Patrick Robbe, LAL Orsay, for the LHCb Collaboration, 16 December 2014 Fixed target in LHCb Patrick Robbe, LAL Orsay, for the LHCb Collaboration, 16 December 2014

Introduction Between 2010 and 2013, LHCb took data in various configurations with LHC beams: pp collisions at 2.76, 7 and 8 TeV center-of-mass energy pPb and Pbp collisions at 5 TeV But also in fixed target configuration: pNe at 87 GeV PbNe at 54 GeV

LHCb experiment Fixed target experiment geometry In the forward region: 2 < h < 5

LHCb VELO (Vertex Locator) Device to measure precisely primary vertices and decay vertices (essential for CP violation measurements) In the LHC vaccuum, 8 mm from the beam Gas target (SMOG) is injected in the VELO

VELO Layout

LHCb luminosity measurements To measure the absolute instantaneous luminosity of the LHC collisions in LHCb: beam imaging method: A gas is injected in the VELO during dedicated periods (van der Meer scans) From the beam-gas vertices, the shapes of the beams are measured Lint = f N1N2/(4psxsy) In normal data taking, the relative luminosity is measured using multiplicity counters calibrated during the scans. The integrated luminosity is obtained summing these counters (with a 3% precision)

Fixed target system The existing system to inject the gas for the luminosity measurement (SMOG) could be re-used for fixed targe physics: Precise vertexing (and LHC filling scheme) allows to separate beam-beam and beam-gas contributions However strong acceptance effects as a function of z No beam One beam Two beams

SMOG For the moment, manual control system and no precise gas pressure measurement: is being solved.

Gas injection For the moment, only local and temporary degradation of vaccuum (~1hour), no longer injections so far

pNe collisions For luminosity measurements, Ne gas is used Data recorded was analysed Dy ~ 4.5: LHCb covers the backward region in the nucleon-nucleon centre-of-mass frame

PbNe collisions Run taken in 2013 (27 minutes), with low multiplicities Clean light hadron signals visible:

Prospects Target types: H and noble gases (He, Ne, Ar, Kr, Xe). He, Ne and Ar already tested. Luminosities: increasing the gas pressure with a factor 10 with respect to now: pA ~ 10/(mb s) PbA ~ 1/(mb s) Operations: No impact on LHC for short run observed in 2013 Longer runs to be checked carefully « Competition » with LHCb standard physics program: No competition for PbA (apart from computing ressources): 1 month of data taking per year Probably difficult to have gas injected during pp collisions: contamination of pp events and output bandwidth limitation (20 kHz after trigger in total). Could expect 1 week of dedicated pA run per year.

New detectors installed end of 2014 Forward and backward (high rapidity) scintillator counters: Increase the rapidity coverage to detect central exclusive processes with large rapidity gaps: gain for diffractive physics that can also be done with fixed targets.

More detailed look at PbAr collisions Study done in collaboration with F. Fleuret (LLR), for charmonium production. Using Ar as gas target gives densities similar to the densities of NA50 In the nucleon-nucleon centre-of-mass frame, -2.2 < y*LHCb < 0.8. Integrated luminosity of ~0.7 nb-1 in one month of data taking.

PbAr event display Full detector simulation on a EPOS event

PbAr multiplicities In most central collisions, ~10 times larger multiplicity than in a pp collisions. Can LHCb work in higher multiplicity environment ? With this factor 10, yes without doubt High multiplicity is a problem for B physics analysis (CP violation) but much less for cross-section measurements LHCb is already routinely running at 3 times higher luminosity than its design Rate is also not a problem: LHCb will work with 20 kHz output rate (after trigger), for PbAr, the interaction rate is 4 kHz (before trigger).

J/y reconstruction prospects From simulation studies (EPOS + Full detector simulation), expect 5x104 J/y reconstructed per year (ie 1 month running) with conservative gas pressure considerations. No MB event selected in our (limited) simulation samples: >7 s signal for 1 year (ie 1 month running). Signal only (with underlying event)

LHCb upgrade plans 2015-2018: Run 2 2018-2020: Upgrade LHCb detector and trigger system: Only one software level of trigger running at 40 MHz, with higher luminosity Improved tracking detectors (VELO with pixels, tracker with scintillating fibers) to cope with higher multiplicities 2020-2030: record 50 fb-1 After 2030: instantaneous luminosity too high for the detector, ideas for evolutions of LHCb after 2030 start to be designed now.

Conclusions SMOG system allows fixed target physics program at LHCb First tests and simulations successful A lot of work still needed to move from test to real physics program (operation in particular) At least, LHCb could be an ideal pilot experiment for future fixed target programs