Study of the central exclusive production of $π^+π^-$, $K^+K^-$ and $p \bar{p}$ pairs in proton-proton collisions at $\sqrt{s} = 510$ GeV with the STAR detector at RHIC
Tomas Truhlar
备注：
3 pages, 4 figures, Proceedings of 40th International Conference on High Energy Physics - ICHEP 2020, July 28 - August 6, 2020, Prague, Czech Republic (virtual meeting)
We report on the measurement of the central exclusive production process $pp \ \rightarrow \ pXp$ in proton-proton collisions at RHIC with the STAR detector at $\sqrt{\mathrm{s}} = 510$ GeV. At this energy, this process is dominated by a double Pomeron exchange mechanism. The tracks of the centrally produced system $X$ were reconstructed in the central detector of STAR, the Time Projection Chamber and the Time of Flight systems, and were identified using the ionization energy loss and the time of flight method. The diffractively scattered protons, moving intact inside the RHIC beam pipe after the collision, were measured in the Roman Pots system allowing full control of the interaction's kinematics and verification of its exclusivity. The preliminary results on the invariant mass distributions of centrally produced $π^+ π^-$, $K^+ K^-$ and $p \bar{p}$ pairs measured within the STAR acceptance are presented.
Feasibility studies of an observation of the exclusive diffractive bremsstrahlung at RHIC at $\sqrt{s} = 200$~GeV and at $\sqrt{s} = 500$~GeV are reported. A simplified approach to the photon and the scattered proton energy reconstruction is used. Influence of possible backgrounds is discussed.
We study the photoproduction of exclusive $2π^+2π^-$ mesons in ultra-peripheral heavy-ion collisions at RHIC and LHC energies. Predictions in photon-nucleus interactions are calculated for various resonances at central and forward rapidities. The recent H1 preliminary data are utilized to improve the description of the poorly known $γp \to 4π^\pm p$ process. We present the comparisons of our results to the available STAR data at RHIC, and made predictions for LHC energies.
We describe a physics program at the Relativistic Heavy Ion Collider (RHIC) with tagged forward protons. The program started with the proton-proton elastic scattering experiment (PP2PP), for which a set of Roman Pot stations was build. The PP2PP experiment took data at RHIC as a dedicated experiment at the beginning of RHIC operations. To expand the physics program to include non-elastic channels with forward protons, like Central Exclusive Production (CEP), Central Production (CP) and Single Diffraction Dissociation (SD), the experiment with its equipment was merged with the STAR experiment a…
We present preliminary differential fiducial cross sections on Central Exclusive Production (CEP, $pp\to pXp$) of two opposite-charge mesons ($X=π^{+}π^{-}$, $K^{+}K^{-}$) in midrapidity region with small squared four-momentum transferred from forward protons, $0.03<|t_{1}|,|t_{2}|<0.2~\textrm{GeV}^{2}$. The process was measured in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) in proton-proton collisions at the center-of-mass energy $\sqrt{s}~=~200~\textrm{GeV}$.
The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) performs studies of diffractive processes with the focus on the exclusive production of particles in central range of rapidity. In 2015 STAR collected 18 pb$^{-1}$ of data in polarized proton+proton collisions at $\sqrt{s}$=200 GeV to measure Central Exclusive Production (CEP) process $pp\to pXp$ through Double Pomeron Exchange (DPE) mechanism. The intact protons moving inside the RHIC beampipe after the collision were measured in silicon strip detectors (SSD), which were placed in the Roman Pot vessels. This enables full contro…
The measurement of the central exclusive production of charged hadron pairs $h^{+}h^{-}$ ($h = π, K, p$) by the STAR experiment at RHIC is reported. The data from proton-proton collisions at $\sqrt{s} = 200$ GeV were used in this study. The pairs of charged hadrons produced in the reaction $pp\to p^\prime+h^{+}h^{-}+p^\prime$ were reconstructed from the tracks in the central detector, while the forward-scattered protons were measured in the Roman Pot system. Differential cross sections were measured in the fiducial region determined by the geometrical acceptance of the experimental setup. They…
We investigate the potentialities offered by the study of J/psi exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected counting rates in both set-ups. Studying Single-Transverse-Spin Asymmetries (A_N) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) E_g(x,xi,t). We evaluate the expected precision on A_N for realistic conditions with the LHCb detector in pH(pol) and PbH(pol…
Measurements of the longitudinal spin transfer, $D_{LL}$, and the transverse spin transfer, $D_{TT}$, of the $Λ(\barΛ$) hyperon in polarized $p$+$p$ collisions are expected to be sensitive to the helicity distribution and transversity distribution of the $s(\bar{s})$ quark in proton and the corresponding polarized fragmentation functions. This contribution presents the new preliminary results of the $Λ(\barΛ$) $D_{LL}$ and $D_{TT}$ using data collected at RHIC-STAR experiment in 2015, with twice larger statistics than previously published results.
