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one of the most important events expected in particle physics in near future is unquestionably the discovery of the higgs boson . the data from the atlas and the cms experiments at the lhc and the dzero and the cdf experiments at the tevatron hint at the existence of a light standard - model - like higgs boson in the mass range of 115 - 130 gev/@xmath1 @xcite . if it is indeed the case , the discovery is expected to be declared within a year or so by the lhc experiments .    in the standard model , the higgs boson is responsible for giving masses to both gauge bosons and matter fermions , via the gauge and yukawa interactions , respectively , upon the spontaneous breaking of the electroweak symmetry . however , unlike the gauge interaction , the mechanism of the spontaneous symmetry breaking and the yukawa interaction have been left untested . as a matter of fact , a higgs doublet with its wine bottle potential , and its yukawa coupling to each matter fermion in the standard model are mere assumptions other than being the minimal mechanism to generate the masses of gauge bosons and fermions . in other words , we know essentially nothing but something must be condensed in the vacuum to give the masses of gauge bosons and fermions . it is well known that the standard model can not describe everything in the universe . an example is the existence of the dark matter which occupies about one - fourth of the energy density in the universe . the non - existence of anti - matter is another example . since the gauge sector of the standard model is well tested , it would be natural to expect that some hints of physics beyond the standard model could be obtained via precise measurements of the higgs boson properties . the lhc experiments are likely to discover the standard - model - like higgs boson . however , their precision is most probably not enough to reveal details of the discovered particle(s ) due to high background environments of proton - proton collisions . thus , precise measurements of the higgs boson properties by an electron - positron collider and its possible options are crucial to uncover its detailed properties which might go beyond the standard model . the international linear collider ( ilc ) has potential to study the properties of the higgs boson(s ) such as coupling strengths to gauge bosons and matter fermions including the top quark with high precision , thereby opening up a window to physics beyond the standard model @xcite . in addition to the @xmath5 collisions , high energy photon - photon collisions are possible at the ilc by converting the electron beam to a photon beam by the inverse compton scattering @xcite . physics and technical aspects of a photon linear collider ( plc ) as an option of the @xmath5 linear collider are described , for instance , in ref . @xcite . a schematic of the plc is shown in fig . the higgs boson properties such as its two - photon decay width and cp properties can be studied in high energy photon - photon interaction and thus the plc plays complementary role to the @xmath5 linear collider . it should also be emphasized that the higgs boson can be singly produced in the @xmath6-channel process so that the required electron beam energy is significantly lower than that for the @xmath5 linear collider . one of the most important observables to be measured in the higgs sector is its self - coupling , since it directly relates to the dynamics of the higgs potential , _ i.e. _ the mechanism of the spontaneous symmetry breaking . for example , a non - standard large deviation in the self - coupling can be direct evidence for strong first - order phase transition of the electroweak symmetry in the early universe @xcite . [ aahheezhh ] ( a ) and ( b ) show diagrams of processes which involve the self - coupling in @xmath0 and @xmath5 interactions . recently , a prospect for studying the self - coupling at the ilc was reported . according to the study , the self - coupling is expected to be measured with precision of 57 % with an integrated luminosity of 2 ab@xmath7 at @xmath8 gev @xcite .     