1. 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

2. 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

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

4. 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

5. VELO Layout

6. 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)

7. 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

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

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

10. 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

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

12. 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.

13. 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.

14. 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.

15. PbAr event display
Full detector simulation on a EPOS event

16. 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).

17. 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)

18. LHCb upgrade plans 2015-2018: Run 2
: 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
: 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.

19. 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