arXiv:1104.5294 CERN-PH-EP-2011-061 Submitted to PLB First LHCf measurement of photon spectra at pseudorapidity >8.8 in LHC 7TeV pp collisions Takashi SAKO (Solar-Terrestrial Environment Laboratory, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University) For the LHCf Collaboration CERN Joint EP/PP/LPCC seminar, 17-May2011, 503-1-001 Council Chamber

Thanks to… CERN, especially LHC crew ATLAS collaboration Michelangelo and LHCC referees Financial support mainly from Japan and Italy

Plan of the talk Motivation The LHCf Experiment History and recent progress in the UHECR observation Hadron interaction models and forward measurements The LHCf Experiment Single photon spectra at 7TeV pp collisions Impact on the CR physics Introduction to on-going works Next plan Further analysis of 0.9 and 7 TeV collision data 14TeV pp / pA, AA collisions Summary

1. Motivation

Frontier in UHECR Observation What limits the maximum observed energy of Cosmic-Rays? Time? Technology? Cost? Physics? GZK cutoff (interaction with CMB photons) >1020eV was predicted in 1966 Acceleration limit

Observations (10 years ago and now) Debate in AGASA, HiRes results in 10 years ago Now Auger, HiRes (final), TA indicate cutoff Absolute values differ between experiments and between methods

Estimate of Particle Type (Xmax) 0g/cm2 Xmax Proton and nuclear showers of same total energy Auger TA Xmax gives information of the primary particle Results are different between experiments Interpretation relies on the MC prediction and has model dependence HiRes

Summary of Current CR Observations Cutoff around 1020 eV seems exist. Absolute energy of cutoff, sensitive to particle type, is still in debate. Particle type is measured using Xmax, but different interpretation between experiments. (Anisotropy of arrival direction also gives information of particle type; not presented today) Still open question : Is the cutoff due to GZK process of protons or heavy nuclei, or acceleration limit in the source? Both in the energy determination and Xmax prediction MC simulation is used and they are one of the considerable sources of uncertainty. Experimental tests of hadron interaction models are indispensable.

What to be measured at colliders multiplicity and energy flux at LHC 14TeV collisions pseudo-rapidity; η= -ln(tan(θ/2)) Multiplicity Energy Flux All particles neutral Most of the energy flows into very forward

2. The LHCf Experiment

The LHCf Collaboration K.Fukatsu, T.Iso, Y.Itow, K.Kawade, T.Mase, K.Masuda, Y.Matsubara, G.Mitsuka, Y.Muraki, T.Sako, K.Suzuki, K.Taki Solar-Terrestrial Environment Laboratory, Nagoya University, Japan H.Menjo Kobayashi-Maskawa Institute, Nagoya University, Japan K.Yoshida Shibaura Institute of Technology, Japan K.Kasahara, Y.Shimizu, T.Suzuki, S.Torii Waseda University, Japan T.Tamura Kanagawa University, Japan M.Haguenauer Ecole Polytechnique, France W.C.Turner LBNL, Berkeley, USA O.Adriani, L.Bonechi, M.Bongi, R.D’Alessandro, M.Grandi, P.Papini, S.Ricciarini, G.Castellini INFN, Univ. di Firenze, Italy K.Noda, A.Tricomi INFN, Univ. di Catania, Italy J.Velasco, A.Faus IFIC, Centro Mixto CSIC-UVEG, Spain D.Macina, A-L.Perrot CERN, Switzerland

Detector Location ATLAS 96mm LHCf Detector(Arm#1) Two independent detectors at either side of IP1 ( Arm#1, Arm#2 ) 96mm Charged particles (+) Neutral particles Beam pipe Protons Charged particles (-) TAN -Neutral Particle Absorber- transition from one common beam pipe to two pipes 　　Slot : 100mm(w) x 607mm(H) x 1000mm(T)

ATLAS & LHCf

LHCf Detectors Imaging sampling shower calorimeters Two independent calorimeters in each detector (Tungsten 44r.l., 1.6λ, sample with plastic scintillators) Arm#1 Detector 20mmx20mm+40mmx40mm 4 XY SciFi+MAPMT Arm#2 Detector 25mmx25mm+32mmx32mm 4 XY Silicon strip detectors

Calorimeters viewed from IP 0 crossing angle 100urad crossing angle η ∞ 8.7 θ [μrad] 310 8.5 Projected edge of beam pipe Geometrical acceptance of Arm1 and Arm2 Crossing angle operation enhances the acceptance

LHCf as EM shower calorimeter EM shower is well contained longitudinally Lateral leakage-out is not negligible Simple correction using incident position Identification of multi-shower event using position detectors

Front Counter Fixed scintillation counter L=CxRFC ; conversion coefficient calibrated during VdM scans

