• K.A. Drees
  BNL, Upton, Long Island, New York
• S.M. White
  CERN, Geneva

Using the vernier scan or Van der Meer scan technique, where one beam is swept stepwise across the other while measuring the collision rate as a function of beam displacement, the transverse beam profiles, the luminosity and the effective cross section of the detector in question can be measured. This report briefly recalls the vernier scan method and presents results from the first RHIC polarized proton run at 250 GeV/beam in 2009.

• S.M. White, R. Alemany-Fernandez, H. Burkhardt, M. Lamont
  CERN, Geneva

Once circulating beams have been established in the LHC the first step towards collisions is to remove the physical separation used to avoid collisions during injection and ramp. A residual separation can remain after the collapsing of the separation bumps. The so-called Van Der Meer method allows for a minimization of this unwanted separation by transversally scanning one beam through the other. The beam sizes at the IP can also be determined by this method and used to give an absolute measurement of the luminosity. We report on how this measurement was implemented and performed in the LHC to optimize and calibrate luminosity.

• J. Stiller
  Heidelberg University, Heidelberg
• H.S. Matis, A. Ratti, W.C. Turner
  LBNL, Berkeley, California
• R. Miyamoto
  BNL, Upton, Long Island, New York
• S.M. White
  CERN, Geneva

The LHC BRAN luminosity detector monitors the high luminosity interaction regions (Atlas and CMS). This chamber, which is an Argon gas ionization detector measures the forward neutral particles from collisions the interaction region. To predict and improve the understanding of the detector's performance, we produced a detailed model of the detector and its surroundings in FLUKA. In this paper, we present the model and results of our simulations including the detector's estimated response to interactions for beam energies of 3.5, 5.0, and 7.0 TeV.

• A. Ratti, H.S. Matis, W.C. Turner
  LBNL, Berkeley, California
• E. Bravin, S.M. White
  CERN, Geneva
• R. Miyamoto
  BNL, Upton, Long Island, New York

The Luminosity Monitor for the LHC is ready for operation during the planned 2009-2010 run. The device designed for the high luminosity regions is a gas ionization chamber, that is designed with the ability to resolve bunch by bunch luminosity as well as survive extreme levels of radiation. The devices are installed at the zero degree collision angle in the TAN absorbers ±140m from the IP and monitor showers produced by high energy neutrons from the IP. They are used in real time as a collider operations tool for optimizing the luminosity at ATLAS and CMS. A photo-multiplier based system is used at low luminosities and also available. We will present early test results, noise and background studies and correlation between the gas ionization and the PMT. Comparison with ongoing modeling efforts will be included.

• Y.I. Levinsen, R. Appleby, H. Burkhardt, S.M. White
  CERN, Geneva

Background and loss rates vary when beams are brought into collisions in the LHC and when the beam separation is varied during luminosity scans. We report on the first observations in the early LHC operation. The observed effects are analyzed and compared with models and simulation.

• R. Bartoldus, I. Aracena, P. Grenier, D.W. Miller, E. Strauss, D. Su
  SLAC, Menlo Park, California
• J. Beringer, P. Loscutoff
  LBNL, Berkeley, California
• H. Burkhardt, S.M. White
  CERN, Geneva
• W. Kozanecki
  CEA, Gif-sur-Yvette
• J. Walder
  Lancaster University, Lancaster

We present results from the measurement of the 3-D luminosity distribution with the ATLAS Inner Detector during early running. The spatial distribution of pp interactions is reconstructed by a dedicated algorithm in the High-Level Trigger that fits tracks and primary event vertices in real time, and by an offline algorithm that takes full advantage of the high tracking efficiency and resolution. The number of vertices provides online monitoring of the instantaneous luminosity, while luminous-centroid motion mirrors IP-orbit and RF-phase drifts. The x, y and z luminous widths reflect the evolution of the transverse and longitudinal emittances. The length scales of the IP orbit bumps, which directly impact the accuracy of the transverse convolved beam sizes measured during van der Meer scans, are calibrated offline against the measured displacement of the luminous centroid; this significantly improves the accuracy of the absolute luminosity calibration. The simultaneous determination, during such scans, of the transverse convolved beam sizes (from the luminosity variation) and of the corresponding luminous sizes can be used to disentangle the transverse IP sizes of the two beams.

• S.M. White, R. Tomás, G. Vanbavinckhove, W. Venturini Delsolaro
  CERN, Geneva

During the first years of operation of the LHC unknown field errors or misalignments could lead to unmatched optics and discrepancies with respect to the model. This could affect some critical parameters such as the luminosity or the lifetime. It is therefore desirable to implement tools which allow for fine tuning of the IP optics and could be used during the commissioning phase of the LHC. In this paper we report on the implementation the performances and the limitations of these commissioning tools.