Search for New Phenomena in Hadron Collisions

1
P. Grannis Rencontres du Vietnam, July
2000
Search for New Phenomenain Hadron Collisions
The nature of Electroweak symmetry breaking is
not understood the Standard Model introduction
of the Higgs scalars, though consistent with
data, is unsatisfactory theoretically, and does
not address cosmological needs. High energy
collisions at the Fermilab p p Tevatron and the
DESY e p HERA colliders offer ways to seek new
phenomena beyond the Standard Model. We report
representative results from H1, ZEUS, CDF and
DO (no new
phenomena are observed !)
2
2
The Standard Model Paradigm
The Standard Model accomplishes electroweak
symmetry breaking, and presently agrees with all
particle physics measurements 1 complex
Higgs scalar doublet W , Z0
get mass ( three Higgs become the longitudinal
W/Z components) All fermions get masses One
remaining observable Higgs boson
Precision measurements at the Z0 (LEP1, SLC), W
(LEP2, Tevatron), top quark (Tevatron), n
scattering give indirect measurement of SM Higgs
mass Mh lt 215 GeV (95 CL) ( June 2000
Susy2000 conference for a particular choice of 2
loop corrections )
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But the SM is not a THEORY of EWSB !
3
SM is an effective theory, up to scale L for new
physics to appear to avoid SM Higgs
inconsistencies. mH 1 TeV/ ln L/v sets
the scale for breakdown of fundamental Higgs
(vHiggs vev 246 GeV)
Higgs self-coupling diverges
Higgs potential develops 2nd min.
Gauge hierarchy problem - mH driven to scale L by
EW loop corrections unless fine tuning of
parameters need cancellation to 30 decimal
places if L Mplanck. Lack of grand unification
-- (SU(3)color X
SU(2)L X U(1)Y couplings dont meet Need a dark
matter candidate - none in SM, but most
Beyond-the-SM theories provide one (e.g. weakly
interacting neutral particle M O(100 GeV) Want
extended CP violation to explain
matter-antimatter asymmetry in universe SM does
not address origin of flavor, pattern of
generations, fermion mass pattern, mixing.
Expect new theory embedding SM in low energy
limit, with associated new phenomena
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Two main classes of BSM models
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Fundamental Higgs scalar (Weak
EWSB) Supersymmetry Extended Poincare group
symmetry between bosons and fermions New mirror
spectrum of particles e e etc. Large no.
new parameters (105 in minimal Susy SM) New
strong dynamics tends to modify precision
measurements -- not seen. Simple models generate
FCNC (e.g. m e g ) and predict low top quark
mass. Models convoluted.
Composite Higgs (New strong
interactions) Technicolor, Topcolor etc. QCD
analogy with new technicolor fermions like quarks
at TeV scale NJL superconductivity condensates,
e.g. tt bound states for Higgs surrogate Strong
WW scattering Fundamental scalars are ugly (QCD
pion is a composite! ) Large number of
unspecified parameters. No a priori
justification strong coupling is QCD inspired