3. Single photon spectra at LHC 7TeV pp collisions

Data Set for this analysis Date : 15 May 2010 17:45-21:23 (Fill Number : 1104) except runs during the luminosity scan. Luminosity : (6.3-6.5)x1028cm-2s-1 (not too high for pile-up, not too low for beam-gas BG) DAQ Live Time : 85.7% for Arm1, 67.0% for Arm2 Integral Luminosity (livetime corrected): 0.68 nb-1 for Arm1, 0.53nb-1 for Arm2 Number of triggers : 2,916,496 events for Arm1 3,072,691 events for Arm2 With Normal Detector Position and Normal Gain MC About 107 pp inelastic collisions with each hadron interaction model, QGSJET II-03, DPMJET 3.04, SYBILL 2.1, EPOS 1.99 and PYTHIA8.145 Only PYTHIA has tuning parameters. The default parameters were used

Event Sample (π0 candidate) Event sample in Arm2 Longitudinal development Note : A Pi0 candidate event 599GeV gamma-ray and 419GeV gamma-ray in 25mm and 32mm tower respectively. Lateral development

Analysis Step.1 : Energy reconstruction Step.2 : Single-hit selection Step.3 : PID (EM shower selection) Step.4 : π0 reconstruction and energy scale Step.5 : Spectra reconstruction

Analysis Step.1 Energy reconstruction : Ephoton = f(Σ(dEi)) (i=2,3,…,13) ( dEi = AQi determined at SPS. f() determined by MC. E : EM equivalent energy) Impact position from lateral distribution Position dependent corrections Light collection non-uniformity Shower leakage-out Shower leakage-in (in case of two calorimeter event) Light collection nonuniformity Shower leakage-out Shower leakage-in

Analysis Step.2 Single event selection Single-hit detection efficiency Multi-hit identification efficiency (using superimposed single photon-like events) Effect of multi-hit ‘cut’ (next slide) Small tower Large tower Arm1 Double hit in a single calorimeter Arm2 Single hit detection efficiency Double hit detection efficiency

Uncertainty in Step.2 Fraction of multi-hit and Δεmulti, data-MC Effect of multi-hit ‘cut’ : difference between Arm1 and Arm2 Effect of Δεmulti to single photon spectra Single / (single+multi), Arm1 vs Arm2

Analysis Step.3 PID (EM shower selection) Select events <L90% threshold and multiply P/ε ε (photon detection efficiency) and P (photon purity) By normalizing MC template L90% to data, ε and P for certain L90% threshold are determined.

Uncertainty in Step.3 Imperfection in L90% distribution Template fitting A Template fitting B Original method ε/P from two methods (Small tower, single & gamma-like) Artificial modification in peak position　(<0.7 r.l.) and width (<20%) (ε/P)B/ (ε/P)A

Analysis Step.4 π0 identification from two tower events to check absolute energy Mass shift observed both in Arm1 (+7.8%) and Arm2 (+3.7%) No energy scaling applied, but assigned the shifts in the systematic error in energy Arm2 Measurement Arm2 MC I.P.1  1(E1) 2(E2) 140m R M = θ√(E1xE2)

Analysis Step.5 Spectra in Arm1, Arm2 common rapidity Enegy scale error not included in plot (maybe correlated) Nine = σine ∫Ldt (σine = 71.5mb assumed)

Combined spectra Weighted average of Arm1 and Arm2 according to the errors

Spectral deformation Suppression due to multi-hit cut at medium energy Overestimate due to multi-hit detection inefficiency at high energy (mis-identify multi photons as single) No correction applied, but same bias included in MC to be compared TRUE/MEASURED TRUE MEASURED True: photon energy spectrum at the entrance of calorimeter

Beam Related Effects Pile-up (7% pileup at collision) Beam-gas BG Beam pipe BG Beam position (next slide) MC w/ pileup vs w/o pileup Crossing vs non-crossing bunches Direct vs beam-pipe photons

Where is zero degree? Beam center LHCf vs BPMSW LHCf online hit-map monitor Effect of 1mm shift in the final spectrum

Comparison with Models

Comparison with Models DPMJET 3.04 QGSJET II-03 SIBYLL 2.1 EPOS 1.99 PYTHIA 8.145

DPMJET 3.04 QGSJET II-03 SIBYLL 2.1 EPOS 1.99 PYTHIA 8.145 None of the models perfectly agree with data. QGSJET II, DPMJET3, PYTHIA8: good agreement in 0.5-1.5TeV at η>10.94 but large difference >2TeV. SIBYLL2 shows good spectral shape >0.5TeV at η>10.94 but only half yield Less deviation at 8.81<η<8.99 but still big difference >2TeV in DPMJET3 and PYTHIA8

4. Impact on the CR physics

π0 spectrum and air shower QGSJET II original Artificial modification Ignoring X>0.1 meson X=E/E0 π0 spectrum at Elab = 1019eV Longitudinal AS development Artificial modification of meson spectra and its effect to air shower Importance of E/E0>0.1 mesons Is this modification reasonable? 30g/cm2

