High-luminosity Large Hadron Collider with laser-cooled isoscalar ion beams

This paper presents one of the case studies of the Gamma Factory initiative -- a proposal of a new operation scheme of ion beams in the CERN accelerator complex. Its goal is to extend the scope and precision of the LHC-based research by complementing the proton-proton collision programme with the high-luminosity nucleus-nucleus one. Its numerous physics highlights include studies of the exclusive Higgs-boson production in photon-photon collisions and precision measurements of the electroweak (EW) parameters. There are two principal ways to increase the LHC luminosity which do not require an upgrade of the CERN injectors: (1) modification of the beam-collision optics and (2) reduction of the transverse emittance of the colliding beams. The former scheme is employed by the ongoing high-luminosity (HL-LHC) project. The latter one, applicable only to ion beams, is proposed in this paper. It is based on laser cooling of bunches of partially stripped ions at the SPS flat-top energy. For isoscalar calcium beams, which fulfil the present beam-operation constrains and which are particularly attractive for the EW physics, the transverse beam emittance can be reduced by a factor of $5$ within the $8$ seconds long cooling phase. The predicted nucleon-nucleon luminosity of $L_{NN}= 4.2 times 10^{34},$s$^{-1}$cm$^{-2}$ for collisions of the cooled calcium beams at the LHC top energy is comparable to the levelled luminosity for the HL-LHC proton-proton collisions, but with reduced pile-up background. The scheme proposed in this paper, if confirmed by the future Gamma Factory proof-of-principle experiment, could be implemented at CERN with minor infrastructure investments.

QED radiative corrections and their impact on H -> tau tau searches at the LHC

In this paper we show that the excess of the tau tau events with respect to the Standard Model background predictions, observed by the ATLAS and CMS collaborations and interpreted as the evidence of the Higgs-boson decay into a pair of tau-leptons, may be accounted for by properly taking into account QED radiative corrections in the modelling of the Z/gamma* -> tau tau background.

The Gamma Factory proposal for CERN

This year, 2015, marks the centenary of the publication of Einsteins Theory of General Relativity and it has been named the International Year of Light and light-based technologies by the UN General Assembly. It is thus timely to discuss the possibility of broadening the present CERN research program by including a new component based on a novel concept of the light source which could pave a way towards a multipurpose Gamma Factory. The proposed light source could be realized at CERN by using the infrastructure of the existing accelerators. It could push the intensity limits of the presently operating light-sources by at least 7 orders of magnitude, reaching the flux of the order of 10^17 photons/s, in the particularly interesting gamma-ray energy domain of 1 < Ephoton < 400 MeV. This domain is out of reach for the FEL-based light sources. The energy-tuned, quasi-monochromatic gamma beams, together with the gamma-beam-driven, high intensity secondary beams of polarized positrons, polarized muons, neutrons and radioactive ions would constitute the basic research tools of the proposed Gamma Factory. The Gamma Factory could open new research opportunities in a vast domain of uncharted fundamental physics and industrial application territories. It could strengthen the leading role of CERN in the high energy frontier research territory by providing the unprecedented-brilliance secondary beams of polarized muons for the TeV-energy-scale muon collider and the polarized- muon-beam based neutrino factory.

On the contribution of the double Drell-Yan process to WW and ZZ production at the LHC

In this paper we investigate consequences of an assumption that the discrepancy of the predicted and observed W+W- production cross sections at the LHC is caused by the missing contribution of the double Drell-Yan process (DDYP). Using our simple model of DDYP of Ref. [1] we show that inclusion of this production mechanism leads to a satisfactory, parameter-free description of the two-lepton mass distribution for 0-jet W+W- events and the four-lepton mass distribution for ZZ events. In such a scenario the Higgs-boson contribution is no longer necessary to describe the data. An experimental programme to prove or falsify such an assumption is proposed.

The LHC excess of four-lepton events interpreted as Higgs-boson signal: background from Double Drell--Yan process?

We construct a simple model of the Double Drell--Yan Process (DDYP) for proton--proton collisions and investigate its possible contribution to the background for the Higgs-boson searches at the LHC. We demonstrate that under the assumption of the predominance of short range, ${cal O}(0.1),$fm, transverse-plane correlations of quark--antiquark pairs within the proton this contribution becomes important and may even explain the observed excess of the four-lepton events at the LHC -- the events interpreted as originating from the Higgs-boson decays: $ H rightarrow ZZ^{*} rightarrow 4l$ and $ H rightarrow WW^{*} rightarrow 2l 2 u$.

Exploring Confinement

This note is an extended version of the contribution to the CERN Council Open Symposium on European Strategy for Particle Physics. It discusses an experimental programme to explore the QCD confinement phenomena at CERN with a new electron-proton and electron-nucleus collider using the existing SPS beams (optionally also the future SPL and PS proton and ions beams) and the polarised electron beam in the range of 5 to 20 GeV from a newly built Energy Recovery Linac.

Ascertaining the origin of the $l u l u$ excess events at the LHC by a change of beam energy

A higher than predicted rate of two leptons plus missing transverse energy events, reported at the summer HEP conferences, can originate from a decay of the Higgs boson into a $WW^{(*)}$ pair, a misjudgement of the rate of SM background processes or a statistical fluctuation. In this paper we discuss a way to resolve this three-fold ambiguity.