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
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LHCb experiment
• Fixed target experiment geometry
• In the forward region: 2 < h < 5
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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
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VELO Layout
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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)
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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 beambeam and beam-gas contributions
– However strong acceptance effects as a function of z
No beam
One beam
Two beams
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SMOG
For the moment, manual control system and no precise gas pressure measurement:
is being solved.
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Gas injection
For the moment, only local and temporary degradation of vaccuum (~1hour), no
longer injections so far
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pNe collisions
• For luminosity measurements, Ne gas is used
• Data recorded was analysed
• Dy ~ 4.5: LHCb covers the backward region in the nucleonnucleon centre-of-mass frame
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PbNe collisions
• Run taken in 2013 (27 minutes), with low multiplicities
• Clean light hadron signals visible:
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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.
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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.
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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.
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PbAr event display
• Full detector simulation on a EPOS event
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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).
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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)
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LHCb Simulation
80
60
40
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0
3000
3020
3040
3060
3080
3100
3120
3140
3160
3180
M(J/ y ) (MeV/c2)
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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.
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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
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