Current Status and Future Prospect of the LHC Experiment

Current Status and Future Prospect of the LHC Experiment

J. TanakaInternational Center for Elementary Particle Physics,the University of Tokyo, Tokyo 113-0033, JAPAN

We present results from the ATLAS and CMS experiments focusing on Higgs searches, wherethe discovery of a Higgs-like particle of around 126 GeV mass was announced on the 4th of July,2012 and various studies has been performed to understand the properties of this new particle. Inaddition the future plan of the LHC experiment is briefly summarized and the potential performanceon some of possible physics with the planned LHC is discussed. All the contents described in thispaper are based on results obtained until the 13th of February, 2013.

I. INTRODUCTION

The ATLAS and CMS experiments have achieved the excellent goal with the first 10 fb−1 at a center-of-mass energy

√s =7 TeV and 8 TeV. The discovery of the Standard Model (SM) Higgs boson [1–3] is one of

the primary goals of the Large Hadron Collider (LHC) [4] program at CERN to understand the mechanism ofelectroweak symmetry breaking and the origin of mass of elementary particles. Both ATLAS and CMS observeda Higgs-like particle at around 126 GeV with a significance of over 5σ [5, 6]. This discovery gives us a phaseshift in the Higgs physics from “search” to “measurement” and the precision measurements of the propertiesof this new particle gets more important to understand the SM Higgs sector and to look for hints of physicsbeyond the SM (BSM). On the other hand, there is no indication of BSM with 7 TeV and 8 TeV direct searches.In Supersymmetry (SUSY) searches, gluinos and squarks with masses below about 1.5 TeV were excluded fortypical SUSY models (e.g. mSUGRA) and natural SUSY scenarios were excluded up to stop (top squark) massof 500−600 GeV. Searches for other new particles and enhancements also excluded various models having themass scale of 1−3 TeV. In 2013 and 2014, LHC is stopped to go to the design energy (

√s =14 TeV) and physics

runs will be restarted in 2015. The energy upgrade is important to discover (heavy) BSM and more data isnecessary for the precise measurements of the Higgs-like particle properties and so on.

II. LHC AND ATLAS/CMS EXPERIMENTS

LHC was operated in proton-proton collisions at√s =7 TeV in 2010 and 2011 and 8 TeV in 2012. The

ATLAS and CMS detectors [7, 8] collected data of ∼6 fb−1 with√s =7 TeV and ∼23 fb−1 with 8 TeV as shown

in Fig. 1. In this paper, results with 7 TeV data taken in 2011 and/or a part of 8 TeV data (up to ∼13 fb−1)are presented.

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III. SEARCH FOR STANDARD MODEL HIGGS

The production cross sections and decay branching ratios for several SM Higgs channels are shown in Fig. 2 [9].At around 126 GeV, where a Higgs-like particle was observed, five dominant decay channels can be investigatedwith reasonable data statistics (up to a few 10 fb−1) and give us measurements of the properties of this newparticle, for example, (relative) couplings to gauge bosons and fermions etc. We briefly summarize analysisand results from both ATLAS and CMS for γγ, ZZ(∗) → `+`−`′+`′−, WW (∗) → `+ν`′−ν̄, bb̄ and ττ channels.Details of analysis are described in Refs cited in each section.

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A. H → γγ channel

The branching ratio of H → γγ is very small, about 0.2% in the mass range of 110−150 GeV while thanksto a good resolution of diphoton invariant mass mγγ (∼1.3% depending on categories), a narrow resonance isobserved on a huge, smooth background as shown in Fig. 3 [6, 10]. Two photon candidates are selected with atransverse momentum (pT) of pT > 40 GeV and 30 GeV at ATLAS and pT > mγγ/3 and mγγ/4 at CMS (/2instead of /3 for VBF-category). Selected events are separated into several categories to improve sensitivitiesfor a global search and specific production processes, for example, VBF and V H. At ATLAS, 12 (10) categoriesfor 8 (7) TeV are introduced, where one for V H (with lepton) and two for VBF and V H (with dijet) while atCMS, 6 (5) categories for 8 (7) TeV, where two for VBF process and multi-variate analysis (MVA) is performed.Four categories out of them are defined based on MVA outputs. The purity of VBF process in the VBF-category is 70−80%. The largest excess with respect to the background-only hypothesis (based on local p0) isobserved (expected) with 6.1 (3.3) standard deviations (σ) at 126.5 GeV by ATLAS and 4.1σ (2.8σ) at 125 GeVby CMS.

