Latest results of the LHCf experiment at LHC

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1 at LHC Uiversity of Cataia ad INFN Sezioe di Cataia, Cataia, Italy I 6 the LHCf experimet has fulfilled its origial goal of measurig the spectra of the eutral particles produced i the very forward directio at LHC at the highest eergy ever available. The mai purpose of these measuremets is ideed to provide the Cosmic Ray ad High Eergy Physics commuities with a missig uique set of iformatio for the improvemet of the hadroic iteractio models used to simulate air showers developmet produced i the iteractio of primary High Eergy Cosmic Rays (HECR) with the Earth atmosphere. The last data sets collected by the LHCf experimet have bee obtaied durig p+p collisios, at a eergy of TeV i the CM frame, ad p+pb collisio, at a eergy of the collidig ucleo pair of 5. TeV ad 8. TeV i the CM frame. A review of the mai results of LHCf ad of the recet ad o-goig activities are preseted. The Europea Physical Society Coferece o High Eergy Physics 5- July, 7 Veice Speaker. O behalf of the LHCf Collaboratio. c Copyright owed by the author(s) uder the terms of the Creative Commos Attributio-NoCommercial-NoDerivatives. Iteratioal Licese (CC BY-NC-ND.).

2 . Itroductio The origi ad properties of Ultra High Eergy Cosmic Rays is a log stadig questio i astroparticle Physics. Dedicated extesive air shower experimets are i place sice may years ad have strogly cotributed to our uderstadig of High ad Ultra High Eergy Cosmic (UHECR) Ray Physics. Recetly, i particular, the Pierre Auger Collaboratio [] ad the Telescope Array Collaboratio [], thaks to the excellet performace of their hybrid detector arrays, are providig us ew excitig observatios of UHECRs. Although these recet results have brought a deeper isight i primary cosmic ray properties, still they are largely affected by the poor kowledge of the uclear iteractios i the earth s atmosphere. I the aalyses of observed data, compariso with Mote Carlo simulatios of air showers is uavoidable, ad the results are sesitive to the choice of hadroic iteractio model used i the simulatio []. A calibratio of the eergy scale i the 5 7 ev eergy rage accessible to LHC provides crucial iput for a better iterpretatio of primary cosmic ray properties, i the regio betwee the kee ad the GZK cut-off. Thaks to the LHC Ru data, some iteractio models have bee updated, ad the dispersio foud i the predictios of air shower observables decreased. However there is still sigificat differece betwee the model predictios especially i the forward regio ad eve some aalyses coclude o-realistic iterpretatios []. The Large Hadro Collider forward (LHCf) experimet was desiged with the aim to provide a calibratio of the hadroic iteractio models i the whole eergy rage spaed by LHC by measurig the eutral forward particle produced i p-p as well as i p-io collisios. More tuigs of the models usig the forward particle productio are ecessary. I fact, the forward particles carry a large fractio of the collisio eergy ad are resposible to determie the shape of air showers [5]. The LHCf experimet has ideed take data i p-p collisios at s =.9,.76, 7 ad TeV ad i p-pb collisios at s = 5. TeV thus collectig a whole set of data of uvaluable importace for the tuig of the models i such a difficult regio.. The LHCf experimet The LHCf experimet is composed by two idepedet positio sesitive electromagetic calorimeters, located o both side of the ATLAS experimet, m away from the LHC-IP iteractio poit, iside the zero-degree eutral absorber (Target Neutral Absorber, TAN). Charged particles from the IP are swept away by the ier beam separatio dipole before reachig the TAN, so that oly photos maily from π decays, eutros ad eutral kaos reach the LHCf calorimeters. Each calorimeter (ARM ad ARM) has a double tower structure, with the smaller tower located at zero degree collisio agle, approximately coverig the regio with pseudo-rapidity η > ad the larger oe, approximately coverig the regio with 8. < η <. Four X-Y layers of positio sesitive detectors (scitillatig fibers i ARM, silico micro-strip detectors i ARM) provide measuremets of the trasverse profile of the showers. The two tower structure allows to recostruct the π decayig i two γs, hittig separately the two towers, hece providig a very precise absolute eergy calibratio of the detectors. I the rage E> GeV, the LHCf

