Extending SUSY reach at the CERN Large Hadron Collider using b-tagging
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Brazilian Journal of Physics, vol. 37, no. 2B, June, 2007 549
Extending SUSY reach at the CERN Large Hadron Collider using b-tagging
P. G. Mercadante,
Instituto de Fı́sica Teórica, Universidade Estadual Paulista,
São Paulo, Rua Pamplona 145, 01405-900, SP, Brazil
J. K. Mizukoshi,
Centro de Ciências Naturais e Humanas, Universidade Federal do ABC,
Rua Santa Adélia 166, 09210-170, Santo André, SP, Brazil
and Xerxes Tata
Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822
Received on 20 November, 2006
We analyze the potential of the CERN Large Hadron Collider on the reach of the focus point (FP) region
in the mSUGRA parameter space. This region, consistent with WMAP results, is characterized by multi-TeV
masses for the superpartners of quarks and leptons and relatively light charginos and neutralinos. Moreover,
since the LSP has a substantial higgsino component, it is expected that the gluino decays predominantly to
third generation quarks, producing a final state with multiple hard b jets. Analyzing events with 6ET + n jets +
tagged b-jets, we show that the LHC reach can improve as much as 20% from current projections. Although we
performed the analysis specifically for the FP region, the b-tagging should be important to enhance the SUSY
signal in a variety of models where a relatively light gluino decays mostly to third generation quarks.
Keywords: SUSY; Supersymmetry; Supergravity; LHC; Jets
I. INTRODUCTION In mSUGRA model R−parity is conserved, consequently
the lightest SUSY particle (LSP), which is the lightest neu-
tralino, is stable and therefore a good candidate for the cold
Supersymmetry (SUSY) [1] is one of the most attractive
dark matter (CDM). Recent measurements of the power spec-
models for new physics beyond the Standard Model. From the
trum of the cosmic microwave background (CMB) from the
theoretical viewpoint one of the main motivation is that SUSY
Wilkinson Microwave Anisotropy Probe (WMAP) and other
cures the quadratically divergent mass correction to the scalar
experiments set the physical matter and baryon densities to
particles. Moreover, in its minimum version, the minimal su-
be [3] Ωm h2 = 0.135+0.008 2
−0.009 and Ωb h = 0.0224 ± 0.0009, re-
persymmetric standard model (MSSM), the gauge couplings
spectively, where h is the Hubble constant in units of 100
unify at the GUT scale.
km/s/Mpc. The excess of non-baryonic matter results in
The MSSM model, however, could contain more than a ΩCDM h2 = 0.1126+0.008
−0.009 . The upper limit derived from this
hundred free parameters due to the introduction of the soft is a true constraint on any stable relic from the Big Bang. Re-
SUSY breaking terms, namely masses, complex phases and cently Baer et al. [4] mapped out the parameter space regions
mixing angles, leading to a model without predictive power. in the mSUGRA model preferred by CDM constraint, which
Nevertheles, if we make several assumptions, e.g. grand- can be probed at the LHC, and are consistent with indirect ex-
unification, we get highly predictive models, like the super- perimental constraints, namely rare decays b → sγ, b → s``,¯
gravity (mSUGRA) model [2]. In this specific model, SUSY and Bs → µ+ µ− . Besides the canonical search ETmiss plus mul-
is assumed to be dynamically broken in a “hidden sector” and tijet and multilepton signal [5] at the LHC, the authors of
communicated to a observable sector (with Standard Model Ref. [4] extended their analysis including the channels with
particles and their superpartners) through gravitational inter- isolated photon or with a leptonically decaying Z boson.
