstructed (e.g. between the measurement [30], where information from secondary vertices from all charm-hadron decays was used, and [172], where D * + mesons were…”

“…Many of the experimental systematic uncertainties are quoted as asymmetric and have been symmetrised before performing a combination. For the measurements [4,30,172], which have not been included in the previous charm combination [60], symmetrisation consisted in taking the largest deviation; no corrections to the central values were applied. For those measurements that have been included in the previous charm combination [60], the symmetrisation remains the same as in [60].…”

“…For those measurements that have been included in the previous charm combination [60], the symmetrisation remains the same as in [60]. 30 …”