Current Status of the LHC Forward (Lhcf) Experiment

32ND INTERNATIONAL COSMIC RAY CONFERENCE,BEIJING 2011

Current status of the LHC forward (LHCf) experiment

T. SAKO1,2,O.ADRIANI3,4,L.BONECHI3,M.BONGI3,G.CASTELLINI3,4, R. D’ALESSANDRO3,4,K.FUKATSU1, M. HAGUENAUER5,T.ISO1,Y.ITOW1,2,K.KASAHARA6,K.KAWADE1,T.MASE1,K.MASUDA1,H.MENJO2, G. MITSUKA1,Y.MURAKI1,K.NODA8,P.PAPINI3, A.-L. PERROT7,S.RICCIARINI3,9,Y.SHIMIZU6,K.SUZUKI1, T. SUZUKI6,K.TAKI1,T.TAMURA10,S.TORII6,A.TRICOMI8,11,W.C.TURNER12 1Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan 2Kobayashi-Maskawa Institute for the Origin of Particles and the Universe Nagoya University, Nagoya, Japan 3INFN Section of Florence, Italy 4University of Florence, Italy 5Ecole-Polytechnique, Palaiseau, France 6RISE, Waseda University, Japan 7CERN, Switzerland 8INFN Section of Catania, Italy 9Centro Siciliano di Fisica Nucleare e Struttura della Materia, Catania, Italy 10Kanagawa University, Japan 11University of Catania, Italy 12LBNL, Berkeley, California, USA '[email protected] ,,&5&9

Abstract: The Large Hadron Collider forward (LHCf) experiment has successfully finished the first phase of data taking at LHC √s = 0.9 and 7 TeV proton-proton collisions in 2010. In this paper, the introduction to LHCf, its first results and future plans are presented.

Keywords: ultra-high-energy cosmic-rays, the Large Hadron Collider, hadron interaction

1 Introduction swept away by the dipole magnet located between IP and the LHCf detector. Figure 2 explains the acceptance in the According to the recent high precision observations from transverse momentum (PT ) as a function of particle energy Auger, HiRes, TA [1, 2, 3, 4, 5, 6, 7], the importance of re- where 310 μrad and 450 μrad correspond to the operations ducing uncertainty in hadron interaction models becomes under 0 and maximum beam crossing angles, respectively. more and more enhanced. To constrain the hadron interac- Each of the LHCf detectors has two compact calorime- tion models in terms of the cosmic-ray physics, the cross ters called ‘tower’ composed of 44 radiation lengths of sections for the very forward particles are indispensable. tungsten plates and 16 sampling layers of plastic scintil- LHCf is motivated to measure particles emitted around 0 lators (figure 3). Four X-Y pairs of position sensitive de- degree of LHC hadron collisions [8]. tectors, SciFi in Arm1 and silicon strip detector in Arm2, are inserted in the calorimeters to identify the incident position of the particle. The small transverse sizes of 2 LHCf Experiment towers (20 mm 20 mm and 40 mm 40 mm in Arm1 and × × 25 mm 25 mm and 32 mm 32 mm in Arm2) are chosen × × LHCf has two independent detectors called Arm1 and to reduce multi-hit in a single tower. The double tower Arm2. They were installed in either side of IP1 (interaction structure allows to identify photon pairs from the decay of point of the ATLAS experiment) as shown in figure 1. At π0 produced in p-p collision [9]. 140 m away from IP there is an installation slot of the detec- The responses of the detectors for high energy particles tors that allows to measure neutral particles emitted in the were tested in the fixed target experiments using electron pseudo-rapidity 1 (η) from 8.7 (8.4 under maximum beam crossing angle operation) to infinity. Charged particles are θ 1. pseudo-rapidity η = -ln(tan( 2 )) where θ is emission angle

Vol. 5, 59 SAKO,T.et al. LHCF STATUS

XF 0.1 1 1.4 450 µrad 1.2

[GeV/c] 1 310µrad gamma 0.8 T

P 0.6 0.4 0.2 0 100 1000 Egamma [GeV]

Figure 2: PT acceptance of LHCf as a function of particle energy. Particles below lines can be detected. The solid and dashed lines correspond to the LHC operations under zero and 140 μrad crossing angle, respectively. Scatter plots are distribution expected for photons in 14 TeV p-p collisions.

