научная статья по теме SEARCH FOR NEW PHYSICS AT LARGE HADRON COLLIDER Физика

Текст научной статьи на тему «SEARCH FOR NEW PHYSICS AT LARGE HADRON COLLIDER»

= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ SEARCH FOR NEW PHYSICS AT LARGE HADRON COLLIDER

©2010 N. V. Krasnikov*, V. A. Matveev

Institute for Nuclear Research, Russian Academy of Sciences, Moscow Received Octiber 24, 2008; in final form, May 14, 2009

We review the search for new physics to be done at Large Hadron Collider, — search for Higgs boson, supersymmetry, and exotic.

1. INTRODUCTION

The SM (Standard Model) [1] which describes within an unprecedental scale of energies and distances the strong and electroweak interactions of elementary particles relays on a few basic principles — the renormalizability, the gauge invariance, and the spontaneous breaking of the underlying gauge symmetry. The principle of the renormalizability [2] which is considered often as something beyond the limits of experimental test is one of the most important (if not the major) ingredients of quantum field theory. The SM gauge group SUc(3) ® SUL(2) ® U(1) is spontaneously broken to SUc(3) ® Uem(1) by the existence of scalar field with nonzero expectation value, leading to massive vector bosons — the W± and Z — which mediate the weak interactions; the photon remains massless. One physical degree of freedom remains in the scalar sector, a neutral scalar boson (Higgs boson) H, which is the last nondiscovered particle of the SM. It should be noted that the existence of the Higgs boson is direct consequence of the renormalizability of the SM model. The SUc(3) gauge group describes the strong interactions (quantum chromodynamics or QCD). The eight vector gluons carry color charges and are selfinteracting. Due to the property of asymptotic freedom, the effective QCD coupling constant as is small for large momentum transfers that allows to calculate reliably deep inelastic cross sections. The fundamental fermions in the SM are leptons and quarks; the left-handed states are doublets under SUl(2) gauge group, while the right-handed states are singlets. There are three generations of fermions, each generation identical except for mass.

Despite the apparent striking success of the SM, there are a lot of reasons why it is not the ultimate theory. In the SM the neutrinos are massless and hence there are no neutrino oscillations. However, there is strong evidence for neutrino oscillations [3] coming from measurements of neutrinos produced in

E-mail: krasniko@ms2.inr.ac.ru

the atmosphere and from a defecit in the flux of electron neutrinos from sun. It is easy to extend the SM to include neutrino masses, however the natural explanation of small neutrino masses is rather untrivial and probably it requires qualitatively new physics. In the SM an elementary Higgs field generates masses for the W, Z, and fermions. For the SM to be consistent the Higgs boson mass should be relatively light mH < < 1 TeV . The tree-level Higgs boson mass receives quadratically-divergent corrections at quantum level 5m2H ~ A2, where A is some ultraviolet cutoff. The natural ultraviolet cutoff in particle physics is the Planck scale MP\ ~ 1019 GeV or Grand Unification scale MG ~ 1014 GeV. Hence, the natural scale for the Higgs boson mass is O(A). To explain the smallness of the Higgs boson mass some delicate cancellation is required that is rather untrivial "fine tuning" or gauge hierarchy problem. At present the supersymmetric solution [4, 5] of the gauge hierarchy problem is the most fashionable one. It predicts that the masses of supersymmetric particles have to be lighter than O(1) TeV. Other possible explanation is based on models with "technicolor" [6]. Also we cannot exclude the possibility that the natural scale of the nature A ~ O(1) TeV. At any rate all solutions of the gauge hierarchy problem predict the existence of new physics at TeV scale. Other untrivial problem is that the SM cannot predict the fermion masses, which vary over at least five orders of magnitude (fermion problem).

The scientific program at the LHC (Large Hadron Collider) which will be the biggest particle accelerator complex ever built in the world consists in many goals. Among them there are two supergoals:

(a) Higgs boson discovery,

(b) supersymmetry discovery.

LHC [7] will accelerate mainly two proton beams with the total energy yfs = 14 TeV. At low luminosity stage (first two—three years of the operation) the luminosity is planned to be Ll = 1033 cm-2 s-1

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with total luminosity Lt = 10 fb 1 per year. At high luminosity stage the luminosity is planned to be Lh = = 1034 cm"2 s"1 with total luminosity Lt = 100 fb"1 per year. Also the LHC will accelerate heavy ions, for example, Pb—Pb ions at 1150 TeV in the centre of mass and luminosity up to 1027 cm"2 s"1. Bunches of protons will intersect at four points where detectors are placed. There are planned to be two big detectors at the LHC: the CMS (Compact Muon Solenoid) [8] and ATLAS (A Toroidal LHC Apparatus) [9]. Two other detectors are ALICE detector [10], to be used for the study of heavy ions, and LHC-B [11], the detector for the study of B-physics.

