Zhiqing Zhang
Results on multi-lepton events at high transverse momenta, isolated lepton events with large missing transverse energy and single W production reported to this conference are based for the first time on the full data samples taken by two colliding experiments, H1 and ZEUS, at HERA. The data correspond to an integrated luminosity of about 1fb-1 from both experiments.
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A summary of the electroweak results at HERA is presented.
Recent results from the H1 and ZEUS experiments are reported on electroweak physics and on searches for new physics. All results are in good agreement with the Standard Model.
The latest results on exclusive vector meson electroproduction from HERA are reviewed. In particular, the new high-statistics measurements of the rho^0 electroproduction are presented and compared to several models.
The latest results from the ep HERA collider both within the Standard (electroweak) Model and beyond are reviewed. Most of the results are based on the full HERA data sample, which corresponds to an integrated luminosity of about 0.5 fb-1 per experiment (H1 or ZEUS).
The H1 and ZEUS collaborations have used the HERA I data to search for physics beyond the SM and to test electroweak physics in electron-proton collisions. The new period of data taking (HERA II) has started and first HERA II analyses become available. An overview is given of recent highlights, including isolated lepton and multi-lepton events, tests of models of new physics and the first systematic search for deviations in all final states with high transverse momentum.
An extended study of exclusive vector meson production in e,p interactions has been performed by the H1 and the ZEUS collaborations at the HERA collider. Recent measurements are reported and discussed within the framework of the dipole model and pQCD.
The exclusive diffractive production of vector mesons and real photons in ep collisions has been studied at HERA in a wide kinematic range. Here the most recent experimental results are presented together with a Regge-type model and projects for new diffractive studies at LHC.
An increased precision of HERA data allows studies of exclusive processes in their transition from soft to hard interactions as well as in the pQCD domain. The recent measurements of vector meson production and Deeply Virtual Compton Scattering (DVCS) performed by the H1 and ZEUS experiments are reported and compared to pQCD expectations.
The HERA experiments have collected interesting luminosity samples with both polarized and unpolarized lepton beams. The polarized CC data are directly sensitive to right handed charged currents. The data are in agreement with the SM predicting their absence. Effects of Z-exchange have been seen in the lepton beam charge and polarization assymetries. A combined electroweak and QCD analysis allows one to measure electroweak parameters with world competitive accuracy.
HERA, the only ep collider, operated in the years 1992-2007 at centre-of-mass energies of 300-320 GeV in Hamburg, Germany. Deep inelastic neutral and charged current scattering at HERA provides the possibility to study the structure of the proton, the dynamics of strong interactions and test quantum chromodynamics (QCD) over a huge kinematic range. Both, neutral and charged current interactions provide complementary information on the QCD and electroweak parts of the Standard Model. In addition, HERA data allow for indirect searches for new phenomena originating at large scales, however, no ev…
New preliminary electroweak results from the HERA lepton-proton collider experiments H1 and ZEUS are presented. These include new high Q^2 neutral current cross section measurements and limits on a possible quark radius, in the search for contact interactions, and on the quark-Z couplings, extracted in combined electroweak and QCD fits. Furthermore, new charged current cross section measurements as a function of the lepton-beam polarisation are presented, as well as charged current measurement results, using the combined HERA I data from H1 and ZEUS. Finally, measurements of the single W boson…
Neutral-current (NC) and charged-current (CC) deep inelastic scattering (DIS) interactions have been studied in electron-proton and positron-proton collisions with longitudinally polarised lepton beams using the H1 and ZEUS detectors at HERA. Measurements of the polarised inclusive cross-sections are presented and compared to the Standard Model (SM) expectations. The extraction of several electroweak parameters using a combined EW-QCD fit of unpolarised data is also discussed.
The exclusive photoproduction reaction gamma p --> Upsilon p has been studied with the ZEUS experiment in ep collisions at HERA using an integrated luminosity of 468 pb^{-1}. The measurement covers the kinematic range 60<W<220 GeV and Q^2<1 GeV^2, where W is the photon-proton centre-of-mass energy and Q^2 is the photon virtuality. These results, which represent the analysis of the full ZEUS data sample for this channel, are compared to predictions based on perturbative QCD.
The energy dependence of exclusive vector meson (V) production is studied using the ZEUS data. The SU(4) universality of V cross sections in the $Q^2+M_V^2$ scale is tested. The energy dependence of the ratio of the cross sections for V production to the total $γ^*p$ cross section is compared with expectations based on pQCD and Regge approaches.