What the classes of models say about each other
Plus suggestions of large-scale compactification
of extra dimensions. String theory motivated
but with observed effects at EW scale ( O(TeV)
solves hierarchy problem by reducing GUT scale.
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SM Higgs searches at Hadron Colliders
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March 2000 LEP SM Higgs limit 108 GeV (95
CL) will probably go up to about 115 GeV
CDF preliminary
Tevatron search for SM Higgs with present 120 pb
-1 does not compete with LEP 2.
In Run 2, can exclude up to 180 GeV with 20 fb-1
discover over some of that region.
20 fb-1
LHC experiments will find SM Higgs (low mass
region most difficult where rely on H gg)
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Susy Higgs
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Two Higgs doublets 5 states (h0,H0,A0,H,H-)
survive after giving W/Z masses. Susy Higgs
sector controlled by mA and tanb ratio of
vevs. For large tanb, decays into down type
quarks or charged leptons are favored.
Rule out large tanb
Charged Higgs searches H gives excess heavy
fermions in top decay through tn, cs, Wbb
decays decay t H b can compete with t W
b. Direct search for t H b, with H t n
gives similar limits for large tanb
H W bb
H t n
H c s
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Susy sparticle searches
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Colored sparticles ( ) are produced
strongly at Tevatron (qq/gg collisions). If
R-parity conserved, the LSP (typically ) is
stable, weakly interacting, so signatures for
typically involve jets and missing
ET. Charginos, neutralinos can occur in cascade
decays of giving rise to multilepton
final states.
Searches are typically done for a specific model,
parameter range, and decay channel
m0 unified scalar mass at GUT scale m1/2
unified fermion mass
Jets ET search (DO) in mSUGRA framework. m0
unified scalar mass m1/2 unified gaugino mass.
Find msquark gt 250 GeV mgluino gt 300 GeV (at
small m0) 95 CL
m1/2
m1/2
squark mass (GeV)
m0
Replot data in plane For tanb 2,
exclude m lt 260 GeV for equal squark/gluino mass.
(tanblt2 excluded for mSUGRA at LEP)
gluino mass (GeV)
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Susy sparticle searches via leptons
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e.g.
Cascade decays through gaugino states can lead to
multilepton final states, relatively free from
background. The can decay to either c or
c-, so can lead to same sign dileptons.
m1/2
DO search dilepton jets ET . For tanb 2,
exclude msquark mgluino lt 255 GeV (95
CL) Extend LEP I for tanb lt 6 comparable to LEP
II at low tanb
m0
CDF search in 2 like sign leptons and 2 jets
exclude in large and smaller mass range.
Exclude for equal mass at about 220 GeV
Several channels give comparable reach
equal mass squark and gluino limit is about 260
GeV
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Susy stop/sbottom searches
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Typically in MSSM, substantial mixing of the Susy
partners of tL and tR ( t1 and t2 ) where the t1
could be the lightest squark. CDF has
searched for t1 c c10 and b
b c10 , extending the mass limits to 120 and
140 GeV, respectively.











t1 c c10
b b c10
m(c10)
m(c10)


Search for t b c1 and c1 l n or c1
W c10





m( n )
Tevatron extends stop/ sbottom limits to higher
mass (but lower n , c10 mass)

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R-parity violating Susy
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If Sparticle number is not conserved in
reactions/decays, LSP is not stable ( typically
take R violation small enough that production
and cascades through the Susy chain are
unaffected). Super potential can have 3
classes of new couplings lijk LiLjEk
lijk QiLjDk lijk UiDjDk (L/Q are
left-handed lepton/squark doublet superfields and
E/D/U are right-handed charged lepton/ d-type/
u-type quark singlet superfields. i,j,k are
generation indices.)
Only 1 type of coupling can be present to
preserve lepton, baryon stability l terms ( B
violating) are difficult at hadron colliders, as
multijet backgrounds are large.
lijk couplings
CDF 4 lepton search limits l121
DO search in three lepton channels limits
l121, l122, l233 to 10-4 - 10-5
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R-parity violating Susy -Tevatron
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lijk couplings



u
u
u
u
d
e


d
c1
c10
D0 l1jk (2 es and 4 jets) rules out equal
mass squark/gluino at 270 GeV
(tanb2), 225 GeV (tanb6).
CDF Two gluino production
g c cL cL e d . Get like sign
electrons to probe l121



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R-parity violating Susy - HERA
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HERA searches for R-violating couplings l ,
assuming c10 is LSP.

H1 limits in l vs. squark mass plane
ZEUS limits in l vs squark mass for various
choices of mSUGRA parameters
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Gauge Mediated Susy
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GMSB allows for supersymmetry breaking in a new
gauge sector, at energy scales much below the
Planck scale. The gravitino, G , is the LSP.
Decays of the next to lightest sparticle c10 (or
t ) to G occur by g (t) transitions. The
chain p p c1 c1- WW- c10 c10
eegg missing ET could explain the CDF
event of this topology.








