### Regular Series

#### Cracow Epiphany Conference on the Physics at the LHC

###### Kraków, Poland; January 8–10, 2014

The Cosmological Constant Puzzle: Vacuum Energies from QCD to Dark Energy

Acta Phys. Pol. B 45, 1269 (2014)

article page: 1269 (contains 11 pages) •

abstract

The accelerating expansion of the Universe points to a small positive vacuum energy density and negative vacuum pressure. A strong candidate is the cosmological constant in Einstein’s equations of General Relativity. The vacuum dark energy density extracted from astrophysics is 10$^{56}$ times smaller than the value expected from the Higgs potential in Standard Model particle physics. The dark energy scale is however close to the range of possible values expected for the light neutrino mass. We investigate this physics in a simple toy model where the chirality of the neutrino is treated by analogy as an Ising-like “spin” degree of freedom.

$D$-meson Nuclear Modification Factor in Pb–Pb Collisions Measured with ALICE

Acta Phys. Pol. B 45, 1281 (2014)

article page: 1281 (contains 9 pages) •

abstract

The ALICE results on open heavy-flavour production in $pp$, $p$–Pb and Pb–Pb collisions are presented, focusing on the $D^0$, $D^+$, $D^{*+}$ and $D_s^+$ meson nuclear modification factors ($R_{AA}$ and $R_{p\rm {Pb}}$). The results obtained from Pb–Pb collisions at $\sqrt {s_{NN}}=2.76$ TeV collected in 2011 indicate a strong suppression of the $D$-meson yield at intermediate/high $p_{\rm T}$ in central collisions. The comparison to results in $p$–Pb collisions suggests that this suppression is a final-state effect due to the presence of the hot and dense medium created in heavy-ion collisions at the LHC.

Heavy-Ion Results from the CMS Experiment

Acta Phys. Pol. B 45, 1291 (2014)

article page: 1291 (contains 19 pages) •

abstract

The Compact Muon Solenoid experiment at the LHC accelerator uses various physics observables to study a hot and dense medium created in heavy-ion collisions. Understanding of the medium properties requires conducting studies for different colliding systems; these include PbPb collisions but also reference $pp$ and $p$Pb data. In this paper, an overview of some selected experimental results concerning collective effects, the jet-quenching phenomenon as well as the suppression of quarkonium states is presented. A special emphasis is put on the results obtained for recently acquired $p$Pb data.

Soft Physics of $p$+Pb and Pb+Pb Collisions from the ATLAS Experiment at the LHC

Acta Phys. Pol. B 45, 1311 (2014)

article page: 1311 (contains 13 pages) •

abstract

Physics studies of $p$+Pb and Pb+Pb collisions in the ATLAS experiment at the LHC range from the studies of a relatively soft sector to the studies of the products of the hard scatterings. In the sector of low transverse momenta, the main areas of interest are the charged particle multiplicities and their scaling with the number of participating nucleons. The collective phenomena arising from the initial collision shape and maintained by the hydrodynamical system evolution from the initial hard scattered quarks and gluons to the hadronized particles are also studied in detail. Recent ATLAS results on the total charged particle multiplicities and spectra in $p$+Pb are reviewed. The observation of elliptic flow in $p$+Pb is also presented supplemented with the novel results on the flow coefficients fluctuations and their relation to the initial state of Pb+Pb collisions.

Search for Associated Production of the Higgs Boson in the $H \rightarrow WW$ Channel with a Fully Leptonic Final State

Acta Phys. Pol. B 45, 1325 (2014)

article page: 1325 (contains 11 pages) •

abstract

A search for the Higgs boson in the $WH$ and $ZH$ associated production modes, with $H\rightarrow WW^{(*)}$ and a fully leptonic final state with electrons or muons, is performed using data collected in 2012 with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to 20.7 fb$^{-1}$ of proton–proton collision data at a center-of-mass energy of 8 TeV. The result obtained with the 2012 data is combined with a search for $WH$, with $H\rightarrow WW^{(*)}$ and a fully leptonic final state with electrons or muons, corresponding to 4.7 fb$^{-1}$ of data recorded at a center-of-mass energy of 7 TeV.