actions. The model is completely specified by just a few pa-
rameters, usually defined at some high scale (frequently taken In this talk we analyze one of region of parameter space
to be ∼ MGUT ). At this scale all scalar fields have a com- favored by the CDM constraint, the so-called focus point (FP)
mon SUSY breaking mass m0 , all gauginos have a mass m1/2 , region [6]. In Fig. 1, taken from Ref. [4], for a fixed value
and all soft SUSY breaking scalar trilinear couplings have of tan β, sign(µ) and A0 , this region is a band just above the
a common value A0 . Electroweak symmetry breaking is as- theoretically excluded region on the right-hand side. In this
sumed to occur radiatively. This fixes the magnitude of the portion of parameter space the LSP has a substantial higgsino
superpotential parameter µ. The soft SUSY breaking bilin- component, thus gluino and squarks decay predominantly into
ear Higgs boson mass parameter (B0 ) can be eliminated in third generation quarks, leading to a final state with high b
favor of tan β = vu /vd (the ratio of vacuum expectation value jet multiplicity. In this scenario, an efficient b-tagging may
of Higgs fields), so that the model is completely specified by improve substantially the discovery reach of supersymmetry
the parameter set: over the canonical search, as we have shown in Ref. [7].
In the next section we show the main features of focus
m0 , m1/2 , A0 , tan β, sign(µ). (1) point, and in the section III we present our main results and550 P. G. Mercadante, J. K. Mizukoshi, and Xerxes Tata
conclusion. We encourage the reader to look at the Ref. [7] weak scale. It is important to mention that this behavior oc-
for further details and discussions. curs when top quark assumes a value mt ∼ 170–180 GeV, a
range in agreement with its experimental value.
Ζ1 not LSP mSugra with tanβ = 30, A0 = 0, µ > 0
1400
1200
~
1000 ETmiss 1
m1/2 (GeV)
2
800
5
600 3
10
400 20
JHEP06(2003)054
40 2
200 0.1 No REWSB
LEP2
0 1000 2000 3000 4000 5000
m0 (GeV)
10 4
mh=114.1GeV aµSUSYx10 Br(b→sγ)x10
FIG. 2: The RG evolution for m2Hu from Ref. [6]. In a) tan β = 10 and
ΩZ~ h = Br(Bs→µ µ )x10
2 + - 7
1
0.094 0.129 1.0 b) tan β = 50 for several values of m0 (in units of GeV), m1/2 = 300
GeV, A0 = 0, and mt = 174 GeV.
Figure 8: The same as figure 6 but for tan β = 30.
FIG. 1: Neutralino relic density constraint on the mSUGRA parame-
region.ter
Inspace for tan
particular, the β = 30,
LHC shouldA0 be
= able
0 and to µ > 0the
cover along
stau with LHC maximal
co-annihilation corridor for The focus point in the m2Hu RGE implies that cm0 in Eq. 3
reach
tan β = and it
30, since contours
yields a of
relicseveral
densitylow
Ω Ze1 henergy
2
observables
< 0.129 for m1/2 . obtained fromwhile is small. Therefore the region of parameter space with m0
1050 GeV [53],
Ref.reach
the LHC [4].for low m0 extends to m1/2 ∼ 1400 GeV. larger than a few TeV is as natural as regions with m0 . 1
Figure 9 illustrates the impact of the low energy observables, this time for tan β = 45 TeV. Moreover, for large tan β, µ becomes nearly a constant at
and µ < 0. The LHC reach is similar to the lower tan β cases, but now the low m 0 and
weak scale and order of |mHu | ∼ mZ , provided that the higher
m1/2 bulk region is firmly excluded by both a µ and BF (b → sγ), which are large negative,
and large positive, respectively. We see, however, that a new region of low relic density
order corrections do not spoil Eq. 2. In Fig. 1 this region is
has opened the Ze1 Ze1 POINT
II.up:FOCUS → f f¯ annihilation corridor, AT
→ A, H PHENOMENOLOGY whereTHE LHC takes
annihilation a band immersed in a portion of the parameter space favored
place especially through the very broad A width [54, 55]. In fact, we see that although the by the CDM constraint, just above the theoretically excluded
A annihilation corridor extends to very large m 1/2 values, the region allowed by WMAP is region on the right-hand side.