and ( b ) : for @xmath9 . higgs boson self - coupling occurs at red points . ]    measurements of the self - coupling at the plc were discussed by several authors @xcite . it has been pointed out that contributions to the self - coupling to the cross - section of fig . [ aahheezhh ] ( a ) and ( b ) are different and measurements in @xmath5 and @xmath0 interactions are complementary from physics point of view . in addition , as a higgs boson pair is directly produced in the @xmath0 interaction , required beam energy is lower , 190 gev as described later , than that for the @xmath5 interaction . this nature is important when considering energy update scenarios of the ilc .    in ref . @xcite , an order - of - magnitude estimation for background processes was presented . however , the cross - section for the @xmath10 boson pair production is @xmath11 orders of higher than that for the higgs boson pair production . the backgrounds from @xmath3 , @xmath12 , and @xmath13 production processes are also large and have the same final state as with the higgs pairs for a low mass higgs boson , which predominantly decays into @xmath14 . given the situation , people had been skeptical about the feasibility of the detection of the higgs pair process at the plc .    in this work , we studied , for the first time , the higgs boson pair creation at the plc extensively with a parameter set of the plc based on an @xmath5 linear collider optimized for the light higgs boson of 120 gev/@xmath1 and the same detector simulation framework as used for the ilc physics analysis . we report details of the analysis , issues and prospects for the measurement of the higgs boson pairs at the plc . in order to choose parameters for the plc , we calculated the statistical sensitivity @xmath15 defined as @xmath16 where , @xmath17 is the deviation of the self - coupling constant from the standard model . the constant of higgs self - coupling @xmath18 can be expressed as @xmath19 , where @xmath20 is the higgs self - coupling constant in the standard model . @xmath21 and @xmath22 are the expected number of events as a function of @xmath17 and that expected from the standard model . @xmath23 and @xmath24 are the cross - section of the higgs boson production as a function of @xmath17 and that of the standard model , while @xmath25 , @xmath26 , @xmath27 , and @xmath28 are the integrated luminosity , the detection efficiency for the signal , the detection efficiency for backgrounds , and the cross - section of background processes , respectively . for @xmath29 and @xmath30 , @xmath15 is written as @xmath31 fig . [ sensitivity ] plots @xmath15 as a function of the center of mass energy of the @xmath0 collision ( denoted @xmath32 hereafter ) for the higgs boson mass of 120 gev@xmath33 with the @xmath0 integrated luminosity of 1000 fb@xmath7 . the cross - section of the signal was calculated according to the formula described in ref . @xcite for the case of @xmath34 and @xmath35 as indicated in fig . [ sensitivity ] . from this result , we found the optimum energy to be @xmath36 270 gev .    ) as a function of @xmath0 collision energy . black and red dots show the @xmath34 and @xmath35 cases . ] the parameters for the electron and the laser beams are summarized in table [ cain ] . it was designed to maximize @xmath0 luminosity at @xmath37 gev based on the tesla optimistic parameters @xcite . the wavelength of the laser was chosen to be 1054 nm , which is a typical wavelength for solid state lasers . the electron beam energy was chosen to maximize @xmath0 luminosity around 270 gev , while keeping the electron beam emittance and the @xmath38 functions at the interaction point the same as the tesla parameters . the luminosity distribution was simulated by cain @xcite , as shown in fig . [ luminosity ] . the @xmath0 luminosity in the high energy region ( @xmath39 ) was calculated to be @xmath40 . .the parameters of electron and laser beams based on tesla optimistic parameters . the polarization of electron beam was assumed to be 100 % . [ cols="^,^,^",options="header " , ]     [ cutsummary_cheat ] we studied the feasibility of the measurement of higgs pair creation at the plc , which is a possible option of the ilc . the optimum center of mass energy of the @xmath0 collision was found to be around 270 gev for the higgs boson with a mass of 120 gev/@xmath1 . we found that the @xmath41 process can be observed with a statistical significance of about @xmath42 for the integrated luminosity corresponding to 5 years of the plc running against the background process which has @xmath11 times larger production cross - section ( @xmath43 ) than the signal and other backgrounds which have the same final state ( @xmath44 , and @xmath45 ) , if each track could be successfully assigned to parent particles ( or partons ) . our analysis showed , for the light higgs boson , improvement of the jet clustering technique is crucial to discriminate the backgrounds by invariant mass information rather than to improve the @xmath46-quark tagging efficiency . this fact is reasonable , because the @xmath47 background turned out to be suppressed by a simple @xmath46-quark tagging scheme since the @xmath10 bosons do not decay into @xmath46-quark pairs , while the @xmath13 and @xmath3 backgrounds can only be suppressed by their mass differences .    