Model uncertainty at LHC energy Very similar!? Forward concentration of x>0.1 π0 π0 energy at √s = 7TeV On going works Air shower simulations with modified π0 spectra at LHC energy Try&Error to find artificial π0 spectra to explain LHCf photon measurements Analysis of π0 events

5. Next Plan Analysis Experiment Energy scale problem to be improved Correction for multi-hit cut / reconstruction for multi-hit event π0 spectrum Hadron 900GeV PT dependence Experiment 14TeV pp collisions pA, AA collisions (only ideas)

14TeV: Not only highest energy, but energy dependence… SIBYLL 7 TeV 10 TeV 14 TeV QGSJET2 7 TeV 10 TeV 14 TeV　(1017eV@lab.) Note: LHCf detector taken into account (biased) Secondary gamma-ray spectra in p-p collisions at different collision energies (normalized to the maximum energy) SIBYLL predicts perfect scaling while QGSJET2 predicts softening at higher energy Qualitatively consistent with Xmax prediction

LHC-COSMIC ? p-Pb relevant to CR physics? CR-Air interaction is not p-p, but A1-A2 (A1:p, He,…,Fe, A2:N,O) Total Neutron Photon LHC Nitrogen-Nitrogen collisions Top: energy flow at 140m from IP Left : photon energy spectra at 0 degree

6. Summary LHCf has measured photon spectra at η>8.8 during LHC 7TeV p-p collisions. Measured spectra are compared with the prediction from various models. None of the models perfectly agree with data Large suppression in data at >2TeV w.r.t. to DPM3, QGS-II, PYTHIA predictions Study on the effect of LHCf measurements to the CR air shower is on-going Further analysis and preparation for next observations are on-going

Backup

CR Acceleration limit

Telescopes to image the fluorescence light (FD) Surface Detectors (SD) to sample particles on ground

Key measurements E0 EM shower E leading baryon Forward spectra (Multiplicity) Cross section Elasticity / inelasticity

Nagoya University LHCf Arm2 LHCf Arm1 ATLAS ALICE LHCb/MoEDAL CMS/TOTEM

Detectors are installed in TAN attached to the vertical manipulators Neutral particles (predominantly photons, neutrons) enter in the LHCf calorimeters Neutral particles

Luminosity Estimation Luminosity for the analysis is calculated from Front Counter rates: The conversion factor CF is estimated from luminosity measured during Van der Meer scan VDM scan Beam sizes sx and sy measured directly by LHCf BCNWG paper https://lpc-afs.web.cern.ch/lpc-afs/tmp/note1_v4_lines.pdf

Operation 2009-2010 With Stable Beam at √s = 900 GeV Total of 42 hours for physics About 105 showers events in Arm1+Arm2 With Stable Beam at √s = 7 TeV Total of 150 hours for physics with different setups Different vertical position to increase the accessible kinematical range Runs with or without beam crossing angle ~ 4·108 shower events in Arm1+Arm2 ~ 106 p0 events in Arm1 and Arm2 Status Completed program for 900 GeV and 7 TeV Removed detectors from tunnel in July 2010 Post-calibration beam test in October 2010 Upgrade to more rad-hard detectors to operate at 14TeV in 2014

Beam test at SPS p,e-,mu σ=172μm σ=40μm Detector Energy Resolution for electrons with 20mm cal. - Electrons 50GeV/c – 200GeV/c Muons 150GeV/c Protons 150GeV/c, 350GeV/c Position Resolution (Silicon) Position Resolution (Scifi) The detector performance was checked by beam tests at SPS. We had exposed the detectors in electron, muon and protons beams with a few hundreds GeV. This is energy resolution of one of the calorimeters as a function of gamma-ray energy. It is in a very good agreement with simulation results and good resolution of 5% for more than 100GeV. And bottom figures show position resolution of scintillating fibers of Arm1 and silicon detectors of Arm2 for 200GeV electrons. σ=172μm for 200GeV electrons σ=40μm for 200GeV electrons

Effect of mass shift Energy rescaling NOT applied but included in energy error Minv = θ √(E1 x E2) (ΔE/E)calib = 3.5% Δθ/θ = 1% (ΔE/E)leak-in = 2% => ΔM/M = 4.2% ; not sufficient for Arm1 (+7.8%) 145.8MeV (Arm1 observed) 135MeV ±3.5% Gaussian probability ±7.8% flat probability Quadratic sum of two errors is given as energy error (to allow both 135MeV and observed mass peak)

π0 mass shift in study Reanalysis of SPS calibration data in 2007 and 2010 (post LHC) <200GeV Reevaluation of systematic errors Reevaluation of EM shower using different MC codes (EPICS, FLUKA, GEANT4) Cable attenuation recalibration(1-2% improve expected) Re-check all 1-2% effects…

Summary of systematic errors