Using the deep inelastic e^+p and e^-p charged and neutral current scattering cross sections previously published, a combined electroweak and QCD analysis is performed to determine electroweak parameters accounting for their correlation with parton distributions. The data used have been collected by the H1 experiment in 1994-2000 and correspond to an integrated luminosity of 117.2 pb^{-1}. A measurement is obtained of the W propagator mass in charged current ep scattering. The weak mixing angle sin^2 theta_W is determined in the on-mass-shell renormalisation scheme. A first measurement at HERA…
Exclusive electroproduction of $ϕ$ mesons has been studied in $e^\pm p$ collisions at $\sqrt{s}=318 \gev$ with the ZEUS detector at HERA using an integrated luminosity of 65.1 pb$^{-1}$. The $γ^*p$ cross section is presented in the kinematic range $2<Q^2<70 \gev^2$, $35<W<145 \gev$ and ${|t|}<0.6 \gev^2$. The cross sections as functions of $Q^2$, $W$, $t$ and helicity angle $θ_h$ are compared to cross sections for other vector mesons. The ratios $R$ of the cross sections for longitudinally and transversely polarized virtual photons are presented as functions of $Q^2$ and $W$. The data are also…
Neutral-current ({NC}) and charged-current ({CC}) deep inelastic scattering ({DIS}) reactions have been studied in $e^+p$ and $e^-p$ collisions using the {H1} and {ZEUS} detectors at {HERA I}. Following the upgrade of the {HERA} accelerator, the {HERA II} program recently started with the first data in $e^+p$ scattering with longitudinally polarised positrons. In this paper, a summary of the electroweak results from {HERA I} and the first measurement of the cross section for $e^+p$ CC {DIS} at a longitudinal polarisation value of 33% from the {ZEUS} collaboration are presented.
The exclusive electroproduction of omega mesons, ep -> e omega p, has been studied in the kinematic range 3<Q^2<20 GeV^2, 40<W<120 GeV and |t|<0.6 GeV^2 with the ZEUS detector at HERA using an integrated luminosity of 37.7 pb^{-1}. The omega mesons were identified via the decay omega -> pi^+pi^-pi^0. The exclusive (ep -> e omega p) cross section in the above kinematic region is 0.108 +- 0.014(stat.) +- 0.026(syst.) nb. The reaction ep -> e phi p, phi -> pi^+pi^-pi^0, has also been measured. The cross sections, as well as the cross-section ratios omega/rho and omega/phi, are presented as a func…
Exclusive production of $ρ^0$ and $J/ψ$ mesons in e^+ p collisions has been studied with the ZEUS detector in the kinematic range $0.25 < Q^2 < 50 GeV^2, 20 < W < 167 GeV$ for the $ρ^0$ data and $2 < Q^2 < 40 GeV^2, 50 < W < 150 GeV$ for the $J/ψ$ data. Cross sections for exclusive $ρ^0$ and $J/ψ$ production have been measured as a function of $Q^2, W$ and $t$. The spin-density matrix elements $r^{04}_{00}, r^1_{1-1}$ and $Re r^{5}_{10}$ have been determined for exclusive $ρ^0$ production as well as $r^{04}_{00}$ and $r^{04}_{1-1}$ for exclusive $J/ψ$ production. The results are discussed in t…
Exclusive rho^0 electroproduction at HERA has been studied with the ZEUS detector using 120 pb^{-1} of integrated luminosity collected during 1996-2000. The analysis was carried out in the kinematic range of photon virtuality 2 < Q^2 < 160 GeV$^2, and gamma^* p centre-of-mass energy 32 < W < 180 GeV. The results include the Q^2 and W dependence of the gamma^* p --> rho^0 p cross section and the distribution of the squared-four-momentum transfer to the proton. The helicity analysis of the decay-matrix elements of the rho^0 was used to study the ratio of the gamma^* p cross section for longitudi…
Exclusive $ρ^{\circ}$ photoproduction has been measured with high statistics by the H1 Collaboration. The kinematical range is $20 < W < 90$ GeV, $|t|<3$ GeV$^2$ and $Q^2<4$ GeV$^2$. Cross sections are measured differentially in $W$ and $t$ and allow the extraction of the Pomeron trajectory, for the first time using $ρ^{\circ}$ photoproduction data of one single experiment. The preliminary result, $α_{\mathbb{P}} (t) = 1.093 \pm 0.003 (stat.) ^{+0.008}_{-0.007} (syst.) + (0.116 \pm 0.027 (stat.) ^{+0.036}_{-0.046} (syst.)$ GeV$^{-2} \cdot t$, deviates significantly from the canonical expectati…
