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Physics Letters B

www.elsevier.com/locate/physletb

Light stop from b-T Yukawa unification

Ilia Gogoladze *1, Shabbar Raza2, Qaisar Shafi

Bartol Research Institute, Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA

ARTICLE INFO

Article history:

Received 31 October 2011

Received in revised form 11 November 2011

Accepted 14 November 2011

Available online 17 November 2011

Editor: A. Ringwald

ABSTRACT

We show that b-T Yukawa unification can be successfully implemented in the constrained minimal supersymmetric model and it yields the stop co-annihilation scenario. The lightest supersymmetric particle is a bino-like dark matter neutralino, which is accompanied by a 10-20% heavier stop of mass ~ 100-330 GeV. We highlight some benchmark points which show a gluino with mass ~ 0.6-1.7 TeV, while the first two family squarks and all sleptons have masses in the multi-TeV range.

© 2011 Elsevier B.V. All rights reserved.

The apparent unification at Mg ^ 2 x 1016 GeV of the three Standard Model (SM) gauge couplings, assuming TeV scale super-symmetry (SUSY), strongly suggests the existence of an underlying grand unified theory with a single coupling constant. The minimal supersymmetric SU(5) model, in addition to unifying the gauge couplings, also predicts unification at MG of the third family bottom (b) quark and tau lepton (t) Yukawa couplings [1]. This b-T Yukawa unification (YU) is to be contrasted with the minimal supersymmetric S0(10) and SU(4)c x SU(2)L x SU(2)R models which predict t-b-T YU [2], where t denotes the top quark. The low energy implications of b-T [3] and t-b-T [4] YU have been discussed in the recent literature. For instance, t-b-T YU is not realized in the mSUGRA/constrained minimal supersymmetric standard model (CMSSM) [5] because of the difficulty of implementing radiative electroweak symmetry breaking (REWSB) [6].

In this Letter we explore the low energy consequences of implementing b-T YU in the CMSSM framework. REWSB in this case is not an issue anymore. We refer to this combination of b-T YU and CMSSM as YCMSSM, the 'Yukawa' constrained version of CMSSM. Among other things, we require that YCMSSM delivers a viable cold dark matter (DM) candidate (lightest stable neutralino) whose relic energy density is compatible with the WMAP measured value [7]. One of our main observations is that the allowed fundamental parameter space of CMSSM is strikingly reduced in the YCMSSM setup. We find that M1/2 ^m0, where M1/2 and m0 denote universal gaugino and scalar soft SUSY breaking masses respectively. Furthermore, b-T YU at the level of 10% or better yields the con-

* Corresponding author.

E-mail addresses: ilia@bartol.udel.edu (I. Gogoladze), shabbar@udel.edu (S. Raza), shah@bartol.udel.edu (Q. Shafi).

1 On leave of absence from: Andronikashvili Institute of Physics, 0177 Tbilisi, Georgia.

2 On study leave from: Department of Physics, FUUAST, Islamabad, Pakistan.

0370-2693/$ - see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2011.11.026

straint 5 TeV < m0 < 20 TeV. The supersymmetric threshold corrections including finite loop corrections to the b quark mass play an essential role here [8].

The lightest supersymmetric particle (LSP) neutralino is essentially a bino, the spin 1/2 supersymmetric partner of the U(1)y gauge boson, which is closely followed in mass by a slightly heavier (next to lightest sparticle for short NLSP) stop, a scalar partner of the top quark. The desired LSP relic abundance is achieved via neutralino stop co-annihilation [9], which in our case requires that the NLSP stop is about 10-20% heavier than the neutralino. The parameter tan p, the ratio of the up and down Higgs VEVs, turns out to lie in a narrow range 35 < tanp < 40. The universal trilinear scalar coupling (A0) is found to satisfy |A0/m0| ~2.3.

We highlight some LHC testable benchmark points with comparable LSP neutralino and light stop masses of around 100-330 GeV, while the corresponding chargino and second neutralino masses are 200-600 GeV and gluino mass ~ 0.6-1.7 TeV. Together with the lightest SM-like Higgs with mass 114-124 GeV, these are the only 'light' (LHC accessible) particles predicted in this NLSP stop scenario with b-T Yukawa unification and neutralino DM. The squarks of the first two families, the heavy stop, the two bottom particles, and the charged sleptons, all have large (multi-TeV) masses.