In MSSM, the tree searches
level Z(seeboson mass at weakA scale
modestgiven
almost entirely accessible to LHC also Ellis et al.,eoss). additional
In the FP region Ze1 , Ze2 , W
e1 are higgsino like, therefore they
by luminosity beyond 100 fb−1 assumed in this study should cover this entire region.
integrated
In addition, the stau co-annihilation corridor remains consistent with WMAP constraints are relatively light and they will be abundantly produced at
the LHC. Since these particles are nearly degenerated, the de-
up to m1/2 values as high as 1200
1 explore − mas2Hushown
m2H GeV tan2 in
β frame2 c) of figure 7, so that LHC should
be able to completelym2Z = thisd region. −
Finally, the µ ∼
HB/FP −m 2 can again
Hu − µ
region 2
(2)
be explored cay product of Ze2 and W e1 will be rather soft leptons, which
up to m1/2 ∼ 7002 GeV at the LHC. tan2 β − 1 are difficult to be separated from the background. In this sce-
In figure 10, we show the same low energy contours in the m 0 vs. m1/2 plane, but
is tan
obtained byµ minimizing thetheHiggs nario, the SUSY signal at the LHC could come from gluino
now for β = 52 and > 0. In this case, low m 0potential. Inregion
and m1/2 bulk particu-
is largely
lar because
excluded the last relation
BF (b → sγ)Brazilian Journal of Physics, vol. 37, no. 2B, June, 2007 551
Rather than perform time consuming scans over the WMAP of W, Z bosons and their subsequent decays to neutrinos, and
favored FP regions of the m0 − m1/2 planes of the mSUGRA from energy mismeasurement.
model for several values of tan β, we have chosen three diverse In Fig. 3 we show the normalized cross sections for a typi-
model lines for our analysis. For each of these model lines, we cal signal, with gluino mass around 1700 GeV, and the back-
take µ > 0 (the sign favored by the result of experiment E821 ground events as a function of the number of jets and b-jets.
at Brookhaven [12]), A0 = 0, and We have used the set of cuts
• m1/2 = 0.295m0 −477.5 GeV with tan β = 30 for Model Nj ≥ 4 ,
Line 1, ETmiss > 400 GeV ,
j1 j2 j3 j4
• m1/2 = 0.295m0 − 401.25 GeV with tan β = 30 for (ET , ET , ET , ET ) > (400, 250, 150, 100)(GeV) ,
Model Line 2, and meff > 1500 GeV ,
• m1/2 = 17
60 m0 −390 GeV with tan β = 52 for Model Line where meff is the scalar sum of the ET of the four hardest jets
3. and ETmiss , and we have obtained the following cross sections
For values of m0 & 1500 GeV, these model lines all lie in the σ(signal) = 1.1 fb ,
WMAP allowed FP region of the mSUGRA parameter space
σ(t t¯) = 5.17 fb ,
delineated in Ref. [4]. The first two model lines have an in-
termediate value of tan β with Model Line 1 being deep in the σ(W + jets) = 13.6 fb ,
FP region while Model Line 2 closer to the periphery of the σ(Z + jets) = 5.68 fb ,
corresponding WMAP region. We choose Model Line 3 again σ(QCD) = 27.7 fb .
deep in the FP region, but with a very large value of tan β to
examine any effects from a very large bottom quark Yukawa For m(g̃) = 1700 GeV, the cross section for the gluino pair
coupling. We take mt = 175 GeV throughout this analysis. production at the LHC is about 2.3 fb. This should be com-
We have checked that for gluino with a mass ∼ 1650 GeV, pared with σ = 1.1 fb we get for the n jets + ETmiss channel,
which is close to the limit that can be probed at the LHC via with n ≥ 4. If we take into account the reconstruction effi-
the usual multijet + multilepton +ETmiss analyses, its branching ciencies, we see that the set of cuts above is rather soft for the
ratio of decay into third generation is around 90%, and around signal, although it reduces significantly the SM backgrounds,
35% of this ratio are due to the decay g̃ → W e −t b̄ + W
e +t¯b. which at this stage it will be dominated by V + jets and QCD
1 1
Usually the decay patterns depend on both tan β and the value events. This is quite encouraging, since these processes usu-
of µ/M1 ( i.e. on how deep we are in the HB/FP region), how- ally have low b-jet multiplicity, if compared with the signal,
ever the total branching fraction for decays to third generation as one can see from Fig. 3b). Besides kinematic variables
quarks is relatively insensitive to these details. Motivated by above, there are others that can help to discriminate the signal
these observations we begin our examination of the inclusive from the backgrounds, namely ST , the transverse sphericity,
b-jet signal for each of the model lines introduced above. ∆φ, the azimuthal angle between ETmiss and the hardest jet, and
∆φb , the azimuthal opening angle between the two hardest b
jets in events with Nb ≥ 2.