The production of $f_{1}$ ($J^{PC} = 1^{++}$) mesons in proton-proton collisions via pomeron-pomeron fusion is discussed. Two ways to construct the pomeron-pomeron-$f_{1}$ coupling are presented. Comparisons with data from the WA102 experiment are made and predictions for RHIC and LHC experiments are given.
A search for central exclusive production of top quark pairs ($\mathrm{t}\bar{\mathrm{t}}$) is presented using collision data collected by CMS and the CMS-TOTEM Precision Proton Spectrometer in 2017. A data-driven method to estimate the background from coincidences of inclusive events and pileup protons is described, as well as the development of a Boosted Decision Tree classifier to separate the exclusive $\mathrm{t}\bar{\mathrm{t}}$ signal from the inclusive $\mathrm{t}\bar{\mathrm{t}}$ background. The first-ever upper limits on the cross section of exclusive $\mathrm{t}\bar{\mathrm{t}}$ are…
We report the first observation of the exclusive decays \bar B \to D^{(*)}K^{*-}, using 9.66 x 10^{6} B \bar{B} pairs collected at the Υ(4S) with the CLEO detector. We measure the following branching fractions: { \cal B}(B^- -> D^0 K^{*-})=(6.1 +- 1.6 +-1.7)x10^{-4}, { \cal B}( \bar{B^0} -> D^+K^{*-})=(3.7 +- 1.5 +- 1.0) x 10^{-4}, { \cal B}( \bar{B^0} -> D^{*+}K^{*-})=(3.8 +- 1.3 +- 0.8) x 10^{-4} and { \cal B}(B^- --> D^{*0} K^{*-})=(7.7 +- 2.2 +- 2.6) x 10^{-4}. The \bar B ->D^*K^{*-} branching ratios are the averages of those corresponding to the 00 and 11 helicity states. The errors shown…
We calculate the inclusive production of a polarized $Λ$ or $\barΛ$ hyperon from the single longitudinally polarized proton and proton ($pp$) collision at RHIC. By comparing the data reported by the STAR Collaboration, we find that this process is sensitive to the polarization of strange and antistrange quarks of the proton in the experimental range. By introducing asymmetric coefficients with the minimization of $χ^2$, we further identify the asymmetry of the polarized strange-antistrange quarks in the proton sea and find that the first moment is $Δs \approx -0.025\pm 0.019$ for strange quark…
Light nuclei have much smaller binding energy compared to the temperature of the system created in heavy-ion collisions. Consequently, the distributions of light nuclei can be used to probe the freeze-out properties, such as correlation volume and local baryon density of the medium created in high-energy nuclear collisions. In this paper, we report the results of deuteron and anti-deuteron production in Au+Au collision at $\sqrt{s_\mathrm{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV, measured by STAR at RHIC. The collision energy, centrality and transverse momentum dependence of th…
The particle ratios $k^+/π^+$, $π^-/K^-$, $\bar{p}/π^-$, $Λ/π^-$, $Ω/π^-$, $p/π^+$, $π^-/π^+$, $K^-/K^+$, $\bar{p}/p$, $\barΛ/Λ$, $\barΣ/Σ$, $ \barΩ/Ω$ measured at AGS, SPS and RHIC energies are compared with large statistical ensembles of $100,000$ events deduced from the CRMC EPOS $1.99$ and the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) hybrid model. In the UrQMD hybrid model two types of phase transitions are taken into account. All these are then confronted to the Hadron Resonance Gas Model. The two types of phase transitions are apparently indistinguishable. Apart from $k^+/π^…
We present results on the Central Exclusive Production of charged particle pairs $h^{+}h^{-}$ ($h = π, K, p$), $pp\to p^\prime+h^{+}h^{-}+p^\prime$, obtained with the STAR experiment at RHIC in proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 200$ GeV. All final-state particles were reconstructed, including forward-scattered protons detected in the Roman Pot system. As a result, the Double Pomeron Exchange (DPE) events were selected and the non-exclusive backgrounds were efficiently rejected. Differential fiducial cross sections were measured as functions of observables relat…
In this paper we study leading neutron production in photon - hadron interactions which take place in $pp$ and $pA$ collisions at large impact parameters. Using a model that describes the recent leading neutron data at HERA, we consider exclusive vector meson production in association with a leading neutron in $pp/pA$ collisions at RHIC and LHC energies. The total cross sections and rapidity distributions of $ρ$, $ϕ$ and $J/Ψ$ produced together with a leading neutron are computed. Our results indicate that the study of these processes is feasible and that it can be used to improve the understa…