The fundamental parameters of CMSSM are

m0, M1/2, tanp, A0, sgn(^) (1)

where sgn(^) is the sign of supersymmetric bilinear Higgs parameter. All mass parameters are specified at MG.

We use the ISAJET 7.80 package [10] to perform random scans over the CMSSM parameter space. ISAJET employs two-loop renor-malization group equations (RGEs) and defines MG to be the scale at which gi = g2. This is more than adequate as a few percent deviation from the exact unification condition g3 = g1 = g2 can

Fig. 1. Evolution of bottom (green) and t (red) Yukawa couplings without (a) and with (b) finite SUSY threshold corrections. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)

be assigned to unknown GUT scale threshold corrections [11]. The random scans cover the following parameter range:

0 < mo < 25 TeV, 0 < M1/2 < 2TeV, 1.1 < tan p < 60, -3 < A0/m0 < 3,

with / > 0 and mt = 173.3 GeV [12]. The results are not too sensitive to one or two sigma variation in the value of mt [13].

In scanning the parameter space, we employ the Metropolis-Hastings algorithm as described in [14]. All of the collected data points satisfy the requirement of REWSB, with the neutralino in each case being the LSP. We direct the Metropolis-Hastings algorithm to search for solutions with 10% or better b-T Yukawa unification (YU). After collecting the data, we impose the experimental mass bounds on all particles [15], and use the IsaTools package [16] to implement the following phenomenological constraints:

mh (lightest Higgs mass) > 114.4 GeV [17],

BR(Bs ^ < 5.8 x 10-8 [18],

2.85 x 10-4 < BR(b ^ sy) < 4.24 x 10-4(2ct) [19],

0.15 ^ BR(Bu ^ TVt)mssm ^ 2.41 (3^) [20],

BR(Bu ^ tvt)sm QcoMh2 = 0.111+0.018(5^) [7].

As far as muon anomalous magnetic moment is concerned, we only require that the model does no worse than the standard model (SM). In Fig. 1(a) we show the evolution of b and T Yukawa couplings without the SUSY threshold corrections for a representative b-T YU solution. It is evident that without suitable SUSY threshold corrections, b-T YU occurs at around 108 GeV. In Fig. 1(b) we show the need for SUSY threshold corrections in order to achieve b-T YU.

The SUSY correction 5mT to the t lepton mass is given by SmT = v cos P5yr. The finite and logarithmic corrections [21] to the t lepton mass are typically small, so that the value of yT at MG is more or less fixed. From Fig. 1(a), to implement yT = yb at MG therefore requires suitable threshold correction (5yb) to the bottom quark [8]. The dominant contribution to 5yb comes from the gluino and chargino loops [8,21]. With our sign conventions (the sign of 5yb is fixed as yb evolves from MG to MZ), a useful approximate formula for the finite one loop correction to 5yb is

given by

o,,finite ,

S yb '

3 m§ + 4 p.

3 ml 8 m2,

Here g3 is the strong gauge coupling, m~g is the gluino mass, At is the stop trilinear coupling, and m\ & (mb + m^)/2, m2 &

(m~t2 + /)/2. b 1, b2 denote the two bottom squarks, f2 is the heavier stop, and we assume that mg ^ m^ ~b2 and mt~1 ^ m^.

In order to achieve b-T YU, yb must receive a negative contribution (-0.2 < 5yb/yb ^ -0.07) from threshold corrections. This is a relatively narrow interval compared to the full range -0.2 < 5 yb/yb < 0.25 in the data that we have collected. The logarithmic corrections to yb are in fact, positive, which leaves the finite corrections to provide for the correct 5yb. Since / > 0, the gluino contribution is positive, and so the contribution from the chargino loop not only has to cancel out the contributions from the gluino loop and the logarithmic correction, it also must provide the correct overall (negative) contribution to 5yb. This is achieved only for suitable large m0 values and large negative At, for which the gluino contribution scales as M1/2/m2 while the chargino contribution scales as At/m0. Note that large values of m0 imply heavy slepton masses, and so b-T YU does not provide any significant SUSY contributions to the muon anomalous magnetic moment.

