Sako, T

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29th International Cosmic Ray Conference Pune (2005) 00, 189192LHCf : A new experiment to study very forward particle emission atLHCT. Sako , Y. Muraki , Y. Itow , K. Masuda , Y. Matsubara , H. Menjo , Y. Obata , K.Tanaka , H. Matsumoto , K. Kasahara, S. Torii, T. Tamura, K. Yoshida, M. Haguenauer,W. C. Turner, O. Adriani  , L. Bonechi  , M. Bongi  , J. Velascoand A. Faus(a) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan(b) Shibaura Institute of Technology, Japan(c) Waseda University, Japan(d) Kanagawa University, Japan(e) Ecole-Polytechnique, France(f) LBNL, Berkeley, USA(g) INFN, Univ. di Firenze, Italy(h) IFIC, Centro Mixto CSIC-UVEG, SpainPresenter: T. Sako (sako@stelab.nagoya-u.ac.jp), jap-sako-T-abs2-he15-oralA new accelerator experiment to study very forward particle emission in the LHC collider, LHCf, is presented.Because a collision of 7 TeV protons corresponds to 	 eV in the laboratory system, our experiment cangive a firm base for simulating air showers generated from ultra high energy cosmic rays. This is indispensableinformation for the ongoing large experiments like TA, Auger and EUSO for the near future. The LHCfdetector is composed of three tower electromagnetic calorimeters and scintillating fibers (SciFi) to measurecore position of the shower. To avoid multi particle hits, each tower has a small cross section (2cm  2cm,3cm  3cm and 4cm  4cm). We have confirmed that we can measure shower energy with  5% resolution evenusing such small calorimeters. A project overview and the results of a beam test carried out in August 2004 atCERN SPS are presented.1. IntroductionDebate over the existence of super GZK cosmic rays has motivated a new set of modern experiments likeTA, Auger and EUSO [1]. All of these experiments are designed to overcome the most crucial issue, namelystatistics. Of course, large efforts are also expended to understand systematic effects of the detectors and theatmosphere in case of fluorescence observations. These experiments are believed to be capable of solving theGZK problem. However, one more important item, the particle interaction cross sections at this energy scale,must be clarified experimentally. To deduce the primary particle energy from air shower observations, MonteCarlo calculations must be carried out assuming a specific interaction model. As summarized by Knapp etal.(2003)[2], different interaction models give different results not only for the GZK problem but also for thechemical composition of cosmic rays in the 10  eV energy range.Models can be studied using accelerators, especially colliders to approach energies as high as possible in thelaboratory system. Because the development of an air shower is dominated by very forward ( 	 	 ; here is the Feynman  ) emitted particles, special detectors must be located near the colliding beam lines. So far,only one such experiment has been carried out by the UA7 collaboration at the laboratory equivalent energyof ﬀ	 ﬂﬁ eV [3]. LHCf is a new such experiment to use Large Hadron Collider (LHC), where collisions of7 TeV protons occur, corresponding to ﬃ	  eV in the laboratory system.In this paper, we present the overview of the LHCf experiment together with the test beam results for theprototype detector carried out in August 2004 at CERN SPS.190 T. Sako et alFigure 1. A schematic view of the LHCf calorimeters. We use 3 different sizes of calorimeter tower cross section. Eachtower consists of tungsten plates (shown as black in the gure), plastic scintillator (gray) and SciFi (also gray but extendedout, 3 pairs for each tower).2. LHCfThe goal of LHCf is to measure the  distribution (energy distribution in practice, where  is the Feynmanparameter) in the very forward region of 7 TeV proton-proton collisions in the LHC. Because the LHC beampipe is separated from one beam pipe into two with Y shape at 140 m from the interaction point, we canplace a detector in the crotch of the Y and observe particle production near zero degrees[4][5]. Because theY chamber is well outside the first beam separation dipole D1, only neutral particle production is observed.The gap between the separated beam pipes of the Y chamber is 96 mm, so our detector must fit in that limitedspace. The detector is composed of 3 towers of sampling shower calorimeters (Figure 1). Each calorimeterconsists of 54 radiation lengths of tungsten plates and 17 plastic scintillators of 3 mm thickness for sampling.The dimensions of each tower are "!$#%&"!$# , '(!)#*+'!)# and ,-!)#%+,!$# in cross section and 25 cm inlongitude. Separation into 3 small towers is chosen to reduce the probability of multi particle hits that wouldprevent us from obtaining the correct particle energy. At the same time, we can reconstruct the invariant massof .0/ when the decay gamma-rays enter different towers. This enables us to obtain a good energy calibration ofthe detector. To measure the shower positions and to identify multi hit events, we also install scintillating fibers(SciFi) in the calorimeters. The SciFi signals are read out by multi-anode PMTs (MAPMT H7546) while thescintillator signals are read by PMT H3164-10 through clear fibers. For the read out of MAPMT, a speciallydesigned ASIC is applied [6].LHCf 191One may question if we can really measure the shower energy correctly using such small calorimeters, es-pecially with "!$#%&"!