DO search in gg missing ET rules out this
interpretation for tanb 2 (there is only mild
tanb dependence). Mc1 gt 150 GeV (95 CL)
CDF has sought the direct GG production
associated with initial state jet radiation.
The limit infers that the GMSB Susy breaking scale




exceeds 217 GeV (mG gt 1.1 x 10-5 eV)
DO has searched for any Susy decay from NLSP to
LSP by g decay (Eg gt 20 GeV) in (g, 2 jets, ET).
Rule out equal mass at 310 GeV.
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Prospects for Susy Discovery at Tevatron Run II
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Run 2a 2 fb-1 at 2 TeV Run 2 total 20
fb-1 increased energy gives
40 increase in s, so effective Susy rates up by
30 in Run 2a. Background/statistics limited
searches increase (Leff) 1/4 (x 2).

• CDF and DO Run 2 detectors are substantially
  improved
• Improved CDF/new DO vertex detectors -- b-tag
  eff. 60
• Improved t ID multi-t important for high
  tanb studies
• dE/dx from silicon, TOF in CDF seek slow,
  highly ionizing tracks (massive stable
  charginos/ staus in AMSB)
• Photon pointing in DO to 2 cm at vertex (GMSB
  signatures)
• Improved CDF calorimetry -- better e ID,
  missing ET
• Improved triggers using tracks, vertices,
  topology

• Some representative estimates (2 fb-1)
• mSUGRA q , g limits to 400 GeV (equal mass)
• R (l coupling) gluino to 500 - 600 GeV
• low tanb charginos to 150 GeV 200 GeV for tanb
  gt 10
• stop limits to 200 GeV
• good possibility for AMSB c1 degenerate with
  c10
• long lived neutralino in GMSB from photon
  pointing