3 detectors have eergy ad positio resolutios for electromagetic showers better tha 5% ad µm, respectively. A detailed descriptio of the LHCf experimetal set-up ad of the expected physics performaces ca be foud i Ref. [6].. Results at s= TeV pp collisios The LHCf experimet has collected data durig the proto-proto collisio periods at s=.9 TeV (9 ad ),.76 TeV (), 7 TeV () ad TeV (5). Photo spectra at pseudorapidity η larger tha 8.8 were published for the proto-proto collisio data take at s=.9 TeV [7] ad 7 TeV [8]. The results were compared with the predictios by several hadroic iteractio models, ad it was foud oe of the models ca perfectly match with the experimetal results. The iclusive eergy spectra of photos produced i p-p collisios at TeV for two pseudorapidity rages together with the predictios of DPMJET.6 [9], EPOS-LHC [], PYTHIA 8. [], QGSJETII- [] ad SIBYLL. [] hadroic iteractio models are show i Fig.. ] - dn/de [GeV /N ie MC/Data Data QGSJET II- EPOS-LHC DPMJET.6 SIBYLL. PYTHIA 8. LHCf s=tev photo η >.9, φ=8 - Ldt=.9b 5 6 ] - dn/de [GeV /N ie LHCf s=tev photo 8.8<η<8.99, φ= - Ldt=.9b 5 6 Figure : Iclusive photo eergy spectra for pseudorapidity regio η >.9 (left) ad 8.8 < η < 8.99 (right). Data are represeted by black poits while MC predictio from several models are represeted by coloured histograms. Gree shaded area represets statistical+systematic errors of data. Spectra are ormalized to the total umber of ielastic collisios. Bottom paels show the ratio of MC predictios to data. MC/Data The results at TeV cofirms what has bee already reported at lower eergies.the LHCf data lie betwee MC predictios but there is ot a uique model with a good agreemet i the whole eergy rage ad i both rapidity regios. I the pseudorapidity rage η >.9, QGSJET ad EPOS presets a good overall agreemet with experimetal data; SIBYLL predicts a lower yield of photos, eve if it features a shape similar to data; PYTHIA spectrum agrees with data util.5 TeV but become harder at higher eergies; DPMJET is geerally harder tha data. I the pseudorapidity rage 8.8 < η < 8.99, EPOS ad PYTHIA spectra agree with data util

4 TeV, while they become harder at higher eergies; SIBYLL has a good agreemet util TeV, the also it becomes harder tha data; QGSJET presets a lower yield of photos, while DPMJET geerally predict a harder spectrum tha experimetal data. A deeper study of the discrepacy has brought to the coclusio that the mai differeces betwee data ad models were attributable to a less-tha-complete uderstadig of the soft hadroic iteractios implemeted i the models as diffractive processes [, 5]. Productio cross sectios of eutros were also published usig the 7 TeV proto-proto collisio data [6]. The result coverig degree productio agle showed a characteristic peak at aroud.5 TeV, ad such high-eergy excess was oly qualitatively explaied by the QGSJET II- model. All the other models studied sigificatly uder-predict the LHCf result. At the off- degree oly the DPMJET model [9] well explaied the LHCf results. The LHCf results showed that the ratio of eergies carried by the eutros to the photos is largest i the experimetal data tha ay model predictios. Results at TeV have ot yet bee published, however the prelimiary spectra obtaied for the forward eutro productio usig the ARM detectors, show i Fig., seems to cofirm the same tred observed at 7 TeV, i particular, the peak at -degree for eergy 5.5 TeV i the data which is reproduced oly qualitatelively by the QGSJET II- model while all the other models studied sigificatly uder-predict the LHCf result. At the off- degree the agreemet betwee data ad models is also poor η > < η < < η < 8.99 LHCf p p s. = TeV QGSJET II EPOS LHC DPMJET.6 PYTHIA 8. SIBYLL. 5 6 Figure : Iclusive eutro eergy spectra of ARM detector for pseudorapidity regio η >.76 (left), 8.99 < η < 9. (ceter) ad 8.8 < η < 8.99 (right). Data are represeted by black poits while MC predictio from several models are represeted by coloured histograms. Grey shaded area represets statistical+systematic errors of data. Spectra are ormalized to the total umber of ielastic collisios. Bottom paels show the ratio of MC predictios to data. This measuremet is particularly importat sice it is expected to provide a importat hit for the problem of cosmic-ray muo excess reported by both Auger [7] ad TA [8]. This excess could be ideed iterpreted i terms of a uderestimatio of the baryo productio i the hadroic iteractio models used to derive the CR spectra [9].