Hydrodynamic Models of Ultrarelativistic Collisions

Acta Phys. Pol. B 45, 1337 (2014)

article page: 1337 (contains 14 pages) •

abstract

We discuss the use of the hydrodynamic model for the description of the evolution of dense matter formed in ultrarelativistic heavy-ion collisions. The collective flow observed in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider is consistent with the assumption that a fireball of strongly interacting matter is formed. Experimental results from $p$–Pb and $d$–Au collisions show similar phenomena, which suggests that collective expansion appears in small systems as well. We review the recent application of the hydrodynamic model to small systems and discuss limitations and possible further checks of this scenario.

Monte Carlo Study of NLO Correction to QCD Evolution Kernel Induced by the Change of the Factorization Scale

Acta Phys. Pol. B 45, 1351 (2014)

article page: 1351 (contains 9 pages) •

abstract

The aim of the present study is to show that the redefinition of the factorization scale $Q_i\to z_i Q_i$ in the ladder can be traded exactly for the NLO correction to the LO evolution kernel, $P(z)\to P(z)+(2C_{\rm F} \alpha _{\rm s}/\pi ){\mit \Delta }(z)$. The above observation was done/exploited in the literature, but the present study demonstrates how this phenomenon is realized within the Markovian Monte Carlo parton shower hence it might be relevant in MC practice.

Calculation of QCD NLO Splitting Functions in the Light-cone Gauge: a New Regularization Prescription

Acta Phys. Pol. B 45, 1361 (2014)

article page: 1361 (contains 10 pages) •

abstract

We report on the progress in calculating NLO DGLAP splitting functions for $x\lt 1$ using the New Principal Value prescription, which is a modification of the standard Principal Value approach proposed by Curci, Furmanski and Petronzio in 1980. The new prescription reproduces the standard results on the inclusive (integrated) level, but simplifies individual contributions and restricts the cancellations between real and virtual diagrams which makes it useful for Monte Carlo simulations.

Exotics Searches with ATLAS and CMS Experiments

Acta Phys. Pol. B 45, 1371 (2014)

article page: 1371 (contains 22 pages) •

abstract

In this paper, we present strategies and results of nine searches for exotic physics performed by the ATLAS and CMS experiments with 2012 $pp$ collision data taken at LHC energy $\sqrt {s}=7$ TeV and 8 TeV. Among them, there are searches from one object to many objects in the final state covering mono-$X$, two leptons, two jets, photon+$X$, microscopic black holes, dibosons, $t \overline {t}$ resonances and vector-like top quark analyses.

About the Role of the Higgs Boson in the Evolution of the Early Universe

Acta Phys. Pol. B 45, 1393 (2014)

