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Ricerche extra-SUSY a LHC


Ricerche extra-SUSY a LHC ... High mass dilepton events in pp collisions mainly from quark-antiquark annihilation. ... Forward-backward asymmetry, ... – PowerPoint PPT presentation

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Title: Ricerche extra-SUSY a LHC

Ricerche extra-SUSY a LHC
  • Incontri di Fisica delle Alte Energie (IFAE)
  • Pavia, April 19th 2006

Lorenzo Menici Università La Sapienza di Roma
Standard Model and beyond
  • The Standard Model (SM) describes a all the
    accelerator based HEP data with great accuracy.
    On the other hand it is not fully theoretically
    satisfactory and there are hints for it to be the
    low energy limit of a more fundamental theory,
  • the hierarchy problem no symmetry preventing the
    scalar masses (Higgs) from getting quadratic
    divergent radiative corrections at all
    perturbative orders
  • it does not include a quantum description of the
  • it does not have a viable candidate for cold DM
    and cannot explain cosmological issues like
    baryon asymmetry and Lcosm
  • it does not take into account massive neutrinos
  • no exact unification of the gauge couplings
    through RG

  • The LHC will have an interesting potential
    already at the beginning
  • vs from 12 to 14 TeV before first shutdown
  • luminosity from 1026 to 1033 in 1st phase. Then
    1034 cm-2 s-1
  • fraction of fb-1 collected in the first year
  • even compared to the latest Tevatron
  • vs 1.8 TeV
  • luminosity latest peaks at 1.21032
  • Here we present the potential at start-up of
    ATLAS and CMS, the two multi-purpose detectors at
    LHC, concerning the possible detection of
    non-SUSY physics Beyond the Standard Model
    (BSM) together with the necessary integrated
    luminosity for the various channels.

  • Among the several on going analysis in the two
    collaborations, in this talk we chose to report
  • Extra gauge bosons (Z, WR) and heavy Majorana
    neutrino N
  • Leptoquarks
  • Lepton Flavour Violation
  • Little Higgs
  • Split SUSY
  • Other relevant signatures of non-SUSY physics BSM
    will have dedicated talks in this session
  • Long-lived Heavy Charged Particles (S. Viganò)
  • Extra Dimensions (F.L. Navarria)
  • New physics with Top events (M. Cobal)
  • New physics with Bottom events (S. Vecchi)

Z heavy neutral gauge bosons
  • Several models (GUT SO(10) and E6, SSM) predict
    an additional heavy neutral gauge boson, referred
    to as Z. No prediction on mZ
  • Z Z assuming they have the same coupling to
    fermions, they do not mix significantly
    (electroweak LEPI)
  • Most recent lower bounds from combined LEPII
  • mZ gt 400-900 GeV/c2 , depending on the model
  • Tevatron RunII expected to explore up to 800
    GeV, depending on scenario

Z ? µµ-
R. Cousins et al. CMS NOTE 2005/002
  • pp ? Z ? µµ- full simulation
  • Dominant irreducible background DY
  • pp ??/Z0 ? µµ- selection on single-µ OR double-µ
    L1 and HLT µ µ tracks originate from same vertex
  • Cuts to suppress reducible background could give
    small improvement (µ isolation, jet veto, µs be
    back to back)
  • No study of systematic errors yet, nor pile up
  • discovery potential starting from 1 fb-1
  • After discovery how to determine the
    theoretical framework Z belongs to?
    Forward-backward asymmetry, study on going at
    CMS, done at ATLAS

Z AFB model discrimination
  • In all models, the Z production cross
  • section as a function of cos? has the
  • typical spin 1 behaviour. High mass
  • dilepton events in pp collisions mainly
  • from quark-antiquark annihilation.