B. H → ZZ(∗) → `+`−`′+`′− channel

This channel has small background since 4 leptons (e and µ) are required and good mass resolu-tions (∼1.5−2%) thanks to precise measurements of muon momenta and electron energy. Higgs boson can-didates are selected by requiring two same-flavor, opposite-sign isolated lepton pairs in an event. These fourleptons are required to have pT >20, 15, 10 and 7/6 GeV at ATLAS and pT >20, 10, 7/5 and 7/5 GeV atCMS (e/µ). Figures 4 show the invariant mass (m4`) distribution of selected 4 leptons and a clear resonance isobserved at around 125 GeV. A peak due to Z with FSR Z∗ is also found at around 91 GeV. In addition CMSadopts MELA (Matrix element likelihood analysis), which uses the fact that the kinematics of this final statecan be described with 7 parameters (5 angles and 2 masses), to improve sensitivities (∼15%). Figures 5 showtwo-dimensional plots of the output (KD) from MELA and m4` for signal and background and the signal eventshave large KD values. The largest excess is observed (expected) with 4.1σ (3.1σ) at 123.5 GeV by ATLAS and4.5σ (5.0σ) at 125.9 GeV by CMS [11].

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C. H →WW (∗) → `+ν`′−ν̄ channel

This channel relatively has a large signal event yield even in a low mass region of around 126 GeV and thebackground processes can be suppressed by requiring two opposite-sign isolated leptons (ee, µµ and eµ at CMSwhile only eµ at ATLAS) and high missing transverse energy Emiss

T . The mass of the Higgs candidates cannotbe reconstructed due to two neutrinos in the final state, hence the transverse mass mT, defined with leptonsmomenta, Emiss

T and its angle, is used as (one of) final discriminant variable(s). In addition since the directionof two leptons from W boson decay are preferentially close, due to the spin quantum numbers of Higgs andW bosons, a small angle between the two leptons, as well as a low invariant mass of two leptons (m``(′)) areexpected. CMS uses two variables mT and m``(′) for the final discriminant while at ATLAS only mT is used forthe final discriminant and cuts on the m``(′) variable are applied. The selected events are separated into 0-jetand 1-jet categories at ATLAS and 0/1 and 2-jet at CMS. Figures 6 (top) show the mT distribution for 0-jetand 1-jet categories at ATLAS [12]. A two-dimensional shape analysis in the (mT, m``(′)) plane is performed for0-jet and 1-jet categories at CMS. Figures 6 (bottom) show results with the one-dimensional bin distributionobtained from the two-dimensional analysis [12]. Non-resonant WW is dominant in the 0-jet category, and top-quark production is dominant in the 1-jet category in these distributions. Because there is no power on massdetermination, the excess is observed in a wide mass range. The observed (expected) significance at 125 GeVis 2.6σ (1.9σ) and 3.1σ (4.1σ) at ATLAS and CMS, respectively.

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D. H → bb̄ channel

Higgs bosons produced in association with a W or Z boson (denoted as V ) are searched with three differentfinal states, WH → `νbb̄, ZH → ``bb̄ and ZH → ννbb̄ by requiring 1-lepton, 2-leptons and 0-lepton, respectively.The event selection is based on the requirement of two b-tagged jets and the kinematic reconstruction of thevector boson. A typical performance of b-tagging used is about 70%, 20% and 1−a few% efficiencies for b, cand light-jets, respectively. Events are separated into several categories by using the transverse momentum ofthe vector boson pVT to improve sensitivities. ATLAS has 5 categories for 1- and 2-lepton and 3 for 0-leptonwhile CMS has 2 categories in only higher pVT region comparing to ATLAS. CMS uses MVA to improve themass resolution of two b-quarks mbb̄. The final discriminant variable is mbb̄ at ATLAS and MVA output at

CMS as shown in Figs. 7. They are only for 0-lepton of the highest pVT category. The observed (expected) 95%CL limit on the cross-section at 125 GeV is 1.8 (1.9) and 2.5 (1.2) times the SM prediction at ATLAS andCMS [13], respectively. Figures 8 also show the mbb̄ distribution in data after subtraction of all backgroundsexcept diboson processes. The data are consistent with the presence of diboson signals with a small contributionfrom 125 GeV SM Higgs boson.