Neutral and charged current deep inelastic ep scattering with longitudinally polarised lepton beams has been studied with the H1 and ZEUS detectors at HERA. The differential cross sections were measured in the range of four-momentum transfer squared, Q^2, up to 50'000 GeV^2, where electroweak effects become clearly visible. The measurements were used to determine the structure function xF_3 and to constrain vector and the axial-vector couplings of the light quarks to the Z^0 boson. The polarisation dependence of the charged current total cross section was also measured. Limits on flavour chang…
Exclusive electroproduction of rho^0 mesons has been measured using the ZEUS detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and 3<Q^2<30 GeV^2. The low-Q^2 data span the range 20<W<90 GeV; the high-Q^2 data cover the 40<W<120 GeV interval. Both samples extend up to four-momentum transfers of |t|=0.6 GeV^2. The distribution in the azimuthal angle between the positron scattering plane and the rho^0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) rho^0 mesons from…
Odderon induced contributions to exclusive photoproduction of $π^{\circ}$, $f_2(1270)$ and $a_2(1320)$ have been searched for at HERA, using the multiphoton decays of these mesons. No indication for such contributions was found, in a kinematic region defined by the average photon-proton centre-of-mass energy $<W> = 200 - 215$ GeV, photon virtualities $Q^2 < 0.01 $GeV$^2$ and $0.02 $GeV$^2 < |t| < 0.3 $GeV$^2$, where t is the squared momentum transfer at the proton vertex. The measured upper limits for the cross sections, $σ(γp\to^{\hspace*{-2ex}\mathbb{O}}π^{\circ}N^{\ast}) < 39$ nb, $σ(γp\to^…
A search for contributions to the reaction ep-> e pi^0 N^* from photon-Odderon fusion in the photoproduction regime at HERA is reported, at an average photon-proton centre-of-mass energy <W> = 215 GeV. The measurement proceeds via detection of the pi^0 decay photons, a leading neutron from the N^* decay, and the scattered electron. No pi^0 signal is observed and an upper limit on the cross section for the photon-Odderon fusion process of sigma(gamma p -> pi^0 N^*) < 49 nb at the 95% confidence level is derived, integrated over the experimentally accessible range of the squared four-momentum tr…
The exclusive production of $ρ^0$ mesons in deep inelastic electron-proton scattering has been studied using the ZEUS detector. Cross sections have been measured in the range $7 < Q^2 < 25$ GeV$^2$ for $γ^*p$ centre of mass (c.m.) energies from 40 to 130 GeV. The $γ^*p \to ρ^0 p$ cross section exhibits a $Q^{-(4.2 \pm 0.8 ^{+1.4}_{-0.5})}$ dependence and both longitudinally and transversely polarised $ρ^0$'s are observed. The $γ^*p \to ρ^0 p$ cross section rises strongly with increasing c.m. energy, when compared with NMC data at lower energy, which cannot be explained by production through so…
The exclusive electroproduction of J/psi mesons, ep->epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15<Q^2<0.8 GeV^2 and 2<Q^2<100 GeV^2 using integrated luminosities of 69 pb^-1 and 83 pb^-1, respectively.The photon-proton centre-of-mass energy was in the range 30<W<220 GeV and the squared four-momentum transfer at the proton vertex |t|<1.The cross sections and decay angular distributions are presented as functions of Q^2, W and t. The effective parameters of the Pomeron trajectory are in agreement with those found in J/psi ph…
A simultaneous Electroweak and QCD fit of electroweak parameters and parton distribution functions to HERA data on deep inelastic scattering is presented. The input data are neutral current and charged current inclusive cross sections measured by the H1 and ZEUS collaborations at the ep collider HERA. The polarisation of the electron beam was taken into account for the ZEUS and H1 data recorded between 2004 and 2007. Results are presented on the vector and axial-vector couplings of the Z boson to u- and d-type quarks. The values are in agreement with Standard Model predictions. The results on …
Inelastic and elastic (exclusive) cross section measurements of vector meson production at HERA are discussed.
A high luminosity upgrade of HERA will allow the measurement of standard model parameters and the neutral current couplings of quarks. These results will have to be consistent with other precision measurements or indicate traces of new physics. The analysis of $W$ production will complement future results of LEP 2 and the Tevatron. We summarize the main results and conclusions obtained by the working group on Electroweak Physics concerning the potential of future experimentation at HERA.
