Narrow Results

In the heavy-quark limit, the valence Fock-state components in the B mesons are described by a set of two light-cone wave functions. We show that these two wave functions obey simple coupled differential equations, which are based on the equations of motion in the Heavy Quark Effective Theory (HQET), and the analytic solutions for them are obtained. The results generalize the recently obtained longitudinal-momentum distribution in the Wandzura–Wilczek approximation by including the transverse momenta. We find that the transverse momentum distribution depends on the longitudinal momentum of the constituents, and that the wave functions damp very slowly for large transverse separation between quark and antiquark.

By extending the statistical distributions to the transverse degree of freedom, we account for a multiplicative factor in the Fermi–Dirac functions of the light quarks, we were led to introduce in a previous work to comply with experiment. We can also get light antiquark distributions, similar to those we proposed earlier.

The study of transverse spin and transverse momentum effects is part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. For these studies, a 160 GeV/c momentum muon beam is scattered on a transversely polarized nucleon target, and the scattered muon and the forward going hadrons produced in DIS processes are reconstructed and identified in a magnetic spectrometer. The measurements have been performed on a deuteron target in 2002, 2003 and 2004, and on a proton target in 2007. The main results obtained measuring single spin asymmetries are reviewed, with particular emphasis on the most recent proton measurements. After two years of spectroscopy measurements with hadron beams, in 2008 and 2009, the Collaboration will resume measurements with the muon beam and a transversely polarized target in 2010.

The phenomenology of multi-particle production of hadrons is reviewed, with emphasis on the results at the Large Hadron Collider (LHC): the first part of the review focuses on the basic kinematic measurements of charged tracks and identified hadrons in minimum bias interactions; the second part reports the short- and long-range correlation studies; the third part concentrates on the underlying event phenomenology in different final states; the fourth part, which constitutes a bottom line on the phenomenology of multiple parton interactions, considers the consequences of multiple hard-scatterings in a single hadron–hadron collisions. Some relevant highlights from the heavy ion program are also discussed.

The parton densities which are dependent on transverse momentum, open a way to understand better the structure of quarks and gluons in a more complete way. We are investigating a method based on the covariant quark model which enables us to extract the transverse momentum dependent (TMD) densities from the usual parton densities which are just dependent on the longitudinal momentum. In continuation, we obtain the dependence of the TMDs on binding energy and the mass of quarks. We do some calculations to obtain the TMDs in the unpolarized case while the mass and binding energy of partons are varying. Considering these effects, the results for TMDs are in good agreement with the results of the recent related models.

A recent search for the lepton flavor violating (LFV) decays of the Higgs boson, performed by CMS collaboration, reports an interesting deviation from the Standard Model (SM). The search conducted in the channel $H\to \mu {\tau }_e$ and $H\to \mu {\tau }_{\mathrm{\text{had}}}$ shows an excess of 2.4$\sigma$ signal events with 19.6 fb${}^{-1}$ data at a center-of-mass energy $\sqrt{s}$ = 8 TeV. On the other hand, a search performed by CMS collaboration for the SM Higgs boson produced in association with a top quark pair $(t\bar{t}H)$ also showed an excess in the same-sign dimuon final state. In this work, we try to find out if these two seemingly uncorrelated excesses are related or not. Our analysis reveals that a LFV Higgs decay $(H\to \mu \tau )$ can partially explain the excess in the same-sign dimuon final state in the $t\bar{t}H$ search, infact brings down the excess well within 2$\sigma$ error of the SM expectation. Probing such non-standard Higgs boson decay is of interest and might contain hints of new physics at the electroweak scale.

Transverse momentum $(p_T)$ distributions of primary charged particles were compared to simulations using the Ultra Relativistic Quantum Molecular Dynamics (UrQMD) transport model and the HIJING 1.0 model in minimum bias p–Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{\text{TeV}}$ in the pseudorapidity $(\eta )$ regions: $\eta <0.3$, $0.3<\eta <0.8$ and $0.8<\eta <1.3$ and in the transverse momentum range $0.5<p_T<20\mathrm{\text{GeV}}/c$. The simulated distributions were then compared with the ALICE data and it was observed that UrQMD predicts systematically higher yields than HIJING 1.0. Both codes cannot describe the experimental data in the range of $0.5<p_T<20\mathrm{\text{GeV}}/c$, though in the region of $p_T>5\mathrm{\text{GeV}}/c$ the model predictions are very close to the experimental results for particles with $\left|\eta \right|<0.3$, $0.3<\eta <0.8$. The ratio of the yield at forward pseudorapidity to that at $\left|\eta \right|<0.3$ was also studied. It was observed that the predictions of the models depend on $\eta$. In the experiment there is no essential difference of yields for particles from the intervals of $\left|\eta \right|<0.3$, $0.3<\eta <0.8$ and $0.8<\eta <1.3$. The differences are significant for the models where the ratios are systematically less than 1. This means that the results are not connected to a medium effect but reflect the Cronin effect. We are led to conclude that the codes cannot take into account satisfactorily the leading effect due to the asymmetric p–Pb fragmentation.