article page: 1393 (contains 21 pages) •

abstract

I review a recent analysis presented in F. Jegerlehner, Acta Phys. Pol. B 45, 1167 (2014); arXiv:1305.6652 [hep-ph], Acta Phys. Pol. B45, 1215 (2014). After the discovery of the Higgs particle the most relevant structures of the SM have been verified and for the first time we know all parameters of the SM within remarkable accuracy. Together with recent calculations of the SM renormalization group coefficients up to three loops, we can safely extrapolate running couplings high up in energy. Assuming that the SM is a low energy effective theory of a cutoff theory residing at the Planck scale, we are able to calculate the effective bare parameters of the underlying cutoff system. It turns out that the effective bare mass term changes sign not far below the Planck scale, which means that in the early universe the SM was in the symmetric phase. The sign-flip, which is a result of a conspiracy between the SM couplings and their screening/antiscreening behavior, triggers the Higgs mechanism. Above the Higgs phase transition the bare mass term in the Higgs potential must have had a large positive value, enhanced by the quadratic divergence of the bare Higgs mass. Likewise the quartically enhanced positive vacuum energy density is present in the symmetric phase. The Higgs system thus provides the large dark energy density in the early universe, which triggers slow-roll inflation, i.e. the SM Higgs is the inflaton scalar field. Reheating is dominated by the decay of the heavy Higgses into (in the symmetric phase) massless top/anti-top quark pairs. The Higgs mechanism stops inflation and the subsequent electroweak phase transition provides the masses to the SM particles in proportion to their coupling strength. The previously most abundantly produced particles are now the heaviest and decay into the lighter ones, by cascading down the CKM-element matrix from top and bottom to normal matter. Baryon-number $B$ violating interactions are naturally provided by Weinberg’s set of close-by dimension 6 four-fermion effective interactions. Since matter is produced originating from the primordial heavy Higgs fields via C and CP violating decays we have actually a new scenario which possibly could explain the baryon-asymmetry essentially in terms of SM physics.

SM and BSM Higgs Results from the CMS Experiment

Acta Phys. Pol. B 45, 1415 (2014)

article page: 1415 (contains 12 pages) •

abstract

Results of CMS Collaboration Higgs boson searches based on the Run 1 LHC data are presented. Details of searches in various Standard Model Higgs decay modes are presented. Results of searches for MSSM Higgs bosons in $\mathrm {\tau \tau }$ and ${b \bar {b}}$ channels are presented as well.

CMS: Performance, Physics, Perspectives

Acta Phys. Pol. B 45, 1427 (2014)

article page: 1427 (contains 19 pages) •

abstract

During the LHC Run-1 (2010–2013) the CMS experiment received almost 30 fb$^{-1}$ of proton–proton data at the energies of $\sqrt {s} =7$ and 8 TeV. The accumulated statistics allow CMS to perform frontier measurements at high-energies. In this paper, the CMS detector and its performance are briefly described. The highlights of the CMS results are given. Selected Higgs physics results, measurements of vector boson, top-quark and jets production, $B/B_s\rightarrow \mu \mu$ and searches for new phenomena are described. The CMS upgrade plans are presented, including ongoing activities during present accelerator shut-down as well as modifications in the LHC Phase-I and beyond.

Overview of Recent Measurements of CP Violation with $B$ Mesons at LHCb

Acta Phys. Pol. B 45, 1447 (2014)

article page: 1447 (contains 21 pages) •

abstract

CP violation describes differences between matter and anti-matter. It is one of the three requirements to produce a matter dominated universe. We are now entering a new era with unprecedented data samples provided by the LHC, which allow to study flavour physics with high precision. LHCb has produced many first and world’s best CP asymmetry measurements, in many different $B$ meson decay modes, which provide a test of the Standard Model. This report presents an overview of recent measurements of CP violation in the $B^0_{(s)}$ system, using the 1 fb$^{-1}$ dataset collected with the LHCb detector.

Charm Production, Mixing and CP Violation

Acta Phys. Pol. B 45, 1469 (2014)

article page: 1469 (contains 11 pages) •

abstract

Recent results on mixing, CP violation and spectroscopy in the charm sector from the LHCb experiment are briefly reviewed.

Double Parton Distribution Functions and Their QCD Evolution Equations

Acta Phys. Pol. B 45, 1481 (2014)

article page: 1481 (contains 12 pages) •

abstract

Double parton distribution functions (DPDFs) are used in the description of double parton scattering. Their QCD evolution equations, known in the leading logarithmic approximation, obey non-trivial momentum and valence quark number sum rules. In this paper, we discuss the QCD evolution of the DPDFs, in particular, the specification of initial conditions for the evolution equations which obey the new momentum sum rules. We also present results on factorization of DPDFs on the product of two single parton distributions.

