Fixed target in LHCb Patrick Robbe LAL Orsay

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 Te. V center-of-mass energy – p. Pb and Pbp collisions at 5 Te. V • But also in fixed target configuration: – p. Ne at 87 Ge. V – Pb. Ne at 54 Ge. V 2

LHCb experiment • Fixed target experiment geometry • In the forward region: 2 < h < 5 3

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 4

VELO Layout 5

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 N 1 N 2/(4 psxsy) • 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) 6

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 and beam-gas contributions – However strong acceptance effects as a function of z No beam One beam Two beams 7

SMOG For the moment, manual control system and no precise gas pressure measurement: is being solved. 8

Gas injection For the moment, only local and temporary degradation of vaccuum (~1 hour), no longer injections so far 9

p. Ne collisions • For luminosity measurements, Ne gas is used • Data recorded was analysed • Dy ~ 4. 5: LHCb covers the backward region in the nucleon centre-of-mass frame 10

Pb. Ne collisions • Run taken in 2013 (27 minutes), with low multiplicities • Clean light hadron signals visible: 11

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: – p. A ~ 10/(mb s) – Pb. A ~ 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 Pb. A (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 k. Hz after trigger in total). Could expect 1 week of dedicated p. A run per year. 12

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. 13

More detailed look at Pb. Ar collisions • Study done in collaboration with F. Fleuret (LLR), for charmonium production. • Using Ar as gas target gives densities similar to the densities of NA 50 • 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. 14

Pb. Ar event display • Full detector simulation on a EPOS event 15

Pb. Ar 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 k. Hz output rate (after trigger), for Pb. Ar, the interaction rate is 4 k. Hz (before trigger). 16

J/y reconstruction prospects • From simulation studies (EPOS + Full detector simulation), expect 5 x 104 J/y reconstructed per year (ie 1 month running) with conservative gas pressure considerations. Signal only (with underlying event) • No MB event selected in our (limited) simulation samples: >7 s signal for 1 year (ie 1 month running). 17

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. 18

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 19