for further improvements , vertex information from @xmath46-tagging analysis must be taken into account in jet clustering , thus they should be coherently developed . efforts in this direction are on - going as a part of the ilc physics study @xcite and significant improvement could be expected in near future .      the authors would like to thank ilc physics working group @xcite for useful discussions , especially y. okada for useful theoretical inputs . this work is supported in part by the creative scientific research grant no . 18gs0202 of the japan society for promotions of science ( jsps ) , the jsps core university program , and the jsps grant - in - aid for science research no . 22244031 , and the jsps specially promoted research no . 23000002 . 99 the atlas collaboration , arxiv:1202.1408v1 [ hep - ex ] ( 2012 ) the cms collaboration , arxiv:1202.1488v1 [ hep - ex ] ( 2012 ) http://www.fnal.gov/pub/presspass/press_releases/2012/higgs-boson-20120307.html a. djouadi , j. lykken , k. mnig , y. okada , m. oreglia , s. yamashita _ et al . _ , `` _ ilc reference design report volume 2  physics at the ilc _ '' ( 2007 ) i. ginzburg , g. kotoki , v. serbo , and v. telnov , pizma ahetf * 34 * 514 ( 1981 ) , jetp lett . * 34 * 491 ( 1982 ) e.boos , a. de roeck , i. f. ginzburg , k. hagiwara , r. d. heuer , g. jikia , j. kwiecinski , d. j. miller , t. takahashi , v. i. telnov , t. rizzo , i. watanabe , p. m. zerwas , arxiv : hep - ph/0103090v1 s. kanemura , y. okada , e. senaha , phys . b * 606 * 361 ( 2005 ) j. tian , slide at the 23rd general meeting at kek ( 2011 ) r. belusevic , g. jikia , phys . d * 70 * 073017 ( 2004 ) e. asakawa , d. harada , s. kanemura , y. okada , k. tsumura , phys . b * 672 * 354 ( 2009 ) e. asakawa , d. harada , s. kanemura , y. okada , k. tsumura , phys . d * 82 * 115002 ( 2010 ) b.badelek _ et al_. , int . 30 , 5097 ( 2004 ) n. maeda , k. fujii , k. ikematsu , y. kurihara , t. takahashi , proceedings of the ilc physics working group meetings at kek , arxiv:1008.0907 [ hep - ph ] h. murayama , i. watanabe , k. hagiwara , kek report 91 - 11 ( 1992 ) g. j. gounaris , j. layssac , p. i. porfyriadis , f. m. renard , arxiv : hep - ph/9909243v3 t. diakonidis , g. j. gounaris , j. layssac , eur . phys . j. c * 50 * , 47 - 52 ( 2007 ) f. yuasa _ et al_. , prog . . suppl . * 138 * ( 2000 ) 18 [ arxiv : hep - ph/0007053 ] s. kawabata , comput . * 88 * ( 1995 ) 309 acfa linear collider working group , arxiv : hep - ph/0109166v1 http://www-jlc.kek.jp/subg/offl/jsf/ t. sstrand , s. mrenna , p. skands , arxiv : hep - ph/0603175 r. yonamine , k. ikematsu , t. tanabe , k. fujii , y. kiyo , y. sumino , h. yokoya , phys . d * 84 * 014033 ( 2011 ) k. monig , acta phys . b * 37 * , no . 4 , 1251 ( 2006 ) w. bartel _ et al_. , z. phys . c * 33*:23 ( 1986 ) c. peterson , t. rognvaldsson , l. lonnblad , comput . * 81 * ( 1994 ) 185 , we used a root - base version of jetnet . http://www.hep.uiuc.edu/home/catutza/root_ to _ jetnet / rtj.html[http://www.hep.uiuc.edu / home / catutza / root _ to _ jetnet / rtj.html ] v. barger , r. phillips , `` _ collider physics _ '' ( addison wesley , 1987 ) t. tanabe , t. suehara , s. yamashita , slide at international workshop on future linear colliders ( lcws11 ) ( 2011 ) t. suehara , t. tanabe , s. yamashita , j. tian , k. fujii , slide at international workshop on future linear colliders ( lcws11 ) ( 2011 )
we studied the feasibility of the measurement of higgs pair creation at a photon linear collider ( plc ) . from the sensitivity to the anomalous self - coupling of the higgs boson 
 , the optimum @xmath0 collision energy was found to be around 270 gev for a higgs mass of 120 gev/@xmath1 . 
 we found that large backgrounds such as @xmath2 and @xmath3 , can be suppressed if correct assignment of tracks to parent partons is achieved and higgs pair events can be observed with a statistical significance of @xmath4 by operating the plc for 5 years .