Charm Meson Production and Double Parton Interactions at the LHC

Acta Phys. Pol. B 45, 1493 (2014)

article page: 1493 (contains 12 pages) •

abstract

We discuss production of open charm mesons in proton–proton collisions at the LHC. The cross section for inclusive production of $c \bar c$ pairs is calculated within the $k_{\perp }$-factorization approach in which a major part of higher-order corrections is believed to be effectively included. We use and test the applicability of several unintegrated gluon distributions. Numerical results of the $k_{\perp }$-factorization approach are compared to NLO pQCD collinear predictions. Inclusive differential distributions in transverse momentum and (pseudo)rapidity of several charmed mesons are presented and compared to recent results of the ALICE, ATLAS and LHCb collaborations. We also examine production of neutral charmed meson–antimeson pairs ($D^0 {\overline D^0}$) in unique kinematics of forward rapidities of the LHCb experiment. Kinematical correlations in azimuthal angle $\varphi _{D^0 {\overline D^0}}$, invariant mass $M_{D^0 {\overline D^0}}$ and rapidity difference $Y_{D^0 {\overline D^0}}$ distributions are calculated. Moreover, we also discuss production of two pairs of $c \bar c$ within a simple formalism of double-parton scattering (DPS). We compare results of calculations of single-parton scattering (SPS) and double-parton scattering (DPS) for production of $c \bar c c \bar c$ and for $D^0$–$D^0$ meson–meson correlations. We compare our predictions for double charm production with recent results of the LHCb Collaboration for azimuthal angle $\varphi _{D^0 D^0}$ distribution, dimeson invariant mass $M_{D^0 D^0}$ and rapidity distance between mesons $Y_{D^0 D^0}$. The obtained results clearly certify the dominance of DPS in the production of events with double charm.

Standard Model and Flavor Physics Measurements with the ATLAS Detector

Acta Phys. Pol. B 45, 1505 (2014)

article page: 1505 (contains 16 pages) •

abstract

This article presents recent results obtained by the ATLAS experiment at the Large Hadron Collider (LHC) regarding the measurements of Standard Model and Flavor Physics phenomena performed with data collected in proton–proton collisions at center-of-mass energies $\sqrt {s}=7$ TeV and $8$ TeV. Presented results include measurements of $W$ and $Z$ boson properties as well as selected QCD phenomena. Finally, a selection of results from the $B$-physics sector is shown.

Alice Experimental Results

Acta Phys. Pol. B 45, 1521 (2014)

article page: 1521 (contains 21 pages) •

abstract

We present selected results devoted to the global properties of dense matter produced in the Pb–Pb collisions at $\sqrt {s_{NN}}=2.76$ TeV measured by the ALICE Collaboration. Several first results on $p$–Pb collisions at $\sqrt {s_{NN}}=5.02$ TeV related to the double ridge structure, identified transverse momentum particle spectra and baryon-to-meson ratio are briefly reviewed. Nuclear modification factor for charged hadrons, identified light and heavy flavors are discussed together with the studies on reconstructed jets.

Mueller–Navelet Jets at the CMS

Acta Phys. Pol. B 45, 1543 (2014)

article page: 1543 (contains 5 pages) •

abstract

In this paper, a brief description of a measurement of Mueller–Navelet dijet angular decorrelations is presented. The measurement is sensitive to effects of BFKL evolution. The experimental results are presented as function of the rapidity separation $y$ between jets, and compared to the predictions of various Monte Carlo models and NLL BFKL calculations.

Modification of $\pi \pi$ Amplitudes and Position of the $\sigma$ Pole

Acta Phys. Pol. B 45, 1549 (2014)

article page: 1549 (contains 10 pages) •

abstract

Meson–meson interaction amplitudes especially $\pi \pi$ scattering amplitudes are often used incorrectly. It also applies to parameters of the $\sigma$ meson. This causes two significant problems: threshold behavior and position of the $\sigma$ pole which is suspicious. We modified multichannel S- and P-wave amplitudes for the $\pi \pi$ scattering, using dispersion relations with imposed crossing symmetry condition. The amplitudes are modified in the low-energy region to improve their consistency with experimental data and the dispersion relations. Agreement with data is achieved for the both amplitudes from the threshold up to 1.8 GeV and with dispersion relations up to 1.1 GeV. Consequences of the applied modifications, e.g. changes of the S-wave lowest-pole positions, are presented.