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Searches for Strong Coupling Phenomena
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Strong coupling models have been proposed, in
analogy with QCD, to avoid fundamental scalars.
A new scale of gauge interactions is envisioned
with a new set of fermions operating at the 1 -
10 TeV scale. These models predict analog
technicolor particles like ordinary p, r, w,
etc. There are typically gauge bosons that can
connect leptons and quarks, yielding the
possibility of color triplet Leptoquark states.
New massive Z bosons are typical. The
technipions, or top quark condensates, play the
role of the Higgs boson, and thus influence WL WL
scattering. However, precision measurements of Z
, W , and top quark properties have not confirmed
the higher order corrections expected in these
schemes, so models have evolved to be quite
different from ordinary QCD.
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Technirho, techniomega
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CDF search for rT W pT , pT b b or b c
D0 search for rT, wT e e- when decay to
pT is forbidden (expect pT /rT nearly
degenerate). Set mass limit of 207 GeV
Heavy Z limits at 690 GeV above 1 TeV in Run
2 Technirho/techniomega limits now at 200 GeV
(would expect them more massive). Run 2 limits
at 500 GeV. Expect Run 2 limits for topgluons
in 1.0 - 1.4 range Top condensate Higgs limits in
Run 2 of 350 GeV
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Leptoquark Searches
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Bosons with lepton and quark number (color
triplet) called Leptoquarks (LQ) arise in
extended models containing new gauge bosons that
connect lepton and quark sectors. Technicolor,
E6 supersymmetry, compositeness models contain
LQs. The experimental suggestion of an excess
of high Q2, high-y events at HERA stimulated LQ
interpretations.
LQ can be formed in s-channel in eq (F0) or
e-q (F2) collisions at HERA. Both types can
be pair produced at Tevatron. Suppression of
FCNC requires that LQs couple to same generation
lepton/quark.
LQ1
q
b
e
e
g
LQ1
l
e,n
LQ1
u/d
u/d
q
b
u,d
HERA s-channel l is Yukawa coupling.
Popular guess l 4paEM 0.3
Tevatron - strong production indep. of Yukawa
coupling l decay BR b 1(0) for e(n) decay.
LQs possible with J 0,1 for 3 generations
F 0, 2
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1st Generation Leptoquark Searches
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Recent H1,ZEUS results improve earlier DO limits
on 1st generation LQ, if l gt 0.1, particularly at
low b.
ZEUS results display the sensitivity for scalar
and vector LQs as a function of Yukawa coupling l
H1/ZEUS exclude scalar LQs up to 280 GeV for EM
strength Yukawa couplings. Tevatron excludes up
to 240 GeV for b1 for any l
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Higher Generation Leptoquark Searches
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2nd generation LQs
DO limits from pp LQ LQ mm jets, mn jets,
nnjets for scalar/vector 2nd generation LQ. e.g.
for Scalars gt 200 GeV (b 1) gt 180 GeV
(b1/2) gt 79 GeV (b 0)
If LQ arises from technirho decay, CDF finds
limit increases up to 174 GeV for b 0. rT
LQ LQ (cn) (cn )
3rd generation LQs
Search for LQ n b jet CDF limit is 148
GeV. In the case that LQs arise from
technirho production with decay into LQ pairs (LQ
bn), the limit is increased as m(rT)
increases
H1 has excluded LQ decays with mixed 1st and 3rd
generation decays (e jet and t jet) (FCNC) up to
275 GeV for equal e/t BR and EM strength Yukawa
coupling.
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Searches for Quark Compositeness
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Quarks and/or leptons could have internal
substructure, as observed at higher levels of
physics (atoms
nuclei proton/neutron quark
) For an effective contact interaction below the
compositeness scale L L g2/L2 h (qL gm
qL) (qL gm qL), get modifications to inclusive
jet cross section. Earlier inclusive jet cross
sections from CDF had a large ET excess that
could be explained by L in the 1.5 - 1.8 TeV
range. DO has set limits on quark
compositeness from large ET dijet angular
distributions that rule out this interpretation.
L gt 2.7 (2.4) TeV for (-)
interference with QCD.
Ratio of jet XS (hjet lt 0.5) / ( 0.5 lt hjet lt
1.0 )
QCD (L ) limit)
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Searches for Quark/Lepton Compositeness
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If quarks and leptons have common constituents,
new contact interactions occur below the scale of
free constitutients L h g2/L2 (e O e) (q O q),
where O is a Lorentz operator, L is the scale
of compositeness, and h is a sign.
HERA experiments seek deviations from DIS at
large Q2 LEP experiments search for deviations in
di-quark production Tevatron experiments seek
modifications to Drell-Yan production.
e
q
LEP
Tevatron
e
q
HERA
The three sets of experiments differ in their
sensitivity to compositeness for different
Lorentz structures O. Limits vary between L gt
2 to 5 TeV at HERA between 4 to
6 TeV for Tevatron, and
between 2 to 7 TeV for LEP
(2 TeV) -1 l/2p 1 am) depending on operator
structure.
Searches for direct evidence of substructure
through excited states HERA limits e gt 230
GeV n gt 160 GeV, q gt 190 GeV. Tevatron
limits on q gt570 GeV. These direct limits are
below those on compositeness scale in contact
interactions.
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Searches for Large Extra Dimensions
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String theories require 6-7 extra spatial
dimensions, previously thought to be compacted at
the Planck scale. Recently, suggestions were
made that compactification might occur for some
of these dimensions at larger scales. For
example, (Randall-Sundrum Antoniadis Dienes
et. al) if compactification radius is at the
EWSB scale (O(TeV)), possibilities exist to
observe a tower of Z -like states at multi
-TeV.
Arkani-Hamed, Dimopoulos, Dvali conjectured that
the fundamental quantum gravity mass (effective
Planck) scale MS could be O(TeV), and the
compactification distance scale of lt mm. In
this model, particle processes could emit
gravitons that propagate into the