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E. H → ττ channel

Higgs bosons decaying into a τ -pair are searched in the H → τlepτlep (so-called ``), H → τlepτhad (`h) andH → τhadτhad (hh) channels, where τlep and τhad denote leptonically and hadronically decaying τ leptons,respectively. Categorization is introduced based on the event topologies; 10(=4/4/2) categories at ATLAS and8(=3/3/2) categories at CMS for (``/`h/hh). Among them, the VBF category is important because S/B is betterby requiring two high pT jets and the typical cut for such jets are mjj > 350− 500 GeV and ∆ηjj > 2.6− 3.5.The purity of VBF process in the VBF category is 70−80%. Figures 9 show the mττ distribution of the VBFcategory for ``/`h/hh channels. The observed (expected) 95% CL limit on the cross-section at 125 GeV is1.9 (1.2) and 1.63 (1.00) times the SM prediction at ATLAS and CMS [14], respectively.

F. Observation and Signal Strength as the SM Higgs

The significance of an excess in the data is quantified with the local p0, the probability that the backgroundcan produce a fluctuation greater than or equal to the excess observed in data. The equivalent formulation interms of number of standard deviations is referred to as the local significance. Figures 10 show the local p0 as a

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function of mH for various channels and the combination of all channels for ATLAS and CMS. The largest localsignificance in the combination is observed (expected) with 7.0σ (5.9σ) at 125 GeV by ATLAS and 6.9σ (7.8σ)at 125.8 GeV by CMS [15].

The best-fit signal strength µ for the combination of all channels is evaluated to be 1.35 ± 0.24 at 125 GeVby ATLAS and 0.88 ± 0.21 at 125.8 GeV by CMS [15], which are consistent with the signal expected from aSM Higgs boson at that mass. The best-fit values of µ for each channel is independently measured as shown inFigs. 11 for ATLAS and CMS at the given mass. Some of channels still have large uncertainties (mainly due tostatistics) and will get better by adding more data and improving analysis.

Figure 11 (right) shows 68% CL contours in the (µqqH+V H , µggH+ttH) plane for each channel [15], whereµqqH+V H represents the coupling to vector bosons and µggH+ttH for top-quark couplings. The bb̄ and ττ channelscan determine µqqH+V H better than µggH+ttH and the ZZ channel has no power on µqqH+V H determinationbecause there is no category for VBF and V H. All the results are consistent with the SM prediction.

G. Mass measurement

The mass of this new particle is measured with H → γγ and H → ZZ(∗) → `+`−`′+`′− channels, which haveexcellent mass resolutions as mentioned before. A mass of mH = 126.6±0.3 (stat)±0.7 (syst) GeV [10] is foundwith the H → γγ channel by ATLAS. A mass of mH = 123.5±0.9±0.3 GeV and 126.2±0.6±0.2 [11] is measuredwith the H → ZZ(∗) → `+`−`′+`′− channel at ATLAS and CMS, respectively. About 3σ difference (tension)between these channels is observed by ATLAS. From the combination of these channels the common mass isevaluated to be mH = 125.2 ± 0.3 ± 0.6 GeV by ATLAS and 125.8 ± 0.4 ± 0.4 GeV by CMS [15]. Figures 12show CL contours for these channels to see which values of µ and mH of a signal hypothesis are simultaneouslyconsistent with data.

H. Spin measurement

ATLAS and CMS have started to check if the observed new particle has spin/parity JP = 0+ by using theH → γγ [10] and H → ZZ(∗) → `+`−`′+`′− [11] channels. In the H → γγ channel, angle θ∗ between a photon

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in the Higgs rest frame and a Higgs lab frame with a few modifications (so-called Collins-Soper frame) is usedand two spin/parity hypotheses are compared: the 0+ SM Higgs and a graviton-like spin-2 state with minimalcouplings (2+

m). Figure 13 (left) shows cos θ∗ distributions after the subtraction of background, profiled witha fit where 0+/2+

m ratio is free. The expected difference between 2+m and 0+ is changed as a function of the

fraction of gluon fusion production and in the observed differences, for any gluon fusion production fraction,data favors the 0+ hypothesis [10]. In the H → ZZ(∗) → `+`−`′+`′− channel, like MELA used by CMS, 5angles and 2 masses are basically used. Figure 13 (right) shows a likelihood ratio between 0− and 0+ and thedata disfavors the 0− hypothesis with CLS of 2.4% [11]. Other various comparisons are found in Refs [10, 11]and the data favors the 0+ hypothesis.