The HERA collider with its center of mass energy of 318 GeV makes it possible to study a wide range of electroweak physics as well as to search for physics beyond the Standard Model (SM). In this article, recent results, obtained by the two collider experiments H1 and ZEUS, are reviewed. The cross sections for inclusive neutral current and charged current processes are shown, and results from a combined electroweak and QCD analysis of the data are discussed. Selected results from searches for physics beyond the SM are presented.
The exclusive photoproduction of J/psi mesons, gamma p->J/psi p, has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20<W<290 GeV, where W is the photon-proton centre-of-mass energy. The J/psi mesons were reconstructed in the muon and the electron decay channels using integrated luminosities of 38 pb^-1 and 55 pb^-1, respectively. The helicity structure of J/psi production shows that the hypothesis of s-channel helicity conservation is satisfied at the two standard-deviation level. The total cross section and the differential cross-section dsigma/dt, where …
The exclusive diffractive production of vector mesons and real photons in ep collisions has been studied at HERA in a wide kinematic range. Here we present the most recent experimental results together with a Regge-type model. We deduce the Pomeranchuk trajectory (Pomeron) by analyzing the HERA data on deeply virtual Compton scattering (DVCS), and then discuss its basic properties, namely its apparent "hardness" and its "non-flat" behavior, different from the claims of some authors.
We present a review of the latest exclusive diffraction results achieved at HERA, discussing in particular the $W$ and $t$ dependence of the measured cross-sections in both DIS and photo-production regimes.
We demonstrate that perturbative QCD allows one to calculate the absolute cross section of diffractive exclusive production of photons at large $Q^2$ at HERA, while the aligned jet model allows one to estimate the cross section for intermediate $Q^2 \sim 2 GeV^2$. Furthermore, we find that the imaginary part of the amplitude for the production of real photons is larger than the imaginary part of the corresponding DIS amplitude by about a factor of 2, leading to the prediction of a significant counting rate for the current generation of experiments at HERA. We also find a large azimuthal angle …
New results on elastic (exclusive) production of vector mesons are presented, mainly on J/ψand phi mesons with emphasis on an interpretation of the data within pQCD.
Results from the HERA experiments, H1 and ZEUS, on $e^\pm p$ deep inelastic scattering (DIS) provide an important contribution to the knowledge of the proton structure and QCD. The data were collected in the years 1994-2000 (HERA I) and 2003-2006 (HERA II) in the center-of-mass energy of $\sqrt{s} = 300$ GeV in 1994-1997 and 319 GeV from 1998. During the HERA II period, the lepton beams were longitudinally polarised. The most recent results on neutral current (NC) and charge current (CC) DIS cross sections from HERA II data are presented. Results on Parton Density Functions (PDFs) and the stro…
A first measurement is presented of exclusive photoproduction of $ρ^0$ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years $2006$ and $2007$ at a centre-of-mass energy of $\sqrt{s}=319$ GeV and correspond to an integrated luminosity of $1.16$ pb$^{-1}$. The $ρ^0$ mesons with transverse momenta $p_T<1$ GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, $x_L>0.35$, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined…
The first five years of operation of the multi-purpose experiments ZEUS and H1 at the electron-proton storage ring facility HERA have opened a new era in the study of vector-meson production in high-energy photon-proton interactions. The high center-of-mass energy available at this unique accelerator complex allows investigations in hitherto unexplored kinematic regions, providing answers to long-standing questions concerning the energy-dependence of the rho, omega, phi, and J/psi production cross sections. The excellent angular acceptance of these detectors, combined with that of specialized …
The parameters of the electroweak theory are determined in a combined electroweak and QCD analysis using all deep-inelastic $e^+p$ and $e^-p$ neutral current and charged current scattering cross sections published by the H1 Collaboration, including data with longitudinally polarised lepton beams. Various fits to Standard Model parameters in the on-shell scheme are performed. The mass of the $W$ boson is determined as $m(W)=80.520\pm 0.115$ GeV. The axial-vector and vector couplings of the light quarks to the $Z$ boson are also determined. Both results improve the precision of previous H1 deter…
At the HERA collider the experiments H1 and ZEUS have studied the exclusive production of vector mesons over a wide kinematical range. A selection of recent measurements of the cross sections has been presented at Moriond QCD 2003.
Recent results from the HERMES experiment at HERA are described. The large data set from Run-I has yielded new information on the helicity structure of the nucleon. The data to be taken in Run-II will deal mainly with the transverse spin structure and with exclusive reactions.