III. RESULTS AND CONCLUSION To quantify the improvement b-tagging makes to the capa-
bilities of the LHC for the detection of SUSY, we have com-
In our analysis we use the program ISAJET 7.69 [13] puted the reach of the LHC for each of the three model lines
to simulate the signal and background events, and a toy introduced above. Towards this end, for every mSUGRA pa-
calorimeter to reconstruct jets and leptons, as described in rameter point that we examined, we generated a set of SUSY
Ref. [9]. Jets events using ISAJET. We also generated large samples of SM
p are found using a cone algorithm with a cone background events. We passed these events through the toy
size ∆R = ∆η2 + ∆φ2 = 0.7. Clusters with ET > 40 GeV
calorimeter simulation mentioned previously, and then ana-
and |η(jet)| < 3 are defined to be jets. Muons (electrons) are
lyzed both the signal and the background for the entire set of
classified as isolated if they have ET > 10 GeV (20 GeV) and
cuts (5 × 5 × 6 × 5 × 34 = 60750 choices in all), given by
visible activity in a cone with ∆R = 0.3 about the lepton direc-
tion smaller than ET < 5 GeV. We identify a hadronic cluster i) N j ≥ 4, 5, 6, 7, 8 ;
with ET ≥ 40 GeV and |η( j)| < 1.5 as a b jet if it also has a B
hadron, with pT (B) > 15 GeV and |η(B)| < 3, within a cone ii) ETmiss (GeV) > 400, 500, 600, 700, 800 ;
with ∆R = 0.5 of the jet axis. We take the tagging efficiency j1 j2 j3 j4
εb = 0.5, and assume that gluon and other quark jets can be iii) (ET , ET , ET , ET ) (GeV) > (400, 250, 150, 100),
rejected as b jets by a factor Rb = 150 (50) if ET < 100 GeV (400, 250, 175, 125), (400, 250, 200, 150), (500, 350,
(ET > 250 GeV) and a linear interpolation in between [14]. 150, 100), (500, 350, 175, 125), (500, 350, 200, 150) ;
The major SM backgrounds to the channel n jets +ETmiss ji
iv) meff (GeV) > [ETmiss + ∑4i=1 ET ]min + 200 × n, n =
signal come from V + jet, V = W ± , Z, t t¯, and QCD produc- 0, 1, 2, 3, 4 ;
tions of light quarks and gluons. In the last case, the ETmiss
arises from neutrinos from c and b quarks, from showering v) ST > 0.0, 0.1, 0.2 ;552 P. G. Mercadante, J. K. Mizukoshi, and Xerxes Tata
0.35
dσ/d N (arbitrary units)
0.3
a) b)
0.25
0.2
0.15
0.1
0.05
0
2 4 6 8 10 12 0 2 4 6 8
N (jets) N (b-jet)
FIG. 3: Normalized cross section distributions for the signal (solid line), t t¯ (dashed), W and Z+jets (dotted), and QCD (dash-dotted).
vi) ∆φ < 180◦ , 160◦ , 140◦ ; luminosity of 100-300 fb−1 and that b-tagging with an effi-
ciency of ∼ 50% remains possible even in the high luminosity
vii) ∆φb < 180◦ , 160◦ , 140◦ ; environment.
viii) Nb ≥ 0, 1, 2 . A few remarks appear to be in order at this point:
Notice that the cuts above are much harder than those used • The statistical significance in Fig. 4 is not significantly
in analysis of the LHC SUSY signal [4]. This is because, improved if the b-tagging efficiency improves to 60%.
unlike in Ref. [4] where the cuts were designed to extract the The reason is that before tagging the signal typically
signal for a wide range of squark and gluino masses, here we contains (on average) 3-4 b quark jets while the back-
focus on the optimization of the signal in the portion of the FP ground typically contains (at most) just two b quark
region with heavy gluinos where the previous strategy fails. jets. As a result, the increased efficiency enhances the
In our analysis, we consider a signal to be observable with b-tagged background more than the signal, and the sta-
a given integrated luminosity if, tistical significance is essentially unchanged.