In this study, we are reporting comprehensive results on ${\pi }^{\pm }$, $K^{\pm }$, $p$ and $\bar{p}$ production in the transverse momentum range of $0<p_T<$ 4 GeV/c at midrapidity of $0<y<$ 0.5 GeV/c, in p–Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. HIJING 1.0 and UrQMD 3.4 event generators are used to perform simulations and the results are compared with the ALICE and RHIC data. It is observed from the comparison that the yields for the baryons are more complex compared to the mesons and the complexity in baryons is due to the striping dynamics (spectators, leading particles of projectiles) of inner nucleus protons and neutrons. Though all the mesons could be produced during the interaction, they have maximum longitudinal momentum $p_L$; baryons and mesons could be produced as a result of decay of massive baryon-resonances. Yields for the ${\pi }^{\pm }$ mesons are greater than the yield for the $K^{\pm }$ mesons. These are the well-known results from the RHIC data, which stated that the Cronin Effect is mainly due to ${\pi }^{\pm }$ mesons that can be produced as a result of multi-particle inner nucleus cascade. There exists the regions where yields for the $K^{\pm }$ mesons and baryons are same that may be due to the appearance of parton nature. The code used in simulation includes the parton dynamics earlier than it is included in the experiment.

Transverse momentum distributions and nuclear modification factor of integrated charged particles yield produced in p+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV are investigated in mid-rapidity regions of $-0.5<y<0$ at one event multiplicity class 0–5% in the transverse momentum range of $0<p_T<$20 GeV/c. Simulations with EPOS-1.99, EPOS-LHC and QGSJETII-04 are compared with the ALICE data. All three models are in good agreement with each other up to $p_T\sim$3 GeV/c for transverse momentum distributions but after that QGSJETII-04 overpredicts the experimental data. EPOS-LHC seems to describe the experimental data quite well as compared to the other two models. The ratios of the kaons to pions and protons to pions are also presented where again EPOS-LHC provides good agreement with the ALICE data. In case of the nuclear modification factor, for (anti) pions and (anti) kaons, the model distribution is around 1, whereas it is greater than 1 in case of (anti) protons which shows Cronin enhancement.

Transverse momentum $(p_T)$ spectra of ${\pi }^{-}$ mesons calculated using ultra-relativistic quantum molecular dynamic (UrQMD) model (Latest version 3.3-p2) simulations have been compared with $p_T$ spectra of ${\pi }^{-}$ mesons, obtained experimentally in interactions of protons beam with carbon nuclei (propane as target) at momentum of 4.2 GeV/c. Spectral temperatures of negative pions obtained in experimental and UrQMD model simulated interactions of protons beam with carbon nuclei have been calculated by fitting both spectra with four different fitting functions, i.e. Hagedorn thermodynamic, Boltzmann distribution, Gaussian and exponential functions. These functions are used commonly for describing hadron spectra and their spectral temperatures. Hagedorn thermodynamic function has been recommended as the most suitable function to extract the temperature of negative pions at above momentum among these four functions.

We present the doubly differential invariant yield of primary charged particles ${\pi }^{+}$, ${\pi }^{-}$, $K^{+}$, $K^{-}$ and $p$, $\bar{p}$ in Xe–Xe collisions at $\sqrt{s_{NN}}$ = 5.44 TeV, as a function of $p_T$ in kinematic region of $0<p_T<15\mathrm{\text{GeV}}/c$ and for the pseudorapidity interval of $\left|\eta \right|<0.8$ by using PYTHIA8 tuned to different Parton Showers like Simple Showers (Time-Like, Space-Like), DIRE and VINCIA Showers. The predictions of hard and soft QCD processes of Parton Showers are then compared with the ALICE data at different centrality classes of 0–5%, 50–60% and 70–80%. The observed deviations are connected with the kinematics involved and are mostly due to the color coherence and soft gluon emission from parton pairs. As PYTHIA8 does not take into account the Cronin effect, thus the deviation at low $p_T$ is due to Cronin effect. The nuclear modification factor is constructed and it is observed that the signal is less than unity or suppression is seen which is coming from Parton Showers.

