Patrick Robbe on behalf of LHCb, LAL Orsay, 2 Dec 2013, SaporeGravis Workshop, Nantes (France)

Status and perspectives in pp and pA-collisions in LHCb for

photoproduction and diffractive processes

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Introduction

• LHCb Detector

• Analysis techniques for Central Exclusive Production at LHCb

• J/y and y(2S) cross-sections

• cc cross-sections

• Future prospects

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LHCb Detector

• Single arm spectrometer• Fully instrumented in 2 < h < 5• Backward track reconstruction in -4 < h < -1.5

JINST 3 (2008) S08005

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LHCb Detector

• Vertex Locator:• 42 silicon strip detectors

surrounding interaction region• Detectors upstream of interaction

point allowing reconstruction of backward tracks

• Scintillating Pad Detector (SPD):• Scintillator plane parallel to beam

axis• Segmented transversally in square

cells• Provides measurement of global

track multiplicity in events (used at the hardware trigger)

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• Main results concerning Central Exclusive Production (CEP), made possible thanks to:– Detector coverage up to high rapidities (h=5)– Low pile-up: large fraction (21% of luminosity) of events with a

single interaction used for simplicity– Flexible trigger:

• Hardware level: muon detectors and calorimeters• Software level: full event information to perform analysis-like selections

– Excellent particle identification, momentum and vertex resolution

– Can detect low momentum particles, and reconstruct hadrons down to almost pT=0.

Diffractive studies at LHCb

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• Elastic process, protons are intact and fly in the beam-pipe• Exchange of colour-less objects (photon, pomeron)

Central exclusive di-muon

Di-photon fusion g-pomeron fusion Di-pomeron fusion

Generators: LPAIR STARLight SuperCHIC[A.G. Shamov, V.I. Telnov, NIM A494 (2002) 51]

[S.R. Klein and J. Nystrand, PRL 92 (2004) 142003]

[L.A. Harland-Lang, V.A. Khoze, M.G. Ryskin, W.J. Stirling, EPJC 65 (2010) 433]

• Process signature: isolated muons in acceptance + rapidity gap• 36pb-1 of 2010 data (7 TeV) [JPhysG 40 (2013) 045001] and 930pb-1 of 2011

data (7 TeV) [NEW: LHCb-PAPER-2013-059]

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• Hardware trigger:– 1 muon with pT>400 MeV, or dimuon with each pT>80 MeV– Number of SPD hits < 20

• Software trigger:– Dimuon with mass > 2.9 GeV, or with mass < 1 GeV and pT <

900 MeV and distance of closest approach < 150 mm.• Offline:

– Two identified muons in 2<h<4.5– No photons, no other forward tracks: Dy=3.5– No backward tracks: Dy=1.7– Dimuon mass in 65 MeV mass window of the J/y and y(2S)

masses.

J/y and y(2S) selection

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Event Display

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Backward tracks veto

All events passing trigger After backward track vetoJPhysG 40 (2013) 045001

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Invariant massesLHCb-PAPER-2013-059

• Fit invariant mass spectra:– Signal: Crystall Ball function, 55985 J/y and 1565 y(2S)– Exponential for combinatorial background

• Background contamination in signal windows:– (0.8±0.1)% for J/y– (17.0±0.3)% for y(2S)

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• J/y from y(2S) decays:– Suppressed by the 2 track requirement– Residual estimated with SuperCHICgenerator:

(2.5±0.2)% of J/y come from y(2S).

• J/y from cc decays: (g undetected)– Suppressed by the requirement of no photon– Residual estimated with SuperCHIC generator:

(7.6±0.9)% of J/y come from cc.

• y(2S) from X(3872) decays: (g undetected):– Estimated: (2.0±2.0)%

Feed-down

LHCb-PAPER-2013-059

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• Largest background from inelastic production with extra particles outside of LHCb

• Estimated using pT2 shapes measured at HERA,

to distinguish « exclusive » and « inelastic » components

• Feed-down pT2 shapes taken from data

• Combinatorial background shapes taken in mass sidebands.

Inelastic contamination

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Inelastic contaminationLHCb-PAPER-2013-059

Purity of sample in exclusively produced:• J/y: (59.2±1.2)%• y(2S): (52±7)%

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Systematic Uncertainties

LHCb-PAPER-2013-059

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• Cross-section x branching fraction in 2<h +, -m m <4.5:

Results (J/y and y(2S))

LHCb-PAPER-2013-059

6.5

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Cross-sections (J/y and y(2S))LHCb-PAPER-2013-059

Predictions from Jones, Martin, Ryskin and Teubner [arXiv: 1307.7099]

J/y

y(2S)

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Cross-sections (J/y and y(2S))LHCb-PAPER-2013-059

Saturation models from Gay Ducati, Griep, Machado [arXiv: 1305.4611] and Motyka and Watt [PRD78 014023]

J/y

y(2S)

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• To compare with HERA measurements, the differential cross-section is calculated in 10 rapidity bins and reweighted by photon flux:

Results (J/y ds/dy)

• Two solutions for W in each rapidity bin

• Possible deviation from power law, which can be explained by higher order effects and saturation effects.

LHCb-PAPER-2013-059

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• Selection: (36pb-1 of 2010 data – 7 TeV)

– Dimuon + g with ET>200 MeV in ECAL,

– No extra track: 194 events• Background:– Inelastic: same method as J/y– Feed-down from y(2S) estimated

from STARLight– Total purity: (39±13)%

• Results

cc production

LHCb-CONF-2011-022

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• With di-muon: – More data available for the cc analysis– Measurements of Y(nS) – 8 TeV data set to be exploited

• Hadronic final states to reconstruct other hadrons: – open charm, – cc hh.➝

• pA/Ap data recorded in 2013 to be analyzed• 25 ns bunch spacing in 2015 will mean lower pile-

up and higher usable integrated luminosity

Future prospects

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• J/y, y(2S) and cc central exclusive production measured at LHCb

• Improved results with larger statistics are imminent

• New ideas for further analyses: active area within LHCb.

Conclusions