√ • We have also checked that with a b-tagging efficiency
a) statistical significance, NS / NB ≥ 5 ;
of 50% and an integrated luminosity of 100 fb−1 , the
b) NS /NB ≥ 0.25 ; signal with ≥ 3 tagged b-jets is rate limited and no in-
crease in the reach is obtained from that shown in the
c) NS ≥ 10 . Figure.
Our results for the LHC reach are shown in Fig. 4, where
• The search strategy proposed here does not use lepton
we plot the largest statistical significance of the signal as we
information at all. We have checked that a transverse
run over the cuts i) to viii) for a) Model Line 1, b) Model
mass cut MT (`, ETmiss ) & 100 GeV on events with at
Line 2, and c) Model Line 3. The solid curves, from low-
least one isolated lepton does not increase the signifi-
est to highest, denote this maximum statistical significance
cance of the signal because the fraction of signal events
without any b tagging requirement, requiring ≥ 1 tagged b-
(after our cuts) with an isolated lepton is not especially
jet, and ≥ 2 tagged b-jets respectively, for an integrated lu-
large.
minosity of 100 fb−1 , while the dotted curves show the cor-
responding results for 300 fb−1 of integrated luminosity that • Although we have not shown this explicitly, we have
may be expected from three years of LHC operation with checked that requiring the presence of additional iso-
the high design luminosity. While, without b-tagging, our lated leptons does not lead to an increase in the reach
reach for gluinos is ∼ 200 GeV smaller than earlier projec- relative to the ≥ 2b channel.
tions [4, 10, 11], we believe that the difference is due to the
SM background estimation. While we use ISAJET, earlier In summary, we have shown that in the FP region of the
works use the code PYTHIA, which does not include shower- mSUGRA model the reach of the LHC as measured in terms
ing of W and Z bosons in QCD events. This difference, how- of gluino masses may be increased by 15-20% by requiring
ever, will not change the main conclusion of this work, that the presence of hard, tagged b-jets in SUSY events. While we
b-tagging will improve the mass reach of gluinos by 15-20%, were mainly motivated in our investigation by the fact that this
provided that LHC experiments can accumulate an integrated part of parameter space is one of the regions compatible withBrazilian Journal of Physics, vol. 37, no. 2B, June, 2007 553
45
Maximum NS / √NB
40 a) b) c)
35 Model Line 1 Model Line 2 Model Line 3
-1
30 100 fb
-1
300 fb
25
20
15
10
5
0
1500 1750 2000 1500 1750 2000 1500 1750 2000
~ ~ ~
m(g) (GeV) m(g) (GeV) m(g) (GeV)
FIG. 4: The maximum value of the statistical significance for the multijet + ETmiss SUSY signal at the LHC as we run over the cuts i) to viii)
for the three model lines introduced in the text, requiring in addition that NS ≥ 10 events and that NS ≥ 0.25NB . The solid lines from bottom
to top denote this statistical significance without any b tagging requirement, with at least one tagged b-jet, and with at least two tagged b-jets
respectively, assuming an integrated luminosity of 100 fb−1 and a tagging efficiency of 50%. The dotted lines show the corresponding result
for an integrated luminosity of 300 fb−1 .
the WMAP data, our considerations may have wider applica- ultimately have good b-tagging capability, we urge that it be
bility. This is in part because the large top Yukawa coupling utilized to maximize the SUSY reach of the LHC.
and, if tan β is large, also the bottom quark Yukawa coupling,
cause the third generation squarks to be lighter than their
siblings in the first two generations, and in part because of
new contributions to gluino decay amplitudes from these large
Acknowledgments
Yukawa couplings [15]. In particular, we may expect that b-
tagging can be applied to models with an inverted squark mass
hierarchy [16], in models with unification of Yukawa cou- This research was supported in part by the U. S. Department
plings, and possibly also in models with non-universal Higgs of Energy under contract number DE-FG-03-94ER40833 and
mass parameters that have recently been re-examined in light by Fundação de Amparo à Pesquisa do Estado de São Paulo
of the WMAP data [17]. (FAPESP). JKM thanks Instituto de Fı́sica da Universidade de
Since the CMS and ATLAS experiments are expected to São Paulo, where part of research was conducted.
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