Arm1 Arm2 @ √s=0.9 TeV 47k 67k @ √s=7 TeV 172M 161M π0 @ √s=7 TeV 345k 676k Figure 3: Schematic view of the LHCf Arm1 (top) and Table 1: Number of particles detected in the LHCf Arm2 (bottom) detectors calorimeters during 2009-2010 operation

May, LHC again operated at 0.9 TeV for a short time. Until 19 July when LHCf stopped operation, LHC has integrated beams below 220 GeV, proton beams below 350 GeV and luminosity up to 350 nb 1 at IP1. The beam crossing angle muon beams below 150 GeV. The performance for elec- − was switched from zero to 100 μrad at the end of June. This tromagnetic showers are well understood using the Monte means the LHCf acceptance was increases from 8.7<η< Carlo simulations [10, 11]. A sub detector called Front ∞ to 8.5<η< at this time. The trigger condition of the Counter composed of two layers of thin plastic scintillators ∞ LHCf data acquisition was set to achieve almost 100% de- was also placed in front of the main calorimeters. A wide tection efficiency for more than 40 and 100 GeV photons physical aperture and low operation threshold allow FC to during 0.9 and 7 TeV collision energies, respectively. The have a high detection efficiency ( 30%) for a single inelas- ∼ total number of high energy particles detected in the LHCf tic collision at √s = 7 TeV. By means of this high efficiency calorimeters at two collision energies is summarized in ta- and constant operation conditions, FC was used to calculate ble 1. LHCf has collected more than 300M and 1M events the luminosity used in the LHCf analysis [12]. More details for single particles (mainly photons and neutrons) and can- of the detectors were reported elsewhere [13, 14]. didate π0, respectively. Figures 4, 5 show a π0 candidate event and invariant mass distribution observed by the Arm2 3 Current Status of LHCf detector.

LHCf has already ﬁnished data taking at 0.9 and 7 TeV p-p 3.2 Analysis collisions in 2010 and removed the detectors from the LHC tunnel. LHCf is capable of determining the spectra (differential cross sections) of neutral particles. The energy spectra of photons observed at 7 TeV collisions are already reported 3.1 Operation at LHC 1 using <1nb− of data [15, 16]. To understand the differ- LHCf has successfully started data taking when LHC ence between data and models in the photon spectra, analy- 0 started proton-proton collisions in December 2009 at sis of π events is now under going and the ﬁrst results are √s = 0.9 TeV. After a winter shutdown, LHC restarted presented in this conference [17]. The impact of photon 0 physics program in the end of March at √s = 7 TeV. In early and π measurements by LHCf on air shower simulation is under investigation. An early study indicates the Xmax

Vol. 5, 60 32ND INTERNATIONAL COSMIC RAY CONFERENCE,BEIJING 2011

TAN

LHCf-1 LHCf-2

TAN

Figure 1: Location of the LHCf detectors in LHC. The structure at the center is the ATLAS experiment. Two LHCf detectors (LHCf1 and LHCf2) are installed in the TAN located 140 m from the interaction point (center of ATLAS).

Small Tower Large Tower 2500 2500

2000 2000

dE [MeV] 1500 dE [MeV] 1500

1000 1000

500 500

0 0 0 2 4 6 8 10121416 0 2 4 6 8 10 12 14 16 Layer Layer

X-View 1st Layer 1000 2st Layer 3rd Layer 500 4th Layer ADC counts

0 0 50 100 150 200 250 300 350 Strip Number Y-View 1st Layer 1000 2st Layer 3rd Layer 500 4th Layer ADC counts

0 0 50 100 150 200 250 300 350 Strip Number Figure 5: Invariant mass spectrum of photon pairs observed Figure 4: An example of the π0 candidate events observed in the Arm2 detector. Clear peak at 135 MeV corresponds 0 in the LHCf Arm2 detector. Top two figures show the lon- to events associating π ’s produced in the collisions and gitudinal developments of the two photon initiated showers immediately decayed into photon pair. observed in the 25 mm and the 32 mm calorimeters. In the middle and bottom panels, transverse X and Y profiles of 4 LHCf Future Plan the showers are shown. 4.1 Operation at 14 TeV pp collision analysis is sensitive to the meson energy spectrum [18]. Air shower simulations using artificially modified meson LHCf is assured to take data at LHC after the major in- energy spectrum within the LHCf experimental errors and crease of the collision energy up to 14 TeV planned in 2014 17 model variation are being studied. where laboratory frame energy corresponds to 10 eV. To Analysis of photon spectra for the 0.9 TeV collision is also take data at higher energy is important not only because it on going. Although the energy resolution is limited, anal- is the highest collision energy but also we can study the en- ysis for the hadron event (presumably neutrons) is also ex- ergy dependence of the hadron interaction. To extrapolate pected to provide an important information to constrain the the knowledge taken at lower energy to the UHECR en- hadron interaction models. ergy range, energy dependence obtained from the identical experiment is useful. In the 14 TeV operation, the radiation dose rate to the detectors becomes almost 10 times higher than that in 7 TeV col-