The latest results of H1 and ZEUS on the electroweak cross sections using polarised lepton beams are presented with the improvements in the parton density functions that result from them. The asymmetry of the neutral current cross sections between e-p and e+p was measured and shows parity violation in agreement with the Standard Model. The dependence of the charged current cross section on the beam polarisation has been obtained and its extrapolation to full polarisation implies the absence of right-handed charged currents. Constraints on the vector and axial vector couplings of the u and d qu…
A first measurement is presented of exclusive photoproduction of $ρ^0$ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years 2006 and 2007 at a centre-of-mass energy of 319 GeV and correspond to an integrated luminosity of 1.16/pb. The $ρ^0$ mesons with transverse momenta $p_T<1$ GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, $x_L>$0.35, are detected in the Forward Neutron Calorimeter having a polar angular acceptance < 0.75 mrad. Single- and double-diff…
The latest measurements of the cross sections for neutral and charged current deep inelastic scattering in e+/-p collisions at HERA with longitudinally polarised lepton beams are presented. The measured cross sections are compared with the predictions of the Standard Model. Next-to-leading-order QCD analyses of HERA data are also presented. The analyses yield the parton distribution functions of the proton and are extended to also extract electroweak parameters. The determinations from HERA are compared with those from LEP and the Tevatron.
The processes of exclusive photo- and electroproduction of vector mesons $ρ^0$(770), $ϕ$(1020) and $J/ψ$(3096) at collision energies $30 GeV<W<300 GeV$ and transferred momenta squared $0<-t<2 GeV^2$ are considered in the framework of a phenomenological Regge-eikonal scheme with nonlinear Regge trajectories in which their QCD asymptotic behavior is taken into account explicitly. By comparison of available experimental data from ZEUS and H1 Collaborations with the model predictions it is demonstrated that corresponding angular distributions and integrated cross-sections in the above-mentioned ki…
The exclusive deep inelastic electroproduction of $ψ(2S)$ and $J/ψ(1S)$ at an $ep$ centre-of-mass energy of 317 GeV has been studied with the ZEUS detector at HERA in the kinematic range $2 < Q^2 < 80$ GeV$^2$, $30 < W < 210$ GeV and $|t| < 1$ GeV$^2$, where $Q^2$ is the photon virtuality, $W$ is the photon-proton centre-of-mass energy and $t$ is the squared four-momentum transfer at the proton vertex. The data for $2 < Q^2 < 5$ GeV$^2$ were taken in the HERA I running period and correspond to an integrated luminosity of 114 pb$^{-1}$. The data for $5 < Q^2 < 80$ GeV$^2$ are from both HERA I a…
The exclusive deep inelastic electroproduction of $ψ(2S)$ and $J/ψ(1S)$ at an $ep$ centre-of-mass energy of 317 GeV has been studied with the ZEUS detector at HERA in the kinematic range $2 < Q^2 < 80$ GeV$^2$, $30 < W < 210$ GeV and $|t| < 1$ GeV$^2$, where $Q^2$ is the photon virtuality, $W$ is the photon-proton centre-of-mass energy and $t$ is the squared four-momentum transfer at the proton vertex. The data for $2 < Q^2 < 5$ GeV$^2$ were taken in the HERA I running period and correspond to an integrated luminosity of 114 pb$^{-1}$. The data for $5 < Q^2 < 80$ GeV$^2$ are from both HERA I a…
We report the measurements of electroweak penguin processes b->d,s gamma and b->sl+l- at the Belle experiment. The analyses are based on 152 million BB-bar events collected at the Upsilon(4S) resonance with the Belle detector at the KEKB e+e- asymmetric-energy collider. The b->d gamma transition is studied through exclusive decays: B0->rho0 gamma, B0->omega gamma and B+ ->rho+ gamma. We apply fully inclusive and semi-inclusive reconstruction to b->s gamma and b->sl+l- processes, respectively. The results on the exclusive b->d gamma and semi-inclusive b->sl+l- measurements are preliminary.