Doubly differential transverse momentum spectra of strange mesons $K^{\ast }{(892)}^0$ and $K^{\ast }{(892)}^{\pm }$ within rapidity interval of $|y|<0.5$ are studied by using Monte Carlo event generators in pp, pPb and PbPb collisions at different center-of-mass energies. The predictions of simulations are compared to the ALICE data. The fitting of doubly differential transverse momentum spectra of ALICE data is performed by using Tsallis distribution function. To check the quality of the fitting, chemical potential is also set to zero value. The extracted values of the non-extensive parameter ($q$), a chemical potential $\mu$ and effective Tsallis temperature $T$ for different particle species have been obtained from the thermodynamically consistent Tsallis distribution function. It is observed that the effective Tsallis temperature increases at different center-of-mass energies. The predictions of the MC event generators, i.e. PYTHIA8 CR and HIJING 2.0 are compared to the ALICE data, deviations are observed which may be due to the reshuffling of parton colors and string fragmentation.

We perform a comprehensive analysis of the pion–photon transition form factor F_πγ(Q²) involving the transverse momentum corrections in the leading order approximation with the present CLEO experimental data, in which the contributions beyond the leading Fock state have been taken into consideration. As is well known, the leading Fock-state contribution dominates of F_πγ(Q²) at large momentum transfer (Q²) region. One should include the contributions beyond the leading Fock state in small Q² region. In this paper, we construct a phenomenological expression to estimate the contributions beyond the leading Fock state based on its asymptotic behavior at Q²→0. Our present theoretical results agree well with the experimental data in the whole Q² region. Then, we extract some useful information of the pionic leading twist-2 distribution amplitude (DA) by comparing our results of F_πγ(Q²) with the CLEO data. By taking best fit, we have the distribution amplitude moments, and all of higher moments, which are closed to the asymptotic-like behavior of the pion wave function.

The shape of the transverse momentum $(p_T)$ distribution of primary charged particles in minimum bias (nonsingle-diffractive) p–Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{\text{TeV}}$ is studied in the pseudorapidity regions: $\left|\eta \right|<0.3$, $0.3<\eta <0.8$ and $0.8<\eta <1.3$ and in the transverse momentum range $0.5<p_T<20\mathrm{\text{GeV}}/c$ using simulated data produced with the HIJING 1.0 code. These are compared with the ALICE data measured by the ALICE detector at the LHC. In the model, the central and forward $\eta$-regions differ more than in the ALICE data and due to this fact HIJING 1.0 cannot describe well the high $p_T$ region in the $p_T$ distributions. The comparison of results from simulation implies that the HIJING 1.0 considered narrower pseudorapidity distribution for the charged particles than it is in the ALICE data. It cannot take into account satisfactorily leading effect due to the asymmetric p–Pb fragmentation.

In 2016 and 2017, the COMPASS Collaboration at CERN collected a large sample of DIS events with a longitudinally polarized 160 GeV/$c$ muon beam scattering off a liquid hydrogen target. Part of the collected data has been analyzed to extract preliminary results for the amplitudes of the modulations in the azimuthal angle of the charged hadrons and for their transverse momentum distributions. These observables give relevant information for the study of the transverse momentum and spin structure of the nucleon. The results, presented here for the first time, have been obtained from part of the data collected in 2016. In the analysis, a new method for the evaluation of the contribution of exclusive diffractive processes has been implemented. The azimuthal asymmetries exhibit strong kinematic dependencies, similar to those observed in COMPASS deuteron measurements. The transverse momentum distributions show smooth exponential trends and are compatible with the previous measurement on deuteron.

We study the effect of isospin degree of freedom, incident energy as well as system mass on the behavior of transverse momentum spectra, dN/p_tdp_t, of neutrons and protons. We find that most of the nucleons suffer soft collisions. The effect of isospin degree of freedom on transverse spectra diminishes with the increase in the incident energy. In Fermi energy region, transverse momentum spectra of both protons and neutrons show sensitivity toward the density dependence of symmetry energy.