Triggering on Delayed Particles in the CMS Detector

Acta Phys. Pol. B 45, 1559 (2014)

article page: 1559 (contains 8 pages) •

abstract

Many of the BSM theories predict existence of massive charged particles with masses greater than $\gtrsim 100$ GeV/$c^2$ and with a decay path greater than a radius of the typical collider detector. The detection of such particles requires dedicated techniques in the trigger system and data analysis algorithms. In the particular case of PAttern Comparator Trigger of the CMS detector (PACT, part of the Level 1 Muon Trigger), one of the important issue is related to massive charged particles which are moving slowly and are delayed in the muon system. The assumption taken throughout the development of PACT about the close to the speed-of-light velocity of particles is irrelevant for massive particles and leads to the signal registration on improper (late) synchronization window of muon chambers electronics. In this note, we present an upgrade proposition for PACT which could allow to detect efficiently delayed particles during Run 2 of the LHC.

Impact of the Insertable B-layer on $b$-tagging Performance for ATLAS Run-II

Acta Phys. Pol. B 45, 1567 (2014)

article page: 1567 (contains 8 pages) •

abstract

The $b$-tagging performance expected for the ATLAS experiment at the Run-II of the Large Hadron Collider (LHC) will be presented hereafter. These studies are based on Monte Carlo simulations of $t\bar {t}$ events. These results allow to quantify the impact on the $b$-tagging performance of both the installation of a new pixel layer and the repairs of several electronics components of the present Pixel Detector. Beforehand, a brief description of the ATLAS experiment at the LHC and of the $b$-tagging algorithms will be given.

Search for Lepton Number Violation at the LHCb. An Update for Majorana Neutrino Search with Like-sign Di-muons: $B^{-}\rightarrow \pi ^{+}\mu ^{-}\mu ^{-}$ Decay

Acta Phys. Pol. B 45, 1575 (2014)

article page: 1575 (contains 6 pages) •

abstract

Lepton number is conserved in the Standard Model, therefore, any evidence for its violation would indicate the existence of New Physics models. This paper presents selected searches of rare decays performed by the LHCb experiment. An update for the studies dedicated to on-shell Majorana neutrinos coupling to muons in the $B^{-}\rightarrow \pi ^{+} \mu ^{-} \mu ^{-}$ decay channel is also presented.

Higgs Results from the ATLAS Experiment (Standard Model and Beyond)

Acta Phys. Pol. B 45, 1581 (2014)

article page: 1581 (contains 15 pages) •

abstract

The discovery of a new particle consistent with the Standard Model-like Higgs boson, with a mass of about 125 GeV, was announced by the ATLAS and CMS experiments on the 4$^{\rm {th}}$ July, 2012. After the discovery, the properties of this new particle have been studied with high precision measurements of its mass, spin and couplings in all the accessible decay channels. In this paper, recent results from the ATLAS experiment are presented, including: $H\rightarrow \gamma \gamma$, $H\rightarrow ZZ^{(*)}\rightarrow 4l$, $H\rightarrow WW^{(*)}\rightarrow l\nu l\nu$, $H\rightarrow \tau \tau$, $H\rightarrow Z\gamma$, $H\rightarrow \mu \mu$ $VH\rightarrow b\bar {b}$, $VH\rightarrow WW^{(*)}$, $ttH\rightarrow \gamma \gamma$ channels. Furthermore, searches for beyond the standard model Higgs bosons are briefly reviewed.