In order to quantify b-T YU, we define the quantity RbT as

RbT = . (4)

min(yb, yT)

In Fig. 2 we present our results in the RbT-m0, RbT-tan/), M1/2-m0 and RbT-/ planes. In panels (a), (b), (d) the gray points are consistent with REWSB and LSP. The orange points satisfy the particle mass bounds, constraints from BR(Bs ^ BR(Bu ^ tvt) and BR(b ^ sy). The blue points form a subset of orange points that satisfies WMAP bounds on X4 DM abundance. The dashed (red) line represents 10% or better b-T YU. In the M1/2-m0 plane the orange color has the same meaning as described earlier, red color represents solutions with 10% or better b-T YU, and the blue points satisfy in addition the WMAP bounds. In the RbT-m0 plane of Fig. 2, we show that in order to have 10% or better b-T YU consistent with the collider bounds, we require m0 > 5 TeV. But if b-T YU is to be compatible with neutralino DM relic density, represented by the blue points in the figure, we can see that m0 > 8 TeV. To better appreciate this result we consider the M1/2 — m0 plane. As we will see later, the neutralino-stop co-annihilation channel is the only solution we have found for neutralino DM compatible with b-T YU. The lighter stop mass

Fig. 2. Plots in RbT-mo, RbT-tanp, M1/2-mo and RbTplanes. In panels (a), (b), (d) the gray points are consistent with REWSB and x° LSP. The orange points satisfy in addition the particle mass bounds, constraints from BR(Bs ^ //+ ), BR(Bu ^ tvt) and BR(b ^ sy). The blue points form a subset of orange points that satisfies WMAP bounds on X0 DM abundance. The dashed (red) line represents 10% or better b-T YU. In M1/2-m0 plane the orange color has the same meaning as described above, red color represents the solution with 10% or better b-T YU, and blue points satisfy the WMAP bounds. (For interpretation of the references to color, the reader is referred to the web version of this Letter.)

can be as light as 100 GeV [15], which means that the neutralino mass has to be of comparable order, in order to implement the stop co-annihilation solution. An ISAJET two loops analysis yields M1/2 > 150 GeV. We can see from Fig. 2(c) that M1/2 > 150 GeV is compatible with 10% or better YU (points in red) for m0 > 8 TeV.

In the RbT-tanp plane (Fig. 2(b)) we have two regions with 10% or better b-T YU. In one region tanp & 1.1, but this is ruled out by the light CP-even higgs mass bound. The second region with successful b-T YU occurs for 34 < tanp < 45. Beyond this range we do not have REWSB if we require 10% or better b-T YU. Imposing, in addition, the neutralino DM requirement (represented by blue points), the allowed region shrinks to 35 < tanp < 40.

The plot in RbTplane shows that for 10% or better YU, ¡ > 3 TeV (orange points). The need for large values of ¡ can be understood by analyzing the threshold effects associated with the b quark. As previously mentioned, the second term in Eq. (3) has to be larger than the first one for successful b-T YU. With tanp squeezed in a relatively narrow interval, and the magnitude of At bounded to avoid color and charge breaking, needs a large ¡ term to obtain the required 8yb. On the other hand, a bino-Higgsino mixed DM scenario requires mx0 & \i\. From Fig. 2(c), the neu-tralino cannot be much heavier than 600 GeV or so for successful b-T YU. But with ¡i > 4 TeV that a bino-Higgsino mixed DM scenario is not realized here.

Next let us consider the A-funnel scenario of CMSSM where one needs mA & 2m^ 0. For large tan p, m2A = 2b/ sin(2p) & b tan p, where b is the soft SUSY breaking Higgs bilinear term. We have

b tanp & 4 TeV for good b-T unification, which implies that mA ^ 2m~0 and thus, the A-funnel scenario is not consistent with 10% or better b-T unification.