hidden dimensions, leading to signatures like
ee- g G monogammas or q q / gg
g G monojets. Also modifications to q q
ee-/g g in hadron collisions, or to changes
in DIS, due to towers of virtual graviton
exchanges.
Several phenomenological calculations of LED
effects differ in parametrization -- dependence
on n, interference
U(1)Y
No hierarchy problem! GUT scale at O(TeV)!
1/coupling
SU(2)L
SU(3)color
MS
MGUT
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Searches for Large Extra Dimensions
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Also, classical gravity would be modified at
short distances R (1/MS) MPl / MS2/n
(R 1013 m for n1 (ruled out!)
0 .7mm for n2, 3 nm
for n3, 10-11 m for n4 ) Cavendish experiments
have recently pushed into the sub-millimeter
regime with no observed deviation from r-2.
n2 also disfavored by supernova and cosmological
effects. (Adelberger, APS meeting April 2000 no
variation to gravity at submillimeter scale)
H1 study of modifications to high Q2 DIS limits
MS to gt 0.48 or gt 0.72 TeV, depending
on sign of interference.
qq ee/gg mass and angular distributions are
modified by the LED effects, depending on MS and
n.
Inter-ference
SM
DO study has set limits on MS for all
phenomenological forms e.g. Han,Lykken,Zhang
n2 MS gt 1.3 TeV n3 1.4
n4 1.2 n5
1.1 n6 1.0 n7
0.95
LED
Comb. MS1 TeV, n4
mee/gg
cosq
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Searches for things not necessarily wanted
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Experiments have also searched for non-standard
new effects some examples --
CDF search for 4th generation b quark b b
Z Mass limit 199 GeV
CDF search for X t t
DO search for bosonic Higgs H gg
DO search for heavy pointlike magnetic monopoles
(seek diphoton radiation) limit 870 GeV for
J1/2
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Searches for things not necessarily wanted
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Large ET leptons observed with large missing ET
at HERA H1 has a sample of 8 events seen, with
background of 2 events (e.g. W production).
ZEUS observation expected.
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Searches for things we dont know about
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The previous search results were all for some
postulated new particle or phenomenon. How do
we search for things for which there is no model?
A formalism for this has been developed by DO --
SHERLOCK . Applied to exclusive final states
e m ET (0,1,2,3) jets
Steps of algorithm 1. Choose exclusive final
states for each,define d kinematic variables
(e.g. ET , S pT (leptons, g,W,Z), S pT (jets).
Do not include topological variables (e.g. mass,
sphericity, as these tend to be dependent on
specific physics model). 2. Make d dimensional
distributions of data and backgrounds,
transforming variables so that background is
uniformly distributed in the unit d-dimensional
hypercube. 3. Define regions R around any set of
N data points (region is that volume closer to
chosen data points than any others) 4.
Calculate probability pNR for background to
fluctuate up to N or greater. Find that
region R for which probability is minimum and
call it pN 5. From an ensemble of Monte Carlo
experiments using known background distributions,
find the fraction of such experiments with
probability lt pN call it PN 6. Find the N for
which PN is minimized P min(PN) 7. Determine
the fraction of MC experiments giving P less than
that observed P . P is the measure of
whether new physics is indicated in the
experiment.
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SHERLOCK study of e m ET ( jets)
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Backgrounds are due to Z/g tt, WW, QCD jet
faking e/m
P distributions with above backgrounds (and MC t
t signal). The background model shows low
probability to account for data, particularly in
2 jets.
1 jet
0 jets
3 jets
2 jets
Using DATA and above backgrounds, the algorithm
identifies the optimum region R for new physics
(e.g. tt ). The probability P for no new physics
is 0.11 (1.2s), indicating top quarks. The 3 tt
events in the conventional analysis are in the
region R chosen. Conventional analysis using
mass and topological variables gave 2.75s excess.
em ET
em ET 1 jet
em ET 2j
em ET 3 jet
Now treating t t as part of background, find
probability that total of known processes explain
the data is P 0.72. No evidence for new
physics !
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Conclusions
The high energy collisions of proton - antiproton
and electron - proton give many opportunities
for observing new physics. Many studies have
been done for new phenomena expected in
Supersymmetry, Strong Coupling models or Large
Extra Dimensions. CDF, DO, H1 and ZEUS have
searched for many other new phenomena, either
based on models, or solely on experimental
signatures. No clear signature for New Physics
yet -- but larger data samples and improved
detectors hold good promise for finding something
before the LHC.