Vol. 5, 61 SAKO,T.et al. LHCF STATUS lisions. To continue operation as long as possible under a [9] H. Menjo et al., Astropart. Phys., 2011, 34, 513-520 minimum radiation damage, LHCf is preparing to upgrade [10] T. Sako, et al., NIM, 2007, A578, 146 the detectors by replacing the plastic scintillators with GSO [11] T.Mase, et al., Proc. 32nd Int.Cosmic Ray Conf., Bei- crystal scintillators [19, 20]. jing, 2011 [12] K.Taki, et al., Proc. 32nd Int.Cosmic Ray Conf., Bei- jing, 2011 4.2 Operation at pA, AA collisions [13] O. Adriani, et al., JINST, 2008, 3, S08006 Because the interaction between cosmic-rays and atmo- [14] O. Adriani, et al., JINST, 2010, 5, P01012 sphere is not p-p collision, data at p-A (ions) and/or A-A [15] O.Adriani, et al., arXiv:1104.5294, 2011 collisions will provide important information to understand [16] G.Mitsuka, et al., Proc. 32nd Int.Cosmic Ray Conf., the nuclear effects or direct calibration for the CR-air inter- Beijing, 2011 action. LHC has succeeded Pb-Pb collisions in 2010 and [17] H.Menjo, et al., Proc. 32nd Int.Cosmic Ray Conf., is planning p-Pb collisions from 2011. Though Pb is too Beijing, 2011 heavy to simulate cosmic-ray or atmosphere, extreme con- [18] K.Kasahara, et al., Proceedings of ‘the Symposium dition can give a crucial information to understand nuclear on the Recent Progress of Ultra-High Energy Cosmic- effect. LHCf is studying a feasibility to take data at p-Pb Ray Observation 2010,’ Nagoya, AIP in press, 2011 collisions in terms of cosmic-ray physics. Also for future [19] K.Kawade, et al., Proc. 32nd Int.Cosmic Ray Conf., possible proposal for light ion collisions (for example Ni- Beijing, 2011 trogen or Carbon) that directly simulate CR-air interaction, [20] T.Suzuki, et al., Proc. 32nd Int.Cosmic Ray Conf., LHCf is studying a feasibility of these collisions by means Beijing, 2011 of MC simulations.

5 Summary

LHCf succeeded the first phase data taking at LHC 0.9 and 7 TeV proton-proton collisions in 2009-2010. The first physics result was published for the photon spectra at 1 η>8.81 by using only less than 1 nb− data. Analysis for π0 is now on going and the first results will be presented in this conference. Further analysis such as neutrons, 0.9 TeV data, PT dependence will be also in our target. LHCf is also assured to take data at 14 TeV collisions. Up- grade of the detectors with radiation resistive GSO scintillators is on going. Combined with the 0.9 and 7 TeV data, energy dependence of hadron interaction can be tested and that is expected to improve the extrapolation of models to the cosmic-ray energy range. Additional possibility to take data at p-A and A-A collisions is also in study to infer the nuclear effect or directly simulate CR-air interaction. Using both existing data and future experiments, LHCf will provide crucial data to constrain hadron interaction models and improve the interpretation of UHECR observations.

References

[1] J. Abraham, et al., Phys. Rev. Lett., 2008, 101 061101 [2] P. Abreu, et al., Astropart. Phys., 2010, 34, 314-326 [3] J. Abraham, et al., PRL, 2010, 104, 091101 [4] R. Abbasi, et al., Astropart. Phys., 2009, 32,53-60 [5] R. Abbasi, et al., arXiv:1002.1444v1, 2010 [6] R. Abbasi, et al., arXiv:0910.418v1, 2009 [7] H. Sagawa, et al., Proceedings of ‘the Symposium on the Recent Progress of Ultra-High Energy Cosmic-Ray Observation 2010,’ Nagoya, AIP in press, 2011 [8] LHCf Technical Design Report, CERN-LHCC-2006- 004

Vol. 5, 62