5 . Summary ad Plas The LHCf experimet has successful take data at various collisio coditio at LHC thus providig a uique set of measuremets i the before-uexplored forward regio to costrait the particle productio relevat for the high eergy Cosmic Ray physics. The so far published photo, eutro ad π results are compared with the iteractio models providig uvaluable iput for the models tuig. The discrepacy foud betwee data ad models may related to the diffractive/odiffractive compoets ad may beefit of combied aalysis betwee LHCf ad ATLAS which will be the aim of future work. I additio, the comparisos betwee the data at differet collisio eergies reveal the scalig or scalig break of the partile spectra. The LHCf results idicate the scalig of photos ad π spectra at s=.76, 7 ad TeV, while the ISR, PHENIX ad LHCf idicate a scalig break of eutro spectra betwee GeV ad TeV. A ew experimet, RHICf, has hece bee prepared to test the scalig (or break) at s = 5 GeV at RHIC. The experimet uses oe of the LHCf detector, ARM, ad has take data at the ed of Jue. Results from RHICf will come i the ear future ad will brig further importat iformatio from Cosmic Ray physics. Refereces [] F. Feu [Pierre Auger Collaboratio], Proc. of 5th It. Cosmic Ray Cof., Bexo, Busa, Korea (7), C7-7-, p [] R. U. Abbasi et al. [Telescope Array Collaboratio], Astropart. Phys. 8 (6) [] Pierre Auger Collaboratio, Phys. Rev. D 9, 5 (). [] K.-H. Kampert ad M. Uger, Astropart. Phys. 5 () 66. [5] K. Akiba et al., J. Phys. G: Nucl. Part. Phys. (6). [6] O. Adriai et al., JINST (8) S86. [7] O. Adriai et al., Phys. Lett. B75 () 98. [8] O. Adriai et al., Phys. Lett. B7 () 8. [9] F.W. Bopp et al., Phys. Rev. C77 (8) 9. [] K. Werer et al., Nucl.Phys.Proc.Suppl (8) [] T. Sjöstad, et al., Comput. Phys. Comm. 78 (8) 85. [] S. Ostapcheko, Phys. Rev. D8 () 8. [] E.-J. Ah et al., Phys. Rev. D8 (9) 9. [] The ATLAS ad the LHCf Collaboratios, ATL-PHYS-PUB-5-8. [5] Q.D. Zhou, Y. Itow, H. Mejo ad T. Sako, Eur. Phys. J. C 77 (7). [6] O. Adriai et al., Phys. Lett. B 75 (5) 6. [7] P. Abreu et al. [The Pierre Auger Collaboratio], Proc. of th It. Cosmic Ray Cof., Beijig (), [arxiv:7.8 [astro-ph.he]]. [8] T. Abu-Zayyad, et al. [TA Collaboratio], Astrophys. J. 768 () L [9] T. Pierog ad K. Warer, Phys. Rev. Lett. (8) 7.