Transverse momentum distributions of primary charged particles have been studied using simulated data from the HIJING 1.0 event generator in the minimum bias p–Pb collisions at $\sqrt{s_{NN}}$ = 0.9, 1.8, 2.76 and 5.02$\mathrm{\text{TeV}}$, in the two forward pseudorapidity ($\eta$) regions: $1.3<\eta <1.8$ and $1.8<\eta <2.3$ and in the transverse momentum range of $0.5<p_T<20\mathrm{\text{GeV/c}}$. The simulated data in the pseudorapidity region of $1.3<\eta <1.8$ at 5.02$\mathrm{\text{TeV}}$ depicts some differences in the region of $p_T$$>$ 2$\mathrm{\text{GeV/c}}$ when compared with CMS data. Model shows systematically higher values than the experimental measurements pointing out absorption effect for the experimental data. It is also observed that with increasing rapidity interval from $1.3<\eta <1.8$ to $1.8<\eta <2.3$ observed differences for the behavior of the transverse momentum distributions are shifted to high transverse momentum region. The nuclear modification factor as a function of transverse momentum is constructed using the HIJING 1.0 code. With incident energy, the values of nuclear modification factor increase, for 0.9 and 1.8 $\mathrm{\text{TeV}}$, the distributions seem to increase, but for 2.76 and 5.02 $\mathrm{\text{TeV}}$, the distributions look flat. Numerically, the value of nuclear modification factor increases with the increase in the number of jets. This result shows that for the considered more forward pseudorapidiry area, the influence of the incident energy dominates and this is the reason that main results in the areas are connected with the leading particles.

We have studied transverse momentum distributions of charged particles produced in pp and Pb–Pb collisions at ${\sqrt{s}}_{NN}=2.76$ TeV and 5.02 TeV in the pseudorapidity interval $\left|\eta \right|<0.8$ and transverse momentum range $0<p_T<20$GeV/$c$. We simulated data using EPOS-LHC, EPOS-1.99 and QGSJETII-04 models. The simulation data is compared with the ALICE experimental data values at ${\sqrt{s}}_{NN}=2.76$ TeV and 5.02 TeV for pp and most central Pb–Pb collisions. It has been observed that, EPOS-LHC and QGSJETII-04 models explain the experimental results for pp collision at ${\sqrt{s}}_{NN}=2.76$ TeV and 5.02 TeV. The behavior of nuclear modification factors has been studied. The simulation codes of all three models EPOS-LHC, EPOS-1.99 and QGSJETII-04 overestimate the experimental results at low transverse momentum interval: $0<p_T<6$ GeV/$c$, for Pb–Pb collisions at ${\sqrt{s}}_{NN}=2.76$ TeV and 5.02 TeV. However, only EPOS-LHC model can explain the experimental data at high transverse momentum in the range: $6<p_T<20$ GeV/$c$. EPOS-1.99 and QGSJETII-04 underestimate in the region of Cronin effect and cannot give satisfactory estimates for the $p_T$ values for which $R_{\mathrm{\text{Pb-Pb}}}$ demonstrates stronger suppression because of the collective parton effect. It can be inferred that these effects are not taken into account in EPOS-1.99 and QGSJETII-04 models. These models, however, satisfactorily explain the ALICE experimental data in the ranges of $p_T$ for which nuclear modification factor $R_{\mathrm{\text{Pb-Pb}}}$ shows rising trend.

The transverse momentum distribution of charged particles formed in Au–Au collisions at Beam Energy Scan (BES) ($\sqrt{S_{NN}}=7.7,11.5,19.6,27,39$GeV) is investigated. In addition, $P_T$ spectra of $\mathrm{\Lambda }$ particle at $\sqrt{S_{NN}}=62.4$GeV were examined. Tsallis distribution is used to extract the temperature, volume and the entropic index from the experimental results at mid-rapidity and zero chemical potential. We measure some particle ratios like $\frac{k^{+}}{{\pi }^{+}}$ and $\frac{\mathrm{\Lambda }}{{\pi }^{-}}$ which are puzzling horn in the experiment and in the thermal model. We conclude that the horn vanished when we used Tsallis distribution, but this does not confirm a solution to the puzzle, which is primarily visible in the experimental results.

The new experimental data on characteristics of the secondary particles associated with the leading protons in inelastic interactions of projectile neutrons and protons with target carbon nuclei at incident momentum 4.2GeV/c (corresponds to incident kinetic energy 3.37GeV) have been presented. The dependencies of the average multiplicities of the charged pions and accompanying protons on longitudinal as well as transverse momenta of the leading protons in $n^{12}$C and $p^{12}$C collisions at 4.2GeV/c have been investigated. The experimental dependencies have been compared systematically with the Monte Carlo calculations using the modified FRITIOF model. On the whole, the kinematical characteristics of the leading protons in $n^{12}$C and $p^{12}$C collisions have proved to be practically the same, even though they are formed as a result of different processes. From a systematic comparison of the experimental data with the model calculations, we could conclude that for a realistic description of the studied experimental dependencies it is probably necessary to take into account the interaction of strings for the case when fragmentation of strings occurs not outside of the nucleus (as it is assumed in the model), but inside and, possibly, on the boundary of the nucleus in the modified FRITIOF model.