In this paper we briefly review the search for new physics to be performed at the LHC. To be precise we review the search for Higgs boson, the search for su-persymmetry and the search for new physics beyond the SM and the Minimal Supersymmetric Standard Model (MSSM). As a rule, we review results based on full simulation of the CMS detector [12].

2. SEARCH FOR STANDARD HIGGS BOSON

The current limit on the SM Higgs boson mass from LEP experiments is mn > 114.4 GeV at 95% C.L. [13]. Analysis of high-precision measurements of electroweak observables lead to indirect upper bound [14] mn < 193 GeV at 95% C.L. on the Higgs boson mass, so within the SM the Higgs boson should be relatively light.

The tree-level Higgs boson couplings to gauge bosons and fermions can be deduced from the SM Lagrangian. Of these, the HW+W", HZZ, and Htjj^ are the most important for the phenomenology. For mn < 2mW Higgs boson decays mainly with (^90%) probability into b quark—antiquark pair and with ^7% probability into t lepton—antilepton pair.

In the heavy Higgs mass regime (2mZ < mn < < 800 GeV), the Higgs boson decays dominantly into gauge bosons. For Higgs boson mass slightly larger than the corresponding gauge boson mass the decay widths into pairs of off-shell gauge bosons play important role.

It should be noted that there are a number of important Higgs couplings which are absent at tree level but appear at one-loop level. Among them the couplings of the Higgs boson to two gluons and two photons are extremely important for the Higgs boson searches at supercolliders.

Typical processes that can be exploited to produce Higgs bosons at the LHC are [15—17]:

(i) gluon fusion: gg — H,

(ii) WW, ZZ fusion: W+W",ZZ — H,

(iii) Higgs-strahlung off W, Z: qqW, Z — W,Z + + H,

(iv) Higgs bremsstrahlung off top: qq,gg — tt + + H.

Gluon fusion plays a dominant role throughout the entire Higgs mass range of the SM, whereas the WW/ZZ fusion process becomes increasingly important with Higgs boson mass rising. The last two reactions are important only for light Higgs masses.

One of the most important reactions for the search for Higgs boson at LHC is

pp — (h — yy) + ..., (1)

which is the most promising one [18] for the search for Higgs boson in the most interesting region 100 < < mn < 150 GeV. The general conclusion is that at 5a level it would be possible to discover Higgs boson for 95 < mn < 145 GeV at low luminosity and at high luminosity the corresponding Higgs boson mass discovery interval is 85 < mn < 150 GeV [ 12].

The signature pp — H — WW* — l+vl'"v' [19] provides the Higgs boson discovery for the Higgs boson mass region between 150 and 180 GeV [12, 18]. Especially important is that the signature H — — WW* — l+vl'"v allows to discover Higgs boson in the mass region around 170 GeV where the branching ratio for H — 4l is small and the use of four lepton signature for the Higgs boson discovery does not help at least for low luminosity. This signature does not require extraordinary detector performance and only requires a relatively low integrated luminosity of about 5 fb"1.

The weak boson fusion channels qq — qqH lead to energetic jets in the forward and backward directions, and the absence of color exchange in the hard process [20—22] that allows to obtain a large reduction of backgrounds from ttq, QCD jets, W and Z production and compensate the smallness of the Higgs weak boson fusion cross section compared to inclusive gg — H. Note that the process of Higgs boson production in the weak boson fusion with forward jet tagging has been considered first for the channels H — ZZ — 4l, 2l2v in [23]. The reaction pp — (H — yy) + 2 forward jets has been investigated at parton level in [20] and at full CMS detector simulation level in [24]. The main conclusion [24] is that the significance S = 5 is reached at the luminosities -100 fb"1 for mn = 115-130 GeV. Additional advantage of this signature is that the ratio of signal to background S/B — 1 in comparison with S/B — 1/15 for inclusive pp — (H — yy) + ... reaction.

The signature H — W* W — l±lE™55 in weak boson fusion mechanism with forward jet tagging has been investigated in [12]. The spin correlations, leading to small opening angles between two charged leptons, are used to suppress the backgrounds. This mode provides the Higgs boson

discovery for 180 > mH > 130 GeV for integral luminosity Lt = 60 fb-1.

The signature H — tt — l + ] + E™ss in weak boson fusion mechanism was studied in [25]. The main conclusion is that for integral luminosity 60 fb-1 the use of this signature allows to discover Higgs boson for mass interval 115 < mH < 135 GeV [25].

The channel H — ZZ* — 4l is the most promising one to observe Higgs boson in the mass range 130—180 GeV. Below 2mZ the event rate is small and the background reduction more difficult, as one of the Zs is off mass shell. In this mass region the width of the Higgs boson is small rH < 1 GeV, and the observed width

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