IV. SEARCH FOR BSM

Two Higgs doublets are required in several models, for example, the Minimal Supersymmetric StandardModel (MSSM), which is an extension of the SM and they are coupled separately to up-type and down-typefermions. This results in five physical Higgs bosons, two of which are neutral and CP -even (h,H), one of whichis neutral and CP -odd (A), and two of which are charged (H±). Since couplings to down-type fermions areenhanced with increasing tanβ for A and either H or h, searches for neutral MSSM Higgs bosons are performedwith the ττ channel. Figures 14 (left) and (middle) show an exclusion region in (mA, tanβ) plan for ATLASand CMS [16], respectively. A low mass region is being closed in all tanβ range. A search for a low mass

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FIG. 12: CL contours in the (µ, mH) plane for the H → γγ and H → ZZ(∗) → `+`−`′+`′− channels and theircombination for ATLAS (left) and CMS (right) [15]. In the CMS plot, the contours correspond to 68% CL.

*|θ|cos

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FIG. 13: Background-subtracted data distributions (left), profiled with a fit where the 0+/2+m ratio is free, in the

H → γγ channel. The fitted ratio value is 0.6 [10]. Distribution of −2 ln(L0−/L0+) (right) for two signal hypothesis in

the H → ZZ(∗) → `+`−`′+`′− channel. The arrow indicates the observed value [11].

charged Higgs, which is produced via t → bH±, is also performed with the H± → τν and cb channels andFigure 14 (right) shows results from the τν channel [17]. In the MSSM mmax

h scenario, a range on tanβ exceptaround 10 is excluded in mH± < 150 GeV. A search for high mass charged Higgs is important in future.

[GeV]Am

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ATLAS

FIG. 14: Exclusion at 95% CL in the (mA, tanβ) plan with the ττ channel for ATLAS (left) and CMS (middle) [16]and with the H± → τν channel at ATLAS [17].

SUSY searches have been performed with many event topologies. In typical SUSY models, for example,mSUGRA, gluinos and squarks can be produced via strong interactions at the LHC and such events are expectedto be observed with event topologies of high pT jets plus a large Emiss

T . Figure 15 (left) shows an exclusionmass region of gluinos and squarks with a simple SUSY model and they are excluded with masses below about1.5 TeV [18].

By considering the discovery of a Higgs-like particle at around 126 GeV and natural SUSY scenarios, the massof stop is expected to be up to about (3−4)×Higgs mass at most. Stop particles are searched in the t̃1 → bχ̃±1 ,

χ̃±1 → W±χ̃01 and t̃1 → tχ̃0

1 channels with various mass values of χ±1 . Figure 15 (right) shows results from theformer channel and most of interested region in the natural SUSY is excluded [19].

Both ATLAS and CMS also search for new particles and enhancements with various BSM, for example,models with extra-dimensions. Such searches also excluded models with a mass scale of roughly 1−3 TeV.

gluino mass [GeV]800 1000 1200 1400 1600 1800 2000 2200 2400

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0L ATLAS-CONF-2013-001

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2L [1208.4305], 1-2L [1209.2102]

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1χm

FIG. 15: Exclusion limits with a simplified MSSM scenario with only strong production of gluinos and first- and second-generation squarks decaying into jets and neutralinos (left) [18] and with various stop searches (right) [19].

V. FUTURE PROSPECT

LHC experiments will be upgraded to perform interesting physics programs as much as possible. LHCaccelerator will be upgraded for the design energy (

√s =14 TeV) and luminosity (1034cm−2s−1 and 100 fb−1

per year) and also to get a higher luminosity of ∼ 5×1034cm−2s−1, which is called “high luminosity” LHC (HL-LHC) and planned from 2022. Table I summarizes the LHC schedule including HL-LHC. The HL-LHC is notapproved yet but is expected to be soon. In the HL-LHC, we will take data until 3000 fb−1, which is necessary toaddress the Higgs self-coupling and so on. In parallel ATLAS and CMS detectors will be and must be upgradedto take data under such higher luminosity conditions. For example, the pixel and silicon strip detectors (andalso TRT at ATLAS) will be replaced due to radiation damages (and occupancy issues) and several electronicswill be replaced and improved to reduce background and improve measurement of jets and missing-energy underhigher pile-up conditions.