We provide Standard Model expectations for the rare radiative decays B->K^* gamma, B->rho gamma and B-> omega gamma, and the electroweak penguin decays B->K^* l^+ l^- and B->rho l^+ l^- at the next-to-leading order (NLO), extending our previous results to b->d transitions. We consider branching fractions, isospin asymmetries and direct CP asymmetries. For the electroweak penguin decays, the lepton-invariant mass spectrum and forward-backward asymmetry is also included. Radiative and electroweak penguin transitions in b->d are mainly interesting in the search for new flavour-changing neutral cu…
Preliminary measurements are reported on the azimuthal single-spin asymmetry of exclusive rho0 mesons for a transversely polarized hydrogen target at HERMES using the 27.6 GeV HERA positron beam. Within the generalized parton distribution (GPD) formalism, this asymmetry is sensitive to the total angular momentum of quarks and gluons in the nucleon. Since the GPD formalism is only valid for mesons produced by longitudinal photons, the transverse target-spin asymmetry of longitudinal rho0 mesons is extracted assuming s-channel helicity conservation and compared to theoretical calculations.
Measurements of Deep-Inelastic Neutral and Charged current interactions are presented in lepton proton scattering at HERA. The measurements are obtained from taken during 1996 to 1999 and consists of $30 \rm{pb^{-1}}$ of $e^+p$ and $16 \rm{pb^{-1}}$ of $e^-p$ data. The addition of the new high statistics electron data with the positron data allows the first extraction of the parity violating structure function $xF_3$ and tests of high-$Q^2$ electroweak effects of the heavy bosons $Z^0$ and $W$ are observed and found to be consistent with the Standard Model expectation.
HERA is the only high energy electron-proton collider in the world today and hence has unique opportunities both to probe the structure of the proton and to search for physics beyond the Standard Model. Results are presented for measurements of both neutral and charged current cross sections, and for searches for exotic processes involving direct electron-quark interactions (leptoquarks and R-parity violating SUSY), generic coupling models (contact interactions) and exclusive final states (isolated leptons and missing PT, single top production and pentaquarks). Exclusion limits on proposed mod…
Recent results from the ep (electron-proton) collider HERA are presented. Jet production and diffraction type reactions allow tests of predictions of Quantum Chromodynamics (QCD). Jet measurements are used to improve the combined H1-ZEUS parton density fits by adding an additional constraint on the gluon distribution in the proton and, at the same time, providing information on strong coupling constant. Measurements of inclusive diffraction and diffractive dijet data are discussed in the context of factorisation in diffraction. Diffractive Upsilon(1S) production was studied to extract the t-sl…
A search for photoproduction events which contain a rapidity gap between the two highest transverse energy jets has been conducted at HERA using the ZEUS detector. The jets have transverse energies greater than 6~GeV, and are separated by pseudorapidity intervals of up to four units. The fraction of events containing a gap is measured as a function of the gap-width. It is expected that this gap-fraction will fall exponentially with the gap-width, until the dominant gap-production mechanism becomes colour singlet exchange, at which point it will plateau. An indication of a plateau in the measur…
Radiative and electroweak penguin mediated decays of $B$ mesons are a great probe for physics beyond the Standard Model of particle physics. Furthermore, recently anomalies on exclusive $b \rightarrow s \ell^{+} \ell^{-}$ processes, which may imply lepton flavor universality violation, have been observed. Belle II experiment sheds light on the anomalies with the measurement of an inclusive analysis in this mode. Thanks to smaller hadronic uncertainties compared with exclusive modes, complementary information can be provided. Belle II is also a unique experiment to search for processes involvin…
The contribution of both inclusive and exclusive cross-section data from HERA to our knowledge of parton distribution functions is reviewed and future prospects are outlined.
HERA, the first electron-proton collider, has been delivering luminosity since 1992. It is the natural extension of an impressive series of fixed-target lepton-nucleon scattering experiments. The increase of a factor ten in center-of-mass energy over that available for fixed-target experiments has allowed the discovery of several important results, such as the large number of slow partons in the proton, and the sizeable diffractive cross section at large $Q^2$. Recent data point to a possible deviation from Standard Model expectations at very high $Q^2$, highlighting the physics potential of H…
A first measurement is presented of exclusive photoproduction of $ρ^0$ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years $2006$ and $2007$ at a centre-of-mass energy of $\sqrt{s}=319$ GeV and correspond to an integrated luminosity of $1.16$ pb$^{-1}$. The $ρ^0$ mesons with transverse momenta $p_T<1$ GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, $x_L>0.35$, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined…
The analysis of events containing multiple high Pt leptons (electrons and muons) produced in ep collisions has been performed with the H1 and ZEUS detectors at HERA, using the full data sets collected by the experiments in the period 1994-2007. Mutually exclusive event topologies containing at least two charged leptons are analysed. The H1 and ZEUS data, corresponding to a total integrated luminosity of about 1 fb^-1, are combined in a common phase space. The observed event yields are compared to the predictions from the Standard Model. In general a good agreement is found, where the expectati…
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