B. Trocme, ATL-PHYS-CONF-2005-014
  • Incoming quark direction unknown, estimated with
    the reconstructed Z direction (see correlation)
  • The prob. e of wrong quark direction (sea )
    is energy dependent ? AFB measured in bins of E
    around Z peak.
  • Wrong q direction cos?? -cos? spoils asymmetry,
    which can be corrected for a posteriori once is e

  • In each bin of cos?, from the
  • observed AFB the corrected
  • AFB is calculated using
  • e(cos?, E), 100 fb-1 used.
  • Results from achiavable with
  • smaller statistics being
  • investigated

cos? (gen)
cos? (reco)
Left-Right symmetric model
Left-Right (LR) symmetric models explains the
origin of the parity violation in weak
interactions (as a result of the spontaneously
broken parity) and predicts the existence of
additional gauge bosons WR and Z. In addition,
heavy right-handed Majorana neutrino states N
arise naturally within LR symmetric model. The Ns
could be partners of light neutrino states,
related to their non-zero masses through the
see-saw mechanism. These reasons make the LR
symmetric model very attractive and the search of
WR, Z and N an important challenge for LHC.
Heavy Majorana neutrino search
S. N. Gnimenko et al. CMS IN 2005/040
  • We can study two kind of processes with WR and
  • pp ? WR ? l Nl X
  • pp ? Z ? Nl Nl X followed by the decay
    Nl ? l j1 j2

Heavy Majorana neutrino search
Suppose a WR mass of 2000 GeV and a Ne mass of
500 GeV. We require events with two isolated
electrons and at least two jets selected and,
using the 4-momenta of jets and electrons, we
calculate Mejj MN and Meejj MWR. Background
from ZW, , Zjets, ZH and WH production.
5s discovery
With 30 fb-1 Ne and WR can be discovered at CMS
for masses up to 2.4 TeV and 4 TeV respectively.
  • Leptoquarks (LQs) are an interesting category of
    exotic colour triplets with couplings to quarks
    and leptons.
  • They are predicted by GUTs, composite models,
    technicolor schemes, E6 models, SUSY models with
    R-parity violation,
  • Inter-generational mixing is not allowed
  • There exist 14 species of LQs, differing by spin
    (scalar/vector), fermion number F 3BL, isospin
    and chirality of the coupling
  • They have fractional electric charge (5/3,
    4/3, 2/3 and 1/3)
  • Two possible decay modes
  • The LQ branching fractions ß B(LQ ? lq)
    depends on the model
  • Consider now the pair production of scalar LQs,
    which proceeds through gg fusion and
    annihilation. TevatronLEPHERA current limit on
    the mass of the first LQ generation MLQ gt 242

(LQ)(LQ) ? lljj
Benekos et al. com-phys-2004-071
  • Pair production of scalar LQs of the first two
    generations (i.e. only eejj and µµjj final states
    are considered here). Topology of the final
    state two high-pT leptons and two high-ET jets.
    Bounds in LQ mass for the cases ß1 and ß0.5.
    The main background arise from QCD processes,
    eliminated requiring
  • isolation of high-pT leptons
  • same-flavour and opposite sign leptons with pT gt
    100 GeV and ? lt 2.5
  • at least two jets with ET gt 70 GeV, SETcalo gt
    570 GeV and ETmiss/SETcalo lt 0.05
  • selecting events with high lepton-jet
    reconstructed invariant mass mlj

(LQ)(LQ) ? eejj 30 fb-1 of int. lum. Selection
with mej-MLQ lt 100 GeV
(LQ)(LQ) ? ??jj
Benekos et al. com-phys-2004-071
  • Pair production of scalar LQs of the third
    generation (i.e. ?t?tjj final state), otherwise
    huge Z(? ??)jets irreducible background fot the
    first two generations. In contrast, for a third
    generation LQ ? ?tb, ATLAS b-tagging allows s/b
    separation. Topology of the final state large
    ETmiss and two high-ET jets. Again ß1 and ß0.5
    cases. Other background from Wjets (l?bb, t?bb),
    (l?bl?b, t?bt?b), ZZ (??bb), ZW (bbl?, bbt?,
    ??jj) and WW (l?jj, t?jj), eliminated requiring
    (besides b-tagging)
  • lepton veto
  • azimuthal constraints on jets (because of
    back-to-back LQ production)
  • at least two jets from b-quarks with ET gt 70 GeV
    and ? lt 5
  • ETmiss gt 400 GeV and mlj gt 180 GeV