$# . Actually, there is some fraction of leakage of shower particles out the side of thecalorimeters. However in Monte Carlo and test beam measurements at the SPS, we found that this fraction isenergy independent and a function of the shower position from the edge of a tower. SciFi is used to determinethe shower position. We first predicted the fraction of shower leakage by using Monte Carlo calculations andverified these calculations experimentally with the prototype test at the CERN SPS in August 2004.3. Prototype Test in 2004Technical questions for LHCf were; 1) can we construct such a compact calorimeter in a limited volume?and 2) can we really measure the shower energy with such a small calorimeter? To clarify these points, weconstructed a prototype LHCf detector and carried out a beam test at the CERN SPS from 26 July to 11 August2004. The detector for these tests consisted of "!)#*1(!)# and ,!)#*2,-!)# towers with the '(!$#%1'(!)# absent.For both towers, SciFi were inserted in 3 places and the shower position was measured. To calibrate the SciFipositioning, we also placed a Silicon tracker in front of the detector and measured the position of each primaryparticle. In the test experiment, we irradiated the calorimeter by muons of 150 GeV/c, electrons of 50, 100,150, 200, 250 GeV/c and protons of 150, 300, 350 GeV/c.In Figure 2, analysis results for electron beam are presented. For panels a) b) and c), the results for 200GeV/celectron are shown. The panel a) shows total number of shower particles for each shower as a function of theDistance from edge (cm)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Number of particles010002000300040005000600070008000900010000a)200GeV/c electron, 2cmDistance from edge (cm)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Number of particles010002000300040005000600070008000900010000b)200GeV/c electron, 2cmNumber of particles4000 5000 6000 7000 8000 9000 1000011000Number of event0100020003000400050006000700080009000c)Solid Line:ExperimentDashed Line:Monte CarloSum NeElectron Energy (GeV/c)0 50 100 150 200 250 300Energy Resolution(%)012345678910 d)Experiment (2cm)Experiment (4cm)Solid Line: Monte Carlo (2cm)Dashed Line: Monte Carlo (4cm)ResolutionFigure 2. a) b) c) Analysis results for 200 GeV/c electron beam irradiated on the 2cm tower. a) Distance from edge versustotal number of particles measured in each shower. b) Leakage corrected plot of a). Correction is done based on a MCcalculation. c) Distribution of number of particles found in the marked area in b) 0.2 cm to 1.0 cm from the edge. d) Energyresolution as functions of the electron energy. See text for more detail.192 T. Sako et aldistance from the edge. As stated above, we can see the effect of leakage when the distance is small. However,the fraction of leakage can be predicted by Monte Carlo calculations as a function of distance from the edgebut independent of shower energy. The panel b) shows the leakage corrected plot of a). The correction appearsto work well. Because the scattering is large for the events near the edge, we decided to use events that arefound to have their shower axes more than 0.2 cm from the edge. This region is indicated by a double arrowin panel b). The distribution of the number of particles after the distance cut is plotted in the panel c) togetherwith the Monte Carlo result. Fitting this distribution with a Gaussian function and 3 of the fit is defined asan energy resolution at that energy. Energy resolutions for all the primary electron energies, experiment andMonte Carlo, 2cm and 4cm towers are plotted in the panel d).As expected from the panel c), we can find good agreement between the experiment and Monte Carlo in thecase of the 2cm tower. In this sense, our prediction for the leakage correction in the small calorimeter isjustified. However, there is a discrepancy between the Monte Carlo and the experiment for the 4cm tower. Wespeculate this is because of the non uniform read out of the scintillation light by the clear fibers. The effect ofthe non uniformity becomes larger for the larger scintillators. Although we corrected for this effect using muondata, the correction may be insufficient and the lateral spread of the shower particles may need to be taken intoaccount. We are trying to understand the cause of this discrepancy and how to correct for it.4. SummaryA new experiment, LHCf, has started. This experiment is motivated by the need to experimentally justify theinteraction model used to estimate the primary energy of ultra high energy cosmic rays. LHCf is planned torun during the first stage of LHC operation in 2007. The concept of the experiment is already approved by theLHC Committee (LHCC) in 2004 and the first technical hurdles have been cleared by the prototype experimentcarried out in 2004. We are now trying to understand the systematic effects found in the 2004 test in detailand improve the detector for the final setup. At the same time, we are investigating the possibility of installinganother detector on the opposite side of the interaction point. The second detector would use silicon detectorsin place of the scintillator. Monte Carlo calculations are also being developed and the expected LHCf resultsfor different interaction models are being calculated.We are grateful to the CERN staffs for the support of the beam test in 2004.References[1] A. V. Olinto, Frontiers of Cosmic Ray Science, 299 (2003).[2] J. Knapp et al., Astroparticle Phys., 19, 77 (2003).[3] E. Pare´ et al., Phys. Lett., B242, 531 (1990).[4] W. C. Turner, E. H. Hoyer and N. V. Mokhov, Proc. of 1998 European Particle Accelerator Conference,Stockholm.[5] D. Li, G. Lambertson and W. C. Turner, Proc. of 2001 IEEE Particle Accelerator Conference, Chicago[6] T Tamura et al., 28th ICRC, Tsukuba (2003) 4, 2189.

LHCf: A new experiment to study very forward particle emission at LHC

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