LIP at DELPHI  

Activities (1986-present)

• Development and start-up of the experiment (1986-1989)
• The first measurements at LEP-I (1989-1995)
• Development of the STIC: the new DELPHI luminosity calorimeter (1992-1995)
• The second phase of LEP and the consolidation of the analysis groups

1986-1989: Development and start-up of the experiment

During a first period, the participation of the portuguese group in DELPHI was centred on the development and startup 
of the experience, namely in the areas of fast electronics, acquisition and analysis software.

In the area of fast electronics and data acquisition, there were specific contributions to:

• the tests of the LTD's - the FASTBUS Lep Time Digitizer's,
• the build of a real time Fastbus diagnostic database,
• development of a fourth level trigger specific language,
• participation in the development of the core software for the central data acquisition,
• fourth level processors (IBM emulators),
• development of Fastbus accelerated processors,
• the development and build of an Fastbus long range optical link and its host interface

In the area of the analysis software, there were specific contributions to:

the core of the data reconstruction program, the event viewing program.

1989-1995: The first measurements at LEP-I

In August 1989 the DELPHI experiment collected the first of 4 million Z⁰ hadronic decays. In the years 1989-1995, the
LEP machine was operated at centre of mass energies close to the Z⁰ mass, and the LIP has concentrated its physics 
analysis activities in the area of Search of New Physics in simple topologies, that is, with leptonic final states.

In this scope LIP has been responsible for the analysis and publications in the channels:

• Search for llV events,
• Final states with only photons,
• Search for a resonance in the photon-photon spectra in the ll final state. 

1992-1995: Development of the STIC - the new DELPHI luminosity calorimeter

In 1992, the LIP DELPHI group has joined a starting project to build a new calorimeter for measuring the luminosity at the interaction point. Using a process with cross-section known and/or calculable to great precision, the Bhabha scattering, one can invert the equation

Nb_EVENTS = Luminosity  x Cross-section x Efficiency      <=>     Luminosity = Nb_EVENTS / (Eff. x C.-S.)

provided the Efficiency can also be determined with great precision (in principle, yes). Experimentally, one needs to determine the efficiency with great precision and to count events during a certain period of time. Theoretically, one needs to access a very precise computed value for the cross section of the process. Then luminosity can be determined using the above equations.

The STIC detector was built with two aims: to reconstruct more efficiently the electrons and separate, if possible, background photons, and to cover a previously unseen space region in the DELPHI detector.

Among other updates to the DELPHI detector, STIC enable to enlarge continuously the covered region down to about 2 degrees in polar angle with the beam pipe. 

STIC is a lead-tile calorimeter, that is, with scintillating tiles (picture on the right) sandwiched between lead plates. The tiles are read by wavelength shifter optical fibers, which pass through the holes in the tiles, down to the photomultipliers in the back of the STIC detector (picture down, with a quarter of the STIC detector in up-down orientation; the particles should arrive at the detector from the bottom of the picture).
	A Picture of STIC in place  at the DELPHI detector

1995-present: The second phase of LEP and the consolidation of the analysis groups

In the end of 1995, LEP has started a second phase of operation, LEP2, with centre of mass energy being increased in steps, first to 130 and 136 GeV in 1995, then to 161 GeV (the threshold for W pair production) and 172 GeV in 1996, to 183 GeV (theshold for double Z⁰ production) in 1997, to 189 GeV in 1998, 192 to 202 GeV in 1999, and around 206 GeV in 2000, with a record energy of 209 GeV (LEP2 was designed to operate at a maximal energy of 200 GeV)!!!

The cross section of produced events is much lower than at the Z⁰ pole, but the luminosity was much higher. The activities of the group were mainly  maintenance of the STIC detector and data analysis in the physics channels:

• Searches for extensions of the Standard Model
  • Search for Heavy and Excited Leptons
  • Search for Fermiophobic Higgses
  • Search for Higgses with Anomalous Couplings
  • Search for Neutralinos and Gravitinos in photonic final states
  • Search for Leptoquarks
  • Search for top quarks within Flavour Changing Neutral Currents
  • Search for 4th generation b' quarks
  • Search Contact Interactions
  • Search for anomalous quartic gauge couplings
• QCD studies
  • Charged multiplicities
  • Multiplicities and spectra of protons, charged and neutral kaons, and Lambdas
  • Differences in the charged multiplicities according to the flavour of the primary quark (the QCD dead cone effect)
  • Colour reconnection between partons from different W's in fully hadronic WW events
• Measurement of the WWgamma cross section
• Measurement of the QED Cross-section for the process e⁺e^- -> gamma gamma (gamma)

Further information can be found in the page of publications, conferences, thesis, etc.

Last update in: 03 Fev 2003 16:31