(LQ)(LQ) ? ??jj 30 fb-1 of int. lum.
Lepton Flavour Violation
Lepton Flavour Violation (LFV) is predicted by
different models of physics BSM (GUT,
Technicolor, String Theory with E symmetry,
leptoquarks, forth generation neutrino models,
SUSY with broken R-parity, MSSM with
non-universal parameters at the GUT scale) and
it could be important in order to give an
explanation to the recent important indications
of neutrino oscillations (mixing also in the
lepton sector?). At the moment no evidence of
LFV from the experimental search of the
most important LFV processes (such as t ? µ?, t ?
µµµ, Z ? µt,). LHC is going to detect LFV
processes or to further lower their possible BRs.
t ? µµµ
M. Biasini et al. CMS NOTE 2002/0037
Main background (see CMS NOTE1997/096) from D B
mesons production
Feynman diagrams giving t ? µµµ in mSUGRA
Little Higgs
  • Little Higgs (LH) is possible way of solving the
    hierarchy problem the Higgs
  • boson is a pseudo-Goldstone boson arising from
    some global symmetry
  • breaking at the TeV scale (usually exploiting
    non-linear sigma models).
  • Several implementation of LH model exist
    (different symmetry groups
  • broken) and they all have in common a set of new
    particles. For the Littlest
  • Higgs case
  • One heavy top particle T with charge 2/3
  • Three scalars F, F and F0
  • Four gauge bosons WH, ZH and AH

T produced at LHC via bq ? Tq small
cross section because of the low b-quark content
in a proton.
The scalar sector is not so favorable because of
the SM background, expecially for the F case.
Extra gauge bosons are the most promising
channels for a first evidence of LH at work in
ZH ? ee-
AH and ZH could be revealed searching for a peak
in the invariant mass distribution of ee- and
µµ-. Consider now ZH its cross section is
proportional to (cot?)2, the mixing angle ? being
the only free parameter of the theory once the ZH
mass is fixed. Here an example of ZH ? ee-
search (with SM Drell-Yan background)
G. Azuelos et al. Eur.Phys.J.C39S2
ZH ? ee-
100 fb-1
300 fb-1
WH ? e?
Also WH production cross section depends on cot?.
WH ? l? is the decay to detect (isolated charged
lepton missing ET). Consider the WH ? e? case
event selected requiring an isolated electron
with pT gt 200 GeV, ? lt 2.5 and ETmiss gt 400
GeV. The main background arises from e?
production via a virtual W.
G. Azuelos et al. Eur.Phys.J.C39S2
WH ? e?
100 fb-1
300 fb-1
Split SUSY
  • Observation if we (at the moment) are forced to
    accept the fine-tuning of the vacuum
  • energy without explanation, the fine-tuning of
    the electroweak scale is much less
  • problematic! Moreover, also in SUSY theories some
    fine-tunings at work (absence of
  • large FCNC effects due to sfermions
    non-observation of light Higgs or charginos at
  • LEP).
  • Split SUSY (SpS) solves these SUSY problems
    giving up electroweak naturalness
  • and featuring
  • heavy sfermions
  • a light Higgs boson (mH lt 200 GeV)
  • long-lived gluino
  • four neutralinos and two charginos
  • At the LHC, the experimental challenge would be
    the observation of R-hadrons
  • (meta-stable color-singlet bound states of the
    long-lived gluino with quarks or
  • gluons), in addition to the search of direct
    production on charginos and neutralinos
  • (identifying their gaugino/Higgsino components
    and their Higgs Yukawa couplings)

R-hadrons detection
R-hadrons can be considered stable from a
detector point of view. Particularly interesting
the case of charged R-hadrons that behave like
slow muon-like particles and could be detected
using Time Of Flight (TOF) methods (see Viganòs
talk). The ATLAS detector would allow the
discovery of R-hadrons with a mass reach of
at least 1300 GeV with few fb-1. For higher
masses we have to face both low production cross
section and trigger problems.
A. C. Kraan et al. hep-ex/0511014
Triggered R-hadrons and Background fot 1 fb-1
Ratio S/vB for different R-hadron masses for 1
  • The subject of this talk is an overview of some
    of the most interesting
  • channels on non-SUSY physics BSM at LHC
  • Many of these signatures (if exist) could be
    revealed with 10-30 fb-1
  • at ATLAS and/or CMS (but also LHCb could play a
    relevant role in searching for new physics!)
  • Other important channels (Extra Dimensions,
    Top/Bottom physics BSM, long-lived heavy charged
    particles) will be discussed afterward in this
  • Other channels have not been discussed due to
    lack of time but are equally important
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