The neutralino-stop co-annihilation channel is compatible with 10% or better b-T unification, as shown by the blue points in Fig. 2. Let see how a light stop mass (m^) of O (100) GeV, is realized for m0 > 8 TeV and Mi/2 < 600 GeV. In this region of the parameter space the diagonal entries of the stop mass matrix to pick up dominant contribution from yt and At couplings, thus making the m~tR entry lighter than its value at MG. In Fig. 3(a) we see that the ratio mt~R /m0 can be as small as 0.05 for large values of A0, which means that at the SUSY scale mtR can be as light as 1 TeV or so, despite the large ~ (8-20) TeV, m0 values. On the other hand, from Fig. 3(b) we see that at SUSY scale, the value of \ At\ can be O (7-20) TeV. The off-diagonal entries -mt(At + ¡ cot p) for the stop quark mass matrix can therefore be of comparable magnitude to the diagonal entries. Because of this, as seen in Fig. 3(b), one of the eigenvalues (m~t1) of the stop quark mass matrix can be 10-20 times smaller than m~tR at the SUSY scale. Thus, with m~t1 O (100) GeV, we can have neutralino-stop co-annihilation compatible with b-T YU. A similar discussion for the stau mass matrix shows that the stau co-annihilation scenario is not realized in YCMSSM.

In Table 1 we present three characteristic benchmark points which satisfy all, especially the dark matter, constraints. Point 1 displays essentially perfect b-T YU solution with RbT = 1, point 2 represents a solution with a relatively light stop mass (~ 114 GeV),

Fig.3. Plots in m-tR/mu — A0 and mf]/m-tR — At planes. The orange points satisfy the particle mass bounds, constraints from BR(Bs ^ BR(Bu ^ tvt) and BR(b ^ sy).

Red color represents the solution with 10% or better b-T YU. Blue color represents the neutralino-stop co-annihilation solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)

Table 1

Point 1 has perfect b-T unification with RbT = 1, point 2 represents a solution with relatively light stop mass 114 GeV), and point 3 represents a solution with a heavier stop mass 330 GeV). The remaining squarks and sleptons all have masses in the multi-TeV range.

Point 1 Point 2 Point 3

m0 15220 10 040 17920

M1/2 177 152 521

tan p 37 39 37

Ac/ma -2.36 -2.32 -2.33

sgn(^) + 1 +1 +1

mh 115 120 115

mH 6036 4566 9752

mA 5997 4537 9688

mH± 6037 4568 9753

mx 0 124, 272 97, 209 290, 592

mx 0 10379, 10379 6836, 6836 12347, 12347

mx ± 275, 10406 211, 6840 598, 1239

mg 796 640 1680

mSL,R 15170, 15214 10 000, 10 030 17892, 17 942

mU,2 153, 5930 114, 4076 328, 7894

m dL,R 15170, 15222 10 000, 10 036 17892, 17 951

m b1,2 6060, 8357 4152, 5752 8097, 11159

m„1 15223 10 041 17929

mt3 12 744 8453 15082

miL,R 15211, 15208 10 032, 10 032 17911, 17909

mT1,2 9843, 12 771 6619, 8474 11801, 15130

°SI (pb) 3.28 x 10-12 5.85 x 10-12 7.93 x 10-13

&sd (pb) 3.90 x 10-12 2.39 x 10-11 1.77 x 10-12

^CDMh2 0.11 0.09 0.1

RbT 1.00 1.02 1.09

and finally point 3 represents a solution with a heavier stop mass (~ 330 GeV). We note that the remaining squarks and sleptons all have masses in the multi-TeV range.

Since the LSP is essentially a pure bino, both its spin-independent and spin-dependent cross sections on nucleons are rather small [22], ~ 10-47-10-48 cm2. Consequently it wont be easy to detect the LSP in direct and indirect experiments.

In summary, we have investigated b-T Yukawa unification in the mSUGRA/CMSSM framework and find that it is consistent with the NLSP stop scenario and yields the desired LSP neutralino relic abundance. This YCMSSM predicts that there are just two 'light' (LHC accessible) colored sparticles, namely the NLSP stop with mass ~ 100-330 GeV, and the gluino which is ~ 600-1700 GeV.

The chargino and a second neutralino are about a factor 2-3 lighter than the gluino. The remaining squarks as well as all sleptons have masses in the multi-TeV range. Regarding the fundamental CMSSM parameters, we find that 5 TeV < m0 < 20 TeV, m0/M1/2 ^ 30-50, tanp ^ 35-40, ~ 3-15 TeV and |A0/m0|~ 2.2-2.4.

Acknowledgements

We thank Rizwan Khalid, Azar Mustafayev and Mansoor Ur Rehman for valuable discussions. This work is supported in part by the DOE Grant No. DE-FG02-91ER40626 (I.G., S.R. and Q.S.) and GNSF Grant No. 07_462_4-270 (I.G.).

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