The central exclusive production of $χ_{c}$ and $χ_{b}$ at the LHC, RHIC and Tevatron energies is analyzed, using the recent unintegrated parton distribution (UGDs) functions available in the package TMDlib. Comparison with data is performed, which tests the underlying assumptions basing the theoretical approach and it can constrain the unintegrated gluon distribution function at the small-$x$ region. Predictions for LHC energies using recent UGDs based in CCFM formalism are provided. It is explored the underlying uncertainties on this production as the choice for the unintegrated gluon distri…
We present the first measurements of the transverse momentum spectra and azimuthal anisotropy of the identified hadrons ($π^{+}$, $π^{-}$, $K^{+}$, $K^{-}$, $p(\bar{p})$, $K^{0}_{s}$, $Λ(\barΛ)$ and $ϕ$) at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV for various collision centralities. These measurements are compared to corresponding results from other BES energies. The bulk properties of the system, like the chemical and kinetic freeze-out conditions and the collectivity extracted from the measured yields of the produced particles are presented. The difference between baryo…
Exclusive vector meson photoproduction associated with a leading baryon ($B = n, Δ^+, Δ^0$) in $pp$ and $pA$ collisions at RHIC and LHC energies is investigated using the color dipole formalism and taking into account nonlinear effects in the QCD dynamics. In particular, we compute the cross sections for $ρ$, $ϕ$ and $J/Ψ$ production together with a $Δ$ and compare the predictions with those obtained for a leading neutron. Our results show that the $V + Δ$ cross section is almost 30 % of the $V + n$ one. Our results also show that a future experimental analysis of these processes is, in princi…
We study the exclusive leptonic and semileptonic $B$ decays $B \to \ell^- \barν$ and $B \to D^{(\ast)} \ell^-\barν$ in the framework of the covariant quark model with built-in infrared confinement. We compute the relevant form factors in the full kinematical momentum transfer region. The calculated form factors are used to evaluate branching fractions and polarization observables of the above transitions. We compare our results with experimental data and results from other theoretical studies.
We use the exclusive baryonic $B$ decays to determine the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $V_{ub}$. From the relation $|V_{ub}|^2/|V_{cb}|^2=({\cal B}_π/{\cal B}_D){\cal R}_{ff}$ based on $B^-\to p\bar p π^-$ and $\bar B^0\to p\bar p D^0$ decays, where $|V_{cb}|$ and ${\cal B}_π/{\cal B}_D\equiv{\cal B}(B^-\to p\bar p π^-)/{\cal B}(\bar B^0\to p\bar p D^0)$ are the data input parameters, while ${\cal R}_{ff}$ is the one fixed by the $B\to p\bar p$ transition matrix elements, we find $|V_{ub}|=(3.48^{+0.87}_{-0.63}\pm 0.40\pm 0.07)\times 10^{-3}$ with the errors corresponding to …
The transverse momentum and rapidity distributions of net-baryons (baryons minus anti-baryons) produced in central gold-gold (Au-Au) collisions at 62.4 and 200 GeV are analyzed in the framework of a multisource thermal model. Each source in the model is described by the Tsallis statistics to extract the effective temperature and entropy index from the transverse momentum distribution. The two parameters are used as input to describe the rapidity distribution and to extract the rapidity shift and contribution ratio. Then, the four types of parameters are used to structure some scatter plots of …
The STAR detector at RHIC, due to its large uniform acceptance and excellent particle identification capabilities, has measured a variety of hadron species ($π^{\pm}$, $K^{\pm}$, $p$, $\bar{p}$, $K^{0}_{S}$, $Λ$, $\barΛ$, $Ξ^{-}$, $\barΞ^{+}$) produced in Au+Au collisions at $\sqrt {s_{NN}}= 7.7$, 11.5, 27 and 39 GeV. These data are part of the Beam Energy Scan (BES) program at RHIC and provide an opportunity to measure the yields and transverse momentum spectra ($p_{T}$) of the particles produced in the collisions. The corresponding measurements allow to study the freeze-out properties and dy…
We study a set of exclusive $B$ and $B_s$ decay modes induced by the rare $b \to s ν\bar ν$ transition in the RS$_c$ model, an extra-dimensional extension of the standard model with warped 5D metric and extended gauge group. We emphasize the role of correlations among the observables, and their importance for detecting the predicted small deviations from the standard model expectations.
Charm quarks, owing to their large mass, are produced predominantly in the initial hard scatterings in heavy-ion collisions, and therefore can be a valuable tool for studying the early time dynamics of these collisions. The rapidity-odd directed flow at mid-rapidity in heavy-ion collisions originates from a tilt in the reaction plane of the thermalized medium caused by the asymmetry between the number of participants from projectile and target nuclei as a function of rapidity. Recently, it has been predicted that the slope of the directed flow at mid-rapidity for $D^0$ mesons, arising from the…
