LHCb

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Coordinates: 46°14′27.64″N 6°5′48.96″E / 46.2410111°N 6.0969333°E / 46.2410111; 6.0969333

Large Hadron Collider
(LHC)
LHC.svg
LHC experiments
ATLAS A Toroidal LHC Apparatus
CMS Compact Muon Solenoid
LHCb LHC-beauty
ALICE A Large Ion Collider Experiment
TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf LHC-forward
MoEDAL Monopole and Exotics Detector At the LHC
LHC preaccelerators
p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)
(not marked) Proton Synchrotron Booster
PS Proton Synchrotron
SPS Super Proton Synchrotron

LHCb (standing for "Large Hadron Collider beauty") is one of seven particle physics detector experiments collecting data at the Large Hadron Collider accelerator at CERN. LHCb is a specialized b-physics experiment, that is measuring the parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark). Such studies can help to explain the Matter-Antimatter asymmetry of the Universe. The detector is also able to perform measurements of production cross sections and electroweak physics in the forward region. Approximately 840 people from 60 scientific institutes, representing 16 countries form the collaboration who built and operate the detector.[1] The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The (small) MoEDAL experiment will share the same cavern.

Physics goals[edit]

The experiment has wide physics program covering many important aspects of Heavy Flavor (both beauty and charm), Electroweak and QCD physics. Six key measurements have been identified involving B mesons. These are described in a roadmap document [2] that form the core physics programme for the first high energy LHC running in 2010–2012. They include:

  • Measuring the branching ratio of the rare Bs → μ+ μ decay.
  • Measuring the forward-backward asymmetry of the muon pair in the flavour changing neutral current Bd → K* μ+ μ decay. Such a flavour changing neutral current cannot occur at tree-level in the Standard Model of Particle Physics, and only occurs through box and loop Feynman diagrams; properties of the decay can be strongly modified by new Physics.
  • Measuring the CP violating phase in the decay Bs → J/ψ φ, caused by interference between the decays with and without Bs oscillations. This phase is one of the CP observables with the smallest theoretical uncertainty in the Standard Model, and can be significantly modified by new Physics.
  • Measuring properties of radiative B decays, i.e. B meson decays with photons in the final states. Specifically, these are again flavour changing neutral current decays.
  • Tree-level determination of the unitarity triangle angle γ.
  • Charmless charged two-body B decays.

The LHCb detector[edit]

The fact that the two b-hadrons are predominantly produced in the same forward cone is exploited in the layout of the LHCb detector. The LHCb detector is a single arm forward spectrometer with a polar angular coverage from 10 to 300 milliradians (mrad) in the horizontal and 250 mrad in the vertical plane. The asymmetry between the horizontal and vertical plane is determined by a large dipole magnet with the main field component in the vertical direction.

LHCb detector among the bending plane

The VELO[edit]

The vertex detector (known as the vertex locator or VELO) is built around the proton interaction region.[3][4] It is used to measure the particle trajectories close to the interaction point in order to precisely separate primary and secondary vertices.

The detector operates at 7 millimetres (0.28 in) from the LHC beam. This implies an enormous flux of particles; The VELO has been designed to withstand integrated fluences of more than 1014 p/cm2 per year for a period of about three years. The detector operates in vacuum and is cooled to approximately −25 °C (−13 °F) using a biphase CO2 system. The data of the VELO detector are amplified and read out by the Beetle ASIC.

RICH1[edit]

The RICH-1 detector (Ring imaging Cherenkov detector) is located directly after the vertex detector. It is used for particle identification of low-momentum tracks.

Main Tracker[edit]

The main tracking system is placed before and after the dipole magnet. It is used to reconstruct the trajectories of charged particles and to measure their momenta. The tracker consists of three subdetectors:

  • The Tracker Turicensis, a silicon strip detector located before the LHCb dipole magnet
  • The Outer Tracker. A straw-tube based detector located after the dipole magnet covering the outer part of the detector acceptance
  • The Inner Tracker, silicon strip based detector located after the dipole magnet covering the inner part of the detector acceptance

RICH2[edit]

Following the tracking system is RICH-2. It allows the identification of the particle type of high-momentum tracks.

Calorimeters[edit]

The electromagnetic and hadronic calorimeters provide measurements of the energy of electrons, photons, and hadrons. These measurements are used at trigger level to identify the particles with large transverse momentum (high-Pt particles).

Muon System[edit]

The muon system is used to identify and trigger on muons in the events.

Data taking[edit]

During the 2011 proton-proton run LHCb recorded a luminosity of 1 fb−1 [5] at a center-of-mass energy of 7 TeV. In 2012 about 2 fb−1 was collected at 8 TeV.[6] These datasets allow to carry out the baseline physics program of the experiment of precision Standard Model tests together with many additional measurements. The analysis led to evidence for the flavour changing neutral current decay Bs → μ μ.[7] This measurement impacts the parameter space of supersymmetry.

See also[edit]

References[edit]

  1. ^ [1], Collaboration webpage
  2. ^ [2], Roadmap for selected key measurements of LHCb
  3. ^ [3], The LHCb VELO (from the VELO group)
  4. ^ [4], VELO Public Pages
  5. ^ [5], 2011 LHC Luminosity Plots
  6. ^ [6], 2012 LHC Luminosity Plots
  7. ^ [7], Arxiv: First evidence for the decay Bs → μ+ μ-

External links[edit]