The precise measurement of the properties of the new boson, in particular couplings, is very importantat the HL-LHC. Figure 16 (left) shows expected precision on the ratio measurements of Higgs boson partialwidths without theory assumptions on the particle content in the Higgs loops or total width. The improvementwith a factor of 2−3 is expected for these measurements with data of 3000 fb−1 [20]. We also studied thepossibility of the Higgs trilinear self-coupling measurement with 3000 fb−1 data in the H(→ bb̄)H(→WW ) andH(→ bb̄)H(→ γγ) channels, which looks promising with further studies in future [20].

Direct BSM searches were studied for the LHC of 300 fb−1 and the HL-LHC. Figure 16 (right) shows bothdiscovery and exclusion regions of gluinos and squarks with masses and mass scales of up to 2−3 TeV can beinvestigated [20].

TABLE I: LHC schedule

2013−14 Shutdown for the design energy (14 TeV)

2015−16√s =13-14 TeV, up to ∼ 1× 1034cm−2s−1, 25-50 fb−1 per year to get ∼ 100 fb−1

2018 Phase-I upgrade for the full design luminosity (100 fb−1 per year)

2019−21√s =13-14 TeV, up to ∼ 2× 1034cm−2s−1, 100 fb−1 per year to get ∼ 400 fb−1

2022 Phase-II upgrade for the high luminosity runs (∼ 5× 1034cm−2s−1)

2023−√s =13-14 TeV, up to ∼ 5× 1034cm−2s−1, take data until > 3000 fb−1

Yκ/Xκ)Yκ/

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Squark-gluino grid, m

ATLAS Preliminary (simulation)

FIG. 16: Expected measurement precision on the ratios of Higgs boson partial widths (left) [20] and exclusion limits anddiscovery reach in a simplified squark-gluino model with massless neutralino (right) [20].

VI. SUMMARY

LHC has finished the first run period with a great success. One of LHC physics goals, “Higgs” discovery, wasachieved in 2012. This Higgs-like boson particle is observed at around 126 GeV. Couplings to gauge bosons andfermions are measured and there is no deviation from the SM Higgs with the present precisions. The spin of thisnew particle is also measured and the data favors 0+. However the present precisions of these measurementsare not good enough to conclude if this new particle is the SM Higgs or a piece of BSM. In addition there is nohint of BSM with direct searches for SUSY etc. With the restart of physics runs in 2015 with

√s=13/14 TeV,

these things will be improved. LHC is a long on-going and interesting project until 3000 fb−1 of data will betaken to address, for example Higgs self-couplings while results from 13/14 TeV collision are important for thedecision of the future experimental particle physics.

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[10] ATLAS Collaboration, ATLAS-CONF-2012-168.[11] ATLAS Collaboration, ATLAS-CONF-2012-169 and CMS Collaboration, CMS-PAS-HIG-12-041.[12] ATLAS Collaboration, ATLAS-CONF-2012-158 and CMS Collaboration, CMS-PAS-HIG-12-042.[13] ATLAS Collaboration, ATLAS-CONF-2012-161 and CMS Collaboration, CMS-PAS-HIG-12-044.[14] ATLAS Collaboration, ATLAS-CONF-2012-160 and CMS Collaboration, CMS-PAS-HIG-12-043.[15] ATLAS Collaboration, ATLAS-CONF-2012-170 and CMS Collaboration, CMS-PAS-HIG-12-045.[16] ATLAS Collaboration, JHEP 02 (2013) 095 and CMS Collaboration, CMS-PAS-HIG-12-050.[17] ATLAS Collaboration, JHEP 06 (2012) 039[18] ATLAS Collaboration, ATLAS-CONF-2012-109.[19] ATLAS Collaboration, Phys. Rev. Lett. 109 (2012) 211802, Phys. Rev. Lett. 109 (2012) 211803, JHEP 11 (2012)

094, ATLAS-CONF-2012-166, EPJC 72 (2012) 2237, Phys. Lett. B720 (2013) 13, ATLAS-CONF-2012-167, ATLAS-CONF-2013-001.

[20] ATLAS Collaboration, ATLAS-PHYS-PUB-2012-001, ATLAS-PHYS-PUB-2012-004, ATLAS-PHYS-PUB-2013-001, ATLAS-PHYS-PUB-2013-002.

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Current Status and Future Prospect of the LHC Experiment