Peripheral, Ultrarelativistic Production of Particles in Heavy Ion Collisions

Acta Phys. Pol. B 45, 1597 (2014)

article page: 1597 (contains 21 pages) •

abstract

The cross sections for the production of two-pions in ultraperipheral ultrarelativistic heavy ion collisions, calculated in the impact parameter Equivalent Photon Approximation (EPA), are presented. Differential distributions in impact parameter, dipion invariant mass, single pion and dipion rapidity, pion transverse momentum and pion pseudorapidity are shown. The $\gamma \gamma \to \pi ^+ \pi ^-$ subprocess constitutes a background to the $A A \to A \rho ^0 (\to \pi ^+ \pi ^-) A$ process, initiated by emission of a photon by one of colliding nuclei. Only a part of the dipion invariant mass distribution associated with the $\gamma \gamma$-collisions can be visible as the cross section for the $A A \to A \rho ^0 A$ reaction is very large. Differential distributions for two $\rho ^0$ meson production in exclusive ultraperipheral, ultrarelativistic collisions via a double scattering mechanism are presented. The cross section for $\gamma A \to \rho ^0 A$ is parametrized based on a calculation from the literature. Smearing of $\rho ^0$ masses is taken into account. The results of calculations are compared to experimental data obtained at RHIC and to the contribution of the two-photon mechanism. The double scattering mechanism populates larger $\rho ^0 \rho ^0$ invariant masses and larger rapidity distances between the two $\rho ^0$ mesons compared to the $\gamma \gamma$ fusion. It gives a significant contribution to the $A A \to A A \pi ^+ \pi ^- \pi ^+ \pi ^-$ reaction. Some observables related to charged pions are presented. The results of our calculation are compared with the STAR Collaboration results for four charged pion production. While the shape in invariant mass of the four-pion system is very similar to the measured one, the predicted cross section constitutes only 20% of the measured one.

Measurement of Hard Probes in $p$+Pb and Pb+Pb Collisions with the ATLAS Detector

Acta Phys. Pol. B 45, 1619 (2014)

article page: 1619 (contains 9 pages) •

abstract

Recent measurements from the ATLAS experiment of high-$p_{\rm T}$ processes in relativistic proton–lead and lead–lead interactions at the Large Hadron Collider (LHC) are presented. In particular, the inclusive jet production results are reviewed including the centrality dependence of the jet nuclear modification factors (NMF) for $p$+Pb and Pb+Pb collisions at energies of $\sqrt {s_{NN}} = 5.02$ TeV and 2.76 TeV, respectively. Additionally, recent results on the electroweak boson rates in Pb+Pb collisions are presented and compared to the theoretical model predictions. The hard probe measurements provide important information on the initial conditions as well as on the properties of the dense medium created in heavy ion collisions.

Current Status of LHC Physics: Precision Theory View

Acta Phys. Pol. B 45, 1629 (2014)

article page: 1629 (contains 23 pages) •

abstract

We discuss the current status of LHC physics from the perspective of precision theory predictions for the attendant QCD and EW higher order corrections. We focus on the interplay between the available data and the predictions for these data viewed in the context of the establishment of baselines for what is needed to exploit fully the discovery potential of the existing LHC data and the data expected by the end of the second LHC run (i.e. , 300 fb$^{-1}$). We conclude that significant improvement in the currently used theoretical predictions will be mandatory. Possible strategies to achieve such improvement are indicated.

Charged Particle Multiplicity in Proton–Lead Collisions at $\sqrt {s_{NN}} = 5.02$ TeV in the ATLAS Experiment

Acta Phys. Pol. B 45, 1653 (2014)

article page: 1653 (contains 10 pages) •

abstract

This note describes ATLAS results on multiplicity of produced charged particles in $p+$Pb collisions at the centre-of-mass energy of 5.02 TeV. The charged particle density was measured in a wide pseudorapidity range as a function of the collision centrality using 2.1 million events which were collected during a “pilot run” in September 2012. Analysis methods, centrality determination and the particle production dependence on the number of nucleons participating in the collisions are presented.

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