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

Fixed target in LHCb

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• 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

Introduction

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

LHCb experiment

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• 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

LHCb VELO (Vertex Locator)

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VELO Layout

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• 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)

LHCb luminosity measurements

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• 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

Fixed target system

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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• 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

pNe collisions

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PbNe collisions

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

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• 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.

Prospects

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• 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.

New detectors installed end of 2014

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• 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.

More detailed look at PbAr collisions

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• Full detector simulation on a EPOS event

PbAr event display

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• 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).

PbAr multiplicities

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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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• 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.

LHCb upgrade plans

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• 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

Conclusions